The role of substituents in a bidentate N,N-chelating ligand on the substitution of aqua ligands from mononuclear Pt(II) complexes

Abstract

The rate of substitution of aqua ligands from three mononuclear platinum(II) complexes, namely [Pt{2-(pyrazol-1-ylmethyl)pyridine}(H2O)2](ClO4)2, [Pt(H 2 Py)]; [Pt{2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine}(H2O)2](ClO4)2, [Pt(dCH 3 Py)] and [Pt{2-[(3,5-bis(trifluoromethyl)pyrazoly-1-ylmethyl]pyridine}(H2O)2](ClO4)2, [Pt(dCF 3 Py)] by thiourea, N,N-dimethylthiourea and N,N,N′,N′-tetramethylthiourea, was studied in aqueous perchloric acid medium of constant ionic strength. The substitution reactions were investigated under pseudo-first-order conditions as a function of nucleophile concentration and temperature using UV/Visible and stopped-flow spectrophotometries. The observed pseudo-first-order rate constants, , for the stepwise substitution of the first and second aqua ligands obeyed the rate law: . The first substitution reaction takes place trans to the pyrazole ligand, while the second entering nucleophile is stabilised at the reaction site trans to the pyridine ligand. The rate of substitution of the first aqua ligand from the complexes followed the order: Pt(dCF 3 Py) > Pt(H 2 Py) > Pt(dCH 3 Py), while that of the second was Pt(H 2 Py) ≈ Pt(dCF 3 Py) > Pt(dCH 3 Py). Lower pK a values were found for the deprotonation of the aqua ligand cis to the pyrazole ring. Density functional theory calculations were performed to support the interpretation of the experimental results.