Nuclear magnetic resonance investigations of a novel intramolecular methylthio replacement process in palladium(II) and platinum(II) complexes of mixed sulfur–phosphine ligands

Abstract

A series of palladium(II) and platinum(II) complexes with the mono- and bis-chelate ligands bis[o-(methylthio)phenyl]phosphine and tris[o-(methylthio)phenyl]phosphine have been prepared and characterised. Despite the potential tridenticity of the first ligand it acts as a P/S monochelate to form square-planar complexes of type [M{PPh(C₆H₄SMe-o)₂}X₂](M = Pd, X = Cl, Br or I; M = Pt, X = Cl). Variable-temperature one- and two-dimensional NMR spectroscopy showed that pyramidal inversion of the co-ordinated sulfur atoms was accompanied by an exchange of pendant and bound methylthio groups. The ΔG^‡ values for both processes were in the range 43–69 kJ mol^–1, being strongly metal and halogen dependent. The bis(chelate) complexes [M{P(C₆H₄SMe-o)₃}₂][ClO₄]₂ are also square planar with metal co-ordination involving one phosphorus and one sulfur of each ligand, the other two pairs of SMe groups being unco-ordinated. The X-ray crystal structure of [Pd{P(C₆H₄SMe-o)₃}₂][ClO₄]₂ shows the co-ordinated S-methyls to be mutually cis with respect to the metallocycle plane. In low-temperature solutions of this complex this cis form is assumed to be predominant. At higher solution temperatures a facile intramolecular exchange of all six methylthio groups occurs. The energy and probable mechanism of this fluxion are reported.