Role of Chelate Substituents and cis σ-Effect on the Rate of Ligand Substitution at Pt(N–N–N) and Pt(N–N–C) Centres†

Abstract

Four complexes of the type [Pt(N–N–X)Cl] (X = N or C) were tailor synthesized for mechanistic studies in methanol. The terdentate ligands included terpy, 4′-Ph-terpy, 4′-(2‴-CF3-Ph)-terpy, and 4′-(2‴-CF3-Ph)-6-Ph-2,2′-bipy. The rate of substitution of the chloro ligand by thiourea, N,N′-dimethylthiourea, and N,N,N′,N′-tetramethylthiourea was studied as a function of nucleophile concentration, temperature and pressure by using a stopped-flow technique. The observed pseudo-first-order rate constants for the substitution reactions obeyed the simple rate law kobs = k2[Nu]. Second-order kinetics and negative activation entropies and volumes support an associative substitution mechanism. At 298 K, the values of the second-order rate constant show a slight dependence on the nature of the moiety attached to the terpy ligand. Changing from a nitrogen σ-donor to a carbon σ-donor in the cis position, results in a deceleration of the substitution rate. The results suggest that the Pt–C bond in the cis position activates the metal centre in a different way than in the trans position.