A novel adenine-based zinc(II) metal-organic framework featuring the Lewis basic sites for heterogeneous catalysis

Article abstract of DOI:10.1016/j.inoche.2016.11.011

Metal-organic frameworks (MOFs), as a new sort of crystalline materials, have attracted lots of interest in many applications during the past decades. Recently, many efforts have been focused on the development of MOFs as heterogeneous catalysis. In this

Full text of DOI:10.1016/j.inoche.2016.11.011

A novel adenine-based zinc(II) metal-organic framework featuring the Lewis
basic sites for heterogeneous catalysis
Shixing Zhang, Hongming He, Fuxing Sun, Nian Zhao, Jianshi Du,
Qinhe Pan, Guangshan Zhu
PII:
DOI:
Reference:
S1387-7003(16)30288-X
doi: 10.1016/j.inoche.2016.11.011
INOCHE 6490
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Inorganic Chemistry Communications
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Accepted date:
4 September 2016
11 November 2016
20 November 2016
Please cite this article as: Shixing Zhang, Hongming He, Fuxing Sun, Nian Zhao, Jianshi
Du, Qinhe Pan, Guangshan Zhu, A novel adenine-based zinc(II) metal-organic framework
featuring the Lewis basic sites for heterogeneous catalysis, Inorganic Chemistry Communi-
cations (2016), doi: 10.1016/j.inoche.2016.11.011
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A novel adenine-based zinc(II) metal-organic framework
featuring the Lewis basic sites for heterogeneous catalysis
Shixing Zhang,a Hongming He,a Fuxing Sun,a Nian Zhao,a Jianshi Du,b* Qinhe Panc*
and Guangshan Zhua
a State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin
University, Changchun 130012, China.
b China Japan Union Hospital, Jilin University, Changchun, 130031, China.
c College of materials and chemical engineering, Hainan University, China.
Corresponding Authors
Fax: +86-431-85168331
E-mail address: zhugs@jlu.edu.cn;
djs3043@126.com;
panqinhe@163.com

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Abstract
Metal-organic frameworks (MOFs), as a new sort of crystalline materials, have
attracted lots of interest in many applications during the past decades. Recently, many
efforts have been focused on the development of MOFs as heterogeneous catalysis. In
this communication, we selected adenine (ad) and tetracarboxylic acid, namely
5,5’-(1,3,6,8-tetraoxobenzo[Imn][3,8]phenanthroline-2,7-diyl)bis-1,3-benzenedicarbo
xylic acid (H4L), as organic linkers to assemble with Zn(II) ions to construct a novel
adenine-based porous MOF. There are three different inorganic clusters in the
framework, including ZnO2N2, Zn2O2N6, and ZnO5N clusters. Interesting, the
resultant porous MOF, namely [H2N(CH3)2]•[Zn4(L)1.5(ad)3(H2O)2]•4DMF, retains
free amino groups in the framework, which can be served as Lewis basic sites to
catalyse Knoevenagel condensation reaction. The catalytic study exhibits that the
as-synthesized MOF with free amino groups can be used as heterogeneous catalysis
with remarkable catalytic efforts and good recycle.
Keywords: metal-organic framework; adenine; amino group; catalysis; Knoevenagel
reaction.
Metal-organic frameworks (MOFs),[1-3] as a burgeoning sort of crystalline
materials, have attracted lots of interest in the past decades, not only due to their
diverse structures, but also because of their widely potential applications, including
magnetism,[4-6] gas sorption,[7-10] drug delivery,[11-15] chemical sensor,[16-20]
optical device,[21-24] and heterogeneous catalysis[25-28] etc. To date, lots of
structures have been successfully prepared through different ligands, metal ions, and
synthetic conditions. From the previous investigations, most of organic ligands in
MOFs are multicarboxylic acids. Nevertheless, more and more scientists pay attention
to the aromatic N-heterocycles, such as adenine, imidazole, triazole, and tetrazolate,
because they are able to generate lots of interesting structures and unique
properties.[29-34] Particularly, adenine can be mixed with multicarboxylic acid
ligands to prepare many available materials, such as bio-MOFs in Rosi’s group.[35-40]

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However, only two previous researches have been reported to obtain adenine-based
porous MOFs by mixing adenine and tetracarboxylic acid ligands.[39, 40] Therefore,
it remains a significant challenge to develop and synthesize such adenine-based
MOFs.
Inspired by the investigations on MOFs with basic functional groups, they always
can be applied as catalysts for the basic catalytic reactions, such as Knoevenagel
condensation reaction. Recently, some MOFs have been reported to catalyse
Knoevenagel condensation reaction.[41-48] Adenine, as an aromatic N-heterocycles,
not only has lots of nitrogen atoms to coordinate with metal ions, but also contains
amino groups as Lewis basic sites. As we know, no previous investigations have been
reported to explore adenine-based MOFs to catalyse the Knoevenagel condensation
reaction. Evidently, it is a quite interesting and big challenge to form new
adenine-based MOFs as basic catalysts for this reaction.
Herein, we selected a tetracarboxylic acid ligand as an organic ligand, namely
5,5’-(1,3,6,8-tetraoxobenzo[Imn][3,8]phenanthroline-2,7-diyl)bis-1,3-benzenedicarbo
xylic acid (H4L).[49] H4L and adenine (ad) are both successfully connected with Zn(II)
ions
to
obtain
a
novel
adenine-based
porous
MOF,
namely
[H2N(CH3)2]•[Zn4(L)1.5(ad)3(H2O)2)]•4DMF (denoted as JUC-188, DMF = N,
N-dimethylformamide). As we expect, the resultant MOF retains free amino groups
(-NH2) as Lewis basic sites in the framework, which can be used as heterogeneous
catalysis to catalyse Knoevenagel condensation reaction. In addition, there are three
different inorganic clusters in JUC-188, including ZnO2N2, Zn2O2N6, and ZnO5N
clusters. The yellow crystals of JUC-188 were successfully obtained via heating these
organic ligands and Zn(NO3)2•6H2O in a mixture of DMF and H2O at 135 °C after 3
days.[50]
From the single-crystal X-ray diffraction analysis,[51] it exhibits that JUC-188
crystallizes in the triclinic space group P-1 and the asymmetric unit contains four
Zn(II) ions, one and a half L4- organic ligands, three adenine ligands and two terminal

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coordinated water molecules (Fig. S1). There are three different Zn(II) clusters in the
structure. Zn1 is coordinated with three nitrogen atoms from three adenine ligands
(Zn1-N = 1.980-2.031 Å), and one oxygen atom from a carboxylic acid group (Zn1-O
= 1.943 Å). In addition, Zn2 has the same coordination environment with Zn1. Zn1
and Zn2 are further linked by adenine linkers to form a binuclear Zn2O2N6 cluster (Fig.
1a). Meanwhile, Zn3 exhibits a distorted octahedral ZnO5N geometry with five
oxygen atoms (Zn3-O = 1.990-2.278 Å) from two carboxylic acids and two
coordinated water molecules, and one nitrogen atom from an adenine molecule
(Zn3-N = 2.059 Å) (Fig. 1b). As shown in Fig. 1c, the other Zn4 is bound with two
oxygen atoms from two carboxylates (Zn4-O = 1.982-2.006 Å) and two nitrogen
atoms from two different adenine molecules (Zn4-N = 2.030-2.037 Å). On the other
hand, the L4- ligand is linked with four different Zn(II) ions (Fig. 1d) and the adenine
ligand is coordinated with three different Zn(II) ions (Fig. 1e). Therefore, the different
Zn(II) clusters can be further linked with the bridging ligands to construct a
three-dimensional (3D) porous coordination framework with widow sizes about 7.5 ×
9 Å (Fig. 1f and Fig. 1g). The guest molecules can be determined from the TGA and
elemental analyses (C, H and N). Interesting, the resultant porous MOF retains free
amino groups as Lewis basic sites in the framework for heterogeneous catalysis (Fig.
1g).
Fig. 1. (a, b, and c) The coordination environments of these Zn(II) ions; the coordination mode of

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L4- ligands (d) and adenine ligands (e); (f and g) viewing the 3D porous framework in different
directions. (The hydrogen atoms are omitted for clarity, and C, gray; N, blue; O, red; Zn, green)
Powder X-ray diffraction (PXRD) pattern of JUC-188 was measured at room
temperature. The characteristic peaks of the as-synthesized sample are closely
matched well with those in the simulated patterns from the single crystal structure
(Fig. 2a). The results confirmed the phase purity of the as-synthesized bulk crystalline
materials. From the thermogravimetric analyses (TGA) results, the as-synthesized
JUC-188 showed a gradual weight loss before 200 °C about 19.78%, corresponding to
the loss of one [H2N(CH3)2] cation, two coordinated water and four DMF guest
molecules (calculated 19.44%) (Fig. 2b). The main framework of JUC-188 began to
collapse with the increasing of heating temperature. The as-synthesized samples were
exchanged with MeOH about three days, and activated at room temperature to obtain
the activated samples. However, the activated samples didn’t have any N2 adsorption,
which was mainly ascribed to the collapse of the skeletal structure after removing the
solvent molecules (Fig. S2). In addition, the PXRD pattern further indicated that the
main framework of the activated samples was collapsed (Fig. S3). As shown in Fig.
S4, the characteristic peaks of adenine and H4L ligands were both found in the FT-IR
spectrum of JUC-188, which undoubtedly exhibited that both organic ligands
coexisted in JUC-188.
Fig. 2. (a) The simulated (black) and as-synthesized (red) PXRD patterns of JUC-188; (b) the
TGA curve of as-synthesized JUC-188.

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Furthermore, we have been systematically studied the catalytic properties of
JUC-188 for the basic catalytic Knoevenagel condensation (Table 1), which is mainly
ascribed to the free amino groups in JUC-188. Before the catalytic reaction, JUC-188
was soaked in toluene for one day to exchange the guest molecules. In a typical
experiment, a mixture of substrate (0.1 mmol), malononitrile (0.11 mmol), and
toluene (2.5 mL) with 10 mg of the catalyst (JUC-188) was putted into a 10 ml glass
reactor, which was stirred at 80 °C. The corresponding products can be monitored by
GC-MS. As shown in Table 1, it summarizes the results of Knoevenagel condensation
of malononitrile with different substrates in the presence of JUC-188 as a solid
catalyst. The results demonstrated that JUC-188 can effectively catalyse small
aldehydes as substrates into their corresponding products (Table 1, entries 1-5). When
1-naphthyl or anthryl groups were introduced into the phenyl ring and prolonged the
reaction time, the yields of the corresponding products were just about 43% (entry 6)
and 21% (entry 7) under the similar reaction condition. Hence, the catalytic efficiency
depends on the molecule interactions between the substrate and the catalytic sites. The
low yields of large-sized substitute molecules may be mainly ascribed to the restricted
diffusion of large molecules into the framework of JUC-188. The results clearly
demonstrate that JUC-188 can serve as an efficient basic catalyst for the Knoevenagel
condensation reaction with a size selective property. Compared with the other
reported MOFs,[41-48] JUC-188 exhibits both high catalytic performance and size
selective property. Furthermore, it also shows that adenine ligands can be applied to
construct MOFs with the Lewis basic sites (-NH2) as heterogeneous catalysis.
Table 1. Knoevenagel condensation of malononitrile with different substrates in the
presence of JUC-188 as a solid catalyst.
Entry
R
Time (h)
Yield (%)

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7
1
2
3
4
5
6
7
Ph
3
97
95
97
98
97
43
21
4-MePh
4-NO2Ph
4-FPh
3
2.5
2.5
2.5
6
4-OMePh
1-naphthyl
anthryl
6
Meanwhile, it did not have any detectable leaching of Zn2+ ions in the reaction
solution after removal of the catalyst by inductively coupled plasma (ICP) analysis.
The results can confirm the essence of the heterogeneous catalysis of JUC-188. In
addition, the recovered catalyst could be reused after filtration from the catalytic
reaction, and washed several times with fresh toluene. As shown in Fig. 3a, the
recovered catalyst can be reused at least four cycles almost without any loss of
activity. In addition, the main framework of JUC-188 can be successfully reserved
after the catalytic reactions, which can be confirmed by PXRD patterns (Fig. 3b).
Fig. 3. The reproducibility (a) and the corresponding PXRD patterns (b) of JUC-188.

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In summary, we successfully synthesized a novel adenine-based porous zinc(II)
MOF featuring the Lewis basic sites (-NH2) as heterogeneous catalysis for the basic
catalytic Knoevenagel condensation reaction. Herein, we anticipate that it is able to
provide a promising approach to obtain such adenine-based MOFs as basic catalysts.
Acknowledgments
We are grateful to the financial support from National Basic Research Program of
China (973 Program, 2012CB821703 and 2014CB931804), Major International
(Regional) and Joint Research Project of NSFC (21120102034), and NSFC Projects
(21531003 and 81373344).
Appendix A. Supplementary data
The asymmetric unit, XRD, N2 sorption and FT-IR spectra are listed in the
supporting information. The CCDC reference numbers 1501692.
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[50]Preparation of [H2N(CH3)2]•[Zn4(L)1.5(ad)3(H2O)2]•4DMF: A mixture of
Zn(NO3)2•6H2O (30 mg, 0.1 mmol), H4L (15 mg, 0.025 mmol), adenine (8 mg,
0.06 mmol) was added in a mixture solution of DMF (4 mL) and H2O (0.5 mL).
Then 0.1 mL HNO3 was added in the mixture. The obtain mixture was sealed into

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a 20 ml Teflon-lined autoclave and heated at 135 °C for three days. The
as-synthesized samples can be obtained after filtration at room temperature.
Finally, the yellow block-shaped crystals were washed with fresh DMF and dried
in air. Yield: 52.9% (based on the H4L). Element analysis (%) Calc. for C74 H67
O24 N23 Zn4 (1921.9): C, 46.20; H, 3.49; N, 16.75. Found: C, 46.57; H, 3.44; N,
16.41. Selected FT-IR data (KBr pellet cm-1): 3365 (br), 1714 (s), 1656 (s), 1572
(s), 1449 (s), 1344 (s), 1248 (s), 1157 (s), 1099 (s), 985 (s), 780 (s), 767 (s), 744
(s), 726 (s), 652 (s), 562 (s), 420 (s).
[51]Crystal data: C74 H67 O24 N23 Zn4: Fw= 1921.9, Triclinic, space group P-1, a =
12.1467(4), b = 18.1091(7), c = 24.1338(9) Å, α = 107.541(2), β = 98.670(2), γ =
1
91.332(2), V = 4990.6(3) Å3, Z = 2, Dc = 1.053 g cm-3, μ = 1.594 mm- , R(int) =
0.0967, Nref = 17002, R1 = 0.0500, wR2 = 0.1148 [I > 2σ(I)], R1 = 0.0763, wR2 =
0.1232 [all data], GOF= 0.925. The crystal data of JUC-188 was collected on a
Bruker SMART CCD diffractometer with Cu Kα (λ = 1.54178 Å) radiation at
100(2) K. The crystal structure was solved by direct methods, which can be
further refined by the full-matrix least-squares on F2 of the crystal data using the
Shelxtl-97 program package.[52] All non-hydrogen atoms were also refined
through anisotropic displacement parameters. However, the framework has lots of
disordered solvent molecules, their corresponding peaks were successfully
removed by using the PLATON/SQUEEZE.[53] In addition, the selected bond
lengths and angles of JUC-188 are both listed in Table S1.
[52]G. M. Sheldrick, SHELXTL 97, Program for Crystal Structure Solution, Univ. of
Göttingen, Göttingen (Germany) (1997).
[53]A. L. Spek, Single-crystal structure validation with the program PLATON, J.
Appl. Cryst. 36 (2003) 7-13.

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Graphical Abstract-Pictogram
A novel adenine-based zinc(II) MOF has been formed by mixing ligands, which can
be used as base catalyst for the Knoevenagel condensation reaction.

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Highlights
1. A novel adenine-based MOF has been obtained by mixing ligands.
2. The resultant MOF retains free amino groups in the framework,
3. It can be used as heterogeneous catalysis for the Knoevenagel condensation
reaction.