Complexation of dimethylmagnesium with alpha-diimines; structural and EPR characterisation of single electron and alkyl transfer products

Treatment of dimethylmagnesium with the a-diimine ligands Ar'N=C(R)C(R)=NAr'[R = naphth-1,8-diyl (1), H (2) CH3 (3); Ar' = 2,6-diisopropylphenyl] in diethyl ether provides the neutral methyl-bridged dimeric complexes [(alpha-diimine-)Mg+(mu-CH3)](2) via single electron transfer (SET) to the coordinated diimine and elimination of a methyl radical. These biradical species have been characterised by EPR spectroscopy and, for the ligand 1, X-ray crystallography. In the presence of THF the reaction of ligand 1 proceeds to the diamagnetic [(ene-1,2-diamide)Mg(THF)(3)] complex in which the diimine ligand has been doubly reduced to an ene-diamide by two successive SET processes. Comparison of the structural data for the free ligand 1 with that obtained for the a-diimine radical anion and ene-diamide complexes shows the expected increases in C-N, and decreases in C-C, bond lengths within the N-C-C-N unit consistent with the progressive reduction of the ligand. In the case of ligand 3, reaction at low temperature provides the complex [Mg(mu(2)-Me){Ar'NC(Me)(2)C(Me)NAr'}](2) in which methyl transfer to a ligand imine carbon atom has occurred. This species has also been structurally characterised. This contrasts with the formation of the radical species at room temperature, and indicates the involvement of an intermediate in which the radical products of the SET process are held in close proximity by the solvent cage. Two competing processes of methyl radical escape and methyl transfer to the ligand account for the formation of the observed products at different temperatures.