A NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIC TECHNIQUE FOR THE CHARACTERIZATION OF LITHIUM ION-PAIR STRUCTURES IN THF AND THF HMPA SOLUTION/

Lithium cations coordinated by HMPA undergo sufficiently slow dynamic exchange on the NMR time scale at low temperature that distinct cation -HMPA complexes can usually be observed. This observation is the basis for a new technique to quickly and unambiguously determine the ion pa ir structure of lithium reagents in THF and THF/HMPA solution using lo w-temperature Li-7 and P-31 NMR. In this paper we describe the procedu res for assigning structures to HMPA-complexed separated and contact l ithium cations. The application of this technique to ion pair structur e determination of several lithium reagents is also presented. The HMP A titrations of fluorenyllithium, trityllithium, and lithium triphenyl mercurate (Ph3HgLi) in THF solution give similar results. All are solv ent-separated ion pairs in THF. The progressive coordination of four H MPA molecules to the lithium cation can be observed in the Li-7 and P- 31 NMR through characteristic chemical shifts and 2J(Li-P) scalar coup ling. Under the same conditions LiI, LiBr, and MeSeLi are contact ion pairs and become almost completely solvent-separated upon addition of 3 equiv of HMPA. Our results show that MeSLi and LiCl are aggregated i n THF, and the addition of HMPA not only breaks down the dimers to mon omers but also causes ion separation on addition of 6 equiv of HMPA. P henyllithium can be deaggregated from dimer to monomer, but even a hig h concentration of HMPA fails to cause significant ion pair separation . The tetrameric methyllithium, on the other hand, undergoes no visibl e dissociation to dimers and monomers on addition of HMPA; only coordi nation of the four corners of the methyllithium tetrahedron with HMPA is observed.