Coordination of a Bifunctional Ligand to a Rhodium(III) Dimethyl Complex:  Lewis Acidity Enhancement by Chelation

The addition of the ambiphilic compound (Me 2 PCH 2 AlMe 2 ) 2 ( 1 ) to Cp*RhMe 2 (DMSO) (DMSO = dimethylsulfoxide) ( 2 ) gives Cp*RhMe 2 (PMe 2 CH 2 AlMe 2 ·DMSO) ( 3 ·DMSO). The addition of Lewis acids (LA) such as La(dbm) 3 (dbm = dibenzoylmethane) and AlMe 3 to a solution of complex 3 ·DMSO gives a competition reaction that results in the formation of LA·DMSO and Cp*RhMe 2 (PMe 2 CH 2 AlMe 2 ) ( 3 ). When heated to 40 °C, complex 3 ionizes to a putative zwitterionic species, Cp*Rh + Me(PMe 2 CH 2 AlMe 3 - ) ( 3 ‘), which is converted to [Cp*Rh(Me)(μ 2 -η 2 -Me 2 PCH 2 )] 2 ( 4 ) irreversibly. Spin saturation transfer experiments demonstrated that the rate of the methyl abstraction by the alane moiety was 0.76 ± 0.09 s - 1, while the rate of abstraction of the methyl in Cp*RhMe 2 (PMe 3 ) by AlMe 3 was 0.10 ± 0.02 s - 1 . The zwitterionic species 3 ‘ could be trapped in solution by addition of PMe 3 to afford both Cp*Rh + Me(PMe 3 )(Me 2 PCH 2 AlMe 3 - ) ( 5 ) and [Cp*Rh + Me(PMe 3 )(Me 2 PCH 2 AlMe 2 )]AlMe 4 - ( 6 ). When compound 1 was added to complex 3 ‘, the formation of the zwitterionic complex Cp*Rh + Me(η 2 -Me 2 PCH 2 Al - Me 2 CH 2 PMe 2 ) ( 7 ) was observed.