Synthesis of a Chiral Crystal Form of MOF-5, CMOF-5, by Chiral Induction

Article abstract of DOI:10.1021/jacs.5b11150

Chiral variants of the prototypal metal-organic framework MOF-5, ∧-CMOF-5 and -CMOF-5, have been synthesized by preparing MOF-5 in the presence of l-proline or d-proline, respectively. CMOF-5 crystallizes in chiral space group P2₁3 instead of F

Full text of DOI:10.1021/jacs.5b11150

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Communication
Synthesis of a Chiral Crystal Form of MOF-5, CMOF-5, by Chiral Induction
Shi-Yuan Zhang, Dan Li, Dong Guo, Hui Zhang, Wei Shi,
Peng Cheng, Lukasz Wojtas, and Michael J. Zaworotko
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b11150 • Publication Date (Web): 25 Nov 2015
Downloaded from http://pubs.acs.org on November 26, 2015
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Synthesis of a Chiral Crystal Form of MOF-5, CMOF-5, by Chi-
ral Induction
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ShiꢀYuan Zhang,†,‡ Dan Li,§ Dong Guo,§ Hui Zhang,§ Wei Shi,‡ Peng Cheng,‡ Lukasz Wojtas,|| and Miꢀ
chael J. Zaworotko*,†
†Department of Chemical & Environmental Science, Materials and Surface Science Institute, University of Limerick, Limerꢀ
ick, Republic of Ireland
‡Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Collaborative Innovation Cenꢀ
ter of Chemical Science and Engineering, Nankai University, Tianjin 300071, P. R. China
§Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R.
China
||Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, Florida 33620, United
States
Supporting Information Placeholder
ABSTRACT: Chiral variants of the prototypal metalꢀorganic
framework MOFꢀ5, ΛꢀCMOFꢀ5 and ꢁꢀCMOFꢀ5, have been synꢀ
thesized by preparing MOFꢀ5 in the presence of Lꢀproline or Dꢀ
proline, respectively. CMOFꢀ5 crystallizes in chiral space group
P213 instead of Fmꢀ3m as exhibited by MOFꢀ5. The phase purity
of CMOFꢀ5 was validated by single crystal and powder Xꢀray
diffraction, IR spectroscopy, TGA, N2 adsorption, microanalysis
and solidꢀstate VCD. CMOFꢀ5 undergoes a reversible single crysꢀ
tal to single crystal phase change to MOFꢀ5 when immersed in a
variety of organic solvents although Nꢀmethylꢀ2ꢀpyrolidone,
NMP, does not induce loss of chirality. Indeed, MOFꢀ5 undergoes
chiral induction when immersed in NMP, affording racemic
CMOFꢀ5.
Chirality is an essential feature of life and plays a vital role in
various chemical and biological processes. Chirality in crystalline
materials manifests itself in applications such as enantioselective
separation1ꢀ4, asymmetric catalysis1ꢀ6 and nonlinear optical behavꢀ
ior7. Metalꢀorganic materials, MOMs,8ꢀ10 can combine chirality
and nanoscale porosity in a single material. Syntheses of chiral
MOMs has thus far tended to focus upon two main approaches:
use of homochiral molecular building blocks, MBBs1, 11ꢀ14; sponꢀ
Figure 1. The synthesis of CMOFꢀ5. The presence of Lꢀ or Dꢀ
taneous resolution of chiral nets comprised of achiral MBBs11, 15ꢀ
proline additives induces chirality and enables the growth and
17. Homochiral MOMs are most typically generated through the
isolation of CMOFꢀ5 crystals. An achiral square grid net,
[Zn(BDC)(NMP)], forms in the absence of proline.
former approach, with homochiral MBBs covalently bonded to
the MOM framework as linking or pendant ligands. Such MOMs
can be synthesized from homochiral starting materials or through
postsynthetic modification (PSM).18ꢀ21 PSM exploits porosity to
attach chiral moieties at organic linkers or unsaturated metal cenꢀ
ters. Homochiral MOMs produced by PSM tend to exhibit reꢀ
duced or lost pore volume but bulk chirality can also be created
through MOMs that exhibit network topology that is inherently
chiral because of the spatial organization of achiral building
blocks. Unfortunately, such MOMs tend to form through spontaꢀ
neous resolution and they are typically racemic conglomerates
(heterochiral).
An approach to synthesis of chiral MOMs that might overcome
the drawbacks of the aforementioned methods is the use of chiral
additives that induce achiral MBBs to generate homochiral
MOMs. Such chiral induction remains poorly understood but it
offers the advantage that in principle it is not limited to particular
MBBs or linkers. Recent examples of this approach include the
synthesis of SIMOFꢀ1 and [Mn3(HCOO)4(adc)] by Morris22 and
Bu23, respectively. Nevertheless, the use of chiral induction to
crystallize chiral MOMs with a particular composition and topolꢀ
ogy remains understudied, especially with respect to high symꢀ
metry MBBs that crystallize in high symmetry space groups.
In this contribution, we demonstrate that MOFꢀ524, a high
symmetry prototypal MOM that is one of the most widely studied
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metalꢀorganic frameworks (MOFs),25ꢀ27 is subject to a phase
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change to a chiral variant during (chiral additives) or after (singleꢀ
crystal to singleꢀcrystal, SCSC, transformation) synthesis. The
existence of the new chiral variant of MOFꢀ5, CMOFꢀ5, is perꢀ
haps surprising given that MOFꢀ5 is comprised from high symꢀ
metry 6ꢀconnected tetranuclear Zn4O clusters linked by 1,4ꢀ
benzenedicarboxylate (BDC) linkers to form a primitive cubic or
pcu network. Figures 1 and 2 illustrate how chiral additives (durꢀ
ing synthesis) or solvent (postꢀsynthesis) can induce the formation
of CMOFꢀ5.
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Figure 3. The formation of CMOFꢀ5 structures: (a) Distorted
metal clusters and BDC ligands. (b) A complete helix turn. Grey
boxes represent Zn4O clusters and the BDC ligands generate the
helical turns. (c) Assembly of helix fragments in one dimension.
(d) Perspective view of CMOFꢀ5. Color code for ΛꢀCMOFꢀ5 and
ꢁꢀCMOFꢀ5: blue and red.
To more fully characterize CMOFꢀ5 and determine the nature
of the guests that reside inside the void spaces, we conducted IR
spectroscopy, nitrogen gas adsorption, thermogravimetric analysis
(TGA) and microanalysis experiments upon MOFꢀ5 and CMOFꢀ5
(see SI for details). IR spectroscopy (Figure S3) revealed the presꢀ
ence of the C=O stretching band of NMP at 1659 cmꢀ1 and the
absence the NꢀH bending band of proline at 1549 cmꢀ1, indicating
that only NMP molecules are present in the channels of as syntheꢀ
sized CMOFꢀ5, asꢀCMOFꢀ5. IR spectroscopy was diagnostic with
respect to CMOFꢀ5 vs. MOFꢀ5 in that there are blue shifts of the
C=O stretching band of BDC (1610 to 1598 cmꢀ1) and the OꢀZn
bending band of the Zn4O cluster (1396 to 1378 cmꢀ1). Elemental
analysis indicated that ∼9.5 NMP per formula unit are incorpoꢀ
rated in the channel, supported by the weight loss of 45.0% obꢀ
served from TGA experiments (Figure S4). The nitrogen gas sorpꢀ
tion isotherm (Figure S5) for activated CMOFꢀ5 at 77 K exhibits
type I behavior with a Langmuir surface area of 4535 m2/g close
to that reported for MOFꢀ528.
Figure 2. Postsynthetic modification, PSM, studies conducted
upon CMOFꢀ5. PSM using most organic solvents converted
CMOFꢀ5 into MOFꢀ5 whereas NMP induced MOFꢀ5 to revert to
a racemic conglomerate of CMOFꢀ5.
ΛꢀCMOFꢀ5 and ꢁꢀCMOFꢀ5 were synthesized from a 1:1:1
mixture of Zn(NO3)2ꢂ6H2O, H2BDC and Lꢀproline or Dꢀproline,
respectively, in Nꢀmethylꢀ2ꢀpyrrolidone (NMP) and water (4:1
v/v) at 120°C. Single crystal Xꢀray diffraction studies (detailed in
SI and Table S1) conducted upon CMOFꢀ5 revealed that it crysꢀ
talizes in the chiral cubic space group P213 and that chirality reꢀ
sults from a distortion of the backbone of the framework. A deꢀ
crease in the unit cell parameter, 25.7726(10) Å in MOFꢀ5 to
25.3688(7) Å in ΛꢀCMOFꢀ5, is accompanied by a corresponding
4.63% decrease in the unit cell volume. The ZnꢀO bond distances
within the Zn4O MBBs are 1.9329(6) Å and 1.909(17)ꢀ1.951(6) Å
in MOFꢀ5 and ΛꢀCMOFꢀ5, respectively. The lower symmetry of
CMOFꢀ5 is illustrated by the waveꢀlike nature of its structure that
results from twisting of adjacent Zn4O clusters by 6.80° (Λꢀ
CMOFꢀ5) and 7.05° (ꢁꢀCMOFꢀ5) vs. MOFꢀ5 (Figures 3 and S1).
The benzene rings between adjacent metal clusters in CMOFꢀ5
are oriented at a non 90° dihedral angle, giving rise to axial chiralꢀ
ity. As a result of disordered BDC ligands, the torsion angles in
ΛꢀCMOFꢀ5 and ꢁꢀCMOFꢀ5, measured by taking the average of
two disordered phenyl planes, are 78.68° and 66.85°, respectively.
It appears that the atropisomerꢀlike bridging mode of the ligands
translates chirality from one cluster to another, thereby affording
the observed chiral framework. To further understand the nature
of chirality in CMOFꢀ5, the building blocks are simplified as a
single handedness helix (Figure 3), which selfꢀassemble to conꢀ
struct the observed helical framework. Powder Xꢀray diffraction
(PXRD) distinguishes between MOFꢀ5 and CMOFꢀ5 thanks to the
lower symmetry of CMOFꢀ5 (Figure S2).
The enantiomeric nature of ΛꢀCMOFꢀ5 and ꢁꢀCMOFꢀ5 was
verified through solidꢀstate vibrational circular dichroism (VCD)
spectra obtained from individual single crystals that had been
converted to KBr pellets. Although single crystals were obtained
from synthesis, enantiomers cannot be identified by appearance
because their morphologies are indistinguishable. Figure 4 reveals
that ΛꢀCMOFꢀ5 and ꢁꢀCMOFꢀ5 exhibit different Cotton effects
in the wavelength range 1800ꢀ1300 nm, supporting the formation
of leftꢀhanded or rightꢀhanded enantiomers from reactions conꢀ
ducted in the presence of Lꢀproline or Dꢀproline, respectively.
VCD measurements were conducted upon a random set of 10
crystals from each crystallization batch (Figure S6 and S7). Enanꢀ
tiomeric excess, ee, (ee = 100 * (Λ ꢀ ꢁ)/(Λ + ꢁ)) of ca. 80% was
observed both ΛꢀMOFꢀ5 and ꢁꢀMOFꢀ5.
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provide a new avenue for both fundamental and applied studies of
chirality in porous crystalline MOMs.
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ASSOCIATED CONTENT
Supporting Information
Information of materials and measurements, synthesis and characꢀ
terization data, supplementary figures. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
xtal@ul.ie
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
M.J.Z. acknowledges the U.S. Department of Energy (DEꢀ
AR0000177) and Science Foundation of Ireland for Award
13/RP/B2549. W.S. and P.C. acknowledge the MOST (973 proꢀ
gram, 2012CB821702), NSFC (21373115, 21331003 and
91422302) and 111 Project (B12015).
Figure 4. Solidꢀstate VCD (top) and IR (bottom) spectra of a
representative single crystal of ΛꢀCMOFꢀ5, ꢁꢀCMOFꢀ5, and
MOFꢀ5, respectively.
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Interestingly, when the same reaction was conducted in the abꢀ
sence of Lꢀproline or Dꢀproline, a new achiral layered squareꢀgrid
compound, [Zn(BDC)(NMP)], was formed. This finding suggests
that the presence of proline is critical for the crystallization of Λꢀ
CMOFꢀ5 or ꢁꢀCMOFꢀ5 under the particular conditions used.
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