CORRELATION OF MACROCYCLE DISTORTION WITH OXIDATION POTENTIALS OF IRON(III) PORPHYRINS - MOLECULAR-STRUCTURE OF THE STERICALLY CROWDED CHLORO-IRON(III) 8,17,18-TETRABROMO-5,10,15,20-TETRAPHENYLPORPHYRIN

The chloro-iron(III) complex of the tetraphenylporphyrin tetrabrominat ed at the antipodal beta-pyrrole positions 17,18-tetrabromo-5,10,15,20 -tetraphenyl)porphyrin] has been synthesized and characterized by spec troscopy and X-ray crystallography. The iron atom is bonded to the chl oride ion and the four pyrrole nitrogens. The Fe-Cl bond distance is 2 .209(4) Angstrom, and the mean value of the two opposite Fe-N-p length s at the brominated pyrrole rings is 2.079(8) Angstrom, whereas the me an value of the two opposite Fe-N-p' bond distances at the non-bromina ted pyrrole rings is 2.031(8) Angstrom. The X-ray structure determinat ion and the analysis of the UV-Vis spectra obtained in solution and on thin films indicate that \FeCl(tpp-Br-4)\ (1) is principally saddle-s haped in the solid state and in solution. Variable-temperature (195-32 5 K) H-1 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 sol ution 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 ox idation potential of 1 increases only by 0.06 V relative to that of th e non-brominated complex \FeCl(tpp)\. The present study, indicates tha t the in crease of the first oxidation potential of 1 is related to th e non-planar distortion of the porphyrin. Relative to the unbrominated derivative \FeCl(tpp)\, this distortion destabilizes the rr system of the macrocycle and thus compensates for the effects of the four elect ron-withdrawing bromine substituents.