Experimental studies of the interconversion of mu-eta(2):eta(2)-peroxo- and bis(mu-oxo)dicopper complexes

The effects of solvent composition, counterion, and temperature on the equi librium between the isomeric complexes [((LCu)-Cu-iPr3)(2)(mu-eta(2):eta(2) -O-2)]X-2 [1(X)(2)] and [((LCu)-Cu-iPr3)(2)(mu-O)(2)]X-2 [2(X)(2); L-iPr3 = 1,4,7-triisopropyl-1,4,7-triazacyclononane; X = PF6-, ClO4-, or SbF6-], wh ich differ with respect to the presence and absence, respectively, of an O- O bond, were examined via solution conductivity and W-vis and resonance Ram an spectroscopic measurements. While previous work (Halfen, J. A.; Mahapatr a, S.; Wilkinson, E. C.; Kaderli, S.; Young, V. G., Jr.; Que, L., Jr.; Zube rbuhler, A. D.; Tolman, W. B. Science 1996, 271, 1397-1400) had shown that 2(X)(2) (X = PF6- or ClO4-) formed exclusively in THF solution at concentra tions >1 mM, new experimental results show that preferential aggregation/pr ecipitation of 2(X)(2) occurs under these conditions. By using lower concen trations and/or X = SbF6-, apparently homogeneous solutions of mixtures of 1(X)(2) and 2(X)(2) in equilibrium in THF and THF/CH2Cl2 mixtures are gener ated. Evaluation of the temperature dependence of the equilibrium in THF al lowed estimation of thermodynamic parameters [Delta H = 0.9(2) kcal mol(-1) and Delta S = 6(1) cal mol(-1) K-1] that are consistent with the small ene rgetic differences between analogous isomers previously calculated using th eoretical methods. Tn addition, by avoiding the selective aggregation/preci pitation of 2(X)(2), the 1(X)(2)/2(X)(2) equilibrium was shown to exhibit a smooth dependence on the THF:CH2Cl2 ratio in solvent mixtures. The observe d gradual shifts in the proportion of the equilibrating isomers as a functi on of solvent and temperature show that subtle environmental factors influe nce the equilibrium in similarly subtle ways, with implications for the pot ential involvement of both types of cores in biological and other catalytic processes.