Six new metal-organic frameworks with multi-carboxylic acids and imidazole-based spacers: Syntheses, structures and properties

Article abstract of DOI:10.1039/c1dt11130b

Six new metal-organic frameworks [Cu(obba)(bimb)·(obbaH ₂)]_n (1), [Cu(obba)(bimb)]_n (2), [Zn ₂(obba)₂(bimb)₂(DMF)₂(H ₂O)_3.5]_n (3), [Ni₃(2,2′,4, 4′-bptcH)₂(bimb)₂(H₂O) ₂·(H₂O)₂]_n (4), [Ni ₂(bimb)₃(H₂O)₆·(aobtc) ·(DMF)₂·(H₂O)₂]_n (5) and [Cd(3,3′,4,4′-bptcH₂)(H₂O)·(bimb)] _n (6), were obtained by reactions of 4,4′-bis(1-imidazolyl) biphenyl (bimb) and multi-carboxylic acids of 4,4′-oxybis(benzoic acid) (obbaH₂), 2,2′,4,4′-biphenyltetracarboxylate acid (2,2′,4,4′-bptcH₄), azoxybenzene-3,3′,5,5′- tetracarboxylic acid (aobtcH₄), and 3,3′,4,4′- biphenyltetracarboxylate acid (3,3′,4,4′-bptcH₄) with corresponding metal salts under hydro/solvothermal conditions, respectively. Complexes 1-3 have entangled structures with different topologies: 1 is a 3-fold interpenetrating NbO three-dimensional (3D) network; 2 is a 3-fold interpenetrating dmp 3D net; 3 is a 6-fold interpenetrating dia 3D chiral net containing rare 1D helical chains with the same handedness. Complex 4 is an uninodal 6-connected network with a Scha?fli symbol of (4⁸6 ⁴8³) based on the trinuclear Ni(ii) subunits, while complexes 5 and 6 are 1D chains. Interestingly, compound 6 represents the rare example of MOFs that exhibit high photocatalytic activity for dye degradation under visible light and shows good stability towards photocatalysis. Complexes 3 and 6 exhibit intense blue emissions in the solid state at room temperature whereas 3 appears to be a good candidate of novel hybrid inorganic-organic NLO material. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt11130b

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PAPER
Six new metal–organic frameworks with multi-carboxylic acids and
imidazole-based spacers: syntheses, structures and properties†
Feng Wang,‡a Xiaohuan Ke,‡a Jinbo Zhao,a Kejian Deng,b Xiaoke Leng,a Zhengfang Tian,c Lili Wen*a,d and
Dongfeng Li*a,d
Received 16th June 2011, Accepted 22nd August 2011
DOI: 10.1039/c1dt11130b
Six new metal–organic frameworks [Cu(obba)(bimb)·(obbaH2)]n (1), [Cu(obba)(bimb)]n (2),
[Zn2(obba)2(bimb)2(DMF)2(H2O)3.5]n (3), [Ni3(2,2¢,4,4¢-bptcH)2(bimb)2(H2O)2·(H2O)2]n (4),
[Ni2(bimb)3(H2O)6·(aobtc)·(DMF)2·(H2O)2]n (5) and [Cd(3,3¢,4,4¢-bptcH2)(H2O)·(bimb)]n (6), were
obtained by reactions of 4,4¢-bis(1-imidazolyl)biphenyl (bimb) and multi-carboxylic acids of
4,4¢-oxybis(benzoic acid) (obbaH2), 2,2¢,4,4¢-biphenyltetracarboxylate acid (2,2¢,4,4¢-bptcH4),
azoxybenzene-3,3¢,5,5¢-tetracarboxylic acid (aobtcH4), and 3,3¢,4,4¢-biphenyltetracarboxylate acid
(3,3¢,4,4¢-bptcH4) with corresponding metal salts under hydro/solvothermal conditions, respectively.
Complexes 1–3 have entangled structures with different topologies: 1 is a 3-fold interpenetrating NbO
three-dimensional (3D) network; 2 is a 3-fold interpenetrating dmp 3D net; 3 is a 6-fold interpenetrating
dia 3D chiral net containing rare 1D helical chains with the same handedness. Complex 4 is an uninodal
6-connected network with a Scha¨fli symbol of (486483) based on the trinuclear Ni(II) subunits, while
complexes 5 and 6 are 1D chains. Interestingly, compound 6 represents the rare example of MOFs that
exhibit high photocatalytic activity for dye degradation under visible light and shows good stability
towards photocatalysis. Complexes 3 and 6 exhibit intense blue emissions in the solid state at room
temperature whereas 3 appears to be a good candidate of novel hybrid inorganic–organic NLO material.
In particular, the reported research demonstrated that the multi-
Introduction
dentate organic ligands with N and/or O-donors as coordination
Recent years have witnessed the rapid development of metal–
sites are effective building blocks in the construction of MOFs,
organic frameworks (MOFs), one new kind of inorganic–organic
thus combining the imidazole-containing ligand with multi-
hybrid species, not only due to their intriguing structures and
carboxylic acids will be a useful strategy for construction of novel
MOFs.10 In this regard, our recent study has been mainly focused
on the construction of new MOFs from accessible imidazole-
containing ligands and multi-carboxylic acids, which show rich
structural features and encouraging photocatalytic properties.11
For example, the Cd-containing MOF with NaCl-type topology
structure, the Gd-containing 3D supramolecular framework and
two isostructural trinodal 4-connected 3D frameworks were found
to be photocatalytically active.
As an extension of our work, a rigid rod-like imidazole-based
spacer 4,4¢-bis(1-imidazolyl)biphenyl (bimb), together with
multi-carboxylic acids, was employed in this study. Herein
we report the syntheses, crystal structures and photocatalytic
properties of six new complexes [Cu(obba)(bimb)·(obbaH2)]n (1),
[Cu(obba)(bimb)]n (2), [Zn2(obba)2(bimb)2·(DMF)2·(H2O)3.5]n (3),
[Ni3(2,2¢,4,4¢-bptcH)2(bimb)2(H2O)2·(H2O)2]n (4), [Ni2(bimb)3-
(H2O)6·(aobtc)·(DMF)2·(H2O)2]n (5) and [Cd(3,3¢,4,4¢-bptcH2)-
(H2O)·(bimb)]n (6), which were obtained by reactions of
bimb and multi-carboxylic acids of 4,4¢-oxybis(benzoic acid)
(obbaH2), 2,2¢,4,4¢-biphenyltetracarboxylate acid (2,2¢,4,4¢-
bptcH4), azoxybenzene-3,3¢,5,5¢-tetracarboxylic acid (aobtcH4),
and 3,3¢,4,4¢-biphenyltetracarboxylate acid (3,3¢,4,4¢-bptcH4) with
unique properties, but also for their potential applications in
catalysis, separation, drug delivery, sensors, gas sorption or storage
and magnetism.1–6 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.7–9
aKey Laboratory of Pesticide & Chemical Biology of Ministry of Education,
College of Chemistry, Central China Normal University, Wuhan, 430079,
P. R. China. E-mail: wenlili@mail.ccnu.edu.cn (L. Wen), dfli@ccnu.edu.cn
(D. Li); Fax: +86 27 67867232; Tel: +86 27 67862900
bKey Laboratory of Catalysis and Materials Science of the State Ethnic
Affairs Commission & Ministry of Education, South-Central University for
Nationalities, Wuhan 430074, P. R. China
cHuanggang Normal Univ, Coll Chem & Appl Chem, Huangguang 438000,
P. R. China
dState Key Laboratory of Coordination Chemistry, Nanjing University,
Nanjing 210093, P. R. China
† Electronic supplementary information (ESI) available: crystallographic
data, additional crystal figures, TGA, PXRD for complexes. CCDC
reference numbers 828134–828138. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt11130b
‡ These authors contributed equally to this work.
11856 | Dalton Trans., 2011, 40, 11856–11865
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corresponding metal salts under hydro/solvothermal conditions,
respectively.
vessel (25 cm3). After being slowly cooled to room temperature,
green crystals of 2 were obtained (yield: 35% based on Cu). Anal.
Calcd. for C32H22CuN4O5: C, 63.41; H, 3.66; N, 9.24%; found: C,
63.46; H, 3.70; N, 9.23%. IR spectrum (cm-1): 3424 w, 3103 m,
2924 m, 2854 m, 1684 w, 1599 s, 1564 s, 1518 s, 1369 s, 1315 m,
1234 s, 1137 m, 1069 s, 964 m, 880 m, 818 s, 771 m, 651 m, 519 w,
498 w.
Experimental
Materials and measurements
Reagents and solvents employed were commercially available and
used as received. Ligands bimb and aobtcH4 were prepared by lit-
erature methods.12 C, H and N microanalyses were carried out with
a Perkin–Elmer 240 elemental analyzer. IR spectra were recorded
on KBr discs on a Bruker Vector 22 spectrophotometer in 4000–
400 cm-1 region. Thermogravimetric analyses were performed on
a simultaneous SDT 2960 thermal analyzer under flowing N2 with
Synthesis of [Zn2(obba)2(bimb)2·(DMF)2·(H2O)3.5]n (3). A mix-
ture of Zn(NO3)2·6H2O (29.7 mg, 0.1 mmol), obbaH2 (25.8 mg,
0.1 mmol), bimb (14.3 mg, 0.05 mmol), DMF (1 mL) and H2O
(1 mL) was placed in a 10 mL glass vial, which was sealed and
heated at 110 ◦C for 2 days, and then cooled to room temperature.
Colorless crystals of 3 were obtained (yield: 74% based on Zn).
Anal. Calcd. for C70H65N10O15.5Zn2. C, 59.00; H, 4.60; N, 9.83%;
found: C, 59.02; H, 4.63; N, 9.87%. IR spectrum (cm-1): 3423 m,
3131 m, 1692 m, 1605 s, 1518 s, 1358 s, 1306 m, 1235 s, 1157 m,
1130 m, 1066 m, 1007 w, 962 m, 944 m, 876 m, 825 s, 782 s, 694 m,
657 m, 624 m, 502 m, 418 w.
◦
a heating rate of 10 C min-1 between ambient temperature and
700 ◦C. The powder XRD data were collected on a Siemens D5005
˚
diffractometer with Cu-Ka radiation (l = 1.5418 A) over the 2q
range of 5–50◦ at room temperature. Fluorescence measurements
were recorded with a Hitachi 850 fluorescence spectrophotometer.
A pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm
was used to generate an SHG signal from samples. The backward
scattered SHG light was collected using a spherical concave mirror
and passed through a filter which transmits only 532 nm radiation.
The solid-state diffuse-reflectance UV/Vis spectra for powder
samples were recorded on a Perkin–Elmer Lambda 35 UV/Vis
spectrometer equipped with an integrating sphere by using BaSO4
as a white standard, whereas the UV/Vis spectra for solution
samples were obtained on a Shimadzu UV 2450 spectrometer.
Photocatalytic experiments: the visible light source was a 500 W
halogen lamp with UV cutoff filter (providing visible light with
l > 400 nm). A suspension of powdered catalyst (50 mg) in
fresh aqueous solution of X3B (50 mL, 6 ¥ 10-6 mol L-1) at
pH 6 were first sonicated for 5 min, and shaken at a constant
rate in the dark overnight (to establish an adsorption/desorption
equilibrium of X3B on the sample surface). At given irradiating
intervals, a series of suspensions of a certain volume were collected
and filtered through a membrane filter (pore size, 0.45 mm) to
remove suspended catalyst particles, and the filtrate was analyzed
on the UV/Vis spectrometer. The organic dye concentration was
estimated by the absorbance at 510 nm, which directly relates to
the structure change of its chromophore.
Synthesis of [Ni3(2,2¢,4,4¢-bptcH)2(bimb)2(H2O)2·(H2O)2]n (4).
A mixture of NiCl2·6H2O (35.7 mg, 0.15 mmol), 2,2¢,4,4¢-bptcH4
(30.2 mg, 0.1 mmol), bimb (28.6 mg, 0.1 mmol) and HCl (6 mol
L-1, a drop) in H2O (5 mL) was placed in a parr Teflon-lined
stainless steel vessel (25 cm3), and then the vessel was sealed and
heated at 180 ◦C for 2 days. After being slowly cooled to room
temperature, light green crystals of 4 were collected (yield: 55%
based on Ni). Anal. Calcd. for C68H50N8Ni3O20: C, 55.36; H, 3.42;
N, 7.60%; found: C, 55.40; H, 3.47; N, 7.58%. IR spectrum (cm-1):
3137 w, 2973 w, 1702 w, 1686 s, 1607 s, 1554 w, 1516 s, 1393 s, 1345
s, 1306 m, 1248 m, 1063 m, 954 s, 818 m, 771 w, 713 m, 659 w,
539 w.
Synthesis of [Ni2(bimb)3(H2O)6·(aobtc)·(DMF)2·(H2O)2]n (5).
A mixture of NiCl2·6H2O (23.8 mg, 0.1 mmol), aobtcH4 (15.6 mg,
0.1 mmol), bimb (28.6 mg, 0.1 mmol), NaOH (16.2 mg, 0.4 mmol)
and DMF (3 mL) was placed in a 10 mL glass vial, which was
sealed and heated at 100 ◦C for 2 days, and then cooled to room
temperature. Pale green crystals of 5 were obtained (yield: 62%
based on Ni). Anal. Calcd. for C76H78N16Ni2O19: C, 55.77; H, 4.80;
N, 13.69%; found: C, 55.72; H, 4.88; N, 13.67%. IR spectrum
(cm-1): 3422 m, 2925 w, 1667 s, 1610 m, 1560 m, 1518 s, 1458 w,
1420 w, 1389 m, 1359 s, 1307 m, 1264 m, 1097 w, 1066 s, 1005 w,
962 m, 937 w, 827 m, 784 m, 736 w, 713 m, 661 m, 531 w, 489 w,
469 w, 415 w, 411 w.
Synthesis of the compounds
Synthesis of [Cu(obba)(bimb)·(obbaH2)]n (1). A mixture of
Cu(NO3)2·3H2O (24.2 mg, 0.1 mmol), obbaH2 (25.8 mg, 0.1 mmol)
and bimb (28.6 mg, 0.1 mmol) in H2O (5 mL) was placed in a parr
Teflon-lined stainless steel vessel (25 cm3), and then the vessel was
sealed and heated at 160 ◦C for 2 days. After being slowly cooled
to room temperature, purple crystals of 1 were collected (yield:
29% based on Cu). Anal. Calcd. for C46H32CuN4O10: C, 63.92; H,
3.73; N, 6.48%; found: C, 63.90; H, 3.79; N, 6.52%. IR spectrum
(cm-1): 3149 w, 1710 w, 1648 m, 1594 s, 1567 w, 1517 m, 1459 w,
1382 s, 1310 w, 1239 s, 1158 m, 1099 w, 1056 w, 1010 w, 963 w, 873
w, 819 w, 777w, 653 w, 523 w.
Synthesis of [Cd(3,3¢,4,4¢-bptcH2)2(H2O)·(bimb)]n (6). A mix-
ture of Cd(NO3)2·4H2O (30.8 mg, 0.1 mmol), 3,3¢,4,4¢-bptcH4
(31.6 mg, 0.1 mmol), bimb (28.6 mg, 0.1 mmol), HCl (6 mol L-1, a
drop) and H2O (3 mL) was placed in a parr Teflon-lined stainless
◦
steel vessel (25 cm3), which was sealed and heated at 170 C for
2 days, and then cooled to room temperature. Colorless crystals
of 6 were obtained (yield: 52% based on Cd). Anal. Calcd. for
C50H34CdN4O17: C, 55.85; H, 3.19; N, 5.21%; found: C, 55.90; H,
3.06; N, 5.20%. IR spectrum (cm-1): 3486 m, 3184 s, 3043 m, 2924
m, 2854 w, 2581 w, 1721 s, 1690 s, 1613 m, 1576 s, 1550 s, 1436
s, 1417 s, 1400 s, 1365 m, 1339 s, 1306 m, 1257 m, 1229 s, 1166
s, 1137 m, 1116 m, 1052 m, 1007w, 950 m, 898 m, 844 w, 832 s,
792 s, 733 m, 685 w, 661 m, 636 m, 585 w, 506 m, 476 w, 429 w,
412 w.
Synthesis of [Cu(obba)(bimb)]n (2). A mixture of Cu(NO3)2·
3H2O (24.2 mg, 0.1 mmol), obbaH2 (25.8 mg, 0.1 mmol), bimb
(28.6 mg, 0.1 mmol), NaOH(8.1 mg, 0.2 mmol) and water (2 mL)
was heated at 180 ◦C for 2 days in a parr Teflon-lined stainless steel
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X-Ray crystallography
Results and discussion
Crystal structures
Suitable single crystals of 1–6 were selected and mounted in air
onto thin glass fibres. 1, 2, 4, and 6 were measured at room tem-
perature on a Bruker SMART APEX CCD-based diffractometer
[Cu(obba)(bimb)·(obbaH2)]n (1). The results of the crystallo-
graphic analysis revealed that the asymmetric unit of 1 consists
of two Cu(II) atoms with occupancy of 0.5 for each, one obba2-
anion, one obbaH2 neutral molecule and one bimb bridge
(Fig. S1a†). As shown in Fig. 1a, both Cu1 and Cu2 are situated
on an inversion centre. The Cu1 and Cu2 atoms are located
in the similar square sphere, coordinated to two carboxylate
oxygen atoms from inequivalent obba2- ligands and two bimb
nitrogen atoms. In addition, Cu1–O1 and Cu1–O1B distances are
˚
with graphite-monochromatic Mo-Ka radiation (l = 0.71073 A).
Data reductions and absorption corrections were performed
with the SAINT and SADABS software packages, respectively.13
X-ray intensity data for 3 and 5 were collected on a Rigaku Saturn
724+ CCD X-ray diffractometer with graphite monochromatic
˚
Mo-Ka radiation (l = 0.71073 A) at 173 K. CrystalClear software
(Rigaku Inc., 2007) was used for data reduction and empirical
absorption correction. The structures of all the crystals were
solved by direct method using SHELXS-97,14 and developed
by conventionally alternating cycles of least-squares refinement
on F2 (SHELXL-97)15 and different Fourier synthesis. All the
non-hydrogen atoms were refined anisotropically. The hydrogen
atom positions were generated theoretically, allowed to ride on
their respective parent atoms and included in the structure factor
calculations with assigned isotropic thermal parameters. To assist
the refinement, several restraints were applied: for 1, 4 and
5, the disordered C25/C26/C28/C29, C21/C22/C24/C25 and
C5/C6/C8/C9 atoms of benzene rings were restrained in order
to obtain reasonable thermal parameters; for 3, the unit cell
includes a large region of disordered solvent molecules, which
could not be modelled as discrete atomic sites. We employed
PLATON/SQUEEZE to calculate the diffraction contribution of
the solvent molecules and, thereby, to produce a set of solvent-
free diffraction intensities.16 Compound 3 was formulated as
[Zn2(obba)2(bimb)2·(DMF)2·(H2O)3.5]n by elemental analysis and
single-crystal X-ray diffraction studies. Crystallographic data and
other pertinent information for 1–6 are summarized in Table 1,
while the selected bond lengths and angles are listed in Table
S1†. More details on the crystallographic studies as well as
atom displacement parameters are presented in the Supporting
Information (see ESI†).
˚
˚
2.927(2) A, Cu2–O5D and Cu2–O5E distances 2.680 A, suggesting
non-negligible interactions between the copper atoms and the
uncoordinated carboxylate oxygen atoms, which can be described
as a semi-chelating coordination mode.17 Consequently, both Cu1
and Cu2 atoms may also be regarded as in an elongated octahedron
environment due to the Jahn–Teller effect of the Cu(II) ion.
As to obba2- anions, the two carboxylates have torsion angles
with the corresponding linking benzene rings ranging from 4.8◦
to 13.7◦. For the obbaH2 neutral moiety, the carboxylate groups
are tilted by the corresponding linking phenyl rings plane with the
dihedral angles between them being 12.2◦ and 10.7◦. The dihedral
angles between the two phenyl rings within the individual obba2- or
obbaH2 moiety vary from 63.7◦ to 69.4◦. The COO ends of obba2-
anions adopt alternatively bidentate-chelating and monodentate
fashions (type I, Scheme 1), linking the copper atoms to generate
˚
1D chains with Cu1 ◊ ◊ ◊ Cu2 separations of 15.020 A, as illustrated
in Fig. S1b†.
Furthermore, rigid bimb ligands bridge two copper metal
ions, with a Cu1 ◊ ◊ ◊ Cu1 (Cu2 ◊ ◊ ◊ Cu2) separation of 17.249 A,
therefore, linking the 1D chains to result in a three-dimensional
(3D) porous structure with large rectangular cavities of size ca.
15.02 ¥ 17.25 A and ca. 15.02 ¥ 16.66 A along the ac and
ab plane, respectively (Fig. 1b and 1c). From the topological
view, if both the obba2- and bimb ligands can be considered
˚
2
2
˚
˚
Table 1 Crystal data and structure refinement for 1–6
1
2
3
4
5
6
Formula
C46H32CuN4O10
864.31
Monoclinic
P21/c
26.9695(15)
14.7086(8)
9.9706(6)
90.00
98.0810(10)
90.00
3915.9(4)
4
1.466
0.626
40329
C32H22CuN4O5
606.08
Orthorhombic
Pnna
9.8501(12)
18.1491(18)
15.1858(14)
90.00
90.00
90.00
2714.8(5)
4
1.483
C64H44N8O10Zn2
1215.86
Orthorhombic
P21212
18.167(4)
26.668(5)
12.453(3)
90.00
90.00
90.00
6033(2)
4
1.339
C68H50N8Ni3O20
1475.29
Orthorhombic
Fddd
16.292(4)
30.712(8)
51.727(14)
90.00
90.00
90.00
25882(12)
16
1.514
C76H78N16Ni2O19
1636.96
Monoclinic
P21/c
8.9784(18)
15.352(3)
27.656(6)
90.00
97.26(3)
90.00
3781.4(14)
2
1.438
0.581
27433
7345
0.1312
0.1974
C50H32CdN4O17
1075.21
Monoclinic
C2/c
28.498(3)
11.7312(11)
12.4676(12)
90.00
95.487(2)
90.00
4149.0(7)
4
1.721
0.618
16659
4306
0.0450
0.0909
Formula weight
Crystal system
Space group
˚
a /A
˚
b /A
˚
c /A
a /◦
b /◦
g /◦
3
˚
V /A
Z
rc /g cm-3
m /mm-1
0.855
14167
2675
0.0478
0.860
18899
0.948
49638
6374
0.0517
Collected reflections
Unique reflections (Rint
R1 [I > 2s(I)]
wR2 (all data)
)
7685
0.0474
0.1100
10875
0.0682a
0.1540a
0.1280
0.1444
a Excluding the solvent molecules and subtracting the contribution of the solvent from the diffraction data using the PLATON/SQUEEZE tool.16
11858 | Dalton Trans., 2011, 40, 11856–11865
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Scheme 1 Coordination modes of obbaH2, 2,2¢,4,4¢-bptcH4 and 3,3¢,
4,4¢-bptcH4 ligands appearing in 1–4 (I–IV) and 6 (V).
Notably, the large voids formed by a single 3D network
allow incorporation of two other identical networks. Therefore,
the overall structure of 1 has three identical 3D nets, which
interpenetrate each other to form a 3-fold interpenetrating 3D
framework belonging to class Ia (Fig. 1e). Hence, the large pore
void spaces of the single net have been reduced significantly. Upon
interpenetration, the compound only retains an effective void
3
˚
volume of 1386.9 A per unit cell, which is 35.4% of the crystal
volume. Interestingly, the free obbaH2 are trapped in the pores as
guest molecules (Fig. S1c†). The protonated carboxylate oxygens
(O6 and O7) of the free obbaH2 are individually involved in strong
hydrogen bonds with the carboxylate oxygens (O4 and O1) of the
triply interpenetrating 3D framework with O ◊ ◊ ◊ O separations of
˚
2.563(3) and 2.612(3) A.
[Cu(obba)(bimb)]n (2). When the raw materials were the same
as those of compound 1 and different reaction conditions were
applied during the synthetic process, compound 2 was formed.
As exhibited in Fig. 2a, the Cu(II) centers adopt tetrahedral
coordination geometry defined by two oxygen atoms from two
carboxylate oxygen atoms and two separate bimb nitrogen atoms.
˚
Furthermore, the Cu1–O1(O1A) distances are 2.501(2) A, which
can also be described as a semi-chelating coordination mode.17
Hence, the coordination environment of the Cu1 atom may be
regarded as a distorted trigonal prismatic sphere. Both obba2- and
bimb are situated on an inversion centre. As to obba2-, the two
carboxylates have torsion angles of 30.9◦ with the corresponding
linking benzene rings and the two benzene planes bridged by the
etheric oxygen atom was tilted by 57◦, which connects the copper
atoms in a bidentate-chelating mode (type II, Scheme 1) to form
Fig. 1 (a) Coordination environments of Cu atoms with hydrogen atoms
and free obbaH2 fragment omitted for clarity, (b) and (c) the porous
3D single framework of 1 along the ac and ab plane, (d) NbO topology
structure for single net of compound 1, purple balls represent the Cu(II)
atoms, (e) the schematic view of 3-fold interpenetration of 1.
˚
1D zigzag chains with a Cu1 ◊ ◊ ◊ Cu1 separation of 14.955 A, like
the chains in 1, as illustrated in Fig. S2†.
Meanwhile, rigid bimb ligands pillar the 1D chains to afford
a three-dimensional (3D) porous structure with large cavities of
as linkers and all Cu(II) centers can be viewed as planar four-
connected nodes, the individual framework of the compound 1
can be simplified as a binodal (4,4)-connected NbO-type topology
with the a Scha¨fli symbol of (658)(6482) for (Cu1)(Cu2),18 see
Fig. 1d.
3
˚
size ca. 45.557 ¥ 17.386 ¥ 14.955 A (based on the Cu1 ◊ ◊ ◊ Cu1
separation) (Fig. 2b). A better insight into the nature of this
intricate framework can be acquired by using a topological
analysis. In 2, each obba2- anion and bimb can be simplified
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by a single 3D network allow the incorporation of another four
identical networks. Therefore, the overall structure of 2 is a 5-fold
interpenetrating 3D framework belonging to class Ia (Fig. 2d).
[Zn2(obba)2(bimb)2·(DMF)2·(H2O)3.5]n (3). To further investi-
gate the effect of the metal center, a Zn(II) salt was utilized in the
reaction for synthesis of 3. Interestingly, crystallographic analysis
showed that 3 crystallizes in a chiral space group P21212. There
are two Zn(II) atoms, two obba2-, and two bimb ligands in the
asymmetric unit of 3 as shown in Fig. S3a†. Both Zn1 and Zn2,
situated in a tetrahedral arrangement, are coordinated by two
oxygen and two nitrogen atoms separately from two obba2- anions
and two bimb ligands, as depicted in Fig. 3a. The carboxylate
groups have torsion angles of 4.1–19.2◦ with the corresponding
linking benzene rings. The salient structural feature of compound
3 is the presence of a 1D helical chainlike structure with 21 helices
˚
along the a-axis with a pitch of 40.54 A, which are formed by the Zn
ions being linked via obba2- ligands in monodentate fashions (type
III, Scheme 1). The dihedral angles between the two phenyl rings
within the individual obba2- ligand are 108.2 and 60.3◦, which
may be significant in forming the helical strand. All the helices
in 3 are of the same handedness, which are connected together
via a rigid bimb linker to afford a 3D chiral porous framework
(Fig. 3b). For simplicity, a similar topological analysis was carried
out to analyze the structure of 3. Each Zn1 and Zn2 ion can
be separately considered as 4-connecting nodes, which link the
bridge ligands into a (4, 4)-connected 66-dia net (Fig. 3c).20 The
single framework of 3 has large enough vacancies to allow the
inclusion of another five identical 3D motifs, therefore, the whole
framework affords a 6-fold interpenetration net of class Ia (Fig.
3d). Due to interpenetration, only small cavities can be observed.
PLATON analysis revealed that the 3D framework structure was
3
˚
composed of voids of 246.7 A , which represent 10.7% per unit
cell volume.
[Ni3(2,2¢,4,4¢-bptcH)2(bimb)2(H2O)2·(H2O)2]n (4). To investi-
gate the influence of the carboxylate ligand on the structure
of the complexes, the reaction of bimb and 2,2¢,4,4¢-bptcH4
with Ni(NO3)2·6H2O was carried out, and a new complex 4
was obtained. As shown in Fig. 4a, although all the Ni(II)
centers have octahedral geometries, the two unique metal centers
exhibit different coordination environments: Ni1, situated on an
inversion centre with occupancy of 0.5, is coordinated to six
carboxylate oxygen atoms (O1, O5, O7, O1A, O5A, O7A) from
two inequivalent 2,2¢,4,4¢-bptcH3-; Ni2 is surrounded by three
carboxylate oxygens (O1, O8, O5A) from two different anions,
two nitrogen atoms (N1, N4A) from distinct bimb ligands and
one coordinated water molecule (O1W).
As to 2,2¢,4,4¢-bptcH3-, the dihedral angle between two phenyl
rings is 104.9◦, the 2- and 2¢-carboxylate groups are tilted by the
plane of corresponding linking phenyl rings with the dihedral
angle between them being 135.2◦ and 28.3◦, respectively. 4-
and 4¢-carboxylate groups have 2.3◦ and 142.2◦ dihedral angles,
respectively, with the plane of corresponding linking phenyl rings.
In 4, each partially deprotonated 2,2¢,4,4¢-bptcH3- moiety bridges
five Ni(II) centers, with two full deprotonated carboxylate groups
Fig. 2 (a) Coordination environments of Cu atoms with hydrogen atoms
omitted for clarity, (b) the porous 3D single framework of 2, (c) dmp
topology structure for single net of compound 2, purple balls represent the
Cu(II) atoms, (d) the schematic view of 5-fold interpenetration of 2.
to be linear connectors; each Cu1 center bridges two bimb
and two obba2- moieties, viewed as considered as 4-connected
nodes in tetrahedral sphere, therefore, the combination of nodes
and connectors suggests an uninodal 4-connected network with
an extended Scha¨fli symbol of 6.6.6.6.6(2).8(3), assigned to the
unusual dmp net according to Wells’ classification, which is very
rare in coordination complexes (Fig. 2c).19 The large voids formed
2
0
2
(2,2¢-COO-) in syn conformation adopting m -h :h modes, and
2
1
1
the full deprotonated 4¢-COO- moiety adopting m -h :h fashions
(type IV, Scheme 1). The 2,2¢,4,4¢-bptcH3- anions thus link with
cadmium ions to form a 1D chain structure (Fig. 4b). Interestingly,
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Fig. 3 (a) Coordination environments of Zn atoms with hydrogen atoms
omitted for clarity, (b) the porous chiral 3D single framework of 3
containing 1D helical chains with the same handedness, (c) dia topology
structure for single net of compound 3, purple balls represent the Zn(II)
atoms, (d) the schematic view of 6-fold interpenetration of 3.
Fig. 4 (a) Coordination environments of Ni atoms with hydrogen atoms
omitted for clarity, (b) 1D chain constructed from 2,2¢,4,4¢-bptcH3-
and Ni2+, (c) the centrosymmetric trinuclear cluster [Ni3(COO)2O4], (d)
complicated 3D structure of 4, (e) the schematic representations of the
6-connected framework of 4 with (486483) topology, blue balls represent
the Ni(II) atoms.
in the case of a 1D chain, two carboxylate groups and four
m2-carboxylate oxygen atoms bridge three nickel(II) atoms to
generate the centrosymmetric trinuclear cluster [Ni3(COO)2O4] as
the secondary building unit (SBU) with Ni1 ◊ ◊ ◊ Ni2 separations of
In addition, it should be noted that there exist rigid bimb
spacers, which establish physical bridges between Ni2 atoms with
˚
Ni2 ◊ ◊ ◊ Ni2 separations of 17.004 A, connecting the 1D chains
˚
3.182 A (Fig. 4c).
to result in a complicated 3D structure (Fig. 4d). The effective
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free volume of 4 was calculated by PLATON analysis as 5.1%
As expected, extensive hydrogen bonding interactions were
found between the guest molecules and the 1D chain: the
coordinated water molecules are involved in stronger hydrogen
3
3
˚
˚
of the crystal volume (1331.1 A out of the 25882 A unit
cell volume), where disordered water molecules reside as labile
ligands.
For better insight into the intricate polymeric framework of
compound 4, a topological analysis is completed. If trinuclear
clusters [Ni3(COO)2O4] act as nodes, and the 2,2¢,4,4¢-bptcH3-
and bimb groups can be simplified to be linear connectors, the
combination of nodes and connectors suggests a uninodal 6-
connected network with a Scha¨fli symbol of (486483), as illustrated
in Fig. 4e.
˚
bonds with lattice water with O ◊ ◊ ◊ O separations of 2.585(8) A,
˚
the acyl group of DMF with O ◊ ◊ ◊ O separations of 2.810(8) A as
well as uncoordinated carboxylate of aobtc4- anions with O ◊ ◊ ◊ O
˚
separations varying from 2.621(7) to 2.743(7) A; weaker hydrogen
bonds were also observed between the carbon atoms of the 1D
chain and the uncoordinated carboxylate aobtc4- moieties with
˚
C ◊ ◊ ◊ O distances varying from 3.163(9) to 3.472(9) A. Besides that,
strong hydrogen bonding interactions also exist between the lattice
water and the aobtc4- anions with O ◊ ◊ ◊ O separations varying from
[Ni2(bimb)3(H2O)6·(aobtc)·(DMF)2·(H2O)2]n (5). When 2,2¢,4,
4¢-bptcH4 was replaced by another carboxylic acid, aobtcH4, in
the reaction, complex 5 was obtained. As shown in Fig. 5a, the
Ni(II) centers adopt a distorted octahedral coordination sphere
that is defined by three terminal water molecules and three
nitrogen atoms from three separate bimb ligands. It is worth noting
that the N N double bond of aobtc4- resides on an inversion
center with the azoxy group disordered over two equally occupied
orientations. An average dihedral angle between the two benzene
rings and the azoxy plane of 34.7◦ was observed.
˚
2.638(9) to 2.732(10) A. All of the hydrogen bonds contribute to
the stability of compound 5.
[Cd(3,3¢,4,4¢-bptcH2)2(H2O)·(bimb)]n (6). Complex 6 was ob-
tained by reaction of Cd(II) salt with bimb and 3,3¢,4,4¢-bptcH4. As
depicted in Fig. 6a, Cd1 is in a distorted pentagonal bipyramidal
coordination sphere, which is defined by six carboxylate oxygen
2-
atoms from four different 3,3¢,4,4¢-bptcH2 and one coordinated
water molecule. As to 3,3¢,4,4¢-bptcH22-, the dihedral angle
between the two phenyl rings is 37.6◦, the 3- and 3¢-carboxylate
groups are tilted by the plane of corresponding linking phenyl
rings with the dihedral angle between them being 49.1◦ and 14.5◦,
respectively. 4- and 4¢-carboxylate groups have 60.9◦ and 92.9◦
dihedral angles, respectively, with the plane of corresponding
linking phenyl rings.
Fig. 5 (a) Coordination environments of Ni atoms with hydrogen atoms
omitted for clarity, (b) guest solvate molecules and aobtc4- anions occupy
the void space of a 1D infinite ladder-like chain in 5.
The bidentate bimb bridges are connected to Ni(II) centers
to finish the 1D infinite ladder-like chain arranged in the
[110] direction: assembled from the bimb ligand to which N5
belongs, located at a crystallographic inversion center, acting
as rungs and the bimb ligand to which N1 belongs serving
as rails. The 1D network is “open” with large rectangular
Fig. 6 (a) Coordination environments of Cd atoms, (b) 1D chains
assembled by 3,3¢,4,4¢-bptcH2 and Cd atoms in 6.
2-
2-
The partially deprotonated 3,3¢,4,4¢-bptcH2 ligand coordi-
nates to Cd atoms with two deprotonated carboxylate groups
2
1
1
respectively adopting m -h :h and mono-dentate coordination
modes (type V, Scheme 1), assembled into a 1D chain along the
c-axis Fig. 6b.
2
˚
channels with dimensions of ca. 18.14 ¥17.78 A based on the
Ni1 ◊ ◊ ◊ Ni1 separation. The lattice water molecules, DMF solvate
molecules and aobtc4- anions occupy the large available space, see
Fig. 5b.
Meanwhile, strong O–H ◊ ◊ ◊ O hydrogen bonds prevail between
the coordinated carboxylate oxygen atoms (O2, O3 and O7) and
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coordination water molecules (O9), as well as the uncoordinated
carboxylate oxygens (O6 and O8). Apparently, the typical O ◊ ◊ ◊ O
˚
distances are in the range of 2.492(3)–3.120(3) A between 1D
chains. More importantly, the bimb moieties are located at an
inversion center, which are included between the 1D chains
(Fig. S4†). The weaker nonclassical hydrogen bonds were observed
between the C–H moieties of bimb and the coordinated carboxy-
late oxygen atom (O1) and the uncoordinated carboxylate oxygen
˚
atom (O6) with the distance varying from 3.190(4) to 3.191(4) A.
The extensive hydrogen bonds may contribute to the stability of
the MOF.
The phase purity of the bulk material was independently
confirmed by powder X-ray diffraction (PXRD). The PXRD
patterns of all as-synthesized products closely match the simulated
ones from the single-crystal data, indicating that products are in a
pure phase (Fig. S5–Fig. S10†).
Fig. 7 UV/Vis diffuse-reflectance spectra of compounds 3, 5 and 6 with
BaSO4 as background.
Obviously, it is still a challenge to obtain the desired MOFs
because they depend highly on the combination of several factors,
such as the coordination geometry of the metal ions, the nature of
the ligands,21,22 the ratio between metal salt and ligand,23 and the
reaction conditions. For example, in our work, from the crystal
structures described above, it is interesting to note that both
the Cu(II) in 1 and 2, as well as Zn(II) in 3 act as 4-connecting
nodes, connecting two bimb and two obba2- ligands, respectively.
However, the different coordination geometry around the central
metal atom resulted in the completely different structures for the
3-fold interpenetrated 3D networks of 1 and 2, as well as the 6-
fold interpenetrated 3D framework of 3. The results imply that the
metal center has a great impact on the structure of the complexes.
In addition, for 1 and 2, the raw materials are the same, but distinct
structures are obtained for different reaction temperatures (160 ◦C
for 1 and 180 ◦C for 2) and different pH values. In contrast to the
common NbO and dia topology of 1 and 3, complex 2 has the un-
usual dmp topology, which is very rare in coordination complexes.
components both in the UV and Vis regions. Obviously, the
absorption bands are around 356, 452 nm for 5 whereas 330 and
486 nm for 6, respectively, which also could be attributed to LMCT.
In the case of 5, one clear additional peak was observed at 627 nm,
which probably originate from the d-d spin-allowed transition
of the d8 (Ni2+) ion. The presence of visible region transitions
motivated us to explore applications of 5 and 6 in heterogeneous
photocatalysis.
The photodegradation experiment under visible light irradia-
tion was carried out after the dark adsorption equilibrium was
achieved. For comparison, the photocatalytic performance of
commercial TiO2 (Degussa P-25) was also assessed under the same
experimental conditions. In addition, control experiments on the
photodegradation of X3B have been carried out. No significant
change in the degradation of X3B was observed in the following
reaction conditions: (1) in the dark; (2) without catalyst.
The distinctly shortened degradation time compared with the
control experiments indicates that catalyst 5 and 6 are active for
the decomposition of X3B under visible light irradiation. The
rate constants for 5 and 6 under visible light irradiation were
found to be 0.073 and 0.104 h-1, respectively, when pseudo-first-
order kinetics were fitted to the experimental data. Notably, the
photocatalytic efficiency of 6 under visible light is higher than
that of 5. The X3B degradation could reach ~60% conversion
within 4 h in the presence of 6. Fig. 8 presents the comparison
Photocatalytic activity
We selected an anionic organic dye, X3B (Scheme 2), as a
model pollutant in aqueous media to evaluate the photocatalytic
effectiveness of the obtained compounds, 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.24 Since the yields of compounds 1 and
2 were not high and the adsorption of X3B on 4 is too strong,
only compounds 3, 5 and 6 were used as potential photocatalysts.
In addition, the diffuse-reflectance UV/Vis spectra reveal that
solid 3, 5 and 6 have different absorption features (Fig. 7). The
main absorptions for 3 were around 328 and 384 nm, belonging
to UV regions, which could be assigned to ligand-to-metal charge
transfer (LMCT). The spectra for 5 and 6 consist of absorption
Fig. 8 Control experiments on the photodegradation of X3B. (a)
X3B/visible light (without catalyst); (b) X3B/compound 6/dark;
(c) X3B/compound 5/dark; (d) X3B/P25 TiO2/visible light; (e)
X3B/compound 5/visible light; (f) X3B/compound 6/visible light.
Scheme 2 Molecular structure of X3B.
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of photocatalytic activities of the samples and P25 under visible
light. Apparently, P25 showed only slight photocatalytic activity
under visible-light irradiation. Notably, the prepared samples of
5 and 6 exhibit good photocatalytic efficiencies under visible light
illumination. After photocatalysis, compounds 5 and 6 display
powder XRD patterns nearly identical to those of the original
compounds, in other words, their stability towards photocatalysis
is good.
useful application in heterogeneous photocatalysis. Remarkably,
compound 6 represents the rare example of MOFs that exhibit
high photocatalytic activity for dye degradation under visible
light and shows good stability towards photocatalysis. In addition,
complexes 3 and 6 exhibit intense blue emissions in the solid state at
room temperature where complex 3 appears to be a good candidate
for a novel hybrid inorganic–organic NLO material.
Acknowledgements
Photoluminescence properties and SHG measurement
This work was financially supported by the National Nature
Science Foundation of China (Nos. 21171062 and 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, Operation of MOE and 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).
It has been reported that d10 complexes show fluorescence proper-
ties and could serve as good candidates for potential photoactive
materials.25 Therefore, the solid-state fluorescent measurements
for 3 and 6 as well as the corresponding ligands were carried out at
room temperature. Intense emission bands were observed at 418,
419 and 421 nm for the bimb, 3,3¢,4,4¢-bptcH4 and obbaH2 with
excitation at 380 nm, respectively. As shown in Fig. 9, emission
bands were observed at 435 and 416 nm for 3 and 6, which may
be a mixture of characters of intraligand and ligand-to-ligand
charge transition (LLCT). And the observed red or blue shift of
the emission maximum between the complexes and the ligands
was considered to mainly originate from the influence of the
coordination of the ligand to metal atom.
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