Oxidation of an o-Iminobenzosemiquinone Radical Ligand by Molecular Bromine: Structural, Spectroscopic, and Reactivity Studies of a Copper(II) o-Iminobenzoquinone Complex

The bis(o-iminobenzosemiquinonato)copper(II) complex 1, containing the radical form [L• SQ]1− arising from the aerial oxidation of the noninnocent ligand 2-anilino-4,6-di-tert-butylphenol, H2L, is readily oxidized by molecular bromine to a bis(o-iminobenzoquinone)copper(II) complex, 2. Thus, a ligand-based oxidative addition is reported for complex 1 containing an electron-rich CuII d9 metal ion. The crystal structure of the synthesized hexacoordinated complex [CuII(LBQ)2Br2] (2) has been determined by X-ray crystallography at 100 K. Variable-temperature (2−290 K) magnetic susceptibility measurements and an X-band electron paramagnetic resonance spectrum establish the spin state to be S t = 1/2 because of localized spin moments mainly in the (d x 2 –y 2 )1 orbital of a CuII d9 ion, indicating clearly the presence of a neutral iminobenzoquinone form, [LBQ]0, of the ligand in 2, as is found also in the X-ray structure. Electrochemical measurements (cyclic voltammograms and coulometry) indicate two successive one-electron reductions of the ligand. The reactivity of complex 2 as an oxidizing agent toward ethanol and triethylamine has been investigated.