Determination of Structures of Solvated Lithium Dialkylamides by Semiempirical

MNDO computational studies of lithium dimethylamide (Me,NLi), lithium diisopropylamide (LDA), and lithium 2,2,6,6-tetramethylpiperidide (LiTMP) solvated by HzO, Me20, THF, phosphoramide ((NH,),P=O), and hexamethylphosphoramide (HMPA) are described. The relative stabilities of monomers, dimers, trimers, open dimers, and anionic triple ions reveal the steric and electronic factors that influence aggregation state. The calculations support previous suggestions that coordinated solvents destabilize cyclic trimers relative to cyclic dimers. The cyclic dimers appear to be most stable the disolvated form (one solvent per lithium). The increased steric demands of N-alkyl moieties cause decreased stabilities of cyclic dimers relative to the monomers, open dimers, and triple ions. The optimized structures of open dimers and triple ions reveal relative rotations of the amide fragments and increased internal N-Li-N bond angles consistent with considerable relief of steric interactions. Direct comparisons between calculated structures of various HMPA and THF solvates of LDA and LiTMP with previously described solution structure studies reveal excellent correlations of theory and experiment.

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