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)2Mg·(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