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

Abstract

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.