Substitution reactions of [Pd(bipy)(malonate)] explored with a different set of ligands: Kinetic and mechanistic interpretation in aqueous medium and at pH 7.4

A brief overview of mechanistic studies of the reactions of Pd(II)-bipy-malonate complex with different set of ligands, viz., (N, S), (S, O) and (S) donor molecules is reported here. The kinetics of complex-formation reactions of three sulphur containing bio-relevant ligands thioglycolic acid[L1],thiourea[L2] and thiosemicarbazide [L3] were studied with innermetallic [Pd(bipy)(malonate)] complex at physiological condition. The effect of the nucleophilicity of the chosen nucleophiles was studied for the reactant complex under pseudo-first order conditions as a function of nucleophile concentration and temperature using stopped-flow technique. This article describes the results obtained for substitution reactions of bi-functional Pd(II) complex with different biomolecules, under varying experimental conditions. The kinetic studies showed that the malonate ring departs from the coordination zone of palladium centre via two-step mechanism. The first step depends on the concentration of the incoming ligand for all the ligands. But in the second step thiourea is ligand dependent where as other two are independent of the ligand concentration. Hence, it can be concluded that the second step is the chelation step for L1 and L3. The mechanism for the substitution of the coordinated malonate molecule is associative, as demonstrated by the negative values of ΔS ≠. Such type of complexes are less toxic than chloro-, which in turn hydrolyses to aqua or aqua complexes as they are prevented from oligomer formation at physiological pH.