Redox-Induced Single-Molecule Magnetism in Mixed-Valent [2 x 2] Co-4 Grid Complexes

Abstract

Homovalent [2 x 2] Co-4 grid complexes (Co4L4H)-L-II(4) (1) and (Co4L4Me)-L-II(4) (2) with two new bis(terdentate) pyrazolate-bridged ligands, HLH = 3,5-bis(6-(1H-pyrazol-1-yl)pyrid-2-yl)pyrazole and HLMe = 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 {N-6} 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 S-T = 0 ground state of 1 and 2. Twofold oxidation, electrochemically or chemically, led to the dimixed-valent [2 x 2] Co-4 grids (Co2Co2L4H)-Co-II-L-III(6) (3) and (Co2Co2L4Me)-Co-II-L-III(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 H-1 and C-13 NMR spectra of paramagnetic 1-4 were recorded and confirmed that the dimixed-valent (Co2Co2III)-Co-II 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 {N-6} 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 (Co2Co2III)-Co-II grids, and both 3 and 4 to display frequency-dependent peaks in the out-of-phase (X) 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 t0 = 6.7 center dot 10-6 and 5.0 center dot 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 (Co2Co2III)-Co-II grids and to structural distortions that are beneficial for SMM properties.