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-(trifluoromethypphenyl)porphyrin vanadyl bis(hexafluoroacetylacetonate), and chromium(III) tetra-phenylporphyrin chloride. The Cu(II) and vanadyl complexes were studied in fluid solution and frozen solution, and the Co(II) and Cr(IIl) 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)2linkage 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry