Ethereal Solvation of Lithium Hexamethyldisilazide: Unexpected Relationships of Solvation Number, Solvation Energy, and Aggregation State

Abstract

Li, 15N, and I3C NMR spectroscopic studies of 6Li-15N labeled lithium hexamethyldisilazide ((6Li,'5N)- LiHMDS) are reported. Mono-, di-, and mixed-solvated dimers are characterized in the limit of slow solvent exchange for a variety of ethereal ligands including the following: tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHl3 2,2-dimethyltetrahydrofuran (2,2-Me2THF), diethyl ether (EtzO), tert-butyl methyl ether (r-BuOMe), n-butyl methyl ether (n-BuOMe), tetrahydropyran (THP), methyl isopropyl ether (i-PrOMe), and trimethylene oxide (oxetane). The ligand exchange is too fast to observe bound and free diisopropyl ether (i-PrZO), tert-amyl methyl ether (Me2(Et)COMe), and 2,2,5,5-tetramethyltetrahydrofuran (2,2,5,5-Me4THF). Exclusively dissociative ligand substitu- tions occur at low ligand concentrations for all ligands except oxetane. Relative free energies and enthalpies of LiHMDS dimer solvation determined for eight ethereal ligands show an approximate inverse correlation of binding energy and ligand steric demand. Mixed solvation is found to be non-cooperative showing there exists little communication between the two lithium sites on the dimer. The different ethereal solvents display a widely varying propensity to cause formation of LiHMDS monomer. The often-cited correlation of reduced aggregation state with increasing strength of the lithium-solvent interaction receives no support whatsoever. The measured free energies of aggregation display a considerable solvent dependence that is traced to solvent-independent enthalpies of aggregation and solvent-dependent entropies of aggregation. LiHMDS monomer solvation numbers derive from solvent- concentration-dependent monomer:dimer proportions. Moderately hindered ethereal solvents afford LiHMDS monomers in trisolvated forms ((MesSi)zNLiS3) whereas THF and oxetane appear to afford considerable concentrations of five-coordinate tetrasolvates ((Me3Si)zNLiS4), The complex relationship between solvation energy and observable aggregation state is discussed in light of solvent-amide and solvent-solvent interactions on both the monomer and the dimer, the combined contributions of solvation enthalpy and entropy, and the complicating intervention of variable solvation numbers.