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

• Published in: Journal of the American Chemical Society Vol. 125; no. 3; pp. 634 - 635
• Main Authors: Zhang, Christiana Xin, Liang, Hong-Chang, Kim, Eun-il, Shearer, Jason, Helton, Matthew E, Kim, Eunsuk, Kaderli, Susan, Incarvito, Christopher D, Zuberbühler, Andreas D, Rheingold, Arnold L, Karlin, Kenneth D
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 22.01.2003
• Subjects: Chemistry; Copper - chemistry; Crystallography, X-Ray; Exact sciences and technology; Inorganic chemistry and origins of life; Kinetics; Kinetics and mechanism of reactions; Molecular Structure; Organometallic Compounds - chemistry; Oxidation-Reduction; Oxygen - chemistry; Cationic complex; Organic solvent; Dinuclear complex; Organic ligand; Electronic effect; Transition metal Complexes; Experimental study; Chemical reactivity; Copper Complexes; Oxo complex; Tridentate ligand
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja028779v

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)]+ (1 R , 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 1 R (ranging from E 1/2 = −270 mV for 1 Cl to −440 mV for 1 MeN vs FeCp2/FeCp2 +) and νCO of the CO adducts of 1 R (ranging from 2093 cm-1 for 1 Cl -CO to 2075 cm-1 for 1 Me2N -CO) display modest but expected systematic shifts. Dioxygen readily reacts with 1 H , 1 Me O, and 1 Me2N , forming the side-on peroxo−CuII 2 complexes [{CuII(R-MePY2)}2(O2)]2+ (2 R , also containing some bis-μ-oxo−CuIII 2 isomer), but there is no reaction with 1 Cl . Stopped-flow studies in dichloromethane show that the formation of 2 Me2N from dioxygen and 1 Me2N 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 2 R 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−CuII 2 complexes [{CuII(R-MePY2)(OH)}2]2+ (3 R ). Product yields increase as the R-group is made more electron-donating, and in some cases are quantitative with 2 Me2N . 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 (k obs = 2(1) × 10-5 s-1 for 2 H to 3(1) × 10-2 s-1 for 2 Me2N ), while methanol oxidation to formaldehyde exhibits an ∼2000-fold increase (k obs = 5(1) × 10-5 s-1 for 2 H to 1(1) × 10-1 s-1 for 2 Me2N ).