Extremely Gradual Spin-Crossover Phenomenon in a Cyano-Bridged Fe−Mo Bimetallic Assembly

Abstract

We report a unique type of spin-crossover phenomenon in a three-dimensional (3-D) Fe−Mo network with a cubic structure, Fe2[Mo(CN)8]·(3-pyCH2OH)8·3H2O (3-py = 3-pyridyl). This compound exhibits an extremely gradual FeII spin-crossover over a wide temperature range, which is more gradual than the crossover according to the Boltzmann distribution. The electronic states at 320 and 50 K are represented as (FeIIhs)2[MoIV(CN)8]·(3-pyCH2OH)8·3H2O and (FeIIhs)0.48(FeIIls)1.52[MoIV(CN)8]·(3-pyCH2OH)8·3H2O, respectively, where hs and ls denote high spin (S = 2) and low spin (S = 0), respectively. The model calculation based on Slichter−Drickamer’s model suggests this extremely gradual spin-crossover can be explained by the contribution of 3-D alternating alignment of hs and ls sites, i.e., −hs−ls−hs−ls−. This system is a strongly correlated system of spin-crossover sites because the spin-crossover FeII sites are directly linked by −NC−Mo−CN− with a high symmetry (FeII sites have one type of symmetry). The elastic interaction due to the volume change in a spin-crossover site isotropically propagates in the whole crystal. Since the CN bridges cannot be disconnected during spin-crossover, a 3-D alternating order of hs and ls sites is considered to be preferable.