Tailoring metal-organic framework catalysts by click chemistry

Article abstract of DOI:10.1039/c2dt11994c

We successively introduce new catalytic centers through click reaction into MOFs and modify their environment by addition of lipophilic groups. The resulting bifunctionalized MOF provides an optimized balance between basicity and lipophilicity and shows outstanding performance for the transesterification reaction. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt11994c

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COMMUNICATION
Tailoring metal–organic framework catalysts by click chemistry†
Marie Savonnet,a,b Aurélie Camarata,a Jerome Canivet,a Delphine Bazer-Bachi,b Nicolas Bats,b
Vincent Lecocq,b Catherine Pinela and David Farrusseng*a
Received 20th October 2011, Accepted 7th December 2011
DOI: 10.1039/c2dt11994c
We successively introduce new catalytic centers through click
reaction into MOFs and modify their environment by
addition of lipophilic groups. The resulting bifunctionalized
MOF provides an optimized balance between basicity and
lipophilicity and shows outstanding performance for the
transesterification reaction.
Metal–organic frameworks (MOFs) are porous crystalline
materials composed of cationic metal systems that behave like
nodes with polytopic organic ligands acting as spacers.1–10
These materials are often viewed as a new class of zeolites due
to their porous structure. MOFs of the most recent generation
present molecular recognition properties11–13 originating from
their considerable dynamic flexibility,14–16 which is usually
under the control of host–guest interactions. It is acknowledged
that MOFs could ultimately mimic enzymes using a “locking”
concept favoring high chemo-, regio- and enantioselectivity.17–22
MOFs could indeed be viewed as potential “artificial enzymes”
combining several properties in a concerted fashion at the nano-
metre scale. Fairly few MOFs bear more than one reactive func-
tion, however. MOFs have already been reported to catalyze a
broad range of organic transformations involving their Lewis
acid nodes as well as their Brønsted acid–base properties.17–18,23
For example, inorganic clusters with unsaturated coordination
(such as HKUST-1) or bridging hydroxyl groups (such as
MIL-53) have been shown to perform Lewis24–26 and Brønsted
type catalysis,27 respectively.
One solution for synthesizing advanced MOFs suitable for
specialized and sophisticated applications is the controlled
Fig. 1 Synthesis of MOF catalysts by “click chemistry”.
addition of more complex functionalities into the porous
network. Through this functionalization, if the physical environ-
ment of the pores and the cavities within the MOFs can be
modified, the interactions with guest species can in turn be
adapted, thereby fine-tuning the chemical reactivity.28 However,
the introduction of reactive chemical functions by self-assembly
methods is not a trivial task and cannot be generalized to all
MOFs.29,30 Various synthetic strategies have been employed
with the aim of achieving MOF post-functionalization, as
detailed in extensive reviews by Cohen et al.31–40 Post-synthetic
modification (PSM) using covalent-type grafting methods has
undergone outstanding progress during the last five years.41–47
We have recently reported an original PSM method starting from
amino derived MOFs (Fig. 1).48–52
The first step consists in converting the amino groups on the
framework walls into their analogous azido (N3). Without iso-
lation nor purification, the desired triazolyl functionalized MOF
materials are obtained by grafting the corresponding alkyne
using “click chemistry”. Using two different MOFs templates,
the DMOF [Zn2(2-amino-terephthalate)2(dabco)] (dabco = 1,4-
diazabicyclo[2.2.2]octane) and the IHM-2 [In(OH)(2-amino-ter-
ephthalate)], we showed that this novel PSM method presents
key benefits for the engineering of MOF: (i) the softness of the
method allows no restriction on the choice of starting amino-
aIRCELYON, University of Lyon 1 – CNRS, 2 avenue Albert Einstein,
F-69626 Villeurbanne Cedex, France. E-mail: david.farrusseng@ircel-
yon.univ-lyon1.fr; Fax: (+33) 4 72 44 53 99
bIFP Energies Nouvelles, BP n°3, 69360 Solaize, France
†Electronic supplementary information (ESI) available: Synthesis,
characterizations and catalysis. See DOI: 10.1039/c2dt11994c
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 3945–3948 | 3945

MOFs and (ii) the grafting yield can be controlled by adjusting
the MOF : alkyne ratio.
Clear proof of the azide formation and of the subsequent full
(3 + 2) cycloaddition can be characterized by liquid 1H NMR on
a quantitative manner (Fig. 2). After the cycloaddition step, new
aromatic shifts of the post-modified compounds 1b and 1c
confirm that the corresponding triazole derivative is formed as
Despite intensive efforts to develop new efficient, selective
and recyclable solid base catalysts such as Layered Double
Hydroxides (LDH),53–55 hydrotalcite, KF and amine supported
compounds, the development of green processes involving basic
catalysts remains a challenge.56–59
In this contribution, we demonstrate that PSM by “click chem-
istry” enables the engineering of MOFs for application in base cat-
alysis. The controlled post-modification of the frameworks
introducing basic and/or lipophilic, i.e. with an affinity towards
fatty molecules, features allows a rational design of catalytic
MOFs for the transesterification reaction.60,61 We show for the first
time that outstanding cooperative catalytic effect can be obtained
by an optimal formulation of our functionalized material. The
base-catalyzed model reaction considered here is the transesterifi-
cation of ethyldecanoate, a fatty ester, in methanol (Scheme 1).
1
the sole product. The post-digestion H NMR spectrum of 1d
shows contributions of both b and c and also of the remaining
azide in a 30 : 30 : 40 ratio. Despite solvent effects on DMOF
crystallinity,48,62 the powder XRD patterns indicate that long-
range order is preserved for all samples (Fig. S4†).
Fig. 2 Liquid 1H NMR of digested MOF samples.
Scheme 1 Transesterification of ethyldecanoate in methanol.
For this reaction to proceed efficiently, both substrates must be
co-adsorbed into the MOF and MeOH should be activated by a
base in order to favour the nucleophilic attack on the carbonyl
group. The DMOF parent structure was selected as the starting
platform since it contains neither Lewis nor Brønsted acid
groups, which could have a catalytic effect, and only slightly
basic amino groups. Two different functional groups were
selected for the post-modification: a 1,2,3-triazolyl substituted
with phenyl, corresponding to b compounds, or tertiary amine at
the 4-position, corresponding to c compounds (Fig. 1). They are
obtained by reacting the DMOF-N3 (1a) with phenylacetylene to
give 1b and with propargylamine to give 1c. The former 1b pre-
sents moderate basicity originating from the triazolyle group
(pKb ≈ 9.4) as well as strong lipophilicity, whereas the latter 1c
is a much stronger base (pKb ≈ 3 for trialkylamines).
Following similar methodology, samples with a variable
degree of modification were prepared. They are obtained by
adjusting the amount of alkyne reactants, phenylacetylene or
diethylpropargylamine, with respect to 1a. There are denoted
hereafter 1b-15, 1b-40 and 1b-80 for degrees of modification
with b groups (phenyl) of 15%, 40% and 80%, respectively, and
1c-40 and 1c-85 for degrees of modification with c groups (ter-
tiary amine) of 40% and 85%, respectively.
These DMOF catalysts were involved in the transesterification
of ethyldecanoate in methanol and the catalytic results are sum-
marized in Table 1. It is worthy to note that the transesterification
conversion obtained using the unmodified DMOF-NH2 (1) as
catalyst is not higher than that obtained without catalyst (c.a.
10%, entries 1 and 2).
Fig. 3 illustrates also how the degree of modification affects
ethyldecanoate conversion in the case of the monofunctionalized
materials. It appears that the activity of these DMOF materials
increases linearly with the degree of modification. The syntheses
and tests were performed twice for 1b-80 in order to verify
A bifunctionalized MOF modified with b groups (phenyl) and
then subsequently modified with c groups (tertiary amine) on the
remaining azido sites was obtained and hereafter denoted as 1d.
The degree of modification of the MOF catalysts is given in
Table 1.
Table 1 Transesterification yield using functionalized MOFs and
reference catalystsa
Entry
Catalyst
b (%)
c (%)
Yield (%)
1
2
3
4
5
6
7
8
9
none
1
1a
1b-40
1b-40b
1b-80
1c-40
1c-85
1d
—
—
—
40
40
80
—
—
30
—
—
—
—
—
—
40
85
30
10
10
10
48
10
80
21
28
84
a Conditions: ethyldecanoate (2.5 mL) is allowed to react in methanol
(10 mL) using 20 mg of DMOF catalyst (∼0.03 mmol of MOF, c.a.
0.06 mmol of –NH2) at 130 °C for 20 h b 2-ethyl-1-butanol is used
instead of methanol.
Fig. 3 The effect of the degree of modification for 1b (◆), 1c (■) and
1d (▲) on ethyldecanoate conversion at 130 °C after 20 h.
3946 | Dalton Trans., 2012, 41, 3945–3948
This journal is © The Royal Society of Chemistry 2012

experimental reproducibility; both times the ethyldecanoate con-
version was approximately 80%.
Moreover, the use of a bulky primary alcohol in the transester-
ification reaction has been also used to confirm that the reaction
takes place in the porous system and not only at the surface.61
The transesterification with 2-ethyl-1-butanol, which is a much
bulkier substrate than methanol, was carried out using the MOF
1b-40 (Scheme 2). As shown in the Table 1, only a low percen-
tage of conversion was obtained in 2-ethyl-1-butanol against
48% in MeOH (entry 5). This indicates that the reaction proceeds
in the porous framework using MeOH, whereas the reaction does
not take place in the pores in the case of a bulky alcohol due to
the molecular sieving property of the MOF. However, even if
uptake experiments conducted with the two alcohols would give
additional information about the sieving effect, the already
reported solvent-dependent flexibility of the DMOF framework48
undermines the reliability of alcohol uptake as a characterization
method.
The DMOF grafted with c groups could have been expected
to show better activity based on the higher basic strength of the
trialkylamine compared to that of the phenyl substituent. Surpris-
ingly, the introduction of lipophilic functions via b exhibits a
more beneficial effect on the catalytic activity. Furthermore, the
corresponding organic linkers found in 1b and 1c, respectively,
linker
b (dimethyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)-tereph-
thalate) and linker c (dimethyl-2-(4-diethylaminomethyl-1H-
1,2,3-triazol-1-yl)-terephthalate), were synthesized. The linker b
and the linker c were tested in the transesterification reaction
under homogeneous conditions and show moderate activity com-
pared to the corresponding grafted MOFs (Table S1†). These
results confirm the moderate activity of the triazolyl group itself.
In contrast, the tandem post-modified 1d shows outstanding
performances compared to the monofunctionalized DMOF
samples. Although 1d contains only 30% of b and 30% of c,
84% conversion was achieved (entry 9). Under similar reaction
conditions, using 1b-40 and 1c-40 with a similar degree of
modification, the conversion only reached 48% and 21%,
respectively (entries 4 and 7). The superior activity of 1d is not
the simple addition of the activities of both 1b-40 and 1c-40 but
can be attributed to a cooperative effect resulting from the com-
bination of both basic and lipophilic groups at a molecular level
(Fig. 4). A high degree of functionalization (≥ 80% in 1b-80
and 1c-85) with lipophilic or basic groups is not necessarily
required but a combination of low modification degrees (30% in
1d) with both basic and lipophilic groups allows the maximum
activity to be reached.
Scheme 2 Molecular sieving effect of the catalysts 1b-40.
Powder XRD shows that the DMOF materials remain mostly
crystalline after catalysis although peak broadening is observed
(Fig. S9†). As discussed previously, it may arise from the intrin-
sic flexibility of the DMOF structure.48,62 Most important, under
catalytic conditions the robustness of the solid towards leaching
was investigated. When the catalyst is filtered off, the reaction
still slightly proceeds but the trend follows that of a reaction per-
formed without catalyst.
As the elemental analyses performed on our samples show the
presence of 1 to 2 wt% of Cu in the solids, we investigated the
effect of copper on our catalytic system. According to the mol-
ecular formula of the functionalized MOFs and to the catalyst
quantity used in the reaction, the amount of copper present in
our tests corresponds to 0.002–0.004 mmol. Although the exact
nature and oxidation state of the copper is not known, it might
be possible that some of the copper remains as CuI(ACN)4PF6 in
the pore after the click chemistry and may act as a catalyst.63
Control experiments carried out with 0.3 mmol of Cu(ACN)4PF6
or Cu(OAc)2, corresponding to 100 times more Cu than is really
contained in our MOFs, showed conversion of 32 and 40%
respectively, lower than that obtained using our functionalized
DMOF.
In conclusion, we were able, in a single framework, to succes-
sively introduce new basic catalytic centers (amino groups) and
lipophilic groups (phenyl groups) in order to enhance the cataly-
tic activity of our solid. Thanks to the strict control of the syn-
thetic conditions, the precise optimization of these parameters
can be achieved. The resulting bifunctionalized MOF, involving
a low degree of modification at each functionalization step,
Fig. 4 The effect of the basic/lipophilic functionalization of MOF cata-
lysts on ethyldecanoate transesterification yield.
In order to assess our hypothesis of a cooperative basic-lipo-
philic effect, liquid adsorption isotherms with an ethyldecanoate/
isooctane mixture were investigated for 1 and for 1b-40, contain-
ing 40% of 4-phenyl-1,2,3-triazolyl functions (Fig. S5†). These
adsorption measurements confirm the stronger lipophilicity of
the latter material. Consequently, we suggest that the higher
apparent activity of 1b compared to 1c should be due to a more
appropriate co-adsorption ratio of both substrates inside the
pores on the more lipophilic MOF. Hence, 1d with a tandem
functionalization reaches an optimal basic/lipophilic balance to
perform the reaction.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 3945–3948 | 3947

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3948 | Dalton Trans., 2012, 41, 3945–3948
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