Kinetic and mechanistic investigation into the influence of substituents on the substitution reactions of Pt(II) NCN-donor complexes

Abstract

The substitution kinetics of cyclometallated platinum(II) complexes [PtL1Cl] (L1 = 1,3-di(2-pyridyl)benzene), [PtL2Cl] (L2 = 3,5-di(2-pyridinyl)-fluorobenzene), [PtL3Cl] (L3 = 2,4-di(2-pyridinyl)-fluorobenzene) and [PtL4Cl] (L4 = 3,5-di(2-pyridinyl)-toluene) with neutral nitrogen-donor nucleophiles imidazole, 1-methylimidazole, 1,2-dimethylimidazole, pyrazole and 1,2,4-triazole were investigated under pseudo-first-order conditions in methanol solution with an ionic strength of 0.1 M. The rate of substitution of the chloride ligand was studied as a function of nucleophile concentration and temperature using stopped-flow spectrophotometric techniques. The observed pseudo-first-order rate constants, kobs, for the substitution reactions obeyed the rate law kobs = k2[Nu]. The reactivity of these complexes follows the order PtL2Cl > PtL3Cl > PtL4Cl > PtL1Cl. The lability of the chloride ligand is influenced by the extent of π-backbonding and also by the σ-trans effect. The reactivity of the nucleophiles depends on their basicity, inductive effect and steric hindrance. Second-order kinetics and negative activation entropies support an associative substitution mechanism. The experimental data are supported by the results of DFT calculations.