Multisignaling detection of cyanide anions based on an iridium(iii) complex: Remarkable enhancement of sensitivity by coordination effect

Article abstract of DOI:10.1039/b9nj00442d

A new molecule, (Z)-4-(4-(dimethylamino)benzylidene)-3-methyl-1-(pyridin-2- yl)-1H-pyrazol-5(4H)-one (dmpp), and its novel heteroleptic iridium(iii) complex, [(ppy)₂Ir(dmpp)]PF₆, were synthesized and characterized. Both dmpp and [(ppy)₂Ir(dmpp)]PF₆ could be used as highly selective and sensitive chemodosimeters for the cyanide anion by the naked eye, owing to the addition of the CN^- to the vinyl group of dmpp. The addition of CN^- to a solution of dmpp induced a change in the solution color from yellow to colorless, and to [(ppy)₂Ir(dmpp)] PF₆ induced a change in the solution color from pink to colorless. Moreover, [(ppy)₂Ir(dmpp)]PF₆ could also act as a phosphorescent and electrochemical probe of CN^-. The green phosphorescence band at 520 nm and a new irreversible oxidation wave at 0.423 V showed up upon the addition of CN^-, indicating that [(ppy) ₂Ir(dmpp)]PF₆ was an excellent multisignaling chemodosimeter of CN^-. Importantly, the Ir complex-based cyanide chemodosimeter [(ppy)₂Ir(dmpp)]PF₆ has much easier and faster detection of CN^- than pure organic molecule dmpp.

Full text of DOI:10.1039/b9nj00442d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Multisignaling detection of cyanide anions based on an iridium(III)
complex: remarkable enhancement of sensitivity by coordination effectw
Bin Lou, Zhu-Qi Chen, Zu-Qiang Bian* and Chun-Hui Huang
Received (in Montpellier, France) 1st September 2009, Accepted 16th October 2009
First published as an Advance Article on the web 20th November 2009
DOI: 10.1039/b9nj00442d
A new molecule, (Z)-4-(4-(dimethylamino)benzylidene)-3-methyl-1-(pyridin-2-yl)-1H-pyrazol-
5(4H)-one (dmpp), and its novel heteroleptic iridium(III) complex, [(ppy)2Ir(dmpp)]PF6, were
synthesized and characterized. Both dmpp and [(ppy)2Ir(dmpp)]PF6 could be used as highly
selective and sensitive chemodosimeters for the cyanide anion by the naked eye, owing to the
addition of the CNÀ to the vinyl group of dmpp. The addition of CNÀ to a solution of dmpp
induced a change in the solution color from yellow to colorless, and to [(ppy)2Ir(dmpp)]PF6
induced a change in the solution color from pink to colorless. Moreover, [(ppy)2Ir(dmpp)]PF6
could also act as a phosphorescent and electrochemical probe of CNÀ. The green phosphorescence
band at 520 nm and a new irreversible oxidation wave at 0.423 V showed up upon the addition
of CNÀ, indicating that [(ppy)2Ir(dmpp)]PF6 was an excellent multisignaling chemodosimeter of
CNÀ. Importantly, the Ir complex-based cyanide chemodosimeter [(ppy)2Ir(dmpp)]PF6 has much
easier and faster detection of CNÀ than pure organic molecule dmpp.
in emission color with changes in the local environment,
Introduction
significant Stokes shifts for easy separation of excitation and
Cyanide is widely used in various industrial processes, such as
emission, and relatively long lifetimes compared to purely
gold mining, electroplating and organic chemical industries.
organic luminophores. In particular, the long lifetime of
Due to its high toxicity, there is a growing interest in the
phosphorescence can eliminate the interference of the
development of cyanide chemodosimeters. The methods used
short-lifetime background and scattering light by using a
for this are mostly colorimetry,1 luminometry,2,3 or lumines-
time-resolved luminescence technology. It is well-known that
cence lifetime.4 Different approaches have also been envisaged
the photophysical properties of iridium(III) complexes are
dependent on the chemical structures of ancillary ligands.6
for the design of chemodosimeters for cyanide detection. Most
of them are based on the coupling of dyes to cyanide binding
sites1a,2a,3i or the use of specific cyanide reactions1b–e,2e,3f,g
specific component to interact with the analyte, the presence of
which take advantage of the strong nucleophilicity of the
When the ancillary ligand of an iridium(III) complex contains a
this analyte can lead to dramatic changes in the photophysical
cyanide anion. Since the chemodosimeters based on specific
and electrochemical properties of the iridium(III) complex.
cyanide reactions always show better selectivity than other
In the present study, on the basis the strong chemical
ones, more and more reaction types have been introduced with
addition of the cyanide anion to an a,b-unsaturated carbonyl
compound, a new molecule, dmpp (1), was prepared for CNÀ
the aim of developing highly sensitive and selective probes for
the cyanide anion. However, there are still several limitations
detection. Moreover, its novel heteroleptic iridium(III)
pertinent to these proposed reaction type probes, among
complex, [(ppy)2Ir(dmpp)]PF6 (2), was also synthesized for
which the worst one is that the reactions could take up to tens
of minutes at low analytic concentrations.1b–e In some cases,
and electrochemical measurement) cyanide detection. It is
multisignaling (UV-vis absorption, phosphorescent emission
the probe systems even need to be heated3f or added with
worthwhile to note that the electron-withdrawing ability of
several hundred-fold excesses of cyanide anions2e in order to
pyrazol-5(4H)-one moiety of 1, and thus the reactivity toward
speed up the reaction.
cyanide anions, could be enhanced by the coordination of the
Recently, the use of phosphorescent iridium(III) complexes
positively-charged Ir(III) center. As a result, the Ir-complex-
as chemodosimeters has attracted considerable interest5
based cyanide chemodosimeter 2 could have much easier and
because of advantageous photophysical properties such as shifts
faster detection for cyanide than 1 and other reported probes
based on specific cyanide reactions.1b–e (see Scheme 1)
Beijing National Laboratory for Molecular Sciences, State Key
Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University,
Beijing, 100871, China. E-mail: bianzq@pku.edu.cn;
Fax: +86 10-62757156; Tel: +86 10-62757156
Experimental section
Materials
w Electronic supplementary information (ESI) available: NOESY
Tetrabutylammonium salts were obtained from Alfa Aesar
spectra of 1, ESI-MS changes of 1 and 2 with cyanide anions, and
and Fluka and used without further purification. 4-(Dimethyl-
UV-Vis spectral change of the probes 1 and 2 with various anions. See
DOI: 10.1039/b9nj00442d
amino)benzaldehyde, potassium hexafluorophosphate and all
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excess of potassium hexafluorophosphate was added. The
suspension was stirred overnight and then filtered to remove
insoluble inorganic salts. The solution was evaporated to
dryness under reduced pressure. It was chromatographed by
CH2Cl2 to afford a brown solid in 70% yield. 1H NMR
(400 MHz, CDCl3 + CD3CN): d 8.91(d, 1H), 8.26 (s, 1H),
7.89–8.02 (m, 6H), 7.72 (d, 2H), 7.65 (s, 1H), 7.58 (d, 1H), 7.49
(s, 1H), 7.46 (m, 1H), 7.18 (t, 1H), 6.97–7.11 (m, 4H),
6.82–6.88 (m, 4H), 6.25 (d, 1H), 6.14 (d, 1H), 3.25 (s, 6H),
1.81 (s, 3H). Anal. calcd for IrC40H34F6N6OP: C, 50.47; H,
3.60; N, 8.83. Found: C, 50.23; H, 3.66; N, 8.79. MS (ESI+):
m/z = 807.3 (M À PF6).
Scheme 1 Structures of probe 1, 2 and the cyanide adduction of 2.
the solvents were obtained from Sinopharm Chemical Reagent
Beijing Co.
Results and discussion
General experiments
Synthesis
1
The H NMR spectra were recorded on an ARX-400 NMR
The preparation of 1 is simple. The precursor, 3-methyl-1-
(pyridin-2-yl)-1H-pyrazol-5(4H)-one,8 was easily obtained in
good yield according to literature procedures by a reaction of
2-hydrazinylpyridine with ethyl acetoacetate. Ligand 1 was
synthesized in a yield of 82% by the microwave-assisted
Knoevenagel condensation.7 The dinuclear cyclometalated
iridium(III) chloro-bridged precursor [ppy2IrCl]2 was synthe-
sized using the same methods as those reported by Nonoyama.9
By bridge splitting reactions of [ppy2IrCl]2, and subsequent
complexation with 1, complex 2 was synthesized as a brown
solid in a yield of 70% and characterized by elemental
spectrometer. Chemical shift data for each signal are reported
in ppm with tetramethylsilane as the internal reference.
Elemental analyses (C, H, N) were performed on a VARIO
EL instrument. Mass spectra were recorded on a Bruker Apex
IV FTMS. UV-vis absorption spectra were measured on a
Shimadzu UV-3100 spectrometer. The photoluminescence
(PL) spectra were recorded on an Edinburgh Analytical
Instruments FLS920 spectrometer.
Electrochemical measurements
Electrochemical measurements were performed with a CHI
600C Chemie’s Autolab. All measurements were carried out in
a one-compartment electrolysis cell under Ar gas, equipped
with a platinum bottom working electrode, a platinum wire
counter electrode, and an Ag/AgCl reference electrode. The
supporting electrolyte was tetrabutylammonium hexafluoro-
1
analysis, H NMR spectroscopy, and ESI-MS spectrometry.
The nature of the reaction between 1 (2) and CNÀ
The probes 1 and 2 are yellow (lmax = 461 nm, log e = 4.72)
and pink (lmax = 526 nm, log e = 4.92) dyes, respectively,
while both of their cyanide adducts are transparent in dilute
solution. It is possible to use probe 2 in ppm concentrations
for naked eye detection of cyanide anions due to its extremely
large e.1
phosphate (Bu4NPF6) with a concentration of 0.10 mol dmÀ3
.
The scan rate was 100 mV sÀ1. The ferrocene–ferrocenium
(Fc+/Fc) potential was measured and used as a reference.
Synthesis of 17
The nature of the reaction of cyanide anions with 1 and 2 is
putative,10 and we monitored the 1H NMR spectral and
ESI-MS changes produced via the addition of B2 equiv.
cyanide anions (as tetrabutylammonium salts) at room
temperature. As shown in Fig. 1, upon the addition of cyanide
anions, the vinylic protons of 1 (Ha) at 7.42 ppm completely
disappear, while a new signal grows in at 5.12 ppm and the
molecular weight of 1 increases by 25 (CNÀ-1) (see ESIw).
Similar changes were found for probe 2 (see both Fig. 1 and
ESIw). These observations clearly indicate that the cyanide
anions are added to the vinyl groups, breaking the conjugation
of the probes.
A 100 mL beaker containing a mixture of 3-methyl-1-(pyridin-2-yl)-
1H-pyrazol-5(4H)-one (3 mmol) and 4-(dimethylamino)-
benzaldehyde (3.4 mmol) was irradiated in a microwave oven
for 6 min at 400 W, and then the dark red oil was dissolved in
CHCl3 with a little CH3OH. After a few days for volatilization,
the red solid was separate out and chromatographed by using
ethyl acetate to afford a red solid in 82% yield. Melting point:
1
174–176 1C. H NMR (400 MHz, CDCl3 + CD3CN): d 8.59
(d, 2H), 8.51 (d, 2H), 8.20 (d, 1H), 7.84 (t, 1H), 7.42 (s, 1H),
7.16 (dd, 1H), 6.79 (d, 2H), 3.16 (s, 6H), 2.37 (s, 3H). Anal.
calcd for C18H18N4O: C, 70.57; H, 5.92; N, 18.29. Found: C,
70.31; H, 5.95; N, 18.17. MS (ESI+): m/z = 306.2.
The NOESY spectrum shows the molecular 1 is (Z)-4-(4-
(dimethylamino)benzylidene)-3-methyl-1-(pyridin-2-yl)-1H-
pyrazol-5(4H)-one.
The reaction rate (detection sensitivity)
In order to confirm the high sensitivity to CNÀ of probe 2,
time-dependent absorption spectral changes of 1 and 2
(20 mM) in the presence of excess cyanide anions (3.0 equiv.
to 1 and 1.6 equiv. to 2) are monitored. As shown in Fig. 2, the
addition of cyanide anions to 1 was rather slow. It took almost
80 min for the complete bleaching of 1 at room temperature,
and the calculated pseudo first order rate constant for cyanide
adduction is 1.32 Â 10À3 sÀ1 (24 1C, inset b in Fig. 2).
Synthesis of 2
A mixture of CH2Cl2 and ethanol (1 : 1, v/v) was added to
a flask containing [(ppy)2IrCl]2 (1 mmol) and compound 1
(1 mmol). The mixture was refluxed for 6 h. The dark red
solution was cooled to room temperature and then a 10-fold
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Fig. 1 1H NMR spectral changes of 1 and 2 produced via the
addition of cyanide anions in a mixture of CDCl3 and CD3CN.
Fig. 3 (a) Titration of 2 (2.0 mM) and (b) titration of 1 (20 mM) with
cyanide anions at room temperature in acetonitrile.
However, the addition of cyanide anion to 2 was too quick to
detect, and the pink color faded out immediately when they
were mixed together. (inset a in Fig. 2). As is expected, the
sensitivity to cyanide anion of probe 2 is improved so
significantly due to the coordination of Ir(III) center, that it
meets the on-line cyanide anion detection.
around 518 nm (19 300 cmÀ1) is characteristic of emission
from the (ppy)2Ir moiety at room temperature.11 The inset
there shows a schematic representation of the mechanism for
the luminescence intensity changes in the system of probe 2
upon the addition of cyanide anions. The cyanide adduction
breaks the conjugation and elevates the triplet state energy
levels (T1) of the dmpp moiety from below 19 200 to
23 400 cmÀ1 (Fig. 5), which is obviously higher than the
excited state of the (ppy)2Ir moiety (19 300cmÀ1) and, thus,
the luminescence of this moiety is no longer quenched. Hence,
complex 2 can act as an excellent phosphorescent probe
for CNÀ.
Photophysical titration tests
A detailed analysis of the absorption changes of 1 and 2
produced upon the addition of CNÀ is shown in Fig. 3.
The probes 1 and 2 are in 20 and 2.0 mM concentrations,
respectively, with 2.0 mM being among the lowest analytic
concentrations for colorimetric cyanide detection.1 Upon the
addition of 2.0 equiv. of cyanide anion to 1 and 1.6 equiv. of
cyanide anion to 2, the absorbance of the solution bleached
completely and the solutions became colorless. The bleaching
of 2 was completed immediately after the addition of cyanide
anion, but the bleaching of 1 was completed only after several
hours. The presence of clear isosbestic points (l = 353 nm for
1 and l = 396 nm for 2) further confirms the simple reaction
process. Hence, both 1 and 2 can act as excellent colorimetric
probes for CNÀ.
Electrochemical titration tests
The electrochemical response of 2 to CNÀ was studied by
cyclic voltammetry. In the absence of CNÀ, complex 2 shows
one reversible oxidation wave at 0.72 V, which can be assigned
to the metal-centered IrIII/IrIV and cyclometalated ligand
oxidation processes,11 one quasi-reversible oxidation wave at
1.05 V, which can be assigned to the dimethylamino group and
one irreversible reduction wave at À1.31 V , which can be
assigned to the ancillary ligand. A significant modification in
the cyclic voltammetry was observed upon addition of increasing
Fig. 4 illustrates the emission spectral changes of 2 upon the
addition of cyanide anions. The rising broad emissive band
Fig. 2 Time-dependent UV-vis spectral changes of probes 1 and 2
(inset a) at 20 mM upon the addition of 3 equiv. cyanide for 1 and
1.6 equiv. for 2 in CH3CN. The inset b shows the calculation of the
rate constant for cyanide addition to probe 1.
Fig. 4 Changes observed in luminescence spectra of 2 (2.0 mM) upon
addition of cyanide at room temperature in acetonitrile. Excitation is
at 396 nm. Inset: schematic representation of the cause of the rising
luminescence.
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Fig. 7 Color changes of 1 (top, 20 mM) and 2 (bottom, 2.0 mM) upon
adding 12 equiv. and 1.6 equiv., respectively, of various anions in
CH3CN.
Fig. 5 The emission spectra of 1, 2, and 1 + CNÀ (B4 equiv.) at
77 K in a EtOH glass in 20 mM concentrations.
the variation in the absorbance and emission intensity were
related to the cyanide concentration (Fig. 8 and Fig. 9).
However, the bleaching for 2 is much slower in the aqueous
solution (heated at 50 1C for 55 min before tests) than that in
acetonitrile, since water can weaken the nucleophilicity of
CNÀvia hydration.1b To circumvent the problem brought on
by water, we tried the chloroform solution of 2 to extract CNÀ
from water. Though it was reported that a biphasic fluoride
(TBAF) capture is very difficult,12 it should be easier for the
large cyanide ions based on Hofmeister effects.13 For example,
shaking a biphasic mixture containing 4.0 mM probe 2 in
5.0 mL CHCl3 and 10 equiv. of TBACN, 0.118 mM MgSO4,
0.689 mM NaCl and 0.182 mM KNO3 in 5.0 mL deionized
water for 1 min results in distinct bleaching of probe 2
(Fig. 10), which makes it possible to easily and quickly detect
levels as low as 40 mM cyanide in pure water. We feel that this
protocol could be usefully applied to all other reaction-type
cyanide probes which exhibit high sensitivity for CNÀ in
organic solutions.
amounts of CNÀ to an acetonitrile solution of 2 (Fig. 6). The
oxidation waves at 1.05 and 0.72 V were shifted to 1.17 and
0.86 V, respectively. The reduction wave at À1.31 decreased
gradually. Importantly, a new irreversible oxidation wave at
0.423 V increased linearly (insert in Fig. 6), which, obviously,
can be assigned to the newborn molecule 2 + CNÀ. Hence,
complex 2 can act as an excellent electrochemical probe
for CNÀ.
Selectivity tests of 1 and 2 for various anions.
Selectivity is also of paramount importance for probes. To test
the selectivity of our probes, mixtures of 1 (20 mM) or 2
(2.0 mM) with various anions were monitored using absorption
and naked-eye measurements (see ESIw). Among the 10 anions
À
tested in CH3CN, namely, CNÀ, FÀ, ClÀ, BrÀ, IÀ, NO3
,
AcOÀ, H2PO4À, HSO4 and ClO4À, both 1 and 2 responded
to CNÀ, resulting in color changes from yellow and pink,
respectivelyÀ, to colorless (Fig. 7). Even 12 equiv. of FÀ, AcOÀ
and H2PO4 can only slightly bleach the probe 1.
À
Conclusion
In summary, probe 1 is a specific chromo chemodosimeter and
probe 2 is a specific lumino-chromo-electro chemodosimeter
for cyanide anions. Probe 2 shows much larger e at lmax and
much faster detection of cyanide anions due to the strong
electron-withdrawing ability of the iridium(III) center than
probe 1 and most previously reported probes based on specific
cyanide reactions.1b–e,2e,3f We also achieved detection of CNÀ
Aqueous cyanide detection
In order to examine the potential use of the excellent probe 2
for the detection of cyanide in aqueous solutions, further
experiments were carried out in the water–acetonitrile 1 : 1 v/v
solution (concentration of 2 in the mixture was 10 mM,
pH B 6.0). As in acetonitrile solutions, it was observed that
Fig. 8 UV-vis spectral changes of the probe 2 (10 mM) seen upon the
addition of cyanide anions in water–acetonitrile 1 : 1 v/v solution after
heating at 50 1C for 55 min. Inset: absorption titration profile at
529 nm versus equivalents of cyanide ions.
Fig. 6 Changes in the cyclic voltammograms of 2 (2 mM) in CH3CN
solution with various amounts of CNÀ (0.14–2.4 mM). Inset: electro-
chemical titration profile of 2 at 0.423 V with CNÀ.
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We thank the National Basic Research Program (2006CB601103)
and the NNSFC (20821091, 20671006, 50772003, 90922004 and
20971006) for financial support, Hong-Wei Li for help in NOESY
text and Dr Kun Hou for language editing.
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