Fine tuning the anion binding properties of 2,6-diamidopyridine dipyrromethane hybrid macrocycles

Article abstract of DOI:10.1021/ja0522938

The synthesis, characterization, and anion-binding properties of a series of 2,6-diamidopyridine dipyrromethane hybrid macrocycles is presented. As part of this work, a new method for effecting the oxidation of dipyrromethane-based macrocycles in organic

Full text of DOI:10.1021/ja0522938

Published on Web 07/21/2005
Fine Tuning the Anion Binding Properties of
2,6-Diamidopyridine Dipyrromethane Hybrid Macrocycles
Jonathan L. Sessler,*,† Evgeny Katayev,†,‡ G. Dan Pantos,† Pavel Scherbakov,‡
Marina D. Reshetova,‡ Victor N. Khrustalev,§ Vincent M. Lynch,† and
Yuri A. Ustynyuk*,‡
Contribution from the Department of Chemistry and Biochemistry and Institute for Cellular and
Molecular Biology, 1 UniVersity Station - A5300, UniVersity of Texas at Austin,
Austin, Texas 78712-0165, Department of Chemistry, M. V. LomonosoV Moscow State
UniVersity, Leninskie Gory, 119899 Moscow, Russian Federation, and A. N. NesmeyanoV
Institute of Organoelement Compounds, 28 VaViloVa str., 117997 Moscow, Russian Federation
Received April 9, 2005; E-mail: sessler@mail.utexas.edu; yust@nmr.chem.msu.su
Abstract: The synthesis, characterization, and anion-binding properties of a series of 2,6-diamidopyridine
dipyrromethane hybrid macrocycles is presented. As part of this work, a new method for effecting the
oxidation of dipyrromethane-based macrocycles in organic solvents has been developed. The macrocyclic
frameworks presented here stand out because of their ease of synthesis and tunable anion-binding
properties. Evidence for anion binding was obtained from UV-vis spectroscopic titrations carried out in
acetonitrile. The results clearly indicate that by changing the flexibility, cavity size, and directionality of
anion-binding moieties in the macrocyclic framework the anion selectivity may be changed dramatically.
These results are in accord with density functional theory molecular modeling calculations performed on
one member of the series.
Introduction
Anion recognition has emerged as an important theme in
modern supramolecular chemistry.1 The synthesis of artificial
receptors with high affinity and selectivity for specific anions
is an ongoing goal of researchers in the field. In addition to
their academic interest, reflecting in part the inherent challenge
of designing systems that can recognize anions of various shapes
and charges with controlled specificity, there are a number of
potential applications that are stimulating interest in this area.2
These run the gamut from anion sensing to drug development
and radioactive and nonradioactive waste remediation. Over the
past 10 years many groups, including our own, have devoted
considerable effort to the synthesis of novel anion receptors.3
The focus of our attention has largely been on pyrrole-based
systems whose properties can be potentially fine tuned. In this
context, we recently reported the pyridine-2,6-dicarboxamide-
dipyrromethane receptor 1 (Figure 1),4 a system that shows
promise as a sulfate anion receptor. In an effort to build on
these findings, we have sought to prepare other pyridine-2,6-
dicarboxamide dipyrromethane macrocyles and wish to report
Figure 1. Pyridine 2,6-dicarboxamide dipyrromethane receptor 14.
here the synthesis of compounds 2-4. These systems display
anion-binding properties very different from those of 1.
Macrocycle 14 displays high selectivity for tetrahedral anions
over spherical, linear, or trigonal planar anions. It is thus a good
candidate for sulfate extraction from nitrate-rich mixtures.4
(3) For recent reviews of work in the anion-binding field, please see the
following, along with references therein: (a) Beer, P. D.; Gale, P. A. Angew.
Chem., Int. Ed. 2001, 40, 486-516. (b) Martinez-Man˜ez, R.; Sanceno´n, F.
Chem. ReV. 2003, 103, 4419-4476. Beer, P. D.; Hayes, E. J. Coord. Chem.
ReV. 2003, 240, 167-189. (c)Best, M. D.; Tobey, S. L.; Anslyn, E. V.
Coord. Chem. ReV. 2003, 240, 3-15. (d) Sessler, J. L.; Camiolo, S.; Gale,
P. A. Coord. Chem. ReV. 2003, 240, 17-55. (e) Llinares, J. M.; Powell,
D.; Bowman-James, K. Coord. Chem. ReV. 2003, 240, 57-75. (f) Bondy,
C. R.; Loeb, S. J. Coord. Chem. ReV. 2003, 240, 77-99. (g) Choi, K.;
Hamilton, A. D. Coord. Chem. ReV. 2003, 240, 101-110. (h) Wedge, T.
J.; Hawthorne, M. F. Coord. Chem. ReV. 2003, 240, 111-128. (i) Lambert,
T. N.; Smith, B. D. Coord. Chem. ReV. 2003, 240, 129-141. (j) Davis, A.
P.; Joos, J.-B. Coord. Chem. ReV. 2003, 240, 143-156. (k) Hosseini, M.
W. Coord. Chem. ReV. 2003, 240, (1-2), 157-166. (l) Gale, P. A. Coord.
Chem. ReV. 2003, 240, 191-221. (m) Wiskur, S. L.; Ait-Haddou, H.;
Anslyn, E. V.; Lavigne, J. J. Acc. Chem. Res. 2001, 34, 963-972.
(4) Sessler, J. L.; Katayev, E.; Pantos, G. D.; Ustynyuk, Yu. A. Chem. Commun.
2004, 1276-1277.
† University of Texas at Austin.
‡ Lomonosov Moscow State University.
§ Newmeyanov Institute of Organoelement Compounds.
(1) (a) Bianchi, A.; Bowman-James K.; Garcia-Espan˜a, E. Supramolecular
Chemistry of Anions; Wiley-VCH: New York, 1997.
(2) (a) Fundamentals and Applications of Anion Separations; Moyer, B. A.,
Singh, R. P., Eds.; Kluwer Academic/Plenum Publishers: New York, 2004.
(b) Camiolo, S.; Gale, P. A.; Sessler, J. L. In Encyclopedia of Supramo-
lecular Chemistry; Atwood, J. L., Steed, J. W., Eds.; Marcel Dekker: New
York, 2004.
9
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J. AM. CHEM. SOC. 2005, 127, 11442-11446
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Diamidopyridine Dipyrromethane Hybrid Macrocycles
A R T I C L E S
Figure 2. Optimized geometries and corresponding binding energies for the mono (+p) and bis (+2p) dihydrogenphosphate complexes of ligand 1 and 2.
The binding energies were calculated using ∆E ) E°(complex) - E°(host) - nE°(guest), where n ) number of anions coordinated by the receptor.
However, it suffers from the disadvantage that it binds dihy-
drogenphosphate more strongly than hydrogen sulfate, albeit
with a 2:1 binding stoichiometry. To design a better hydrogen
sulfate receptor we performed density functional theory (DFT)
calculations on a series of related macrocycles (see Results and
Discussion section). Previous experience in the expanded
prophyrin field coupled with the molecular modeling results led
us to choose macrocycle 2 as our synthetic target. In this
macrocycle, the pyridine-2,6-dicarboxamide backbone is re-
tained, while the dipyrromethane subunit is modified. Such a
modification, it was thought, would allow the basic structure
of the system, and hence its anion-binding properties, to be
further tuned via reduction and oxidation. As described below,
this approach, which yields structures 3 and 4, respectively, does
give rise to receptor systems with substantially altered molecular
recognition characteristics.
multiple, disparate NH-anion interactions are possible. To the
extent such a rationalization is true, macrocycle 2 should display
a greater selectivity for hydrogen sulfate relative to dihydro-
genphosphate. This is because the conformation of 2 should be
“frozen” as the result of substituting the â-positions of the
pyrrole unit and “inserting” a tolyl group into the “meso-like”
position of the dipyrromethane subunit.
To provide support for the above suggestions, we performed
molecular modeling of macrocycles 1 and 2 and their respective
dihydrogenphosphate complexes. Modeling studies were per-
formed using DFT calculations in the gas phase. Optimized
structures of the free ligands 1 and 2 appeared to be unsym-
metrical. Ligand 2 can exist in two different conformations,
where the tolyl and pyridine fragments are either in trans or cis
positions relative to the o-phenylene rings. The modeling
indicates the trans conformation has the lower energy. Therefore,
all subsequent modeling studies were carried out starting with
2 in this conformation.
Results and Discussion
Anion-Receptor Complex Modeling Studies. The strong
affinity for tetrahedral anions, such as sulfate and phosphate,
displayed by 1 is most readily explained in terms of a
geometrical complementarity between the hydrogen-bonding
sites of the receptor and the bound tetrahedral anions. Such a
conclusion, while not rigorously proved, is consistent with the
prior findings of Bowman-James et al. made in the context of
studying related macrocyclic frameworks.5 However, such a
generalized explanation fails to account for the relatively high
affinity and 2:1 binding stoichiometry seen in the case of
dihydrogenphosphate. The existence of two binding sites for
this particular anion could reflect the inherent flexibility of 1,
especially within the dipyrromethane subunit, that, in turn, might
allow NH hydrogen bond donors to orient is such a way that
Optimization of the 1:1 dihydrogenphosphate complexes
formed from receptors 1 and 2 revealed in both cases the
existence of two binding sites for each ligand, an “endo” site
(under the macrocycle) and an “exo” site (above the macro-
cycle), as shown in Figure 2. While the binding geometries are
similar in both cases, the difference in energies between the
(5) (a) Hossain, Md. A.; Llinares, J. M.; Powell, D.; Bowman-James, K. Inorg.
Chem. 2001, 40, 2936-2937. (b) Clifford, T.; Danby, A.; Llinares, J. M.;
Mason, S.; Alcock, N. W.; Powell, D.; Aguilar, J. A.; Garcia-Espan˜a, E.;
Bowman-James, K. Inorg. Chem. 2001, 40, 4710-4720. (c) Kang, S. O.;
Llinares, J. M.; Powell, D.; VanderVelde, D.; Bowman-James, K. J. Am.
Chem. Soc. 2003, 125, 10152-10153. (c) Hossain, Md. A.; Kang, S. O.;
Powell, D.; Bowman-James, K. Inorg. Chem. 2003, 42, 1397-1399. (d)
Hossain, Md. A.; Kang, S. O.; Llinares, J. M.; Powell, D.; Bowman-James,
K. Inorg. Chem. 2003, 42, 5043-5045. (e) Kang, S. O.; Hossain, Md. A.;
Powell, D.; Bowman-James, K. Chem. Commun. 2005, 328-330.
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A R T I C L E S
Sessler et al.
Scheme 1. Reduction and Oxidation of Macrocycle 2
Scheme 2. Improved Synthesis of Precursor 8
exo and endo coordination modes of 1 and 2 are 1.48 and 18.45
kcal/mol, respectively, in favor of the exo site (Figure 2).
Interestingly, the modeling indicates that for both receptors only
three oxygen atoms of the dihydrogenphosphate anion are
involved in hydrogen-bonding interactions, (A and B in Figure
2, inset), while the fourth one is directed away from the binding
sites. A similar arrangement is observed in the solid state of
2‚H2SO4, as inferred from the single-crystal X-ray diffraction
analysis (vide infra).
The structures 1+p (C and D, Figure 2) and 2+p(exo) (I)
are characterized by strong hydrogen-bonding interactions
between one of the OH groups of dihydrogenphosphate and the
two amide NHs and an imine fragment. In the structure 2+p-
(endo) (H), a different binding mode is observed, where the
two pyrrole fragments lie in the same plane and are coordinated
to the bound dihydrogenphosphate anion, albeit weakly as the
result of steric involving the methyl and tolyl substituents.
Optimization of the complexes containing two bound anions
revealed two reasonable binding modes in the case of compound
1 but only one likely mode in the case of compound 2. Of the
1:2 binding modes predicted for 1 and dihydrogenphosphate,
structure F is the more stable, being 2.36 kcal/mol lower in
energy than structure E. This prediction is not surprising since
structure F incorporates the same binding mode for the “first”
bound anion as does the most stable (predicted) 1:1 complex.
The second bound dihydrogenphosphate anion in the 1:2
complex is calculated as being bound to one of the amide NH
protons. The predicted structure of 2 (G) with two dihydrogen-
phosphate anions follows the same pattern.
The above results lead us to several key predictions. First,
they underscore the fact that the dipyrromethane fragment in 1
is very flexible. In particular, the pyrrole rings can rotate along
the C-C bond between the pyrrole ring and the meso carbon,
with the net result that two coordination sites, exo and endo,
are produced that differ little in energy both before and after
H2PO4- complex formation. A second related prediction is that
macrocycle 2, being less flexible, gives rise to an exo coordina-
tion site that is lower in energy than the corresponding endo
alternative, at least when bound to dihydrogenphosphate.
Therefore, a 1:2 binding mode with dihydrogenphosphate is
unlikely to be observed in the case of 2. A third prediction
arising from these calculations is that, because of the same steric
considerations, receptor 2 will bind the first (and presumably
only) equivalent of dihydrogenphosphate less well than 1.
Synthesis. Precursor 8 is one of the key intermediates
required for the synthesis of macrocycles 1-4. In our initial
work,4 this species was prepared according to a literature
procedure.6 This method proved to be tedious and slightly
inefficient (1 week reaction time; 23% overall yield; 2 steps
from commercially available starting materials). We have now
discovered what we believe is a better procedure. It is sum-
marized in Scheme 2. Briefly, commercial o-phenylenediamine
5 was converted to the corresponding mono-BOC-protected
derivative 6 by dissolving in a dioxane-water mixture contain-
ing 2 equiv of HCl, followed by the addition of (BOC)2O and
Na2CO3. The mono-BOC-protected o-phenylenediamine 6 was
then reacted with pyridine-2,6-dicarbonyl dichloride to yield
intermediate 7. After deprotection with trifluoroacetic acid in
dichloromethane, intermediate 8 was obtained in high purity.
By use of this procedure, we were able to synthesize compound
7 in three steps from commercially available precursors, in 1
day, in 80% overall yield.
The pyrrolic precursor 9 for the synthesis of macrocycles 2-4
was synthesized using standard conditions.4 Specifically, ethyl
4-methyl-3-n-propylpyrrole-2-carboxylate was reacted with
4-methylbenzaldehyde in dichloromethane in the presence of
methanesulfonic acid as a catalyst. The corresponding dipyr-
romethane derivative was subject to decarboxylation under basic
conditions and subsequently formulated using triethyl ortho-
formate in trifluoroacetic acid. This gave 9 in 90% overall yield.
The H2SO4 salt of macrocycle 2 was then synthesized by heating
a methanolic solution of 8 and 9 at reflux for 15 min in the
(6) Picard, C.; Arnaud, N.; Tisne`s, P. Synthesis 2001, 1471-1478.
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Diamidopyridine Dipyrromethane Hybrid Macrocycles
A R T I C L E S
Figure 3. UV-vis spectra of macrocycles 1-4 in CH3CN. 1, λmax [nm]
(ꢀ in M-1 cm-1) 300 (19 000); 2, λmax [nm] (ꢀ in M-1 cm-1) 340 (32 000);
3, λmax [nm] (ꢀ in M-1 cm-1) 244 (28 000); 4, λmax [nm] (ꢀ in M-1 cm-1
306 (30 000), 535 (21 000).
)
Figure 4. ORTEP-POVray rendered view of 2‚H2SO4. Solvent molecules
and most hydrogen atoms have been removed for clarity. Thermal ellipsoids
are scaled to the 50% probability level.
presence of 2.2 equiv of H2SO4. Compound 2 was obtained as
the “free base” by quenching the reaction mixture with an excess
of triethylamine. The overall yield for the last two, condensation
and deprotection, steps was 90%.
For the synthesis of macrocycles 3 and 4, the free base form
of 2 was used as the starting material (Scheme 1). While the
synthesis of 3 proved to be straightforward via reduction with
NaBH4 in a CH2Cl2-MeOH mixture, the oxidation of 2 to form
4 proved challenging. Initial attempts were made using the
standard organic oxidizing agents, 2,3-dichloro-5,6-diocyano-
quinone (DDQ) and chloranil. Although a wide range of reaction
conditions were tested, neither DDQ nor chloranil yielded the
desired macrocycle 4. Rather, only starting material or decom-
position products were obtained. Ferrocenium hexafluorophos-
phate, an oxidant that was used successfully in the synthesis of
metal-free texaphyrin,7 showed promise, as inferred from mass
spectrometric analysis. However, it too failed to yield isolable
quantities of 4 once the crude reaction mixtures were subject
to workup and attempted purification. On the other hand, the
use of manganese-based oxidants in acetonitrile allowed this
transformation to be carried out successfully. For instance, in
initial studies it was found that treatment of 2 with MnO2 in
CH3CN at room temperature for 12 h yielded the desired product
(4) in 35% yield. By use of similar conditions, but switching to
a stronger oxidant, KMnO4, improves the yield to 75%.
Compounds 2-4 were characterized using standard spectro-
scopic techniques (cf. Supporting Information). Here, the UV-
vis spectral features proved especially diagnostic. Whereas
macrocyles 1 and 2 give rise to very similar absorption profiles,
with λmax at 300 and 340 nm (Figure 3), the reduced macrocycle
3 displays a λmax at 244 nm, while compound 4 displays two
absorptions at 306 and 535 nm, respectively.
Figure 5. ORTEP-POVray rendered view of 4. A molecule of CH3CN is
hydrogen bonded to one of the amide NH (N7). Most hydrogen atoms have
been removed for clarity. Thermal ellipsoids are scaled to the 50%
probability level.
2-
oxygen atoms of SO4 are hydrogen bonded in an unsym-
metrical bifocal manner to a pyrrole NH and a protonated imine
NH as follows: N1-O2 (2.709 Å, 178.0°), N4-O2 (2.789 Å,
155.84°), N10-O4 (2.753 Å, 150.28°), and N15-O4 (2.725
Å, 160.71°). The complex is stabilized by an aditional hydrogen
bond between the amide NH proton and the SO42- anion, N22-
O3 (2.889 Å, 145.94°). An interesting observation is that the
fourth oxygen atom of the sulfate anion is not involved in
hydrogen-bonding interactions with atoms from the macrocycle
but rather with a water molecule.
Proof for the proposed structures of macrocycles 2 and 4 came
from single X-ray diffraction analyses of their diprotonated and
neutral forms, respectively. X-ray diffraction analysis (Figure
4) revealed that 2‚H2SO4 adopts an up-down picket fence (Z-
shape) structure, with the pyridine moiety and the 4-methyl-
phenyl group acting as the “pickets”. The two imine nitrogens,
N1 and N15, are protonated and participate in hydrogen-bond
In the case of 4, X-ray diffraction analysis (Figure 5) revealed
an L-shaped structure in which the pyridine moiety is orthogonal
to the plane formed by N3, N4, N5, and N6. The 4-methylphenyl
group is also orthogonal both to the N3, N4, N5, and N6 planes
and the pyridine ring. Interestingly, the two sCHdNs moieties
adopt an endo-exo arrangement with respect to the inner
macrocyclic core. There are two hydrogen bonds present in the
molecule. One is between a pyrrole NH proton and the pyrrole
-
interactions with the bound (formal) SO4 anion. Two of the
(7) Hannah, S.; Lynch, V. M.; Gerasimchuk, N.; Magda, D.; Sessler, J. L.
Org. Lett. 2001, 3, 3911-3914.
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A R T I C L E S
Sessler et al.
Table 1. Affinity Constants (M-1) for the Binding of Anions by
Receptors 1-4 as Determined from UV-Vis Spectroscopic
Titrations Carried out in CH3CN at 23 °Ca
Scheme 3. Synthesis of Macrocycle 2 (H2SO4 Salt)
receptor
anion
1b
2
3
4
Br-
c
c
c
c
c
c
c
2 760 ( 380
-
NO3
c
Cl-
2 000 ( 20
116 000 ( 11 000 c
67 000 ( 9 900
CH3COO- 38 000 ( 3 000 12 600 ( 450
c
c
c
-
HSO4
H2PO4
64 000 ( 2 600 108 000 ( 17 000 47 00 ( 960
34 2000; 26 000d 29 000 ( 1 900 15 500 ( 1 750
-
a The anions studied were in the form of their tetrabutylammonium salts.
The R2 values for the curves fits used to determine the affinity constants
range between 0.979 and 0.997. b Affinity constants reported in ref 4. c No
apparent binding as reflected in the lack of changes observed in the UV-
vis spectral titrations upon anion addition. d Stepwise, 2:1 (anion: receptor)
binding observed; the values refer to those for the first and second binding
events, respectively.
via a series of single C-C and C-N bonds, is also rather
flexible. Thus, in contrast to its congeners, receptor 3 is not
expected to provide as good control over the directionality of
its hydrogen-bond donor moieties. Such features are expected
to make it more suitable for the coordination of spherical anions.
Macrocycle 4 is the most rigid of the anion receptors
considered in this study. As observed from the single-crystal
X-ray analysis (vide supra) the dipyrromethene unit is flat, a
fact that is consistent with the presence of an extended
imine-like nitrogen, N4-N5 (2.709 Å, 138.67°), while the other
is between an amide NH proton and a solvent (CH3CN)
molecule, N7-N1A (3.083 Å, 134.38°).
Anion-Binding Studies. Anion-binding studies were per-
formed using UV-vis spectroscopic titrations. To maintain
consistency with previously reported results, the measurements3
were carried out in CH3CN using the tetrabutylammonium salts
of the anions being studied. Binding stoichiometries were
determined using the Job plot method. Table 1 summarizes our
findings.
conjugation pathway and a strong absorption band (at λmax
)
535 nm) in its UV-vis spectrum. On the other hand, as the
result of oxidation, receptor 4 has fewer hydrogen bond donor
atoms than 2 or 3. Not surprisingly, therefore, it is a weaker
anion-binding agent. In fact, receptor 4 binds only bromide in
acetonitrile, and weakly at that. It displays no interaction with
the other anions studied, as judged from the UV-vis titrations.
An inspection of Table 1 reveals that the nature of the
macrocycle and its geometry play crucial roles in modulating
the observed anion affinities. Receptors 1 and 2 display
selectivity for tetrahedral anions. However, the dihydrogen
phosphate-hydrogen sulfate selectivity is reversed. This fact
can be explained in terms of the different structures involved.
Whereas macrocycle 1 possesses a larger cavity and, presum-
ably, a more flexible geometry, due to the presence of the gem-
dimethyl moiety, in receptor 2 the presence of the tolyl group
in the meso position, acts as a “pseudo lid” for the macrocycle
cavity. This group is also expected to freeze the conformation
of the macrocycle as the result of steric interactions involving
the methyl groups in the 4,4′ positions of the dipyrromethane
subunit. Receptor 1 is also found to bind dihydrogenphosphate
in a 2:1 anion:host mode,4 while 2 binds this same anion with
a 1:1 binding stoichiometry, as judged by Job plot studies (cf.
Supporting Information). Such a change in stoichiometry is
predicted based on these same geometric considerations and is
supported by the DFT calculations (vide supra). The same
reasoning, higher rigidity combined with higher steric demand,
also provides a rationale for why 2 displays greater selectivity
for hydrogensulfate, as compared to 1, under identical conditions
of study.8
Conclusions
The pyridine 2,6-dicarboxamide dipyrromethane macrocyles
1-4 represent a new class of anion receptors with tunable
properties. As the Ka values in Table 1 confirm, modifications
in the basic structure of the macrocyclic can have a dramatic
effect on the anion binding affinities and selectivities. The
straightforward synthesis of these systems means they may find
use in “real world” applications, including sulfate anion extrac-
tion protocols. On the other hand, their structural features also
lead us to predict that they could serve as possible ligands for
metal coordination. Studies along these lines are currently in
progress.
Acknowledgment. This project was supported by the U.S.
Department of Energy Office of Science (Grant No. DE-FG02-
04ER63741 to J.L.S.), the National Science Foundation (CHE-
0107732 to J.L.S.), and by the Russian Foundation for Basic
Research (Grant Nos. 02-03-32101 and 05-03-32684 to Y.A.U.)
and is part of an ongoing collaboration involving the Sessler
research group at The University of Texas at Austin and those
of Dr. Bruce Moyer at The Oak Ridge National Laboratory and
Professor Kristin Bowman-James at the University of Kansas.
We would like to acknowledge Dr. Dmitri Laikov for making
available the quantum chemistry software “PRIRODA-04”.
Macrocycle 3 displays a relatively strong interaction with
chloride anion and weaker interactions with the rest of the test
anions. This selectivity reflects, we believe, the “hydrogen-rich”
nature of this receptor, which makes its inner cavity better suited
for the binding of smaller, spherical anions. System 3, linked
Supporting Information Available: General methods and
materials, synthetic experimental, representative Job plots, UV-
vis titrations, corresponding binding isotherms, and details of
affinity constant determinations as well as X-ray crystallographic
information including CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org.
(8) Another factor that could affect the anion binding selectivity of macrocycles
1 and 2 are the minor electronic effects resulting from the changes in the
â-pyrrolic substitution patterns. However, on the basis of DFT calculations,
we conclude that it is changes in the rigidity of the macrocycle that are
mainly responsible for the observed differences in anion selectivity.
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