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

The reactivity of Rh(III) half-sandwich complex Cp*RhMe2(kappa(P)-mu Al-mePMe2CH2AlMe2) 2 (Cp* = eta(5)-pentamethylcyclopentadienyl) with ethylene was investigated. Coordination of ethylene (or ethylene-d(4)) gave a mixture of products including as principal species zwitterionic complexes Cp*Rh+Me(C2H4)(PMe2CH2AlMe3-) 4 (or 4-d(4)) 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 center dot C2H4) (5). Heating this reaction mixture at 50 degrees C results in the generation of propene (or propene-d(3,4)), methane (or methane-d(1)), and trace amounts of butene, as organic materials, as well as previously characterized [Cp*RhMe(mu(2)-eta(2)(P,C)-PMe2CH2)](2) (10) and rhodium-(I) species. Two different pathways for the reactivity of the zwitterionic pi-complex were investigated by density functional theory (DFT). It is likely that propene is formed by beta-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.