High-field lithium-7 nuclear magnetic resonance spectroscopic and cryoscopic relative molecular mass studies on solutions of amido- and imido-lithium compounds

Abstract

High-field lithium-7 n.m.r. spectroscopy and molecular mass measurements are shown to provide an important guideline into the nature of the species present in benzene and toluene solutions of a representative selection of amido- and imido-lithium derivatives. The compounds have solid-state structures ranging from hexameric to tetrameric clusters {[Ph(Bu^t)CNLi]₆(3), [(Me₂N)₂CNLi]₆(4), [Ph(Me₂N)CNLi]₆(5), to (Ph₂CNLi·NC₅H₅)₄(2)(NC₅H₅= pyridine)}, to trimeric and dimeric rings {[(PhCH₂)₂NLi]₃(6) and (Bu^t₂CNLi·hmpa)₂(1)(hmpa = hexamethylphosphoramide), [(PhCH₂)₂NLi·OEt₃]₂(7), with probably n= 2 for [PhN(H)Li·hmpa]_n(8), and [(C₆H₁₁)₂NLi·hmpa]_n(10)}, and to a likely monomer {[(Buⁿ)(C₅H₅N)Li·₂(NC₅H₈)]_n(9)}, yet even more diverse behaviour has been detected in solution. Thus while some of the compounds essentially retain their solid association states [hexameric (3), (4), (5), dimeric (8), and monomeric (9)], others engage in concentration-dependent equilibria also involving monomeric species [tetrameric (2), trimeric (6), dimeric (1) and (10)], and one complex [dimeric (7)] loses donor on dissolution before rearranging to a trimer and a monomer.