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Mononuclear Ni(III) Complexes [NiIII(L)(P(C6H3-3-SiMe3-2-S)3)]0/1- (L = Thiolate, Selenolate, CH2CN, Cl, PPh3):  Relevance to the Nickel Site of [NiFe] Hydrogenases

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posted on 25.12.2006, 00:00 by Chien-Ming Lee, Ya-Lan Chuang, Chao-Yi Chiang, Gene-Hsiang Lee, Wen-Feng Liaw
The stable mononuclear Ni(III)−thiolate complexes [NiIII(L)(P(C6H3-3-SiMe3-2-S)3)]- (L = SePh (2), Cl (3), SEt (4), 2-S-C4H3S (5), CH2CN (7)) were isolated and characterized by UV−vis, EPR, IR, SQUID, CV, 1H NMR, and single-crystal X-ray diffraction. The increased basicity (electronic density) of the nickel center of complexes [NiIII(L)(P(C6H3-3-SiMe3-2-S)3)]- modulated by the monodentate ligand L and the substituted groups of the phenylthiolate rings promotes the stability and reactivity. In contrast to the irreversible reduction at −1.17 V (vs Cp2Fe/Cp2Fe+) for complex 3, the cyclic voltammograms of complexes [NiIII(SePh)(P(o-C6H4S)3)]-, 2, 4, and 7 display reversible NiIII/II redox processes with E1/2 = −1.20, −1.26, −1.32, and −1.34 V (vs Cp2Fe/Cp2Fe+), respectively. Compared to complex 2 containing a phenylselenolate-coordinated ligand, complex 4 with a stronger electron-donating ethylthiolate coordinated to the Ni(III) promotes dechlorination of CH2Cl2 to yield complex 3 (kobs = (6.01 ± 0.03) × 10-4 s-1 for conversion of complex 4 into 3 vs kobs = (4.78 ± 0.02) × 10-5 s-1 for conversion of complex 2 into 3). Interestingly, addition of CH3CN into complex 3 in the presence of sodium hydride yielded the stable Ni(III)−cyanomethanide complex 7 with a NiIII−CH2CN bond distance of 2.037(3) Å. The NiIII−SEt bond length of 2.273(1) Å in complex 4 is at the upper end of the 2.12−2.28 Å range for the NiIII−S bond lengths of the oxidized-form [NiFe] hydrogenases. In contrast to the inertness of complexes 3 and 7 under CO atmosphere, carbon monoxide triggers the reductive elimination of the monodentate chalcogenolate ligand of complexes 2, 4, and 5 to produce the trigonal bipyramidal complex [NiII(CO)(P(C6H3-3-SiMe3-2-S)3)]- (6).

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