Electron transfer coupled spin transition in cyano-bridged mixed-valence {FeIII2FeII2} molecular squares.

The research on electron transfer coupled spin transition regulating the valence state and spin state transition of metal ions is promising and challenging. Herein, we report a cyano-bridged {FeIII2FeII2} molecular square complex, {[Fe(Tp)(CN)] [Fe(bnbpen)]}(ClO)·8CHOH (1·8CHOH, bnbpen = ,'-bis-(2-naphthylmethyl)-,'-bis(2-picolayl)-ethylenediamine), and its free of solvents form (1). Combined single-crystal X-ray diffraction, temperature-dependent infrared (IR) spectra, magnetic measurements, and Mössbauer spectra reveal that 1·8CHOH and 1 exhibit reversible one-step and two-step electron transfer coupled spin transition (ETCST) with temperature change, between the low-temperature state {Fe(μ-CN)Fe} (LS = low spin, HS = high spin) and the high-temperature state {Fe(μ-CN)Fe}, respectively.