Abstract
The design and synthesis of metal-organic frameworks has stirred up explosive interest, owing to the possibilities of obtaining a wide range of structures with diverse applications in many fields such as gas storage and/or separation, actuators and catalysis. In this study, we synthesised three new types of metal-organic frameworks: {[(Cu)(CN)(Me4bpz)].MeCN} denoted as 1MeCN, {[Fe2(BDC)4(Me4bpz)2].DMF} denoted as 2DMF and {[(Cu)2(CN)2(Me4bpz)].MeCN.H2O}n denoted as 3MeCN, to investigate the solid-state dynamics, mechanical properties and catalysis. The prime aim was to provide an in-depth understanding of the framework response upon temperature variation and determine key framework features suitable for click chemistry.
The first phase of the study involved the synthesis of the ligand 3,3',5,5'-tetramethyl-4,4'-bipyrazole (Me4bpz). The molecular structure was determined using 1H and 13C nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction analysis. Thereafter, we constructed a copper-(I) metal-organic framework, {[(Cu)(CN)(Me4bpz)].MeCN},1MeCN, using a solvothermal method. The framework structure was confirmed by single-crystal X-ray diffraction. The framework constituent units involved and thermal stability were determined by Fourier transform infrared spectroscopy and thermogravimetric analysis, respectively. We further assessed the framework response using variable temperature single-crystal X-ray diffraction on heating from 100-298 K at 20 K intervals. The thermal expansion coefficients were calculated using the principal axis strain calculator program. In the temperature range 100-160 K, the framework was revealed to undergo a phase transition accompanied by a change in space group (Pna21 to Ima2). Upon continuous heating of 1MeCN in the temperature range 160-298 K, the thermal expansion coefficients were derived. The results suggested that the framework exhibited negative thermal expansion along the a-axis (αa = -13 MK-1) and positive thermal expansion along the b-axis (αb = 106 MK-1) and c-axis (αc = 45 MK-1). The volumetric expansion of αv = 148 MK-1 was observed over the temperature range 160-298 K.
In the second phase of our study, we synthesised an iron-(II) metal-organic framework from the combination of terephthalic acid (BDC) and Me4bpz as mixed ligands and iron-(II) perchlorate as a metal precursor. This resulted in {[Fe2(BDC)4(Me4bpz)2].DMF}, 2DMF, as determined by single- crystal X-ray diffraction analysis. The phase purity and thermal stability were confirmed by
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powder X-ray diffraction and thermogravimetric analysis. Single-crystal X-ray diffraction analysis revealed a pseudo-octahedral coordination geometry for FeII with a paddlewheel unit. Using Mössbauer spectroscopy, we confirmed the FeII oxidation state in the framework and measurements from 7-300 K revealed an unusual temperature dependence in the relative line intensities of the quadrupole doublet, attributable to lattice vibrational anisotropy (Goldanskii-Karyagin effect).
In the final phase of the study, a copper-(I) metal-organic framework {[(Cu)2(CN)2(Me4bpz)] MeCN.H2O}n, 3MeCN, was synthesised and the molecular structure was elucidated by single-crystal X-ray diffraction. The crystal structure shows two distinct Cu coordination geometries, tetrahedral and trigonal planar and bears carbene-like features. The 3MeCN metal-organic framework was confirmed to undergo redox reactions using cyclic voltammetry which showed the catalyst to be active for more than two cycles. This further motivated us to conduct a copper azide-alkyne cycloaddition reaction using 3MeCN as a heterogeneous catalyst. In room temperature reactions with 10 mol% catalyst loading, 3MeCN proved to be an efficient catalyst, with yields of up to 62% of 1,4-phenyl-1,2,3-triazoles. The metal-organic framework 3MeCN displayed catalytic performance because it possesses CuI active sites and accessible voids.