Chromogenic signalling of water content in organic solvents by hydrazone-acetate complexes

Article abstract of DOI:10.1016/j.dyepig.2011.07.019

New chromogenic probe systems for the signalling of the water content in representative water-miscible organic solvents (acetonitrile and tetrahydrofuran) were investigated. The effect of the water content in organic solvents on the complex formation of h

Full text of DOI:10.1016/j.dyepig.2011.07.019

Dyes and Pigments 92 (2012) 1199e1203
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Chromogenic signalling of water content in organic solvents
by hydrazoneeacetate complexes
*
Youn Hwan Kim, Myung Gil Choi, Hyun Gyu Im, Sangdoo Ahn, Il Wun Shim, Suk-Kyu Chang
Department of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
New chromogenic probe systems for the signalling of the water content in representative water-miscible
organic solvents (acetonitrile and tetrahydrofuran) were investigated. The effect of the water content in
organic solvents on the complex formation of hydrazone dye with acetate ions was utilized for the
signalling. The hydrazoneeacetate system exhibited a pronounced chromogenic signalling behaviour
that could be detected by eye in response to the changes in water content in such common water-
miscible organic solvents as acetonitrile and tetrahydrofuran. Prominent colour changes were
observed for up to 1 and 2% water content in acetonitrile and THF, respectively. Detection limits of the
anthracene-based hydrazoneeacetate system for determination of the water content in acetonitrile and
tetrahydrofuran were 0.037 and 0.071%, respectively. The 7-hydroxycoumarin-based hydrazoneeacetate
system showed somewhat less sensitive signalling behaviour, with respective detection limits of 0.12 and
0.63% in acetonitrile and tetrahydrofuran. The designed hydrazoneeacetate systems could be used as
a simple and convenient chromogenic probes for the determination of the water content in represen-
tative organic solvents of acetonitrile and tetrahydrofuran.
Received 20 February 2011
Received in revised form
11 July 2011
Accepted 19 July 2011
Available online 5 August 2011
Keywords:
Water content
Organic solvent
Hydrazoneeacetate complex
Chromogenic signalling
Ratiometry
Probe
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
known to act as ligands for a variety of metal ions, including lan-
thanide(III) ions such as yttrium(III), lanthanum(III), and lutetiu-
Determination of water content in organic solvents is very
important in various fields of chemistry and industrial processes
[1e5]. Water content in varying substances, such as common
solvents, petroleum products, foods, and drugs are generally
determined by Karl Fischer titration [6,7] or gas chromatography
[8]. However, optical methods are more desirable than these
conventional analytical techniques in terms of convenience and
instrumental requirements. There have been many reports on the
development of optical sensing systems employing various func-
tional dye molecules [9,10] for the determination of water content
in organic solvents, including merocyanines [11e13], flavones [14],
chalcone [15], 3-hydroxychromone [16], naphthalimides [17,18],
and indole derivatives [19].
Hydrazones are widely used in many fields of chemistry. In
practical syntheses, hydrazones, acting as synthetic equivalents of
the carbonyl compounds, are often preferred nitrogen analogues of
aldehydes and ketones [20]. Hydrazones possessing an azomethine
imine group constitute an important class of compounds for new
drug development [21]. Hydrazone and its derivatives are also
m(III) [22e24]. For application towards metal ion binding,
Sakamoto et al. reported the extraction of alkali metal ions by
benzo-crown ethers bearing an ionizable hydrazone group [25].
More recently, metalehydrazone complexes that exhibit vapo-
chromic behaviour toward vapor molecules of alcohols, acetoni-
trile, dimethylformamide, dimethylsulfoxide, and 1,4-dioxane were
investigated [26].
On the other hand, hydrazone derivatives are known to selectively
interact with fluoride and acetate ions through hydrogen bonding [27].
There are many fluoride or acetate-selective hydrazone derivatives
based on the various molecular framework of 5-bromosalicylaldehyde
[28], indolin-2-one [29], 9-anthracenecarboxaldehyde [30], and 1,10-
phenanthroline-2,9-dicarboxaldehyde [31]. A Ru(II)-based phenan-
throline-5,6-dione containing a quinonehydrazone group that can be
transformed to an azophenol form in the presence of fluoride ion has
been reported [32]. However, due to the hydrogen bonding nature of
the interaction, the recognition and/or sensing of the anions occurred
only in noncompetitive organicsolvents such as acetonitrile and DMSO
[28]. In aqueous solution, the hydrogen bonding interaction between
the hydrazone and fluoride or acetate was significantly weakened due
to the presence of competing water molecules; the signalling thus
became very weak or no longer possible [28].
* Corresponding author. Tel.: þ82 2 820 5199; fax: þ82 2 825 4736.
E-mail address: skchang@cau.ac.kr (S.-K. Chang).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.07.019

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Y.H. Kim et al. / Dyes and Pigments 92 (2012) 1199e1203
We have designed a new chromogenic signalling system for the
formed was filtered and washed with ethanol several times and
dried under vacuum to yield a red powder, 2 (350 mg, Yield ¼ 95%).
determination of the water content in organic solvents by using the
effects of water on the complex formation of hydrazone with
acetate ions. Among the acetate and fluoride chromogenic candi-
dates for the hydrazone, acetate was chosen as a component of
the probe system because hydrazones are known to interact with
“Y”-shaped acetate ions more strongly than fluoride ions [30].
Using hydrazoneeacetate complexes, variations in the water
content in the water-miscible organic solvents of acetonitrile and
tetrahydrofuran (THF) were sensitively visualized by ratiometry, as
well as by visually discernible colour changes.
Decompose at 230 ꢀC. 1H NMR (600 MHz, DMSOed6)
d 9.28 (s, 1H),
8.86 (d, J ¼ 3.0 Hz, 1H), 8.39 (dd, J ¼ 3.0 and 9.6 Hz, 1H), 8.01
(d, J ¼ 9.6 Hz,1H), 7.86 (d, J ¼ 9.6 Hz,1H), 7.66 (d, J ¼ 8.4 Hz, 1H), 6.97
(d, J ¼ 8.4 Hz, 1H), 6.34 (d, J ¼ 9.6 Hz, 1H). 13C NMR (150 MHz,
DMSOed6) d 160.8, 160.1, 153.7, 145.2, 145.0, 144.3, 137.7, 131.8, 130.4,
130.3, 123.4, 116.7, 113.8, 112.6, 112.0, 107.3. IR (KBr); n(NeH and OeH)
3246, n(CeH) 3091, n(C]O) 1730, n(C]N) 1607, vðNO Þ 1506 and
2
1325 cmꢁ1. HRMS (EI, DIP); m/z calcd for C16H10N4O7 [Mþ]: 370.0549,
found 370.0594.
2. Experimental section
2.4. Titration experiments
2.1. General
Stock solutions of 1 and 2 were prepared in acetonitrile and THF.
Solutions of hydrazoneeacetate complex were prepared by
respective addition of 10 equiv and 100 equiv of acetate ions to stock
solutions of 1 and 2 in acetonitrile. In THF, 10 equiv of acetate ion
was used to obtain the desired hydrazoneeacetate complex. Final
concentrations of 1 and 2 were 2.0 ꢂ 10ꢁ5 M in both solutions. Job’s
plot was obtained by plotting the absorbance at the maximum
wavelength (536 nm for 1 and 502 nm for 2), using varying amounts
of 2.0 ꢂ 10ꢁ5 M of 1 or 2 and acetate ion under the conditions of:
[hydrazone] þ [acetate] ¼ 2.0 ꢂ 10ꢁ5 M.
All solvents (acetonitrile, THF with 250 ppm 2,6-di-tert-
butyl-4-methylphenol as inhibitor, and methanol) were
purchased from SigmaeAldrich Chemical Co. as ‘anhydrous’
grade. 9-Anthracenecarboxaldehyde, 7-hydroxycoumarin, 2,4-
dinitrophenylhydrazine, trifluoroacetic acid (TFA), hexamethy-
lenetetramine (HMTA), and tetrabutylammonium acetate were
purchased and used without further purification. 1H NMR
(300 MHz, 600 MHz) and 13C NMR (75 MHz, 150 MHz)
spectra were obtained on a Varian Gemini-2000 and VNS
spectrometer. Mass spectral data were obtained with a Micro-
mass Autospec mass spectrometer. UVevis spectra were recor-
ded with a Jasco V-550 spectrophotometer. Compound 1 was
prepared by the reaction of 9-anthracenecarboxaldehyde with
2,4-dinitrophenylhydrazine following the reported procedure
[30].
3. Results and discussion
The hydrazone derivative of anthracene aldehyde
1 was
prepared by the reaction of 9-anthracenecarboxaldehyde with 2,4-
dinitrophenylhydrazine in ethanol following the reported proce-
dure [30]. Hydroxycoumarin-based hydrazone 2 was prepared
similarly from 7-hydroxycoumarin-8-aldehyde, which was
prepared by formylation of 7-hydroxycoumarin (HMTA, TFA) [33]
(Scheme 1).
First, the UVevis spectral behaviour of 1 in the presence of
acetate ions in acetonitrile was measured. Hydrazone 1 showed
strong absorption bands at 360 and 426 nm in acetonitrile (Fig. 1).
Upon treatment with 100 equiv of acetate ion as the tetrabuty-
lammonium salt, the absorption spectrum changed significantly;
the absorption bands at 360 and 424 nm decreased while a new
strong band at 536 nm developed. Concomitantly, the solution
colour changed from yellow to red (left inset of Fig. 1). This chro-
mogenic behaviour is, as is well-known, due to the complex
formation by the hydrogen bonding interaction of 1 with acetate
ions [30]. As shown in the right inset of Fig. 1, a constant signal was
2.2. Synthesis of 7-hydroxycoumarin-8-carboxaldehyde [33]
To a solution of 7-hydroxycoumarin (1.6 g, 9.9 mmol) in
dichloromethane (20 mL) was added acetic anhydride (10.8 g,
106 mmol) and pyridine (15 mL). The reaction mixture was stirred at
room temperature for 12 h and the volatiles were evaporated under
reduced pressure. The residue was then dissolved in water,
and extracted with ethyl acetate. The organic phase was separated,
and washed with brine and water, and evaporated to obtain a
solid residue. The product was purified by crystallization from
ethyl acetate to yield 7-acetoxycoumarin. To
a solution of
7-acetoxycoumarin (1.5 g, 7.35 mmol) in trifluoroacetic acid
(10 mL) in an ice-cooled bath was added hexamethylenetetramine
(1.5 g,10.7 mmol). The reaction mixture was heated under reflux for
24 h. Volatiles of the reaction mixture were evaporated under
reduced pressure, and water (30 mL) was added to the residue. The
mixture was heated and stirred at 60 ꢀC for half an hour. After
cooling the mixture, the yellow precipitate formed was filtered and
washed with water to give the 7-hydroxycoumarin-8-aldehyde as
a yellow powder (1.06 g, 76% yield). mp 190 ꢀC (decompose). 1H
N
O2
N
O2
N
O2
N
O2
H
O
HN
N
NH2
N
H
In EtOH
NMR (300 MHz, DMSOed6)
7.43 (d, J ¼ 8.7 Hz, 1H), 6.46 (d, J ¼ 8.7 Hz, 1H), 5.95 (d, J ¼ 9.0 Hz,
1H). 13C NMR (75 MHz, DMSOed6)
189.7, 173.3, 160.4, 158.4, 145.1,
d
10.28 (s, 1H), 7.76 (d, J ¼ 9.0 Hz, 1H),
1
d
134.2, 119.6, 111.0, 106.6, 105.7. IR (KBr); n(OeH) 3332, n(CeH) 3086,
n(C]O) 1730 and 1656 cmꢁ1. HRMS (EI, DIP); m/z calcd for C10H6O4
[Mþ]: 190.0266, found 190.0265.
N
O2
N
O2
N
O2
O
H
HN
N
NH2
H
O
O
O
N
H
H
O
2.3. Synthesis of 2
O
O
In EtOH
To a solution of 7-hydroxycoumarin-8-carboxaldehyde (190 mg,
1.0 mmol) in ethanol (10 mL) was added 2,4-dinitrophenylhydrazine
(218 mg, 1.1 mmol); the reaction mixture was then heated under
reflux for 5 h. After cooling to room temperature, the precipitate
2
Scheme 1. Preparation of hydrazone derivatives of anthracene and 7-
hydroxycoumarin aldehyde.

Y.H. Kim et al. / Dyes and Pigments 92 (2012) 1199e1203
1201
Fig. 1. UVevis spectra of 1 and 1 in the presence of varying amount of acetate in
acetonitrile. [1] ¼ 2.0 ꢂ 10ꢁ5 M. Equiv of acetate ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0,
10, 20, 40, 60, 80, and 100.
Fig. 2. Changes in UVevis spectra of 1eOAc system with varying water content in
acetonitrile. [1] ¼ 2.0 ꢂ 10-5 M, [OAcꢁ] ¼ 2.0 ꢂ 10-3 M. Water % ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5,
0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0.
obtained with approximately 50 equiv of acetate ions. The Job’s plot
showed that the binding stoichiometry of 1 with the acetate ion
was 1:1 (Figure S1, Supplementary data). From this change in the
concentration-dependent UVevis spectra, the association constant
Kassoc for the 1eOAc system in acetonitrile was estimated as
7.16 ꢂ 104 Mꢁ1 [34].
the water content gave a well-correlated relationship (right inset of
Fig. 2). Particularly, the changes in the lower water content region
(<1%) were pronounced, and could be used as a calibration curve
for determination of the water content in acetonitrile within this
range. The response was quite pronounced and the transition of the
solution colour from red to yellow in response to the changes in
water content less than 1% could be easily discernible with the
naked eye (left inset of Fig. 2). From this plot, the detection limit for
determination of the water content in acetonitrile was estimated as
0.037%.
7-Hydroxycoumarin-derived hydrazone 2 also exhibited similar
chromogenic signalling behaviour toward the water content in
acetonitrile. Hydrazone 2 showed a strong absorption band at
385 nm in acetonitrile (Fig. 3). Upon treatment with 1 equiv of
acetate as the tetrabutylammonium salt, the absorption spectrum
changed significantly; the absorption band at 385 nm decreased
while a new strong band at 502 nm developed. Concomitantly, the
solution colour changed from yellow to red (left inset of Fig. 3). The
strength of the complex formation of 7-hydroxycoumarin hydra-
zone 2 with acetate was found to be larger than that of anthracene
derivative 1, and the titration of 2 with acetate reached a saturation
state much earlier (approximately 1 equiv of acetate ions) than 1.
The binding stoichiometry of the 2eOAc system was also estimated
to be 1:1 by a Job’s plot (Figure S2, Supplementary data). From the
Next, the effect of the water content on formation of the 1eOAc
complex in acetonitrile was studied. When 5% water was added to
the 1eOAc system, which was obtained upon treatment of 1 with
100 equiv of acetate ions in acetonitrile, the UVevis spectrum
changed to nearly that of 1 only. This transformation was due to the
breaking of the hydrogen bonding interaction between 1 and the
acetate ions by water molecules (Scheme 2) [28]. In the presence of
water molecules, the 1eOAc complex was no longer stable and,
upon dissociation of the acetate ions, free 1 was regenerated. This
observation suggests that the chromogenic behaviour of 1eOAc in
response to the changes in water content could be used as a probe
for signalling of water content in organic solvents.
Based upon this observation, a systematic investigation of the
signalling of the water content in acetonitrile was executed. The
changes in absorption behaviour of the 1eOAc system, which
was obtained by addition of 100 equiv of acetate ions to 1
(2.0 ꢂ 10ꢁ5 M), as a function of the water content in acetonitrile,
were measured (Fig. 2). As the water content increased, the
absorption behaviour was progressively converted from the spec-
trum of 1eOAc to that of 1 only. Concomitantly, the solution colour
progressively changed from red for 1eOAc to yellow for free 1; this
could be easily discerned by eye.
The changes in the absorption spectrum of the 1eOAc system as
a function of water content could be conveniently analyzed by
ratiometry using the ratio of the two absorbances at 536 and
424 nm. The plot of the absorbance ratio A536/A424 as a function of
Fig. 3. UVevis spectra of 2 and 2 in the presence of varying amounts of acetate in
acetonitrile. [2] ¼ 2.0 ꢂ 10ꢁ5 M. Equiv of acetate ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.5, 5.0, and 10.
Scheme 2. Signalling of water content in organic solvents by the 1eOAc system.

1202
Y.H. Kim et al. / Dyes and Pigments 92 (2012) 1199e1203
of acetate increased from 2 to 10 equiv, the titration profile was
affected slightly; increased amounts of acetate resulted in a slightly
less sensitive response toward changes in the water content in
acetonitrile. However, the titration results of the 2eOAc system
with the water content using 10 and 100 equiv of acetate were
nearly identical. This implies that the signalling by the 2eOAc
system was relatively insensitive to the variations in acetate
concentration between 10 and 100 equiv, which is favorable for
practical application of the hydrazoneeacetate signalling system.
We used more than a stoichiometric amount of acetate for the
complex formation of 1 or 2 with acetate ions. Although, the sig-
nalling sensitivity became somewhat lower when using excess
acetate, the uncertainty introduced by using exact equivalent of
acetate for the assay of water content would be mitigated as can be
seen in Fig. 5.
Next, signalling of the water content in another important
organic solvent THF was studied. Hydrazone 1 showed quite similar
chromogenic behaviour toward the water content in THF, which is
comparable to acetonitrile. Upon treatment with acetate ions, the
absorption bands of free 1 at 359 and 426 nm in THF decreased
steadily and a new strong band due to the 1eOAc complex at
554 nm appeared (Figure S3, Supplementary data). As the water
content in THF increased, the absorption spectrum of 1eOAc
progressively transformed to 1 only, with a prominent colour
change from red to yellow that could be readily observed by eye
(Fig. 6). Water signalling behaviour of 1eOAc could be analyzed
ratiometrically using the ratio of two characteristic absorbances at
554 and 426 nm (inset of Fig. 6). Significant changes in the absor-
bance ratio A554/A426 were observed up to 1% water in THF. From
the water content-dependent changes in A554/A426, the detection
limit of the 1eOAc system for determination of the water content in
THF was estimated as 0.071%. Relevant signalling behaviour of the
2eOAc system for probing water content in THF are given in the
Supplementary data (Figures S4 and S5, Supplementary data).
Finally, the effect of polar impurities on the signalling of water
by the present probe system was measured. Methanol was used as
a representative polar impurity. In the presence of 0.5% methanol,
significantly reduced responses were observed for 1eOAcꢁ system
in acetonitrile (Figure S6, Supplementary data). This observation is
due to the fact that the present water signalling system is based
upon the competition of water and acetate ions for the complex
formation with the hydrazone chromophore by hydrogen bonding.
Polar and strongly hydrogen bonding methanol molecules signifi-
cantly interact with the hydrazone probe and affect the signalling of
water content in acetonitrile.
Fig. 4. Changes in UVevis spectra of 2eOAc system with varying water content in
acetonitrile. [2] ¼ 2.0 ꢂ 10-5 M, [OAcꢁ] ¼ 2.0 ꢂ 10-4 M. Water % ¼ 0, 0.5, 1.0, 1.5, 2.0, 2.5,
3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, and 10.
concentration-dependent absorption changes, the association
constant Kassoc of the 2eOAc system was estimated to be
1.74 ꢂ106 Mꢁ1 in acetonitrile. As other hydrazone sensors, although
interference from fluoride might be present, coumarin based
hydrazone 2 could function as a new sensor for the selective
detection of acetate ions.
Next, the signalling of the water content in acetonitrile by the
2eOAc system was investigated (Fig. 4). As the water content
increased, the UVevis absorption spectrum of 2eOAc, obtained
by treatment of 2 with 10 equiv of acetate ions in acetonitrile,
changed steadily to that of 2 only. The solution colour concomi-
tantly changed from red to yellow (left inset of Fig. 4). In this case,
the ratiometric analysis afforded a good calibration curve for esti-
mation of the water content up to 2% in acetonitrile. However, due
to the difference in binding strength of 2eOAc compared with
1eOAc, somewhat different signalling behaviour was observed.
A higher water content was required to convert the 2eOAc system
to 2, and a prominent signalling useful for the determination of the
water content was observed for up to 2% water. The detection limit
of the 2eOAc system for determination of the water content was
0.12% in acetonitrile.
To gain further insight into the signalling behaviour of the
hydrazoneeacetate system, the effect of the acetate concentration
on the signalling of the water content in acetonitrile of the more
stable 2eOAc system was elucidated (Fig. 5). As the concentration
1
5
7
4
2 equiv of acetate
3
0.8
0.6
0.4
0.2
0
2
1
0
6
5
4
3
2
1
0
10 equiv of acetate
100 equiv of acetate
0
0.5
1
1.5
2
% Water
% Water
300
400
500
Wavelength (nm)
600
700
0
2
4
6
8
10
% Water
Fig. 6. Changes in UVevis spectra of 1eOAc system with water content in THF.
[1] ¼ 2.0 ꢂ 10ꢁ5 M, [OAcꢁ] ¼ 2.0 ꢂ 10ꢁ4 M. Water % ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7,
0.8, 0.9, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, and 10.
Fig. 5. Effect of acetate concentration on UVevis titration of 2eOAc system with water
in acetonitrile. [2] ¼ 2.0 ꢂ 10ꢁ5 M. Ratio ¼ A502/A385
.

Y.H. Kim et al. / Dyes and Pigments 92 (2012) 1199e1203
1203
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4. Conclusion
A new chromogenic signalling system for the determination of the
water content in organic solvents was developed. Signalling was
based on competition of water molecules with the hydrogen bonding
interaction of acetate with hydrazone. Hydrazones were derived from
9-anthracenealdehyde and 7-hydroxycoumarin-8-carboxaldehyde. A
pronounced chromogenic behaviour, detectable by eye, was observed
for both compounds. The solution colour changed from red of the
hydrazoneeacetate complex to yellow of the hydrazone itself.
Prominent colour changes were observed for up to 1 and 2% water
content in acetonitrile and THF, respectively. The anthracene-based
hydrazone was found to be more sensitive toward changes in water
content than the 7-hydroxycoumarin derivative. The designed
hydrazoneeacetate system could be used as a convenient ratiometric
probe for measurement of the water content in common water
miscible organic solvents acetonitrile and THF.
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.dyepig.2011.07.019.
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