Concealed cyclotrimeric polymorph of lithium 2, 2, 6, 6- tetramethylpiperidide unconcealed: X-ray crystallographic and NMR spectroscopic studies

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Abstract

Lithium 2, 2, 6, 6-tetramethyl-piperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cy-clotetrameric (LiTMP) solid-state structure. Made by a new approach through transmetalation of Zn(TMP) with tBuLi in n-hexane solution, a crystalline polymorph of LiTMP has been uncovered. X-ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P6/m and exhibited a discrete cyclotrimeric (C) structure with a strictly planar (LiN) ring containing three symmetrically equivalent TMP chair-shaped ligands. The molecular structure of (LiTMP) was redeter-mined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN) ring possessing pseudo (non-crystallographic) C symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low-temperature (freezer at - 35°C) cooling of the reaction solution favoured (LiTMP); whereas high-temperature (bench) storage favoured (LiTMP). Routine H and C NMR spectroscopic studies in a variety of solvents showed that (LiTMP) and (LiTMP) exist in equilibrium, whereas H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes. 
LanguageEnglish
Pages14069-14075
Number of pages7
JournalChemistry - A European Journal
Volume19
Issue number42
Early online date9 Sep 2013
DOIs
Publication statusPublished - 11 Oct 2013

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Polymorphism
Lithium
Nuclear magnetic resonance
X rays
Thymidine Monophosphate
Crystallization
lithium 2,2,6,6-tetramethylpiperidide
Temperature
Organometallics
Molecular structure
Ligands
Cooling
Chemical analysis

Keywords

  • amides
  • crystal structures
  • lithium
  • NMR spectroscopy
  • polymorphism

Cite this

@article{4f1a9848781c4eb498c82f9e3de3e40b,
title = "Concealed cyclotrimeric polymorph of lithium 2, 2, 6, 6- tetramethylpiperidide unconcealed: X-ray crystallographic and NMR spectroscopic studies",
abstract = "Lithium 2, 2, 6, 6-tetramethyl-piperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cy-clotetrameric (LiTMP) solid-state structure. Made by a new approach through transmetalation of Zn(TMP) with tBuLi in n-hexane solution, a crystalline polymorph of LiTMP has been uncovered. X-ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P6/m and exhibited a discrete cyclotrimeric (C) structure with a strictly planar (LiN) ring containing three symmetrically equivalent TMP chair-shaped ligands. The molecular structure of (LiTMP) was redeter-mined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN) ring possessing pseudo (non-crystallographic) C symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low-temperature (freezer at - 35°C) cooling of the reaction solution favoured (LiTMP); whereas high-temperature (bench) storage favoured (LiTMP). Routine H and C NMR spectroscopic studies in a variety of solvents showed that (LiTMP) and (LiTMP) exist in equilibrium, whereas H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes. ",
keywords = "amides, crystal structures, lithium, NMR spectroscopy, polymorphism",
author = "Eva Hevia and Kennedy, {Alan R.} and Mulvey, {Robert E.} and Ramsay, {Donna L.} and Robertson, {Stuart D.}",
note = "This is the accepted version of the following article: Concealed cyclotrimeric polymorph of lithium 2, 2, 6, 6- tetramethylpiperidide unconcealed: X-ray crystallographic and NMR spectroscopic studies Hevia, E., Kennedy, A. R., Mulvey, R. E., Ramsay, D. L. and Robertson, S. D. (2013), Concealed Cyclotrimeric Polymorph of Lithium 2,2,6,6-Tetramethylpiperidide Unconcealed: X-Ray Crystallographic and NMR Spectroscopic Studies. Chem. Eur. J., 19: 14069–14075. doi: 10.1002/chem.201302709",
year = "2013",
month = "10",
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TY - JOUR

T1 - Concealed cyclotrimeric polymorph of lithium 2, 2, 6, 6- tetramethylpiperidide unconcealed

T2 - Chemistry - A European Journal

AU - Hevia, Eva

AU - Kennedy, Alan R.

AU - Mulvey, Robert E.

AU - Ramsay, Donna L.

AU - Robertson, Stuart D.

N1 - This is the accepted version of the following article: Concealed cyclotrimeric polymorph of lithium 2, 2, 6, 6- tetramethylpiperidide unconcealed: X-ray crystallographic and NMR spectroscopic studies Hevia, E., Kennedy, A. R., Mulvey, R. E., Ramsay, D. L. and Robertson, S. D. (2013), Concealed Cyclotrimeric Polymorph of Lithium 2,2,6,6-Tetramethylpiperidide Unconcealed: X-Ray Crystallographic and NMR Spectroscopic Studies. Chem. Eur. J., 19: 14069–14075. doi: 10.1002/chem.201302709

PY - 2013/10/11

Y1 - 2013/10/11

N2 - Lithium 2, 2, 6, 6-tetramethyl-piperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cy-clotetrameric (LiTMP) solid-state structure. Made by a new approach through transmetalation of Zn(TMP) with tBuLi in n-hexane solution, a crystalline polymorph of LiTMP has been uncovered. X-ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P6/m and exhibited a discrete cyclotrimeric (C) structure with a strictly planar (LiN) ring containing three symmetrically equivalent TMP chair-shaped ligands. The molecular structure of (LiTMP) was redeter-mined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN) ring possessing pseudo (non-crystallographic) C symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low-temperature (freezer at - 35°C) cooling of the reaction solution favoured (LiTMP); whereas high-temperature (bench) storage favoured (LiTMP). Routine H and C NMR spectroscopic studies in a variety of solvents showed that (LiTMP) and (LiTMP) exist in equilibrium, whereas H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes. 

AB - Lithium 2, 2, 6, 6-tetramethyl-piperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cy-clotetrameric (LiTMP) solid-state structure. Made by a new approach through transmetalation of Zn(TMP) with tBuLi in n-hexane solution, a crystalline polymorph of LiTMP has been uncovered. X-ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P6/m and exhibited a discrete cyclotrimeric (C) structure with a strictly planar (LiN) ring containing three symmetrically equivalent TMP chair-shaped ligands. The molecular structure of (LiTMP) was redeter-mined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN) ring possessing pseudo (non-crystallographic) C symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low-temperature (freezer at - 35°C) cooling of the reaction solution favoured (LiTMP); whereas high-temperature (bench) storage favoured (LiTMP). Routine H and C NMR spectroscopic studies in a variety of solvents showed that (LiTMP) and (LiTMP) exist in equilibrium, whereas H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes. 

KW - amides

KW - crystal structures

KW - lithium

KW - NMR spectroscopy

KW - polymorphism

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JO - Chemistry - A European Journal

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