STRUCTURE OF LITHIUM HEXAMETHYLDISILAZIDE (LIHMDS) IN THE PRESENCE OF HEXAMETHYLPHOSPHORAMIDE (HMPA) - SPECTROSCOPIC AND COMPUTATIONAL STUDIES OF MONOMERS, DIMERS, AND TRIPLE IONS

Li-6 and N-15 NMR spectroscopic studies of Li-6 and N-15 isotopically labeled lithium hexamethyldisilazide ([Li-6]LiHMDS and [Li-6, N-15]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 N-15 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-d8. Complex equilibria are observed in LiHMDS/HMPA/THF mixtures using one-dimensional Li-6, N-15, and P-31 NMR spectroscopy in conjunction with Li-6-N-15 and P-31-Li-6 heteronuclear multiple quantum correlation spectroscopic methods. Stepwise solvation of monomers, dimers, and triple ions [e.g., R2N-Li-NR2-//+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.