A homochiral porous metal-organic framework for highly enantioselective heterogeneous asymmetric catalysis

Article abstract of DOI:10.1021/ja052431t

A homochiral porous noninterpenetrating metal-organic framework (MOF), 1, was constructed by linking infinite 1D [Cd(μ-Cl)2]n zigzag chains with axially chiral bipyridine bridging ligands containing orthogonal secondary functional groups. The secondary ch

Full text of DOI:10.1021/ja052431t

Published on Web 06/04/2005
A Homochiral Porous Metal-Organic Framework for Highly Enantioselective
Heterogeneous Asymmetric Catalysis
Chuan-De Wu, Aiguo Hu, Lin Zhang, and Wenbin Lin*
Department of Chemistry, CB#3290, UniVersity of North Carolina, Chapel Hill, North Carolina 27599
Received April 14, 2005; E-mail: wlin@unc.edu
Zeolites are used for the production of numerous chemical
intermediates and household products.1 The importance of chirality
in biological processes has prompted significant efforts on develop-
ing chiral zeolites for applications in stereoselective processes. Few
enantiopure zeolites have been obtained to date, and none have
found applications in stereoselective processes.2 Porous metal-
organic frameworks (MOFs) are zeolite analogues with permanent
porosity and absorption capacity.3 The mild conditions typically
used to synthesize MOFs allow the rational design of novel
functional materials.3,4 Judicious choices of building blocks can lead
to chiral porous MOFs that can be used for heterogeneous
asymmetric catalysis.5 Herein we wish to report the synthesis of a
Figure 1. Crystal structure of 1. Cyan, green, red, blue, gray, and white
represent Cd, Cl, O, N, C, and H atoms, respectively. (a) Schematic
representation of the 3D framework of 1 as viewed slightly off the a-axis.
The 3D network is formed by linking the [Cd(µ-Cl)2]n SBUs (zigzag chains
shown in purple) with the L ligands shown in yellow sticks and pairs of
blue sticks. (b) Space-filling model of 1 as viewed down the a-axis showing
the large chiral 1D channels (∼1.6 × 1.8 nm).
highly porous homochiral MOF and its application in heterogeneous
asymmetric catalysis with activity and stereoselectivity rivaling its
homogeneous counterparts.
Our strategy for creating catalytically active chiral porous MOFs
utilizes chiral bridging ligands containing orthogonal functional
groups. The primary functional groups can be linked by metal-
connecting units to form extended networks, whereas the orthogonal
secondary chiral groups can then be used to generate asymmetric
catalytic sites (Chart 1). This synthetic strategy leads to ideal
heterogeneous asymmetric catalysts in which both the catalytic sites
and the secondary environments around them are identical through-
out the solid. Compared to other immobilization approaches,6 the
present strategy allows the synthesis of heterogeneous asymmetric
catalysts with higher catalyst loading and more accessible catalytic
centers.
Figure 2. (a) Space-filling presentation of the tightly paired L ligands via
strong hydrogen bonding and π‚‚‚π interactions. (b) Schematic representa-
tion of the active (BINOLate)Ti(OiPr)2 catalytic sites in the open channels
of 1. Cyan, green, red, blue, gray, and yellow represent Cd, Cl, O, N, C,
and Ti atoms, respectively.
Chart 1
the 1D zigzag chain further coordinates to two pyridyl groups of
the L ligands and connects adjacent 1D [Cd(µ-Cl)2]n SBUs to form
a noninterpenetrating 3D network with very large chiral channels
of ∼1.6 × 1.8 nm cross-section along the a-axis (measured from
van der Waals surfaces). Interestingly, two of the three L ligands
in each unit cell pair up via strong hydrogen bonding between
neighboring hydroxy groups (O‚‚‚O distance of 2.78 Å) and π‚‚‚π
interactions between pyridyl and naphthyl rings (the nearest C‚‚‚C
distance of 3.27 Å) and between the naphthyl rings (the nearest
C‚‚‚C distance of 3.32 Å). The dihydroxy groups for these two L
ligands are shielded from the open channels by the naphthyl rings
(Figure 2a). The third L ligand of 1 is not involved in supramo-
lecular interactions, and the dihydroxy group points to the open
channel. PLATON calculations indicate that 1 contains 54.4% void
space that is accessible to the solvent molecules.9 TGA analysis
showed that 1 lost 18.8% of its mass in the 20-250 °C range,
corresponding to the loss of all solvent molecules (expected 18.7%).
X-ray powder diffraction demonstrated that the framework structure
of 1 was maintained upon the removal of all the solvent molecules.
Axially chiral bridging ligand (R)-6,6′-dichloro-2,2′-dihydroxy-
1,1′-binaphthyl-4,4′-bipyridine, L, which contains the bipyridyl
primary functionality and orthogonal chiral 2,2′-dihydroxy second-
ary functionality,7 was used to construct homochiral porous MOFs.
Colorless crystals of [Cd3Cl6L3]‚4DMF‚6MeOH‚3H2O, 1, were
obtained in high yield by slow diffusion of diethyl ether into a
mixture of L ligand and CdCl2 in MeOH/DMF for 3 days.8
1
crystallizes in the triclinic P1 space group with three Cd atoms,
six chloride atoms, and three L ligands in each unit cell. Octahe-
drally coordinated Cd(II) centers in 1 are doubly bridged by the
chlorides to form 1D zigzag [Cd(µ-Cl)2]n chains that serve as
secondary building units (SBUs) (Figure 1). Each Cd(II) center in
9
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J. AM. CHEM. SOC. 2005, 127, 8940-8941
10.1021/ja052431t CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Ti(IV)-Catalyzed ZnEt2 Additions to Aromatic Aldehydesa
aldehyde 4′-G2′-PhCHO, presumably because it cannot access the
catalytic sites as a result of its large diameter. This set of
experiments unambiguously demonstrates that 1‚Ti is a true
heterogeneous asymmetric catalyst with both ZnEt2 and aromatic
aldehyde accessing the catalytic sites via the open channels.
As illustrated in Figure 2, we believe that only one-third of all
L ligands in 1 react with Ti(OiPr)4 to generate the active catalyst
because the other two L ligands are tightly paired via strong
hydrogen-bonding and π‚‚‚π stacking interactions.11 Heterogeneous
asymmetric diethylzinc addition reactions with 1‚Ti were therefore
carried out with catalyst loading lower (13 mol %) than that of the
homogeneous control experiments (20 mol %). The fact that
complete conversions were observed for all the small aromatic
aldehydes in these heterogeneous reactions demonstrates the high
catalytic activity of 1‚Ti. The 1‚Ti system is also more enantiose-
lective in diethylzinc addition reactions (by >20% ee) than
analogous heterogeneous catalysts derived from structurally ill-
defined zirconium phosphonates.12
BINOL/Ti(OiPr)4
1‚Ti
Ar
conv %
ee %
conv %
ee %
1-Naph
Ph
>99
>99
>99
>99
>99
>99
>99
95b
94
88
86
84
80
75
78
67b
>99
>99
>99
>99
>99
73
93
83
80
80
88
77
81
-
4-Cl-Ph
3-Br-Ph
4′-G0OPh
4′-G1′OPh
4′-G1OPh
4′-G2′OPh
63
0
Taken together, we have designed a highly active and enantio-
selective asymmetric catalyst 1‚Ti via creating readily accessible
uniform catalytic sites inside a porous MOF. The modular nature
of the present synthetic strategy should allow the incorporation of
chiral bridging ligands with a diverse range of primary and
secondary functionalities to lead to new chiral porous MOFs for
practically useful heterogeneous asymmetric catalysis.
aAll the reactions were conducted with 13 mol % of 1 or 20 mol % BINOL
and excess amounts of Ti(OiPr)4 at room temperature for 12 h. Conv %
were determined by GC or NMR, while ee % values were determined on
chiral GC or HPLC for all the secondary alcohols except for 4′-G2′OPh
whose ee % was determined by NMR spectrum of its Mosher’s ester. b With
40 mol % BINOL.
Acknowledgment. We acknowledge financial support from
NSF. W.L. is an Alfred P. Sloan Fellow, an Arnold and Mabel
Beckman Young Investigator, a Cottrell Scholar of Research Corp,
and a Camille Dreyfus Teacher-Scholar.
CO2 adsorption isotherms measured at 273 K indicated that the
evacuated sample of 1 possesses permanent porosity with a specific
surface area of 601 m2/g and a pore volume of 0.26 mL/g.
Supporting Information Available: Experimental procedures,
analytical data, 13 figures, 1 table, and crystallographic data (CIF, PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
We have used 1 for heterogeneous asymmetric catalysis by taking
advantage of the readily accessible chiral dihydroxy groups. Ti-
(OiPr)4 can react with the chiral dihydroxy groups in BINOL or its
analogues to afford Lewis acidic (BINOLate)Ti(OiPr)2 compounds
which are active catalysts for the addition of ZnEt2 to aromatic
aldehydes to afford chiral secondary alcohols.10 Treatment of 1 with
excess Ti(OiPr)4 indeed led to an active catalyst (designated as 1‚
Ti) for the ZnEt2 addition reactions (Figure 2b). Specifically, 1‚Ti
catalyzes the addition of ZnEt2 to 1-naphthaldehyde to afford (R)-
1-(1-naphthyl)propanol with complete conversion and 93% enan-
tiomeric excess (ee) (Table 1). This level of ee rivals that of the
homogeneous analogue under similar conditions (94% ee). 1‚Ti also
catalyzes the addition of ZnEt2 to a range of other aromatic
aldehydes with complete conversion and ee values comparable to
those of the homogeneous analogue. 1‚Ti represents a unique highly
enantioselective asymmetric catalyst derived from a homochiral
MOF. The only prior literature report gave a modest ee of 8% for
a transesterification process catalyzed by a homochiral MOF.5
No secondary alcohol product was obtained when 1-naphthal-
dehyde was treated with ZnEt2 in the presence of the supernatant
from a mixture of 1 and Ti(OiPr)4, indicating heterogeneous nature
of the present catalyst system. To ascertain that ZnEt2 and aromatic
aldehydes are accessing the internal (BINOLate)Ti(OiPr)2 sites via
the open channels, we have designed a series of aromatic aldehydes
of varying sizes. Molecular mechanics simulations indicated that
these aromatic aldehydes containing Fre´chet-type dendrons have
an estimated size ranging from ∼0.8 to ∼2.0 nm. As expected, the
efficiency for homogeneously catalyzed ZnEt2 additions is not
affected by the dendron size (Table 1). In contrast, the yields of
ZnEt2 addition products catalyzed by 1‚Ti greatly depend on the
dendron size: as the size of the dendritic aldehyde increases, the
yield of the ZnEt2 addition product steadily decreases (>99, 73,
64, and 0%). No ZnEt2 addition product was observed for the largest
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