Acentric and chiral four-connected metal-organic frameworks based on the racemic binaphthol-like chiral ligand of 4-(1-H(or methyl)-imidaozol-1-yl) benzoic acid

Article abstract of DOI:10.1039/c0ce00789g

By modulating the synthetic strategy based on changing substituents on the ligand and solvent used in the synthesis, a series of metal organic frameworks (MOFs) of two chiral binaphthol-like ligands of 4-(1H-imidaozol-1-yl)benzoic acid (HIBA) and 4-(1H-2-

Full text of DOI:10.1039/c0ce00789g

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PAPER
Acentric and chiral four-connected metal–organic frameworks based on the
racemic binaphthol-like chiral ligand of 4-(1-H(or methyl)-imidaozol-1-
yl)benzoic acid†
*
Ke-Hui Cui, Shi-Yan Yao, Hai-Qin Li, Yan-Tao Li, Hai-Ping Zhao, Chun-Jie Jiang and Yun-Qi Tian
*
Received 29th October 2010, Accepted 10th February 2011
DOI: 10.1039/c0ce00789g
By modulating the synthetic strategy based on changing substituents on the ligand and solvent used in
the synthesis, a series of metal organic frameworks (MOFs) of two chiral binaphthol-like ligands of
4-(1H-imidaozol-1-yl)benzoic acid (HIBA) and 4-(1H-2-methylimidazol-1-yl)benzoic acid (HMIBA)
with various interpenetrating topologies have been synthesized under solvothermal conditions:
Zn(IBA)2 (1), Co(IBA)2 (2), Cd(IBA)2$H2O$1.7DMF (3), Cd(MIBA)2$H2O$1.3DMF (4),
Co(MIBA)2$H2O$1.3DMF (5) and Cd(MIBA)2$0.5H2O$1.2DMA (6) (DMF ¼ N,N-
dimethylformamide and DMA ¼ N,N0-dimethylacetamide). X-ray diffraction study reveals that
compounds 1 and 2 crystallize in tetragonal crystal system with chiral space groups P4122 and P4322,
respectively, indicating that they are a pair of structural enantiomers, having a twofold interpenetrating
(4,4)-net. Compound 3 crystallizes in orthorhombic crystal system and non-centrosymmetric space
group Pca21, which can be defined as an abnormal fourfold [2 + 2] interpenetrating diamond net. The
isostructural compounds 4 and 5 crystallize in tetragonal crystal system and space group P4212,
representing a chiral structure and a normal mode of fourfold interpenetrating diamondoid net.
Compound 6 crystallizes in non-centrosymmetric space group Aba2 and exhibits a normal mode of
fourfold diamondoid interpenetrating net.
similar structural features of zeolitic primary building units of
TO4 (T ¼ tetrahedral Si or Al, O–T–O ¼ 109.5ꢀ, T–O–T ¼ 145ꢀ,
in which the T–O bonds could freely rotate), the metal (zinc(II),
cobalt(II) and cadmium(II)) imidazolate frameworks not only
have adopted the zeolitic topologies, but also have developed
several elegant unprecedented zeolite-like structures.6i All of
these have formed a large family of ZMOFs. Besides the ZMOFs
formed by four-coordinated metal ions and imidazole (or
substituent imidazole) ligands, the ZMOFs derived from eight-
coordinated metal ions and 4,5-imidazoledicarboxylic acid
(IDCA)6e have also been realized owing to the four dentate
ligands functioning as the linkers of eight-coordinated nodes in
four-connected nets. However, due to the rotational inflexibility
of the chelate bonds between the ligands and metal atoms, this
motif suffers from limited zeolitic topological varieties.
Introduction
Metal–organic frameworks (MOFs) are crystalline solids
constructed from metal ions and organic ligands, and have
attracted much attention from chemists and material scientists
owing to their promising applications in magnetism,1 catalysis,2
luminescence and chemical sensing,3 as well as gas adsorption
and/or separation.4 However, any of these applications has to
meet the requirement of material thermal robustness. Since the
first porous MOF reported by Robson and others in the early
1990s,5 the newly discovered porous materials have developed
from the first generation of fragile MOFs to the current third
generation of thermally stable ones involving dynamic channels.4
This great progress mainly depends on the rational design and
reticular synthesis of the MOFs, of which the zeolitic metal–
organic frameworks (ZMOFs)6 are among the most eye-catching
for approaching third generation porous MOFs. Owing to the
To create novel MOFs having zeolitic structures, another
imidazole derivative of 4-(1H-imidaozol-1-yl)benzoic acid
(HIBA) may be worthy of consideration. MOFs based on HIBA
have scarcely been reported7 where the ligand combines with six-
coordinated divalent metal ions in forming four-connected nets.
We expect that ZMOFs can also be realized by using HIBA
ligand and the divalent metal ions (Scheme 1).
Institute of Chemistry for Functionalized Materials, College of Chemistry
and Chemical Engineering, Liaoning Normal University, 116029 Dalian,
PR China. E-mail: jiangcj@lnnu.edu.cn; yqtian@lnnu.edu.cn; Fax: +86-
411-82156858; Tel: +86-411-82159141
† Electronic supplementary information (ESI) available. CCDC
reference numbers 795326–795333 for HIBA, HMIBA and 1–6. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0ce00789g
Moreover, the ligand shows a dihedral angle between the
imidazolyl and benzoyl rings that affords the ligand with
binaphthol-like atropisomeric chirality8 (Scheme 2b). Thus, by
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CuSO4$5H2O (0.0025 g, 1.0 ꢂ 10ꢁ5 mol) were ground sufficiently
in an agate mortar. Then, the powdered reactant mixture was
transferred into a 30 mL Teflon-lined autoclave, which was
sealed and heated at 200 ꢀC for 10 h. After cooling the reaction
system to room temperature, the reaction mixture was dissolved
in water, and then, filtered off. The filtrate was adjusted with
dilute hydrochloric acid for pH 2.0–3.0 and evaporated to
dryness in vacuum. This raw product was recrystallized in
ethanol and 0.78 g colorless needle crystals of HIBA$HCl$H2O
were obtained (Yield: 62%, based on 4-bromobenzoic acid). IR
(cmꢁ1, KBr): 3438 (w), 3266 (m), 1702 (vs), 1608 (m), 1536 (m),
1426 (w), 1386 (m), 1335 (m), 1240 (s), 1106 (m), 1036 (w),
935 (w), 792 (m). Elementary Anal. Calcd for C10H11N2O3Cl: C,
49.50; H, 4.57; N, 11.54%. Found: C, 51.06; H, 4.97; N, 11.86%.
HIBA$HCl$H2O was prepared in a procedure almost the same
as above, where the imidazole was replaced by 2.05 g (0.025 mol)
2-methylimidazole. 0.87 g product was obtained (yield: 68%
based on 4-bromobenzoic acid). IR (cmꢁ1, KBr): 3356 (m),
3155 (m), 2787 (s), 1703 (vs), 1605 (s), 1515 (m), 1406 (m),
1236 (vs), 916 (w), 856 (m), 756 (m). Elementary Anal. Calcd for
C11H13N2O3Cl: C, 51.47; H, 5.10; N, 10.91%. Found: C, 51.10;
H, 5.27; N, 10.56%.
Scheme 1 (a) The coordinate orientation similarity between HIBA and
imidazole (b) The schematic illustration of the four-connected nodes
based on the six-coordinated metal (M) and the ligand of HIBA
(or HMIBA) (only three neighboring M-atoms are selected for clarity).
Synthesis of Co(IBA)2 (1)
A
mixture of Co(NO3)2$6H2O (0.0291 g, 0.1 mmol),
HIBA$HCl$H2O (0.0243 g, 0.1 mmol), and DMF (6 mL) was
stirred at room-temperature for 10 min and then was placed into
a 10 mL Teflon-linꢀed stainless steel container, which was sealed
and heated at 200 C for 12 h. After cooling to room tempera-
ture, blue block crystals of 1 were collected (Yield 60%, based on
Co). Elementary Anal. Calcd for C20H14N4O4Co: C, 55.44; H,
Scheme 2 (a) A schematic illustration of the synthesis of HIBA$HCl and
HMIBA$HCl. (b) The binaphthol-like atropisomeric chirality for the
ligands of HIBA and HMIBA
a rational synthesis based on this ligand, chiral ZMOFs could be
realized that is essentially significant for porous materials.9
Therefore, HIBA and 4-(1-methyl-imidazol-1-yl)benzoic acid
(HMIBA) were synthesized according to earlier literature10
(Scheme 2a). And by using the ligands, six MOFs were obtained
under solvothermal conditions. Herein, we report the synthesis
and characterization of these MOFs.
3.26; N, 12.93%. Found: C, 55.61; H, 3.16; N, 12.85%. IR (cmꢁ1
,
KBr): 3415 (m), 3124 (m), 1612 (vs), 1570 (s), 1523 (s), 1375 (vs),
1240 (m), 1057 (m), 969 (w), 835(m), 787 (m).
Synthesis of Zn(IBA)2 (2)
A
mixture of Zn(NO3)2$6H2O (0.0297 g, 0.1 mmol),
Experimental
HIBA$HCl$H2O (0.0243 g, 0.1 mmol) and DMF (6 mL) was
stirred at room-temperature for 10 min, and then was placed into
a 10 mL Teflon-lined autoclave, which was sealed and heated at
Materials and physical measurements
ꢀ
70 C for 11 days. After cooling to room temperature, colorless
Solvents and starting materials for synthesis were obtained
commercially and used as received without further purification.
The IR spectra were recorded from KBr pellets in the range of
4000–400 cmꢁ1 on a FT-IR spectrometer TENSOR 27 (see the
ESI, Fig. S1 and S2†). The C, H and N elemental analyses were
conducted using a Perkin-Elmer 240C elemental analyzer. The
XRD analyses were carried out on a Bruker D8 Advance (see the
ESI, Fig. S3†) and the Thermogravimetric Analysis (TGA) was
needle crystals of 2 were collected (Yield 52% based on Zn).
Elementary Anal. Calcd for C20H14N4O4Zn: C, 54.63; H, 3.21;
N, 12.74%. Found: C, 54.46; H, 3.39; N, 12.86%. IR (cmꢁ1, KBr):
3436 (m), 3120 (m), 1619 (s), 1529 (m), 1374 (s), 1123 (m),
1071 (m), 846 (m), 775 (m).
Synthesis of Cd(IBA)2$H2O$1.7DMF (3)
ꢀ
performed under a nitrogen stream with a heating rate of 10 C
A
mixture of Cd(NO3)2$4H2O (0.0308 g, 0.1 mmol),
minꢁ1 by using a Perkin-Elmer Diamond Thermogravimetric
HIBA$HCl$H2O (0.0243 g, 0.1 mmol) and DMF (6 mL) was
stirred at room-temperature for 10 min, the mixture was trans-
ferred into a 10 mL Teflon-lined autoclave, which was then
Analyzer (see the ESI, Fig. S4†).
Preparation of binaphthol–like ligands
ꢀ
sealed and heated at 130 C for 2 days. After cooling to room
The synthesis of HIBA$HCl$H2O was performed according to
the literature method for preparation of 1,3,5-tri(7-azaindol-1-
yl)benzene.10 A mixture of K2CO3 (1.93 g, 0.014 mol), 4-bro-
mobenzoic acid (1.00 g, 0.005 mol), imidazole (1.71 g, 0.025 mol),
temperature, colorless block crystals of 3 were isolated by
filtration. (Yield 45%, based on Cd). Elementary Anal. Calcd for
C22.85H20.65N4.95O6.7Cd: C, 46.98; H, 3.57; N, 11.87%. Found: C,
45.68; H, 3.21; N, 11.32%. IR (cmꢁ1, KBr): 3423 (m), 3126 (w),
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1664 (m), 1600 (s), 1554 (s), 1525 (s), 1399 (vs), 1303 (m),
1250 (w), 1058 (m), 858 (w), 781 (m), 748 (m), 659 (w).
summarized in Table S1 (see the ESI†) and Table 1, respectively.
Selected bond lengths and angles are listed in Table S2.†
Synthesis of Cd(MIBA)2$H2O$1.3DMF (4)
Results and discussion
A mixture of Cd(NO3)2$4H2O (0.0308 g, 0.1 mmol), HMI-
BA$HCl$H2O (0.0509 g, 0.2 mmol) and DMF (6 mL) was stirred
at room-temperature for 10 min. Then, the mixture was trans-
ferred into a 10 mL Teflon-lined autoclave which was sealed and
heated at 130 ꢀC for 2 days. After cooling to room temperature,
colorless rod crystals of 4 were isolated by filtration. (Yield 45%,
based on Cd). Elementary Anal. Calcd for C25.9H27.9O6.1N5.3Cd:
C, 49.96; H, 4.53; N, 11.93%. Found: C, 49.44; H, 4.18; N,
11.45%. IR (cmꢁ1, KBr): 3455 (m), 3126 (w), 3008 (w), 2933 (w),
2858 (w), 1677 (s), 1600 (s), 1548 (s), 1406 (vs), 1303 (m),
1097 (m), 865 (m), 788 (m), 742 (m), 671 (w).
Syntheses and characterization of the ligands
The ligand of HIBA was synthesized by modulating the literature
reported method as illustrated in Scheme 2. In order to obtain the
derivatives of HIBA with alkyl substituents on the imidazolyl
ring; expecting that the alkyl substituent could function as the
template or structure-directing group, we used 2-alkyl (alkyl ¼
methyl, ethyl and propyl) imidazole in place of 2-H-imidazole.
However, because of the domain of the SN2 mechanism in the
reaction, a large substituent on the imidazolyl ring serves as an
obstacle in hindering the substitution reaction. Therefore, by the
present synthetic routine, only ligands of HIBA and HIMBA can
be prepared and they were obtained as the salts of HIBA$HCl and
HMIBA$HCl. X-ray single-crystal diffraction study on the
ligands confirms that they have the binaphthol-like atropisomeric
chirality and show dihedral angles of 37ꢀ and 50ꢀ between the
imidazolyl and benzoyl rings, respectively. Despite the ligands
being chiral, they were prepared as racemic mixtures in a ratio of
R : S ¼ 1 : 1 (see ESI S5†). And the chirality resolution for the
ligands was not performed due to racemization of the compounds
readily taking place under solvothermal conditions, under which
their coordination polymers will then be synthesized.
Synthesis of Co(MIBA)2$H2O$1.3DMF (5)
Blue rectangle crystals of 5 (Yield 52% based on Co) were
obtained by adopting the same procedures for the synthesis of 4,
where Co(NO3)2$6H2O (0.0291 g, 0.1 mmol) was used instead
of
Cd(NO3)2$4H2O.
Elementary
Anal.
Calcd
for
C26H29.5O6.3N5Co: C, 54.69; H, 5.22; N, 12.27%. Found: C,
54.31; H, 4.98; N, 11.87%. IR (cmꢁ1, KBr): 3479 (m), 3136 (w),
3080 (w), 2931 (w), 2861 (w), 1674 (s), 1604 (s), 1546 (s), 1417
(vs), 1306 (m), 1094 (w), 862 (m), 790 (m), 740 (m), 678 (w).
Syntheses of compounds 1–6
Synthesis of Cd(MIBA)2$0.5H2O$1.2DMAC (6)
The syntheses of the compounds was carried out under
solvothermal conditions, in which the hydrochloric salts of the
ligands were used. Thus, the solvent utilized should be DMF and
DMA since they can digest the extra acid during the sol-
vothermal reaction. The metal ions employed in the synthesis are
divalent six-coordinated transition metal cations because they
can incorporate with the ligands to from the neutral four-con-
nected frameworks. So far, only the frameworks of Co(II), Zn(II)
and Cd(II) can be obtained and they generate six MOFs, repre-
senting four different structural types. X-ray powder diffraction
patterns were experimentally obtained on the bulk samples of
1–6 and they are in agreement with those simulated from their
X-ray single-crystal data, revealing that the structures of the
single crystal 1–6 represent those of the bulk samples.
A mixture of Cd(NO3)2$4H2O (0.0308 g, 0.1 mmol), HIM-
BA$HCl$H2O (0.0509 g, 0.2 mmol) and DMAC (6 mL) was
stirred at room-temperature for 10 min, then it was transferred
intoꢀa 10 mL Teflon-lined autoclave that was sealed and heated at
100 C for 4 days. After cooling to room temperature, colorless
cross-shaped crystals of 6 were isolated by filtration. (Yield 60%,
based on Cd). Elemental Anal. Calcd for C30.8H37.8N6.2 O6.2Cd:
C, 52.36; H, 5.40; N, 12.30%. Found: C, 51.89; H, 4.84; N,
11.94%. IR (cmꢁ1, KBr): 3429 (m), 3114 (w), 3069 (w), 2927 (w),
1602 (s), 1547 (s), 1400 (vs), 1311 (m), 1106 (w), 857 (m), 786 (m),
738 (w), 677 (w).
Single-crystal X-ray crystallography
X-ray single-crystal diffraction data for both of the two free
ligands and their coordination polymers 1–6 were collected on
a Bruker SMART CCD diffractometer with graphite-mono-
Crystal structures of Co(IBA)2 (1) and Zn(IBA)2 (2)
ꢀ
chromatized Mo-Ka radiation (0.71073 A) at 293(2) K. Data
Both of the compounds 1 and 2 crystallize in tetragonal and have
almost the same crystal parameters. Therefore, they are enan-
tiomeric crystals with space groups P4122 and P4322, respec-
tively. In the structures of 1 (or 2), Co(II) (or Zn(II)) ions are
four-coordinated, while the carboxyl groups of the ligands are
monodentate and they associate with imidazolyl nitrogen atoms
in linking the metal cations (Fig. 2a, 2b) into a two-dimensional
sheet of (4,4)-network. Two-fold of such networks interpenetrate
(Fig. 1c) and are stacked into the individual single crystal. With
the Flack factors of 0.04(3) for 1 and 0.01(4) for 2, the two
determined single crystals are individually characterized as the
pure chiral enantiomers (Fig. 1a and 1b) and their chirality
originates from the enantiopure ligands of (R)-HIBA and
integration and reduction were processed with SAINT11 soft-
ware. An empirical absorption correction was applied to the
collected reflections with SADABS12 using XPREP.13 The
structure was solved by the direct method using SHELXTL14 and
refined on F2 by the full matrix least-squares technique using the
SHELXL-97 program package. All non-hydrogen atoms were
refined with anisotropic thermal parameters. Because guest
solvent molecules (DMF or DMA) in the channels of 3–6 were
highly disordered, the diffused electron densities resulting from
them were removed by the SQUEEZE routine in PLATON15 and
the results were attached to the CIF file. Details of the structural
analysis of the two chiral ligands and compounds 1–6 are
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Table 1 Crystallographic data and structure refinement summary for compounds 1–6
Compound
1
2
3
4
5
6
Formula
Formula
weight
Crystal
system
Space
C20H14N4 O4Co
433.28
C20H14N4 O4Zn
439.72
C22.9H20.7N5.0 O6.7Cd
584.19
C25.9H28N5.3 O6.1Cd
622.58
C26H29.5N5 O6.3Co
570.98
C30.8H37.8N6.2 O6.2Cd
706.47
Tetragonal
Tetragonal
Orthorhombic
Tetragonal
Tetragonal
Orthorhombic
P4122
P4322
Pca21
P4212
P4212
Aba2
group
ꢀ
a/A
8.0814(3)
8.0814(3)
28.0836(4)
90
90
90
1834.11 (10)
0.04(3)
4
1.569
0.976
11555/2343
8.0674(4)
8.0674(4)
28.0367(6)
90
90
90
1824.71(13)
0.01(4)
4
1.601
1.007
9881/1771
19.0367(16)
8.2300(7)
18.2963(15)
90
90
90
2866.5(4)
0.63(3)
4
1.458
0.994
11412/4623
18.8725(15)
18.8725(15)
8.0221(13)
90
90
90
2857.2(6)
0.36(6)
4
1.463
1.013
14296/2503
18.5718(9)
18.5718(9)
7.7776(8)
90
90
90
2682.6(3)
0.47(3)
4
1.421
1.074
12408/2128
15.573(2)
19.416(3)
19.664(3)
90
90
90
5945.8(15)
0.10(4)
8
1.406
1.048
11183/3653
ꢀ
b/A
ꢀ
c/A
a (ꢀ)
b (ꢀ)
g (ꢀ)
3
ꢀ
V/A
Flack
Z
Dc/g cmꢁ3
GOF
Reflns
collected/
unique
Rint
0.0034
0.0329/0.0691
0.0699
0.0435/0.1008
0.0277
0.0282/0.0686
0.1077
0.0462/0.0605
0.0503
0.051/0.128
0.0688
0.045/0.078
R1/wR2
[I > 2s(I)]
(S)-HIBA, of which compound 1 or 2 consists. It should be noted
that the compounds 1 and 2 have been earlier reported by Chen7d
and Sun,7b who had occasionally determined the single crystals of
1 and 2 with the space group P4322 and P4122, the enantiomeric
crystals of 1 and 2 with the space groups P4122 and P4322,
respectively. This result reveals that the racemic form ligands of
HIBA (R) and HIBA (S) can be induced by the metal cations of
cobalt(II) (or zinc(II)) into a recognition-driven self-assembly
of the enantiopure chiral crystals of (R)-1 (with space group
P4322) and (S)-1 (with space group P4122) (or (R)-2 (with space
group P4322) and (S)-2 (with space group P4122)), respectively.
However, these individual enantiopure single crystals are mixed
in a ratio of 1 : 1, which are racemic in bulk.
Fig. 2 (a) The schematic views of the [2 + 2] interpenetration dia-
mondoid net. (b) A single distorted adamantane cage in 3, which consists
of six (S)-IBAꢁ (red), six (R)-IBAꢁ (blue) and ten Cd metal ions (pink)
(hydrogen atoms have been omitted for clarity). (c) Topological diagram
of framework 3, in which the curved links represent the non-linear ligand
of IBAꢁ.
Cd(IBA)2$H2O$1.7DMF (3)
Compound 3 crystallizes in orthorhombic system, space group
Pca21. In the crystal, the metal atoms are six-coordinated and the
carboxyl groups are bidentate and associated with imidazolyl
Fig. 1 (a) 2D (4, 4)-network for 1 (or 2) with space group P41222. (b) 2D (4, 4)-network for 1 (or 2) with space group P4322. (c) Space-filing diagram of
the (4,4)-network interpenetration in 1 or 2.
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nitrogen atoms in a six-coordinate mode just as that illustrated in
Scheme 1b, forming the expected four-connected net. However,
it shows an interpenetrating network consisting of four inde-
pendent equivalent diamondoid nets with the net separation in
ꢀ
ꢀ
Cd/Cd distance of 12.390 A along the a-direction and 12.536 A
along the c-direction (Fig. 2a), respectively. This fourfold net
refers to the ‘‘abnormal’’ interpenetrating diamond-like net16
which can be described as a [2 + 2] interpenetrating diamondoid
net (Fig. 2a, c) that is similar to the eightfold [4 + 4] inter-
penetrating diamond network.17 Unlike compounds 1 and 2,
compound 3 is achiral because each of its single crystals is con-
structed by equal proportions of (R)-IBA and (S)-IBA (Fig. 2).
Fig. 4 (a) The ball-and-stick (top) and the topological (bottom)
diagrams of the normal four-fold interpenetrating diamond-like network
of 6. (b) Toplogical diagram of 3D framework of 6 in which the curved
links represent the non-linear ligand of IMBAꢁ.
Cd(MIBA)2$H2O$1.3DMF (4) and Co(MIBA)2$H2O$1.3DMF
(5)
unit must be lacking in, at least, one of the 3 structural features6g
owned by the zeolitic primary building unit. We can see in Scheme 1
that the chelating double bonds between the carboxyl groups and the
metal atoms have made the four-connected frameworks of M(IBA)2
and M(MIBA)2 lose its flexibility of the Si–O bond-like rotation.
Although there are also single bonds, such as the C–C (between
carboxyl group and benzoic ring) and C–N (between benzoic and
imidazolyl rings) bonds, in the ligands of IBAꢁ and MIBAꢁ, they do
not have the rotational flexibility due to the conjugating effect
between the groups. Despite the building unit illustrated in Scheme 1
possessing the Si-like tetrahedral node and Si–O–Si-like angle,
however, without the flexibility of the Si–O bond-like rotation, it is
still difficult to form the zeolitic structures with the versatility of
Si–Si–Si-like angles.
Both of the compounds 4 and 5 crystallize in tetragonal, space
group P4212. In the crystals, the metal atoms are also six-coor-
dinated and the carboxyl groups have also the same coordination
mode as that illustrated in Scheme 1. With the same four-con-
nected diamond-like net (Fig. 3a), the two compounds charac-
terize not only as the chiral, but also as the isostructural motifs.
However, with the Flack parameters of 0.32(10) for 4 and 0.48(5)
for 5, the two compounds are almost or totally racemic in the bulk.
Topologically, although the compounds of 4 and 5 also exhibit
a fourfold interpenetrating diamond-like net (Fig. 3b), they
represent a ‘‘normal’’ interpenetration mode16 (Fig. 3b) that are
commonly found in the diamond-like interpenetrating MOFs.
Cd(MIBA)2$0.5H2O$1.2DMA (6)
Also, in reviewing the MOFs demonstrated above, 50% of the
structures (Table 2) are chiral that reveals the use of a racemic
chiral ligand being more likely to obtain the chiral structures and
chiral recognition of the racemic ligands can be realized by
forming enantiopure single crystals of 1 (or 2). However, in order
to obtain the homochiral MOFs of the ligands, chiral separation
of the ligands is still necessary because these enantiopure single
crystals of 1 and 2 are still racemic in the bulk that are optically
equal to the racemic twin chiral crystals of 4 and 5, in which
chirality is transmitted from the chiral enantiomeric ligands.
Additionally, compared with the chiral ligands that contain the
asymmetric carbon atoms, the binaphthol-like ligands are more
rigid and more likely to form the robust MOFs. Therefore, design
and synthesis of enantiopure binaphthol-like ligands should be an
effective strategy for realizing the robust chiral porous MOFs.
Like the other ZMOF synthesis, the solvent used can function
as the structure-directing agent that is especially useful to the
ligand with alkyl substituent groups, for example, different
structural Cd(MIBA)2 of 4 and 6 were generated by using DMF
and DMA as solvents. But, by using the solvents only one
structural Cd(IBA)2 of 3 was synthesized.
Compound 6 crystallizes in the orthorhombic system, space
group Aba2. In the structure of 6, the coordination mode of the
metals and ligands is the same as that of 3, 4 and 5, while the
framework topology is also the fourfold diamond-like inter-
penetrating net of the ‘‘normal’’ mode (Fig. 4). With the achiral
framework, compound
6 contains equal proportions of
(R)-MIBA and (S)-MIBA ligands which are related symmetri-
cally by the mirror-planes in the single crystals.
In review of the MOFs demonstrated above, two 2D layer struc-
tures and four 3D frameworks with diamond-like topology based on
the HIBA and HMIBA were obtained that were obviously not the
expected ZMOFs. With the uninodal net and vertex symbol of
626262626262, dia topology represents the uniform and non-porous
frameworks. If a primary building unit has the strong tendency to
form the four-connected framework with dia topology, this building
Thermal stabilities of 3–6
In order to examine the thermal stabilities of 3–6, TGA experi-
ments were performed. The TGA data of compounds 3–6
depicted in Fig. S4† show that each of the compounds has a two-
Fig. 3 (a) The single adamantane cage in 4 and 5 that consists of
enantiopure (R)-MIBAꢁ (or (S)-MIBAꢁ) and metal ions. (b) The four-
fold interpenetration of diamondoid networks in crystals of 4 and 5.
(c) Topological diagram of framework 4 and 5, in which the curved links
represent the non-linear ligand of IMBAꢁ.
ꢀ
step of ca. 19–22% weight-loss before 170 C, corresponding to
the departure of H2O and DMF molecules. There is no weight-
loss or obvious heat change observed in the temperature range of
3436 | CrystEngComm, 2011, 13, 3432–3437
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Table 2 General view and comparison of 2–6
CoL2
CdL2
Ligands L
Solvent
Single-Crystal
Space group
Configuration of L
Absolute configuration
Network
Degree and mode of
interpenetration
HIBA
DMF
1 and 2
P4122
S
Chiral complex
(4,4)
2
Normal
HMIBA
DMF
5
P4212
S and R
Twin Crystal
Diamond
4
HIBA
DMF
3
Pca21
S:R ¼ 1 : 1
Mesomer
Diamond
4
HMIBA
DMF, DMA
4
HMIBA
DMA
6
Aba2
S:R ¼ 1 : 1
Mesomer
Diamond
4
P4212
S and R
Twin Crystal
Diamond
4
Normal
[2 + 2]
Normal
Normal
172–350 ꢀC, indicating that the guest-free frameworks can
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ꢀ
maintain their framework integrity below 350 C.
ꢁ
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Conclusion
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In summary, six MOFs based on the particularly designed
binaphthol-like chiral ligands of HIBA and HMIBA have been
synthesized under solvothermal conditions. Owing to the struc-
ture-directing effect of the substituent on the imidazolyl ring and
the solvent used in the synthesis, different interpenetrating dia-
mond-like structures were obtained. However, none of ZMOF-
type structures based on the ligands can be synthesized due to the
MOFs lack of the Si–O bond-like rotational flexibility. As the
binaphthol-like chiral ligands, despite the ligands of HIBA and
HMIBA are used as synthesized in the racemic form, 50% of
MOFs obtained are chiral that reveals the binaphthol-like
ligands being another likely choice for the rational design and
synthesis of chiral MOFs with robust structures. Nevertheless,
the search for new binaphthol-like ligands in construction of the
chiral ZMOFs is still under way.
Acknowledgements
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The authors gratefully acknowledge the financial support from
the Foundation for the Author of National Excellent Doctoral
Dissertation of PR China (FANEDD) (200733) and the National
Natural Science Foundation of China (20741003 and 20771007).
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