Metallation of the isopropenyl group by platinum(II): mechanism of formation of isomeric σ-allylic and σ-vinylic six-membered chelate rings. X-Ray structure of the σ-vinylic complex di-µ-acetato-bis-{[2-(o-diphenylarsinophenyl)propenyl–C′As]platinum(II)}

Abstract

The (o-isopropenylphenyl)diphenylarsine (ipa) complex [Pt(ipa)(acac)][BF₄](acac = acetylacetonate) reacts with nucleophiles including acetate to give [[graphic omitted]sPh₂}(acac)](1) in which the ligand is bonded as a σ-allylic group. The complex [PtCl₂(ipa)] reacts with silver acetate to give the complex [{[graphic omitted]sPh₂}₂(µ-O₂CMe)₂](2) shown from X-ray analysis to have the deprotonated olefin bonded as a σ-vinyl group. Crystals are monoclinic, space group C2/c, a= 31.633(5), b= 11.345(5), c= 18.450(5)Å, β= 141.37(2)°, U= 4 133.6 Å₃, Z= 4, and R= 0.059 for 2 093 reflections. The σ-allylic complex (1) is converted to the σ-vinylic complex (2) by acetic acid. Mechanisms for the formation, protonation, and interconversion of the two isomeric forms of the metallated olefin have been deduced; carbonium ion complexes are strongly indicated as reactive intermediates. The σ-allylic and σ-vinylic forms appear to be the consequence of intermolecular and intramolecular attack respectively of a nucleophile on the co-ordinating olefin.