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

, , , , und . Chemistry - A European Journal 20 (18): 5414--5422 (2014)


Ni(QM)2, QM = 4,6-di-tert-butyl-N-(2-methylthiomethylphenyl)-o-iminobenzoquinone, is a singlet diradical species with approx. planar configuration at the tetracoordinate metal atom and without any Ni-S bonding interaction. One-electron oxidn. results in addnl. 2-fold Ni-S coordination (dNi-S$\approx$2.38 \AA) to produce a complex cation of Ni(QM)2(PF6) with hexacoordinate NiII 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(QM)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(QM)2(ClO4)2 to exhibit a similar NiN2O2S2 framework as the monocation. However, the two tridentate (O,N,S) ligands are now equiv. according to NiII(QM0)22+. Cyclic voltammetry reflects the qual. structure change on the 1st, but not the 2nd oxidn. of Ni(QM)2, and spectroelectrochem. reveals a pronounced dependence of the 800-900 nm absorption on the solvent and counterion. Redn. of the neutral form occurs in an electrochem. reversible step to yield an anion with an intense near-IR absorption at 1345 nm (e = 10400 M-1 cm-1) and a conventional g factor splitting for a largely metal-based spin (S = 1/2 ), suggesting a (QM.-)NiII(QM2-)- configuration with a tetracoordinate metal atom with antiferromagnetic NiII-(QM.-) interactions and symmetry-allowed ligand-to-ligand intervalence charge transfer (LLIVCT). Calcns. were used to understand the Ni-S binding activity as induced by remote electron transfer at the iminobenzoquinone redox system. on SciFinder(R)

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