Detection of Mercury(II) and Silver(I) Ions
Dagny Dagmara Konieczna, Amelié Blanrue and René Wilhelm
Department of Chemistry, University of Paderborn, Warburgerstr. 100, 33098 Paderborn, Germany
Reprint requests to Prof. Dr. René Wilhelm. Fax: +49 5251 603245.
E-mail:rene.wilhelm@uni-paderborn.de
Z. Naturforsch.2014,69b, 596 – 604 / DOI: 10.5560/ZNB.2014-4014 Received January 31, 2014
Two imidazol(in)ium-dithiocarboxylates have been investigated as sensors for the detection of mer- cury ions and silver ions. They could be applied as colorimetric chemosensors for the detection of Hg2+and Ag+. Furthermore, an additional sensory input was found by a colorimetric change of a two-phase system from the organic phase into the aqueous phase. Due to different colors at dif- ferent ratios of the betaines and Hg2+it is possible to estimate the concentration of Hg2+with the
”naked eye”.
Key words:Carbenes, Imidazolium-dithiocarboxylates, Mercury, UV/Vis, Betaines
Introduction
Imidazol(in)ium-dithiocarboxylate betaines, formal adducts of carbenes and CS2are composed of a pos- itively charged imidazolium ring and a negatively charged CS2group tilted almost perpendicular to the ring [1–4].These betaines were first reported by Win- berg and Coffman in 1965 [5]. Thereafter, their chem- istry, like their application in [3+2] cycloaddition re- actions with electron-deficient alkynes, was studied by several groups [6–22]. The compounds are known to be stable [1–4] contrary to their CO2 and COS analogs [1,4]. In addition, the synthesis of several metal complexes with these betaines as ligands have been reported [23–32], and recently a few investi- gations of reactions catalyzed by metal complexes of these betaines have been described [33–35]. More- over, the group of Delaude found that these compounds could be used for probing the stereoelectronic parame- ters ofN-heterocyclic carbenes [36].
Recently, we reported the application of symmet- ric imidazolinium-dithiocarboxylates as organocata- lysts for the TMSCN addition on aldehydes [37] and of enantiopure analogs as organocatalysts for an asym- metric Staudinger reaction [38]. In addition, we pre- pared new ionic liquids via methylation of the cor- responding betaines, resulting in red cations, which
showed different absorptions maxima depending on the solvent and the lipophilicity of the anions [39]. In order to further investigate the behavior of these be- taines, we present here the evaluation of this class of compounds as sensors for different metal ions. Hg2+
ions for example possess a high affinity to sulfur, thus these betaines offer the possibility to serve as ligands for binding with Hg2+due to the CS2group [32]. Fur- thermore, a visual color change can be expected, since the binding of Hg2+to CS2would change the electron density of the CS2unit and hence change the charge- transfer system between the CS2group and the imida- zol(in)ium ring.
Mercury is a highly toxic element, accumulating readily as inorganic mercury compound in the envi- ronmental water, where it can be converted into the even more toxic methylated mercury, which may en- ter the food chain by eating contaminated fish [40,41].
A simple and straight forward method to detect mer- cury contaminations, without the need of a large ex- perimental setup and instrumentation, is the appli- cation of colorimetric chemosensors [42–44]. Sev- eral types of chemosensors for Hg2+ have been developed, and their number has increased contin- uously [45–53]. A larger number of examples of these chemosensors are based on ferrocenes [54, 55], rhodamines [56–58], naphthalimides [59,60],
© 2014 Verlag der Zeitschrift für Naturforschung, Tübingen·http://znaturforsch.com
cyclodextrins [67,68].
Here we present the investigation of a new simple straightforward motif for a sensor for mercury(II) and silver(I) cations.
Results and Discussion
Two different betaines were prepared according to Scheme1. Starting from the literature-known imida- zolinium salt 1 or the commercially available imida- zolium salt 2, the corresponding carbenes were pre- pared with KOtBu as the base. The in-situ prepared carbenes were reacted with CS2to the desired betaines 3and4.
Ideally, a chemosensor for Hg2+should be applied in an aqueous system. Because the prepared betaine 4 was not or only slightly soluble in water, the first experiments were carried out in methanol. One equiv- alent of Hg(NO3)2was added to a methanol solution of each betaine. Compound4generated the most sig- nificant color change from red to purple (Fig.1). This appears plausible, since the π-electron system in4 is more delocalized.
The selectivity towards Hg2+was tested by measur- ing UV/Vis absorption spectra for other metal nitrates
N N Bn
Bn S S 3
N N
Bu
Me S S 4 N
N Bn
Bn 1
N N
Bu
Me 2
Cl
KOtBu CS2 THF
KOtBu CS2 THF BF4
Scheme 1. Synthesis of carbene CS2adducts.
which verify that only Hg forms a strong mercury complex by the decrease and the shift of the absorp- tion maximum at about 350 nm toward shorter wave- lengths. In addition, a marginal increase and shift to- ward longer wavelengths of the absorption maximum in the visible range at about 540 nm was observed. The other metal ions did not promote significant changes in the UV/Vis spectra. In order to confirm the for- mation of a complex between4and Hg2+,1H NMR and13C NMR spectra were measured in CD3OD be- fore and after the addition of Hg(NO3)2as illustrated in Figs.2and3. The comparison of the spectra shows peak shifts caused by the formed metal complex. The
13C spectrum also provides the information how Hg2+
binds to4. After adding mercury, the signal of the CS2 group in the13C NMR spectrum shifted upfield from 226 to 216 ppm, which corroborates the presumption that the binding of4to Hg2+ion takes placesviathe CS2group.
E D
B C
A
Fig. 1 (color online). UV/Vis spectra of 4without and in the presence of various metal ions (1 equiv.) in MeOH. The concentration of the measured solution was 10−5mol L−1. The photograph of color changes of 4in MeOH solution (10−2mol L−1) with 1 equiv. of different metal ions. A: –;
B: Zn2+; C: Co2+; D: Cd2+; E: Hg2+.
1 2
N N 3
4 5 6 7
S S Hg2+
1 2
N N 3
4 5 6 7
S S A
B
1 2 1 2
3
3 4
4
5
5 6
6 7
7
Fig. 2.1H NMR spectra of4(10−2mol L−1) before (A) and after the addition of Hg2+(B), recorded in CD3OD.
For clarifying how many equivalents of Hg2+ions impact the color change, 4 was treated with 2 and 4 equiv. of Hg2+. While the doubling of equivalents leads to an intensity decrease of the absorption band at about 350 nm and an increase at about 540 nm in the visible range, the quadrupling affects the absorp- tion only marginally. A significant visual color change was not observed.
Because3was not soluble in methanol, acetonitrile was used as solvent, to which 1 equiv. of various metal nitrates was added. Similar to 4, only Hg2+ caused a significant color change from yellow to pink. This has also been confirmed by the UV/Vis spectra shown in Fig.4. The addition of Zn2+, Co2+and Cd2+did not result in a significant change in the UV/Vis spectra.
Next, the colorimetric behavior of 3 was investi- gated in a two-phase mixture of water and CHCl3. As
shown in the inset of Fig.5, compound3shows a yel- low color in the chloroform phase with an absorption maximum centered at about 360 nm and in the visi- ble range at about 430 and 500 nm. After adding an equimolar amount of aqueous Hg(NO3)2solution and shaking the mixture, the intensity of the absorption band in the near UV range decreased, and the absorp- tion maximum slightly shifted to a longer wavelength.
The change in the UV/Vis spectra can also be seen vi- sually by the color change from yellow to pink, ob- viously caused by the interaction of Hg2+ and 3 to a complex at the phase boundary, which remains in the organic phase.
Compound 4 was also investigated with regard to the detection of Hg2+in an H2O-CHCl3solution. The photography in Fig.6shows the two-phase mixture of 4with and without Hg2+. In the latter case the organic
1 2
N N4 3
5 6 7
8 9
S S Hg2+
1 2
N 4N 3
5 6 7
8 9
S S
B
9 9
4
4
1 2
1 2 5 3
3
6 7 8
6 7
8
5
Fig. 3.13C NMR spectra of4(10−2mol L−1) before (A) and after the addition of Hg2+(B) in CD3OD.
phase shows a red color, while the aqueous phase is colorless to light yellow. The slight coloration to yel- low is probably due to a minor solubility of4in water or to a small amount of CHCl3in the aqueous phase.
This is only temporarily, and the color disappears after a few hours. Surprisingly, the addition of Hg2+caused a color change to colorless in the organic phase and to pink in the aqueous phase. Thus,4 forms a complex with Hg2+at the phase border, which remains in the aqueous phase. The corresponding UV/Vis spectra are shown in Fig.6, which shows that the absorption band of the organic phase disappears after adding water en- riched with Hg2+. In contrast, the absorption band of the aqueous phase increases and shifts to shorter wave- lengths in the near UV range, and a new absorption band appears at about 550 nm.
To determine the visible detection limit, two sam- ples of4 were prepared in H2O-CHCl3 with a con- centration of 10−3mol L−1. One sample was enriched with 1 equiv. of Hg2+, shown in Fig.7. A resulting color change was observed in both phases, however it was very pale. While the organic phases differ only slightly from each other, the aqueous phase of contam- inated water can be distinguished from pure H2O by the color change to light pink.
Because a concentration of4of 10−3mol L−1was too low for a significant color change, further inves- tigations were carried out with4in the concentration of 10−2mol L−1 in CHCl3-H2O. The photograph in Fig.8 shows the resulting two-phase mixtures, which were enriched in the following sequence with 0.1, 0.2, 0.3, 0.4 and 0.5 equiv. of Hg2+. For comparison, the
H I J
F G
Fig. 4 (color online). UV/Vis spectra of 3(10−5mol L−1) without and in the presence of different metal ions. The photograph of color changes of 3 in acetonitrile solution (10−2mol L−1) by addition of 1 equiv. of different metal ions. F: – ; G: Zn2+; H: Co2+; I: Cd2+; J: Hg2+.
M N
Fig. 5 (color online). UV/Vis spectra of3(10−5mol L−1) in a two-phase mixture of H2O and CHCl3 without and with Hg2+(1 equiv.). The photograph shows the color changes of 3(10−2mol L−1) in H2O and CHCl3. M: – ; N: Hg2+.
K L
Fig. 6 (color online). UV/Vis spectra of 4(10−5mol L−1) in a two phase mixture of H2O and CHCl3 without and with Hg2+ (1 equiv.). Photograph of color changes of 4 (10−2mol L−1). K: – ; L: Hg2+.
O P
Fig. 7 (color online). Photograph of color changes of 4 (10−3mol L−1) in H2O and CHCl3before (O) and after the addition of 1 equiv. Hg2+(P).
sample without Hg2+is also shown. With the increas- ing addition of Hg2+, the organic phase becomes red to colorless, while the aqueous phase turns from yel- lowviared to pink. Moreover, it is demonstrated that half of Hg2+ to 4 shows the same color change as such, when using the same ratio. The full discoloring of the organic phase suggests that one Hg2+ion is bind- ing to two molecules of4. The corresponding UV/Vis spectra correspond to the visual color changes. With
K Q R S T U Fig. 8 (color online). Photograph of color changes of 4 (10−2mol L−1) by addition of different amounts of Hg2+. K: – ; Q: 0.1 equiv.; R: 0.2 equiv.; S: 0.3 equiv.; T: 0.4 equiv.;
U: 0.5 equiv. Hg2+.
Fig. 9 (color online). UV/Vis spectra of the aqueous phase (A) of 4(10−5mol L−1) in a mixture of H2O and CHCl3 with different amounts of Hg2+.
increasing Hg2+ concentration, the absorption bands appear in the case of the aqueous phase and disappear in the organic phase (see Figs.9and10).
The validity of the Lambert-Beer Law was exam- ined by plotting the absorption against different con- centrations for a selected wavelength (see Supporting Information available online; see note at the end of the paper for availability). The linearity of the graph showed that only one Hg dye complex is involved in the color change.
The selectivity towards Hg2+ was investigated by the addition of various metal nitrates ( Co2+, Zn2+, Cd2+, Cu2+, Ag+ and Pb2+) dissolved in H2O to a solution of4in CHCl3. Since Cu2+, Ag+and Pb2+
have a similar chemical behavior as Hg2+, it is diffi- cult to develop sensors reacting selectively with Hg2+
ions [69–72]. Silver complexes with ligands incorpo-
Fig. 10 (color online). UV/Vis spectra of the organic phase of4in a mixture of H2O and CHCl3with different amounts of Hg2+.
rating a CS2moiety have been reported in the litera- ture [73,74]. From Fig.11it can be seen that only the addition of Ag+und Hg2+causes a significant color change in both phases. The presence of Ag+ causes an immediate color change from colorless to deep red in the aqueous phase, also confirmed by UV/Vis spec- tra in Fig.11. While the absorption maxima of the or- ganic phase centered at about 360 nm and in the visible range at about 540 nm disappear, the absorption of the aqueous phase increases, resulting in a broad absorp- tion band with a shoulder in the visible range between 500 and 600 nm. Even though Ag+ provides a color change with4, it can be distinguished from Hg2+.
Conclusion
It has been shown that the imidazolium- dithiocarboxylate4shows the potential as a chemosen- sor for Hg2+ and Ag+. The betaine has not been investigated before for this application. The observed phase change with betaine4 with silver and mercury complexes may lead to new sensors based on these betaines which are capable to detect two different metal species simultaneously, either remain in the organic phase or transfer into the aqueous phase. In addition, due to the different colors at different ratios of the betaine and Hg2+, it is possible to estimate the concentration of Hg2+ with the “naked eye”.
Considering the low sensitivity and selectivity of
V W X Y Z AA L
Fig. 11 (color online). UV/Vis spectra of4(10−5mol L−1) in a two phase mixture of H2O and CHCl3 without and with 1 equiv. of Hg2+. Photograph of color changes of 4 (10−2mol L−1) in H2O and CHCl3by addition of 1 equiv. of different metal ions. V: Co2+; W: Cd2+; X: Zn2+; Y: Pb2+; Z: Cu2+; AA: Ag+; L: Hg2+.
the investigated betaines towards Hg2+/Ag+, future investigations will focus on the preparation of new betaines incorporating fluorescent moieties. Addition- ally, the displacement of the absorption maxima in Fig.9is currently under investigation.
Experimental Section
General methods
The reactions were carried out under argon atmosphere using standard Schlenk line techniques. Tetrahydrofuran was distilled from sodium benzophenone ketyl. Infrared spectra were recorded on a Vector 22 FT-IR from Bruker. The ab- sorption of solids was measured by potassium bromide pel- lets, the absorption of liquids by using a thin layer between sodium chloride plates.1H and13C NMR spectra were taken on AMX 400 (400 MHz), AC 250 P (200 MHz) or Advance 500 (500 MHz) spectrometers from Bruker. Mass spectra were recorded on a MS 5889 B instrument from Hewlett Packard. UV/Vis spectra were recorded with a Cary 50 spec- trometer of Varian. The measurements were performed at
room temperature. The reactions were followed by thin layer chromatography on silica gel precoated plates (Merck TLC silica gel F254). Column chromatography was performed us- ing silica gel 60. Salt2andN,N0-dibenzylenediamine were purchased from Aldrich.
1,3-Dibenzylimidazolinium tetrafluoroborate (1)
N,N0-Dibenzylethylenediamine (0.74 mL, 3.16 mmol, 1 eq.), triethyl orthoformate (0.53 mmol, 3.16 mmol, 1 eq.) and ammonium tetrafluoroborate (331 mg, 3.16 mmol, 1 eq.) were heated for 2 h at 120◦C in a closed vessel. Recrys- tallization in dry ethanol gave a colorless solid (953 mg, 2.8 mmol, 89 %). M. p. 84◦C. – IR (KBr):v=3092, 1649, 1457, 1443, 1373, 1304, 1208, 1058, 704 cm−1. –1H NMR (200 MHz, CDCl3): δ =8.54 (s, 1 H, NCHN), 7.34 (m, 10 H, H-Ar), 4.67 (s, 4 H, NCH2Ar), 3.74 ppm (s, 4 H, 2 NCH2CH2). – 13C NMR (50 MHz, CDCl3): δ =157.7 (NCHN), 132.5, 129.4, 129.2, 129.0, 52.4, 47.8 ppm. – MS (ESI, 0 V): m/z(%) = 251 (100) [M]+. – Anal. for C17H19N2BF4: calcd. C 60.38, H 5.66, N 8.28; found C 59.93, H 5.69, N 8.37.
1,3-Dibenzylimidazolinium-2-dithiocarboxylate (3) 1,3-Dibenzylimidazolinium tetrafluoroborate (1) (3.16 mmol, 1 eq.) was dissolved in THF (2 mL), and KOtBu (531 mg, 4.74 mmol, 1.5 eq.) was added. After stirring for 30 min CS2 (0.95 mL, 15.8 mmol, 5 eq.) was added. The mixture became reddish. Water and CH2Cl2 were added, and the aqueous layer was extracted 3 times with CH2Cl2. The organic layers were dried (Na2SO4), and the solvent was removed by evaporation. The obtained product was isolated by column chromatography with CH2Cl2to give a red solid (653 mg, 2 mmol, 63 %). Spectral data were consistent with literature values [7].13C NMR data are not provided in the literature and are given here.
–13C NMR (100 MHz, CDCl3):δ =224.9 (CS2), 167.0, 133.0, 129.2, 129.1, 129.0, 51.2, 46.0 ppm.
3-Butyl-1-methylimidazolium-2-dithiocarboxylate (4) 3-Butyl-1-methylimidazolium chloride (145) (330 mg, 1.9 mmol, 1 eq.) was stirred with KOtBu (320 mg, 2.89 mmol, 1.5 eq.) in THF overnight. CS2 (0.6 mL, 9.5 mmol, 5 eq.) was added to give a deep-red solution.
The product was isolated by column chromatography with CH2Cl2 to give a red oil (344 mg, 1.6 mmol, 84 %). – IR (NaCl):v=3104, 2960, 2933, 1575, 1508, 1266, 1056, 911, 732 cm−1. – 1H NMR (400 MHz, CDCl3):δ =6.85 (s, 2 H, NCH=CHN), 4.06 (t,J=7.6 Hz, 2 H, NCH2), 3.73(s, 3 H, NCH3), 1.89 – 1.74 (m, 2 H, NCH2CH2), 1.41 – 1.19 (m, 2 H, NCH2 CH2CH2), 0.87 ppm (t, J=7.3 Hz, 3 H, CH3). – 13C NMR (100 MHz, CDCl3): δ =224.7 (CS2),
113 (41), 104 (18), 83 (20), 71 (67), 57 (79). – HRMS:
m/z = 237.0489 (calcd. 237.0496 for C9H14N2S2Na, [M+Na]+).
A plot of the Lambert–Beer correlation is given as Sup- porting Information available online (DOI: 10.5560/ZNB.
2014-4014).
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