Chromogenic amides of pyridine-2,6-dicarboxylic acid as anion receptors

Article abstract of DOI:10.1080/10610278.2012.695789

The synthesis of simple, chromogenic pyridine-2,6-dicarboxylic acid amides, derivatives of isomeric nitroanilines and aminonitrophenols, and their ion binding properties are described. The ligands' response to ionic species was examined by naked eye and was studied with the use of UV-vis spectroscopy in DMSO and its mixture with water. The effect of the localisation and the type of the substituents in aromatic rings were discussed. ¹H NMR experiments were carried out to probe the mechanism of anion recognition, i.e. complexation via hydrogen bond formation versus ligand deprotonation. A selective response of N,N-bis(2-hydroxy-4-nitrophenyl)pyridine-2,6-dicarboxamide (L5) towards dihydrogen phosphate was found in both DMSO and DMSO-water (95:5) solvent mixture. The structure of N,N-bis(2-hydroxy-5-nitrophenyl)pyridine-2,6- dicarboxamide (L4) was confirmed by X-ray crystallography.

Full text of DOI:10.1080/10610278.2012.695789

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Supramolecular Chemistry
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Chromogenic amides of pyridine-2,6-dicarboxylic acid
as anion receptors
Ewa Wagner-Wysiecka a & Jarosław Chojnacki b
a Department of Chemical Technology , Chemical Faculty, Gdansk University of Technology ,
Narutowicza11/12, 80-233 , Gdansk , Poland
b Department of Inorganic Chemistry , Chemical Faculty, Gdansk University of Technology ,
Narutowicza11/12, 80-233 , Gdansk , Poland
Published online: 20 Jun 2012.
To cite this article: Ewa Wagner-Wysiecka & Jarosław Chojnacki (2012) Chromogenic amides of pyridine-2,6-dicarboxylic acid
as anion receptors, Supramolecular Chemistry, 24:9, 684-695, DOI: 10.1080/10610278.2012.695789
To link to this article: http://dx.doi.org/10.1080/10610278.2012.695789
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Supramolecular Chemistry
Vol. 24, No. 9, September 2012, 684–695
Chromogenic amides of pyridine-2,6-dicarboxylic acid as anion receptors
Ewa Wagner-Wysieckaa* and Jarosław Chojnackib
aDepartment of Chemical Technology, Chemical Faculty, Gdansk University of Technology, Narutowicza11/12, 80-233 Gdansk, Poland;
bDepartment of Inorganic Chemistry, Chemical Faculty, Gdansk University of Technology, Narutowicza11/12, 80-233 Gdansk, Poland
(Received 19 January 2012; final version received 9 May 2012)
The synthesis of simple, chromogenic pyridine-2,6-dicarboxylic acid amides, derivatives of isomeric nitroanilines and
aminonitrophenols, and their ion binding properties are described. The ligands’ response to ionic species was examined by
naked eye and was studied with the use of UV–vis spectroscopy in DMSO and its mixture with water. The effect of the
localisation and the type of the substituents in aromatic rings were discussed. 1H NMR experiments were carried out to probe
the mechanism of anion recognition, i.e. complexation via hydrogen bond formation versus ligand deprotonation. A selective
response of N,N0-bis(2-hydroxy-4-nitrophenyl)pyridine-2,6-dicarboxamide (L5) towards dihydrogen phosphate was found
in both DMSO and DMSO–water (95:5) solvent mixture. The structure of N,N0-bis(2-hydroxy-5-nitrophenyl)pyridine-2,6-
dicarboxamide (L4) was confirmed by X-ray crystallography.
Keywords: chromogenic amides; anion recognition; hydrogen bonds; UV–vis spectroscopy; NMR spectroscopy
1. Introduction
receptors have been exhaustively discussed in compre-
hensive reviews (4a–i). The anion–ligand interaction may
be of electrostatic nature, formation of hydrogen bonds,
binding anions by electron-deficient Lewis acidic centres,
anion–p, p–p interactions or of sum of all these (or part
of them) relating to both the host and the guest as well as
their surroundings.
After several decades, when very broad interest in
supramolecular chemistry was connected with metal cation
complexation, the present time seems to be a ‘golden age’
for anion coordination chemistry. There are a number of
reasons for still growing interest in anion recognition.
Anions play an important role in many biological
processes, e.g. carrying genetic information (polyanionic
DNA); a majority of enzyme substrates and cofactors are
anionic. There is also a dark side of ubiquitous anions.
Some may disadvantageously influence the environment
and human health or even life. For example, phosphates
and nitrates as fertilisers cause eutrophication of natural
waters (1); metabolites of nitrates are known to be
carcinogenic for many years (2). Fluorides widely used in
toothpastes may also cause serious diseases (3a, b).
Although the nature is the most perfect synthetic and self-
monitoring laboratory, the design and synthesis of artificial
anion receptors allows us to mimic natural systems and
learn about mechanisms of biochemical processes, and also
gives a useful analytical tool for anion detection and
determination.
One of the most popular constituents of building
blocks for anion receptors are groups that are able to form
hydrogen bond(s) (5). Such interactions utilising NZH
hydrogen donor groups are found in many types of
artificial anion receptors as calyx[n]pyrroles (6), urea and
thiourea derivatives (7), sulfonamides (8) and amides (9a,
b). A series of amide anion receptors, containing among
others 2,6-diamidopyridine and 1,3-diamidobenzene as
building blocks, have been described by Jurczak and co-
workers (10a–d). Pyridine, pyrrole, imidazole or more
sophisticated heterocyclic residues are often introduced
into an anion receptor forming primary or supplementary
parts of the ligand bearing hydrogen bond acceptor(s)
and/or donor(s) (11, 12). Besides NZH protons, also other
centres such as OZH (13a–c) or CZH (14a–c) may be
involved in hydrogen bond formation. Such systems, i.e.
utilising OH and CONH groups, occur in natural anion
binding sites of peptides (15a, b).
Anionic species are challenging targets for recognition
studies because of several features, such as their wide
range of sizes and shapes, diverse hydrophobicity, pH-
dependent charge, etc. There are many reasons why the
design of sufficiently selective anion receptors is not
always easy. This topic and many other important aspects
related to anion complexation by different types of
Data about anion–receptor interaction may be rapidly
acquired by enriching the ligand structure with chromo-
phore residue(s), resulting in the formation of chromo-
genic molecule. Due to cooperation of receptor and
*Corresponding author. Email: ewa.wagner-wysiecka@pg.gda.pl
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2012 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2012.695789
http://www.tandfonline.com

Supramolecular Chemistry
chromophoric parts, ion binding could be observed with a 2. Results and discussion
685
‘naked eye’. It also enables simple tracing of complexation
process using UV–vis spectroscopy. A considerable
number of publications in recent years concern chromo-
genic anion receptors, suggesting a major interest in this
area. Different types of chromogenic anion receptors have
been described in review articles by Suksai and Tuntulani
2.1 Synthesis and characterisation of ligands L1–L5
Ligands L1–L5 were prepared using a facile reaction of
commercially available 2,6-pyridine dicarboxylic acid
chloride with isomeric nitroanilines or aminonitrophenols
in DMF and in the presence of triethylamine. The general
synthetic route is shown in Figure 1.
´
(4e, 16). Another review article by Martinez-Man˜ez and
´
Sancenon (17) has presented both chromogenic and
Pure compounds were obtained after crystallisation
from methanol; in most cases, the yield was high (about
90%). Their structures were confirmed by high-resolution
mass spectra and 1H NMR, 13C NMR and FTIR
spectroscopy. Among all presented ligands, to the best
our knowledge – on the basis of Chemical Abstracts, only
L3 has been reported in literature until now. The reported
method, where acetonitrile (25) was used as a solvent, gives
a significantly lower yield (52% for L3) of the desired
compounds.
fluorogenic anion receptors showing different design
principles for anion sensing. Colorimetric anion sensors
differ from each other not only by the arrangement of
receptor site, but also by types of chromogenic residues
that are responsible for transduction of recognition
information into optical signal. Several of these com-
pounds may act as both colorimetric and fluorescent anion
chemosensors. For examples of different types of anion
receptors, see Refs (18–26).
In this paper, the synthesis of simple chromogenic
pyridine-2,6-dicarboxylic acid (dipicolinic acid) amides
L1–L5 (Figure 1) is presented. These receptors were
investigated as colorimetric and ‘naked eye’ sensors for
ionic species, mainly anions, in DMSO, an aprotic solvent,
and its mixture with water. The effect of the localisation
and the type of the benzene ring substituents are discussed,
and the possible mechanism of anion recognition is
proposed.
2.2 Complexation studies
Compounds presented in Figure 1, until now, were not
examined as chromogenic anion sensors, except L3, for
which colorimetric anion-sensing properties and mechan-
ism of anion recognition were studied (25, 26). Three
pseudo-polymorphs of L3 were characterised by X-ray
crystallography (27). It has also been reported (28) that L3
can efficiently bind to ruthenium(II).
O
O
N
NH
HN
L1 o-NO (89%)
2
H2N
L2 m-NO (83%)
2
L3 p-NO (90%)
2
NO2
NO2
NO
2
Cl
Cl
O
O
N
N
NO2
H2N
HO
O
O
NH
HN
NO2
O2N
Et3N, DMF, 60°C, 12h
L4 (67%)
OH
HO
H2N
HO
O
O
N
NO2
NH
HN
L5 (95%)
O2N
NO2
OH
HO
Figure 1. Synthetic route leading to chromogenic diamides L1–L5.

686
E. Wagner-Wysiecka and J. Chojnacki
Here, we report metal cation and anion binding studies The comparison of the spectral changes upon titration
of L1–L3 with TBAF and TBAOH in DMSO is shown in
Figure 3(a)–(d).
for ligands L1–L5. Complexation investigations were
carried out using UV–vis spectrophotometry in DMSO as
a solvent. The studied metal cations were alkali (Liþ, Naþ,
Kþ), alkaline earth (Mg2þ, Ca2þ, Sr2þ, Ba2þ) and heavy
The colour changes increase in the order
L3 . L1 . L2. It is in agreement with the observed
colour changes in the presence of an excess of fluorides.
From the titration course it may be concluded for all cases
that more than one equilibrium exists under titration
conditions. The appearance of a new band, of a relatively
large bathochromic shift and not a sharp isosbestic point for
L3 and L1, as well as the trend of changes for L2 may
suggest that two-step deprotonation of the ligands is
responsible for colour changes rather than anion com-
plexation via hydrogen bond formation. The other
possibility is the ligand–anion interaction via hydrogen
bond formation at low TBAF concentration and deprotona-
tion of the ligand in the presence of an excess of basic
fluorides. The most probable deprotonation process is
supported by the fact that almost identical spectral changes
for the investigated ligands were observed when titrated
with TBAOH and TBAF (Figure 3(d)). Relatively low
concentrations of the reactants, which are required for
typical UV–vis titrations, indicate that deprotonation is
more favoured than it is at higher concentration levels (both
the ligand and the respective salt), where the deprotonation
degree is smaller (13b, 29). Thus, further studies on the
nature of ligand–anion interactions were carried out by
divalent metal cations (Co2þ, Ni2þ, Zn2þ, Cu2þ, Pb2þ
)
used in the form of perchlorates. No significant changes in
absorption spectra, recorded in DMSO, were found in the
presence of all studied metal cations. It means that these
cations used as counterions should not affect anion
complexation, although in the studies presented only tetra-
n-butylammonium (TBA) salts were used for anion
complexation. To study the anion–ligand interactions,
anions of different sizes and shapes were chosen: spherical
(halides), linear (thiocyanate), V-shaped (simple carbox-
ylates: acetate, benzoate, tosylate), trigonal-planar
(nitrate) and tetrahedral (dihydrogen phosphate, hydrogen
sulphate, perchlorate). Anion complexation was examined
in pure DMSO and mixed DMSO–water (95:5, v/v)
solvent system.
As anion recognition via hydrogen bonding is not
always easy to differentiate from deprotonation of NZH
and/or OZH residues, experiments with tetra-n-butylam-
monium hydroxide (TBAOH) were also carried out.
Preliminary studies of the complexation mechanism were
performed using 1H NMR spectroscopy.
For derivatives of isomeric nitroanilines L1–L3,
spectral and colour changes were observed in DMSO only
in the presence of fluorides among the above listed salts.
Halide anions, other than fluorides, do not cause colour
change, even if used in large excess (saturated solutions).
The pale yellow (L1 and L3) or almost colourless (L2)
solution in DMSO turned intensively yellow after fluoride
addition (50 equiv.) with the most pronounced changes for
the para-substituted nitroaniline derivative (L3) (Figure 2).
The smaller amounts of tetra-n-butylammonium fluoride
(TBAF) (1 equiv.) cause a visible, but less significant colour
change for L3. For better visualisation of colour changes of
L1–L3 in the presence of fluorides, a large excess is more
favourable. Almost the same trends were observed upon
gradual addition of TBAOH. The observable colour
changes for L3 could be easily explained by the electron-
withdrawing effect of the nitro group in relation to the para
amide moiety, strongly influencing acidity of NZH
protons.
1
analysing the H NMR titration of L3 (4.62 £ 1023 M)
with TBAF, i.e. at a concentration level of about two orders
of magnitude higher than that for UV–vis spectroscopic
experiments. Spectra were registered in DMSO-d6. As a
model compound, L3 was chosen, due to the most
significant colour and spectral changes. Figure 4(a)
displays 1H NMR spectra (range 8–17 ppm) of L3 titration
with fluoride salt.
The NZH signal at 11.44 ppm broadens and dramati-
cally shifts downfield upon addition of fluoride ions. In the
presence of 1 equiv. of TBAF, the NZH signal appeared at
15.6 ppm as a broad singlet, suggesting existence of an
intramolecular hydrogen bonding between fluoride anion
and ligand, in which NZH amide protons are involved.
Moreover, aromatic proton signals of the neighboring CZH
appearing as a doublet at 8.47 ppm for free ligand are also
shifted downfield (8.55 ppm). Upon addition of 1.2 equiv.
of fluorides the NZH signal appeared at 16.1 ppm as a
distorted singlet and, finally, in the presence of ,2 equiv. of
fluorides, this signal was observed at 16.3 ppm (t,
J ¼ 119 Hz) and did not shift (upon further increase of
titrant concentration). This signal may be attributed to HF22
formation under measurement conditions. Considering that
both amide NH moieties in U-shaped, 2,6-pyridinedia-
mides preferably exist in a syn–syn conformation (30a–c)
and pyridine nitrogen is involved in formation of
intramolecular hydrogen bond, the mechanism of fluoride
binding proposed here is shown in Scheme 1. In the
Figure 2. Colour changes of L1–L3 (8.1 £ 1025 M) observed
upon addition of 50-fold excess of TBAF and TBAOH.

Supramolecular Chemistry
687
(a)
(b) 1.0
0.8
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Above 12 eq
12 eq
0.6
0.4
5 eq
1
2
4 eq
0.2
0.0
300
350
400
450
500
550
600
300
350
400
450
500
550
Wavelength [nm]
Wavelength [nm]
TBAOH
TBAF
(c)
(d) 0.8
0.6
1.0
0.8
0.6
0.4
0.2
0.0
'Free' ligand
0.4
Above 7 eq
7 eq
0.2
0.0
300
350
400
450
500
550
300
350
400
450
500
550
Wavelength [nm]
Wavelength [nm]
Figure 3. Changes in absorption spectra of ligands L1–L3 upon titration with fluorides (TBAF): (a) L1 (8.02 £ 1025 M), TBAF
(0–1.30 £ 1023 M); (b) L2 (1.86 £ 1025 M), TBAF (0–2.01 £ 1024 M); (c) L3 (1.75 £ 1025 M), TBAF (0–3.24 £ 1023 M) and
(d) the comparison of changes in the presence of fluorides and hydroxide for L3 (1.14 £ 1025 M), TBAF (2.66 £ 1023 M) and TBAOH
(1.75 £ 1025 M) in DMSO.
presence of fluoride anion, intramolecular hydrogen bonds
NZH· · ·N are profitably replaced by a stronger NZH· · ·F
intermolecular hydrogen bond. Additionally, due to rigidity
and planarity of the 2,6-bis(carbamoyl)pyridine residue,
CaromZH protons are able to participate in fluoride binding
as CaromZH· · ·F weak hydrogen bonds (cf. 13a–c, 27). As a
consequence, the L3·F2 formed complex is fully
symmetric. It manifests in pattern of aromatic proton
signals in 1H NMR spectrum (see Section 4).
From the NMR titration experiment, the stoichiometry
and the stability constant value of the complex formed
were estimated using molar ratio (Figure 4(b)) and Scott’s
method. The molar ratio plot indicates the formation of a
1:1 complex which is satisfactorily confirmed by Scott’s
(a)
0 eq F
0.25 eq F
0.5 eq F
0.75 eq F
1.0 eq F
16.5
16.0
15.5
15.0
14.5
14.0
13.5
13.0
12.5
12.0
11.5
11.0
10.5
10.0
9.5
9.0
8.5
8.0
(b)
(c)
17
16
15
14
13
12
11
0.5
0.4
0.3
0.2
0.1
R=0.99957
0.0
0.5
1.0
cF/cL
1.5
2.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
cFx102 [mol/dm–3
]
Figure 4. Parts of 1H NMR spectra (8–17 ppm) of L3 (4.62 £ 1023 M) titration with fluorides in DMSO: (a) changes in the chemical
shift of the NZH proton signal; (b) molar ratio plot and (c) Scott’s plot.

688
E. Wagner-Wysiecka and J. Chojnacki
O
O
O
O
F–
N
N
NH
NH
H
HN
HN
F–
NO2
O2N
H
O2N
H
NO2
H
Scheme 1. Proposed mechanism of fluoride binding by L3.
linear plot (Figure 4(c)). The stability constant value for
the L3·F2 complex was estimated from Scott’s equation
(31a) which is a modification of the Benesi–Hildebrand
(31b) method:
TBAOH solution were also carried out. For both ligands
L4 and L5, colour changes in the presence of 1 equiv. of
TBA salts were almost identical for fluorides, acetates,
benzoates and hydroxide. In contrast to L4, noticeable, by
the naked eye, but less visible changes were found for
dihydrogen phosphate in the case of L5. Colour changes in
the presence of 1 equiv. of the respective TBA salts and
TBAOH for ligands L4 and L5 are shown in Figure 5(a)
and (b). Comparing the above-mentioned ligands L1–L3,
where a relatively large excess of fluoride is needed to
observe remarkable colour changes, for L4 and L5, colour
changes at equimolar concentrations of ligand and the
respective anion were observed. It may be attributed to
the presence of phenolic OH groups in the structure of the
investigated compounds.
½Aꢀ
1
½Aꢀ
;
¼
þ
D0K D0
D
where [A ] is the molar concentration of anions, D is the
observed change in chemical shift for a given anion
concentration and D0 is the chemical shift change between
a complex and the ‘free’ ligand. The equation was solved
by graphical method, where the slope for K and the
intercept for D0 are required. For ligand L3, the value of
stability constant for the 1:1 fluoride complex in DMSO
was estimated as log K ¼ 3.28.
Comparison of the colour changes for ligand L5 in the
presence of TBA salts, i.e. fluorides, acetates and
benzoates, and TBAOH suggests that dominating process
might be deprotonation. There are four dissociable protons
in the studied amides: two phenolic and two from amide
groups. In the case of L4, colour changes in the presence of
TBAOH are not as spectacular (large excess cause orange
colour of ligand solution) as for ligand L5, where colour
changes from pale yellow to orange (at 1 equiv. of
fluorides), it is red and purple to navy blue in the extreme
case. For both ligands, the colour changes in the presence
of TBA salts were associated with the bathochromic shift
of absorption bands in UV–vis spectra. The shift is larger
for L5 than for L4. Such behaviour is often observed for
the ionised chromophore molecules. The difference in
colour changes for ligands L4 and L5 may be explained,
e.g. by the different location of the electron-withdrawing
nitro group in relation to OH and NH groups.
Similar experiments carried out in pure DMSO were
done for ligands L1–L3 in DMSO–water (95:5, v/v)
mixture. In the water-containing system, colour changes
were observed only in the case of TBAF and TBAOH
(excess, added as solid) with the most evident colour
change for ligand L3. Under titration conditions, in mixed
solvent system, negligible changes were observed in
absorption spectra of L1 and L2 upon addition of TBAF
solution. Better perceptible changes, however less
spectacular than in pure DMSO, were found upon L3
titration with fluorides (data not shown). L3 in DMSO–
water (95:5, v/v) mixture shows absorption maximum at
332 nm. Upon titration, it shifts slightly, resulting in
lmax ¼ 344 nm and a new band of lower intensity appears
successively at ,430 nm. One isosbestic point (341 nm)
indicates two species under equilibrium and the stoichi-
ometry obtained from the molar ratio plot (data not shown)
1:2 (L:F) suggests ligand deprotonation rather than
fluoride complexation under conditions of spectrophoto-
metric titration.
Even though visible changes might suggest deproto-
nation rather than complexation of L4 and L5, UV–vis
titration with TBA salts was carried out in DMSO.
Changes upon titration with acetate (Figure 6(a)) and
dihydrogen phosphate (Figure 6(b)) are exemplified for
ligand L5 (because of more significant spectral changes).
Changes upon titration with fluorides and benzoates
were similar to those observed for acetates; however,
there are most distinctive for fluorides and next for
acetates and benzoates. The smallest changes were
observed for dihydrogen phosphates. This trend is in
agreement with the basicity order of the studied anions:
It was expected that ligands having additional
hydrogen bond donors (e.g. OH groups; L4, L5) will
influence not only anion–ligand interaction but also
spectral changes caused by the presence of electron
donor/acceptor residues located in different positions of
the molecules. In DMSO, among all the investigated
anions, significant spectral and colour changes of L4 and
L5 solutions were observed in the presence of fluorides,
dihydrogen phosphates, acetates and benzoates. To
exclude or confirm deprotonation, experiments with

Supramolecular Chemistry
689
Figure 5. Colour changes of ligands in DMSO: (a) L4 (1.87 £ 1024 M) and (b) L5 (2.19 £ 1024 M) solutions in the presence of 1 equiv.
of TBA salts.
F2 . CH3COO2 .BzO2 . H2PO24 . Figure 6(c) shows
registered limiting spectra and Figure 6(d) shows molar
ratio plots for the respective ligand–anion pairs. From
molar ratio plots it is evident that the systems equilibrate
more or less significantly at different concentrations of
the added salts. It seems that under titration conditions
the main process is deprotonation of the ligand in the
case of fluorides and acetates, whereas for benzoates and
dihydrogen phosphates (for 1:1 stoichiometry) binding
via hydrogen bond formation might be taken into
consideration. Stability constant values (log K) for 1:1
complexes calculated with the OPIUM (32) program for
benzoates and dihydrogen phosphates are 2.83 ^ 0.01
and 2.51 ^ 0.06, respectively.
(a) 1.4
1.2
(b)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
550
600
650
300
350
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(c)
(d) 0.6
0.5
'Free' ligand
0.4
–
F–
H2PO4
0.3
CH3COO–
F– (500 nm)
H2PO4– (430 nm)
CH3COO– (495 nm)
BzO– (470 nm)
0.2
BzO–
0.1
0.0
300
350
400
450
500
550
600
650
0
1
2
3
4
5
Wavelength [nm]
CA/CL
Figure 6. Changes in absorption spectra upon titration of L5 (4.75 £ 1025 M) with (a) acetates (0–2.03 £ 1024 M); (b) dihydrogen
phosphates (0–9.99 £ 1025 M) as TBA salts in DMSO; (c) the limiting spectra obtained during titration of L5 with fluorides, acetates,
dihydrogen phosphates and benzoates and (d) molar ratio plots for the respective anions in DMSO.

690
E. Wagner-Wysiecka and J. Chojnacki
A
B'
B
O
O
N
3'
1'
3
1
NH
D
HN
2
2'
D
O2N
OH
C
HO
C
NO2
L5
9.6
9.6
1,1'
3,3'
B,B'
2,2'
'Free' ligand
D
C
A
12.0
11.8
11.6
11.4
11.2
11.0
11.0
10.8
10.8
10.6
10.4
10.4
10.2
10.0
9.8
9.4
9.4
9.2
9.2
9.0
9.0
8.8
8.6
8.4
8.4
8.2
8.2
8.0
8.0
7.8
7.6
7.6
7.4
7.4
7.2
7.2
1eq fluorides
12.0
11.8
11.6
11.4
11.4
11.2
11.2
10.6
10.2
10.0
9.8
8.8
8.6
7.8
1 eq benzoates
12.0
12.0
11.8
11.6
11.0
11.0
10.8
10.8
10.6
10.6
10.4
10.4
10.2
10.2
10.0
10.0
9.8
9.8
9.6
9.6
9.4
9.4
9.2
9.2
9.0
9.0
8.8
8.8
8.6
8.6
8.4
8.4
8.2
8.2
8.0
8.0
7.8
7.8
7.6
7.6
7.4
7.4
7.2
7.2
1 eq dihydrogen phosphates
11.8
11.6
11.4
11.2
Figure 7. 1H NMR spectra of L5 (2.60 £ 1023 M) in the presence of 1 equiv. of fluorides, benzoates and dihydrogen phosphates as TBA
salts in DMSO-d6.
To evaluate the possible mechanism of anion–receptor
1
L5 interaction, H NMR (DMSO-d6) spectra were taken
respectively. Proton A of pyridine ring was observed as a
triplet with the same chemical shift found for free ligand
(8.40 ppm). From the changes of chemical shifts for the
respective protons, it may be deducted that benzoate
protons might be involved in ligand–anion interactions (in
a spectrum of pure TBA benzoate aromatic protons are
sharp signals at 7.80–7.78 and 7.19–7.18 ppm). Opposite
to the anions mentioned above, the chemical shifts alter
differently in the case of dihydrogen phosphates.
The largest change in chemical shift for aromatic protons
is found for protons 1,10 (Dd ¼ þ0.19 ppm) and 3,30
(Dd ¼ þ0.12 ppm). 2,20 protons are shifted downfield
(Dd ¼ –0.16 ppm). It indicates that dihydrogen phosphate
anion mainly interacts with L5 ligand (or its dianion) via
formation of OZH· · · ·O2 hydrogen bonds, whereas the
N–H protons are involved to a smaller degree compared to
small spherical fluorides. The differences may reflect
steric effect of the relatively bulky, tetrahedral anion.
The proposed models of L5–dihydrogen phosphate anion
interaction is shown in Scheme 2(a),(b).
for pure sample and after addition of 1 equiv. of guests
(Figure 7).
In the presence of 1 equiv. of the salt protons from the
OH residues (C) are not observed, suggesting probable
deprotonation. Nevertheless, considering the low intensity
of the OH proton signal for the free ligand might be hardly
observed in spectra registered in the presence of TBA
salts. From downfield shifting of the NZH protons (D)
signal in the presence of anions, formation of H-bond
might be deducted. The largest shift was observed for
fluorides (Dd ¼ þ0.72 ppm), then for benzoates
(Dd ¼ þ0.32 ppm) and acetates (Dd ¼ þ0.28 ppm). The
smallest shift of NZH signal was observed for dihydrogen
phosphate (Dd ¼ þ0.02 ppm). Signals of aromatic protons
are shifted upfield in the presence of fluorides, acetates
(data not shown, see Section 4) and benzoates; however,
the nature of these changes is different for benzoates and
fluorides. In the presence of fluorides, signals of all
aromatic protons are shifted upfield (similarly as in the
presence of TBAOH, see Section 4). Protons 1,10 and 2,20
are observed as a broad singlet at 7.48 ppm while signal of
3,30 protons is shifted to 8.41 ppm. For benzoates, more
complicated spectrum was obtained, because of additional
broad signals of aromatic protons form the anionic form of
benzene derivative. Protons 3,30 of the amidophenolic
residue were slightly shifted downfield to 8.48 ppm
(Dd ¼ –0.06 ppm), whereas 1,10and 2,20 protons are
observed as multiplets of integration corresponding to
one and three protons at 7.58–7.52 and 7.51–7.46 ppm,
Considering colour changes of solution, in the presence
of 1 equiv. TBA salts, high selectivity for L4 and L5 was
not stated in DMSO. Nonetheless, it was found that for L5
the use of large excess of TBA salts (50-fold) enables visual
differentiation among the fluorides, acetates, dihydrogen
phosphates and benzoates (see Figure 8). This is in contrast
to L4, where changes in absorption spectra were also
observed in the presence of the mentioned anions; however,
no special selectivity towards any of them was found. In all
cases, colour changes to intensive yellow which is
connected with new absorption maximum formation at

Supramolecular Chemistry
691
(a)
(b)
O
O
O
O
N
P
N
P
NH
OH
HN
HO
NH
O–
HN
–O
O–
O
O–
O
H
H
O
O
NO2
O2N
O2N
NO2
OH
HO
Scheme 2. Proposed models of L5–dihydrogen phosphate anion interaction based on hydrogen bonds.
the ,450 nm of different intensity depending on the kind of
the added anion salt. Colour and spectral changes for L4
(a)
and L5 in the presence of 50-fold excess of TBA salts are
compared in Figure 8(c),(b). Under such conditions, ligand
L5 may work as a naked-eye anion sensor where the most
probable anion recognition mechanism is based on both
ligand deprotonation and different types of deprotonated
ligand–anion interactions dependent on the type and anion
F–, H2PO4
–
(b)
CH3COO–
1.1
basicity.
To extend the possible application for anion sensing,
similar experiments with L5 were carried out in a mixed,
1.0
BzO–
0.9
0.8
'Free' ligand
and rest of anions
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
water-containing, solvent system, i.e. DMSO–water
(95:5). Colour and spectral changes in this system are
shown in Figure 9. In the presence of both 1 and 50 equiv.
of TBA salts and TBAOH, selective response towards
dihydrogen phosphate was found. Additionally, at higher
concentrations of TBA salts, new absorption bands appear
allowing differentiation of ligand’s response towards
hydroxide (l ¼ 494 nm), fluorides (l ¼ 485 nm), acetates
(l ¼ 480 nm) and benzoates (l ¼ 470 nm) in the water-
containing solvent system.
OH–
300
350
400
450
500
550
600
650
Wavelength [nm]
(c)
2.3 X-ray studies
Slow evaporation of methanol from the equimolar mixture
of L4 and TBAF resulted in formation of crystals suitable
for the X-ray diffraction study. The substance crystallises
(d)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
n
þ
ꢀ
"Free" ligand
in triclinic P1 space group with four Bu4N cations and
two anions present in the unit cell (Z ¼ 2).
From the obtained results it is evident that in a solid
state, NZH protons, although directed towards the cavity
centre, are not engaged in ligand–anion interaction
(Figure 10). The molecule of L4 is not planar and fluoride
anion is located above the plane of the molecule. Mean
plane defined by the six pyridine atoms forms dihedral
angles of 34.138 and 9.898 with pending C1ZC6 and
C14ZC19 hydroxynitrophenyl rings, respectively.
–
H2PO4
BZO–
OH–
F–
CH3COO–
˚
The measured distance O· · ·F 2.8 A formally corre-
sponds to weak hydrogen bond; however, OH protons are
not observed in the Fourier electron density map, which
might suggest that one or two OH groups are deprotonated.
Charge balance indicates that if a fluoride anion is present
then one group is OH and the second is O2. The presence
of HF molecule and phenolate dianion is unlikely because
the acidity of HF is higher than the acidity of phenol.
300
350
400
450
500
550
600
650
Wavelength (nm)
Figure 8. L4 and L5 (8.5 £ 1025 M) interactions with anions:
(a) and (c) colour changes of ligands’ solution; (b) and (d)
changes in absorption spectra observed upon addition of 50-fold
excess of the respective TBA salts and hydroxide in DMSO
(registered in quartz cuvettes of 2 mm path length).

692
E. Wagner-Wysiecka and J. Chojnacki
(a)
(c)
(b)
(d)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.8
'Free' ligand
0.6
'Free' ligand
BzO–
OH–
F–
0.4
AcO–
F–
AcO–
BzO–
OH–
–
0.2
H2PO4
–
H2PO4
0.0
300
400
500
600
300
400
500
Wavelength [nm]
600
700
Wavelength [nm]
Figure 9. Colour and spectral changes of L5 in DMSO–water (95:5). (a) and (b) L5 (2.9 £ 1024 M) in the presence of 1 equiv. of TBA
salts; (c) and (d) L5 (9.9 £ 1025 M) in the presence of 50 equiv. of TBA salts (registered in quartz cuvettes of 2 mm path length).
Figure 10. X-ray crystal structure of L4 crystals formed in the presence of TBAF (nBu4N cations and H-atoms are omitted for clarity).
Crystal data and details of structure refinement for
L4·F2 are presented in Table 1.
4. Experimental
4.1 General procedures and materials
All chemicals were purchased from commercial sources
and were used without further purifications. Perchlorate
salts (Caution!) should be regarded as potentially explosive
and handled with care. Dimethylformamide (DMF) for
syntheses was dried over A4 molecular sieves A4. Thin
layer chromatography: aluminium sheets covered with
3. Conclusions
A simple, high-yielding method for preparation of
chromogenic diamides L1–L5 derived from isomeric
nitroanilines and aminonitrophenols was presented. For the
obtainedligands,studiesofanionrecognitionanditspossible
nature were carried out in DMSO and its mixture with water.
N,N0-Bis(nitrophenyl)pyridine-2,6-dicarboxamides undergo
mainly deprotonation at lower concentrations. However,
as shown for L3, fluorides are able to interact with this ligand
via hydrogen bond formation at higher concentration of both
the ligand and TBAF. N,N0-Bis(2-hydroxy-4-nitrophenyl)-
pyridine-2,6-dicarboxamide (L5) shows dihydrogen phos-
phate selective response in both DMSO and its mixture with
water. It extends possible application of this compound as
dihydrogen phosphate receptor. L5 may also be used at high
salt concentration as naked-eye sensor towards selected
anions in DMSO and to some extent in the water-containing
solvent system. Such selectivity enables preparation of dry
tests for rapid anion detection.
1
60F254 were from Merck, Darmstadt, Germany. H NMR
spectra were recorded at 200 or 500 MHz and 13C NMR at
125 MHz on a Varian instrument. Chemical shifts are
reported as d (ppm) values in relation to tetramethylsilane.
FTIR spectra were recorded on a Mattson Genesis II
instrument. UV–vis spectra were recorded on an UNICAM
UV 300 apparatus in 1.0 cm and 2 mm quartz cells. Mass
spectrometry was conducted on an AMD-604 apparatus (EI
method, 70 eV) and Mariner (ESI method).
Ion binding studies wereperformed by UV–vis titration
of the ligand solution in DMSO with the respective TBA
salt (for anions) or with the perchlorates (for metal cations).
The stability constant values were calculated with the use of

Supramolecular Chemistry
Table 1. Crystal data and details of structure refinement for L4·F2.
693
Empirical formula
Formula weight
Temperature
2ðC16H36NþÞ; C19H11N5O82; F2
941.24
295(2) K
˚
0.71073 A
Triclinic
Wavelength
Crystal system
Space group
Unit cell dimensions
ꢀ
P1
˚
a ¼ 9.2891(10) A, a ¼ 99.069(9)8
˚
b ¼ 12.4727(15) A, b ¼ 90.159(8)8
˚
c ¼ 25.736(3) A, g ¼ 108.167(10)8
3
˚
2793.44 A
Volume
Z
Density (calculated)
Absorption coefficient
F(0 0 0)
2
1.119 Mg/m3
0.078 mm21
941.24
Theta range for data collection
Index ranges
2.41–25.108
29 . h . 11; 214 . k . 14; 230 . l . 26
17,492
Reflections collected
Independent reflections
Completeness to theta ¼ 25.108
Refinement method
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I . 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
9931; 2470 (I . 2s(i))
0.999
Full-matrix least squares on F 2
9926/416/631
0.910
R1 ¼ 0.0966, wR2 ¼ 0.2323
R1 ¼ 0.2983, wR2 ¼ 0.3395
3
˚
0.363 and 20.178 e/A
the OPIUM (32) program on the basis of titration
experimental data.
258C): d ¼ 7.78 (2H, t, J ¼ 8.1 Hz; Ar-H); 8.07 (2H, dd,
J1 ¼ 8.3 Hz J2 ¼ 1.5 Hz; Ar-H); 8.35–8.39 (3H, m;
Ar-H); 8.46 (1H, s; Ar-H); 8.48 (1H, s; Ar-H); 8.99 (2H,
t, J ¼ 1.9 Hz; Ar-H); 11.37 ppm (2H, s; NH); 13C NMR
(125 MHz, DMSO-d6, t ¼ 608C): d ¼ 162.2; 148.4; 148.2;
140.2; 139.3; 130.2; 126.9; 125.8; 118.8; 114.9 ppm;
FTIR (KBr): n ¼ 519; 597; 665; 736; 817; 880; 1000;
1071; 1150; 1243; 1299; 1348; 1424; 1484; 1259; 1587;
1602; 1677; 3078; 3109; 3312 cm21; UV–vis (DMSO):
4.2 Synthesis
4.2.1 General procedure for pyridine-2,6-dicarboxylic
acid amides
Pyridine-2,6-dicarboxylic acid chloride (1 mmol) was
added in portions to a solution of nitroamine or to the
respective nitroaminophenol (2 mmol) and triethylamine
(2.2 mmol) in DMF (20 ml). The mixture was heated for
12 h at 608C. The cooled reaction mixture was diluted with
water. The solid was filtered off and washed with water,
and crystallised from methanol to obtain pure products.
N,N0-Bis(2-nitrophenyl)pyridine-2,6-dicarboxamide
(L1): 89%, yellow solid, Rf ¼ 0.49 (methylene chloride);
mp 245–2468C; 1H NMR (500 MHz, DMSO, 258C):
d ¼ 7.51 (2H, t, J ¼ 8.7 Hz; Ar-H); 7.89 (2H, t,
J ¼ 8.3 Hz; Ar-H); 8.17–8.20 (4H, m; Ar-H); 8.37–8.44
(3H, m; Ar-H); 11.75 ppm (2H, s; NH); 13C NMR
(125 MHz, DMSO, 608C): d ¼ 162.4; 148.7; 141.5; 135.7;
132.5; 126.6; 126.3; 126.2; 125.3 ppm; FTIR (KBr):
n ¼ 649; 683; 745; 786; 1001; 1069; 1146; 1231; 1238;
1337; 1440; 1501; 1538; 1608; 1698; 3096; 3288; 3361;
3376 cm21. UV–vis (DMSO): l1(11) ¼ 261 (2.2 £ 104),
l2(12) ¼ 345 (5.9 £ 103); HRMS (EI): m/z calcd for
C19H13N5O6 [Mþ]: 407.08658; found 407.08535.
l
max(1max) ¼ 278 (3.0 £ 104); HRMS (EI): m/z calcd for
C19H13N5O6 [Mþ]: 407.08658; found 407.08459.
N,N0-Bis(4-nitrophenyl)pyridine-2,6-dicarboxamide
(L3): 90%, yellow solid, Rf ¼ 0.61 (CH2Cl2/acetone
100:1); mp . 3058C; 1H NMR (500 MHz, DMSO-d6,
258C): d ¼ 8.26 (4H, d, J ¼ 7.7 Hz; Ar-H); 8.35–8.39
(5H, m; Ar-H); 8.48 (2H, d, J ¼ 7.8 Hz; Ar-H); 11.44 ppm
(2H, s; NH); 13C NMR (125 MHz, DMSO-d6, t ¼ 608C):
d ¼ 162.2; 148.3; 144.2; 143.2; 140.2; 126.1; 124.7;
120.5 ppm; FTIR (Nujol): n ¼ 502; 640; 681; 752; 855;
1000; 1112; 1247; 1332; 1409; 1503; 1536; 1598; 1695;
3080; 3114; 3329 cm21; UV–vis (DMSO): lmax(1-
max) ¼ 333 (3.3 £ 104); HRMS (EI): m/z calcd for
C19H13N5O6: 407.08658 [Mþ]; found: 407.08567.
N,N0-Bis(4-nitrophenyl)pyridine-2,6-dicarboxamide
aromatic proton signals in the presence of 0.75 equiv. of
TBAF, L3.F2: 1H NMR (500 MHz, DMSO-d6, 258C):
d ¼ 8.21 (4H, d, J ¼ 9.1 Hz); 8.28 (1H, t, J ¼ 7.3 Hz);
8.36 (2H, d, J ¼ 7.8 Hz) 8.45 (4H, d, J ¼ 8.5 Hz);
14.78 ppm (broad s).
N,N0-Bis(3-nitrophenyl)pyridine-2,6-dicarboxamide
(L2): 83%, pale yellow solid, Rf ¼ 0.69 (CH2Cl2/acetone
100:1); mp . 3058C; 1H NMR (500 MHz, DMSO-d6,

694
E. Wagner-Wysiecka and J. Chojnacki
N,N0-bis(2-hydroxy-5-nitrophenyl)pyridine-2,6-dicar-
not indicated otherwise. The stability constant values were
estimated with the use of the OPIUM (32) program on the
basis of titration data. Absorption spectra of more
concentrated ligand solutions were registered in a quartz
cuvettes of 2 mm path length.
boxamide (L4): 67%, grey solid, Rf ¼ 0.56 (CH2Cl2/
metanol 15:2); mp . 3058C; 1H NMR (200 MHz, DMSO-
d6, 258C): d ¼ 7.13 (2H, d, J ¼ 8.9 Hz; Ar-H); 8.02 (2H,
dd, J1 ¼ 8.9 Hz, J2 ¼ 2.7 Hz; Ar-H); 8.32–8.46 (3H, m;
Ar-H); 9.09 (2H, d, J ¼ 2.7 Hz; Ar-H); 10.43 (2H, s; NH);
,12 ppm (,1.3H, broad s, OH); 13C NMR (125 MHz,
DMSO-d6, t ¼ 608C): d ¼ 161.3; 154.2; 148.4; 140.5;
139.5; 125.9; 125.5; 121.4; 116.1; 114.8 ppm; FTIR
(KBr): n ¼ 641; 748; 829; 893; 948; 1001; 1077; 1284;
1340; 1440; 1494; 1538; 1595; 1692; 3094; 3125; 3356;
4.4 X-ray structural analysis of L4 crystal
Crystals of L4 were obtained by slow methanol
evaporation from equimolar mixture of L4 and TBAF.
X-ray measurements were carried out on a KM4CCD
kappa-geometry diffractometer equipped with a Sapphire-
2 CCD detector. Enhanced X-ray Mo Ka radiation source
with a graphite monochromator was used. Determination
of the elemental cell and data collection were carried out at
room temperature 295(2) K. The preliminary calculations
were made using the CrysAlis software package (Oxford
Diffraction, 2008) (33). The structure was solved by direct
method and refined by full-matrix least-squares procedure
based on F 2. Absorption correction was applied in the
form of multi-scan procedure. Because of the weak
diffraction power of all specimens and poor ratio of
observed/collected reflections, all non-hydrogen atoms
were refined in isotropic approximation (by ISOR shelx
command). Final calculations were carried out using the
SHELX-97 program package (34). Two terminal atoms
C26–C27 and C50–C51 in n-butyl chains were refined as
disordered over two positions with probabilities
0.668(11)/0.332(11).
3503 cm21
;
UV–vis (DMSO):
l
max(1max) ¼ 294
(2.4 £ 104); HRMS (ESI negative-ion): m/z calcd for
C19H12N5O8[M–H]2: 438.06804; found:438.06695.
N,N0-Bis(2-hydroxy-4-nitrophenyl)pyridine-2,6-dicar-
boxamide (L5): 95%, yellow solid, Rf ¼ 0.67 (CH2Cl2/-
metanol 15:2); mp . 3058C; 1H NMR (200 MHz, DMSO-
d6, 258C): d ¼ 7.77 (2H, d, J ¼ 2.4 Hz; Ar-H); 7.86 (2H,
dd, J1 ¼ 8.9 Hz, J2 ¼ 2.03 Hz; Ar-H); 8.38–8.50 (3H, m;
Ar-H); 8.54 (2H, d, J ¼ 8.9 Hz; Ar-H); 10.54 (2H, s; NH);
,11.50 ppm (,1.3H, s; OH); 13C NMR (125 MHz,
DMSO-d6, t ¼ 608C): d ¼ 161.2; 148.4; 147.4; 143.3;
140.7; 132.4; 125.8; 119.2; 115.4; 109.2 ppm; FTIR
(KBr): n ¼ 744; 813; 870; 888; 999; 1084; 1118; 1217;
1267; 1339; 1431; 1510; 1534;1595; 1663; 1699; 3093;
3371; 3418 cm21; l1(11) ¼ 317 (1.5 £ 104), l2(12) ¼ 362
(2.1 £ 104); HRMS (ESI negative-ion): m/z calcd for
C19H12N5O8 [M–H]2: 438.06804; found: 438.06975.
N,N0-Bis(2-hydroxy-4-nitrophenyl)pyridine-2,6-dicar-
boxamide (L5) in the presence of 1 equiv. of TBA acetate:
1H NMR (500 MHz, DMSO-d6, 258C): d ¼ 7.55 (6H,
broad distorted d; Ar-H); 8.32–8.37 (2H, distorted t;
Ar-H); 8.40–8.46 (1H, distorted d; Ar-H); ,10.82
(broad s; NH).
5. Supporting Information Available
CIF file for compound L4 was deposited with the
Cambridge Crystallographic Data Centre as supplemen-
tary publication number CCDC 857718 (CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK; E-mail: deposit@
ccdc.cam.ac.uk).
N,N0-Bis(2-hydroxy-4-nitrophenyl)pyridine-2,6-dicar-
boxamide (L5) in the presence of 0.75 equiv. TBAOH: 1H
NMR (500 MHz, DMSO-d6, 258C): d ¼ 6.83 (1H,
distorted s; Ar-H); 6.91–6.97 (2H, d; Ar-H); 8.18–8.34
(6H, m; Ar-H). Proton signals attributed to OH and NH in
a free ligand are not observed upon addition of 0.5 equiv.
of TBA (at 0.25 equiv., a very weak and broad signal is
observed at 10.98 ppm).
Acknowledgements
E.W.-W. acknowledges support from sources for science in the
year 2010–2011, Grant No. N N204 137438. E.W.-W. also
thanks Prof. Elz˙bieta Luboch and Prof. Jan. F. Biernat for the
valuable discussion and students Monika Wilamowska and
Karolina Hak for their contribution to synthetic work.
4.3 Complexation studies
The stock solutions, for working solution preparation, of
the ligands (,1024 M), metal perchlorates and TBA salts
(,1022 M) were prepared by weighing the respective
quantities and dissolving in volumetric flasks in DMSO
(ACS Spectrophotometric Grade, Sigma–Aldrich,
Steinheim, Germany). Deionised water (18 MV cm,
Hydrolab, Poland) was used for the preparation of
DMSO–water mixtures. Titrations were carried out in
1 cm path length quartz cuvettes keeping constant volume
of the ligand solution (2.3 ml). Titration step was 0.01 ml if
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