Size-dependent catalysis by DABCO-functionalized Zn-MOF with one-dimensional channels

Article abstract of DOI:10.1039/c1dt11274k

Lewis basic DABCO-functionalized 3D-like metal-organic framework, Zn-MOF, catalyzes nitroaldol (Henry) reaction of 4-nitrobenzaldehyde with nitroalkanes in a size-dependent manner. Small nitroalkanes give rise to higher conversion than larger ones. This M

Full text of DOI:10.1039/c1dt11274k

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Size-dependent catalysis by DABCO-functionalized Zn-MOF with
one-dimensional channels†
Ja-Min Gu, Wan-Seok Kim and Seong Huh*
Received 5th July 2011, Accepted 23rd August 2011
DOI: 10.1039/c1dt11274k
Lewis basic DABCO-functionalized 3D-like metal–organic
framework, Zn-MOF, catalyzes nitroaldol (Henry) reaction
of 4-nitrobenzaldehyde with nitroalkanes in a size-dependent
manner. Small nitroalkanes give rise to higher conversion
than larger ones. This MOF-based heterogeneous catalyst is
very robust and can be recycled several times without losing
its activity.
Metal–organic frameworks (MOFs) consist of metal ions or metal
clusters as nodes and various organic linkers, including strut
Fig. 1 (a) A framework view along the a-axis. 1D channels are clearly
visible. (b) A Connolly surface with a probe radius of 1.0 A shown along
the b-axis. The yellow arrows indicate the free Lewis basic N atoms of
DABCO ligands.
ligands, as bridging ligands which interconnect the nodes.1 The
application of functional MOFs is an important area of current
research for nanoporous materials.2 Many organic reactions can
be efficiently catalyzed by MOFs. There are numerous MOF-
catalyzed organic reactions reported in the literature.3 The catalyt-
ically active centers are metal ions or functional organic linkers.
A survey of the literature, however, indicates that most of the
reported catalysis by MOFs so far is related to the first case.
Lewis acidic transition metal catalysis is a good example. Despite
the great potential of heterogeneous MOF catalysis, the latter
case is seldom found in the literature.4 For example, Telfer et al.
recently invented a new method for the introduction of proline-
based organocatalyst moieties into MOFs using a thermolabile
protecting group.4b Although a number of new methods for
functionalizing the bridging ligands are being developed,5 the
incorporation of catalytically active Lewis basic organic functional
groups is still challenging.3a Catalytically active metal complexes
could also be incorporated into bridging ligands through postsyn-
thetic modifications.6
˚
a smaller dimension based on the closest atom-to-atom distances
7
˚
˚
(ca. 6.48 A ¥ 5.32 A). The potential strut linker, DABCO, uses
only one N atom instead of the two available N atoms to coordinate
axial positions of the paddlewheel Zn2 moiety of 1. As a result,
the Zn-MOF 1 unusually contains open Lewis basic sites located
in an orderly way in 1D channels. The exposed N atoms of
DABCO ligands are chemically accessible, indicated by the high
CO2 adsorption enthalpy value.7 Also, the Zn-MOF stack did
not disintegrate during the activation process at 120 ◦C based on
the powder X-ray diffraction (PXRD) study. Stimulated by these
interesting results, we envisioned that the exposed DABCO ligands
inside 1D microporous channels might catalyze organic reactions
without disintegration of the porous structure.
Here we report the nitroaldol (Henry) reaction8 between 4-
nitrobenzaldehyde and nitroalkanes with different molecular sizes
catalyzed by the as-prepared Zn-MOF 1. We systematically varied
We recently reported an interesting Zn-MOF, [Zn(3,3¢-
˚
the molecular sizes of nitrolalkanes (nitromethane, 5.16 A ¥ 5.10
TPDC)]·DMF·2H2O (1), containing
a C2h-symmetric or-
˚
˚
˚
˚
˚
˚
A; nitroethane, 5.52 A ¥ 6.49 A; 2-nitropropane, 6.75 A ¥ 6.68 A;
ganic linker, terphenyl-3,3¢-dicarboxylate (3,3¢-TPDC), and 1,4-
diazabicyclo[2,2,2]octane (DABCO).7 The crystal structure of 1
indicated a two-dimensional (2D) layered structure. Each 2D layer
tightly packs to result in robust 3D-like microporous framework
with well-defined 1D channels as shown in Fig. 1. The 1D channels
contain cage-like open spaces interconnected with windows having
˚
˚
1-nitropropane, 5.52 A ¥ 7.38 A; and nitrocyclohexane, 6.75 A ¥
8.92 A) to elucidate the reaction mechanism and reaction center.
˚
Their physical dimensions were estimated by using Chem3D15
(Fig. S1†). If the nitroaldol reaction is catalyzed by the DABCO
anchored into 1D channels, the reaction yield would be inversely
proportional to the molecular size of nitroalkanes due to the
substrate size selection effect of the microporous 1D channels
as often observed in zeolite catalysis.9 If so, nitromethane will
give rise to the highest yield among five nitroalkane nucleophiles.
Furthermore, this reactivity trend will corroborate the 3D-like
framework robustness even though the Zn-MOF 1 is a 2D layered
structure (Fig. S2†).
Department of Chemistry and Protein Research Center for Bio-
Industry, Hankuk University of Foreign Studies, Yongin, 449-791, Korea.
E-mail: shuh@hufs.ac.kr; Fax: 82 31 330 4566; Tel: 82 31 330 4522
† Electronic supplementary information (ESI) available: Experimental
details, molecular size calculation, and crystal structure. See DOI:
10.1039/c1dt11274k
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The as-prepared Zn-MOF 1 was synthesized according to our
previous method.7 We employed the as-prepared Zn-MOF 1 as a
catalyst precursor without any prior treatments. We expected that
the solvate molecules, DMF and H2O, could be easily exchanged
with the reactant, nitroalkanes, during the nitroaldol reaction. The
as-prepared Zn-MOF 1 is quite stable in the solid state in air for a
prolonged period of time.
basic moieties. Unusually, the reaction center in our system is
an exposed DABCO ligand which is a Lewis base. To the best of
our knowledge, the current system is the first example of nitroaldol
reaction catalyzed by the Lewis basic sites of MOFs. Furthermore,
the 3D-like Zn-MOF 1 exhibited substrate selectivity based on
their molecular sizes.
Stimulated from this size-dependent catalysis by Zn-MOF 1,
we also tested the recyclability of the heterogenenous Zn-MOF
1 catalytic system. To test recyclability of the system, we scaled
up the reaction for easy separation of crystalline Zn-MOF 1 after
the reaction. As shown in Fig. 3, the catalytic system maintained
constant reactivity for four consecutive reactions. The Zn-MOF 1
catalyst was simply filtered off and reused for the next reaction. No
significant loss of activity has been observed during the recycling.
Therefore, the Zn-MOF 1 is also a good catalytic system for
recycling. In order to check the heterogeneity of the catalytic
reaction, the hot filtrate obtained after the reaction was analyzed
by inductively coupled plasma atomic emission spectroscopy (ICP-
AES). As expected, a negligible amount of Zn2+ ion was detected
(~ 11 ppm).
The Zn-MOF 1 indeed catalyzed the nitroaldol reaction
between 4-nitrobenzaldehyde and nitroalkanes to produce b-
nitro alcohols as shown in Fig. 2. No dehydrated nitroalkene
products were observed and the conversions were negligible for
all nitroalkanes in the absence of the Zn-MOF 1.10 Thus, it is
confirmed that the nitroaldol reaction is efficiently catalyzed by the
Zn-MOF 1. More importantly, the reaction yield systematically
depends on the molecular size of the nitroalkanes. For example,
when nitrocyclohexane was used, the conversion was merely 12%
compared with the conversion of 80% for nitromethane. In the case
of nitroethane, the conversion was 34%. The conversion decreased
in the order of nitromethane (80%) > nitroethane (34%) > 2-
nitropropane (30%) > 1-nitropropane (19%) > nitrocyclohexane
(12%). Therefore, the conversions are inversely proportional to the
molecular size of nitroalkanes. Nitromethane (pKa = 10.24) is less
acidic than nitroethane (pKa = 8.60), 1-nitropropane (pKa = 8.98),
and 2-nitropropane (pKa = 7.7–7.8).11 Thus the higher conversion
for nitromethane can be attributed to the size effect rather than
the electronic effect. Since the crystal structure of the Zn-MOF
1 is known,7 we could calculate turnover number (TON) of this
system. The respective TONs for nitromethane, nitroethane, 2-
nitropropane, 1-nitropropane, and nitrocyclohexane are 48.2, 20.5,
18.1, 11.4, and 7.2 for 120 h.
Fig. 3 Recycling test for nitroaldol reaction of 4-nitrobenzaldehyde with
nitromethane catalyzed by Zn-MOF 1. Reaction conditions: Zn-MOF 1
50 mg (0.083 mmol), 4-nitrobenzaldehyde 5.0 mmol, and nitromethane
10 mL at 60 ◦C for 120 h.
To verify the framework robustness, the powder X-ray diffrac-
tion (PXRD) pattern of the recovered Zn-MOF 1 after the fourth
recycling experiment was obtained. The pattern indicated that the
framework was robust enough to exhibit a very similar diffraction
pattern of the desolvated sample of the as-prepared 1. More
interestingly, a very weak diffraction peak at 2q = 5.56◦ was also
observed (Fig. 4(d)). This peak was previously observed at 2q =
5.38◦ for the desolvated sample after CHCl3-exchange for gas
sorption measurement (Fig. 4(c)).7 Thus, the solvate molecules
in the as-prepared Zn-MOF 1 were exchanged with the reactant,
nitroalkanes, during the catalytic reaction as we expected.
Additionally, IR spectra for both the as-prepared Zn-MOF 1
and the four-time recycled Zn-MOF 1 indicated overwhelmingly
identical patterns as depicted in Fig. 5 except for two new bands
observed at 1557 and 1351 cm-1. This fact further corroborates
the framework stability even after recycling four times. The two
newly appeared bands can be assigned to N–O asymmetric and
symmetric stretching modes of nitromethane, respectively, and the
nitromethane molecules are thought to be captured in the 1D
nanochannels. Neat nitromethane exhibited N–O asymmetric and
symmetric stretching modes at 1552 and 1376 cm-1, respectively
Fig. 2 Nitroaldol reaction between 4-nitrobenzaldehyde and varying
molecular sizes of nitroalkanes catalyzed by Zn-MOF 1 under the
same condition. Reaction conditions: Zn-MOF 1 10 mg (0.0166 mmol),
4-nitrobenzaldehyde 1.0 mmol, and nitroalkanes 10 mL at 60 ◦C for 120 h.
There is only one previous report on the MOF catalysis for
nitroaldol reaction.12 In that case, however, the reaction was
catalyzed by the Lewis acidic Cu(II) centers. As shown in this
example, the generation of Lewis acidic sites in MOFs, mostly
open metal sites, is more popular than the introduction of Lewis
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Scheme 1 Cyanosilylation of 4-nitrobenzaldehyde with TMSCN cat-
alyzed by Zn-MOF 1. Reaction conditions: Zn-MOF 1 10 mg (0.0166
mmol), 4-nitrobenzaldehyde 1.0 mmol, and TMSCN 1.0 mmol in 10 mL
dry toluene at 50 ◦C for 24 h.
of the as-prepared Zn-MOF 1 relative to 4-nitrobenzaldehyde,
the same reaction gave 77% cyanohydrin. Thus, the Zn-MOF
1 is also catalytically active for cyanosilylation of aldehyde.
Pretreatment of the Zn-MOF 1 catalyst was not required.
Therefore, we do not need a laborious pretreatment of the
catalyst precursor before catalytic cyanosilylation.14a–c On the
other hand, HKUST-1, Cu3(BTC)2(H2O)3·xH2O where BTC is a
1,3,5-benzenetricarboxylate, should be dehydrated at an elevated
temperature for the generation of active catalytic center which can
activate aldehyde substrates during cyanosilylation.
Fig. 4 PXRD patterns for the simulation from X-ray crystallography (a),
as-prepared Zn-MOF 1 (b), CHCl3-exchanged and desolvated Zn-MOF 1
at 120 ◦C under high vacuum (c), and the retrieved Zn-MOF 1 after four
times of nitroaldol reaction in nitromethane (d).
In summary, we have developed a Zn-MOF catalytic system for
nitroaldol reaction with a high substrate size-selectivity by using
the partially coordinated DABCO ligand to Zn ion. The uncoor-
dinated nitrogen atoms serve as Lewis basic catalytic centers for
nitroaldol reaction and cyanosilylation of 4-nitrobenzaldehyde.
The unprecedented substrate size dependency can be accounted
for by the fact that the nitroaldol reaction exclusively takes place
inside microporous 1D channels. The Zn-MOF catalytic system
maintained its activity for several repeated reactions. This indicates
the structure of the Zn-MOF is very robust. This framework
robustness of the 3D-like 2D layered Zn-MOF represents a novel
MOF-based catalytic system.
This work was supported by Hankuk University of Foreign
Studies Research Fund of 2011.
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10828 | Dalton Trans., 2011, 40, 10826–10829
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©