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

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 X4BQ (X = Cl, Br) in aprotic solvents to give tetranuclear products ]NCuX[4Y2. When X = Cl or Br, Y is the corresponding 3, 4, 5, 6-tetrahalocatecholate, which bridges pairs of copper(II) centers in [NCuX]4Y2. When X = I, Y appears to be the corresponding semiquinone radical in mixed valence products ]NCuI[4Y2. Products [NCuX]4Y2 differ from those with Y = O or CO3 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 CO3 at room temperature) lead to stronger electronic coupling of the copper(II) centers in [NCuX]4Y2. Products [NCuX]4Y2 also differ from those with Y = O or CO3 in being electrochemically active at a platinum electrode in aprotic solvents. The formal potential for [NCuX]4Y2 reduction decreases sharply on changing X from Cl or Br to I, which also suggests semiquinone-copper(I) character for [NCuI]4Y2 complexes. Differences between the electronic spectra of [NCuX]4Y2 with X = Cl or Br and X = I are also consistent with a mixed valence formulation for [NCuI]4Y2 products.