Displacement of Neutral Nitrogen Donors by Chloride in AuCl 3 (am) (am = Pyridines and Amines): Kinetics and DFT Calculations Show the Effects of Basicity and π‐Acceptor Ability

The kinetics of the process AuCl 3 (am) + Cl – → AuCl 4 – + am (am = sp 2 N‐donor isosteric pyridines with different π systems and sp 3 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, k 2 , are very sensitive to the nature of the leaving group, and plots of log  k 2 against the p K 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 sp 3 N‐donors > “normal” pyridines > “π‐extended” pyridines. This result, with the support of ground‐state DFT calculations on the AuCl 3 (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)