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Mononuclear [NiII(L)(P-(o-C6H4S)2(o-C6H4SH))]0/1- (L = Thiolate, Selenolate, PPh3, and Cl) Complexes with Intramolecular [Ni···S···H···S]/[Ni···H···S] Interactions Modulated by the Coordinated Ligand L:  Relevance to the [NiFe] Hydrogenases

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posted on 06.03.2006, 00:00 by Chien-Hong Chen, Gene-Hsiang Lee, Wen-Feng Liaw
The shift of the IR νS-H frequency to lower wavenumbers for the series of complexes [NiII(L)(P-(o-C6H4S)2(o-C6H4SH))]0/1- (L = PPh3 (1), Cl (6), Se-p-C6H4-Cl (5), S-C4H3S (7), SePh (4)) indicates that a trend of increasing electronic donation of the L ligands coordinated to the Ni(II) center promotes intramolecular [Ni−S···H−S] interactions. Compared to the Ni···S(H) distance, in the range of 3.609−3.802 Å in complexes 1 and 47, the Ni···S(CH3) distances of 2.540 and 2.914 Å observed in the [NiII(PPh3)(P(o-C6H4S)2(o-C6H4-SCH3))] complexes (8a and 8b, two conformational isomers with the chemical shift of the thioether methyl group at δ 1.820 (−60 °C) and 2.109 ppm (60 °C) (C4D8O)) and the Ni···S(CH3) distances of 3.258 and 3.229 Å found in the [NiII(L)(P(o-C6H4S)2(o-C6H4-SCH3))]1- complexes (L = SPh (9), SePh (10)) also support the idea that the pendant thiol protons of the Ni(II)−thiol complexes 1/47 were attracted by both the sulfur of thiolate and the nickel. The increased basicity (electronic density) of the nickel center regulated by the monodentate ligand attracted the proton of the pendant thiol effectively and caused the weaker S···H bond. In addition, the pendant thiol interaction modes in the solid state (complexes 1a and 1b, Scheme ) may be controlled by the solvent of crystallization. Compared to complex 1a, the stronger intramolecular [Ni−S···H−S] interaction (or a combination of [Ni−S···H−S]/[Ni···H−S] interactions) found in complexes 47 led to the weaker S−H bond strength and accelerated the oxidation (by O2) of complexes 47 to produce the [NiY(L)(P(o-C6H4S)3)]1- (L = Se-p-C6H4−Cl (11), SePh (12), S-C4H3S (13)) complexes.