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Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks
Published on: June 9, 2023
Redox-Induced Single-Molecule Magnetism in Mixed-Valent [2 × 2] Co4 Grid Complexes
Jin Tong1, Serhiy Demeshko1, Michael John1
1Institute of Inorganic Chemistry, Georg-August-University Göttingen , Tammannstrasse 4, 37077 Göttingen, Germany.
Abstract:
Homovalent [2 × 2] Co4 grid complexes [Co(II)4L(H)4](BF4)4 (1) and [Co(II)4L(Me)4](BF4)4 (2) with two new bis(terdentate) pyrazolate-bridged ligands, HL(H) = 3,5-bis(6-(1H-pyrazol-1-yl)pyrid-2-yl)pyrazole and HL(Me) = 4-methyl-3,5-bis(6-(1H-pyrazol-1-yl)pyrid-2-yl)pyrazole, were synthesized and comprehensively characterized. X-ray crystallographic analyses of 1 and 2 showed a square arrangement of cobalt(II) ions, each metal ion in distorted octahedral {N6} coordination created by two terdentate compartments from orthogonal ligand strands. Magnetic measurements revealed the metal ions' high-spin (S = 3/2) state and moderate antiferromagnetic interactions to give an overall diamagnetic ST = 0 ground state of 1 and 2. Twofold oxidation, electrochemically or chemically, led to the dimixed-valent [2 × 2] Co4 grids [Co(II)2Co(III)2L(H)4](BF4)6 (3) and [Co(II)2Co(III)2L(Me)4](BF4)6 (4). ESI mass spectrometry confirmed the ruggedness of all grids 1-4 and their integrity in solution, and analysis of the UV/vis/NIR spectra suggested moderate electronic coupling. Solution (1)H and (13)C NMR spectra of paramagnetic 1-4 were recorded and confirmed that the dimixed-valent Co(II)2Co(III)2 grids are charge-localized on the NMR time scale. X-ray crystallography revealed significant structural grid distortion upon oxidation, with the low-spin Co(III) at opposite corners and in almost octahedral {N6} environment but the remaining Co(II) ions' coordination sphere approaching a trigonal prism. SQUID studies showed the high-spin Co(II) to be magnetically isolated in the dimixed-valent Co(II)2Co(III)2 grids, and both 3 and 4 to display frequency-dependent peaks in the out-of-phase (χ") component of the alternating-current (ac) magnetic susceptibility data, characteristic for single molecule magnet (SMM) properties (with energy barriers Ueff/kB = 16 and 26 K, and relaxation times τ0 = 6.7·10(-6) and 5.0·10(-6) s for 3 and 4, respectively). The results are discussed in terms of cooperative effects that originate from elastic coupling within the grids, mediated by the rigid bridging ligands. These effects synergistically lead to the preferred trans configuration of the dimixed-valent Co(II)2Co(III)2 grids and to structural distortions that are beneficial for SMM properties.
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