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

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 pKa 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, 69451 Weinheim, Germany, 2007)