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

Abstract

The relative free energy changes (lanthanum cation basicity, LaCB[L2]) for the reaction [La(OMe)2]L 2+ ⇌ 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 ΔLaCB[L2] of acetophenone is described in terms of the Yukawa–Tsuno equation, ΔG = ρ(σ° + r+ Δ σ R+), 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) 2+] 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) 2+ and the carbonyl oxygen atom and, in part, to the long distance between La(OMe) 2+ and the substituent. Contrary to the ρ 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 © 2010 John Wiley & Sons, Ltd.

Azo-Coupling Reactions of Para-X-Benzenediazonium Cations with 3-Ethoxythiophene in Acetonitrile

“…Careful examination of literature showed that deviations from Hammett plots based on standard σ‐values have been rarely reported in S E Ar reactions, but have been commonly observed in S N Ar and S N 2 process. Jamaoui and co‐workers, for example, investigated the reactions of 4‐chloro‐7‐nitrobenzofurazan with 4‐X‐substituted anilines (X = OH, OCH 3 , CH 3 , H, I Cl and CN) in dimethyl sulfoxide (Me 2 SO) solution at 25°C, and the curved Hammett plot has been interpreted in terms of a change in mechanism from a polar S N Ar process for less basic nucleophiles (X = I, Cl and CN) to a single‐electron transfer for more basic nucleophiles (X = OH, OCH 3 and CH 3 ) .…”