Charge distribution in Schiff-base biquinone complexes. Electrochemical, spectroelectrochemical and electron spin resonance studies of complexes of CoIII, NiII, ZnII, CuII, and CdII with the N-(2′-hydroxy-3′,5′-di-t-butylphenyl)-4,6-di-t-butyl-o-benzoquinone imine ligand system in CH2Cl2

Abstract

Five different Schiff-base biquinone complexes were examined by electrochemical, spectroelectrochemical, and e.s.r. methods: [M^II(hbqi)₂] and [Co^III(hbqi)(hbsqi)][M =Ni, Zn, Cu, or Cd; hbqi is the anion of N-(2′-hydroxy-3′,5′-t-di-butylphenyl)-4,6-di-t-butyl-o-benzoquinone imine and hbsqi is singly reduced hbqi, N-(2′-hydroxy-3′,5′-di-t-butylphenyl)-4,6-di-t-butyl-o-benzosemiquinone iminate(2–)]. Each compound was characterized by e.s.r. and u.v.–visible spectroscopy as well as by electrochemistry and spectroelectrochemistry in CH₂Cl₂ containing 0.1 mol dm^–3 NBuⁿ₄ClO₄. The first one-electron oxidation and the first one-electron reduction of the complexes both occur at ligand-based orbitals. The voltammetric and e.s.r. data are self consistent and indicate that an oxidized hbqi ligand dissociates from [M^II(hbqi)₂] after a one-electron abstraction. Singly oxidized [Co^III(hbqi)(hbsqi)] does not undergo a loss of ligand and [Co^III(obsq)(hbsqi)]⁺ is electrogenerated where obsq is singly oxidized hbqi. This complex does not exhibit coupling between the two unpaired electrons. The one-electron reduction of [M^II(hbqi)₂](M = Ni, Zn or Cd) produces a stable [M^II(hbqi)(hbsqi)]^– species in which the unpaired electron resides on the hbsqi ligand. The stable reduction product of [Co^III(hbqi)(hbsqi)] is identified as [Co^III(hbsqi)₂]^–, a species in which the two unpaired electrons independently interact with the cobalt nucleus and two hydrogens. Finally, e.s.r. spectra suggest that a structural change in [Cu^II(hbqi)₂] occurs upon going from the solid state to methylene chloride solutions at low temperature.