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Copper−Dioxygen Adducts and the Side-on Peroxo Dicopper(II)/Bis(μ-oxo) Dicopper(III) Equilibrium:  Significant Ligand Electronic Effects

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posted on 03.04.2006, 00:00 authored by Lanying Q. Hatcher, Michael A. Vance, Amy A. Narducci Sarjeant, Edward I. Solomon, Kenneth D. Karlin
The variation of ligand para substituents on pyridyl donor groups of tridentate amine copper(I) complexes was carried out in order to probe electronic effects on the equilibrium between μ-η22-(side-on)-peroxo [CuII2(O22-)]2+ and bis(μ-oxo) [CuIII2(O2-)2] species formed upon reaction with O2. [CuI(R-PYAN)(MeCN)n]B(C6F5)4 (R-PYAN = N-[2-(4-R-pyridin-2-yl)-ethyl]-N,N,N-trimethyl-propane-1,3-diamine, R = NMe2, OMe, H, and Cl) (1R) vary over a narrow range in their CuII/CuI redox potentials (E1/2 vs Fe(cp)2+/0 = −0.40 V for 1NMe2, −0.38 V for 1OMe, −0.33 V for 1H, and −0.32 V for 1Cl) and in C−O stretching frequencies of their carbonyl adducts, 1RCO:  ν(C−O) = 2080, 2086, 2088, and 2090 cm-1 for R = NMe2, OMe, H, and Cl, respectively. However, within this range of electronic properties for 1R, dioxygen reactivity is significantly affected. The reaction of 1Cl or 1H with O2 at −78 °C in CH2Cl2 gives UV−vis and resonance Raman spectra indicative of a μ-η22-(side-on)-peroxo dicopper(II) adduct (2R). Compound 1OMe reacts with O2, yielding equilibrium mixtures of side-on peroxo (2OMe) and bis(μ-oxo) (3OMe) species. Oxygenation of 1NMe2 leads to the sole generation of the bis(μ-oxo) dicopper(III) complex (3NMe2). A solvent effect was also observed; in acetone or THF, increased ratios of bis(μ-oxo) relative to side-on peroxo complex are observed. Thus, the equilibrium between a dicopper side-on peroxo and bis(μ-oxo) species can be tuned by ligand designspecifically, more electron donating ligands favor the formation of the latter isomer, and the peroxo/bis(μ-oxo) equilibrium can be shifted from one extreme to the other within the same ligand system. Observations concerning the reactivity of the dioxygen adducts 2H and 3NMe2 toward external substrates are also presented.

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