文摘
Homovalent [2 × 2] Co4 grid complexes [CoII4LH4](BF4)4 (1) and [CoII4LMe4](BF4)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 {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 [CoII2CoIII2LH4](BF4)6 (3) and [CoII2CoIII2LMe4](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 1H and 13C NMR spectra of paramagnetic 1–4 were recorded and confirmed that the dimixed-valent CoII2CoIII2 grids are charge-localized on the NMR time scale. X-ray crystallography revealed significant structural grid distortion upon oxidation, with the low-spin CoIII at opposite corners and in almost octahedral {N6} environment but the remaining CoII ions’ coordination sphere approaching a trigonal prism. SQUID studies showed the high-spin CoII to be magnetically isolated in the dimixed-valent CoII2CoIII2 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 CoII2CoIII2 grids and to structural distortions that are beneficial for SMM properties.