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*RhMe 2 (κ P -μ Al-Me PMe 2 CH 2 AlMe 2 ) 2 (Cp* = η 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(C 2 H 4 )(PMe 2 CH 2 AlMe 3 − ) 4 (or 4 - d 4 ) and [Cp*Rh + Me(C 2 H 4 )(PMe 2 CH 2 AlMe 3 − )][AlMe 3 ] ( 4′ ), after the abstraction of a Rh-methyl group by the pendant Lewis acid, and Cp*RhMe 2 (PMe 2 CH 2 AlMe 2 ·C 2 H 4 ) ( 5 ). Heating this reaction mixture at 50 °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(μ 2 -η 2 (P,C)-PMe 2 CH 2 )] 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(PMe 2 CH 2 AlMe 2 ) 11 is most likely responsible for the reductive elimination of methane.