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

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.