Substitution reactions of cis-platinum(II) complexes containing bidentate N,N-donor pyridinecarboxamide ligands with different substituents

Abstract

Substitution reactions of [2-(pyridinecarboxamide)dichloride Pt(II)] [PtCl₂], [N-phenyl-(2-pyridinecarboxamide)dichloride Pt(II)] [PhPtCl₂], [N-(4-methylphenyl)-2-pyridinecarboxamide)dichloride Pt(II)] [CH₃PhPtCl₂], [N-(4-methoxyphenyl)-(2-pyridine-carboxamide)dichloride Pt(II)] [CH₃OPhPtCl₂] and [N-(4-fluorophenyl)-(2-pyridinecarboxamide)dichloride Pt(II)] [FPhPtCl₂], with nucleophiles; thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU) were studied under pseudo first-order conditions. The rates of substitution were investigated as a function of nucleophile concentration and temperature using stopped-flow and UV-visible absorption spectrophotometers. Substitutions of the two coordinated chloride ligands of the Pt(II) complexes occur consecutively, with the first substitution occurring opposite the coordinated pyridyl. The observed pseudo first-order rate constants regressed linearly with concentration of the incoming nucleophiles according to the equation k_obs = k₂[Nu]. The highest substitution rates were measured for PtCl₂, which is attributed to the unsubstituted non-leaving carboxamide ligand. The Pt(II) center of this complex is the most electrophilic as a result of the strong withdrawal of electron density through π-resonance by the carboxamide group. The introduction of a 4′-substituted phenyl group on the amido N of the carboxamide reduces the reactivity of the complexes due to proportional electronic effects from the ancillary substituents on the phenyl ring as well as its conformational disposition with respect to the plane of the complex. The order of reactivity of studied nucleophiles is TU > DMTU > TMTU. The substitution is associatively activated as supported by the negative entropy of activation values for the reactions.

Keywords:

• N-(4-substituted-phenyl)-(2-pyridine-carboxamide)
• pyridine carboxamide
• Pt(II) complexes
• ligand substitution
• consecutive reactions

Acknowledgements

The authors gratefully acknowledge financial support from the University of KwaZulu-Natal and the National Research Foundation.

Disclosure statement

No potential conflict of interest was reported by the authors.