| The Fe-Fe hydrogenases, which are efficient enzyme in hydrogen production than other types of hydrogenases, have inspired chemists in the bioinorganic community to synthesize close mimics of their active sites in the search for hydrogen production catalysts. The active site, wherein the catalytic process takes place, adopts a bis-octahedral butterfly geometry, highly resembling the well-known organometalic complex [Fe2(μ-SR)2(CO)6-nLn]. The major challenge is now to explore the enzymatic catalytic mechanism and search the synthetic competitive catalysts that function with hydrogenase-like capability. In this paper, a series of diiron disulfide complexes were synthesized as the structural and functional models of the Fe-Fe hydrogenase active site.To investigate the role of the secondary amine of azadithiolat bridge in the active site of hydrogenase, alkylsilyl was chosen to protect the amine and achieve the formation of the complex [(μ-SCH2NHCH2S)Fe2(CO)6] (5). In addition, a serial of the monosubstituted complexes (μ-SCH2)2NH[Fe2(CO)5L] (L = PPh3 (7), PMe2Ph (8), PMe3(9), P(OEt)3(10)) and one disubstituted complexe [(μ-SCH2NHCH2S)Fe2(CO)4 {P(OEt)3}2](11) were prepared. The crystal structure of the complex 7 showed that the N-H bond is in axial position and close to the higher electron-density Fe atom. This may indicate that the hydrogen on the bridged-N atom plays a very important role in catalyzing the reversible reaction of protons and electrons to molecular hydrogen. Proton reduction catalyzed by 9 was studied by cyclic voltammtry in the presence of HOTf. The first reductive peak at -1.88V was shifted by 0.4V towards more positive potential when HOTf was added, which is consistent with the protonation occurred on the N-bridging head. All of the features were indicative of catalytic proton reduction.On the basis of the particular dinuclear system, modifications of ligand substituents can improve hydrophilicity and water solubility. Three new complexes (μ-SCH2)2NCH2CH2CH3 [Fe2(CO)6-nLn](n=1, L=PTA, 17, n=2, L=PTA, 18, n=1, L=DAPTA, 19) were prepared as the models of the iron hydrogenase active site through controlled CO displacement of (μ-SCH2)2NCH2CH2CH3[Fe2(CO)6] (16) with PTA and DAPTA. The Fe-Fe distances in 17 (2.5505(6) A) and 18 (2.5381(8) A) are somewhat longer thanthe structural data of Fe-Fe bonds (2.49 - 2.51 A) found in other diiron azadithiolates and closer to those in DdHase and CpHase (ca. 2.6 A). Electrochemical properties of the complexes were studied by cyclic voltammetry in acetonitrile or a mixture of acetonitrile and water in the presence of acetic acid. The current sensitivity of the reduced species to acid concentration in the presence of H2O is greater than that in the pure CH3CN solution. Protonation of the three complexs only occurred at the bridging-N atom, rather than the tertiary nitrogen atom on the PTA or DAPTA ligands.Three complexes [(μ-SCH2CH2CH2S)Fe2(CO)5L] (L = PPh2-NHC2H4N(CH3)2(26), PPh2-NH(2-NH2C6H4)(27), PPh2[2-CH2N(CH3)2C6H4](28)) were prepared in which pendant nitrogen atom were utilized as a proton relay. The reduction potentials of complexes 26 and 27 show the anodic shifts 172 mV and 170 mV respectively in the presence of HOTf, which is consistent with the protonation occurred on the nitrogen atom of the phosphine ligand. All of the features were indicative of catalytic proton reduction.
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