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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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position of phenyl substituents occupying the opposite meso positions 5
and 15. In an early work, these authors discovered that the Glaser-Hay
oxidative coupling of the Zn(II) complex of 51 in pyridine produced the
cyclic trimer 53 in 47% yield (Figure 29).65 They also showed that this
covalent trimer was an excellent receptor for 2,4,6-trispyridyl-l,3,5-
triazine 54 with = 10' mol " 'L. They subsequently reasoned that this
trispyridine could play the role of a template for formation of the
covalent trimer. Accordingly, when they performed the above reaction on
the bisacetylenic porphyrin 52 in dichloromethane, using air/CuCl/TMEDA
(TMEDA is the tetramethylethylenediamine ligand) as the oxidant, and in
the presence of 54, they obtained the free-base cyclic
50
Figure 28. Dimerization of bis-porphyrin 49 around a tetraamine template.
The aryl substituents have been omitted in the dimer 50.
trimer after acidic treatment in 52% yield. Interestingly, when the same
reaction was performed in the presence of 4,4'-bipyridine, the cyclic
dimer was selectively obtained, in 70% yield.66 The same procedure was
applied to bisacety-
40 / Noncovalent Multiporphyrin Assemblies
17
51 : R = H 54
52 : R = CH2C02Me
53
Figure 29. Cyclotrimerization of the Zn(ll) complex of porphyrin 51. The
porphyrin /^-substituents have been omitted in the covalent trimer 53.
The same reaction performed on the Zn(ll) complex of porphyrin 52, but in
the presence of the template 54, also afforded a covalent trimer such as
53.
lenic-5,15-dioxoporphyrins (Figure 30).67 Because the Zn(II) complexes of
dioxoporphyrins have a very strong affinity for pyridine ligands,
multicomponent complexes, like 55 of Figure 30, are readily assembled.
Consequently, when the Zn(II) complex of 56 was subjected to the Glaser-
Hay coupling reaction in the presence of 54, the covalent trimer 57 was
obtained quantitatively. The same was true for the dimer formed in the
presence of 4,4'-bipyridine. The case of the tetramer will be discussed
in Section III.A.3.
3. Porphyrins Gathered by a Porphyrin Assembling Unit
Two cases can be envisioned when the external bridging ligand is simply a
porphyrin, and these different systems are schematically represented in
Figure 31. In the first case (a, b), the porphyrin bridge is a
metallo^orphyvm, which has free axial coordination sites for porphyrins
functionalized with a pendant, monodentate ligand. Therefore the maximal
number of porphyrins that can be gathered in this case is two. In the
second (c-f), the porphyrin bridge is a polytopic ligand thanks to its
OT<?.vo-(4-pyridyl) substituents. These pendant ligands coordinate
metalloporphyrins through their
axial positions. Up to four metalloporphyrins can therefore be gathered
at a central, free-base porphyrin core, leading to a pentameric
aggregate. In both cases, the porphyrins that are directly connected have
mutual orthogonal orientations.
The different free-base porphyrins used as assembling units are mainly
(4-pyridyl)-substituted TPPs, as represented in Figure 32. The mono
substituted is 5-(4-pyridyI)-10, 15, 20-triphenylporphyrin 45, the
disubstituted are 5, 10-bis(4-pyridy I )-l 5,20-diphenyl porphyrin 58
(cis form), and 5,15-bis(4-pyridyl)-10,20-diphenylporphyrin 59 (trans
form). The trisubstituted is 5,10,15-tris(4-pyridyl)-20-phe-nylporphyrin
60, and the tetrasubstituted is 5,10,15,20-tetrakis(4-pyridyl)porphyrin
61. Also used were (3-pyridyl)-substituted TPPs: 5-(3-pyridyl)-10,15,20-
triphenylporphyrin 62 is the monosubstituted compound, and 63 (5,10-bis-
(3-pyridy 1)-15,20-diphenylporphyrin) and 64 (5,15-bis(3-pyridyl)-10, 20-
diphenylporphyrin) are respectively the cis- and frans-disubstituted
compounds.
Sanders and coworkers68 cited unpublished work where two [Ru(CO)]
porphyrins are bridged by the free-base
5,15-bis(4-pyridyl)-7,13,17,23-tetramethy 1-8,12,18,22-tetra-
ethylporphyrin. Fleischer and Schachter46 characterized a similar trimer
in which two Zn(II)-porphyrins are bridged by porphyrin 59.
Imamura and coworkers69-70 assembled one or two [Ru(CO)] porphyrin
fragments using tetraarylporphyrins 45 and 59. They also took the
symmetrical approach, that is, they assembled two tetraarylporphyrins 45
at a central Ru(II)-porphyrin core. The different compounds (dimer 65,
trimers 66 and 67) are shown in Figure 33. A comparison of 'H NMR spectra
of two complementary trimers, 66 and 67 proves interesting; the
anisotropic effect of the two axially coordinated Ru(II)-porphyrins in
the former is stronger than the effect of the free-base porphyrins in the
latter. UV-vis spectroscopy shows that the Ru(II) porphyrins of 66 behave
independently. These results are in sharp contrast with the fact that
many porphyrin dimers having face-to-face orientations show changes in
their electronic absorption properties as compared to the parent
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