Lithium Diisopropylamide Solvated by Monodentate and Bidentate Ligands: Solution Structures and Ligand Binding Constants

6Li and 15N NMR spectroscopic studies of lithium diisopropylamide ([6Li]LDA and [6Li,15N]LDA) in toluene/pentane solutions containing a variety of monoand polydentate ligands are reported. LDA forms exclusively dimers in the presence of n-BuOMe, Et2O, t-BuOMe, THF, 2-methyltetrahydrofuran, 2,2-dimethyltetrahydrofuran, tetrahydropyran, dimethoxyethane, N,N,N′,N′-tetramethylethylenediamine, and MeOCH2CH2NR2 (NR2 ) NMe2, NEt2, pyrrolidino). Addition of 1,2-dipyrrolidinoethane and (2-pyrrolidinoethyl)dimethylamine provides monomer-dimer mixtures. Treatment of LDA with trans-N,N,N′,N′-tetramethylcyclohexanediamine (TMCDA) or trans-1-(dimethylamino)-2-isopropoxycyclohexane in hydrocarbons afford exclusively monomers. Sparteine binds only reluctantly, giving a mixture of unsolvated oligomers and monomer. Competitions of the ethereal ligands vs TMCDA afford binding constants and associated free energies for dimer solvation which are correlated with those obtained previously for lithium hexamethyldisilazide.