Process for preparing metal-organic frameworks, metal-organic frameworks, and uses thereof

Abstract

A process for preparing a mixed-metal metal-organic framework. The process comprises providing a hydrogen bonded metal organic framework (HM1OF); soaking the hydrogen bonded metal organic framework (HM1OF) in a metal (M2)-comprising solution; and forming a mixed-metal MOF (M1OF-M2), where M1 and M2 are different metals.

Description

PROCESS FOR PREPARING METAL-ORGANIC FRAMEWORKS, METAL-ORGANIC FRAMEWORKS, AND USES THEREOF FIELD 5

[0001] The present disclosure relates generally to metal organic frameworks, for example mixed-metal metal-organic frameworks. BACKGROUND

[0002] A class of coordination polymers are metal-organic frameworks (MOFs). By 10 the IUPAC definition, MOFs are coordination networks containing organic ligands and metal nodes or clusters that are potentially porous [Batten, S. R.; Champness, N. R.; Chen, X.-M.; Garcia-Martinez, J.; Kitagawa, S.; Öhrström, L.; O’Keeffe, M.; Suh, M. P.; Reedijk, J. Terminology of Metal-Organic Frameworks and Coordination Polymers (IPUAC Recommendations 2013). Pure Appl. Chem. 2013, 85 (8), 1715–1724. 15 https: / / doi.org / 10.1007 / 978-1-4684-0904-8_6.]. Large variability in choice of the ligand and metal allows for rational design of frameworks, giving MOFs the potential to be tailored for many applications. Porosity, although not a requirement, is one of the hallmarks of MOF materials. MOFs can be highly ordered owing to the reversibility of the metal-ligand coordination bonds but single crystallinity is not required. To exploit the stability and further20 improve their properties, a secondary metal ion can be incorporated into chromium(III)- phosphonate MOFs that can lead to improved gas separation performances, catalytic activity, sensing ability, magnetic properties, photoactive properties, without sacrificing framework stability. 25 SUMMARY

[0003] In one or more embodiments as described herein, there is provided: 1. A process for preparing a mixed-metal metal-organic framework, the process comprising: providing a hydrogen bonded metal organic framework (HM1OF); 30 soaking the hydrogen bonded metal organic framework (HM1OF) in a metal (M2)- comprising solution; and forming a mixed-metal MOF (M1OF-M2), where M1and M2are different metals. 2. The process of embodiment 1, wherein soaking the hydrogen bonded metal organic 35 framework (HM1OF) in a metal (M2)-comprising solution comprises:- 1 -soaking the hydrogen bonded metal organic framework (HM1OF) in the metal (M2)- comprising solution; and forming a mixed-metal hydrogen bonded metal organic framework (HM1OF-M2). 5 3. The process of any previous embodiment, wherein soaking the HM1OF in the metal (M2)-comprising solution comprises transmetallation of M1in the HM1OF with M2of the metal (M2) comprising solution to form the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2). 10 4. The process of any previous embodiment, wherein the transmetallation comprises exchanging between >0% and <100% of M1in the HM1OF with M2of the metal (M2)- comprising solution. 5. The process of any previous embodiment, wherein soaking the HM1OF in the metal 15 (M2)-comprising solution comprises proton exchange between the HM1OF and the M2of the metal (M2) comprising solution to form the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2). 6. The process of any previous embodiment, wherein the proton exchange comprises 20 exchanging between >0% and <100% of protons in the HM1OF with the M2of the metal (M2)-comprising solution. 7. The process of any previous embodiment, wherein soaking the HM1OF in the metal (M2)-comprising solution comprises 25 soaking the HM1OF in a first metal (M2)-comprising solution; and soaking the HM1OF in at least a second metal (M2)-comprising solution; wherein the metal M2of the first solution is different from the metal M2of the second solution. 30 8. The process of any previous embodiment, wherein the metal (M2)-comprising solution comprises a one or more metal (M2) salts, one or more organometallic (M2) compounds, a hydrate thereof, a solvate thereof, or a combination thereof.- 2 -9. The process of any previous embodiment, wherein soaking the HM1OF in the metal (M2)-comprising solution comprises soaking the HM1OF in a metal (M2)-comprising solution; the metal (M2)-comprising solution comprising two or more metal (M2) salts, two or 5 more organometallic (M2) compounds, hydrates thereof, solvates thereof, or a combination thereof, and the metal (M2) of each metal (M2) salt and / or each organometallic (M2) compound is a different metal (M2). 10 10. The process of any previous embodiment, wherein forming a mixed-metal MOF (M1OF-M2) comprises desolvating the mixed-metal hydrogen bonded metal organic framework (HM1OF- M2) at, or above 50⁰C; at, or above 75⁰C; or at, or above 100⁰C to form the mixed-metal MOF (M1OF-M2). 15 11. The process of any previous embodiment, wherein providing the hydrogen bonded metal organic framework (HM1OF) comprises: providing a hydrogen-bonded organic framework (HOF); combining the hydrogen-bonded organic framework (HOF) with a metal (M1)- 20 comprising solution; and forming the hydrogen bonded metal organic framework (HM1OF). 12. The process of any previous embodiment, wherein the metal (M1) comprises Cr3+, Co2+, Ni2+, Na+, K+, Li+, Mn2+, Fe2+, Ca2+, Mg2+, Sr2+, Ba2+, hydrates thereof, solvates thereof, 25 or a combination thereof. 13. The process of any previous embodiment, wherein the metal (M2) comprises a transition metal or post-transition metal, such as a transition metal or a post-transition metal with a +2 oxidation state or higher; a lanthanide; a metalloid; hydrates thereof, solvates 30 thereof, or a combination thereof. 14. The process of any previous embodiment, wherein the hydrogen bonded metal organic framework (HM1OF) comprises one or more guest molecules for templating pore structure, pore size distribution, surface area, and / or crystallinity of the formed mixed-metal 35 MOF (M1OF-M2).- 3 -15. The process of any previous embodiment, wherein the one or more guest molecules comprises a solvent of the metal (M1)-comprising solution; a solvent of the metal (M2)-comprising solution; and / or 5 water, acetone, acetonitrile, methanol, ethanol, isopropanol, acetic acid, ethylene glycol, ethyl acetate, nitrobenzene, nitromethane, toluene, an ammonium compound, CO2, CO, methane, ethylene, propane, propene, acetylene, H3PO4, H2SO4, HCI, HBr, formic acid, H2CO3, HNO3, ortho-xylene, meta-xylene and para-xylene; or amines, alcohols, ethers, esters, ketones, substituted or unsubstituted aromatics, substituted or unsubstituted 10 polyaromatics; or any salts thereof; or any combination thereof. 16. The process of any previous embodiment, wherein the hydrogen-bonded organic framework (HOF) comprises an organic phosphonate ligand, optionally substituted with one or more alkyl groups, amino groups, halogens, nitro groups, alcohol groups, or a 15 combination thereof. 17. The process of any previous embodiment, wherein the hydrogen bonded metal organic framework (HM1OF) has the structure: {[Cr(H2O)6]2[H2L3]^(CH3OH)2(C3H6O)2}n; {[Cr(H2O)6]2[H2L3]^(H2O)}n; or {[Ni(H2O)6]2[H4L3]^C3H6O^2H2O^CH3OH}n. 20 18. The process of any previous embodiment, wherein mixed-metal MOF (M1OF-M2) has a M2 / M1ratio of: about 0 to about 100. 19. The process of any previous embodiment, wherein mixed-metal MOF (M1OF-M2) 25 has the structure:Cr0.27Cu2H3.19L3^5H2O; Cr0.2 2Cu2.5H2.6L3^4H2O; or Ni0.9Cr1.5H1.6L3^3H2O. 20. A mixed-metal metal-organic framework (M1OF-M2) prepared by the process of any one of embodiments 1-19. 30 21. The mixed-metal metal-organic framework (M1OF-M2) of the previous embodiment, having the structure: [M1]a[M2]b[HxLn]c, wherein 35 M1and M2are different metals,- 4 -a and b is metal content where a and b are between 0 and 6, x is a protonation state of ligand L between 0 and 12, n is a number label given to the ligand, and c is number of ligands per unit cell in the formula between 0 and 6. 5 22. A mixed-metal metal-organic framework (MOF) prepared by the process of any one of embodiments 1-19, useful for adsorption, gas separation, catalysis, sensing, magnetic solids, luminescent solids, fluorescent solids, ion conduction, ion exchange, or a combination thereof. 10 23. A method of uptaking at least one substance into a mixed-metal MOF prepared by the process of any one of embodiments 1-19, the method comprising contacting the mixed- metal MOF with the at least one substance under conditions for uptaking the at least one substance into the mixed-metal MOF. 15 24. The method of embodiment 23, wherein the at least one substance comprises carbon dioxide, nitrogen, hydrogen, a transition metal, a post-transition metal, a lanthanide, a metalloid, or a combination thereof. 20 25. The method of any previous embodiment, wherein uptaking the at least one substance is for the purpose of its storage, absorption, or separation, or a combination thereof. BRIEF DESCRIPTION OF THE FIGURES 25

[0004] Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

[0005] FIG.1 depicts a.) CO2capacity as a function of Ni(II) mol% in MM-MOF-74 and b.) the corresponding surface areas after exposure to 95% relative humidity. The black lines represent the predicted behaviour of the MOF assuming a linear correlation, whereas 30 the red points are experimentally determined results.114

[0006] FIG.2 depicts a.) Crystal structure of H-CrL3. The blue polyhedral represent the chromium(III) hexaaqua clusters, b.) the structure of H-CrL3as seen from the a-axis, c.) the hydrogen bonding (grey dashed lines) motif between a hexaaqua cation and phosphonate ligands, and the O --- O distances in angstroms.- 5 -

[0007] FIG. 3 depicts H-CrL3structure showing the a.) (011ത) hkl planes and b.) PXRD patterns for the HMOF and MOF, with the pink asterisks representing the (011ത) planes in the simulated pattern.

[0008] FIG.4 depicts TGA and DSC of a.) H-CrL3and b.) CrL3. 5

[0009] FIG.5 depicts FTIR of the HMOF, H-CrL3, and MOF, CrL3.

[0010] FIG.6 depicts CO2isotherm for CrL3at 273K. The inset is the calculated NLDFT PSD.

[0011] FIG.7 depicts a.) CO2isotherms at various temperatures for CrL3, and the virial fits for each isotherm. B.) The enthalpy of adsorption for CrL3. 10

[0012] FIG.8 depicts a.) CO2and N2adsorption isotherms for CrL3 at 273K and b.) the IAST selectivity for CO2over N2in a 15 / 85 mixture at 273K.

[0013] FIG.9 depicts a.) PXRD and b.) CO2isotherms at 273K (inset is NLDF PSD) for CrL3and CrL3after soaking in Cu(NO3)2solution for 5 days.

[0014] FIG. 10 depicts a.) A sample EDX spectrum for CrL3-Cu1and b.) the 15 atomic% for each spectrum, along with the averaged Cu / Cr and P / Cu ratio.

[0015] FIG. 11 depicts a.) A sample EDX spectrum for CrL3-Cu2and b.) the atomic% for each spectrum, along with the averaged Cu / Cr and P / Cu ratio.

[0016] FIG.12 depicts EDX mapping of CrL3-Cu2, showing the distribution of P, Cr, and Cu. 20

[0017] FIG.13 depicts PXRD patterns for H-CrL3, CrL3, and the Cu(II) doped MOFs.

[0018] FIG.14 depicts FTIR spectra of the various CrL3compounds.

[0019] FIG.15 depicts kinetic studies showing a.) the amount of Cr(III) and Cu(II) in the material as a function of time and b.) the amount of Cr(III) found in the mother liquor solution over time. 25

[0020] FIG. 16 depicts a.) PXRD monitoring the metal exchange on H-CrL3over time. The asterisk represents the (10-1) hkl planes, shown in b.).

[0021] FIG. 17 depicts the various coordination geometries of Zn(II) in [Zn7(L1)3(H2O)7]n·[Zn5(L1)3(H2O)5]n.273

[0022] FIG.18 depicts PXRD pattern for CrL3-hy compared to H-CrL3and CrL3. 30

[0023] FIG. 19 depicts CO2adsorption isotherms at 273K for CrL3and CrL3-Cu doped MOFs.

[0024] FIG.20 depicts N2isotherm for CrL3-Cu1at 77K, with the NLDT PSD in the inset.- 6 -

[0025] FIG. 21 depicts a.) CO2isotherms for CrL3-Cu2at various temperatures, along with their virial fits and b.) the calculated enthalpy of adsorption as a function of CO2loading.

[0026] FIG.22 depicts a.) CO2and N2isotherms for CrL3-Cu1at 273K and b.) the 5 selectivity of CO2over N2in a 15 / 85 mixture as calculated by IAST.184

[0027] FIG.23 depicts a, c, e) CO2isotherms at 273K for pristine MOF and after stability test, and b, d, f) their respective PXRD patterns.

[0028] FIG.24 depicts PXRD patterns for H-CrL3after soaking in a 0.06M solution of Ni(NO3)2·6H2O for 1, 2, and 3 days. 10

[0029] FIG.25 depicts crystal structure of H-NiL3a.) as seen from the a-axis, b.) the hydrogen bonding (grey dashed lines) motif of the hexaaqua nickel(II) clusters, and c.) the structure of H-NiL3as seen from the b-axis. The hexaaqua nickel(II) clusters are represented by green polyhedral, which hydrogen bond to the phosphonate ligands. The cavities are filled with acetone, water, and methanol molecules. 15

[0030] FIG.26 depicts IR spectra for H-NiL3and NiL3. H-NiL3is significantly more hydrated than NiL3owing to the hexaaquanickel(II) clusters in the HMOF.

[0031] FIG.27 depicts a) crystal structure of the MOF, NiL3, the dehydrated version of H-NiL3, as first reported by Stock et al.267b.) The octahedral Ni(II) centers in the MOF bridged by two phosphonate ligands and six coordinated water molecules. 20

[0032] FIG.28 depicts a.) PXRD patterns for simulated and experimental H-NiL3, NiL3, and for CAU-46, reported by Stock et al.267The (002) plane of the H-NiL3is depicted with a blue asterisk, and the (200) plane for CAU-46 is shown with a pink asterisk. B.) The (002) hkl plane shown in H-NiL3, and c.) the (200) plane for CAU-46.

[0033] FIG.29 depicts PXRD patterns for H-NiL3 after soaking in various solvents. 25

[0034] FIG.30 depicts a) A sample EDX spectrum for NiL3-Cr and b.) the Cr, Ni, and P values.

[0035] FIG.31 depicts EDX mapping for NiL3-Cr, showing Ni, Cr, and P dispersion.

[0036] FIG.32 depicts PXRDs for the various Ni(II) / Cr(III) compounds.

[0037] FIG.33 depicts FTIR of the HMOF, H-NiL3, and all its derivatives. 30

[0038] FIG. 34 depicts CO2isotherm at 273K for NiL3-Cr, with the NLDFT PSD given in the inset.

[0039] FIG.35 depicts a.) CO2isotherms for NiL3-Cr at different temperatures and b.) the calculated enthalpy of adsorption for CO2as a function of loading.

[0040] FIG. 36 depicts a.) CO2and N2isotherms of NiL3-Cr at 273K and b.) the 35 selectivity for CO2over N2in a 15 / 85 mixture.- 7 -

[0041] FIG.37 depicts a.) CO2isotherm at 273K and b.) PXRD patterns for pristine NiL3-Cr and after soaking in water for 5 days.

[0042] FIG. 38 depicts ac crystal structure and formula for chromium(III) HMOF and the structure of the ATP ligand. 5

[0043] FIG.39 depicts powder Xray patterns showing retention of order and subtle changes in Zn exchanged HMOFs / MOFs.

[0044] FIG.40 depicts powder Xray Patterns showing retention of order and subtle changes in Fe exchanged HMOFs / MOFs.

[0045] FIG 41 depicts powder Xray Patterns showing retention of order and 10 changes in Cu exchanged Cr MOF.

[0046] FIG.42 depicts crystal structure and formula for nickel(II) HMOF with the structure of the ATP linker.

[0047] FIG. 43 depicts powder Xray Patterns showing shift in pattern of Cr exchanged Ni MOF (yellow). 15

[0048] FIG. 44. depicts impact on CO2gas sorption (273K) of secondary metal exchange in Cr MOFs.

[0049] FIG. 45 depicts synthesis of the ligand 2,4,6-tri-( phenylene-4-phosphonic acid)-s-triazine (or H6PPT) 20 DETAILED DESCRIPTION

[0050] Unless defined otherwise, all technical and scientific terms used herein have the meaning as commonly understood in the art.

[0051] As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context dictates otherwise. 25

[0052] "Substituted" refers to a compound having one or more substituents, otherwise referred to as functional groups, whose presence does not prevent, hinder, or otherwise interfere with the desired reaction or interaction - but may facilitate, support, or accelerate the desired reaction or interaction. Examples of substituents include alkyl, alkenyl, alkynyl, aryl, heteroaryl, cycloalkyl, Si(alkyl)3, Si(alkoxy)3, alkoxyl, amino, 30 alkylamino, alkenylamino, amide, hydroxyl, thioether, alkylcarbonyl, alkylcarbonyloxy, arylcarbonyloxy, alkoxycarbonyloxy, aryloxycarbonyloxy, carbonate, alkoxycarbonyl, aminocarbonyl, alkylthiocarbonyl, phosphate, phosphate ester, phosphonato, phosphinato, acylamino, imino, sulfhydryl, a!kylthio, arylthio, thiocarboxylate, dithiocarboxylate, sulfate, sulfato, sulfonate, sulfamoyl, sulfonamide, nitro, nitrile, azido, heterocyclyl, ether, ester, 35 silicon-containing moieties, thioester, or a combination thereof.- 8 -

[0053] H-bonded metal-organic phosphonate frameworks (HMOF), comprising highly aquated (otherwise referred to as hydrated) metal ions as building units, may be used as precursors to form metal organic frameworks (MOFs). Upon heating or otherwise 5 desolvating a HMOF, its metal ions dehydrate, or desolvate and form metal phosphonate coordination bonds with functional groups on its organic phosphonate ligand.

[0054] HMOFs tend to be dynamic in structure, in that they can adapt their pores to shrink wrap different molecular guests (otherwise referred to as guest molecules). A "guest molecule” or "template molecule" refers to molecules that fill void spaces (pores) in 10 porous materials; and as a HMOF dehydrates or desolvates around a guest molecule, the guest molecule may determine the pore structure, pore size distribution, surface area, selectivity, crystallinity, and / or enthalpy of adsorption of the MOF that results from the HMOF. For example, upon heating a HMOF in the presence of guest molecules, pores of the HMOF can be imprinted with the structure of the guest. 15

[0055] As HMOFs are based on hydrogen bonded aquo / solvated complexes, it can be possible to introduce at least a second metal into HMOF structure to form a mixed-metal HMOF. This may be accomplished through one of two routes, or a combination of both. The first route may involve exchange of the aquated or solvated first metal ion in the HMOF for a second metal. This may be the result of stronger, direct coordination of the second 20 metal relative to the first, or the formation of stronger H-bonds in the aquated / solvated form of the second metal. The second route may involve the incorporation of a second metal ion by ion exchange with residual protons on the phosphonate ligand of the HMOF, rather than physical exchange with the first metal. This may occur by forming direct coordination bonds with the phosphonate ligand of the HMOF, or by forming new H-bonds with the framework 25 in the aquated HMOF.

[0056] The role of the first metal, if it is completely exchanged, may involve positioning the ligands in space and creating pores into which exchange with the second metal can occur. If the first metal remains in the dehydrated or desolvated structure, its role may involve forming a robust skeletal backbone of the HMOF framework. In examples 30 where the MOF formed from the second metal alone may not have sufficient stability to enable practical use, the robust skeletal backbone of the HMOF framework may otherwise facilitate formation of a MOF comprising this second metal. As the organic phosphonate ligands of HMOFs are already positioned in space by the first metal, the resulting structure of the mixed-metal HMOF may have a higher degree of order, and possibly even a different 35 structure, than if it were assembled directly in the presence of the second metal ion.- 9 -

[0057] Addition of a second metal may be used to impart new functions. The second metal may alter pore size and shape, and may also provide open metal sites for gas molecules to interact. This may impact gas sorption properties (for example, see Figure 7.7). This may also lead to other functions, such as catalysis, imparting of colour or 5 photoemissive properties (for example, for sensing applications), or magnetic properties.

[0058] Generally, the present disclosure describes a process for preparing a mixed- metal metal-organic framework. The present disclosure also generally provides a mixed- metal metal-organic framework. Further, the present disclosure generally provides a 10 method of uptaking at least one substance into a mixed-metal MOF.

[0059] In one or more examples, the present disclosure generally provides: 1. A process for preparing a mixed-metal metal-organic framework, the process comprising: providing a hydrogen bonded metal organic framework (HM1OF); 15 soaking the hydrogen bonded metal organic framework (HM1OF) in a metal (M2)- comprising solution; and forming a mixed-metal MOF (M1OF-M2), where M1and M2are different metals. 2. The process of example 1, wherein soaking the hydrogen bonded metal organic 20 framework (HM1OF) in a metal (M2)-comprising solution comprises: soaking the hydrogen bonded metal organic framework (HM1OF) in the metal (M2)- comprising solution; and forming a mixed-metal hydrogen bonded metal organic framework (HM1OF-M2). 25 3. The process of any previous example, wherein soaking the HM1OF in the metal (M2)-comprising solution comprises transmetallation of M1in the HM1OF with M2of the metal (M2) comprising solution to form the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2). 30 4. The process of any previous example, wherein the transmetallation comprises exchanging between >0% and <100% of M1in the HM1OF with M2of the metal (M2)- comprising solution. 5. The process of any previous example, wherein soaking the HM1OF in the metal 35 (M2)-comprising solution comprises- 10 -proton exchange between the HM1OF and the M2of the metal (M2) comprising solution to form the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2). 6. The process of any previous example, wherein the proton exchange comprises 5 exchanging between >0% and <100% of protons in the HM1OF with the M2of the metal (M2)-comprising solution. 7. The process of any previous example, wherein soaking the HM1OF in the metal (M2)-comprising solution comprises 10 soaking the HM1OF in a first metal (M2)-comprising solution; and soaking the HM1OF in at least a second metal (M2)-comprising solution; wherein the metal M2of the first solution is different from the metal M2of the second solution. 15 8. The process of any previous example, wherein the metal (M2)-comprising solution comprises a one or more metal (M2) salts, one or more organometallic (M2) compounds, a hydrate thereof, a solvate thereof, or a combination thereof. 9. The process of any previous example, wherein soaking the HM1OF in the metal 20 (M2)-comprising solution comprises soaking the HM1OF in a metal (M2)-comprising solution; the metal (M2)-comprising solution comprising two or more metal (M2) salts, two or more organometallic (M2) compounds, hydrates thereof, solvates thereof, or a combination thereof, and 25 the metal (M2) of each metal (M2) salt and / or each organometallic (M2) compound is a different metal (M2). 10. The process of any previous example, wherein forming a mixed-metal MOF (M1OF- M2) comprises 30 desolvating the mixed-metal hydrogen bonded metal organic framework (HM1OF- M2) at, or above 50⁰C; at, or above 75⁰C; or at, or above 100⁰C to form the mixed-metal MOF (M1OF-M2). 11. The process of any previous example, wherein providing the hydrogen bonded 35 metal organic framework (HM1OF) comprises:- 11 -providing a hydrogen-bonded organic framework (HOF); combining the hydrogen-bonded organic framework (HOF) with a metal (M1)- comprising solution; and forming the hydrogen bonded metal organic framework (HM1OF). 5 12. The process of any previous example, wherein the metal (M1) comprises Cr3+, Co2+, Ni2+, Na+, K+, Li+, Mn2+, Fe2+, Ca2+, Mg2+, Sr2+, Ba2+, hydrates thereof, solvates thereof, or a combination thereof. 10 13. The process of any previous example, wherein the metal (M2) comprises a transition metal or post-transition metal, such as a transition metal or a post-transition metal with a +2 oxidation state or higher; a lanthanide; a metalloid; hydrates thereof, solvates thereof, or a combination thereof. 15 14. The process of any previous example, wherein the hydrogen bonded metal organic framework (HM1OF) comprises one or more guest molecules for templating pore structure, pore size distribution, surface area, and / or crystallinity of the formed mixed-metal MOF (M1OF-M2). 20 15. The process of any previous example, wherein the one or more guest molecules comprises a solvent of the metal (M1)-comprising solution; a solvent of the metal (M2)-comprising solution; and / or water, acetone, acetonitrile, methanol, ethanol, isopropanol, acetic acid, ethylene 25 glycol, ethyl acetate, nitrobenzene, nitromethane, toluene, an ammonium compound, CO2, CO, methane, ethylene, propane, propene, acetylene, H3PO4, H2SO4, HCI, HBr, formic acid, H2CO3, HNO3, ortho-xylene, meta-xylene and para-xylene; or amines, alcohols, ethers, esters, ketones, substituted or unsubstituted aromatics, substituted or unsubstituted polyaromatics; or any salts thereof; or any combination thereof. 30 16. The process of any previous example, wherein the hydrogen-bonded organic framework (HOF) comprises an organic phosphonate ligand, optionally substituted with one or more alkyl groups, amino groups, halogens, nitro groups, alcohol groups, or a combination thereof. 35- 12 -17. The process of any previous example, wherein the hydrogen bonded metal organic framework (HM1OF) has the structure: {[Cr(H2O)6]2[H2L3]^(CH3OH)2(C3H6O)2}n; {[Cr(H2O)6]2[H2L3]^(H2O)}n; or {[Ni(H2O)6]2[H4L3]^C3H6O^2H2O^CH3OH}n. 18. The process of any previous example, wherein mixed-metal MOF (M1OF-M2) has a 5 M2 / M1ratio of: about 0 to about 100. 19. The process of any previous example, wherein mixed-metal MOF (M1OF-M2) has the structure:Cr0.27Cu2H3.19L3^5H2O; Cr0.2 2Cu2.5H2.6L3^4H2O; or Ni0.9Cr1.5H1.6L3^3H2O. 10 20. A mixed-metal metal-organic framework (M1OF-M2) prepared by the process of any one of examples 1-19. 21. The mixed-metal metal-organic framework (M1OF-M2) of the previous example, having the structure: 15 [M1]a[M2]b[HxLn]c, wherein M1and M2are different metals, a and b is metal content where a and b are between 0 and 6, 20 x is a protonation state of ligand L between 0 and 12, n is a number label given to the ligand, and c is number of ligands per unit cell in the formula between 0 and 6. 22. A mixed-metal metal-organic framework (MOF) prepared by the process of any one 25 of examples 1-19, useful for adsorption, gas separation, catalysis, sensing, magnetic solids, luminescent solids, fluorescent solids, ion conduction, ion exchange, or a combination thereof. 23. A method of uptaking at least one substance into a mixed-metal MOF prepared by30 the process of any one of examples 1-19, the method comprising contacting the mixed- metal MOF with the at least one substance under conditions for uptaking the at least one substance into the mixed-metal MOF.- 13 -24. The method of example 23, wherein the at least one substance comprises carbon dioxide, nitrogen, hydrogen, a transition metal, a post-transition metal, a lanthanide, a metalloid, or a combination thereof. 5 25. The method of any previous example, wherein uptaking the at least one substance is for the purpose of its storage, absorption, or separation, or a combination thereof.

[0060] Guest Molecule

[0061] A "guest molecule” or "template molecule" refers to molecules that fill void 10 spaces (pores) in porous materials. Guest molecules can be important for the formation of a MOF from a HMOF, because the metal and ligand may dehydrate or desolvate around the guest molecule, such that the guest molecule may determine the pore structure, pore size distribution, surface area, selectivity, crystallinity, and / or enthalpy of adsorption of the resultant MOF. Guest molecules can act as a medium to transfer heat, and can occupy 15 pores as the dehydrating or desolvating MOF structure contracts. The resulting MOF structure may thus be impacted by the size, shape and chemical functionalities of these molecules. A pore may not be precisely templated, as heating rate can also impacts structure. As such, a guest or template molecule may be selected to achieve specific structure and porosity. A guest molecule may be interchangeable. A guest molecule 20 may be neutral. A guest molecule may be charged. A guest molecule may be a solvent molecule. A guest molecule may be water. A guest molecule may be a metal ion, such as copper ions. A guest molecule may be organic molecules, organic solvents, organic liquids, inorganic acids and bases, inorganic complexes, or a combination thereof.

[0062] A guest molecule or template molecule may include a solvent of the herein 25 described metal (M1)-comprising solution. A guest molecule or template molecule may include a solvent of the herein described metal (M2)-comprising solution. A guest molecule or template molecule may include water, acetone, acetonitrile, methanol, ethanol, isopropanol, acetic acid, ethylene glycol, ethyl acetate, nitrobenzene, nitromethane, toluene, an ammonium compound, CO2, CO, methane, ethylene, propane, propene, 30 acetylene, H3PO4, H2SO4, HCI, HBr, formic acid, H2CO3, HNO3, ortho-xylene, meta-xylene and para-xylene; or amines, alcohols, ethers, esters, ketones, substituted or unsubstituted aromatics, substituted or unsubstituted polyaromatics; or any salts thereof; or any combination thereof.

[0063] Metals- 14 -

[0064] As described herein, the process for preparing the mixed-metal MOF (M1OF-M2) comprises a metal (M2)-comprising solution.

[0065] The metal (M2)-comprising solution may comprise a metal (M2) salt, or hydrates thereof, or solvates thereof, or a combination thereof. The metal (M2)-comprising 5 solution may comprise one or more metal (M2) salts, or hydrates thereof, or solvates thereof, or a combination thereof. When the metal (M2)-comprising solution comprises more than one metal (M2) salt, the metal (M2) of each metal (M2) salt may be the same metal (M2) or a different metal (M2). The metal (M2)-comprising solution may comprise an organometallic (M2) compound, or hydrates thereof, or solvates thereof, or a combination 10 thereof. The metal (M2)-comprising solution may comprise one or more organometallic (M2) compounds, or hydrates thereof, or solvates thereof, or a combination thereof. When the metal (M2)-comprising solution comprises more than one organometallic (M2) compound, the metal (M2) of each organometallic (M2) compound may be the same metal (M2) or a different metal (M2). The metal (M2)-comprising solution may comprise an organometallic 15 (M2) compound, as such an organozinc compound. The metal (M2)-comprising solution may comprise an organometallic (M2) compound, as such an organozinc compound, that reacts to form a metal (M2) salt in-situ. The metal (M2)-comprising solution may comprise one or more metal (M2) salts, one or more organometallic (M2) compounds, , or hydrates thereof, or solvates thereof, or a combination thereof. 20

[0066] The metal (M2) of the metal (M2)-comprising solution may comprise a transition metal, or hydrates thereof, or solvates thereof, or a combination thereof. The metal (M2) of the metal (M2)-comprising solution may comprise a transition metal, such as a transition metal with a +2 oxidation state or higher. The metal (M2) of the metal (M2)- comprising solution may comprise a post-transition metal, or hydrates thereof, or solvates 25 thereof, or a combination thereof. The metal (M2) of the metal (M2)-comprising solution may comprise a post-transition metal, such as a post-transition metal with a +2 oxidation state or higher. The metal (M2) of the metal (M2)-comprising solution may comprise a lanthanide, or hydrates thereof, or solvates thereof, or a combination thereof. The metal (M2) of the metal (M2)-comprising solution may comprise a metalloid, or hydrates thereof, or solvates 30 thereof, or a combination thereof. The metal (M2) of the metal (M2)-comprising solution may comprise a transition metal, post-transition metal, a lanthanide, a metalloid, , or hydrates thereof, or solvates thereof, or a combination thereof.

[0067] As described herein, the process for preparing the mixed-metal MOF (M1OF-M2) comprises a metal (M1) salt. As described herein, the process for preparing the 35 mixed-metal MOF (M1OF-M2) comprises a metal (M1)-comprising solution. The metal (M1)- 15 -of the metal (M1) salt may comprise Cr3+, Co2+, Ni2+, Na+, K+, Li+, Mn2+, Fe2+, Ca2+, Mg2+, Sr2+, Ba2+, hydrates thereof, solvates thereof, or a combination thereof. The metal (M1) of the metal (M1)-comprising solution may comprise Cr3+, Co2+, Ni2+, Na+, K+, Li+, Mn2+, Fe2+, Ca2+, Mg2+, Sr2+, Ba2+, hydrates thereof, solvates thereof, or a combination thereof. 5

[0068] The M2 / M1ratio in the mixed-metal MOF (M1OF-M2) may be about 0 to about 100. The ratio may depend on pore size and structure. The ratio may depend on intended use of the MOF, or properties of the metal. For example, if a photoactive metal is used, only a trace amount of said metal may be needed, whereas for a catalytic center, there may be a full, or nearly full exchange. 10

[0069] Ligands

[0070] The process as described herein further comprises forming the hydrogen bonded metal organic framework (HM1OF) by combining a hydrogen-bonded organic framework (HOF) with a metal (M1)-comprising solution. The HOF comprises organic ligands. The organic ligands may include phosphonate ligands. The HOF may comprise an 15 organic phosphonate ligand, optionally substituted with one or more alkyl groups, amino groups, halogens, or a combination thereof.

[0071] The organic ligands may be a phosphonate ligand comprising the phosphonate group(s) P(O)(OR)2and / or P(O)(OH)(OR), wherein R may be Et, Me, iPr, alkyl, cycloalkyl, cyclic amides, phenyl, benzyl, and other aromatic derivatives including 20 imidazoles, triazoles, the phosphonate ligand being derived from any one or more of the following phosphonic acid ligands, wherein the aryl rings of said phosphonic acid ligands may be substituted with one or more alkyl groups, amino groups, halogens, nitro groups, alcohol groups, or a combination thereof:- 16 -25 / 36or a salt thereof. The HOF may comprise 1,1,2,2-Tetrakis[4-phosphonophenyl]ethylene (H8L3).

[0072] Hydrogen Bonded Metal-Organic Framework Precursors 5

[0073] Hydrogen bonded metal organic frameworks (HMOFs) are hydrogen bonded networks consisting of metal clusters and ligands; for example, hydrated or- 26 -solvated metal clusters and phosphonate ligands, such as hexaaquachromium(III) clusters and phosphonate ligands. HMOFs may act as "precursors" to MOFs because they can be heated to remove the waters or solvent around the metal centers (e.g., dehydrated or desolvated) to induce metal-ligand bond formation to yield a MOF. Further, the water or 5 solvent around the metal centers may act as guest molecules in the HMOF. This may result in the HMOF being templated by said guest molecules, which can result in eventual formation of a MOF comprising pore sizes and distribution tuned for applications such as catalysis, sensing, magnetic properties, photoactive properties, gas sorption and separation, and / or imaging. 10

[0074] As described herein, the process for preparing the mixed-metal MOF (M1OF-M2) may comprise providing a hydrogen-bonded organic framework (HOF) as described herein; combining the hydrogen-bonded organic framework (HOF) with a metal (M1)-comprising solution as described herein; and forming the hydrogen bonded metal organic framework (HM1OF). 15

[0075] As described herein, the process for preparing the mixed-metal MOF (M1OF-M2) comprises soaking the hydrogen bonded metal organic framework (HM1OF) in a metal (M2)-comprising solution. Soaking the hydrogen bonded metal organic framework (HM1OF) in the metal (M2)-comprising solution comprises forming a mixed-metal hydrogen bonded metal organic framework (HM1OF-M2). 20

[0076] Soaking the HM1OF in the metal (M2)-comprising solution may comprise soaking the HM1OF in a first metal (M2)-comprising solution; and soaking the HM1OF in at least a second metal (M2)-comprising solution; where the metal M2of the first solution is different from the metal M2of the second solution. Soaking the HM1OF in the metal (M2)- comprising solution may comprise soaking the HM1OF in a metal (M2)-comprising solution, 25 where the metal (M2)-comprising solution comprises two or more metal (M2) salts, two or more organometallic (M2) compounds, hydrates thereof, solvates thereof, or a combination thereof, and the metal (M2) of each metal (M2) salt and / or each organometallic (M2) compound is a different metal (M2).

[0077] Soaking the HM1OF in the metal (M2)-comprising solution may comprise 30 transmetallation of M1in the HM1OF with M2of the metal (M2) comprising solution to form the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2). Transmetallation may comprise exchanging between >0% and <100% of M1in the HM1OF with M2of the metal (M2)-comprising solution. Soaking the HM1OF in the metal (M2)-comprising solution may comprise proton exchange between the HM1OF and the M2of the metal (M2) 35 comprising solution to form the mixed-metal hydrogen bonded metal organic framework- 27 -(HM1OF-M2). The proton exchange may comprising exchanging between >0% and <100% of protons in the HM1OF with the M2of the metal (M2)-comprising solution.

[0078] Timing and temperature are conditions that may vary depending on the HMOF being soaked. The soaking temperature may generally need to be lower than the 5 dehydration or desolvation temperature (e.g., below 70 °C) to ensure the secondary metal can be incorporated into the HMOF before it is dehydrated or desolvated. Soaking time may range between several hours to several days. Depending on the HMOF, the metal- comprising solution may also need to be refreshed occasionally (e.g., discarded and replenished with new metal solution). 10

[0079] In some examples, selection of heating rates, solvents (e.g., guest molecules), soaking times and temperatures may tune the porosity of the HMOF, and thus tune the resultant MOF for a particular application. Preparation of the HMOFs as described herein may be performed at either room temperature (about 22 to about 25 °C) or lower, and may include mixing solutions of ligand and metal-comprising solutions. The preparation 15 may also include the addition of, or presence of guest molecules. The guest molecules may be separately added molecules or solvents. The guest molecules may be solvent molecules of the solutions described herein, or may be water. The guest molecules may be sourced from the hydration and / or solvation spheres of the metals, metal salts, organometallic compounds described herein. 20

[0080] Preparation of the mixed-metal MOFs as described herein may require heating the HMOF, for example as a dry powder or by heating in solution of a guest molecule (e.g., a solvent, such as methanol), at a temperature between about 50 °C to about 180 °C, depending on the dehydration or desolvation conditions of each specific HMOF and guest molecule (e.g., solvent) being used. A heating rate of a fast vs slow 25 heating rate may have an impact on the final mixed-metal MOF. For example, the heating rate may impact a mixed-metal MOF's porosity (surface area, pore size distribution), adsorption properties (enthalpy of adsorption, selectivity), crystallinity; or may determine defect structure including open metal sites and / or uncoordinated PO3groups, that may impact the mixed-metal MOF's porosity (surface area, pore size distribution), adsorption 30 properties (enthalpy of adsorption, selectivity), crystallinity. Though, in general, the more porous the HMOF, the less of a dependence on heating rate there may be; whereas, if there is not an easy egress pathway for solvent, the heating rate may be important.

[0081] Transmetallation as described herein may comprise exchanging between >0% and <100% of M1in the HM1OF with M2of the metal (M2)-comprising solution. For 35 example, a HMOF as described herein may comprise Ni in its hydrated metal cluster.- 28 -HMOFs comprising Ni may more readily undergo metal transfer with metals comprising a 2+or 3+charge than HMOFs comprising another metal, due to a relatively lower stability of Ni in HMOFs. For example, when a HMOF comprises Ni, said Ni could be exchanged with any one or more of a transition metal or post-transition metal, such as a transition metal or 5 a post-transition metal with a +2 oxidation state or higher; a lanthanide; a metalloid; or a combination thereof.

[0082] The hydrogen bonded metal organic framework (HM1OF) as described herein may have the structure {[Cr(H2O)6]2[H2L3]^(CH3OH)2(C3H6O)2}n; {[Cr(H2O)6]2[H2L3]^(H2O)}n; or {[Ni(H2O)6]2[H4L3]^C3H6O^2H2O^CH3OH}n. 10

[0083] Mixed-Metal Metal Organic Frameworks

[0084] Mixed-metal MOFs as described herein may be prepared by providing a hydrogen bonded metal organic framework (HM1OF) as described herein; soaking the hydrogen bonded metal organic framework (HM1OF) in a metal (M2)-comprising solution 15 as described herein; and forming the mixed-metal MOF (M1OF-M2), where M1and M2are different metals. The mixed-metal MOF (M1OF-M2) may be formed by complete or partial metal exchange. The complete or partial metal exchange may occur with metals including Cr3+, Co2+, Ni2+, Na+, K+, Li+, Mn2+, Fe2+, Ca2+, Mg2+, Sr2+, Ba2+, Eu3+, Gd3+, and / or La3+.

[0085] Preparing the mixed-metal MOFs as described herein may be further20 prepared by forming a mixed-metal hydrogen bonded metal organic framework (HM1OF- M2); and dehydrating or desolvating the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2) at, or above 100⁰C to form the mixed-metal MOF (M1OF-M2). Synthesizing the mixed-metal MOFs via said dehydration or desolvation route may result in relatively highly stable MOFs that have applications in catalysis, sensing, magnetic 25 properties, photoactive properties, gas sorption and separation, and / or imaging.

[0086] The mixed-metal metal-organic framework (M1OF-M2) as described herein may have the structure: [M1]a[M2]b[HxLn]c, 30 where M1and M2are different metals, a and b is metal content where a and be are between 0 and 6, x is a protonation state of ligand L between 0 and 12, n is a number label given to the ligand, and c is number of ligands per unit cell in the formula between 0 and 6.

[0087] The mixed-metal MOF (M1OF-M2) as described herein may have the structure Cr0.27Cu2H3.19L3^5H2O; Cr0.2 2Cu2.5H2.6L3^4H2O; or Ni0.9Cr1.5H1.6L3^3H2O. 35- 29 -

[0088] Applications

[0089] Mixed-metal MOFs as described herein may be useful for adsorption, gas separation, catalysis, sensing, magnetic solids, luminescent solids, fluorescent solids, ion conduction, ion exchange, or a combination thereof. When used for adsorption 5 applications, the mixed-metal MOF may act as a porous sorbent. When used for gas separation applications, the mixed-metal MOF may act as a sorbent in a solid bed, or be incorporated into a membrane. When used for catalysis applications, the mixed-metal MOF may have catalytic metals for M1and / or M2, may have acid groups that act as catalysts, and / or may act as a porous support to house other catalysts. When used for sensing 10 applications, the mixed-metal MOF may comprise metal ions or photoactive ligands that could bind to a target and where the binding results in a colour change. When used for magnetic applications, the mixed-metal MOF may comprise paramagnetic M1or M2metals that show coupling or could be used to grow magnetic nanoparticles. When used for luminescent and / or fluorescent applications, the mixed-metal MOF may be formed with 15 emissive metal ions or ligands. When used for ion conduction applications, the mixed-metal MOF may comprise acid groups that can conduct protons; or may additionally comprise proton carriers that can conduct ion exchanged in the MOF’s pores; for example, Li ions could be exchanged in the pores for protons. When used for ion exchange applications, the mixed-metal MOF may comprise protons or cations in its pores that could be exchanged 20 for other metal ions; for example for remediation applications.

[0090] Mixed-metal MOFs as described herein may be useful as a solid support. For example, the mixed-metal MOFs may act as robust porous solids that can be used to grow or deposit other solids or molecules thereon. Further, the mixed-metal MOFs may act as robust porous solids that can provide a higher surface area. 25

[0091] Mixed-metal MOFs as described herein may be useful as a solid support for ion exchange or ion conduction. Mixed-metal MOFs as described herein may be useful for emissive applications. For example, M2of the mixed-metal MOF may be an emissive metal (e.g. Eu3+) or a combination of metals (e.g., 2 or more lanthanides) that may be used to tune for a specific emitted wavelength. The mixed-metal MOFs may also become emissive 30 due to the choice of ligand or via an inclusion of a dye.

[0092] Mixed-metal MOFs as described herein may be useful for a method of uptaking at least one substance into a mixed-metal MOF, the method comprising contacting the mixed-metal MOF with the at least one substance under conditions for uptaking the at least one substance into the mixed-metal MOF. The at least one substance may comprise 35 carbon dioxide, nitrogen, hydrogen, a transition metal, a post-transition metal, a lanthanide,- 30 -a metalloid, or a combination thereof. Uptaking the at least one substance may be for the purpose of its storage, absorption, or separation, or a combination thereof. Uptaking the at least one substance may be the result of a pore surface’s inherent tendency to adsorb ions. Uptaking the at least one substance may result from adsorption, proton exchange, metal 5 and / or ion exchange, any one of which may be driven by concentration gradients or charges on species being taken up.

[0093] To gain a better understanding of the invention described herein, the following examples are set forth. It should be understood that these examples are for 10 illustrative purposes only. Therefore, they should not limit the scope of this invention in anyway.

[0094] EXAMPLES

[0095] Example 1 - Synthesis of a Robust Cr(III) / Cu(II) Mixed-Metal MOF by 15 Transmetallation of a Cr(III) – Phosphonate HMOF

[0096] Introduction: Mixed-Metal MOFs

[0097] Hydrogen bonded metal-organic framework precursors can provide an opportunity to synthesize many MOFs with different properties depending on how they are dehydrated. From heating the HMOF in a solvent to vary the dehydration rate, a single 20 HMOF can yield a family of MOFs, where each one is potentially better suited for a different application. In addition, chromium(III)-phosphonate MOFs show impressive stability, which is perhaps one of the more attractive feature of these MOFs as stability is uncompromisable for practical applications. To exploit the stability and further improve their properties, a secondary metal ion can be incorporated into chromium(III)-phosphonate MOFs that can 25 lead to improved gas separation performances without sacrificing framework stability.

[0098] Investigations into mixed-metal materials suggest that they may perform better than the parent, monometallic MOF owing to the synergistic effects that can occur from having two different types of metals in the framework.253,254,255,256For example, changing or adding a second metal can introduce open metal sites that improve gas 30 sorption properties of the framework, or make the MOF catalytically active.

[0099] A study done by Walton et al. showed that the stability of Mg-MOF-74 can be tuned by introducing a secondary metal in the framework.114MM-MOF-74 (MM for mixed metal) was synthesized using the ligand, 2,5-dioxidoterephthalate (DOT), with varying amounts of Ni(II) and Mg(II). As shown in Figure 1, the monometallic Mg-MOF-74 has a 35 high CO2capacity, at 0.55 mol / mol M2+. Unfortunately, exposure to 95% relative humidity- 31 -results in almost complete loss of surface area. Ni(II) can be doped into the parent Mg(II) MOF to yield a MM MOF. The authors show that doping in 16 mol% Ni(II) into the MOF causes a slight decrease in the CO2 capacity. However, the Ni(II) / Mg(II) MOF retains 80% of its surface area after undergoing the same stability test as the pure Mg-MOF-74. This 5 study suggested that introducing only a small percentage of Ni(II) to the Mg(II) MOF can significantly enhance stability without strongly affecting the CO2capacity.

[0100] Thus, incorporating a secondary metal into chromium(III)-phosphonate materials can introduce new and improved functionality while maintaining high framework stability due to the chromium(III) metal. Cu(II) frameworks are known to possess high CO210 capacities owing to the presence of open metal sites, which can act as high energy sites for adsorption.257However, these open metal sites can also be weak points that are susceptible to hydrolytic attack, making these MOFs less stable in humid environments.258,259

[0101] Thus, described herein are developments to retain the impressive gas 15 sorption properties exhibited by Cu(II) MOFs, while maintaining framework stability by synthesizing mixed metal Cu(II) / Cr(III) phosphonate MOFs. Incorporating a secondary metal may improve the performance of a MOF and the framework stability may be tuned. For example, as shown in Figure 23, increasing the amount of Cu in a Cr-phosphonate framework can increase CO2capacity. Figure 23a shows a framework with the highest Cu 20 and lowest Cr content. This material had the highest CO2capacity (owing to high Cu content), but also had poor stability (owing to low Cr content). In contrast, Figure 23e shows a framework with the lowest Cu and highest Cr content. This material was stable, but with a low CO2capacity. A material that performed in-between these two MOFs is shown in Figure 23c, involving a framework which had moderate Cu and Cr content and thus, the 25 material had improved CO2capacity compared to the parent pure Cr-MOF, but with enhanced stability compared to the higher Cu-content MOF. Other properties that may be imparted by incorporating a secondary metal may include catalytic activity, sensing ability, magnetic properties, photoactive properties, or a combination thereof.

[0102] Direct Synthesis of Mixed-Metal MOFs 30

[0103] A simplistic method to synthesizing a mixed-metal MOF is to do so directly using a one-pot synthesis. This is done by dissolving both metal salts and ligand and mixing either at room temperature or hydrothermally at elevated temperatures and pressures. However, the main disadvantage to this method is the lack of control over framework formation. Different metals have different reactivities, making it difficult to simultaneously 35 incorporate the two or more metals in an orderly fashion.- 32 -

[0104] For metals that exhibit vastly different reactivities, delicate control tends to be required. As shown by Serre et al., Cr(III) is kinetically inert, whereas Fe(III) is much more reactive.261To control the reactivity of the two metals and prevent formation of a monometallic phase, the less reactive Fe0was used in a one pot synthesis with Cr(III) 5 nitrate, hydrofluoric acid, and the ligand, terephthalic acid (BDC). This resulted in the mixed metal MOF, MIL-53(Cr-Fe), with a formula of Cr0.6Fe0.4(OH)0.7F0.3(BDC)^(H2O). The bimetallic MOF showed different gas sorption properties compared to either pure chromium or iron MOF, implying the opportunity to tune the bimetallic MOF for target gas sorption behaviours. 10

[0105] Mixed Metal MOFs Obtained from Post Synthetic Modification

[0106] An alternative approach to the direct synthesis route is to design mixed- metal MOFs via post synthetic modification (PSM). First described in 2007 by Cohen et al., PSM is a chemical derivatization of a material after it has already been formed.262,263The advantage of this method is the incorporation of building blocks with specific functional 15 groups that may be difficult to obtain by the direct method due to synthetic limitations. Metal exchange by PSM typically involves soaking the parent MOF in a metal solution and allowing the secondary metal to diffuse through the pores to exchange with the original metal.

[0107] Several factors need to be considered when optimizing conditions for 20 successful metal exchange in MOFs. The first is that the MOF needs to be sufficiently porous to allow for diffusion of metal ions into and out of the material. Secondly and perhaps the most obvious factor to consider is the choice of metals ions. The preferred coordination environments of all metals play a significant role in determining the success of the exchange. If the incoming metal adopts a geometry that is incompatible with the metal 25 geometry of the parent MOF, this can lead to structural strain that hinders the incorporation of the secondary metal. A good example of this is shown by Volkmer et al. on the MOF, MFU-4l.264This MOF consists of triazole linkers coordinated to five Zn(II) clusters, one of which is octahedrally coordinated and the other four are tetrahedral. Soaking MFU-41l in a CoCl2DMF solution results in isostructural replacement of the tetrahedral Zn(II) ions only. 30

[0108] Another requirement for successful metal exchange is for the MOF to be porous enough so that there is room for solvated metal ions to diffuse into and out of the material. Solvent choice also needs to be considered as it can affect the thermodynamics of the metal exchange reaction. The parent metal should be easily solvated to facilitate the outgoing of the metal, whereas the secondary metal should be poorly solvated so that there 35 is a driving force for the metal to become incorporated into the structure. Dincǎ et al. did a- 33 -thorough study on this for the exchange of Zn(II) ions in MOF-5 with Ni(II).265By soaking MOF-5 in a DMF solution of Ni(NO3)2^6H2O at various temperatures, the authors were able to determine the Gibbs free energy, which was then used to determine the enthalpy and entropy of exchange. It was found that ΔH and ΔS were both positive values. The increase 5 in entropy is hypothesized to be due to the release of Zn(II) ions from the MOF into the solution during the exchange process. Meanwhile, the positive enthalpy implies that the exchange reaction is thermodynamically unfavourable based on the solvation environments of Zn(II) and Ni(II). Ni(II) has a high ligand field stabilization energy and is easily solvated in DMF, whereas Zn(II) has a considerably lower ligand field stabilization 10 energy, making it less favourable for Zn(II) ions to leave the MOF and enter into the solution. However, exchanging Zn(II) with Ni(II) in MOF-5 is still possible if a large excess of Ni(II) is present, which forces the exchange by Le Chatelier’s principle. The authors further proved this by redoing the metal exchange in acetonitrile instead of DMF. The results showed that the entropy of the exchange was similar to that of the exchange in 15 DMF, but the enthalpy of exchange was much lower. Calculations showed that Ni-MeCN interaction is much weaker than that of Ni-DMF, implying that there is a driving force for Ni(II) to form coordination bonds within the MOF rather than to be solvated by weak Ni- MeCN bonds. These findings were supported by Zhang and Wang et al. who also found that solvent can greatly affect the metal exchange reaction.20720

[0109] The stability of the MOF must also be considered. Metals that are not highly connected to the ligand and possess terminal solvent ligands can more easily exchange with an incoming metal as there are fewer bonds that need to be broken and reformed during the transmetallation. Usually, PSM metal exchanges are performed on less robust MOFs consisting of labile coordination bonds as there is a thermodynamic driving force to 25 incorporating a secondary metal that forms stronger coordination bonds that could stabilize the MOF. However, incorporating an inert metal such as Cr(III) can be difficult due to its slow ligand exchange rate. This was the case when Zhou et al. attempted to exchange Mg(II) in PCN-426-Mg with Cr(III) ions.206Conventional PSM by soaking the MOF in a solution of Cr(NO3)3·9H2O or CrCl3did not result in metal exchange. Instead, the authors 30 soaked crystals of PCN-426-Mg in a CrCl2solution to achieve full Mg(II) to Cr(II) exchange, and then suspended the Cr(II) MOF in DMF while bubbling air into the solution. This oxidized all Cr(II) ions to Cr(III) to achieve PCN-426-Cr(III), a pure Cr(III) MOF that is isostructural to the parent PCN-426-Mg MOF. The same authors were able to exchange Fe(III) ions in the MOF PCN-333(Fe) with Cr(III) ions, but high temperatures (150⁰C) were 35 required and the Cr(III) solution needed to be replenished to promote the exchange.266- 34 -

[0110] Incorporating Cu(II) into a Cr(III)-Phosphonate Framework

[0111] Cr(III) and Cu(II) tend to be considered very different metal ions, with different reaction kinetics and oxidation states. This makes can make it difficult to control formation of a uniform, mixed-metal MOF from a direct synthesis approach. Thus, the PSM 5 approach was deemed more appropriate to synthesizing a Cu(II) / Cr(III) phosphonate MOF. However, Cr(III)-phosphonate MOFs tend to be very robust, making it unlikely for the metal- ligand bonds to be displaced by exchanging with a secondary metal. In addition, doping Cr(III) ions into a Cu(II)-phosphonate framework can be difficult given the kinetic inertness of Cr(III) ions in solution. Instead, the goal was to exchange the Cr(III) ions in the Cr(III)- 10 phosphonate HMOF with Cu(II). The adaptable nature of the HMOF allows for incorporation of Cu(II) ions into the framework that are otherwise unfavourable when doping Cu(II) into a Cr(III)-phosphonate MOF. It was hypothesized that the driving force in this exchange reaction would be displacement of the Cr(III)-phosphonate hydrogen bonds by Cu(II)- phosphonate coordination bonds. In this sense, incorporating a secondary metal acts as 15 another method of dehydration in converting HMOF to MOF. As herein described, the bimetallic MOFs, CrL3-Cu1, CrL3-Cu2, and CrL3-Cu-hy were prepared, each with varying amounts of Cu(II) / Cr(III) ratios in the framework. This directly affected the gas sorption properties and stability of each of these compounds. This offered insight to a mechanism of transmetallation on a hydrogen bonded system between chromium(III) hexaaqua cations 20 and a tetratopic phosphonate ligand, H8L3. The HMOF used, H-CrL3, was chosen due to the large, 1D channels seen in the crystal structure that would provide space for diffusion of metal ions into and out of the framework.

[0112] Experimental

[0113] Synthesis of ligand (H8L3) in CrL325

[0114] All starting materials were purchased from commercial suppliers (Alfa Aesar and Sigma Aldrich). Ethanol was dried 4Å molecular sieves and backfilled with Ar. DCM and pyridine were dried by placing in 3Å molecular sieves and backfilled with Ar. All other chemicals were used without further purification. All glassware was also placed in an oven overnight prior to use. Tetrahydrofuran (THF) was dried by distillation over sodium metal 30 and benzophenone indicator. The collected solvent was transferred to a Schlenk flask containing 4 Å molecular sieves and backfilled with argon (Ar). The ligand H8L3has been reported before using a different method.267However, due to availability of starting materials, a different procedure was used to synthesize H8L3,as described below.

[0115] Synthesis of 1,1,2,2-Tetrakis(4-methoxyphenyl)ethane- 35 -

[0116] Zinc (19.57g, 299mmol) was placed in a round bottom flask and the system was evacuated and backfilling with argon three times. Anhydrous THF (70mL) was added, and the mixture was put on an ice bath. Once cooled, TiCl4 (6.4mL, 58mmol) was added dropwise and the reaction was then heated to reflux at 72⁰C for 2 hours before being left to 5 cool again on ice. In an addition funnel, 4,4’-dimethoxybenzophenone (7.30g, 29mmol) was added by dissolving in anhydrous THF (140mL). The solution was slowly added to the round bottom flask over 2 hours and the entire mixture was refluxed overnight. Afterwards, the reaction was quenched with K2CO3(10%, 120mL dissolved in water) and the THF was removed by rotary evaporation. The product was extracted with DCM, dry loaded onto a 10 silica plug, and eluted using a 1:2 mixture of DCM / hexanes. After removing the solvent, a crystalline white powder was obtained (4.43g, 66%).1H NMR (400 MHz, CDCl3): δ = 3.77 ppm (s, 12sH, Ar-O-CH3), δ = 6.64 – 6.68 ppm (d, 8H, Ar-H), δ = 6.94 – 6.97 ppm (d, 8H, Ar-H).

[0117] Synthesis of 1,1,2,2-Tetra(p-hydroxyphenyl)ethylene 15

[0118] 1,1,2,2-Tetrakis(4-methoxyphenyl)ethene (5.30g, 11.7mmol) was added to a round bottom flask and the flask was evacuated and backfilled three times with argon. Anhydrous DCM (160mL) was added, and the mixture was left to cool on ice before boron tribromide (3mL, 31.59mmol) was added dropwise. The mixture changed colour from yellow to purple and was left to stir overnight. To quench the reaction, HCl (100mL, 1.0M) 20 was added slowly, resulting in a yellow precipitate. The DCM was removed by rotary evaporation and the product was extracted using ethyl acetate (4 x 35mL). The organic layers were dried with Na2SO4, and the ethyl acetate removed to obtain a purple precipitate. The solid was recrystallized twice with a 50:50 mixture of water and acetone to yield a crystalline white powder (4.32g, 93%). 25

[0119] Synthesis of 1,1,2,2-Tetra(4’-trifluoromethanesulfonate)ethylene

[0120] 1,1,2,2-Tetra(p-hydroxyphenyl)ethylene (4.32g, 10.90mmol) was placed in a round bottom flask and the flask was evacuated and backfilled with argon three times. Dried DCM (190mL) was added and the mixture was left to cool on ice while stirring. Triflic anhydride (11mL, 65.50mmol) was slowly added, along with anhydrous pyridine (8mL, 30 98.10mmol). The reaction was left to stir overnight, resulting in a pink solution with white precipitate. Afterwards, the orange reaction was quenched with ice water (60mL) and the product extracted with DCM (3 x 30mL). The organic layers were washed with brine and water (3 x 30mL each) and dried over Na2SO4. The solvent was removed by rotary evaporation and the product was dry loaded on to a silica plug. Extraction using a 1:9 35 mixture of ethyl acetate / hexanes yielded a white product (8.19g, 81.4%).1H NMR (400- 36 -MHz, CDCl3): δ = 7.19 – 7.21 ppm (d, 3H, Ar-H), δ = 7.32 – 7.35 ppm (d, 3H, Ar-H).19F NMR (400 MHz, CDCl3): δ = -73.0 ppm (Ar-OSO2CF3).

[0121] Synthesis of Tetra(diethylphosphono)phenylethylene

[0122] To a round bottom flask, 1,1,2,2-Tetra(4’- 5 trifluoromethanesulfonate)ethylene (5.00g, 5.40mmol), Pd(OAc)2(0.12g, 0.54mmol), 1,1'- bis(diphenylphosphino)ferrocene (dppf, 0.60g, 1.10mmol), and potassium acetate (0.21g, 2.20mmol) were added. The flask was then evacuated and backfilled with argon three times and dry toluene (50mL), diisopropylethylamine (11.2mL, 64.80mmol), and diethyl phosphite (6.1mL, 47.5mmol) were added. The yellow solution was left to reflux for 18 hours, resulting 10 in a brown solution. After cooling to room temperature, the toluene was removed by rotary evaporation, the product was extracted with CHCl3(35mL x 3), and dried over MgSO4. After dry loading onto a silica plug, the product was eluded with CHCl3, with increasing amounts of MeOH (up to 2% near the end of the plug). The solvent was removed, and the resulting brown oil was stirred in diethyl ether to remove the precipitated dppf. The solution was then 15 evaporated, and the product was stirred in pentane to obtain the product as an off-white powder (3.47g, 73.3%).1H NMR (400 MHz, CDCl3): δ = 1.31 – 1.35 ppm (t, 24H, Ar-P-O- CH2-CH3), δ = 4.06 – 4.18 ppm (m, 16H, Ar-P-O-CH2-CH3).31P NMR (400 MHz, CDCl3): δ = 18.02 ppm.

[0123] Synthesis of 1,1,2,2-Tetrakis[4-phosphonophenyl]ethylene (H8L3) 20 added to around bottom flask, which was then evacuated and backfilled three times with argon. Anhydrous DCM (175mL) was then added, and the mixture was left to cool on ice. Then, boron tribromide (3.6mL, 30mmol) was added dropwise and the reaction was left to stir 25 overnight. To quench the reaction, the mixture was put on ice and water (100mL) was added, resulting in a white precipitate. The DCM and water were removed by rotary evaporation, resulting in an orange oil. The oil was stirred in diethyl ether for several days, and fresh diethyl ether was replenished each day to yield an off-white powder (1.58g,- 37 -97.3%).1H NMR (400 MHz, CDCl3): δ = 7.06 – 7.09 ppm (m, 8H, Ar-H), δ = 7.44 – 7.50 ppm (m, 8H, Ar-H).31P NMR (400 MHz, CDCl3): δ = 12.65 ppm.

[0125] Synthesis and Characterization of H-CrL3 {[Cr(H2O)6]2[H2L3]^(CH3OH)2(C3H6O)2}n5

[0126] Cr(NO3)3·9H2O (0.264g, 0.66mmol) was dissolved in 66mL water. In a separate flask, H8L3(0.0938g, 0.144mmol) was dissolved in a solution containing 44mL methanol and 22mL water. From each solution, 1mL was taken and combined in a vial. This vial was then placed in a larger vial containing 2mL acetone for vapour diffusion. The entire vial was placed in a fridge overnight to yield purple crystals that were 10 crystallographically characterized as the HMOF, {[Cr(H2O)6]2[H2L3]}n; H-CrL3, with disordered guests within the pores.

[0127] Single crystals of H-CrL3were mounted by freezing to the tip of a glass capillary using Paratone oil. Experiments were conducted at 173K on a Bruker SMART instrument with a CuKα X-ray source and equipped with an APEX-II detector. The intensity 15 data collection was performed in the - scanning mode with the goniometer and detector angular settings were optimized. The crystallographic information can be found in Table 2.

[0128] Bulk synthesis of H-CrL3was achieved by dissolving Cr(NO3)3·9H2O (0.36g, 0.9mmol) in 44mL of water. In a separate flask, H8L3 (0.28g, 0.43mmol) was dissolved in a solution containing 29mL of methanol and 15mL of water. The metal and ligand solutions 20 were combined, resulting in a cloudy solution, which is then placed in a larger container containing 6mL of acetone for vapour diffusion. The flask was placed in a fridge to yield a light purple powder. The powder was then filtered and soaked in a fresh solution of methanol for one day to obtain pure phase of H-CrL3.

[0129] Synthesis of CrL325

[0130] The dry powder of H-CrL3was placed in an oven that was pre-heated to 120⁰C. After leaving in the oven for 6 hours, the powder turned green. Full dehydration was confirmed by PXRD (Figure 3), EA (Table 1), TGA (Figure 4), and IR (Figure 5).

[0131] Synthesis of CrL3-Cux

[0132] The dry, purple powder of H-CrL3was soaked in a solution of 30 Cu(NO3)2·2.5H2O in methanol (10mL of 0.01M for CrL3-Cu1or 0.03M for CrL3-Cu2) for 24 hours, yielding a bright green powder. The powder was filtered and soaked in a fresh solution of methanol overnight before being filtered again and used for further experiments.

[0133] The one-pot synthesis to yield CrL3-hy was performed by combining Cr(NO3)3·9H2O (0.012g, 0.03mmol), Cu(NO3)2·2.5H2O (0.007g, 0.03mmol), and H8L335 (0.02g, 0.03mmol) in a 23mL autoclave, along with water (5mL) and methanol (5mL). The- 38 -autoclave was heated to 120 ⁰C over a course of 2 hours, held at this temperature for 6 hours, and then cooled over 48hours. The resulting product was a green powder which was collected by vacuum filtration and then washed with methanol.

[0134] Instrumentation 5

[0135] ICP-OES analysis

[0136] MOFs were digested by heating to 90⁰C overnight in aqua regia and then diluting with water to obtain a 2% HNO3solution for ICP-OES analysis. The mother liquor was evaporated to dryness and then dissolved in 800mL of a 2% HNO3solution for ICP- OES analysis. See section 2.4.3 for details. Formulas for all bimetallic MOFs were 10 determined by combining ICP-OES and EA data.

[0137] Gas sorption analysis

[0138] All gas sorption analyses were conducted on a Accelerated Surface Area & Porosimetry System (ASAP) 2460 supplied by Micromeritics Instruments Inc. The dry sample (~40mg) was loaded into a glass analysis tube. The MOFs were put under vacuum 15 (~ 10-6mbar) and activated at 120⁰C overnight. After activation, the samples were manually backfilled with N2before being transferred to the analysis port. Here, they were evacuated for a further 120 min before the analyses started.

[0139] Table 1: Elemental analysis of H-CrL3and CrL3. Compound %C %H Experimental Theoretical Experimental Theoretical 20

[0141] Structure and Dehydration of H-CrL3

[0142] Cr(NO3)3·9H2O was dissolved in an aqueous solution and combined with a methanol / water solution of H8L3to yield purple crystals. X-ray crystallography revealed the structure of H-CrL3, which was a hydrogen bonding network between 25 hexaaquachromium(III) cations and H2L3and exist in a 2:1 metal to ligand ratio. Chains of [Cr(H2O)6]3+clusters are pillared by the tetraphosphonate ligand, resulting in large 1-D pores of approximately 12.8Å by 13.5Å size, including van der Waals radii, between the ligands. There are disordered solvent molecules within these pores that were not able to be elucidated crystallographically. The network displays extensive charge assisted- 39 -hydrogen bonds, with short acceptor-donor (O --- O between the aquo and phosphonate ligands) distances within the range of 2.523 – 2.727 Å (Figure2).

[0143] By combining the crystallography data with EA (Table 1), a formula of {[Cr(H2O)6]2[H2L3]^(CH3OH)2(C3H6O)2}nwas determined for H-CrL3. TGA showed a large 5 mass loss (20.9%) up to 100⁰C and DSC showed three endothermic peaks, corresponding to loss of acetone and methanol within the pores of the HMOF, and the water around the hexaaquachromium(III) clusters as the sample is heated (Figure 4). Bulk powder was synthesized as discussed above and the HMOF was washed with methanol, giving an EA with a formula of {[Cr(H2O)6]2[H2L3]^H2O}n. The experimental PXRD pattern collected for the 10 bulk powder matched that of the simulated pattern, indicating the phase purity of the powder (Figure 3).

[0144] Table 2: Crystallographic information for H-CrL3. Compound H-CrL3Chemical formula C52H84Cr4O48P8

[0145] 3, powder and 15 placed in a pre-heated oven at 120⁰C for 6 hours to obtain CrL3. This is associated with a colour change from purple to green. As shown by the PXRD pattern in Figure3, the MOF- 40 -is highly ordered. The peak at 5.98⁰ 2theta in the simulated PXRD pattern is for the (011ത) Bragg planes, which correspond to the hexaaquachromium(III) layers within the structure and are approximately 14.7Å apart. Upon dehydration, the water ligands around the metal are removed and replaced by phosphonate groups, causing the (011ത) planes to shift to a 5 slightly higher 2theta of 7.04⁰, corresponding to a contraction of 1.9Å between the chromium-chromium planes.

[0146] TGA (Figure 4) was also used to confirm dehydration, which showed that the HMOF has a significant mass loss of 20.9% up to 100⁰C corresponding to water around the chromium(III) clusters and solvent molecules. In contrast, TGA for CrL3shows only 3% 10 mass loss up to 100⁰C, suggesting only solvent molecules are present in H-CrL3. This was also corroborated by EA data (Table1) which suggests only methanol and water are present in the MOF, and FTIR data (Figure 5) that shows significantly less water in the MOF compared to the HMOF.

[0147] MOFs using this H8L3ligand have been reported before.267,268,269One MOF 15 containing Ti(IV) and H8L3was porous, with a BET surface area of 382 m2 / g, and the structure showed a moderate degree of order.270However, those that consist of divalent and trivalent metals were only porous to H2O and not to N2or CO2, as the pores were too small to be measured by CO2or N2, and their structures could only be elucidated by Rietveld refinement using PXRD data. In contrast, PXRD and gas sorption of CrL3(after 20 activating at 120⁰C overnight) showed that the MOF is both highly crystalline and porous. Gas sorption of H-CrL3could not be obtained because the HMOF dehydrates under vacuum at room temperature. The MOF, CrL3, is non-porous to N2isotherm at 77K, but is porous to CO2(Figure 6; Figure 8). The CO2isotherm at 273K reveals the MOF has a BET surface area of 56 m2 / g (Langmuir: 157 m2 / g), with a capacity of 1.3mmol / g at 1200mbar. 25 A very uniform NLDFT PSD was determined from the CO2isotherm, with a dominant PSD around 10.5Å and a less prominent PSD around 8.5Å.

[0148] It was also determined for CrL3that the enthalpy of adsorption (Figure 7) at low CO2loadings is -37.1 kJ / mol, indicating the presence of open metal sites.246This may be due to the quick heating rate used which results in defects that can expose the surface 30 of the metal. As the CO2loading increases and the high energy sites are depleted, the enthalpy of adsorption decreases. The exposed metal sites within the pores results in a high IAST CO2 selectivity over N2 in a 15 / 85 mixture at 273K, with a value of 56.7 at 1 bar (Figure 8).

[0149] The HMOFs and MOFs as described herein are prepared by a process 35 comprising transmetallation (e.g., doping metals into a metal-comprising MOF). As such,- 41 -the HMOFs and / or MOFs as described herein - even if they comprise metals and ligands that have been previously reported, such as the ligand H8L3and / or the metals Ni(II), Co(II), Al(III), Ga(III), Fe(III), and Ti(IV) - are not expected to produce the same MOFs as previously report. This is because, for the HMOFs and / or MOFs as described herein: (i) 5 they would comprise mixed metal frameworks, whereas those previously reported have been monometallic; (ii) even if full metal exchange occurred, the exact same coordination geometry of the metals in the parent MOF is unlikely to be completely maintained by an incoming secondary metal, which may result in defects and / or less crystalline MOFs; and / or (iii) metals with different oxidation states may be exchanged (e.g. 2+ metals being 10 exchanged with 3+ metals), which would require the ligands to reorient and charge balance, resulting in different MOFs.

[0150] Metal Exchange Experiments

[0151] Post Synthetic Modification of CrL3

[0152] Attempts to post synthetically exchange Cr(III) in the MOF, CrL3, with Cu(II) 15 were unsuccessful. Expectedly, the MOF too stable to undergo any changes to accommodate Cu(II), as the result would be Cu(II) – phosphonate bonds that are higher in energy than the Cr(III) – phosphonate coordination bonds in the parent MOF. ICP-OES results showed no trace of Cu(II) after soaking CrL3in a Cu(NO3)2·2.5H2O solution for 5 days, and no changes were observed in the PXRD or gas sorption data (Figure 9). NLDFT 20 PSD was also the same as the original CrL3after soaking in the metal solution for 5 days, indicating that no metal adsorption occurred either. The CrL3MOF sits in a deep thermodynamic well that makes it resistant to structural changes. Hence, PSM metal exchange experiments were attempted on H-CrL3instead as it is a metastable state that is adaptable to incoming guests. 25

[0153] Post Synthetic Modification of H-CrL3

[0154] Observing the large 1D channels in H-CrL3, it was expected that the HMOF would be a good candidate for metal exchange of Cr(III) with Cu(II) ions. Powders of H- CrL3were washed thoroughly with methanol and then placed in 0.01M and 0.03M methanol solutions of Cu(NO3)2·2.5H2O over a course of 24 hours to generate CrL3-Cu1and CrL3- 30 Cu2, respectively. During this time, the powders changed from a light blue colour to bright green, visually suggesting either dehydration of the HMOF, or incorporation of Cu(II) ions into the network. After soaking the powders in methanol overnight to wash out any metal ions trapped in the pores, the powders were digested for ICP-OES analysis. The results indicated that a significant amount of Cr(III) ions were exchanged with Cu(II), which has a 35 Cu(II) / Cr(III) ratio of 7.253 for CrL3-Cu1and 11.878 for CrL3-Cu2(Table 3). Metal exchange- 42 -and not metal adsorption was confirmed by analysing the mother liquor after soaking the HMOF, which showed there was a significant amount of Cr(III) in the Cu(II) solution. By combining this data with EA (Table 3) and EDX (Figure 10 and Figure 11), a formula of Cr0.27Cu2H3.19L3·5H2O was obtained for CrL3-Cu1, and Cr0.22Cu2.5H2.6L3·4H2O for CrL3-Cu2. Below is a sample calculation for the ICP-OES data for CrL3-Cu2.

[0155] 1.5mg of the MOF after metal exchange was digested in 100mL of a 2% HNO3solution for ICP-OES. Assuming the formula of Cr0.22Cu2.5H2.6L3·4H2O (MW=889.31 g / mol), the expected amount of Cr(III) from the MOF is 0.193 ppm. This compares well to the experimentally determined Cr(III) concentration of 0.164 ppm.

[00156] 1.5^^^^ ^^^^^^ ൈ^^^^ ଼଼ଽ.ଷ^^ ൈ ^.ଶଶ^^^ ^^ ^^^^ ெைி ൈ ହ^.ଽଽ^ ^^^^ ^^ ൈ ^ ^.^^ൌ 0.193^^^^^^ ^^^^^^^^^^^^^^^^ ^^^^^^

[0157] Likewise, the amount of Cu(II) in in the MOF is expected to be 2.680 ppm, which is comparable to the experimentally determined Cu(II) concentration of 2.948 ppm.

[00158] 1.5^^^^ ^^^^^^ ൈ^^^^ ଼଼ଽ.ଷ^^ ൈ ଶ.ହ^^^ ^௨ ^^^^ ெைி ൈ ^ଷ.ହହ^ ^^^^ ^௨ ൈ ^ ^.^^ൌ 2.680^^^^^^ ^^^^^^^^^^^^^^^^ ^^^^^^

[0159] Table 3: ICP-OES and EA results for CrL3, CrL3-Cu1, and CrL3-Cu2. CrL3-Cu CrL CrL3-23Cu1CrL3-Cu2mother liquor%C (Experimental) 40.68 35.11 35.16 -- %H (Theoretical) 3.03 3.58 3.25 --. ,3. Cu(NO3)2·2.5H2O. Given that the formula for the HMOF is {[Cr(H2O)6]2[H2L3]^(H2O)}n(MW = 984.56 g / mol) as calculated by EA, the total starting amount of Cr(III) in the HMOF is calculated to be 3.253mg:

[0161] 30.8^^^^ ^^^^^^^^ ൈ ^^^^ ଶ ^^^ ^^^ ଽ଼ସ.ହ^^ ൈ ூூூ^ ^^^^ ுெைி ൈ ହ^.ଽଽ^ ^^^^ ^^ൌ 3.253^^^^ ^^^^^^^^^^^^^^^^ ^^^^^^^^

[0162] The mother liquor from the exchange experiment was evaporated to dryness and then redissolved in 800mL of a 2% nitric acid solution. ICP-OES analysis showed that a significant amount of chromium was present. This data can be used to determine that 2.538 mg of Cr(III) that was exchanged out of the MOF. Subtracting this number from the 3.253 mg of total starting amount of Cr(III) calculated above shows that 0.715 mg (22%) of Cr(III) remains unexchanged. In other words, 78% of Cr(III) has been exchanged with Cu(II) during the transmetallation.

[0163] The ICP-OES experiments were complimented by EDX results, which showed an average Cu / Cr ratio of 11.15, and a P / Cu ratio of 2.46. The original H-CrL3structure is a 2:1 Cr : ligand ratio, with a 6+ charge from the two Cr(III) metals and a 6- charge on the ligand to charge balance. Because Cu(II) has a 2+ charge, up to three Cu(II) could be exchanged with the two Cr(III) to charge balance with the ligand. ICP-OES results suggest that there are 2.5 moles of Cu(II) for every 0.22 moles of Cr(III), showing that the metal exchange went almost to completion. EDX results show that the ratio of P to Cu is 2.18, suggesting approximately 1 ligand for every two Cu(II) metals, which supports the ICP-OES results. Additionally, EDX mapping (Figure 12) shows that Cr(III) and Cu(II) metals are uniformly distributed.

[0164] The same calculations were performed on CrL3-Cu1to yield a formula of Cr0.22Cu2.5H2.6L3·4H2O. Notedly, CrL3-Cu1has a lower Cu(II) content and higher Cr(III) content than CrL3-Cu2after soaking H-CrL3in a lower concentration of Cu(NO3)2·2.5H2O. This is expected as increasing the concentration of the secondary metal shifts the equilibrium further towards the transmetallation process via Le Chatelier’s principle.265

[0165] PXRD (Figure 13) of CrL3-Cu1and CrL3-Cu2are similar, and show that the structures are ordered but not as crystalline as the pure Cr(III) MOF, CrL3. For bothmaterials, there is a peak at 7.04⁰ 2θ that is also present in CrL3which is indicative of dehydration. This suggests that soaking H-CrL3in a Cu(II) solution results in a metal exchange that also triggers a dehydration, whereby the HMOF is converted into a MOF. FTIR data (Figure 14) supports this, as the broad peaks around 2500 cm-1to 3500 cm-1corresponding to O-H stretching frequencies is substantially decreased in both CrL3-Cu1and CrL3-Cu2as compared to H-CrL3.

[0166] Kinetic Studies on Metal Exchange of H-CrL3

[0167] To probe the mechanism of metal exchange, kinetic experiments were performed on CrL3-Cu2, where H-CrL3was soaked in 0.03M solutions of Cu(NO3)2·2.5H2O for varying amounts of time. ICP-OES analysis (Figure 15) showed that the metal exchange process is rapid and within the first 30 minutes, 23.7% of the Cr(III) has been replaced by Cu(II). Importantly, the amount of Cr(III) found in the mother liquor supports the fact that there is metal exchange occurring and not simply Cu(II) adsorption into the framework. The rapid metal exchange continues and within the first 1.5 hours of soaking, 66.8% of Cr(III) has been replaced by Cu(II). Then after soaking for 24 hours, 78% of Cr(III) has been replaced and subsequent soaking does not result in further exchange with Cu(II).

[0168] The ICP-OES and PXRD results are used to propose a possible mechanism of metal exchange. However, to confirm the following claims, a combination of in-situ experiments would be required. For example, UV-Vis would help in monitoring the colour change during the exchange, and IR would be useful in observing any changes in bonding characteristics. The following proposal, is therefore, hypothesized based on the current data presented here.

[0169] The rapid metal exchange that occurs within the first two hours is facilitated by the large channels in H-CrL3 that allow for diffusion of both the incoming Cu(II) ions and outgoing of Cr(III) ions. Although the PXRD pattern for the HMOF after soaking for 30 minutes (Figure 16) does not change significantly compared to the original pattern of H- CrL3, there is a slight decrease in crystallinity, along with broadening of the peak at 7.24⁰ 2θ. This peak corresponds to the (10-1) hkl planes in H-CrL3and represents the distances between the centers of the ligands. The broadening can be explained by likelihood that the Cu(II) ions that have displaced some of the exposed hexaaquachromium(III) clusters, which does not occur uniformly across the material. The HMOF tends to be adaptable and can undergo structural shifts in the presence of strongly interacting guest molecules. In this case, H-CrL3adapts to the incoming Cu(II) ions by breaking the hydrogen bonds between [Cr(H2O)6]3+and phosphonate ligands in favour of forming Cu(II) – phosphonate coordination bonds. In addition to this enthalpic driving force, there is also an entropicincentive as Cr(III) ions and solvent molecules are liberated into the solution. This is monitored by PXRD, which shows a decreasing intensity of the (01-1) hkl planes at 5.84⁰ 2θ (corresponding to the chains of hexaaquachromium(III) clusters as discussed in section 5.3.1) along with an increase in intensity at 7.24⁰ 2θ as soaking time increases.

[0170] Notedly, increasing soaking time past 24 hours does not result in further Cr(III) exchange with Cu(II). Initially, Cu(II) does not displace all the Cr(III) ions simultaneously. Instead, there is a gradual replacement of a few Cr(III) ions first, but the geometry of the outgoing hexaaquachromium(III) cluster cannot be perfectly maintained by the incoming Cu(II) ion. The geometry around the metal is distorted and the hydrogen bonds between neighbouring [Cr(H2O)6]3+and phosphonate ligands are no longer optimal. To accommodate this, there are two possibilities that can occur. The first is that there is rapid exchange of Cr(III) with Cu(II) because of the enthalpic driving force to form the Cu(II) – phosphonate coordination bonds compared to the Cr(III) – phosphonate hydrogen bonds as mentioned earlier. The second is that the distortion around the secondary sphere of the hexaaquachromium(III) is so severe that a dehydration is triggered, where the enthalpic contributions of forming Cr(III) – phosphonate coordination bonds is substantial enough to overcome the kinetic barrier associated with the [Cr(H2O)6]3+ion. ICP-OES data suggests that a mixture of both scenarios occurs for CrL3-Cu2. Initial exchange of Cu(II) is easy, but as more Cr(III) ions become replaced by Cu(II) ions, the framework becomes more distorted, and that triggers the dehydration of the remaining Cr(III) centers. Compared to the hydrogen bonded Cr(III) ion, the coordinated Cr(III) metal is too thermodynamically stable to be exchanged, and so subsequent soaking in Cu(II) solution does not result in further transmetallation. This explains the rapid exchange that occurs initially, followed by a sudden plateau in the amount of Cr(III) that can be exchanged after 2 hours of soaking.

[0171] The choice of Cu(II) ions facilitated the success of the transmetallation. The Irving-Williams series states that the stability of 3d octahedral divalent transition metal complexes increases across the periodic table as crystal field stabilization energy increases.271It is well known that this series peaks at Cu(II) owing to its ability to undergo Jahn-Teller distortion to further stabilize the complex. Although Cu(II) is also predominantly found in octahedral geometry, the Jahn-Teller effect allows the metal to adopt other geometries as well, such as tetrahedral or square planar.272This makes Cu(II) more accommodating for different geometries in the metal exchange process. This is an important characteristic as H-CrL3is expected to undergo dramatic structural shifts. This can also be seen as attempts were made to exchange Cr(III) with Ni(II), Fe(III), and Zn(II) in H-CrL3, but only Cu(II) proved to be successful.

[0172] This phenomenon was also observed by Zhang et al. in their Zn(II) based MOF, [Zn7(L)3(H2O)7]n·[Zn5(L)3(H2O)5]n, where L is N-phenyl-N’-phenyl bicyclo[2,2,2]oct-7- ene-2,3,5,6-tetracarboxdiimide tetracarboxylic acid.273The various Zn(II) metals are in square pyramidal, tetrahedral, and octahedral geometry (Figure 17). Metal exchanges were attempted for Ni(II), Co(II) and Cu(II), with only Cu(II) being successful. Similar findings were discovered by Zou et al. for MOF-14, another Zn(II)-based MOF.274Only partial (<40%) exchange was observed for Co(II) and Ni(II) after three months, whereas 95% of exchange was achieved for Cu(II) within three days. In addition, attempts at reintroducing Zn(II) ions into the Cu(II) analogue resulted in only 38% exchange after 3 months.

[0173] Direct Synthesis of Cr(III) / Cu(II) MOF (CrL3-Cu-hy)

[0174] A direct synthesis of a Cr(III) / Cu(II) MOF was attempted by combining equimolar amounts of Cr(NO3)3·9H2O, Cu(NO3)2·2.5H2O, and H8L3with water and methanol in an autoclave. After heating at 120⁰C, a green powder was obtained, identified as CrL3- Cu-hy (hydrothermal). The PXRD pattern (Figure 18) of CrL3-Cu-hy is similar to that of CrL3-Cu1and CrL3-Cu2, and ICP-OES analysis showed that CrL3-Cu-hy contains both Cu(II) and Cr(III) in a 0.53 Cu(II) / Cr(III) ratio.

[0175] Typically, synthesis of H-CrL3by mixing solutions of Cr(NO3)3·9H2O and H8L3at room temperature results in a cloudy solution that precipitates overtime into H-CrL3. Thus, combining the Cu(II), Cr(III), and ligand components in an autoclave can result in initial formation of the HMOF H-CrL3. Then, Cu(II) ions diffuse through the HMOF and become incorporated via the same mechanism discussed above. However, the amount of Cr(III) that can be exchanged with Cu(II) is limited as the temperature of the reaction increases, causing the HMOF to dehydrate. This results in formation of irreversible Cr(III) – phosphonate coordination bonds that make exchange with Cu(II) unfeasible. This is evidenced by the low amounts of Cu(II) in the final product despite having the same PXRD pattern as CrL3-Cu1and CrL3-Cu2, which both have significantly higher amounts of Cu(II) in their frameworks.

[0176] Gas sorption Analysis

[0177] The successful synthesis of the Cr(III) / Cu(II) mixed metal phosphonate MOFs prompted studies to see how doping H-CrL3with Cu(II) affects the gas sorption properties. As seen in Figure 19, there is a clear trend where the CO2capacity is related to the amount of Cu(II) in the MOF. CrL3-Cu2has the highest Cu(II) content in the framework and subsequently has the highest CO2capacity at 273K of all the other MOFs. Compared to the original pure Cr(III) MOF, CrL3, which adsorbs 1.32 mmol / g of CO2at 1.2 bar, CrL3-Cu2has nearly double the capacity, adsorbing 2.50 mmol / g. Unlike the other MOFs, CrL3- Cu2is porous to N2at 77K (Figure 20), with a Type II isotherm and BET surface area of 112 m2 / g (Langmuir = 161 m2 / g). The MOF has a fairly uniform PSD, with two main distributions centered around 10Å and 13Å. CrL3-Cu1has the second highest Cu(II) content and hence the second highest capacity of 2.08 mmol / g, followed by CrL3-Cu-hy, which has the lowest Cu(II) content and only a slightly higher CO2capacity of 1.54 mmol / g compared to the parent MOF, CrL3. This trend can be attributed to an increasing amount of Cu(II) ions in the MOFs, which act as high energy sites for polarizing CO2. Compared to the pure Cr(III) parent MOF, CrL3, the Cu(II) / Cr(III) bimetallic MOFs shows improved gas sorption performance.

[0178] The high CO2capacity exhibited by CrL3-Cu2prompted further gas sorption analysis. The enthalpy of adsorption was found to be high and ranges between -33.1 kJ / mol and -49.89 kJ / mol (Figure 21). This is in the realm of unsaturated metal sites and supports the theory that defects are formed to generate open metal sites during the metal exchange process. This has a direct result on the IAST selectivity,184which shows that for CO2over N2in a 15 / 85 mixture at 273K, CrL3-Cu2has a high selectivity value of 310 at 1 bar (Figure 22).

[0179] Stability Tests

[0180] Exposed Cu(II) sites in a MOF can help improve CO2adsorption properties, but the applications go beyond gas separation. Owing to the metal’s non-toxic nature, abundance and catalytic properties, Cu(II) – based MOFs have widespread use in catalysis and sensors, among others.275,276However, one of the drawbacks to Cu(II) MOFs is their instability. The d9electron configuration results in Jahn-Teller distortion and as a result, the axial positions of Cu(II) are labile and can undergo rapid ligand exchange.110This decreases the stability of many Cu(II) based MOFs and limits their applications. Hence, the mixed-metal approach discussed herein can be used to increase the stability of Cu(II) MOFs without sacrificing the function. It was hypothesized that the inertness of the Cr(III) metal would work synergistically with the Cu(II) metal, resulting in a framework that is robust with enhanced gas sorption properties compared to the parent Cr(III) MOF.

[0181] CrL3-Cu2consists primarily of Cu(II) ions, and only small amounts of Cr(III), with a Cu(II) / Cr(III) ratio of 11.4. Although this MOF shows the best CO2capacity of all the MOFs discussed herein, the abundance of Cu(II) sites and lack of Cr(III) ions tends to make CrL3-Cu2unstable to water. This can be seen when exposing the MOF to 85% relative humidity (RH) for 24 hours. Although the PXRD pattern that shows crystallinity is maintained, gas sorption results show a significant decrease in CO2capacity at 1 bar and273K, from 2.50 mmol / g in the pristine MOF to 1.77 mmol / g (Figure 23). An abundance of Cu(II) sites in the framework may act as weak points for hydrolytic attack, and the small quantity of Cr(III) that is present in the framework may not be insufficient to maintaining framework integrity.

[0182] In contrast, CrL3-Cu-hy has only a 0.53 Cu(II) / Cr(III) ratio, which results in a CO2capacity that is only slightly enhanced compared to the parent CrL3MOF. However, owing to the much higher Cr(III) content in the framework, CrL3-Cu-hy is substantially more stable, and can be left in water for 1 week without change to its porosity or crystallinity (Figure 23).

[0183] Thus, CrL3-Cu1, with a Cu(II) / Cr(III) ratio of 7.4, acts as a good medium between the previous two MOFs as this material has substantially higher CO2capacity compared to the parent MOF, but also exhibits stability after soaking in water for 7 days. In these bimetallic MOFs, there is a balance that exists between the amount of Cu(II) required to enhance gas sorption capabilities and the amount of Cr(III) that must be maintained in the framework to make the MOF robust. Exposed Cu(II) sites are vulnerable points of hydrolytic attack that can lead to framework decomposition, but with enough support from inert metals throughout the framework, the structural integrity of the MOF can still be maintained in the presence of water.

[0184] Conclusions

[0185] Incorporating secondary metals into MOFs can add complexity and significantly improve function of the material. The presence of two different metals within a single framework can result in synergistic effects that can enhance the MOF properties or introduce new functions that were not possible with the homonuclear analogue. Cu(II) based MOFs are widely explored in the literature owing to their applications in catalysis, drug deliver, and impressive gas sorption capabilities. However, Cu(II) coordination bonds are labile and are thus susceptible to hydrolytic attack that leads to framework decomposition. Hence, there is merit in improving the stability of Cu(II) based MOFs.

[0186] Above is explored the synthesis of bimetallic Cu(II) / Cr(III) phosphonate MOFs with the intention that Cu(II) will enhance the gas sorption properties of the MOF while the inert Cr(III) ions increase framework stability. This was attempted by using a PSM approach, where Cu(II) ions are doped into a Cr(III) framework. However, given the incredibly robust nature of Cr(III) – phosphonate MOFs, PSM on the MOF is not feasible. Instead, the PSM approach was performed on the HMOF due to its adaptable nature. The hydrogen bonds are labile so that the framework can accommodate incoming guests, making for a more facile route to transmetallation.

[0187] Used was HMOF, H-CrL3, which consists of [Cr(H2O)6]3+clusters that hydrogen bond to the tetratopic phosphonate ligand, H8L3. The framework possesses large, 1D channels that allow for easy diffusion of incoming and outgoing metal ions, which facilitates the metal exchange process. It was found that soaking the HMOF in 0.01M and 0.03M solutions of Cu(NO3)2·2.5H2O results in successful metal exchange to yield the MOFs CrL3-Cu1and CrL3-Cu2, respectively. A hydrothermal synthesis was also performed, where equimolar mixtures of Cr(III), Cu(II), and H8L3were combined and heated to generate the bimetallic MOF CrL3-Cu-hy.The adaptable nature of the Cr(III) – phosphonate HMOFs was demonstrated as the metal exchange process was facilitated due to the labile hydrogen bonds in the HMOF. Parallel experiments performed on the Cr(III) – phosphonate MOF were unsuccessful. Kinetic studies were also performed on CrL3-Cu2to elucidate the mechanism of transmetallation, which showed that the reaction is rapid and that full exchange of the Cr(III) ions with Cu(II) was not possible.

[0188] ICP-OES analyses show that the Cu(II) / Cr(III) ratio in these MOFs increase in the order of CrL3-Cu-hy < CrL3-Cu1< CrL3-Cu2. This was considered important as there is a direct correlation between the CO2capacity and the Cu(II) content of the MOFs. CrL3- Cu2,with the highest Cu(II) content, exhibits the highest CO2capacity of 2.50 mmol / g. This is followed by CrL3-Cu1, with a capacity of 2.08 mmol / g, and then CrL3-Cu-hy, with a capacity of 1.54 mmol / g.

[0189] The improved CO2capacity is a result of the Cu(II) content, but this also has an effect on the stability of the network. Although CrL3-Cu2has the highest gas capacity, it also suffers from framework instability as the amount of Cu(II) substantially outnumbers that of Cr(III). The MOF was found to be not stable in 85% RH for 24 hours. In contrast, CrL3-Cu1 and CrL3-Cu-hy have lower CO2 capacities but were more robust, maintaining crystallinity and gas sorption properties after soaking in water for 7 days. Thus, the MOF may be tuned to contain varying amounts of Cu(II) by soaking in different concentrations of Cu(NO3)2·2.5H2O. The optimal Cu(II) / Cr(III) ratio can be explored to yield a framework with the highest CO2capacity while still maintaining framework integrity.

[0190] Dehydration method may be used to yield stable and ordered MOFs; for example Cr(III) – phosphonate HMOFs . The HMOF tends to be highly amenable to templates and so dehydrating via different methods can result in different MOFs, each with their own unique properties. HMOFs can be further exploited by incorporation of a secondary metal to yield highly stable bimetallic MOFs with improved function. The metal exchange process demonstrated herein may act as another method of dehydration, adding to the list of the many opportunities offered by the [Cr(H2O)6]3+- phosphonate HMOFs.

[0191] Example 2 - Synthesis of a Robust Ni(II) / Cr(III) Phosphonate MOF via a Ni-HMOF Intermediate

[0192] Introduction

[0193] Example 1 has shown that Cu(II) could be doped into the HMOF, H-CrL3, to yield a bimetallic Cu(II) / Cr(III) MOF. Depending on the Cu(II) / Cr(III) ratio, the MOF can exhibit enhanced gas sorption properties compared to the homometallic Cr(III) MOF, and is stable to water. Other attempts at exchanging Cr(III) ions in H-CrL3with other metals, such as Fe(III), Ni(II), and Zn(II) had been attempted but only Cu(II) proved successful. Without wishing to be bound by theory, it was considered that Cu(II) proved successful because of its ability to undergo Jahn-Teller distortion, which can facilitate metal exchange.

[0194] Direct synthesis of bimetallic MOFs consisting of metals other than Cu(II) had also been attempted via hydrothermal synthesis, but the resulting products showed only Cr(III) in the final MOF structure. Clearly, formation of the monometallic Cr(III) – phosphonate MOF was the energetically favourable product, with only Cu(II) being incorporated into the framework possibly due to its ability to undergo Jahn-Teller distortion. The stability presented by Cr(III) – phosphonate MOFs is attractive and incorporating other metals other than Cu(II) can further their applications. Ni(II) in particular has applications in catalysis for CO2methanation and catalysis.277,278However, Ni(II) MOFs can be plagued by instability in humid conditions due to the labile coordination bonds that are susceptible to hydrolytic attack. As shown in Example 1, incorporation of Cr(III) ions into the framework can enhance stability while maintaining function of the secondary metal in the MOF.

[0195] Hence, an alternative approach was proposed and is herein described whereby Cr(NO3)3·9H2O was doped into a [Ni(H2O)6]2+based HMOF. Phosphonate ligands can form crystalline materials when coupled with mono or divalent metals. This has resulted in many examples of divalent metal phosphonate MOFs, where their structures have been elucidated crystallographically.196However, Ni(II) is unique as it has a d8electron configuration in an octahedral geometry so that, similar to what is observed in Cr(III), there tends to be substantial crystal field stabilization energy.111As a result, hexaaquanickel(II) is inert and retains its aquo ligands in the presence of phosphonates, generating a [Ni(H2O)6]2+– phosphonate HMOF.

[0196] Herein, the Ni(II) based HMOF was synthesized using the same H8L3phosphonate ligand described in Example 1. Interestingly, doping Cr(III) ions into this HMOF resulted in an HMOF-to-HMOF transmetallation, generating the Ni(II) / Cr(III) bimetallic HMOF, H-NiL3-Cr. This HMOF was then dehydrated to yield the mixed metalMOF, NiL3-Cr, with likely exposed metal sites. Whereas the parent Ni(II) MOF dissolves in water, NiL3-Cr was stable after soaking in water for 3 days. Although this MOF was not initially found to be particularly porous, this highlights a method in which Ni(II) / Cr(III) phosphonate MOFs can be synthesized for applications beyond gas adsorption.

[0197] Experimental

[0198] Refer to Example 1 for the synthetic procedure of H8L3.

[0199] Synthesis of H-NiL3, {[Ni(H2O)6]2[H4L3]^C3H6O^2H2O^CH3OH }n

[0200] Ni(NO3)2·6H2O (0.174g, 0.60mmol) was dissolved in 40mL of water. In a separate flask, H8L3(0.196g, 0.3mmol) was dissolved in a mixture consisting of 26mL methanol and 13mL water. The solutions were combined and placed in a larger flask containing 6mL acetone for vapour diffusion. After leaving at room temperature for 2 days, large green crystals of H-NiL3were collected. X-ray crystallography determined a chemical formula of {[Ni(H2O)6]2[H4L3]·(H2O)2(CH3OH)(C3H6O)}n(see Table 6 for crystallographic information and Figure 25 for the crystal structure). This was corroborated by EA data, which yielded a formula of {[Ni(H2O)6]2[H4L3]^C3H6O^2H2O^CH3OH }nfor the bulk powder (Table 4).

[0201] Synthesis of NiL3, {Ni2H4L3·3H2O}n

[0202] The dry, green powder of H-NiL3was placed in an oven that was pre-heated to 100⁰C. After leaving in the oven for 6 hours, the powder turned yellow. Dehydration was confirmed by IR (Figure 26) and PXRD (Figure 28). A formula of {Ni2H4L3^3H2O}nwas obtained by EA (Table 4).

[0203] Table 4: Elemental analysis for H-NiCrL3and NiL3MOF. Compound %C %H Experimental Theoretical Experimental Theoretical

[0204] Synthesis of H-NiL3-Cr and NiL3-Cr

[0205] The dry, light green powder of H-NiL3(41mg) was placed in a solution of Cr(NO3)3·9H2O dissolved in methanol (0.03M, 10mL). After soaking for 24 hours at room temperature, the light green powder became dark green. The powder was filtered and left to soak in fresh methanol for another 24 hours to remove any excess metal trapped in thepores. After filtering, H-NiL3-Cr was obtained. To convert H-NiL3-Cr to NiL3-Cr, the dry powder of H-NiL3-Cr was placed in a preheated oven at 100⁰C. After leaving in the oven for 6 hours, NiL3-Cr was obtained.

[0206] Instrumentation

[0207] ICP-OES analysis

[0208] MOFs were digested by heating to 90⁰C overnight in aqua regia and then diluting with water to obtain a 2% HNO3solution for ICP-OES analysis. The mother liquor was evaporated to dryness and then dissolved in 400mL of a 2% HNO3solution for ICP- OES analysis. Formulas for NiL3-Cr were determined by combining ICP-OES, EDX and EA data. See Section 2.4.3 for details.

[0209] Gas sorption analysis

[0210] All gas sorption analyses were conducted on a Accelerated Surface Area & Porosimetry System (ASAP) 2460 supplied by Micromeritics Instruments Inc. The dry sample (~40mg) was loaded into a glass analysis tube. The MOFs were put under vacuum (~ 10-6mbar) and activated at 120⁰C overnight. After activation, the samples were manually backfilled with N2before being transferred to the analysis port. Here, they were evacuated for a further 120 min before the analyses started.

[0211] Results and Discussion

[0212] Doping Ni(II) into H-CrL3

[0213] In an attempt to synthesize a Ni(II) / Cr(III) phosphonate MOF and based on the results of the Cu(II) / Cr(III) phosphonate MOFs described in Example 1, Cr(III) exchange with Ni(II) was attempted on the HMOF, H-CrL3. Powders of H-CrL3were soaked in methanol solutions of Ni(NO3)2·6H2O ranging in concentrations of 0.01M, 0.03M and 0.06M. After replenishing the Ni(II) solutions for a consecutive three days, no change was observed in the PXRD patterns (Figure 24), and ICP-OES data showed that no Ni(II) was incorporated into the HMOF. The PXRD patterns indicate a decrease in crystallinity over time and is possibly due to slow dehydration. The HMOFs are a kinetic product due to the kinetic inertness of the Cr(III) metal, and are typically left in a fridge overnight to slow the rate of ligand exchange between the aquo ligands and phosphonate. However, these exchange reactions were performed at room temperature, which can facilitate dehydration of the HMOF.

[0214] As shown in Figure 2, each proton on the aquo ligands are involved in charge assisted hydrogen bonds, resulting in 12 hydrogen bonds around each Cr(III) center. For a secondary metal to exchange with the [Cr(H2O)6]3+clusters, each of these hydrogen bonds must be broken and new bonds between the secondary metal and ligandneed to form. Nickel phosphonates are themselves less common, for example compared to copper phosphonates, which, would suggest the bonding to nickel is less favoured. The cumulative strength of the hydrogen bonds in the HMOF poses a significant energy barrier that inhibits incorporation of Ni(II) into the structure.

[0215] The difference observed between doping H-CrL3with Ni(II) and Cu(II) can be attributed to the Irving William Series.271As discussed in Example 1, Cu(II) exhibits the strongest metal-ligand interactions for all 3d divalent metals due to its ability to undergo Jahn-Teller distortion.279Ni(II) coordination complexes, in contrast, do not exhibit the same stability, reducing the enthalpic driving force of the metal exchange. The coordination geometry differences between Ni(II) and Cu(II) also plays a role in the transmetallation process. Whereas Cu(II) can adopt tetrahedral, square planar, or octahedral geometries, Ni(II) is more restrictive in its bonding and favours octahedral geometry. This makes Ni(II) less accommodating in adapting to a new coordination environment and prevents exchange with the Cr(III) metals.

[0216] Evidently, there was not a sufficient driving force to incorporate Ni(II) into H- CrL3, even at the HMOF stage. Hence, an alternative strategy was proposed, where Cr(III) ions could be doped into a Ni(II) HMOF. It was hypothesized that the more thermodynamically favourable Cr(III)-phosphonate coordination bonds, or even more charge-assisted hydrogen bonds, would facilitate the exchange. Additionally, the Ni(II)- coordination bonds are expected to be weaker than Cu(II)-coordination bonds according to the Irving-Williams series, providing a stronger enthalpic driving force for a successful exchange.

[0217] Crystal Structure of H-NiL3

[0218] Rather than forming a MOF, combining a solution of Ni(NO3)2·6H2O with a solution of H8L3at room temperature yielded H-NiL3, a hydrogen bonded network between hexaaquanickel(II) cations and phosphonate ligands (Figure 25). Each hydrated Ni(II) cluster hydrogen bonds to another hexaaquanickel(II), and are arranged in chains that run along the b-axis. These chains are separated by the ligands, which stack offset from each other, and experience extensive hydrogen bonding with the metal ions, water and methanol. The metal to ligand ratio is 2:1, with 1D channels approximately 9.6Å by 10.1Å (including van der Waals radii) that run along the a-axis and are filled by ordered acetone and water molecules.

[0219] Ni(II) exhibits a high hydration enthalpy (-2105 kJ / mol) compared to other divalent cations (e.g. -1996, -2100, -2046 kJ / mol for Co(II), Cu(II), and Zn(II), respectively) owing to its small charge to size ratio.280,281,282In addition, Ni(II) is a d8metal and possessesstrong crystal field stabilization energy in an octahedral environment. This results in a slower ligand exchange rate compared to other first row divalent metal ions with an octahedral configuration (Table 5), and hence a metastable HMOF phase is crystallized in the presence of phosphonate ligands. This is a similar phenomenon as to what is observed for the Cr(III) HMOFs, but it should be highlighted that the Ni(II) HMOF may not be as robust as the Cr(III) HMOFs. Not only is Cr(III) a d3metal that also exhibits high crystal field stabilization energy, but it is trivalent, with a much higher charge to radius ratio, resulting in stronger hydrogen bonds than Ni(II) hydrogen bonds. In other words, both [Cr(H2O)6]3+and [Ni(H2O)6]2+cations are kinetically inert, but the Cr(III) analogue may possess more thermodynamic stability than the Ni(II) complex..

[0220] Table 5: Rate constant for water exchange on first row divalent metal ions.111MetalExchange rate constant, k (298K) (s-1)Mn2+2.1 ൈ 107Fe2+4.4 ൈ 106Co2+3.2 ൈ 106Ni2+3.2 ൈ 104Cu2+5.7 ൈ 109Cr3+2.4 ൈ 10-6

[0221] Bhaumik et al. have also reported a structure consisting of Ni(II) and H8L3, but the material was synthesized under hydrothermal conditions and high temperatures, resulting in a MOF, with Ni(II) ions directly coordinated to the phosphonate ligand.269The material was non-porous as hexaaquanickel(II) cations filled all void spaces within the structure. In contrast, H-NiL3consists entirely of hydrogen bonds between [Ni(H2O)6]2+and H8L3.

[0222] Table 6: Crystallographic information for H-NiL3. Compound H-NiL3Chemical formula C30H58Ni2O28P4Crystal colour Green Fw; F(000) 1108.02; 2312.0 T (K) 173 wavelength (Å) 1.54178 Space group Pna21a (Å) 10.7252(2) b (Å) 13.0374(3)

[0223] 3

[0224] H-NiL3can be easily dehydrated by heating to remove the aquo ligands around the metal, converting the HMOF into the MOF by forcing Ni(II)-phosphonate coordination bonds. The resulting MOF, NiL3, is yellow, compared to the green colour of the HMOF. The dehydration can be seen by a change in the PXRD pattern. As seen in Figure 28, the experimental PXRD pattern for H-NiL3matches that of the simulated pattern well. The peak at 5.36⁰ 2theta corresponds to the (002) hkl planes (Figure 28), which run parallel to the hexaaquanickel(II) chains and bisect the center of the H4L3ligand. When dehydrated, the (002) planes contract and this peak is shifted to 6.28⁰, corresponding in a contraction of 0.34Å. Unfortunately, TGA data could not be obtained to give quantitative information about the dehydration, but EA results (Table 4) and IR data (Figure 26) were consistent with dehydration in converting H-NiL3to NiL3.

[0225] In 2020, Stock et al. published a structure of CAU-46 ([Ni2(H4L3)(H2O)6]^4H2O), a MOF consisting of Ni(II) and H8L3. The stoichiometry between CAU-46 and H-NiL3are the same, with a 2:1 metal to ligand ratio.267By combining Ni(II) cations with the same H8L3ligand but under hydrothermal conditions, a structure very similar to H-NiL3was synthesized (Figure 27). The hydrothermal conditions involved combining Ni(NO3)2*6H2O (0,27mL, 1mol / L) and 58.7mg of ligand in a 2mL reactor, along with 0.73mL of water. The reaction was ramped over 6 hours to 180 °C, held at this temperature for 48 hours, and then cooled to room temperature over 6 hours.

[0226] Except for the hydrated Ni(II) centers in H-NiL3, CAU-46 was a MOF that was isostructural to H-NiL3. In CAU-46, there were NiO6edge-sharing polyhedra, resulting in Ni2O10 clusters containing 6 water molecules with each cluster coordinated to 4 ligands. In contrast, the Ni centers in H-NiL3were all hydrated in the form of Ni(H2O), and each of the water molecules around the Ni hydrogen bonded to the ligands rather than being directly coordinated, like in CAU-46. The divalent nickel centers form dimers that are bridged by two phosphonate groups. Each Ni(II) is coordinated to one other phosphonate group, and are also coordinated to three water molecules. The simulated PXRD pattern of CAU-46 showed that the most prominent peak at 6.1⁰ 2theta corresponds to the (200) hkl plane that runs through the center of the ligands, parallel to the Ni(II) chains. This peak matched the experimental pattern on NiL3well, further demonstrating the dehydration of H- NiL3to NiL3.

[0227] In addition to heating, H-NiL3can be dehydrated by soaking in various solvents. As seen in Figure 29, the HMOF is preserved if soaked in acetone or ethanol, but in the presence of DMF or methanol, dehydration occurs. This effect is hypothesized to be due to the differences in dielectric constants of the solvents. The dielectric constant is directly related to the solvent’s polarity, where the higher the dielectric constant, the more polar a solvent is. For the solvents presented here, the dielectric constant follows the order of DMF (37) > methanol (33) > ethanol (24) > acetone (21).283It can then be deduced that the solvents with higher dielectric constant is more able to attract the water from the hexaaquanickel(II) clusters, which triggers a dehydration. Thus, to preserve the hydrogen bonded network for the metal exchange reactions, ethanol was used a solvent for all experiments.

[0228] Doping Cr(III) into H-NiL3

[0229] Previous experiments showed that incorporating Ni(II) into H-CrL3was not successful, prompting the investigation of incorporating Cr(III) ions into a Ni(II) framework instead. After soaking powders of H-NiL3overnight in a solution of Cr(NO3)3·9H2O, the crystals changed from a light green colour to a dark green. As shown by ICP-OES data, Ni(II) is present in the mother liquor of the exchange reaction, confirming that Cr(III) was exchanged with Ni(II) rather than being adsorbed into the framework.

[0230] Table 7: ICP-OES and EA data for H-NiL3and metal exchange analogues. NiL3-Cr Mother H-NiL3NiL3-Cr liquorICP-OES Ni(II) (ppm)3. All formulas determined by ICP-OES were also corroborated by EA data.

[0232] 1.7mg of the NiL3-Cr MOF was digested in 100mL of a 2% HNO3solution 5 for ICP-OES (see above for details). Assuming a formula of Ni1Cr1.5L3^4H2O (MW = 848.80 g / mol), the expected concentration of Ni(II) in the MOF is 1.058 ppm, which matches the experimental Ni(II) concentration of 1.069 ppm well.

[00233] 10 [0023 Cr(III) in the MOF would be 1.562ppm, similar to the experimental concentration of 1.518 ppm.

[00235]

[0236] By following the same digestion and dilution method for ICP-OES analysis, 2.1 mg of H-NiL3(MW = 1110.146 g / mol) theoretically should yield a Ni(II) concentration of 2.220 ppm, which is similar to the 2.295 ppm concentration found experimentally.

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[0238] The mother liquor from the exchange experiment was evaporated to dryness and then dissolved in 400mL of a 2% HNO3solution. The ICP-OES data shows that 69.5% of the Ni(II) from the original H-NiL3 has been exchanged. EDX analysis was also performed, which supported the ICP data. As shown in Figure 30, the average Cr(III) / Ni(II) ratio is 1.30, comparable to the 1.42 ratio found by ICP and the 1.67 determined theoretically. Additionally, EDX mapping (Figure 31) shows even dispersion of Ni(II), Cr(III), and P throughout the powder.

[0239] Interestingly, the PXRD pattern (Figure9) of H-NiL3-Cr is similar to that of the original Ni(II) HMOF, H-NiL3. After soaking in a Cr(III) solution, the powder is less crystalline, but, surprisingly, the peak at 5.36⁰ 2θ remains, suggesting that the HMOF structure remains intact after the exchange. However, there is an emergence of a new peak at 6.48⁰ 2θ which aligns with the prominent peak in the dehydrated Ni(II) MOF, NiL3. This indicates the onset of dehydration, although FTIR (Figure 33) shows there is still a substantial amount of water in the framework.

[0240] Given that the ICP-OES data shows there truly was a replacement of Ni(II) ions with Cr(III), it is possible that this is actually an HMOF-to-HMOF transmetallation. Both Cr(III) and Ni(II) are kinetically inert, keeping them both in the hexaaqua state at room temperature. However, the hydrogen bonding between [Cr(H2O)6]3+and phosphonates is stronger than that of [Ni(H2O)6]2+due to the higher charge density on Cr(III). This acts as an enthalpic driving force that drives the metal exchange. In the case of CrL3-Cu, incorporation of Cu(II) immediately triggered a rapid transmetallation, where the Cr(III) hydrogen bonds are exchanged with more enthalpically favourable Cu(II) coordination bonds. In contrast, the exchange of hydrogen bonds between [Ni(H2O)6]2+and [Cr(H2O)6]3+are much more reversible and so there is not as much strain on the framework and hence, the HMOF does not fully dehydrate. However, the geometries around the metals will be slightly distorted as the components rearrange to form optimal bonding interactions. This triggers a slight dehydration as evidenced by the small peak in the PXRD pattern at 6.48⁰ 2θ. Quickly heating H-NiL3-Cr by placing the powder in a preheat oven at 120⁰C results in the MOF, NiL3-Cr, the completely dehydrated analogue of the bimetallic HMOF. This is associated with a change in the PXRD pattern, and the loss of the broad peak between2500 cm-1to 3500 cm-1that correspond to -OH groups in FTIR. This data together supports the notion that this metal exchange is via an HMOF-to-HMOF transformation. The original H-NiL3 HMOF is a light green colour, but after the exchange experiment, the powders become a deep green colour. The [Cr(H2O)6]3+complex is associated with a purple colour, which can be detected by UV-Vis experiments.

[0241] Gas Sorption Properties

[0242] Gas sorption analyses were performed on NiL3-Cr. The microporous MOF is non-porous to N2at 77K but exhibits CO2porosity at 273K, yielding a BET surface area of 60 m2 / g (Langmuir = 226 m2 / g) (Figure 34). NLDFT PSD also shows that the pores are very uniform, which is expected given the ordered structure with 1D channels of H-NiL3. To further probe the adsorption properties of the bimetallic MOF, the enthalpy of adsorption was calculated. Figure 35 shows that the ΔHadsat low CO2loadings is very high, with a value of -45.3 kJ / mol. As expected, the fast dehydration from H-NiL3-Cr to yield NiL3-Cr results in open metal sites. The high enthalpy of adsorption is not uncommon in MOFs with open metal sites and is comparable to that of Cr3-Cu2discussed in Chapter 5.284However, the high energy sites provided by the open metal sites also increases the affinity for N2, resulting in an IAST selectivity of 16.2 for CO2over N2at 273 in a 15 / 85 mixture (Figure 36).184

[0243] Stability Tests

[0244] H-NiL3can be dehydrated to yield the pure Ni(II) MOF, NiL3. However, the MOF is not stable. Placing powders of the NiL3in water results in immediate rehydration back to the HMOF, as seen by a colour change from yellow to green, followed by dissolution of the solid within 1 hour. In contrast, the bimetallic MOF, NiL3-Cr, is substantially more stable than the parent monometallic Ni(II) MOF. The PXRD pattern and CO2isotherms show that even after soaking NiL3-Cr in water for 3 days, structural integrity and crystallinity is maintained. The high Cr(III) content in the MOF is important for stabilizing the MOF. FTIR also shows that NiL3-Cr does not rehydrate after soaking in water, as evidenced by the lack of significant OH stretching between 2500 cm-1and 3500 cm-1(Figure 33). Also see Figure 14, which depicts PXRD patterns for pristine NiL3-Cr and after soaking in water for 5 days.

[0245] Conclusions

[0246] Herein, a HMOF, H-NiL3, based on [Ni(H2O)]62+and the phosphonate ligand, H8L3, has been reported. This HMOF was found to not be stable and dehydrates easily in specific solvents. Even after converting to the MOF, NiL3, the material was found not to be stable and dissolved in the presence of water. Hence, H-NiL3was used as an intermediatewhereby doping with Cr(III) can result in a bimetallic Ni(II) / Cr(III) MOF with enhanced stability.

[0247] H-NiL3-Cr was successfully synthesized by soaking H-NiL3 in a solution of Cr(III) ions, and the exchange of Ni(II) with Cr(III) was confirmed by ICP-OES. Interestingly, PXRD and IR suggests that the transmetallation appears to occur in an HMOF-to-HMOF fashion owing to the kinetic inertness of [Cr(H2O)]63+and [Ni(H2O)6]2+. The enthalpic driving force is the formation of stronger Cr(III) hydrogen bonds with H8L3compared to the Ni(II) hydrogen bonds, but the kinetic barrier associated with ligand exchange is too great to overcome. Hence, the result is possibly an HMOF that consists of both hexaaquanickel(II) and hexaaquachromium(III) clusters. H-NiL3-Cr can then be dehydrated to yield the bimetallic MOF, NiL3-Cr. Without wishing to be bound by theory, it was considered that charged metals, such as M2+ / 3+metals, may exchange in a Ni-HMOF due to a relatively lower stability of Ni in the HMOF.

[0248] Compared to the parent monometallic Ni(II) MOF, NiL3-Cr shows enhanced stability. Whereas NiL3dissolves in water after 1 hour, NiL3-Cr can be soaked in water for at least 3 days without change to its gas sorption capacity and crystallinity. In addition, the enthalpy of adsorption shows that there are exposed metal sites in the MOF, which can make for potentially interesting catalytic applications. Additionally, kinetic studies similar to what was done in Example 1 can provide insight into the metal exchange mechanism. Along with this comes the variables can be adjusted to see what effects it has in the conversion of H-NiL3to H-NiL3-Cr. For example, the concentration, soaking time, and temperature can all affect how the bimetallic MOF forms. The amount of Ni(II) / Cr(III) required in the MOF to balance stability and function is also an important aspect to consider.

[0249] The method of doping Cr(III) ions into a Ni(II) HMOF shows bimetallic Ni(II) / Cr(III) phosphonate MOFs can be made by using HMOFs as an intermediate.

[0250] Example 3 - Mixed-Metal MOFs Formed From H-Bonded MOFs Via Proton Exchange

[0251] As HMOFs are based on hydrogen bonded aquo / solvated complexes, it can be possible to introduce at least a second metal into the HMOF structure to form a mixed- metal HMOF that may then be desolvated to form a mixed-metal MOF.

[0252] This may be accomplished through one of two routes, or a combination of both.

[0253] The first route may involve exchange of the aquated or solvated first metal ion in the HMOF for a second metal. The second route may involve the incorporation of a second metal by ion exchange with residual protons on the phosphonate ligand of the HMOF, rather than physical exchange with the first metal. This may occur by forming direct coordination bonds with the phosphonate ligand of the HMOF, or by forming new H-bonds with the framework in the aquated HMOF.

[0254] This proton exchange was investigated with Cr3+HMOFs and Ni2+HMOFs. A factor in facilitating or undergoing the proton exchange was considered to be the degree of protonation of the HMOF / phosphonic acid ligand.

[0255] Examples of Proton-Metal Exchange

[0256] Starting Cr(III) HMOF is shown in Figure 38.

[0257] Procedure: Cr(III) HMOF was soaked in 0.01M MeOH solution of second metal Zn(NO3)2•xH2O, Fe(NO3)3•xH2O, or Cu(NO3)3•xH2O. Solution was replenished once daily for 5 days at room temperature. Powdered samples were digested by soaking in aqua regia, and then diluting with water to obtain a 2% HNO3solution for ICP-OES. Mother liquor solutions were evaporated to dryness and then dissolved in a 2% HNO3solution for ICP- OES.

[0258] For results, see Table 8-10 and Figures 39-44.

[0259] It was observed that metal-metal exchanges using Ni2+, Cu2+, Zn2+, Fe3+on a pre-formed, dehydrated Cr(III)-MOF did not occur (Table 8, 9). This suggests that mixed- metal MOF (M1OF-M2) formation may occur via metal exchange at the H-bonded stage and / or a proton-metal exchange at the HMOF stage for these metals more so than via a metal-metal exchange at the pre-formed MOF stage.

[0260] It was observed that apparent proton-metal exchanges occurred using Zn2+, Fe3+on the Cr(III)-HMOF (Table 8), as Fe and Zn were incorporated into the structure of Cr(III)-HMOF while the amount of Cr did not substantially change before and after the exchange attempt. As such, without being bound by theory, it was considered that the exchange mechanism occurring for Fe and Zn was a proton-metal exchange mechanism, as opposed to a metal-metal exchange mechanism.

[0261] It was observed that metal-metal exchanges using Cu2+on Cr(III)-HMOF did occur (Table 9); and that metal-metal exchange using Cr3+on Ni(II)-HMOF also occured (Table 10).

[0262] Table 8. Exchange of Zn2+and Fe3+for Cr3+ / H+in final MOFs by exchanging at HMOF or MOF stageSample Cr (ppm) Zn (ppm) Fe (ppm) Comment l r s , t l e r l 3 l r l

[0263] Table 9. Exchange of Ni2+and Cu2+for Cr3+ / H+in final MOFs first, and then by exchanging Cu2+at HMOF stage. For synthesis, see Example 1, Experimental Section. For exchange protocol, see Example 1, Results and Discussion Section. Concentration (ppm) CommentCr Cu NiCr(III)-MOF 1.076 N / A 0.000 Cr(III)-MOF + Ni 0.957 N / A 0.001 No Nickel exchange with preformed MOF Cr(III)-MOF + Cu 0.843 0.008 N / A No Copper exchange with preformed MOF No exchange observed. It was considered that Ni wouldn’t exchange as the Cr(III)-HMOF + Ni N / Asecond metal because it may result in a more unstable mixed- metal MOF. In other words, it was not expected that a 2+ ion would displace a 3+ ion. Cr(III)-HMOF + 0.164 1. Significant Cu exchange but Cu 948 N / A retention of some Cr

[0264] Table 10. Exchange of Cr3+for Ni2+ / H+in final MOF by exchanging at HMOF stage. For synthesis, see Example 2, Experimental Section. For exchange protocol, see Example 2, Results and Discussion Section. Cr Ni CommentNi(II)-HMOF 0.007 2.295Ni(II)-HMOF + Cr 1.518 1.069 Clear mixed metal system

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[0266] The synthesis of the ligand 2,4,6-tri-( phenylene-4-phosphonic acid)-s- triazine (or H6PPT) was performed through the nitrile trimerization, Michaelis-Arbuzov reaction and McKenna reaction, by using trifluoromethanesulfonic acid, triethylphosphite at 160ºC and bromotrimethylsilane followed by water, respectively.

[0267] The preparation of the Ni-Cr-HMOF to Ni-Cr-MOF was accomplished by combining dropwise a solution of Ni(NO3)2^6H2O (0.2 mmol) in 0.25 mL of water into asolution of H6PPT (0.1 mmol) dissolved in a mixture consisting of 9 mL isopropanol and 3 mL water. The immediate precipitate was white. After 24 hours at room temperature, the mother liquor was removed and new solvent (3:1 isopropanol:water) was added. Then, a solution of Cr(NO3)3^9H2O (0.1 mmol) in 0.25 mL of water was added dropwise. An 5 immediate greyish precipitate was formed.

[0268] After 24 hours, the mother liquor was removed and new solvent (3:1 isopropanol:water) was added. Then, the HMOF mixture was placed in an oven that was pre-heated to 130ºC. After 6 hours, the obtained MOF was dark green. Figure 45.

[0269] In order to study the effect of the pH, the same preparation took place but 10 with the addition of NaOH 0.02M (1.95 mL) to the solvent solution, increasing the pH to 7.

[0270] The embodiments described herein are intended to be examples only. Alterations, modifications, and / or variations can be effected to the particular embodiments by those of skill in the art. The scope of the claims should not be limited by the particular embodiments set forth herein, but should be construed in a manner consistent with the 15 specification as a whole.

[0271] The aspects, embodiments, and / or examples of the present disclosure being thus described, it should be recognized that said aspects, embodiments, and / or examples may be varied in ways that do not depart from the spirit and scope of the present disclosure, and that said variations are intended to be included within the scope of the 20 following claims.

[0272] All publications, patents and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication patent, or patent application was specifically and individually indicated to be incorporated by reference.

Claims

WHAT IS CLAIMED IS:

1. A process for preparing a mixed-metal metal-organic framework, the process comprising: providing a hydrogen bonded metal organic framework (HM1OF); soaking the hydrogen bonded metal organic framework (HM1OF) in a metal (M2)- comprising solution; and forming a mixed-metal MOF (M1OF-M2), where M1and M2are different metals.

2. The process of claim 1, wherein soaking the hydrogen bonded metal organic framework (HM1OF) in a metal (M2)-comprising solution comprises: soaking the hydrogen bonded metal organic framework (HM1OF) in the metal (M2)- comprising solution; and forming a mixed-metal hydrogen bonded metal organic framework (HM1OF-M2).

3. The process of any previous claim, wherein soaking the HM1OF in the metal (M2)- comprising solution comprises transmetallation of M1in the HM1OF with M2of the metal (M2) comprising solution to form the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2).

4. The process of any previous claim, wherein the transmetallation comprises exchanging between >0% and <100% of M1in the HM1OF with M2of the metal (M2)-comprising solution.

5. The process of any previous claim, wherein soaking the HM1OF in the metal (M2)- comprising solution comprises proton exchange between the HM1OF and the M2of the metal (M2) comprising solution to form the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2).

6. The process of any previous claim, wherein the proton exchange comprises exchanging between >0% and <100% of protons in the HM1OF with the M2of the metal (M2)-comprising solution.

7. The process of any previous claim, wherein soaking the HM1OF in the metal (M2)- comprising solution comprises soaking the HM1OF in a first metal (M2)-comprising solution; andsoaking the HM1OF in at least a second metal (M2)-comprising solution; wherein the metal M2of the first solution is different from the metal M2of the second solution.

8. The process of any previous claim, wherein the metal (M2)-comprising solution comprises a one or more metal (M2) salts, one or more organometallic (M2) compounds, a hydrate thereof, a solvate thereof, or a combination thereof.

9. The process of any previous claim, wherein soaking the HM1OF in the metal (M2)- comprising solution comprises soaking the HM1OF in a metal (M2)-comprising solution; the metal (M2)-comprising solution comprising two or more metal (M2) salts, two or more organometallic (M2) compounds, hydrates thereof, solvates thereof, or a combination thereof, and the metal (M2) of each metal (M2) salt and / or each organometallic (M2) compound is a different metal (M2).

10. The process of any previous claim, wherein forming a mixed-metal MOF (M1OF-M2) comprises desolvating the mixed-metal hydrogen bonded metal organic framework (HM1OF-M2) at, or above 50⁰C; at, or above 75⁰C; or at, or above 100⁰C to form the mixed-metal MOF (M1OF- M2).

11. The process of any previous claim, wherein providing the hydrogen bonded metal organic framework (HM1OF) comprises: providing a hydrogen-bonded organic framework (HOF); combining the hydrogen-bonded organic framework (HOF) with a metal (M1)-comprising solution; and forming the hydrogen bonded metal organic framework (HM1OF).

12. The process of any previous claim, wherein the metal (M1) comprises Cr3+, Co2+, Ni2+, Na+, K+, Li+, Mn2+, Fe2+, Ca2+, Mg2+, Sr2+, Ba2+, hydrates thereof, solvates thereof, or a combination thereof.

13. The process of any previous claim, wherein the metal (M2) comprises a transition metal or post-transition metal, such as a transition metal or a post-transition metal with a +2 oxidation state or higher; a lanthanide; a metalloid; hydrates thereof, solvates thereof, or a combination thereof.

14. The process of any previous claim, wherein the hydrogen bonded metal organic framework (HM1OF) comprises one or more guest molecules for templating pore structure, pore size distribution, surface area, and / or crystallinity of the formed mixed-metal MOF (M1OF-M2).

15. The process of any previous claim, wherein the one or more guest molecules comprises a solvent of the metal (M1)-comprising solution; a solvent of the metal (M2)-comprising solution; and / or water, acetone, acetonitrile, methanol, ethanol, isopropanol, acetic acid, ethylene glycol, ethyl acetate, nitrobenzene, nitromethane, toluene, an ammonium compound, CO2, CO, methane, ethylene, propane, propene, acetylene, H3PO4, H2SO4, HCI, HBr, formic acid, H2CO3, HNO3, ortho-xylene, meta-xylene and para-xylene; or amines, alcohols, ethers, esters, ketones, substituted or unsubstituted aromatics, substituted or unsubstituted polyaromatics; or any salts thereof; or any combination thereof.

16. The process of any previous claim, wherein the hydrogen-bonded organic framework (HOF) comprises an organic phosphonate ligand, optionally substituted with one or more alkyl groups, amino groups, halogens, nitro groups, alcohol groups, or a combination thereof.

17. The process of any previous claim, wherein the hydrogen bonded metal organic framework (HM1OF) has the structure: {[Cr(H2O)6]2[H2L3]^(CH3OH)2(C3H6O)2}n; {[Cr(H2O)6]2[H2L3]^(H2O)}n; or {[Ni(H2O)6]2[H4L3]^C3H6O^2H2O^CH3OH}n.

18. The process of any previous claim, wherein mixed-metal MOF (M1OF-M2) has a M2 / M1ratio of: about 0 to about 100.

19. The process of any previous claim, wherein mixed-metal MOF (M1OF-M2) has the structure:Cr0.27Cu2H3.19L3^5H2O; Cr0.22Cu2.5H2.6L3^4H2O; or Ni0.9Cr1.5H1.6L3^3H2O.

20. A mixed-metal metal-organic framework (M1OF-M2) prepared by the process of any one of claims 1-19.

21. The mixed-metal metal-organic framework (M1OF-M2) of the previous claim, having the structure: [M1]a[M2]b[HxLn]c, wherein M1and M2are different metals, a and b is metal content where a and b are between 0 and 6, x is a protonation state of ligand L between 0 and 12, n is a number label given to the ligand, and c is number of ligands per unit cell in the formula between 0 and 6.

22. A mixed-metal metal-organic framework (MOF) prepared by the process of any one of claims 1-19, useful for adsorption, gas separation, catalysis, sensing, magnetic solids, luminescent solids, fluorescent solids, ion conduction, ion exchange, or a combination thereof.

23. A method of uptaking at least one substance into a mixed-metal MOF prepared by the process of any one of claims 1-19, the method comprising contacting the mixed-metal MOF with the at least one substance under conditions for uptaking the at least one substance into the mixed- metal MOF.

24. The method of claim 23, wherein the at least one substance comprises carbon dioxide, nitrogen, hydrogen, a transition metal, a post-transition metal, a lanthanide, a metalloid, or a combination thereof.

25. The method of any previous claim, wherein uptaking the at least one substance is for the purpose of its storage, absorption, or separation, or a combination thereof.

26. A ligand consisting or comprising of 2,4,6-tri-( phenylene-4-phosphonic acid)-s-triazine.

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