Lithium ion solvation: Amine and unsaturated hydrocarbon solvates of lithium hexamethyldisilazide (LiHMDS)

Li-6, N-15, and C-13 NMR spectroscopic studies of Li-6-N-15 labeled lithium hexamethyldisilazide ([Li-6,N-15]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.