A homochiral microporous hydrogen-bonded organic framework for highly enantioselective separation of secondary alcohols

Article abstract of DOI:10.1021/ja4129795

A homochiral microporous hydrogen-bonded organic framework (HOF-2) based on a BINOL derivative has been synthesized and structurally characterized to be a uninodal 6-connected {3³5⁵6⁶7} network. This new HOF exhibits not only a permanent porosity with the BET of 237.6 m ² g^-1 but also, more importantly, a highly enantioselective separation of chiral secondary alcohols with ee value up to 92% for 1-phenylethanol.

Full text of DOI:10.1021/ja4129795

Communication
pubs.acs.org/JACS
A Homochiral Microporous Hydrogen-Bonded Organic Framework
for Highly Enantioselective Separation of Secondary Alcohols
Peng Li,† Yabing He,† Jie Guang,† Linghong Weng,‡ John Cong-Gui Zhao,† Shengchang Xiang,§
and Banglin Chen*,†
†Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States
‡Department of Chemistry, Fudan University, Shanghai, 200433, PR China
§College of Chemistry and Material, Fujian Normal University, Shangsan Road, Cangshang Region, Fuzhou 350007, PR China
S
* Supporting Information
not only to assemble a series of structurally porous HOFs12 but
ABSTRACT: A homochiral microporous hydrogen-
bonded organic framework (HOF-2) based on a BINOL
derivative has been synthesized and structurally charac-
terized to be a uninodal 6-connected {3355667} network.
This new HOF exhibits not only a permanent porosity
with the BET of 237.6 m2 g−1 but also, more importantly, a
highly enantioselective separation of chiral secondary
alcohols with ee value up to 92% for 1-phenylethanol.
also to achieve the first porous HOF-1 for the highly selective
separation of C2H2/C2H4 at room temperature.9 Given the fact
that 1,1′-bi-2-naphthol (BINOL) is a very powerful organic
backbone for asymmetric induction,13−15 we incorporated the
(R)-BINOL scaffold into 2,4-diaminotriazinyl hydrogen-bond-
ing motif to synthesize a new chiral organic building block for
the self-assembly of HOFs (Scheme 1).
Scheme 1. Organic Building Block To Build HOF-2
orous hydrogen-bonded organic frameworks (HOFs)1,2
Phave lagged significantly behind metal−organic frame-
works (MOFs)3,4 in terms of their development on their
framework design, topological rationalization, and functional
exploration, although the concepts of constructing such porous
materials by making use of hydrogen bonding and coordination
bonding, respectively, were proposed during the same time
period as those of MOFs.5,6 Such a situation is mainly due to
the weaker hydrogen-bonding interactions within HOFs, which
are typically not strong enough to stabilize the frameworks and
establish their permanent porosities.7 It is, therefore, not
surprising that only recently a few HOFs have been shown to
possess permanent porosities,8−11 although a large number of
HOFs have been structurally characterized and reported in the
literature. Nevertheless, it is expected that the establishment of
these few porous HOFs will initiate the rebounding interest in
the exploration of functional porous HOF materials for their
potential applications in gas storage and separation, sensing,
and heterogeneous catalysis. Thus, further research endeavors
to figure out the basic and strong hydrogen-bonding motifs to
stabilize the frameworks, rationalizing that the basic principles
for constructing the frameworks of the desired topologies and
controlling the pore sizes, dimensions, and functionalities of the
porous HOF materials for their diverse applications are
warranted. Specifically, new hydrogen-bonding motifs need to
be explored for the construction of porous HOFs and examined
comprehensively on their universal applicability and feasibility
to build a series of expanded HOFs whose pore sizes and
functionalities can be systematically tuned and explored for
their applications.
Herein we report the crystal structure of this new homochiral
HOF-2 of the uninodal 6-connected {3355667} network
topology. More importantly, this homochiral HOF-2 exhibits
highly enantioselective separation of chiral secondary alcohols,
with the enantiometric excess (ee) up to 92% for 1-
phenylethanol because of the specific recognition of HOF-2
for the 1-phenylethanol molecule.
The chiral organic building block, shown in Scheme 1, can be
readily synthesized in 52% yield by reacting known chiral nitrile
with dicyandiamide (see Scheme S1 in the Supporting
Information [SI]).
Single crystals of HOF-2 suitable for X-ray diffraction study
can be easily obtained by slow evaporation of a solution of the
organic building block in 2-methoxyethanol at room temper-
ature. The purity of HOF-2 was confirmed by 1H NMR
spectroscopy, thermogravimetric analysis (TGA), and powder
X-ray diffraction (PXRD) (Figures S1−S3 in the SI).
The single-crystal X-ray structure of HOF-2 reveals that it
crystallizes in a chiral space group P65.16 HOF-2 is a three-
dimensional porous material in which each organic building
block is connected with six neighboring ones by multiple strong
hydrogen bonds (Table S1 in the SI) among the 2,4-
The hydrogen-bonding motif of 2,4-diaminotriazinyl (DAT),
pioneered by Wuest, is of particular interest for constructing
functional porous HOFs.6f This motif has recently been utilized
Received: December 20, 2013
Published: January 3, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
diaminotriazine groups (Figure 1a. Topologically, it can be
viewed as a uninodal 6-connected {3355667} network (Figure
2a is only slightly shifted from that of the as-synthesized HOF-2
crystals (Figure S3 in the SI), indicating that the framework is
retained. TGA shows that HOF-2a is thermally stable up to 350
°C (Figure S2 in the SI). The porosity of HOF-2a was
evaluated by CO2 gas sorption at 196 K. The type I isotherm
shows a very sharp uptake at P/P0 < 0.1, which indicates a
microporous material. The CO2 isotherm gives an apparent
Brunauer−Emmett−Teller (BET) surface area of 237.6 m2/g
for HOF-2a, corresponding to a pore volume ∼0.13 mL/g
(Figure S5 in the SI). Its permanent porosity was further
confirmed by C2H2 sorption isotherm at 273 K (Figure S6 in
the SI).
The chiral porous nature of HOF-2 has enabled us to
examine its potential for enantioselective separation of small
alcohols, 1-phenylethanol (1-PEA), 1-(4-chlorophenyl)ethanol
(1-(4Cl-PEA)), 1-(3-chlorophenyl)ethanol (1-(3Cl-PEA)), 2-
butanol (2-BUT), 2-pentanol (2-PEN), 2-hexanol (2-HEX),
and 2-heptanol (2-HEP) at room temperature. As shown in
Table 1, HOF-2 shows different recognition behaviors for these
Table 1. Resolution of Racemic Secondary Alcohols with
a
HOF-2a at Room Temperature
b,c
entry
secondary alcohols
ee (%)
1
2
3
4
5
6
7
1-phenyethanol
1-(4-chlorophenyl)ethanol
2-butanol
92
79
77
Figure 1. X-ray crystal structure of HOF-2 featuring (a) multiple
hydrogen bonding (light-blue dashed lines) among adjacent units to
form three-dimensional hydrogen-bonded organic framework exhibit-
ing 1D hexagonal pores with the diameter of about 4.8 Å along the c
axis and (b) the uninodal 6-connected {3355667} network topology. X-
ray crystal structure of HOF-2⊃R-1-PEA indicating (c) the
enantiopure R-1-PEA molecules residing in the channels of the
framework along the c axis and (d) the chiral cavities of the framework
for the specific recognition of R-1-PEA which is further enforced by
the hydrogen-bonding interactions among the −OH groups of R-1-
PEA (green molecule) and oxygen atoms of the diethoxy groups from
the HOF-2 framework. Comparison of X-ray crystal structures of (e)
HOF-2⊃S-1-PEA and (f) HOF-2⊃R-1-PEA, indicating the different
recognition of the HOF-2 for these two enantiomers (C, gray; H,
white; N, pink; O, red).
1-(3-chlorophenyl)ethanol
2-pentanol
66
48
2-hexanol
<10
2-heptanol
b
<4
a
c
See SI for experimental details. Determined by HPLC. R isomers
are preferred.
small alcohols. For aromatic secondary alcohols, HOF-2
exhibits higher chiral separation for 1-phenylethanol (1-PEA)
(ee of 92%, entry 1) than its derivatives 1-(4Cl-PEA) (ee of
79%, entry 2) and 1-(3Cl-PEA) (ee of 66%, entry 4), while for
the aliphatic secondary alcohols, the HOF-2 shows enantiose-
lective separation selectivity in the order of 2-BUT > 2-PEN >
2-HEX > 2-HEP (entries 3, 5−7). HOF-2 systematically
displays higher enantioselective separation for aromatic
secondary alcohols than for aliphatic secondary alcohols. The
significantly high enantioselective separation of HOF-2 for the
chiral aromatic 1-PEA (ee of 92%) is remarkable.17 In fact,
HOF-2 is superior to our best demonstrated MOF (ee of
82%)18 for such an industrially important separation, given the
fact that the chiral secondary alcohols are valuable inter-
mediates in the synthesis of a variety of pharmaceutical,
agricultural, and fine chemicals.19,20
In order to rationalize why HOF-2 exhibits highly
enantioselective separation for 1-PEA, we characterized the X-
ray single-crystal structure of the 1-PEA included HOF-2,
which is prepared by soaking single crystals of the as-
synthesized HOF-2 in racemic 1-PEA. The crystal structure
clearly indicates that HOF-2 exclusively encapsulates R-1-PEA
from the racemic mixture of the 1-PEA (Figure 1c) to form the
HOF-2⊃R-1-PEA. Furthermore, the specific recognition of
HOF-2 for R-1-PEA is not only attributed to the confinement
of the chiral pockets induced by the chiral BINOL scaffolds but
is also enhanced by the strong hydrogen bonding among the
hydroxy (−OH) groups of R-1-PEA molecules and oxygen
atoms of the diethoxy groups of the HOF-2 framework (d(O3−
1b). There exist one-dimensional hexagonal pores with a
diameter of about 4.8 Å based along the c axis, taking into
account the van der Waals radii (Figure 1a,b). The chiral pore
spaces within the framework encapsulate a certain amount of
disordered 2-methoxyethanol and H2O solvent molecules. The
solvent-accessible void space of porous HOF-2 is 54.3% by
PLATON analysis. The homochiral channels are formed by
rotational axes that are composed of building blocks 2 linked
with each other by quadruple hydrogen bonds (Figure S4 in the
SI). The framework is further enforced by multiple aromatic
π−π interactions among the organic building blocks. The
collaborative hydrogen bonding and aromatic π−π interactions
have featured the HOF-2 as the rare example of potentially
robust HOFs.
Before performing the adsorption properties test and the
chiral alcohol separation experiment, the guest solvent
molecules in HOF-2 were removed by solvent exchange with
diethyl ether, and the resulting samples were further activated at
room temperature under high vacuum for 24 h to obtain
desolvated HOF-2a. Such activation completely removed the
guest molecules, as confirmed by 1H NMR spectroscopy
(Figure S7 in the SI). The PXRD pattern of the activated HOF-
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Journal of the American Chemical Society
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ASSOCIATED CONTENT
■
S
* Supporting Information
(19) Liu, Y.; Xuan, W.; Cui, Y. Adv. Mater. 2010, 22, 4112−4135.
(20) Li, G.; Yu, W.; Ni, J.; Liu, T.; Sheng, E.; Cui, Y. Angew. Chem.,
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Crystallographic data, TGA, PXRDs, HPLC plots, isotherm fit
parameters, gas sorption isotherms, and additional figures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
banglin.chen@utsa.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the awards from the Welch
Foundation (AX-1730 for B.C. and AX-1593 for J.C.-G.Z.).
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