Mononuclear Nickel(Ⅲ) and Nickel(Ⅱ) Thiolate Complexes with Intramolecular S-H Proton Interacting with Both Sulfur and Nickel: Relevance to the [NiFe]/[NiFeSe] Hydrogenases

摘要

Mononuclear, distorted square planar [Ni~(Ⅱ)(ER)(P(o-C_6H_4S)_2(o-C_5H_4SH))]~- (ER = SePh (1), 2-S-C_4H_3S (2)) with a S-H proton directly interacting with both nickel and sulfur atoms were prepared by reaction of [Ni(CO)(SePh)_3]~-/[Ni(CO)(2-S-C_4H_3S)_3]~- and P(o-C_6H_4SH)_3, individually. The presence of combinations of intramolecular [Ni-S...H-SR]/[Ni...H-SR] interactions was verified in the solid state by the observation of an IR ν_(Sh) stretching band (2273 and 2283 cm~(-1) (KBr) for complexes 1 and 2, individually) and ~1H NMR spectra (δ 8.079 (d) (CD_2CI_2) and 8.39 (d) (C_4D_8O) ppm (-SH) for complexes 1 and 2, respectively) and subsequently confirmed by X-ray diffraction study. The exo-thiol proton (o-C_6H_4SH) in complexes 1 and 2 was identified as a D_2O exchangeable proton from NMR and IR studies and was quantitatively removed by Lewis base Et_3N to yield Ni(Ⅱ) dimer [Ni~(Ⅱ)(P(o-C_6H_4S)_3)]_2~(2-) (5). Instead of the ligand-based oxidation to form dinuclear Ni(Ⅱ) complexes and dichalcogenide, oxidation of THF-CH_3CN solution of complexes 1 and 2 by O_2 resulted in the formation of the mononuclear, distorted trigonal bipyramidal [Ni~(Ⅲ)(ER)(P(o-C_6H_4S)_3)]~- (ER = SePh (3), 2-S-C_4H_3S (4)) accompanied by byproduct H_2O identified by ~1H NMR, respectively. The 4.2 K EPR spectra of complexes 3 and 4 exhibiting high rhombicities with three principal g values of 2.304, 2.091, and 2.0 are consonant with Ni(?) with the odd electron in the d_(Z~2) orbital. Complex 3 undergoes a reversible Ni~(Ⅲ/Ⅱ) process at E_(1/2) = -0.67 V vs Ag/AgCl in MeCN.