Experimental and theoretical studies of the binding interaction between copper(I) cation and the carbonyl group

The free energy changes (ΔG, copper cation basicity) for the reaction L2Cu+ = Cu+ + 2L were obtained in the gas phase for m- and p-substituted acetophenones based on the measurement of ligand-exchange equilibria using an FT-ICR spectrometer. A plot of the relative copper cation basicities (ΔCCB[L2Cu+]) against the ΔGBs (proton basicities) showed a limited linear relationship for meta substituents and para electron-withdrawing groups with a slope of 1.24. Negative deviations observed for strong π-donor substituents suggested that the resonance effect of these substituents is reduced in the Cu+ complex compared to that in the protonated acetophenone. Based on the correlation analysis using the Yukawa-Tsuno equation, ΔG=ρ(σ°+rΔσ¯R+), the substituent effect on ΔCCB[L2Cu+] was characterized by a ρ value (kJ mol−1σ¯−1 unit) of −56.2 and an r value of 0.60. The ρ value of −37.5 for monomeric complex (LCu+) could be evaluated from a ρ value for L2Cu+ by using a correction coefficient of 1.5 given by the theoretical calculations. In comparison with the corresponding results for other Lewis cation basicity of the acetophenone system, it has been shown that the ρ value decreases in the order of H+ (−48.5) > Me3Si+ (−46.0) > Me3Ge+ (−40.6) > Cu+ (−37.5) > Li+ (−34.7). The decrease in the ρ value indicates that the positive charges at the Lewis cation moiety of the adduct ion increases in this order. This is consistent with increasing ionic (ion-dipole interaction) nature of the bonding interaction between Lewis cations and the carbonyl oxygen atom. The r value that is a measure of the degree of charge-delocalization into the aromatic π-system decreases also in the same order.