A new π-acidity scale for several N-donor heterocycles as ligands in neutral gold(III) complexes

Abstract

A computational DFT approach for the comparison of the π-acceptor character of some N-donor heterocycles L {L = pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io)} in neutral AuCl 3L complexes is reported. The electronic properties of these ligands have been tuned by adding methyl and/or trifluoromethyl groups in various positions. Linear relationships between the Mulliken charge of the AuCl 3 fragment in AuCl 3L and the computed proton affinity (PA) of the heterocycle were obtained for all the considered ligands. The different slopes found on changing the N-donor type represent a measure of the π-acidity of these nitrogen ligands once coordinated to AuCl 3, and as a consequence the π-acceptor ability scale pyridines ≈ pyrimidines < imidazoles < pyrazoles ≈ isoxazoles has been derived. Moreover, on the basis of the metal fragment charge variation, a minimum proton affinity value for a meaningful interaction between the ligands and the AuCl 3 fragment has been estimated.