To obtain a better understanding of the effects of ortho-halogen and related substituents on the phenyl rings of TPPH2 and (TPP)Fe(III) complexes, a series of unsymmetrically substituted tetraphenylporphyrins have been synthesized. In each of these complexes one phenyl ring bears halogen(s) on one (or both) ortho position(s), while the other three phenyl rings carry para-methoxy substituents. The free-base porphyrins were characterized by UV−visible and 1H NMR spectroscopy. The resonance of the pyrrole protons closest to the phenyl ring bearing the ortho substituent, Ha, of the free-base porphyrins shows a progressive shift to higher shielding as the atomic radius and Hammett substituent constant σ p of the substituent increases. However, the fact that 2,6-substitution has a similar effect as 2-substitution suggests that size effects are more important than through-bond electronic effects. Equilibrium constants, β2 III, for addition of N-methylimidazole to the series of complexes (o-X)(p-OCH3)3(TPP)FeCl and (2,6-X2)(p-OCH3)3(TPP)FeCl were measured in chloroform at 25 °C. log(β2 III) increases in the order F < Cl < Br < I < F2 < CF3 < Cl2 < Br2, and all β2 III values for mixed substituent porphyrins except the monofluoro-containing complex are larger than the β2 III for the reaction between the symmetrical non-ortho-substituted parent compound, (p-OCH3)4(TPP)FeCl, and NMeIm. Hence, ortho-halogen and -CF3 substituents increase the values of β2 III in order of increasing size, with the 2,6-disubstituted phenyls causing an increase of β2 III by more than a factor of 2 over that for 2-substituted phenyls. In the strongly solvating solvent dimethylformamide, where dissociation of the anion has already taken place, the opposite order of log(β2 III) is observed (F > CF3 > Cl2 ≫ p-OCH3). Both sets of equilibrium constant data suggest that ortho-halogens and -CF3 groups are electron donating, which we believe is due to direct overlap between the electron clouds of the halogens and the π system of the porphyrin. This direct overlap of electron clouds decreases the Lewis acidity of Fe(III), both making it easier for Cl- to dissociate from the Fe+Cl- starting material in CHCl3 and making the complex less stable; the former contribution is more important in CHCl3, while the latter becomes evident in DMF. The 1H NMR spectra of the same series of low-spin Fe(III) complexes show a decrease in the spread of the pyrrole-H resonances in the order of Cl2 > F2 > Br2 ≫ F > Cl > Br ∼ CF3 ≫ I. This order suggests that the apparent electron-donating characteristics of the substituents decrease in the listed order, which is quite different from that derived from the values of log(β2 III) measured in chloroform solution. The differences probably result from the fact that these two physical properties are sensing different effects of the ortho substituents:  log(β2 III) values probe changes in the σ basicity of the pyrrole nitrogens and hence the Lewis acidity of the metal, while the spread of pyrrole-H resonances probes changes in the π el...

As part of an effort to assign completely the 'H resonances of the pyrrole protons of model haems, the NOESY spectra of a series of unsymmetrically phenyl-substituted derivatives of a low-spin paramagnetic haem model complex tetraphenylporphyrinatoiron(II)bis(N-methylimidazole),[ TPPFe(N-MeIm), 1 +, for which the COSY spectra have been reported previously, were recorded in CD,CI, or CDCI, at temperatures of 30 to -35 O C using a variety of mixing times. Four phenomena of importance to the understanding of the cross-correlation patterns obtained in the NOESY spectra of these paramagnetic complexes were observed: (1) NOE cross peaks can be detected for these intermediate molecular weight complexes, even in non-viscous solvents; (2) for some complexes only one set of NOE cross peaks different from those previously observed in the COSY maps were obtained, whereas for other complexes two sets were observed; (3) cross correlations between protons in the same pyrrole ring (i.e. cross peaks previously observed in the COSY maps) were observed in the NOESY spectra of some complexes, but not of others; and (4) at ambient temperature, where axial ligand exchange is detectable through cross peaks between the methyl resonances of free and coordinated N-methylimidazole, additional sets of cross correlations between pairs of pyrrole protons, not observed at -35 "C, were detected. Each of these phenomena is discussed and questions are raised concerning the cross-relaxation pathways leading to the observed NOESY maps of these paramagnetic complexes. At the present state of development of two-dimensional NMR studies of these paramagnetic model haem complexes, the electron density distribution in the orbital utilized for spin delocalization can be delineated (with some cautionary notes), but there still remains some ambiguity in the assignment of the four pyrrole proton resonances of these mono-ortho-substituted tetraphenylporphyrinatoiron(II1) complexes.

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