Design of Rigidified μ-(9-Fluorenethiolate) {FeFe} Hydrogen Evolving Catalysts

Abstract

The hexacarbonyl diiron complex [Fe₂(μ-9-fluorenethiol)₂(CO)₆] (1) has been synthesized and characterized by various spectroscopic techniques as well as quantum chemical calculations. The molecular structure for complex 1 was determined by single-crystal X-ray diffraction and was supported by density functional theory (DFT) calculations. Complex 1 crystallized in the tetragonal P4̅2₁m crystal system with both fluorene moieties anti to each other. According to a QM conformational search, this corresponds to the lowest energy conformer. The complex displayed electrocatalytic activity for proton reduction in the presence of organic acids (acetic acid and trifluoroacetic acid), which was investigated by cyclic voltammetric (CV) and controlled-potential coulometric (CPC) experiments and calculations. Bulk electrolysis of complex 1 in the presence of an acid resulted in significant hydrogen evolution. DFT calculations demonstrated that complex 1 undergoes a one-electron metal-based reduction with a calculated redox potential in excellent agreement with experiment (at a low potential of −1.32 V), while a two-electron reduction occurs at a more negative potential of −1.70 V. Due to the rigidity of the μ-bridging 9-fluorenethiolates and structural integrity of 1 during reduction events, a two-electron-reduction mechanism followed by protonation of a terminal iron hydride species appears feasible.