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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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While the reaction mixture contains several calixpyrroles containing more
than one ester functional group (as evidenced by mass spectrometry), the
monoester
II could nonetheless be isolated in 12% yield after column
chromatography. .Subsequent hydrolysis of 11 yielded the monoacid 12.
VI. Properties of Functionalized Calixpyrroles
A. SELF-ASSEMBLY OF "meso-HOOK" CARBOXYLATE 12
The deprotonated form of compound 12 contains both an anionic carboxylate
group and an anion-receptor calixpyrrole macrocycle. X-ray quality
crystals of the calixpyrrole carboxylate 12 were obtained by slow
evaporation of a dichloromethane solution of 12 in the presence of excess
tetrabutylanimonium fluoride hydrate. Interestingly, the crystals did not
contain any fluoride anions within the lattice. Rather, the structure was
found to consist of the tetrabutylanimonium caIix|4]pyrrole carboxylate
salt. The structure of the salt also revealed that the calixpyrrole
carboxylate self-assembles in the solid state with the carboxylate of one
calixpyrrole bound to the pyrrolic array of an adjacent calixpyrrole and
vice versa/''' This forms a dimeric cyclic structure as indicated in
Figure 13. In this
13
266
Sessler and Gale
Figure 13. X-ray crystal structure of (12-H 1 )2 self-assembled dimer.
The calixpyrrole macrocycles both adopt the cone conformation as is
observed in other calixpyrrole anion complexes. Crystal structure
originally published in Sessler, J. L.; Andrievsky, A.; Gale, P. A.;
Lynch, V. Angew. Chem. Int. Ed. Engl. 1996, 35, 2782. Diagram produced
using data from the Cambridge Crystallographic Database.
case, the calix[4]pyrrole adopts the cone conformation as was found in
the other structurally characterized calixpyr-role-anion complexes. Four
hydrogen bonds from the pyrrole groups bind the carboxylate "tail" of a
second functionalized calixpyrrole unit and thus serve to stabilize the
overall self-assembled structure.
Rotating Frame Nuclear Overhauser Effect Spectroscopy (ROESY), NMR, a
technique that allows exchange processes to be observed when applied to
the trimethylam-monium salt of 12 in dichloromethane-c/?, provided
evidence for aggregation in solution (Figure 14). Two resonances were
observed between 7.0 and 7.5 ppm
corresponding to noncomplexed pyrrole NH protons. Another resonance was
observed at approximately 11 ppm. This was assigned to bound-pyrrole-NH
protons, in exchange with the unbound NH resonances. These ROESY analyses
are thus consistent with dimerization occurring extensively in solution,
a conclusion that is also supported by FAB MS studies. In any event, to
the best of our knowledge, this self-assembling calix[4]pyrrole system
(12) represents the first and only example wherein purely anionic
subunits are seen to self-assemble both in solution and in the solid
state (see below).
12 11 10 9
Figure 14. ROESY NMR spectrum of the trimethylammonium salt of 12 in
dichloromethane-d2 showing exchange peaks. These are consistent with a
monomer-dimer exchange process occurring in solution.
45/Calixpyrroles: Novel Anion and Neutral Substrate Receptors
267
B. ANION-BINDING PROPERTIES OF ^-SUBSTITUTED CALIX[4]PYRROLES
The groups on the periphery of the calixpyrrole have a profound effect on
the anion-binding ability of the core of the macrocycle.62 Solution-
anion-binding properties of the /)-octamethoxy derivative 7 and the /-
octabromo derivative 10 have been studied in dichloromethane-c/? using 'H
NMR titration techniques (Table 3). Compound 7 displays lower stability
constants than does the corresponding "/?-ftee" analogue 2, presumably
due to the electron donating effects of the methoxy groups (this
decreases the acidity of the pyrrole-NH protons and therefore lowers the
stability of the calixpynole-anion complex formed). In contrast, compound
10 displays substantially higher stability constants with anions than
does its "/?-free" control (compound 1) as a result, presumably, of the
electron withdrawing effect of the bromine substituents.
VII. Calix[4]pyrrole Conjugates
A. CALIXPYRROLE-BASED SENSORS
The synthesis of new molecular devices designed to sense and report the
presence of a particular substrate is an area of analytical chemistry
that is currently attracting intense interest. Metallocene units have
previously been appended to calixarenes64-65 and other receptor
molecules66 to generate conjugates that have been shown to be effective
Table 3. Stability Constants (M n) for Compounds 7 and 10 with Anionic
Substrates0 in dichloromethane-d2 at 298
Anion Compound
7 10
Fluoride 1.7 (0.2) x ID2 2.7 ( 0.4) x 104/>
Chloride < 10 4.3 ( 0.6) x 10 '
Dihydrogenphosphate N.D. 6.5 ( 0.4) x 102
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