Kinetics and mechanism of the reaction between dimethyl sulphoxide and dichloro(pyridine-2-carboxylato)platinate(II) anion and the cis-trans isomerization of the products. X-Ray crystal structure of trans-(O,S)chloro(dimethyl sulphoxide)(pyridine-2-carboxylato)platinum(II)

Abstract

The reaction between [Pt(pyca)Cl₂]^–(pyca = pyridine-2-carboxylate) and Me₂SO in methanolwater (95 : 5, v/v) at 40 °C gives a mixture of the two isomeric forms of [Pt(pyca)(Me₂SO)Cl] which undergo a subsequent slow sulphoxide-catalysed isomerization to form an equilibrium mixture. The two isomers, trans-(N,S) and trans-(O,S)[(1) and (2) respectively], have ben independently synthesized and the isomer (2) characterized by a single-crystal X-ray analysis. Crystals of (2) are monoclinic, space group P2₁/n with Z= 4 in a unit cell of dimensions a= 9.748(4), b= 13.870(6), c= 8.1 54(4)Å and β= 100.37(3)°. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares to R= 0.034 for 1 700 observed reflections. The co-ordination of the platinum atom is square planar, by a chlorine atom, a sulphur atom from the Me₂SO, and by the nitrogen and one oxygen atom from pyridine-2-carboxylate acting as a chelating ligand. The kinetically controlled product ratio [1]/[2]= 3.8 relates to the greater trans effect of N and differs considerably from the thermodynamic ratio of 0.38. Chloride also catalyses the isomerization but eventually displaces the dimethyl sulphoxide. The relative reactivities show that the chloride-catalysed isomerization cannot proceed through a reversible replacement of Me₂SO by Cl^–.