Coordination of Ethylene to a Zwitterionic Rh(III) Half-Sandwich Complex: Influence of Ambiphilic Ligands on Reactivity

Abstract

The reactivity of Rh(III) half-sandwich complex Cp*RhMe2(κP-μAl-MePMe2CH2AlMe2) 2 (Cp* = η5-pentamethylcyclopentadienyl) with ethylene was investigated. Coordination of ethylene (or ethylene-d4) gave a mixture of products including as principal species zwitterionic complexes Cp*Rh+Me(C2H4)(PMe2CH2AlMe3−) 4 (or 4-d4) and [Cp*Rh+Me(C2H4)(PMe2CH2AlMe3−)][AlMe3] (4′), after the abstraction of a Rh-methyl group by the pendant Lewis acid, and Cp*RhMe2(PMe2CH2AlMe2·C2H4) (5). Heating this reaction mixture at 50 °C results in the generation of propene (or propene-d3,4), methane (or methane-d1), and trace amounts of butene, as organic materials, as well as previously characterized [Cp*RhMe(μ2-η2(P,C)-PMe2CH2)]2 (10) and rhodium(I) species. Two different pathways for the reactivity of the zwitterionic π-complex were investigated by density functional theory (DFT). It is likely that propene is formed by β-hydride elimination from a cationic Rh-propyl fragment that is generated either by insertion of ethylene into a Rh−C bond or by a nucleophilic attack of the methyl-aluminate fragment on coordinated ethylene. After release of propene, the neutral complex Cp*RhHMe(PMe2CH2AlMe2) 11 is most likely responsible for the reductive elimination of methane.