Promoting π‑Facial Interactions in Phenyl-Substituted 1,8-Bis(silylamido)naphthalene Alkaline Earth Complexes

Bimetallic 1,8-bis(silylamido)naphthalene alkaline earth
complexes
[(R3L)Ae]2 ([R3L]2– = [1,8-{(R3Si)N}2C10H6)]2–, where R3 =
Ph2Me, Ae = Ca (1), Sr (2), and
Ba (3); R3 = Ph3, Ae = Ca (4), Sr (5), and Ba (6) were prepared via protonolysis reactions of the phenyl-substituted proligands Ph3LH2 and Ph2MeLH2 with [AeN″2]2 (N″
= [N(SiMe3)2]−) in benzene.
X-ray crystallographic analysis showed that 1, 2, and 4 crystallize as nitrogen-bridged dimers.
Conversely, 5 and 6 display a naphthalene-bridged
motif, while the structure of 3 is intermediate between
the two distinct classes. NMR spectroscopic analysis of isolated samples
of 1–6 in thf-d8 confirmed their conversion into the monomeric thf-d8 adducts [(R3L)Ae(thf-d8)n]; crystallographic
verification of the structural motif was provided by the X-ray crystal
structure of [(Ph3L)Sr(thf)3] (7). The structural range of dimers 1–6 was influenced by the electron-withdrawing nature of the
phenyl substituents of the ligand and the ability to form “soft”
multihaptic π-facial interactions with the metal ions, which
was preferential for the larger Sr2+ and Ba2+ cations as well as the relative strength of the metal-N bonds. This
has been rationalized through complementary computational studies.
This work provides insight into the structure and bonding preferences
of heavy alkaline earth complexes with rigid bis(amido) ligands.