Topology-guided design of an anionic bor-network for photocatalytic [Ru(bpy)3]2+ encapsulation

Article abstract of DOI:10.1039/c5cc08614k

An anionic metal-organic framework, PCN-99, has been synthesized through a topology-guided strategy; its underlying bor-net is realized by the use of a tetrahedral [In(COO)₄]^- node and a judiciously designed trigonal planar linker. In light of its anionic nature, the inherent cuboctahedral cage and 1D channel make PCN-99 an excellent matrix to encapsulate the photocatalytic [Ru(bpy)₃]²⁺.

Full text of DOI:10.1039/c5cc08614k

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DOI: 10.1039/C5CC08614K
Journal Name
ARTICLE
Topology-Guided Design of An Anionic bor-Network for
2+
Photocatalytic [Ru(bpy) ] Encapsulation
3
a
a
a
a
a
Xuan Wang , Weigang Lu , Zhi-Yuan Gu , Zhangwen Wei , Hong-Cai Zhou *
Received 00th January 20xx,
Accepted 00th January 20xx
An anionic metal-organic framework, PCN-99, has been synthesized through a topology-guided strategy; its underlying bor-
net is realized by the use of a tetrahedral [In(COO)
4
]-node and a judiciously designed trigonal planar linker. In light of its
DOI: 10.1039/x0xx00000x
anionic nature, the inherent cuboctahedral cage and 1D channel make PCN-99 an excellent matrix to encapsulate the
www.rsc.org/
2+
photocatalytic [Ru(bpy) ] .
3
strong electrostatic interactions to the guest molecules and, in most
21, 22
cases, function as templates for post-synthetic ion exchange;
these unique properties make ionic MOFs promising for chemical
Introduction
23, 24
25, 26
sensing
or ideal as highly efficient heterogeneous matrices.
The utilization of metal-organic frameworks (MOFs) for
heterogeneous catalysis has been a hot topic in the past two
Nevertheless, the incorporation of large photoactive species has
rarely been studied likely due to the lack of ionic MOFs with large
1-5
decades. MOFs are well-defined structures that can achieve
exceptionally high porosity, which endows them with the capacity of
hosting not only densely packed but also precisely positioned active
sites for heterogeneous catalysis. To realize such an assembly of
2
7, 28
pore apertures.
Only a few ionic MOFs with cavities at
2
9, 30
mesoporous scale have been reported to date.
General approaches to obtain MOFs with desired topology and
a larger cavity involve extension of the organic linkers. In this work,
a careful inspection of the net topology of the target framework may
provide more insights into rational material design. With regard to
“molecular reactors”, a variety of suitable MOF scaffolds to
6
, 7
accommodate catalytic centers of various sizes are needed.
significant amount of effort has been dedicated to rationally design
MOFs with desired porosity and controlled cavity size and shape to
A
ionic MOFs, indium-based MOFs are commonly constructed with
+
4
, 5,
either cationic building blocks [In
3
O(COO)
6
]
or anionic building
tune the environment surrounding the active center of a catalyst.
-
21, 27, 31-40
8
blocks [In(COO)
4
] .
For the sake of incorporation of cationic
]- is our choice. Topologically, the [In(COO)
Most recently, the rapid development of MOFs with permanent
mesoporous cavities has prompted the study of encapsulation of
2+
-
[
Ru(bpy)
3
] , [In(COO)
4
4
] ,
8
a 4-connected tetrahedral node, could potentially form various high-
bulky catalysts and ensuing catalytic properites. The widely studied
photoactive cation [Ru(bpy)
41
2+
symmetry nets when combined with other highly symmetric nodes.
3
]
(bpy = bipyridine), in particular, is
The extension of btc (btc = 1,3,5-benzenetricarboylate) could
theoretically produce an isorecticular ionic MOFs with larger
such a catalyst, which displays potential in both light harvesting and
9
photocatalysis. The difficulty in the recycling of photocatalyst,
30, 35
1
0, 11
cavities.
Therefore, tritopic carboxylates are chosen as the
however, has delayed its practical applications.
One approach to
inorganic building units. Herein, we report the synthesis of an anionic
MOF PCN-99 (PCN = porous coordination polymer) with a pore cavity
circumvent this issue is to design and synthesize heterogeneous
photocatalysts, in which the catalytic centers are trapped in the solid
2+
2
+
3
at the mesoporous scale, spontaneous encapsulation of [Ru(bpy) ]
form. [Ru(bpy)
ligand design
3
] encapsulation in MOFs have been studied through
6
,
7, 12-16
10, 17-20
through ion exchange, and its photocatalysis in oxidative
hydroxylation of arylboronic acids.
and in situ entrapment
. However,
through post-synthetic ion exchange is
2
+
3
encapsulation of [Ru(bpy) ]
relatively understudied, which is puzzling because the photocatalytic
cation can be encapsulated easily and effectively.
Ionic MOFs are porous crystalline materials having either
positively- or negatively-charged frameworks with trapped
Results and discussion
Initially, the development of TATB (4,4',4"-s-triazine-2,4,6-triyl-
-
4
counterions inside the cavities. The charged frameworks provide tribenzoate), a trigonal planar linker, with 4-connected [In(COO) ]
was attempted in the expectation of forming such an anionic MOF
with large cavities, but to no avail. Similar endeavours have been
reported with the achievement of a microporous anionic MOF. This
could be ascribed to the free rotation of carboxyphenyl rings in TATB,
which prevents the three carboxylates from staying coplanar with
each other (Figure 1a). Compared to the orientation preference of
TATB, BTB (4,4’,4’’-benzene-1,3,5-triyl-tribenzoate) is even less likely
a.Department of Chemistry, Texas A&M University, College Station, Texas 77842-
012, USA.
Electronic Supplementary Information (ESI) available: [details of supplementary
information about full experimental details, crystallographic data, TGA, and NMR
spectra of products from aerobic oxidative hydroxylation reaction. CCDC
42
3
b.
1
431184]. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x
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Journal Name
must be adopted for the linker configuration. Therefore, guided by
View Article Online
DOI: 10.1039/C5CC08614K
topological analysis using the known connectivity of metal nodes, a
tritopic ligands 10, 15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole-
3
,8,13-tricarboxylate acid (H
3
DCTA) was designed and synthesized.
(
Figure 1b, Supporting information Section 2) Solvothermal reaction
of In(NO
3
)
2
·H
2
O, H
3
DCTA, and tetrafluoroboric acid in a mixture of
o
N,N-dimethylformamide (DMF) and ethanol at 120 C for 4 days
yielded colourless cubic single crystals of PCN-99 (Supporting
information Section 3).
In PCN-99, each indium center is coordinated by four carboxylate
groups of DCTA linkers, making the framework negatively charged
(
Figure 1d). The counterions interact weakly with the framework and
25, 27
can be easily exchanged.
A small octahedral cage is formed by six
indium units and four DCTA linkers (Figure 1g). In addition, a large
cuboctahedral cage comprised of twelve indium units and four
linkers leads to a mesoporous cavity (Figure 1h, more detailed
structural information in supporting information Section 4). Such a
2
9, 30
large pore is rarely reported for indium-based ionic MOFs.
Moreover, this cage-based structure can help minimize the leaching
of guest molecules, which was found in some MOFs used for enzyme
43
immobilization. At the topological level, PCN-99 adopts the bor
topology, in which the connection of the coplanar tritopic linker
-
DCTA and the [In(COO)
4
] node give rise to not only large cages but
also 1D channels (Figure 1f). This eases the diffusion of the substrates
during catalysis. Overall, this unique structural architecture imparts
PCN-99 great potential in large catalyst encapsulation. Due the large
pore aperture and ionic nature, the activation of PCN-99 for nitrogen
adsorption measurement was not successful.
Figure 1. Design strategy of PCN-99; a) flexible conformation of
TATB and b) coplanar conformation of DCTA; c), e) assembly of
a T
d
and coplanar trigonal nodes into a bor network; d), f)
-
combination of 4-connected [In(COO)
4
]
node（blue
tetrahedral） and 3-conneted DCTA linker (red triangle)
produces PCN-99; g) octahedron and h) cubooctahedron cages
in PCN-99.
To evaluate the anionic nature of PCN-99, single crystals of PCN-
9 were immersed in solutions of [Ru(bpy) ]Cl or congo red,
3 2
9
respectively (Figure 2a, Supporting information Section 5). The
colour of the congo red solution was retained after 24 hours, while
2+
the colour of the [Ru(bpy)
3
] solution gradually turned from yellow
to have a coplanar conformation. Under the combination with
to colourless, and the originally colourless PCN-99 at the bottom of
-
[
In(COO)
4
] , three nets were interwoven to provide a jcy framework
2+
vial turned orange. The decrease of [Ru(bpy)
3
] concentration, a
2
1
with micropores. In order to minimize the structural variation
caused by the flexibility of linkers, a strictly coplanar conformation
dramatic drop at 454 nm, is demonstrated by UV-Vis absorption
Figure 2. a) UV-Vis absorbance changes over time for PCN-99 in solutions of congo red and [Ru(bpy)
3 2
]Cl , respectively; b) UV-Vis
absorbance for PCN-99 in a mixture of [Ru(bpy) ]Cl and congo red in an equal molar; c) PXRD spectra of simulated pattern from crystal
3
2
3
structure (black), as-synthesized PCN-99 (red), and Ru(bpy) @PCN-99 (blue)
2
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spectra, which is consistent with the trapping of cationic [Ru(bpy)
in the pores of PCN-99. Powder X-ray Diffraction (XRD) studies Ru(bpy)
3
]2+ [Ru(bpy)
3
]Cl
2
in homogenous solution (Table 1).. In the catalysis of
View Article Online
DOI: 10.1039/C5CC08614K
3
@PCN-99, the influence of the Lewis acidity of metal cluster
showed evidence that the crystallinity was well preserved during and was excluded (Supporting information Section 8). As a result, the
after the guest inclusion (Figure 2c). The high absorption selectivity catalytic activity of Ru(bpy)
3
@PCN-99 is due to the efficient
active sites within the unique bor
2+
2+
of [Ru(bpy)
3
]
owing to the strong electrostatic interactions over encapsulation of [Ru(bpy)
3
]
congo red was also witnessed in a equimolar-mixed environment. topology. Interestingly, comparable product conversions were also
The diminishing absorption peak at 454 nm corresponds to the observed for the second use of catalyst with a small amount of
2+
inclusion of [Ru(bpy)
3
]Cl
2
while the consistent peak at 530 nm Ru(bpy)
3
leaching due to the partial decomposition of MOFs under
indicated the retention of congo red in the solution (Figure 2b, basic condition (supporting information Section 9).
supporting information Section 5).
For practical applications, a reversible ion-exchange process is
Arylphenols have been recognized as versatile intermediates in commonly required.27, 45 Modulation of the relative concentration of
the chemical and pharmaceutical industry. Since [Ru(bpy)
displayed excellent catalytic activities in aerobic oxidative consequently leads to release and recapture of the original guests.
3 2
]Cl has different guest molecules shifts the dynamic equilibrium and
27
6, 44
hydroxylation of arylboronic acids,
Ru(bpy) @PCN-99 in this type of chemistry as a heterogeneous Ru(bpy)
catalyst within the scope of arylboronic acids (Supporting adding the same volume of saturated NaNO
it is interesting to apply The release experiment was performed on the fully loaded
2+
3
3
@PCN-99. The release of [Ru(bpy)
3
]
was triggered by
3
+
solution (Figure 3) . The
could be achieved by
2
information Section 7). All conversions of the substrates exhibited virtually complete release of [Ru(bpy)
relatively high efficiency (Table 1, supporting information Section 5b increasing the volume of saturated NaNO
and Section7). Due to the fact that the framework structure impedes original 10 mL after 23 hours with the assist of gentle stirring (Figure
3
]
3
solution to 20 mL from the
2+
the access of substrates to a certain extent, so a reduced catalytic 3). As the [Ru(bpy)
3
] was replaced by the sodium ions, the yellow
activity was observed for Ru(bpy) @PCN-99 compared to colour of the crystals gradually faded.
3
Table 1. Visible-light-induced aerobic oxidative hydroxylation of
arylboronic acid.
[a][b]
Conclusions
In this study, a trigonal coplanar linker was designed using
topology guidance to produce an anionic bor-MOF PCN-99. In
light of its anionic nature, the selective encapsulation of
2+
[
3 3
Ru(bpy) ] through ion exchange imparts Ru(bpy) @PCN-99
Entry
R-
Catalyst
[Ru(bpy) ]Cl
Ru(bpy) @PCN-99
[Ru(bpy) ]Cl
Ru(bpy) @PCN-99
[Ru(bpy) ]Cl
Ru(bpy) @PCN-99
Conversion
71.9%
59.0%
89.4%
65.4%
with heterogeneous photocatalytic activity toward the aerobic
hydroxylation of arylboronic acid. Not only does this work
exemplify a successful approach to customize MOF structures
to accommodate a bulky photocatalyst, it also provides insights
into the construction of ionic MOFs with large cavities. The post-
synthetic ion-exchange can be a technically straightforward
alternative to the modification of other ionic MOFs with
catalytic units—as such, it could be tested with other types of
ionic MOFs and catalytic ions. The specific matrix PCN-99 in this
work is also interesting for its luminescence properties, which
may be useful for chemical sensing.
1
2
3
4
5
6
COOMe
COOMe
Me
Me
CHO
3
2
3
3
2
3
3
2
70.8%
45.9%
CHO
3
[
(
a] isolated yield.[b] Reaction Condition: 1a (0.5mmol), catalyst
2 mol%), Pr
lamp irradiation, and open to air for 48 hour.
i
2
NEt (2.0 equiv), DMF (5 mL), 36W fluorescence
Figure 3: a) UV-Vis absorbance of [Ru(bpy)3]2+ at different time after the addition of 20 mL of saturated NaNO
solution during the
3
2+
release experiment; b) trapping and releasing of [Ru(bpy)3] in PCN-99 in one-ion-change and release cycle.
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Acknowledgements
The organic synthesis and structural studies of this research
was supported as part of the Hydrogen and Fuel Cell Program
under Award Number DE-EE-0007049. The UV study and 23. J.-S. Qin, S.-R. Zhang, D.-Y. Du, P. Shen, S.-J. Bao, Y.-Q. Lan and Z.-
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of Energy, ARPA-e (DE-AR0000249) program, the Office of Naval
Research (N00014-14-1-0720) and the Welch Foundation
through a Robert A. Welch chair in Chemistry to HCZ. The
authors also acknowledge the financial supports of Texas A&M
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Laboratory at Department of Chemistry, Texas A&M University
were purchased with funds provided by the National Science
Foundation (CHE-9807975, CHE-0079822, and CHE-0215838).
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