Heterobimetallic [NiFe] Complexes Containing Mixed CO/CN Ligands: Analogs of the Active Site of the [NiFe] Hydrogenases.

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging, and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni( N S)Fe(CO)(CN)], [Ni( S)Fe(CO)(CN)], and [Ni( N S)Fe(CO)(CN)] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)(CN)} unit. X-ray crystallographic studies on [Ni( N S)Fe(CO)(CN)], supported by DFT calculations, are consistent with a solid-state structure containing distinct molecules in the singlet ( S = 0) and triplet ( S = 1) states. Each cluster exhibits irreversible reduction processes between -1.45 and -1.67 V vs Fc/Fc and [Ni( N S)Fe(CO)(CN)] possesses a reversible oxidation process at 0.17 V vs Fc/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a NiFe formulation for [Ni( N S)Fe(CO)(CN)]. The singly occupied molecular orbital (SOMO) in [Ni( N S)Fe(CO)(CN)] is based on Ni 3d and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a NiFe formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni( N S)Fe(CO)(CN)] and its [Ni( N S)] precursor, together with calculations on the oxidized [Ni( N S)Fe(CO)(CN)] and [Ni( N S)] forms suggests that the binding of the {Fe(CO)(CN)} unit to the {Ni(CysS)} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.