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

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 AuCl3L 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 AuCl3 fragment in AuCl3L 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 AuCl3, 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 AuCl3 fragment has been estimated.

Graphical abstractA computational DFT approach for the comparison of the π-acceptor character of some N-donor heterocycles L {L = pyridines, pyrimidines, imidazoles, pyrazoles and isoxazoles} in neutral AuCl3L complexes is reported. The π-acceptor ability scale pyridines ≈ pyrimidines < imidazoles < pyrazoles ≈ isoxazoles has been derived. Moreover, the minimum proton affinity value for a meaningful interaction between the ligands and the AuCl3 fragment has been estimated.Figure optionsDownload full-size imageDownload as PowerPoint slide