Synthesis of Triazole and Tetrazole-Functionalized Zr-Based Metal-Organic Frameworks Through Post-Synthetic Ligand Exchange

Summary

Post-synthetic ligand exchange (PSE) is a versatile and powerful tool for installing functional groups into metal-organic frameworks (MOFs). Exposing MOFs to solutions containing triazole- and tetrazole-functionalized ligands can incorporate these heterocyclic moieties into Zr-MOFs through PSE processes.

Abstract

Metal-organic frameworks (MOFs) are a class of porous materials that are formed through coordination bonds between metal clusters and organic ligands. Given their coordinative nature, the organic ligands and strut framework can be readily removed from the MOF and/or exchanged with other coordinative molecules. By introducing target ligands to MOF-containing solutions, functionalized MOFs can be obtained with new chemical tags via a process called post-synthetic ligand exchange (PSE). PSE is a straightforward and practical approach that enables the preparation of a wide range of MOFs with new chemical tags via a solid-solution equilibrium process. Furthermore, PSE can be performed at room temperature, allowing the incorporation of thermally unstable ligands into MOFs. In this work, we demonstrate the practicality of PSE by using heterocyclic triazole- and tetrazole-containing ligands to functionalize a Zr-based MOF (UiO-66; UiO = University of Oslo). After digestion, the functionalized MOFs are characterized via various techniques, including powder X-ray diffraction and nuclear magnetic resonance spectroscopy.

Introduction

Metal-organic frameworks (MOFs) are three-dimensional porous materials that are formed through coordination bonds between metal clusters and multi-topic organic ligands. MOFs have garnered significant attention due to their permanent porosity, low density, and ability to associate organic and inorganic components, which enables diverse applications^1,2. Moreover, the vast range of metal nodes and strut organic linkers offer MOFs theoretically unlimited structural combinations. Even with identical framework structures, MOFs' physical and chemical properties can be modified through ligand functionalization with chemical tags. This modification process offers a promising route to tailor the properties of MOFs for specific applications^{3,4,5,6,7,8,9}.

Both the pre-functionalization of ligands prior to MOF synthesis and post-synthetic modification (PSM) of MOFs have been employed to introduce and/or modify functional groups in MOF ligands^10,11. In particular, covalent PSMs have been extensively studied to introduce new functional groups and generate a range of MOFs with diverse functionalities^12,13,14. For instance, UiO-66-NH₂ can be converted to amide-functionalized UiO-66-AMs with different chain lengths (ranging from the shortest acetamide to the longest n-hexyl amide) through acylation reactions with appropriate acyl halides (such as acetyl chloride or n-hexanoyl chloride)^15,16. This approach demonstrates the versatility of covalent PSMs to introduce specific functional groups onto MOF ligands, paving the way for a broad range of applications.

In addition to covalent PSMs, post-synthetic ligand exchange (PSE) is a promising strategy for modifying MOFs (Figure 1). Since MOFs are composed of coordination bonds between metals and ligands (such as carboxylates), these coordination bonds can be replaced with external ligands from a solution. Exposing MOFs to a solution containing the desired ligand with chemical tags can be incorporated into the MOFs via PSE^{17,18,19,20,21,22}. Since the PSE process is accelerated by the existence of coordinative solvents, the phenomenon is also called solvent-assisted ligand exchange (SALE)^23,24. This approach offers a flexible and facile method for functionalizing MOFs with a wide range of external ligands, enabling a broad spectrum of applications^{25,26,27,28,29}.

Figure 1: Synthesis of triazole and tetrazole-functionalized H₂BDC ligands and preparation of triazole- and tetrazole-functionalized UiO-66 MOF through PSE. Please click here to view a larger version of this figure.

The progress of the PSE process can be controlled by adjusting the ligand ratio, exchange temperature, and time. Notably, room temperature PSE can be employed to obtain functionalized MOFs by exchanging ligands from a solution into MOF solids²⁰. The PSE strategy is particularly useful for introducing both thermally unstable functional groups (such as azido groups) and coordinative functional groups (such as phenol groups) into MOF structures¹⁸. In addition, the PSE strategy has been applied to various MOFs with metal and coordination bond variations. This exchange is a universal process in the chemistry of MOFs^30,31,32. In this study, we present a detailed protocol for PSE to obtain functionalized MOFs from pristine, non-functionalized MOFs, and we provide a characterization strategy to confirm successful functionalization of the MOFs. This method demonstrates the versatility and convenience of PSE for modifying MOFs with diverse functional groups.

Tetrazole-containing benzene-1,4-dicarboxylic acid (H₂BDC-Tetrazole)³³, and triazole-containing benzene-1,4-dicarboxylic acid (H₂BDC-Triazole) are synthesized as target ligands and utilized in the PSE of UiO-66 MOFs to obtain novel, coordination-free, triazole-containing MOFs. Both triazoles and tetrazoles possess acidic N-H protons on their heterocyclic rings and can coordinate with metal cations, thus enabling their use in constructing MOFs^34,35. However, there are limited studies on incorporating coordination-free tetrazoles and triazoles into MOFs and related structures. In case of triazole-functionalized Zr-MOFs, UiO-68 type MOFs were investigated to photophysical properties through direct solvothermal synthesis with benzotriazole functionalities³⁶. For tetrazole-functionalized Zr-MOFs, the mixed direct synthesis was employed³³. These heterocycle-functionalized MOFs could provide potential coordinating sites in MOF pores for catalysis, selective molecular uptake by binding affinity, and energy-related applications, such as proton conduction in fuel cells.

Protocol

The reagents required to prepare MOFs and the ligands are listed in the Table of Materials.

1. Setting up the post-synthetic ligand exchange (PSE) process

1. Completely dry the pre-synthesized UiO-66 MOFs under vacuum to remove any unreacted metal-salts and ligands in the pores, and remaining solvent residues overnight.
   NOTE: See Supplementary File 1 for the synthesis procedure of UiO-66 MOFs.
2. Prepare functionalized ligands, H₂BDC-Triazole, and H₂BDC-Tetrazole (see Supplementary File 1 for the preparation process; Supplementary Figure 1 and Supplementary Figure 2 for characterization) in an isolated state and fully dry under vacuum overnight.
3. Prepare a 4% potassium hydroxide (KOH) aqueous solution by dissolving potassium hydroxide in deionized water.
4. Perform the PSE process in 20 mL scintillation vials with polypropylene (PP) caps (see Table of Materials).
5. Measure the H₂BDC-Triazole (23.3 mg, 0.1 mmol) or H₂BDC-Tetrazole (23.4 mg, 0.1 mmol) and place into the scintillation vial.
6. Dissolve the H₂BDC ligand in aqueous solution. Use a glass pipette to transfer 1.0 mL of the 4% KOH aqueous solution into the scintillation vial containing BDC-Triazole or BDC-Tetrazole.
   CAUTION: The 4% KOH aqueous solution is highly basic. Avoid all forms of contact and wear personal protective equipment.
7. Sonicate the mixture until all the solids have fully dissolved.
8. Neutralize the solution to pH 7. Use a glass pipette to transfer 1 M hydrochloric acid (HCl) aqueous solution with stirring into the scintillation vial containing dicarboxylate until pH 7 is reached. Measure the pH either by pH paper (and its color) or a pH meter.
   NOTE: It is crucial to maintain a pH of 7 during the exchange process, since MOFs are typically unstable under basic conditions (>pH 7), and the BDC ligand is not soluble under acidic conditions (<pH 7). Approximately 1.5 mL of 1 M HCl aqueous solution is required to neutralize BDC-Triazole or BDC-Tetrazole-containing solution.
9. Add MOFs to the dicarboxylate solution and incubate. Add UiO-66 MOF (33 mg, 0.02 mmol) to the scintillation vial containing pH 7 solution.
   NOTE: At pH 7, UiO-66 MOF particles are not soluble in water, resulting in their suspension in water.
10. Incubate the scintillation vial containing the MOF and ligand in a shaker at 120 rpm and at room temperature for 24 h.
    ​NOTE: Stirring can be used as an alternative to shaker-incubation for the PSE process. However, caution should be taken, since physical contact between the magnetic stirring bar and MOFs can result in cracking or breaking of the MOF particles.

2. Isolating the exchanged MOF and washing process

1. After incubation, isolate the solid MOF from the mixture by centrifugation (1,166 x g, 5 min, room temperature).
2. Add fresh methanol (10 mL) to the obtained solid MOF and shake the mixture to form a heterogeneous mixture to dissolve the remaining unexchanged BDC ligands.
3. Isolate the solid that was isolated by centrifugation (1,166 x g, 5 min, room temperature).
4. Repeat steps 2.2-2.3 two more times for a total of three washing cycles.
5. Fully dry the exchanged MOF solid under a vacuum for overnight after the last washing.

3. Characterization of the MOF by powder x-ray diffraction (PXRD)

1. Transfer approximately 10 mg of the exchanged MOF solid to a PXRD sample holder (see Table of Materials).
2. Place the sample holder in the diffractometer.
3. Collect the PXRD pattern (Figure 2) in the 2θ range from 5° to 30°.
4. Compare the obtained data to the parent UiO-66 MOF and the simulated pattern.

4. Characterization of the MOF by nuclear magnetic resonance (NMR) after digestion

1. Transfer approximately 30 mg of the exchanged MOF solid to a fresh 4 mL vial.
2. Use a micropipette to transfer 400 µL of DMSO-d₆ to the MOF sample.
3. Use a micropipette to transfer 200 µL of 4.14 M NH₄F/D₂O solution to a DMSO-d₆ suspension of MOF powder.
   NOTE: An aqueous solution of approximately 40% HF can be used instead of the NH₄F/D₂O solution. In this case, a significant H₂O peak is observed in the NMR. However, clearer digestion is possible in the case of HF digestion.
   CAUTION: HF is highly toxic to the body and central nervous system. All forms of contact should be avoided, work should be performed in a fume hood, and personal protective equipment should be worn.
4. Sonicate the heterogeneous mixture for 30 min until the MOF is dissolved in the dimethyl sulfoxide (DMSO)-D₂O mixed solvent after digestion.
5. Remove any remaining insoluble solids by filtering the solution through a polyvinylidene difluoride (PVDF) syringe filter (Φ 13 mm, 0.45 µm pore; see Table of Materials) while transferring it from the 4 mL vial to an NMR tube.
6. Place the NMR tube in the NMR machine. Collect the NMR data (Figure 3).

Results

The successful synthesis of exchanged UiO-66 MOFs, UiO-66-Triazole, and UiO-66-Tetrazole produced colorless microcrystalline solids. Both H₂BDC-Triazole and H₂BDC-Tetrazole ligands also exhibited a colorless solid state. The standard method used to determine the success of the exchange involved measuring the PXRD patterns and comparing the crystallinity of the sample with pristine UiO-66 MOF. Figure 2 displays the PXRD patterns of exchanged UiO-66-Triazole and UiO-66-Te...

Discussion

The PSE process with functionalized BDC ligands toward Zr-based UiO-66 MOFs is a simple and versatile method to obtain MOFs with chemical tags. The PSE process is best conducted in aqueous media, requiring the initial step of solvating the ligand in an aqueous medium. When using pre-synthesized BDC with functional groups, direct dissolution in a basic solvent, such as a 4% KOH aqueous solution, is recommended. Alternatively, sodium or potassium salt of benzene-1,4-dicarboxylate may be used. Neutralization to pH 7 is crit...

Materials

Name	Company	Catalog Number	Comments
2-Bromoterephthalic acid	BLD Pharm	BD5695	reagent for BDC-Triazole
Azidotrimethylsilane	Simga Aldrich	155071	reagent for BDC-Triazole
Bis(triphenylphosphine)palladium(II) dichloride	TCI	B1667	reagent for BDC-Triazole
Copper(I) cyanide	Alfa-Aesar	12135	reagent for BDC-Tetrazole
Copper(I) iodide	Acros organics	20150	reagent for BDC-Triazole
Digital Orbital Shaker	Daihan Scientific	SHO-1D	PSE
Formic Acid	Daejung chemical	F0195	reagent for BDC-Tetrazole
Hybrid LC/Q-TOF system	Bruker BioSciences	maXis 4G	HR-MS
Lithum hydroxide monohydrate	Daejung chemical	5087-4405	reagent for BDC-Triazole
Magnesium sulfate	Samchun chemical	M1807	reagent for BDC-Triazole
Methyl alcohol	Daejung chemical	M0584	reagent for BDC-Tetrazole
N,N-Dimethylformamide	Daejung chemical	D0552	reagent for BDC-Tetrazole
Nuclear Magnetic Resonance Spectrometer-500 MHz	Bruker	AVANCE 500MHz	NMR
Polypropylene cap (22 mm, Cork-Backed Foil Lined)	Sungho Korea	22-200	material for digestion
Potassium cyanide	Alfa-Aesar	L13273	reagent for BDC-Tetrazole
PVDF Synringe filter (13 mm, 0.45 µm)	LK Lab Korea	F14-61-363	material for digestion
Scintillation vial (20 mL, borosilicate glass)	Sungho Korea	74504-20	material for digestion
Sodium azide	TCI	S0489	reagent for BDC-Tetrazole
Sodium bicarbonate	Samchun chemical	S0343	reagent for BDC-Triazole
Tetrabutylammonium fluoride (1 M THF solution)	Acros organics	20195	reagent for BDC-Triazole
Triethylamine	TCI	T0424	reagent for BDC-Triazole
Triethylamine hydrochloride	Daejung chemical	8628-4405	reagent for BDC-Tetrazole
Trimethylsilyl-acetylene	Alfa-Aesar	A12856	reagent for BDC-Triazole
Triphenylphosphine	TCI	T0519	reagent for BDC-Triazole
X RAY DIFFRACTOMETER SYSTEM	Rigaku	MiniFlex 600	PXRD
Zirconium(IV) chloride	Alfa-Aesar	12104	reagent for BDC-Tetrazole