A bifunctional metal–organic framework platform for catalytic applications

Article abstract of DOI:10.1016/j.poly.2018.11.003

DUT-71 (DUT – Dresden University of Technology), a copper paddle wheel based framework, was used as a platform for postsynthetic modification to introduce specific catalytically active sites and to enhance the catalytic activity for fine chemicals product

Full text of DOI:10.1016/j.poly.2018.11.003

Accepted Manuscript
A bifunctional metal-organic framework platform for catalytic applications
Philipp Müller, Volodymyr Bon, Irena Senkovska, Khoa D. Nguyen, Stefan
Kaskel
PII:
S0277-5387(18)30720-4
https://doi.org/10.1016/j.poly.2018.11.003
POLY 13549
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
31 May 2018
9 October 2018
1 November 2018
Please cite this article as: P. Müller, V. Bon, I. Senkovska, K.D. Nguyen, S. Kaskel, A bifunctional metal-organic
framework platform for catalytic applications, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.11.003
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A bifunctional metal-organic framework platform for catalytic applications
Philipp Müller, Volodymyr Bon, Irena Senkovska,* Khoa D. Nguyen, and Stefan Kaskel**
Technische Universität Dresden, Inorganic Chemistry, Bergstr. 66, 01062 Dresden, Germany;
*irena.senkovska@chemie.tu-dresden.de
**stefan.kaskel@tu-dresden.de
Abstract
DUT-71 (DUT - Dresden University of Technology), a copper paddle wheel based framework, was used
as a platform for postsynthetic modification to introduce specific catalytically active sites and to enhance
the catalytic activity for fine chemicals production using bifunctional N-donor ligands such as dabco (1,4-
diazabicyclo[2.2.2]octan) and 3,5-bis(1H-imidazol-1-yl)arene derivatives resulting in the corresponding
networks DUT-72 and DUT-90. The intact network structure after functionalisation was confirmed by
1
powder X-ray diffraction, the degree of functionalization was analysed by H NMR, thermal and
elemental analysis. As model reaction, the C-C coupling reaction of Henry type catalysed by Lewis acids
in presence of organic base was chosen. The influence of the functionalisation degree, solvent, and
reaction temperature on the catalytic activities was studied. Finally, yields up to 77% of 2-nitro-1-(4-
nitrophenyl)ethan-1-ol in the reaction of 4-nitrobenzaldeyde with nitromethane were achieved using a
dabco functionalised network DUT-72.
Keywords: coordination polymer; Henry reaction; copper paddle wheel; post synthetic modification
Introduction
The crystalline character combined with a high pore volume accessible for guest molecules
make metal-organic frameworks suitable for catalytic applications.1–3 Because of the easy
integration of active centres of interest into the cluster or the ligand, a catalyst can be designed in
a modular way. It is the main reason for the wide applicability of this type of materials in a broad
range of catalytic reactions, including C-C or C-X (X = N, O, S) coupling reactions.4
Depending on the network composition and functional groups presented, various kinds of
reactions are feasible.5 By introduction of active palladium species into the cluster, ligand or as
guest species in the pore system, the catalytic conversion in Suzuki-Miyaura,6 Sonogashira7 or
Mizoroki-Heck8,9 coupling were successful. Also transformations catalysed by copper were often
reported, since the copper-based paddle wheel cluster is the most prominent node in MOF
chemistry, offering catalytically active open metal sites. Amongst others, C-C coupling
reactions10,11 or various types of condensation12 or addition13 reactions can be catalysed by
copper paddle wheel based MOFs.
Recently, we reported a DUT-71 (Cu4(mpbatb)2(H2O)4) MOF, based on copper paddle wheels
and a tetratopic 1,3-phenylebis(azanetriyl)tetrabenzoate (mpbatb) ligand, as a platform for
introducing various active molecular building blocks post synthetically.14,15 As an example, the
integration of 1,3-bis(1H-imidazol-1-yl)benzene (1,3-bib) or 3,6-bis(pyridine-4-yl)1,2,4,5-tetrazine
(bpta) in combination with 1,4-diazabicyclo[2.2.2]octane (dabco) ligands increases the
robustness of the network for desolvation and enables to use DUT-71 based materials for
sensing14 and gas adsorption.15 In particular, the functionalised structures facilitate the detection

of alcohols in the ppm range or the identification of the solvents of different polarity.14 The
disposition for functionalisation renders DUT-71 as an ideal candidate for the integration of
predefined catalytically active centres. Post-synthetic modification is nowadays recognized as a
powerful tool in MOF chemistry16,17 to create new catalysts.18,19
The Henry reaction is one example of a C-C coupling reaction catalysed by copper under the
addition of a base such as dabco.20 In the course of the reaction the α-carbon of an aliphatic nitro
compound reacts with a carbonyl compound to a β–nitro alcohol under the formation of a C-C
bond. Products of the Henry reaction are useful intermediates for the synthesis of complex
organic molecules. The resulting nitro alcohol can act as functionalised starting material for
subsequent transformations such as oxidation or reduction.
Scheme 1: Model catalytic reaction investigated.
The structure of DUT-71 contains two crystallographically independent paddle wheel clusters.
The paddle wheels arranged along c-axis have alternating inter-paddle wheel distances of 8.0
and 13.8 Å (Figure 1a). The accessible metal sites of remaining paddle wheels point into the
pores. DUT-71 is ideal to bind dabco in a way, that only one nitrogen atom is coordinated to the
paddle wheel (monodentate). The non-coordinating nitrogen can consequently participate in the
Henry reaction, making use of the bifunctionality: the copper open site and the pending nitrogen
of the organic base dabco. To evaluate this concept, the Henry reaction of 4-nitrobenzaldehyde
with nitromethane to 2-nitro-1-(4-nitrophenyl)ethan-1-ol was chosen as a test reaction (Scheme
1).
Experimental
1. MOFs synthesis
DUT-71, DUT-72 and DUT-90 MOFs were synthetized as reported elsewhere14,15 (for more
details see ESI.
2. Catalysis
For a typical experiment the MOF catalyst was transferred into a GC vial and the vial was placed
in a Pyrex or Schlenk tube (Fig. S11 ESI). 4-Nitrobenzaldehyde (102.7 mg, 0.679 mmol)
dissolved in 6.500 mL nitromethane or in 6.500 mL ethanol/nitromethane mixture (mixture
composition: 0.364 mL (414 mg, 10 eq) nitromethane and 6.136 mL ethanol) was added and the
reaction mixture was heated to the desired temperature, indicated in the Table S1 of ESI.
To identify and characterise the products of the reactions, they were isolated after defined
reaction time periods and analysed by high performance liquid chromatography (HPLC). HPLC
measurements were performed on ELITE LACHROM System from VWR/HITACHI using a UV
L2400 detector. For data evaluation EZCHROM ELITE software from AGILENT
TECHNOLOGIES was applied. The chiral column CHIRALPAK ID (diameter 4.6 mm, length 250
mm, particle size 5 m) was used with following parameters for baseline separation: wavelength
220 nm; composition of the mobile phase: n-hexane/isopropanol (85/15); flow: 1 ml min−1.
Retention times for enantiomers: 20.8 and 26.8 min, respectively.
The 2-nitro-1-(4-nitrophenyl)ethan-1-ol (1) was synthetized, purified and used for the HPLC
calibration. The yields of 1 in the reaction was determined using obtained calibration curve. If 1-
(diethoxymethyl)-4-nitrobenzene (2) or 1-nitro-4-(2-nitrovinyl)benzene (3) appeared as a side
1
products, they could be detected as additional signals in the chromatogram. Moreover, the H
13
and C NMR analysis was performed to identify the compounds. The yield of 2 and 3 was not
determined.

Results and discussion
To create bifunctional, copper/base containing catalyst, DUT-71 (Fig. 1a) crystals were soaked in
a dabco solution at 80 °C for 7 days resulting in the formation of DUT-72 (Figs. 1b and 2a).
Figure 1: a) Crystal structure of DUT-71 along b axis. b) Crystal structure of DUT-72 along c axis.
[14,15]
Since the structure of the DUT-71 derivatives contains two independent paddle wheel clusters
a
b
(paddle wheel Cu2 consists of Cu2 and Cu1 atoms; paddle wheel Cu2 consists of two symmetry
equivalent Cu3 atoms, Figs. 1, S12), dabco can be distributed between four Cu atoms inside the
MOF upon post synthetic modification. The maximal, theoretically possible loading (coordination
ability) of the available Cu sites with dabco in DUT-71 (Cu4(mpbatb)2) is 3.5 molecules per 4 Cu
a
b
atoms (ideal composition Cu4(mpbatb)2(dabco)3.5 or [Cu2 (dabco)1.5][Cu2 (dabco)2](mpbatb)2).
In DUT-72 and DUT-90 we expect a full occupancy of the small Cu - Cu gap (8 Å gap in Fig. 1a,
Figs. 2a, S12) by dabco first, because of favourable two-fold coordination mode. The remaining
dabco should be coordinated in a monodentate fashion to the paddle wheels (Table 1, column 3)
pointing into the pores (Fig. 2a, S12) and can potentially act as base in the catalysis.
The real compositions of MOFs obtained after post-synthetic modification (content of dabco and
1,3-bib derivatives) were determined on the basis of elemental and thermogravimetric analysis
results (Table 1, Figs. S8 – S9 ESI). The real composition of DUT-72 was estimated to be
Cu4(mpbatb)2(dabco)1.45, pointing on some dabco deficit in the structure and some Cu sites left
unoccupied by dabco (Table 1, column 2). Accordingly, the 2.05 cooper atoms per formula unit
still accessible (linker free) and potentially available as Lewis acidic catalytic sites. So, the ratio
between the Lewis acidic catalytic sites and base (dabco coordinated in a monodentate fashion)

amounts to 2.15 in DUT-72 (Table 1, column 4), pointing on the excess of Cu sites in respect to
dabco amount.
a)
b)
Figure 2: Part of the crystal structure of: a) DUT-72; b) DUT-90 along b axis.
Table 1: Analysis of Cu and dabco contents in the investigated MOFs given per formula unit.
Open
Monodentate
coordinated
dabco (b)
(linker free)
copper
sites (a)
4
ratio
(a) to (b)
DUT-71
Cu4(mpbatb)2
DUT-72
Cu4(mpbatb)2(dabco)1.45
DUT-90
0
n.a.
2.15
0.48
2.05
0.65
0.95
1.35
Cu4(mpbatb)2(1,3-bib)0.5(dabco)1.85
In DUT-90, the large gap (13.8 Å, Fig. 1a) is occupied by additional, nitrogen containing ligand
(1,3-bib) (Fig. 2b), making it possible to encircle more precisely the position of possible catalytic
a
sites in this compound in comparison to DUT-72, because all Cu2 paddle wheels are involved
into the coordination of two-fold coordinating ligands (dabco in small gape, 1,3-bib in large gape).
In this compound, catalytically active sites as well as monodentate coordinated dabco molecules
b
should more probably come from Cu2 paddle wheels only (Fig. 2b). In case of DUT-90, lager

amount of dabco coordinated in a monodentate fashion should be present according to the
elemental analysis (Tab. 1).
To evaluate the impact of crystallographically different Cu sites (regarding the coordination and
chemical environment) as well as dabco in the catalytic reaction, the obtained materials were
applied as catalyst in a model Henry reaction (Scheme 1).
Initially, catalytic tests were performed in nitromethane (at 100 °C), acting as solvent and as
reactant at the same time. The reaction was terminated after 72 h. The products were isolated
and analysed by HPLC (for more details see experimental section).
The DUT-72 and DUT-90 turned out to be more active catalysts then DUT-71, producing 43%
and 45% of 2-nitro-1-(4-nitrophenyl)ethan-1-ol (1, Scheme 2),respectively. The DUT-71 yields
only 14% (Fig. 3). For DUT-71 and DUT-90 a stepwise increase in yield after 30 h of reaction
was observed, pointing on some induction period.
Figure 3: MOF-catalysed production of 2-nitro-1-(4-nitrophenyl)ethan-1-ol using DUT-71 (■),
DUT-72 (♦) and DUT-90 () as catalyst in nitromethane at 100 °C.
DUT-71 has the highest concentration of accessible Cu sites (Table 1), but the absence of dabco
leads to the low yield in comparison to DUT-72 and DUT-90. In case of DUT-71, the activation of
the nitromethane takes place, obviously, only by a thermal treatment, since the reaction was
performed at high temperature (100 °C). In DUT-72 the amount of potentially accessible Cu-sites
is almost three times higher than in DUT-90 at comparable dabco amount (Table 1), so we
conclude that slightly higher loading of manodentate coordinated dabco in DUT-90 leads to the
slightly better performance in the catalytic reaction.
Thus, the positive influence of dabco, post-synthetically introduced into DUT-72 and DUT-90
could be clearly identified.
OH
O
NO2
NO2
O
O2N
O2N
O2N
(1)
(2)
(3)
Scheme 2: Products formed in the catalytic tests.
To evaluate the effect of solvent, ethanol was added to the reaction system in the next trials.
Utilizing DUT-71 at room temperature (RT), the formation of 1-(diethoxymethyl)-4-nitrobenzene
(2, Scheme 2) was observed. The same reaction product was detected in small amounts in a

control reaction, when only 4-nitrobenzaldehyde and nitromethane were stirred in ethanol at RT
without catalyst, pointing on low activity of the Cu sites of MOFs under such conditions.
For further test the temperature was increased to 80 °C. For DUT-72 and DUT-90 a yield of 1 up
to 50% was detected after 8 h of reaction (Fig. 4). Interestingly, longer reaction time leads to
decrease in the amount of 1 formed (Fig. 4). After isolation of the products, 1-nitro-4-(2-
nitrovinyl)benzene (3, Scheme 2) as a side product was detected. Obviously, ethanol accelerates
the reaction of 4-nitrobenzaldehyde to 2-nitro-1-(4-nitrophenyl)ethan-1-ol (1) and facilitates a
secondary elimination reaction of 2-nitro-1-(4-nitrophenyl)ethan-1-ol (1) to 1-nitro-4-(2-
nitrovinyl)benzene (3). The reason for the side reaction is the protic character of ethanol as
solvent, which is able to act as proton donor interacting with OH group of 1 to form water. Such
elimination was not observed if the reaction was performed in aprotic nitromethane.
Figure 4: Formation of 2-nitro-1-(4-nitrophenyl)ethan-1-ol using DUT-72 (♦) and DUT-90 () as
catalyst in ethanol at 80 °C.
To evaluate temperature dependence for the formation of 3, the reaction temperature was
decreased stepwise. If the temperature was decreased to 50 °C, 74% yield of 1 for DUT-90 and
77% of 1 for DUT-72 were obtained.
Figure 5: Formation of 2-nitro-1-(4-nitrophenyl)ethan-1-ol (1) in ethanol for DUT-72 (♦) and DUT-
90 () at 50 °C (black) and at 25 °C (grey).
Longer reaction times (up to 126 h) show no significant improvement in the yields (Fig. 5).
Obviously, lowering the reaction temperature suppresses the secondary reaction of 1.
At 25 °C a maximal 2-nitro-1-(4-nitrophenyl)ethan-1-ol 1 yield of 47% for DUT-72 and 45% for
DUT-90 were observed after 126 h of reaction (Fig. 5). Also no side products were detected.
Interestingly, the type of catalyst critically influences the products formed in the reaction, giving,
to some extent, the possibility to design the catalyst. For example amines immobilized on silica-
alumina are able to convert an aldehyde with nitromethane to a 1,3-dinitroalkane species.21 Such
a product was not observed in our studies. Also MOFs are able to catalyse the reaction to
different products, depending on the nature of the catalyst applied. For example amine

functionalised MIL-101(Cr) and amine functionalised UiO-66 (Zr based MOF) guide the reaction
to elimination, forming product 3 (Scheme 2).22-25 Interestingly, a reaction catalysed by UiO-67
(isoreticular to UiO-66) functionalised with an urea group stops at alcohol formation step (yield
67%).26 Dabco containing Zn-MOF27 (yield 60%) or pyridine containing Cu-MOF28 (yield 85%)
were reported to produce the alcohol derivatives.
To analyse the stability of DUT-72 and DUT-90 during the catalysis, recycling tests were
performed for reactions at 25 °C and 50 °C.
Figure 6: Recycling tests for DUT-72 and DUT-90 at RT and 50 °C after 96 h reaction time.
At 25 °C, a certain decrease in yield of 1 was observed during three catalytic cycles for both
catalysts (from 48 % to 35 % for DUT-72 and from 45% to 35% for DUT-90). At 50 °C the yield
decreases more drastically in the third cycle (from 76 to 41% for DUT-72) than in the second
cycle (Fig. 6), although PXRD analysis of the reused catalysts reveals the retained crystallinity
after 3 runs (Figs. S1 - S6).
Thus, the results obtained in this study indicate a high sensitivity of the MOF catalysed reaction
towards solvent nature and temperature. Advantageous opportunity to terminate the reaction
selectively at the alcohol formation stage opens the possibility to use it as starting material for
further oxidation of the hydroxyl group to a ketone29 or for the reduction of the nitro group to an
amine.30
Figure 7: Introducing of the chiral information into the DUT-90 framework by ligand
functionalisation.
Since examples of enantioselective Henry reaction catalysed by MOFs are not reported in the
literature yet, chirality was introduced into the DUT-90 by replacing the 1,3-bib ligand by a chiral
analogue (Fig. 7) to form chir-DUT-90 ([Cu4(mpbatb)2(chir-1,3-bib)0.5(dabco)1.7) according to
PXRD analysis (Fig. S7) and NMR data (Fig. S10). Only few reports about preferable formation
of diastereomers by coupling with e.g. nitroethane are published.31-34
Catalytic tests using chir-DUT-90 were carried out in ethanol at 0 °C, 25 °C, and 50 °C. After
three days yields of 34%, 37%, and 40% of 1, respectively, were obtained. However no
enantiomeric excess was observed for the desired product. A possible explanation could be the
wide gap in-between chiral information and the catalytic active sites within the network.

Conclusions
Post synthetic modification is a versatile tool for the introduction of desired functionalities into
MOF structures containing open metal sites. The crystallinity of MOFs allows precise crystal
engineering as well as prediction of molecules suitable for introduction into a certain MOF. In this
work we could show that the intrinsic catalytic activity of MOFs can be enhanced by post
synthetic modification. The introduction of dabco gives a bifunctional catalyst and leads to the
yield of 2-nitro-1-(4-nitrophenyl)ethan-1-ol up to 77% in Henry reaction. By deliberately choosing
the right solvent and temperature, the selectivity of the reaction product could be controlled.
Acknowledgements
DFG is gratefully acknowledged for funding (SPP 1362). Khoa D. Nguyen would like to thank the
Vietnamese Government for a doctoral fellowship (Project 911). Christel Kutzscher and Silvia
Raschke are acknowledged for the support during HPLC measurements.
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catalyst:
DUT-71
up to 14%
O
OH
H
NO2
O2N
+
O2N
NO2
Henry reaction
up to 43%
DUT-71 was used as a platform for postsynthetic modification to enhance the catalytic activity of MOF in Hanry
catalyst:
DUT-72
reaction.