Correlation of macrocycle distortion with oxidation potentials of iron(III) porphyrins: molecule structure of the sterically crowded chloro-iron(III) 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin

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-N_p lengths at the brominated pyrrole rings is 2.079(8) Å, whereas the mean value of the two opposite Fe-N_p′ 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.