The π-acceptor effect in the substitution reactions of tridentate N-donor ligand complexes of platinum(ii): a detailed kinetic and mechanistic study

Abstract

The nucleophilic substitution reactions of complexes [Pt{4′-(2′′′-CH3-phenyl)-2,2′:6′,2′′-terpyridine}Cl]CF3SO3, [CH33PhPtCl], [Pt{4′-(2′′′-CH3-phenyl)-6-(3′′-isoquinoyl)-2,2′bipyridine}Cl]SbF6, [CH33PhisoqPtCl], [Pt{2-(2′-pyridyl)-1,10-phenanthroline}Cl]Cl, [pyPhenPtCl], and [Pt(terpyridine)Cl]+, [PtCl] with a series of nucleophiles: thiourea (TU), N,N-dimethylthiourea (DMTU), N,N,N,N-tetramethylthiourea (TMTU), I−, Br−, and SCN− were studied in 0.1 M LiCF3SO3 in methanol (in the presence of 10 mM LiCl). The reactivity of the investigated complexes follows the order pyPhenPtCl > PtCl > CH33PhPtCl > CH33PhisoqPtCl. The lability of the chloride ligand is dependent on the strength of π-backbonding properties of the spectator ligands around the platinum centre. The experimental data is strongly supported by DFT calculations. The dependence of the second-order rate constants on concentration of the nucleophiles as well as the large negative values reported for the activation entropy (ΔS‡) confirmed an associative mechanism of substitution.