Microsolvation and 13C−Li NMR Coupling
Rudolf Knorr,* Thomas Menke, Kathrin Ferchland, Johann Mehlstäubl, and David S. Stephenson
The empirical expression 1JCLi = L[n(a + d)]−1 is proposed; it claims a reciprocal dependence of the NMR coupling constant 1J(13C, Li) in a C−Li compound on two factors: (i) the number n of lithium nuclei in bonding contact with the observed carbanion center and (ii) the sum (a + d) of the numbers a of anions and d of donor ligands coordinated at the Li nucleus that generates the observed 1JCLi value. The expression was derived from integrations of separate NMR resonances of coordinated and free monodentate donor ligands (t-BuOMe, Et2O, or THF) in toluene solutions of dimeric and monomeric 2-(α-aryl-α-lithiomethylidene)-1,1,3,3-tetramethylindan at moderately low temperatures. This unusually slow ligand interchange is ascribed to steric congestion in these compounds, which is further characterized by measurements of nuclear Overhauser correlations and by solid-state structures of the dimers bearing only one donor per lithium atom (d = 1). Increasing microsolvation numbers d are also accompanied by typical changes of the NMR chemical shifts δ (positive for the carbanionic 13Cα, negative for Cpara and p-H). The aforementioned empirical expression for 1JCLi appears to be applicable to other cases of solvated monomeric, dimeric, or tetrameric C−Li compounds (alkyl, alkenyl, alkynyl, and aryl) and even to unsolvated (d ≈ 0) trimeric, tetrameric, or hexameric organolithium aggregates, indicating that 1JCLi might serve as a tool for assessing unknown microsolvation numbers. The importance of obtaining evidence about the 13C NMR C−Li multiplet splitting of both the nonfluxional and fluxional aggregates is emphasized.