The water-soluble argentivorous molecule: Ag+-π interactions in water

Article abstract of DOI:10.1039/c3ob40125a

Ag⁺-π interactions between Ag⁺ ions and a water-soluble tetra-armed cyclen bearing aromatic side-arms (tetracesium 4,4′,4′′,4′′′-((1,4,7,10- tetraazacyclododecane-1,4,7,10-tetrayl)tetrakis(methylene))tetrabenzoate, Cs₄L) are reported. The structure of the Ag⁺ complex with Cs₄L was examined using cold ESI-MS, and ¹H NMR and UV spectroscopies. It is found that when it forms Ag⁺ complexes in water, Cs₄L behaves like an insectivorous plant (Venus flytrap).

Full text of DOI:10.1039/c3ob40125a

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The water-soluble argentivorous molecule:
Ag+–π interactions in water†
Cite this: Org. Biomol. Chem., 2013, 11,
4265
Yoichi Habata,*a,b Yoko Okeda,b Mari Ikedaa,b and Shunsuke Kuwaharaa,b
Ag+–π interactions between Ag+ ions and a water-soluble tetra-armed cyclen bearing aromatic side-arms
(tetracesium 4,4’,4’’,4’’’-((1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetrakis(methylene))tetrabenzo-
ate, Cs4L) are reported. The structure of the Ag+ complex with Cs4L was examined using cold ESI-MS, and
1H NMR and UV spectroscopies. It is found that when it forms Ag+ complexes in water, Cs4L behaves like
an insectivorous plant (Venus flytrap).
Received 21st January 2013,
Accepted 2nd May 2013
DOI: 10.1039/c3ob40125a
www.rsc.org/obc
groups in the side-arms (3) and another is via a tetra-armed
Introduction
cyclen bearing bromophenyl groups in the side-arms (5).
1
Metal-cation–π interactions have been a topic of much interest
Structures of 3 and 5 were confirmed by H and 13C NMR
in the past decade.1–5 However, reports of metal-cation–π inter- spectroscopies, elemental analysis, and X-ray crystallography
actions in water are very rare. Only one case of metal-cation–π (Fig. S1, S2, S5, S6, S20–24, and Table S5 in the ESI†). As we
interactions in water has been reported, i.e., between p-sulfo- previously reported, the protons at the 2′-/6′-positions of the
natocalix[4]arene and several metal cations.6 In that system, aromatic side-arms shifted to a higher field in the 1H NMR
Tl+–π interactions were suggested, but no specific metal- titration experiments, when the side-arms cover the Ag+ ion
cation–π interactions were reported for Na+ and Ag+ ions.6 It incorporated in the cavity ligand. As shown in Fig. S22 in the
was also reported that Ag+–π interactions are very difficult in ESI,† the 2′-/6′-protons (Hb protons) of the aromatic side-arms
water, because Ag+ ions are much more hydrated than other in 3 shifted to a higher field by ca. 0.87 ppm. On the other
monovalent cations. Recently, we reported that tetra-armed hand, the 3′-/5′-protons (Ha protons) shifted to a lower field by
cyclens with aromatic side-arms behave like an insectivorous ca. 0.08 ppm.
plant (Venus flytrap) when they form complexes with Ag+ ions,
The X-ray structure of the 3–AgCF3SO3 complex (Fig. 1)
i.e., the aromatic side-arms cover the Ag+ ions incorporated in shows that the hydrogens at the 2′-/6′- and 3′-/5′-positions are
the ligand cavities, and no conformational changes are located in the shielded and deshielded areas of the next aro-
observed when other metal cations are added.7 We called the matic side-arms. The C7–Ag1 distance is 3.326 Å. The distance
tetra-armed cyclen an “argentivorous molecule”.8 Here, we is comparable with those of anthracene-cryptand–Ag+ and
report Ag+–π interactions in water with a water-soluble tetra- anthracene-diphosphine–Ag+ systems.9 The X-ray structure
armed cyclen (Cs4L).
suggests that the Ag+ ions interact with the aromatic side-arms
with η2-hapticity. The X-ray results support the 1H NMR
Results and discussion
Cs4L was prepared via two routes, as shown in Scheme 1: one
route is via a tetra-armed cyclen bearing methyl benzoate
aDepartment of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama,
Funabashi, Chiba 274-8510, Japan. E-mail: habata@chem.sci.toho-u.ac.jp;
Fax: +81 47 472 4322
bResearch Centre for Materials with Integrated Properties, Toho University,
2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
†Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of compounds, titration experiments, X-ray structures (PDF). CCDC
919399–919402. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ob40125a
Scheme 1 Synthetic procedure.
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Fig. 3 ORTEP diagram of Cs4L. Two water and two DMSO molecules bind to
cesium ions.
Fig. 1 Spacefilling diagram (left, side view) and ORTEP diagram (right, top
view) of 3–AgCF3SO3 (left). The CF3SO3 anion is omitted. Meshed hydrogens are
the protons at the 2’- and 6’-positions of the side-arms.
Fig. 4 Cold ESI-MS of a 1 : 1 mixture of Cs4L and AgCF3SO3 at 298 K (in H2O–
CH3OH (1/7, v/v)).
fragment ion peaks arising from [Cs4L + H+]+ and [Cs4L + Cs+]+
were observed.
Fig. 2 The LUMO and HOMOs calculated by the DFT method [B3LYP/3-21G(*)]
using the X-ray structures of the Ag+ complex with 3 (isosurface value is
0.032 au). LUMO[+4] (mesh) and HOMO[−11], HOMO[−12], HOMO[−12], and
HOMO[−14] (solid).
To examine the structure of the Ag+ complex with Cs4L
including DMSO and water molecules, cold ESI-MS of a
1 : 1 mixture of Cs4L and AgCF3SO3 was conducted in a mixture
of H2O–CH3OH (1/7, v/v). Cold ESI-MS is an excellent tool for
confirming the presence of unstable species and/or solvated
species.11 The fragment ion peaks arising from [Cs4L + Ag+]+,
titration experiments. Fig. 2 shows the LUMO and HOMOs cal-
culated by the DFT method [B3LYP/3-21G(*)] using the X-ray
structures of the Ag complex with 3. The result indicates the
interactions between LUMO of Ag+ and HOMOs and the aro-
matic side-arms.
The sodium salt of the ligand (Na4L) was very difficult to
purify, because Na4L and NaOH precipitate together. On the
other hand, the cesium salt of the ligand (Cs4L) was easy to
purify by recrystallization from a mixture of DMSO and water.
We, therefore, used cesium salt of the ligand. The structure of
Cs4L was confirmed by 1H, 13C NMR, and IR spectroscopies,
cold ESI-MS, elemental analysis, and X-ray crystallography
(Fig. S3, S4, S8–S10, and Table S1 in the ESI†).
[2Cs4L + 2Ag+]2+, [Cs4L + Ag+ + 2DMSO]+, and [Cs4L + Ag+
+
DMSO + 3CH3OH + 6H2O]+ (or [Cs4L + Ag+ + 2DMSO + 7H2O]+)
were observed at m/z = 1345, 1349, 1501, and 1627, respectively
(Fig. 4). It is important to note that the fragment ion peaks
at m/z = 1345–1349 are overlapped by the [Cs4L + Ag+]+
and [2Cs4L + 2Ag+]2+ ions. The patterns of the fragment
ion peaks agree with the theoretical distributions (see
Fig. S11a and 11b in the ESI†). The mass spectrometry
data suggest that the AgCF3SO3 complex with Cs4L is a 1 : 1
complex.
Titration experiments using 1H NMR were carried out in
D2O. We reported that7 unusual higher-field shifts of the
protons at the 2′-/6′-positions in the aromatic side-arms are
observed upon the addition of equimolar amounts of Ag+ ions
when the aromatic side-arms cover the Ag+ ions incorporated
in the ligand cavities. If the aromatic side-arms of Cs4L cover
the Ag+ ions in water, the protons at the 2′-/6′-positions in the
aromatic side-arms should shift to a higher field because the
2′-/6′-protons are located in the shielded area of the next
As shown in Fig. 3 (and Fig. S8 in the ESI†), two water and
two DMSO molecules bind to the Cs+ ions in the Cs4L. The
Cs–O distances are in the range of 3.050–3.443 Å and are compar-
able with the typical Cs–O distances of cesium benzoate
derivatives.10 The presence of water and DMSO molecules was
1
also observed in the H NMR and IR spectra (Fig. S3 and S10
in the ESI†) and elemental analyses. In cold ESI-MS, the
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Fig. 6 AgCF3SO3-induced UV spectral changes and titration plots at 230 nm
(in water). [Cs4L] = 5.0 × 10−5 mol L−1
.
Fig. 5 AgCF3SO3-induced 1H NMR spectral changes (in D2O). Ha (uncom-
plexed, blue ; complexed, blue ) and Hb (uncomplexed, red ; complexed, red
) mean the protons at 3’-/5’- and 2’-/6’-positions in the side-arms, respectively
(298 K).
Nonlinear least-squares analyses of the titration profiles
(absorbance versus equivalents of Ag+ added, see Fig. S25 in
the ESI†) clearly indicated the formation of a 1 : 1 complex,
and allowed us to estimate the association constants defined
as eqn (1).12
aromatic side-arm in solution. Fig. 5 shows the results of the
1
AgCF3SO3-induced H NMR titration experiments in D2O (aro-
matic region). Upon the addition of equimolar amounts of Ag+
ions, as the intensities of the doublet at δ 7.8 ppm and the
doublet at δ 7.2 ppm decrease, the intensities of the doublet at
δ 7.85 ppm and the doublet at δ 6.5 ppm increase. The chemi-
cal shift changes stopped at a ratio of 1 : 1. When AgPF6 and
AgNO3 were used instead of AgCF3SO3, the protons at the
2′-/6′-positions shifted by ca. −0.7 ppm (see Fig. S13 and S14,
and Table S2 in the ESI†). As presented above, the X-ray structure
of the 3–AgCF3SO3 complex indicates that the Ha protons are
located in the deshielded area of the next aromatic side-arms.
The Ha protons in the Cs4L–Ag+ complex, therefore, would be
Cs4L þ Agþ $ ½Cs4L–Agþꢀ
K ¼ ½Cs4L–Agþꢀ=½Cs4Lꢀ½Agþꢀ
ð1Þ
From the UV spectral data, the log K value of the complex
was estimated to be ca. 10.9 (Fig. S26 in the ESI†). This result
strongly supports Ag+–π interactions between the aromatic
side-arms and Ag+ ions in water.
The complexing properties of Cs4L toward several metal
ions were examined based on metal-ion-induced UV spectral
changes in water. When equimolar amounts of Ag+ were
added, significant spectral changes were observed, as shown
in Fig. 7 and 8, whereas no spectral changes were observed
upon the addition of equimolar amounts of Li+, Na+, K+, Mg2+,
Ca2+, Sr 2+, Ba2+, Co2+, Ni2+, Zn2+, and Cd2+ ions. In contrast, a
1
also shifted to a lower field. The H NMR spectra show that (i)
higher-field shifts (ca. −0.7 ppm) of the protons at the 2′-/6′-
positions are observed in water and (ii) Cs4L forms a 1 : 1
complex with Ag+ ions. These 1H NMR titration experiments
strongly support that the aromatic side-arms in Cs4L cover the
Ag+ ions incorporated in the ligand cavity as we expected. On
the other hand, no spectral changes were observed upon the
addition of equimolar amounts of Li+, Na+, K+, Mg2+, Ca2+, Sr 2+
,
Ba2+, Co2+, Ni2+ ions (Table S3 in the ESI†). When Pb2+and
Zn2+ ions were added, slight changes were observed. [Fig. S17
and S18 in the ESI† show Pb2+ and Zn2+-ions-induced 1H NMR
spectral changes, respectively. 1H–1H HOHAHA NMR
(Fig. S19†) was used to assign the Ha and Hb protons in a
1 : 0.5 mixture of Cs4L and Pb(NO3)2.]
Titration experiments using UV spectra were carried out to
confirm the Ag+–π interactions in water. Fig. 6 shows the Ag+-
ion-induced UV spectral changes of Cs4L. A decrease and an
increase in the absorbance at 230 nm (λmax) and shorter wave-
lengths were observed, respectively, with an isosbestic point at
220 nm, upon the addition of Ag+ ions. An inflection point was
observed at 1.0 (=[Ag+]–[Cs4L]), showing a 1 : 1 complex.
Fig. 7 Metal-ion-induced UV spectral changes (in water).
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Experimental
Melting points were obtained with a Mel-Temp capillary appar-
atus and not corrected. FAB-MS was performed using a JEOL
1
600 H spectrometer. H NMR spectra were measured in CDCl3
on a JEOL ECP400 (400 MHz) spectrometer. Cold ESI-MS were
recorded on a JEOL JMS-T100CS spectrometer. UV-vis spectra
were recorded on a JASCO V-650 spectrometer. CD spectra were
recorded on a JASCO J-820 spectrometer. Cyclen was pur-
chased from Macrocyclics. All reagents were standard analyti-
cal grade and were used without further purification.
Synthesis of tetramethyl 4,4′,4′′,4′′′-[1,4,7,10-tetra-
azacyclododecane-1,4,7,10-tetrayltetrakis(methylene)]-
tetrabenzoate (3)
Fig. 8 Changes of ε (Δε) at 215 nm upon the addition of various metal cations
([cations] : [Cs4L] = 1 : 1).
After a mixture of cyclen (1) (0.268 g, 1.56 mmol), methyl 4-for-
mylbenzoate (2) (2.23 g, 13.6 mmol), and NaBH(OAc)3 (2.92 g,
13.8 mmol) in 1,2-dichloroethane (15 mL) was stirred for 5
days at rt under a nitrogen atmosphere, saturated aqueous
NaHCO3 was added. The organic layer was separated, and the
aqueous layer was extracted with CHCl3 (100 mL × 3). The com-
bined organic layer was washed with water, dried over Na2SO4,
and concentrated. The residue was recrystallized from aceto-
nitrile (5 mL) to give 3.
Yield 1.02 g (91%). Mp. 196.0–197.2 °C (dec.). FAB-MS
(matrix DTT : TG = 1 : 1, m/z), 766 ([M + 1]+, 100%); 1H NMR
(CDCl3) 7.92 (d, J = 8.2 Hz, 8H), 7.39 (d, J = 8.2 Hz, 8H), 3.92 (s,
12H), 3.44 (s, 8H), 2.66 (s, 16H). 13C NMR (CDCl3) 167.3, 145.7,
129.7, 129.0, 128.9, 60.1, 53.6, 52.2. Anal. Calcd for
C44H52N4O8: C, 69.09; H, 6.85; N, 7.32. Found: C, 68.85;
H, 6.74; N, 7.16.
slight UV spectral change was observed on the addition of Pb2+
ions. To investigate the spectral change upon the addition of
Pb2+ ions, Pb2+-induced UV spectral changes were observed
using a mixture of cyclen and cesium 4-methylbenzoate (1/4,
v/v) or cesium 4-methylbenzoate as ligands (Fig. S15 and S16
in the ESI†). In both cases, an increase at λmax (230 nm) was
observed upon the addition of Pb2+ ions. It is well known that
carboxylate binds to Pb2+ ions.13 The UV spectral changes in
Cs4L resulted from the binding of the COO− groups in the
side-arms to Pb2+ ions. Two types of proton signals at the
2′-/6′- and 3′-/5′-positions appeared in the Pb2+-induced 1H NMR
1
titration experiments (Fig. S17 in the ESI†). The H NMR data
strongly support binding of the COO− groups. These results
indicate that sensing using UV spectroscopy with Cs4L pro-
vides Ag+ selectivity.
Synthesis of tetracesium 4,4′,4′′,4′′′-(1,4,7,10-
tetraazacyclododecane-1,4,7,10-tetrayltetramethanediyl)-
tetrabenzoate (Cs4L)
Conclusions
After a mixture of 3 (0.0377 g, 0.0493 mmol) and powdered
NaOH (0.180 g, 4.50 mmol) in ethanol (55 mL) was refluxed
for 1 day under a nitrogen atmosphere, solvent was evaporated
in vacuo. The residue was recrystallized from methanol (15 mL)
to give the crude product of Na4L including NaOH. The solid was
In conclusion, we have demonstrated that a water-soluble
tetra-armed cyclen with aromatic side-arms behaves like an
insectivorous plant (Venus flytrap) when forming Ag+ com-
plexes. We confirmed that the conformational changes are the
result of Ag+–π interactions between the Ag+ ions and the aro-
matic side-arms in water. The unique property of this water-
soluble argentivorous molecule shows an application for new
supramolecular systems of Ag+–π interactions (Fig. 9).
dissolved in water and then 10 mL of aqueous HCl (2 mol L−1
)
was added. The residual precipitate was washed with
CHCl3 (10 mL) and water (10 mL) to give H4L·4HCl as white
powders. After H4L·4HCl was dried in vacuo, the powders were
dissolved in 10 mL of DMSO, and then a saturated aqueous
CsOH solution was added until the pH reaches 8 to give Cs4L
as white powders.
Yield 0.0449 g (29%). Mp. 224 °C (dec.). Cold ESI-MS (H2O–
CH3OH = 1 : 7, m/z), 1237 [Cs4L + H+]+), 1369 ([Cs4L + Cs+]+).
1H NMR (D2O) 7.79 (d, J = 8.0 Hz, 8H), 7.19 (d, J = 8.0 Hz, 8H),
3.61 (s, 8H), 2.83 (s, 16H). 13C NMR (D2O) 176.3, 136.8, 131.1,
130.1, 59.5, 49.0, 39.9. IR (KBr disk, cm−1) 3413, 2928, 2791,
1597, 1550, 1397, 1012. Anal. Calcd for C40H40Cs4N4O8
+
2DMSO + 4H2O: C, 36.08; H, 4.13; N, 3.83. Found: C, 36.28;
H, 4.11; N, 3.77.
Fig. 9 Image of the water-soluble argentivorous molecule.
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Synthesis of 1,4,7,10-tetrakis(4-bromobenzyl)-1,4,7,10-tetra-
azacyclododecane (5)
CD3OD = 0.75/0.005, v/v) 7.99 (d, J = 7.9 Hz, 8H), 6.52 (d, J =
7.9 Hz, 8H), 4.03 (s, 12H), 3.16 (broad-s, 16H), 2.45 (broad-s,
8H). 13C NMR (CDCl3–CD3OD = 0.75/0.005, v/v) 166.2, 143.4,
130.8, 130.6, 129.7, 58.9, 52.6, 50.1. The 13C signals of the
CF3SO3 anion were not observed under the condition.
After a mixture of cyclen (1) (0.346 g, 2.01 mmol) and 4-bromo-
benzaldehyde (4) (3.32 g, 17.9 mmol) in 1,2-dichloroethane
(28 mL) was stirred for 2 hours at rt under a nitrogen atmos-
phere, NaBH(OAc)3 (2.92 g, 13.8 mmol) was added and stirred
for 1 day under a nitrogen atmosphere. Saturated aqueous
NaHCO3 was added to the reaction mixture and then the
residue was extracted with CHCl3 (100 mL × 3). The organic
layer was separated and combined. The organic layer was
washed with water, dried over Na2SO4, and concentrated
in vacuo. The residue was recrystallized from acetonitrile
(5 mL) to give 5 as white crystals.
1H NMR titration experiments
1H NMR titration experiments were carried out at 298 K by the
addition of 0.2–2.0 (or 0.5–2.0) equivalents of metal salts
(LiNO3, NaNO3, Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, Co(NO3)2,
Ni(NO3)2, AgNO3, AgCF3SO3, AgPF6, and Zn(NO3)2: 0.001 mmol
μL−1) in D2O (CDCl3–CD3OD for 3) to Cs4L (0.01 mmol/
0.65 mL in D2O, CDCl3–CD3OD for 3).
Yield 1.38 g (81%). Mp. 184.8–186.0 °C. FAB-MS (matrix m-
NBA, m/z) 845 ([M − 3]+, 4%), 847 ([M − 1]+, 12%), 849 ([M + 1]+,
19%), 851 ([M + 3]+, 13%), 853 ([M + 5]+, 4%). 1H NMR
(CDCl3) 7.37 (d, J = 8.2 Hz, 8H), 7.16 (d, J = 8.2 Hz, 8H), 3.33 (s,
8H), 2.61 (s, 16H). 13C NMR (CDCl3) 139.2, 131.4, 130.9, 120.8,
59.8, 53.4. Anal. Calcd for C36H40Br4N4: C, 50.97; H, 4.75;
N, 6.60. Found: C, 50.88; H, 4.75; N, 6.35.
X-ray structure determination
Crystals of the Cs4L, 3, 3–AgCF3SO3 complex, and 5 were
mounted on top of a glass fiber, and data collection was per-
formed at 100–173 K. Data were corrected for Lorentz and
polarization effects, and absorption corrections were
applied.14 Structures were solved by a direct method and sub-
sequent difference-Fourier syntheses.15 All non-hydrogen
atoms were refined anisotropically and hydrogen atoms were
placed at calculated positions and then refined using Uiso(H) =
1.2Ueq(C). The crystallographic refinement parameters of the
complexes are summarized in Tables S1 and S4–S6 in the ESI.†
Synthesis of tetracesium 4,4′,4′′,4′′′-((1,4,7,10-
tetraazacyclododecane-1,4,7,10-tetrayl)tetrakis(methylene))-
tetrabenzoate (Cs4L)
n-Butyl lithium (1.6 mol L−1 in hexane, 3.0 mL) was added to 5
(0.846 g, 0.997 mmol) in absolute THF (60 mL) at −78 °C
under an argon atmosphere. After dry ice (>44 mg) was added
to the reaction mixture, the flask was stirred for 1 h at rt.
Water (10 mL) and aqueous HCl (2 mol L−1, 10 mL) were
added to the reaction mixture, and then the residual precipi-
tate was washed with CHCl3 (15 mL) and water (15 mL) to give
H4L·4HCl as white powders. After H4L·4HCl was dried
in vacuo, the powders were dissolved in a mixture of DMSO
(15 mL) and methanol (150 mL). To the solution was added
dropwise a saturated aqueous CsOH solution until the pH
reaches 8 to give Cs4L as white powders.
Acknowledgements
This research was supported by Grants-in-Aid (08026969 and
11011761), a High-Tech Research Center project (2005–2009),
and the Supported Program for Strategic Research Foundation
at Private Universities (2012–2016) from the Ministry of Edu-
cation, Culture, Sports, Science and Technology of Japan, and
the Futaba Memorial Foundation, for Y.H.
Yield 0.899 g (62%).
Notes and references
Synthesis of cesium 4-methylbenzoate
A saturated aqueous CsOH solution was added dropwise to
4-methylbenzoic acid (0.96 g, 5.07 mmol) in methanol (70 mL)
until the pH reaches 8. The residual precipitate was washed
with methanol.
Yield 70%. H NMR (D2O) 7.76 (d, J = 8.2 Hz, 2H), 7.28 (d,
J = 8.2 Hz, 2H), 2.36 (s, 3H). Anal. Calcd for C8H7O2Cs + 1.5H2O:
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1
C, 34.13; H, 3.04. Found: C, 34.29; H, 3.24.
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Preparation of the 3–AgCF3SO3 complex
Compound 3 (9.20 mg, 0.025 mmol) in chloroform (1 mL) was
added to AgCF3SO3 (15.0 mg, 0.025 mmol) in methanol
(1 mL). Acetonitrile and 1,2-dichloroethane (0.5 mL) were
added to the mixture. Crystals were obtained quantitatively on
evaporation of the solvent.
Anal. Calcd for C45H52N4AgF3O11S: C, 52.89; H, 5.13;
N, 5.48. Found: C, 53.22; H, 5.24; N, 5.33. 1H NMR (CDCl3–
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2013, 52, 2542; (c) Y. Habata, Y. Oyama, M. Ikeda and 10 (a) D. Braga, S. D’Agostino and F. Grepioni, CrystEngComm,
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8 “Argentivorous” is different from “argentophilic”. “Argento-
philic” is used in the sense of Ag+–Ag+ interactions. For
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(c) F. Wiesbrock and H. Schmidbaur, Inorg. Chem., 2003,
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Struct., 1994, 321, 251; (e) N. Kasuga, S. Nakahama,
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