Amino substituted Cu3(btc)2: A new metal-organic framework with a versatile functionality

Article abstract of DOI:10.1039/c2cc36220a

A new amino substituted tricarboxylate linker and the new metal-organic framework Cu₃(NH₂btc)₂ have been synthesised. The new MOF shows good adsorption properties and is suitable for postsynthetic modification to form an a

Full text of DOI:10.1039/c2cc36220a

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Amino substituted Cu3(btc)2: a new metal–organic framework
with a versatile functionalityw
¨
Katharina Peikert, Frank Hoffmann and Michael Froba*
Received 27th August 2012, Accepted 27th September 2012
DOI: 10.1039/c2cc36220a
A new amino substituted tricarboxylate linker and the new
metal–organic framework Cu3(NH2btc)2 have been synthesised.
The new MOF shows good adsorption properties and is suitable
for postsynthetic modification to form an amide functionalised
framework.
Cu3(btc)2 (also known as HKUST-1; btc = 1,3,5-benzene-
tricarboxylate) is one of the first and best-known MOFs. Since
its publication10 by Chui et al. in 1999 there have been
continuous efforts to improve the synthesis and activation
procedure of Cu3(btc)2. Most studies have focused on gas
adsorption properties.11 Ingleson et al. did PSM studies on
Cu3(btc)2 by coordinating 4-(methylamino)-pyridine (4-map)
to the Cu centres. Thus, they introduced accessible secondary
amine functionalities that react with nitric oxide (NO) to form
N-diazenium diolates (so-called NONOates).12 Since NO is an
extremely important biological signaling molecule, there is great
interest in developing NO storage and delivery materials.13
Besides the storage as NONOate a second central method for
storing NO in MOFs is the coordination to ‘‘open’’, i.e.
coordinatively unsaturated metal centres.
Metal–Organic Frameworks (MOFs) are crystalline, inorganic–
organic hybrid materials, which are characterised by their
porosity and the associated high specific surface areas.1
MOF structures can be tailored quite easily by combining
different metal ions or metal–oxygen clusters and organic
ligands of different size and shape. This results in a variety
of properties, such as tuneable pore sizes,2 large inner pore
volumes and specific chemical properties of the pore walls.3
MOFs can also be designed to contain ‘‘open’’ metal centres,
which exhibit high affinities towards certain gases. These
properties turn MOFs into prime candidates for use as gas
storage and separation materials.4
In spite of the great research interest in Cu3(btc)2, there is
only one report on functionalisation of the btc ligand. Very
recently, Cai et al. presented two copper frameworks con-
structed from H3btc that has been functionalised with methyl
and ethyl groups.14 Although the reaction conditions for the
MOF synthesis were the same as those reported for the
Cu3(btc)2 synthesis11a the resulting frameworks exhibit a
different topology.
The classical way to incorporate functional groups into a
MOF is the modification of the organic ligand with specific
substituents before synthesising the MOF itself. This approach
has resulted in MOFs with functional groups such as –Br,
–NH2, –CH3, –OH, –OC3H7, and –C2H4, known for example
from the IRMOF2 or MIL5 series. Another route for obtaining
functionalised MOFs is the postsynthetic modification (PSM).
Since the first detailed studies in 2007 by Wang and Cohen6
this route is a growing and promising approach in the area of
MOF chemistry. One of the advantages of this synthetic
methodology is the possibility to create functional groups
inside the MOF which are not stable under the solvothermal
conditions in which the majority of MOFs are synthesised
usually. Furthermore, it is possible to prepare topologically
identical, but functionally different frameworks.7 PSM studies
show that this approach can lead to improved gas adsorption8
and catalytic properties.9
Herein, we present the synthesis and characterisation of the
new amino substituted tricarboxylate linker 2-amino-1,3,5-
benzenetricarboxylate (NH2btc) as well as the synthesis, struc-
ture and properties of the resulting metal–organic framework
Cu3(NH2btc)2, which we called UHM-30 (UHM = University
of Hamburg Materials) (see Fig. 1). The new MOF is iso-
structural to Cu3(btc)2 and opens a way to a functionalised
Cu3(btc)2 series, as we offer amino groups inside the MOF on
the one hand and could show the accessibility of these groups
by PSM on the other hand. In this regard the amino group
exhibits great potential for further modification. In the literature
Department of Chemistry, Institute of Inorganic and Applied
Chemistry, University of Hamburg, Martin-Luther-King-Platz 6,
20146 Hamburg, Germany. E-mail: froeba@chemie.uni-hamburg.de;
Fax: +49 40 42838 6348; Tel: +49 40 42838 3100
w Electronic supplementary information (ESI) available: Synthetic
details, characterisation of all compounds, crystallographic details,
and adsorption isotherms. CCDC 896694. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc36220a
Fig. 1 Visualisation of the construction principle of Cu3(NH2btc)2.
c
11196 Chem. Commun., 2012, 48, 11196–11198
This journal is The Royal Society of Chemistry 2012

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a multitude of modification reactions of primary amino groups
in MOFs is reported. A few examples are the incorporation of
amides,15 N-diazenium diolates,16 alcohols17 and azides.18 In
contrast to the IRMOF and MIL series our structure enables
the combination of ‘‘open’’ metal sites and functional groups
in one material. In this context it should be possible to store
NO in this material via chemisorption as NONOate and via
physisorption at the ‘‘open’’ metal sites. Different release
kinetics for the chemisorbed and physisorbed NO species are
conceivable.
Fig. 2 Postsynthetic modification of Cu3(NH2btc)2 with acetic
anhydride.
The synthesis of the new linker could be accomplished in a
five-step reaction starting with 2-fluoro-1,3,5-trimethylbenzene.
Solvothermal reaction of NH2btc and Cu(NO3)2Á3H2O in
dimethylacetamide (DMA)–water (H2O) led to greenish crystals
of Cu3(NH2btc)2 in high yields.w
Single X-ray crystal structure analysis reveals that
Cu3(NH2btc)2 is isostructural to Cu3(btc)2.w According to
the three possible orientations of the linker during the incor-
poration into the framework the amino group shows a posi-
tional disorder over three sites, all with a site occupation factor
(s.o.f) of 1/3 (for details, see ESIw). To remove the solvent used
during the synthesis Cu3(NH2btc)2 was activated via solvent
exchange. Complete activation could be ensured by thermo-
gravimetric and differential thermal (TG-DTA) studies
coupled with mass spectrometry (Fig. S2, ESIw). The N2
physisorption measurement shows a typical type-I isotherm
(Fig. S4, ESIw). The analysis of the isotherm reveals a specific
surface area of SBET = 1834 m2 gÀ1 (calculated from the
adsorption branch and in the relative pressure interval from
0.001 to 0.025) and a micropore volume of Vpore = 0.69 cm3 gÀ1
(d o 2 nm calculated at p/p0 = 0.18). These values are very
similar to the values known for Cu3(btc)2.11c Both low (up to
1 bar) and high pressure (up to 45 bar) volumetric CO2 and
CH4 adsorption measurements were carried out at 298 K.
Under the same pressure conditions H2 adsorption isotherms
were measured at 77 K (Fig. S5 and S6, ESIw). The results are
summarised in Table 1. The physisorption measurements for
Cu3(NH2btc)2 show similar adsorption properties to those of
Cu3(btc)2. Whereas the results for CO2 and CH4 at 45 bar
show slightly lower adsorption capacities, the values at 1 bar
are slightly improved compared to those of Cu3(btc)2.19
To prove the accessibility of the amino groups for post-
synthetic modification the solvent exchanged MOF was trea-
ted with acetic anhydride to build an amide functionality
(Fig. 2). A typical reaction was carried out with 77.3 mg
MOF in CHCl3 at 55 1C for 24 h, followed by extensive
washing with methanol. The amount of acetic anhydride was
varied from 1 equivalent up to 20 equivalents relating to the
chemical amount of amino groups. The modification was
confirmed by nuclear magnetic resonance (NMR) spectroscopy
Fig. 3 1H NMR spectra of digested Cu3(NH2btc)2 (black) and modified
samples by the use of different amounts of acetic anhydride (coloured).
The blue circles mark the unmodified NH2btc, the red ones show the
modified linker. The signals are normalised to the peak at 8.5 ppm.
spectra of the modified samples show a highfield shift of the
aromatic proton signal and a corresponding signal for the
acetic methyl group (Fig. 3). By comparing the relative
integrated areas of the aromatic resonances between the
modified and unmodified linkers the degree of conversion of
the amino groups in the framework could be determined.
Depending on the concentration of acetic anhydride it is
possible to convert up to 92% of the amino groups. The
results are summarised in Table 2.
ESI-MS studies gave additional evidence of the modifica-
tion as the sodium adduct of the amide could be detected
(Fig. S7, ESIw). As evidenced by accompanied powder XRD
measurements (Fig. 4) the degree of crystallinity of the frame-
work decreases with increasing degree of conversion of the
amino groups. This observation is in agreement with the
results obtained for the BET surface areas of the function-
alised MOFs, which are summarised in Table 2. As the
measurements show the porosity of the framework is reduced
by increasing degree of conversion. The material treated with
1
and electrospray mass spectrometry (ESI-MS). The H NMR
Table 1 H2, CH4 and CO2 uptake in Cu3(NH2btc)2 at low and high pressures compared to the literature known values of Cu3(btc)2
Fluid
H2
CH4
CO2
Cu3(NH2btc)2 uptake 1 bar
Max. uptake
Cu3(btc)2 uptake 1 bar
Max. uptake
2.28 wt%
3.35 wt% (21 bar)
2.27 wt%19a
1.04 mmol gÀ1
5.26 mmol gÀ1
8.66 mmol gÀ1 (43 bar)
0.98 mmol gÀ1 (295 K)19b
—
11.7 mmol gÀ1 (23 bar)
4.69 mmol gÀ1 (295 K)19b
15.8 mmol gÀ1 (22 bar)19e
19d
3.53 wt% (22 bar)19c
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11196–11198 11197

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Table 2 Conversion degree of Cu3(NH2btc)2 to Cu3(NHCOCH3btc)2
depending on the acetic anhydride (Ac2O) concentration and the
resulting BET surface areas of Cu3(NHCOCH3btc)2
as NONOate and via physisorption at the ‘‘open’’ metal sites.
The preparations for NO adsorption, storage and release
experiments are in process.
This work was supported by the Deutsche Forschungsge-
meinschaft (DFG: Fr 1372/18-2) as part of the priority
program 1362 (Porous metal–organic frameworks). We thank
Matthias Rogaczewski for support in the laboratory.
NH2 : Ac2O
Conversiona (%)
SBET (m2 gÀ1
)
1 : 1
1 : 2
1 : 4
1 : 6
1 : 8
1 : 10
1 : 15
1 : 20
14
26
42
55
66
73
87
92
1149
1137
1124
943
481
640
—
Notes and references
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a
The percent conversion values are the arithmetic average of three
independent experiments.
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15 or 20 equivalents of acetic anhydride even loses its porosity
completely. This leads to the assumption that the framework is
destructed by the treatment with higher amounts of acetic
anhydride than 10 equivalents. This result is not very surprising,
since acetic acid is built during the PSM reaction and Cu3(btc)2
is known to be not stable in acid media. However, it is possible
to get materials with a tuneable degree of conversion up to
70%, which still show an acceptable porosity. The thermal
stability of the modified samples slightly increases relative to
the unmodified Cu3(NH2btc)2 (Fig. S8, ESIw).
In summary, we have successfully synthesised a new tri-
carboxylic linker and the resulting amino substituted MOF
Cu3(NH2btc)2. The MOF shows good adsorption properties
for a series of gases. The accessibility of the amino groups
could be successfully proved by postsynthetic modification
with acetic anhydride. The amount of acetic anhydride was
varied from 1 equivalent up to 20 equivalents relating to the
amount of amino groups. The modification was verified by
1H NMR and ESI-MS analysis. XRD and N2 physisorption
studies showed the breakup of the framework by the use of
acetic anhydride concentrations higher than 10 equivalents.
However, it was possible to get porous materials with a
tuneable degree of conversion up to 70%. The combination
of ‘‘open’’ metal sites and accessible amino functionalities in
this new MOF reveals great potential for use as NO storage
material, as it should be possible to store NO via chemisorption
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c
11198 Chem. Commun., 2012, 48, 11196–11198
This journal is The Royal Society of Chemistry 2012