Lithium Ion Solvation: Amine and Unsaturated Hydrocarbon Solvates of Lithium Hexamethyldisilazide (LiHMDS)

6Li, 15N, and
13C NMR spectroscopic studies of
6Li−15N labeled lithium
hexamethyldisilazide
([6Li,15N]LiHMDS) solvated by more than 20 different mono-, di-, and
trialkylamines (and ammonia) are described. LiHMDS
dimers solvated by the least hindered trialkylamines and most
dialkylamines exchange ligands by a dissociative
mechanism that is sufficiently slow to observe discrete mono-, di-, and
mixed-solvated dimers. Dimers solvated by
the hindered trialkylamines and unhindered monoalkylamines undergo
rapid ligand substitutions by relatively rapid
dissociative and associative mechanisms (respectively). Mono- and
disolvated dimers can be observed for the
monoalkylamines at <1.0 equiv of ligand (per Li). The monomers
that form at elevated trialkylamine concentrations
are suggested to be di- and trisolvated. The relationship between
ligand structure and lithium amide aggregation
state is a complex and sensitive function of amine alkyl substituents.
The dialkylamines prove to be remarkably
similar to dialkyl ethers as ligands for the LiHMDS dimer despite
pronounced differences expected for nitrogen-
and oxygen-based coordination. A greater relative promotion of
monomer formation by the dialkylamines than the
dialkyl ethers can be traced to disproportionate monomer stabilization
by the amines. Hydrocarbon-dependent
aggregation effects are discussed in terms of primary and secondary
shell solvation.