The effect of the chelate ligand on the rate of chloride substitution from Pt(II) complexes with picolyl and isoquinolinylcarboxamide moieties on the non-leaving ligand

Abstract

Substitution reactions of four Pt(II) complexes, namely; N-(2-picolyl)picolinamide-platinum(II) chloride (Pt3), N-(8-quinolyl)pyridine-2-carboxamide platinum(II) chloride (Pt4), N-(8-quinolyl)-3-isoquinolinecarboxamide platinum(II) chloride (Pt5) and N-(8-quinolinyl)-1-isoquinolinecarboxamide platinum(II) chloride (Pt6), were studied with biorelevant nucleophiles, viz. thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU) under pseudo first order conditions as a function of concentration and temperature using the stopped flow spectrophotometer. The observed pseudo first order rate constants for the substitution reactions obey the rate law kobs = k2[Nu]. The reactivity of the studied complexes depends on strength of π-backbonding of the attached ligands, which is enhanced by the strong electron withdrawing nature of the carboxamide group on the non-leaving ligands of Pt(II) complexes. Quinoline moieties of Pt5 and Pt6 lie out-of-the square plane, resulting in enhanced reactivity due to the aided entrapment of the nucleophile by the non-planar groups of the ligand. Negative activation entropies and positive enthalpies of activation support an associative mode of activation.