Cu-mediated solid-state reaction in a post-functionalized metal-organic framework

Article abstract of DOI:10.1039/c2ce00017b

We serendipitously found that a functionalized MOF containing grafted copper species can react with itself without solvent in order to modify the chemical functions present on its walls. The use of EPR techniques allows new insight into the copper-mediated azide reduction.

Full text of DOI:10.1039/c2ce00017b

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Post-synthetic modification of
coordination networks
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Discovery, development, and functionalization of Zr(IV)-based metal–organic
frameworks
Min Kim and Seth M. Cohen
CrystEngComm, 2012, DOI: 10.1039/C2CE06491J
Controlled modification of the inorganic and organic bricks in an Al-based MOF by
direct and post-synthetic synthesis routes
Tim Ahnfeldt, Daniel Gunzelmann, Julia Wack, Jürgen Senker and Norbert Stock
CrystEngComm, 2012, DOI: 10.1039/C2CE06620C
Conversion of primary amines into secondary amines on a metal–organic framework
using a tandem post-synthetic modification
Andrew D. Burrows and Luke L. Keenan
CrystEngComm, 2012, DOI: 10.1039/C2CE25131K
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COMMUNICATION
Cu-mediated solid-state reaction in a post-functionalized metal–organic
framework{
a
a
b
c
d
Marie Savonnet, J e´ r oˆ me Canivet, Serge Gambarelli, Lionel Dubois, Delphine Bazer-Bachi, Vincent
d
Lecocq, Nicolas Bats and David Farrusseng*
d
a
Received 5th January 2012, Accepted 29th February 2012
DOI: 10.1039/c2ce00017b
We serendipitously found that a functionalized MOF contain-
ing grafted copper species can react with itself without solvent
in order to modify the chemical functions present on its walls.
The use of EPR techniques allows new insight into the copper-
mediated azide reduction.
Chemical reaction in the solid state without external reagents is
11–14
rarely reported inside MOFs
even if it is generally acknowl-
edged that well-defined isolated reactive single sites in MOFs will
enable insight to be gained into reaction mechanisms.
Very recently, azide reductions have been reported using
organometallic systems based on copper. The nature of the
active centers and mechanism are unclear. Qiao et al. reported
the Cu-catalyzed amination of halogenoarenes using sodium
azide as amino source to give the corresponding ortho-
Reactions in the solid state are defined as chemical processes in
which one or more solid reaction products are formed with
diffusion of elementary particles within their interior, as compre-
1
15
hensively reviewed by Cohn. The layer of reaction products
functionalized aromatic amines. The most efficient system
increases by chemical reaction with the reactants adjacent to its
boundaries. Solid-state reactions are known from thermal
intracrystalline conversions (isomerizations or loss of volatile
fragments), photoreactions, gas–solid reactions, and solid–solid
was a combination of copper iodide with dimethyl ethylenedia-
mine and caesium carbonate in ethanol. In the work of Sajiki
1
6
2 3
et al., ethyl-4-azidobenzoate was reacted with CuF –Et N in
dimethylacetamide to give the corresponding aniline derivative
in low yields. In a similar way, Ramana and Goriya reported the
use of Cu(II) species combined with sodium ascorbate and
proline to obtain the complete reduction of 4-azidonitroben-
2
reactions.
The reactivity in the solid state shows strict correlation with
crystal packing because molecular migrations within the crystal are
required for all reactions. Fujita extensively studied solid-state
reactivity of compounds hosted in a robust crystalline porous
1
7
zene. From these three studies, we can assume that the
reduction can be performed in the presence of both copper and
the nitrogen-based ligand. Nevertheless, the oxidation state and
coordination of copper species are still unclear.
3–9
structure, developing the concept of molecular flasks. In this
concept, organic guests are trapped in discrete porous nanocon-
tainers, such as organometallic self-assembled cages, creating
micro-environments and allowing specific reactions of the guest.
Using this concept, Fujita and co-workers were able to get
structural snapshots of chemical transformation intermediates
In this work, we report for the first time an intrareticular reaction
in a MOF. The azido reduction which takes place in the microporous
cavity is performed by copper-coordinated triazolyl moieties,
introduced by postsynthetic click reaction into the MOF framework
18–21
(Scheme 1).
1
0
The nature and oxidation state of heterogenized
thanks to in situ X-ray analyses. They mounted a crystal of a
porous coordination polymer containing a primary amine trapped
into the framework on an X-ray diffractometer. With the amine-
containing crystal being exposed to a flow of aldehyde solution,
they were able to isolate the hemiaminal intermediate of the imine
formation by regularly cooling the crystal at 90 K and collecting
diffraction data.
copper complex are characterized by cutting-edge EPR techniques.
In previous reports, we described an original functionalization
2
1
method starting from amino-containing MOFs. The first step
consists of converting the amino groups on the framework walls into
3
their analogous azido (N ) groups. Without isolation or purification,
a
IRCELYON, University of Lyon 1—CNRS, 2 avenue Albert Einstein,
F-69626, Villeurbanne Cedex, France.
E-mail: david.farrusseng@ircelyon.univ-lyon1.fr;
Fax: (+33) 4 72 44 54 36
b
CEA, INAC, SCIB, Laboratoire de R e´ sonance Magn e´ tique (UMR-E 3
CEA-UJF), 17 rue des Martyrs, 38054 Grenoble Cedex 09, France
c
CEA, INAC, SCIB, Laboratoire de Reconnaissance Ionique (UMR-E 3
CEA-UJF), 17 rue des Martyrs, 38054 Grenoble Cedex 09, France
d
IFP Energies Nouvelles, BP nu3, 69360, Solaize, France
Scheme 1 Copper-mediated intrareticular reduction of MOF azido
{ Electronic supplementary information (ESI) available: Syntheses,
characterizations and catalysis. See DOI: 10.1039/c2ce00017b
functions to their amino analogues.
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm, 2012, 14, 4105–4108 | 4105

the desired triazolyl-functionalized MOF materials are obtained by
grafting the corresponding functionalized terminal alkyne using the
copper-mediated Huisgen cycloaddition known as ‘‘click chemistry’’.
To the best of our knowledge, we report here the first self-reaction of
a metal–organic framework containing different functional groups
respectively, in the presence of Cu(CH
the obtained compounds 1 to 4 were analyzed by liquid H NMR
of the samples dissolved in a DCl–D O–DMSO-d solution.
3 4 6
CN) PF in acetonitrile. All
1
2
6
1
As shown in Fig. 1, the liquid H NMR spectrum of the azido
intermediate 2 reveals the full conversion of the amino moieties
into the corresponding azido moieties according to the appearance
of new aromatic broad signals at 7.7–7.8 ppm. This coincides with
the disappearance of the three aromatic signals of 1 at 7.6, 7.8 and
7.9 ppm. The NMR spectrum of 3a illustrates that the triazole
with amino substituent was successfully formed following the
above described method. It is noteworthy that the isolated single
peak at 8.6 ppm is assigned to the proton of the triazolyl ring. The
integrations of the signals show a grafting rate of 20%, similar to
that found for 4a. The assignment of new aromatic shifts for 4a
22–28
introduced by postsynthetic modification.
This study focuses on (In) MIL-68-NH which is isostructural
2
2
9–32
with MIL-68
formed by indium octahedra and 2-aminoterephthalates (BDC–
NH ) as bridging linkers. It is composed of hexahedral and
triangular 1D-channels with apertures of 16 and 6 A respectively.
In) MIL-68-NH (1) was prepared according to the previously
and has a one-dimensional rod-shaped structure,
2
˚
(
2
33
patented method. It was obtained by precipitation reaction at
room temperature. Afterwards the as-synthesized MOF was
purified by multiple DMF treatments at 160 uC and soaked in
21
was previously described.
N adsorption isotherms confirm that
2
21
CH Cl
2
2
for 24 h. According to our seminal work, 1 was treated
the porosity is maintained throughout the modification. However
PXRD analyses show a slight decrease in crystallinity (see ESI{).
Elemental analysis shows that copper species from the Cu(I) salt
used for the click reaction are present in both 3a and 4a
samples. From these analyses, 1.3 wt% of copper remains in 3a,
corresponding to one copper atom for two triazolyl functions,
whereas 3 wt% remains in 4a. The higher amount of copper
blocked in 4a can be explained by the pore blocking due to phenyl
substituents on the MOF framework.
with tBuONO and TMSN3 in THF overnight at room
temperature to give the corresponding azido intermediate 2
(Table 1). The functionalized MOFs 3a and 4a were then obtained
by adding an excess of propargylamine and phenylacetylene
a
Table 1 Functional composition of compounds 1 to 4
Structure
Amino (%) Azido (%) Triazolyl (%)
In order to assess the nature of copper species confined in
the frameworks of both 3a and 4a, continuous wave electron
paramagnetic resonance spectroscopy (CW-EPR), pulsed EPR, and
superconducting quantum interference device (SQUID) magnet-
ometer measurements were performed on the samples. For 3a and
1
2
3
3
4
a
100
—
—
—
20
20
20
27
4
a, susceptibility measurements showed the presence of 0.96 6 10
27
mol of S = 1/2 spins per mg and 2.1 6 10 mol of S = 1/2 spins per
mg respectively (ESI{). Since Cu(II) is the only paramagnetic species
which can be found in these systems, these experiments indicated
—
100
80
70
0.61 wt% and 1.32 wt% for 3a and 4a respectively. These results,
combined with elemental analyses, allow us to establish that our
solids contain both Cu(I) and Cu(II) species in approximately a 1 : 1
a
—
b
b
10
b
a4b
—
80
1
Fig. 1 H NMR spectra of compounds 1 to 4. Typical peaks of BDC–
1
b
According to H NMR analysis. After treatment at 100 uC under
2
NH are marked by circles and triazolyl proton signals are indicated by
vacuum.
triangles.
4
106 | CrystEngComm, 2012, 14, 4105–4108
This journal is ß The Royal Society of Chemistry 2012

ratio. In order to further characterise these paramagnetic species,
CW-EPR spectra were recorded (Fig. 2). The EPR spectrum of 3a is
characteristic of a Cu(II) ion in one site with a well-defined
environment. In particular, copper hyperfine coupling is easily seen
Indeed, the nitrogen atoms on the triazolyl ring and the free
terminal amino group of 3a provide a good environment for
copper chelation as a N,N-donating group whereas only weakly
coordinating triazolyl moieties are present in 4a. We can thus
postulate the existence of both copper(I) and (II) triazolylmetha-
namine complexes anchored into the MOF 3a, as described in
Scheme 2.
34,35
and spectral features are well pronounced.
In sharp contrast with the previous result, the EPR spectrum
of 4a is broader and almost featureless with no visible copper
hyperfine coupling. In addition to the central line, this spectrum
exhibits a very broad feature which can be tentatively attributed to
copper clusters. All these characteristics point to a non-specific
Cu(II) species which can even be present in the framework as ill-
defined aggregates.
Compounds 3b and 4b were obtained by treating their parent
compounds 3a and 4a under vacuum at 100 uC overnight
(Table 1). Surprisingly, NMR spectra reveal that a partial
3
reduction of the aromatic –N moieties, to give the aromatic –
NH moieties, occurs during the treatment of 3a and leads to the
2
These conclusions can be substantiated by an additional pulsed
EPR experiment with a HYSCORE sequence. A HYSCORE
sequence is used to observe hyperfine coupling in paramagnetic
systems and is particularly suited for detecting nuclei with I > 1/2
formation of 3b (Fig. 1). Integration of signals shows that the new
ratio between the functional groups in 3b is azido : amino :
triazolyl = 70 : 10 : 20, the percentage of triazolyl being, as
expected, the same for both 3a and 3b. Nevertheless, the structural
integrity and the porosity of 3b are strongly affected by this
intrareticular reaction (see ESI{). The same procedure was applied
to 4a to give 4b. However, no sign of the azide reduction appeared
in the NMR spectrum of 4b. According to these results, the
reduction of the azido occurs more easily in 3a, which contains a
well-defined copper complex (Scheme 2).
14
spin, like N. It is able to sort three types of nuclei interacting with
the Cu(II) ion: strongly and weakly coupled nuclei and ‘‘distant’’
nuclei, which are characterized by very low hyperfine constants.
The latter correspond to peaks on the diagonal of the (+,+)
quadrant, whereas the strongly and weakly coupled nuclei
36,37
appear in the (2,+) and (+,+) quadrants, respectively.
The
HYSCORE spectrum of 3a is well defined and exhibits several
With copper-mediated reduction of azide into amines being
15–17
already reported under homogeneous conditions,
features which can be unambiguously attributed to at least one
we assumed
1
4
1
strongly coupled N and several weakly coupled H, some
that the solvent-free azide reduction proceeds via the involvement
of the MOF-supported Cu complex described in Scheme 2.
In order to assess our hypothesis, we performed model reactions
under homogeneous conditions. As a representative substrate, we
used dimethyl azidoterephthalate, similar to the MOF building
block, in the presence of copper salt and ligand in THF at room
temperature (Scheme 3).
features probably belonging to a coordinating water molecule
34,35,38
Fig. 3).
(
It was impossible to repeat these pulsed EPR
experiments for 4a where no spin echo signal can be detected. One
possible explanation is that Cu(II) ions are localized in non-specific
sites and ill-defined aggregates have very short relaxation times.
The results summarized in Table 2 show that copper(I) species
combined with an excess of N-containing ligands allow the
completion of the azide reduction (entries 2 and 5). Too low an
amount of amino ligand leads to uncompleted reactions (entries 3
and 4), and copper(II) precursors show no activity (entries 7 and 8).
It is noteworthy that neither copper(I) nor amine can alone
promote the reduction (entries 5 and 6) and that reducing the
Fig. 2 Comparison of CW-EPR spectra of 3a (black) and 4a (red).
Scheme 2 MOF-supported copper(I) triazolylmethanamine complex in
2 3
3a (R = NH or N ).
Fig. 3 X-band HYSCORE spectrum of 3a.
Scheme 3 Liquid-phase copper-mediated aryl azide reduction.
CrystEngComm, 2012, 14, 4105–4108 | 4107
This journal is ß The Royal Society of Chemistry 2012

7
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Table 2 Liquid-phase Cu-mediated reduction of dimethyl azidoter-
ephthalate
a
2
b
Entry
Cu source (eq.)
Ligand (eq.)
Yield (%)
1
2
3
4
5
6
7
8
a
Cu(ACN)
Cu(ACN)
Cu(ACN)
Cu(ACN)
Cu(ACN)
None
4
4
4
4
4
PF
PF
PF
PF
PF
6
6
6
6
6
(0.25)
(0.5)
(0.5)
(0.5)
(0.5)
Propylamine (5.5)
Propylamine (5.5)
Propylamine (2.75)
Propylamine (1)
None
Propylamine (5.5)
Propylamine (5.5)
Propylamine (5.5)
50
100
10
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,1
,1
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Cu(acac) (0.5)
2
CuO (0.5)
1
Reaction conditions: dimethyl azidoterephthalate (0.09 mmol) in
THF (3 mL) with copper and ligand stirred for 12 h at room
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1
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1
1
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4 6
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1
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2
0 Y. Goto, H. Sato, S. Shinkai and K. Sada, J. Am. Chem. Soc., 2008,
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1 M. Savonnet, D. Bazer-Bachi, N. Bats, J. Perez-Pellitero, E. Jeanneau,
1
2
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the reduction of azido occurs via the oxidation of the copper(I)
triazolylmethanamine complex in its vicinity, to give the
corresponding amino and N . After N removal, protons of Cu-
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2
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2
2
2
coordinated water molecules (Scheme 2) are transferred to the
remaining nitrogen to form the aryl amine. Indeed, Cu(II) dosing
using a SQUID magnetometer shows an increase of the quantity
of this ion under reaction conditions (100 uC under vacuum, see
ESI{) since Cu(II) quantity increases from 0.61 wt% to 0.64 wt%.
In conclusion, we report here for the first time the self-reactivity
of a MOF through the reaction between two of its functionalities
anchored on its walls. A MOF-supported copper complex,
introduced by click chemistry and characterized by EPR, was
found to reduce some of the remaining azido functions in the
MOF in the solid state by gentle heating under vacuum, the
temperature used being much lower than that of the azide self-
decomposition. Moreover, this case study could allow new insight
into the mechanism of the copper-mediated azide reduction.
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108 | CrystEngComm, 2012, 14, 4105–4108
This journal is ß The Royal Society of Chemistry 2012