Synthesis of phosphine derivatives of [Fe₂(CO)₆(μ-sdt)] (sdt = SCH₂SCH₂S) and investigation of their proton reduction capabilities

The reactions of [Fe₂(CO)₆(μ-sdt)] (1) (sdt = SCH₂SCH₂S) with phosphine ligands have been investigated. Treatment of 1 with dppm (bis(diphenylphosphino)methane) or dcpm (bis(dicyclohexylphosphino)methane) affords the diphosphine-bridged products [Fe₂(CO)₄(μ-sdt)(μ-dppm)] (2) and [Fe₂(CO)₄(μ-sdt)(μ-dcpm)] (3), respectively. The complex [Fe₂(CO)₄(μ-sdt)(κ²-dppv)] (4) with a chelating diphosphine was obtained by reacting 1 with dppv (cis-1,2-bis(diphenylphosphino)ethene). Reaction of 1 with dppe (1,2-bis(diphenylphosphino)ethane) produces [{Fe₂(CO)₄(μ-sdt)}₂(μ-κ¹-dppe)] (5) in which the diphosphine forms an intermolecular bridge between two diiron cluster fragments. Three products were obtained when dppf (1,1′-bis(diphenylphosphino)ferrocene) was introduced to complex 1; they were [Fe₂(CO)₅(μ-sdt)(κ¹-dppfO)] (6), the previously known [{Fe₂(CO)₅(μ-sdt)}₂(μ-κ¹-κ¹-dppf)] (7), and [Fe₂(CO)₄(μ-sdt)(μ-dppf)] (8), with complex 8 being produced in highest yield. Single crystal X-ray diffraction analysis was performed on compounds 2, 3 and 8. All structures reveal the adoption of an anti-arrangement of the dithiolate bridges, while the diphosphines occupy dibasal positions. Infra-red spectroscopy indicates that the mono-substituted complexes 5, 6, and 7 are inert to protonation by HBF₄.Et₂O, but complexes 2, 3, 4 and [Fe₂(CO)₅(μ-sdt)(κ¹-PPh₃)] (9) show shifts of their ν_(C-O) resonances that indicate that protons bind to the metal cores of the clusters. Addition of the one-electron oxidant [Cp₂Fe]PF₆ does not lead to any discernable shift in the IR resonances. The redox chemistry of the complexes was investigated by cyclic voltammetry, and the abilities of complexes to catalyze electrochemical proton reduction were examined.