Influence of pendant amine of phosphine ligands on the structural, protophilic, and electrocatalytic properties of diiron model complexes related to [FeFe]-hydrogenases

Abstract

To explore the influence of pendant amine of phosphine ligands on the structural, protophilic, and electrochemical properties of diiron model complexes related to [FeFe]-hydrogenases, one new aminophosphine-substituted diiron oxadithiolate (odt) complex Fe2(μ-odt)(CO)5{Ph2P(CH2NMe2)} (1) and its new reference analogue Fe2(μ-odt)(CO)5{Ph2P(CH2Ph)} (2) were synthesized and characterized through elemental analysis, FT-IR as well as NMR spectroscopies, and X-ray crystallography. Notably, a comparative study on the protonation and electrochemistry of 1 and 2 is performed in the absence or presence of strong acid CF3CO2H and weak acid CH3CO2H by using in situ spectroscopic techniques (IR and NMR) and cyclic voltammetry (CV). The protonation study reveals that complexes 1 and 2 are treated with excess CF3CO2H to form a small amount of their respective Fe-protonated products [(μ-H)Fe2(μ-odt)(CO)5{Ph2P(CH2NMe2)}](CF3CO2) ([1(μH)]+) and [(μ-H)Fe2(μ-odt)(CO)5{Ph2P(CH2Ph)}](CF3CO2) ([2(μH)]+), but they are unreactive with excess CH3CO2H. The CV investigation suggests that complexes 1 and 2 are found to be active biomimetic electrocatalysts for proton reduction to hydrogen (H2) with CF3CO2H and CH3CO2H, in which complex 1 shows a slightly greater turnover frequency (TOF) value for H2 evolution in contrast to its reference 2.