Oxidation of tetranuclear copper(I) complexes [(Pip)nCuX]4; n=1 or 2, Pip=Piperidine and X=Cl or Br, with two equivalents of tetrachloro-1,2-benzoquinone (TClBQ) and sequentional oxidation with one equivalent of TClBQ followed by dioxygen in aprotic media: Models for intermediates in catalytic catechol–copper(II) systems

Abstract

Copper(I) halides react quantitatively with piperidine in dioxygen-free CH2Cl2 or PhNO2 to form tetranuclear copper(I) complexes [(pip)nCuX]4; n=1 or 2 and X=Cl or Br. These complexes are very soluble in CH2Cl2 and PhNO2 and completely reduce tetrachloro1,2-benzoquinone (TClBQ) and O2 to the 3,4,5,6-tetrachlorocatecholato ligand (Cat) and the oxo form. The stable solid complexes [(pip)nCuX]4Cat2 and [(pip)nCuX]4CatO are closely related to the intermediate, bridging μ-catecholato complex, which is formed during the mono-oxygenation and oxidation of PhOH by oxytyrosinase. Cryoscopic and analytical data for these complexes indicate that [(pip)nCuX]4Cat2 and [(pip)nCuX]4CatO are discrete tetranuclear species. Electronic transition spectra in the near i.r. with high molecular absorptivity are diagnostic for a tetranuclear ‘Cu4X4’ core structure. The electronic transitions are more likely to be due to charge transfer between a minimum of three halo ligands to the copper(II) center. The room temperature e.p.r. spectra of [(pip)nCuX]4Cat2 and [(pip)nCuX]4CatO in CH2Cl2 are isotropic with four hyperfine lines. The room temperature solid state e.p.r. spectra of these complexes show an axial spectra with dx2−y2 ground state, suggesting a square pyramidal arrangement around copper(II) centers in all n = 1 complexes and an elongated tetragonal octahedral arrangement around copper(II) centers in all n = 2 complexes. Cyclic voltammetry measurements show that they are more likely to be irreversible and show slight quasi-reversibility when X=Br and n = 2. Constant potential electrolysis indicate that the number of electrons consumed are equal to four which will be due to the reduction of four copper(II) species to copper(I).