Displacement of Neutral Nitrogen Donors by Chloride in AuCl3(3R-py)(3R-py=meta-Substituted Pyridine):Comparison between meta-and para-Substituted Pyridines by Kinetics and DFT Calculations

摘要

The kinetics of the process AuCl3(3R-py)+Cl~-->AuCl_4~-+3R-py(3R-py=one of a number of mefa-substituted pyridines covering a wide range of basicity)have been studied in methanol at 25°C.The reactions obey the usual two-term rate law observed in the substitution reactions of 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 of ca.-0.6 already found for para-substituted pyridines(4R-py).Until now the two groups of bases have been considered to behave in the same manner in their displacement from Au~(III)and Pt~(II)by various nucleophiles but,on the contrary,the reactivity of the two classes of N donors is slightly different and follows the order:4R-py>3R-py.This kinetic result is explained on the basis of an energetic difference between the frontier orbitals of the AuCl3(3R-py)and AuCl3(4R-py)derivatives.