Mechanism of Alkyl Migration in Diorganomagnesium 2,6-Bis(imino)pyridine Complexes: Formation of Grignard-Type Complexes with Square-Planar Mg(II) Centers

Abstract

Dialkylmagnesium compounds [MgR2L2] (R = n-Bu, L = none or R = Bn, L = THF) react with 2,6-bis(imino)pyridines (BIP) to afford different types of Mg(II) alkyl complexes, depending on the nature of R. For R = n-Bu, thermally stable products resulting from selective alkyl transfer to the pyridine nitrogen (N1) atom are obtained. However, NMR studies showed that the reaction of [Mg(Bn)2THF2] with iPrBIP at −65 °C leads to a thermally unstable product arising from benzyl migration to position C2 in the pyridine ring. Above +5 °C, this compound rearranges, cleanly yielding a mixture of two isomeric complexes, in which the benzyl group has migrated to positions C3 or C4 of the central ring, respectively. Similar isomeric mixtures were obtained when [Mg(Bn)2THF2] was reacted with iPrBIP or MesBIP at room temperature. Such mixtures are thermally stable below 80 °C, but at this temperature, the 3-benzyl isomer converts into the thermodynamically favored 4-benzyl product, albeit not quantitatively. An alternate route was devised for the selective syntheses of the latter type of compounds. The X-ray diffraction structure of one of them provided an unusual example of a square-planar alkylmagnesium(II) center.