SOLUTION STRUCTURE OF LITHIUM BENZENESELENOLATE AND LITHIUM DIPHENYLPHOSPHIDE - NMR IDENTIFICATION OF CYCLIC DIMERS AND MIXED DIMERS

The solution structures of lithium benzeneselenoate (PhSeLi) and lithi um diphenylphosphide (Ph(2)PLi) in ether, THF, and mixtures with HMPA were studied by multinuclear NMR techniques. Lithium benzeneselenoate is aggregated in ether, as established by observation of Li-7-Se-77 co upling in both the Li-7 and Se-77 NMR spectra (J(Li-Se) = 18 Hz). It i s probably dimeric, although a cyclic trimer could not be definitively excluded. The titration of PhSeLi with HMPA in ether produces initial ly a triply bridged dimer la ((PhSeLi)(2)(HMPA)(3)) and eventually (wi th excess HMPA) a separated ion pair. In THF the reagent is likely a c ontact monomeric species, as judged by Li and Se delta values and the sequence of ion pair formation during an HMPA titration. A mono HMPA c ontact ion pair and then a separated ion pair species is formed. Lithi um diphenylphosphide is also dimeric in ether. Higher cyclic oligomers could be definitively ruled out by the analysis of the virtually coup led multiplet for the ipsp phenyl carbon signal, resulting from large (>120 Hz) P-P J coupling between the lithium-bridged phosphorus nuclei . In THF, Ph(2)PLi appears to be monomeric; treatment with HMPA leads to formation of a mono HMPA contact ion pair, followed by ion separati on. When PhSeLi and Ph(2)PLi are mixed in ether, a statistical mixture of a heterodimer and the two homodimers is formed. The heterodimer (P hSeLi.Ph(2)PLi) was characterized by C-13, P-31, Li-7, and Se-77 NMR s pectroscopy. It shows coupling between Li and P ((1)J(Li-P) 46 Hz), be tween Li and Se ((1)J(Se-Li) = 18 Hz), and between the lithium-bridged Se and P nuclei ((2)J(Se-P) = 91 Hz). Higher cyclic oligomers can be strictly ruled out for the mixed oligomer on the basis of the NMR spec tra.