Structure–Property Relationships of Fe₄S₄ Clusters

In this theoretical study, the sensitivity of Fe 4 S 4 cluster properties, such as potential energy, spin coupling, adiabatic detachment energy, inner‐sphere reorganization energy, and reactivity, to structural distortions is investigated. [Fe 4 S 4 (SH) 4 ] 3−/2−/1− model clusters anchored by fixed hydrogen atoms are compared with Fe 4 S 4 clusters coordinated by ethyl thiolates with fixations according to cysteine residues in crystal structures. For the model system, a dependence of the ground‐state spin‐coupling scheme on the hydrogen–hydrogen distances is observed. The minima of the potential energy surface of [Fe 4 S 4 (SH) 4 ] 2−/1− clusters are located at slightly smaller hydrogen–hydrogen distances than those of the [Fe 4 S 4 (SH) 4 ] 3− cluster. For inner‐sphere reorganization energies the spin‐coupling scheme adopted by the broken‐symmetry wave function plays an important role, since it can change the reorganization energies by up to 13 kcal mol −1 . For most structures, [Fe 4 S 4 (SR) 4 ] 2− and [Fe 4 S 4 (SR) 4 ] 1− (R=H or ethyl, derived from cysteine) favor the same coupling scheme. Therefore, the reorganization energies for this redox couple are relatively low (6–12 kcal mol −1 ) compared with the 2−/3− redox couple favoring different spin‐coupling schemes before and after electron transfer (14–18 kcal mol −1 ). However, one may argue that more reliable reorganization energies are obtained if always the same spin‐coupling pattern is enforced. All theoretical observations and insights are discussed in the light of experimental results distilled from the literature.