Potassium Hexamethyldisilazide (KHMDS): Solvent-Dependent Solution Structures

Abstract

Solution structures of potassium hexamethyldisilazide [KHMDS] and labeled [ 15 N]KHMDS were examined using a number of analytical methods including 29 Si NMR spectroscopy and density functional theory computations. A combination of 15 N− 29 Si couplings, 29 Si chemical shifts, and the method of continuous variations reveals dimers, monomers, and ion pairs. Weakly coordinating monofunctional ligands such as toluene, N,N-dimethylethylamine, and Et 3 N afford exclusively dimers. 1,3-Dioxolane, THF, dimethoxyethane, hexamethylphosphoramide, and diglyme provide dimers at low ligand concentrations and monomers at high ligand concentrations. N,N,N′,N'-Tetramethylethylenediamine and N,N,N′,N'-tetramethylcyclohexanediamine provide exclusively dimers at all ligand concentrations at ambient temperatures and significant monomer at −80 °C. Studies of 12-crown-4 ran into technical problems. Equimolar 15crown-5 forms a dimer, whereas excess 15-crown-5 affords a putative ion pair. Whereas equimolar 18-crown-6 also affords a dimer, an excess provides a monomer rather than a solvent-separated ion pair. [2.2.2]cryptand affords what is believed to be a contact-ionpaired cryptate. Solvation was probed using largely density functional theory (DFT) computations. Thermally corrected energies are consistent with lower aggregates and higher solvates at low temperatures, but the magnitudes of the computed temperature dependencies were substantially larger than the experimentally derived data.