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N N =
- N N-
w ë ,
¦ - NH y'
12 A "•
Figure 19. Examples of porphyrins (31-33) containing pendant pyridine
arms having different orientations, and the aggregates (34-36) that they
generate, represented schematically.
Figure 20. Dimerization of the Mn(lll) porphyrin 37. In the dimer 38,
the substituents have been omitted for clarity.
Other dimers of Fe(lll)-porphyrin-like compounds were described by
Baleii and coworkers.! These authors studied the coordination chemistry
of oxophlorins, which are the lirst intermediates of the oxidative
degradation of porphyrins into verdoheme and btliverdin. They found that
coordination of a metal, Fe(lll) lor example, to (he hydroxy tautomer 39,
prevented further degradation of the oxoph-lorin, thanks to dimerization
of the molecule in basic solution (compound 40 of Figure 21), in a manner
similar to that of the system observed by Goff and coworkers.X-ray
crystallography of the ln(lll) analogue''4 showed that the
Figure 21. Dimerization of the Fe(lll) meso hydroxy porphyrin 39. In
the dimer 40, the substituents have been omitted for clarity.
interporphyrin distance was 3.25 A, placing these two aromatic units
within ë-ë contacts. Very interestingly, in the case of the Fe(lII) dimer
(40), they observed that a one-electron chemical oxidation of the system
left intact the dimeric structure of the molecule. Moreover, the two
porphyrins of the dimer were equivalent, the unpaired electron being
delocalized over both macrocycles. The close proximity of the two
macrocycles apparently facilitates (he oxidation of the dimer and allows
for extensive delocalization. These results are highly relevant with
regard to the biological function of the SP.
With very different aims, related to cooperativity effects, but still
involving Fe-porphyrins, Traylor anti his
Chambron et al.
coworkers55 prepared an Fe protoporphyrin IX modified with a flexible
side chain bearing a terminal 4-pyridyl group (41) (Figure 22). They
showed that this system, in the presence of an external ÿ-acceptor ligand
[CO for Fe(II), CN _ for Fe(III)], was able to form the dimer 42 thanks
to cooperativity effects (Figure 23). The affinity of these 5-coordinate
Fe porphyrins for a sixth axial ligand being stronger than the affinity
of 4-coordinate Fe porphyrins for a fifth ligand provides the driving
force for formation of the dimer containing hexacoordinated Fe atoms,
through the various intermediates represented in Figure 23.
Cyclic trimers were described by Wojaczynski and Latos-Grazynski.56-
58 These authors prepared M(III)(TPP)C1 complexes (M = Fe, Mn, and a
nontransition metal, Ga) bearing a benzyl-protected hydroxy group at one
/5-position of the porphyrin macrocycle. They showed that subsequent
treatment of these compounds with NaOH produced cyclic trimers. The case
of Fe(III)56 is shown in Figure 24 where 43 is the monomer precursor and
44 the trimer. Due to the high affinity of Fe(III) for an anionic oxygen
donor atom, the Fe atom is 5-coordinate. The monomers being asym-
R - (ÑÍ2)2Ñ02ÑÍç
Figure 22. Structural formula of iron porphyrin 41.