Alkali metal and magnesium enamides from metallation of the alkyl ligands [(2-Pyr)(SiMe3)CH2] and [6-Me-(2-Pyr)(SiMe3)CH2]:: a solid state and ab initio study

The alkali metal complexes [rNa . (pmdta)](2), 1, [rK . (pmdta)](2), 2, [rNa . (tmeda)](2), 3, [r'Li . (pmdta)], 4, and the magnesium complex, [(r)(2)Mg . (hmpa)(2)], 5, formed on metallation of the monosilylated ligands [(2-Pyr)(SiMe3)CH2] (=rH) and [6-Me-(2-Pyr)(SiMe3)CH2] (=r'H), rather than being metal alkyls, are all metal enamides in the solid state, as evidenced by single crystal X-ray diffraction studies. Even in the presence of the bi- and tri-dentate donors tmeda (=N,N,N',N'-tetramethylethylenediamine) and pmdta (=N,N,N',N',N " -pentamethyldiethylenetriamine) the heavier alkali metal complexes are found to be dimeric. The Li complex is monomeric and adopts the enamide configuration despite the presence of additional steric bulk on introduction of a Me group on the 6-position of the pyridyl ring. This preference for an enamide configuration, rather than that of carbanion or aza-allyl, has been studied by ab initio MO calculations. These confirmed that as the coordination environment of the metal increases the enamide form rather then the aza-allylic form becomes the most energetically favoured configuration.