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

摘要

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 Ni-S bonding interaction. One-electron oxidation results in additional twofold Ni-S coordination (d_(Ni-s) ≈ 2.38 ?) to produce a complex cation of [Ni(Q_M)_2](PF_6) 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](PF_6) 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](CIO_4)_2 to exhibit a similar NiN_2O_2S_2 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 spectroelectro-chemistry reveals a pronounced dependence of the 800-900 nm 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 1345 nm (?= 10400 m~(-1) cm~(-1)) and a conventional g factor splitting for a largely metal-based spin (5= 1/2), suggesting a [(Q_M~(·-))Ni~(II)(Q_M~(2-))]~- configuration with a tetracoordinate metal atom with antiferro magnetic Ni~(II)-(Q_M~(·-)) interactions and symmetry-allowed ligand-to-ligand intervalence charge transfer (LLIVCT). Calculations are used to understand the Ni-S binding activity as induced by remote electron transfer at the iminobenzoquinone redox system.