Experimental and theoretical studies of the binding interaction between copper(l) cation and the carbonyl group substituent effect on the stability of (acetophenone)2CU+ complex in the gas phase

The free energy changes (AG, 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 (Delta CCB[L2Cu+]) against the AGBs (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 pi-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, A G = p(sigma degrees + r Delta 5 sigma R--(+)), the substituent effect on Delta CCB[L2Cu+] was characterized by a to value (kJ mol(-1) sigma(-1) unit) of -56.2 and an r value of 0.60. The p value of -37.5 for. monomeric complex (LCu+) could be evaluated from a p 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 p value decreases in the order of H+ (-48.5) > Me-3 Si+ (-46.0) > Me3Ge+ (-40.6) > Cu+ (-37.5) > Li+ (-34.7). The decrease in the p 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 pi-system decreases also in the same order. (C) 2007 Elsevier B.V. All rights reserved.