A dynamic nuclear magnetic resonance study of trimethylplatinum(IV) halide complexes of 1,1,2,2- tetrakis(methylthio)ethane. Part 2. High-temperature fluxionality

Abstract

The complexes [PtXMe₃{(MeS)₂CHCH(SMe)₂}](X = Cl, Br, or I) in C₆D₅NO₂ solution exist as trans and cis-1 isomers which, in the temperature range 333–423 K, exhibit novel fluxionality. This involves a trans⇌cis-1 interconversion which proceeds via a mechanism which exchanges all co-ordinated and unco-ordinated S–methyl environments and all Pt–methyl environments. The dynamics of this process were measured by ¹H n.m.r. band-shape analysis and two-dimensional exchange spectroscopy. Both methods yielded energy data in good agreement, ΔG^‡(363 K) values being in the range 88.7–91.2 kJ mol^–1 for the three halogen complexes. Since ΔG^‡ values based on S–methyl and Pt–methyl exchanges were identical within experimental error a single concerted mechanism is operating. This is, in essence, a 1,3-metal-pivot process resulting from a rearrangement of individual Pt–S bonds and involving 109° clockwise or anticlockwise twists of the pendant –CH(SMe)₂ group about its adjacent C–C bond, bringing the other gem S-methyl into co-ordination with the metal. The fluxion proceeds via a highly non-rigid seven-co-ordinate platinum(IV) intermediate which effects exchange of all Pt–methyl environments. This is the first example of a metal-pivot process in mononuclear platinum(IV) complexes.