Integration of rigid and flexible organic parts for the construction of a homochiral metal-organic framework with high porosity

Article abstract of DOI:10.1039/c4cc09821h

Presented is a pair of homochiral metal-organic frameworks built from mixed ligands integrating rigid and flexible organic parts, and each compound shows high porosity and can be used for enantioselective separation of racemic 1-phenethylalcohol and methyl lactate. This journal is

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Integration of Rigid and Flexible Organic Parts for the Construction of
Homochiral Metal-Organic Framework with High Porosity
Zhong-Xuan Xu,ab Yan-Xi Tan,a Hong-Ru Fu,a Yao Kang,a Jian Zhang *a
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
Presented is a pair of homochiral metal-organic frameworks
built from mixed ligands integreting rigid and flexible organic
parts, and each compound shows high porosity and can be
10 used for enantioselective separation of racemic 1-
phenethylalcohol and methyl lactate.
Homochiral metal-organic frameworks (HMOFs) have attracted
more attentions for their promising application in enantioselective
separation, asymmetry catalysis, and so on.1-3 Although great
15 progresses have made in the past ten years, the design and
synthesis of HMOFs with high porosity is still a huge challenge.
Self-assembly of enantiopure organic ligands with metal ions is
the most effective synthesis method to construct HMOFs.3
However, an ideal enantiopure ligand should have easy
20 availability, low price, coordination variety and rigidity or semi-
rigidity. Therefore, rational design and selection of chiral ligands
has become a key factor to get multifunctional HMOFs.4
Nature amino acids, which are inexpensive, nontoxic and
readily available, may be the ideal enantiopure linkers for the
25 formation of HMOFs. However, for the flexible nature of amino
acids, HMOFs from amino acids that possess 3D architecture as
well as permanent porosity are always limited.5 In order to
overcome this limitation, we have modified the -NH2 group of
proline with 1,3,5-benzenetricarboxylic acid (H3BTC) and
30 synthesized a pair of proline derivatives ((S)-H3PIA and (R)-
H3PIA) (Scheme 1). Containing proline and isophthalate units,
the (S)-H3PIA and (R)-H3PIA are semi-rigid chiral ligands and
have provided a new approach to design and construct 3D
HMOFs with large porosity.6 Furthermore, adding rigid auxiliary
35 ligands (e.g. 4,4’-bipyridine) to support the porous structures of
such HMOFs is also an effective approach.7 So we believe the
combination of semi-rigid links ((S)-H3PIA or (R)-H3PIA) and a
rigid auxiliary ligand can synthesize HMOFs with larger porosity
and specific functions.
Figure 1. The coordination environment in 1-D.
40
According to the above mentioned synthetic strategy, we chose
a C3 symmetric rigid ligand, 1,3,5-tri(1H-imidazol-1-yl) benzene
(TIB), to help (S)-H3PIA or (R)-H3PIA to build HMOFs. In this
contribution, a pair of Cd-based HMOFs has been gained by
using mixed DMF and H2O solvents, namely [Cd9((R)-
45 PIA)6(TIB)4(H2O)12].3H2O (1-D) and [Cd9((S)-PIA)6(TIB)4
(H2O)12].3H2O (1-L).† Compound 1-D (1-L) exhibits three-
dimensional porous structure with unique nanocages and trigonal
helical channels.
Because compounds 1-D and 1-L are enantiomers, we mainly
50 focused on the structural details of 1-D. X-ray crystallography
reveals that compound 1-D crystallizes in the chiral space group
P4332 with a Flack parameter of -0.001(14).‡ There are two
independent Cd(II) ions in the asymmetric unit of 1-D (Figure 1).
Cd1 has distorted pentagonal bipyramid geometry, which is
55 coordinated by five carboxylate O atoms from three (R)-PIA
ligands, one N atom from TIB ligand and a water O atom. Cd2
shows distorted trigonal bipyramid geometry, and it is
coordinated by two carboxylate O atoms, two N atoms from two
TIB ligands and a water O atom. The (R)-PIA ligand acts as a μ4-
60 linker and has two types of carboxylate groups with different
coordination modes: (a) the carboxylate group from proline unit
acts in bidentate bridging mode to link Cd1 and Cd2; (b) the
carboxylate group of isophthalate unit only links a separate Cd
ion (Cd1 or Cd2). Due to the unique coordination modes of (R)-
65 H3PIA ligand, a trigonal trinuclear Cd (Cd2, Cd1, Cd2) unit
Cd3(CO2)6 is formed (Figure S1). In this Cd3(CO2)6 unit, the
Cd1···Cd2 distance is 4.425 Å. Some guests in the pores are
O
O
Chiral Component
(Proline Unit)
HO
OH
O
O
N
N
Achiral Component
( isophthalate unit)
HO2C
CO2H
(R)-H3PIA
HO2C
CO2H
(S)-H3PIA
Scheme 1. The enantiopure linkers: (S)-H3PIA and (R)-H3PIA.
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DOI: 10.1039/C4CC09821H
Figure 4. a) The left-handed helical channel in 1-D; b) the 3D Cd-TIB
framework in 1-D; c) the srs net of Cd-TIB framework in 1-D.
Figure 2. a) The A-cage- in 1-D; b) the B-cage substructure in 1-D; c) the
cages in 1-D.
Figure 5. The solid-state CD spectra of bulk samples of 1-D and 1-L.
isophthalate units and three Cd1 centers (Figure 2c). Such cage-
by-cage mode leads to the generation of the 3D framework of 1-
D (Figure 3).
Since the structure of 1-D contains two distinct ligands (TIB
and (R)-PIA), it is easy to divide the whole structure into two
parts and further simplify this complicated topological net. If only
the connectivity between TIB and Cd2 centers is considered, a 3D
Cd-TIB framework with helical channels is generated (Figure 4a-
Figure 3. The 3D framework of 1-D (The yellow balls represent the
cages.).
20
disorder and cannot be identified. The contribution of the
disordered guests was subtracted from the reflection data by the
SQUEEZE method.
25 b). The existence of trigonal helical channels is the second
remarkable structural feature of 1-D. In this Cd-TIB framework,
the Cd2 ions are linked by the TIB ligands to form an infinite left-
handed helical chain running along the [111] direction in 1-D.
Such a helical chain just has a trigonal channel with inner
30 diameter of 11 Å and pitch lengths of 51.3 Å (Figure 4a). Finally,
this Cd-TIB framework can be reduce into a 3-connected srs net
with point (Schläfli) symbol of (105·105·105) (Figure 4c). If the
whole framework of 1-D is considered, both (R)-PIA and TIB
ligands are 3-connected nodes, and each Cd3(CO2)6 unit is a 10-
35 connected node. The whole framework of 1-D can be
topologically represented as a (3,3,10)-connected net with point
(Schläfli) symbol of (414.610.812.109)3 (43)10.
The first outstanding structural feature of 1-D is the presence
of two types of cages in the structure (A-cage and B-cage; Figure
2). Each A-cage with inner diameter of 8 Å consists of three
Cd3(CO2)6 units and six (R)-PIA fragments (Figure 2a). It is
interesting to note that all these (R)-PIA fragments are only rigid
isophthalate units of (R)-PIA ligands and do not involve chiral
10 proline units. However, the A-cage is still a chiral cage, because
we can’t find any necessary symmetry operations in it. The B-
cage with inner diameter of 16 Å consists of ten Cd1, three Cd2,
a TIB ligand, three (R)-PIA ligands and six (R)-PIA fragments
(Figure 2b). The (R)-PIA fragments with proline units in B-cage
15 are different from those in A-cage. Each A-cage links two B-
cages by sharing of two triangular windows formed from three
5
The solid-state circular dichroism (CD) measurements with
2
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Table 1 Enantioselective sorption of 1-D (1-L) toward racemic 1-
phenethylalcohol and methyl lactate a
b
Entry
Substrate
Adsorbent
VieweAertic(l%e O)nline
DOI: 10.1039/C4CC09821H
OH
1
20.3 (S)
1-L
1-D
1-L
1-D
OH
2
3
4
21.2 (R)
33.0 (S)
34.8 (R)
OH
O
O
OH
O
O
Figure 6. The N2 sorption isotherms of 1-D at 77k.
a For details, see the Experimental section in ESI. b Determined by
GC (letters in brackets specify the preferable isomer), and the
deviations for the ee values are less than 5%.
KCl plates between 200 and 600 nm are performed to further
demonstrate the homochirality of 1-D and 1-L (Figure 5). The
CD spectrum for the bulk sample of 1-D exhibits an obvious
positive CD signal at 243 nm, revealing its homochiral nature. A
mirror image is observed for 1-L, confirming that compounds 1-
D and 1-L are enantiomers.
aState Key Laboratory of Structural Chemistry, Fujian Institute of
40 Research on the Structure of Matter, the Chinese Academy of Sciences,
Fuzhou, Fujian 350002, P. R. China. E-mail: zhj@fjirsm.ac.cn
bDepartment of Chemistry, Zunyi Normal College, Zunyi, Guizhou
563002, P. R. China.
5
45 † Synthesis of [Cd9((R)-PIA)6(TIB)4(H2O)12].3H2O (1-D) : the mixture of
Cd(CH3COO)2·2H2O (0.2 mmol, 0.053 g), (R)-H3PIA (0.032 g, 0.1 mmol),
TIB (0.2 mmol, 0.055 g), DMF (3 ml) and H2O (1 ml) was sealed in a 20
ml vial and heated to 100 oC for 3 days. After cooling to room-
temperature, the colourless polyhedral crystals 1-D were separated from
50 the mixture by filtration (Yield: 42%). Similar procedure was used to
synthesis compound 1-L (Yield: 45%).
The co-existenc of cages and helical channels in the structure
of 1-D makes it a porous framework with solvent accessible
volume of about 49.2% per unit cell as calculated by PLATON.
10 The permanent porosity of 1-D was further demonstrated by the
N2 gas sorption at 77K. The desolvated sample of 1-D shows
type-I sorption isotherms (Figure 6), and the Langmuir and
Brunauer-Emmett-Teller (BET) surface areas for 1-D are 1327.9
m2/g and 955.8 m2/g, respectively. To the best of our knowledge,
15 HMOFs with such high surface area are rarely reported.
The presence of high porosity in 1-D (1-L) prompts us to
explore their application on enantioselective separation of
racemic compounds. After screening some racemic alcohols, our
studies identified that 1-D (1-L) can enantioselectively separate
20 1-phenethylalcohol and methyl lactate (Table 1, Figures S6-7). It
is obvious that 1-D has high affinity to (R)-phenethylalcohol or
(R)-methyl lactate and 1-L is good for (S)-phenethylalcohol or
(S)-methyl lactate. Such enantioselective adsorption should be
attributed to hydrogen-bonding interactions between the
25 framework and racemic agents that make the corresponding chiral
molecules orderly dock in the chiral channels.8
In summary, by employment of a pair of predesigned proline
derivative ligands ((R)-PIA and (S)-PIA) to assemble with TIB
and Cd2+ ion, a pair of HMOFs with permanent porosity were
30 successfully synthesized. It is unusual that the cages and helical
channels are presented in the homochiral framework together.
Both HMOFs have promising application on enantioselective
separation of racemic alcohols. The results reveal a successful
strategy on the construction of stable HMOFs with high porosity
35 via organic ligands integreting rigid and flexible parts.
‡ Crystal data for 1-D: C48H36Cd3N10O18, M = 1378.10, Cubic, a = b = c =
29.59550(10) Å, V = 25922.51(15) Å3, T = 100(2) K, space group P4332,
Z = 12, 12945 reflections measured, 4716 independent reflections (Rint
=
55 0.0354). The final R1 value was 0.0557 (I > 2σ(I)). The final wR(F2) value
was 0.1518 (I > 2σ(I)).The final R1 value was 0.0816 (all data). The final
wR(F2) value was 0.1663 (all data). The goodness of fit on F2 was 0.982
and the Flack parameter was 0.001(14).
§
Electronic Supplementary Information (ESI) available: [Experimental
60 details, IR spectra, TGA, powder X-ray diffraction patterns, and CIF file
(CCDC-1031194 (1-D) ]. See DOI: 10.1039/b000000x/
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