Tuning copper-dioxygen reactivity and exogenous substrate oxidations via alterations in ligand electronics

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 E 1/2 = -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, △H- -20.3(6) KJ mol-1,△S=-219(3) J mol -1 K-1 Solutions of 2 R readily oxidize exogenous substrates (9,10- dihydroanthracene → N- methlaniline and formaldehyde, benzyl alcohol→ benzaldehyde, benzhydrol→ benzophenone, and methanol→ formaldehde), forming the bis -μ-hydroxo-Cu II2 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 the strongest ligand donor (i.e., R=Me 2N). 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)× S-1 for 2Me2N), while methanol oxidation to formaldehyde exhibits an 2000- fold increase ( K obs= 5(1)×10-5 S-1 for 2H to 1(1)×10-1 S-1 for 2Me2N). Copyright © 2003 American Chemical Society.