Biomimetics of the [FeFe]-hydrogenase enzyme: Identification of kinetically favoured apical-basal [Fe2(CO)4(μ-H){κ2-Ph2PC(Me2)PPh2}(μ-pdt)]+ as a proton-reduction catalyst

Abstract

Reaction of [Fe2(CO)6(μ-pdt)] with the small bite-angle diphosphine 2,2′-bis(diphenylphosphino)propane gave the chelated complex [Fe2(CO)4{κ2-Ph2PC(Me2)PPh2}(μ-pdt)]. This exists in solution as a mixture of non-interconverting dibasal and apical-basal isomers which slowly rearrange to the bridged isomer, [Fe2(CO)4{μ-Ph2PC(Me2)PPh2}(μ-pdt)], upon heating. X-ray structures of the dibasal and bridged isomers reveal an increase of ca. 19° in the PCP bond angle upon diphosphine movement from chelated to bridged positions. To probe the relative stability of these isomers, DFT calculations have been carried out and the bridged isomer is found to lie 3.8 and 1.3 kJ mol−1 lower in energy than the dibasal and apical-basal chelated isomers respectively. Protonation of the bridged isomer with HBF4·Et2O is slow and gives an unstable product. In contrast, both chelated isomers protonate rapidly and cleanly to initially yield apical-basal [Fe2(CO)4(μ-H){κ2-Ph2PC(Me2)PPh2}(μ-pdt)][BF4], which rearranges slowly to the dibasal isomer. The latter has been crystallographically characterized, protonation resulting in only very minor metric changes with the iron–iron bond length and diphosphine coordination being essentially unchanged. Electrochemical studies have been carried out in MeCN, and for the chelated isomers separate redox features are seen for the dibasal and apical-basal isomers. The chelated isomers are proton reduction catalysts in acetonitrile in the presence of HBF4·Et2O. Proton reduction occurs at −1.58 V via the kinetically favoured apical-basal hydride cation. DFT calculations have been used to study the mechanism of formation of H2 and are consistent with competing CECE and CEECC mechanisms, the branch point being the protonation or one-electron reduction of the 35-electron species [Fe2(CO)4(μ-H){κ2-Ph2PC(Me2)PPh2}(μ-pdt)].