ISSN:
1432-1327
Keywords:
Key words Porphyrin conformation
;
Oxidation potentials
;
Molecular orbital calculations
;
Heme proteins
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Abstract The chloro-iron(III) complex of the tetraphenylporphyrin tetrabrominated at the antipodal β-pyrrole positions [(7,8,17,18-tetrabromo-5,10,15,20-tetraphenyl)porphyrin] has been synthesized and characterized by spectroscopy and X-ray crystallography. The iron atom is bonded to the chloride ion and the four pyrrole nitrogens. The Fe-Cl bond distance is 2.209(4) Å, and the mean value of the two opposite Fe-Np lengths at the brominated pyrrole rings is 2.079(8) Å, whereas the mean value of the two opposite Fe-Np′ bond distances at the non-brominated pyrrole rings is 2.041(8) Å. The X-ray structure determination and the analysis of the UV-Vis spectra obtained in solution and on thin films indicate that |FeCl(tpp-Br4)| (1) is principally saddle-shaped in the solid state and in solution. Variable-temperature (195–325 K) 1H NMR spectroscopy confirms the high-spin state (S=5/2) of the iron(III) center and indicates that the saddle-shaped conformation of 1 is maintained in solution. EPR spectra obtained in frozen CH2Cl2 solution and in the solid state show a rhombic symmetry with g values of 6.25, 5.70 and 1.99. Kadish et al. have shown that the one-electron oxidation potential of 1 increases only by 0.06 V relative to that of the non-brominated complex |FeCl(tpp)|. The present study indicates that the increase of the first oxidation potential of 1 is related to the non-planar distortion of the porphyrin. Relative to the unbrominated derivative |FeCl(tpp)|, this distortion destabilizes the π system of the macrocycle and thus compensates for the effects of the four electron-withdrawing bromine substituents.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s007750050181
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