Synthetic, Structural, Mechanistic, and Theoretical MO Studies of the Alkali-Metal Chemistry of Dibenzylamine and Its Transformation to 1,3-Diphenyl-2-azaallyl Derivatives

Philip C. Andrews, David R. Armstrong, Daniel R. Baker, Robert E. Mulvey, William Clegg, Lynne Horsburgh, Paul A. O'Neil, David Reed

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Dibenzylamido anions ((PhCH2)2N-) can be transformed into 1,3-diphenyl-2-azaallyl anions by the assistance of PMDETA- ((Me2NCH2CH2)2NMe) complexed Li+, Na+, or K+ cations. The heavier alkali-metal cations give only the trans, trans conformation of the azaallyl anion, in contrast to the lighter Li+ cation, which yields two crystalline conformers, the trans, trans and an unknown species. Ab initio MO geometry optimizations on model Li and Na complexes intimate that it is the relative tightness of the contact ion pair structures which dictates this distinction with Li+ having more influence on the conformation and stability of the anion than Na+, which forms a much looser contact ion pair more akin to the “free” anion. On the basis of kinetic 1H NMR studies, combined with X-ray crystallographic data, the amido → azaallyl conversion can be explained in terms of a two-step process involving β-elimination of a metal hydride followed by hydride metalation of the produced imine PhCH2N=C(H)Ph. This process appears to be initiated by deaggregation of the metallodibenzylamine to an intermediate monomeric structure, accomplished by solvation. The nature and degree of solvation required depend on the particular M+ cation involved. Three new crystal structures are revealed in the course of this study. All are based on familiar four-membered (N-M)2 rings, but whereas the sodium complex [{(PhCH2)2NNa·TMEDA}2] and the lithium complex [{(PhCH2)2NLi·THF2] are both discrete dimers, unique [{[(PhCH2)2NLi·(dioxane)}], isolated as its toluene hemisolvate, is a polymer composed of linked dimeric units and so is the first dibenzylamido alkali-metal species to have an infinitely extended structure.

Original languageEnglish
Pages (from-to)427-439
Number of pages13
Issue number1
Publication statusPublished - 1 Jan 1995
Externally publishedYes

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