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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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than the monomers, as expected for electronic - interactions between
aromatic rings in close proximity. However, comparison of the EPR spectra
of the oxidized monomer and dimer suggested that the odd electron in the
dimeric radical cation might not be delocalized over both rings. This is
different from what is observed in the oxidized special pair of the
photosynthetic RC.
D. MOLECULAR BOXES FOR PORPHYRINS
The molecular squares based on [Re(CO)3Cl] comers and Zn(II)-porphyrin
edges were shown to be ideal receptors for the complexation of porphyrins
containing 4-pyridyl substituents at meso positions.92 For example
5,10,15,20-tetrakis(4-pyridyl)porphyrin 61 was complexed with an
association constant of Ka = A x 107mol~ 'L in the penta-mer 101 built
from the Zn(II) complex of 95 and [Re(COb] subunits (Figure 57).
Fluorescence quenching of the Zn(II)-porphyrin was virtually complete,
and was used as a diagnostic of host-guest interaction. An organic
receptor analogue made of Zn(II) porphyrin subunits was prepared in 13%
yield by intramolecular cyclization of a linear Zn(II) porphyrin tetramer
around the porphyrin template 61.96 The resulting assembly (102) had a
binding constant even greater: 2 x 10lomol ~~ 1L, but smaller than
expected on the basis of results obtained with Zn-porphyrin-based
receptors. Complexation freezes out the receptor in an idealized D2d
symmetric conformation, as represented in Figure 58.
This host-guest system was characterized by X-ray crystallography
(Figure 59).97 The same authors made a large macrocycle containing two
alternating Zn(II)-por-phyrins and pyromellitimide moieties.68 This
molecule was shown to be a receptor for rra/i.v-bis(4-pyridyl)porphyrin
59, which was bound with Kd = 2.6 x 105 mol ~ 'L in the complex 103 of
Figure 60. Binding was due both to coordination of the porphyrin to the
Zn(II)-centers of the porphyrins contained by the receptor and to -
interactions between the free-base porphyrin guest and the
pyromellitimide walls. Irradiation of the free-base porphyrins was
followed by efficient electron transfer to the pyromellitimide acceptors.
40/Noncovalent Multiporphyrin Assemblies
31
51
97
Figure 55. A cyclic trimer (97) of the Zn(ll) complex of porphyrin 51,
assembled at tram-Pt(PEts)? complex metal fragments. The /^-substituents
of the porphyrin rings have been omitted for clarity. The resulting
molecular box binds guest 98 with a high affinity constant.
E. COMBINATION OF COORDINATION BONDS AND HYDROGEN BONDS
Ogoshi and coworkers4'4 designed and studied a self-assembled porphyrin
trimeric system, the construction of which relies on hydrogen bonding and
coordination bonds. The assembled system, which gathers four components,
is shown in Figure 61. The free-base porphyrin 104, a doublebridged
porphyrin, recognizes dialkyl tartrates selectively, via 4-point hydrogen
bonds. When the dialkyl tartrate is functionalized with alkyl chains
bearing pendant 3-pyridyl groups, as in 105, it can bridge two
(Rh(OEP)CI] moieties 106. This tartaric acid-bridged dimer 107, can now
interact with the cradle-like porphyrin 104. producing the supramolecular
assembly 108. The association constant between 104 and the dimer 107 is
. = 3 x 103 mol - 'L. which is much smaller than that of 104 and
uncomplexed 105 (greater than I( mol 1L). This is probably due to the
steric hindrance brought by the Rh(IIJ)-porphyrin components of the
system.
Figures 62 and 63 show other systems combining hydrogen and
coordination bonds. Whitesides and co-workers1'9 assembled porphyrins on
hydrogen-bonded aggregates based on the complementary interaction between
isocyanuric acid and melamine. Using the trismelamine tripod molecule 109
bearing pendant imidazole ligands.
they could gather three Z.n(TPP) porphyrins 110 by coordination of the
imidazole to the Zn(Il) atoms. The assembly 111 of Figure 62 was fixed in
a more or less defined conformation by interaction with the isocyanuric
acid derivative 112. As shown in Figure 63, replacement of the
isocyanuric acid linker by a molecule (113) in which two isocyanuric acid
moieties are covalently bridged by a rigid nieta phenylene spacer formed
assemblies in which two groups of three porphyrins each are linked solely
by hydrogen-bond arrays, as in 114.
IV. Rotaxanes and Catenanes Incorporating Two or Several
Noncovalently Bound Porphyrins
The Hrst porphyrin-incorporating |2|-rotaxanes were made only recently
(1992),1 <)0-101 the tetrapyrrole nuclei being efficient bulky blocking
groups, preventing dethreading of the string-like component from the ring
through which it was threaded. However, these compounds do not belong to
the family of molecules covered by this chapter since a covalent bond
pathway exists between the porphyrin units. This restriction also applies
to most of the other [2]-
Chambron et al.
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