Reactivity of neutral nitrogen donors in square-planar d 8 metal complexes: The system chloro(2,2′:6′,2″-terpyridine)platinum(II) cation with five-membered N-donor heterocycles in methanol

Abstract

The kinetics of the forward and reverse steps of the reaction [Pt(terpy)Cl] + + nu ⇌ [Pt(terpy)(nu)] 2+ + Cl − (terpy = 2,2′:6′,2″-terpyridine, nu = one of a number of thiazoles, oxazole, isoxazole, imidazole, pyrazole and 3,5-dimethylpyrazole, covering a wide range of basicities) have been studied in methanol at 25 °C. Both forward and reverse reactions obey the usual two-term rate law observed in square-planar substitution. The second-order rate constants for the forward reactions, k 2 f , show a slight dependence upon the basicity of the entering nu, while the steric hindrance due to the presence of one methyl group in the α position to the nitrogen markedly decreases the reactivity. The second-order rate constants for the reverse reactions, k 2 r , are very sensitive to the nature of the leaving group and a plot of log k 2 r against the p K a of the conjugate acids of the unhindered five-membered N-donors is linear with a slope of −0.51. The results are compared with data from the literature regarding a series of pyridines reacting with the [Pt(terpy)Cl] + cation under the same experimental conditions. Both in the forward and in the reverse reaction, the reactivity depends not only upon the ligand basicity but also upon the nature of the nucleophile in the order: (thiazoles, oxazole, isoxazole, imidazole, pyrazoles) > pyridines for the entry of N-donors and on the contrary for the displacement by Cl −. Steric retardation, due to the presence of a methyl group in the α position to the nitrogen, is remarkably lower for five-membered N-donors if compared to pyridines both in the forward and in the reverse reaction.