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

David R. Armstrong, Pablo Garcia-Alvarez, Alan R. Kennedy, Robert E. Mulvey, Stuart D. Robertson

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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.

LanguageEnglish
Pages6725-6730
Number of pages6
JournalChemistry - A European Journal
Volume17
Issue number24
DOIs
Publication statusPublished - Jun 2011

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Alkali Metals
Alkali metals
Amides
Discrete Fourier transforms
Molecular structure
Nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance
Lithium
X rays
Dimers
Metals
Sodium
Oligomers
Monomers
Ligands
Molecules

Keywords

  • alkali metals
  • amides
  • crystal structures
  • density functional calculations
  • NMR spectroscopy
  • crystal structure
  • mixture analysis
  • orbital methods
  • solid-state
  • open dimers
  • basis sets
  • complexes

Cite this

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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.",
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AU - Garcia-Alvarez, Pablo

AU - Kennedy, Alan R.

AU - Mulvey, Robert E.

AU - Robertson, Stuart D.

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AB - 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.

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KW - crystal structure

KW - mixture analysis

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KW - basis sets

KW - complexes

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