Single-crystal to single-crystal cross-linking of an interpenetrating chiral metal-organic framework and implications in asymmetric catalysis

Article abstract of DOI:10.1002/anie.201003377

Support framework: Post-synthesis modification of an interpenetrating Zn-based chiral metal-organic framework with Ti(OiPr)₄ results in a Lewis acidic catalyst (see picture; gray C, white H, red O, green Ti) with modest enantioselectivity in the asymmetric addition of diethylzinc to aldehydes. The Ti(OiPr)₄-treated framework contains single-crystal to single-crystal cross-linking of the two interpenetrating networks.

Full text of DOI:10.1002/anie.201003377

Communications
DOI: 10.1002/anie.201003377
Metal–Organic Frameworks
Single-Crystal to Single-Crystal Cross-Linking of an Interpenetrating
Chiral Metal–Organic Framework and Implications in Asymmetric
Catalysis**
Liqing Ma, Chuan-De Wu, Marcela M. Wanderley, and Wenbin Lin*
Metal–organic frameworks (MOFs) have received extensive
interest in the past decade because of their interesting
Herein we report the synthesis and characterization of two
interpenetrating chiral MOFs [Zn (L)(dmf)-
(H O)]·2EtOH·4.3DMF·H O (1, where L is (R)-2,2’-dieth-
2
[1]
properties. MOFs exhibit exceptional gas-uptake capacity
2
2
[2]
owing to their extreme porosity. The ability to incorporate
desired functional groups allows MOFs to perform in a
oxy-1,1’-binaphthyl-4,4’,6,6’-tetrabenzoate) and [Zn (L’)-
2
(dmf)(H O)]·2EtOH·4.3DMF·4H O (2, L’ = (R)-2,2’-dihy-
2
2
[3]
[4]
variety of applications, such as catalysis, chemical sensing,
droxy-1,1’-binaphthyl-4,4’,6,6’-tetrabenzoate), which are con-
structed from dizinc SBUs and chiral tetracarboxylate ligands.
More importantly, we observed unprecedented single-crystal
to single-crystal crosslinking of the two interpenetrating
[
5]
and drug delivery. In particular, MOFs represent ideal
candidates as heterogeneous catalysts by simultaneously
imparting porosity and introducing catalytic sites into
MOFs. Unlike other catalyst heterogenization strategies, the
MOF-derived heterogeneous catalysts can remain single-
crystalline, thus providing a unique opportunity for detailed
structural interrogation and therefore delineating the rela-
tionships between the MOF structures and their catalytic
activities and selectivities.
networks in 2 by Ti(OiPr) to lead to intermolecular [Ti-
4
(BINOLate) ] complexes that exhibit modest enantioselec-
2
tivity in catalyzing the addition of diethylzinc to aromatic
aldehydes to afford chiral secondary alcohols. This result
provides unambiguous structural identification of an immo-
bilized homogeneous catalyst and has significant implications
in rational design of MOF-based heterogeneous asymmetric
catalysts.
Moderate hydrolytic and thermal stabilities of most
MOFs limit the scope of reactions that can be heterogene-
ously catalyzed by MOFs. We have recently focused our
efforts on the design of chiral porous MOFs for heteroge-
neous asymmetric catalysis as most asymmetric catalytic
reactions are carried out under mild conditions in aprotic
MOF 1 was synthesized by heating a mixture of ZnI and
2
(R)-H L in DMF/EtOH at 908C for five days, while MOF 2
4
was produced by heating ZnI and (R)-H L’ in DMF/EtOH at
2
4
1008C for one week (Scheme 1). The formulae of 1 and 2 were
established by single-crystal X-ray diffraction studies, NMR
analysis, and thermogravimetric analysis (TGA).
[
6]
solvents. In order for chiral MOFs to be useful asymmetric
catalysts, they must possess large nanometer-scale open
channels for the facile transport of sterically demanding
substrates and products. We have recently synthesized
isoreticular chiral MOFs based on tetracarboxylate bridging
ligands derived from 1,1’-bi-2-naphthol (BINOL) and copper
paddle-wheel secondary building units (SBUs), and observed
the remarkable dependence of the enantioselectivities of the
addition of Et Zn to aromatic aldehydes on the MOF open-
2
7]
[
channel sizes. In order to expand the scope of applications of
such chiral tetracarboxylate ligands, and to further under-
stand the relationships between framework structures and
catalytic activities, we have used these ligands in combination
with other metal-connecting points or metal-cluster SBUs.
Scheme 1. Synthesis of homochiral MOFs 1 and 2.
[
+]
[+]
[
*] Dr. L. Ma, Dr. C.-D. Wu, M. M. Wanderley, Prof. W. Lin
Department of Chemistry, CB#3290
University of North Carolina
Chapel Hill, NC 27599 (USA)
Fax: (+1)919-962-2388
Crystals of 1 and 2 are isostructural and crystallize in the
[8]
orthorhombic I2 2 2 space group. Here we only discuss
1
1 1
structure 2, which contains two Zn atoms, one L’ ligand, one
water molecule, and one DMF molecule in the framework in
each asymmetric unit. The two Zn atoms have distinct
coordination environments: one atom coordinates to four
carboxyl oxygen atoms of four different L’ ligands and one
water molecule in square-pyramidal geometry, and the other
atom coordinates to three carboxyl oxygen atoms and one
DMF molecule in a distorted tetrahedral geometry. The two
distinct Zn atoms are triply bridged by carboxylate groups to
E-mail: wlin@unc.edu
Homepage:
http://www.chem.unc.edu/people/faculty/linw/wlindex.html
+
[
] These authors contributed equally to this work.
[
**] We thank NSF (CHE-0809776) for financial support and Kathryn
deKrafft and Dr. Banu Kesanli for experimental help.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003377.
8244
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Angew. Chem. Int. Ed. 2010, 49, 8244 –8248

Angewandte
Chemie
form {Zn (m -CO ) (m -CO )} SBUs. The fourth carboxylate
2
2
2
3
1
2
group of L’ coordinates to the square-pyramidal Zn atom in a
monodentate fashion. Consequently, each L’ ligand, as well as
each Zn unit, acts as a four-connecting linker that leads to a
2
3
D network (Figure 1). Topological analysis of 2 reveals a
6
four-connected uninodal net with a Schlꢀfli symbol of {6 },
which is a known topological type of unc (one of the uninodal
[
9]
4
-connected 3D nets).
Figure 2. X-ray powder diffraction patterns for 2. From bottom to top:
simulated PXRD from the crystal structure; pristine sample; evacuated
sample; evacuated sample resoaked in DMF/EtOH; sample treated
with Ti(OiPr) (denoted as Ti-loaded); and sample recovered after the
4
catalytic Et Zn addition reaction. The inset shows photographs of
2
colorless 2 and orange Ti-2.
work distortion, 1 and 2 are highly porous, as shown by N and
2
H adsorption experiments.
2
The N gas-sorption isotherms were measured at 77 K in
2
order to evaluate the permanent porosity of 1 and 2. Fresh
crystals of 1 or 2 were washed with a DMF/EtOH (1:1 v/v)
mixture followed by a brief wash with CH Cl . Compound 1
2
2
or 2 was then evacuated at 608C for 16 h under vacuum to
Figure 1. a) Representation of coordination modes of ligand L’ (simpli-
fied as a distorted orange tetrahedron) with zinc atoms (blue
polyhedron); b) schematic representation of the network connectivity
of 2 as viewed down the a axis; c,d) space-filling model of 2 as viewed
down the a and b axis, respectively. Zn blue, O red, C gray, H white; all
atoms of the other interpenetrating network are shown in green.
remove the remaining solvent molecules before N adsorption
2
measurements. As shown in Figure 3, both 1 and 2 show type I
The non-interpenetrating unc network of 2 possesses an
extremely large void space with the largest channel dimen-
2
sions of approximately 1.5 ꢁ 3.0 nm along the a axis. To avoid
the formation of such an open structure, 2 forms a twofold
interpenetrated network (Figure 1c). The shortest distance
between the naphthyl rings of the two interpenetrating
networks is 4.05(2) ꢂ. Even after twofold interpenetration,
2
2
still possesses large channels (ca. 1.5 ꢁ 2.0 nm ) along the
a axis. PLATON calculations indicate that the effective
3
volume for solvent molecules in framework 1 is 7932.7 ꢂ
Figure 3. N (* =1, ~ =2) and H (^ =1,
symbols)/desorption (open symbols) isotherms of 1 and 2 at 77 K.
&
=2) adsorption (filled
2
2
per unit cell, which is 49.2% of the crystal volume. Compound
3
2
has a void space of 7825.5 ꢂ per unit cell, which is 51.6% of
the unit-cell volume. TGA analyses indicated solvent weight
losses of 34.9% and 38.8% for 1 and 2, respectively.
adsorption isotherms, thus indicating microporous structure
for these frameworks. By assuming a monolayer coverage of
The phase purity of 1 and 2 is supported by a good match
between powder X-ray diffraction (PXRD) patterns of bulk
samples of 1 and 2 and those simulated from their single-
crystal structures. The evacuated sample of 2 shows peak
broadening at 2q values larger than 158, thus suggesting slight
framework distortion upon solvent removal; this phenom-
N and applying the Langmuir model, we found that the
2
apparent Langmuir surface areas of evacuated samples of 1
2
À1
2
À1
and 2 are 1481 m g and 1847 m g , respectively (the P/P =
0
0.001 to 0.1 pressure range was selected). The BET surface
2
À1
2
À1
area is 1335 m g for 1 and 1657 m g for 2. The calculated
[10]
enon is not uncommon for MOFs with elongated ligands.
BET surface areas based on grand canonical Monte Carlo
2
À1
2
À1
The crystallinity of 1 and 2 is recovered after soaking the
desolvated samples in a DMF/EtOH (1:1 v/v) solvent mixture
for 24 h at room temperature (Figure 2). In spite of frame-
(GCMC) simulations are 1720 m g and 1877 m g for 1
and 2, respectively (Figures S5 and S7 in the Supporting
Information). The structural distortion of desolvated samples
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8245

Communications
IV
of 1 and 2 is supported by the smaller observed pore sizes,
charge transfer excitation in Ti naphthoxide complexes. This
which are reduced by approximately 2 ꢂ compared to those
calculated from GCMC simulations (Figures S6 and S8 in the
Supporting Information). To further characterize the gas-
sorption properties of 1 and 2, hydrogen-uptake experiments
were carried out. At 1 atm and 77 K, 1 and 2 exhibit high
hydrogen uptakes of 1.1 and 1.3 wt%, respectively.
color change indicates the reaction of the BINOL moieties in
2 with Ti(OiPr) to form catalytically active titanium BINO-
4
Late species. Interestingly, when the structure of Ti-2 was
resolved, the formation of intermolecular [Ti(BINOLate)2]
species by single-crystal to single-crystal crosslinking of the
two interpenetrating networks in 2 was revealed (Fig-
ure 4a,b). Although a number of single-crystal to single-
We have also evaluated the accessibility of the open
channels in 1 and 2 to large molecules, as this factor is
important for asymmetric catalytic reactions. The uptake of
Brilliant Blue R-250 dye by 1 and 2 are 8.0 and 10.6 wt%.
These dye uptake amounts correspond to effective dye
concentrations of 131 and 167 mm in the open channels of 1
and 2, as compared to a dye concentration of 24 mm in the
original MeOH solution. The fact that 1 and 2 can host a dye
2
molecule with approximate dimensions of 1.8 ꢁ 2.2 nm sug-
gests their ability to transport large substrate and product
molecules in asymmetric catalytic reactions.
We evaluated potential applications of 2 in heterogeneous
asymmetric catalysis. The dihydroxy groups in 2 can be
transformed into an active Lewis acidic catalyst by post-
[
6,7,11]
synthesis modification with Ti(OiPr)4.
As shown in
Table 1, Ti(OiPr) -treated 2 is highly active in the addition
4
Table 1: Addition of diethylzinc to aromatic aldehydes catalyzed by 2/
[
a]
Ti(OiPr)4.
Entry
Ar
Conv. [%]
ee [%]
1
2
3
4
Ph
4-Cl-C H4
4-Br-C H
6 4
1-naphthyl
Ph
Ph
99
99
99
98
99
99
17.4
12.1
11.1
21.8
29.8
<2
6
[
b]
c]
Figure 4. a) Space-filling model of framework Ti-2. Ti(OiPr) blue,
2
5
6
[
O red, C gray, H white; all atoms of the other interpenetrating network
are shown in green. b) Stick and line model representation showing
the X-ray structure of the intermolecular [(OiPr) Ti(BINOLate) ] species
[
a] Reactions were carried out in toluene (1 mL), with 13 mol% of 2 with
respect to the amount of BINOL. [b] With 39 mol% of 2. [c] With
3 mol% of 1.
2
2
formed by crosslinking the two BINOL moieties in the two inter-
1
penetrating networks. Partially occupied Ti(OiPr) fragments for the
2
Ti1/T1A parts are omitted for clarity. Zn green, O red, C gray, H white.
c) ChemDraw structure of the intramolecular [(OiPr) Ti(BINOLate)]
2
species. d) ChemDraw structure of the intermolecular [(OiPr) Ti-
2
of diethylzinc to aromatic aldehydes to afford secondary
alcohols. However, only very modest enantioselectivity was
observed in these reactions (17.4% ee for benzaldehyde at
(
BINOLate) ] species.
2
1
3 mol% catalyst loading). This result is in stark contrast to
our earlier results from MOF-catalyzed diethylzinc additions,
which gave very high ee values (up to 93% ee), which rival the
values obtained with corresponding homogeneous cata-
crystal transformations of MOFs have been reported in recent
years, these results represent the first example of converting a
MOF to a catalytically active material in a single-crystal to
[
6,7]
[12,13]
lysts.
single-crystal fashion.
The proximity of the two inter-
Structural analysis and dye-uptake studies indicated that
penetrating networks has led to short distances between OH
the channels of the doubly interpenetrating networks in 2 are
groups from L’ ligands in different networks, thus allowing the
accessible to aldehyde and Et Zn reagents. In order to
reaction of Ti(OiPr) with two different BINOL moieties in
2
4
understand the origin of the low enantioselectivity, single-
crystal diffraction studies were carried out on Ti(OiPr)4-
treated 2 (denoted as Ti-2), which was obtained by the
treating single crystals of 2 suspended in toluene with
Ti(OiPr)4 for three days. Compound 2 turned to from
colorless to light yellow and then orange upon treatment
the two adjacent L’ ligands to form intermolecular [(OiPr) Ti-
2
(BINOLate) ] complexes (Figure 4b,c). The [(OiPr) Ti-
2
2
(BINOLate) ] complex is disordered in three possible posi-
2
tions. Ti1 (1/4 occupancy) sits in general positions, and Ti2 (1/
2 occupancy) sits on the two-fold axis along the c axis
(Figure S13 in the Supporting Information). Every pair of L’
with Ti(OiPr) (Figure 2, inset), as a result of ligand-to-metal
ligands shares a total of one Ti(OiPr) fragment in Ti-2.
4
2
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Angewandte
Chemie
Although Ti-2 has the same space group as 2, careful
comparisons of their structures revealed significant changes in
unit-cell dimensions and molecular conformations of the L’
ligands. The a and b axes of Ti-2 increase by 3.1% and 18.8%,
while the c axis shrinks by 12% when compared to those of 2.
parts. The rational design of MOF-derived catalysts can be
greatly facilitated with the delineation of the relationship
between their structures and catalytic properties.
While the BINOL moieties in 2 adopt a transoid conforma- Experimental Section
tion with a dihedral angle of 1118 between the naphthyl
groups, the BINOL moieties adopt a cisoid conformation with
a dihedral angle of 78.78 in Ti-2. In the structure of 2, the
hydroxy groups between adjacent BINOL moieties from two
interpenetrating networks have the shortest hydroxy O–O
distance of 3.74(2) ꢂ. This O–O distance is shortened to
Synthesis of [Zn
L(DMF)(H O)]·4.3DMF·2EtOH·H O (1): A mix-
2 2 2
ture of ZnI (31.9 mg, 0.10 mmol), H L (41.1 mg, 0.05 mmol), DMF
2
4
(
3
2 mL), and EtOH (2 mL) was heated in a capped vial at 908C for
days. Colorless crystals of 1 were filtered, and briefly dried on filter
paper at room temperature. Yield: 46 mg (63%). [Zn L’(DMF)-
2
(
H O)]·4.3DMF·2EtOH·4H O (2) was similarly synthesized in 51%
2
2
yield.
3
.52(2) ꢂ in Ti-2. In contrast, the two hydroxy groups in the
Diethylzinc addition experiments: Fresh prepared crystals of 2
9.2 mg, 6.3 mmol) were washed repeatedly with anhydrous MeOH,
(
same BINOL moiety in 2 have an O–O distance of 4.16(2) ꢂ,
which is too far away to simultaneously coordinate to a Ti
center to form a [(OiPr) Ti(BINOLate) ] complex. As a
consequence of the crosslinking of the two interpenetrating
networks by the Ti centers, the other inter-BINOL hydroxy
O–O distance is shortened from 4.46(2) ꢂ to 3.06(2) ꢂ and
CH Cl , and toluene. Ti(OiPr) (19 mL, 63 mmol) was added with
2
2
4
vigorous stirring to the crystals of 2 suspended in toluene (1 mL).
2
2
After 1 h, Et Zn (1.0m in toluene, 147 mL, 147 mmol) and benzalde-
2
hyde (5 mL, 49 mmol) were added. The mixture was allowed to stir
overnight, and then quenched by the addition of 1 mL of 3m HCl. The
organic layer was passed down a short silica column and analyzed by
GC.
also engages in forming intermolecular [(OiPr) Ti-
2
(
BINOLate) ] complexes (Figure S13 in the Supporting
2
Received: June 3, 2010
Published online: September 21, 2010
Information).
Space-filling models indicated the presence of substrate-
accessible open channels in the framework Ti-2 after the
Keywords: adsorption · asymmetric catalysis ·
crystal engineering · heterogeneous catalysis ·
metal–organic frameworks
formation of intermolecular [Ti(BINOLate) ] species. We
.
2
believe that the intermolecular [Ti(BINOLate) ] species
2
should be less enantioselective in Et Zn addition reactions
2
because of the smaller steric influence of the naphthyl rings
on the Ti center. As a result, 2/Ti(OiPr) exhibits much lower
4
enantioselectivity than our previously reported MOF systems
that can form desired intramolecular Ti BINOLate species
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revealed that 2 underwent single-crystal to single-crystal
crosslinking upon treatment with Ti(OiPr) to form intermo-
4
lecular [Ti(BINOLate) ] complexes that are less enantiodis-
2
criminating than intramolecular titanium BINOLate cata-
lysts. This work highlights the important influence of frame-
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8247

Communications
[
8] The determination of the unit cell and data collections for
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SMART CCD diffractometer. Data for Ti-2 was collected on a
Bruker SMART APEX II diffractometer. Crystallographic data
for 1: Orthorhombic, space group I2 2 2 , a = 13.0884(3), b =
1
1 1
3
2
0
8.7131(7), c = 42.8771(11) ꢂ, V= 16113.6(7) ꢂ , Z = 8, 1
=
calcd
À3
À1
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,
m(MoKa) = 0.635 mm
.
R1(I>2s(I)) = 0.0713,
wR2(I>2s(I)) = 0.1513, Flack parameter= 0.00(3) and GOF =
.062. Crystallographic data for 2: Orthorhombic, space group
I2 2 2 , a = 12.7526(10), b = 26.9066(19), c = 44.199(3) ꢂ, V=
1
1
1 1
3
À3
À1
1
5166(2) ꢂ , Z = 8, 1
= 0.861 gcm , m(MoKa) = 0.672 mm .
calcd
14, 8812 – 8821; d) B. D. Chandler, G. D. Enright, K. A. Udachin,
R1(I>2s(I)) = 0.0492, wR2(I>2s(I)) = 0.1011, Flack parame-
ter= 0.10(1) and GOF = 0.890. Crystallographic data for Ti-2:
Orthorhombic, space group I2 2 2 , a = 13.1451(4), b =
S. Pawsey, J. A. Ripmeester, D. T. Cramb, G. K. Shimizu, Nat.
Mater. 2008, 7, 229 – 235.
1
1 1
3
[13] Related catalytic reactions have recently been reported for
molecular solids, see: a) A. N. Sokolov, D.-K. Bu cˇ ar, J. Baltru-
saitis, S. X. Gu, L. R. MacGillivray, Angew. Chem. 2010, 122,
3
1.9683(10), c = 39.8933(12) ꢂ, V= 16344.0(9) ꢂ , Z = 8,
À3
À1
1calcd = 0.805 gcm
,
m(CuKa) = 1.409 mm
.
R1(I>2s(I)) =
0.0956, wR2(I>2s(I)) = 0.2442, Flack parameter= 0.10(6) and
4369 – 4373; Angew. Chem. Int. Ed. 2010, 49, 4273 – 4277; b) Z.
GOF = 1.223. CCDC 772189, 772190, and 778882 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
9] a) V. A. Blatov, A. P. Shevchenko, V. N. Serezhkin, Russ. J.
Coord. Chem. 1999, 25, 453 – 465; b) M. OꢃKeeffe, N. E. Brese,
Acta Cryst. A, 1992, A48, 663 – 669.
Huang, P. S. White, M. Brookhart, Nature 2010, 465, 598 – 601.
14] L. Pu, H.-B. Yu, Chem. Rev. 2001, 101, 757.
15] The Ti content (Ti/Zn = 0.42) determined by the ICP-MS
analysis is slightly higher than that determined from the X-ray
structure (Ti/Zn = 0.25), thus suggesting that there might be
[
[
[
some disordered, unreacted Ti(OiPr) in the open channels of Ti-
4
2.
8248 www.angewandte.org
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Angew. Chem. Int. Ed. 2010, 49, 8244 –8248