SYNTHESIS AND PROPERTIES OF MIXED-VALENCE COPPER MOLECULES (MU-Y)N4CU(II)2CU(I)2X4 (N = N,N-DIETHYLNICOTINAMIDE, Y = 3,4,5,6-TETRACHLOROCATECHOLATE, X = CL, BR, I) AND THE PRODUCTS AND KINETICS OF THEIR REACTIONS WITH DIOXYGEN IN NITROBENZENE

Tetranuclear copper(l) complexes [NCuX]4 (N = N.N-diethylnicotinamide; X = Cl, Br, 1 (Ia-c, respectively)) react stoichiometrically with equimolar 3,4,5,6-tctrachloro-l.2-benzoquinone (C14BQ) under dinitrogen at room temperature in methylene chloridc or nitrobenzene to give tetranuclear mixed-valence copper products (μ-Y)N4Cu2Cu,2X4 (Va-c, respectively), where Y is 3.4.5.6-tetrachlorocatecholate. The electronic spectra of Va-c contain maxima in the region 750‧820 nm that are due to their coppcr(ll) ccntcrs. The maximum molar absorptivities decrease as X is changed from Cl to Br to I and the corresponding wavelengths decrease when X is changed from Br to Cl to I. Molecules Va-c are oxidized by dioxygen in ambient nitrobenzene with stoichiomelry Δ[VI]/[02] = 2.0 ± 0.1 to give tetranuclear mono(oxo)mono(catecholato)copper(II) complexes (μ-O.μ-Y)[NCuX]4 (Vla-c, respectively). Products VI have larger maximum molar absorptivities than the respective products V in the 600‧900-nm region. Absorptivity of VI is larger with X = Br than with X = Cl and smaller with X = I. The spectra of (μ-Y)2[NCuX]4 (Y = O (II), C03. or 3,4,5,6-tetrachlorocatecholate (IV)) with fixed X = Cl or Br are very similar. Rhombic EPR spectra with four hypcrfinc lines for solid mixed-valence complexes Va-c at 300 K indicate trigonal-pyramidal geometry for their copper(II) centers with no evidence for significant copper(l)-copper(II) interactions. The EPR spectra of Vla-c in methylene chloride are isotropic at 300 K and rhombic with more than four hypcrfine lines at 130 K, indicating more than one kind of copper(II) site. Comparison of the solid-state EPR spectra of (μ-0)2[NCuCl]4 (IIa), IVa, Va, and VIa at 300K indicates that (a) IIu contains square-pyramidal copper(II), (b) solid IVa has different copper(II) centers despite having only one kind of bridge, and (c) the distinct copper(II) centers of Via can be observed in the solid state. Cyclic voltammograms of Va and Vla-c at a Pt electrode in ambient methylene chloride arc quasi-reversible (Ef = 0.51, 0.52, 0.61, and 0.59 V vs SCE, respectively).Complex Va is oxidized by 02 to Via with a third-order rate law, as found for the oxidation of copper(I) dimers [LCuX]2, where L is an N,N,‧.N.‧-tetraalkyl diamine. Comparison of the kinetic data indicates assembly of an activated complex containing one 02 and two Va molecules as the rate-determining step. By contrast, the corresponding oxidations of Vb and Vc are second-order processes, like those of complexes [NCuX]4, N3Cu3M(NS)X4, and N3Cu3M(NS)2X4 (NS = monoanionic 5-methyl isopropylidenehydrazinccarbodithioate). The slowest steps appear to involve electron transfer from Vb and Vc to 02. Respective intermediates IXb and IXc are rapidly reduced by excess Vb or Vc to the corresponding oxocopper(ll) products VIb and Vi. © 1990, American Chemical Society. All rights reserved.