Correlated Coordination and Redox Activity of a Hemilabile Noninnocent Ligand in Nickel Complexes

The compound [Ni(Q(M))(2)], Q(M)=4,6-di-tert-butyl-N-(2-methylthiomethylphenyl)-o-iminobenzoquinone, is a singlet diradical species with approximately planar configuration at the tetracoordinate metal atom and without any NiS bonding interaction. One-electron oxidation results in additional twofold NiS coordination (d(NiS)approximate to 2.38 angstrom) to produce a complex cation of [Ni(Q(M))(2)](PF6) with hexacoordinate Ni-II and two distinctly different mer-configurated tridentate ligands. The O,O-trans arrangement in the neutral precursor is changed to an O,O-cis configuration in the cation. The EPR signal of [Ni(Q(M))(2)](PF6) has a very large g anisotropy and the magnetic measurements indicate an S=3/2 state. The dication was structurally characterized as [Ni(Q(M))(2)](ClO4)(2) to exhibit a similar NiN2O2S2 framework as the monocation. However, the two tridentate (O,N,S) ligands are now equivalent according to the formulation [Ni-II(Q(M)(0))(2)](2+). Cyclic voltammetry reflects the qualitative structure change on the first, but not on the second oxidation of [Ni(Q(M))(2)], and spectroelectrochemistry reveals a pronounced dependence of the 800-900nm absorption on the solvent and counterion. Reduction of the neutral form occurs in an electrochemically reversible step to yield an anion with an intense near-infrared absorption at 1345nm (epsilon=10400M(-1)cm(-1)) and a conventional g factor splitting for a largely metal-based spin (S=1/2), suggesting a [(Q(M)(.-))Ni-II(Q(M)(2-))](-) configuration with a tetracoordinate metal atom with antiferromagnetic Ni-II-(Q(M)(.-)) interactions and symmetry-allowed ligand-to-ligand intervalence charge transfer (LLIVCT). Calculations are used to understand the NiS binding activity as induced by remote electron transfer at the iminobenzoquinone redox system.