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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

Andrews, Philip C. and Armstrong, David R. and Raston, Colin L. and Roberts, Brett A. and Skelton, Brian W. and White, Allan H. (2001) 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. Journal of the Chemical Society, Dalton Transactions, 1472 (7). pp. 996-1006.

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Abstract

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.