ETHEREAL SOLVATION OF LITHIUM HEXAMETHYLDISILAZIDE - UNEXPECTED RELATIONSHIPS OF SOLVATION NUMBER, SOLVATION ENERGY, AND AGGREGATION STATE

Li-6, N-15, and C-13 NMR spectroscopic studies of Li-6-N-15 labeled li thium hexamethyldisilazide ([Li-6, N-15]-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 follo wing: tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 2,2-di methyltetrahydrofuran (2,2-Me(2)THF), diethyl ether (Et(2)O), tert-but yl methyl ether (t-BuOMe), n-butyl methyl ether (n-BuOMe), tetrahydrop yran (THP), methyl isopropyl ether (i-PrOMe), and trimethylene oxide ( oxetane). The ligand exchange is too fast to observe bound and free di isopropyl ether (i-Pr2O), tert-amyl methyl ether (Me(2)(Et)COMe), and 2,2,5,5-tetramethyltetrahydrofuran (2,2,5,5-Me(4)THF). Exclusively dis sociative ligand substitutions occur at low ligand concentrations for all ligands except oxetane. Relative free energies and enthalpies of L iHMDS dimer solvation determined for eight ethereal ligands show an ap proximate inverse correlation of binding energy and ligand steric dema nd. Mixed solvation is found to be non-cooperative showing there exist s little communication between the two lithium sites on the dimer. The different ethereal solvents display a widely varying propensity to ca use formation of LiHMDS monomer. The often-cited correlation of reduce d aggregation state with increasing strength of the lithium-solvent in teraction receives no support whatsoever. The measured free energies o f aggregation display a considerable solvent dependence that is traced to solvent-independent enthalpies of aggregation and solvent-dependen t entropies of aggregation. LiHMDS monomer solvation numbers derive fr om solvent-concentration-dependent monomer:dimer proportions. Moderate ly hindered ethereal solvents afford LiHMDS monomers in trisolvated fo rms ((Me(3)Si)(2)NLiS3) whereas THF and oxetane appear to afford consi derable concentrations of five-coordinate tetrasolvates ((Me(3)Si)(2)N LiS4). The complex relationship between solvation energy and observabl e 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.