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

Abstract

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 3L complexes {AuX 3 = AuBr 3, AuCl 3, trans-AuCl(CN) 2, Au(CN) 3} is reported. Linear relationships between the AuX 3 Mulliken charge in AuX 3L 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 π-acidity of these nitrogen ligands once coordinated to the metal centre, by the consequence a π-acceptor ability scale has been derived. The π-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 AuX 3 and L fragments in the AuX 3L 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 < AuCl 3 < AuBr 3 and this last result has been explained on the basis of the metal fragments relative hardness.