Displacement of neutral nitrogen donors by chloride in AuCl3(am) (am = pyridines and amines): Kinetics and DFT calculations show the effects of basicity and π-acceptor ability

Article abstract of DOI:10.1002/ejic.200700457

The kinetics of the process AuCl₃(am) + Cl^- → AuCl₄^- + am (am = sp² N-donor isosteric pyridines with different π systems and sp³ amines; they cover a wide range of basicity) have been studied in methanol at 25°C. The reactions obey the usual two-term rate law observed in substitutions on square-planar complexes. The second-order rate constants, k₂, are very sensitive to the nature of the leaving group, and plots of log k₂ against the pK_a of the conjugate acids are linear, with the same slope, -0.68, for both normal pyridines and pyridines with a more extended π system, such as 4-cyanopyridine, isonicotinic acid, methyl isonicotinate and 4-acetylpyridine. The reactivity of the considered N-donors is different and follows the order sp³ N-donors > normal pyridines > π-extended pyridines. This result, with the support of ground-state DFT calculations on the AuCl₃(am) derivatives, is explained on the basis of an Au-N bond enforcement due to an increased π-back-donation contribution. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.