A metal-organic framework material that functions as an enantioselective catalyst for olefin epoxidation

Article abstract of DOI:10.1039/b600408c

A new microporous metal-organic framework compound featuring chiral (salen)Mn struts is highly effective as an asymmetric catalyst for olefin epoxidation, yielding enantiomeric excesses that rival those of the free molecular analogue. Framework confinement of the manganese salen entity enhances catalyst stability, imparts substrate size selectivity, and permits catalyst separation and reuse. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b600408c

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A metal–organic framework material that functions as an
enantioselective catalyst for olefin epoxidation{
So-Hye Cho,{a Baoqing Ma,{a SonBinh T. Nguyen,*a Joseph T. Hupp*a and Thomas E. Albrecht-Schmittb
Received (in Berkeley, CA, USA) 10th January 2006, Accepted 9th May 2006
First published as an Advance Article on the web 23rd May 2006
DOI: 10.1039/b600408c
A new microporous metal–organic framework compound
featuring chiral (salen)Mn struts is highly effective as an
asymmetric catalyst for olefin epoxidation, yielding enantio-
meric excesses that rival those of the free molecular analogue.
Framework confinement of the manganese salen entity
enhances catalyst stability, imparts substrate size selectivity,
and permits catalyst separation and reuse.
Crystalline metal–organic framework (MOF) compounds, espe-
cially those exhibiting zeolite-like properties such as high internal
surface area and microporosity, comprise a promising emerging
class of functional materials.1 Among the functions most often
envisioned is chemical catalysis.2 The notion is that MOF-based
catalysts may be able to replicate some of the key features of
zeolitic catalysts (e.g. single-site reactivity, pore-defined substrate
size and shape selectivity, easy catalyst separation and recovery,
and catalyst recyclability) while incorporating reactivity and
properties unique to molecular catalysts. One important property
of many molecular catalysts that has yet to be demonstrated with
purely zeolitic catalysts is enantioselectivity. Herein, we report that
a microporous MOF containing chiral (salen)Mn struts is highly
Fig. 1 A POVRAY view of a single network unit for 1. Yellow
effective as an asymmetric catalyst for olefin epoxidation. The
polyhedra represent the zinc ions. Carbon: grey; oxygen: red; nitrogen:
observed enantiomeric excesses (ee) rival those of the free
blue; chloride: green; manganese: brown.
molecular catalyst. At the same time, framework confinement
enhances catalyst stability, imparts substrate size selectivity, and
permits catalyst separation and reuse.3–5
Employed as a catalytic strut was (R,R)-(2)-1,2-cyclohexa-
nediamino-N,N9-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)MnIIICl,
L.6 Since MOFs based exclusively upon metal–pyridine
bonding tend to collapse if evacuated, L was incorporated
instead in a more robust pillared paddlewheel structure, 1,
containing pairs of zinc ions together with biphenyldicarboxyl-
ate (bpdc) as the second ligand.7 Obtained by sealed-vial
solvothermal synthesis in dimethylformamide (DMF)§, 1 has
the formula Zn2(bpdc)2L?10DMF?8H2O and crystallizes in the
triclinic P1 space group (Fig. 1) as an interpenetrating pair of
networks (Fig. 2").
aDepartment of Chemistry and the Institute for Environmental Catalysis,
Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208,
USA. E-mail: stn@northwestern.edu; j-hupp@northwestern.edu;
Notwithstanding the interpenetration, solvent occupies 57% of
Fax: 847-491-7713; Tel: 847-467-3347
the volume of 1 as determined by PLATON. Notably, the ligands
bDepartment of Chemistry and Biochemistry, Auburn University, AL,
L of the paired networks are parallel to each other with cyclohexyl
and tert-butyl groups protruding along the [100] direction. As
such, the channel in the crystallographic b direction is essentially
blocked, leaving distorted-rectangular and rhombic channels in
36830, USA
{ Electronic supplementary information (ESI) available: TGA analysis,
XRPD patterns for 1 and general procedure for catalyst recycling, leaching
and competitive substrate selectivity studies. Syntheses and characteriza-
tion of compounds 2 and 3. See DOI: 10.1039/b600408c
{ These authors contributed equally to this work.
˚
the c and a directions with dimensions of 6.2 6 15.7 A and
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Fig. 2 Space-filling diagram of 1 showing network interpenetration and
framework openings. Channels are viewed down crystallographic c (left)
and a (right).
Fig. 3 Plots of total turnover number versus time for epoxidation of 2,2-
dimethyl-2H-chromene catalyzed by 1 (blue squares) and L (magenta
circles).
˚
however, only minor selectivity degradation was observed (82% ee
for 1 vs. 88% ee for free L). Enantioselectivity is known to decrease
when electron-withdrawing substituents are introduced.9 Thus, a
potential alternative explanation for the modest decrease in
enantioselectivity is the electronic effect arising from binding
pyridyl groups to zinc cations.
6.2 6 6.2 A, respectively. Importantly, diagonal displacement of
the networks leaves all MnIII sites accessible to the channels. The
˚
shortest distance between Mn ions on different networks is 7.2 A.
Thermogravimetric analyses (TGA) of 1 showed that 40% of its
mass is lost in the temperature range 25–200 uC, consistent with
solvent loss as calculated from structural data (40.2%); little
additional loss occurs until T exceeds 360 uC. Powder X-ray
diffraction (PXRD) measurements showed that the evacuated
compound retains crystallinity. However, diffraction peaks are
shifted, implying structural distortions. Notably, resolvation by
DMF reverses the shifts. TGA measurements with resolvated
material confirmed that porosity is retained. In contrast, we have
previously observed that when shifts in diffraction peaks for a
pillared paddlewheel compound are irreversible, resolvation (as
measured by TGA) does not occur.7
Inlight of the persistence of the framework compound’s catalytic
activity, its recyclability was examined. Remarkably, after three
cycles no loss of enantioselectivity and only a small loss of activity
were observed (Table 1). Recycling was accompanied by MOF
particle fragmentation and a decrease, therefore, in average particle
size. Evaluation of the product solution by inductively coupled
plasma (ICP) spectroscopy after removal of MOF particles showed
that between 4 and 7% of the manganese initially present in the
framework material was lost per cycle – either as molecular species
or as particles too small to be removed by filtration through Celite.
Measurements of reaction progress after removal of MOF particles
by filtration established that the remaining small quantity of
dissolved manganese did not catalyze the reaction.
The catalytic activity of 1 toward asymmetric epoxidation was
examined with 2,2-dimethyl-2H-chromene as substrate and 2-(tert-
butylsulfonyl)iodosylbenzene (a soluble compound) as oxidantI:
To determine whether catalysis by 1 occurs predominantly
within the channel-containing material or instead on the external
surface, competitive size selectivity studies were performed (see
ESI{). A large porphyrinic substrate, 2 (too large to enter the
channels of 1), and a small substrate, ethyl 4-vinyl benzoate, 3,
were mixed in 1 : 1 olefinic unit ratio and reacted with 2-(tert-
butylsulfonyl)iodosylbenzene in the presence of 1. Initially, the
ratio of small to large substrate reactivity was 2, but at 45%
conversion the reactivity ratio had increased to y18. Control
experiments with free L as a homogeneous catalyst gave a time-
independent 1 : 1 reactivity ratio. Evidently, at the beginning a
Fig. 3 compares the framework’s reactivity with that of free L.
As is often seen for homogeneous epoxidation catalysts, L is
initially highly effective, but loses much of its activity after the first
few minutes. After a few hours, essentially all activity is lost. In
contrast, the framework-immobilized catalyst exhibited close to
constant reactivity, culminating in nearly four times the number of
turnovers seen for L by the time the experiment was terminated at
3.4 h. For (salen)Mn complexes, loss of catalytic activity typically
is associated with oxidation of the salen ligand. If salen oxidation is
facilitated by reactive encounters with other catalyst molecules,
immobilization should prevent encounters and extend the catalyst
lifetime. TGA evaluation of MOF particles after catalysis,
followed by evacuation and re-immersion in DMF for two days,
established that the material remained porous (see ESI,{ Fig. S3).
For Jacobsen-type catalysts like L, the flexibility of the salen
complex is believed to be important in achieving enantioselec-
tivity.8 We were concerned, therefore, that framework immobiliza-
tion might strongly attenuate the selectivity of L. Remarkably,
Table 1 Recyclability of 1 in the asymmetric epoxidation of 2,2-
dimethyl-2H-chromenea
Entry
Cycleb
Yield [%]c
TON
ee [%]d
1
2
3
a
1st
2nd
3rd
71
71
66
1430
1420
1320
82
82
82
Reaction performed in a conical vial under ambient conditions
using magnetic stirring. Molar ratio olefin/oxidant/catalyst = 4000/
b
2000/1. After each cycle,
1 was separated by centrifugation,
thoroughly washed and reused in a freshly made reaction mixture.
c
GC yield after 2 h using undecane as an internal standard.
Determined using a Supelco b-DEX 120 chiral GC column.
d
2564 | Chem. Commun., 2006, 2563–2565
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Zn2(bpdc)2(L)?3H2O;. calc. (%): C 58.95, H 4.76, N 4.17; found (%): C
58.33, H 4.39, N 4.85. Note that the synthesis employs an excess of L.
Initially, stoichiometric amounts of L were used. While the desired
compound (1) was obtained, samples invariably were contaminated with
white microcrystals of the cubic MOF, (Zn4O)(bpdc)3.
significant fraction of the framework-based catalysis occurs on the
MOF surface, but at later stages catalysis occurs chiefly in the
MOF interior – perhaps because of surface catalyst over-oxidation
and inactivation. If so, then intentionally poisoning or destroying
the outermost catalytic sites, as is sometimes done with zeolites,
could be a useful strategy for eliciting selectivity. Indeed, pre-
treatment of 1 by exposure to solution containing oxidant, but
lacking substrate, led to a three-fold increase in the substrate size
selectivity measured at 10% conversion.
" Crystal data: compound 1, C96H144Zn2MnClN14O28, M = 2163.38,
˚
triclinic, P1, a = 15.1376(18), b = 15.2092(18), c = 26.300(3) A, a =
3
˚
73.271(2), b = 77.508(2), c = 82.596(2)u, U = 5647.2(12) A , Z = 2, Dc =
1.272 Mg m23, m = 0.630 mm21, F(000) = 2288, GoF = 0.797. R1 and wR2
are 0.0638 and 0.1396, respectively, for 1477 parameters and 20334
reflections [I . 2s(I)]. The data were collected on a Bruker SMART1000
˚
CCD with Mo-Ka radiation (l = 0.71073 A) at 120(1) K. The structures
were solved by direct methods and refined by a full matrix least squares
technique based on F2 using the SHELXL97 program. CCDC 284675. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b600408c.
I General procedure for the asymmetric epoxidation catalyzed by 1 and L:
to a 3.7-mL screw-thread vial (15 6 45 mm) were added a dichloro-
methane solution (2 mL of a 2.5 mM solution of substrate) of 2,2-dimethyl-
2H-chromene (5.0 6 1023 mol) and undecane (35.7 mg) as an internal
standard. Crystals of 1 (1.1 mg, containing 5.0 6 1027 mol of L) were
placed in the vial along with a micro stir bar. The oxidant, 2-(tert-
butylsulfonyl)iodosylbenzene (85 mg, 2.5 6 1024 mmol), was added to the
solution to start the reaction. The same quantity of oxidant was added 15
more times at 10 min intervals (total amount added = 4.0 6 1023 mmol).
Aliquots (20 mL) of the reaction mixture were taken periodically over 3.4 h,
filtered through a silica plug (60 mg) and washed with dichloromethane
(5 mL). The filtrate was analyzed by GC and chiral GC for yield and
enantioselectivity, respectively.
For the comparison study with L, the same [chromene + undecane]
solution was mixed with L (0.32 mg, 5.0 6 1027 mol) providing a brown
homogeneous solution. 2-(tert-Butylsulfonyl)iodosylbenzene (85 mg, 2.5 6
1024 mmol), was added in the same manner as mentioned above. Aliquots
(40 mL) of the reaction mixture were removed via syringe and passed
through a plug of silica (120 mg), washed with dichloromethane (8.0 mL)
and the combined filtrate was analyzed via GC and chiral GC.
1 (a) D. Bradshaw, J. B. Claridge, E. J. Cussen, T. J. Prior and
M. J. Rosseinsky, Acc. Chem. Res., 2005, 38, 273–282; (b) J. L. Rowsell
and O. M. Yaghi, Microporous Mesoporous Mater., 2004, 73, 3–14; (c)
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O. M. Yaghi, M. O’Keeffe, N. M. Ockwig, H. K. Chae, M. Eddaoudi
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N. Rosi, D. Vodak, J. Wachter, M. O’Keefe and O. M. Yaghi, Science,
2002, 295, 469–472.
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3 While this paper was in preparation a report appeared on homochiral-
MOF catalyzed conversions of aldehydes to secondary alcohols. See:
C. D. Wu, A. Hu, L. Zhang and W. Lin, J. Am. Chem. Soc., 2005, 127,
8940–8941.
4 There are also two early reports of MOF-based asymmetric Lewis acid
catalysis with very low, but measurable ee (y8% and ,5%). See: (a)
J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K. Kim,
Nature, 2000, 404, 982–986; (b) O. R. Evans, H. L. Ngo and W. Lin,
J. Am. Chem. Soc., 2001, 123, 10395–10396.
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6 (a) G. A. Morris and S. T. Nguyen, Tetrahedron Lett., 2001, 42,
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In summary, asymmetric catalytic oxidation behavior has been
demonstrated with a paddlewheel-stabilized MOF material. In
comparison to the free catalyst, framework-immobilization confers
multiple advantages: higher stability, easier separation, recycl-
ability, and substrate size selectivity. By varying the metal center
and ligand structure of the catalytic strut, a spectrum of usefully
heterogenized molecular catalysts should be obtainable. Of
particular interest may be multi-site catalysts that exploit crystal-
line channel geometries to enhance chemical selectivity.
We gratefully acknowledge the U. S. National Science
Foundation, the American Association of University Women
(fellowship for S. H. C.) and the Institute for Environmental
Catalysis at Northwestern University for financial support. We
also thank the Basic Energy Sciences Program, Office of Science,
US Department of Energy (Grant No. DE-FG02-01ER15244) for
support at the initial stages of the study.
Notes and references
§ Preparation of Zn2(bpdc)2(L)?10DMF?8H2O (1): in a small vial were
mixed Zn(NO3)2?6H2O (12 mg, 0.04 mmol), H2bpdc (7.2 mg, 0.03 mmol)
and L (34 mg, 0.05 mmol) with dimethylformamide (DMF, 6 mL). The vial
was capped and heated at 80 uC in an oil-bath for one week, over which
time brown block-shaped crystals slowly formed. The crystals were
collected by filtration and washed with DMF several times. The crystals,
which were insoluble in water and common organic solvents (ethanol,
acetonitrile, acetone, chloroform and DMF), were left to air-dry for 0.5 h
before being analyzed by XRPD and TGA. Elemental analysis for freshly
synthesized 1 gave a formula of Zn2(bpdc)2(L)?10DMF?8H2O, which is
consistent with TGA data and powder X-ray diffraction analysis; calc. (%):
C 53.24, H 6.66, N 9.06; found (%): C 52.97, H 6.36, N 9.07. Elemental
analysis for evacuated samples at 100 uC overnight gave a formula of
9 (a) T. Katsuki, Coord. Chem. Rev., 1995, 140, 189–214; (b) M. Palucki,
N. S. Finney, P. J. Pospisil, M. L. Guler, T. Ishida and E. N. Jacobsen,
J. Am. Chem. Soc., 1998, 120, 948–954.
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Chem. Commun., 2006, 2563–2565 | 2565