A supramolecular tubular nanoreactor

Article abstract of DOI:10.1002/chem.201402612

The extremely strong noncovalent complexation between the rigid host of phthalocyanine-bridged β-cyclodextrins and the amphiphilic guest carboxylated porphyrin is employed to construct a hollow tubular structure as a supramolecular nanoreactor. A representative coupling reaction occurs in the hydrophobic interlayers of the tubular walls in pure water at room temperature, leading to an enhancement of ten times higher reaction rate without any adverse effect on catalytic activity and conversion.

Full text of DOI:10.1002/chem.201402612

DOI: 10.1002/chem.201402612
Communication
&
Nanoreactors
A Supramolecular Tubular Nanoreactor
Zhi-Qiang Li, Ying-Ming Zhang, Yong Chen, and Yu Liu*[a]
between permethylated b-cyclodextrin (PMCD) dimers with
rigid silicon(IV) phthalocyanine cores (1) and carboxylated por-
phyrin bearing long hydrophobic tails (2), followed by the
supramolecular assembly of 1/2 couples (Scheme 1). In this
nanostructure, the rigid-core-tethered host (1) of silicon(IV)
phthalocyanine axially bridged with two permethylated b-cy-
clodextrins (PMCD) can adjust the curvature of organic nano-
tubes to avoid any undesirable formation of micelles or vesi-
cles,[8] whereas carboxylated porphyrin 2 not only provides the
anchoring sites for catalytic centers, but also offer a hydropho-
bic microenvironment to readily accommodate the active bind-
ing domains. As a result, the tubular nanostructure can pro-
mote highly efficient catalytic reactions under environmentally
benign conditions.
Abstract: The extremely strong noncovalent complexation
between the rigid host of phthalocyanine-bridged b-cyclo-
dextrins and the amphiphilic guest carboxylated porphyrin
is employed to construct a hollow tubular structure as
a supramolecular nanoreactor. A representative coupling
reaction occurs in the hydrophobic interlayers of the tubu-
lar walls in pure water at room temperature, leading to an
enhancement of ten times higher reaction rate without
any adverse effect on catalytic activity and conversion.
In the last two decades, nanometer-scaled hollow structures
have attracted growing interest owing to their fascinating
properties in material conversion and delivery,[1] and construc-
tion protocols for various self-assembled hollow nanostruc-
tures with cage-like, cavity-like, vesicle-like, tube-like, and
porous topology have been successfully established.[2] Among
them, tubular nanostructures possess several considerable ad-
vantages compared with nanostructures with other topolo-
gies.[3] Firstly, the tubular structures may have reduced weight,
better mechanic stability, and larger surface-to-volume area.
Secondly, the open-ended feature of tubular arrays enables
not only the simultaneous interactions of cylindrical internal
and external surfaces with substrates, but also the delivery of
exogenous products, which jointly maximize the catalytic per-
formance.[4] These advantages are also used by natural systems
such as in the coating protein of the tobacco mosaic virus, cy-
toplasmic microtubules, and endoplasmic reticulum.[5] Typically,
the enveloping membrane of chloroplast in some plants can
form vesicle or tubular structures, and the biosynthesis of
lipids takes place under the catalysis of enzymes attached to
the surface of these hollow structures.[6] However, the potential
of artificial tubular nanostructures as reactors has barely been
uncovered, despite the fact that these tubular nanostructures,
which have tunable diameters ranging from the micro- down
to nano-scale, can provide spatially confined reaction cham-
bers and channels for materials synthesis and substance ex-
change. Herein, we report a hollow tubular nanostructure con-
structed through reasonably strong host–guest associations[7]
The dimeric host, 1, possessing a silicon(IV) phthalocyanine
core, was prepared according to the reported method,[9] and
the amphiphilic guest, 2, with hydrophobic tail was synthe-
sized by the reaction of phenolic hydroxyl-substituted porphy-
rin with a benzyl bromide derivative bearing alkyl chains under
basic conditions (Figures S1–S3, see the Supporting Informa-
tion). Subsequently, UV/Vis spectroscopic experiments were
performed to quantitatively investigate the host–guest binding
behaviors in aqueous solution. As shown in Figure 1, the incor-
poration of 1 with 2 underwent a clear complexation-induced
bathochromic shift from 680 to 689 nm, accompanied by
a clear isosbestic point at 685 nm. Similarly, the absorbance
changes of 1 became steady in the presence of 1 equivalent of
2, indicating 1:1 host–guest binding stoichiometry between
1 and 2 (Figure 1, inset).[10] In addition, the complex stability
constant (KS) was calculated to be 1.3ꢀ107 mÀ1 by analyzing
the sequential changes in absorbance intensity (DA) of 1 at
varying concentrations of 2 by using a nonlinear least-squares
curve-fitting method.[11] Moreover, the peak at m/z 1619.4413
in the mass spectrum was assigned to [1+2À3K+]3À (Fig-
ure S4, see the Supporting Information). These results jointly
reveal that the porphyrin backbone of 2 was concurrently as-
sociated with two PMCD cavities of 1 to form a highly stable
inclusion complex in water, and more importantly, this ex-
tremely strong binding would facilitate the eventual formation
of supramolecular tubules as described below (Scheme 2).
Direct morphological information of the tubular nanostruc-
ture based on the host–guest complexation of 1 with 2 was
provided by scanning electron microscopy (SEM) and transmis-
sion electron microscopy (TEM). As can be discerned from
Figure 2A, SEM images of an air-dried equimolar solution of
1 and 2 exclusively display cylindrical tubules with an open-
ended hollow space. Moreover, TEM images gave detailed
structural information, in which the aggregates constructed
from complex 1·2 clearly show hollow tubular structures with
[a] Z.-Q. Li, Dr. Y.-M. Zhang, Prof. Dr. Y. Chen, Prof. Dr. Y. Liu
Department of Chemistry, State Key Laboratory of
Elemento-Organic Chemistry, Nankai University
Collaborative Innovation Center of Chemical Science and Engineering
Tianjin 300071 (P.R. China)
E-mail: yuliu@nankai.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402612.
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Scheme 1. Structural illustration of compounds 1, 2, and permethyl b-cyclodextrin (PMCD).
of guest molecule 2 was 4.1 nm, the wall thickness of the
nanotubes essentially corresponds to bilayer array of
a
complex 1·2 along the vertical section of the tubular wall
(Figure S5, see the Supporting Information). In addition, the
powder X-ray diffraction (XRD) spectrum of the nanotubes ex-
hibited a diffraction peak with a d spacing of 3.6 nm (2q=
2.468), which could be assigned to the size of complex 1·2
along the axial direction of the nanotubes (Figure S6, see the
Supporting Information). Therefore, by combining this informa-
tion, we can reasonably infer the structure of the nanotubes is
as follow: the interior and exterior surfaces of the nanotubes
are composed of the highly stable PMCD/porphyrin-associated
units with rigid phthalocyanine spacers, whereas the hydro-
phobic alkyl chains interlace with each other in the middle of
the tubular walls (Scheme 2).
Figure 1. UV/Vis spectral changes of Q band of 2 (5.0ꢀ10À6 m) upon addi-
tion of 0–1.3 equivalents of 1 (from a to n) in pH 7.2 phosphate buffer solu-
tion at 258C. Inset: the absorption changes versus 1/2 molar ratio recorded
at l=680 nm.
1
The self-assembly in solution was investigated by H NMR,
DOSY, and dynamic light scattering (DLS) experiments. In
1H NMR spectroscopy, the aromatic protons of free 2 showed
dramatic broadening of the
peaks, but the proton signals
became sharp and clear in the
presence of 1 (Figure S7, see the
Supporting Information). These
large disparities in the NMR peak
patterns suggest that the forma-
tion of the inclusion complex
2·PMCD can greatly deter the
negatively charged porphyrins
from p-aggregation in water.[12]
Compared with the diffusion co-
efficient of the individual com-
Scheme 2. Schematic representation of the tubular self-assembly. A) Building blocks of 1 and 2 in the formation
of the nanotube wall. B) Self-assembly of the nanotube wall. C) Supramolecular nanotube with a bilayer array of
1·2 complex. D) Test reaction catalyzed by Pd@NT.
pounds
1
(1.87ꢀ10À10 m2 sÀ1
)
and 2 (4.03ꢀ10À10 m2 sÀ1) from
DOSY experiments, the complex
uniform size (Figure 2B and 2C). On the basis of the TEM anal-
ysis, the average inner and outer diameters of the obtained
nanotubes were around 182 and 200 nm, respectively, with
a wall thickness of 9 nm. Considering that the stretched length
1·2 gave
a
relatively low diffusion coefficient (1.52ꢀ
10À10 m2 sÀ1) at the same concentration (1.0ꢀ10À3 m), indicat-
ing that the complex 1·2 can diffuse as one entity with
a slower diffusion rate and a higher molecular weight (Fig-
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Figure 2. Morphological characterization of the tubular assembly. A) Typical SEM and B) typical TEM images of the tubular assembly (inset: magnified image
of tubules with 100 nm scale bar). C) Diameter size distribution of the tubular assembly obtained from TEM image statistics. D) Typical SEM and E) typical TEM
images of Pd@NT. F) The corresponding EDX spectrum of Pd@NT (inset: magnified image of Pd@NT wall with 2 nm scale bar).
ure S8, see the Supporting Information) than the individual
components.[13] In addition, the DLS results for complex 1·2
also display an average hydrodynamic radius of 1.2 mm, corre-
sponding to the existence of large-scaled aggregates in aque-
ous solution (Figure S9, see the Supporting Information). More-
over, microscopy experiments were also performed to obtain
morphological information about the supramolecular assembly
in solution and the solid state. The cryo-TEM image (Fig-
ure S10a, see the Supporting Information) shows the mutual
spatial arrangement of assembly 1·2 as tubular structures with
an average diameter of around 200 nm, which is consistent
with the corresponding value measured from Figure 2. Addi-
tionally, the SEM images of freeze-dried samples (Figure S10b)
also exhibited typical tubular structures with a similar size. In
contrast, as shown in Figure S11a in the Supporting Informa-
tion, only amorphous morphological structures could be ob-
served in 1. It is also confirmed that the amphiphilic com-
pound 2 can self-assemble to a closed vesicular structure with
a diameter of around 200 nm, which is much smaller than the
supramolecular nanotube 1·2 (Figures S12 and S13, see the
Supporting Information). These results jointly demonstrate the
formation of tubular assemblies of 1·2.
see the Supporting Information). For example, palladium chlo-
ride (PdCl2) in aqueous solution could be conveniently intro-
duced onto the interior and exterior surfaces of the nanotube
to form Pd2+-loaded nanotubes (Pd@NT) through the electro-
static interaction between the carboxylic acid groups and
metal catalysts. Tubular nanostructures similar to those in Fig-
ure 2A and B were observed in the SEM and TEM images of
Pd@NT, suggesting that the nanotube remained in its original
structure after attaching Pd2+ (Figure 2D and 2E). In addition,
energy-dispersive X-ray (EDX) analysis demonstrated the exis-
tence of Pd2+ on the tubular surfaces (Figure 2F), giving the
characteristic peaks of Pd2+ attached on the interior and exte-
rior surfaces. Further TEM analysis provided direct information
about the structure of the Pd@NT wall. The selected area elec-
tron diffraction (SAED) pattern gave the polycrystalline struc-
ture of the Pd@NT wall (Figure S15, see the Supporting Infor-
mation), and the high-resolution TEM image of the edge of the
Pd@NT wall presented lattice fringes of 0.22 nm, which coincid-
ed with the lattice structures of Pd2+ (Figure 2F, inset). Conse-
quently, it is reasonable to deduce that the surfaces of the
hollow microstructures have been successfully loaded with
Pd2+ catalyst. In addition, from inductively coupled plasma
(ICP) analysis, the content of the Pd2+ ions was measured as
2.62 mgLÀ1 in a 3ꢀ10À5 molLÀ1 solution of Pd@NT; that is, one
Pd2+ ion was carried by 1.23 complexes of 1·2 on average.
After the ability of the nanotubes to be loaded with metal
catalysts was validated, Suzuki–Miyaura coupling reactions,
which consist of ligand exchange followed by a reductive cou-
pling reaction between aryl halides and phenylboronic acid de-
From the structural features of the tubular nanostructures,
one can see that there were numerous pendant carboxylic
acid groups located at the periphery of the porphyrins, groups
that could act as anchoring sites for various metal-based cata-
lysts. Moreover, the zeta potential of the tubular self-assembly
was measured as À34.5 mV and this negatively charged sur-
face would facilitate the binding of metal catalysts (Figure S14,
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rivatives,[14] were selected to explore the catalytic capability of
the metal-loaded nanotubes, Pd@NT. Some representative cat-
alytic results for reactions preformed at ambient temperature
and pressure in water without the need of any organic cosol-
vent are listed in Table 1. As shown in Table 1, the catalytic ac-
tivity of Pd@NT is applicable to various substrates, ultimately
leading to the excellent isolated yields (93–99%) in only one
reaction rate, and only 35% conversion to the cross-coupling
product was achieved even after extending the reaction time
to two hours (Figure 3, b). A control experiment using pristine
PdCl2 in the presence of potassium benzoate was carried out
to mimic any effect from the carboxylic acid groups on Pd@NT.
No obvious change in yield was observed with potassium ben-
zoate and a similar yield to the previous pristine PdCl2 experi-
ment (33%) was observed. In addition, a moderate yield of iso-
lated product of up to 80% was obtained by using a THF/
water mixed solvent system to increase the solubility of the
starting reactants (Figure 3, c). Presumably, the lack of efficient
supporting materials blocks the sufficiently close contact of re-
actant molecules to catalysts, which limits any further increases
in the yield.[16] Moreover, the individual compound 2 was
found to self-assemble to closed vesicular structures (Fig-
ures S12 and S13), which showed a significantly lower conver-
sion ratio than Pd@NT (Figure 3, d). In particular, within the ini-
tial 15 min, the conversion ratio of Pd-loaded vesicle 2 was less
than 50% of the corresponding value of Pd@NT under the
same conditions. A possible reason may be that the catalytic
reaction only occurred at the exterior surface of the fully
sealed vesicles, leading to the decreased catalytic activity com-
pared with Pd@NT, where both interior and exterior surfaces of
the nanotubes actively participate in the catalytic reaction (Fig-
ure S13). Moreover, no catalytic activity was observed by using
Table 1. Suzuki–Miyaura coupling reaction catalyzed by Pd@NT.
Entry
R1
R2
Yields of isolated product [%]
cycle 1
cycle 2
cycle 3
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
96
94
97
99
98
96
97
93
95
99
98
94
95
94
97
99
98
96
95
94
95
97
99
95
96
94
96
98
97
95
96
93
95
96
97
93
OCH3
CH3
CN
NO2
COCH3
H
OCH3
CH3
CN
NO2
COCH3
9
10
11
12
the individual component 1 as it lacked the loaded Pd2+
.
These results confirm that the spatial arrangement of supra-
molecular tubules plays a crucial role in the promotion of the
coupling reaction. Furthermore, a plot of ln[4-nitrobromoben-
zene] versus reaction time was essentially linear (Figure S16,
see the Supporting Information), thus attesting to a pseudo-
first-order reaction.[17] The slope of the straight line provided
the value of the observed rate constant (kobs). The kobs value of
the Pd@NT-catalyzed coupling reaction was calculated as
1.63ꢀ10À3 sÀ1, which was 10 times higher than the corre-
sponding control groups.
hour, which was a comparably higher efficiency than results
under the same experimental conditions.[15] Moreover, the
Pd@NT catalyst can be easily recovered and reused for many
catalytic cycles without any loss of reactivity and conversion
ratio.
Taking the coupling reaction of 4-nitrobromobenzene with
phenylboronic acid as an example, some control experiments
were carried out to prove the prominent enhancement in the
Pd@NT-catalyzed coupling reactions. It was found that Pd@NT
can greatly accelerate the chemical reaction rates, and 95%
conversion was readily obtained in 30 min (Figure 3, a). In com-
parison, the use of pristine PdCl2, containing the same Pd2+
content, instead of Pd@NT resulted in a dramatically decreased
The binding geometry involving the tubular nanoreactor
1
was further confirmed by H NMR spectra. By comparison with
the spectra of complex 1·2, a broadened peak in the range
from 6.8 to 7.5 ppm was assigned to the characteristic signals
of biphenyl derivative of 4-biphenylcarbonitrile in D2O (Fig-
ure S17, see the Supporting Information). As shown in the ro-
tating-frame Overhauser effect spectroscopy (ROESY) experi-
ment, a clear nuclear Overhauser enhancement (NOE) cross-
correlation between the protons of 4-biphenylcarbonitrile and
alkyl chains was observed (peaks A, Figure S18, see the Sup-
porting Information). In addition, the coupling reaction was
also performed with a wide range of Pd2+ concentrations (the
mole ratio of Pd2+ to substrate was fixed at 3%). It is shown
that a relatively high yield could be obtained even when the
concentration of complex 1·2 in Pd@NT was as low as 2ꢀ
10À5 m (Table S1, see the Supporting Information). In this case,
through a calculation of the binding constant of 1 and 2 (1.3ꢀ
107 mÀ1), as well as the concentrations of host and guest, more
than 94% of complex 1·2 still exists in solution. Furthermore,
to investigate the advantage of the nanotube in the coupling
reaction, 2 equivalents of PMCD as the competitive host was
Figure 3. Suzuki–Miyaura reaction of 4-nitrobromobenzene with phenylbor-
onic acid catalyzed by: a) Pd@NT in pure water, b) PdCl2 in pure water,
c) PdCl2 in 1:1 THF/water mixed solvent, and d) Pd-loaded vesicle 2.
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image, the tubular structure was seriously destroyed in the
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dimers and amphiphilic guests in water. Structurally, the preor-
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ous dispersion of active binding sites. The introduction of car-
boxylic acid groups immobilizes the metal catalysts on the sur-
faces of the tubules. Functionally, the resulting Pd@NT display
enhanced catalytic activity for the Suzuki–Miyaura cross-cou-
pling reaction, even for those substrates with low water solu-
bility, with the added benefits of using environmentally friend-
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will continue to energize and expand the exciting uses of
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Experimental Section
Preparation of tubular assembly Pd@NT
The freeze-dried 1:1 inclusion complex of
1 and 2 (99 mg,
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0.02 mmol) were added into distilled water (10 mL), then PdCl2
(18 mg, 0.1 mmol) was added. The mixture was stirred for 12 h at
room temperature in the dark. The suspension was centrifuged at
8000 rpm for 10 min to remove the undissolved PdCl2. Subse-
quently, the Pd-loaded organic nanotube (Pd@NT) was purified by
dialysis (molecular weight cutoff 3500) in distilled water several
times until the water outside the dialysis tube contained a negligi-
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ble amount of Pd2+
.
Acknowledgements
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We thank the 973 Program (2011CB932502) and the National
Natural Science Foundation of China (NNSFC) (Nos. 20932004,
91227107, 91027007, 21272125, and 21102075) for financial
support.
Keywords: cyclodextrin · nanoreactors · nanotubes · self-
assembly · supramolecular catalysts
Received: March 14, 2014
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COMMUNICATION
&
Nanoreactors
Z.-Q. Li, Y.-M. Zhang, Y. Chen, Y. Liu*
&& – &&
A Supramolecular Tubular
Nanoreactor
Reaction in a nanotube: A tubular
nanoarchitecture is constructed by the
noncovalent complexation of phthalo-
cyanine-bridged bis(b-cyclodextrin)s and
amphiphilic porphyrin. The nanotube is
utilized as a supramolecular nanoreactor
to drive a Pd-catalyzed coupling reac-
tion within its hydrophobic interlayers
under environmentally friendly condi-
tions.
&
&
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!