Zn2+ cation triggers self-assembly of cyclen into a stable metallogel

Article abstract of DOI:10.1007/s11458-010-0109-5

A novel type of ligands contained 1,4,7,10-tetrazacyclododecane and functionalized by two azobenzene moieties grafted with two alkyl chains was designed and synthesized. The ligands with long alkyl chains can form metallogels in the presence of Zn²⁺ cations. The formation of metallogel was followed by NMR and electronic spectral detection. The morphology of the xerogels is varied with the equivalent of Zn²⁺ cations and the concentration of the gel. Spectral and structural analysis indicated that the driving forces of the gel formation were attributed to intermolecular hydrophobic interactions between alkyl chains and π-π interaction between azobenzenes.

Full text of DOI:10.1007/s11458-010-0109-5

Front. Chem. China 2010, 5(2): 184–192
DOI 10.1007/s11458-010-0109-5
RESEARCH ARTICLE
27]. Kimura et al. have done a lot of work in cyclen
compounds in the field of biochemistry [28–33]. Zn2+-cyclen
derivatives have been found to be useful in elucidating and
understanding the intrinsic properties of substrate or inhibitor
recognition by zinc ions at the active centers of carbonic
anhydrase and carboxypeptidase [28–33]. However, all of
these works were carried out solely in a solution; the
aggregation properties of those complexes rarely attracted
attention.
Zn2+ cation triggers self-
assembly of cyclen into a
stable metallogel
Tianmin SHU, Junchen WU, Ying ZOU,
Keyin LIU, Liqing CHEN and Tao YI (✉)
Organogels are of increasing interest in recent years
because of their numerous potential applications, such as
hardeners of solvents, adaptive materials, drug delivery
systems, and sensors [34–47]. Most of the gels are assembled
by means of hydrogen bond, hydrophobic interaction, π-π
stacking, and Van der Waals forces [48–52]. So far there have
been some researches in the field of metallogel [53–58]. For
instance, organometal gels have been reported by MacLa-
chlan’s group that zinc salphen complexes can form stable
gels in certain solvents [59]. Also, Xu’s group investigated
that calix[4]-arene can gelate DMSO in the presence of Pd2+
[60,61]. A metallogel of palladium pincer complex has been
reported for catalytic application [62]. In those metallogels,
the center metal ions are generally posited at a planer
environment with four coordination numbers in most cases,
which may be in favor of the formation of the gel aggregate.
To our knowledge, there is few investigation of organogels
containing cyclen complexes in which the metal cations are
not in a planar position. Our strategy makes use of cyclen,
which is connected with 2,4,6-trichloro-1,3,5-triazine featur-
ing azobenzene with flexible trails, to produce different
supramolecular specie (Scheme 1). It has been reported that
azobenzene chromophore has an excellent response to
external stimulation because of the N = N trans and cis
interconvertion when illuminated by ultraviolet radiation and
undo [63–71]. Here, we report that the novel cyclen
derivatives containing azobenzene can form metallogel by
the induction of Zn2+. The reaction shows color change in
response to UV light.
A novel type of ligands contained 1,4,7,10-tetrazacy-
clododecane and functionalized by two azobenzene
moieties grafted with two alkyl chains was designed and
synthesized. The ligands with long alkyl chains can
form metallogels in the presence of Zn2+ cations. The
formation of metallogel was followed by NMR and
electronic spectral detection. The morphology of the
xerogels is varied with the equivalent of Zn2+ cations
and the concentration of the gel. Spectral and structural
analysis indicated that the driving forces of the gel
formation were attributed to intermolecular hydro-
phobic interactions between alkyl chains and π-π
interaction between azobenzenes.
Keywords cyclen, metallogel, self-assembly, azoben-
zene
1 Introduction
Supramolecular architectures that are formed by artificial self-
assembly of small molecules under equilibrium conditions
have great potentials for selective guest inclusion and
molecular recognition [1–7]. Crown ethers are particular
charming in the field of supramolecular architectures because
of their simplest but most attractive ligand combining with
cation (Na+, K+, Cs+) [8,9]. Such performance results in
potential usage as ion-carrier [10,11]. Moreover, macrocyclic
polyamines, like1,4,7,10-tetrazacyclododecane (cyclen) deri-
vatives, can be combined with compatible cation-diameter,
such as Zn2+, Cu2+, Ni2+, Co2+, and Pd2+, into a stable host-
guest system [12–17]. It has been proven that Zn2+ complexes
of macrocyclic tetraamine derivatives, such as Zn2+-cyclen,
are good models for Zn2+ enzymes. They form 1∶1 complexes
with anions, including phosphate monoesters, imidates (e.g.,
thymine), and thiolates in aqueous solution at neutral pH [18–
2 Experiment
2.1 General
All starting materials are acquired from commercial suppliers.
4-aminophenol, acetic anhydride, and 1,4,7,10-tetraazacyclo-
dodecane were obtained from Sinopharm Chemical Reagent
Co., Ltd. (Shanghai); di-tert-butyl dicarbonate were obtained
from Aldrich. Other solvents were provided by Shanghai No.1
chemical reagent. 1H NMR and 13C NMR spectra were
recorded on a Mercuryplus NMR spectrometer at 400 and
100 MHz, respectively. Proton chemical shifts are reported in
Received December 9, 2009; accepted February 19, 2010
Department of Chemistry and Laboratory of Advanced Materials, Fudan
University, Shanghai 200433, China
E-mail: yitao@fudan.edu.cn
© Higher Education Press and Springer-Verlag Berlin Heidelberg 2010

Zn2+ cation triggers self-assembly of cyclen into a stable metallogel
185
K2CO3 (3.3 g, 24.17 mmol) in acetone (50 mL) at room
temperature. The reaction mixture was stirred for 48 h under
nitrogen atmosphere. Then the solvent was removed under
reduced pressure, and the residue was subjected to column
chromatography (PE/EA = 4/1, v/v) on silica gel to yield 4b as
1
an orange powder. Yield 43%. Mp: 95°C–98°C. H NMR
(400 MHz, CDCl3): d 7.94–7.89 (q, 8H), 7.29–7.27 (d, 4H),
7.02–6.99 (d, 4H), 3.90 (s, 6H), 3.55 (bs, 4H), 3.38–3.35 (bs,
12H), 1.43 (s, 9H), 1.35 (s, 18H); 13C NMR (100 MHz,
CDCl3):d 171.9, 162.1, 153.7, 150.2, 147.0, 124.8, 123.7,
122.3, 114.2, 80.4, 80.0, 69.5, 55.6, 53.9, 50.2, 49.8, 31.8,
29.3, 28.5, 28.4.
Ligand 4a was obtained through a same process as 4b by
1
reaction between 2a and 3. Yield 51%. Mp: 63°C–65°C. H
NMR (400 MHz, CDCl3): d 7.92–7.86 (q, 8H), 7.29–7.27 (d,
4H), 7.00–6.97 (d, 4H), 4.04 (t, 4H), 3.56 (bs, 4H), 3.39–3.35
(bs, 12H), 1.84–1.79 (m, 4H), 1.43 (s, 9H), 1.36 (s, 18H),
1.31–1.27 (m, 52H), 0.88 (t, 6H). 13C NMR (100 MHz,
CDCl3): d 172.1, 162.0, 156.3, 153.9, 150.4, 147.1, 124.6,
123.8, 122.4, 116.0, 115.0, 80.5, 80.2, 68.6, 50.6, 50.3, 49.9,
32.1, 29.9, 29.8, 29.8, 29.8, 29.6, 29.5, 29.4, 28.7, 28.6, 26.2,
22.9, 14.2.
Scheme 1 Chemical structure of the ligands and complexes.
parts per million (ppm) downfield from tetramethylsilane
(TMS). High-resolution mass spectra were recorded using a
4700 peoteomic analyzer (Applied biosystems, USA). Melt-
ing points were determined on a hot-plate melting point
apparatus XT4-100A without being corrected.
2.4 General synthesis of 1a–1b
Acetyl chloride (5 mL) was carefully added into methanol
(20 mL) under stirring in ice water bath. The mixture was
withdrew from the bath after continuous stirring at 0°C for
3 min. Then compound 4a–b dissolved in THF (as little
as possible) was slowly added into it. The reaction mixture
was stirred at room temperature overnight and then neutra-
lized with aqueous LiOH$H2O to pH 8–9. The precipitated
product was filtrated and dried in vacuum to yield the raw
production, followed by chromatograph on silica gel
(chloroform/methanol = 50/1, v/v) to yield 1 as an orange
solid.
2.2 The synthesis of the ligands
Compound 2 was synthesized according to Ref. [72].
1
2a Mp: 109°C–110°C. H NMR (400 MHz, CDCl3): d
7.90–7.85 (t, 4H), 7.00–6.94 (q, 4H), 4.03 (t, 2H), 1.81 (m,
2H), 1.33–1.26 (m. 26H), 0.88(t, 3H); 13C NMR (100 MHz,
CDCl3): d 161.6, 158.0, 124.9, 124.8, 124.6, 116.0, 116.0,
115.0, 68.6, 31.1, 29.9, 29.8, 29.8, 29.7, 29.6, 29.5, 29.4,
26.2, 22.9, 14.2.
1a: Mp: 153°C–157°C. 1H NMR (400 MHz, CDCl3):
7.90–7.86 (q, 8H), 7.28–7.26 (d, 4H), 7.00–6.98 (d, 4H), 4.03
(t, 4H), 3.81 (s, 4H), 2.98–2.94 (d, 8H), 2.71 (s, 4H), 2.00 (s,
2H), 1.82 (t, 6H), 1.41–1.26 (m, 50H), 0.87 (t, 6H); 13C NMR
(100 MHz, CDCl3): 172.0, 167.6, 161.8, 153.3, 150.4, 146.7,
124.8, 123.5, 122.3, 114.7, 68.4, 53.7, 51.5, 50.5, 47.8, 47.6,
42.3, 31.9, 29.7, 29.6, 29.4, 29.2, 26.0, 22.7, 14.1, HR-MS
calcd for C67H102N11O4 [M + H]+: 1124.8116, found:
1124.8042.
1
2b Mp: 134°C–136°C. H NMR (400 MHz, CDCl3): d
7.90–7.83 (q, 4H), 7.00–6.99 (d, 2H), 6.95–6.92 (d, 2H), 3.89
(s, 3H), 1.81 (m, 2H), 1.33–1.26 (m. 26H), 0.88 (t, 3H); 13C
NMR (100 MHz, CDCl3): d 161.6, 157.9, 147.1, 147.0,
124.6, 124.4, 115.8, 114.2, 55.6.
Compound 3 was synthesis based on Ref. [73]. Mp:
178°C–180°C. 1H NMR (400 MHz, CDCl3): d 3.73 (bs, 4 H),
3.49–3.42 (m, 12 H), 1.47 (s, 9 H,), 1.43 (s, 18 H); 13C NMR
(100 MHz, CDCl3): d 170.2, 165.0, 157.8, 156.4, 80.8, 80.6,
51.8, 50.7, 50.0, 49.2, 28.7, 28.6.
1
1b: Mp: 190°C–192°C. H NMR (400 MHz, CDCl3): d
7.91–7.88 (q, 8H), 7.28–7.26 (d, 4H), 7.02–6.99 (d, 4H), 3.90
(s, 6H), 3.00–2.99 (m, 8H), 2.74 (w, 4H); 13C NMR
(100 MHz, CDCl3): d 172.0, 166.8, 162.3, 153.4, 150.5,
146.9, 124.9, 123.7, 122.3, 114.3, 55.6. HR-MS calcd for
C37H42N11O4[M + H]+: 704.3421, found: 703.9861.
2.3 Synthesis of 4a–4b
3 was slowly added (0.5 g, 0.81 mmol) in 15 mL acetone into
a mixed solution of 2b (0.55 g, 2.42 mmol) and anhydrous
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Tianmin SHU, Junchen WU, Ying ZOU, Keyin LIU, Liqing CHEN et al.
2.5 Gelation test of organic fluids
recorded on a UV-Vis 2550 spectroscope (Shimadzu).
Scanning electron microscope (SEM) images of the xerogels
were obtained using an SSX-550 (Shimadzu) with an
accelerating voltage of 15 kV. Samples were prepared by
spinning the gels on glass slices and coating with Au.
Microspectral images were obtained using an OLYMPUS
IX81 confocal laser scanning microscope equipped with a
60X oil immersion objective lens. X-ray powder diffraction
(XRD) spectra were carried out by a D/max-γB X-ray
scattering instrument (Rigaku Industrial Corporation) using
Cu Kα radiation source (l: 0.1542 nm) with the power of
40 kV and 60 mA at room temperature.
The gelators and the solvents were put in a septum-capped test
tube and were heated until the solid was dissolved. The
sample vial was cooled to 25°C and left for 2 h under ambient
conditions. Qualitatively, gelation was considered successful
if no sample flow was observed upon inverting the container
at room temperature.
2.6 Techniques
1H NMR titration spectra were acquired on a DMX 500 NMR
spectrometer at 500 MHz (Bruker). UV-Vis spectra were
Scheme 2 Synthesis process of the ligands.
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Zn2+ cation triggers self-assembly of cyclen into a stable metallogel
187
3 Results and discussion
3.1 Gelation studies
The ligands 1a and 1b were synthesized according to Scheme
2. The target molecules were characterized by 1H, 13C NMR,
and high-resolution mass spectroscopy. In particular, 1b is the
consult compound that could provide us the model on how
Zn2+ react with the cyclen. Ligands 1a and 1b has nice
solubility in chloroform but has poor solubility in other
common solvents at room temperature. However, 1a of low
concentration ( < 10 mg$mL–1) generates viscous solution in
DMSO, acetone, and toluene after a heating-cooling process,
but yields a deposit with higher concentration. This means
that even though 1a incline to gelation in those solvent, the
balance of the driving force for aggregation is not in an
optimized state. It is interesting that 1a can form a stable gel in
the presence of Zn2+ ion in DMSO. In fact, Zn-1a has a better
solubility in DMSO than 1a. As an example, a clear solution
can not be observed even heating 10 mg of 1a in 0.5 mL
DMSO to 150°C. However, a clear yellow solution was
obtained when 1.0 equiv of Zn(ClO4)2 was added into this 1a
solution at about 100°C. When clear solution was slowly
cooled down to room temperature, a stable gel was obtained
(Figure 1(a)). The gel-sol transition temperature (Tg) of Zn-1a
has been measured at different concentrations (Figure 1(b)). It
is clear that the higher concentrated the gel is, the higher the
Tg is. Tg values of 20 and 80 mg$mL–1 gel are 14.5°C and
48°C, respectively.
1
Figure 2 Partial H NMR (500 MHz) spectroscopy of 1a under
Zn2+-titration in d-6DMSO at 60°C ([1a]: 12 mg$mL–1, Zn2+ from
0 equiv to 2.0 equiv).
ring of 1,4,7,10-tetrazacyclododecane is the reactive spot,
which has strong interaction with Zn2+. For example, the
intensity of 1H NMR signal of 1a became stronger as the Zn2+
was added; it would indicate the enhanced solubility of the
complex. Meanwhile, a notable shift of amino proton H1 at
4.69 ppm to high field (4.57 ppm) was observed. At the same
time, the chemical shift of H2 has shifted downfield notably
from 3.63 to 3.66 ppm. The chemical shifts of H3, H4, and H5
protons are also moved downfield likewise. The NMR data
indicated that the isolated-pair electron density of nitrogen
atom must decline as interacting with Zn2+ and therefore
increase the shield effect on amino proton (H1) and cause the
chemical shift moving to high field. In contrast, the protons of
methylene (H2–H4) in the cyclen shifted downfield. The
chemical shift had little change after the Zn2+ reached to 1.0
The efficiency that Zn-1a gelates DMSO may be based on
the combination of the cyclen ring to metal ion. Such
combination allows aggregation of the complex by inter-
molecular π stacking, possibly enhanced by van der Waals
interactions between the alkyl chains. Zn2+-titration 1H NMR
spectral studies of Zn-1a at 60°C were performed to probe the
role of cyclen ring in the gelation process (Figure 2). By
adding Zn2+ from 0.25 equiv to 1.0 equiv, it is clear that the
Figure 1 (A) Photographs of 1a (10 mg) in DMSO (0.5 mL) at room temperature, a is in the absence of Zn2+, b is in the presence of Zn
(ClO4)2$6H2O (1.0 equiv). (B) Concentrations variation Tg in gels of (1.0) Zn-1a.
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Tianmin SHU, Junchen WU, Ying ZOU, Keyin LIU, Liqing CHEN et al.
equiv, which means that the cyclen ring reacted with Zn2+ as aggregation. From the absorption spectra of 1b and Zn-1b in
molar ratio 1∶1. DMSO solution (Figure 3(c) and 3(d)), we can see that the
To characterize the function of the azobenzene chromo- formation of the complex does not cause distinct change in the
phore, the UV-Vis spectra were investigated. Two films of 1a absorption spectra. Thus, the drastic shift of the absorption in
with and without Zn2+ were made about 16.5 mm thick. Then Zn-1a gel should be deduced by the gelation. We also noted
the absorption spectra of the two films were measured before that the UV irradiation did not cause the gel to sol
and after irradiation of light (365 nm) at the same circum- transformation as mentioned in some previous report
stance. From the spectra, we can find that 1a has a notable [64,65], even though the color of the gel changed from
absorption decrease at 370 nm (π-π*) and an absorption brown to nut-brown (Figure 3(b), inset). This may be due to
increase at about 450 nm (n-π*) under irradiation, indicating a incomplete photo isomerization in the gel state.
photo isomerization of azobenzene group (Figure 3(a)).
However, the π-π* absorption of Zn-1a belonging to the
3.2 The morphology of the gels
trans isomer has a blue-shift of 60 nm by comparison with
that of 1a even though the n-π* absorption for the cis isomer
To gain a better insight into the molecular organization in the
gel specimens, the morphology of the xerogels of Zn-1a in
DMSO were analyzed by SEM. Honeycomb-like three-
dimensional vesicles of about 4–6 µm in diameter were
observed in the deposition of the ligand without Zn2+
posited at the same wavelength (450 nm) (Figure 3(b)). It
indicates that there is strong aggregate between trans
azobenzene groups in the gel state. Further study was done
related to short-chain compound 1b to give more evidence for
Figure 3 Absorption spectra of the thin films of (a) 1a and (b) Zn-1a (16.5 µm); and the solution of (c) 1b and (d) Zn-1b (c = 1 Â 10–3
mg$mL–1, l = 0.1 cm) in DMSO upon 365 nm irradiation.
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Zn2+ cation triggers self-assembly of cyclen into a stable metallogel
189
(Figure 4(a)), with regular intertexture holes of about 0.2– From the diffraction pattern of 1a, d values of 9.08 and
0.5 µm in size distributed on the surface of the spheres. After 4.49 nm (d/1: d/2) suggests a lamellar structure [74,75].
the addition of Zn2+, the honeycomb structure was destroyed However, only one clear peak at 7.66 nm was observed in the
to flakes when the ratio of Zn2+ was up to 0.4 equiv small angle range for the xerogel of Zn-1a. This diffraction
(Figure 4(b), 4(c)). With further addition of Zn2+, fibrous length is just twice of the extended molecular length from the
intertexture morphology was observed in Zn-1a complex with center Zn ion to the long alkyl chain in CPK model (Figure 6
a ratio of 1∶1 (Figure 4(d)–(f)). The morphology change of the (b)). This data can not give the definite structure of the gel but
complex with concentration was further studied by optical can provide the relationship between molecules in the
micro image, as shown in Figure 5. In a concentration lower aggregation network with the assistance of spectral analysis.
than the critical gelation concentration, spherical morphology Before being coordinated to Zn2+ ion, the ligands
was observed, which may represent the initial stage of the aggregated to two dimensional structures by hydrophobic
gelation. With the increase of concentration, we observed the interaction between alkyl chains and hydrogen bonds
morphology evolution from flakes to fibers.
between triazine and amines. However, the hydrogen bond
was weakened when Zn2+ was coordinated to the ligand. The
hydrophobic interaction between the long alkyl chains and
π-π interaction between azobenzene groups became the
main driving force for the aggregation. The H type
aggregation between azobenzene groups was evidenced by
3.3 Structural study of the gel
The structure and dimensions of the gel network of Zn-1a and
the powder of 1a were studied by X-ray diffraction (Figure 6).
Figure 4 SEM images of Zn(m)-1a: (a) m = 0 equiv, (b) m = 0.2 equiv, (c) m = 0.4 equiv, (d) m = 0.6 equiv, (e) m = 0.8 equiv and (f) m =
1.0 equiv from DMSO (40 mg$mL–1).
Figure 5 Micro image of the gels (Zn-1a) with concentration of (a) 10, (b) 20, (c) 40, and (d) 80 mg$mL–1, respectively.
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Tianmin SHU, Junchen WU, Ying ZOU, Keyin LIU, Liqing CHEN et al.
Figure 6 (a) Powder X-ray diffraction spectra of 1a powder and Zn-1a xerogel from DMSO. (b) The perspective aggregation of Zn-1c
gel in DMSO.
the absorption spectra. Thus the complexes form one-
dimensional fiber structure by hydrophobic interaction and
π-π interactions with the structure parameter of 7.66 nm, as
shown in Figure 6(b).
Tao YI received her B.S. (1987), M. S.
(1990) and Ph. D. degrees (1998) from
Peking University, and worked there in
1990–1999. Following two years as a JSPS
postdoctoral fellow at Kyoto University
(1999–2001), she spent the other two years
in University of Tokyo and Kyushu
University. She worked in Paris University
4 Conclusions
(XI) as a red position researcher of CNRS
In conclusion, a novel type of ligands containing cyclen
ring and two azobenzene groups grafted with two long alkyl
chains was designed and synthesized. In the presence of
Zn2+, the complex gelates DMSO into a stable metellogel.
The morphology of the xerogels is varied with the
equivalent of Zn2+ cation and the concentration of the gel.
from 2003 to 2004 and jointed Fudan University in 2004. She is now
a professor at Department of Chemistry, Fudan University. Her
research interests focus on responsive supramolecular soft matters
and optical functional materials.
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