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

Mononuclear, distorted square planar [NiII(ER)(P(o-C6H4S)2(o-C6H4SH))]- (ER = SePh (1), 2-S−C4H3S (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−C4H3S)3]- and P(o-C6H4SH)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 νSH stretching band (2273 and 2283 cm-1 (KBr) for complexes 1 and 2, individually) and 1H NMR
spectra (δ 8.079 (d) (CD2Cl2) and 8.39 (d) (C4D8O) ppm (−SH) for complexes 1 and 2, respectively) and
subsequently confirmed by X-ray diffraction study. The exo-thiol proton (o-C6H4SH) in complexes 1 and 2
was identified as a D2O exchangeable proton from NMR and IR studies and was quantitatively removed
by Lewis base Et3N to yield Ni(II) dimer [NiII(P(o-C6H4S)3)]22- (5). Instead of the ligand-based oxidation to
form dinuclear Ni(II) complexes and dichalcogenide, oxidation of THF−CH3CN solution of complexes 1
and 2 by O2 resulted in the formation of the mononuclear, distorted trigonal bipyramidal [NiIII(ER)(P(o-C6H4S)3)]- (ER = SePh (3), 2-S−C4H3S (4)) accompanied by byproduct H2O 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(III) with the odd electron in the dz2 orbital. Complex
3 undergoes a reversible NiIII/II process at E1/2 = −0.67 V vs Ag/AgCl in MeCN.