Size-selective Lewis acid catalysis in a microporous metal-organic framework with exposed Mn2+ coordination sites

Article abstract of DOI:10.1021/ja800669j

Treatment of selected aldehydes and ketones with cyanotrimethylsilane in the presence of the microporous metal-organic framework Mn₃[(Mn₄Cl)₃BTT₈(CH₃OH)₁₀]₂ (1, H3BTT = 1,3,5-ben

Full text of DOI:10.1021/ja800669j

Published on Web 04/10/2008
Size-Selective Lewis Acid Catalysis in a Microporous
Metal-Organic Framework with Exposed Mn2+ Coordination Sites
Satoshi Horike, Mircea Dincaˇ, Kentaro Tamaki, and Jeffrey R. Long*
Department of Chemistry, UniVersity of California, Berkeley, California 94720
Received January 27, 2008; E-mail: jrlong@berkeley.edu
The extraordinary combination of chemical tunability and high
internal surface area has generated widespread interest in metal-
organic frameworks.1 One of the more prominent applications pur-
sued for these materials has been their possible use in high-density
hydrogen storage.2 Here, efforts to increase the enthalpy of H2 adsorp-
tion have led to a number of microporous frameworks bearing coor-
dinatively unsaturated metal centers.3 Importantly, such crystalline
solids with well-defined pores and surface-isolated Lewis acid sites
could also potentially serve as size- or shape-selective heteroge-
neous catalysts, in a manner similar to zeolites.4 Reports of catalytic
studies in metal-organic frameworks have, however, been relatively
scarce.5,6 While these have included some significant successes in
enantioselective catalysis,5a,b there has been little focus thus far on
probing selectivity with substrates of comparable size to the pore
dimensions.5d With this in mind, we undertook investigations of the
catalytic activity of the recently discovered sodalite-type compound
Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 (1; H3BTT ) 1,3,5-benzene-
tristetrazol-5-yl),3e wherein Mn2+ ions exposed on the surface of
the framework might serve as potent Lewis acids. Herein, we show
that compound 1 indeed catalyzes the cyanosilylation of aromatic
aldehydesandketones,aswellasthemoredemandingMukaiyama-aldol
reaction. Moreover, in each case, a pronounced size-selectivity
effect consistent with the pore dimensions is observed.
Compound 1 is a thermally stable microporous solid exhibiting
a cubic network of 7 and 10 Å pores that affords a BET surface
area of 2100 m2/g (see Figure 1).3e The 10 Å pore system is readily
accessible and presents a surface lined with two different types of
MnII sites. Site I has one chloride and four bridging tetrazolate
ligands, with a sixth methanol ligand that projects into the pore
present only 83% of the time. Site II is bound by just two tetrazolate
ligands and accounts for one-ninth of the total manganese in the
compound. Both types of coordinatively unsaturated metal centers
are well-positioned to interact with guest molecules that enter the
framework pores, suggesting that 1 should function as an active
heterogeneous catalyst for Lewis acid promoted reactions.
Figure 1. A portion of the crystal structure of 1 showing the two different
types of MnII sites exposed within its three-dimensional pore system of 10 Å
wide channels.3e Orange, green, gray, and blue spheres represent Mn, Cl, C,
and N atoms, respectively; H atoms and bound MeOH molecules are omitted
for clarity. Site I is five-coordinate, while site II is only two-coordinate; the
separation between them is 3.420(8) Å.
Table 1. Results for the Cyanosilylation of Carbonyl Substrates in
the Presence of 1a
entry
Ar
R
time (h)
yield (%)b
To compare the catalytic activity of 1 with that of other metal-
organic frameworks, we first probed the cyanosilylation of carbonyl-
functionalized organic substrates. This reaction provides a conve-
nient route to cyanohydrins, which are key derivatives in the synthe-
sis of fine chemicals and pharmaceuticals.7Notably, catalysis of the
transformation had previously been explored using metal-organic
frameworks, but only relatively low conversion rates were ob-
served.6 Our experiments employed a 1:2 molar ratio of selected
aromatic aldehydes and cyanotrimethylsilane in dichloromethane at
room temperature. As shown in Table 1, a loading of 11 mol % of
solid 1 led to a 98% conversion of benzaldehyde after 9 h. This
represents a significant improvement over the best prior result for
metal-organic frameworks: a yield of 69% obtained after 16 h of
exposure to a series of lanthanide bisphosphonates.6b Importantly,
removal of 1 by filtration after only 4 h completely shut down the
reaction, affording only 40% total conversion upon standing for
20 h. This demonstrates that no homogeneous catalyst species exists
in the reaction solution. In addition, infrared spectroscopy of the
1
2
3
4
5
6
phenyl
H
9
9
98
90
19
18
28
1
1-naphthyl
4-phenoxyphenyl
biphenyl
H
H
H
CH3
CH3
9
9
24
24
phenyl
biphenyl
a Reaction conditions: Me3SiCN (3 mmol), aldehyde/ketone (1.5 mmol),
CH2Cl2 (5 mL), 1 (0.04 g, 0.006 mmol), room temperature, under N2.
b Determined by 1H NMR based on the carbonyl substrate.
catalyst impregnated with an aliquot of the reaction solution
revealed two C-O stretches at 1698 and 1686 cm-1. These
correspond to free and MnII-bound benzaldehyde, respectively,
confirming that activation of the substrate occurs at the unsaturated
MnII sites within 1.
To probe whether activation of the carbonyl species occurs inside
the pores or on the surface of the solid catalyst, substrates of
increasing dimension were tested. Unlike the lanthanide bisphos-
phonates, which exhibit lamellar structures that can swell to
9
5854 J. AM. CHEM. SOC. 2008, 130, 5854–5855
10.1021/ja800669j CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Results for the Mukaiyama-Aldol Reaction in the
Presence of 1a
was greatly retarded at -45 °C, however, affording only an 8%
conversion for the same reaction after 6 h. Notably, addition of 1
accelerated the reaction considerably at this lower temperature, with
a conversion of 51% being observed after 6 h. This indicates that
the MnII centers are active even in the presence of a highly coordi-
nating solvent such as DMF. In fact, the 51% conversion is much
higher than the 16% conversion observed in dichloromethane after
6 h at 25 °C. Although further mechanistic investigations would
be necessary to elucidate the cause of this conversion enhancement,
we tentatively ascribe it to a cumulative effect wherein the MnII
centers activate the carbonyl substrate and DMF molecules activate
the silyl enolate through loose association with the Si-O bond.
The foregoing results demonstrate that the microporous metal-
organic framework 1, featuring a high concentration of Lewis acidic
MnII sites on its internal surfaces, can catalyze both the cyanosi-
lylation of aromatic aldehydes and the Mukaiyama-aldol reaction
in a size-selective fashion. Further studies will focus on modifying
1 toward enantioselective versions of these and other reactions, as
well as on investigating the differences arising upon exchange of
MnII for other reactive metal centers.3f,g
entry
aldehyde
silyl enolate
time (h)
solvent
yield (%)b
1
A1
A2
A1
A1
A1
A1
SE1
SE1
SE2
SE3
SE1
SE1
99
99
99
99
6
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DMF
63
24
2
3
<1
<1
8d
4
5c
6c
6
DMF
51
a Reaction conditions: silyl enolate (2 mmol), aldehyde (1 mmol),
solvent(5 mL), 1 (0.04 g, 0.006 mmol), room temperature, under N2.
b Determined by 1H NMR based on the aldehyde. c Reaction at -45 °C.
d No catalyst added.
accommodate a wide range of substrates,6b a significant size-
selectivity effect is observed with 1. As evident from Table 1, the
molecular dimensions of 1-naphthaldehyde, 9.7 × 8.4 Å2,8 allow
it to diffuse swiftly through the pores, such that its conversion to
the corresponding cyanosilylate reaches 90% after 9 h. In contrast,
the conversion yields for 4-phenoxybenzaldehyde and 4-phenyl-
benzaldehyde, with molecular dimensions of 13.3 × 7.3 and 13.1
× 6.7 Å2, respectively, are reduced to below 20% under similar
conditions. Given the expectation of comparable reactivity, this
suggests that the 10 Å wide pore systems in 1 are too small to
readily accommodate the transition state geometry required for
activating these substrates. A size-selectivity effect is also apparent
in the conversion of selected ketones. Due to their innate reduced
reactivity, ketones typically give lower yields than aldehydes under
similar conditions. Thus, while conversion of acetophenone to the
cyanosilylated product reached only 28% after 24 h, conversion
of 4-acetylbiphenyl, a larger ketone with dimensions 13.9 × 6.7
Å2, was still below 1% after 24 h. Together with recent reports on
the oxidation of sulfides5d and the Knoevenagel condensation,5e
this represents one of the first demonstrations of size-selective
catalysis in a metal-organic framework.
Encouraged by the high yields observed for the cyanosilylation
reactions, we tested the catalytic activity of 1 toward a transforma-
tion typically requiring stronger Lewis acids. The Mukaiyama-aldol
reaction involves the reaction of an aldehyde (A) with a silyl enolate
(SE) and is one of the most fundamental tools for the selective for-
mation of carbon-carbon bonds.9 Significantly, this reaction is cata-
lyzed only by very active Lewis acid catalysts,10 and previous
attempts to employ metal-organic frameworks as catalysts proved
unsuccessful.6c In a test reaction, a solution of benzaldehyde (A1)
in CH2Cl2 was treated with methyltrimethylsilyldimethylketene acetal
(SE1) in the presence of 16 mol % of 1. As shown in Table 2, the
observed yield at room temperature was 63% after 99 h, which is
comparable with yields obtained for cation-exchanged ZSM-5 or
Y-zeolites.11 In 1, however, an unprecedented size-selectivity was
realized, with the use of 4-tert-butylbenzaldehyde (A2) reducing
the conversion to only 24% and reactions of benzaldehyde with
the larger silyl enolates SE2 and SE3 resulting in no conversion.
To test the activity of the MnII sites in the presence of coordi-
nating solvents, the Mukaiyama-aldol reaction was also performed
in N,N-dimethylformamide (DMF). DMF and certain polar solvents
are known to act as Lewis bases that can accelerate the Mukaiyama-
aldol reaction at room temperature, even in the absence of Lewis
acids.12 Indeed, for the reaction of A1 with SE1 in neat DMF at
25 °C, we observed yields of up to 80% after only 1 h. The rate
Acknowledgment. This research was funded by DoE Grant
No. DE-FG03-01ER15257. We thank the Yamada Science Foun-
dation for partial support of S.H., and JSR Corporation for support
of K.T.
Supporting Information Available: Experimental procedures,
plots demonstrating catalyst recyclability and conversion ratios, an IR
spectrum of 1 after absorbing benzaldehyde, powder X-ray diffration
data for as-synthesized and postreaction samples of 1. This material
is available free of charge via the Internet at http://pubs.acs.org.
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