Phenyl-functionalized Diiron Propanedithiolate Complexes with 1,3-Bis(diphenylphosphine)propane Ligand: Chelated and Bridged Isomers as Proton-reduction Catalysts

Reaction of the diiron propanedithiolate complex [μ-(SCH2)2CHC6H5]Fe2(CO)6 (A) with 1,3-bis(diphenylphosphine)propane (dppp) in refluxing xylene yielded a chelated complex [(μ-SCH2)2CHC6H5]Fe2(CO)4(κ2-dppp) (1) and a bridged isomer [(μ-SCH2)2CHC6H5]Fe2(CO)4(μ-dppp) (2), while in refluxing toluene or in hot MeCN in the presence of Me3NO·2H2O gave the chelated isomer 1 as the major or sole product. Protonation of complex 1 upon addition of HBF4·Et2O gave a bridging hydride (μ-H)[(μ-SCH2)2CHC6H5]Fe2(CO)4(κ2-dppp)(BF4) (3). The structures of all complexes were fully characterized by spectroscopic methods and for complexes 1 and 2 by X-ray crystallography. In the solid state, the chelated complex adopts an apical–basal diphosphine arrangement and exhibits a six-membered ring in approximate chair conformation. The bridged isomer presents an expected cisoid dibasal geometry and the bond length of Fe–Fe is longer than that in chelated isomer, reaching to 2.61 Ǻ. The electrochemical investigation of both isomers in MeCN-[NBu4][PF6] in the presence of HBF4 showed that the catalytic efficiency of the bridged isomer is no less than that of the chelated isomer but better than that of the analogue [(μ-SCH2)2CHC6H5]Fe2(CO)4(μ-dppm). Interestingly, isomer 1 was converted to isomer 2 in refluxing xylene. An electron-transfer-catalyzed isomerization between one isomer and the other was not observed.