Metal-nitroxyl interactions. 53. Effect of the metal-nitroxyl linkage on the electron-electron exchange interaction in spin-labeled complexes of copper(II), low-spin cobalt(II), vanadyl, and chromium(III)

Kundalika M. More, Gareth R. Eaton, Sandra S. Eaton, Olga H. Hankovszky, K. Hideg

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Abstract

To examine the effect of several metal-nitroxyl linkages on the magnitude of the electron-electron interaction, EPR spectra were obtained for two spin-labeled isoquinolines and nine spin-labeled pyridines bound to copper(II) bis(hexafluoroacetylacetonate), cobalt(II) tetrakis(p-(trifluoromethyl)phenyl)porphyrin, vanadyl bis(hexafluoroacetylacetonate), and chromium(III) tetraphenylporphyrin chloride. The Cu(II) and vanadyl complexes were studied in fluid solution and frozen solution, and the Co(II) and Cr(III) complexes were studied in frozen solution. The values of the metal-nitroxyl electron-electron coupling constant, J, for the isoquinoline complexes were about a factor of 10 smaller than for complexes of spin-labeled pyridines with the same linkage between the heterocyclic ring and the nitroxyl. The differences in the value of J for isomers of the same ligand indicated that the value of J is strongly dependent on the conformation of the pyridine-nitroxyl linkage. Addition of a carbonyl group to the metal-nitroxyl linkage caused the value of J to decrease by a factor of 10-70. When the 2-isomer could not form a chelate ring, the value of J for the copper complex of the 2-isomer was about twice as large as for the 4-isomer. A saturated (CH2)2 linkage to the 2-carbon of the nitroxyl ring resulted in about the same strength of exchange interaction as an unsaturated (CH=CH) linkage to the 3-carbon. When the pyridine was attached directly to the 2-carbon of the nitroxyl ring, the exchange interaction was sufficiently large that only a lower limit on the value of J was obtained. The barrier to rotation about a single bond in four of the ligands was about 5 kcal/mol.

Original languageEnglish
Pages (from-to)1734-1743
Number of pages10
JournalInorganic Chemistry
Volume28
Issue number9
Publication statusPublished - 1989

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Vanadates
Exchange interactions
Chromium
Cobalt
linkages
Copper
chromium
cobalt
Metals
copper
Electrons
pyridines
Isomers
isomers
metals
electrons
Pyridines
rings
interactions
Carbon

ASJC Scopus subject areas

  • Inorganic Chemistry

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Metal-nitroxyl interactions. 53. Effect of the metal-nitroxyl linkage on the electron-electron exchange interaction in spin-labeled complexes of copper(II), low-spin cobalt(II), vanadyl, and chromium(III). / More, Kundalika M.; Eaton, Gareth R.; Eaton, Sandra S.; Hankovszky, Olga H.; Hideg, K.

In: Inorganic Chemistry, Vol. 28, No. 9, 1989, p. 1734-1743.

Research output: Contribution to journalArticle

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abstract = "To examine the effect of several metal-nitroxyl linkages on the magnitude of the electron-electron interaction, EPR spectra were obtained for two spin-labeled isoquinolines and nine spin-labeled pyridines bound to copper(II) bis(hexafluoroacetylacetonate), cobalt(II) tetrakis(p-(trifluoromethyl)phenyl)porphyrin, vanadyl bis(hexafluoroacetylacetonate), and chromium(III) tetraphenylporphyrin chloride. The Cu(II) and vanadyl complexes were studied in fluid solution and frozen solution, and the Co(II) and Cr(III) complexes were studied in frozen solution. The values of the metal-nitroxyl electron-electron coupling constant, J, for the isoquinoline complexes were about a factor of 10 smaller than for complexes of spin-labeled pyridines with the same linkage between the heterocyclic ring and the nitroxyl. The differences in the value of J for isomers of the same ligand indicated that the value of J is strongly dependent on the conformation of the pyridine-nitroxyl linkage. Addition of a carbonyl group to the metal-nitroxyl linkage caused the value of J to decrease by a factor of 10-70. When the 2-isomer could not form a chelate ring, the value of J for the copper complex of the 2-isomer was about twice as large as for the 4-isomer. A saturated (CH2)2 linkage to the 2-carbon of the nitroxyl ring resulted in about the same strength of exchange interaction as an unsaturated (CH=CH) linkage to the 3-carbon. When the pyridine was attached directly to the 2-carbon of the nitroxyl ring, the exchange interaction was sufficiently large that only a lower limit on the value of J was obtained. The barrier to rotation about a single bond in four of the ligands was about 5 kcal/mol.",
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