Extractants that leach, bind, and release transition-metal elements and methods of use thereof

Abstract

A leachant / extractant for selective separation of a transition-metal element is provided, wherein the extractant includes a 7,7'-diamido-2,2'-diindolylmethane backbone and a plurality of aromatic pendant groups, wherein each aromatic pendant group of the plurality of aromatic pendant groups is covalently linked to the 7,7'-diamido-2,2'-diindolylmethane backbone through an amide bond. Also provided is a method of using such extractant for selective separation of a transition-metal element, wherein the method includes leaching the transition-metal element from a feedstock to produce a metallate of the transition-metal element, extracting the metallate using the extractant to form an extractant-metallate host-guest adduct, and recovering, using a stripping agent, the transition-metal element from the extractant-metallate host-guest adduct.

Description

Attorney Docket No. 0073605-001091EXTRACTANTS THAT LEACH, BIND, AND RELEASE TRANSITION-METAL ELEMENTS AND METHODS OF USE THEREOFCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Application Serial No. 63 / 735,495, filed on December 18, 2024, and entitled “EXTRACTANTS THAT LEACH, BIND, AND RELEASE FOR METAL SEPARATIONS”, the entirety of which incorporated herein by reference.FIELD OF THE INVENTION

[0002] The present invention generally relates to extractants for separation of transition-metal elements and methods of use thereof.BACKGROUND

[0003] Transition-metal elements are versatile and indispensable building blocks across many scientific and industrial fields such as metallurgy, electronics, catalysis, energy storage, dyes / pigments, and transportation, among others. Extraction of transition-metal elements from ores is typically performed through a sequence of leaching, separation / extraction, and deposition steps. Leaching is a process where an insoluble feedstock (e.g., an ore) including a mixture of several elements is made soluble by reacting with one or more leaching agents. Separation / extraction is a process where a soluble element of interest is separated from one or more other elements that became soluble in the leaching process. Organic molecules are often added to selectively interact with one of the elements over the others, and these interactions are often strategically engineered to increase the degree of separation. After one or multiple separation / extraction steps, deposition occurs where the separated elements are stripped from the solution and any organic molecules that were used in the separation / extraction process. See FIG. 1.

[0004] Several existing methods for extracting transition-metal elements are based on such sequential operation. These methods are often plagued by low efficiencies, limited yields, safety hazards, and excessive costs related to e.g., construction and maintenance of instruments, among other drawbacks. For instance, the chemical reagents and / or experimental conditions used in each of the leaching, separation / extraction, and deposition steps are typically different from one another, which hinders the efficiency of separation and leads to a limited yield of transition-metal element(s). One of such examples is the Mond process (also known as the carbonyl process, see FIG. 2), which was used to extract Ni before the end of the 19thcentury. Specifically, an insoluble feedstock containing nickel oxide(s) reacts with carbon monoxide (CO) to form a Ni(CO)4, which is a gaseous,Attorney Docket No. 0073605-001091 volatile, and highly poisonous coordination complex. This coordination complex subsequently undergoes thermal decomposition to form metallic Ni and carbon monoxide (CO).

[0005] Solution-based means for extraction of transition-metal elements remain limited and underexplored. One such example is a liquid-liquid extraction method based on phenolic oximes, which are applied in 25% of the world’s copper production. Nevertheless, this method is complicated by several side reactions, such as the dimerization and / or deprotonation of phenolic oximes and the coordination between Cu(II) and Cl' that form [CuCh]2'. Suitable alternatives to phenolic oximes remain limited.

[0006] Further, despite the tendency of transition-metal elements to form coordination complexes, Only a few successful strategies exist in selectively capturing transition-metal elements in their complexed forms. 7,7'-Diureido-2,2'-diindolylmethanes represented a promising scaffold for constructing supram olecul ar “ligands” for specific recognition and binding of non-metal, polyatomic anions. For example, Dydio et al. discloses a molecule that specific binds to organic oxyanions such as phosphates (Dydio, Zielinski, and Jurczak, “7,7'-Diureido-2,2'-diindolylmethanes: Anion Receptors Effective in a Highly Competitive Solvent, Methanol”, Org. Lett. 2010; 12(5): 1076-1078). However, this molecule has not been used to bind to anions that contain transition-metal elements.

[0007] Robust, cost-effective, and environmentally friendly solutions for extraction of transition-metal elements remain to be explored.SUMMARY OF THE DISCLOSURE

[0008] An aspect of the present disclosure is a leachant / extractant for selective separation of a transition-metal element. The extractant includes a 7,7’-diamido-2,2’-diindolylmethane backbone. The extractant further includes a plurality of aromatic pendant groups, wherein each aromatic pendant group of the plurality of aromatic pendant groups is covalently linked to the 7,7’-diamido- 2,2’-diindolylmethane backbone through an amide bond.

[0009] In some embodiments, the leachant / extractant can selectively bind to a metallate of the transition-metal element to form an extractant-metallate host-guest adduct, preferably with a binding constant of at least 10,000 M'1, more preferably with a binding constant of at least 30,000 M'1, even more preferably with a binding constant of at least 100,000 M’1. In some embodiments, the leachant / extractant can bind to a metallate of the transition-metal element predominantly in a 1 : 1 stoichiometry.

[0010] In some embodiments, the extractant-metallate host-guest adduct, upon treatment using a stripping agent, releases the transition-metal element from the extractant.Attorney Docket No. 0073605-001091

[0011] In some embodiments, the stripping agent can include a 1,3 -dicarbonyl, such as without limitation acetyl acetone. In some embodiments, the stripping agent can include an acid. In some embodiments, the acid can include one or more of HBF4 etherate, H2SO4, and H3PO4.

[0012] In some embodiments, the metallate to which the leachant / extractant binds is an oxometallate having a formula of MOm”’ or a halogenometallate having a formula of MXm11', wherein M denotes the transition-metal element; n denotes an integer of at least 1 and no greater than 3, preferably 2; X denotes F, Cl, Br, or I, preferably Cl; and m denotes 4 or 6, preferably 4.

[0013] In some embodiments, the metallate to which the leachant / extractant binds can be a tetraalkyl ammonium salt. In some embodiments, the alkyl chain of the tetraalkylammonium salt can include 1-6 carbon atoms. In some embodiments, the tetraalkylammonium salt can be a tetrabutylammonium salt, such as without limitation a tetra-n-butylammonium salt. In some embodiments, the tetraalkylammonium salt can be a tetraethyl ammonium salt.

[0014] In some embodiments, the metallate to which the leachant / extractant binds can be an oxometallate having a formula of MC2’ or a halogenometallate having a formula of MCU2’, wherein M denotes the transition-metal element.

[0015] In some embodiments, the transition-metal element can be a Group 6, Group 9, Group 10, or Group 11 transition-metal element. In some embodiments, the transition-metal element can be selected from the group consisting of Cr, Mo, W, Co, Ni, Pd, Pt, and Cu. In some embodiments, the transition-metal element can be Cr, Mo, W, Co, Ni, Pd, Pt, or Cu. In some embodiments, the transition-metal element can be Mo or W.

[0016] In some embodiments of the leachant / extractant, at least one pendant aromatic group of the plurality of aromatic pendant groups can be a pyridinyl group, a / -butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, / -butylphenyl group, or pyrrolyl group are independently and optionally substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

[0017] In some embodiments of the leachant / extractant, at least one pendant aromatic group of the plurality of aromatic pendant groups can be a pyridinyl group, a / -butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, z-butylphenyl group, or pyrrolyl group are independently and optionally substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl groupAttorney Docket No. 0073605-001091 having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of-O-, -S-, an amine (-NH-, -NR-, etc., wherein R denotes a substituent), a carbonyl, an amide, and an ester.

[0018] Tn some embodiments of the leachant / extractant, at least one pendant aromatic group of the plurality of aromatic pendant groups can be a 2-pyridinyl group, a 4- / -butylphenyl group, or a 2- pyrrolyl group.

[0019] In some embodiments of the leachant / extractant, at least one pendant aromatic group of the plurality of aromatic pendant groups can include one or more N-heterocycles selected from the group consisting of a substituted or unsubstituted imidazole, a substituted or unsubstituted benzimidazole, a substituted or unsubstituted pyrazole, a substituted or unsubstituted indazole, a substituted or unsubstituted oxazole, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted isoxazole, a substituted or unsubstituted benzisoxazole, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothiazole, a substituted or unsubstituted triazole, a substituted or unsubstituted benzotriazole, a substituted or unsubstituted oxodiazole, a substituted or unsubstituted benzoxadi azole, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadi azole, a substituted or unsubstituted tetrazole, a substituted or unsubstituted oxatriazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted pentazole, a substituted or unsubstituted oxatetrazole, a substituted or unsubstituted thiatetrazole, a substituted or unsubstituted pyridine, a substituted or unsubstituted quinoline, a substituted or unsubstituted pyrazine, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted naphthyridine, a substituted or unsubstituted benzodiazine, and a substituted or unsubstituted diazanaphthalene, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted quinazoline, a substituted or unsubstituted pyridazine, a substituted or unsubstituted phthalazine, a substituted or unsubstituted cinnoline, a substituted or unsubstituted triazine, a substituted or unsubstituted benzotriazine, a substituted or unsubstituted tetrazine, a substituted or unsubstituted purine, and a substituted or unsubstituted pteridine, wherein the one or more N-heterocycles are optionally fused with a substituted orAttorney Docket No. 0073605-001091 unsubstituted benzene, furan, pyrrole, thiophene, indole, naphthalene, anthracene, phenathrene, chryscene, or pyrene.

[0020] In some embodiments of the leachant / extractant, one or more hydrogen atoms of the 7,7’-diamido-2,2’-diindolylmethane backbone can be independently substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; and a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

[0021] In some embodiments of the leachant / extractant, one or more hydrogen atoms of the 7,7’-diamido-2,2’-diindolylmethane backbone can be independently substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of-O-, -S-, an amine (-NH-, -NR-, etc., wherein R denotes a substituent), a carbonyl, an amide, and an ester.

[0022] Another aspect of the present disclosure is a method for selective separation of a transition-metal element. The method includes leaching the transition-metal element from a feedstock to produce a metallate of the transition-metal element. The method further includes extracting the metallate using an extractant to form an extractant-metallate host-guest adduct. The extractant includes a 7,7’-diamido-2,2’-diindolylmethane backbone, and a plurality of aromatic pendant groups, wherein each aromatic pendant group of the plurality of aromatic pendant groups is covalently linked to the 7,7’-diamido-2,2’-diindolylmethane backbone through an amide bond. The method further includes recovering, using a stripping agent, the transition-metal element from the extractant-metallate host-guest adduct. In some embodiments, the method can further include recycling the extractant after recovery of the transition-metal element.

[0023] In some embodiments of the method, leaching the transition-metal element can include treating the feedstock with a basic solution.Attorney Docket No. 0073605-001091

[0024] In some embodiments, the stripping agent can include a 1,3 -dicarbonyl, such as without limitation acetyl acetone. In some embodiments of the method, the stripping agent can include an acid. In some embodiments, the acid can include one or more of HBF4 etherate, H2SO4, and H3PO4.

[0025] In some embodiments of the method, the metallate can be dissolved in a first solution phase, the extractant can be dissolved in a second solution phase immiscible with the first solution phase. Accordingly, extracting the metallate using the extractant can include transferring the metallate from the first solution phase to the second solution phase upon forming the extractant- metallate host-guest adduct.

[0026] In some embodiments of the method, the first phase can be an aqueous phase, and the second phase can be an organic phase.

[0027] In some embodiments of the method, the extractant can selectively bind to the metallate with a binding constant of at least 10,000 M'1, more preferably with a binding constant of at least 30,000 M'1, even more preferably with a binding constant of at least 100,000 M'1. In some embodiments, the extractant can bind to a metallate of the transition-metal element predominantly in a 1 : 1 stoichiometry.

[0028] In some embodiments, the metallate can be an oxometallate having a formula of h40m or a halogenometallate having a formula of MXm11', wherein M denotes the transition-metal element; n denotes an integer of at least 1 and no greater than 3, preferably 2; X denotes F, Cl, Br, or I, preferably Cl; and m denotes 4 or 6, preferably 4.

[0029] In some embodiments, the metallate can be a tetraalkylammonium salt. In some embodiments, the alkyl chain of the tetraalkylammonium salt can include 1-6 carbon atoms. In some embodiments, the tetraalkylammonium salt can be a tetrabutyl ammonium salt, such as without limitation a tetra- / 7-butylammonium salt. In some embodiments, the tetraalkyl ammonium salt can be a tetraethylammonium salt.

[0030] In some embodiments, the metallate can be an oxometallate having a formula of MO42' or a halogenometallate having a formula of MCI42', wherein M denotes the transition-metal element.

[0031] In some embodiments, the transition-metal element can be a Group 6, Group 9, Group 10, or Group 11 transition-metal element.

[0032] In some embodiments, the transition-metal element can be selected from the group consisting of Cr, Mo, W, Co, Ni, Pd, Pt, and Cu. In some embodiments, the transition-metal element can be Cr, Mo, W, Co, Ni, Pd, Pt, or Cu. In some embodiments, the transition-metal element can be Mo or W.Attorney Docket No. 0073605-001091

[0033] In some embodiments, at least one pendant aromatic group of the extractant can be a pyridinyl group, a / -butylphcnyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, z-butylphenyl group, or pyrrolyl group are independently and optionally substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; and a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

[0034] In some embodiments of the method, at least one pendant aromatic group of extractant can be a pyridinyl group, a Z-butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, Z-butylphenyl group, or pyrrolyl group are independently and optionally substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of -O-, -S-, an amine (-NH-, - NR-, etc., wherein R denotes a substituent), a carbonyl, an amide, and an ester.

[0035] In some embodiments, at least one pendant aromatic group of the extractant can be a 2- pyridinyl group, a 4-Z-butylphenyl group, or a 2-pyrrolyl group.

[0036] In some embodiments, at least one pendant aromatic group of the extractant can include one or more N-heterocycles selected from the group consisting of a substituted or unsubstituted imidazole, a substituted or unsubstituted benzimidazole, a substituted or unsubstituted pyrazole, a substituted or unsubstituted indazole, a substituted or unsubstituted oxazole, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted isoxazole, a substituted or unsubstituted benzisoxazole, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothi azole, a substituted or unsubstituted triazole, a substituted or unsubstituted benzotri azole, a substituted or unsubstituted oxodiazole, a substituted or unsubstituted benzoxadi azole, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadi azole, a substituted or unsubstituted tetrazole, a substituted or unsubstituted oxatriazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted pentazole, a substituted or unsubstituted oxatetrazole, a substituted or unsubstituted thiatetrazole, aAttorney Docket No. 0073605-001091 substituted or un substituted pyridine, a substituted or unsubstituted quinoline, a substituted or unsubstituted pyrazine, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted naphthyridine, a substituted or unsubstituted benzodiazine, and a substituted or unsubstituted di azanaphthalene, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted quinazoline, a substituted or unsubstituted pyridazine, a substituted or unsubstituted phthalazine, a substituted or unsubstituted cinnoline, a substituted or unsubstituted triazine, a substituted or unsubstituted benzotriazine, a substituted or unsubstituted tetrazine, a substituted or unsubstituted purine, and a substituted or unsubstituted pteridine, wherein the one or more N-heterocycles are optionally fused with a substituted or unsubstituted benzene, furan, pyrrole, thiophene, indole, naphthalene, anthracene, phenathrene, chryscene, or pyrene.

[0037] In some embodiments, one or more hydrogen atoms of the 7,7’-diamido-2,2’- diindolylmethane backbone of the extractant can be independently substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; and a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

[0038] In some embodiments, one or more hydrogen atoms of the 7,7’-diamido-2,2’- diindolylmethane backbone of the extractant can be independently substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of-O-, -S-, an amine (-NH-, -NR- etc., wherein R denotes a substituent), a carbonyl, an amide, and an ester.

[0039] These and other aspects and features of nonlimiting embodiments of the present invention will become apparent to those skilled in the art upon review of the following description of specific nonlimiting embodiments of the invention in conjunction with the accompanying drawings.Attorney Docket No. 0073605-001091BRIEF DESCRIPTION OF THE DRAWINGS

[0040] For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings.

[0041] FIG. 1 depicts an exemplary schematic showing a traditional approach for transporting and depositing a metal using a ligand through a sequence of leaching, separation / extraction, and deposition steps. Specifically, the process involves conversion of solid feedstock to form an intermediate of a gas phase, transporting the intermediate through the gas phase, and depositing / solidifying the metal while recycling / reusing the transporting ligands.

[0042] FIG. 2 depicts an exemplary schematic showing the Mond process for extraction of metallic Ni, in accordance with details of FIG. 1.

[0043] FIGS. 3A-B depict exemplary schematics showing the general approach of the present disclosure for extraction of transition-metal elements in the liquid / solution phase. Specifically, the approach involves conversion of solid feedstock to form an intermediate of a liquid / solution phase, transporting the intermediate through the liquid phase, and depositing the transition-metal element while recycling the transporting ligands.

[0044] FIGS. 4A-B depict exemplary schematics of extraction of Cu(II) using phenolic oximes and complications associated thereto. This process is specifically complicated by several side reactions, such as the dimerization and / or deprotonation of phenolic oximes and the coordination between Cu(II) and CT that form [CuCh]2’.

[0045] FIGS. 5A-B depict exemplary schematics showing the interaction between the extractants described herein and oxometallates, [MCE]2-(FIG. 5A), and chlorometallates, [MCE]2' (FIG. 5B), of transition-metal elements, M.

[0046] FIG. 6A depicts an exemplary schematic showing the extraction of Mo(VI) using the extractant described herein and acetylacetone (acacH) as a stripping agent. First, the extractant leaches Mo from solid MoOa to form a soluble extractant-MoCE2' adduct. Acetyl acetone, upon tautomerization to form its enol tautomer, releases MoO42’ from the extractant by forming an insoluble, octahedral [Mode] species, thereby regenerating the extractant.

[0047] FIG. 6B depicts an exemplary schematic showing the extraction of Cu(II) using the extractant described herein.

[0048] FIG. 7A depicts an exemplary schematic of a general chemical formula of the extractant described herein. The extractant includes a 7,7’-diamido-2,2’-diindolylmethane backbone.Attorney Docket No. 0073605-001091

[0049] FIG. 7B depicts an exemplary schematic of an extractant-MoCE2' complex, wherein the pendant aromatic groups of the extractant are 2-pyrrolyl groups.

[0050] FIG. 8 depicts three exemplary chemical structures of the extractants described herein, wherein the pendant aromatic groups are 2-pyrrolyl groups (L3), 2-pyidinyl groups (L4), and 4- / - butylphenyl groups (L5), respectively.

[0051] FIG. 9 depicts an exemplary schematic depicting the interaction between L5 and MoC>42’ to produce an L5-MoO42' adduct.

[0052] FIG. 10 depicts an exemplary structure, based on X-ray crystallography, of the L5- MoO42’ adduct described in FIG. 9. [nBu4]+counterions are omitted for clarity.

[0053] FIG. 11 depicts an exemplary, partially redacted structure, based on X-ray crystallography, of the extractant-MoC2" adduct described herein. The extractant-MoCM2-adduct is stable upon isolation, and its solution-phase structure is stable as well.

[0054] FIG. 12 depicts stacked 'l l NMR spectra (400 MHz, CD3CN) of L5 titrated with increasing molar equivalents of MoCh2'. The change in peak position and linewidth over the course of titration suggests the formation of an L5-MoC>42' adduct.

[0055] FIG. 13 depicts an exemplary schematic depicting the process of stripping MoC2' from an extractant-MoO42‘ adduct using a stripping agent.

[0056] FIG. 14 depicts stacked 'H NMR spectra (400 MHz, CD3CN) of an extractant-MoO42' adduct titrated with increasing molar equivalents of a stripping agent. The change in peak position and linewidth over the course of titration suggests the release of Mod ' from the extractant-MoO42' adduct and the regeneration of the extractant.

[0057] FIG. 15 depicts an exemplary schematic depicting the process of stripping MoO42" from the L5-MOO42' adduct using HBF4 etherate (HBF4 Et2O) as a stripping agent. L5 is regenerated as a BF4- salt, which is a monoanionic blue solid.

[0058] FIGS. 16A-B depict two sets of stacked 'H NMR spectra (400 MHz, CD3CN) of the L5- MOO42’ adduct titrated with increasing molar equivalents of HBF4 etherate. The change in peak position and linewidth over the course of titration suggests the release of MoC>42’ from the L5- MOO42’ adduct and the regeneration of the L5. Throughout the course of titration, the color of the solution changes from yellow to blue and dark blue.

[0059] FIG. 17 depicts an exemplary photograph showing a H2MOO4 precipitate that forms upon treatment of the L5-MoO42’ adduct with HBF4 etherate.Attorney Docket No. 0073605-001091

[0060] FIGS. 18A-C depict stackedXH NMR spectra from a titration of L5 with increasing molar equivalents of MoOr' to form the L5-MoO42' adduct, followed by a titration of the L5-MoC>42' adduct with increasing molar equivalents of HBF4 etherate (FIG. 18A), H2SO4 (FIG. 18B), and H3PO4 (FIG. 18C), respectively. The change in peak position and linewidth over the course of titrations suggests the formation of an L5-MoC>42’ adduct and the subsequent release of MoO42’ from the L5-MOC>42’ adduct, which regenerates free L5.

[0061] FIG. 19 depicts stacked 'H NMR spectra (500 MHz, CD3CN) of L5 titrated with increasing molar equivalents of WO42’. The change in peak position and linewidth over the course of titration suggests the formation of an L5-WO42' adduct.

[0062] FIG. 20 depicts an exemplary structure, based on X-ray crystallography, of the L5- WO42adduct described in FIG. 19, . [nBu4]+counterions are omitted for clarity.

[0063] FIG. 21 depicts stacked 'H NMR spectra (500 MHz, CD3CN) of L5-MoO42’ and L5- WO adducts. The match between the two spectra suggests structural similarity between the two adducts.

[0064] FIG. 22 depicts an exemplary schematic depicting the interaction between L3 and MOO42’ to produce an L3-MoC>42’ adduct. An exemplary structure, based on X-ray crystallography, of the L3-MOO42‘ adduct is shown on the right.

[0065] FIG. 23 depicts an exemplary schematic depicting the interaction between increasing molar equivalents of L3 and a fixed amount of MoCh2’, which sequentially produces L5-MoO42' adducts in 1 : 1 and 2: 1 stoichiometries. Ku and K21 are the binding constants of the two adducts, respectively. (Ku x K21 > 105M'2) The two binding constants are too large to be resolved or deconvoluted using NMR spectroscopy.

[0066] FIG. 24 depicts a simulated plot between the change in chemical shift of 'H NMR peaks and the molar ratio between the initial concentrations (as indicated by the subscript “0”) of L5 (host) and MOO42’ (guest).

[0067] FIG. 25 depicts an exemplary schematic depicting the interaction between increasing molar equivalents of L4 and a fixed amount of MoCh2’, which sequentially produces L4-MoO42' adducts in 1 : 1 and 2: 1 stoichiometries. Ku and K21 are the binding constants of the two adducts, respectively. The 1 : 1 adduct is strongly preferred over the 2:1 adduct (Ku = 160,000 M’1, and K21 = 3,000 M’1).Attorney Docket No. 0073605-001091

[0068] FIG. 26 depicts stacked 'H NMR spectra of L4 titrated with increasing molar equivalents of MoO42'. The change in peak position and linewidth over the course of titration suggests the formation of an L4-MoO42’ adduct.

[0069] FIG. 27 depicts stacked 'H NMR spectra of L5 titrated with increasing molar equivalents of [NiCU]2’. The change in peak position and linewidth over the course of titration suggests the formation of an L5-[NiC14]2’ adduct.

[0070] FIG. 28 depicts a comparison of theXH NMR spectra (400 MHz, CD3CN) of L5 titrated with increasing molar equivalents of MoC>42’ and the 'H NMR spectrum of a mixture of L5 and 1 molar equivalent of [NiCh]2’. While it is speculated that it might have been the Cl’ that interacts with L5, the exact mechanism for the interaction between L5 and [NiCh]2' is still a study in progress.

[0071] FIGS. 29-31 depict stacked1H NMR spectra of L5 titrated with increasing molar equivalents of [CoC14]2’(FIG. 29), [NiCh]2’, and [CuCh]2’, respectively. While it is speculated that it might have been the Cl’ that interacts with L5, the exact mechanism for the interaction between L5 and [CoCh]2', [NiCh]2’, or [CuCh]2’ is still a study in progress.DETAILED DESCRIPTION

[0072] The present disclosure describes a new class of extractants for selective separation of transition-metal elements in solution phase (i.e., solvo-metallurgy). The extractants described herein are capable of performing substantially all of the following three functions: (1) leaching transitionmetal elements from their respective feedstocks; (2) separating / extracting the transition-metal elements from the leachate with high affinity and / or specificity; and (3) releasing the transitionmetal elements to regenerate the extractant. See FIGS. 3A-B. The extractants described herein require less instrumentation (and maintenance thereof), demand a smaller footprint to deploy, and can achieve effective separation through fewer purification cycles.

[0073] The present disclosure further provides the first crystallographic characterization of the host-guest adducts formed between the extractants and the metallates to which they bind with high specificity and / or affinity.

[0074] The extractants described herein provide alternatives to extractants such as phenolic oximes (FIGS. 4A-B) and can greatly alter and improve the technoeconomic viability in separating transition-metal elements.

[0075] To facilitate the understanding of this invention, a number of terms are defined below and throughout the disclosure. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to whichAttorney Docket No. 0073605-001091 this invention belongs. In case of conflict, the present specification, including definitions, will control. The terminology herein is used to describe specific embodiments of the invention, but their usage does not limit the invention, except as outlined in the claims. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0076] It is to be understood that any aspect and / or element of any embodiment of the method(s) described herein or otherwise may be combined in any way to form additional embodiments of the method(s), all of which are within the scope of the method(s).

[0077] Where a process is described herein, those of ordinary skill in the art will appreciate that the process may operate without any user intervention. In another embodiment, the process includes some human intervention (e.g., a step is performed by or with the assistance of a human).

[0078] As used herein, including the claims, the phrase “at least some” means “one or more” and includes the case of only one. Thus, e.g., the phrase “at least some ABCs” means “one or more ABCs” and includes the case of only one ABC.

[0079] As used herein, including the claims, the term “at least one” should be understood as meaning “one or more” and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with “at least one” have the same meaning, both when the feature is referred to as “the” and “the at least one”.

[0080] As used herein, the term “portion” means some or all. Therefore, for example, “a portion of X” may include some of “X” or all of “X”. In the context of a conversation, the term “portion” means some or all of the conversation.

[0081] As used herein, including the claims, the phrase “using” means “using at least” and is not exclusive. Thus, e.g., the phrase “using X” means “using at least X”. Unless specifically stated by use of the word “only”, the phrase “using X” does not mean “using only X”.

[0082] As used herein, including the claims, the phrase “based on” means “based in part on” or “based, at least in part, on” and is not exclusive. Thus, e.g., the phrase “based on factor X” means “based in part on factor X” or “based, at least in part, on factor X”. Unless specifically stated by use of the word “only”, the phrase “based on X” does not mean “based only on X”.

[0083] In general, as used herein, including the claims, unless the word “only” is specifically used in a phrase, it should not be read into that phrase.Attorney Docket No. 0073605-001091

[0084] As used herein, including the claims, the phrase “distinct” means “at least partially distinct”. Unless specifically stated, distinct does not mean fully distinct. Thus, e.g., the phrase “X is distinct from Y” means that “X is at least partially distinct from Y” and does not mean that “X is fully distinct from Y”. Thus, as used herein, including the claims, the phrase “X is distinct from Y” means that X differs from Y in at least some way.

[0085] It should be appreciated that the words “first”, “second”, and so on, in the description and claims, are used to distinguish or identify, and not to show a serial or numerical limitation.

[0086] Similarly, letter labels (e.g., “(A)”, “(B)”, “(C)”, and so on, or “(a)”, “(b)”, and so on) and / or numbers (e.g., “(i)”, “(ii)”, and so on) are used to assist in readability and to help distinguish or identify, and are not intended to be otherwise limiting or to impose or imply any serial or numerical limitations or orderings. Similarly, words such as “particular”, “specific”, “certain”, and “given”, in the description and claims, if used, are to distinguish or identify, and are not intended to be otherwise limiting.

[0087] As used herein, including the claims, the terms “multiple” and “plurality” mean “two or more,” and include the case of “two”. Thus, e.g., the phrase “multiple ABCs” means “two or more ABCs” and includes “two ABCs”. Similarly, e.g., the phrase “multiple PQRs” means “two or more PQRs” and includes “two PQRs”.

[0088] The present invention also covers the exact terms, features, values, and ranges, etc., in case these terms, features, values, and ranges, etc., are used in conjunction with terms such as “about”, “around”, “generally”, “substantially”, “essentially”, “at least”, etc. Thus, e.g., “about 3” or “approximately 3” shall also cover exactly 3, and “substantially constant” shall also cover exactly constant.

[0089] It should be understood that every maximum numerical limitation given throughout this specification includes every lower numerical limitation, as if such lower numerical limitations were expressly written herein. Every minimum numerical limitation given throughout this specification will include every higher numerical limitation, as if such higher numerical limitations were expressly written herein. Every numerical range given throughout this specification will include every narrower numerical range that falls within such broader numerical range, as if such narrower numerical ranges were all expressly written herein.

[0090] As used herein, unless stated otherwise, the terms “about” or “approximately” refer to a value that is within 10% above or below the value being described.Attorney Docket No. 0073605-001091

[0091] As used herein, including the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. In other words, terms such as “a”, “an”, and “the” are not intended to refer to only a singular entity but include the general class of which a specific example may be used for illustration.

[0092] Throughout the description and claims, the terms “comprise”, “including”, “having”, “contain”, and their variations should be understood as meaning “including but not limited to” and are not intended to exclude other components unless specifically so stated.

[0093] It will be appreciated that variations to the embodiments of the invention can be made while still falling within the scope of the invention. Alternative features serving the same, equivalent, or similar purpose can replace features disclosed in the specification, unless stated otherwise. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features.

[0094] Use of exemplary language, such as “for instance”, “such as”, “for example” (“e.g.,”), and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless specifically so claimed.

[0095] While the invention has been described in connection with what is presently considered to be the most practical and embodiments thereof are further described in the examples below, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[0096] The following description sets forth various examples along with specific details to provide a thorough understanding of claimed subject matter. It will be understood by those skilled in the art, however, that claimed subject matter may be practiced without one or more of the specific details disclosed herein. Further, in some circumstances, well-known methods, procedures, systems, and / or components have not been described in detail in order to avoid unnecessarily obscuring claimed subject matter. The illustrative embodiments described in the detailed description and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, can beAttorney Docket No. 0073605-001091 arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure.Extractant

[0097] As used herein, an “extractant” is a compound that selectively binds, captures, and isolates a chemical species of interest from a mixture containing such chemical species of interest. The chemical species of interest described herein primarily refers to a chemical species that includes a transition-metal element, such as without limitation a solvated form or a coordination complex of a transition-metal cation. Specifically, an extractant can function by selectively associating with a chemical species containing the transition-metal element, through covalent and / or noncovalent interactions, preferably noncovalent interactions such as coordinate covalent bonds, electrostatic attractions / ion pairing, hydrogen bonds, ion-dipole interactions, dipole-dipole interactions, dipole- induced dipole interactions, van der Waals forces, and / or hydrophobic contacts / London dispersion forces, among others. In some embodiments, an extractant can function by selectively associating with a chemical species containing the transition-metal element, through a combination of covalent and noncovalent interactions. As a result of such interaction(s), the extractant can at least partially encapsulate the chemical species of interest to form a host-guest adduct and isolate the chemical species of interest from the rest of the mixture. It should be noted that the extractant described herein can function as both a leachant and an extractant. Accordingly, the terms “extractant” or “leachant / extractant” may be used interchangeably throughout the present disclosure.

[0098] In some embodiments, the leachant / extractant described herein can be considered or function as a ligand. As used herein, a “ligand” is a chemical species capable of binding and / or stabilizing another chemical species, through coordinate covalent bond or specific noncovalent interactions.

[0099] As used herein, a “host-guest adduct” is a chemical composition that results from an association, typically a noncovalent association, between a first chemical species and a second chemical species, wherein the first chemical species at least partially encapsulates the second chemical species, thereby at least partially isolating the second chemical species from its chemical environment. The first chemical species typically has a larger size and includes a binding site or binding portion specific for the second chemical species, and is therefore referred to as the “host”, whereas the second chemical species typically has a smaller size that fits into the binding site or binding portion of the host, and is therefore referred to as the “guest”.Attorney Docket No. 0073605-001091

[0100] A host-guest adduct can separate back to the first and second chemical species upon interacting with a third chemical species and / or upon experiencing one or more changes in its chemical environment such as temperature, pressure, ionic strength, and / or the like.Metallate

[0101] In some embodiments, the leachant / extractant described herein can selectively bind to a metallate of the transition-metal element to form an extractant-metallate host-guest adduct. As used herein, a “metallate” is a complex anion that includes a metal bonded to two or more atoms or groups of atoms. Nonlimiting examples of metallates include oxometallates (which includes O2'), thiometallates (which includes S2‘), cyanometallates (which includes CN’), halogenometallates (which includes F', Cl', Bf, or I"), tetranitratometallates, tetrahydroxometallates, etc.

[0102] As used herein, a “complex” is a chemical species that includes at least one coordinate covalent bond, where one or more electron-rich, Lewis basic species donate their electron density to one or more electron-deficient, Lewis acidic metal atoms or cations.

[0103] In some embodiments, the metallate to which the leachant / extractant binds can be an oxometallate having a formula of MOm"’, wherein M denotes the transition-metal element; n denotes an integer of at least 1 and no greater than 3, preferably 2; and m denotes 4 or 6, preferably 4. Nonlimiting examples of such oxometallates include MoCL2', WCh2', and / or the like. See FIG. 5A, FIG. 6A.

[0104] In some embodiments, the metallate to which the leachant / extractant binds can be a halogenometali ate having a formula of MXmn’, wherein M denotes the transition-metal element; n denotes an integer of at least 1 and no greater than 3, preferably 2; X denotes F, Cl, Br, or I, preferably Cl; and m denotes 4 or 6, preferably 4. Nonlimiting examples of such halogenometallates include [CoCh]2', [NiCL]2', [CuCL]2', and / or the like. See FIG. 5B, FIG. 6B.

[0105] In some embodiments, the metallate to which the leachant / extractant binds can have a tetrahedral geometry. In some embodiments, the metallate to which the leachant / extractant binds can have an octahedral geometry. In some embodiments, the metallate to which the leachant / extractant binds can have a square planar geometry.

[0106] In some embodiments, the metallate to which the leachant / extractant binds can be an oxometallate having a formula of MO42' or a halogenometallate having a formula of MCI42', wherein M denotes the transition-metal element. See FIGS. 5A-B, 6A-B.

[0107] In some embodiments, the transition-metal element can be a Group 6, Group 9, Group 10, or Group 11 transition-metal element. In some embodiments, the transition-metal element can beAttorney Docket No. 0073605-001091 selected from the group consisting of Cr, Mo, W, Co, Ni, Pd, Pt, and Cu. In some embodiments, the transition-metal element can be Cr, Mo, W, Co, Ni, Pd, Pt, or Cu. In some embodiments, the transition-metal element can be Mo or W.

[0108] In some embodiments, the metallate to which the leachant / extractant binds can be a tetraalkylammonium salt. In some embodiments, the alkyl chain of the tetraalkylammonium salt can include 1-6 carbon atoms. In some embodiments, the tetraalkylammonium salt can be a tetrabutylammonium salt, such as without limitation a tetra-n-butylammonium salt. In some embodiments, the tetraalkylammonium salt can be a tetraethylammonium salt.

[0109] In some cases, the nonpolar nature of the alkyl chains and the large polarizability of tetraalkylammonium ions can increase the solubility of an extractant-metallate host-guest adduct in organic solvents or solvent mixtures, thereby facilitating the transport of extractant-metallate hostguest adduct from an aqueous phase to an organic phase.7.7’-diamido-2,2’-diindolylmethane backbone

[0110] The leachant / extractant includes a 7,7’-diamido-2,2’-diindolylmethane backbone (see FIG. 7A). This diindolylmethane backbone was selected as a building block for the extractant described herein at least because of the two following reasons. First, this diindolylmethane backbone has a bent molecular shape, which can effectively “wrap around” and encapsulate a metallate.Second, this diindolylmethane backbone contains four N-H groups, each of which can function as a hydrogen bond donor that selectively associate with a hydrogen bond acceptor or an electronegative element of the metallate.[OHl] In some embodiments, the 7,7’ -diamido-2, 2’ -diindolylmethane backbone described herein can be further functionalized in order to fine-tune the solubility of the extractant and / or the extractant-metallate host-guest adduct. Such substitution can be used to increase or maximize the partition the host-guest adduct in an organic solvent or solvent mixture over an aqueous phase.

[0112] In some embodiments of the leachant / extractant, one or more hydrogen atoms of the 7,7’-diamido-2,2’-diindolylmethane backbone can be independently substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; and a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

[0113] In some embodiments of the leachant / extractant, one or more hydrogen atoms of the 7,7’-diamido-2,2’-diindolylmethane backbone can be independently substituted by a straight-chainAttorney Docket No. 0073605-001091 alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of-O-, -S-, an amine (-NH-, -NR-, etc., wherein R denotes a substituent), a carbonyl, an amide, and an ester.Aromatic Pendant Group

[0114] The leachant / extractant further includes a plurality of aromatic pendant groups (see FIG. 7A, group R). As used herein, a “pendent group” is a group that at least partially protrudes from a chemical structure to which it is covalently attached. As used herein, an “aromatic pendant group” is a pendant group that includes at least one aromatic structure or moiety with (4n + 2) pi electrons, where n is an integer of least 0, in accordance with the Huckel’s rule. Each aromatic pendant group of the plurality of aromatic pendant groups is covalently linked to the 7,7’-diamido-2,2’- diindolylmethane backbone through an amide bond. The steric bulk, molecular geometry, polarizability, and / or polarity of the pendant group can fine-tune the selectivity and / or strength of the binding interaction between the extractant and the metallate.

[0115] In some embodiments of the leachant / extractant, at least one pendant aromatic group of the plurality of aromatic pendant groups can be a pyridinyl group, a / -butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, Z-butylphenyl group, or pyrrolyl group are independently and optionally substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

[0116] In some embodiments of the leachant / extractant, at least one pendant aromatic group of the plurality of aromatic pendant groups can be a pyridinyl group, a Z-butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, Z-butylphenyl group, or pyrrolyl group are independently and optionally substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or moreAttorney Docket No. 0073605-001091 hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of-O-, -S-, an amine (-NH-, -NR- etc., wherein R denotes a substituent), a carbonyl, an amide, and an ester.

[0117] In some embodiments of the leachant / extractant, at least one pendant aromatic group of the plurality of aromatic pendant groups can be a 2-pyridinyl group, a 4- / -butylphenyl group, or a 2- pyrrolyl group. See FIG. 8.

[0118] In some embodiments of the leachant / extractant, at least one pendant aromatic group of the plurality of aromatic pendant groups can include one or more N-heterocycles selected from the group consisting of a substituted or unsubstituted imidazole, a substituted or unsubstituted benzimidazole, a substituted or unsubstituted pyrazole, a substituted or unsubstituted indazole, a substituted or unsubstituted oxazole, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted isoxazole, a substituted or unsubstituted benzisoxazole, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothiazole, a substituted or unsubstituted triazole, a substituted or unsubstituted benzotriazole, a substituted or unsubstituted oxodiazole, a substituted or unsubstituted benzoxadiazole, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadi azole, a substituted or unsubstituted tetrazole, a substituted or unsubstituted oxatriazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted pentazole, a substituted or unsubstituted oxatetrazole, a substituted or unsubstituted thiatetrazole, a substituted or unsubstituted pyridine, a substituted or unsubstituted quinoline, a substituted or unsubstituted pyrazine, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted naphthyridine, a substituted or unsubstituted benzodiazine, and a substituted or unsubstituted diazanaphthalene, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted quinazoline, a substituted or unsubstituted pyridazine, a substituted or unsubstituted phthalazine, a substituted or unsubstituted cinnoline, a substituted or unsubstituted triazine, a substituted or unsubstituted benzotriazine, a substituted or unsubstituted tetrazine, a substituted or unsubstituted purine, and a substituted or unsubstituted pteridine, wherein the one or more N-heterocycles are optionally fused with a substituted orAttorney Docket No. 0073605-001091 unsubstituted benzene, furan, pyrrole, thiophene, indole, naphthalene, anthracene, phenathrene, chryscene, or pyrene.Binding Constant and Binding Stoichiometry

[0119] It should be noted that the numerical value of the binding constant between the leachant / extractant and the metallate can be case-dependent and vary based on the identity of the extractant and the metallate. The extractant can preferably bind to a metallate with a binding constant of at least 10,000 M’1, more preferably with a binding constant of at least 30,000 M’1, even more preferably with a binding constant of at least 100,000 M’1.

[0120] In some embodiments, the leachant / extractant can bind to a metallate of the transitionmetal element preferentially and / or predominantly in a 1 : 1 stoichiometry. As a nonlimiting example, L4 (see FIG. 8) can preferentially bind to MoO42" in a 1 : 1 stoichiometry to form an L4-MoO42’ adduct with a binding constant of Ku = 160,000 M’1; in contrast, when L4 binds to MoCh2' in a 2: 1 stoichiometry, the binding constant, K21, becomes much smaller (3,000 M'1). As another nonlimiting example, the extractant can preferentially bind to WO42' in a 1: 1 stoichiometry to form an extractant- WO adduct with a binding constant of Ku = 30,000 M’1; in contrast, when the extractant binds to WO in a 2:1 stoichiometry, the binding constant, K21, becomes much smaller (2,200 M’1). See FIGS. 23-26.Stripping Agent

[0121] In some embodiments, the extractant-metallate host-guest adduct, upon treatment using a stripping agent, can release the transition-metal element from the leachant / extractant. As used herein, a “stripping agent” is a reactant or chemical composition that separates a chemical species, which includes a transition-metal element of interest, from an extractant bound or associated thereto.

[0122] In some embodiments, a stripping agent can compete with a leachant / extractant in selectively binding to a chemical species that includes a transition-metal element, such as a metallate, thereby displacing the leachant / extractant. In some embodiments, the stripping agent can include a 1,3 -dicarbonyl, such as without limitation acetyl acetone. An 1,3 -dicarbonyl, upon tautomerization to form its respective enol, can selectively bind to the transition-metal element or metallate and displace the extractant originally associated thereto. See FIGS. 6A, and 13-14.

[0123] In some embodiments, a stripping agent can react with a chemical species that includes a transition-metal element, such as a metallate, thereby decreasing its binding affinity towards the leachant / extractant and releasing it from the leachant / extractant. In some embodiments, the stripping agent can include an acid. The acid can include any acid deemed suitable or relevant by a person ofAttorney Docket No. 0073605-001091 ordinary skill in the art, in view of the entirety of the present disclosure. In some embodiments, the acid can include one or more of HBF4 etherate, H2SO4, and H3PO4. As a nonlimiting example, an acid can react with the MoCk2' center of an extractant-MoCh2' adduct to form insoluble H2MOO4, thereby releasing it from the extractant as a precipitate. See FIGS. 15, 16A-B, 17, 18A-C.Method for Selective Separation of a Transition-Metal Element

[0124] The method for selective separation of a transition-metal element includes leaching the transition-metal element from a feedstock to produce a metallate of the transition-metal element.

[0125] In some embodiments of the method, leaching the transition-metal element can include treating the feedstock with a basic solution. Such basic solution can be applied either concurrently with or separately from the extractant. As a nonlimiting example, for extraction of Mo, such basic solution can convert insoluble MoOs to soluble MoCE2-, which is a metallate that can subsequently be captured by an extractant. This treatment step can be performed using a solution of any type of base or basic compound deemed suitable or relevant by a person of ordinary skill in the art in view of the entirety of the present disclosure. Nonlimiting examples of such base(s) or basic compound(s) include NaOH, KOH, Ca(OH)2, tetraalkylammonium hydroxide, etc.

[0126] The method further includes extracting the metallate using the extractant described herein to form an extractant-metallate host-guest adduct.

[0127] The method further includes recovering, using a stripping agent, the transition-metal element from the extractant-metallate host-guest adduct, in accordance with details described elsewhere in this disclosure.

[0128] It should be noted that the method described herein, including any step(s) thereof, can be performed under any temperature and / or pressure seemed suitable or relevant by a person of ordinary skill in the art, in view of the entirety of the present disclosure. In some embodiments, one or more steps of the method can be performed at a temperature of at least 5 °C and no greater than 70 °C. In some embodiments, one or more steps of the method can be performed at a temperature of at least 15 °C and no greater than 45 °C. In some embodiments, one or more steps of the method can be performed at room temperature. In some embodiments, one or more steps of the method can be performed under ambient pressure.

[0129] It should also be noted that the method described herein, including any step(s) thereof, can be performed using any suitable mode of operation as recognized by a person of ordinary skill in the art, in view of the entirety of the present disclosure. In some embodiments, one or more steps ofAttorney Docket No. 0073605-001091 the method can be performed as a batch process. In some embodiments, one or more steps of the method can be performed as a continuous process, such as without imitation under continuous flow.

[0130] In some embodiments of the method, the metallate can be dissolved in a first solution phase, the extractant can be dissolved in a second solution phase immiscible with the first solution phase. Accordingly, extracting the metallate using the extractant can include transferring the metallate from the first solution phase to the second solution phase upon forming the extractant- metallate host-guest adduct.

[0131] The first phase or the second phase can include any protic / aprotic solvent or combination of solvents deemed suitable by a person of ordinary skill in the art in view of the entirety of the present disclosure. Non limiting examples of such solvents include methanol, ethanol, acetone, acetonitrile, diethyl ether, dimethyl sulfoxide, N,N-dimethylformamide, 1,4-di oxane, hexane(s), benzene, toluene, cyclohexhane(s), chloroform, dichloromethane, ethyl acetate, etc.

[0132] In some embodiments of the method, the first phase can be an aqueous phase, and the second phase can be an organic phase. In some embodiments, the first phase can include a mixture of water with one or more polar organic solvents miscible with water, such as without limitation methanol, ethanol, acetone, acetonitrile, diethyl ether, dimethyl sulfoxide, N,N-dimethylformamide, 1,4-di oxane, etc.EXAMPLEMethods

[0133] General Procedures. All manipulations were performed under N2 using standard Schlenk or inert atmosphere glovebox techniques. Glassware was oven-dried before use. NMR spectra were collected using a Bruker 400 or 500MHz spectrometer. Deuterated solvents were purchased from Cambridge Isotope Laboratories and stored over molecular sieves in Schlenk tubes inside inert atmosphere glovebox. CD3CN was refluxed over CaH overnight then distilled onto molecular sieves. CH2CI2 was dried by passage through activated alumina and molecular sieve columns under N2 (a Grubbs-type solvent purification system by Glass Contour). IR spectra were collected using Bruker ALPHA II spectrometer via transmission on attenuated total reflectance (ATR) with a diamond ATR crystal. X-ray data were measured on a Bruker Kappa APEXII Duo system equipped with a Incoatec Microfocus IpS (Cu Ka, X= 1.54178 A) and a multi-layer mirror monochromator at the X-ray facility of The Pennsylvania State University. Elemental analysis were performed at the University of Rochester.Attorney Docket No. 0073605-001091

[0134] Synthetic Procedures.

[0135] Synthesis of Bisindole Ligands

[0136] 1, l-bis-(3-methyl-7-nitro-lH-indole-2-yl)-propane was synthesized according to P.Dydio, T. Zielinski and J. Jurczak, Chem. Commun. 2009, 45604562.

[0137] The following is an alteration of the reported synthesis for the ligands described herein.

[0138] Under an N2 atmosphere, l,l-bis-(-3-methyl-7-nitro-lH-indole-2-yl)-propane (329 mg, Immol) was added to a fisher porter apparatus. The solid was suspended in dry methanol (20 mL) and 5% palladium on charcoal was added (0.1 g). The reaction vessel was purged with 20 atm of H2 gas six times to remove any N2 atmosphere. The reaction was sealed with 20 atm of H2 gas and stirred overnight. After completion, the vessel was vented and 0.5 g of celite was added directly to reaction mixture. The resulting slurry was filtered over a celite pad to prevent any palladium on charcoal from entering the reaction mixture. The solvent was evaporated, and the crude amine was used immediately.

[0139] General Procedure for Preparation of Ligands

[0140] Triethylamine was added (0.42 mL, 3 mmol) to the cooled (0 °C) solution of crude diamine (1 mmol) in dry CH2Q2 (60 mL). Subsequently, appropriate acid chloride was added slowly dropwise to the stirred solution under a N2 atmosphere. The cooling bath was removed, and stirring was continued overnight. The organic layer was washed with NaHCOa (2 x 50mL), water (50 mL), dried over MgSCh, and filtered, and solvent was evaporated therefrom. The crude product was purified by column chromatography on a silica gel, using a CH2CI2 : methanol (1%) mixture as the eluent. See Scheme 1 below.Scheme 1 : General Procedure for Preparation of LigandsAttorney Docket No. 0073605-001091

[0141] Synthesis of pyridine-armed ligand (L4)

[0142] 2-picolinoyl chloride (0.41 g, 2.5 mmol) was used as the acyl reagent and synthesized as described above. The reaction yielded 0.492 g of the product, which is recrystallized from liquidliquid layering with an ethyl acetate-hexanes mixture to produce white crystals.

[0143] Synthesis of tertbutyl-phenylated ligand (L5)

[0144] 4-tert-butylbenzoyl chloride (0.57 mL, 2.5 mmol) was used as the acyl reagent. The reaction yielded 0.550 g of the product.

[0145] Synthesis of [L5] - [MoO4] isolated adduct.

[0146] Under a N2 atmosphere, tetrabutylammonium molybdate (0.05 g, 1 mmol) was dissolved in ImL of dry acetonitrile. The resulting mixture was fdtered through syringe filter directly into L5 (0.05 g, 1 mmol). The mixture was again filtered through clean syringe filter. Vapor diffusion of diethyl ether (35 mL) was carried out, and clear block shape crystals suitable for X-ray crystallography were grown.

[0147] Compounds described herein are characterized by1H,13C, and other types of NMR spectroscopy; infrared spectroscopy (IR); high-resolution mass spectrometry (HRMS); and X-ray crystallography, in accordance with details described throughout the present disclosure.Results

[0148] Referring now to FIG. 7A, FIG. 7A depicts an exemplary schematic of a general chemical formula of the extractant described herein. The extractant includes a 7,7’-diamido-2,2’- diindolylmethane backbone.Attorney Docket No. 0073605-001091

[0149] Referring now to FIG. 7B, FIG. 7B depicts an exemplary schematic of an extractant- MOO42’ complex, wherein the pendant aromatic groups of the extractant are 2-pyrrolyl groups (L3).

[0150] Referring now to FIG. 8, FIG. 8 depicts three exemplary chemical structures of the extractants described herein, wherein the pendant aromatic groups are 2-pyrrolyl groups (L3), 2- pyidinyl groups (L4), and 4- / -butylphenyl groups (L5), respectively.

[0151] Referring now to FIG. 9, FIG. 9 depicts an exemplary schematic depicting the interaction between L5 and MoC>42' to produce an L5-MoO42' adduct.

[0152] Referring now to FIG. 10, FIG. 10 depicts an exemplary structure, based on X-ray crystallography, of the L5-MoO42' adduct described in FIG. 9. [nBiu] counterions are omitted for clarity.

[0153] Referring now to FIG. 11, FIG. 11 depicts an exemplary, partially redacted structure, based on X-ray crystallography, of the extractant-MoOi2' adduct described herein. The extractant- MOO42’ adduct is stable upon isolation, and its solution-phase structure is stable as well.

[0154] Referring now to FIG. 12, FIG. 12 depicts stackedJH NMR spectra (400 MHz, CD3CN) of L5 titrated with increasing molar equivalents of MoO42'. The change in peak position and linewidth over the course of titration suggests the formation of an L5-MoC>42' adduct.

[0155] Referring now to FIG. 13, FIG. 13 depicts an exemplary schematic depicting the process of stripping MoO42' from an extractant-MoCh2' adduct using a stripping agent.

[0156] Referring now to FIG. 14, FIG. 14 depicts stacked1H NMR spectra (400 MHz, CD3CN) of an extractant-MoO42' adduct titrated with increasing molar equivalents of a stripping agent. The change in peak position and linewidth over the course of titration suggests the release of MoO42' from the extractant-MoCh2" adduct and the regeneration of the extractant.

[0157] Referring now to FIG. 15, FIG. 15 depicts an exemplary schematic depicting the process of stripping MoCh2' from the L5-MoC>42’ adduct using HBF4 etherate (HBF4 Et2O) as a stripping agent. L5 is regenerated as a BF4' salt, which is a monoanionic blue solid.

[0158] Referring now to FIGS. 16A-B, FIGS. 16A-B depict two sets of stacked1H NMR spectra (400 MHz, CD3CN) of the L5-MoO42’ adduct titrated with increasing molar equivalents of HBF4 etherate. The change in peak position and linewidth over the course of titration suggests the release of MoO42' from the L5-MoC>42' adduct and the regeneration of the L5. Throughout the course of titration, the color of the solution changes from yellow to blue and dark blue.

[0159] Referring now to FIG. 17, FIG. 17 depicts an exemplary photograph showing a H2MOO4 precipitate that forms upon treatment of the L5-MoC>42’ adduct with HBF4 etherate.Attorney Docket No. 0073605-001091

[0160] Referring now to FIG. 18A-C, FIGS. 18A-C depict stacked!H NMR spectra from a titration of L5 with increasing molar equivalents of MoO-T’ to form the L5-MoC>42' adduct, followed by a titration of the L5-MoO42‘ adduct with increasing molar equivalents of HBF4 etherate (FIG.18 A), H2SO4 (FIG. 18B), and H3PO4 (FIG. 18C), respectively. The change in peak position and linewidth over the course of titrations suggests the formation of an L5-MoO42' adduct and the subsequent release of MoC>42’ from the L5-MoO42‘ adduct, which regenerates free L5.

[0161] Referring now to FIG. 19, FIG. 19 depicts stackedXH NMR spectra (500 MHz, CD3CN) of L5 titrated with increasing molar equivalents of WC>42'. The change in peak position and linewidth over the course of titration suggests the formation of an L5-WO42’ adduct.

[0162] Referring now to FIG. 20, FIG. 20 depicts an exemplary structure, based on X-ray crystallography, of the L5-WO42’ adduct described in FIG. 19, . [nBu4]+counterions are omitted for clarity.

[0163] Referring now to FIG. 21, FIG. 21 depicts stacked 'l l NMR spectra (500 MHz, CD3CN) of L5-MOO42‘ and L5-WO42' adducts. The match between the two spectra suggests structural similarity between the two adducts.

[0164] Referring now to FIG. 22, FIG. 22 depicts an exemplary schematic depicting the interaction between L3 and MoC2' to produce an L3-MoO42' adduct. An exemplary structure, based on X-ray crystallography, of the L3-MoO42' adduct is shown on the right.

[0165] Referring now to FIG. 23, FIG. 23 depicts an exemplary schematic depicting the interaction between increasing molar equivalents of L3 and a fixed amount of MoO42', which sequentially produces L5-MoC>42' adducts in 1: 1 and 2: 1 stoichiometries. Ku and K21 are the binding constants of the two adducts, respectively. (Ku x K21 > IO5M'2)

[0166] Referring now to FIG. 24, FIG. 24 depicts a simulated plot between the change in chemical shift of 'H NMR peaks and the molar ratio between the initial concentrations (as indicated by the subscript “0”) of L5 (host) and MoO42’ (guest).

[0167] Referring now to FIG. 25, FIG. 25 depicts an exemplary schematic depicting the interaction between increasing molar equivalents of L4 and a fixed amount of MoO42’, which sequentially produces L4-MoC>42' adducts in 1 : 1 and 2: 1 stoichiometries. Ku and K21 are the binding constants of the two adducts, respectively. The 1 : 1 adduct is strongly preferred over the 2: 1 adduct (Ku = 160,000 M1, and K21 = 3,000 M1).Attorney Docket No. 0073605-001091

[0168] Referring now to FIG. 26, FIG. 26 depicts stackedNMR spectra of L4 titrated with increasing molar equivalents of MoO42'. The change in peak position and linewidth over the course of titration suggests the formation of an L4-MoO42’ adduct.

[0169] Referring now to FIG. 27, FIG. 27 depicts stacked!H NMR spectra of L5 titrated with increasing molar equivalents of [NiCU]2'. The change in peak position and linewidth over the course of titration suggests the formation of an L5-[NiC14]2' adduct.

[0170] Referring now to FIG. 28, FIG. 28 depicts a comparison of theNMR spectra (400 MHz, CD3CN) of L5 titrated with increasing molar equivalents of MoO42' and theJH NMR spectrum of a mixture of L5 and 1 molar equivalent of [NiCh]2'.

[0171] Referring now to FIGS. 29-31, FIGS. 29-31 depict stacked 'H NMR spectra of L5 titrated with increasing molar equivalents of [CoC14]2"(FIG. 29), [NiCh]2", and [CuCh]2-, respectively.

[0172] It should be understood that modifications to the embodiments disclosed herein can be made to meet a particular set of design criteria. For instance, the number of or configuration of components or parameters may be used to meet a particular objective.

[0173] It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teachings of the disclosure. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternative embodiments may include some or all the features of the various embodiments disclosed herein. For instance, it is contemplated that a particular feature described, either individually or as part of an embodiment, can be combined with other individually described features, or parts of other embodiments. The elements and acts of the various embodiments described herein can therefore be combined to provide further embodiments.

[0174] It is the intent to cover all such modifications and alternative embodiments as may come within the true scope of this invention, which is to be given the full breadth thereof. Additionally, the disclosure of a range of values is a disclosure of every numerical value within that range, including the end points. Thus, while certain exemplary embodiments of the device and methods of making and using the same have been discussed and illustrated herein, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

PCT / US25 / 60067 17 December 2025 (17.12.2025)Attorney Docket No. 0073605-001091CLAIMS1. An extractant for selective separation of a transition-metal element, comprising: a 7,7’-diamido-2,2’-diindolylmethane backbone; and a plurality of aromatic pendant groups, wherein each aromatic pendant group of the plurality of aromatic pendant groups is covalently linked to the 7,7’-diamido- 2,2’-diindolylmethane backbone through an amide bond.

2. The extractant according to claim 1, wherein the extractant selectively binds to a metallate of the transition-metal element to form an extractant-metallate host-guest adduct, preferably with a binding constant of at least 10,000 M’1.

3. The extractant according to claim 2, wherein the extractant-metallate host-guest adduct, upon treatment using a stripping agent, releases the transition-metal element from the extractant.

4. The method according to claim 3, wherein the stripping agent comprises a 1,3- di carbonyl or an acid.

5. The method according to claim 4, wherein the acid comprises one or more of HBF4 etherate, H2SO4, and H3PO4.

6. The extractant according to any one of claims 2-5, wherein the metallate is an oxometallate having a formula of MOm11' or a halogenometallate having a formula of MXm11', wherein:M denotes the transition-metal element; n denotes an integer of at least 1 and no greater than 3;X denotes F, Cl, Br, or I, preferably Cl; and m denotes 4 or 6.

7. The extractant according to any one of claims 2-5, wherein the metallate is a tetraalkyl ammonium salt.

8. The extractant according to any one of claims 2-5, wherein the metallate is an oxometallate having a formula of MC>42' or a halogenometallate having a formula of MCh2', wherein M denotes the transition-metal element.

9. The extractant according to any one of claims 1-5, wherein the transition-metal element is a Group 6, Group 9, Group 10, or Group 11 transition-metal element.

10. The extractant according to claim 9, wherein the transition-metal element is selected from the group consisting of Cr, Mo, W, Co, Ni, Pd, Pt, and Cu.

11. The extractant according to any one of claims 1-5, wherein the transition-metal element is Mo or W.PCT / US25 / 60067 17 December 2025 (17.12.2025)Attorney Docket No. 0073605-00109112. The extractant according to any one of claim 1-5, wherein at least one pendant aromatic group of the plurality of aromatic pendant groups is a pyridinyl group, a t- butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, / -butylphenyl group, or pyrrolyl group are independently and optionally substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; and a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

13. The extractant according to any one of claims 1-5, wherein at least one pendant aromatic group of the plurality of aromatic pendant groups is a pyridinyl group, a t- butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, / -butylphenyl group, or pyrrolyl group are independently and optionally substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of -O-, - S- , an amine, a carbonyl, an amide, and an ester.

14. The extractant according to any one of claims 1-5, wherein at least one pendant aromatic group of the plurality of aromatic pendant groups is a 2-pyridinyl group, a 4- / -butylphenyl group, or a 2-pyrrolyl group.

15. The extractant according to any one of claims 1-5, wherein at least one pendant aromatic group of the plurality of aromatic pendant groups comprises one or more N- heterocycles selected from the group consisting of a substituted or unsubstituted imidazole, a substituted or unsubstituted benzimidazole, a substituted or unsubstituted pyrazole, a substituted or unsubstituted indazole, a substituted or unsubstituted oxazole, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted isoxazole, a substituted or unsubstituted benzisoxazole, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothi azole, a substituted orPCT / US25 / 60067 17 December 2025 (17.12.2025)Attorney Docket No. 0073605-001091 unsubstituted triazole, a substituted or unsubstituted benzotri azole, a substituted or unsubstituted oxodiazole, a substituted or unsubstituted benzoxadi azole, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadi azole, a substituted or unsubstituted tetrazole, a substituted or unsubstituted oxatriazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted pentazole, a substituted or unsubstituted oxatetrazole, a substituted or unsubstituted thiatetrazole, a substituted or unsubstituted pyridine, a substituted or unsubstituted quinoline, a substituted or unsubstituted pyrazine, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted naphthyridine, a substituted or unsubstituted benzodiazine, and a substituted or unsubstituted diazanaphthalene, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted quinazoline, a substituted or unsubstituted pyridazine, a substituted or unsubstituted phthalazine, a substituted or unsubstituted cinnoline, a substituted or unsubstituted triazine, a substituted or unsubstituted benzotriazine, a substituted or unsubstituted tetrazine, a substituted or unsubstituted purine, and a substituted or unsubstituted pteridine, wherein the one or more N-heterocycles are optionally fused with a substituted or unsubstituted benzene, furan, pyrrole, thiophene, indole, naphthalene, anthracene, phenathrene, chryscene, or pyrene.

16. The extractant according to any one of claims 1-5, wherein one or more hydrogen atoms of the 7,7’-diamido-2,2’-diindolylmethane backbone are independently substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; and a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

17. The extractant according to any one of claims 1-5, wherein one or more hydrogen atoms of the 7,7’-diamido-2,2’-diindolylmethane backbone are independently substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl,PCT / US25 / 60067 17 December 2025 (17.12.2025)Attorney Docket No. 0073605-001091 or alkynyl group, or the aryl group are optionally substituted by one or more of -O-, - S- , an amine, a carbonyl, an amide, and an ester.

18. A method for selective separation of a transition-metal element, comprising: leaching the transition-metal element from a feedstock to produce a metallate of the transition-metal element; extracting the metallate using an extractant to form an extractant-metallate host-guest adduct, wherein the extractant comprises: a 7,7’-diamido-2,2’-diindolylmethane backbone; and a plurality of aromatic pendant groups, wherein each aromatic pendant group of the plurality of aromatic pendant groups is covalently linked to the 7,7’-diamido-2,2’-diindolylmethane backbone through an amide bond; and recovering, using a stripping agent, the transition-metal element from the extractant- metallate host-guest adduct.

19. The method according to claim 18, wherein leaching the transition-metal element comprises treating the feedstock with a basic solution.

20. The method according to claim 18, wherein the stripping agent comprises a 1,3- di carbonyl or an acid.

21. The method according to claim 20, wherein the acid comprises one or more of HBF4 etherate, H2SO4, and H3PO4.

22. The method according to any one of claims 18-21, wherein: the metallate is dissolved in a first solution phase; the extractant is dissolved in a second solution phase immiscible with the first solution phase; and extracting the metallate using the extractant comprises transferring the metallate from the first solution phase to the second solution phase upon forming the extractant-metallate host-guest adduct.

23. The method according to claim 22, wherein the first phase is an aqueous phase, and the second phase is an organic phase.

24. The method according to any one of claims 18-21, wherein the extractant selectively binds to the metallate with a binding constant of at least 10,000 M’1.

25. The method according to any one of claims 18-21, wherein the metallate is an oxometallate having a formula of MOm11' or a halogenometallate having a formula of MXm11', wherein:PCT / US25 / 60067 17 December 2025 (17.12.2025)Attorney Docket No. 0073605-001091M denotes the transition-metal element; n denotes an integer of at least 1 and no greater than 3;X denotes F, Cl, Br, or I, preferably Cl; and m denotes 4 or 6.

26. The method according to any one of claims 18-21, wherein the metallate is a tetraalkyl ammonium salt.

27. The method according to any one of claims 18-21, wherein the metallate is an oxometallate having a formula of MC>42' or a halogenometallate having a formula of MCU2’, wherein M denotes the transition-metal element.

28. The method according to any one of claims 18-21, wherein the transition-metal element is a Group 6, Group 9, Group 10, or Group 11 transition-metal element.

29. The method according to any one of claims 18-21, wherein the transition-metal element is selected from the group consisting of Cr, Mo, W, Co, Ni, Pd, Pt, and Cu.

30. The method according to any one of claims 18-21, wherein the transition-metal element is Mo or W.

31. The method according to any one of claims 18-21, wherein at least one pendant aromatic group of the plurality of aromatic pendant groups is a pyridinyl group, a t- butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, / -butylphenyl group, or pyrrolyl group are independently and optionally substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; and a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

32. The method according to any one of claims 18-21, wherein at least one pendant aromatic group of the plurality of aromatic pendant groups is a pyridinyl group, a t- butylphenyl group, or a pyrrolyl group, wherein one or more hydrogen atoms of the pyridinyl group, / -butylphenyl group, or pyrrolyl group are independently and optionally substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of thePCT / US25 / 60067 17 December 2025 (17.12.2025)Attorney Docket No. 0073605-001091 straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of -O-, - S- , an amine, a carbonyl, an amide, and an ester.

33. The method according to any one of claims 18-21, wherein at least one pendant aromatic group of the plurality of pendant aromatic groups is a 2-pyridinyl group, a 4- / -butylphenyl group, or a 2-pyrrolyl group.

34. The method according to any one of claims 18-21, wherein at least one pendant aromatic group of the plurality of pendant aromatic groups comprises one or more N- heterocycles selected from the group consisting of a substituted or unsubstituted imidazole, a substituted or unsubstituted benzimidazole, a substituted or unsubstituted pyrazole, a substituted or unsubstituted indazole, a substituted or unsubstituted oxazole, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted isoxazole, a substituted or unsubstituted benzisoxazole, a substituted or unsubstituted thiazole, a substituted or unsubstituted benzothiazole, a substituted or unsubstituted isothiazole, a substituted or unsubstituted benzisothi azole, a substituted or unsubstituted triazole, a substituted or unsubstituted benzotri azole, a substituted or unsubstituted oxodiazole, a substituted or unsubstituted benzoxadi azole, a substituted or unsubstituted thiadiazole, a substituted or unsubstituted benzothiadi azole, a substituted or unsubstituted tetrazole, a substituted or unsubstituted oxatriazole, a substituted or unsubstituted thiatriazole, a substituted or unsubstituted pentazole, a substituted or unsubstituted oxatetrazole, a substituted or unsubstituted thiatetrazole, a substituted or unsubstituted pyridine, a substituted or unsubstituted quinoline, a substituted or unsubstituted pyrazine, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted naphthyridine, a substituted or unsubstituted benzodiazine, and a substituted or unsubstituted di azanaphthalene, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted quinazoline, a substituted or unsubstituted pyridazine, a substituted or unsubstituted phthalazine, a substituted or unsubstituted cinnoline, a substituted or unsubstituted triazine, a substituted or unsubstituted benzotriazine, a substituted or unsubstituted tetrazine, a substituted or unsubstituted purine, and a substituted or unsubstituted pteridine, wherein the one or more N-heterocycles are optionally fused with a substituted or unsubstituted benzene, furan, pyrrole, thiophene, indole, naphthalene, anthracene, phenathrene, chryscene, or pyrene.PCT / US25 / 60067 17 December 2025 (17.12.2025)Attorney Docket No. 0073605-00109135. The method according to any one of claims 18-21, wherein one or more hydrogen atoms of the 7,7’-diamido-2,2’-diindolylmethane backbone are independently substituted by one or more of a halogen atom; a nitrile group; a hydroxyl group; a carboxyl, carboxylate, or ester group; a sulfonic acid, sulfonate, sulfonate ester, sulfuric acid, or sulfate group; and a phosphonic acid, phosphonate, phosphoric acid, or phosphate group.

36. The method according to any one of claims 18-21, wherein one or more hydrogen atoms of the 7,7’-diamido-2,2’-diindolylmethane backbone are independently substituted by a straight-chain alkyl, alkenyl, or alkynyl group having 1-10 carbon atoms, a branched or cyclic alkyl, alkenyl, or alkynyl group having 3-10 carbon atoms, or an aryl group having 6-12 carbon atoms, wherein one or more hydrogen atoms of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of a hydroxyl, an a thiol, an amine, a nitrile, a carboxyl, a sulfonate ester, a sulfonate ion, and a halogen, and / or wherein one or more -CH2- or -CH- of the straight-chain alkyl, alkenyl, or alkynyl group, the branched or cyclic alkyl, alkenyl, or alkynyl group, or the aryl group are optionally substituted by one or more of -O-, - S- , an amine, a carbonyl, an amide, and an ester.

37. The method according to any one of claims 18-21, further comprising recycling the extractant after recovery of the transition-metal element.

38. Use of the extractant according to any one of claims 1-5 for separation of a transitionmetal element.

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