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

The addition of the ambiphilic compound (Me2PCH2AlMe2)(2) (1) to Cp*RhMe2(DMSO) (DMSO = dimethylsulfoxide) (2) gives Cp*RhMe2(PMe2CH2AlMe2 center dot DMSO) (3 center dot DMSO). The addition of Lewis acids (LA) such as La(dbm)(3) (dbm = dibenzoylmethane) and AlMe3 to a solution of complex 3 center dot DMSO gives a competition reaction that results in the formation of LA center dot DMSO and Cp*RhMe2(PMe2CH2AlMe2) (3). When heated to 40 degrees C, complex 3 ionizes to a putative zwitterionic species, Cp*Rh+Me(PMe2CH2AlMe3-) (3'), which is converted to [Cp*Rh(Me)(mu(2)-eta(2)-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)(Me2PCH2AlMe3-) (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(eta(2)-Me2PCH2Al-Me2CH2PMe2) (7) was observed.