Hydrogen Evolution Reaction Catalysed by [FeFe] Hydrogenase Active Site Models Incorporating Aminophosphine Ligands

Two diiron complexes [Fe₂(μ-SC₆H₄CH₃-p)₂(CO)₅(P(NEt₂)(OMe)₂)] 1 and [Fe₂(μ-SC₆H₄CH₃-p)₂(CO)₅{PPh(NEt₂)(κ¹-SC₆H₄CH₃-p)}] 2 with aminophosphine ligands have been synthesized, characterized spectroscopically and studied for electrocatalytic proton reduction activity. Aminophosphine ligands possess basic N-sites, are strong σ-donors, and offer the possibility of altering substituents on both phosphorus and nitrogen atoms, thus, such ligands could be modulated to tune the electronic properties of the metallic core for improving catalysis. The phosphine ligands, P(NEt₂)₃ and PPh(NEt₂)₂ used for the synthesis of complexes 1 and 2 converted to P(NEt₂)(OMe)₂ and PPh(NEt₂)(κ¹-SC₆H₄CH₃-p), respectively due to alcoholysis/solvolysis during the synthesis of the complexes. Complexes 1 and 2 crystallized in the orthorhombic (P2₁2₁2₁) and monoclinic (P2₁/c) crystal systems, respectively. Proton reduction activity observed for both the complexes with acetic acid and trifluoroacetic acid as proton sources in acetonitrile was confirmed by cyclic voltammetric (CV) and coulometric experiments. The values of k_obs (s⁻¹) were found to be 310, 490 in acetic acid and 95, 69 in trifluoroacetic acid for complexes 1 and 2, respectively. Based on the CV measurements, an ECEC (E = Electrochemical and C = Chemical) mechanistic cycle was speculated for the electrocatalytic proton reduction by complexes 1 and 2 in the presence of acetic acid.