Structural Insight into Guest Binding Sites in a Porous Homochiral Metal-Organic Material

Article abstract of DOI:10.1021/jacs.5b06760

An enantiomeric pair of chiral metal-organic materials (CMOMs) based upon mandelate (man) and 4,4′-bipyridine (bpy) ligands, [Co₂(S-man)₂(bpy)₃](NO₃)₂·guest (1S·guest) and [Co₂(R-man)₂

Full text of DOI:10.1021/jacs.5b06760

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Article
Structural Insight into Guest Binding Sites in
a Porous Homo-chiral Metal-Organic Material
Shi-Yuan Zhang, Lukasz Wojtas, and Michael J. Zaworotko
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b06760 • Publication Date (Web): 07 Sep 2015
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Structural Insight into Guest Binding Sites in a Porous Homo-
chiral Metal-Organic Material
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Shi-Yuan Zhang,†,‡ Lukasz Wojtas, ‡ and Michael J. Zaworotko*,†
† Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, Florida 33620,
United States.
‡ Department of Chemical & Environmental Science, University of Limerick, Limerick, Republic of Ireland.
ABSTRACT: An enantiomeric pair of chiral metal-organic materials (CMOMs) based upon mandelate (man) and 4,4’-
bipyridine (bpy) ligands, [Co2(S-man)2(bpy)3](NO3)2∙guest, (1S∙guest) and [Co2(R-man)2(bpy)3](NO3)2∙guest, (1R∙guest),
has been prepared. The cationic frameworks exhibit 1D chiral channels with dimensions of 8.0 Å × 8.0 Å. The pore chem-
istry is such that chiral surfaces lined with nitrate anions and phenyl groups create multiple binding sites for guest and/or
solvent molecules. The performance of 1S and 1R with respect to resolution of racemic mixtures of 1-phenyl-1-propanol
(PP) was studied by varying time, temperature and the use of additives. Selectivity towards PP was determined by chiral
HPLC with ee values of up to 60%. The binding sites and host-guest interactions were investigated through single crystal
X-ray structural analysis of guest exchanged 1S and 1R. Crystallographically observed structural changes (e.g. absolute
configuration of the three PP binding sites switch from R, R and S to R, R and R/S) correlate with experimentally observed
ee values of 33% and 60% for variants of 1S that contain PP and different solvent molecules, 1S∙PPex and 1S∙PPex', respec-
tively. That manipulation of guest solvent molecules, which in effect serve as cofactors, can modify chiral sites and in-
crease enantioselectivity is likely to aid in the design of more effective CMOMs and processes for chiral separations.
separation. The benchmark performance is exhibited by
1. INTRODUCTION
M’MOF-7, which affords ee up to 82.4% for resolution of
That metal-organic materials (MOMs)1a can exhibit per-
manent porosity has attracted considerable attention in
the past 15 years.1 An aspect of MOMs that exploits both
their porosity and their fine-tunable chemical features is
their ability to undergo guest exchange or be transformed
by post-synthetic modification (PSM). For example, guest
exchange enables MOMs to serve as “crystal sponges” for
structure eludication2 and chiral MOMs (CMOMs) have
been studied in the context of enantioselective separa-
tion3 and asymmetric catalysis4. PSM5 can modify pore
chemistry to enhance gas sorption performance6 or can be
used to access otherwise inaccessible MOMs.7 The most
typical approach to generate CMOMs involves homochi-
ral molecular building blocks (CMBBs)8 as opposed to
relying upon spontaneous resolution of CMOMs sus-
tained by achiral MBBs9. CMOMs with homochiral MBBs
covalently bonded to the framework as linking or pendant
ligands have been synthesized using a variety of ligands
including L-aspartate10, L-lactate11, L-alanine derivatives12,
L-leucine derivatives13, L-camphorate derivatives14, D-
tartrate derivatives3c, BINOL derivatives15 and Schiff base
derivatives16. However, the relative cost of homochiral
species and racemization during synthesis17 has limited
the development of CMOMs relative to MOMs in gen-
eral18. Moreover, if there is a lack of control over pore
chemistry, size and shape then chirality in a framework
does not necessarily translate into binding sites that ena-
ble strong performance in the context of enantioselective
1-phenylethanol.16a However, a hydrogen bonded network,
HOF-2a, exhibits an ee of 92%.15 In most instances much
lower ee values are observed.10-12,14
Whereas the development of CMOMs in terms of net-
work design and functionalization is well addressed,13,19
the nature of the interactions that promote enantioselec-
tive separation by CMOMs is understudied. Simply put,
the combination of porosity and chirality is not on its
own enough to enable strong enantioselectivity. This is
partly because of the dearth of single crystal X-ray struc-
tural studies of host-guest interactions in CMOMs since
such studies require retention of crystallinity after guest
exchange and crystallographically observable guest mole-
cules10,14. Indeed, there are very few structural studies that
reveal the intermolecular interactions between CMOMs
and chiral guests15,20. Herein we report the synthesis and
crystal structures of a pair of novel and robust CMOMs,
[Co2(S-man)2(bpy)3](NO3)2∙guest (1S∙guest) and [Co2(R-
man)2(bpy)3](NO3)2∙guest (1R∙guest). Our study reveals
that relatively high ee values can be achieved through
confined space that exploits the homochirality of the
MBBs through van der Waal forces, hydrogen bonding
interactions and π-π stacking interactions. Further, the
role that solvent can play as a cofactor at binding sites is
delineated.
2. EXPERIMENTAL SECTION
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lated PP had indeed been released. The filtrates were com-
2.1. Materials and Synthesis: All reagents and solvents
were commercially available and used as received.
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bined and analyzed by chiral HPLC to determine ee values
and UV-vis was used to determine loading. The resulting
crystals were dried in air and weighed (weights ranged from
0.03~0.04 g). A standard calibration curve for PP was gener-
ated (Scheme S1) and the following formula used to calculate
2.1.1. Synthesis of 1S∙NB and 1R∙NB. A 5 mL methanol
solution of 0.4 mmol Co(NO3)2∙6H2O (120 mg) and 0.4 mmol
enantiopure mandelic acid (S isomer for 1S and R isomer for
1R, 60.8mg) was layered above a 5 mL nitrobenzene (NB)
solution of 0.3 mmol bpy (46.8 mg). The buffer solution of a
5mL 1:1 methanol/nitrobenzene was layered between the top
and the bottom layers to allow slow diffusion for 7 days. Red
rectangular prismatic crystals were obtained in ~50% yield.
2.1.2. Synthesis of solvent exchanged variants 1S∙guest
and 1R∙guest. Crystals of as-synthesized 1S∙NB and 1R∙NB
were exchanged with DCM daily for 5 days affording 1S∙DCM
and 1R∙DCM. Desolvated 1S and 1R were obtained from
1S∙DCM and 1R∙DCM, respectively, under vacuum. 1S∙CH
and 1R∙CH were prepared by immersing crystals of 1S and 1R
in cyclohexane (CH) for 7 days. Single crystals of 1S∙PPex
were prepared by soaking 1S∙DCM in racemic PP for 7 days.
Another variant, 1S∙PPex', was prepared by soaking desolvat-
ed 1S in racemic PP in the presence of 200 μL 90%
MeOH/H2O for 5 days.
the loading of PP: ꢀꢁꢂꢀꢃꢄꢅꢀꢆꢇꢈꢉ ꢊꢊ ꢋꢅꢌꢉꢍ ꢑ 100%. The chiral
ꢎꢈꢉꢎꢏꢉꢈꢇꢀꢐ ꢊꢊ ꢋꢅꢌꢉꢍ
resolution procedure is expressed in the form of a flow chart
in Scheme S2. HPLC data for the resolution of PP are pre-
sented in Figures S9-53.
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3. RESULTS AND DISSCUSSION
2.2. Characterization.
2.2.1. Physical measurements: Powder X-Ray diffraction
was performed on a Bruker D8 Venture using Cu-Kα radia-
tion (λ = 1.5418 Å). Thermogravimetric analysis was per-
formed using a TA Instruments TGA-Q50 at a constant rate
of 5ºC/min from 25ºC to 800ºC. UV-vis spectra were meas-
ured using a JASCO J-715. FT-IR spectra were recorded on a
Perkin-Elmer Spectrum Two spectrometer. HPLC measure-
ments were carried out on a Shimadzu HPLC system with
Chiralcel OD-H column with a flow rate of 1 mL/min.
2.2.2. Crystallographic studies: As-synthesized 1S∙NB
and 1R∙NB, guest exchanged 1S∙CH, 1S∙PPex and 1S∙PPex'
crystals are chosen for single crystal X-ray diffraction study.
The data were collected on a Bruker D8 Venture PHOTON
100 CMOS system equipped with a Cu Kα INCOATEC Imus
micro-focus source (λ = 1.54178 Å, T = 100(2) K). In all cases
indexing was performed using APEX2.21 Data integration and
reduction were performed using SaintPlus 6.01.22 Absorption
correction was performed by multi-scan method implement-
ed in SADABS.23 Space groups were determined using XPREP
implemented in APEX2. Structures were solved using Patter-
son Method (SHELXS-97), expanded using Fourier methods
and refined on F2 using nonlinear least-squares techniques
with SHELXL-97 contained in APEX2 and WinGX v1.70.0124-27
programs packages. Crystallographic data for the as-
synthesized and guest exchanged CMOMs are summarized
in Tables S1 and S2, respectively. Refinement details concern-
ing the PP guest molecules are given in SI.
Figure 1. Left: The 1D chiral chains linked by (a) (S)-
mandelate in 1S and (c) (R)-mandelate in 1R. Right: Projec-
tion of the structures of 1S (b) and 1R (d) from above the bc
plane. Hydrogen atoms, nitrate anions and solvent molecules
are omitted for the sake of clarity.
1S∙NB and 1R∙NB were prepared by slow diffusion of a
solution of Co(NO3)2∙6H2O and (S)-mandelic acid or (R)-
mandelic acid, respectively, in MeOH onto 1:1 metha-
nol/nitrobenzene that had been layered over a nitroben-
zene solution of bpy. Single crystal X-ray diffraction anal-
ysis of 1S∙NB and 1R∙NB reveal that they are isostructural,
crystallizing in chiral space group P21. The structure of
1S∙NB is sustained by Co2+ ions linked by (S)-mandelate
anions so as to form 1D chiral chains running parallel to
the a axis (Figure 1a). These chains are cross-linked by
bpy linkers in the other two directions to form a 3D net-
work with bnn topology (Figure 1b). In 1R∙NB the struc-
ture is of the opposite chirality (Figures 1c,d). The pore
size of the 1D channels in 1S∙NB and 1R∙NB are defined by
the length of bpy linkers (Figure S1): ca. 8.0 Å × 8.0 Å after
subtracting van der Waals radii. Pore chemistry and
2.3. Chiral Resolution of PP.
The crystals used to study chiral resolution were obtained
from layering and used as synthesized. 1S∙DCM and 1R∙DCM
were immersed in 1 mL racemic PP with no stirring or shak-
ing for various time periods and temperatures as detailed in
Tables 1 and S3. Desolvated materials 1S and 1R were treated
by
a
similar procedure except varying amounts of
MeOH/H2O solutions were used as additives (Tables 2 and
S4). After specific time periods, crystals were filtered and
washed with CH (6 × 1 mL) to remove the residual PP from
the surface of the crystals. DCM was then used to successive-
ly extract PP from the crystals (8 × 0.5 mL). The resulting
extracts were monitored by TLC to ensure that all encapsu-
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ings of 96% and 95%, respectively, were observed. In-
shape is controlled by the chiral mandelate linkers and
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nitrate counterions, resulting in uneven pore surfaces.
The void volume of the pores was calculated using
PLATON21 to be 33% of the unit cell volume. The pores of
the as-synthesized crystals of 1S∙NB and 1R∙NB are occu-
pied by nitrobenzene. The positions and binding sites of
NB inside the channels are shown and described in Figure
S2.
creasing MeOH/H2O from 10 to 50 μL resulted in ee val-
ues remaining largely unchanged (~30% for 1S and 38%
for 1R). However, the loading of PP decreased from 99%
(98%) to 82% (86%) for 1S (1R), indicating competition
for the PP guest binding sites from MeOH/H2O, acceler-
ated amorphization or both. The highest ee value (60%)
was observed with 200 μL of 90% methanol solution, but
the loading was only 33%. Nevertheless, to our knowledge
this performance is 1.7× higher than reported for any oth-
er porous MOM12. Further experiments were conducted at
40 °C for 1d (Table S4). The ee values were observed to
improve gradually as the MeOH/H2O ratio was increased.
However, once again the loading of PP dropped, this time
from 98% to 28%.
We examined the ability of 1S∙DCM and 1R∙DCM to re-
solve 1-phenyl-1-propanol (PP) following procedures de-
tailed in the experimental section. We selected PP for
study because it is an important intermediate in the syn-
thesis of pharmaceutical and parasiticide compounds.3,28
PP exchanged 1S materials were characterized by PXRD
(Figure S4) and FT-IR (Figure S5). Table 1 reveals that
1S∙DCM and 1R∙DCM that had been soaked in racemic PP
for 7d exhibit higher ee values, 32% and 30%, respectively,
than samples exposed to PP for shorter time periods. The
loading amount of PP was also observed to increase grad-
ually, from 82% to 96%, within 5d. However, a further 2d
of exposure resulted in decreased loading of PP. We at-
tribute this effect to partial loss of crystallinity of the bulk
sample as suggested by broadening of PXRD peaks (Fig-
ure S4). PP/additive exposed crystals also appeared to
start losing crystallinity within 5 days. Conversely, PXRD
profiles of 1S soaked in CH were unchanged after one
month (Figure S3). Thermogravimetric analysis (TGA)
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Table 2. Resolution of PP by 1S and 1R at Room Tem-
perature for 5 days
(S)-PP:
(R)-PPc
33:67
30:70
35:65
35:65
34:66
25:75
20:80
67:33
69:31
Additive
[μL, %]b
ee
Loading
[%]d
CMOMa
[%]c
0, 0
10, 50
10, 90
50, 50
50, 90
100, 90
200, 90
0, 0
34
40
30
30
32
50
60
34
38
38
38
38
46
52
96
99
97
83
82
72
33
1S
°
indicates that 1S is stable to 200 C (Figure S6). When
resolution of PP was conducted at 40°C ee values re-
mained ~26% from 1d to 5d (Table S3). However, the load-
ing of PP started to decrease after only 3 days. These re-
sults indicate that 5 days at room temperature are optimal
conditions for conducting separations since slow amor-
phization thereafter reduces loading.
95
98
97
86
86
70
30
10, 50
10, 90
50, 50
50, 90
100, 90
200, 90
69:31
1R
69:31
Table 1. Resolution of PP by 1S∙DCM and 1R∙DCM at
Room Temperature over Different Time Periods
69:31
72:28
76:24
CMOM
Time (S)-PP:(R)-PPa ee [%]a
Loading [%]b
a
1d
3d
5d
7d
1d
41:59
39:61
39:61
34:66
58:42
61:39
62:38
65:35
18
22
22
32
16
82
88
96
70
83
88
97
71
The desolvated material was obtained from 1S·DCM and
b
1R·DCM under vacuum.
Total volume (left) of an x%
1S∙DCM
(right) MeOH aqueous solution (v/v) used as additive in 1 mL
racemic PP. c Determined by HPLC analysis using a chiral
d
OD stationary phase. Total amount of released PP was de-
termined by UV-vis according to a PP calibration curve.
3d
5d
7d
22
24
30
That 1S and 1R retain crystallinity after solvent/guest
exchange for at least several days enabled us to use single
crystal X-ray crystallography to study the nature of the
interactions between various guest molecules and the
pore surfaces of 1S and 1R. For example, soaking crystals
of 1S in cyclohexane resulted in 1S∙CH, in which CH mol-
ecules lie in ordered positions and interact with phenyl
groups and nitrate ions (Figure S7).
1R∙DCM
a
Determined by HPLC analysis using a chiral OD station-
b
ary phase. Total amount of released PP was determined by
UV-vis according to PP calibration curve.
To test the effect of additives upon performance, reso-
lution experiments using 1S∙DCM and 1R∙DCM desolvat-
ed by vacuum were conducted. 1S and 1R were soaked in
racemic PP in the presence of varying amounts of
MeOH/H2O at the optimal conditions found for pure PP,
i.e. room temperature for 5 days. As detailed in Table 2, in
the absence of MeOH/H2O, ee values of 34% with load-
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1S∙PPex, a DCM molecule participates in a hydrogen
1
bonded ring (Figure 2b) involving the nitrate anion, the
second PP and the third PP. Additional close contacts
between DCM and PP molecules with Cl∙∙∙H-Cethyl and C-
H∙∙∙πphenyl distances of 3.516 and 3.616 Å occur. In the other
disordered position (Figure 2d), the third PP interacts
with the hydroxyl group of the second PP (O-H∙∙∙O, 2.981
Å) and DCM (π∙∙∙H-C, 3.433 Å) and results in it orienting
nearly parallel with respect to the bc plane. The third PP
molecule is resolved as the R enantiomer, meaning that
the maximum ee value according to the crystal structure
is 33% (a selectivity of 2:1 for (R)-PP over (S)-PP). In con-
trast, disordered water/methanol molecules in 1S∙PPex'
participate in a cyclic hydrogen bonded ring (Figure 3b)
involving a nitrate anion with the second and third PP
molecules. This arrangement forces the third PP molecule
to orient closer to perpendicular with respect to the bc
plane. Figure 3d reveals that intermolecular hydrogen
bonding interaction (C-H∙∙∙O, 2.356 Å) between the se-
cond and the third PP molecules enables these PP mole-
cules to lie in close proximity.
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Figure 2. Locations of the three crystallographically inde-
pendent PP molecules (colored magenta, green, and blue)
in 1S∙PPex. The third PP molecule is disordered over two
positions (a, b and c, d).
Figure 3. Locations of the three crystallographically inde-
pendent PP molecules (colored magenta, green, and blue) in
1S∙PPex'. The second and the third PP molecules are disor-
dered over two positions (a, b and c, d).
In order to better understand the enantioselectivity of
1S towards racemic PP we also determined the single crys-
tal structures of 1S∙PPex and 1S∙PPex'. The unit cells of
1S∙PPex and 1S∙PPex' are doubled those of 1S∙NB and
1S∙CH. Figures 2 and 3 provide insight as to why there is
doubling of the unit cell and into why 1S-PPex and
1S∙PPex' bind (R)-PP in preference to (S)-PP. There are
three distinct PP binding sites. The first and second bind-
ing sites are similar. Specifically, hydroxyl groups from PP
molecules form O-H∙∙∙O hydrogen bonds with nitrate
anions with contact distances of 3.023/2.833 Å and
2.934/2.892 (2.665) Å in 1S∙PPex and 1S∙PPex', respective-
ly. The most notable difference between the two struc-
tures is the direction, position and conformation of the
third PP molecule. The third PP molecule is disordered
over two positions as illustrated in Figures 2 and 3. In
Figure 4. Perspective view of the binding sites occupied by
the third PP molecule in 1S∙PPex (a) and 1S∙PPex' (b). The
color of the mesh represents the element which generates
the corresponding part of the surface. C orange, O red, N
blue, Cl green, H white. The first and second binding sites
are illustrated in Figure S8.
The change in orientation of the third PP could be re-
sponsible for the enantioselectivity for (S)-PP in 1S∙PPex
relative to (R)-PP in 1S∙PPex'. Figures 4 illustrate that the
binding sites are distinctly different even though most of
the components around the binding site are unchanged.
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Coordination Polymers: Design, Analysis and Application; Royal
Further, the hydroxyl groups of the third PP are fixed in
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Society of Chemistry: Cambridge, U.K., 2009. (c) MacGillivray, L.
R. Metal–Organic Frameworks: Design and Application; John
Wiley & Sons: Hoboken, NJ, 2010.
both structures through hydrogen bonding to a nitrate
anion and a water molecule. However, the DCM and wa-
ter/methanol molecules in 1S∙PPex and 1S∙PPex', respec-
tively, profoundly impact the shape of the binding site.
Specifically, the DCM molecule compresses the width of
the cavity (Figure 4a) and prevents the phenyl group from
aligning perpendicular to bc plane. Conversely, the wa-
ter/methanol molecule reduces the length of the cavity
(Figure 4b) and prevents the phenyl group from lying
parallel to the bc plane. The overall effect of these struc-
tural changes is that there is higher enantioselectivity for
the third PP in 1S∙PPex'. This effect is reminiscent of
changing an enzyme’s preference for cofactors.
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1S∙PPex' exhibits 60% ee for PP according to HPLC
analysis but this is lower than expected from our crystal-
lographic analysis. However, lower than expected ee val-
ues have also been observed by other groups,10,29-30 pre-
sumably because of disorder of guest molecules in host
channels. Gaining an understanding for the reasons for
this lack of specificity of the chiral binding sites is neces-
sary in order to design materials with even better ee per-
formance.
4. CONCLUSION
We report the single step synthesis of a pair of robust
CMOMs from commercially available chemicals. Thanks
to the 1D homochiral channels within 1S and 1R enanti-
oselective recognition towards PP was observed with ee
values of up to 60%. Our study of the crystal structures of
variants of 1S∙PP reveals how DCM and water/methanol
molecules can play an important role in affecting the
shape of the binding sites for PP. Indeed, the manipula-
tion of guest solvent molecules, which in effect serve as
cofactors, can be used to modify a PP binding site and
increase the overall enantioselectivity. Further studies will
be conducted to address the effect of pore size and pore
chemistry upon enantioselectivity towards PP and other
chiral guest molecules.
(11) (a) Dybtsev, D. N.; Nuzhdin, A. L.; Chun, H.; Bryliakov, K.
P.; Talsi, E. P.; Fedin, V. P.; Kim, K. Angew. Chem. Int. Ed. 2006,
45, 916. (b) Suh, K.; Yutkin, M. P.; Dybtsev, D. N.; Fedin, V. P.;
Kim, K. Chem. Commun. 2012, 48, 513.
(12) Lin, L; Yu, R; Wu, XY; Yang, WB; Zhang, J. Inorg. Chem.
2014, 53, 4794.
(13) Kuang, X.; Ma, Y.; Su, H.; Zhang, J.; Dong, Y.-B.; Tang, B.
Anal. Chem. 2014, 86, 1277.
(14) Li, Z.-J.; Yao, J.; Tao, Q.; Jiang, L.; Lu, T.-B. Inorg. Chem.
2013, 52, 11694.
ASSOCIATED CONTENT
Supporting Information. Information of characterization
data, supplementary schemes, tables and figures. This mate-
rial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
(15) Li, P.; He, Y.; Guang, J.; Weng, L.; Zhao, J.; Xiang, S.; Chen,
B. J. Am. Chem. Soc. 2014, 136, 547.
Corresponding Author
xtal@ul.ie
(16) (a) Das, M.; Guo, Q.; He, Y.; Kim, J.; Zhao, C.-G.; Hong, K.;
Xiang, S.; Zhang, Z.; Thomas, K.; Krishna, R.; Chen, B. J. Am.
Chem. Soc. 2012, 134, 8703. (b) Yuan, G.; Zhu, C.; Xuan, W.; Cui,
Y. Chem. Eur. J. 2009, 15, 6428. (c) Li, G.; Yu, W.; Cui, Y. J. Am.
Chem. Soc. 2008, 130, 4582.
(17) (a) Sang, R.-L.; Xu, L. Chem. Commun. 2013, 49, 8344. (b)
Jhu, Z.-R.; Yang, C.-I.; Lee, G.-H. CrystEngComm, 2013, 15, 2456.
(c) Liang, X.-Q.; Li, D.-P.; Li, C.-H.; Zhou, X.-H.; Li, Y.-Z.; Zuo, J.-
L.; You, X.-Z. Cryst. Growth Des. 2010, 10, 2596.
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy
(DE-AR0000177) and Science Foundation of Ireland for
Award 13/RP/B2549. We thank Prof. Peter Zhang’s group for
their assistance with collection of HPLC data.
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