Di/mono-nuclear iron(i)/(ii) complexes as functional models for the 2Fe2S subunit and distal Fe moiety of the active site of [FeFe] hydrogenases: protonations, molecular structures and electrochemical properties

Abstract

Di/mono-nuclear iron(I)/(II) complexes containing conjugated and electron-withdrawing S-to-S linkers, [{(μ-S)₂(C₄N₂H₂)}Fe₂(CO)₆] (1), [{(μ-S)₂(C₄N₂H₂)}Fe₂(CO)₅(PMe₃)] (1P), and [{(μ-S)₂(C₄N₂H₂)}Fe(CO)₂(PMe₃)₂] (2) were prepared as biomimetic models for the 2Fe2S subunit and distal Fe moiety of the active site of [FeFe] hydrogenases. The N atoms in the heterocyclic pyrazines of 1 and 2 were protonated in the presence of proton acid to generate one and two hydrides, [1(NH)]⁺ CF₃SO₃⁻, [2(NH)]⁺ CF₃SO₃⁻, and [2(NH)₂]²⁺ (CF₃SO₃⁻)₂, respectively. The protonation processes were evidenced by in situ IR and NMR spectroscopy. The molecular structures of the protonated species [1(NH)]⁺ CF₃SO₃⁻ and [2(NH)₂]²⁺ (CF₃SO₃⁻)₂ together with their originating complexes 1 and 2, and the mono-PMe₃ substituted diiron complex 1P were identified by X-ray crystallography. The IR and single-crystal analysis data all suggested that the electron-withdrawing bridge, pyrazine, led to decreased electron density at the Fe centers of the model complexes, which was consistent with the electrochemical studies. The cyclic voltammograms indicated that complex 1 exhibited a low primary reduction potential at −1.17 V vs. Fc–Fc⁺ with a 270 mV positive shift compared with that of the benzene-1,2-dithiolate (bdt) bridged analogue [(μ-bdt)Fe₂(CO)₆]. Under the weak acid conditions, complexes 1 and 2 could electrochemically catalyze the proton reduction. More interestingly, the mononuclear ferrous complex 2 showed two catalytic peaks during the formation of hydrogen, confirming its potential as a catalyst for hydrogen production.