Abstract
[NiFe] hydrogenase containing [Ni-Fe] active center has been suggested to be the catalytic site for hydrogen activation. The catalytically active form Ni-C of [NiFe] hydrogenase was proposed to exist as the [(Scys-H)(Scys)NiIII(μ-Scys)2(μ-H) Fe(CO)(CN)2] with a hydride (H－) bridging between the Ni and Fe atoms. In model study, Reaction of complex [NiIII(OR)P(C6H3-3-SiMe3-2-S)3]– (R = Ph or CH2Ph) with HBpin (pin = OCMe2CMe2O) in THF at –80℃ undergoes HBpin-promoted hydride-alkoxide metathesis reaction to produce the thermally unstable [NiIII-H]-containing [PPN][NiIII(H)(P(o-C6H3-3-SiMe3-2-S)3)] (1) characterized by low-temperature UV-vis, EPR. Insertion of CS2 into the thermally unstable Ni(III)-hydride [PPN][Ni(H)(P(o-C6H3-3-SiMe3-2-S)3)] (1) generates [NiIII-1-S2CH]-containing [PPN][NiIII(1-S2CH)(P(o-C6H3-3- SiMe3-2-S)3)] (2). Reaction of the d-THF solution of complex 1 and CDCl3 yielding the very characteristic CHDCl2 identified by 1H NMR (δ 5.46 (t) ppm (C4D8O)) and the known [NiIII(Cl)(P(o-C6H3-3-SiMe3-2-S)3)]– also supports the existence of [NiIIIH]-containing complex 1 at –80 ℃. Both the tetradentate [P(o-C6H3-3-SiMe3-2-S)3]3–-supported Ni(III)-imidazole [Ni(Im)(P(o-C6H3-3-SiMe3-2-S)3)] (Im = imidazole) (3) and Ni(III)-methylimidazole [NiIII(MeIm)(P(o-C6H3-3-SiMe3-2-S)3)] (6) (MeIm = methylimidazole) catalyze oxidation of O2 to yield O2 along with [NiII(Im)(P(o-C6H3-3-SiMe3-2-S)3)]– (4) and [NiII(MeIm)(P(o-C6H3-3-SiMe3-2-S)3)] (7), respectively. Complex 4, composed of the electron-rich [NiII(P(o-C6H3-3-SiMe3-2-S)3)] motif and proton-provider imidazole-coordinate ligand, provide an optimum electronic condition to activate O2 producing imidazolate-bridged [{Ni(P(o-C6H3-3-SiMe3-2-S)3)}2(-Im-H)]– (Im-H = deprotonated imidazole) (5) and H2O, in contrast to the reversible interconversion between Ni(III)-methylimidazole-containing complex 6 and Ni(II)-methylimidazole- containing complex 7 under the presence of O2- and O2, respectively. That is, redox shuttling between the Ni(III) and Ni(II) states of [NiII/III(Im)(P(o-C6H3-3- SiMe3-2-S)3)]0/1 regulates superoxide-to-oxygen-to-H2O transformation, presumably, via the proposed transition state [{Ni(P(o-C6H3-3-SiMe3-2-S)3)}2(Im)2(2-O2)]. The results gained in this study may lend support to the suggestion that imidazole ligand could function as proton transfer to the nascent peroxide in the substrate reduction step of the NiSOD mechanism (Nired + O2- + 2H+ → Niox + H2O2).