Robust molecular bowl-based metal-organic frameworks with open metal sites: Size modulation to increase the catalytic activity

Article abstract of DOI:10.1021/acs.inorgchem.5b00185

Herein, two stable lead(II) molecular-bowl-based metal-organic frameworks and their micro- and nanosized forms with open metal sites were presented. These materials could act as Lewis acid catalysts to cyanosilylation reaction. Moreover, the catalytic per

Full text of DOI:10.1021/acs.inorgchem.5b00185

Communication
pubs.acs.org/IC
Robust Molecular Bowl-Based Metal−Organic Frameworks with
Open Metal Sites: Size Modulation To Increase the Catalytic Activity
Lin Liu,† Zheng-Bo Han,*,† Shi-Ming Wang,† Da-Qiang Yuan,*,‡ and Seik Weng Ng§
†College of Chemistry, Liaoning University, Shenyang 110036, P. R. China
‡State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou 350002, P. R. China
§Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
S
* Supporting Information
the activity depends on the surface area and substrate transport.
ABSTRACT: Herein, two stable lead(II) molecular-bowl-
based metal−organic frameworks and their micro- and
nanosized forms with open metal sites were presented.
These materials could act as Lewis acid catalysts to
cyanosilylation reaction. Moreover, the catalytic perform-
ances are size-dependent, with the catalyst with nanosized
form being 1 order of magnitude more efficient than those
with micro- and millisized forms.
Deliberately scaling down the size of the catalyst and accessing a
micro- or nanosized catalyst is an efficient strategy to formulate
materials with a high density of catalytically active sites.8
With those parameters in mind, we choose 5-tert-butylisoph-
thalic acid as the ligand to construct the molecular-bowl-based
MOFs. Upon adjustment of the synthetic procedure, 5-tert-
butylisophthalic acid reacted with lead(II) nitrate in N,N-
dimethylformamide (DMF) to yield [Pb4(C12H12O4)4(DMF)4]·
0.5H2O (1) and in DMA to yield an analogue of 1,
[Pb4(C12H12O4)4(H2O)4]·2DMA·4H2O (2). Interestingly,
through slight changes in the the reaction conditions, the
micro- and nanosized forms of 1 were isolated. The present study
will demonstrate that the catalytic properties of molecular-bowl-
based MOFs are governed by the size of the catalysts.
Both 1 and 2 belong to the polar P4nc space group (Table S1 in
the Supporting Information). The lead(II) sites in the molecular-
bowl building block are O,O-chelated by the carboxylate arms of
two adjacent 5-tert-butylisophthalate dianions (Figure 1a,b). In
1, the lead(II) atom is also coordinated by a DMF molecule. In 2,
the lead(II) atom is disordered over two positions (with the
disorder precluding the location of the coordinated water
molecule in the refinements); the disordered metal center is
coordinated by an aqua ligand (Figure S1 in the SI). The crystal
structure of 1 is described here in more detail (that of
isostructural 2 is not due to disorder). Carboxylate chelation is
asymmetrical [Pb−O 2.414(5), 2.666(5) Å and 2.419(5),
2.648(5) Å]. The tetranuclear unit lies along the 4-fold axis
along the c axis, featuring calix[4]arene-like motifs that are
stacked over each other in a manner similar to that of paper cups
(Figure 1c). These units are further connected through weak
interactions [the Pb−O distances are 2.791(5) and 2.796(5) Å;
Figure S2 in the SI] from other adjacent molecular-bowl units of
the carboxylate groups to generate an infinite 3D structure
(Figure 1d). Thus, the geometry can be regarded as ψ-
antiprismatic, with the electron lone pair occupying one of the
points (Figure S3 in the SI).9 The lower rim, when described in
terms of a circle, has a diameter of 10.90 Å (on the basis of a Pb···
Pb distance of 9.66 Å) because the diameter of the upper rim is
11.85 Å (on the basis of a Ctert···Ctert distance of 10.50 Å; Figure
S4 in the SI). After the coordinated and guest solvent molecules
owl-shaped molecules like calixarene or cyclodextrin
B
analogues, which have the fascinating architectures and
potential applications especially in molecular vessels, chemo-
sensors, drug delivery, catalysis, etc., have attracted considerable
attention.1 Efforts are concentrated in not only the organic
synthesis area devoted to exploring new molecular containers but
also in the development of a new strategy for the construction of
bowl-shaped metal−organic hybrids; among them, the noble-
metal-based bowl-shaped systems have been systematically
studied.2 For the purpose of wide application of this kind of
building block, the active sites need to be further studied.
Choosing proper organic ligands is the vital factor in
constructing molecular-bowl-based complexes. The following
aspects should be taken into account at least: first, the ligand
should own a nonuniform distribution of coordination active
sites that give the possibility of forming an asymmetric
coordination mode;3 second, one end of the ligand needs to
have a large coordination inactive group to provide certain steric
hindrance, inducing the form of the entrance/large end of the
bowl.4
The metal−organic frameworks (MOFs) constitute an
important class of Lewis acid heterogeneous catalysts.5 It is
difficult to prepare thermally stable MOFs with open metal sites
because the thermal stability is not a feature of coordinately
unsaturated metal centers.6 However, to our knowledge,
molecular-bowl-based MOFs with both high thermal stability
and high efficient catalytic activity have not been documented.
From the catalytic point of view, researchers have devoted time
to the development of an easily preparative, recoverable, and
recyclable catalyst with higher activity for Lewis acid catalytic
reaction.7 It would be an artful strategy to combine the active
metal sites with the bowl-shaped building block to devise an
efficient catalyst. Moreover, heterogeneous catalysts often
display size-dependent physical and chemical properties because
Received: January 28, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00185
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Table 1. Cyanosilylation Reaction Catalyzed by Different
a
Sized 1
a
Conditions: a mixture of catalyst (0.02 mmol), aldehydes (4 mmol),
and cyanotrimethylsilane (8 mmol) in CH2Cl2(5 mL) was stirred at 40
°C. Based on GC analysis. TOF = % conversion (mol of substrate/
mol of catalyst per h).
b
c
Figure 1. Structural overview for 1. Molecular-bowl building block with
thermal ellipsoids of 50% probability viewed along the c (a) and b (b)
axes. “Bowl-in-bowl” mode of 1 (c). 3D framework viewed along the c
axis (d).
7). However, for the larger-size aromatic aldehydes, 4-
benzyloxybenzaldehyde, only a low conversion of 49.5% (TOF,
33.0 h−1; entry 10) was achieved. According to the
aforementioned results, the cyanosilylation reactions with
larger-size aromatic aldehydes were also catalyzed by activated
1. In such cases, the active sites are most likely on the surface of
the catalyst. Although the larger-size aldehydes would be
hindered to access the open metal sites in the cavity of 1, the
active sites on the surface of the catalyst would catalyze the
reaction.
Micro- and nanosized forms of 1 were introduced to further
confirm that the active sites were located on the surface. With
regard to our molecular-bowl-based structure, there would be a
higher density of activated sites on the surface of the catalyst
when the particle size was scaled down. The strategy for reducing
the particle size of 1 was carried out by using PEG-10000 as the
surfactant. In the absence of surfactant, 1 was grown to microrods
(1a) with a diameter of 1 0.5 μm (Figures 2a and S15 in the SI).
are omitted, both 1 and 2 present 3D porous frameworks (Figure
S5 in the SI). Therefore, thermal activation would leave metal
sites totally exposed to the channels to generate a high density of
open metal sites.
The thermal stability of 1 and 2 was checked with
thermogravimetric analysis (TGA; Figures S6 and S7 in the SI)
and variable-temperature powder X-ray diffraction (XRD;
Figures S8 and S9 in the SI). After thermal activation, 1 and 2
can be converted to Pb4(C12H12O4)4 with the loss of coordinated
DMF or H2O, and solvent-free molecular-bowl-based MOFs are
stable up to 300 °C. Meanwhile, lead(II) centers exist in a lower
coordination state. Comparison of the powder XRD and TGA
patterns between 1 and 2 after thermal treatment at 250 °C
(Figure S10 in the SI) also confirmed that the residual skeleton of
the complexes is the same. Therefore, the activated catalyst was
prepared using 1 with no special indication in the following
sections.
The cyanosilylation reaction provides a convenient route to
cyanohydrins, which are key derivatives in the synthesis of fine
chemicals and pharmaceuticals.10 Several examples of metal−
organic hybrids have been observed to have catalytic activity to
the cyanosilylation reaction.7d,11 The catalyst 1 was used in three
forms: a millisized rodlike crystal (1, which was obtained by
conventional synthesis; Figure S11 in the SI), a microrod form
(1a), and a nanorod form (1b). That the substrates were
converted to their cyanosilylated products was verified by gas
chromatography (GC) analyses (Figure S12 in the SI). At first,
catalyst 1 was used, and the crystal shape was not damaged in the
reaction processes (Figure S13 in the SI). As shown in Table 1, a
loading of 0.5 mol % 1 led to a 99.5% conversion of benzaldehyde
after 3 h (entry 1). This represents significant progress compared
to previous reports (Table S2 in the SI).5a,12 To probe whether
catalysis occurs inside the cavity or on the surface of the catalyst,
substrates of different dimensions were tested (Table 1 and
Figure S14 in the SI). After 3 h, 1-naphthaldehyde had a
conversion of 99.0% (TOF, 66.0 h−1; entry 4) and p-
anisaldehyde had a conversion of 84.1% (TOF, 56.1 h−1; entry
Figure 2. Scanning electron microscopy images of microrods 1 (a) and
nanorods 1 (b).
With addition of the surfactant, the size of 1 changed drastically.
The diameter of the resulting nanorods (1b) is 160 50 nm
(Figures 2b and S15 in the SI). The XRD patterns of 1a and 1b
were in accordance with that of crystalline 1 (Figure S16 in the
SI); the differences between the peak widths can be attributed to
the size effect. TGA (Figure S17 in the SI) revealed that the
thermal stabilities of 1a and 1b are the same as that of 1. Through
B
DOI: 10.1021/acs.inorgchem.5b00185
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Notes
reduction of the particle size, the superficial area was enlarged
dramatically. The size-dependent effect of the catalyst was carried
out under the same reaction condition as those of 1. As shown in
Table 1, the conversion and TOF values were obviously
enhanced when using 1a and 1b as the catalyst. Also, the
detected catalytic efficiency of 1b was much higher than that of
1a. Taking the benzaldehyde group as example, the TOF of 1b
was 266.1 h−1 and that of 1a was 133.1 h−1 (entries 2 and 3). The
efficiency of catalysts 1a and 1b is more efficient than that of 1
(Figure S18 in the SI).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was granted financial support from the National
Natural Science Foundation of China (Grants 20871063 and
21271096).
REFERENCES
(1) (a) Gil-Ramιrez, G.; Escudero-Adan
Ballester, P. Angew. Chem., Int. Ed. 2008, 47, 4114−4118. (b) Zhang, Y.
Y.; Shen, X. Y.; Weng, L. H.; Jin, G. X. J. Am. Chem. Soc. 2014, 136,
15521−15524. (c) Yoshizawa, M.; Klosterman, J. K. Chem. Soc. Rev.
2014, 43, 1885−1898.
■
́
́
, E. C.; Benet-Buchholz, J.;
The catalyst to the series of cyanosilylation reactions could be
evaluated from two aspects. First, the reactants could diffuse into
the channels of the molecular-bowl-based MOFs. Therefore,
when using 1 as the catalyst, larger sizes of aldehydes were
difficult to diffuse into the channels, which results in relatively
low conversion and TOF values. In other words, the crystalline 1
highlights the diffusion effect. Second, there were considerable
open metal sites on the surface of the activated catalyst. So, when
1a and 1b were used, whatever size of aldehydes all performed
high conversion. The unactivated 1 was employed as the catalyst
for the cyanosilylation reaction. Because the channels of 1 were
jammed by coordinated DMF and only a limited surface of
unsaturated coordinated lead(II) was exposed to the substrate,
the conversion and TOF values were relatively low (Figure S18
in the SI). A filtration test was carried out to demonstrate that
there was no catalytically active species leaching into the system
(Figure S19 in the SI). Compared with the raw catalyst, a new IR
peak at 1686 cm−1 emerged and is attributed to the lead(II)-
bonded benzaldehyde.13 It was also revealed that the active site of
the catalyst is lead(II) (Figure S20 in the SI). The stability and
recyclability of activated 1 was also tested. The results of powder
XRD further confirmed the structural integrity of 1 after catalytic
tests (Figure S21 in the SI), thus suggesting the high stability of
this material in the cyanosilylation reaction. Also importantly,
there was no loss of activity of the catalyst after five cycles of
reactions (Figure S22 in the SI).
(2) (a) Yu, S.-Y.; Huang, H.; Liu, H. B.; Chen, Z. N.; Zhang, R.; Fujita,
M. Angew. Chem., Int. Ed. 2003, 42, 686−690. (b) Jiang, X. F.; Hau, F. K.;
Sun, Q. F.; Yu, S.-Y.; Yam, V. W. W. J. Am. Chem. Soc. 2014, 136, 10921−
10929. (c) Xie, T.-Z.; Guo, C.; Yu, S.-Y.; Pan, Y.-J. Angew. Chem., Int. Ed.
2012, 51, 1177−1181.
(3) (a) Zhao, D.; Timmons, D. J.; Qiang, D.; Zhou, H. C. Acc. Chem.
Res. 2011, 44, 123−133. (b) Natarajan, S.; Mahata, P. Chem. Soc. Rev.
2009, 38, 2304−2318.
(4) Ning, G. H.; Xie, T. Z.; Pan, Y. J.; Li, Y. Z.; Yu, S. Y. Dalton Trans.
2010, 39, 3203−3211.
(5) (a) D’Vries, R. F.; de la Pena-O’Shea, V. A.; Snejko, N.; Iglesias, M.;
̃
Gutier
5792. (b) Gan
́
rez-Puebla, E.; Monge, M. A. J. Am. Chem. Soc. 2013, 135, 5782−
́
dara, F.; Gomez-Lor, B.; Gutierrez-Puebla, E.; Iglesias, M.;
́
Monge, M.; Proserpio, D.; Snejko, N. Chem. Mater. 2008, 20, 72−76.
(c) Ma, L.; Falkowski, J. M.; Abney, C.; Lin, W. Nat. Chem. 2010, 2,
2838−2846.
(6) (a) Ogura, M.; Nakata, S.; Kikuchi, E.; Matsukata, M. J. Catal. 2001,
199, 41−47. (b) Concepcion-Heydorn, P.; Jia, C.; Herein, D.; Pfander,
N.; Karge, H. G.; Jentoft, F. C. J. Mol. Catal. A: Chem. 2000, 162, 227−
246.
́
̈
(7) (a) Li, C.; Liu, Y. Bridging Heterogeneous and Homogeneous
Catalysis: Concepts, Strategies, and Applications, 1st ed.; Wiley-VCH:
New York, 2013; pp 351−390. (b) Chui, S. S. Y.; Lo, S. M. F.; Charmant,
J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148−1150.
̌
(c) Horike, S.; Dinca, M.; Tamaki, K.; Long, J. R. J. Am. Chem. Soc. 2008,
In conclusion, two lead(II) molecular-bowl-based MOFs have
been prepared with a facile approach. The lead(II) centers of
both compounds adopt unsaturated coordination modes, which
exhibited high stability under thermal activation and catalytic
conditions. The cyanosilylation reaction was catalyzed by the
open metal sites both on the surface and inside the channels of
the catalyst. The size of 1 was modulated simply by changing the
reaction conditions. Most notably, the catalytic activity of 1 for
cyanosilylation can be enhanced by scaling down the particle size.
This approach provides a new strategy for the design of a highly
efficient heterogeneous catalyst for Lewis acid catalyzed
reactions.
130, 5854−5855. (d) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.;
Nguyen, S. T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450−1459.
(8) (a) Lin, W.; Rieter, W. J.; Taylor, K. M. L. Angew. Chem., Int. Ed.
2009, 48, 650−658. (b) Wee, L. H.; Lohe, M. R.; Janssens, N.; Kaskel, S.;
Martens, J. A. J. Mater. Chem. 2012, 22, 13742−13746. (c) Yang, G.;
Wei, Y.; Xu, S.; Chen, J.; Li, J.; Liu, Z.; Yu, J.; Xu, R. J. Phys. Chem. C
2013, 117, 8214−8222. (d) Sun, Q.; Ma, Y.; Wang, N.; Li, X.; Xi, D.; Xu,
J.; Deng, F.; Yoon, K. B.; Oleynikov, P.; Terasaki, O.; Yu, J. J. Mater.
Chem. A 2014, 2, 17828−17839.
(9) Briand, G. G.; Smith, A. D.; Schatte, G.; Rossini, A. J.; Schurko, R.
W. Inorg. Chem. 2007, 46, 8625−8637.
(10) (a) Higuchi, K.; Onaka, M.; Izumi, Y. Bull. Chem. Soc. Jpn. 1993,
66, 2016−2032. (b) Gregory, R. J. H. Chem. Rev. 1999, 99, 3649−3682.
(c) North, M. Tetrahedron: Asymmetry 2003, 14, 147−176. (d) Thir-
upathi, B.; Patil, M. K.; Reddy, B. M. Appl. Catal., A 2010, 384, 147−153.
(11) (a) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem.
Soc. 1994, 116, 1151−1152. (b) Evans, O. R.; Ngo, H. L.; Lin, W. B. J.
Am. Chem. Soc. 2001, 123, 10395−10396.
(12) (a) Kumar, G.; Gupta, R. Inorg. Chem. 2013, 52, 10773−10787.
(b) Phuengphai, P.; Youngme, S.; Gamez, P.; Reedijk, J. Dalton Trans.
2010, 39, 7936−7942.
(13) Morsali, A.; Mahjoub, A. R.; Darzi, S. J.; Soltanian, M. J. Z. Anorg.
Allg. Chem. 2003, 629, 2596−2599.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format, materials and
characterization, experimental section, supplementary structure
figures, and supplementary physical characterizations and
catalysis section. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: ceshzb@lnu.edu.cn.
*E-mail: ydq@fjirsm.ac.cn.
C
DOI: 10.1021/acs.inorgchem.5b00185
Inorg. Chem. XXXX, XXX, XXX−XXX