Transition metal–carbon bonds. Part XXVI. Allylic and olefin complexes of platinum(II)

Abstract

The complexes [Pt(all)₂], (all = allyl or 2-methylallyl) are shown to be mononuclear and to exist as cis-trans-mixtures with symmetrically bonded π-allylic groups. With dry hydrogen chloride [Pt(allyl)₂] gives the tetranuclear [PtCl(allyl)]₄ with bridging (or µ-bonded) allyl groups and bridging chlorines. With thallous acetylacetonate, [PtCl(allyl)]₄ gives the binuclear [Pt(acac)(allyl)]₂ with bridging allyl ligands. In contrast, [Pt(2-methylallyl)₂] reacts with dry hydrogen chloride to give the binuclear [PtCl(2-methylallyl)]₂ which has bridging chlorines and symmetrically bonded π-2-methylallyl groups. This complex exists as a mixture having both cis- and trans-arrangements of the 2-methylallyl ligands with respect to the Pt₂Cl₂-bridging system (as shown by low-temperature ¹H n.m.r. spectroscopy and double resonance). Complexes of type [PtCl(all)]_x react with neutral ligands, L (isoquinoline, pyridine, tertiary phosphines or tertiary arsines), to give mononuclear complexes [PtCl(all)L] having asymmetrically bonded π-allylic ligands, or cationic species [Pt(all)L₂]⁺. [Pt(2-methylallyl)₂] reacts with EPh₃(E = P or As) to give [Pt(σ-2-methylallyl)₂(EPh₃)₂] with σ-bonded 2-methylallyl ligands. Several complexes of the type [PtCl(all)]₂(all = alkyl-substituted allylic ligand) were made by heating olefin complexes of the type [PtCl₂(olefin)]₂ with 50% aqueous acetic acid. Complexes of the type [PtX(all)]₂, [Pt(all)(acac)] and [Pt(all)(C₅H₅)] are also described. Mononuclear complexes show sharp satellites in their ¹H n.m.r. spectra due to coupling with platinum-195 but the satellites in some of the spectra of the binuclear complexes, [PtX(all)]₂, are broadened or not observable. With [PtCl(2-methylallyl)]₂ the satellite widths show complex behaviour with temperature. Double resonance experiments on [PtCl(2-methylallyl)]₂ show that the platinum-195 relaxation time must be short compared with the relaxation time of the syn-protons. Some rate processes which occur in these allylic-platinum complexes were studied qualitatively by ¹H n.m.r. spectroscopy.