Kinetics and mechanistic investigation of the substitution from dinuclear Pt(II) complexes bridged by N,N’-bis(2-pyridylcarboxamide)phenylenediamines

Abstract

The rates of substitution of the chloride ligand in dinuclear complexes, [Pt₂(N,N'-bis(2-pyridylcarboxamide)-1,3-phenylenediamine)Cl₄] (1), [Pt₂(N,N'-bis(2-pyridylcarboxamide)-1,4-phenylenediamine)Cl₄] (2), [Pt₂(N,N'-bis(3-isoquinolylcarboxamide)-1,3-phenylenediamine)Cl₄] (3) and [Pt₂(N,N'-bis(3-isoquinolylcarboxamide)-1,4-phenylenediamine)Cl₄] (4), by three bio-relevant nucleophiles, thiourea (TU), N,N-dimethylthiourea (DMTU) and N,N,N’,N’-tetramethylthiourea (TMTU), were investigated. The mononuclear analogue [Pt(N-phenylpyridine-2-carboxamide)Cl₂] (5) was included to compare the results. The kinetics of the reactions were studied under pseudo first-order conditions in a methanol solution (I = 0.1 M LiCl) as a function of concentration and temperature using the stopped-flow spectrophotometer. The observed pseudo first-order rate constants for the substitution reactions of all the complexes were calculated from two well-separated steps and obey the rate law k_{obs (1}^st_/2^nd₎ = k_{2 (1}^st_/2^nd₎[Nu]. The reactivity of the complexes decreases in the order 2 > 1 > 4 > 3 > 5. The conformational symmetry of the complexes as controlled by the phenylenediamine bridges, steric hindrance due to the bridges as well as the σ-donor capacity of the coordinated groups around the metal centers influence the reactivity of the dinuclear complexes. The low enthalpy (ΔH^#) and negative intrinsic entropy (ΔS^#) values support an associative mechanism of substitution. The kinetic data are supported by DFT calculations.

Keywords:

• Platinum complexes
• carboxamide
• substitution reactions
• mechanistic

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors gratefully acknowledge financial support from the University of KwaZulu-Natal and the National Research Foundation (NRF) of South Africa.