Magnesium–Ligand Cooperation in Breaking the O–H and C–H Bonds of Water and Diazoalkane

In our previous paper, we reported that the reaction
of a tridentate
nacnac ligand with a pendant picolyl group, with KHMDS and MgI2, resulted in the formation of a homoleptic hexacoordinate
magnesium compound. Here, we show that the analogous reaction of the
ligand with CH3MgBr led to a heteroleptic magnesium bromide
species (1). Attempts to generate the magnesium hydride
species from 1 led to the dearomatization of the pyridine
ring, and the resulting product was a magnesium hydroxide (3) presumably generated due to an adventitious amount of water. The
reaction of the ligand with nBu2Mg afforded
a unique dearomatized magnesium species (2) in high yield.
Theoretical calculations reveal the presence of a nonbonding orbital
on the magnesium, susceptible to nucleophilic attack. Indeed, the
reaction of 2 with H2O/D2O cleaves
the O–H/D bond via magnesium–ligand cooperation and
generates a magnesium hydroxide (4 and 5). In addition, 2 reacts with Me3SiCHN2 and cleaves the C–H bond to generate another unusual,
well-defined magnesium compound with a bridging isocyanide moiety
(6) via migration of the SiMe3 group from
the carbon to the nitrogen atom. The latter can be described as a
dimer of magnesium isocyanamide. DFT calculations were performed to
understand the electronic structures of the synthesized molecules.