Spin Crossover in a Hexaamineiron(II) Complex: Experimental Confirmation of a Computational Prediction

Single crystal structural analysis of [Fe^II (tame)₂ ]Cl₂ ⋅MeOH (tame=1,1,1-tris(aminomethyl)ethane) as a function of temperature reveals a smooth crossover between a high temperature high-spin octahedral d⁶ state and a low temperature low-spin ground state without change of the symmetry of the crystal structure. The temperature at which the high and low spin states are present in equal proportions is T_1/2 =140 K. Single crystal, variable-temperature optical spectroscopy of [Fe^II (tame)₂ ]Cl₂ ⋅MeOH is consistent with this change in electronic ground state. These experimental results confirm the spin activity predicted for [Fe^II (tame)₂ ]²⁺ during its de novo artificial evolution design as a spin-crossover complex [Chem. Inf.

Model: 2015, 55, 1844], offering the first experimental validation of a functional transition-metal complex predicted by such in silico molecular design methods. Additional quantum chemical calculations offer, together with the crystal structure analysis, insight into the role of spin-passive structural components. A thermodynamic analysis based on an Ising-like mean field model (Slichter-Drickammer approximation) provides estimates of the enthalpy, entropy and cooperativity of the crossover between the high and low spin states.