Issue 12, 2018

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

Abstract

Lithium amides are versatile C–H metallation reagents with vast industrial demand because of their high basicity combined with their weak nucleophilicity, and they are applied in kilotons worldwide annually. 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 the aggregation of lithium amides to form cubes or ladders from the polar Li–N covalent metal amide bond along the ring stacking and laddering principle. Deaggregation, however, is more governed by the Li←N donor bond to form amine adducts. The geometry of the solid state structures already 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 calculations based on the quantum theory of atoms in molecules (QTAIM) and DFT calculations using energy decomposition analysis (EDA). This new approach allows for the grading of electrostatic Li+N, covalent Li–N and donating Li←N bonding, and provides a way to modify traditional widely-used heuristic concepts such as the −I and +I inductive effects. The electron density ρ(r) and its second derivative, the Laplacian ∇2ρ(r), mirror the various types of 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 an 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−1), followed by the bond dissociation energy (−420.9 kcal mol−1). Partitioning the interaction energy into the Pauli (ΔEPauli), dispersion (ΔEdisp), electrostatic (ΔEelstat) and orbital (ΔEorb) terms gives a 71–72% ionic and 25–26% covalent character of the Li–N bond, different to the old dichotomy of 95 to 5%. In this regard, there is much more potential to steer the reactivity with various substituents and donor solvents than has been anticipated so far.

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

Supplementary files

Article information

Article type
Edge Article
Submitted
18 Dec 2017
Accepted
30 Jan 2018
First published
08 Feb 2018
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2018,9, 3111-3121

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, 9, 3111 DOI: 10.1039/C7SC05368A

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