To what extent can the Pt–C bond of a metallacycle labilize the trans position? A temperature- and pressure-dependent mechanistic study

Abstract

The orthoplatinated complex [Pt{C₆H₃X(CH₂NMe₂)}(NC₅H₄SO₃-3)(H₂O)](X = H 1a or 3-MeO 1b) was designed for mechanistic studies in water. The aqua ligand is located trans to the Pt–C bond of the phenyl group which lies in the platinum(II) co-ordination plane. The rates of substitution of the aqua ligand by a series of nucleophiles (Nu)(Cl^–, Br^–, I^–, N₃^–, SCN^–, thiourea, N,N′-dimethylthiourea or N,N,N′,N′-tetramethylthiourea) were studied as a function of concentration, pH, temperature, and pressure by using a stopped-flow technique. The pK_a value of the aqua ligand in 1a is 9.75 ± 0.05 and the observed pseudo-first-order rate constants for the substitution reaction are given by k_obs=k₁[Nu]+k_–1. The k_–1 term arises from the reverse solvoiysis reaction and is insignificant for stronger, S-donor nucleophiles. The values of k₁ are ca. four orders of magnitude higher than the corresponding rate constants for anation reactions of [Pt(dien)(H₂O)]²⁺(dien = diethylenetriamine) and close to the rate constants for anation of [Pd(dien)(H₂O)]²⁺. The effect is largely due to a strong decrease in ΔH^‡. An analysis of the activation parameters, ΔH^‡, ΔS^‡ and ΔV^‡, clearly shows that the substantial rate increase is not associated with a changeover in mechanism and that the substitution process is still associative in nature.