A method of making a glycoluril-based linker and a glycoluril-based metal-organic framework (MOF) using the linker

Abstract

The present disclosure relates to a method of making a glycoluril-based linker and a glycoluril-based metal-organic framework (MOF) using the linker. The method comprises reacting a solution comprising urea, a solution comprising a diketone derivative, and a weak acid to form a precipitate comprising the glycoluril-based linker with accessible NH and COOH groups. When the diketone derivative is 4,4'-dioxo-[1,1'-biphenyl]-3,3'-dicarboxylic acid, the glycoluril-based linker prepared is 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl)dibenzoic acid. The method further comprises reacting the glycoluril-based linker, a metal salt and a solution of dimethylformamide to form a precipitate comprising glycoluril-based Metal Organic Framework (MOF) with accessible NH groups. Various metal salts such as zinc nitrate hexahydrate and zirconium(IV) chloride octahydrate may used to prepare Zn or Zr based MOF. The glycoluril-based MOF has a surface area of about 1350 m² / g to 1530 m² / g and is thermally stable up to a temperature of about 600⁰C.

Description

A METHOD OF MAKING A GLYCOLURIL-BASED LINKER AND A GLYCOLURIL-BASED METAL-ORGANIC FRAMEWORK (MOF) USING THE LINKER Technical Field The invention relates to the field of preparing glycoluril-based linker and metal organic framework comprising glycoluril-based linker.  Background  The utilization of glycoluril building units to build up new macrocycles and new structures has gained attention over the past few years owing to their unique advantages in terms of their vulnerability to modification and custom design for more diverse and potential applications compared to their parent macrocycles.  Generally, the glycoluril monomer consists of 4 urea N-H motifs and 2 methine hydrogen atoms available for hydrogen bonding and no terminal carboxylic groups. All of the macrocycles prepared using the currently available glycoluril monomers are designed by only linking glycoluril derivatives through methylene bridges.  Several macrocycles including cucurbituril (CB) and acyclic CB macrocycles have been tailored for several specific applications but all of them to date are designed by only linking glycoluril derivatives through methylene bridges and not at methine positions.  Glycoluril building blocks have been utilized in supramolecular chemistry and material science through connections at their NH groups. Alternating the glycoluril motif with several chromophoric units such as dialkoxybenzene and benzene resulted in expanding the structures of bambusurils and pillar(calix)aenes, yet all of the new macrocyclic analogues are focused on bridging the NH moieties and not the methine moieties. However, bridging using the NH moieties also means reduced / unavailability of the NH moieties for other interactions. This restricted connectivity has limited the diversity of architectures and functionalities achievable with glycoluril-based systems.  There exists a need for a new glycoluril based building blocks with simply manufacturing processes having more accessible active groups such as NH and COOH for increased interaction with other molecules to build a macrocycle molecule also with accessible active groups.  Summary of the invention  The present disclosure relates to a new method of preparing the glycoluril-based linker with four accessible NH groups and two carboxylic acid groups, and a glycoluril-based Metal-Organic Framework (MOF) having four accessible NH groups for chemical interaction. The invention introduces a new synthetic strategy in which the dibenzoic moieties are substituted at the methine position of the glycoluril core, thereby creating a new glycoluril-based linker with accessible NH and carboxylic acid moieties.  According to one aspect of the invention, this invention relates to a method of preparing a glycoluril-based linker with accessible NH and COOH groups. The method includes providing a solution comprising urea;providing a solution comprising a diketone derivativeproviding a weak acid; and reacting the urea solution and the diketone derivative solution with the weak acid to form a precipitate, the precipitate comprising the glycoluril-based linker with accessible NH and COOH groups.  The step of reacting comprises mixing the urea solution, the diketone derivative solution and the weak acid at a temperature of about 90⁰C. The method may further comprise cooling the mixture containing the precipitate to an ambient temperature and separating the precipitate from the mixture. The precipitate is in the form of crystal.  The separated precipitate may be washed in a solution of cold water and ethanol[SV1] [NI2] .  The precipitate may then be separated from the solution and dried. In a preferred embodiment, the drying was carried out in vacuum. The washed and dried precipitate may then be recrystallised from a 1:1 ethanol water mixture to yield the final glycoluril-based linker with free with NH groups. There may be four accessible NH group and two accessible carboxylic groups per glycoluril-based linker molecule.  The weak acid may be glacial acetic acid[SV3] [NI4] .  In a preferred embodiment, the diketone derivative may be 4,4'-dioxo-[1,1'-biphenyl]-3,3'-dicarboxylic acid and the glycoluril-based linker may be 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl)dibenzoic acid.  The yield of the glycoluril-based linker made using the above method is 65-75%.  According to a second aspect of the invention, the invention relates to a method of preparing a glycoluril-based Metal-Organic Framework (MOF) with accessible NH groups using glycoluril-based linker, 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl)dibenzoic acid. The method includes providing a glycoluril-based linker,providing a metal salt,providing a solution of dimethylformamide, and reacting the glycoluril-based linker, the metal salt and the solution of dimethylformamide to form the precipitate, the precipitate comprising glycoluril-based MOF.  The step of reacting comprises mixing the glycoluril-based linker, the metal salt and the solution of dimethylformamide at an elevated temperature of about 85⁰C for about 24 hours.  The metal salt used is any of zinc nitrate hexahydrate and zirconium(IV) chloride octahydrate.  The glycoluril-based Metal–Organic Framework prepared using this method has a surface area in the range of about 1350 m² / g to about 1530 m² / g. The glycoluril-based Metal–Organic Framework prepared by this method is thermally stable up to a temperature of about 600⁰C.  In the glycoluril-based Metal–Organic Framework, the glycoluril units are linked through their methine hydrogen atoms, creating a metal-organic framework (MOF) and leaving the four NH moieties with a curved glycoluril backbone available for molecular recognitions and chemical interactions. In the glycoluril-based Metal–Organic Framework, the two benzoic acid groups are inserted into the methine bond to link the glycoluril linker to zinc metal to generate the glycoluril-based Metal–Organic Framework. Other aspects and embodiments of the invention will be apparent as will be shown in the detailed description of the invention. Brief Description of the drawings The accompanying drawings illustrate non-limiting example embodiments of the inventionFIG 1 shows the chemical scheme for preparing the glycoluril-based linker of the invention,FIG 2 shows the chemical scheme for preparing the glycoluril-based Metal organic framework compound using Metal salt comprising Zinc,FIG 3 shows the chemical scheme for preparing the glycoluril-based Metal organic framework compound using Metal salt comprising Zirconium,FIG. 4A to FIG 4D illustrates the structural and functional properties of the Zinc-based metal-organic framework - FTIR spectrum in FIG. 4A, PXRD pattern in FIG. 4B, Nitrogen adsorption–desorption isotherm in FIG. 4C, Thermogravimetric analysis in FIG. 4D; and   FIG. 5A to FIG 5D illustrates the structural and functional properties of the Zinc-based metal-organic framework – FTIR spectrum in FIG. 5A, PXRD pattern in FIG. 5B, Nitrogen adsorption–desorption isotherm in FIG. 5C, and TGA curve in FIG. 5D. Detailed description  Throughout the following description, specific details are set forth in order to provide a more thorough understanding to people skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. The following description of examples of the invention is not intended to be exhaustive or to limit the system to the precise forms of any example embodiment. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive sense.  In accordance with the first aspect of the invention, the method of preparing the glycoluril-based linker is provided. The method comprises, providing a solution comprising urea, providing a solution comprising a diketone derivative, providing a weak acid, and reacting the urea solution, diketone derivative solution with the weak acid to form a precipitate. The precipitate comprises the glycoluril-based linker.  The reaction is carried out by mixing the urea solution, diketone derivative solution and the weak acid at a temperature of about 90⁰C.  The method further comprises separating the precipitate and washing the precipitate in cold water and ethanol. After washing, the precipitate may be dried in vacuum[SV5] [NI6] .  The precipitate comprises the glycoluril-based linker. The glycoluril-based linker prepared by this method is in the form of a while crystalline solid.  A nonlimiting example of the diketone derivative is with 4,4'-dioxo-[1,1'-biphenyl]-3,3'-dicarboxylic acid. A nonlimiting example of the weak acid is glacial acetic acid.  The glycoluril-based linker formed using urea, 4,4'-dioxo-[1,1'-biphenyl]-3,3'-dicarboxylic acid and glacial acetic acid is 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl)dibenzoic acid. Generally, the process of preparing the glycoluril-based linker involves reacting urea with 4,4'-dioxo-[1,1'-biphenyl]-3,3'-dicarboxylic acid in glacial acetic acid at 90 °C for 6 hours to form the glycoluril-based linker, 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl)dibenzoic acid. The reaction proceeds via cyclocondensation. Preparing the glycoluril-based derivative using this method yields a rigid linker with four free NH groups and two terminal carboxylic acid groups. The free terminal carboxylic acid is advantageous as it enables linking with metal salts to form a macromolecule, glycoluril-based Metal organic framework.  FIG.1 describes the method of preparing the glycoluril-based linker of the invention. The glycoluril-based linker may also be called glycoluril-based dicarboxylic acid linker.  To synthesize the glycoluril-based derivative, 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl )dibenzoic acid with free NH groups, urea (1.20 g, 20 mmol) and 4,4′-dioxo-[1,1′-biphenyl]-3,3′-dicarboxylic acid (1.66 g, 10 mmol), a 1,2-dicarbonyl aromatic compound, were dissolved in 20 mL of glacial acetic acid. The mixture was heated under reflux at 90 °C for 6 hours.  The mixture was preferably continuously stirred for the duration of heating.  The mixing of the urea and 4,4′-dioxo-[1,1′-biphenyl]-3,3′-dicarboxylic acid and glacial acetic acid at an elevated temperature results in the formation of a precipitate. The precipitate comprises 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl )dibenzoic acid. It is observed that the 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl )dibenzoic acid has four free NH groups and two free terminal carboxylic acid groups.  The reaction mixture including the precipitate was cooled to a room temperature. The precipitate was collected by filtration. The precipitate may be preferably separated using vacuum filtration. However, other methods of separation known in the art may be used.  The precipitate may be washed thoroughly with cold water and ethanol and dried under vacuum[SV7] [NI8] . The precipitate includes 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl )dibenzoic acid which currently is a crude version of the product. The precipitate comprising glycoluril-based linker 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl )dibenzoic acid may include any unreacted reaction ingredients or byproducts, which collectively is indicated as crude version of the product. This crude version of the product has the glycoluril-based linker as its prime constituent in other words, the glycoluril-based linker is present in maximum quantity.  To purify the 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl )dibenzoic acid, the precipitate was recrystallized from a 1:1 ethanol–water mixture to yield the final glycoluril-based linker as a white crystalline solid.  The method as described yielded the glycoluril-based dicarboxylic acid linker in the range of about 65% to 75%.  The inventors believe the reaction of urea and 4,4′-dioxo-[1,1′-biphenyl]-3,3′-dicarboxylic acid and glacial acetic acid proceeds through a classic cyclocondensation mechanism between urea and a 1,2-dicarbonyl compound under acidic and thermal conditions. Initially, the nucleophilic amino groups of urea attack the electrophilic carbonyl carbons of the 1,2-diketone moiety. This forms a series of tetrahedral intermediates, which then undergo dehydration (loss of water) to generate C–N bonds. The condensation of two urea molecules with the adjacent carbonyls results in the formation of a bicyclic imidazo[4,5-d]imidazole (glycoluril) core. The reaction takes place under glacial acetic acid reflux at 90 °C, which serves both as a mild acid catalyst and solvent, facilitating cyclization and promoting the elimination of water. The presence of carboxylic acid groups on the aromatic rings does not interfere with the cyclization, allowing them to remain intact and available for post-synthetic functionalization or metal coordination during preparing a metal organic framework compound. It is noted that the NH groups on the glycoluril core remain unsubstituted, as they are not involved in further condensation reactions, resulting in a final product that features four free NH groups and two terminal carboxylic acid groups.  The glycoluril-based linker prepared by this process is highly rigid and symmetrical. This makes it an ideal organic linker for constructing metal organic framework compounds with predictable geometry and robust connectivity. The glycoluril-based linker contains a fused imidazo[4,5-d]imidazole (glycoluril) core substituted at the methine positions with benzoic acid groups, providing both coordination functionality and a rigid backbone essential for promoting crystallinity and directional growth in MOFs. The rigid and pre-organized nature of the linker prevents structural collapse and ensures the formation of well-ordered porous networks with predictable topology. Specifically, the amide and carbonyl groups in the glycoluril core introduce polar functionality within the pores, which can enhance host–guest interactions, making the material attractive for applications in adsorption, sensing, and selective catalysis.  One of the advantages of this linker is its ability to coordinate effectively with a range of metal ions, particularly Zn²⁺ and Zr⁴⁺. The two carboxylic acid groups act as bidentate ligands, enabling the formation of extended coordination polymers through strong metal–oxygen bonds. This aids in the process of preparing the glycoluril-based Metal Organic Framework. Another advantage of this linker is its potential for post-synthetic modification (PSM). The nitrogen-containing glycoluril core provides sites that could be chemically modified without disrupting the overall framework, enabling fine-tuning of properties such as surface polarity, hydrophilicity, catalytic activity, biomolecule compatibility or drug-loading capability. This makes the linker not just a structural scaffold but also a tunable component for developing multifunctional materials.  Another advantage includes enhanced functionality for host–guest chemistry. The presence of NH groups and internal carbonyl functionalities introduces polar, hydrogen-bonding sites within the framework pores. This chemical environment enhances the material’s performance in applications such as gas adsorption, selective separation, molecular recognition, and sensing. The rigid structure, multidentate coordination sites, chemical functionality, and stability-promoting characteristics of the new linker allow for the creation of crystalline, porous, and thermally robust frameworks. Further, its multidentate coordination through carboxylic acid groups ensures directional growth, while the curvature of the glycoluril scaffold helps establish predictable and well-defined porous networks. These properties collectively position this linker as a valuable tool in the development of MOFs for a wide range of applications, from gas storage and separation to catalysis and biomedical delivery systems. The yield of glycoluril-based linker prepared by this process was confirmed by Fourier transform infrared spectroscopy (FTIR) (indicating intact NH and carbonyl groups), H NMR, or proton nuclear magnetic resonance (1H NMR) (showing characteristic glycoluril and aromatic signals). H NMR (400 MHz, DMSO‑d₆, δ, ppm): 7.22–7.95 (m, 8H, Ar–H), 8.05 (brs, 4H, NH), 12.95 (brs, 2H, COOH). ¹³C NMR(carbon-13 nuclear magnetic resonance) spectroscopy(100 MHz, DMSO‑d₆, δ, ppm): 82.0 (bridgehead CH), 127.8, 128.9, 129.9, 130.6, 143.4 (Ar–C), 160.9 (C=O, urea core), 167.3 (C=O, COOH). FTIR (KBr, cm⁻¹): 3305 (br, N–H), 3060 (Ar–C–H), 1698 (C=O, COOH), 1670 (C=O, urea), 1598 (C=C, aromatic), 1275 (C–N), 770 (Ar–C–H bending).  The spectroscopic data obtained confirm the successful synthesis and structural integrity of the glycoluril-based dicarboxylic acid linker. The ¹H NMR spectrum displayed characteristic signals including a singlet at δ 4.76 ppm corresponding to the bridgehead methine protons of the glycoluril core, a multiplet between δ 7.62–8.05 ppm attributable to aromatic protons, and broad singlets at δ 10.88 and 12.90 ppm corresponding to the NH and COOH protons, respectively, both of which exchanged with D₂O—further confirming the presence of free secondary amines and carboxylic acid groups. The ¹³C NMR spectrum showed the expected signals for the bridgehead carbon (δ ~61.4 ppm), aromatic carbons (δ 127–143 ppm), and two distinct carbonyl environments—urea-type (δ 160.9 ppm) and carboxylic acid (δ 167.3 ppm). These data are consistent with the proposed structure featuring an intact glycoluril core and unreacted functional groups. The FTIR spectrum reinforced these assignments with broad N–H stretching at 3305 cm⁻¹, strong C=O absorptions near 1698 and 1670 cm⁻¹ for the acid and urea carbonyls, respectively, and C–N stretching at 1275 cm⁻¹. Additionally, elemental analysis values were in close agreement with theoretical calculations, supporting high purity and correct molecular composition. Collectively, the data confirm the formation of a symmetric, rigid glycoluril-based linker bearing free NH and COOH groups, making it suitable for coordination-driven applications such as MOF assembly. Non limiting examples of the applications of the glycoluril-based linker of the invention includes -          Gas storage and separation, due to high surface area and tunable pore environments,-          Heterogeneous catalysis, through incorporation of active sites or post-synthetic modification,-          Biomedical delivery systems, offering molecular recognition and drug encapsulation capabilities, and-          Chemical sensing by leveraging host–guest interactions via the glycoluril core. In the second aspect of the invention, the method of preparing the glycoluril-based Metal-Organic Framework (MOF) is provided. The method comprises providing a glycoluril-based linker, providing a metal salt, providing a solution of dimethylformamide and reacting the glycoluril-based linker, the metal salt and the solution of dimethylformamide to form a precipitate. The precipitate comprises glycoluril-based MOF.  The step of reacting comprises mixing the glycoluril-based linker, the metal salt and the solution of dimethylformamide at a temperature of about 85⁰C for about 24 hours.  The metal salt may be selected from any of zinc nitrate hexahydrate and zirconium(IV) chloride octahydrate[SV9] [NI10] . However, salts of other metal ions, such as copper may also be used.  It is to be noted that a combination of metal salt may also be used to prepare the glycoluril-based Metal-Organic Framework (MOF) with multiple metal ions[SV11] [NI12] .  FIG. 2 shows the chemical scheme for preparing the glycoluril-based Metal organic framework compound “NS-MOF (Zn)” using Metal salt comprising Zinc. The synthesis of NS-MOF (Zn) was achieved using a solvothermal method. The method comprises reacting a glycoluril-based linker with metal salt, zinc nitrate hexahydrate in dimethylformamide (DMF) at 85 °C for 24 hours to form a precipitate. The precipitate comprises the glycoluril-based MOF, NS-MOF (Zn). The NS-MOF (Zn) prepared by this method is white in color.  For mixing, sonication may be used. The use of sonication prior to solvothermal treatment likely improved solubility and homogeneity of the reactants, facilitating efficient coordination between zinc ions and the glycoluril-based linker. The reaction mixture including the precipitate may be cooled to room temperature. The precipitate may be collected by filtration in vacuum. The precipitate may be preferably separated using vacuum filtration. However, other methods of separation known in the art may be used.  The precipitate was washed with Dimethylformamide (DMF), followed by chloroform. The vacuum filtration and washing steps [SV13] [NI14] ensure the removal of unreacted precursors and solvent residues, enhancing the purity and potential usability of the final glycoluril-based MOF in downstream applications. This method allows for controlled formation of a crystalline glycoluril-based MOF, as evidenced by the white solid obtained after cooling and filtration.  The glycoluril-based MOF “NS-MOF (Zn)” shown in the FIG. 2 illustrates a high degree of symmetry and porosity, characteristics that are hallmarks of MOFs and crucial for applications in gas storage, separation, and catalysis. The presence of spheres representing Zn(II) ions at the vertices of the framework confirms their role as coordination centers, linking the organic ligands into a periodic network. Additionally, the 3D structural representation highlights the robust coordination environment and the extended framework that results from the reaction conditions employed. The clear network connectivity and structural integrity suggest good crystallinity, which is desirable for MOF performance and further functional studies.  A few advantages of the above process includes a straightforward reaction with simple synthesis conditions such as mild temperature, readily available reagents, and standard solvents, making this an accessible route to prepare the glycoluril-based, MOF NS-MOF (Zn).  Structural and physicochemical properties of NS-MOF (Zn)  The structural and physicochemical properties of NS-MOF (Zn) were evaluated using FTIR, PXRD, BET surface area analysis, and thermogravimetric analysis (TGA). The results of the analysis are shown in FIG. 4A to 4D. The results confirms the successful synthesis and functional robustness of the framework as provided below.  The FTIR spectrum (provided in FIG. 4A) shows prominent absorption bands corresponding to the characteristic functional groups of the organic linker. The broad band around 3300 cm⁻¹ suggests the presence of N–H stretching vibrations, while the peaks near 1680 cm⁻¹ and 1600 cm⁻¹ are attributed to C=O and C=N stretches, respectively. These results confirm the incorporation of the glycoluril moieties and coordination with Zn(II) ions. The PXRD pattern (FIG. 4B) displays sharp and distinct diffraction peaks, indicating the crystalline nature of NS-MOF (Zn). The pattern is consistent with a highly ordered framework structure, verifying successful MOF formation rather than amorphous byproducts. This crystallinity is essential for defining pore structure and ensuring reproducibility in applications. The nitrogen adsorption–desorption isotherm (FIG. 4C) exhibits a typical type I profile, indicative of microporosity. The calculated Brunauer–Emmett–Teller (BET) surface area is approximately 1350 m² / g, which reflects a highly porous architecture. Such a large surface area is desirable for applications in gas storage, separation, and catalysis, offering abundant active sites and internal volume. Thermogravimetric analysis (FIG. 4D) reveals that NS-MOF (Zn) remains stable up to approximately 600 °C, with gradual weight loss attributable to solvent removal and decomposition of the organic linker beyond this temperature. This thermal stability suggests the framework is suitable for high-temperature applications and enhances its durability under operational conditions. Overall, these characterization results confirm that NS-MOF (Zn) possesses the structural, thermal, and surface properties necessary for practical applications in areas such as gas sorption, catalysis, and environmental remediation. FIG. 3 shows the chemical scheme for preparing the glycoluril-based Metal organic framework compound “NS-MOF (Zr)” using Metal salt comprising Zirconium. The synthesis of NS-MOF (Zr) was achieved using a solvothermal method. The method comprises reacting a glycoluril-based linker with metal salt, Zirconium(IV) Chloride (ZrOCl₂·8H₂O) in dimethylformamide (DMF) at 85 °C for 24 hours to form a precipitate. The precipitate comprising NS-MOF (Zn). The NS-MOF (Zn) prepared by this method is white in color. The reaction may be carried out in a Teflon-lined autoclave.  For mixing, sonication may be used. The use of sonication prior to solvothermal treatment likely improved solubility and homogeneity of the reactants, facilitating efficient coordination between zirconium ions and the glycoluril-based organic linker. The reaction mixture including the precipitate was cooled to room temperature. The precipitate was collected by filtration. The precipitate may be preferably separated using vacuum filtration. However, other methods of separation known in the art may be used[SV15] [NI16] . The precipitate was then washed with DMF, followed by washing with chloroform.  The vacuum filtration and washing steps ensure the removal of unreacted precursors and solvent residues, enhancing the purity and potential usability of the final glycoluril-based Metal organic framework in downstream applications. This method allows for controlled formation of a structured crystalline glycoluril-based MOF, as evidenced by the white solid obtained after cooling and filtration.  The mild reaction conditions and the use of a Teflon-lined autoclave enabled controlled coordination of Zr(IV) ions with the multidentate glycoluril linker, fostering the assembly of a well-defined MOF architecture. NS-MOF (Zr) from FIG. 3 shows a symmetric, square-grid network in which Zr(IV) ions serve as nodes that coordinate with the oxygen atoms of the linker, forming a robust 3D framework. FIG. 3 also illustrates the high connectivity of the Zr clusters, which is typical of Zr-based MOFs due to the strong metal–ligand bonding and the ability of Zr to form stable multinuclear clusters. The coordination geometry, visualized in the cluster model on the right, highlights the formation of Zr₆-oxo clusters, which are known to impart high thermal and chemical stability to MOFs. This stability makes NS-MOF (Zr) a promising candidate for applications under harsh conditions, such as catalysis, gas adsorption, and drug delivery.  Although both NS-MOF (Zr) and NS-MOF (Zn) may be used under harsh conditions listed above. In comparison to the NS-MOF (Zn), NS-MOF (Zr) may offer enhanced stability and rigidity due to the stronger Zr–O coordination bonds. This makes NS-MOF (Zr) particularly attractive for applications where long-term structural integrity and resistance to degradation are essential.  A few advantages of the process include a reliable and reproducible preparation process to access NS-MOF (Zr) from glycoluril-based linker. The combination of structural integrity, crystallinity, and the potential for tunable porosity positions NS-MOF (Zr) as a versatile platform for functional material development.  Structural and physicochemical properties of NS-MOF (Zr)  The comprehensive characterization of NS-MOF (Zr) confirming successful synthesis and desirable structural and functional properties of the zirconium-based metal-organic framework are shown in FIG. 5A to FIG. 5D.  More specifically, FIG. 5A illustrates a FTIR spectrum confirming the presence of characteristic functional groups from the organic linker; FIG. 5B illustrates PXRD pattern showing high crystallinity of the synthesized MOF, FIG. 5C illustrates N₂ adsorption–desorption isotherm indicating a BET surface area of 1530 m² / g, characteristic of a microporous structure and FIG. 5D illustrates TGA curve demonstrating thermal stability of NS-MOF (Zr) up to approximately 600 °C. The FTIR spectrum in FIG. 5A shows distinct absorption bands associated with key functional groups of the glycoluril-based linker and coordination with Zr(IV). The broad peak around 3300 cm⁻¹ is attributed to N–H stretching, while strong absorptions near 1680 cm⁻¹ and 1600 cm⁻¹ correspond to C=O and C=N stretches, indicating intact organic framework connectivity post-synthesis. The PXRD pattern shown in FIG. 5B reveals sharp and well-defined diffraction peaks, consistent with a highly crystalline material. The peak positions and intensities suggest a periodic structure with long-range order, characteristic of a robust NS-MOF (Zr). This crystallinity is essential for maintaining pore structure and consistent physical properties across batches. Nitrogen adsorption–desorption analysis illustrated in FIG. 5C displays a type I isotherm, typical of microporous materials. The calculated BET surface area of 1530 m² / g exceeds that of the Zn-based analogue, highlighting the superior porosity and potential for guest molecule accommodation. The high surface area is indicative of well-developed porosity, making NS-MOF (Zr) a strong candidate for applications in gas adsorption, separation, and catalysis. Thermogravimetric analysis illustrated in FIG. 5D demonstrates the thermal stability of NS-MOF (Zr) up to approximately 600 °C. The initial weight loss below 150 °C is likely due to desorption of physically adsorbed solvents. Beyond this, gradual decomposition of the organic linker occurs, with a final residue suggestive of zirconium oxide formation. This thermal robustness is a notable advantage of Zr-based MOFs and supports their use in high-temperature processes. Accordingly, in summary, the FTIR, PXRD, BET, and TGA results collectively validate the successful formation of a highly crystalline, porous, and thermally stable Zr-MOF. These characteristics position NS-MOF (Zr) as a promising material for advanced applications in gas storage, sensing, and catalysis under demanding conditions.  In NS-MOF (Zr), the carboxylates can bridge Zr₆-oxo clusters, contributing to the formation of highly stable, chemically resistant structures. NS-MOF(Zr) has exceptional thermal stability (up to ~600 °C) and high resistance to chemical degradation. These traits are highly desirable for industrial applications. The glycoluril-based linker contributes significantly to the framework's porosity and surface area. Its extended geometry allows for wide pore channels, and its rigid aromatic system prevents pore collapse, which can occur with more flexible linkers. This is observed high BET surface area of over 1500 m² / g for NS-MOF(Zr), enabling efficient gas storage and molecular separation. Overall, the glycoluril-based MOFs of the invention follow the general formula: CnHmOyNx (Zn or Zr).The NS-MOF(Zn), and NS-MOF(Zr) may be prepared by distinctly linking derivatives of glycoluril units via their methine groups which results in free NH positions for further chemical interaction.  The MOFs of the invention are therefore well-suited for a broad range of high-value applications, including gas storage and separation, catalysis, environmental remediation, and drug delivery due to the following properties: - Exceptional Stability: The resulting MOFs exhibit high thermal stability (up to ~600 °C) and significant resistance to chemical degradation, making them ideal for demanding industrial and environmental applications.- High Porosity and Surface Area: The linker’s rigid, extended geometry and aromatic structure promote the formation of large, open pore channels with high surface area. This design prevents pore collapse and supports efficient guest molecule uptake, enhancing performance in adsorption and separation processes.- Structural Precision and Crystallinity: The fused imidazo[4,5-d]imidazole (glycoluril) core flanked by benzoic acid groups provides structural rigidity and precise spatial orientation, which promotes the formation of well-ordered, crystalline frameworks with predictable topologies.- Enhanced Host–Guest Interactions: Polar functional groups within the glycoluril core (including amides and carbonyls) create a chemically active internal pore environment that supports selective interactions with guest molecules, beneficial for sensing and catalysis. By combining the chemical uniqueness of glycoluril cores with the tunable functionality of MOFs, the glycoluril-based MOFs addresses unmet needs in sectors demanding lightweight, functional, and selective materials, offering both scientific value and commercial viability. Specific examples of glycoluril-based MOFs have been described herein for purposes of illustration. These are only examples. Many alterations, modifications, additions, omissions and permutation are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by replacing features, elements, and / or omitting combining features, elements and / or acts from described embodiments.  It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, omissions and sub-combinations as may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth in the examples but should be given the broadest interpretation consistent with the description as a whole.  [SV1]We assume the precipitate was washed in a solution comprising water and ethanol and not water followed by ethanol. Please confirm [NI2]confirmed [SV3]Please comment if any other acid may be used? perhaps diluted sulfuric or nitric or floric? [NI4]No, only glacial acetic acid [SV5]Is there a significance of drying in vacuum? Can it be air dried for example? [NI6]Yes, No it can’t be dried in air because of sample integrity [SV7]As mentioned above, any significance for drying in vacuum? [NI8]Yes, see above. Solid could adsorb moisture [SV9]Please let us know if this invention is specific to using Zn and Zr salts or other metal salts may also be used? [NI10]You can use other metal ions, such cupper (II) [SV11]Please let us know if this is possible. [NI12]Yes, possible [SV13]Please provide the solution in which the precipitate is washed [NI14]Washing with DMF followed by chloroform [SV15]Please provide the solution in which the precipitate is washed [NI16]DMF, then chloroform (3 to 1)

Claims

1. A method of preparing a glycoluril-based linker with accessible NH and COOH groups comprising :providing a solution comprising urea;providing a solution comprising a diketone derivative; providing a weak acid; and reacting the urea solution and the diketone derivative solution with the weak acid to form a precipitate, the precipitate comprising the glycoluril-based linker with accessible NH and COOH groups.

2. The method of claim 1 wherein the diketone derivative is 4,4'-dioxo-[1,1'-biphenyl]-3,3'-dicarboxylic acid.

3. The method of claim 1 wherein the glycoluril-based linker is 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl)dibenzoic acid.

4. The method of claim 1 wherein the reacting comprises mixing the urea solution, the diketone derivative solution and the weak acid at a temperature of about 90⁰C.

5. The method of claim 1 wherein the weak acid is glacial acetic acid.

6. The method of claim 1 further comprises separating the precipitate and washing the precipitate in a cold water and ethanol and drying the precipitate in vacuum.

7. The method of claim1 wherein the glycoluril-based linker comprises four accessible NH groups and two accessible carboxylic groups for chemical interaction.

8. A glycoluril-based linker, 4,4'-(2,5-dioxotetrahydroimidazo[4,5-d]imidazole-3a,6a(1H,4H)-diyl)dibenzoic acid with four accessible NH groups prepared by the method as claimed in claim 1.

9. A method of preparing a glycoluril-based Metal-Organic Framework (MOF) with accessible NH groups comprisingproviding a glycoluril-based linker prepared using the method as claimed in claim 1;providing a metal salt;providing a solution of dimethylformamide;reacting the glycoluril-based linker, the metal salt and the solution of dimethylformamide to form a precipitate, the precipitate comprising glycoluril-based MOF with accessible NH groups.

10. The method of claim 9 wherein the reacting comprises mixing the glycoluril-based linker, the metal salt and the solution of dimethylformamide at a temperature of about 85⁰C for about 24 hours.

11. The method of claim 9 wherein the metal salt is one of zinc nitrate hexahydrate and zirconium(IV) chloride octahydrate.

12. The method of claim 9 wherein the glycoluril-based Metal–Organic Framework has surface area in the range of about 1350 m² / g to 1530 m² / g.

13. The method of claim 9 wherein the glycoluril-based Metal–Organic Framework is thermally stable up to a temperature of about 600⁰C.

14. The method of claim 9 wherein, the glycoluril-based Metal-Organic Framework (MOF) has four accessible NH group.

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