Gas‐phase basicities of acetophenones toward lanthanum cation [La(OMe)]

The relative free energy changes (lanthanum cation basicity, LaCB[L2]) for the reaction [La(OMe)2]L ⇌ La(OMe) + 2L were determined in the gas phase for m- and p-substituted acetophenones based on the measurement of ligand exchange equilibria using an FT-ICR mass spectrometer. The substituent effect on ΔLaCB[L2] of acetophenone is described in terms of the Yukawa–Tsuno equation, ΔG = ρ(σ° + r+ Δ σ), with a ρ value of −11.2 and an r+ value of 0.49. From this result, a ρ value of −7.0 and an r+ value of 0.49 were estimated for the monomeric complex [LLa(OMe)] with the aid of theoretical calculations. This ρ value was found to be significantly smaller than that for protonation, and even smaller than Li+ basicity. Such a small ρ value has been attributed to the largely ionic (ion–dipole interaction) nature of the bonding interaction between La(OMe) and the carbonyl oxygen atom and, in part, to the long distance between La(OMe) and the substituent. Contrary to the ρ value, the r+ value is identical in both La(OMe) and Li+ basicities, suggesting that the r+ value of 0.49 can be regarded as a limiting one in a series of Lewis cation basicities of the acetophenone system, H+ (0.86) > Me3Si+ (0.75) > Me3Ge+ (0.71) > Cu+ (0.60) > Li+ = La(OMe) (0.49). Since the binding interaction between La(OMe) or Li+ and a neutral ligand is mostly electrostatic, the moderate r+ was interpreted to result from the redistribution of the induced positive charge within the acetophenone moiety upon binding with a metal ion rather than transfer of positive charge from a metal ion to the aromatic moiety. Copyright © 2010 John Wiley & Sons, Ltd.