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

• Published in: International journal of mass spectrometry 2007-11, Vol.267 (1), p.205-214
• Main Authors: Than, Soe, Maeda, Hideaki, Irie, Maki, Kikukawa, Kiyoshi, Mishima, Masaaki
• Format: Article
• Language: English
• Subjects: Acetophenones; Copper(I) cation basicity; Ligand-transfer equilibrium; Substitutent effect
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/j.ijms.2007.02.037

The free energy changes (Δ G, copper cation basicity) for the reaction L 2Cu + = 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[L 2Cu +]) 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, Δ G = ρ ( σ ° + r   Δ σ ¯ R + ) , the substituent effect on ΔCCB[L 2Cu +] was characterized by a ρ value (kJ mol −1 σ ¯ − 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 L 2Cu + 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) > Me 3Si + (−46.0) > Me 3Ge + (−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.