Assembly of a series of MOFs based on the 2-(m-methoxyphenyl)imidazole dicarboxylate ligand

Article abstract of DOI:10.1039/c2dt32727a

The coordination features of an imidazole dicarboxylate ligand, 2-(3-methoxyphenyl)-1H-imidazole-4,5-dicarboxylic acid (m-H₃MOPhIDC) has been explored. Consequently, seven coordination polymers, namely [Sr(m-HMOPhIDC)(H₂O)]_n (1), [Sr(m-H₂MOPhIDC) ₂]_n (2), [Cd₃(m-H₂MOPhIDC) ₂(m-HMOPhIDC)₂(H₂O)₂]_n (3), [Cu(m-HMOPhIDC)(phen)]_n (phen = 1,10-phenanthroline) (4), [Cd₂(m-HMOPhIDC)₂(phen)₂]_n (5) [Cd₂(m-HMOPhIDC)₂(2,2′-bipy)₂] _n (2,2′-bipy = 2,2′-bipyridine) (6) and [Co(m-HMOPhIDC)(H₂O)₂]_n (7) have been hydro(solvo)thermally synthesized by fine control over synthetic conditions, and structurally characterized. X-ray single-crystal analyses reveal that these polymers indicate rich structural chemistry ranging from one-dimensional (4-7), two-dimensional (1 and 3) to three-dimensional (2) structures, and the m-H ₃MOPhIDC ligand in these polymers can be singly deprotonated or doubly deprotonated, and coordinates to metal ions by various modes. The thermal and fluorescence properties of the complexes 1-7 have been determined as well. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt32727a

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Assembly of a series of MOFs based on the
2-(m-methoxyphenyl)imidazole dicarboxylate ligand†
Cite this: Dalton Trans., 2013, 42, 4613
Zhi-Fang Xiong, Rui-Min Gao, Zheng-Kun Xie, Bei-Bei Guo, Li Li, Yan-Yan Zhu and
Gang Li*
The coordination features of an imidazole dicarboxylate ligand, 2-(3-methoxyphenyl)-1H-imidazole-4,5-
dicarboxylic acid (m-H3MOPhIDC) has been explored. Consequently, seven coordination polymers,
namely [Sr(m-HMOPhIDC)(H2O)]n (1), [Sr(m-H2MOPhIDC)2]n (2), [Cd3(m-H2MOPhIDC)2(m-HMOPhID-
C)2(H2O)2]n (3), [Cu(m-HMOPhIDC)(phen)]n (phen = 1,10-phenanthroline) (4), [Cd2(m-HMOPhIDC)2-
(phen)2]n (5) [Cd2(m-HMOPhIDC)2(2,2’-bipy)2]n (2,2’-bipy = 2,2’-bipyridine) (6) and [Co(m-HMOPhIDC)
(H2O)2]n (7) have been hydro(solvo)thermally synthesized by fine control over synthetic conditions, and
structurally characterized. X-ray single-crystal analyses reveal that these polymers indicate rich structural
chemistry ranging from one-dimensional (4–7), two-dimensional (1 and 3) to three-dimensional (2) struc-
tures, and the m-H3MOPhIDC ligand in these polymers can be singly deprotonated or doubly deproto-
nated, and coordinates to metal ions by various modes. The thermal and fluorescence properties of the
complexes 1–7 have been determined as well.
Received 15th November 2012,
Accepted 14th December 2012
DOI: 10.1039/c2dt32727a
www.rsc.org/dalton
acceptors.5,7–11 In this context, imidazole-4,5-dicarboxylic acid
(H3IDC) is an excellent candidate for assembling novel
Introduction
The construction of metal–organic frameworks (MOFs) has MOFs.12–20 Besides being partially or fully deprotonated to
grown rapidly in chemistry and material science fields over the H2IDC−, HIDC2− and IDC3− anions, the ligand H3IDC also
past two decades because of their promising applications in exhibits flexible coordination modes.21 Recently, people have
gas storage and capture, heterogeneous catalysis materials and had great interest in introducing different substituent units
so on.1,2 To date, although people have been making dramatic into the 2-position of H3IDC, such as ethyl,22,23 propyl,24
progress in the research of the optimum strategy and manipu- phenyl,25 hydroxymethyl,26 or pyridyl27–33 groups, and studied
lation in the self-assembly of MOFs,3–5 it is still a big challenge the influence of the substituent on the coordination abilities
to achieve target MOFs which are designed in advance. Many of the corresponding imidazole dicarboxylate ligands. More
factors such as the organic ligands, coordination preference of recently, based on our previous attractive results of MOFs
the central metal ion, crystallization conditions, metal/ligand bearing 2-phenyl-1H-imidazole-4,5-dicarboxylic acid,25 we
ratio and the reaction solvent system can significantly influ- adopted the methoxyl unit to modify the phenyl group, and
ence the final products of MOFs. Among them, the organic further explored the coordination features of the related
ligands play an extremely important role in directing the ligand.9e,f The studies showed that 2-(4-methoxyphenyl)-1H-
extended structure of the resulting complexes. Thus, consider- imidazole-4,5-dicarboxylic acid (p-H3MOPhIDC) can build up
able efforts have been devoted to design or choose the multi- interesting MOFs.9e,f Prompted by these findings, we hope to
functional bridging ligands.6
probe the substituent effect of the methoxyl unit on the 3-posi-
Recently, the N-heterocyclic dicarboxylate ligands have tion of the phenyl group (2-(3-methoxyphenyl)-1H-imidazole-
attracted much interest in the preparation of new MOFs under 4,5-dicarboxylic acid (m-H3MOPhIDC)) from both theoretical
hydro(solvo)thermal conditions due to their versatile coordi- and experimental aspects.
nation modes and potential hydrogen-bonding donors and
At the same time, there are no reports of MOFs con-
structed from the m-H3MOPhIDC ligand. Herein, we will
report the hydro(solvo)thermal syntheses and structural
determinations of one 2D polymer [Sr(m-HMOPhIDC)(H2O)]n
(1), one 3D polymer [Sr(m-H2MOPhIDC)2]n (2), one 2D
polymer [Cd3(m-H2MOPhIDC)2(μ3-HMOPhIDC)2(H2O)2]n (3),
and four 1D polymers [Cu(HMOPhIDC)(phen)]n (phen = 1,10-
phenanthroline) (4), [Cd2(m-HMOPhIDC)2(phen)2]n (5),
Department of Chemistry, Zhengzhou University, Zhengzhou 450052, Henan,
P. R. China. E-mail: gangli@zzu.edu.cn
†Electronic supplementary information (ESI) available: The X-ray crystallo-
graphic files in CIF format. CCDC 895949–895953 for 1–5, 903675 and 903676
for complexes 6, and 7, respectively. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt32727a
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Preparation of crystalline [Sr(m-H2MOPhIDC)2]n (2)
A mixture of m-H3MOPhIDC (52.4 mg, 0.2 mmol), Sr(NO3)2
(42.3 mg, 0.2 mmol), Et3N (0.056 mL, 0.4 mmol) and CH3OH–
H2O (3 : 4, 7 mL) was sealed in a 25 mL Teflon-lined bomb,
which was heated to 150 °C for 96 h. After the mixture was
cooled to room temperature, yellow crystals of 2 were obtained,
washed with methanol and water, and dried in air. Yield: 64%
(based on Sr). Anal. calcd for C24H16N4O10Sr: C, 47.41; H, 2.65;
N, 9.22%. Found: C, 47.82; H, 2.43; N, 9.07%. IR (cm−1, KBr):
3621 m, 3223 w, 2963 m, 1593 s, 1556 w, 1490 s, 1420 w, 1384
w, 1261 s, 1056 w, 800 s, 628 m.
Preparation of crystalline [Cd3(m-H2MOPhIDC)2-
(m-HMOPhIDC)2(H2O)2]n (3)
A
mixture of m-H3MOPhIDC (52.4 mg, 0.2 mmol), Cd
Scheme 1 Coordination modes of m-H3−nMOPhIDCn− anions (n = 1 or 2).
(NO3)2·4H2O (61.6 mg, 0.2 mmol), pyridine (0.2 mL), and de-
ionized water (7 mL) was sealed in a 25 mL Teflon-lined bomb
and heated at 160 °C for 96 h, and then cooled to room temp-
erature. Colorless strip-shaped crystals of 3 were collected,
washed with distilled water and dried in air. Yield: 53% (based
on Cd). Anal. calcd for C24H19N4O11Cd1.5: C, 40.70; H, 2.70; N,
7.91%. Found: C, 40.58; H, 2.82; N, 8.22%. IR (cm−1, KBr):
3440 s, 2847 w, 1570 s, 1446 w, 1374 w, 1245 w, 1221 w, 837 s,
699 s.
[Cd2(m-HMOPhIDC)2(2,2′-bipy)2]n (2,2′-bipy = 2,2′-bipyridine)
(6) and [Co(m-HMOPhIDC)(H2O)2]n (7). Single crystal X-ray
diffractions reveal that one or two hydrogen atoms can
be removed from the ligand m-H3MOPhIDC forming
m-H2MOPhIDC− and m-HMOPhIDC2− anions, and the depro-
tonated m-H3−nMOPhIDCn− (n = 1 or 2) could coordinate to
metal ions in various modes (Scheme 1).
Preparation of crystalline [Cu(m-HMOPhIDC)(phen)]n (4)
A mixture of m-H3MOPhIDC (52.4 mg, 0.2 mmol), Cu(ClO4)2
(52.4 mg, 0.2 mmol), phen (39.6 mg, 0.2 mmol) and Et3N
(0.056 mL, 0.4 mmol) and CH3OH–H2O (3 : 4, 7 mL) was
sealed in a 25 mL Teflon-lined bomb and heated at 160 °C for
72 h, and then cooled to room temperature. Blue crystals of 4
were collected, washed with distilled water and dried in air.
Yield: 53% (based on Cu). Anal. calcd for C24H16N4O5Cu: C,
57.19; H, 3.20; N, 11.12%. Found: C, 57.48; H, 3.02; N, 11.21%.
IR (cm−1, KBr): 3423 w, 3036 w, 1588 s, 1445 w, 1423 s, 1221 w,
839 s, 722 s.
Experimental section
General details
All chemicals were of reagent grade quality obtained from
commercial sources and used without further purification.
The organic ligand m-H3MOPhIDC was prepared according to
the literature procedure.34
C, H and N microanalyses were carried out on a FLASH EA
1112 analyzer. IR data were recorded on a BRUKER TENSOR
27 spectrophotometer with KBr pellets in the 400–4000 cm−1
region. TG-DSC measurements were performed by heating the
crystalline sample from 20 to 850 °C at a rate of 10 °C min−1 in
the air on a Netzsch STA 409PC differential thermal analyzer.
Fluorescence spectra were characterized at room temperature
by a F-4500 fluorescence spectrophotometer.
Preparation of crystalline [Cd2(m-HMOPhIDC)2(phen)2]n (5)
A mixture of m-H3MOPhIDC (52.4 mg, 0.2 mmol), Cd-
(NO3)2·4H2O (61.6 mg, 0.2 mmol), phen (39.6 mg, 0.2 mmol)
and Et3N (0.056 mL, 0.4 mmol) and CH3OH–H2O (3 : 4, 7 mL)
was sealed in a 25 mL Teflon-lined bomb and heated at 155 °C
for 96 h, and then cooled to room temperature. Colorless strip-
shaped crystals of 5 were collected, washed with distilled water
and dried in air. Yield: 53% (based on Cd). Anal. calcd for
C48H32N8O10Cd2: C, 52.14; H, 2.89; N, 10.13%. Found: C,
52.48; H, 2.52; N, 10.42%. IR (cm−1, KBr): 3446 m, 2834 m,
1635 s, 1471 s, 1386 s, 1234 s, 1123 s, 846 s, 709 m.
Preparation of crystalline [Sr(m-HMOPhIDC)(H2O)]n (1)
A mixture of m-H3MOPhIDC (52.4 mg, 0.2 mmol), Sr(NO3)2
(42.3 mg, 0.2 mmol), Et3N (0.056 mL, 0.4 mmol) and CH3CN–
H2O (3 : 4, 7 mL) was sealed in a 25 mL Teflon-lined bomb and
heated at 150 °C for 96 h. The reaction mixture was then
allowed to cool to room temperature. Block-shaped crystals of
1 were collected, washed with acetonitrile and dried in air.
Preparation of crystalline [Cd2(m-HMOPhIDC)2(2,2′-bipy)2]n (6)
A
mixture of Cd(NO3)2·6H2O (30.8 mg, 0.1 mmol), m-
Yield: 86% (based on Sr). Anal. calcd for C12H10N2O6Sr: C, H3MOPhIDC (26.2 mg, 0.1 mmol), 2,2′-bipy (15.6 mg,
39.40; H, 2.73; N, 7.66%. Found: C, 39.82; H, 2.91; N, 7.48%. 0.1 mmol) and CH3OH–H2O (3 : 4, 7 mL), Et3N (0.056 mL,
IR (cm−1, KBr): 3620 s, 3225 w, 1593 s, 1529 s, 1262 s, 1178 w, 0.4 mmol) was sealed in a 25 mL Teflon-lined bomb and
867 m, 800 w, 727 s, 472 m.
heated at 155 °C for 120 h, and then cooled to room
4614 | Dalton Trans., 2013, 42, 4613–4624
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temperature. Colorless strip-shaped crystals of 6 were col- observed reflections and 381 variable parameters for 3, 5322
lected, washed with distilled water and dried in air. Yield: 53% observed reflections and 309 variable parameters for 4, 8932
(based on Cd). Anal. calcd for C44H32N8O10Cd2: C, 49.97; H, observed reflections and 591 variable parameters for 5, 6013
3.02; N, 15.12%. Found: C, 49.78; H, 2.76; N, 15.42%. IR observed reflections and 580 variable parameters for 6, 3201
(cm−1, KBr): 3445 m, 2832 m, 1593 s, 1471 w, 1388 s, 1235 s, observed reflections and 209 variable parameters for 7. All cal-
1123 s, 835 w, 764 s.
culations were performed using the SHELX-97 crystallographic
software package.9a Selected bond lengths and bond angles,
and hydrogen bond parameters for 1–7 are listed in Tables S1
and S2,† respectively.
Preparation of crystalline [Co(m-HMOPhIDC)(H2O)2]n (7)
A mixture of m-H3MOPhIDC (26.2 mg, 0.2 mmol), CoCl2·6H2O
(23.8 mg, 0.1 mmol) Et3N (0.056 mL, 0.4 mmol) and CH3CN–
H2O (3 : 4, 7 mL) was sealed in a 25 mL Teflon-lined bomb and
heated at 155 °C for 120 h, and then cooled to room tempera-
ture. Pink crystals of 7 were collected, washed with distilled
water and dried in air. Yield: 53% (based on Co). Anal. calcd
for C12H12N2O7Co: C, 40.58; H, 3.43; N, 7.93%. Found: C,
40.78; H, 3.52; N, 7.63%. IR (cm−1, KBr): 3364 w, 2835 w, 1574
s, 1467 s, 1405 m, 1233 s, 1125 s, 848 m, 709 w.
Quantum chemical calculation
The optimized geometry, NBO charge distributions of the free
ligand m-H3MOPhIDC were given by the GAUSSIAN 03 suite of
programs.35 And all calculations were carried out at the B3LYP/
6-311++G(d, p) level of theory.
Results and discussion
X-ray crystallography
Synthesis
Crystal data for compounds 1–7 are contained in Table 1. The deprotonation process and diversity of the coordination
Single crystals of 1–7 were selected and mounted on a glass modes of the ligand p-H3MOPhIDC have been discussed pre-
fiber. All data were collected at room temperature using the viously and eleven corresponding fascinating polymers have
ω–2θ scan technique and corrected for Lorenz-polarization been reported by our laboratory.9e,f It was found that the posi-
effects. A correction for secondary extinction was applied.
tion and type of functional groups of this kind of organic
The seven structures were solved by direct methods and ligand are crucial for the generation of frameworks. To
expanded using the Fourier technique. The non-hydrogen further predict the m-methoxyl substituent effect on the
atoms were refined with anisotropic thermal parameters. m-H3MOPhIDC ligand, we calculated the optimized geometries
Hydrogen atoms were included but not refined. The final cycle and NBO charge distributions of the free ligand m-
of full-matrix least squares refinement was based on 3033 H3MOPhIDC on the B3LYP/6-311++G(d, p) level of theory.35
observed reflections and 200 variable parameters for 1, 2803 The computed results reveal that the negative NBO charges are
observed reflections and 179 variable parameters for 2, 4835 mainly distributed on the oxygen and nitrogen atoms in the
Table 1 Crystallographic data for compounds 1–7
1
2
3
4
5
6
7
Formula
fw
Crystal system
Crystal size
(mm)
Space group
a (Å)
b (Å)
C
12H10N2O6Sr
C24H18N4O10Sr C48H38N8O22Cd3 C24H16N4O5Cu C48H32N8O10Cd2 C44H32N8O10Cd2 C12H12N2O7Co
365.84
610.04
Tetragonal
0.20 × 0.18 ×
0.16
1416.06
Triclinic
0.23 × 0.22 ×
0.21
503.95
1105.62
Orthorhombic
0.40 × 0.30 ×
0.20
1057.58
Monoclinic
0.30 × 0.20 ×
0.10
355.17
Monoclinic
0.20 × 0.18 ×
0.16
Orthorhombic
0.18 × 0.18 ×
0.16
Triclinic
0.20 × 0.18 ×
0.16
ˉ
ˉ
P1
P21/c
I41/a
P1
Pbcn
Pbca
P21
16.085(3)
6.6736(10)
12.681(2)
90
111.524(2)
90
16.7894(12)
16.7894(12)
17.349(3)
90
90
90
9.145(3)
10.039(3)
14.935(4)
70.431(3)
72.443(4)
73.837(3)
1207.5(6)
1.947
16.936(6)
12.348(5)
20.245(8)
90
90
90
19.014(2)
21.244(2)
25.136(3)
90
90
90
9.3619(6) A
22.6503(15)
9.6061(6)
90
94.8710(10)
90
8.0394(14)
8.8500(15)
9.4594(16)
90.930(2)
97.192(2)
102.019(2)
652.47(19)
1.808
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
1266.3(3)
1.919
4890.4(9)
1.657
4234(3)
1.581
10 153.5(19)
1.447
2029.6(2)
1.731
Dc (Mg m−3
)
Z
4
8
1
8
8
2
2
μ (mm−1
)
4.289
2.272
1.407
1.078
0.899
1.120
1.356
Reflns
collected/
unique
11 482/3033
R(int) = 0.0480 R(int) = 0.0843
14 668/2803
6067/4171
R(int) = 0.0689
27 074/5322
R(int) = 0.0747
48 939/8922
R(int) = 0.0700
10 798/6013
R(int) = 0.0193
8956/3201
R(int) = 0.0342
Data/restraints/ 3033/0/200
parameters
2803/0/179
4171/0/370
5322/0/309
8922/0/591
6013/2/588
3201/0/214
R
0.0351
0.0852
0.985
0.0468
0.1074
1.051
0.0487
0.1109
0.916
0.0407
0.1123
0.768
0.0676
0.1894
1.037
0.0188
0.0439
1.032
0.0362
0.0901
1.042
Rw
GOF on F2
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The amount of pyridine is important for the formation of
complex 3. The addition of pyridine (2.5 mmol) has produced
crystals of 3 successfully, while a different amount of pyridine
has failed to produce suitable crystals or has given products
with low yield. We have also attempted to replace pyridine with
Et3N (2.5 mmol or 0.4 mmol) in the synthesis of 3, only
leading to the formation of some unidentified powder. This
fact implies that pyridine has been adopted mainly to adjust
pH values and benefit the crystallization of complex 3.
When auxiliary ligands phen and 2,2′-bipy were introduced
into the reaction system, three chain polymers 4, 5 and 6 can
be obtained. Obviously, these chelating ligands coordinate to
the metal ions and occupy some coordination sites. Thus, a
Scheme 2 The optimized geometries of the free ligands p-H3MOPhIDC and
m-H3MOPhIDC (the blue ball represents N atom, the red ball represents O atom
and the gray ball represents C atom).
similar situation has been observed in the previous coordi-
25
nation polymer [Cd2(μ2-PhHIDC)2(phen)2]n
2-phenyl-1H-imidazole-4,5-dicarboxylic acid).
(PhH3IDC =
The pH values of the reaction solution are adjusted by Et3N,
however pyridine is more suitable for preparation of 3. The
free ligand m-H3MOPhIDC. The NBO charges are −0.0637 for reactions of metal salt and the imidazole dicarboxylate ligand
O1, −0.647 for O2, −0.665 for O3, −0.597 for O4, −0.463 for all in a molar ratio of 1 : 1 have produced crystals of 1–7 suc-
N6, and −0.496 for N7 in the free ligand m-H3MOPhIDC cessfully, while other stoichiometries have failed to produce
(Scheme 2). The methoxyl group plays the role of electron suitable crystals or have given products with low yields (about
donor and the oxygen has a wealth of electron. In comparison 2.5%).
with the free ligand p-H3MOPhIDC,9e the NBO charge distri-
In conclusion, through the careful control of reaction con-
butions of carboxyl oxygen and imidazole nitrogen atoms all dition, such as pH values, solvent, metal-to-ligand molar ratio,
slightly increase because of the introduction of a methoxyl etc., the reproducibility of our experiment is good.
group into the meta position of the benzene ring. These
The IR spectra reveal characteristic absorption bands for
values indicate that the oxygen and nitrogen atoms of the water molecules, carboxyl groups, phenyl, imidazolyl and
m-H3MOPhIDC ligand also have potential coordination ability pyridyl rings. Polymers 1–7 exhibit strong characteristic
and various coordination modes under appropriate reaction absorption bands in the range of 1384–1610 cm−1 due to
conditions. This can be confirmed by our present experimental νas(COO−) and νs(COO−) vibrations. The strong and broad
results. In polymers 1, 3–7, the imidazole-H and COO-H are bands centered at 3620 and 3440 cm−1 are attributed to the
removed from the organic ligand m-H3MOPhIDC to form a νO–H stretching of coordinated water in polymers 1 and 3. The
m-HMOPhIDC2− unit. The singly deprotonated m-H2MOPhIDC− frame vibrations of the phenyl ring in 1–7 vary from 1400 to
and doubly deprotonated m-HMOPhIDC2− can be found in 1700 cm−1
.
The strong bands between 1588 cm−1 and
polymer 2. Our experimental results demonstrate that the 1670 cm−1 in all these polymers imply the CvN and CvC
organic compound m-H3MOPhIDC is also a promising ligand stretching bands of imidazole ring.
like p-H3MOPhIDC to build amazing MOFs, which show
Crystal structure of crystalline polymer [Sr(m-HMOPhIDC)-
(H2O)]n (1)
various coordination modes (Scheme 1) and strong coordi-
nation ability. To understand the coordination modes of
m-H3MOPhIDC, we have analyzed the Cambridge Structure Complex 1 exhibits a 2D framework, which is constructed from
Database, and found about more than four hundred com- alternate 1D helical chains bridged by the carboxyl groups.
plexes bearing H3IDC and its 2-position-substituted deriva-
It is worth noting that each m-HMOPhIDC2− anion in 1
tives. Through comparison of the similarities and differences shows an uncommon coordination mode, namely μ5-kO:kO,O′:
of the coordination modes about the imidazole dicarboxylate kO′,O′′:kO′′,O′′′:kO′′′ (Scheme 1a), which is different from the
ligands in CCDC, we found that each coordination mode of μ2-kO:kO′ coordination mode in the previous polymer [Sr(μ5-
m-H3MOPhIDC can be found in previous different imidazole H2PhIDC)2(H2O)4]·2H2O.9a As shown in Fig. S1,† the asym-
dicarboxylate-based complexes.9,22–27 This suggests that no metric unit of 1 consists of one Sr(II) ion, five m-HMOPhIDC2−
matter what kinds of 2-position substituent the derivatives of anions, one coordinated water molecule. Each Sr(II) ion is co-
H3IDC contain, they all show diversity of coordination modes.
ordinated by nine oxygen atoms from five share μ5-m-HMO-
The different structural complexes 1 and 2 have been PhIDC2− anions and one water molecule showing
a
obtained in high yield by hydrothermal reaction of Sr(NO3)2 polyhedron coordination geometry. Neighboring Sr atoms are
with H3MOPhIDC in different solvents. This indicates that linked by m-HMOPhIDC2− anions to form the 1D left-handed
solvent is crucial for the formation of the two complexes. and 1D right-handed helical chains. The distance of the two
However, the role of solvent in the construction of polymers 1 adjacent intrachain Sr⋯Sr is 4.4593 Å. Surprisingly, the left-
and 2 is ambiguous.
and right-handed helix chains are hierarchical helix of the
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Fig. 1 The layer structure of polymer 1 constructed by the left- and right-
handed helix chains (benzene rings omitted for clarity).
helix, and very like a DNA double helix (Fig. 1a). As shown in
Fig. 1b, six Sr(II) atoms and six m-HMOPhIDC2− units compose
a distorted hexagonal ring. Consequently, these distorted hexa-
gonal rings are connected into a grid sheet by two oxygen
atoms from the carboxyl groups.
The Sr–O bond lengths vary from 2.4828(18)–2.803(2) Å,
which are comparable to the previously reported values.12 The
bond angles around each Sr(II) ion vary from 47.05(6) to
171.81(7)°. The m-HMOPhIDC2− ligands link neighbor Sr(II)
ions forming helical chains along the crystallographic a-axis.
The other oxygen atoms of the m-HMOPhIDC2− ligands bridge
these helices to get a sheet structure. Moreover, the hydrogen
bonds (Table S2†) occurring at O(2)–H(1W)⋯N(3)#6 and N(2)–
H(2W)⋯O(2)#1 enhance the stability of complex 1.
Crystal structure of crystalline polymer [Sr(m-H2MOPhIDC)2]n (2)
Complex 2 features a 3D framework. The central Sr(II) ion is
eight-coordinated with six oxygen (O2B, O2C, O2D, O2E, O4
and O4A) and two nitrogen atoms (N1 and N1A) from six indi-
vidual μ3-m-H2MOPhIDC− anions (Fig. S2†). The Sr–O bond
distances ranging from 2.488(3) to 2.773(3) Å and Sr–N bond
distance being 2.737(3) Å are consistent with the previously
reported values.36–38 Obviously, the metal sites in 2 also
exhibit polyhedron coordination geometry.
Interestingly, the organic ligand exhibits a different coordi-
nation mode from that in polymer 1. They all adopt one type
of coordination mode, namely μ3-kN,O:kO′:kO′ (Scheme 1b) in
2. The coordination mode for the imidazole dicarboxylate
ligand in polymer 2 is rather unusual. One imidazole nitrogen
atom takes part in the coordination to the Sr(II) atom. So far,
only three related reports have been found in the
literature.36–38 As shown in Fig. 2a, each m-HMOPhIDC2−
Fig. 2 (a) The ladder-like chain along the b-axis. (b) The layer constructed from
m-HMOPhIDC2− and Sr(II) atoms. (c) The 3D framework along the b-axis built by
the layers and m-HMOPhIDC2− units (H atoms omitted for clarity). (d) Schematic
anion bridges three Sr(II) atoms to form step-shaped infinite illustrating the topology of the 3D framework of compound 2. (The organic
ligands are represented by pink spheres, Sr ions by green.)
chains (Fig. 2a). The four strontium atoms form a parallelo-
gram in which the two side lengths are 3.6072 and 9.4490 Å,
respectively, and one of the interior angles is 62.676°. These
parallelograms with a common edge connection form a 11.2672 Å. Finally, π–π interactions of the parallel benzene
ladder-like 1D chain. Furthermore, these infinite chains are rings between layers increase the stability of complex 2.
linked by μ3-m-H2MOPhIDC− ligands to build up many layers
From a topological viewpoint, the m-HMOPhIDC2− and the
(Fig. 2b). It is noteworthy that adjacent layers are pillared by two Sr ions can be viewed as a 2-connected node and a 8-con-
the μ3-m-H2MOPhIDC− ligands to result in a three-dimensional nected node. Thus, the 3D framework of 2 can be described as
supramolecular structure with microporous channels (Fig. 2c). a (2,8)-connected topology with the point symbol of (44.1224)-
The microporous channel diameter is about 6.2122 Å × (4)4 (Fig. 2d).
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Crystal structure of crystalline polymer [Cd3(m-H2MOPhIDC)2-
(m-HMOPhIDC)2(H2O)2]n (3)
coordination environments are slightly different. The Cd1 is
chelated by two m-HMOPhIDC2− ligands (Fig. 3a) with O,N
chelating donors (N3C, O6C, N3 and O6) to form two five-
membered chelating rings. The Cd1 center displays a distorted
octahedral environment, in which the equatorial plane com-
prises two nitrogen atoms and two carboxylate oxygen atoms
from two distinct doubly deprotonated m-HMOPhIDC2−
ligands, and two oxygen atoms from the other two m-HMO-
PhIDC2− ligands occupy the apical sites. For Cd2, there is a
little difference: water molecules participate in the coordi-
nation by providing an oxygen atom instead of one hydroxyl
group. It should be noted that the chelating effects of both
organic ligands are the main effect responsible for the distor-
tion in the coordination geometries of both two Cd(II) atoms.
The Cd(1)–O(1) (2.363(5) Å) and Cd(1)–O(6) (2.387(4) Å) bond
distances are longer than that of Cd(2)–O(9)#4 (2.283(4) Å) and
Cd(2)–O(2) (2.299(4) Å), and slightly shorter than that of Cd(2)–
O(9)#1 (2.408(4) Å). Both Cd1 and Cd2 are held together by the
imidazole ring and one carboxylate group. The N and O atoms
behave as a bidentate ligand to yield a binuclear metallocycle.
The organic ligand likes one bridge. Each binuclear unit is sur-
rounded via five organic ligands. These binuclear units are
connected into a layer by N and O atoms chelating as double
bridges (Fig. 3c). Two bond angles (N3C–Cd1–O6 and N4–Cd2–
O9A being 107.02(16)° and 71.43(16)°, respectively) are respon-
sible for the obvious distortion from the octahedral environ-
ment. The Cd–L (L = O or N) distances are in the range of
2.283(4)–2.408(4) Å. The m-HMOPhIDC2− ligands link neigh-
boring Cd(II) ions to form a 1-D wavy chain (Fig. 3b), which
comprise a plane like paper cut for a window decoration along
the b-axis, as shown in Fig. 3c. Within the same [Cd3(μ2-m-
H2MOPhIDC)2(μ3-m-HMOPhIDC)2(H2O)2]n chain, the dihedral
angles between phenyl ring C6–C11 (the mean deviation from
the plane is 0.000 Å) and imidazole ring N1–N2 (the mean
deviation from the plane is 0.0195 Å), and the phenyl ring
C18–C23 and other imidazole ring N3–N4 (the mean deviation
from the plane is 0.0378 Å) are 57.417° and 27.603°, respect-
ively. Finally, such layers are packaged into an interpenetrating
3D supramolecular architecture through weak π–π interactions
and hydrogen bond (Fig. 3d).
Single-crystal X-ray diffraction studies reveal that the architec-
ture of 3 is a sheet structure. As illustrated in Fig. 3a, it con-
tains two crystallographically independent Cd(II) ions and six
organic ligands per asymmetric unit. Interestingly, singly
deprotonated m-H2MOPhIDC− and doubly deprotonated
m-HMOPhIDC2− coexist in 3, and they adopt two types of
coordination modes, namely μ3-kN,O:kN,O:kO (Scheme 1c) and
μ2-kN,O:kO (Scheme 1d), respectively. Each organic ligand
serves as bridges to link Cd atoms by N,O-chelating, O-bridging
and N′,O′-chelating coordination modes into a sheet structure.
Although both Cd1 and Cd2 are six-coordinated, their
Crystal structure of crystalline polymer [Cu(m-HMOPhIDC)-
(phen)]n (4)
Single crystal X-ray diffraction studies reveal that 4 is an in-
finite chain structure. As illustrated in Fig. S3,† the asymmetri-
cal unit of 4 comprises one Cu2+ cation, one phen unit and
one singly deprotonated μ2-m-HMOPhIDC2− anion. The
m-HMOPhIDC2− ligand adopts one type of coordination mode,
namely μ2-kN,O:kN′,O′ (Scheme 1e). The central Cu2+ cation is
six-coordinated by two nitrogen atoms (N1 and N2) from phen,
two oxygen atoms (O3 and O6A) from carboxylate groups and
another two nitrogen atoms (N3 and N4A), showing a slightly
distorted octahedral geometry. The bond distances of Cu–N
vary from 1.987(2) to 2.2853(18) Å while the Cu–O length is
1.9966(16) Å, which are partially different from the previously
Fig. 3 (a) Coordination environments of Cd(II) atom in polymer 3 (H atoms
omitted for clarity). (b) The chain along the a-axis based on Cd(II) and m-HMO-
PhIDC2− units. (c) The layer constructed from μ3-m-HMOPhIDC2− and the chain
(H atoms and benzene ring omitted for clarity). (d) The 3-D supramolecular
network of 3. (e) The simplified topological consideration of 3.
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Fig. 4 (a) The chain along the a-axis based on Cu(II) and m-HMOPhIDC2− units.
(b) The 3D supramolecular structure of polymer 4 supported by π–π interactions
and hydrogen bonds.
reported values.39 The bond angles around the central Cu2+
atom are in the range of 83.12(6) to 177.25(7)°.
The adjacent organic μ2-m-HMOPhIDC2− units are almost
vertical in the infinite uniform chain of 4. Each μ2-m-HMO-
PhIDC2− anion connects the neighboring Cu(II) ions in N,O-
chelation or N′,O′-chelation to obtain a chain along the b-axis
with auxiliary ligand 1,10-phenanthroline arranged regularly
in the side chain (Fig. 4a). The intrachain neighboring Cu⋯Cu
distance is 6.1740(25) Å. Each chain packs with the others
through π–π interactions (the dihedral angle between the two
phen rings being 0.000°) to extend a layer along the ab axes.
Furthermore, the layers are further assembled into a 3D supra-
molecular structure, with π–π interactions and hydrogen bonds
increasing structural stability (Fig. 4b).
Fig. 5 (a) Coordination environments of Cd(II) atoms in polymer 5 (H atoms
omitted for clarity). (b) 1D chain along the a-axis based on Cu(II) and m-HMO-
PhIDC2− units. (c) The 3D supramolecular structure of 5 supported by the π–π
interactions.
Crystal structure of crystalline polymers [Cd2(m-HMOPhIDC)2-
(phen)2]n (5) and [Cd2(m-HMOPhIDC)2(2,2′-bipy)2]n (6)
Polymers 5 and 6 are 1-D zigzag chains and contain very The Cd2 atom is also six-coordinated by two phen nitrogen
similar compositions: Cd(II) ions, m-HMOPhHIDC2− anions, atoms (N5 and N6), two nitrogen atoms (N1 and N9) and two
chelating N-containing ligands, phen or 2,2′-bipy. Therefore, oxygen atoms (O9 and O1) from two μ2-m-HMOPhIDC2−
the structure of 5 was selected and described in detail and per- anions. The Cd–O and Cd–N bond lengths are in the range of
tinent points for the polymer 6 will be just mentioned wher- 2.379(7)–2.416(8) Å and 2.243(8)–2.381(9) Å, respectively, which
ever appropriate.
In polymer 5, the asymmetric unit consists of two Cd(II) around the central Cd(II) ion vary from 71.0(3) to 170.5(3)°.
ions, two phen units and two individual μ2-m-HMOPhIDC2−
It is noteworthy that the Cd atoms are joined by the doubly
are consistent with the previous values.22 The bond angles
anions (Fig. 5). Each imidazole dicarboxylate ligand adopts μ2- deprotonated μ2-m-HMOPhIDC2− ligands to generate an infi-
kN,O:kN′,O′ mode (Scheme 1e), connecting two Cd(II) ions to nite corrugated-shaped chain along the a-axis (Fig. 5b). The
generate a zigzag chain. The two Cd(II) ions assume similar intrachain Cd⋯Cd separations are 6.3948 Å and 6.4333 Å,
distorted octahedral environments. Cd1 is coordinated by two respectively. Notably, a layer is constructed by π–π interactions
nitrogen atoms (N3 and N4) from a phen unit, two carboxylato between the chains. The two phen rings are approximately
oxygen atoms (O4 and O6) and two nitrogen atoms (N2 and parallel and the distance between them is 3.7052 Å. Finally,
N8) from two imidazole rings. N3, N8, O4 and O6 occupy the intermolecular π–π interactions and hydrogen bonds lead to
equatorial sites, and N2, O6 are located at the axial positions. the formation of a 3D network (Fig. 5c).
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interactions (the distance between two staggered phenyl rings
of m-HMOPhHIDC2− anion is 7.1661 Å) (Fig. 6c).
Crystal structure of crystalline polymer [Co(m-HMOPhIDC)-
(H2O)2]n (7)
Single-crystal X-ray structural analysis reveals that 7 is a 1D
chain. As illustrated in Fig. S3,† the asymmetric unit of 7 com-
prises one Co2+ cation, one double deprotonated μ2-m-HMO-
PhIDC2− anion. The ligand adopts one type of coordination
mode, namely μ2-kN,O:kN′,O′ (Scheme 1e). The Co(II) atom is
six-coordinated, adopting a slightly distorted octahedral geo-
metry (Fig. S4†). The equatorial plane is defined by two nitro-
gen atoms (N1, N1B) and two oxygen atoms (O5, O5B) from
two individual m-HMOPhIDC2− ligands. The axial positions
are occupied by two water molecules. The bond distances of
Co–O/N vary from 2.0648 (2)–2.207(2) Å, which are partially
different from the previously reported values.9c The bond
angles around the central Co2+ atom are in the range of 78.64(7)
to 180.00°. Each μ2-m-HMOPhIDC2− anion connects the neigh-
boring Co(II) ions in N,O-chelation or N′,O′-chelation to obtain
a chain along the b-axis (Fig. 7a). In addition, the neighboring
Co⋯Co separation is 6.5794(8) Å within the chain. Finally,
such 1-D infinite chains are further connected and assembled
into an interpenetrating 3-D solid-state network through the
π–π interactions and hydrogen bonds (Fig. 7b).
Thermogravimetric analyses
To characterize the polymers more fully in terms of thermal
stability, their thermal behaviors were studied by the thermo-
gravimetric analysis (TGA). The TG curves have been obtained
Fig. 6 (a) Coordination environments of Cd(II) atoms in polymer 6 (H atoms
omitted for clarity). (b) 1D chain along the c-axis based on Cd(II) and m-HMO-
PhIDC2− units. (c) The 3D supramolecular structure of 6.
Complex 6 is assembled in a 1D chain with two Cd(II)
atoms, two m-HMOPhHIDC2− anions and two 2,2′-bipy in each
asymmetric unit. As shown in Fig. 6a, Cd1 is ligated by two
nitrogen atoms (N3 and N4) from 2,2′-bipy and two oxygen
atoms (O3 and O10) from two individual μ2-m-HMOPhHIDC2−
units in the quasi-plane, and two nitrogen atoms (N2 and N8)
from two individual μ2-m-HMOPhHIDC2− in axial positions.
Cd2 has a similar coordination environment to Cd1. The bond
angles around Cd(II) ion range from 69.47(13) to 166.96(15)°.
Therefore the two Cd(II) ions are both in seriously distorted
octahedral geometries. Interestingly, the doubly deprotonated
m-HMOPhHIDC2− anions all adopt μ2-kN,O:kN′,O′ (Scheme 1e)
in 5 and 6. And the 2,2′-bipy ligands are arranged alternately
beside the chain (Fig. 6b), and the dihedral angle between two
adjacent 2,2′-bipy rings is 33.419°. Similar to 5, intermolecular
hydrogen bonds link infinite parallel chains to form the layers,
Fig. 7 (a) 1D chain along the b-axis based on Co(II) and m-HMOPhIDC2− units.
which assemble in a 3D structure through weak π–π stacking (b) The 3D supramolecular structure of 7.
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840 °C. The final blue residue is CuO (observed 15.44%, calcu-
lated 15.78%).
For polymer 5, the first weight loss of 2.24% in the region
of 32.8–323.7 °C corresponds to the loss of the m-methoxyphe-
nyl group (calculated 2.81%). The further weight loss of
37.49% in the temperature range of 323.7–410.4 °C can be
attributed to the decomposition of phen molecule. It keeps
losing weight from 410.4 to 540.3 °C corresponding to the col-
lapse of other parts of m-HMOPhIDC2− units (observed
36.69%, calculated 36.42%) remaining parts of m-HMOPhIDC2−
unit. The final white residue of 23.58% is close to the calcu-
lated 23.23% based on 2CdO.
For polymer 6, the first weight loss of 49.69% (calculated
49.54%) in the region of 341.2–442.3 °C can be attributed to
the decomposition of the collapse of m-methoxyphenyl group.
The further weight loss of 25.84% corresponds to the two 2,2′-
bipy molecules in the temperature range of 442.3–500 °C
Fig. 8 The TG curves for complexes 1–7.
under flowing air for the crystalline samples of polymers 1–7 (observed 25.84%, calculated 29.53%). The final white residue
(Fig. 8).
For polymer 1, the first weight loss of 5.14% (calculated
of 24.47% is close to the calculated 24.28% based on 2CdO.
For polymer 7, the first weight loss of 12.29% in the region
4.93%) occurs at about between 164.4 °C and 373.9 °C, which of 44.6–170 °C corresponds to the loss of the carboxyl group
is attributed to the loss of coordinated water molecules. The (calculated 12.67%). The further weight loss of 3.77% in the
second weight loss of 59.37% between 373.9 °C and 560 °C temperature range of 170–278.9 °C can be attributed to the
corresponds to the loss of the m-HMOPhIDC2− unit. The water molecule. It keeps losing weight from 278.9 to 480 °C
remaining weight of 36.32% corresponds to the percentage corresponding to the collapse of other parts of m-HMOPhIDC2−
(calculated 35.49%) of the Sr and O components, indicating units (observed 56.12%, calculated 61.18%). The final rose
that the final product is SrO.
residue of 21.25% is close to the calculated value of 21.09%
Polymer 2 is stable up to 161 °C, then loses weight from based on CoO.
161 to 198.5 °C (observed 10.2%, calculated 10%) correspond-
In conclusion, the thermal data of complexes 1–7 are in
ing to two methoxy groups. Subsequently, a plateau region is reasonable agreement with the crystal structure analyses.
observed from 198.5 to 373.0 °C. It keeps losing weight from
Photoluminescent properties
373.0 to 567.3 °C corresponding to the decomposition of the
remaining group of m-HMOPhIDC2− units (observed 23.50%,
Metal–organic coordination complexes constructed from metal
calculated 22.88%). A white amorphous residue is 2SrO
(observed 35.4%, calculated 34.10%).
atoms and conjugated organic ligands are candidates for
hybrid photoactive materials with potential applications.40 The
solid-state luminescent emission spectra of polymers 1–7 and
the free m-H3MOPhIDC ligand were studied at room tempera-
ture. As shown in Fig. 9, the free m-H3MOPhIDC ligand shows
For polymer 3, the weight loss corresponding to the release
of two methoxyphenyl units is observed from 157.8 to 259.2 °C
(observed 7.92%, calculated 8.72%). Then an advanced degra-
dation process takes place after 259.2 °C. It continues to
decompose upon further heating and undergoes a slow weight
loss of 46.00% covering the temperature from 259.2 to
418.6 °C, which corresponds to the destruction of two phen
molecules and four carboxyl groups. It keeps losing weight
from 418.6 to 490.5 °C corresponding to decomposition of the
remaining groups of the imidazole dicarboxylate ligand
(observed 19.56%, calculated 18.36%). Finally, a plateau region
is observed from 490.5 to 840 °C. The remaining weight corres-
ponds to the formation of 1.5CdO (observed 26.52%, calcu-
lated 27.2%).
Polymer 4 exhibits an initial weight loss from 247.4 to
397.8 °C, with the observed weight loss of 42.27% correspond-
ing to the release of one phen molecule and one carboxyl
group (observed 22.24%, calculated 21.26%). After that an
additional weight loss of 41.43% up to 542 °C shall be attribu-
ted to the gradual decomposition of other parts of the organic
Fig. 9 The solid-state photoluminescent spectra of the free m-H3MOPhIDC
ligand. Finally, a plateau region is observed from 640 to ligand and polymers 1, 2, 3, 5, and 6.
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luminescence with the emission maximum at 376 nm by selec- J1210060 and J1103309), and Program for New Century Excel-
tive excitation at 280 nm, which may be attributed to the lent Talents in University (NCET-10-0139) and the Natural
ligand-centered π* → n electronic transitions. The emission Science Foundation of Henan Education Department
spectra of 1 (λem = 360 nm, λex = 300 nm), 2 (λem = 360 nm, λex (2009A150028 and 2011A150029).
= 300 nm), 3 (λem = 393 nm, λex = 295 nm), 5 (λem = 441 nm, λex
= 280 nm) and 6 (λem = 464 nm, λex = 336 nm) show lumines-
cent properties. Compared with the free m-H3DMPhIDC
ligand, blue shifts of 16 nm in 1 and 2 have been observed. We
References
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charge transfer (LLCT) nor metal-to-ligand charge transfer
(MLCT) in nature. Therefore, the emission of 1 and 2 may be
ascribed to a charge transfer transition between ligands and
metal centers, namely ligand-to-metal charge transfer (LMCT),
similar to the reported results on coordination polymers with
benzene dicarboxylate ligands.39 Further, it can be observed
that the emission spectra for 3, 5 and 6 exhibit red-shifts with
respect to the free ligand may be due to the chelating and/or
bridging effect of the deprotonated m-HMOPhIDC2− ligands.
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Current beautiful structures further confirm the stirring
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Acknowledgements
We gratefully acknowledge the financial support by the
National Natural Science Foundation of China (21071127,
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