Synthesis of methyl-palladium(II) and -platinum(II) complexes containing labile chelates and olefinic tertiary phosphine ligands: intramolecular insertion of an olefin into a palladium–carbon bond, and activation of a carbon–hydrogen bond by a platinum(II) complex

Abstract

The square planar complexes [M(Me)(L–L)(L)]{M = Pd, L = dpvp [PPh₂(C₆H₄CHCH₂-o)], L–L = acac (acetylacetonate), bzsac (1-phenyl-3-sulfanylbut-2-en-1-onate) or pyca (pyridine-2-carboxylate), L = PPh₂(C₆H₄Et-o), L–L = acac or pyca, L = P(CH₂–CHCH₂)Ph₂, L–L = pyca; M = Pt, L = dpvp, L–L = bzsac or pyca, L = PPh₂(C₆H₄Et-o), L–L = pyca} have been synthesised and their thermal behaviour studied. Heating the palladium–dpvp complexes in benzene leads to intramolecular migratory insertion of the vinyl double bond of the phosphine into the Pd–Me bond to give complexes with a five-membered palladacyclic ring, [Pd(L–L){PPh₂(C₆H₄CHCH₂Me)}]. In contrast, warming the platinum complex [PtMe(pyca)(dpvp)] leads to a cyclometallation reaction forming a platinacycle with a platinum–alkenyl bond [Pt(pyca){PPh₂(C₆H₄CCH₂)}]. The platinum complex appears to react by oxidative addition of the vinyl C–H bond of dpvp to Pt^II; methane is eliminated during the reaction. Although no such species has been identified, kinetic evidence suggests that the two thermal processes occur via a common four-co-ordinate intermediate containing a ‘dangling’ pyca ligand and with dpvp chelated through the phosphorus and the vinyl double bond [M(Me)(pyca){PPh₂(C₆H₄CHCH₂)}].