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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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conformation in solution. The w<?.w-methyl groups are then arranged in
either axial or equatorial positions and therefore resonate al different
frequencies (Figure 5). At room temperature this inequivalence is not
observed because the complexation-decom-plexation rate is fast on the NMR
timescale. By contrast, at lower temperatures this process is slow enough
that (he cone conformation becomes delectable.
The pyrrole NH resonance also splits into two as the temperature is
lowered in the presence of fluoride anions (Figure 6a). One possible
cause for this would be a coupling of the l4F nucleus of the bound-
fluoride anion to the (spin 1/2) NH protons. To test this hypothesis, a
|l,F NMR
Figure 5. Generalized structure of a meso-octamethylcalix[4]pyrrole-anion
complex illustrating the adopted cone conformation. The meso-methyl
groups may be grouped according to whether they are found in axial or
equatorial environments.

J nh-f - 39.5 Hz 193K
JCZc= 39.5 Hz

12.8 12.6 ppm

-91.6 -91.8 -92,0 -92.2 -92,4 ppm
Figure 6. NMR spectra recorded at 193 in dichloromethane-d2
illustrating coordination induced coupling between the pyrrole NH protons
of 1 and the bound fluoride anion () = 39.5 Hz) (a) 'h NMR and (b) 19F
NMR spectra. (Reprinted with permission from Chem. Commun. 1998, 1.
Copyright (c) 1998 The Royal Society of Chemistry).
Sessler and Gale
spectrum of the fluoride complex was acquired at 193 (Figure 6b). The
bound-fluoride-anion resonance was found to be a quintet (with a coupling
constant of 39.5 Hz) confirming the proposed anion-HN-coordination-
derived coupling effect.
The coordination of small neutral molecules (e.g., short-chain alcohols
and simple mono-amides) is a particular challenge. These species lack net
charge and contain few functional groups that could participate in
hydrogen bonding or other noncovalent bonding interactions. The
calix[4]pyrroles are capable of forming hydrogen-bonded complexes with
such species.36 Proton NMR titration techniques were used to evaluate the
binding affinity of "!e.vo-octamethylcalix[4]pyiTole with a series of
alcohols and amides. Benzene-d6 solutions of calix[4]pyrrole 1 were
treated with the substrates listed in Table 2, and the shifts in the NH
proton of the calixpyrrole noted. As in the case of the anion recognition
studies, stability constants were then calculated using the EQNMR
computer program. It was found that the data were best fit to a 1:1
receptor/neutral substrate-binding model. A trend in stability constants
is evident across both the alcohol and amide series, in which the
constants uniformly decrease with increasing bulk near the substrate
oxygen atom. For the amides, the stability constants are sensitive to
changes in substituents on the nitrogen atom and on the carbonyl carbon.
Thus, calix[4]-pyrrole 1 has a higher affinity for a secondary formamide
(jV-formylglycine ethyl ester) than for a tertiary one (N,N-
dimethylformamide, DMF), and forms a more stable complex with DMF than
with /V,/V-diinethylacetamide. The low value of 2.20.1 M~' recorded
for 1,1,3,3-tetramethylurea is consistent with the presence of two sets
of jV-bound methyl groups serving to hinder severely the approach of the
amide oxygen atom to the calixpyrrole-NH hydrogen-bonding array.
Table 2. Stability Constants (M 1) for Compound 1 with Neutral Substrates
in Benzene-d6 Solution at 298
Substrate added KatM-1)
Methanol 12.7 ( 1.0)
Ethanol 10.7 (0.7)
Benzyl alcohol 9.7 (0.7)
Isopropyl alcohol 7.0 ( 0.4)
.ver-Butanol 6.2 (0.4)
iV-Fonnylglycine ethyl ester 13.3 (1.0)
iV.iV-Dimethylformainide 11.3 (0.8)
iV.iV-Dimethylacetamide 9.0 ( 0.9)
1.1,3.3-Tetramethylurea 2.2 (0.1)
Dimethyl sulfoxide 16.2 ( 1.1)
1,2-Dimethylimidazole 5.4 (0.3)
Acetone 2.2 ( 0.2)
Nitromethane h ---
Source: Dala from Allen. W. E.; Gale. P. A.: Brown. . .; Lvneh. V. .:
Seller. J. L. J. Am. Chem. Soc. 1996. II S'. 12471.
"In hen/-f/h al 298 K. For each lilralion. the concentration of 1 was
held constant (at ca. 4x10 ? M) as aliquots of the substrate in benzene-
";/fl (cci. I M) were added.
1 this instance, the induced shifts in the NH prolon(s) of 1 were loo
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