Structures and photocatalytic activities of metal-organic frameworks derived from rigid aromatic dicarboxylate acids and flexible imidazole-based linkers

Article abstract of DOI:10.1016/j.inoche.2011.05.017

Three metal-organic frameworks based on the connectivity co-effect between rigid aromatic dicarboxylic acids and flexible linear linkers have been synthesized and characterized. [Cd(3-NO₂-bdc)(bbi)]_n (1) and [Co(3-NO₂-bdc)(bbi)]_n (2) (3-NO₂-bdcH ₂ = 3-nitro-1,2-benzenedicarboxylic acid, bbi = 1,1′-(1,4- butanediyl)bis(imidazole)) are isostructural, which exhibit two-dimensional (2D) layer frameworks; [Co(3,5-pdc)(bbi)2H₂O]_n (3) (3,5-pdcH_{2 =} pyridine-3,5-dicarboxylic acid) has 2D 6-connected network with a Sch?fli symbol of (3⁴4⁶5³). In addition, compounds 1 and 2 exhibit good photocatalytic activities under visible irradiation, and their different photocatalytic activities have also been analyzed based on the diffuse-reflectance UV/Vis spectra.

Full text of DOI:10.1016/j.inoche.2011.05.017

Inorganic Chemistry Communications 14 (2011) 1347–1351
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Structures and photocatalytic activities of metal-organic frameworks derived from
rigid aromatic dicarboxylate acids and flexible imidazole-based linkers
a,
Hua Qu a, Ling Qiu b, Xiao-Ke Leng a, Miao-Miao Wang a, She-Ming Lan a, Li-Li Wen a, , Dong-Feng Li
⁎
⁎
a
Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, P.R. China
b
Key Laboratory of Nuclear Medicine, Ministry of Health, Jiangsu Institute of Nuclear Medicine, Wuxi 214063, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three metal-organic frameworks based on the connectivity co-effect between rigid aromatic dicarboxylic
acids and flexible linear linkers have been synthesized and characterized. [Cd(3-NO2-bdc)(bbi)]n (1) and [Co
(3-NO2-bdc)(bbi)]n (2) (3-NO2-bdcH2 =3-nitro-1,2-benzenedicarboxylic acid, bbi=1,1′-(1,4-butanediyl)bis
(imidazole)) are isostructural, which exhibit two-dimensional (2D) layer frameworks; [Co(3,5-pdc)(bbi)
2H2O]n (3) (3,5-pdcH2 = pyridine-3,5-dicarboxylic acid) has 2D 6-connected network with a Schäfli symbol
of (344653). In addition, compounds 1 and 2 exhibit good photocatalytic activities under visible irradiation,
and their different photocatalytic activities have also been analyzed based on the diffuse-reflectance UV/Vis
spectra.
Received 27 April 2011
Accepted 19 May 2011
Available online 27 May 2011
Keywords:
Metal-organic frameworks
Photocatalytic activity
Diffuse-reflectance UV/Vis spectra
© 2011 Elsevier B.V. All rights reserved.
Metal-organic frameworks (MOFs) are a relatively new class of
crystalline coordination polymers, which have potential in a myriad of
applications, including gas storage, chemical separation, heteroge-
neous catalysis and optical, electronic, magnetic materials [1–15].
Nowadays, much effort has also been devoted to developing new
photocatalytic materials based on metal-organic frameworks, which
is motivated largely by a demand for solving pollution problems in
view of their potential applications in the green degradation of
organic pollutants [16–23].
In this regard, our recent study has been mainly focused on the
synthesis and characterization of new MOFs from accessible raw
materials, which show rich structural features and encouraging
photocatalytic properties [24,25]. As an extension of our work, herein,
3-nitro-1,2-benzenedicarboxylic acid (3-NO2-bdcH2) or pyridine-3,5-
dicarboxylic acid (3,5-pdcH2) were chosen as the starting materials
together with divalent metal ions, with the aid of flexible bidentate
linker 1,1′-(1,4-butanediyl)bis(imidazole) (bbi). Utilizing hydrother-
mal technique, three new MOFs, [Cd(3-NO2-bdc)(bbi)]n (1) , [Co(3-
NO2-bdc)(bbi)]n (2) and [Co(3,5-pdc)(bbi) 2H2O]n (3) have been
obtained [26,27]. In addition, photocatalytic activities of compounds 1
and 2 under visible irradiation were studied, and their different
activities have been analyzed based on the diffuse-reflectance UV/Vis
spectra.
Compound 1 crystallizes in the orthorhombic Pbca space group. As
shown in Fig. 1a, Cd1(II) is coordinated to three carboxylate oxygen
atoms from two inequivalent 3-NO2-bdc2− and two different bbi
nitrogen atoms. Furthermore, the Cd1–O2 distance is 2.680 Å,
suggesting a non-negligible interaction with the uncoordinated
carboxylate oxygen atom, which can be described as a semi-chelating
coordination mode [28]. Hence, the coordination environment of
Cd1 atom may be also regarded as a distorted trigonal prismatic
coordination sphere. The average Cd–O and Cd–N distances are 2.335
and 2.231 Å, respectively, both of which are in the normal range. Each
fully deprotonated 3-NO2-bdc2− ligand coordinates to two Cd atoms,
with two carboxylate groups adopting μ1-η1: η1 and μ1-η1: η0 modes.
The 3-NO2-bdc2− ligand thus links Cd ions to form a 1D zigzag chain
along b-axis, as illustrated in Fig. S1. The 1D chain is further stacked
with bidentate bbi spacers, thus giving rise to the formation of a 2D
layer framework (Fig. 1b). In 1, the bbi spacers adopt cis conformation
with imidazole groups twisted by 96º, which establish physical bridge
between Cd atoms, imposing Cd···Cd separations of 8.875 Å. It is
worth mentioning that compound 1 is a rare MOF in which bbi ligands
can adopt cis geometry; in the case of the previously reported
coordination framework based on bbi, most bbi spacers exhibit trans
geometry [29–31].
The weaker nonclassical hydrogen bonds were observed between
C–H moieties and the coordinated carboxylate oxygen atoms (O1, O3,
O4) as well as the uncoordinated carboxylate oxygen atom (O2),
with the distance varying from 2.996(4) to 3.280(4)Å. The extensive
hydrogen bonds may contribute for the stability of the MOF.
Similar cell parameters with the same space group Pbca and the
results of crystallographic analysis confirm that 1 and 2 are iso-
structural. In compound 2, the Co(II) ion is in a slightly distorted
⁎
Corresponding authors at: Key Laboratory of Pesticide & Chemical Biology of
Ministry of Education, College of Chemistry, Central China Normal University, Wuhan
430079, PR China. Tel.: +86 27 67862900; fax: +86 27 67867232.
E-mail addresses: wenlili@mail.ccnu.edu.cn (L.-L. Wen), dfli@mail.ccnu.edu.cn
(D.-F. Li).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.017

1348
H. Qu et al. / Inorganic Chemistry Communications 14 (2011) 1347–1351
A better insight into the nature of this framework can be acquired
by using topological analysis. Each Co node is coordinated by five
bridging ligands but linked to six nearest neighbors, ligands 3,5-pdc2−
and bbi can be simplified to be connectors; therefore, the combination
of nodes and connectors suggests a 6-connected network with a
Schäfli symbol of (344653) (Fig. 2c).
Herein, we selected an anionic organic dye X3B (Scheme 1), as a
model pollutant in aqueous media to evaluate the photocatalytic
effectiveness of compounds 1, 2 and 3, considering that X3B is
commonly used as a representative of widespread organic dyes that
are very difficult to decompose in waste streams under UV or visible-
light irradiation [33,34]. The photodegradation experiment under
visible light irradiation was carried out after the dark adsorption
equilibrium achieved according to the previous reported literature
[24,25]. In addition, control experiments on the photodegradation of
X3B have been carried out. The distinctly shortened degradation time
compared with the control experiments indicates that both catalyst 1
and 2 are active for the decomposition of X3B in the presence of visible
light irradiation. When pseudo-first-order kinetics was fitted with the
experimental data, for 1 and 2, the rate constant under visible light
irradiation was found to be 0.0875 and 0.1137 h−1, respectively
(Figs. 3 and 4, Figs.S4 and S5). Obviously, the X3B is degraded about 60
and 80% in the presence of 1 and 2 after 9 h irradiation, which is
comparable with those of the reported uranyl-organic assembly
compounds [17]. In addition, the number of moles for degradated X3B
per gram metal atom in the presence of 1 and 2 is 6.64×10−6 and
16.61×10−6 mol/g, respectively (Fig. S6).
However, the decomposition rate of X3B under visible light
conditions for compound 3 is much slower than those of compounds
1 and 2, as depicted in Fig. S7. Therefore, only the photoactivities of
compounds 1 and 2 were studied in detail.
In order to study the photocatalytic reaction mechanism, the
photodegradation of X3B was carried out in the presence of tert-butyl
alcohol (TBA), a widely used ·OH scavenger [35,36]. The presence of
TBA greatly depressed the photodegradation rate of X3B on catalyst 1
(Fig. 3, curve b) and 2 (Fig. 4, curve b), that is, the relevant rate
constant for 1 and 2 was sharply individually decreased from 0.0875
to 0.0333 h−1 and from 0.1137 to 0.0340 h−1 in the presence of TBA
under visible light. The ·OH quenching experiment result suggests
that the photodegradation of X3B on catalyst 1 and 2 is predominately
through attack of ·OH radicals, which is similar to the photocatalytic
reaction mechanism reported in our previous study. To exclude the
possibility that the photocatalytic properties of 1 and 2 result from
dissolved molecular or oligomeric fragments of solid catalysis in the
photocatalytic process, another control experiments were conducted.
The reaction suspensions after 9 h of irradiation were filtered to
remove the solid catalyst particles, and fresh X3B was added into the
respective filtrates for catalysis testing. Without solid catalyst in the
reaction system, the fresh X3B was not degraded during another 9 h
of irradiation, which indicates that the solution contains no photo-
catalytically active fragments. Clearly the photocatalytic activities
arise solely from the solid 1 and 2. In addition, the stability of
compounds 1 and 2 was monitored using PXRD during the course of
photocatalytic reactions (Figs. S8 and S9). After photocatalysis,
compounds 1 and 2 display a powder XRD pattern nearly identical
to that of the original compounds, implying the stability towards
photocatalysis for the complexes is good.
Fig. 1. (a) Coordination environments of Cd atoms with hydrogen atoms omitted for
clarity; (b) the 2D layer structure of 1.
tetrahedral environment, coordinated by two carboxylate oxygen
atoms from two different 3-NO2-bdc2− and two different bbi nitrogen
atoms (Fig. S1). The average Co–O and Co–N distances are 2.001 and
2.030 Å, respectively. Each fully deprotonated 3-NO2-bdc2 ligand
coordinates to two Co atoms, with both carboxylate groups adopting
μ1-η1: η0 modes. The 3-NO2-bdc2− ligand connects Co ions to form a
1D chain, which is further stacked with cis bbi bridges, resulting in a
2D layer framework similar to 1(Fig. S2). The resulting 2D structure is
cross-linked by hydrogen-bond interactions between C-H groups
from bbi and carboxylate oxygen atoms, thus leading to the formation
of a 3D supramolecular architecture.
Compound 3 crystallizes in the monoclinic P21/n space group. As
displayed in Fig. 2a, the Co(II) metal center in a distorted trigonal
bipyramidal coordination sphere, which is defined by two oxygen
donors and one nitrogen atom from three distinct 3,5-pdc2− ligands
in the equatorial positions while axial positions are furnished by two
nitrogen donors from two individual bbi ligands. The average Co–O
and Co–N distances are 2.077 and 2.125 Å; both are somewhat longer
to those seen in 2, which is consistent with the fact that bond lengths
in a tetrahedral geometry are generally shorter than those in other
geometries. Each fully deprotonated 3,5-pdc2− anions act as μ3-bridge
linking three Co centers via two unidentate carboxylate groups and
one nitrogen moiety, leading to a 2D porous layer (Fig. S3). In
addition, it should be noted that bbi spacers establish physical bridge
between Co atoms with Co···Co separations of 7.491 Å, resulting in
the formation of a complicated 2D structure with channels along a-
and b- axes, which are occupied by water molecules (Fig. 2b). The
effective free volume of 3 was calculated by PLATON analysis as 13.9%
of the crystal volume (268.8 out of the 1933.6 Å3 unit cell volume),
where water molecules reside as labile ligands [32].
Although 1 and 2 are isostructural, different central metal ions
between 1 and 2 may lead to distinct bandgap size, which may give
rise to the discrepancy in their photocatalytic activity. The diffuse-
reflectance UV/Vis spectra reveal that solid 1 and 2 have different
absorption features (Fig. 5). The main UV absorption bands are around
383(347), 391 (346) nm for 1 and 2, respectively, which can be
assigned to ligand-to-metal charge transfer (LMCT). To obtain the
precise values of band gap from the absorption edges, the point of
inflection in the first derivatives of the absorption spectrum was used.

H. Qu et al. / Inorganic Chemistry Communications 14 (2011) 1347–1351
1349
a
b
c
Fig. 2. (a) Coordination environments of Co atoms with hydrogen atoms omitted for clarity; (b) the 2D porous structure of 3; (c) The topology structure of 3.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
The values of the band gap obtained from corresponding transitions
are 3.23 and 3.17 eV for 1 and 2, respectively. In the case of 2, one clear
additional peak was observed at 646 nm, which probably originate
from the d-d spin -allowed transition of the d7 (Co2+) ion. Clearly, the
a
b
Cl
N
N
OH
HN
c
N
Cl
N
N
0
1
2
3
4
5
6
7
8
9
Time/h
SO3Na
NaO3S
Fig. 3. Control experiments on the photodegradation of X3B. (a) X3B/compound 1/dark;
(b) X3B/compound 1/tert-butyl alcohol/visible light; (c) X3B/compound 1/visible light.
Scheme 1. Molecular structure of X3B.

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H. Qu et al. / Inorganic Chemistry Communications 14 (2011) 1347–1351
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
different photocatalytic activities have also been analyzed based on
the diffuse-reflectance UV/Vis spectra.
a
Acknowledgments
b
This work was financially supported by the National Nature
Science Foundation of China (Nos. 20801021), Program for New
Century Excellent Talents in University of China (NCET-10-040),
Program for Distinguished Young Scientist of Hubei Province
(2010CDA087), CCNU from the Colleges' Basic Research and Opera-
tion of MOE, Program for Innovation Group of Hubei Province of China
(2009CDA048) and the Program for Changjiang Scholars and
Innovative Research Team in University (PCSIRT, No. IRT0953).
c
0
1
2
3
4
5
6
7
8
9
Appendix A. Supplementary material
Time/h
Crystallographic data (CIF file) have been deposited at the
Cambridge Crystallographic Data Center, CCDC Nos. 765739 for 1,
765738 (for 2), and 765737 (for 3). These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre viawww.
ccdc.cam.ac.uk/ data_request/cif. The structure analysis of compound
2, control experiments on the photodegradation of X3B for compound
3, TG curves, fluorescent emission spectra of complex 1 in solid state
at room temperature, photocatalytic experiments details, the pseudo-
first-order kinetics fitted with the experimental data, and time
conversion plots of X3B disappearance per gram metal atom in the
presence of 1 and 2 are shown in the supporting information.
Supplementary data to this article can be found online at doi:10.1016/
j.inoche.2011.05.017.
Fig. 4. Control experiments on the photodegradation of X3B. (a) X3B/compound 2/dark;
(b) X3B/compound 2/tert-butyl alcohol/visible light; (c) X3B/compound 2/visible light.
band gaps of 1 and 2 follow the order 1N2 and the degradation rate of
X3B follow the reverse order.
As shown in Figs. S10–S12, compounds 1 and 2 exhibit similar
thermal decomposition behavior, which are respectively stable up to
240 and 269 °C where the organic groups start to decompose, and
consecutive decompositions suggest the total destruction of the
framework, then residue of CdO (obsd 27.17%, calcd 25.09%) for 1 and
CoO (obsd 17.67%, calcd 16.35%) for 2 remained. For 3, the weight loss
of 8.11% below 85 °C (calcd 7.99%) corresponds to the loss of two free
water molecules per formula. A plateau region is observed for 3 from
188 to 317 °C and consecutive decompositions suggest the total
destruction of the framework.
The solid-state excitation emission spectra of 1 have been studied
at room temperature (Fig. S13). The strongest emission peak for the
free ligand 3-NO2-bdcH2 is at 420 nm with the excitation peak at
235 nm, which is attributed to the π*–n transitions [37]. The strongest
excitation peaks for 1 are at 359 nm and emission spectra mainly
show strong peaks at 388 nm. Therefore, it is reasonable to ascribe the
luminescence of compound 1 to be an intraligand transition.
In summary, three metal-organic frameworks based on the
connectivity co-effect between rigid 3-NO2-bdcH2 (or 3,5-pdcH2)
and flexible linear ligands bbi have been synthesized and character-
ized. 1 and 2 are isostructural, which exhibit two-dimensional (2D)
layer framework; compound 3 has 2D 6-connected network with a
Schäfli symbol of (344653). In addition, compounds 1 and 2 exhibit
good photocatalytic activities under visible irradiation, and their
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[26] Synthesis of [Cd(3-NO2-bdc)(bbi)]n (1). A mixture of Cd(NO3)2 4H2O (0.0623 g,
0.2 mmol), 3-NO2-bdcH2 (0.0423 g, 0.20 mmol), bbi (0.0389 g, 0.20 mmol), NaOH
(0.0159 g, 0.40 mmol) and H2O (3 mL) was placed in a parr Teflon-lined stainless
steel vessel (25 cm3), and then the vessel was sealed and heated at 140 °C for 3 days.
After the mixture was slowly cooled to room temperature, coloress crystals of 1 were
obtained (yield: 36% based on Cd). IR (KBr) cm–1: 3438 m, 3131 m, 2933 m, 1599 s,
1525 s, 1458 s, 1392 s, 1350 s, 1293 m, 1242 m, 1157w, 1111 s, 1090 s, 1073w,
1034w, 942 s, 925 s, 852 s, 835 s, 789 s, 764 s, 713 s, 656 s, 626 m, 552 m, 451 m,
421 m. Synthesis of [Co(3-NO2-bdc)(bbi)]n (2). Compound 2 was prepared in the
same way as 1, using Co(Ac)2·4H2O (0.2 mmol) instead of Cd(NO3)2 4H2O at 130 °C
for 3 days. Purple crystals of 2 were obtained (yield: 45% based on Co). IR (KBr) cm–1
3421 s, 3119 m, 1622 s, 1593 s, 1561 m, 1525 s, 1460 s, 1360 s, 1294 m, 1254 m,
: