Oxidation of tetranuclear copper(I) complexes [NCuX] 4 (N= N, N-diethylnicotinamide; X=Cl, Br or I) with tetrahalo-1,2-benzoquinones in aprotic media: models for intermediates in catalytic catechol-copper(II) systems

Abstract

Tetranuclear copper(I) complexes [NCuX] 4 (N= N, N-diethylnicotinamide; X=Cl, Br or I) react stoichiometrically with two mol of 3,4,5,6-tetrahalo-1,2-benzoquinones X 4BQ (X=Cl, Br) in aprotic solvents to give tetranuclear products [NCuX] 4Y 2. When X=Cl or Br, Y is the corresponding 3,4,5,6-tetra- halocatecholate, which bridges pairs of copper(II) centers in [NCuX] 4Y 2. When X=I, Y appears to be the corresponding semiquinone radical in mixed valence products [NCuI] 4Y 2. Products [NCuX] 4Y 2 differ from those with Y=O or CO 3 in being EPR active in methylene chloride solution at room temperature. Isotropic EPR spectra indicate a very flexible coordination environment for copper(II) and exhibit a sharp signal for a coordinated semiquinone radical when X=I. Lower signal intensity with decreasing temperature suggests that more rigid bridging structures (as exist with Y=O or CO 3 at room temperature) lead to stronger electronic coupling of the copper(II) centers in [NCuX] 4Y 2. Products [NCuX] 4Y 2 also differ from those with Y=O or CO 3 in being electrochemically active at a platinum electrode in aprotic solvents. The formal potential for [NCuX] 4Y 2 reduction decreases sharply on changing X from Cl or Br to I, which also suggests semiquinone-copper(I) character for [NCuI] 4Y 2 complexes. Differences between the electronic spectra of [NCuX] 4Y 2 with X=Cl or Br and X=I are also consistent with a mixed valence formulation for [NCuI] 4Y 2 products.