1,2-Ene dithiolate bridged diiron carbonyl-phosphine and -phosphite complexes in relevance to the active site of [FeFe]-hydrogenases: Synthesis, characterization and electrocatalysis

Abstract

A series of binuclear {FeIFeI} complexes, such as, [Fe2{μ-6,7-qdt}(CO)6], [Fe2{μ-diph-6,7-qdt}(CO)6] and [Fe2{μ-btdt}(CO)6] have recently been reported by us. These complexes have now been examined to explore structural, electronic and electrochemical effects on substituting one or two CO group(s) with other donor ligands, e.g., phosphine and phosphite ligands. Mono-phosphine substituted compounds [Fe2{μ-6,7-qdt}(CO)5PPh3] (1), [Fe2{μ-diph-6,7-qdt}(CO)5PPh3] (2) and [Fe2{μ-btdt}(CO)5PPh3] (7) are synthesized by the reactions of [Fe2{μ-6,7-qdt}(CO)6], [Fe2{μ-diph-6,7-qdt}(CO)6] and [Fe2{μ-btdt}(CO)6] respectively with PPh3 in the presence of Me3NO. Interestingly, treatment of [Fe2{μ-6,7-qdt}(CO)6], [Fe2{μ-diph-6,7-qdt}(CO)6] and [Fe2{μ-btdt}(CO)6] with controlled amount of P(OEt)3 affords the mono-substituted phosphite derivatives [Fe2{μ-6,7-qdt}(CO)5P(OEt)3] (3), [Fe2{μ-diph-6,7-qdt}(CO)5P(OEt)3] (4), [Fe2{μ-btdt}(CO)5 P(OEt)3] (8) respectively. The same reaction with an excess amount of P(OEt)3 affords the di-substituted phosphite derivatives [Fe2{μ-6,7-qdt}(CO)4{P(OEt)3}2] (5), [Fe2{μ-diph-6,7-qdt}(CO)4{P(OEt)3}2] (6) and [Fe2{μ-btdt}(CO)4{P(OEt)3}2] (9) correspondingly. These new complexes 1–9 have been characterized by IR, 1H, 13C, and 31P{1H} NMR and mass spectroscopy including elemental analysis. The solid state structures for all compounds have been determined by single-crystal X-ray structure analyses. The electrochemistry of 1–9 was performed by cyclic voltammetry to evaluate the effects of phosphine and phosphite ligands on the reduction potentials of the all carbonyl di-iron model complexes [Fe2{μ-6,7-qdt}(CO)6], [Fe2{μ-diph-6,7-qdt}(CO)6] and [Fe2{μ-btdt}(CO)6]. The electrocatalytic activities of model complexes 7–9 toward proton reduction of a strong acid p-HOTs have been described.