Biomimetic Model Studies on Ni/Fe-Containing Metalloenzymes: [NiFe] Hydrogenase and Fe-Containing Nitrile Hydratase

• Main Authors: LEE CHIEN-MING, 李建明
• Other Authors: LIAW WEN-FENG
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/61796168678264927167

博士 === 國立清華大學 === 化學系 === 93 === Mononuclear, distorted square planar [NiII(SePh)(P(o-C6H3RS)2(o-C6H3RSH))]- (R = H (I-1), SiMe3 (II-la or II-lb)) with a S－H proton interacting with both nickel and sulfur atoms (Type I) or an intramolecular [Ni…H－S] interaction (Type II) were prepared by reaction of [Ni(CO)(SePh)3]- and P(o-C6H4SH)3/ P(C6H3-3-SiMe3-2-SH)3, respectively. The presence of intramolecular [Ni…H－S]/[Ni－S…H－S] interac- tions was verified in the solid state by the observation of the IR vS－H band (Table 4-1) and subsequently confirmed by X-ray diffraction study. The exo-thiol proton in I-1, II-la and II-lb was identified as a D2O exchangeable proton from NMR and IR stu- dies. Instead of the ligand-base oxidation to form dinuclear Ni(II) complexes and diphenyl diselenide, oxidation of THF-CH3CN solution of I-1, II-la and II-lb by O2 resulted in the formation of the mononuclear, distorted trigonal bipyramidal [NiIII(Se- Ph)(P(o-C6H3RS)3)]- (R = H (I-2), SiMe3 (II-2)) accompanied by byproduct H2O identified by 1H NMR, individually. Importantly, the reversible transformation from [Ni(III)－S] (II-2) to [Ni(II)…H－S] (II-1a or II-1b) was observed by reaction of II-2 and LiAlH4, followed by the addition of H2O (H+ source). More sterically hindered and stable complex II-2 reacts with exogenous CO to produce [NiII(CO) (P(C6H3-3-SiMe3-2-S)3)]- (II-3). Complex II-2 undergoes dechlorination in CH2Cl2 solution to generate [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]- (II-4) which acts as a precur- sor for synthesis of [NiIII(SEt)(P(C6H3-3-SiMe3-2-S)3)]- (II-5) via ligand metathesis. The EPR spectra of I-2, II-2, II-4 and II-5 exhibiting high rhombicities with three principal g values of 2.30, 2.09 and 2.0 are consonant with Ni(III) with the odd elec- tron in the dz2 orbital. The cyclic voltammetric response of I-2, II-2 and II-5 reveals a reversible NiIII/II process (Table 4-1). When II-1 is dissolved in CH2Cl2 at room temperature, a slow reaction ensues to give the mononuclear [Ni(Cl)(P(C6H3-3-Si- Me3-2-S)2(C6H3-3-SiMe3-2-SH))]- (II-6) after separation of dinuclear neutral [Ni(P- (C6H3-3-SiMe3-2-S)(C6H3-3-SiMe3-2-SH) (C6H3-3-SiMe3-2-��-S))]2 (II-7) by diethyl ether. Two types of crystalline products, orange-red plate (II-6a) and red-brown block (II-6b), were isolated upon precipitation of II-6 with different solvent pairs. Com- pared to the vS－H stretching frequencies among complexes II-1a, II-1b, II-6a, II-6b and II-7, the vS－H stretching frequencies are disturbed by changing terminal donor ligand (Table 4-1). Upon injecting O2 into a THF solution of II-7, the neutral dinu- clear [NiIII(P(C6H3-3-SiMe3-2-S)2 (C6H3-3-SiMe3-2-��-S))]2 (II-8) is isolated and iden- tified by single crystal X-ray diffraction studies. Electrochemical studies reveal that II-8 undergoes one-electron reduction to a [Ni(III)Ni(II)] species that is further re- duced to a [Ni(II)Ni(II)] species. Some of those results are relative to the structure, reactivity, and spectroscopic properties of the nickel site of bimetallic Ni－Fe active site of [NiFe]H2ase. The five-coordinated iron-thiolate nitrosyl complexes [(NO)Fe(S,S-C6H4)2]- (20), and [(NO)Fe(S,S-C6H4)2]2- (III-1) have been isolated and structurally characterized. Iron-thiolate nitrosyl complex 20 containing {Fe(NO)}6 core triggers sulfur oxyge- nation by O2 to yield the S-bonded monosulfinate iron species [(NO)Fe(S,SO2- C6H4)(S,S-C6H4)]- (21). In contrast, there is to a certain extent an attack of O2 on the •NO radical of complex III-1 containing {Fe(NO)}7 core leading to the formation of complex 20 accompanied by the minor products, [Fe(S,S-C6H4)2]22- and [NO3]- (9％). Treatment of 1 equiv of [EtS]- and complex 21 in CH3CN-THF yields [(NO)Fe- (S,SO2-C6H4)(S,S-C6H4)]2- (III-2) along with (EtS)2 identified by 1H NMR. Com- pared to III-1, complex III-2 with the less electron-donating sulfinate ligand coordinated to {Fe(NO)}7 core were oxidized by O2 to yield 21. Obviously, the elec- tronic perturbation of the {Fe(NO)}7 core caused by the coordinated sulfinate in III-2 may serve to regulate the reactivity of III-2 toward O2. The iron-sulfinate nitrosyl species with {Fe(NO)}6/7 core exhibit the photolabilization of sulfur-bound [O] moiety under irradiation. The interconversion between complexes 20, 21, III-1 and III-2 (Scheme 4-3) may provide some clues to the transformation pathways between the active and inactive NO-bound forms of Fe-containing nitrile hydratase (Scheme 4-4).