MIXED-DIMER FORMATION BETWEEN DIMERIC LITHIUM 2,6-DIMETHYLPHENOLATE AND THE LITHIUM-SALTS OF STRONG ACIDS

The C-13, Li-7, and Li-6 NMR spectra of lithium 2,6-dimethyl-, 4-bromo-2,6-dimethyl-, and 2,4,6-tribromophenolates in 1,3-dioxolane and THF and in the presence of LiClO4, LiBPh4, LiCl, and lithium p-toluenesulfonate are reported for a wide range of temperature and concentration. It is established that all systems give rise to mixed dimers. The extent of mixed-dimer formation is shown to depend on the basicity of the phenolate ion, the cation solvation power of the solvent, and the nature of the anion. DELTA-H (kcal mol-1) and DELTA-S (cal mol-1 deg-1) for lithium 2,6-dimethylphenolate are 0, 14 (LiClO4/dioxolane); 0, 7 (LiClO4/THF); and -1.2, -1.3 (LiBPh4/dioxolane), respectively. The C-13(para) chemical shifts for mixed dimers of lithium 2,6-dimethylphenolate are strongly dependent on the nature of the anion. The structures of the mixed aggregates are postulated to be ion pairs of the type [Li2P]+X- that are contact, partially solvent separated, or fully solvent separated depending on the nature of X and the solvent. Exchange of X- between Li2PX and free salt is rapid on the C-13 NMR time scale at -100-degrees-C. Exchange of phenolate between dimeric phenolate and mixed dimer is a slower process that appears to involve the intermediacy of monomeric lithium phenolate.