Substituent effects on the choice of the orbital preferred for electron spin delocalization in two paramagnetic low-spin iron(III) porphyrins: Mapping the spin density distribution at the pyrrole positions by 1H COSY and NOESY techniques

Article abstract of DOI:10.1021/ja00092a032

In order to elucidate the distribution of unpaired electron spin density within the porphyrin π orbitals of two unsymmetrically substituted derivatives of (tetraphenylporphyrinato)iron(III), [(p-Cl)(p-NEt₂)₃TPPFe(N-MeIm)₂]-Cl and [(p-NEt₂)(p-Cl)₃TPPFe(N-MeIm)₂]Cl (NEt₂ = diethylamino; N-MeIm = N-methylimidazole), ¹H COSY and NOESY spectra were acquired. The cross peak patterns observed in the 2-D maps of these complexes allow assignment of the proton resonances in the same pyrrole ring (H_a,H_b and H_c,H_d) and of those that are closest to each other in adjacent pyrrole rings (H_b,H_c). Using these assignments, it is possible to delineate the pattern of unpaired electron spin delocalization in the porphyrin ring. This pattern can be unambiguously correlated to the relative electronic effects of the two types of phenyl substituents present, thereby permitting the electron-donating or -withdrawing effects of other substituents to be predicted. In order to test if the pattern of delocalized spin density derived from the 2-D NMR studies can be modeled, simple Hu?ckel molecular orbital calculations were performed. Although only intended to provide a qualitative understanding of the effects of electron-donating and -withdrawing substituents, these most basic calculations provided significant results. Using well-established Hu?ckel parameters, together with lowering or raising the energy of the p_z orbital of one meso-carbon atom to account for the electronic nature of the unique substituent, an electron density distribution was obtained that fully supports the ¹H resonance assignments based on the cross peaks observed in the COSY and NOESY maps of these complexes. This indicates that more sophisticated theoretical calculations may lead to a quantitative description of the effects of porphyrin substituents on the pattern of spin delocalization in synthetic and naturally occurring hemes and their byproducts. In support of this conclusion, it should be noted that even the simplest Hu?ckel calculations described herein have already proved valuable in explaining the pattern of spin delocalization observed in the substrate-bound form of the enzyme heme oxygenase, which catalyzes the stereospecific α-meso bridge cleavage of hemin to yield biliverdin-IXα (Herna?ndez, G.; Wilks, A.; Paolesse, R.; Smith, K. M.; Ortiz de Montellano, P. R.; La Mar, G. N. Biochemistry, in press).