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

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posted on 1996-03-06, 00:00 authored by Brett L. Lucht, David B. Collum
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.