Synthesis of an Enantipure Tetrazole-Based Homochiral CuI,II-MOF for Enantioselective Separation

Article abstract of DOI:10.1021/acs.inorgchem.6b02514

An enantipure (1S)-1-(5-tetrazolyl)ethylamine (5-eatz) ligand was first employed to construct a homochiral porous metal-organic framework, {[Cu^I₂Cu^II(5-eatz)₂(CN^-)(H₂O)]⁺[NO₃^-]}·[DMF] (1), with unusual ligand-unsupported Cu···Cu interactions. Such a compound shows high stability in water and common organic solvents. Remarkably, it has high enantioselectivity toward (R,S)-1-phenylethanol and (R,S)-1-phenylpropanol with enantiomeric excess values of up to 42% and 48%, respectively.

Full text of DOI:10.1021/acs.inorgchem.6b02514

Communication
pubs.acs.org/IC
Synthesis of an Enantipure Tetrazole-Based Homochiral CuI,II-MOF
for Enantioselective Separation
Juan Liu,†,‡ Fei Wang,*,† Qing-Rong Ding,† and Jian Zhang*,†
†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, P. R. China
‡University of Chinese Academy of Sciences, 100049 Beijing, P. R. China
S
* Supporting Information
Scheme 1. Synthesis Route of Compound 1
ABSTRACT: An enantipure (1S)-1-(5-tetrazolyl)-
ethylamine (5-eatz) ligand was first employed to construct
a homochiral porous metal−organic framework,
−
{[CuI2CuII(5-eatz)2(CN−)(H2O)]+[NO3 ]}·[DMF] (1),
with unusual ligand-unsupported Cu···Cu interactions.
Such a compound shows high stability in water and
common organic solvents. Remarkably, it has high
enantioselectivity toward (R,S)-1-phenylethanol and
Compound 1 crystallizes in the tetragonal space group
(R,S)-1-phenylpropanol with enantiomeric excess values
P212121. There are three Cu ions, two 5-eatz ligands, one CN−,
of up to 42% and 48%, respectively.
one H2O molecule, and a NO3− in the asymmetric unit. Both CuI
and CuII exist in the structure, which can be proven by the X-ray
photoelectron microscopy pattern (Figure S2). CuI1 and CuI3
n recent years, homochiral metal−organic frameworks
I(MOFs) have emerged as ideal platforms for enantiomer
separation and asymmetric catalysis because of their high
porosity, size, and shape selectivity.1−4 Basically, the combination
of porosity and chirality is not on its own enough to enable strong
enantioselectivity. To design homochiral MOFs as enantiose-
lective adsorbents, the driving force to efficiently discriminate
two opposite enantiomers should be considered.5,6 A homochiral
MOF should have chiral recognition sites that can work
synergistically to induce enantioselective separation.7,8 Thus,
the synthesis of a desired homochiral MOF for efffciently
enantioselective separation should consider the interactions
between the chiral framework and chiral guests.
show three-coordinate modes, while CuII2 has an octahedral
geometry (Figure 1a). CuI1 (or CuI3) is coordinated by two 5-
In this work, we choose an enantipure tetrazole9 to assemble
with a Cu ion, targeting a homochiral MOF, {[CuI2CuII(5-
eatz)2(CN−)(H2O)]+[NO3 ]}·[DMF] [1; 5-eatz = (1S)-1-(5-
−
tetrazolyl)ethylamine], for enantioselective separation of race-
mic alcohols. Compound 1 has 1D channels, and it can effectively
separate racemic 1-phenylethanol or 1-phenylpropanol. More-
over, it can be readily recycled and reused without any apparent
loss of performance.
The purple crystals of 1 were solvothermally synthesized at
100 °C for 2 days with a mixture of 5-eatz, NaOH, K3[Fe(CN)6],
and Cu(NO3)2 in the mixed solvent of DMF, H2O, and EtOH
(Scheme 1). Here, K3[Fe(CN)6] can dissociate CN−, which is a
bridging ligand in compound 1. The structure and composition
of 1 were characterized by single-crystal X-ray diffraction and
elemental analyses. Its phase purity and thermal stability were
studied by powder X-ray diffraction (PXRD) and thermogravi-
metric analysis (TGA), respectively. Compound 1 presents good
stability in aqueous solution and other organic solvents (Figure
S1).
Figure 1. (a) Coordination environment of 1. (b) Wave layer in 1. The
green and violet colors highlight the Z-type chains. (c) 3D framework of
1 with 1D channels along the a axis. (d) Clear presentation of NO3− and
H2O molecules in the channel.
eatz ligands and a CN− ligand. The octahedral CuII2 is chelated
−
by two 5-eatz ligands and coordinated by one NO3 and one
H2O molecule (Figure 1a). The CuII(5-eatz)2 subunits are
connected to a Z-type chain by two CuI ions. The resulting chains
are further linked into a layer by CN− ligands (Figure 1b).
Finally, the wavelike layers are connected into a 3D framework by
Received: October 20, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02514
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
ligand-unsupported Cu···Cu interactions. Strong Cu···Cu
interactions, with a distance of 2.695 Å, are observed between
the layers. The 3D framework of 1 shows 1D channels along the a
axis. The size of each channel is ca. 8.0 Å × 7.0 Å (Figure 1c). To
Because the window of the 1D channel is too narrow, access of
large molecules is likely impossible. Thus, enantioselective
separation may occur on the chiral surface of compound 1. Thus,
the reason why 1 can enantioselectively separate aromatic
alcohols but not aliphatic alcohols is comprehensible. For
aromatic alcohols, enantioselective separation can be explained
by the three-point rule.7 Interactions between the surface of 1
and aromatic alcohols may include π···π interaction, hydrogen-
bonding interaction, and stereochemical interaction of a chiral
carbon. The 5-eatz ligands on the surface of 1 tend to adsorb R-
configuration aromatic alcohols (Figure 3). For aliphatic
−
emphasize, there are NO3 and H2O molecules hanging along
the wall of the 1D pore, which can form hydrogen bonding with
the guest (Figure 1d). The void volume of the pores was
calculated using PLATON to be 27% of the unit cell volume. The
CO2 gas adsorption was measured to evaluate the porosity of 1.
The adsorption isotherm curves show that 1 can take up 30, 40,
and 92 cm3 g−1 CO2 gas at 292, 273, and 195 K, respectively
(Figure S3). The low adsorption capacities indicate that 1 has a
low superficial area.
Because 1 is a homochiral MOF, the enantioselective
separation of 2-butanol, 2-methyl-2,4-pentadiol, (R,S)-1-phenyl-
ethanol, and (R,S)-1-phenylpropanol was explored (Tables S1−
S4). Interestingly, the gas chromatography (GC) results reveal
that the enantioselectivity of 1 toward is different from that of
aliphatic alcohols. As shown in Table 1, the ee values of aromatic
Table 1. Enantioselective Separation for Racemates at Room
a
Temperature
Figure 3. Possible interactions between the chiral framework and chiral
aromatic alcohols.
alcohols, the interactions between 1 and the aliphatic alcohols
include merely hydrogen bonding, which could not agree with
the three-point rule. Thus, the R and S configurations can both
interact with 1 through hydrogen bonding. Finally, no
enantioselectivity for aliphatic alcohols can be achieved.
In conclusion, a homochiral MOF with an enantipure 5-eatz
ligand and unusual ligand-unsupported Cu···Cu interactions has
been successfully synthesized. It has high enantioselectivity
toward (R,S)-1-phenylethanol and (R,S)-1-phenylpropanol.
Also, enantioselectivity is attributed to interactions between
the surface of 1 and the aliphatic alcohols.
a
For details, see the Experimental Section in the Supporting
b
Information. Determined by GC (letters in brackets specify the
preferable isomer), and the deviations for the ee values are less than
5%.
alcohols, (R,S)-1-phenylethanol and (R,S)-1-phenylpropanol,
released by 1 can reach 42% and 48%, respectively. However, 1
shows low ee percentage values toward 2-butanol and 2-methyl-
2,4-pentadiol. The recycle experiments reveal that the
enantioselective efficiency maintains the same level with no
reduction after four cycles. Meanwhile, the structure of 1 is intact
after recycled experiments (Figure 2b).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02514.
Experimental details, TGA diagram, and PXRD (PDF)
CIF file (CCDC 1510073) (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: wangfei04@fjirsm.ac.cn.
*E-mail: zhj@fjirsm.ac.cn.
■
ORCID
Fei Wang: 0000-0001-8432-0009
Jian Zhang: 0000-0003-3373-9621
Figure 2. (a) ee values of chiral samples in chiral separation recycled
experiments. (b) PXRD patterns of 1 after recycled experiments.
B
DOI: 10.1021/acs.inorgchem.6b02514
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(9) Liu, J.; Wang, F.; Liu, L. Y.; Zhang, J. Interpenetrated Three−
dimensional Copper−Iodine Cluster with Enantiopure Porphyin−like
Templates. Inorg. Chem. 2016, 55, 1358−1360.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Science Foundation of
China (Grants 21425102, 21521061, and 21573236).
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DOI: 10.1021/acs.inorgchem.6b02514
Inorg. Chem. XXXX, XXX, XXX−XXX