Synthesis, structure and DFT analysis of the THF solvate of 2-picolyllithium : a 2-picolyllithium solvate with significant carbanionic character

Brouillet, Etienne V. and Kennedy, Alan R. and Krämer, Tobias and Mulvey, Robert E. and Robertson, Stuart D. and Stewart, Alexander and Towie, Stephen (2020) Synthesis, structure and DFT analysis of the THF solvate of 2-picolyllithium : a 2-picolyllithium solvate with significant carbanionic character. Zeitschrift fur Anorganische und Allgemeine Chemie, 646 (13). pp. 726-733. ISSN 0044-2313 (https://doi.org/10.1002/zaac.202000006)

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Abstract

Previous studies of different solvates of 2-methylpyridyllithium (2-picolyllithium) have uncovered electronic structures corresponding to aza-allyl and enamido resonance forms of the metallated pyridine-based compounds. Here, we report the synthesis and characterisation of [2-CH2Li (THF)2C5H4N], a new THF solvate. X-ray crystallographic studies reveal a dimeric arrangement featuring a non-planar eight-membered [NCCLi]2ring, in which the primary cation-anion interaction is between the Li centre and the C atom of the deprotonated methyl group [length, 2.285(2) Å], suggesting a new carbanionic resonance structure for this 2-picolyllithium series. The significant carbanionic character of [2-CH2Li (THF)2C5H4N] was confirmed by gas-phase DFT calculations [B3LYP/6-311+G(d)] with the calculated electron density interrogated by means of quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. For comparison these computational analyses were also performed on the literature structures of[2-CH2Li(2-Picoline) C5H4N] and [2-CH2Li (PMDETA)C5H4N]. In a reactivity study, [2-CH2Li (THF)2C5H4N] was found to undergo nucleophilic addition to pyridine to generate dipyridylmethane in a good yield.

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