Intra- and intermolecular hydrogen bonds in pyrrolylindandione derivatives and their interaction with fluoride and acetate: Possible anion sensing properties

Article abstract of DOI:10.1021/jp4072179

The series of push-pull compounds containing the pyrrole ring as a donor and the 1,3-indandione derived moieties as the acceptor unit were synthesized, and strong intramolecular hydrogen bonding in their molecules was studied. In the presence of fluoride and acetate anions their solutions undergo color changes. It was shown by NMR, UV-vis, and quantum chemical calculations including AIM analysis that all these compounds undergo solvent-assisted rupture of the intramolecular hydrogen bond followed by the formation of a strong intermolecular hydrogen bond with fluoride and acetate anions which finally abstract a proton from the pyrrole ring. The insensitivity of the studied compounds to other anions (Cl, Br, HSO₄, PF₆) is consequence of the instability of the corresponding hydrogen-bonded complexes.

Full text of DOI:10.1021/jp4072179

Article
pubs.acs.org/JPCA
Intra- and Intermolecular Hydrogen Bonds in Pyrrolylindandione
Derivatives and Their Interaction with Fluoride and Acetate: Possible
Anion Sensing Properties
Mark V. Sigalov,*,† Bagrat A. Shainyan,‡ Nina N. Chipanina,‡ and Larisa P. Oznobikhina‡
†Department of Chemistry, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel
‡A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Division of Russian Academy of Sciences, 664033 Irkutsk, Russia
S
* Supporting Information
ABSTRACT: The series of push−pull compounds containing the pyrrole ring as a donor and the
1,3-indandione derived moieties as the acceptor unit were synthesized, and strong intramolecular
hydrogen bonding in their molecules was studied. In the presence of fluoride and acetate anions
their solutions undergo color changes. It was shown by NMR, UV−vis, and quantum chemical
calculations including AIM analysis that all these compounds undergo solvent-assisted rupture of
the intramolecular hydrogen bond followed by the formation of a strong intermolecular hydrogen
bond with fluoride and acetate anions which finally abstract a proton from the pyrrole ring. The
insensitivity of the studied compounds to other anions (Cl, Br, HSO4, PF6) is consequence of the
instability of the corresponding hydrogen-bonded complexes.
analyses, and experimentally using NMR and UV−visible
spectroscopy. The choice of the objects of research was based
on the fact that, in comparison with the parent compound 1,
compound 2 has an extended conjugation system, and
compounds 3 and 4 contain substituents with stronger electron
withdrawing ability than the carbonyl oxygen atom and higher
ability to charge delocalization. Taking into account the color
change induced in solutions of these compounds by fluoride
and acetate anions, one can consider them as potential optical
sensors. The special classical study of sensing properties was
not accomplished here, but in order to understand their nature,
the intermolecular complexes with various anions were
calculated. The relevant research in the field of sensing anions
for these and similar compounds is underway now.
Pyrrole-based anion receptors now attract a great deal of
interest that is reflected in hundreds publications and reviews
during the past decade. Most of known anion receptors contain
pyrrolic moiety incorporated into macrocycle (expanded
porphyrins and their heteroatomic analogues, porphyrinogens,
calix(4)pyrroles like 6,6−10 bipyrroles and linear oligopyrroles
such as prodigiosin analogue 7 and its derivatives,11
dipyrrolylquinoxalines 8,12 numerous amide- and urea-function-
alized pyrroles 9,13 and some derivatives of indole 10 and
carbazole.14,15 Their activity as anion sensors is based mainly on
hydrogen bonding toward most anions and, in the case of
INTRODUCTION
■
Pyrrolylidene derivatives of 1,3-indandione 1 and 3 (Scheme 1)
possess strong intramolecular N−H···O hydrogen bonds in
nonpolar solvents and are able to form intermolecular
hydrogen bonds with polar basic solvents.1 Strong H-bonding
1
is witnessed by large low-field H NMR shift of the H-bonded
proton (>13 ppm), similar to those in structurally close model
compounds 5,2 which were shown by X-ray analysis to have a
3
1
short N···O distance of 2.60−2.62 Å. Note that H NMR
chemical shifts of the H-bonded proton in compounds 5 agree
satisfactorily well with the correlation of geometric and NMR
parameters described in ref 4. In spite of the absence of X-ray
data for 1, its NMR behavior allows for a conclusion about the
presence of strong hydrogen bonding in this and similar
compounds.
Recently, we have found that solutions of these compounds
in different organic solvents (DMSO, acetonitrile, dichloro-
methane) undergo color change in the presence of fluoride
ion.5 This may be indicative of the rupture of the intra-
molecular hydrogen bond N−H···O followed by formation of
intermolecular hydrogen bond or deprotonation and appear-
ance of the anion with more extended π-conjugated system.
The present work reports on the study of intramolecular
hydrogen bond in extended series of compounds 1−4 both
theoretically, using DFT calculations and topological AIM
Received: July 21, 2013
Revised: October 2, 2013
Published: October 4, 2013
© 2013 American Chemical Society
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Scheme 1. Compounds Studied in This Work (1−4) and Model Compound 5
strongly basic anions, like fluoride, on deprotonation of the
pyrrole NH group.
compound, compared with the indandione or tricyanoethylene
derivatives.
EXPERIMENTAL AND COMPUTATIONAL
METHODS
■
Synthesis of compounds 1 and 3, their physical properties, and
their spectra are described in ref 1.
2-((1H-Pyrrol-2-yl)methylene)-5,6-benz[f ]-1,3(2H)-in-
dandione (2). 5,6-Benz[f]-1,3(2H)-indandione (0.324 g, 1.65
mmol) and 1H-pyrrole-2-carbaldehyde (0.157 g, 1.65 mmol)
were suspended in 10 mL of acetic acid, and 3 drops of
concentrated HCl were added under vigorous stirring. Then
the mixture was refluxed for 45 min and cooled to room
temperature. The reaction mixture was poured into 50 mL of
water, the precipitate formed filtered, dried, and purified by
column chromatography with dichloromethane as an eluent to
give 0.32 g (70%) of 2 as an orange solid. Mp 212−214 °C.
Anal. Found, %: C 79.33; H 4.22; N 5.20. C18H11NO2. Calcd,
%: C, 79.11; H, 4.06; N, 5.13. 1H NMR, δ, ppm, CDCl3: 13.49
bs, 1H, NH; 8.42 s, 1H and 8.41 s, 1H − H-4, H-9; 8.09 m, 2H,
H-5,H-8; 7.83 s, 1H, CH; 7.69 m, 2H, H-6, H-7; 7.46 m, 1H,
H-5′; 7.14 m, 1H, H-3′; 6.67 m, 1H, H-4′. 13C NMR, δ, ppm,
CDCl3: 191.67, 190.38, 136.70, 136.34, 136.17, 135.93, 133. 90,
131.97, 130.52, 130.33, 128.92, 127.36, 123.48, 123.32, 114.80.
(Z)-2-((1H-Pyrrol-2-yl)methylene)-[1,2′-biindenyli-
dene]-1′,3,3′(2H)-trione (4). To the suspension of [1,2′-
biindenylidene]-1′,3,3′(2H)-trione (bindone) (0.32 g, 1.16
mmol) in 4 mL of acetic anhydride at 60 °C was added 1H-
pyrrole-2-carbaldehyde (0.11 g, 1.16 mmol). The mixture was
stirred overnight and cooled. The precipitated dark-violet solid
was filtered, washed with cold acetic anhydride, and dried. Yield
0.31 g (76%). Mp 240−242 °C. Anal. Found, %: 78.43; H,
3.64; N 3.72. C23H13NO3. Calcd, %: C, 78.62; H, 3.73; N, 3.99.
1H NMR, δ, ppm, CDCl3: 13.48 bs, 1H, NH; 8.84 d, 1H, H-10;
8.00 s, 1H, H-6; 8.00 (overlapped), 1H, H-20 or H-17; 7.96 d,
1H, H-17 or H-20; 7.85 d, 1H, H-13; 7.81 m, 2H, H-18, H-19;
7.69 t, 1H, H-11; 7.63, t, 1H, H-12; 7.53 m, 1H, H-5; 7.12 m,
1H, H-3; 6.58 m, 1H, H-4. 13C NMR, δ, ppm, CDCl3: 192.55,
190.83, 189.42, 163.23, 143.12, 141.75, 140.77, 139.49, 139.42,
The derivatives 6a−c modified by electron acceptor
substituents10 strongly bind various anions (Cl−, AcO−,
HP2O73−) because of formation of several hydrogen bonds.
Binding of the fluoride ion is also based on hydrogen bonding
at low ratios [F−]/[6], but at higher concentrations of the base
the deprotonation of the substituted (activated) pyrrole motif
takes place. Indole derivatives 10a−c undergo deprotonation
under the action of anions, but 10a is distinguished by its high
selectivity toward fluoride, whereas other compounds show
3−
approximately equal responses to F−, AcO−, and HP2O7
ions.15 It was explained by lower NH acidity of the former
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Table 1. Selected B3LYP/6-311(d,p) Optimized Geometric Parameters (in Å) for Compounds 1−4 in Gas Phase and DMSO
Solution (PCM Model)
gas
DMSO
compd
O···H−N
C6−C7
C7−C9
C5−C6
O···H−N
C6−C7
C7−C9
C5−C6
1
2
3
4
1.797
1.781
1.754
1.759
1.370
1.372
1.383
1.381
1.487
1.488
1.468
1.465
1.411
1.409
1.408
1.409
1.826
1.812
1.783
1.796
1.374
1.376
1.387
1.385
1.482
1.482
1.462
1.459
1.407
1.407
1.404
1.405
137.99, 134.57, 132.98, 131.66, 131.11, 130.08, 128.53, 123.25,
122.95, 122.69, 120.52, 114.66.
Table 2. Wiberg Bond Indices Calculated for Trans-
Conformers of Molecules 1−4
Calculations of the optimized geometries, total energies, and
free energies of compounds 1−4, their anions, and their H-
complexes in the gas phase and solution were performed at the
DFT level of theory using B3LYP potential and 6-311(d,p)
basis set. Solvent effects were taken into account using the
polarizable continuum model (PCM) and supermolecule−
polarizable continuum model (SM−PCM) with DMSO as a
solvent. No restrictions were imposed on the geometry
optimization. All calculated minima were checked by frequency
calculations; no imaginary frequencies were found. Simulation
of the proton transfer between the pyrrolyl part of 1B and the
acetate anion was performed by scanning the N···H distance for
the fixed N···O distance of 2.60 Å. The AIM analysis was
performed at the MP2/6-311++G(d,p) level of theory.
Chemical shifts were calculated with the GIAO method at
the B3LYP/6-311(d,p) optimized geometry. All computations
were performed with the Gaussian 09 program package.16
UV−vis spectra for 1 × 10−4 M solutions of pure compounds
1, 2, 4 in acetonitrile and with addition of Bu4NF were
recorded on a Perkin-Elmer Lambda 35 UV/vis spectrometer
for the molar ratio compound/Bu4NF from 1:1 to 1:200.
1H and 13C NMR spectra were recorded on a Bruker DMX-
500 spectrometer at working frequencies of 500.13 MHz (1H)
compd
gas
DMSO
1
2
3
4
0.8090
0.8092
0.8030
0.8077
0.7894
0.7888
0.7894
0.7887
Table 3. Properties of the BCP (NH···O) and Hydrogen
Bond Energies EHB in the Cis-Conformers 1A−4A in Gas
Phase and DMSO Solution (B3LYP/6-311G** Optimized
Geometry)
a
a
b
compd
ρ(r)
∇2ρ(r)
Medium: Gas
EHB
1
2
3
4
3.71
3.86
4.06
4.06
12.33
12.64
13.69
13.52
10.3
10.9
11.9
11.8
1
and 125.1 MHz (13C). H and 13C NMR chemical shifts are
reported in parts per million relative to TMS.
Medium: DMSO Solution (PCM) (ε = 46.7)
1
2
3
4
3.97
3.57
3.81
3.70
11.76
12.06
13.16
12.74
9.3
9.8
RESULTS AND DISCUSSION
■
10.9
10.5
Theoretical Calculations. Evidently, the anion sensing
properties of the studied compounds are determined by their
ability to form intermolecular N−H···B hydrogen bond after
the rupture of the intramolecular N−H···OC hydrogen bond
and 180° flip of the 2-pyrrolyl group. Therefore, theoretical and
experimental investigation of both types of hydrogen bonding
in compounds 1−4 is a key point of the present study.
Geometry. Most important geometric parameters of
compounds 1−4 in the vicinity of the N−H···OC hydrogen
bond are shown in Table 1. The H···O distance in compounds
1 and 2 is 0.04 Å longer than those in 3 and 4, indicating a
stronger hydrogen bond in the latter compound. The order of
the O −H···N bond lengths in Table 1 (3 ≥ 4 > 2 > 1)
coincides with the order of the H-bond energies as determined
from the AIM analysis (vide infra, Table 3).
Medium: DMSO Solution (SM + PCM) (ε = 46.7)
1
2
3
4
2.44
2.50
2.74
2.62
8.85
9.11
5.9
6.1
7.0
6.6
10.41
9.79
a
MP2(full)/6-311++G** calculated electron density [ρ(rc), a.u.] and
b
its Laplacian [∇2ρ(rc), a.u.], in BCP (O···HN). The H-bond energy
[EHB, kcal/mol].
At the same time, neither the Mulliken (−0.385, −0.391,
−0.387, −0.391) nor the electrostatic charges (−0.517, −0.531,
−0.499, −0.513) on the H-bonded carbonyl oxygen atoms in
compounds 1−4, respectively, correlate with the corresponding
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Figure 1. Structure of cis- and trans-complexes of 1−4 with DMSO
(see Table 4).
Figure 2. Solution of compound 2 in DMSO in the presence of 10
−
−
equiv of anions (from left to right): none, F−, Cl−, Br−, HSO4 , PF6 ,
−
CH3CO2 .
hydrogen bond lengths from Table 1. Moreover, judged from
the chemically reasonable electrostatic charges, the carbonyl
groups in 3 and 4 seem to have a somewhat lower basicity
which is in apparent contradiction with the shorter hydrogen
bond in these molecules. To remove this discrepancy, one
should keep in mind that the strength of the hydrogen bond
N−H···OC depends also on the acidity of the NH group in
the same molecule. We tried to estimate the NH acidity by
calculating the Wiberg bond indices of the N−H bond in the
absence of interaction of the NH proton with the carbonyl
oxygen, that is, for the trans-conformers (Table 2).
However, although these indices correctly reproduce the
increase of NH acidity by transferring a molecule from the gas
phase to DMSO solution, in both phases they are practically
constant within the series. Taking into account that the NH
acidity is favored not only by weakening of the N−H bond but
also by extension of the conjugation system of the rest of the
molecule, we have considered the geometry of the chain of
conjugation between the electron donating 5-pyrrolyl group
and the aromatic part of molecules 1−4. As a criterion of the
degree of extension of conjugation, the difference between the
C7−C9 and C6−C7 bond distances can be used. This value
decreases by >0.032 Å, from ∼0.12 Å for molecules 1 and 2 to
∼0.08 Å for molecules 3 and 4. This means that the hydrogen
bond in the studied compounds belongs to the resonance-
assisted type,4 and the conjugation is more efficient and the
hydrogen bond is stronger in molecules 3 and 4.
chelate ring as indicated by the ring critical point [RCP (3,
+1)].
According to the electron density ρ(r) values and their
corresponding Laplacian ∇2ρ(r), the positive electron energy
Ee(r), and the ∇2ρ(r) value in BCP (NH···O), the hydrogen
bond (NH···O) in the molecules 1−4 both in the gas phase and
in DMSO solution has medium strength (that is, fall in the
range 4−15 kcal/mol, Table 3) and generally follows the order
3 ≥ 4 > 2 > 1. In agreement with the experimental data (1H
NMR, vide infra) both optimized geometry and the AIM
parameters show moderate (by 1.0−1.3 kcal/mol) weakening
of the hydrogen bond in terms of PCM model calculations and
a larger one (by 3−4 kcal/mol) in terms of combined PCM +
SM approach. Note that molecule 1 forms a bifurcate hydrogen
bond with DMSO in the intramolecularly H-bonded cis-
conformation (complex 1A−DMSO) and an intermolecular H-
bond in the trans-conformation (complex 1B−DMSO) (Figure
1).
In a recent paper1 we have shown experimentally the
existence of equilibrium between the hydrogen-bonded cis-
conformers and nonbonded trans-conformers of compounds 1
and 3 in polar solvents such as acetone, pyridine, and DMSO,
whereas in acetonitrile and methylene chloride only the
hydrogen-bonded cis-form was observed. This allowed us to
conclude that cis−trans equilibrium is assisted by intermo-
lecular complexation with a basic solvent, in particular, with
AIM Analysis. The properties of the N−H···O hydrogen
bond in molecules 1−4 were studied not only by geometric
criteria but also by quantum chemical topological analysis. The
AIM method was used to find the bond critical point [BCP (3,
−1)] of the N−H···O hydrogen bond to evaluate its energy and
to identify the formation of the corresponding seven-membered
1
DMSO. The spectral evidence was the broadening of the H
NMR H-3 signals and the existence of cross-peaks between H-6
on the one hand and NH and H-3 on the other hand, in 2D
NOESY spectra. The same spectral features are observed for
compounds 2 and 4 (Figure 11 and Figure S1).
Table 4. Structure and Conformational Equilibria of Compounds 1−4 and Their Complexes with DMSO (See Figure 1)
compd
1A
r1, Å
r2, Å
2.308
r3, Å
ΔG°, kcal/mol
A−DMSO/B−DMSO (∼300 K)
1.797
1.984
4.53
0.12
∼100:0
55:45
1A−DMSO
1B−DMSO
2A
1.771
1.737
1.711
1.733
1.781
1.971
5.10
0.85
∼100:0
80:20
2A−DMSO
2B−DMSO
3A
2.314
2.246
2.282
1.754
1.929
6.89
0.67
∼100:0
75:25
3A−DMSO
3B−DMSO
4A
1.759
1.958
4.87
1.39
∼100:0
91:9
4A−DMSO
4B−DMSO
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Figure 5. UV−vis spectra of compound 2 with fluoride ion in MeCN:
1/Bu4NF = 1:0 (1), 1:2 (2), 1:6 (3), 1:10 (4), 1:30 (5), 1:50 (6),
1:100 (7), 1:150 (8).
Figure 6. UV−vis spectra of compound 1 with acetate ion in MeCN:
1/Bu4Ac = 1:0 (1), 1:1 (2), 1:50 (3), 1:200 (4), 1:400 (5), 1:500 (6),
1:600 (7), 1:700 (8), 1:1000 (9).
Figure 3. UV−vis spectra of compounds 1, 2, 4 in MeCN (C = 1 ×
10−4 mol/L).
Figure 7. UV−vis spectra of compound 4 with acetate ion in MeCN:
4/Bu4NAcO = 1:0 (1), 1:10 (2), 1:20 (3).
Figure 4. UV−vis spectra of compound 1 with fluoride ion in MeCN:
1/Bu4NF = 1:0 (1), 1:1 (2), 1:10 (3), 1:20 (4), 1:30 (5), 1:50 (6),
1:100 (7), 1:200 (8).
gas phase (ΔE = 8.1 kcal/mol for 1) and in DMSO solution
(5.2 kcal/mol, PCM model). Note that these values differ from
the H-bond energies estimated from the AIM analysis (Table
3), thus proving that they include not only hydrogen bond
energies but also various other interactions like dipole−dipole,
The H-bonded cis-conformations of compounds 1−4 are
much more stable than their trans-conformations both in the
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1
Figure 8. H NMR spectra of 1 in DMSO-d6 (a) and with 20-fold excess of Bu4NF in 1:1 CD2Cl2/DMSO-d6 mixture at 260 K (b).
1
Figure 9. H NMR spectrum of 1 in CD2Cl2 (a) and in DMSO-d6 (b) in the presence of various amounts of Bu4NF.
1
electrostatic, and hydrogen bonding between C−H-3 and
carbonyl in the trans-conformation.
other cases neither UV−vis nor H NMR spectra show any
changes.
Intense long-wave absorption bands in the UV−vis spectra of
The position of the A−B cis ⇆ trans equilibrium determined
by the free energy difference (ΔG°) is strongly shifted to the
left in the gas phase, whereas for the complexes of the two
species with DMSO (A−DMSO and B−DMSO) the
concentration of the trans-conformation B−DMSO is sub-
stantial or even almost equal to that of the cis-conformation A−
DMSO (Table 4).
solution of compounds 1, 2, and 4 in acetonitrile have maxima
at 427, 451, and 345 nm, respectively (Figure 3). Addition of
fluoride ion to the solutions of compounds 1 and 2 in the molar
ratio from 1:1 to 1:200 results in a decrease of the intensity of
these bands and appearance and increase of the intensity of the
bathochromically shifted bands (457 and 492 nm) due to
electronic transitions in the corresponding anions (Figures 4
and 5).
Interaction with Anions. UV−Visible Spectroscopy
−
Study. The addition of various anions (Cl−, Br−, HSO4 ,
The absence of clearly defined isosbestic points in the spectra
in Figures 4 and 5 points to the participation of more than two
forms in the reaction of substrates 1 and 2 with Bu4NF (e.g.,
the starting neutral molecule and its anion in the cis- and trans-
conformation). The maximum at 469 nm in the spectra of
compound 2 with Bu4NF in Figure 5 may just be suggestive of
PF6 , F−, AcO−) in the form of the corresponding
−
tetrabutylammonium salts (always the same thereinafter) to
the DMSO solutions of compounds 1−4 was studied. It was
found that the addition of the fluoride and, to a lesser extent,
acetate ion causes color change as shown in Figure 2. In all
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Figure 10. Low-field (left) and aromatic (right) parts of the 1H NMR spectra of 1 in DMSO-d6 upon addition of 0 (a), 1 (b), 2 (c), and 5 (d) equiv
of Bu4NF.
the appearance of the trans anionic conformation in the
equilibrium.
The acetonitrile solution of compound 4 reacts instantly with
fluoride ion, turning from light-brown to blue-green and then
to red-brown.
Addition of tetrabutylammonium acetate to the acetonitrile
solution of compound 1 results in similar changes of the
spectrum of the neutral molecule as those occurring with
fluoride ion (Figure 6) except for the fact that the equilibrium is
strongly shifted toward the neutral form. As a result, even with
large excess of acetate ion (1:1000) the maximum belonging to
the anion of compound 1 (457 nm) is displayed only as a
shoulder on the long-wave wing of the band of the starting
molecule.
No changes occur in the spectrum of compound 4 in
acetonitrile solution with equimolar amount of acetate ion.
With 10-fold excess of AcO− (Figure 7, spectrum 2) the
intensity of the band at 345 nm decreases; a larger excess of the
acetate ion (Figure 7, spectrum 3) leads to a further drop in
intensity of this band, an increase of intensity of the bands at
412 and 572 nm, and appearance of an absorption band at 600
nm, responsible for the green color of the solution.
NMR Spectroscopy Study. 1H NMR spectra of the solutions
of compounds 1, 2, and 4 in DMSO-d6 upon addition of excess
Bu4NF confirm the N-deprotonation of the pyrrole ring
(Figures 8, 11, and S2). The NH signal at 12.9 ppm disappears
−
and a new triplet signal corresponding to HF2 ion arises at
16.1 ppm. Delocalization of the negative charge over the whole
molecule leads to a high-field shift of all protons except pyrrolic
H-3, which moves to low field by about 1.2 ppm.
Dramatic changes of the 1H NMR spectrum upon the
formation of the anion deserve special consideration. Unlike
neutral compound 1, the cis-conformation of anion 1A is
unstable because of repulsive interaction between the negatively
charged nitrogen and the carbonyl oxygen. On the contrary, the
trans-conformation is stabilized by the C−H···OC hydrogen
bond as proved by a large low-field shift of H-3 (8.52 ppm)
with respect to those of H-4 and H-5 (6.37 and 7.6 ppm,
respectively):
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Table 5. Geometric Parameters (in Å) of the Complexes 1−4 with Anions
anion
Cl
Br
HSO4
AcO
a
a
b
b
compd
r1
r2
r3
r1
r2
r3
r1
r2
r3
r1
r2
r3
1A−X−
1B−X−
2A−X−
2B−X−
3A−X−
3B−X−
4A−X−
4B−X−
1.835
2.712
1.830
2.884
1.920
2.590
2.573
2.421
2.446
2.020
2.307
2.338
2.193
2.245
2.192
2.190
2.177
2.183
2.367
2.358
2.354
2.360
1.887
1.782
1.722
1.737
1.610
1.576
1.546
1.561
1.823
1.791
1.800
2.680
2.689
1.820
1.792
1.801
2.865
2.832
2.861
1.899
1.884
1.903
2.017
1.998
2.012
2.690
a
b
Short contact H-5−Cl(Br). External component of bifurcate hydrogen bond for HSO4 and AcO.
more stable complex [1B···HF2]− having the calculated NH
chemical shift of 18.2 ppm (see Figure 9c).
Titration of 1 with Bu4NF in CD3CN solution results in a
similar signal appearing at 12.4 ppm, which disappears at the
same excess of fluoride. However, the calculated NH chemical
shift of 1B−MeCN is only 10.8 ppm (due to both a weaker
coordination and the anisotropy contribution of the CN
bond) so that the experimental shift of 12.4 ppm cannot be
obtained by averaging between 1B and 1B−MeCN, and one
has to assume the presence of an additional species in the
equilibrium with the NH chemical shift larger than 12.4 ppm.
In our opinion, this may be complex [1B···HF2]− (δNH 18.2
ppm). Then the question arises: Why is it that titration with
Bu4NF in DMSO does not require invoking the hypothesis of
the presence of the fluoride complex 1B−···HF, whereas in
acetonitrile it cannot be avoided? Of course, in DMSO the
complex 1B−···HF may also be involved in the equilibrium, but
its role must be very limited because of much higher stability of
the complex of 1B with DMSO than with MeCN.
Titration of 1 in CD2Cl2 was not carried out because the
appearance of deprotonation product is observed only at large
excess of fluoride and amounts to only 10%.
The observed solvent dependence may be explained by the
existence of a significant amount of the trans-conformation in
DMSO (see above), its small concentration in acetonitrile, and
its complete absence in dichloromethane.
The B3LYP/6-311G(d,p) energy of anion 1 in the trans-
conformation (1B) is 3.9 kcal/mol lower than that of the cis-
conformation.
Deprotonation of compound 1 in CD2Cl2 and CDCl3 occurs
only partially: addition of 60-fold excess Bu4NF gives a mixture
containing ∼10% of the anion and ∼90% of neutral 1 (Figure
9).
In DMSO-d6, gradual addition of Bu4NF (Figure 9) results in
broadening and decreasing the NH signal of 1 at 12.8 ppm,
which completely disappears in the presence of 3 equiv of
Bu4NF. Besides, a new signal appears at 12.0 ppm upon the
addition of 0.5 equiv of Bu4NF, reaching the maximum integral
intensity with 1 equiv of Bu4NF and disappearing also at 3-fold
excess of Bu4NF.
One can suppose that this new signal belongs to the trans-
form of 1, hydrogen-bonded with DMSO (1B−DMSO) and
existing in equilibrium with the nonbonded one (1B). The
calculated NH chemical shift of 1B is 8.6 ppm, and that of 1B−
DMSO is 13.2 ppm. The experimental chemical shift of 12.0
corresponds (assuming good agreement between calculated and
experimental chemical shifts) to the constant of the equilibrium
1B + DMSO ⇆ 1B−DMSO equal to ∼3, which seems quite
reasonable because of the high concentration of DMSO. The
addition of Bu4NF facilitates transformation of 1A to 1B
because of deprotonation of 1 by fluoride ion and free rotation
about the C−Cpyrr bond. However, with large excess of
DMSO (as a solvent) the equilibrium 1B−···HF + DMSO ⇆
1B−DMSO + F− must be shifted to the right. This means that
upon addition of up to 1 equiv of Bu4NF the fluoride ion plays
the role of a mediator and its complexes with 1B can be ignored
(see Figure 8b). Further addition of Bu4NF leads to
deprotonation of 1B and an increase of concentration of the
The interaction of 1 with excess of [Bu4N]OAc in DMSO
and acetonitrile, as was mentioned above, results in
deprotonation and formation of the anion which is proved by
the identity of the 1H NMR spectra in the presence of excess of
[Bu4N]OAc and Bu4NF.
The behavior of 2 in the presence of fluoride and acetate ions
in the same three solvents is similar to that of 1. The 1H NMR
spectral patterns corresponding to the pyrrole part of both
anions are identical (Figures 8 and 11).
However, the spectral patterns of aromatic moieties of the
two anions are different (Figure 10). Whereas the signals of
four aromatic protons of anion 1 form unsymmetric ABCD
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and bromide ions show the existence of short contacts between
H-5 and halide ion (2.68−2.71 and 2.83−2.88 Å, respectively).
The intramolecular N−H···O hydrogen bond length in these
complexes remains the same as in free molecules. Sulfate and
acetate ions form with 1A−4A complexes possessing bifurcate
hydrogen bonds with the internal components significantly
longer than in the neutral precursors.
Table 6. Complex Formation Energy ΔEZP, Free Energy
ΔG°, and Intermolecular Hydrogen Bond Energy E(HB) for
Interaction of 1−4 with Snions
ΔEZP
,
ΔG,
E(HB),
kcal/mol
complex
kcal/mol
kcal/mol
1A−Cl
1B−Cl
1A−Br
1B−Br
1A−HSO4
1B−HSO4
1A−AcO
2.8
−1.5
2.6
8.2
5.8
5.3
Short contacts between pyrrolic hydrogen and X− of 1B−
4B−X− smaller than the sum of their van der Waals radii were
found in all cases. AIM analysis reveals the existence of BCP in
N−H···X fragment in all cases except Br−. The values of
hydrogen bond energy of anion complexes of trans-conformers
1B−4B derived from BCP are given in Table 6.
7.9
−1.3
−2.5
−10.8
−6.6
−15.9
−29.4
−40.5
2.6
5.1
−
6.7
1.2
6.8
3.1
1B−AcO (1B−−AcOH)
1A−−HF
−5.3
−23.3
−33.2
8.3
19.0 (25.7)
As was mentioned above, the estimated acidity of NH
hydrogen atoms is approximately the same for 1−4 (Table 2).
The geometric (bond lengths) and energetic parameters of
hydrogen bonds of 1B−4B with chloride anion are very close,
and the same is true for geometry of bromide complexes. For
acetate complexes, hydrogen bonds of 2B−4B are also very
similar, both in energy (E(HB) = 21.4−23.6 kcal/mol, Table 6)
and in geometry (r3 = 1.546−1.576 Å, Table 5). In the case of
sulfate complexes, both energies and bond lengths are quite
different (6.8−12.2 kcal/mol and 1.722−1.887 Å, respectively).
Because of additional hydrogen bond between carbonyl oxygen
in 1B and 2B and sulfate OH group (Figure 11), the “main”
hydrogen bond N−H···O became weakened and more bent.
For similar complexes of sulfate with 3B and 4B additional
hydrogen bond cannot be formed. It should be mentioned,
however, that the calculated free energy of complexation ΔG°
for all these complexes (both cis- and trans-, except trans-
complexes with acetate and, in the cases of 3B and 4B, for
sulfate anions) is positive, and hence, their concentration is very
small. This explains the insensitivity of the studied substrates to
chloride, bromide, and sulfate ions (Figure 2). The acetate
complexes of 1A−4A are also unstable in spite of rather large
negative ΔE values, whereas their trans-counterparts are very
stable (Table 6). One can conclude that the studied pyrrole
derivatives show the acetate sensing ability only being in the
trans-conformation. Geometry optimization reveals a two-well
potential for the hydrogen atom transfer in the acetate
complexes with 1B−4B simulating the process of deprotona-
tion: one minimum corresponds to a complex of NH-pyrrole
with acetate and has the N−H bond of 1.07−1.08 Å, and the
second one corresponds to a complex of pyrrolium anion with
acetic acid and has the N−H distance of 1.59−1.60 Å (Figures
12 and S3). The difference of zero-point corrected energies
between the two wells is 1.3 kcal/mol for 1, 1.07 kcal/mol for
2, 0.06 kcal/mol for 3, and 0.38 kcal/mol for 4 in favor of the
N−H···OAc complex.
1B−−HF
2A−Cl
33.3
2B−Cl
2A−Br
−1.7
2.5
4.3
5.1
8.0
2B−Br
−1.6
−2.0
−7.9
−6.6
−40.9
−16.3
−29.5
2.4
4.3
−
2A−HSO4
2B−HSO4
2A−AcO
7.5
1.9
8.3
3.5
2B−AcO (2B−−AcOH)
2A−−HF
−34.0
−6.3
−23.0
7.8
21.4 (24.1)
2B−−HF
3A−Cl
31.4
3B−Cl
3A−Br
−1.3
2.4
4.6
5.6
8.2
3B−Br
−1.2
−3.0
−11.5
−7.5
−31.0
−16.8
−32.9
2.6
4.4
−
3A−HSO4
3B−HSO4
3A−AcO
7.0
−1.1
3.2
12.2
3B−AcO (3B−−AcOH)
3A−−HF
−36.6
−7.0
−25.5
3.1
30.9 (21.7)
23.6
3B−−HF
4A−Cl
4B−Cl
4A−Br
−1.6
2.4
4.4
5.5
7.7
4B−Br
−1.5
−2.8
−12.9
−7.1
−16.5
−32.5
−27.9
4.3
−
4A−HSO4
4B−HSO4
4A−AcO
7.0
−0.5
3.4
11.4
4B−AcO (4B−−AcOH)
4A−−HF
−5.6
−25.7
−35.1
22.4 (21.9)
30.0
4B−−HF
spin system, the similar protons of anion 2 form symmetric
AA′BB′ spin system (two multiplets at 7.57 and 8.08 ppm).
Besides, H-4 and H-9 of the central benzene ring give one
singlet, whereas in the neutral molecule they are nonequivalent.
This may be the result of fast rotation of naphthindandione
moiety around the C−C bond, which in turn arises from a
larger than in 1 polarization of this bond due to more electron
acceptor character of the aromatic fragment.
Fluoride ion abstracts the NH proton from molecules 1−4 in
both the cis- and trans-conformations to form H-bonded
complexes of pyrrolium anion with HF with large negative
values of the total energy and free energies of complexation.
CONCLUSION
Complexes with Anions, Their Geometry, and Energy.
Interaction of 1−4 in their cis- and trans-conformations with
halide (Cl, Br, F), sulfate, and acetate ions was studied by
quantum chemical (DFT) calculations. The geometric and
energetic parameters of the complexes are collated in Table 5
(except those with fluoride anion, which in fact are complexes
of anions 1B−4B with HF) and Table 6.
■
The strength of the intramolecular hydrogen bond N−H···O in
the series of compounds 1−4, estimated as 10.3−11.9 kcal/mol
in gas phase and methylene chloride solution, diminishes by
about 1 kcal/mol in DMSO. This weakening paves the way to
equilibrium between cis- and trans-conformers about the C−C
bond supported by the intermolecular hydrogen bonding of
pyrrolyl NH group with the solvent molecules. The presence of
the trans-form in polar solvents facilitates complexation with
Optimized structures (PCM model, solvent DMSO) of the
complexes formed by the cis-conformers 1A−4A with chloride
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1
Figure 11. H NMR spectra of 2 in DMSO-d6 in the absence (a) and with 20-fold excess of Bu4NF (b).
ASSOCIATED CONTENT
■
S
* Supporting Information
1
1H NMR spectrum of 4 in DMSO-d6, H NMR spectrum of 4
with excess Bu4NF, potential curve simulating proton transfer
in the complex 1B−AcO−, and full citation of ref 16. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
REFERENCES
■
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