Reactivity of cis-platinum(II) complexes with 2-(4-substituted)phenylthiomethyl)quinoline nonleaving ligands toward thiourea nucleophiles

Abstract

The sequential substitution of aqua ligands from [Pt{2-(phenylthiomethyl)quinoline}(H2O)2]CF3SO3, Pt(L1), [Pt{2-(4-tert-butylphenylthiomethyl)quinoline}(H2O)2]CF3SO3 Pt(L2), and [Pt{2-(4-fluorophenylthiomethyl) quinoline}(H2O)2]CF3SO3 Pt(L3) by thiourea nucleophiles (Nu) was studied under pseudo–first-order conditions as a function of concentration and temperature using stopped-flow and UV-visible spectrophotometric techniques. The observed pseudo–first-order rate constants for the substitutions can be described by the rate law: kobs(1/2) = k2(1/2)[Nu], where the subscript denotes the consecutive substitution steps. The first aqua ligand was substituted opposite to the strong σ-trans-directing thioether followed by the aqua ligand opposite to the quinoline or pyridine moieties. Second-order rate constants, k2(1st), for the substitution of the first aqua ligand by thiourea nucleophiles ranged between 9 and 22 M−1 s−1 for Pt(L1), 86 and 326 M−1 s−1 for Pt(L2), and 58 –287 M−1 s−1 for Pt(L3). The ranges of the second-order rate constant, k2(2nd), were always lower than the k2(1st)’s and are 0.3-9 M−1 s-1 for Pt(L1), 2-20 M−1 s−1 for Pt(L2), and 0.3-5 M−1 s−1 for Pt(L3). Aqua substitution from Pt(L2) is slower for both steps than from Pt4, its pyridyl derivative from our previous study. A Job's method of continuous variation plot suggests a species with a metal-to-nucleophile ratio of 1:3 as the ultimate product of the chloride substitution from the Pt(II) complexes by the incoming thiourea nucleophiles. The high and negative activation entropy and low and positive activation enthalpy values support an associative mechanism of activation, characteristic of substitution reactions occurring in square-planar complexes.