Molecular structures of THF-solvated alkali-metal 2,2,6,6-tetramethylpiperidides finally revealed: x-ray crystallographic, DFT, and NMR (including DOSY) spectroscopic studies

Armstrong, David R. and Garcia-Alvarez, Pablo and Kennedy, Alan R. and Mulvey, Robert E. and Robertson, Stuart D. (2011) Molecular structures of THF-solvated alkali-metal 2,2,6,6-tetramethylpiperidides finally revealed: x-ray crystallographic, DFT, and NMR (including DOSY) spectroscopic studies. Chemistry - A European Journal, 17 (24). pp. 6725-6730. ISSN 1521-3765 (https://doi.org/10.1002/chem.201100523)

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Abstract

The often studied THF solvates of the utility alkali-metal amides lithium and sodium 2,2,6,6-tetramethylpiperidide are shown to exist in the solid state as asymmetric cyclic dimers containing a central M2N2 ring and one molecule of donor per metal to give a distorted trigonal planar metal coordination. DFT studies support these structures and confirm the asymmetry in the ring. In C6D12 solution, the lithium amide displays a concentration-dependent equilibrium between a solvated and unsolvated species which have been shown by diffusion-ordered NMR spectroscopy (DOSY) to be a dimer and larger oligomer, respectively. A third species, a solvated monomer, is also present in very low concentration, as proven by spiking the NMR sample with THF. In contrast, the sodium amide displays a far simpler C6D12 solution chemistry, consistent with the solid-state dimeric arrangement but with labile THF ligands.

ORCID iDs

Armstrong, David R., Garcia-Alvarez, Pablo, Kennedy, Alan R. ORCID logoORCID: https://orcid.org/0000-0003-3652-6015, Mulvey, Robert E. ORCID logoORCID: https://orcid.org/0000-0002-1015-2564 and Robertson, Stuart D. ORCID logoORCID: https://orcid.org/0000-0002-9330-8770;
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