Substituent effects of tertiary phosphines on the structures and electrochemical performances of azadithiolato‐bridged diiron model complexes of [FeFe]‐hydrogenases

Abstract

In this work, a new series of azadithiolato‐bridged diiron complexes [Fe2(μ‐adtNOH)(CO)5{P(C6H4R)3}] (adtNOH = (SCH2)2N(C6H4CH2CH2OH‐p) and R = Cl‐p, 1; Me‐m, 2; OMe‐p, 3 supported by tertiary phosphines, which may be considered as the active site models of [FeFe]‐hydrogenases, has been prepared in 73–81% yields by the Me3NO‐assisted decarbonylating reactions of all‐CO precursor [Fe2(μ‐adtNOH)(CO)6] (A) with several tertiary phosphines [(P(C6H4R)3] in MeCN at room temperature. All the new complexes 1–3 are fully characterized by elemental analysis, spectroscopic techniques (Fourier transform‐infrared spectroscopy [FT‐IR] and nuclear magnetic resonance [NMR]), and especially for 1 and 3 by X‐ray crystallography. The IR and 31P{1H} NMR spectroscopic analyses have shown that the electron density of diiron center in 1–3 may be adjusted by the introduction of the different substituents (R) of the P(C6H4R)3 ligands into the [2Fe2S] cluster. The X‐ray crystallographic investigation has displayed that the typical Fe‐Fe, Fe‐P, and Fe‐C distances of 1–3 are significantly influenced by the electronic effects of the P(C6H4R)3 phosphines coordinated apically to one Fe atom, whereas their Papical‐Fe‐Fe and Capical‐Fe‐Fe angles are mainly controlled by the steric interaction between the phosphine ligands and the dithiolate bridges. Further, the electrochemical performances of 1–3 are studied and compared in the absence or presence of acetic acid (HOAc) as proton source by using cyclic voltammetry, suggesting that they are active for the electrocatalytic proton reduction to hydrogen (H2).