Mononuclear Nickel(III) Complexes [Ni^III(OR)(P(C₆H₃-3-SiMe₃-2-S)₃)]⁻ (R = Me, Ph) Containing the Terminal Alkoxide Ligand: Relevance to the Nickel Site of Oxidized-Form [NiFe] Hydrogenases

The unprecedented nickel(III) thiolate [NiIII(OR)(P(C6H3-3-SiMe3-2-S)3)]− [R = Ph (1), Me (3)] containing the terminal NiIII−OR bond, characterized by UV−vis, electron paramagnetic resonance, cyclic voltammetry, and single-crystal X-ray diffraction, were isolated from the reaction of [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]− with 3 equiv of [Na][OPh] in tetrahydrofuran (THF)−CH3CN and the reaction of complex 1 with 1 equiv of [Bu4N][OMe] in THF−CH3OH, respectively. Interestingly, the addition of complex 1 into the THF−CH3OH solution of [Me4N][OH] also yielded complex 3. In contrast to the inertness of complex [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]− toward 1 equiv of [Na][OPh], the addition of 1 equiv of [Na][OMe] into a THF−CH3CN solution of [NiIII(Cl)(P(C6H3-3-SiMe3-2-S)3)]− yielded the known [NiIII(CH2CN)(P(C6H3-3-SiMe3-2-S)3)]− (4). At 77 K, complexes 1 and 3 exhibit a rhombic signal with g values of 2.31, 2.09, and 2.00 and of 2.28, 2.04, and 2.00, respectively, the characteristic g values of the known trigonal-bipyramidal NiIII [NiIII(L)(P(C6H3-3-SiMe3-2-S)3)]− (L = SePh, SEt, Cl) complexes. Compared to complexes [NiIII(EPh)(P(C6H3-3-SiMe3-2-S)3)]− [E = S (2), Se] dominated by one intense absorption band at 592 and 590 nm, respectively, the electronic spectrum of complex 1 coordinated by the less electron-donating phenoxide ligand displays a red shift to 603 nm. In a comparison of the NiIII−OMe bond length of 1.885(2) Å found in complex 3, the longer NiIII−OPh bond distance of 1.910(3) Å found in complex 1 may be attributed to the absence of σ and π donation from the [OPh]-coordinated ligand to the NiIII center.