Self-assembly of a homochiral nanoscale metallacycle from a metallosalen complex for enantioselective separation

Article abstract of DOI:10.1002/anie.200704347

(Chemical Presented) Only half may enter: A homochiral porous nanoscale metallacycle has been efficiently self-assembled from semiflexible enantiopure metallosalen complexes with complementary coordination motifs. Single crystals of the macromolecule (see structure; C gray, Zn purple, O red, N blue) show a reversible and controllable dynamic behavior. Particularly, the metallacycle can be used to resolve small racemic alcohols with high enantioselectivity (over 99.0% ee).

Full text of DOI:10.1002/anie.200704347

Angewandte
Chemie
DOI: 10.1002/anie.200704347
Porous Assemblies
Self-Assembly of a Homochiral Nanoscale Metallacycle from a
Metallosalen Complex for Enantioselective Separation**
Gao Li, Weibin Yu, Jia Ni, Taifeng Liu, Yan Liu, Enhong Sheng, and Yong Cui*
Coordination-driven self-assembly provides unique opportu-
nities to design and prepare ordered arrays of molecules and
clusters, and has led to significant progress in the construction
of metal–organic assemblies with potentially exploitable
functions, from ion exchange to gas storage to separations
to catalysis.[1,2] Of particular interest to us are elegant works
on coordination-linked porphyrin arrays that demonstrate the
feasibility of constructing a large family of meso-metallopor-
phyrin cycles and boxes based on complementary coordina-
tion interactions between coordinatively unsaturated central
metal atoms and coordinating sidearms.[3] This assembling
process is especially effective for the construction of discrete
cyclic arrays, in that appropriately designed components are
almost automatically self-assembled to form large arrays, thus
providing an associated entropic advantage.[3c] However, such
a direct approach has thus far received less attention in other
supramolecular systems.[4,5]
(Figure 1).[6a] Single crystals of the robust metallomacrocycle
show a reversibly and controllably dynamic behavior induced
by external stimuli, and remarkably, can be used to separate
small racemic alcohols with an enantioselectivity of up to
99.8% ee.
Metallosalen-based architectures (salen = N,N’-ethylene-
bis(salicylideneaminato)) have diverse potential applications
in catalysis and separations,[6,7] which motivated us to examine
the self-assembly of enantiopure complementary metallosa-
len complexes through coordination, with the aim of gen-
erating well-defined enzyme-like chiral cavities and function-
alities for enantioselective processes. With a few notable
exceptions, metal–organic assemblies have not been explored
for chirotechnology.[8,9]
Herein, we report the efficient self-assembly of a chiral
porous macromolecule from the semiflexible pyridyl-func-
tionalized metallosalen [ZnL]using coordination bonds
[*] G. Li, W. Yu, T. Liu, Y. Liu, Prof. Dr. Y. Cui
School of Chemistry and Chemical Technology
Shanghai Jiao Tong University
Figure 1. Assembly of a [Zn4L4] macrocycle from self-complementary
{ZnL} building blocks.
800 Dongchuan Rd., Shanghai 200240 (China)
Fax: (+86)21-5474-1297
E-mail: yongcui@sjtu.edu.cn
Heating Zn(NO3)2·6H2O and H2L (1:1) in a mixture of
DMF and CH3CN afforded light-yellow, rod-like crystals in
high yield. The product is stable in air and insoluble in water
and common organic solvents, and was formulated as
[Zn4L4]·4CH3CN (1·4CH3CN) on the basis of elemental
analysis, IR spectroscopy, and thermogravimetric analysis
(TGA).
A single-crystal X-ray diffraction study performed on
1·4CH3CN reveals a metallomacrocycle constructed from
four [ZnL]molecules by the complementary coordination of
the pyridyl groups to the metal centers (Figure 1).[10] Com-
pound 1·4CH3CN crystallizes in the tetragonal chiral space
group P42, with a crystallographic C2 axis passing through the
center; thus, half of the tetramer is in the asymmetric unit.
Each Zn center adopts a square-pyramidal geometry with the
W. Yu, Prof. E. Sheng
College of Chemistry and Material Science
Anhui Normal University
Wuhu, Anhui 24100 (China)
J. Ni
Center Laboratory, Shantou University
Shantou, Guangdong 505063 (China)
[**] This work was supported by NSFC-20671062, NCET-05-0395, Key
Project of Basic Research of Shanghai (05JC14020), Shanghai
Pujiang Program (B4727A), Shuguang Program (06SG12), and the
State Key Laboratory of Structural Chemistry. We also thank the
“Instrumental Analysis Center” of Jiao Tong University for the use of
the spectrometers.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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equatorial plane occupied by the N2O2 donors of one L ligand
and the apical position by one pyridine group of another L
11.8974(5) and 11.9302(5) Š, and between opposite zinc
atoms 15.0783(6) and 15.4947(6) Š. Compound 1 is a rare
nanoscale chiral molecular container that has been crystallo-
graphically characterized.[11]
À
À
ligand. The Zn N and Zn O bond lengths range from
2.045(6) to 2.123(6) Š and from 1.944(6) to 1.993(4) Š,
respectively, while the zinc centers are located 0.44 and
0.42 Š above the plane formed by N2O2 coordination toward
the axial pyridyl group. Each {ZnL} unit thus uses one
terminal pyridyl group to coordinate to a Zn atom, thus
building a tetramer with one pyridine group remaining
uncoordinated. The coordinated pyridine groups are almost
coplanar with the basal salen plane defined by the ZnN2O2
core and two attached aromatic rings with small dihedral
angles of 11.4 and 15.38, while the uncoordinated pyridine
groups are twisted by 33.9 and 36.58 from coplanarity.
The two edges of adjacent [Zn4L4]tetramers are related
by a 42 screw axis and thus are ideally positioned to interact by
p–p stacking between the extended p-conjugated parts of
{ZnL} units (shortest plane–plane separation: 3.37 Š). Each
tetramer is surrounded by four adjacent analogues of the
same handedness through four pairs of such p–p stacking
interactions. As a result, the atoms in the {ZnL} conjugated
system are polarized alternately; all the pairs of atoms facing
each other in the two conjugated systems are oppositely
polarized and hence should attract each other strongly.[12] The
face-to-face intermolecular p–p interactions in 1 thus lead to
a neutral, porous 3D structure (Figure 3). Equilateral-trian-
gle-shaped zigzag channels with each side measuring 7.0 Š
The four pairs of tert-butyl groups of the {ZnL} units are
organized such that they partly cover two sides of the cyclic
tetramer along its crystallographic twofold axis, thus generat-
ing four hydrophobic pockets, each of which is occupied by
one CH3CN molecule (Figure 2). The four guest molecules
interact with each other to afford a distorted tetrahedron. The
guest-templating effect in 1 is inferred from the presence of
weak hydrophobic interactions between the ligand methyl
groups and solvent methyl groups. The maximum size of the
cyclic tetramer is 3.1 nm, while the diagonals of the aperture
¯
result from stacking of the tetramers along the [110]or [11 0]
2
on each face are 5.5 ” 8.2 Š (based on the atomic van der
Waals radii). The separations between adjacent zinc atoms are
Figure 3. a) A view of the packing of [Zn4L4] macrocycles along the
c axis; b) Space-filling model showing the open channels along [110]
direction within the 3D supramolecular structure of 1 (the included
guest molecules are omitted for clarity).
direction. Furthermore, rhombus-shaped channels with
2
dimensions of 4.0 ” 5.5 Š result from the arrangement of
four adjacent tetramers in the (110) planes. This supramolec-
À
ular structure is further stabilized by CH···p interactions (C
H···p 2.92 and 3.04 Š) between the butyl groups and the
coordinated pyridine rings of adjacent tetramers and CH···p
(2.85 Š) and CH···N (3.09 Š) interactions between the
uncoordinated pyridyl groups and aromatic rings of the
Figure 2. a) A view showing the inclusion of four CH3CN molecules in
1; b) Space-filling model of 1 (the cavity is highlighted by a yellow
sphere, and the guest molecules are omitted for clarity).
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{ZnL} units. Calculations using the PLATON program
indicate that 1 has 14.5% of its total volume occupied by
solvent molecules.[13] TGA revealed that the guest molecules
could be removed at 80–1508C (Figure S12 in the Supporting
Information).
itive-binding studies indicated 1 selectively includes alcohols
in the order methanol > ethanol > 1-propanol > 1-butanol,
but does not include chlorocarbons and nonpolar molecules.
More interestingly, 1 exhibits excellent selectivity for the
inclusion of acetonitrile over methanol and ethanol
(Figure S13 in the Supporting Information). Notably, exper-
imental information on the diffusion of guests in porous
molecules and solids is scarce.[14] A detailed understanding of
the dynamic process will no doubt allow systems with
unmatched physical properties and functionalities to be
designed.[15,16]
To investigate whether enantioselective separation can be
achieved with the metallacycle, the crystalline sample of 1 was
soaked in neat racemic 2-butanol in a sealed vial at 408C
(Scheme 1). After two days, most crystals remained trans-
parent with no apparent fracturing. The single-crystal struc-
A remarkable feature of 1 is that it undergoes reversible
single-crystal to single-crystal structural transformations in
response to guest removal and uptake. After the guests have
been removed completely by heating at 408C in a vacuum for
2 h, the individual crystals remain transparent with no
apparent fracturing, but their color transforms from light
yellow to deep yellow. The single-crystal structure revealed
that the four CH3CN molecules escaped. The cell volume
decreases by 2.3%; the shorter a axis contracts, while the long
c axis remains unchanged (Table S1 in the Supporting
Information). Although the tetramer skeleton is intact, the
{ZnL} units undergo obvious rotational rearrangements upon
desolvation. In particular, the dihedral angles between the
coordinated pyridine groups and the salen basal planes
decrease from 11.4 and 15.38 in 1·4CH3CN to 7.8 and 8.88 in
1, owing to the axial rotation of the pyridine rings. As a result,
intermolecular p–p interactions in the evacuated sample are
strengthened significantly, even leading to an unprecedented
color transformation of the crystals. This change can be
detected from their visible spectra, which exhibits a bath-
ochromic shift from 353 for 1·4CH3CN to 391 nm for the
apohost 1 (Figure S14 in the Supporting Information).
Scheme 1. Selective inclusion of (R)-2-butanol into the cavity of 1.
This crystal transformation is reversible and controllable.
Upon exposure of the apohost 1 to the vapor of guest
molecules such as CH3CN, CH3OH, and EtOH for two days at
room temperature, the crystals reversibly transform back to
the tetramers with stoichiometric uptake of the solvent. The
bulk integrity of each individual crystal is maintained, as
confirmed by powder XRD experiments, and the color reverts
back to light yellow. The cell volume with CH3CN reverts
back, and the volume with EtOH increases by 3.9% and 1.7%
(both the a and c axes expand) relative to the evacuated 1 and
1·4CH3CN. From 1·4CH3CN to 1·4EtOH, the dihedral angles
between the coordinated pyridine rings and the salen basal
planes increase as expected upon increasing the guest size
(14.9 and 16.18 for 1·4EtOH). However, further studies have
demonstrated that the cavity of 1 can only accommodate two
molecules of 1-propanol and one molecule of 1-butanol, 2-
butanol, or 3-methyl-2-butanol per tetramer and cannot
accommodate 2-pentanol and 2-hexanol. A single-crystal
structure of 1·2-butanol (see below) shows that the degree of
rotation of the coordinated pyridine rings to the basal salen
plane (dihedral angles 11.2 and 11.38) is smaller than those of
1·4CH3CN and 1·4EtOH. The above results suggest that the
potential barrier for the rotation of the pyridine rings in single
crystals of 1 is very small and can be readily overcome by the
inclusion of guest molecules. Whereas the amount of included
guest is mainly determined by the cavityꢀs volume, the host
preferentially forms thermodynamically stable adducts
through a process of lower energy barrier with guest
molecules that fit the cavity in size and shape. Therefore,
the dynamic motion of pyridine groups in 1 can be regulated
by guest sorption and also endows the macromolecule with
special molecular-recognition ability. For example, compet-
ture clearly revealed that (R)-2-butanol is included in the
cavity of 1 and the complex adduct is isostructural to 1. This
inclusion of 2-butanol is further supported by elemental
analysis, TGA, and powder XRD (Figure S11 and S12 in the
Supporting Information). Chiral GC analysis of the desorbed
2-butanol yielded an ee value of 99.8%, and the absolute
R configuration was confirmed by comparing its optical
rotation with that of the standard sample. Similar enantiose-
lective inclusion was observed for racemic 3-methyl-2-buta-
nol, whereby compound 1 exhibited remarkable sorption
toward the R enantiomer over the S enantiomer as well. The
ee value of the desorbed 3-methyl-2-butanol was determined
to be 99.6% by chiral GC analysis. The resulting solid of 1
could be recycled and directly reused for the second cycle of
resolving racemic 3-methyl-2-butanol with retention of crys-
tallinity but with a small decrease in enantioselectivity
(91.6% ee for the second cycle; Figure S15 in the
Supporting Information). An alternative approach to recycle
1 is to recycle the {ZnL} units by using dilute HCl to dissociate
the used powder and then regenerate 1 by crystallization.
Further investigations on resolutions of other molecules and
optimal recycling conditions are underway.
In conclusion, this work illustrates the feasibility of
producing a homochiral nanosized metallacycle from metal-
losalens with complementary coordination motifs. The
dynamic cavity and chiral functionality presented by 1 make
it an excellent host for a variety of guests with selectivity.
Particularly, it can resolve small racemic alcohols with high
enantioselectivity. The tunability of such a modular approach
promises to lead to a variety of metallosalen-based nano-
structures with unique and useful enantioselective functions.
Angew. Chem. Int. Ed. 2008, 47, 1245 –1249
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Communications
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Experimental Section
1·4CH3CN and 1:
A mixture of Zn(NO3)2·6H2O (14.9 mg,
0.05 mmol), H2L (29.5 mg, 0.05 mmol), DMF (3 mL), and CH3CN
(4 mL) was sealed in a 10-mL vial with a screw cap and heated at 808C
for 12 h. The mixture was then cooled to room temperature, and light-
yellow rod-like crystals were collected, washed with ether, and dried
in air. Yield: 29.6 mg, 86% based on Zn. Elemental analysis (%) calcd
for C160H180N20O8Zn4: C 69.31, H 6.54, N 10.10; found: C 69.04,
H 6.46, N 10.01. The guest molecules can be readily removed to
generate 1 by evacuating 1·4CH3CN at 408C for 2 h.
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A typical guest-exchange experiment: Freshly evacuated crystals
of 1 (0.10 mmol, 25.5 mg) and neat EtOH (5 mL) or a mixture of
EtOH (2 mL) and CH2Cl2 (3 mL) were placed in a 10-mL capped vial
at 408C for two days. The resulting sample was then isolated by
filtration and washed several times with ether. Based on elemental
analysis and TGA, the product can be formulated as 1·4EtOH.
Powder XRD indicates that crystals of 1 remain almost intact without
loss of crystallinity. Similarly, the guest-exchanged products can be
formulated as 1·4(guest) for methanol, 1·2(guest) for 1-propanol, and
1·(guest) for 1-butanol, 2-butanol, and 3-methyl-2-butanol. These
formulations were also confirmed by single-crystal X-ray crystallog-
raphy for the ethanol and 2-butanol products.
Separation experiment: An evacuated sample of 1 (50 mg) and
racemic 2-butanol (5 mL) or 3-methyl-2-butanol (5 mL) were put in a
sealed vial at 408C for three days. The solid samples were filtered,
washed with diethyl ether, and then loaded into a distillation setup
and gently heated to 608C under vacuum while the volatile
components were collected with a liquid nitrogen bath. Optical
purity of the condensates was determined by GC using a Chiral-G-TA
capillary column: 99.8% ee (R) for acetylated 2-butanol and
99.6% ee (R) for 3-methyl-2-butanol. Optical rotation: [a] = À14.7
and À3.2 degcm3 gÀ1 dmÀ1 (c = 10 gcmÀ3 in CH3OH, 208C) for 2-
butanol and 3-methyl-2-butanol, respectively. Other experimental
details are provided in the Supporting Information.
Received: September 20, 2007
Published online: January 4, 2008
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Keywords: chirality · enantioselectivity · macrocycles ·
.
self-assembly · zinc
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[10]Crystal data for 1·4CH3CN: Tetragonal, space group P42, a =
25.9432(13), c = 10.9287(12) Š, V= 7355.6(10) Š3, Z = 2, T=
298 K,
F(000) = 2928,
1calcd = 1.252 gcmÀ3
,
m(MoKa) =
0.708 mmÀ1 (l = 0.71073 Š), R1 = 0.0614, wR2 = 0.1280, Flack
parameter= 0.034(19), GOF = 1.030. 1: Tetragonal, space group
P42, a = 25.949(4), c = 10.677(2) Š, V= 7189(2) Š3, Z = 2, T=
298 K,
F(000) = 2752,
1calcd = 1.205 gcmÀ3
,
m(MoKa) =
0.720 mmÀ1 (l = 0.71073 Š), R1 = 0.0745, wR2 = 0.1333, Flack
parameter= 0.01(3), GOF = 0.995. 1·4EtOH: Tetragonal, space
group P42, a = 26.098(4), c = 10.981(2) Š, V= 7479(2) Š3, Z = 2,
T= 298 K, F(000) = 2960, 1calcd = 1.240 gcmÀ3
,
m(MoKa) =
0.698 mmÀ1 (l = 0.71073 Š), R1 = 0.0670, wR2 = 0.0718, Flack
parameter= 0.04(3), GOF = 0.825. 1·2-butanol: Tetragonal,
space group P42, a = 25.9703(7), c = 10.7390(6) Š, V=
7243.0(5) Š3, Z = 2, T= 298 K, F(000) = 2936, 1calcd
=
1.230 gcmÀ3
,
m(MoKa) = 0.717 mmÀ1 (l = 0.71073 Š), R1 =
0.0549, wR2 = 0.0950, Flack parameter= 0.00(3), GOF = 0.953.
CCDC-651775, 651776, 651777, and 651778 contain the supple-
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obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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