Coordination of a bifunctional ligand to a rhodium(III) dimethyl complex : Lewis acidity enhancement by chelation

Abstract

The addition of the ambiphilic compound (Me2PCH2AlMe2)2 (1) to Cp*RhMe2(DMSO) (DMSO ) dimethylsulfoxide) (2) gives Cp*RhMe2(PMe2CH2AlMe2‚DMSO) (3‚DMSO). The addition of Lewis acids (LA) such as La(dbm)3 (dbm ) dibenzoylmethane) and AlMe3 to a solution of complex 3‚DMSO gives a competition reaction that results in the formation of LA‚DMSO and Cp*RhMe2(PMe2CH2AlMe2) (3). When heated to 40 °C, complex 3 ionizes to a putative zwitterionic species, Cp*Rh+Me(PMe2CH2AlMe3 -) (3′), which is converted to [Cp*Rh(Me)(µ2 -η2 -Me2PCH2)]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*RhMe2(PMe3) by AlMe3 was 0.10 ( 0.02 s-1 . The zwitterionic species 3′ could be trapped in solution by addition of PMe3 to afford both Cp*Rh+Me(PMe3)(Me2- PCH2AlMe3 -) (5) and [Cp*Rh+Me(PMe3)(Me2PCH2AlMe2)]AlMe4 - (6). When compound 1 was added to complex 3′, the formation of the zwitterionic complex Cp*Rh+Me(η2 -Me2PCH2Al-Me2CH2PMe2) (7) was observed.