A 1H nuclear magnetic resonance study of the mechanism of the reaction between cis-dichlorobis(dimethyl sulfoxide)-platinum(II) and nitrogen donors

Abstract

The product of the reaction between equimolar amounts of cis-[Pt(dmso)₂Cl₂](dmso = dimethyl sulfoxide) and heterocyclic nitrogen bases (L) in nitromethane depends upon the steric requirements of the base. Strongly hindered ligands such as 2-methylquinoline and 2,6-dimethylpyridine give cis-[Pt(dmso)(L)Cl₂], while the less bulky 2-chloropyridine, 2,4-dimethylpyridine and pyridine give the corresponding trans isomer which later isomerises very slowly to the cis form. The analogous cationic bis(dimethyl sulfoxide) complexes have been prepared as their tetrafluoroborate salts, cis-[Pt(dmso)₂(L)Cl][BF₄] and their reactions with chloride in nitromethane solution have been studied by ¹H NMR spectroscopy. It is concluded that these ionic cations are intermediates in the replacement of dimethyl sulfoxide by L and that all of the stages take place with complete retention of configuration. The reaction products are accounted for in terms of an interplay between the normal trans effect advantage of chloride over the nitrogen donors and the steric repulsion arising from a 2,6-disubstituted pyridine in the axial site of the trigonal-bipyramidal intermediate and/or transition states.