Massimiliano Bonivento

A comparison of nucleophilicity and lability of thiazoles and pyridines in substitution reactions at platinum(II) complexes

The second order rate constants, k2, for the displacement of chloride by nitrogen donors L (L = thiazoles, pyridines, amines) from [Pt(S-S)Cl2] [S-S = 1, 2-bis-(phenylsulfanyl) ethane] and [Pt(SNS)Cl]+ [SNS=2, 6-bis- (methylsulfanylmethyl)pyridine] are linearly related to their proton basicity according to a free-energy relationship of the type logk2 = α(pKa) + β. In the reverse reactions, i.e. the displacement by chloride of the coordinated nitrogen donors, the lability depends not only upon ligand basicity but also upon the nature of the group to be displaced in the order thiazoles < pyridines. This behaviour, attributed to different π interactions in the ground state, is confirmed in the solvolytic displacement of the organic bases from complexes of the type trans-[Pt(dmso)(L)Cl2]. Steric retardation, owing to the presence of a methyl group ortho to the nitrogen in the aromatic rings, is considerably less for entering thiazoles as compared to pyridines.

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

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.