Transition metal ion induced hydrogelation by amino-terpyridine ligands

Article abstract of DOI:10.1039/c4ob01867b

Hydrogelation behavior of two amino-terpyridine ligands in the presence of divalent metal ions in water was studied in detail. The effect of ligand structure and different counter anions on the gel morphologies was also explored. This journal is

Full text of DOI:10.1039/c4ob01867b

Organic &
Biomolecular Chemistry
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Transition metal ion induced hydrogelation by
amino-terpyridine ligands†
Cite this: Org. Biomol. Chem., 2014,
2, 8836
1
a
a,b
a
Sandip Bhowmik, Biswa Nath Ghosh and Kari Rissanen*
Received 1st September 2014,
Accepted 29th September 2014
DOI: 10.1039/c4ob01867b
www.rsc.org/obc
1
7
Hydrogelation behavior of two amino-terpyridine ligands in the larger macromolecular structures. However, due to low solu-
presence of divalent metal ions in water was studied in detail. The bility in water, very few studies exist about their self-assem-
9
,18,19
effect of ligand structure and different counter anions on the gel bling properties in aqueous media.
morphologies was also explored.
Recently, we discovered selective gelation behavior of an
amino-terpyridine ligand upon binding Hg(II) ion. The results
were particularly intriguing considering the lack of any specific
In the past few decades, there has been great interest in eluci-
dating mechanisms responsible for self-assembly of small
molecules into hierarchical nanostructures. Supramolecular
gelators have attracted particular attention because of the
20
Hg(II) binding motif on the gelator. When analogues terpyri-
dines were studied in details for clues regarding metal
induced gelation, identity of metal ions as well as subtle
changes in the ligand structure was found to have critical
impact on the overall gelation process. In certain cases these
gels have interesting properties such as pyrophosphate detec-
1
expanding usage of gels in fields such as optoelectronics,
2
3
medicines, organic–inorganic hybrid materials etc. Although,
4
most commercial gels are polymeric in nature, one great
advantage of supramolecular gels lies in the dynamic nature of
the assemblies which allow structural modulations at the
molecular level. Several such functional materials have been
reported in recent literature where the physical properties of
21
tion at nanomolar concentrations.
Herein we report interesting gelation behaviors of 4′-(4-
N,N′-dimethylaminophenyl)-2,2′:6′,2″-terpyridine (L1) and 4′-
(
4-aminophenyl)-2,2′:6′,2″-terpyridine (L2) (Scheme 1) in presence
5
the material could be modified by applying external stimuli
of divalent transition metal ions and how small changes in the
gelator molecule led to significant changes in the self-assem-
bly process.
6
7
8
such as pH, light, chemical agents etc., thus greatly expand-
ing the scope of such systems for target oriented applica-
tions. Among various chemical stimuli applied to modify gel
behavior, cations and anions have been the most common
22
Ligand L1 was synthesized following a literature method.
Ligand L2 was synthesized by the Suzuki cross-coupling reac-
tion between 4′-chloro-2,2′:6′,2″-terpyridine and 4-amino-
phenylboronic acid pinacol ester in dimethoxyethane solvent in
the presence of Na CO using Pd(PPh ) as catalyst (Scheme 2).
9
–12
regulators.
In cases of hydrogelators with metal binding
motifs, introduction of metal ions in water was shown to have
profound effects on the self-assembly process. In many of the
metallogels, the formation of the 3D gel network was found to
be solely dependent on the presence of a particular metal
2
3
3 4
Uniform conditions were used for gelation experiments
with both the ligands. L1 and L2 were solubilized in an
aqueous solution of 0.15 N HCl. At this pH, electrostatic inter-
actions between partially protonated ligands and water were
1
3–15
ion.
Also, in some cases, gelators have been deliberately
16
designed to have metal-binding motifs to enhance selectivity.
Terpyridines have been extensively used to complex tran-
sition metal ions and have often served as building blocks for
too strong to allow self-assembly and resulted in clear solu-
20,21
tions. However, as observed previously,
metal coordination
a
Department of Chemistry, Nanoscience Center, University of Jyväskylä, P.O. Box 35,
40014 Jyväskylä, Finland. E-mail: kari.t.rissanen@jyu.fi; Fax: +358 14 2602501;
Tel: +358-50-5623721
b
Department of Chemistry, Veer Surendra Sai University of Technology, Burla,
Sambalpur 768018, Odisha, India
†
Electronic supplementary information (ESI) available: Experimental details,
gelation procedure, X-ray single crystal structure analysis. CCDC 1021539–1021543.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ob01867b
Scheme 1 Chemical structures of the terpyridine ligands L1 and L2.
8836 | Org. Biomol. Chem., 2014, 12, 8836–8839
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Scheme 2 Synthesis of ligand L2.
Table 1 Gelation studies of L1 and L2 (8 mM) with different divalent
metal (8 mM) ions (S = solution, P = precipitate, G = gel)
Ligand Mg(II) Ca(II) Mn(II) Fe(II) Ni(II) Co(II) Cu(II) Zn(II) Cd(II) Hg(II)
L1
L2
S
S
S
S
P
P
P
P
P
P
P
P
G
P
G
G
G
G
P
G
significantly altered solubility and charge polarization in the
complexes, leading to self-assembly and eventual gelation.
Several divalent metal ions were screened for the ability to
induce gelation (Table 1).
Although, most of the metal ions caused precipitation of
2
1
the resulting complexes, it was found that addition of Zn(II)
and Cd(II) to both ligands led to gelation. More interesting
results were observed for Cu(II) and Hg(II) cations. Cu(II) was
found to induce gelation upon complexation with L1 but not
with L2, whereas Hg(II) showed the exact reverse behavior
(Fig. 1). This was quite intriguing considering only minor
structural differences exist between the two ligands.
The metal to ligand ratio was optimized to 1 : 1 for all the
gelation experiments. It was observed that the acidic environ-
ment and hence the protonation of the amino terminal was
crucial for gel formation. It can also be assumed that π–π
stacking interactions as well as the salt bridge interactions
would play a major role in the self-assembly of the complexes.
However the properties of the metal ion that differentiate
between precipitation and gelation are not absolutely clear at
this point. Although the trends for both the ligands suggested
that gelation is favored in case of larger cations where
hydration is weak enough to allow other interactions. During
the course of our investigation, all the gels formed were found
to be thermo-irreversible and thixotropic.
Fig. 2 SEM images of xerogels of (a) Zn(II):L1, (b) Zn(II):L2, (e) Cd(II):L1
and (f) Cd(II):L2; TEM images of xerogels of (c) Zn(II):L1, (d) Zn(II):L2,
(
g) Cd(II):L1 and (h) Cd(II):L2.
elucidated by SEM (scanning electron microscopy) and TEM
transmission electron microscopy) techniques. Although all
(
gel networks mostly comprised of thin fibers, distinct struc-
tural differences were observed in case of each metal ion. Signi-
ficant morphological variations were also observed between the
two ligands L1 and L2 for the same metal ion (Fig. 2).
To better understand the nucleation and propagation of
the self-assembly in different cases, the gel morphology was
The dried Zn(II) hydrogels (xerogels) with both L1 and L2
showed uniform fibrous morphology in both SEM and TEM.
While TEM images of the gels in both cases revealed dense
network of thin fibers (∼50 nm width), presence of larger
aggregates (formed by higher order assemblies of smaller
fibers) were discovered through SEM analysis. However, dis-
tinct structural differences were obtained in case of Cd(II) gels.
While Cd(II) gel of L1 showed typical fibrous structure
(
∼80–100 nm width), both SEM and TEM analysis showed Cd(II):
Fig. 1 Hydrogels of (a) L1 with Cu(II), Zn(II), Cd(II) and (precipitate with)
Hg(II) (left to right); (b) L2 with Cu(II) (precipitate), Zn(II), Cd(II) and Hg(II)
L2 xerogel to be consisting of unusual flake like aggregates
(
left to right).
(small flat fibers of 120–150 nm width).
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Both Cu(II):L1 and Hg(II):L2 gels also have fibrous mor-
phology. However the fibers were found to be much thinner in
case of Cu(II):L1 gel than Hg(II):L2 gel (see ESI† for SEM and
TEM images).
To better understand the structural anomalies and varying
nature of interactions responsible for such behavior, crystal
structures of the parent complexes were analyzed in detail.
Due to extreme difficulty in obtaining the single crystals under
exactly the same conditions which led to the gelation, the
corresponding hydrogels were dissolved in DMF and slowly
evaporated to afford single crystals of the coordination com-
2
1
plexes CuCl
and HgCl (L2). The crystal structures confirmed the formation
of 1 : 1 complexes in which the metal centers adopt distorted
trigonal bipyramidal N Cl coordination. Fig. 3a shows the
(L1), while structure of CdCl (L2) is
2 2 2 2 2
(L1), ZnCl (L1), CdCl (L1), ZnCl (L2), CdCl (L2)
2
3
2
crystal structure of CdCl
depicted in Fig. 3b.
2
2
Although the coordination geometry around the metal Fig. 4 Crystal packing of ZnCl
2
(L2) showing π⋯π (yellow dotted bond)
and N–H⋯Cl interactions (red dotted bond) (top) and the dimeric motif
in the crystal packing of CuCl (L1) showing π⋯π interactions (bottom).
centers in all the complexes is identical, some minor differ-
2
ences were observed when comparing the structures of the
complexes. While the ligand L1 was found to be planar in all
the three complexes, the aniline moiety of ligand L2 in each of
the three complexes was found to be distorted from the planar-
ity. Analysis of crystal packing of metal complexes of L2 revealed
the presence of π⋯π interactions between the terpyridine
domain of the neighboring complexes as well as N–H⋯Cl
interactions (between the terminal Cl atom and adjacent anili-
nic H-atoms) (Fig. 4). However, the crystal packing of metal
complexes of L1 was mainly governed by π⋯π interactions
(
Fig. 4).
The evidence from the crystal packing of the complexes
suggested that the weak interactions such as π⋯π interactions
between neighboring complexes might play an important role
in the self-assembly. In addition, the N–H⋯Cl interactions
have also influenced the self-assembly of L2 complexes.
Anions were found to have a profound effect on gelation
process, especially in cases where metal coordination is
2
3
involved. Dong et al. recently demonstrated that the fiber
−
dimensions can be regulated by Cl ions controlling the
24
charge distribution on π-conjugated molecules.
This
prompted us to investigate the effect of anions on the gelation
process. For this purpose, the gelation experiments were
repeated by dissolving ligands L1 and L2 in 0.15 N of different
acids (only strong acids were selected to ensure complete dis-
sociation and to maintain constant counter anion concen-
2
5
trations ). Gelation occurred uninterrupted with Zn(II), Cd(II)
and Hg(II) complexes irrespective of the counter anion
present. However, in case of Cu(II), gelation (with ligand L1)
Fig. 3 (a) ORTEP plot of the molecular structure of CdCl
bond distances (Å) and angles (°): Cd(1)–N(1) 2.371(4), Cd(1)–N(2) 2.302(3),
2
(L1). Selected
2
5
Cd(1)–N(3) 2.353(4), Cd(1)–Cl(1) 2.4202(14), Cd(1)–Cl(2) 2.4548(13), was found to be critically influenced by the presence of anions
Cl(1)–Cd(1)–Cl(2) 114.16(5), N(2)–Cd(1)–N(1) 69.14(13), N(2)–Cd(1)–N(3)
and only happened in the presence of HCl and HBr (Fig. 5).
6
9.64(13); (b) ORTEP plot of the molecular structure of CdCl
Selected bond distances (Å) and angles (°): Cd(1)–N(1) 2.341(2), Cd(1)–N(2)
.299(2), Cd(1)–N(3) 2.350(2), Cd(1)–Cl(1) 2.4762(8), Cd(1)–Cl(2) 2.4238(8),
Cl(1)–Cd(1)–Cl(2) 119.76(3), N(2)–Cd(1)–N(1) 69.75(9), N(2)–Cd(1)–N(3)
0.40(8). Thermal ellipsoids are shown at 50% probability level.
2
(L2).
To investigate the role of anions further, the following
study was conducted. Addition of 1 equivalent of Cu(ClO ) to
4
2
2
3 2 4
L1 (dissolved individually in 0.15 N of HNO , H SO , HBr,
7
HClO and p-toluenesulfonic acid) in presence of 1 equivalent
4
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(
KR. grant no. 256259, 26337, 265328 and 263256) is kindly
acknowledged for financial support.
Notes and references
1
2
3
4
5
6
L. Korala, Z. Wang, Y. Liu, S. Maldonado and S. L. Brock,
ACS Nano, 2013, 7, 1215–1223.
P. K. Vemula, J. Li and G. John, J. Am. Chem. Soc., 2006,
1
28, 8932–8938.
S. Bhowmik, T. Gorai and U. Maitra, J. Mater. Chem. C,
014, 2, 1597–1600.
H. A. Aliyar, P. D. Hamilton and N. Ravi, Biomacromolecules,
004, 6, 204–211.
2
Fig. 5 Cu(II):L1 (8 mM/8 mM) system after dissolving L1 in HCl, HBr,
2
H
2
SO
4 3 4 3 3 3 3
, HNO , HClO , p-toluenesulfonic acid, CF SO H and CH SO H
E. Carretti, M. Bonini, L. Dei, B. H. Berrie, L. V. Angelova,
P. Baglioni and R. G. Weiss, Acc. Chem. Res., 2010, 43, 751–760.
Y. Hisamatsu, S. Banerjee, M. B. Avinash, T. Govindaraju
and C. Schmuck, Angew. Chem., Int. Ed., 2013, 52, 12550–
(
left to right).
12554.
7
8
9
J. Eastoe, M. Sánchez-Dominguez, P. Wyatt and
R. K. Heenan, Chem. Commun., 2004, 2608–2609.
S. Banerjee, R. K. Das and U. Maitra, J. Mater. Chem., 2009,
19, 6649–6687.
O. Kotova, R. Daly, C. M. G. dos Santos, M. Boese,
P. E. Kruger, J. J. Boland and T. Gunnlaugsson, Angew.
Chem., Int. Ed., 2012, 51, 7208–7212.
1
1
0 T. Tu, W. Fang, X. Bao, X. Li and K. H. Dötz, Angew. Chem.,
Int. Ed., 2011, 50, 6601–6605.
1 J. Park, J. H. Lee, J. Jaworski, S. Shinkai and J. H. Jung,
Inorg. Chem., 2014, 53, 7181–7187.
Fig. 6 Cu(II):L1 (8 mM/8 mM) Gels in presence of 8 mM NaCl after dis-
solving L1 in HNO
to right).
3
, H
2
SO
4
, HBr, HClO
4
and p-toluenesulfonic acid (left 12 M. George and R. G. Weiss, Acc. Chem. Res., 2006, 39, 489–497.
13 A. Kumar, M. Dubey, A. Kumar and D. S. Pandey, Chem.
Commun., 2014, 50, 10086–10089.
1
4 A. Y.-Y. Tam and V. W.-W. Yam, Chem. Soc. Rev., 2013, 42,
1540–1567.
of Cl− (NaCl), always led to gelation irrespective of the large
excess of other counter anions in the system (Fig. 6).
15 M.-O. M. Piepenbrock, G. O. Lloyd, N. Clarke and
−
The results confirmed the importance of Cl ion in the
J. W. Steed, Chem. Rev., 2010, 110, 1960–2004.
gelation process. Although the reason for this interesting gela- 16 T. Sato, M. Ebara, S. Tanaka, T.-A. Asoh, A. Kikuchi and
tion behavior is not well understood, the smaller size of Cu(II) T. Aoyagi, Phys. Chem. Chem. Phys., 2013, 15, 10628–10635.
ion and hence stronger polarisability might have a critical 17 A. Wild, A. Winter, F. Schlütter and U. S. Schubert, Chem.
impact on the self-assembly. Soc. Rev., 2011, 40, 1459–1511.
In conclusion, the self-assembly behavior of divalent metal 18 L. Sambri, F. Cucinotta, G. D. Paoli, S. Stagni and
complexes of two simple amino-terpyridine ligands L1 and L2 L. D. Cola, New J. Chem., 2010, 34, 2093–2096.
in water was explored in detail. It was observed that hydrogela- 19 A. Griffith, T. J. Bandy, M. Light and E. Stulz, Chem.
tion depended critically on the metal ions and also on minor Commun., 2013, 49, 731–733.
changes in the ligand structure. The crystal packing of both 20 B. N. Ghosh, S. Bhowmik, P. Mal and K. Rissanen, Chem.
the ligand-complexes showed definite differences that might Commun., 2014, 50, 734–736.
have led to the morphological changes in the corresponding 21 S. Bhowmik, B. N. Ghosh, V. Marjomäki and K. Rissanen,
gel structures. The observation of the role of the metal ion in J. Am. Chem. Soc., 2014, 136, 5543–5546.
controlling the dimensions of such aggregated structures was 22 J. Wang and G. S. Hanan, Synlett, 2005, 1251–1254.
intriguing. Given that the terpyridine based gelators were not 23 J. W. Steed, Chem. Soc. Rev., 2010, 39, 3686–3699.
explored until recently, this study could provide valuable 24 B. Dong, T. Sakurai, Y. Bando, S. Seki, K. Takaishi,
insights into the dynamics of such system and would help
creating new class of metal ion responsive materials.
M. Uchiyama, A. Muranaka and H. Maeda, J. Am. Chem.
Soc., 2013, 135, 14797–14805.
Mr Petri Papponen, Mr Hannu Salo and Dr Rakesh Put- 25 The gelation experiment with H
2
SO
4
solution of L1 was
2
−
treddy are kindly acknowledged for their help in TEM, SEM
and X-ray measurements, respectively. Academy of Finland
done in presence of 0.075 M Na SO to balance the SO
concentration.
2
4
4
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Org. Biomol. Chem., 2014, 12, 8836–8839 | 8839