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

Di/mono-nuclear iron(I)/(II) complexes containing conjugated and electron-withdrawing S-to-S linkers, [{(mu-S)(2)(C4N2H2)}Fe-2(CO)(6)] (1), [{(mu-S)(2)(C4N2H2)}Fe-2(CO)(5)(PMe3)] (1P), and [{(mu-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 [(mu-bdt)Fe-2(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.