Electron Paramagnetic Resonance Study of Guest Molecule-Influenced Magnetism in Kagome Metal–Organic Framework

Abstract

We present a continuous-wave electron paramagnetic resonance (EPR) study of Cu(ipa)(H 2 O) (ipa stands for isophthalate ligand) metal−organic framework (MOF) that is based on the kagome topology. The main structural unit of this compound is a copper paddle-wheel (PW) consisting of two Cu 2+ ions interconnected via four carboxylate groups in the syn−syn fashion. X-and Q-band EPR measurements of this MOF allow us to probe distinct magnetic interactions of different magnitudes that originate from these dinuclear units. EPR experiments performed at different temperatures reveal that two Cu 2+ ions in a single PW are antiferromagnetically coupled to the total spin S = 0 state with the exchange coupling constant J = −285 cm −1 . The fine structure pattern observed in the spectra indicates the thermal population of the excited triplet S = 1 spin states of the Cu 2+ pairs. Spectral simulations are used to determine the fine and hyperfine structure tensors of the PW units. At higher temperatures a collapse of the fine structure to a single EPR line is observed, indicating the interdinuclear exchange of the S = 1 spin states mediated via the isophthalate linkers. Comparison of the EPR results with the structural X-ray diffraction data reveals that the dehydration of the compound opens an additional spin triplet exchange path joining apical positions of the PWs. We estimated the effective exchange coupling constants in the as-synthesized and dehydrated forms of the compound by simulating the observed powder patterns. It is shown that the additional path is reversibly closed after the dehydrated sample is allowed to interact with air, demonstrating a peculiar relationship between the adsorbed guest molecules and magnetic properties of kagome MOFs even at room temperature.

Structural and magnetic properties of spin-1/2 dimer compound Cu2(IPA)2(DMF)(H2O) with a large spin gap

“…29,30 The compound tetrakis(µ 2 benzoato-O,O )-bis(dimethyl sulfoxide)dicopper(II) which also has paddle-wheel like dimer units but with a different ligand show one-dimensional character with a exchange coupling J/k B 405 K. 31 It undergoes a FM ordering at T C 9 K. Recently, another compound Cu(IPA)(H 2 O) is reported to have paddlewheel like spin dimers coupled through IPA ligand, similar to our compound. The crystal lattice forms a Kagomé structure with a very large spin gap of ∆/k B 410 K. 32 From the above examples it is clear that though all these compounds are having paddle-wheel like spin dimers but different connecting ligands between dimers lead to diverse ground states. Thus, in these compounds one can tune the ground state properties and even the magnitude of the spin gap/exchange coupling by simply changing/modifying the connecting ligand.…”

“…It undergoes a FM ordering at T 9 C K. Recently, another compound Cu(IPA)(H 2 O) is reported to have paddle-wheel like spin dimers coupled through IPA ligand, similar to our compound. The crystal lattice forms a Kagome ´ structure with a very large spin gap of / ∆ k 410 B K [33]. From the above examples is compounds are having paddle-wheel like spin dimers but different connecting ligands between dimers lead to diverse ground states.…”

Control of evolution of porous copper-based metal–organic materials for electroreduction of CO2 to multi-carbon products

“…30 Therefore, the Q-band EPR spectra of TCNQ@HKUST-1 at 100 K and of TCNQ@CuMOP at 293 K were chosen to compare with the un-modified parent materials. The EPR spectrum of HKUST-1 contains three types of EPR signals: 30,31 (i) a sharp signal due to monomeric, s = 1/2 Cu( ii ) sites (feature A), which in [Cu II 2 (OOCR) 4 ] paddlewheel-MOFs has been attributed to extra-framework uncoupled Cu( ii ) centres formed during the synthesis; (ii) excited S = 1 intra-binuclear triplet state of the [Cu II 2 (OOCR) 4 ] paddlewheel (feature B), spread over a wide field range due to a zero-field splitting | D | of ca. 0.325 cm −1 (Table S1 and Fig.…”

“…30 Therefore, the Q-band EPR spectra of TCNQ@HKUST-1 at 100 K and of TCNQ@CuMOP at 293 K were chosen to compare with the un-modified parent materials. The EPR spectrum of HKUST-1 contains three types of EPR signals: 30,31 (i) a sharp (Table S1 and Fig. S3, ESI †), and (iii) a broad isotropic feature (feature C) arising from inter-dimer exchange ( J 0 E 1 cm À1 ; Table S1, ESI †) between neighbouring S = 1 populated [Cu II 2 (OOCR) 4 ] paddlewheels (Fig.…”

Exploring Defect-Engineered Metal–Organic Frameworks with 1,2,4-Triazolyl Isophthalate and Benzoate Linkers

“…Such tools are X-ray photoelectron spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and X-ray absorption spectroscopy (XAS). 12,[45][46][47][48][49][50]51 For example, studies by Fang et al, 12 Marx et al, 51 and Kjaervik et al, 52 utilized XPS, FT-IR, and XAS measurements to demonstrate the influence of doping in HKUST-1 with the modified linker 2,5-pyridinedicarboxylate (PyDC) in defectengineered HKUST-1. EPR methods have been shown to be powerful tools for studying the local structure of triazolyl benzoate-based MOFs.…”