Nitrophenyl derivatives of pyrrole 2,5-diamides: Structural behaviour, anion binding and colour change signalled deprotonation

Article abstract of DOI:10.1039/b210848h

Two new pyrrole 2,5-diamide clefts have been synthesized containing 4-nitrophenyl or 3,5-dinitrophenyl groups appended to the amide positions. The 3,5-dinitrophenyl derivative has been shown to deprotonate in the presence of fluoride, which in acetonitride solution, gives rise to a deep blue colour.

Full text of DOI:10.1039/b210848h

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Nitrophenyl derivatives of pyrrole 2,5-diamides: structural
behaviour, anion binding and colour change signalled
deprotonation†
Salvatore Camiolo, Philip A. Gale,* Michael B. Hursthouse and Mark E. Light
School of Chemistry, University of Southampton, Southampton, UK SO17 1BJ.
E-mail: philip.gale@soton.ac.uk; Fax: 44-23-8059-6805; Tel: 44-23-8059-3332
Received 7th November 2002, Accepted 16th December 2002
First published as an Advance Article on the web 28th January 2003
Two new pyrrole 2,5-diamide clefts have been synthesized containing 4-nitrophenyl or 3,5-dinitrophenyl groups
appended to the amide positions. The 3,5-dinitrophenyl derivative has been shown to deprotonate in the presence
of fluoride, which in acetonitrile solution, gives rise to a deep blue colour.
Et3N and DMAP which afforded the receptors in 43 and 11%
yields respectively.
Introduction
The selective recognition of anionic species by organic host
molecules is a rapidly developing area of supramolecular
chemistry.1 We have recently been investigating the anion com-
plexation properties of 2,5-diamide-substituted pyrroles (e.g.
1a), simple receptors that show oxo-anion selectivity.2 We found
that introduction of electron withdrawing chlorine substituents
to the 3- and 4- positions of the pyrrole ring, significantly
enhanced the affinity of the receptors (e.g. 1b) for anions such
as chloride,3 but more basic anions would deprotonate the
pyrrole NH group leading to the formation of interlocked
pyrrole anion dimers.3,4 We decided to introduce electron with-
drawing groups to the amide positions in an attempt to enhance
the affinity of the receptor without increasing the acidity of the
pyrrole NH proton to such an extent that deprotonation would
occur upon addition of anions. We therefore synthesized com-
pounds 2 and 3 containing nitro-aromatic moieties and investi-
gated their binding properties upon addition of anionic guests.
In most cases, these receptors were found not to deprotonate
and enhanced binding as compared to compound 1a was
observed. However, addition of fluoride to receptor 3 did cause
deprotonation, resulting (in acetonitrile) in the formation of a
deep blue coloured solution. Colorimetric sensors for fluoride
are particularly desirable as putative sensors for the nerve gas
Sarin (GB) (isopropyl methylphosphonofluoridate) which loses
a fluoride anion during hydrolysis.5
Crystals of compound 2 suitable for analysis by single crystal
X-ray diffraction were obtained by recrystallisation from
hot acetonitrile.6 The crystals were very small and hence the
diffraction weak, leading to a very high merging R factor. The
asymmetric unit contains one and a half molecules. The crystal
structure does not reveal the formation of dimers in the solid
state which has been observed with a number of other similar
molecules (Fig. 1).2 This may be due to extensive π-stacking in
Fig. 1 (a) The crystal structure of compound 2. Colour key: carbon =
green, nitrogen = blue, oxygen = red. (b) Side view of the π-stacking
interaction.
this crystal. Crystals of both compounds 2 and 3 were obtained
from DMSO solutions of the receptors‡. The crystal structures
reveal a DMSO bound to the pyrrole NH and one of the amide
NH groups with the receptors both adopting a “semi-cleft con-
formation” as has been previously observed with compound 12
(Fig. 2).7,8 When crystallised from a dichloromethane solution,
receptor 3 forms sheets that extend along the Ϫ1 0 1 plane via
pyrrole NH–ON(nitro) hydrogen bonds (Fig. 3).9
Results and discussion
Compounds 2 and 3 were synthesised by reaction of either
4-nitro- or 3,5-dinitroaniline with 3,4-diphenyl-1H-pyrrole-2,5-
dicarbonyl dichloride in dichloromethane in the presence of
Interestingly, in DMSO-d6–0.5% water solution, the amide
NH proton resonances of 1, 2 and 3 are at 9.36, 10.19 and 11.29
† Electronic supplementary information (ESI) available: 1H NMR, 13
NMR and mass spectra for compounds 2 and 3, NMR anion titration
profiles in DMSO-d6–0.5% water. See http://www.rsc.org/suppdata/ob/
b2/b210848h/
C
‡ CCDC reference number(s) 195412-195416. See http://www.rsc.org/
suppdata/ob/b2/b210848h/ for crystallographic files in .cif or other
electronic format.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 4 1 – 7 4 4
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receptors 2 and 3 making it impossible to calculate an associ-
ation constant based on the shift of this proton resonance.
Under the same conditions, addition of anions to solutions
of receptor 3 in DMSO-d6–0.5% water gave unusual results.
Surprisingly, upon addition of fluoride and benzoate, no
significant shift in the NH resonance was observed (with the
resonance disappearing at higher anion concentrations).
However the CH protons on the nitro-aromatic ring do shift
downfield in what appears to be a three-stage process for
fluoride (Fig. 4). Upon addition of the first two equivalents of
Fig. 4 NMR titration curve of compound 3 with fluoride anions in
DMSO-d6–0.5% water.
Fig. 2 Crystal structures of the DMSO solvates of (a) compound 2
and (b) compound 3. Colour key: carbon = green, nitrogen = blue,
oxygen = red, sulfur = yellow.
anion a curve is obtained that is similar to that observed during
the titration of compound 1b with fluoride in dichloro-
methane.2 Further addition of anions causes a downfield
shift until the plateau is reached after the addition of three
equivalents of fluoride. This behaviour seems to be consistent
with a three step process that may be hypothesized as follows:
the first equivalent of fluoride is coordinated by the receptor
(causing a significant downfield shift); the second equivalent
promotes the deprotonation process3,4 and the third equivalent
of fluoride is coordinated by the deprotonated receptor with the
participation of the phenyl CH groups that, in this receptor,
are particularly acidic because of the presence of the electron
withdrawing groups present in the phenyl ring.
X-Ray crystallographic analysis of crystals of 3 grown by
slow evaporation of a dichloromethane–methanol solution of
the receptor in the presence of excess tetrabutylammonium
fluoride, revealed the presence of an adventitious chloride
anion coordinated to receptor 3.12 The crystals were very small
and hence the diffraction weak, leading to a very high merging
R factor. The structure confirms that the receptor has been
deprotonated (two tetrabutylammonium counter cations are
present) and reveals a chloride anion bound by two CH–Cl and
two NH–Cl interactions in the plane of the three rings with
C(H)–Cl hydrogen bonds being in the range 3.377(11)–
3.397(12) Å and N(H)–Cl hydrogen bonds in the range
3.385(13)–3.403(12) Å. The chloride–pyrrolic nitrogen separ-
ation is 3.500(8) Å. We assume that if an amide had in fact
deprotonated, the chloride would not be bound in its current
position (Fig. 5).
For benzoate, the shift of the CH protons could be fitted to a
1 : 1 binding model that corresponds to an association constant
of 4200 MϪ1. With chloride, the amide NH protons do shift
allowing an association constant of 53 MϪ1 to be calculated.
Addition of dihydrogen phosphate gave a continuous downfield
shift of the amide NH protons.
We also investigated the possibility of using these receptors
as colorimetric sensors for anions in acetonitrile (Fig. 6). In this
solvent, the receptors are not soluble, however addition of
anions solubilises the host (in all cases except the addition of
bromide to compound 2). In the case of receptor 2, a yellow
colour appears with the anions that are bound most strongly (in
DMSO-d6 solution) i.e. fluoride, benzoate and dihydrogen
phosphate. However, with compound 3, an intense blue colour
Fig. 3 The crystal structure of compound 3 showing NH–ON
hydrogen bonds (phenyl groups in the 3- and 4- positions of the pyrrole
rings have been omitted for clarity). Colour key: carbon = green,
nitrogen = blue, oxygen = red.
ppm respectively indicating the increasing degree of de-shield-
ing of this proton as the number of nitro groups increases.
Proton NMR titrations were performed in order to assess
the affinity of the receptors for anions.10 Addition of solutions
of anions (as their tetrabutylammonium salts) to solutions of
receptor 2 in DMSO-d6–0.5% water and fitting of the resultant
titration curves gave association constants of 1245 MϪ1 for
fluoride, 39 MϪ1 for chloride and 4150 MϪ1 for benzoate. The
titration curve found with dihydrogen phosphate could not be
fitted to a simple 1 : 1 binding model. This may be due to
oligomerisation of the phosphate anions in solution11 or
alternatively simply to the formation of a variety of complexes
with differing stoichiometries. Unfortunately, the pyrrole NH
proton frequently disappears (presumably due to exchange
processes) upon addition of anions to the solution for both
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 4 1 – 7 4 4
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lographic asymmetric unit contains two half pyrrole anions and
a single tetrabutylammonium cation. The pyrrole 2,5-diamide
anions interlock as has been observed previously in 3,4-dichlo-
ropyrrole 2,5-diamides with NH ؒ ؒ ؒ NϪ distances in the range
2.97–3.08 Å.3,4 Interestingly when this material is dissolved in
dichloromethane it maintains the dark red colour in solution.
However a variety of colours are observed when the compound
is dissolved in more polar solvents: blue in acetonitrile or acet-
one, purple in methanol and violet in water. UV/VIS analysis
carried out on both an acetonitrile solution of this material and
compound 3 in the presence of excess fluoride, revealed identi-
cal UV/VIS spectra (with a maximum at 598 nm). Con-
sequently, we believe that the deep blue colour observed upon
addition of an excess of fluoride anions to acetonitrile solutions
of compound 3 is due to deprotonation of the receptor by the
fluoride anion.
The strategy of attaching electron withdrawing groups3,14
to an anion receptor to enhance anion affinity has worked in
this case with the affinity for benzoate increasing from 560 MϪ1
for compound 1a to 4150 MϪ1 for compound 2 and 4200 MϪ1
for compound 3. With our previous 3,4-dichloropyrrole-based
systems, benzoate and dihydrogen phosphate were shown to
deprotonate the receptors3 however in this case only receptor
3 undergoes a deprotonation process upon anion addition
and then only with fluoride. A number of research groups
are currently synthesizing new colorimetric anion sensors.15 We
have shown that deprotonation of a functionalised pyrrole
provides a selective colorimetric method of sensing fluoride in
acetonitrile solution.
Fig. 5 The crystal structure of 3-Hϩ binding chloride via NH and CH
hydrogen bonds (two tetrabutylammonium counter cations have been
omitted for clarity). Colour key: carbon = green, nitrogen = blue,
oxygen = red, chloride = light green.
Experimental
J-values are given in Hz.
N,NЈ-Bis(4-nitrophenyl)-3,4-diphenyl-1H-pyrrole-2,5-di-
carboxamide (2):
Fig. 6 Solutions of (a) receptor 2 and (b) receptor 3 (2 mM) in
acetonitrile with various anionic guests (added as their
tetrabutylammonium salt at a concentration of 20 mM). In the absence
of an anion, the receptors are not soluble in this solvent but are
solubilised upon addition of the anion (with the exception of receptor 2
and bromide).
3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid16 (1.0 g, 3.3
mmol) was suspended in SOCl2 (20 cm3) and refluxed
overnight. The reaction was allowed to cool and the excess of
SOCl2 was removed in vacuo. A brown solid formed that was
dissolved in CH2Cl2 (30 cm3) and Et3N (0.74 g, 7.3 mmol) and
DMAP (5 mg, 0.04 mmol) were added. After addition of
4-nitroaniline (0.98 g, 7.15 mmol), the mixture was stirred at
room temperature for 72 hours. The solvent was removed in
vacuo and acetonitrile (20 cm3) was added to the red solid. A
pale yellow compound crystallised, that was collected and
washed with acetonitrile (2 × 5 cm3). After drying, 770 mg of
the desired compound was obtained (43%). δH (300 MHz;
DMSO-d6; Me4Si) 7.26–7.35 (10H, m, Arom.), 7.85 (4H, d,
J 9.1, NHCCH ), 8.33 (4H, d, J 9.1, NO2CCH), 10.19 (2H, s,
CONH), 12.91 (1H, s, NH). δC (75 MHz; DMSO-d6; Me4Si)
119.1, 125.0, 126.9, 127.8, 128.1, 130.5, 133.2, 142.4, 144.8,
159.3. m/z (ESϩ) 1095 (M ϩ Mϩ). HRMS (ESϪ): 546 (MϪ), ∆ =
0.7 ppm. Found: C, 64.51; H, 3.75; N, 12.10%. Mϩ 0.2DCM
requires C, 64.26; H, 3.82; N, 12.41%. Mp >320 ЊC.
is observed upon addition of fluoride anions in acetonitrile. We
believe this colour is due to a deprotonation process caused by
fluoride acting as a base and subsequent charge transfer inter-
action between the deprotonated pyrrole and the nitroaromatic
functionalities. In order to confirm this, receptor 3 was treated
with 20 equivalents of TBAOH and, after removing the excess
base, a dark red material was crystallised from a mixture of
diethyl ether–dichloromethane. The crystal structure of this
material reveals that it is indeed the deprotonated ligand, crys-
tallised as the tetrabutylammonium salt (Fig. 7).13 The crystal-
N,NЈ-Bis(3,5-dinitrophenyl)-3,4-diphenyl-1H-pyrrole-2,5-di-
carboxamide (3):
3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid16 (1.5 g, 4.9
mmol) was suspended in SOCl2 (25 cm3) and refluxed
overnight. The reaction was allowed to cool and the excess of
SOCl2 was removed in vacuo. A brown solid formed that was
dissolved in CH2Cl2 (50 cm3) and Et3N (1.21 g, 12 mmol) and
DMAP (5 mg, 0.04 mmol) were added. After addition of 3,5-
dinitroaniline (1.88 g, 10 mmol), the mixture was stirred at
room temperature for 72 hours. The solvent was removed
in vacuo and acetonitrile (100 cm3) was added. A white solid
corresponding to the desired compound slowly crystallised
from the hot acetonitrile solution. The solid was collected,
Fig. 7 The X-ray crystal structure of the tetrabutylammonium salt
of 3-Hϩ. Colours represent individual components of the assembly.
Certain hydrogen atoms have been omitted for clarity.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 4 1 – 7 4 4
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washed with acetonitrile (2 × 10 cm3) and dried in vacuo (348
mg, 11%). δH (300 MHz; DMSO-d6; Me4Si) 7.26–7.33 (10H, m,
Arom.), 8.63 (2H, s, Arom.), 9.13 (4H, s, Arom.), 11.29 (2H, s,
CONH), 13.34 (1H, s, NH). δC (75 MHz; DMSO-d6; Me4Si)
112.5, 118.8, 126.7, 127.5, 130.1, 130.7, 133.4, 141.4, 148.2,
159.2. m/z (ESϪ) 636 (MϪ), 672 (M ϩ ClϪ), 749 (MϪ ϩ TFA).
HRMS (ESϪ): 636 (MϪ), ∆ = 2.1 ppm. Found C, 56.20; H, 2.62;
N, 15.35%. M requires C, 56.52; H, 3.00; N, 15.38%. Mp
>320 ЊC.
8 Crystal data for 3–2DMSO colourless plate, C34H31N7O12S2, Mr =
¯
793.78, T = 120(2) K, triclinic, space group P1, a = 8.608(5),
b = 15.212(5), c = 15.257(5) Å, α = 67.786(5), β = 84.475(5), γ =
73.886(5)Њ, V = 1776.8(13) Å3, ρcalc = 1.484 g cm Ϫ3, µ = 0.225 mmϪ1
,
Z = 2, reflections collected: 19499, independent reflections: 6028
(Rint = 0.1126), final R indices [I > 2σI]: R1 = 0.0559, wR2 = 0.0998,
R indices (all data): R1 = 0.1424. wR2 = 0.1242.
9 Crystal data for 3 colourless prism, C30H19N7O10, Mr = 637.52,
T = 120(2) K, monoclinic, space group P21/c, a = 20.3034(11),
b = 10.1636(5), c = 13.4570(6) Å, β = 104.057(2)Њ, V = 2693.8(2) Å3,
ρcalc = 1.572 g cm Ϫ3, µ = 0.122 mmϪ1, Z = 4, reflections collected:
17978, independent reflections: 4716 (Rint = 0.0965), final R indices
[I > 2σI]: R1 = 0.0550, wR2 = 0.1074, R indices (all data): R1 =
Acknowledgements
0.1256. wR2
= 0.1308. Close contacts exist between both
O7 ؒ ؒ ؒ N7(i) (2.763(7) Å) and O6 ؒ ؒ ؒ O10(i) (2.760(7) Å).
We would like to thank the EPSRC for a project studentship
(to S. C.) and for use of the X-ray facilities at the University of
Southampton. P. A. G. would like to thank the Royal Society
for a University Research Fellowship.
We would like to thank Christopher J. Woods for the
excellent cover illustration.
Presumably the former interaction contains an electrostatic
component, and this combined with
a potential CH ؒ ؒ ؒ O
interaction (C14 ؒ ؒ ؒ O10(i) = 3.34(5) Å) bring O6 and O10I into
close proximity. (i) = x, 0.5 Ϫ y, 0.5 ϩ z.
10 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.
11 M. E. Light, S. Camiolo, P. A. Gale and M. B. Hursthouse, Acta
Crystallogr. Sect. E: Struct. Rep. Online, 2001, 57, o727.
12 Crystal data for bistetrabutylammonium (3–Hϩ) ClϪ colourless
block, C62H90N9O10Cl, Mr = 1156.88, T = 120(2) K, triclinic,
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¯
space group P1, a = 13.408(5), b = 15.847(5), c = 17.023(5) Å,
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α = 73.042(5), β = 74.733(5), γ = 68.046(5)Њ, V = 3159.9(18) Å3,
ρcalc = 1.216 g cm Ϫ3, µ = 0.123 mmϪ1, Z = 2, reflections collected:
18298, independent reflections: 7903 (Rint = 0.2876), final R indices
[I > 2σI]: R1 = 0.1330, wR2 = 0.1941, R indices (all data): R1 =
0.4834. wR2 = 0.2967.
13 Crystal data for tetrabutylammonium salt of 3- Hϩ, red block,
C46H54N8O10, Mr = 878.97, T = 120(2) K, monoclinic, space
group C2/c, a = 33.3632(3), b = 15.2635(3), c = 23.1686(3) Å, β
= 130.061(1), V = 9029.98(19) Å3, ρcalc = 1.293 g cm Ϫ3, µ =
0.093 mmϪ1, Z = 8, reflections collected: 42281, independent
reflections: 7954 (Rint = 0.0497), final R indices [I > 2σI]: R1 =
0.0382, wR2 = 0.0903, R indices (all data): R1 = 0.0498. wR2 =
0.097.
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6 Crystal data for 2 colourless plate, C30H21N5O6, Mr = 547.52,
T = 120(2) K, orthorhombic, space group Pbcn, a = 32.285(2), b =
11.8982(6), c = 19.8475(11) Å, V = 7624.2(8) Å3, ρcalc = 1.431 g cm Ϫ3
,
µ = 0.102 mmϪ1, Z = 12, reflections collected: 40433, independent
reflections: 5946 (Rint = 0.4075), final R indices [I > 2σI]: R1 = 0.0941,
wR2 = 0.1043, R indices (all data): R1 = 0.3726. wR2 = 0.1634.
7 Crystal data for 2–3(DMSO) yellow plate, C36H36N5O9S3, Mr
=
¯
778.88, T = 120(2) K, triclinic, space group P1, a = 11.0659(2),
b = 12.0400(2), c = 15.9608(4) Å, α = 84.462(1), β = 81.333(1),
γ = 64.306(1)Њ, V = 1893.21(7) Å3, ρcalc = 1.366 g cm Ϫ3, µ = 0.256
mmϪ1, Z = 2, reflections collected: 19454, independent reflections:
6390 (Rint = 0.0621), final R indices [I > 2σI]: R1 = 0.0452, wR2 =
0.1118, R indices (all data): R1 = 0.0675. wR2 = 0.1222.
16 M. Friedman, J. Org. Chem., 1965, 30, 859.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 4 1 – 7 4 4
744