Size-selective catalysts in five functionalized porous coordination polymers with unsaturated zinc centers

Article abstract of DOI:10.1039/c7nj02021j

The five reported structural isomorphic porous coordination polymers (PCPs) 1-5, namely, [Zn(L)(ip) (1), Zn(L)(aip) (2), Zn(L)(hip) (3), Zn(L)(nip) (4), and Zn(L)(HBTC) (5) (L = N⁴,N^4′-di(pyridine-4-yl)biphenyl-4,4′-dicarboxamide, H₂ip = isophthalic acid, H₂aip = 5-aminoisophthalic acid, H₂hip = 5-hydroxyisophthalic acid, H₂nip = 5-nitroisophthalic acid, H₃BTC = 1,3,5-benzenetricarboxylic acid)] were used to catalyze the acetylation of phenol. All these heterogeneous catalysts exhibit good catalytic efficiency and size-selectivity toward the acetylation of phenols owing to their unsaturated metal centers, non-coordinated amide, and suitable channel size and shape. Among them, 2 displays the highest catalytic activity and excellent cooperative catalysis due to the presence of basic non-coordinated amide groups.

Full text of DOI:10.1039/c7nj02021j

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Size-selective catalysts in five functionalized porous coordination
polymers with unsaturated zinc centers
Dongsheng Denga, Hui Guoa, Baoming Jia*, Weizhou Wanga, Lufang Maa and Feng Luob*
a Henan Key Laboratory of FunctionꢀOriented Porous Materials, and College of Chemistry and
Chemical Engineering, Luoyang Normal University, Luoyang 471934, P. R. China College of Biology,
b Chemistry and Material Science, East China Institute of Technology, Fuzhou, Jiangxi, P. R. China.
(* Corresponding author Tel.: +86 379 65523821; fax: +86 379 65523821.
Eꢀmail address: dengdongsheng168@sina.com; lyhxxjbm@126.com)
Abstract: The five reported structural isomorphic porous coordination polymers (PCPs) 1–5, namely,
[Zn(L)(ip) (1), Zn(L)(aip) (2), Zn(L)(hip) (3), Zn(L)(nip) (4), and Zn(L)(HBTC) (5) (L =
N4,N4’ꢀdi(pyridineꢀ4ꢀyl)biphenylꢀ4,4’ꢀdicarboxamide, H2ip
=
isophthalic acid, H2aip
=
5ꢀaminoisophthalic acid, H2hip = 5ꢀhydroxyisophthalic acid, H2nip = 5ꢀnitroisophthalic acid, H3BTC
= 1,3,5ꢀbenzenetricarboxylic acid)] were used to catalyze acetylation of phenol. All these
heterogeneous catalysts exhibit good catalytic efficiency and sizeꢀselectivity toward acetylation of
phenols owing to their unsaturated metal centers, nonꢀcoordinated amide, and suitable channel size
and shape. Among them, the 2 displays highest catalytic activity and excellent cooperative catalysis
due to the present of basic nonꢀcoordinated amide groups.
1. Introduction
Catalysts based on rigid porous or channel frameworks possess the merits of both homogeneous
and heterogeneous catalysts, and surmount all the limitations of the homogeneous catalysts, such as
difficulty in separation and recovery and disposal of spent catalysts.1ꢀ4 Considered to be promising
analogues of inorganic zeolites, porous coordination polymers (PCPs) constructed by inorganic ions
and organic linkers are nowadays at the front of materials research because of their intriguing
structural architecture and outstanding functional properties.5ꢀ8
Generally, porous coordination polymers as heterogeneous catalysts should own the following
features:9,10 1) the three distinct components of PCPs, namely metal nodes, organic ligands and pores
can all be easily tailored for their catalytic activities due to the modular nature and facile tenability of
PCPs, combining the merits of inorganic and organic chemistry; 2) the open accessible channels or
pores are beneficial for trapping substrates or activating of reactants, and the wellꢀdefined crystalline
structure of PCPs is useful for elucidating structureꢀfunction relationships; 3) the existence of
coordinatively unsaturated metal sites (UMS) or the introduction of functional organic sites (FOS) can

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be used to decorate channel surfaces of PCPs; 4) as a solid materials, PCPs catalysts must be easy to
separate from reaction system and can be reused for the next run, and thus reducing waste and
contamination.
The exploiting of PCPs as heterogeneous catalysts has been accompanied by the massive growth
of PCPs structural and functional complexity.11ꢀ14 Among these studies, it is noted that UMS or FOS is
indispensable in the catalytic system for the C–C bond formation reaction, where UMS or FOS can
activate reaction substrates so as to promote the reaction smoothly. Meanwhile, a lot of reports have
revealed that the unique merits of PCPs based UMS or FOS catalysis exceeds purely organic or
inorganic systems.15ꢀ17 For example, Kitagawa and coꢀworkers have designed and synthesized a 3D
PCPs functionalized with amide groups.9 They showed that these amide groups locating on the
channel surface can interact with guest molecules and selectively promote the Knoevenagel
condensation reaction based on the relationship between the size of the reactants and the pore window
of host. Next, Long and other group revealed that a 3D Mn PCPs was used as Lewis acidꢀtype catalyst
to catalyze cyanosilylation reaction with good substrateꢀsizeꢀselectivity.18 Lately, Duan and
coꢀworkers have isolated biꢀfunctionalized PCPs for sequential multistep catalysis.19
Inspired by the above reaction principle, as an extension to our previous investigation,20,21 we
pursue to evaluate the catalytic performance of the five Zn(II) PCPs with both UMS and different FOS
in the framework, which are anticipated to be endowed with satisfactory activity for acetylation of
phenols. As is well known, the acetylation of phenols is an important nucleophilic reaction for the
synthetic organic chemist, which is often carried out under homogeneous conditions with acetic acid
or acetyl chloride or anhydride, in the presence of basic catalysts such as pyridine or triethylamine.22,23
One drawback of this method is that baseꢀsensitive substrates may result in decomposition. To address
this issue, some PCPs have been found to be an ideal catalyst for the acetylation of phenols.24,25 For
example, Moorthy et. al. demonstrated that the use of some PCPs based on a twisted tetrapyridylbiaryl
ligand as catalysts, acetylation reactions of various phenols proceeded smoothly, and affording the
corresponding products with excellent yields.26 Thus, the development of suitable PCPs for this
reaction is a top of great interest.
Herein, the present work aims at investigating the catalytic efficiency of different types of
functional groups exposed in the pores toward the acetyl reactions, so as to find which material can
exhibit cooperative effect on catalytic capacity or not. As a cooperative catalyst, 2 displays excellent
catalytic ability for the acetylation of phenols. Based on the aboveꢀmentioned fact, we think that the
presence of basic amine groups, the full accessible porosity, together with a high thermal and chemical
stability can make 2 an excellent nucleophilic catalyst for acetylation of phenols.
2. Experimental Section

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2.1. General procedures
All chemicals were of reagent grade quality obtained form commercial sources and used as
received unless special notice. Thermogravimetric analyses (TGA) were carried out on a SDTQ600
differential thermal analyzer with a heating rate of 10 °C/min in a N2 atmosphere. Powder Xꢀray
diffraction (PXRD) data were recorded with a Bruker D8ꢀADVANCE Xꢀray diffractometer by using
1
CuKα radiation. H NMR spectra were measured on a Varian UNITY/NOVA 400 NMR spectrometer
using CDCl3 as the solvent at room temperature. Chemical shifts are given in δ relative to TMS. The
coupling constants J are given Hz.
2.2. The syntheses of 1–5
The polymers of 1–5 were prepared according to the literature method.21 A mixture of Zn(NO3)2
(38 mg, 0.2 mmol), L (79 mg, 0.2 mmol), multiꢀcarboxylate ligand (H2ip (33 mg, 0.2 mmol), or H2aip
(36 mg, 0.2 mmol), or H2hip (36 mg, 0.2 mmol), or H2nip(42 mg, 0.2 mmol), or H3BTC(42 mg, 0.2
mmol) and DMF/H2O (10 mL, 5:1) was sealed in a 23 mL Teflonꢀlined stainless steel container and
heated at 120 °C for 2 days, and then cooled to room temperature at a rate of 3°C/h. The collected
crystals were treated under vacuum at 100 °C for 12 h to obtain the full evacuated framework. The
yields for 1–5 were 88%, 84%, 87%, 88% and 86% based on Zn, respectively.
2.3. Typical procedure for acetylation of phenols using 2 as catalyst
A capped vial was charged with acetic anhydride (1.5 mmol), phenol (0.5 mmol), catalyst (0.01
mmol) and CH2Cl2 (1 mL). The reaction mixture was stirred for the required time, and was monitored
by TLC analysis. After completion of the reaction, 10 mL of ethyl acetate was added to the reaction
mixture. The mixture was stirred for another 5 min. Then the catalyst was isolated by filtration and
washed thoroughly ethyl acetate, and the filtrate was thoroughly washed with aqueous Na2CO3
solution to remove the unreacted anhydride and acetic acid. The organic layer was dried over
anhydrous Mg2SO4 and evaporated to obtain the corresponding pure products. All isolated pure
products were fully characterized by NMR analysis. The recovered catalyst was washed with ethyl
acetate, dried, and reused without further purification or regeneration.
2.4. Experimental procedure for reusability tests
The reusability of 2 was tested for acetylation of phenols. When the reaction finished, the catalyst
was isolated by filtration and washed thoroughly ethyl acetate, and the resulting catalyst was dried at
80°C for 4 h under vacuum, then reused directly without further purification for the second run with
fresh acetic anhydride and phenol. It was used for four consecutive runs with minor loss in activity.
Moreover, the recovered catalysts were characterized by the Xꢀray powder diffraction and showed
identical results to those of the fresh samples.

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3. Results and Discussion
The previous crystal structural analyses revealed that the compounds 1–5 are structural
isomorphs.21 In the structures of 1–5, the unique Zn2+ ion is tetraꢀcoordinated by two L ligand N
atoms and two carboxylate O atoms in a distorted tetrahedral geometry, as observed in compound 2
(Fig. 1a). Moreover, the 1D regular hexagonal channels along the c axis are observed in their
frameworks (Fig. 1b), which not only possess abundant amide groups but also is decorated by
different functional groups of –H, –OH, –NH2, –NO2, and –COOH, respectively. The pore diameters
are estimated in the range of 7.0–9.4 Å. Therefore, the five zinc PCPs have both coordinatively
unsaturated Lewis acid (Zn2+) centers, basic centers (amide groups), as well as other different organic
functional groups (Fig. 1). These features imply that the presence of coordinatively unsaturated Zn(II)
atoms allows their use as a Lewis acid, and the different FOS located around the cavity of the PCPs
would provide a significant catalyzing force for special reactions. Because the zinc polymers (1–5)
have different FOS, it would be interesting to see how the different functional features affect the
catalytic behavior. As is well known, the thermal stability of catalysts is an important factor in
evaluating their catalytic capacity. Therefore, it is necessary to investigate the thermal stability of 1–5.
In this regard, thermogravimetric analysis experiments were performed for 1–5 under a N2
atmosphere. As shown in Figure S1, the thermograms of 1–5 show similar profiles. The five
compounds are thermally very stable up to 270 °C. Because there are no solvent molecules in the
frameworks, there is a plateau region ranging from 25 to 330 °C in 1, 380 °C in 2, 375 °C in 3,
365 °C in 4, and 370 °C in 5, followed by a sudden decrease in the weight, suggesting the onset of
the decomposition temperature of the compounds (see ESI† for details). The result reveals that these
five compounds possess excellent structural stability.

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Organicsite
Lewisacidsite
(a)
(b)
Fig. 1. Catalyst 2 structure: a) Coordination environment of Zn(II) in 2. b) 1D hexagonal windows in 1–5
functionalized by different organic groups.
3.1. Screening of catalysts.
In our catalytic strategy, the acetylation of pꢀbromophenol and acetic anhydride in CH2Cl2 was
chosen as a mode reaction to evaluated the catalytic activity of 1–5, and the results are depicted in
Fig.2. It can be seen from the plots that catalyst 2 yields the product almost quantitatively after 10 h,
while the yields for catalysts 1, 3, 4 and 5 were lower than 97, 79, 78 and 74%, respectively. The
results demonstrated that all the five PCPs display good catalytic efficiency toward the acetylation of
phenols because both the coordinatively unsaturated Zn centers (similar to the previous literature27)
and the amide groups are introduces as trigger sites to achieve efficient guest interactions, a powerful
catalytic driving force, and good signal responding communications, similar to the previous report.26
Taken together, the acetylation of phenols were coꢀcatalyzed by Zn Lewis acids and amide groups
synergistically. However, the active differences found among the five PCPs cannot be attributed to the

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differences in diffusion rate, because the five frameworks contained similar pore openings, but must
be due to difference in the catalytic capacity of the active organic groups. Indeed, within the five PCPs,
the coordinatively unsaturated Zn centers can be used as Lewis acid sites to activate reaction
substrates, and the amide groups can behave as baseꢀtype catalytic driving force to promote the
acetylation of phenols due to their two types of hydrogen bonding sites: the ꢀNH moiety acts as an
electron acceptor and the carbonyl group acts as an electron donor. It must be pointed out that, in 2,
the pendent NH2 groups present at the pore surface make 2 show a higher basicity than that of the
other PCPs. Therefore, among the five zinc PCPs, ꢀNH moiety in 2 can form the strongest hydrogen
bond with phenol, resulting in an increase of the nucleophilicity of phenol, which is benefit for the
completion of the acetylation reaction. The results can be explained by the early studies involving the
deuterium isotope effect and kinetic investigations that intermolecular hydrogen bonds play a key role
in the acetylation of phenols.25 Accordingly, hydrogenꢀbonded phenols are better nucleophiles, and
stronger hydrogen bonding results in more nucleophilicity and reactivity. Notably, the organization of
the amide and amine groups in 2 may be precisely adjusted in such a way that they can behave
synergistically as cooperative catalysts, similar to that of our previous report.20 Together with the
coordinatively unsaturated Zn centers and the strongest hydrogen bonding interaction among –CONH,
–NH2 and phenol, the compound 2 displays the highest catalytic activity among the five compounds,
similar to that of the previous reports.28ꢀ30 Meanwhile, the lower activity of 5 might be ascribed to the
decrease of base by the acid pendent COOH groups. Also, the activity observed for 2 catalyzed
acetylation of phenols is superior to those found for [Zn(L)(NO3)2] (10 wt% loading, 98% yield and
24 h).31 Based on the aboveꢀmentioned fact, the following acetylation of phenols was explored by
employing 2 as a heterogeneous catalyst.
Fig. 2. Plots of yields versus time for the acetylation of pꢀbromophenol in CH2Cl2: comparison among
the catalytic performance of 1–5.

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3.2. The optimization of reaction parameters.
The optimization of the reaction conditions such as the effects of catalyst amount and solvents was
carried out in a model acetylation system with 2 as the catalyst, and the results are collected in Table 1.
The increase of the amount of catalyst from 1.0 to 3.0 mol% enhances the product yield from 80 to
99% but a further increase in the amount of catalyst leads to no significant increase in catalytic
activity.
To select the most suitable solvent, experiments with various solvents (CH2Cl2, THF, CH3CN) and
solventꢀfree have been carried out. Although the reaction was found to finish in neat acetic anhydride
in 2 h, that in CH2Cl2 containing 3 equivalents of acetic anhydride was found to be complete in 10 h.
The worst solvent is CH3CN (80% yield) for 48 h.
Table 1. Optimization of parameters of acetylation of phenol catalyst 2.
O
OH
O
Catalyst
(CH3CO)2O
r. t
Br
Br
entry
solvent
catalyst amount (mol%)
Time (h)
yield (%)a
1
2
3
4
5
6
7
solventꢀfree
2
2
2
99
99
83
80
4
dichloromethane
tetrahydrofuran
acetonitrile
10
48
48
6
2
2
solventꢀfree
0%
1%
3%
dichloromethane
dichloromethane
10
10
80
99
a Yield of pure isolated product.
3.3. The scope of acetylation of phenols
The remarkably simple and convenient reaction conditions, easy work up, and high yield for the
transformation prompted us to extend the scope of 2 as heterogeneous catalyst for the acetylation of a
wide variety of phenols. The results are summarized in Table 2. The results reveal that, the acetylation
of various phenols proceeds in a facile manner to give the corresponding aryl acetates in quantitative
yields except the acetylation of 4ꢀphenylphenol. The substrates containing electronꢀwithdrawing

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groups were found to be acetylated faster than those containing electronꢀdonating groups. For example,
compared with the acetylation of pꢀbromophenol with oꢀnitroꢀpꢀmethylphenol, the latter proceeds
faster (10 h vs 8 h, entries 4 and 9, Table 2). This can be explained by the mechanism of acetylation of
phenols. According to Bonner’s studying,32 the oꢀnitroꢀpꢀmethylphenol can form stronger hydrogen
bond with acetic anhydride than that of pꢀbromophenol, and thus behaves as better nucleophiles for
increasing reaction rate to achieve better product yield. Furthermore, in this catalytic system, a
sizeꢀselectivity effect is also observed toward increasingly larger substrate sizes. For example, the
conversion for pꢀphenylphenol with molecular dimension of 9.8 × 4.3 Å is reduced to 12% after 48 h
(Table 2, entry 13). However, a small amount of pyridine is added the above system, which can
promote the reaction to completion. When a larger substrate 2ꢀisobutylꢀ4ꢀphenylnaphthol is present,
the reaction do not work under similar conditions (Table 2, entry 14). Since the pore sizes of 2 are 7.0
× 8.7 Å which exhibits the confined sizes and cavity, the largest substrate (9.9 × 10.0 Å) might be
excluded in the catalyst due to difficulty in diffusion into the channels of 2. This also suggests that the
open active catalytic sites within the PCPs are not readily accessed by this particular substrate.
The assembly of the coordinatively unsaturated Zn centers and FOS into a robust PCPs can not
only avoid their deactivations and but also create confined voids with high density of the accessible
Lewis acid sites and active functional organic sites, resulting in a very efficient catalytic capacity that
is not a simply linear sum of those of the pure components.
3.4. Control experiments
Considering the possibility of those ligands containing amide, or amine, or pyridyl groups to
catalyze
acetylation
of
phenols,
we
performed
control
experiments
with
N4,N4’ꢀdi(pyridinꢀ4ꢀyl)biphenylꢀ4,4’ꢀdicarboxamide or 5ꢀaminoisophthalic acid as catalysts instead of
2. As in the case of 2, the reaction of pꢀbromophenol with acetic anhydride in the presence of L or
H2aip afforded the same products with little lower yields under same reaction conditions (e.g., 20% for
H2aip, 76% for L vs 99% for 2). However, when L and H2aip were used as catalysts, all the acetylation
reaction did not show any selectivity toward the reaction substrates. It is also noted that, the process
involving 2 was heterogeneous, catalytic reaction in the presence of the two aboveꢀmentioned ligands
was partially homogeneous, but without achieving complete dissolution of the two ligands. Therefore,
compared to the two ligands, higher activity of 2 may be caused by much better accessibility of the
catalyst’s active sites. Taken together, these results indicate that the catalytic activity of 2 is superior
to its corresponding components.

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Table 2. Acetylation of various phenols catalyzed by 2
O
OH
O
Catalyst 2 mol%
R
(CH3CO)2O
R
CH2Cl2, r. t
entry
1
substrate
moluclar size
time yielda
OH
14
14
12
10
99
99
99
99
OH
2
3
4
5
6
OH
Cl
OH
OH
Br
14
14
99
99
Cl
OH
OH
7
8
9
12
16
8
99
99
99
Cl
O
OH
OH
NO2
OH
10
11
12
18
24
24
99
98
98
OH
HO
OH
OH
13
OH
48
12

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OH
14
48
trace
a Yield of pure isolated product.
3.5. Heterogeneity of 2
To further verify whether 2 is a true heterogeneous catalyst or not, a procedure similar to that of
our study and others was followed. We performed a controlled experiment, removed the catalyst by
centrifugation, and the catalyst free solution was kept stirring under the same conditions and
monitored by NMR analysis along the time to determine whether metal component dissolved from the
solid structure of 2. After removal of catalyst 2, the conversion within 6 h only increased 2%, which
could be attribute to the background reaction. The result confirms the assumption of a heterogeneous
mechanism.
3.6. Recycle performance of 2.
Finally, in order to perform the catalyst recycling experiment, the used catalyst 2 (separated by
filtration) was washed with ethyl acetate and dried at 80°C under vacuum. It was then reused for the
acetylation of phenols as described above. The catalyst 2 was recycled in four consecutive
experiments, and its activity remained essentially the same. As mentioned before, a corresponding
product yield of 99% is achieved in the first run. In the second, third, and fourth run, the product
yields, determined under the same conditions, decrease to 97%, 96%, and 94%, respectively. We have
performed powder Xꢀray diffraction of catalyst 2 before and after the reaction and observed only a
slight difference in peak after the reaction, as depicted in Fig. 3. These experiments are indicative that
the frameworks essentially remains intact during the catalytic process. The recycle characterization
and structural stabilities of other compounds are shown in Fig. S2ꢀS5 (see ESI† for details).

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Fig. 3. XRD patterns of 2: (a) the simulated XRD pattern, (b) the experimental one, (c) after the first recycling,
(d) after the second recycling, and (e) after the fourth recycling.
Conclusions
In conclusion, all the PCPs 1–5 exhibit good sizeꢀselective catalysis for the acetylation of phenols.
They effectively catalyzes the reaction of various phenols with acetic anhydride, and producing the
corresponding phenyl acetates in excellent yields, depending on the nucleophile of the substrates.
Among them, 2 is the most effective catalyst. Most importantly, the present study also provides further
evidence that the porous coordination polymers with two different functionalities decorated the porous
wall can be utilized as effective heterogeneous cooperative catalysts.
ACKNOWLEDGMENT
We gratefully acknowledges the financial support from the Natural Science Foundation of China
(Grant No. 21272109, 21372112, and 21371089).
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