Solution and Computed Structure of o-Lithium N,N-Diisopropyl-P,P-diphenylphosphinic Amide. Unprecedented Li−O−Li−O Self-Assembly of an Aryllithium

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• 作者：Ignacio Fernndez ; Pascual Oa-Burgos ; Josep M. Oliva ; Fernando Lpez Ortiz
• 刊名：Journal of the American Chemical Society
• 出版年：2010
• 出版时间：April 14, 2010
• 年：2010
• 卷：132
• 期：14
• 页码：5193-5204
• 全文大小：444K
• 年卷期：v.132,no.14(April 14, 2010)
• ISSN：1520-5126

文摘

The structural characterization of an ortho-lithiated diphenylphosphinic amide is described for the first time. Multinuclear magnetic resonance (¹H, ⁷Li, ¹³C, ³¹P) studies as a function of temperature and concentration employing 1D and 2D methods showed that the anion exists as a mixture of one monomer and two diastereomeric dimers. In the dimers the chiral monomer units are assembled in a like and unlike manner through oxygen−lithium bonds, leading to fluxional ladder structures. This self-assembling mode leads to the formation of Li₂O₂ four-membered rings, a structural motif unprecedented in aryllithium compounds. DFT computations of representative model compounds of ortho-lithiated phosphinic amide monomer and Li₂C₂ and Li₂O₂ dimers with different degrees of solvation by THF molecules showed that Li₂O₂ dimers are thermodynamically favored with respect to the alternative Li₂C₂ structures by 4.3 kcal mol⁻¹ in solvent-free species and by 2.3 kcal mol⁻¹ when each lithium atom is coordinated to one THF molecule. Topological analysis of the electron density distribution revealed that the Li₂O₂ four-membered ring is characterized by four carbon−lithium bond paths and one oxygen−oxygen bond path. The latter divides the Li−O−Li−O ring into two Li−O−Li three-sided rings, giving rise to two ring critical points. On the contrary, the bond path network in the Li₂C₂ core includes a catastrophe point, suggesting that this molecular system can be envisaged as an intermediate in the formation of Li₂O₂ dimers. The computed ¹³C chemical shifts of the C−Li carbons support the existence of monomeric and dimeric species containing only one C−Li bond and are consistent with the existence of tricoordinated lithium atoms in all species in solution.