Complexes of ferric iron have long been recognized for their ability to exhibit spin crossover and potential for use in sensing, switching, and memory devices. In the dinuclear iron complexes, a spin transition can be accompanied by change in oxidation state of the transition metals. Therefore, metalloenzymes containing dinuclear iron clusters could demonstrate spin evolution during the redox process. We studied the [Fe2S2(SCH3)4]2- complex that structurally mimics the Fe2S2-ferredoxin active site involved in the electron transfer in metalloproteins. We show that the small rearrangement of thiomethyl groups in [Fe2S2(SCH3)4]2- can facilitate nonadiabatic transitions between singlet and triplet states. These transitions are assumed to occur at the minimum energy crossing point (MECP) that also acts as a bifurcation point on the energy paths connecting three conformers. Two competitive pathways lead from the MECP to two singlet conformers (1B and 1C) with different orientations of methyl groups, as shown in Figure 1a. Density functional calculations predict similar barrier heights for both pathways. However, the minimal energy path connects the MECP with the higher-energy conformer 1B, as illustrated in Figure 1b. The nonadiabatic transitions are mediated by spin-orbit coupling with the magnitude of 148 cm-1 at MECP obtained with complete active space configuration interaction method. We calculated probabilities and rates of the singlet-triplet transitions using nonadiabatic transition state theory with Landau-Zener and Delos formulas. The values of the rate constants strongly depend on the barriers heights, while density of rovibrational states have no significant effect.

Computational study of spin crossover in the Fe2S2-ferredoxin active site model

251st National Meeting (2016)

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