Effect of Ligand Modulation on Metal-to-Metal Electron Transfer in a Series of [FeCo] Molecular Square Complexes.

A series of three new cyanide-bridged [FeCo] molecular square complexes, namely, {[Fe(Tp*)(CN)][Co(L)]}(BF)·2DMF (L = bik (), bik* (), and vbik (); Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bik = bis(1-methyl-1-imidazol-2-yl)ketone, bik* = bis(1-ethyl-1-imidazol-2-yl)ketone, and vbik = bis(1-vinyl-1-imidazol-2-yl)ketone; DMF = dimethylformamide) were synthesized and characterized by single-crystal X-ray diffraction analyses and by magnetic, electrochemical, and spectroscopic measurements. Magnetic studies reveal that all three complexes exhibit temperature-induced metal-to-metal electron transfer (MMET) from a high-spin Co(II) center to a low-spin Fe(III) center, transforming a high-temperature paramagnetic {Fe-CN-Co} ground state into a low-temperature diamagnetic {Fe-CN-Co} state with a decrease in the temperature from 300 to 100 K. Complexes and show the interconversion of the paramagnetic {Fe-CN-Co} ground state into a diamagnetic {Fe-CN-Co} state in a single-step transition with values of 180 and 186 K, respectively, while a two-step MMET with value of 214 and 178 K was observed for complex .