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

Three hexacarbonyl diiron dithiolate complexes [Fe2 (CO)6 (μ-(SCH2 )2 X)] with different substituted bridgeheads (X=CH2 , CEt2 , CBn2 (Bn=CH2 C6 H5 )), 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 (pdtEt2 , pdtBn2 ) 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.