The influence of the enantiomeric ratio of an organic ligand on the structure and chirality of metal-organic frameworks

Article abstract of DOI:10.1039/c4cc06190j

We have prepared three distinct polyamino acid-based metal-organic frameworks (MOFs) with different chiralities and porosities using the same chemistry, by simply modifying the enantiomeric ratio of the chiral organic ligand used. This journal is

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The influence of the enantiomeric ratio of an organic
ligand on the structure and chirality of Metal-Organic
Frameworks
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
a,b
Received 00th January 2012,
Accepted 00th January 2012
Iván Burneo, Kyriakos C. Stylianou, Inhar Imaz and Daniel Maspoch
DOI: 10.1039/x0xx00000x
www.rsc.org/
We have prepared three distinct polyamino acid-based metal- chirality of the resulting MOFs. Chiral ligands are commonly used to
1
7
synthesise homochiral porous MOFs. They can be also synthesised
from achiral building blocks in which their arrangement within the
organic frameworks (MOFs) with different chirality and
porosity using the same chemistry, by simply modifying the
enantiomeric ratio of the chiral organic ligand used.
18
structure is chiral. These approaches have recently attracted great
interest because of the potential for chiral MOFs in enantioselective
19
20
separation
and asymmetric catalysis.
Prior to our study,
Metal–organic frameworks (MOFs) are crystalline porous solids
constructed by linking metal ions/clusters to organic ligands through
isostructural homochiral or racemic MOFs have been prepared by
combining Cu(II) or Zn(II) with 4,4’ꢀbipyridine and Dꢀ or DLꢀ
1
coordination bonds. Currently, MOFs are among the most attractive
21
camphoric acid, respectively. Also, researchers have reported that
porous materials due to their diverse compositions and structures,
which are characterised by ordered arrays of pores that can be used
Zn(II)ꢀCo(II)ꢀbased MOFs and amorphous solids are obtained by
22
using racemic and enantiopure metalloligands, respectively, and
2
3
4
5
for gas storage, separation, catalysis, drug capture and delivery,
that the framework interpenetration in Cu(II)ꢀbased MOFs can be
controlled by using Rꢀ or racemicꢀ2,2ʹꢀdiethoxyꢀ1,1ʹꢀbinaphthylꢀ
6
7
imaging and sensing. The strategic design of the organic ligands to
be used (known as the linker design approach) has proven critical
8
4
,4ʹ6,6ʹꢀtetrabenzoate ligands and solvent molecules of different
for tuning the structure of MOFs and thus, for achieving the desired
23
sizes.
However, to the best of our knowledge, there are no
functionality for
a target application. This is because the
previous reports showing the influence of the enantiomeric ratio (er)
of chiral organic ligands in the formation of MOFs.
9
characteristics of the ligands can influence the structural topology,
internal pores (surface and volume), and other properties of the
resulting MOFs. Several variables have already been identified and
have since been incorporated into the ligandꢀdesign process,
including the geometry and length, the postꢀsynthetic addition of
9
1
0
1
1
certain functional groups, and the use of functional ligands with
1
2
13
preꢀdetermined properties such as fluorescence and chirality. For
example, researchers have shown that the pore size (from 14 to 98
Å) of a series of Mg(II)ꢀbased MOFs isostructural to MOFꢀ74 (or
CPOꢀ27) can be tuned by increasing the length of the organic ligand
(
starting with dihydroxyꢀterephthalate and going up to eleven
14
phenyl rings), without modifying its functional groups.
Alternatively, some authors have reported enhancing the catalytic
activity of MOFs by postꢀsynthetically incorporating Lewis acid
1
5
metal centres on the pore surfaces. Lastly, researchers have
demonstrated that the luminescence properties of Zn(II)ꢀbased
MOFs can be tailored according to the organisation of the
fluorescent stilbene dicarboxylic acid ligand around the Zn(II),
leading to stronger or weaker ligandꢀligand chromophore
Figure 1. Schematic illustration of the enantiopure ligands Rꢀ and Sꢀ
BTAsp. Under identical conditions, the reaction of Cu(II) with RꢀBTAsp
alone, or mixtures of S- and R-BTAsp, or SꢀBTAsp alone affords the three
distinct MOFs R, RS and S (bottom), respectively, which differ by chirality
and/or porosity.
16
interactions.
Herein we report that the enantiopurity of a chiral ligand used in
MOF synthesis can be exploited to influence the structure and
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Our strategy relies on the design of two enantiomorphs that act
as enantiomerically pure ligands with opposite chirality, when
used alone, or as a racemic mixture of ligands, when an equimolar
ratio is used. They can also be mixed in different ratios to prepare
solutions of different er (hereafter, S:R). Both ligands, (1S,3S,5S)ꢀ
benzene tricarbonyl triꢀ(aspartic acid) (SꢀBTAsp) and (1R,3R,5R)ꢀ
benzene tricarbonyl triꢀ(aspartic acid) (RꢀBTAsp) (Figure 1),
combine the rigidity of the phenyl ring (blue) with the
coordination ability, chirality and flexibility of the aspartate
carboxylic groups (S, orange; R, green; Figure 1). For detailed
information on the synthesis and characterisation of
enantiomerically pure Sꢀ and RꢀBTAsp, see section S1 and Figures
S1ꢀ2. We demonstrate that the reaction of Cu(II) with either
enantiomerically pure ligand (Sꢀ or RꢀBTAsp), or with the racemic
mixture RSꢀBTAsp, under identical reaction conditions yields
three 3D polyamino acidꢀbased MOFs: two isostructural and
homochiral, nonꢀporous (but with large void volumes) MOFs with
opposite optical activity (hereafter, S and R); and a third, achiral,
MOF (hereafter, RS), which contains 1D pores accessible for gas
uptake (Figure 1). We also show that the formation and purity of
the MOFs are directly affected by the er of the Sꢀ/RꢀBTAsp
mixture used.
Figure 2. (aꢀd) Crystal structure of S. a) Binding mode of the ligand Sꢀ
BTAsp. b) Single cavity forming the 3D channel system, in which the
purple ball illustrates the internal diameter (7.0 Å). c) Single large cage
delimited by 8 SꢀBTAsp ligands and 18 Cu(II) paddlewheels. The yellow
ball represents the internal void volume (1340 Å ) within the cage. d)
Packing of S along the cꢀaxis, showing the organisation of the large cages
3
(
yellow balls) and cavities (purple balls) forming the channel system. (eꢀh)
Crystal structure of RS. e) Coordination of BTAsp ligand to nine Cu(II)
centres. f) Leftꢀ or rightꢀhanded helices made by connecting the Cu(II) ions
to either SꢀBTAsp or RꢀBTAsp, respectively, along the bꢀaxis. g) Stick
representation of one pore channel (dimensions: of 9.1 Å x 4.3 Å), and h)
Connolly surface representation of RS viewed in 3D. Atom colour code:
Cu: sky blue; C: grey; O: red; N: blue; and H: yellow.
Reaction of Cu(NO ) ·6H O and SꢀBTAsp in a slightly acidic
3
2
2
(pH 6) aqueous solution at room temperature for 24 hours afforded
octahedral crystals (yield: 67%; section S1 and Figures S3 and
ꢀ1
ꢀ1
signals are at 1623 cm and 1417 cm , which primarily correspond
to asymmetric and symmetric stretching bands, respectively, of the
S9). Singleꢀcrystal XRD revealed a 3D network of formula [Cu (Sꢀ
3
BTAsp)(H O) ]·12.75H O, S, which crystallises in the chiral F23
ꢀ1
2
3
2
carboxylate groups; and at 1537 cm , which is attributed to the
space group (Table S1). The basic unit of S is a Cu paddlewheel
2
bending of NꢀH bond. Both frameworks were also studied using
ECD by disassembling them under acidic conditions (pH 2.0;
unit, in which each Cu(II) is coordinated to four carboxylate O
atoms, from four adjacent aspartate functions, and to one H O
2
Figures 3b). For
S and R, identical ECD signals of their
molecule. Each carboxylate group of the aspartate residue bridges
corresponding enantiomeric Sꢀ or RꢀBTAsp ligands, respectively,
were observed (Figure S10).
two Cu(II) centres within the Cu paddlewheel unit; consequently, a
2
single SꢀBTAsp ligand binds to twelve Cu(II) centres (Figure 2a).
Within S, the aspartate residues of a single SꢀBTAsp have the same
conformation, as confirmed by measurement of the side chain
dihedral angle: ꢀ66.65° (Figures S4 and S5, and Table S2). The
specific folding of the aspartate groups of SꢀBTAsp, and the
coordination around the Cu(II)ꢀpaddlewheel unit, of together create a
To explore the influence of the enantiopurity of Rꢀ and SꢀBTAsp
ligands on MOF synthesis, we first prepared a racemic mixture using
equimolar amounts (er = 5:5) of the two ligands (Figure S2), and
then reacted it with Cu(II), using the same experimental conditions
as for the isolation of S and R. In contrast with the formation of
octahedral crystals of S or R after 24 h, intergrown needleꢀlike
crystals began to form after 15 to 17 days (yield: 35%; purity
confirmed by PXRD, elemental analysis and SEM; Figures S3 and
S9). Singleꢀcrystal XRD revealed the formation of a new 3D MOF,
of formula [Cu (RSꢀBTAsp)(H O) ]·7.5H O, RS, which crystallises
in the centrosymmetric Pcnm orthorhombic space group (Table S1).
Within RS, there are three distinct Cu(II) ions: Cu#1, which has
square pyramidal geometry; and Cu#2 and Cu#3, both of which have
square planar geometry (Figure S11). Each aspartate group of the
BTAsp within RS is bound to three Cu(II) ions, giving a total
connectivity per ligand of nine Cu(II) ions (Figure 2e). A striking
difference observed between S or R and RS, is that the sideꢀchain
dihedral angles of the three aspartate groups within a single BTAsp
ligand in RS vary (73.47º, ꢀ60.42º and ꢀ71.74°; Figures S4 and S5,
and Table S2). In RS, the coordination of the BTAsp ligand around
the three distinct Cu(II) leads to the formation of columnar leftꢀ and
rightꢀhanded helices (Figure 2f), in which the benzene rings of the
BTAsp are stacked along the bꢀaxis, introducing weak πꢀπ stacking
interactions (distance: 4.0 Å; Figures 2f and S11ꢀ13). This
organisation generates 1D channels (dimensions: 9.1 Å x 4.3 Å,
considering vdW radii) running along the bꢀaxis (Figures 2gꢀh). The
3
D structure that incorporates two types of void volumes: i) a 3D
channel system, formed of cavities (internal diameter: 7.0 Å,
considering van der Waals radii (vdW); Figures 2b and S6)
connected by small pores (dimensions: 7.0 Å x 3.5 Å, considering
vdW radii) and ii) larger, isolated cages, in which the shortest
separation between their centroid and their vdW surface is 6.8 Å,
3
2
4
2
3
indicating that a sphere with a volume of 1340 Å can fit inside them
(Figures 2c and S7). The large cages are not accessible, as the largest
window aperture is found to be between HOH···OCNH and
CH···HCH, at a distance of 3.4 Å and 0.5 Å, respectively, including
vdW radii of the atoms (Figure S8). The 3D channel system and the
large cages combine for a total void space of 52% per unit cell
24
(Figure 2d).
We prepared R using the same conditions as for S, except that
instead of SꢀBTAsp, we used RꢀBTAsp (yield: 70%; obtained as a
pure phase, as confirmed by elemental analysis and SEM; Figure
S3). Interestingly, the PXRD patterns demonstrated that both S and
1
7b
R are isostructural (Figure S9).
In addition, the infrared (IR)
spectra of S and R are nearly identical, also confirming that they are
isostructural (Figure 3a). The opposite chirality between S and R
was confirmed by solidꢀstate Vibrational Circular Dichroism (VCD):
their chirality is defined by the coordination network between the
Cu(II) ions and the carboxylate groups, which have specific
absorptions in the infrared region (Figure 3a). For the VCD spectra,
a mirrorꢀimage relationship is observed between the enantiomeric
pairs of S and R (Figure 3a). The most relevant inverted VCD
channels are filled with four H O molecules coordinated to Cu#3
2
(
Figures 2gꢀh, and S12 and 13) and 7.5 guest H O molecules. The
2
24
total accessible void volume of RS is 32% per unit cell, and the
calculated pore volume in RS is 0.181 cm /g.
3
To further evaluate the importance of the enantiopurity of the
BTAsp ligand in the resulting MOF, we systematically prepared
2
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different mixtures of Sꢀ and RꢀBTAsp (from 9:1 to 1:9, including the consistent with the TGA data and with the PXRD pattern of the
o
enantiopure 10:0 and 0:10), and then reacted each mixture with activated S after lyophilisation at ꢀ50 C and 0.07 mbar (Figure S18),
Cu(II) under the conditions used for the isolation of S, R and RS. We showing that the collapse of S (and therefore, of R) occurs gradually,
observed that the use of a slight excess of either Sꢀ or RꢀBTAsp (7:3 upon the loss of H O molecules. Contrariwise, the VTꢀPXRD
2
and 6:4, or vice versa) always led to the formation of the achiral RS patterns of RS show that it loses much of its crystallinity at a far
phase, just as when synthesised with the equimolar mixture (5:5). higher temperature (200 ºC) than does S.
For all reactions, the formation and purity of RS was confirmed by
Field Emission SEM (FESEM), PXRD (Figure 3d) and ECD after
disassembly of the product at pH 2 (at which point we could not
detect any signal by ECD; Figure S14). Contrariwise, when a large
excess of either Sꢀ or RꢀBTAsp (9:1 and 8:2, or vice versa) was
used, we observed mainly the formation of octahedral crystals
indicative of the homochiral S or R, respectively, as confirmed by
both PXRD (Figure 3d) and ECD (Figure 3b). However, the
presence of lowꢀintensity Bragg reflections (2θ: 8.3º and 13.2º),
As expected, type II isotherms (77 K and 1 bar) of S and of R
2
5
together with the different ECD intensity maxima, revealed that a
small amount of RS also had formed together with S or R.
To quantify the amounts of S, R and RS formed in the
aforementioned reactions, we disassembled the product crystals at
pH 2, and then determined the corresponding enantiomeric excess
(ee) of the free Sꢀ or RꢀBTAsp. We used an analytical method
based on calculation of the anisotropy factor: the gꢀfactor, a
concentrationꢀindependent parameter derived from the absorbance
2
6
and ellipticity of the chiral ligand (section SI9). In a first step,
this method was validated by confirming the linearity of the gꢀ
factor calculated in pure samples of Sꢀ or RꢀBTAsp in the working
concentration range (0.04 mg/mL to 0.06 mg/mL; Figure S15). The
2
analytical calibration curve (R = 0.994) was measured by
Figure 3. a) IR (top) and VCD (bottom) spectra of S, R and RS. b) ECD
spectra collected after the disassembly of the MOF crystals synthesised
using er of 10:0, 9:1 and 8:2 (and vice versa) of SꢀBTAsp/RꢀBTAsp and er
calculating the gꢀfactor of different mixtures of Sꢀ and RꢀBTAsp
from 9:1 to 1:9, including the enantiopure 10:0 and 0:10), and then
(
5
:5. c) Plot of gꢀvalues against enantiomeric excess of SꢀBTAsp/RꢀBTAsp.
plotting them against the enantiomeric excess (ee). As shown in
Figure 3c, the straight line nearly intercepts the “x” axis at 0% ee—
the value expected for the racemic mixture. To test this method, a
series of control experiments was performed, in which the ee of
pure samples of S, R and RS was measured. The gꢀvalues of the
enantiopure S and R, and of the racemic RS, were ꢀ2.4 ± 0.4, 2.8 ±
.4 and 0.2 ± 0.4, respectively, corresponding to the expected 99.3 ±
.7 ee (SꢀBTAsp), 97.5 ± 7.9 ee (RꢀBTAsp) and 0.7 ± 8.8 ee (Rꢀ
BTAsp), respectively (Figure 3c). Finally, the gꢀvalues of the MOF
crystals resulting from the reactions run with a large excess of either
isomer (9:1 and 8:2, and vice versa) were measured. These values
were found to be: ꢀ1.7 ± 0.4 [9:1; 71.4 ± 7.2 ee (SꢀBTAsp)], ꢀ1.8 ±
The triangles represent the gꢀvalues calculated from the MOFs synthesised
using er of 0:10 (blue), 1:9 (sky blue), 2:8 (purple), 5:5 (red), 8:2 (dark
yellow), 9:1 (light green) and 10:0 (green). d) PXRD patterns and FESEM
images of the MOFs resulting when various ratios of SꢀBTASp/RꢀBTAsp
were used in the reaction with Cu(II).
revealed that they are nonꢀporous to N . This is due to their low
2
0
8
stability upon removal of the guest H O molecules, as confirmed by
2
TGA and PXRD (Figure S18). In contrast, RS (once activated by
solvent exchange with chloroform for 72 hours (Figure S19) and
27
subsequent outgassing at 40 ºC overnight) is porous to N (92.7
2
3
2
0
cm /g at 0.96 bar; BET surface area: 263 m /g at 0.05>p/p >0.3) at
7 K, for which it showed a reversible typical typeꢀI isotherm
Figure S20a). The pore volume for RS, derived from the N
7
0
.4 [8:2; 75.5 ± 6.4 ee (SꢀBTAsp)], 1.9 ± 0.5 [2:8; 65.2 ± 11.0 ee (Rꢀ
(
2
2
8
BTAsp)] and 2.0 ± 0.5 [1:9; 67.6 ± 10.8 ee (RꢀBTAsp)]. Importantly, adsorption branch using the DubininꢀRadushkevich equation, was
3
these ee values, together with the previously described PXRD data, found to be 0.142 cm /g, which is somewhat smaller than the value
3
demonstrate that both ligands indeed participate in the construction (0.181 cm /g) estimated from the static crystal structure. The typeꢀI
of the MOFs in these reactions. Here, if one assumes that a N isotherm of RS prompted us to investigate its CO sorption and to
2
2
combination of S and RS is simultaneously formed when 9:1 and 8:2 assess the strength of the interactions between RS and CO . RS is
2
3
ratios are used (or R and RS for the 1:9 and 2:8 ratios), then the porous to CO (79.8 cm /g at 195 K and 0.85 bar) at 195 K, showing
2
percentage of S relative to RS (or R relative to RS for the 1:9 and 2:8 a typeꢀI isotherm (Figure S20b). At 273 K and 0.85 bar, RS adsorbed
3
ratios) is simply given by the corresponding ee value (~ 65ꢀ75%).
38.3 cm /g of CO , whereas at 295 K and 0.85 pressure, it adsorbed
2
3
Thermogravimetric analysis (TGA) of isostructural S and R 25.5 cm /g of CO . Isosteric heats of adsorption (Q ) of RS for CO
2
st
2
2
9
indicated
a continuous weight loss of 25.1% and 26.3%, were calculated from the virialꢀtype expression, using the
o
respectively, from 30 C to 200 ºC, which we attributed to the loss of adsorption branches of the isotherms measured at 273 K and 295 K
o
all H O molecules (calcd. 27.7%; Figure S16). Above 200 C, both (Figure S21). At zero coverage, Q was calculated to be 27.5 kJ/mol,
2
st
frameworks decompose in multiple steps. The TGA profile of RS but it gradually decreased to 18.5 kJ/mol at high loadings.
o
o
shows a weight loss of 11.2% from 30 C to 200 C, which we
ascribed to the removal of the 7.5 guest H O molecules (calcd. opposite enantiomeric ligands can influence the final structure
In conclusion, we have demonstrated that the ratio of two
2
1
4.6%; Figure S16). Also, RS begins to decompose stepwise at 200 of the resulting MOFs. We showed that by simply modifying
o
C. To further evaluate the thermal stability of S and RS, TGA data the ratio of
were complemented with in-situ Variable Temperature PXRD (VTꢀ of the resulting MOF can be tuned between the three structures
PXRD) experiments performed in a closed capillary tube. VTꢀPXRD
SꢀBTAsp/RꢀBTAsp in a MOF reaction, the structure
R, RS and S, which exhibit different porosity and chirality
revealed that S (and therefore, R) slowly collapses and becomes an properties. The systematic identification of new parameters that
amorphous solid above 100 ºC (Figure S17). This finding is determine MOF structures is important, as it will enable greater
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This work was supported by the MINECOꢀSpain under the
project PN MAT2012ꢀ30994 and EU FP7 ERCꢀCo 615954.
I.B. thanks SENESCYTꢀEcuador for a PhD fellowship, I.I.
thanks MINECO for a Ramón y Cajal grant and K.C.S. thanks
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5 A. E. Holmes, S. Zahn and J. W. Canary, Chirality, 2002, 14, 471ꢀ
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†
Electronic Supplementary Information (ESI) available: Synthetic details
and crystallographic information. CCDC 1005432 ( ), 1005433 ( ) and
005434 (RS). For ESI and crystallographic data in CIF format see
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ChemComm
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DOI: 10.1039/C4CC06190J
Ivan Burneo, Kyriakos C.
Stylianou, Inhar Imaz and
Daniel Maspoch
The modification of the
The Influence of the
Enantiomeric Ratio of an
Organic Ligand on the
Structure and Chirality of
Metal-Organic
enantiomeric ratio of a
chiral ligand resulted in
the isolation of three
distinct MOFs.
Frameworks

ChemComm
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DOI: 10.1039/C4CC06190J
4
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