The employment of a hydrophilic tris(morpholino)phosphineligand in diiron propane-1,3-dithiolate complexes for potentially water-soluble iron-only hydrogenase active-site mimics

Abstract

A tris(morpholino)phosphine (TMP) ligand was introduced into the diiron dithiolate complexes in order to improve the hydro- and protophilicity of the iron-only hydrogenase active site models. Mono- and di-TMP substituted diiron complexes, (μ-pdt)[Fe(CO)₃][Fe(CO)₂(TMP)] (2, pdt = 1,3-propanedithiolato) and (μ-pdt) [Fe(CO)₂(TMP)]₂ (3), were synthesized and spectroscopically characterized. The coordination configuration of 3 was determined by single X-ray analysis. Temperature-dependent ¹H and ³¹P NMR spectroscopy studies provided insight into the interconversion of the irondithiacyclohexane ring and the rotation of the [Fe(CO)₂PR₃] moieties for 2 and 3 in solution. The electrochemical properties of 2 and 3 were investigated in pure CH₃CN and CH₃CN–H₂O mixtures in the absence and presence of acetic acid. Hydrogen production and the dependence of current on acid concentration indicated that complexes 2 and 3 exhibited electrocatalytic activities for proton reduction in both pure and H₂O-containing CH₃CN solutions. The current sensitivities, i.e., electrocatalytic activities, were demonstrated to be greater in CH₃CN–H₂O mixtures than in pure CH₃CN. The most effective electrocatalytic activities of 2 and 3 were observed with 10% added water.