Normal vs. Inverted Ordering of Reduction Potentials in [FeFe]-Hydrogenases Biomimetics: Effect of the Dithiolate Bulk

Three hexacarbonyl diiron dithiolate complexes [Fe-2(CO)(6)(mu-(SCH2)(2)X)] with different substituted bridgeheads (X=CH2, CEt2, CBn2 (Bn=CH2C6H5)), have been studied under the same experimental conditions by cyclic voltammetry in dichloromethane [NBu4][PF6] 0.2 M. DFT calculations were performed to rationalize the mechanism of reduction of these compounds. The three complexes undergo a two-electron transfer whose the mechanism depends on the bulkiness of the dithiolate bridge, which involves a different timing of the structural changes (Fe-S bond cleavage, inversion of conformation and CO bridging) vs redox steps. The introduction of a bulky group in the dithiolate linker has obviously an effect on normally ordered (as for propanedithiolate (pdt)) or inverted (pdt(Et2), pdt(Bn2)) reduction potentials. Et -> Bn replacement is not theoretically predicted to alter the geometry and energy of the most stable mono-reduced and bi-reduced forms but such a replacement alters the kinetics of the electron transfer vs the structural changes.