Copper(l)-Dioxygen Reactivity of [(L)Cu1]^+ (L = Tris(2-pyridylmethyl)amine): Kinetic/Thermodynamic and Spectroscopic Studies Concerning the Formation of Cu-O2 and CU2-O2 Adducts as a Function of Solvent Medium and 4-Pyridyl Ligand Substituent Variations

The kinetic and thermodynamic behavior of O2-binding to Cu(l) complexes can provide fundamental understanding of copper(l)/dioxygen chemistry, which is of interest in chemical and biological systems. Here we report stoppedflow kinetic investigations of the oxygenation reactions of a series of tetradentate copper(l) complexes [(L^Cuʹ" (MeCN)+ (1R, R=H, Me, tBu, MeO, Me2N) in propionitrile (EtCN), tetrahydrofuran (THF), and acetone. The syntheses of 4-pyridyl substituted tris(2-pyridylmethyl)amine ligands (LR) and copper(l) complexes are detailed. Variations of ligand electronic properties are manifested in the electrochemistry of 1R and V(CO) of [(LR)Cu-CO]^ complexes. The kinetic studies in EtCN and THF show that the O2-reactions of 1R follow the reaction mechanism established for oxygenation of 1H in EtCN (J. Am. Chem. Soc. 1993, 115, 9506), involving reversible formation (K1/K-1) of [(LR)Cu-)+ (2R) which further reacts (K2/K-2) with 1R to form the 2:1 Cu2O2 complex [{(LR)CU11}2(O^2-)]2+ (3R). In EtCN, the rate constants for formation of 2R (K1) are not dramatically affected by the ligand electronic variations. For R = Me and tBu, the kinetic and thermodynamic parameters are very similar to those of the parent complex (1H); e.g., K1 is in the range 1.2x10^4 to 3.1 x 10^4 M^-1 s^-1 at 183 K. With the stronger donors R = MeO and Me2N, more significant effects were observed, with the expected increase in thermodynamic stability of resultant 2R and 3R complexes, and decreased dissociation rates. The modest ligand electronic effects manifested in EtCN are due to the competitive binding of solvent and dioxygen to the copper centers. In THF, a weakly coordinating solvent, the formation rate for 2H is much faster (>100 times) than that in EtCN, and the thermodynamic stabilities of both the 1:1 (K1) and 2:1 (beta = K1K2) copper-dioxygen species are much higher than those in EtCN (e.g., for 2H, deltaH (Ki) =-41 kJ mol^-1 in THF versus -29.8 kJ mol^-1 in EtCN,- for 3H, deltaH (beta =-94 kJ mol^-1 in THF versus -77 kJ mol^-1 in EtCN). In addition, a more significant ligand electronic effect is seen for the oxygenation reactions of 1MeO in THF compared to that in EtCN; the thermal stability of superoxo- and peroxocopper complexes are considerably enhanced using L^(MeO compared to L^H In acetone as solvent, a different reaction mechanism involving dimeric copper(l) species [(L^R)2Cu]^2+ is proposed for the oxygenation reactions, supported by kinetic analyses, electrical conductivity measurements, and variable-temperature NMR spectroscopic studies. The present study is the first systematic study investigating both solvent medium and ligand electronic effects in reactions forming copper-dioxygen adducts.