Meso-protons

Columns 3—5 of Table 9 show a comparison of the wavelength or wavenumber of the a-band and the chemical shift of the meso protons of the porphyrin ring. There is a correlation between these data the energy (expressed as va) of the a-band and the shielding of the meso protons increase in the series [29a] < [29b] < [29c] < [29d] < [29e] < [29f] < [ 29i]. The absorption spectra are of a Ayper-disturbed hypso type (Fig. 7), and the latter type allows the application of transmission modes A, D, or E (Fig. 1) for an interpretation of the observed cis effect of the axial ligands L on the spectral properties of the porphinato ligand (OEP). 

Table 9. Comparison of the wavelengths (A.a) and wavenumbers (va) of the or-bands, the chemical shifts (8) of the porphyrin meso-protons, and the Os /Os111 redox potentials (E1/2) in a series of osmochromes Os(OEP)L2 ]29a-k).

Table 13. Further series (a—h) of iron or ruthenium porphyrins showing cis effects exerted by the axial ligands X or L on the wavelength of the a-band (or (3-band in Series d, e and g) and the chemical shift (6) of the porphyrin meso-protons (Series c, h o-proton in c). The metal II/III-redox potentials (Ej/2) are also given. For abbreviations, see Table 2... 

Fig. 8. NMR spectra at the meso proton region of the dipor-phyrin in CDC1 (A), and in CD CN (B). The greater scattering of the meso protons simply reflects a tighter interaction of the two porphyrin rings in the more polar CD CN solvent.

Similar 6 values for syn-and meso—protons but marked differences for those of anti protons... 

Proton exchange at the meso position of corrole is facile, and the deuteration is complete in 15 min in trifluoroacetic acid at room temperature. Under similar condition, porphyrin does not exchange the meso protons. 

Electrophilic proton exchange occurs readily for sapphyrin (39) and meso deuteration is completed in trifluoroacetic acid and at room temperature overnight.274 The substitution proceeds at a moderate rate for two of the meso protons but very slowly for the other two in the dioxa analogue... 

Fig. 19. Temperature dependence of the isotropic shifts of the resonances due to the methyl groups and meso protons of Fe(OMC) in chloroform. Labels refer to the assignments reported in Table 11. Taken from Ref. [28] with permission...

The resonances at high-field (D, E) have been attributed to the residual protons of deuteriated pyridine axially ligated to iron. This assignment should however be corrected on the basis of the recent data reported by Vogel et al. [27] and the resonances should be attributed to the meso-protons of the macrocycle (see Table 13). 

The resonances due to the meso protons appear as two signals in the spectra of all rhodium complexes. In most cases their relative intensity is 2 1 (5,15-H and 10-H) with the 5,15-H resonating at lower field. However, when the axial ligand is PPh3 or AsPh3 the situation is reversed and it is the 10-H which resonates at lower field. 

Since no variations of the spectra have been observed between 300 and 340 K, in a concentration range between 10-2 and 10- M, the existence of conformational equilibria has been ruled out. The variation of the intensities of the meso-proton resonances has then been attributed to a contribution of the shielding effect of the phenyl rings of the axial ligands to that of the macrocycle. 

In the 80 MHz H-NMR spectrum of Sn(OMC)Cl, and in the analogous germanium complex, the meso-protons appear as two signals while in the spectra of the acetate derivatives a single resonance has been observed. No linewidth data have been reported and the results have been explained considering that the behavior of acetate as a bidentate ligand, discussed in Sect. 4.2, would introduce distortions into the molecule causing accidental isochrony of the meso-proton resonances. 

Data relative to the 400 MHz H-NMR spectra of cobalt corrolates are reported in Table 16. The strong shift due to the macrocycle ring current demonstrates that the presence of the meso-phenyl substituents does not cause the loss of aromaticity. The meso-protons of the monophenyl derivative resonate at the same chemical shift value observed for the meso-unsubstituted complex. 

A general feature of weso-phenyl substituted Co3 + corrolates is then that the planarity of the macrocyclic ligand is maintained in solution the pattern shown by the resonances due to the peripheral methyl groups in fact are indicative of the existence of a C2 symmetry axis the direct pyrrole-pyrrole bond typical of a planar corrole skeleton, confirmed also by the signals due to the meso-protons, if present. 

The stronger diamagnetic shielding of the resonances of the meso-protons in the NMR spectrum of [Co(CN)2(A2ODC)] with respect to the corresponding tetradehydrocorrin complex is consistent with the existence of an extended it conjugation. 

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