Homogeneous oxidative coupling catalysts:: stoichiometry and characterization of the first stable oxotetranuclear solids [(PiP)nCuX]4O2 (n=1 or 2, Pip = piperidine, X = Cl, Br, I)

Copper(I) halides react quantitatively with piperidine (Pip) in dioxygen-free methylene chloride or nitrobenzene to form tetra-nuclear copper(I) complexes [(Pip)(n) CuX]4; n = 1 or 2, X = Cl, Br or I. These complexes are very soluble and completely reduce dioxygen to dioxo bridging ligand, with stoichiometry, Delta[Cu(I)]/Delta[O-2] = 4.0. The stable oxo solids [(Pip)(n)CuX](4)O-2 mimic tyrosinase copper protein. They act as a homogeneous oxidative coupling catalysts for phenols. Electronic transition spectra in the near infrared with high molecular absorptivity are diagnostic for tetranuclear "Cu4X4" core structure. The electronic transitions are more likely due to charge transfer between a minimum of three halo ligands and copper(II) center. The room temperature EPR spectra of [(Pip)(n)CuX](4)O-2 in methylene chloride are isotropic with four hyperfine lines. The room temperature solid-state EPR spectra of [PipCuX](4)O-2 show an axial spectra with d(x2-y2) ground state, suggesting square pyramidal arrangement of the five coordinated ligands around copper(II) centers. Cyclic voltammetry measurements show that they are more likely irreversible in character and show slight quasi-reversability when X = Br or I. Constant potential electrolysis indicate that the number of electrons consumed are equal to four electrons which will be due to the reduction of four copper(II) to copper(I). (C) 2003 Published by Elsevier B.V.