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Benchmarking lithium amide versus amine bonding by charge density and energy decomposition analysis arguments

Abstract

Lithium amides are versatile C–H metallation reagents of vast industrial demand because of their high basicity combined with weak nucleophilicity, applied worldwide annually in kilotons. The nuclearity of lithium amides, however, modifies and steers reactivity, region- and stereo-selectivity and product diversification in organic syntheses. In this regard, it is vital to understand Li–N bonding as it causes aggregation of lithium amides to cubes or ladders along the ring stacking and laddering principal from the polar Li–N covalent metal amide bond as well as deaggregation from the Li←N donor bond to amines. Already the geometry of the solid-state structures suggests that there is σ- and π-contribution to the covalent bond. To quantify the mutual influence we investigated [{(Me2NCH2)2(C4H2N)}Li]2 (1) by means of experimental charge density investigation based on the Quantum Theory of Atoms in Molecules (QTAIM) and DFT calculations based on energy decomposition analysis (EDA). This new approach allows grading of electrostatic Li+N–, covalent Li–N and donating Li←N bonding and provides a handle to modify traditional widely-used heuristic concepts like e. g. the –I and +I inductive effects. The electron density ρ(r) and its second derivative, the Laplacian ∇^2 ρ(r), mirror the various bonding. Most remarkably, from the topological descriptors, there is no clear separation of the lithium amide bonds from the lithium amine donor bonds. The computed Natural Partial Charges for lithium are only +0.58, indicating optimal density supply from the four nitrogen atoms while the Wiberg bond orders of about 0.14 au suggest very weak bonding. The interaction energy between the two pincer molecules (C4H2N)22- with the Li22+ moiety is very strong (ca. -628 kcal/mol) followed by the bond dissociation energy (-420.9 kcal/mol). Partitioning of the interaction energy into the Pauli (ΔEPauli) dispersion (ΔEdisp), electrostatic (ΔEelsta) and orbital (ΔEorb) terms gives a 71-72 % ionic and 25-26 % covalent character of the Li–N, different to the old dichotomy of 95 to 5 %. In this light, there is much more potential to steer reactivity with various substituents and donor solvents than anticipated so far.

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Publication details

The article was received on 18 Dec 2017, accepted on 30 Jan 2018 and first published on 08 Feb 2018


Article type: Edge Article
DOI: 10.1039/C7SC05368A
Citation: Chem. Sci., 2018, Accepted Manuscript
  • Open access: Creative Commons BY-NC license
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    Benchmarking lithium amide versus amine bonding by charge density and energy decomposition analysis arguments

    F. Engelhardt, C. Maaß, D. M. Andrada, R. Herbst-Irmer and D. Stalke, Chem. Sci., 2018, Accepted Manuscript , DOI: 10.1039/C7SC05368A

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