The roles of ligands proton affinity, π-back donation and metal fragment hardness on the Au-N bond in N-donor heterocycles gold(III) complexes

A DFT study on the Au-N interaction for some groups of N-donor heterocycles L {L = pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io)} in neutral AUX(3)L complexes {AUX(3) = AuBr3, AuCl3, trans-AuCl(CN)(2), Au(CN)(3)} is reported. Linear relationships between the AUX(3) Mulliken charge in AUX(3)L. and the computed proton affinity (PA) of the heterocycle were found for all the considered ligands. The different slopes found on changing the N-donor species represent a measure of the pi-acidity of these nitrogen ligands once coordinated to the metal centre, by the consequence a pi-acceptor ability scale has been derived. The pi-acceptor ability of the 5-membered N-donor ligands resulted in all the cases greater than that of the 6-membered N-heterocycles. The proton affinity average value corresponding to a zero charge of the AuX3 and L fragments in the AUX(3)L species has been estimated. This parameter represents the minimum PA value for the formation of a bond between the N-heterocycles and gold(III) and it does not depend on the electronic features of the coordinated ligands. The sensitivity of the AUX(3) fragments towards ligands PA variations follows the order Au(CN)(3) < trans-AuCl(CN)(2) < AuCl3 < AuBr3 and this last result has been explained on the basis of the metal fragments relative hardness. (C) 2009 Elsevier Ltd. All rights reserved.