| The iron-only hydrogenases can catalyze the reduction of proton to hydrogen at extraordinarily high rates. The active site, wherein the catalytic chemistry takes place, adopts a bi-octahedral butterfly geometry, highly resembling the well-known organometalic complex [Fe2(μ-SR)2(CO)6-xIx]. Because of the simple structure and high efficiency, biomimetic models related to the active site of the Fe-only hydrogenase are of particular interest to bioinorganic chemists. The major challenge is now to understand the enzymatic catalytic mechanism and search the synthetic competitive catalysts that function with hydrogenase-like capability. Herein, a series of diiron disulftde complexes were synthesized as the structural and functional models of the Fe-only hydrogenase active site.To investigate the role of the bridging nitrogen heteroatom in the active site of hydrogenase, the ADT bridged diiron complex [{(μ-SCH2)2N(4-CH3C6H4)}Fe2(CO)6] (3) and its /Modophenylisocyanide disubstituted complex [{(μ-SCH2)2N(4-CH3C6H4)}Fe2(CO)4(4-IC6H4NC)2] (5) were prepared. The IR spectra show that the iodophenylisocyanide ligand has better electron-donating ability relative to CO ligand. Analysis of cyclic voltammetric curve indicates that complex 5 can catalyze the proton reduction to hydrogen at ca. -1.43 V vs Fc+/Fc.The CO ligand of [(μ-pdt)Fe2(CO)6] (6) was replaced by the strong electron-donating ligand tris(N-pyrrolidinyl)phosphine (P(N(C4H8)3) to afford an asymmetrically monosubstituted complex [(μ-pdt)Fe2(CO)5P(NC4H8)3] (7). IR and electrochemical studies have indicated that the P(NC4H8)3 ligand has better electron-donating ability than that of those phosphine ligands, such as PMe3, PTA (1,3, 5-triaza-7-phosphaadamantane), PMe2Ph PPh3, and P(OEt)3. Analysis of cyclic voltammetric curve indicates that complex 7 can catalyze the reduction of protons to hydrogen at its Fe0Fe1 level in the presence of HO Ac. The formation of a bridging CO group during the reduction of 7 at -1.98 V has been identified using IR spectroelectrochemical techniques. A plausible ECCE mechanism was proposed.In the search for synthetic competitive catalysts that function with hydrogenase-like capability in mild conditions, Ph2PPyr monosubstituted diiron complex [(μ-pdt)Fe2(CO)5(Ph2PPyr)] (13) and disubstituted complex [(μ-pdt)Fe2(CO)5(Ph2PPyr)2] (14), as well as PPyr3 monosubstituted diiron complex [(μ-pdt)Fe2(CO)5(PPyr3)] (15) and disubstituted complex [(μ-pdt)Fe2(CO)5(PPyr3)2] (16) were prepared. IR spectra of CO bands for complexes 13-16 are shifted to higher energy relative to "traditional" phosphine ligands, such as PPh3, PMe3, PTA, indicating that N-pyrrolyl phosphines are better π-acceptor. The reduction potentials of these complexes show the anodic shifts relative to other phosphine...
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