Gas-phase basicities of acetophenones toward lanthanum cation [La(OMe)2+]

The relative free energy changes (lanthanum cation basicity, LaCB[L-2]) for the reaction [La(OMe)(2)]L-2(+) (sic) La(OMe)(2)(+) + 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 Delta LaCB[L-2] of acetophenone is described in terms of the Yukawa-Tsuno equation, Delta G = rho(sigma degrees + r(+) Delta(sigma) over bar (+)(R)), with a rho value of -11.2 and an r(+) value of 0.49. From this result, a rho value of -7.0 and an r(+) value of 0.49 were estimated for the monomeric complex [LLa(OMe)(2)(+)] with the aid of theoretical calculations. This rho value was found to be significantly smaller than that for protonation, and even smaller than Li+ basicity. Such a small rho value has been attributed to the largely ionic (ion-dipole interaction) nature of the bonding interaction between La(OMe)(2)(+) and the carbonyl oxygen atom and, in part, to the long distance between La(OMe)(2)(+) and the substituent. Contrary to the rho value, the r(+) value is identical in both La(OMe)(2)(+) 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)(2)(+) (0.49). Since the binding interaction between La(OMe)(2)(+) 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 (C) 2010 John Wiley & Sons, Ltd.