Chiral binaphthylbisbipyridine-based copper(I) coordination polymer gels as supramolecular catalysts

Article abstract of DOI:10.1039/b926936c

A novel class of chiral coordination polymer organogels, with multi-stimuli responsive properties and suitable for use as catalysts in 1,3-dipolar Huisgen cycloaddition reaction, are reported.

Full text of DOI:10.1039/b926936c

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Chiral binaphthylbisbipyridine-based copper(I) coordination polymer gels
as supramolecular catalystsw
Yabing He, Zheng Bian,* Chuanqing Kang, Yanqin Cheng and Lianxun Gao*
Received (in Cambridge, UK) 22nd December 2009, Accepted 1st March 2010
First published as an Advance Article on the web 29th March 2010
DOI: 10.1039/b926936c
A novel class of chiral coordination polymer organogels, with
multi–stimuli responsive properties and suitable for use as
catalysts in 1,3-dipolar Huisgen cycloaddition reaction, are
reported.
stoichiometry. The use of less than one equivalent of
Cu(CH3CN)4BF4 resulted in a sol being formed. In addition,
the counterions of copper(I) can influence the gelation. CuCF3SO3
and CuI only afforded solutions or precipitates, respectively.
Slightly surprisingly, we found that gel Cu(I)ꢀ1 could also be
formed via sonicating the suspension of 1 and Cu(CH3CN)4BF4
in CH2Cl2–CH3CN at room temperature. The formed gels
were stable at ambient temperature and no crystallization or
dissolution happened after months.
Recently, studies on metallogels have received much interest
from chemists because the presence of the metal ions may
endow the gel unusual functionalities such as spectroscopic,
magnetic, redox and catalytic properties.1 It has been reported
that some discrete metal complexes may self-assemble into
gel fibers by intermolecular hydrogen bonding, p-stacking,
solvophobic, and metal–metal interactions.2 Furthermore,
the use of multidentate bridging ligands or rare earth metal
ions can also result in the formation of a 3D cross–linked gel
network.3 However, very few metallogels based on a 1D
coordination polymer are known.4 For example, Kimizuka
et al. have described that Co(II) and lipophilic 1,2,4-triazole
assemble into a linear polymer structure leading to the
gelation.5 Lee et al. have reported stimuli–responsive hydrogels
from silver coordination polymers.6
When hydrophilic methoxymethyl sidechains in 1 were
replaced by hydrophobic ethyl or hexyl groups, a gel in
CH3CN–CH2Cl2 (v/v, 1/1) was still formed with MGC of
9.0 mM for Cu(I)ꢀ2 and 14.7 mM for Cu(I)ꢀ3, respectively.
Cu(I)ꢀ4 required a longer time to produce a gel (MGC,
16.2 mM), while Cu(I)ꢀ5 completely lost the gelation ability
in the mixed solvent, indicating the length of spacer unit
between bipyridine and binaphthyl ring in the ligand could
affect the gelation property.
All attempts to obtain single crystals of Cu(I)ꢀ1 failed.
However, postulation of the polymerization of ligands and
Cu(I) ions by tetrahedral coordination and the proper assembling
of the 1D polymeric chains into higher-order organized fibers
could easily explain the gel formation (Fig. 2). No gelation was
observed when 6 was used instead of 1, also suggesting that the
formation of the 1D coordination polymer was necessary for
the gelation.
As we know, 2,20-bipyridine is one of the most important
ligands in coordination chemistry.7 Especially, bisbipyridines
have been shown to form helical complexes with a range of
transition metal ions.8 Recently, Lutzen’s group testified that
¨
enantiomerically pure dinuclear double-stranded helicates
may be formed by diastereoselective self-assembly of the
binaphthylbisbipyridine ligands and copper(I) ion.9 In this
communication, we report a novel self-assembly pattern
between chiral binaphthyl-based bisbipyridine and copper(I)
ion as a 1D metal coordination polymer organogel.
The coordination behavior of 1 towards copper(I) was first
investigated by 1H NMR spectroscopy. Compared to the
1H NMR spectrum of the free ligand, the proton signals of
Cu(I)ꢀ1 in solution became broad and were significantly
shifted, indicating the formation of metal coordination bonds
and a very dynamic behavior on the NMR time scale.10
Lowering the temperature led to more ill-resolved signals,
The ligands 1–6 were synthesized by a Suzuki cross-coupling
reaction (Scheme 1, see ESIw). All of the analytical data are in
full agreement with the chemical structures presented. Very
interestingly, when the complexation of equimolar 1 and
Cu(CH3CN)4BF4 was performed in hot CH3CN–CH2Cl2
(v/v, 1/1) solution, a dark red gel phase was formed on cooling
to room temperature, with the minimum gel concentration
(MGC) of 10 mM, which is above the upper solubility limit for
the ligand in the mixed solvent. Complexation also successfully
gelated CH3CN–THF and CH3CN–dioxane mixed solvents at
room temperature. The formation of the gel was dependent on
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Graduate School of Chinese Academy of Science, 130022, Changchun,
China. E-mail: bianzh@ciac.jl.cn, lxgao@ciac.jl.cn;
Fax: +86 431 85697831; Tel: +86 85262265
w Electronic supplementary information (ESI) available: Synthetic
procedures and characterization of 1–6, Gel tests, DSC, FTIR, MS,
UV, CD, SEM, CV, NMR. See DOI: 10.1039/b926936c
Scheme 1
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exciton coupling theory and by comparison with related
literatures,9,12 we assigned a L configuration at the copper(I)
center. This led to a proposed overall P-helical polymeric
structure for the complex Cu(I)ꢀ1 (Fig. 2).
The properties of gel Cu(I)ꢀ1 were characterized by various
analytical techniques. Differential scanning calorimetric (DSC)
analysis of gel Cu(I)ꢀ1 showed no endothermic transition when
heating to 100 1C, indicating the gel was stable below this
temperature. The MALDI-TOF-MS spectrum of the complex
displayed the base peak at m/z = 897.24663, corresponding
to [Cu(I)ꢀ1]+ (calcd. 897.24966), indicating the polymer
underwent severe fragmentation.13 The characteristic C–N
stretching peaks of the bipyridine in ligand 1, observed at
1586, 1458 cmꢂ1 in the IR spectrum, shifted to 1593, 1468 cmꢂ1
in xerogel, suggesting the coordination of the bipyridines.14
The absorption spectra of gel Cu(I)ꢀ1 exhibited the typical
MLCT absorption band at 452 nm, which is attributable to the
tetrahedrally coordinated Cu(I)ꢀ(2,20-bipyridine)2 complex.15
In addition, the maximum absorption of gel Cu(I)ꢀ1 red-shifted
13 nm compared to diluted solution (CH3CN–CH2Cl2,
2 ꢃ 10ꢂ6 M), indicating that p–p stacking interactions between
1D polymeric chains could play an important role in the
formation of gel-fibers (Fig. 4a). Scanning electron micro-
scopic (SEM) images (Fig. 4b) of xerogel Cu(I)ꢀ1 revealed the
existence of typical nanofibers, responsive for the observed
gelation.
Fig.
1
Phase transition behavior of gel Cu(I)ꢀ1 triggered by
sonication, temperature, chemical redox reaction and solvent.
Fig. 2 A schematic representation of the possible self-assembly
processes of the coordination polymer gel Cu(I)ꢀ1.
presumably resulting from a slower exchange process or the
formation of new species. The coordination reaction of 1 and
copper(I) was further monitored through both UV-vis and
circular dichroism (CD) spectroscopic titration. As depicted in
Fig. 3, clear spectral changes were observed upon the addition
of copper(I) aliquots to solutions of the compound 1. The
existence of unideal isosbestic points suggested the formation
of oligomers or polymers in the complexation process between
1 and copper(I).11 The Job plot analysis exhibited the 1 : 1
complexation of Cu(I) and 1.
The redox-responsive properties of gel Cu(I)ꢀ1 were
measured.16 The oxidant NOBF4 could turn the dark red gel
into the pale green solution, and afterward, ascorbic acid was
used to regenerate the gel (Fig. 1, see ESIw). UV-vis spectra
revealed that the MLCT absorption band at 452 nm
disappeared in the oxidized solution sample and appeared
again in the re-formed gel. Therefore, it is obvious that the
redox state of copper ion plays a crucial role in the stability of
gel system. In addition, no collapse was observed when various
solvents, including CH2Cl2, CH3CN, CH3OH, THF, acetone,
hexane, H2O, DMF and HOAc, were placed over the metal-
logel, and no Cu(I) was leached out of the metallogel as
confirmed by inductively coupled plasma-mass spectrometry
(ICP-MS) analysis. However, the metallogel completely
dissolved in pyridine (Fig. 1).
The CD spectra of two copper(I) complexes of 1 and its
enantiomer showed they were identical mirror images, indicating
the expected formation of supramolecular enantiomers. The
complex Cu(I)ꢀ1 and the free ligand showed a rather dominant
Cotton effect in the 235 to 275 nm region, which is caused by
the (P)-configured chiral binaphthyl axis. More important was
that the first positive Cotton effect assigned to the ligand-
centered transition appeared at 375 nm (Fig. 3). Applying
Using Huisgen 1,3-dipolar cycloaddition (‘‘click’’ reaction)
as a model reaction, we explored the catalytic ability of these
metallogels (Table 1).17 Initial experiments were performed
with 1 mol% xerogel Cu(I)ꢀ3 as the catalyst and with benzyl
azide and ethynylbenzene as reactants in different solvents at
Fig. 3 Titration curves of 1 solution (2 ꢃ 10ꢂ4 M (a), 1 ꢃ 10ꢂ5 M (b))
in CH3CN–CH2Cl2 (v/v, 1/1) with Cu(CH3CN)4BF4 by CD (a) and
UV-vis (b) spectroscopy.
Fig. 4 UV-vis absorption spectra (a) of Cu(I)ꢀ1 in CH3CN–CH2Cl2
(v/v, 1/1) solution (2 ꢃ 10ꢂ6 M) and gel (1.6 ꢃ 10ꢂ2 M), and the SEM
image (b) of xerogel Cu(I)ꢀ1.
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Table 1 ‘‘Click’’ reaction catalyzed by xerogela
catalysts to asymmetric reaction and atom-transfer radical
polymerization are ongoing.
This work was supported by grants from the National
Natural Science Foundation of China (20502024).
Notes and references
Entry Catalyst Azide Alkyne Solvent
Time/h Yield (%)b
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1
2
3
4
5
6
7
8
Cu(I)ꢀ3
Cu(I)ꢀ3
Cu(I)ꢀ3
Cu(I)ꢀ3
Cu(I)ꢀ1
Cu(I)ꢀ2
Cu(I)ꢀ4
Cu(I)ꢀ3
Cu(I)ꢀ3
Cu(I)ꢀ3
Cu(I)ꢀ3
Cu(I)ꢀ3
Cu(I)ꢀ3
7
7
7
7
7
7
7
7
7
7
7
7
7
8a
8a
8a
8a
8a
8a
8a
8b
8c
8d
8e
8f
CH2Cl2
CH3CN
18
18
64
59
56
100 (97)
78
73
CH3CN–H2Oc 18
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H2O
H2O
H2O
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H2O
H2O
H2O
18
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18
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18
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18
8
97
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9
10
11
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13
8a
a
Reaction condition: alkyl azide (1.0 mmol), phenylacetylene
b
(1.2 mmol), xerogel (0.01 mmol), solvent (2.0 mL), RT, air. GC
c
yield (isolated yield in parenthesis) v/v, 1 : 1.
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room temperature in air. Using water as the solvent gave the
best result. Screening the catalysts gave the excellent result for
xerogel Cu(I)ꢀ3. 1 mol% xerogel Cu(I)ꢀ3 effectively promoted
the reaction in water at room temperature in air, giving
quantitative conversion (entry 4). The long lipophilic carbon
chain in xerogel Cu(I)ꢀ3 could provide an organic micro-
environment in the aqueous phase, thus facilitating the
diffusion of the substrates from surrounding media to the
catalytic sites in the gel fibers.18 The xerogel Cu(I)ꢀ3 was
recovered by filtration and reused in the next three cycles of
the reaction of 7 with 8a without appreciable loss of reactivity,
giving 9a in 100%, 100%, and 99.5% yield, respectively. No
copper(I) ion was detected in aqueous phase by ICP-MS
analysis, indicating the heterogenous catalysis. We also
examined the reaction of various alkynes with benzyl azide
catalyzed by xerogel Cu(I)ꢀ3. Aryl alkynes gave excellent
results (entry 8–10), while alkyl alkynes gave inferior outcomes
(entry 11–12). Using Cu(I)ꢀ(2,20-bipyridine)2 as catalyst was
accompanied by a rapid color change from red to green,
suggesting poor support of Cu(I) by bipyridine. However, no
color change was observed for xerogel Cu(I)ꢀ3 and catalytic
activity remained almost unchanged on staying in air for at
least one year, demonstrating significant stabilization of Cu(I)
in the complex, which was further confirmed by cyclic
voltammetry (CV) experiments. The redox potential of
9 J. Bunzen, T. Bruhn, G. Bringmann and A. Lutzen, J. Am. Chem.
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Ed., 1999, 38, 302–322.
13 ESI-MS spectra of CH3CN–CH2Cl2 (v/v, 1 : 1) solution
(5 ꢃ 10ꢂ6 M) of the complex Cu(I)ꢀ1 did not provide any valuable
information.
14 A. Westcott, C. J. Sumby, R. D. Walshaw and M. J. Hardie,
New J. Chem., 2009, 33, 902–912.
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Cu(I)/Cu(II) in
a
CH3CN–CH2Cl2 solution of Cu(I)ꢀ3
showed a dramatic increase of 1.20 V compared to that of
Cu(I)ꢀ(2,20-bipyridine)2.
In conclusion, we described
a novel class of metal
coordination polymer gels, which were not only responsive
to various stimuli such as heat, sound and redox, but also well
suited as self-supported stable supramolecular catalysts for
‘‘click’’ reaction. Further applications of the supramolecular
ꢁc
This journal is The Royal Society of Chemistry 2010
3534 | Chem. Commun., 2010, 46, 3532–3534