Five Coordinate M(II)-Diphenolate [M = Zn(II), Ni(II), and Cu(II)] Schiff Base Complexes Exhibiting Metal- and Ligand-Based Redox Chemistry

Five-coordinate ZnII, NiII, and
CuII complexes containing pentadentate N3O2 Schiff base ligands [1A]2– and [1B]2– have been synthesized
and characterized. X-ray crystallographic studies reveal five coordinate
structures in which each metal ion is bound by two imine N-donors,
two phenolate O-donors, and a single amine N-donor. Electron paramagnetic
resonance (EPR) spectroscopic studies suggest that the N3O2 coordination spheres of [Cu­(1A)] and [Cu­(1B)] are retained in CH2Cl2 solution
and solid-state superconducting quantum interference device (SQUID)
magnetometric studies confirm that [Ni­(1A)] and [Ni­(1B)] adopt high spin (S = 1) configurations.
Each complex exhibits two reversible oxidation processes between +0.05
and +0.64 V vs [Fc]+/[Fc]. The products of one- and two-electron
oxidations have been studied by UV/vis spectroelectrochemistry and
by EPR spectroscopy which confirm that each oxidation process for
the ZnII and CuII complexes is ligand-based
with sequential formation of mono- and bis-phenoxyl radical species.
In contrast, the one-electron oxidation of the NiII complexes
generates NiIII products. This assignment is supported
by spectroelectrochemical and EPR spectroscopic studies, density functional
theory (DFT) calculations, and the single crystal X-ray structure
of [Ni­(1A)]­[BF4] which contains Ni in a five-coordinate
distorted trigonal bipyramidal geometry.