Study of stereodynamics by variable-temperature platinum-195 NMR spectroscopy. Diastereomerism in platinum(II) thioether complexes and solvent effects

Abstract

New (PtCl3(thioether)) complexes have been synthesized with the following thioethers as unidentate ligands: N-formyl-DL-homomethionine (FHMetH), N-acetyl-L-methionine (AcMetH), N-acetyl-S-methyl-DL-cysteine (AcMeCysH), DL-3-(methylthio)-1,2-propanediol (MTPD), and DL-3-(methylthio)-2-butanone (MTB). These complexes constitute a series in which the length of the (CH2)/sub x/ chain connecting the chiral carbon and sulfur atoms is varied systematically: x = 3,2,1,1, and 0, respectively. Each of the complexes exists in two diastereomeric forms, which are related by intramolecular inversion of configuration at the coordinated sulfur atom. The diastereomers are clearly evident in the ZVPt NMR spectra, whose dependence on temperature yields the G/sup double dagger/ values for inversion. Stereodynamic processes involving all but the simplest thioether ligands and processes resulting in subtle changes in molecular structure proved intractable by the common H and TC NMR methods. The barrier to inversion depends on the solvating ability of the medium in an interesting way. Small amounts of diglyme in aqueous solution reduce the difference in chemical shifts between the ZVPt NMR peaks of the diastereomers and thus lower the coalescence temperature, but they do not affect the barrier significantly, Large amounts of diglyme, however, lessen the stabilizing effect of hydration upon the thioether complex and lower the barrier. In unidentate complexes, whichmore » have flexible structures, the chiral C atom provides virtually no discrimination between the two configurations of the chiral S atom irrespective of the length of the (CH2)/sub x/ chain between the two atoms. Significant discrimination is evident, however, in the bidentate complex cis-(PtCl2(MeCysH)). Steric constraint, such as that provided by chelation, seems to be a prerequisite for chiral discrimination.« less