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

The substitution kinetics of cyclometallated platinum(II) complexes [PtL (1) Cl] (L (1) = 1,3-di(2-pyridyl)benzene), [PtL (2) Cl] (L (2) = 3,5-di(2-pyridinyl)-fluorobenzene), [PtL (3) Cl] (L (3) = 2,4-di(2-pyridinyl)-fluorobenzene) and [PtL (4) Cl] (L (4) = 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, k (obs), for the substitution reactions obeyed the rate law k (obs) = k (2)[Nu]. The reactivity of these complexes follows the order PtL (2) Cl > PtL (3) Cl > PtL (4) Cl > PtL (1) Cl. The lability of the chloride ligand is influenced by the extent of pi-backbonding and also by the sigma-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.