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Tuning Copper−Dioxygen Reactivity and Exogenous Substrate Oxidations via Alterations in Ligand Electronics

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journal contribution
posted on 21.12.2002, 00:00 by Christiana Xin Zhang, Hong-Chang Liang, Eun-il Kim, Jason Shearer, Matthew E. Helton, Eunsuk Kim, Susan Kaderli, Christopher D. Incarvito, Andreas D. Zuberbühler, Arnold L. Rheingold, Kenneth D. Karlin
Copper(I)−dioxygen adducts are important in biological and industrial processes. For the first time we explore the relationship between ligand electronics, CuI−O2 adduct formation and exogenous substrate reactivity. The copper(I) complexes [CuI(R-MePY2)]+ (1R, where R = Cl, H, MeO, Me2N) were prepared; where R-MePY2 are 4-pyridyl substituted bis[2-(2-pyridyl)ethyl]methylamine chelates. Both the redox potential of 1R (ranging from E1/2 = −270 mV for 1Cl to −440 mV for 1MeN vs FeCp2/FeCp2+) and νCO of the CO adducts of 1R (ranging from 2093 cm-1 for 1Cl-CO to 2075 cm-1 for 1Me2N-CO) display modest but expected systematic shifts. Dioxygen readily reacts with 1H, 1MeO, and 1Me2N, forming the side-on peroxo−CuII2 complexes [{CuII(R-MePY2)}2(O2)]2+ (2R, also containing some bis-μ-oxo−CuIII2 isomer), but there is no reaction with 1Cl. Stopped-flow studies in dichloromethane show that the formation of 2Me2N from dioxygen and 1Me2N proceeds with a k = 8.2(6) × 104 M-2 s-1 (183 K, ΔH = −20.3(6) kJ mol-1, ΔS = −219(3) J mol-1 K-1). Solutions of 2R readily oxidize exogenous substrates (9,10-dihydroanthracene → anthracene, tetrahydrofuran (THF) → 2-hydroxytetrahydrofuran (THF−OH), N,N-dimethylaniline → N-methylaniline and formaldehyde, benzyl alcohol → benzaldehyde, benzhydrol → benzophenone, and methanol → formaldehyde), forming the bis-μ-hydroxo−CuII2 complexes [{CuII(R-MePY2)(OH)}2]2+ (3R). Product yields increase as the R-group is made more electron-donating, and in some cases are quantitative with 2Me2N. Pseudo-first-order rate constants for THF and methanol oxidation reactions demonstrate a remarkable R-group dependence, again favoring the strongest ligand donor (i.e., R = Me2N). For THF oxidation to THF−OH a nearly 1500-fold increase in reaction rate is observed (kobs = 2(1) × 10-5 s-1 for 2H to 3(1) × 10-2 s-1 for 2Me2N), while methanol oxidation to formaldehyde exhibits an ∼2000-fold increase (kobs = 5(1) × 10-5 s-1 for 2H to 1(1) × 10-1 s-1 for 2Me2N).