Structure of lithium hexamethyldisilazide in the presence of hexamethylphosphoramide. Spectroscopic and computational studies of monomers, dimers, and triple ions

Abstract

Li and I5N NMR spectroscopic studies of 6Li and I5N isotopically labeled lithium hexamethyldisilazide ( (6Li)LiHMDS and (6Li,15N)LiHMDS) confirm Kimura and Brown's earlier conclusions that LiHMDS exists as a monomer-cyclic dimer mixture in THF, a single cyclic oligomer of indeterminate structure in toluene, and a mixture of two or more cyclic oligomers in pentane.42 I5N zero-quantum NMR spectroscopy provides the distinction of the THF-solvated dimers from the higher oligomers but provides ambiguous results for the cyclic oligomer observed in toluene-dg. Complex equilibria are observed in LiHMDS/HMPA/THF mixtures using one-dimensional 6Li, 15N, and 31P NMR spectroscopy in conjunction with 6Li-15N and 31P-6Li heteronuclear multiple quantum correlation spectroscopic methods. Stepwise solvation of monomers, dimers, and triple ions (e.g., R2N-Li-NRz-//+LiS4) are readily monitored. Excess (>2.0 equiv per Li) HMPA affords a disolvated monomer and a tetrasolvated triple ion as the limiting structures. Enthalpies calculated using semiempirical (MNDO) computational methods reveal trends that are in good qualitative agreement with experiment.