Mechanism of the reaction of [Mo(η-C6H6)(Me2PCH2CH2PMe2)(η-C2H4)H]+ with PMe3 giving [Mo(η-C6H5Et)(dmpe)(PMe3)H]+: metal-to-ring migration of an ethyl group

Abstract

The cis-ethylenehydride compounds [Mo(η-C₆H₆)(L₂)(η-C₂H₄)H]PF₆[L₂= Me₂PCH₂CH₂PMe₂(dmpe) or o-C₆H₄(EMe₂)₂(E = P, pdmp, or As, pdma)] react with trimethylphosphine to give the η-ethylbenzene compounds [Mo(η-C₆H₅Et)(L₂)(PMe₃)H]PF₆. Detailed studies of the mechanism of this reaction show that there is an initial reaction, at low temperatures, giving isomeric mixtures of thermally unstable η-cyclohexadienyl derivatives [Mo(η-C₆H₇)(dmpe)(PMe₃)(η-C₂H₄)]⁺. The combined evidence from n.m.r. spectra, including kinetic studies, and from the preparation and reactions of intermediates shows that the η-ethylbenzene compound is formed by the sequence [Mo(η-C₆H₆)(dmpe)(η-C₂H₄)H]⁺→[Mo(η-C₆H₆)(dmpe)(PMe₃)Et]⁺→[Mo(η-C₆H₆Et-endo)(dmpe)(PMe₃)(solvent)]⁺→[Mo(η-C₆H₅Et)(dmpe)(PMe₃)]+ H⁺→[Mo(η-C₆H₅Et)(dmpe)(PMe₃)H]⁺. The following new compounds have been prepared and characterised: [Mo(η-C₆H₆){P(OMe)₃}₂(η-C₃H₅)] PF₆, [Mo(η-C₆H₆)(pdmp)(η-C₃H₅)]PF₆, [Mo(η-C₆H₆)(pdma)(η-C₃H₅)]PF₆, [Mo(η-C₆H₆)(pdmp)(η-C₂H₄)], [Mo(η-C₆H₆)(pdma)(η-C₂H₄)], [Mo(η-C₆H₆)(pdmp)(η-C₂H₄)H]PF₆, [Mo(η-C₆H₆)(pdma)(η-C₃H₅)]PF₆, [Mo(η-C₆H₆)(pdmp)(η-C₂H₄)], [Mo(η-C₆H₆)(pdma)(η-C₂H₄)], [Mo(η-C₆H₆)(pdmp)(η-C₂H₄)H]PF₆, [Mo(η-C₆H₆)(pdma)(η-C₂H₄)H]PF₆, [Mo(η-C₆H₅Et)(pdmp)(PMe₃)H]PF₆, [Mo(η-C₆H₅Et)(pdma)(PMe₃)H]PF₆, [Mo(η-C₆H₆)(dmpe)(PMe₃)D]PF₆, [Mo(η-C₆H₆)(dmpe)(η-C₂H₄)], [Mo(η-C₆H₆Et-endo)(dmpe)₂]PF₆, [Mo(η-C₆H₆)(dmpe){P(OMe)₃}H]PF₆, [Mo(η-C₆H₆)(dmpe)(CO)H]PF₆, [Mo(η-C₆H₆)(dmpe)(2,6-Me₂C₆H₃ NC)]PF₆, [Mo(η-C₆H₅Me)(dmpe)(η-C₂H₄)H]PF₆, [Mo(C₆H₄Me-2-Et-1)(dmpe)(PMe₃)H]PF₆, [Mo(C₆H₄Me-3-Et-1)(dmpe)(PMe₃)H]PF₆, [Mo(C₆H₅Me-1-Et-6-endo)(dmpe)₂]PF₆, [Mo(C₆H₅Me-2-Et-6-endo)(dmpe)₂]PF₆, and [Mo(η-C₆H₆Et-endo)(pdmp)₂]PF₆.