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

• Published in: Journal of the American Chemical Society Vol. 132; no. 14; pp. 5193 - 5204
• Main Authors: Fernández, Ignacio, Oña-Burgos, Pascual, Oliva, Josep M, Ortiz, Fernando López
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 14-04-2010
• Subjects: Computer Simulation; Lithium - chemistry; Lithium Compounds - chemistry; Magnetic Resonance Spectroscopy; Models, Chemical; Molecular Structure; Organometallic Compounds - chemistry; Solutions; Stereoisomerism
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja910556a

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.