s‑Block Metal Dibenzoazepinate Complexes: Evidence for Mg–Alkene Encapsulation

The dibenzo­[b,f]­azepinate (DBAP)
complexes (DBAP)­Li·(THF)3, (DBAP)2Mg·(THF)2, and (DBAP)2Ca·(THF)3 could be
isolated as highly air-sensitive compounds in yields of 93%, 72%, and 48%, respectively. Crystal structures
of these THF adducts reveal monomeric complexes in which the degree
of ring puckering depends on the nature of the metal. The most extreme
deviation from planarity is found for the most covalent bound metal,
Mg, but in all cases no interaction between the metal and the azepine
CC bond is observed. The THF-free complex [(DBAP)2Mg]2, which could be obtained in 77% yield, crystallizes
as an unusual dimer with three bridging and one terminal DBAP ligand.
The bridging DBAP ligands are highly bent and span a cavity in which
a Mg2+ ion is bound through three alkene–Mg interactions
with an average Mg···C distance of 2.794(3) Å.
Theoretical calculations support these contacts. A combination of
AIM and NPA analyses shows polarization of the alkene π-electron
density toward the metal (vertical polarization) but also demonstrates
a polarization of electron density toward the C atom closest to Mg
(horizontal polarization). Such metal–alkene interactions and
implicit CC bond polarization are key features in main group
metal catalyzed alkene conversions.