Cu(ii)4L4 coordination-driven molecular container: A reusable visual colorimetric sensor for Ag(i) ions

Article abstract of DOI:10.1039/c4cc00118d

A Cu₄L₄ square-like molecular container which can be a reusable visual sensor for Ag⁺ is reported. The present results can be a useful stepwise approach for the construction of the heterometallic supramolecular complexes with potential applications. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc00118d

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Cu(II)4L4 coordination-driven molecular container:
a reusable visual colorimetric sensor for Ag(I)
ions†
Cite this: Chem. Commun., 2014,
50, 4721
Received 6th January 2014,
Accepted 17th March 2014
Jian-Ping Ma,‡ Shen-Qing Wang,‡ Chao-Wei Zhao, Hai-Ying Wang and
Yu-Bin Dong*
DOI: 10.1039/c4cc00118d
www.rsc.org/chemcomm
A Cu4L4 square-like molecular container which can be a reusable difficult due to the inherent difficulty in the preparation of the
visual sensor for Ag+ is reported. The present results can be a useful multidentate organic spacers with specific recognizing sites for the
stepwise approach for the construction of the heterometallic metal ions. Furthermore, the reported metallamacrocycles are
supramolecular complexes with potential applications.
mainly used to trap and recognize organic guest molecules and
sometimes counter ions;1d meanwhile they are rarely used as a host
Metal-directed self-assembly of discrete two- and three-dimensional to incorporate heavy metal cations, furthermore, to be the sensors to
molecular containers can accommodate various guest species such detect heavy metal cations.
as neutral molecules and ionic species, and often draw them to
As we know, Ag+ is very harmful to human health as an
reveal some new properties.1 Among various coordination-driven environmental heavy metal pollutant.5 Therefore, the development
molecular containers, molecular squares can be the most typical of an Ag+ sensor, especially a direct, convenient and selective Ag+-
and representative of coordination-driven containers. Since the first detector is very significant. To date, various methods, for example
spontaneous formation of the square-like Pd2+-complex in 1990,2 atomic absorption spectroscopy, inductively coupled plasma-mass
numerous squares and related metal-cornered structures have been spectroscopy, and electrochemical methods based on ion-selective
successfully synthesized. In general, the end-capped M-components electrodes, have been developed.6 More recently, organic species,7
(M = Pd2+ and Pt2+) used for the construction of molecular squares molecular complexes,8 nanomaterials,9 quantum dots10 and DNA-
based on linear rigid bidentate ligands are usually four-coordinated based materials11 are adopted as the fluorescent and electrochemical
square planar, which is naturally required for the formation of sensors to detect Ag+. To our knowledge, sensors for metal cations
square-like metallamacrocycles. Besides Pd2+ and Pt2+, the other based on discrete coordination-driven molecular containers are
kinds of metal-cornered squares, however, have attracted much less extremely rare. As we know, the most practical and convenient
attention.3 The preparation of other metallasquares such as Cu2+ sensor candidates are the naked-eye colorimetric ones. Compared
molecular squares is very hard, which might be limited by the to the fluorescent and electrochemical sensors, discrete coordination-
preferred metal node coordination geometry together with the driven molecular containers that exhibit naked-eye colorimetric
specific ligand design.4
response, visible to the naked eye, after incorporating a specific type
In most cases, the organic linkers used for construction of of metal ions are unprecedented.
metallasquares are the 4,40-bipy-type of organic ligands which
In this contribution, we describe a triazole-bridged ligand
contain no free heteroatomic donors except the coordinating sites. (LH) capped by NO cheating groups12 which can bind Cu2+ ions
On the other hand, the insertion of uncoordinated heteroatoms into a square-like metallamacrocycle (1) with uncoordinated
such as N, O and S would endow the resulted molecular containers nitrogen donors and amino groups (Scheme 1). Moreover,
more functionalities. The synthesis of such ligands, however, is very compound 1 can be a reusable naked-eye colorimetric sensor
to detect Ag+ ion. The sensing mechanism is also investigated.
Ligand LH, generated from 2,5-bis(4-aminophenyl)-4-amino-
College of Chemistry, Chemical Engineering and Materials Science, Collaborative
1,2,4-triazole and acetylacetone, is capped by two terminal
Innovation Center of Functionalized Probes for Chemical Imaging, Key Laboratory
of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, N,O-chelating groups (ESI†). Neutral macrocyclic compound 1
Jinan 250014, P. R. China. E-mail: yubindong@sdnu.edu.cn
was isolated as a light brown powder by mixing LH and
Cu(OAc)2 (molar ratio, 1 : 1) in a mixture of MeOH and CH2Cl2
in the presence of KOH in quantitative yield (Scheme 1).§
† Electronic supplementary information (ESI) available: LH synthesis, the ORTEP
figure of 2, UV-vis spectra, TGA, the CSI-MS spectrum of 2, Job’s plot and crystal
data of 2. CCDC 978498. For ESI and crystallographic data in CIF or other
Cold-spray ionization mass spectroscopy (CSI-MS) provides
substantial evidence for the formation of a Cu4L4 structure.
electronic format see DOI: 10.1039/c4cc00118d
‡ These authors contributed equally to this work.
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Scheme 1 Synthesis and the 3D molecular modeling of 1 (the model
structure was obtained by the Gauss View program).
Fig. 2 Up: color change of 1 in the presence of various representative
metal ions, from left to right: blank, Ag+, Co2+, Ni2+, Zn2+, Cd2+, Cr3+, Mg2+
,
Ca2+, Fe3+, Pb2+, Na+ and K+. [1] = 10ꢂ4 M. [Metal ion] = 3.75 ꢁ 10ꢂ4 M.
Solvent: MeOH; bottom: UV-vis bar diagrams showing the change in the
absorbance for 1 (10ꢂ4 M) in the presence of above representative metal
ions (3.75 ꢁ 10ꢂ4 M) in MeOH at a lmax of 505 nm, and Ag+ with different
counter ions.
To determine if the counter ions used can affect the Ag+ recognition
based on 1, different silver salts such as AgSbF6, AgNO3, AgSO3CF3,
AgClO4, and AgCOOCF3 were used to perform the parallel reactions.
The adsorption spectra showed that the different anions have no
interaction with 1 in methanol solution (Fig. 2). Furthermore, the
UV-vis absorption spectra of 1 in the presence of other kinds of
metal ions mentioned above were also recorded. No change in the
spectral pattern was observed except Cr3+, indicating that Cu4L4
binds preferentially to Ag+ (Fig. S3 and S4, ESI†). Based on above
Fig. 1 CSI-MS spectrum of 1 obtained from MeOH/CH2Cl2. The inset
shows the calculated (top) and the experimental (bottom) isotopic dis-
tribution of the square species [Cu4L4ꢀ4CH3OHꢀ5H2O + H+].
As shown in Fig. 1, the molecular peak at m/z 2185.8 was observation, the metallamacrocycle of 1 could be a selective visual
observed due to the formation of the [Cu4L4ꢀ4CH3OHꢀ5H2O + H+] sensor for Ag+ ions.
species, thereby confirming that the Cu2+ cations in solution are
As indicated above, the color of methanol solution of 1 imme-
chelated by four deprotonated ligand L (Scheme 1). The existence diately changed to red as Ag+ was added to it. However, the red color
of co-crystallized MeOH and H2O solvent molecules was further did not deepen with increasing Ag+ concentration when the ratio of
confirmed by thermogravimetric analysis (TGA). TGA revealed that Ag+ to 1 exceeded 4 : 1. A detailed absorption titration study of Ag+
the observed solvent molecule weight loss is 10.6% (calculated was conducted using a 9.37 ꢁ 10ꢂ5 M solution of 1 in MeOH. As
10.0% based on Cu4L4ꢀ4CH3OHꢀ5H2O, Fig. S1, ESI†).
shown in Fig. 3a, at the concentration range of 0 to 3.75 ꢁ 10ꢂ4 M,
As mentioned above, the obtained Cu4L4 metallamacrocycle the absorption intensity at 505 nm of 1 increased upon the addition
contains four triazole moieties with uncoordinated N atoms. Such of Ag+ ions. The absorption intensity, however, gradually decreased
a nucleophilic molecular pocket would be expected to be an ideal when the concentrations of Ag+ was higher than 3.75 ꢁ 10ꢂ4 M. So,
receptor to trap a specific type of metal cations based on their size the maximum absorption was observed at the Ag+ concentration of
and charge. Following this idea, compound 1 was used as a macro- 3.75 ꢁ 10ꢂ4 M. At this point, the molar ratio between Cu4L4 (1) and
cyclic host to bind different metal cations, including IA, IIA and Ag+ is 1 : 4 (Fig. 3b). The binding constant, Ka, of the Ag+ to the Cu4L4
other some other common transition metal ions.
host was found to be 5.27 ꢁ 1016 based on Job’s plot (Fig. 3c and
Interestingly, our initial investigations showed that the color Fig. S4, ESI†).13
change of the methanol solution of 1 from light green to red
Notably, the Ag+-responded color change is reversible (Fig. 4a).
upon addition of Ag+ aqueous solution was readily visible to the As shown in Fig. 4b, upon addition of Brꢂ (4 equiv. to 2) to 2, the
naked eye (Fig. S2, ESI†). It suggested that Ag+ ions were trapped by maximum absorbance in the UV-vis spectrum at 505 nm dis-
the heteroatom-rich Cu4L4 host. It is different from Ag+, as the color appeared. Meanwhile, the color of the system went back to light
of methanol solution containing 1 did not show basically any color green (Fig. 4a), indicating that the Cu4L4 host was regenerated. On
change upon addition of each metal cation, i.e., Co2+, Ni2+, Zn2+, the other hand, the maximum absorbance was completely recovered
Cd2+, Cr3+, Mg2+, Ca2+, Fe3+, Pb2+, Na+ and K+, which is further upon addition of Ag+ (4 equiv. to 1) to the above system, and
confirmed by the absorption spectra (Fig. 2 and Fig. S2, ESI†). the corresponding red color of the solution reappeared (Fig. 4a).
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Fig. 5 ORTEP figure (displacement ellipsoids drawn at the 30% probability
level) and side view of 2. The photograph of crystals 2 is inserted.
Fig. 3 (a) Absorption spectral response of 1 upon addition of Ag+ ions in
methanol; (b) the diagram of absorption variation with [Ag+] and (c) Job’s
plot of CAg/C (C = CAg + C1) versus absorbance.
A side view of 2 (Fig. 5) reveals that the {Ag4O2}2+ cluster core is
situated in the center of the macrocycle, and deviates from the
ꢂ
square plane ca. 1 Å. The SbF6 ions are at the side of the molec-
ular square. Besides single-crystal analysis, the ESI-MS spectrum
(Fig. S7, ESI†) of 2 recorded in DMSO/MeOH displays multiple
charged cations, confirming the formation of the Cu2+–Ag+ hetero-
metallic species.
As mentioned above (Fig. 3), the maximum UV-vis absor-
bance of 2 was observed when the ratio of Cu4L4 to Ag+ reached
1 : 4, which is well consistent with the single crystal structure.
Upon addition of more Ag+, the UV-vis absorption intensity
gradually decreased which might be caused by the collapse of
the Cu4L4 macrocyclic structure due to the ion-exchange
between Cu2+ nodes and the excess of Ag+ ions.
In summary, we have shown that the formation of a soluble
tetrametallic square-like Cu2+-macrocycle based on a triazole brid-
ging bis-didentate ligand is possible. The obtained Cu2+-macrocycle
with eight uncoordinated heterocyclic N-donors is very sensitive and
selective toward Ag+ over twelve other important ions as demon-
strated by UV-vis titrations. More importantly, it can be a naked-eye
sensor to clearly detect Ag+ (visual limit, 10ꢂ4 M) under ambient
conditions. In addition, this selective sensing mechanism is studied.
Single-crystal analysis reveals that the Ag+-species was trapped in the
Cu2+-host as a {Ag4O2}2+ cluster core, which well agrees with the
UV-vis titration. Notably, the adsorption of Ag+-species based on 1 is
reversible, and the empty Cu4L4 host can be easily regenerated.
On the other hand, the present results can be a useful stepwise
approach for the construction of the heterometallic supramolecular
complexes with potential applications.
Fig. 4 (a) Reversible Ag+ encapsulation based on 1 in MeOH/H2O; and (b)
corresponding UV-vis spectra. The corresponding photographs of 1 and 2
are inserted.
It is worth noting that the empty Cu4L4 host cannot be regenerated
upon addition of Clꢂ (4 equiv. to 2), because the Ksp (1.8 ꢁ 10ꢂ10
)
)
value of AgCl is much higher than that of AgBr (5.35 ꢁ 10ꢂ13
(Fig. S5, ESI†).
In order to understand the mechanism for this interesting visual
colorimetric Ag+ sensing, supramolecular complex Ag+@Cu4L4 (2)
was isolated and its structure was determined by single-crystal X-ray
diffraction analysis (ESI,† and Fig. S6). The deep red single crystals
(Fig. 5) of 2 ({[Cu4L4Ag4(O2)](SbF6)2}ꢀ4(CH2Cl2)ꢀ4H2O) were obtained
by the combination of 1 and AgSbF6 (molar ratio = 1 : 4) in a MeOH/
CH2Cl2 mixed-solvent system at ambient temperature(§). Complex 2
crystallizes in the monoclinic space group P2/c. As shown in Fig. 5,
the four tetrahedral Cu2+ nodes are bridged by four bis-didentate
deprotonated ligands to form a square-like molecular structure, in
Notes and references
§ Synthesis of Cu4L4ꢀ4CH3OHꢀ5H2O (1). A solution of Cu(OAc)2ꢀH2O
(80 mg, 0.4 mmol) in MeOH (30 mL) was added to a solution of LH
which the Cu2+ꢀꢀꢀCu2+ distances are in a range of 14.353–14.763 Å. (172 mg, 0.4 mmol) and KOH (22.4 mg, 0.4 mmol) in CH2Cl2/MeOH
(20/20 mL). The mixture was stirred for 3 h at room temperature. After
The eight N-donors on the central 1,2,4-triazole moieties face toward
removal of most of the solvent under vacuum, Et2O (50 mL) was added
the center of the square; furthermore, bind a {Ag4O2}2+ cluster core
to precipitate 1 as a brown crystalline powder in 96% yield. IR (KBr
through eight Ag–N bonds. The Ag–N bond distances are in a range pellet cmꢂ1): 3279(m), 1599(vs), 1563(vs), 1510(vs), 1474(m), 1396(vs),
of 2.161(2)–2.225(2) Å. The geometry of the {Ag4O2}2+ cluster core is
1331(vs), 1276(vs), 1185(s), 1017(s), 927(m), 940(m), 831(w) and
747(m). Elemental analysis (%) calcd for C100H122N24O17Cu4: C 54.89,
sort of a tetragonal pyramid, in which one oxygen donor occupies
H 5.58, N 15.37; found: C 54.67, H 5.39 and N 15.45. Synthesis
the axial positions with a long Ag–O bond distance of 2.6388(8) Å. of {[Cu4L4Ag4(O2)](SbF6)2}ꢀ4(CH2Cl2)ꢀ4H2O (2). A solution of AgSbF6
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(20 mg, 0.058 mmol) in MeOH (8 mL) was slowly added to a solution of
K. Takatoshi, K. Yuriko, H. Osamu, I. Yashihisa and S. Kiyoko, Clin.
Chem., 2001, 47, 763.
1 (32 mg, 0.015 mmol) in MeOH/CH2Cl2 (10 mL, VCH Cl : VMeOH = 9 : 1),
2
2
and the red solutions were left for about ten days at ambient tempera-
ture, and the deep red crystals of 2 were obtained. IR (KBr pellet cmꢂ1):
3422(s), 2025(w), 1627(m), 1504(w), 1467(w), 1471(w), 1362(w), 1310(w),
1160(s), 1105(s), 1056(s), 931(w), 806(w), 788(w) and 750(w). Elemental
analysis (%) calcd for {[Cu4L4Ag4(O2)](SbF6)2}ꢀ1.5H2O (C96H99N24O11.5-
Cu4Ag4Sb2F12): C 39.35, H 3.41 and N 11.47; found: C 39.33, H 3.38 and
N 11.52. X-ray single-crystal analysis was performed on a Aglent
Supernova CCD-based diffractometer system (Mo Ka radiation, l =
0.71073 Å). The raw data frames were integrated into reflection intensity
files using CrysAlisPro (Version 1.171.36.32), which also applied correc-
tions for Lorentz and polarization effects. The final unit cell parameters
are based on the least-squares refinement of 4817 reflections from the
data set with I 4 5(s) I. Analysis of the data showed negligible crystal
decay during the data collection. No correction for absorption was
applied. These data can be obtained from the ESI.† Crystal data for 2:
{[Cu4(C24H24N6O2)4Ag4(O2)](SbF6)2}ꢀ4(CH2Cl2)ꢀ4H2O, Mr = 1453.57, mono-
clinic, P2/c, a = 18.7240(8) Å, b = 11.4725(8) Å, c = 39.3968(12) Å, b =
96.363(3)1, V = 8912(5) Å3, Z = 4, T = 100(2) K, rcalcd = 1.309 g cmꢂ3 and
final R [I 4 2s(I)]: R1 = 0.0534, wR2 = 0.1100.
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