UNUSUALLY STABLE PEROXOCOPPER COMPLEXES - STOICHIOMETRY, PRODUCTS AND KINETICS OF OXIDATION OF THE DIMERIC COPPER(I) COMPLEX [LCUBR]2 (L=N,N'-DIETHYLETHYLENEDIAMINE) BY DIOXYGEN IN METHYLENE-CHLORIDE FROM -51-DEGREES-C TO 30-DEGREES-C

Copper(I) bromide dissolves in deoxygenated methylene chloride and nitrobenzene solutions of equimolar N,N'-diethylethylenediamine (DEED) to give the colorless copper(I) dimer [(DEED)CuBr]2 (D). Dioxygen uptake, analytical, cryoscopic, spectral and kinetic data show that D is oxidized to the blue tetranuclear mixed valence peroxocomplex [(DEED)CuBr]4O2 (A) at temperatures from -51 to 30-degrees-C. The rate law is d[A]/dt=k(DL)[O2][D]2 with activation parameters DELTAH(DL)not-equal = -1.8 +/- 0.4 kcal mol-1 and DELTAS(DL)not-equal = -38 +/- 5 cal deg-1 mol-1 at 25-degrees-C. These parameters resemble those for third-order oxidation of the copper(I) dimer [(TEED)CuCl]2 (TEED is N,N,N',N'-tetraethylethylenediamine) to the peroxocopper product [(TEED)CuCl]4O2 at low temperatures. They are completely different from those for direct, third-order oxidation of [LCuX]2 dimers to the oxocopper(II) products [LCuX]2O at ambient temperatures because of slow transfer of the third electron from copper(I) to O2 in ambient [(DEED)CuBr]2/O2 and low temperature [(TEED)CuCl]2/O2 reactions. As observed in the [(TEED)CuCl]2/O2 system, primary product A relaxes to a different tetranuclear copper complex B, with A thermodynamically favored at higher temperatures up to -17-degrees-C. First-order decomposition of A to give 2 mol of the oxocopper(II) product [(DEED)CuBr]2O (C) has an exceptionally long half-life of 3.2 +/- 0.1 h at 25-degrees-C. The resistance of A to intramolecular copper(I) --> peroxide electron transfer with L = DEED and X = Br is attributed to (i) hydrogen bonding between the N-H groups of DEED and bound peroxide and (ii) stabilization of copper(I) by Br.