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  Study on the heteroatom influence in pyridine-based nitronyl nitroxide biradicals with phenylethynyl spacers on the molecular ground state

Rajadurai, C., Ivanova, A., Enkelmann, V., & Baumgarten, M. (2003). Study on the heteroatom influence in pyridine-based nitronyl nitroxide biradicals with phenylethynyl spacers on the molecular ground state. Journal of Organic Chemistry, 68(26), 9907-9915.

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Rajadurai, C.1, Author           
Ivanova, A.1, Author           
Enkelmann, Volker1, Author           
Baumgarten, Martin1, Author           
Affiliations:
1MPI for Polymer Research, Max Planck Society, ou_1309545              

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 Abstract: Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (2), and monoradicals, 4-(5-bromopyridine-3-ylethynyl)-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (3), 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (4), and 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)pyridine (5), were synthesized and investigated by ESR and UV-vis spectroscopy. The solution EPR measurements of the biradicals gave well-resolved, nine-line spectra with exact half line spacing as compared to monoradicals (giso = 2.0067) with isotropic line spacing \aN\= 7.36 G. This indicates strong, intramolecular exchange coupling (J >> 7 × 10-4 cm-1; J/aNN >> 1) of the biradicals with in the limit of EPR. The temperature dependence on the Δms = ±2 signal intensity of biradicals follow Curie behavior down to 4 K ascertaining the triplet ground state or its near-degeneracy with the singlet state. UV-vis studies of 1-5 show characteristic differences in the extinctions of n-π* transitions around 600 nm. Both biradicals 1 and 2 were crystallized in monoclinic space groups C2/c and P21/a with the intraradical distances 1.54 and 1.47 nm, respectively. Computational studies of the biradicals 1, 2, and 1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]benzene (T) were performed by the AM1/CAS(8,8) method to calculate the singlet-triplet (ΔEST) energy difference and the spin density distribution. Results show that the position of the pyridyl nitrogen in 1 and 2 in comparison with T does not alter the triplet ground-state spin multiplicities supporting the obtained experimental results.

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Language(s): eng - English
 Dates: 2003
 Publication Status: Issued
 Pages: -
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 Rev. Type: -
 Identifiers: eDoc: 60543
Other: P-03-171
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Title: Journal of Organic Chemistry
  Alternative Title : J. Org. Chem.
Source Genre: Journal
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Pages: - Volume / Issue: 68 (26) Sequence Number: - Start / End Page: 9907 - 9915 Identifier: -