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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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metallomacrocydes having various shapes, mainly those of squares, but an
aggregate displaying a triangular array of porphyrins has also been
synthesized. The principle for formation of these systems is represented
in Figure 52, and is in part an extension of the principle of
construction of square metallomacrocydes based on 4,4'-bipyridyl ligands
initially invented by Fujita and coworkers,89 and further developed by
Stang and coworkers.90
a. Porphyrins Form Corners of Squares
The porphyrins now contain two 4-pyridyl meso substituents in cis
orientations {i.e., positions 5 and 10), as in 58. The shape of the
aggregate will depend on the nature of the complex metal fragment as
shown schematically in Figure 52 (compare a and c). Some real examples
are displayed in Figure 53.91 When the complex metal fragment is ds-
[PdCl2] or cis-[PtCl2], a dimer is obtained. Compound
93 is the dimer based on Pd apexes. In contrast, when the complex metal
linker is trans~[PdCl2], a square-shaped tetramer is obtained in which
the porphyrins form the comers and the fra/w-[PdCl2] subunits occupy the
edges. This compound (94) is much less stable than the dimer analogues
and decomposes into uncharacterized multimers. Electronic interactions
between the porphyrins were evidenced by lower extinction coefficients,
broadening and red shifts in the absorption bands, and were shown to be
stronger in the case of the tetramers than in the case of the dimers.
40 / Noncovalent Multiporphyrin Assemblies
29
M- <

T
M
- M-
M
1
Figure 52. Cyclic multiporphyrin assemblies based on porphyrins (empty
squares) containing two monodentate pendant ligands (arrows) and
nonporphyrinic metals as assembling units. The pendant arms are in c/s
[(a) and (c)] or trans [(b) and (d)] relative orientations.
93

94
Figure S3. A porphyrin dimer (93) based on coordination of porphyrin 58
to ds-PdCI2 complex metal fragments, and a porphyrin tetramer (94) based
on coordination of the same porphyrin 58 to trans-PdCI;, complex metal
fragments.
b. Porphyrins Form Edges oj Squares
To promote the formation of square metallomacrocyeles where the
porphyrins form the edges, it is necessary to use porphyrins having two
4-pyridyl meso substituents in trans orientations (i.e., positions 5 and
15) like 59, hut based on
porphyrin 95, and complex metal fragments offering cis coordinating
positions. Accordingly, squares based on square planar metal fragments
like Pt or Pd(dppp) or cis-[PtCbJ,4041 or octahedral metal fragments like
[Re(CO);ClJ,42 were prepared. In the case of the assembly 96, containing
c(.v-Pt(dppp) comers, the porphyrins were
30
Chambron et al.
95
R = (CH2)5CH3
96
Figure 54. A porphyrin tetramer (96) based on coordination of porphyrin
95 to c/s- Pt(dppp) complex metal fragments.
shown to be coplanar with the Pt(dppp) moieties, as represented in Figure
54.90 Each porphyrin moiety has part of the ring located inside its
square.
c. Triangular Porphyrin Aggregate
As demonstrated by Sanders and coworkers, the bisacetylenic porphyrin
51 may be used as a monomer not only in Glaser-Hay coupling reactions to
generate covalent cyclic oligomers,6566 but also in condensation
reactions with fm/i.y-PUPEbjiCb, in order to form porphyrin-based
metallomacrocydes (Figure 55).91 The cyclic trimer 97 was the major
isolated product, obtained in 16% yield. This trimer binds 2,4,6-
trispyridyl-l,3,5-triazine 54 with a moderate binding constant of 3 x
107mol~'L. This is due to the fact that this tridentate ligand is too
small to fit perfectly into the cavity of the trimer. The "complex as
ligand" guest 98, which is larger than 54 by 3.7 A, was designed and
synthesized, and shown to bind the cyclic trimer with a quite high
binding constant, the lower limit being 10l0mol_lL. However, despite this
high binding affinity, 98 did not prove successful as a template in the
preparation of the trimer.
C. METAL-BRIDGED PORPHYRIN DIMER
Krishnan and coworkers94'95 designed and synthesized TPP analogues (99)
in which 15-crown-5 ether moieties are anchored to the meso phenyl
substituents (Figure 56a). The porphyrins were metalated at the
tetrapyrrole macrocycle with Mg(II), VO(IV), Cu(II), Ni(II), Zn(II), and
Mn(III), to ensure that the oxidation processes are not metal-centered.
The porphyrins were monomeric in solution but dimerization occurs to
afford 100 upon addition of +, which forms sandwich-type complexes of
1:2 stoichiometry with benzo-15-crown-5, as depicted in Figure 56b. The
porphyrins are held together by four metal cations which bridge them by
coordination to their pendant crown ether complexing subunits. This
dimerization is unique, and its principle is schematically represented in
Figure 56c. It was observed that the dimers were more easily oxidized
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