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

Tools

Hevia, Eva and Kennedy, Alan R. and Mulvey, Robert E. and Ramsay, Donna L. and Robertson, Stuart D. (2013) Concealed cyclotrimeric polymorph of lithium 2, 2, 6, 6- tetramethylpiperidide unconcealed : X-ray crystallographic and NMR spectroscopic studies. Chemistry - A European Journal, 19 (42). pp. 14069-14075. ISSN 1521-3765 (https://doi.org/10.1002/chem.201302709)

[thumbnail of Robertson-etal-CEJ2013-concealed-cyclotrimeric-polymorph-of-lithium-2,2,6,6]
Preview
 Text. Filename: Robertson_etal_CEJ2013_concealed_cyclotrimeric_polymorph_of_lithium_2_2_6_6.pdf
Accepted Author Manuscript
Download (869kB)| Preview

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.

ORCID iDs

Hevia, Eva ORCID logoORCID: https://orcid.org/0000-0002-3998-7506, Kennedy, Alan R. ORCID logoORCID: https://orcid.org/0000-0003-3652-6015, Mulvey, Robert E. ORCID logoORCID: https://orcid.org/0000-0002-1015-2564, Ramsay, Donna L. and Robertson, Stuart D. ORCID logoORCID: https://orcid.org/0000-0002-9330-8770;
  • Item type:Article
    ID code:46010
    Dates:
    Date
    Event
    11 October 2013
    Published
    9 September 2013
    Published Online
    Notes: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
    Subjects:Science > Chemistry
    Department:Faculty of Science > Pure and Applied Chemistry
    Depositing user: Pure Administrator
    Date deposited:20 Nov 2013 10:42
    Last modified:20 Nov 2024 01:09
    Related URLs:
    URI:https://strathprints.strath.ac.uk/id/eprint/46010
CORE (COnnecting REpositories)