Tripodal Amido Boron and Aluminum Complexes

The synthesis and structural elucidations of novel boron and aluminum complexes incorporating the tripodal triamido
[N3]3- ligand framework that is hypothesized to promote the preorganized pyramidal geometry for high Lewis acidity
are reported. Salt metathesis between the in situ-generated trianionic lithium complexes of the tripodal amido
ligands with BCl3 leads to boranes HC[SiMe2N(4-MeC6H4)]3B (1) and MeSi[SiMe2N(4-MeC6H4)]3B (2); however,
substitution of the N-Ar group with the bulky tBu affords the unexpected non-boron-containing LiCl adduct {[HC(SiMe2NtBu)2(SiMeNtBu)]Li3(Et2O)Cl}2 (3) via apparent elimination of MeBCl2. The products derived from the salt
metathesis reaction with AlCl3 are determined by the reaction medium:  while the reaction in a hexanes−ether
mixture or toluene affords solvated salt adduct HC[SiMe2N(4-MeC6H4)]3Al·ClLi(Et2O)2 (4) or salt adduct HC[SiMe2N(4-MeC6H4)]3Al·ClLi (5), respectively; the addition of a small amount of THF produces a mixture of complexes
HC[SiMe2N(4-MeC6H4)]3Al·(THF) (6, major) and HC[SiMe2N(4-MeC6H4)]3Al(OCHCH2)·Li(THF)2 (7, minor). The
desired complex 6 can be exclusively formed using HC[SiMe2N(4-MeC6H4)]3Li3·(THF)3 and the hexanes−ether
mixture solvent. The molecular structures of complexes 1, 3, 5, 6, and 7 have been elucidated by X-ray diffraction
studies. The structure of 1 shows an approximately trigonal pyramidal geometry at B with no significant N−B p−p
π-interactions. The strong salt adduct and solvate formation of the tripodal amido Al complex, as well as its similarity
to the strong Lewis acid Al(C6F5)3 in the THF adduct and enolaluminate formation and structure, indicate the
desired core structure [N3]Al is indeed highly Lewis acidic.