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

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