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 σp 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, β2III, 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(β2III) increases in the order F < Cl < Br < I < F2 < CF3 < Cl2 < Br2, and all β2III values for mixed substituent porphyrins except the monofluoro-containing complex are larger than the β2III 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 β2III in order of increasing size, with the 2,6-disubstituted phenyls causing an increase of β2III 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(β2III) 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(β2III) 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(β2III) 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 electron density distribution in the e(π) orbitals and/or differences in the energy separation of those two orbitals of the porphyrin ring.
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Feb 1, 1999
E Espinosa + 2
Open Access