Scorpionate-type coordination in MFU-4l metal-organic frameworks: Small-molecule binding and activation upon the thermally activated formation of open metal sites

Article abstract of DOI:10.1002/anie.201310004

Postsynthetic metal and ligand exchange is a versatile approach towards functionalized MFU-4l frameworks. Upon thermal treatment of MFU-4l formates, coordinatively strongly unsaturated metal centers, such as zinc(II) hydride or copper(I) species, are gene

Full text of DOI:10.1002/anie.201310004

.
Angewandte
Communications
DOI: 10.1002/anie.201310004
Small-Molecule Binding
Scorpionate-Type Coordination in MFU-4l Metal–Organic
Frameworks: Small-Molecule Binding and Activation upon the
Thermally Activated Formation of Open Metal Sites**
Dmytro Denysenko, Maciej Grzywa, Jelena Jelic, Karsten Reuter, and Dirk Volkmer*
Abstract: Postsynthetic metal and ligand exchange is a versatile
approach towards functionalized MFU-4l frameworks. Upon
thermal treatment of MFU-4l formates, coordinatively strongly
unsaturated metal centers, such as zinc(II) hydride or cop-
another well-established MOF family with interesting chemi-
sorption properties. CPO-27-Fe, for example, showed rever-
sible chemisorption of O2 and N2 with initial heats of
À1
adsorption of 41 and 35 kJmol , respectively. However, O
2
I
per(I) species, are generated selectively. Cu -MFU-4l prepared
chemisorption was reversible at À628C, whereas at room
in this way was stable under ambient conditions and showed
fully reversible chemisorption of small molecules, such as O2,
N , and H , with corresponding isosteric heats of adsorption of
temperature the irreversible formation of a dimeric iron(III)
[
8]
peroxide was observed. A weak chemisorption of hydrogen
with well-defined 1:1 stoichiometry (metal sites/H ) was
2
2
2
À1
5
3, 42, and 32 kJmol , respectively, as determined by gas-
reported for CPO-27-Ni, with an initial heat of adsorption
À1
sorption measurements and confirmed by DFT calculations.
of 13.5 kJmol , which ranks among the highest values
I
[9]
Moreover, Cu -MFU-4l formed stable complexes with C H
reported for a MOF material. In contrast, compound 5,
2
4
and CO. These complexes were characterized by FTIR
spectroscopy. The demonstrated hydride transfer to electro-
philes and strong binding of small gas molecules suggests these
novel, yet robust, metal–organic frameworks with open metal
sites as promising catalytic materials comprising earth-abun-
dant metal elements.
presented herein, shows fully reversible binding of O , N , and
2
2
H molecules under ambient conditions. To the best of our
2
knowledge, no copper(I)-containing metal–organic frame-
work has previously been shown to possess similar chemi-
sorption properties, although IR investigations on monomeric
CuCl embedded in an Ar matrix gave first experimental proof
I
of the formation of a Cu –H complex, in which H binds in
2
2
[
10]
[11]
M
etal–organic frameworks with open metal sites have been
a side-on coordination mode. The chemisorption of N2
[1]
[12]
proposed for many different applications, such as catalysis,
and H2
on copper-exchanged zeolites has also been
[
2]
[3]
[4]
gas storage and capture, separation, and sensing. How-
ever, most known MOFs with free metal centers are only able
to bind typical Lewis bases, such as CO or H O, whereas the
described previously. For these microporous compounds,
I
I
Cu –N and Cu –H adducts were characterized by spectro-
2
2
scopic methods (mainly IR spectroscopy). However, presum-
2
2
I
activation of unreactive small molecules, such as O , N , and
ably as a result of the relatively low Cu content, no accurate
2
2
H , still remains a challenge.
values of N or H binding energies were reported.
2
2
2
[
5]
[5]
[6]
Molecular dioxygen, dinitrogen, and dihydrogen
We herein describe a new approach towards such highly
active metal centers in a metal–organic framework based on
complexes have attracted significant attention in coordination
chemistry. Only very few stable MOFs with open metal
centers have been described that are able to bind these
2
À
MFU-4l, constructed from bistriazolate BTDD ligands and
6
+
[13]
{Zn Cl } building units (Figure 1). Coordination frame-
5
4
II
2
molecules. A structural analogue of CuBTC, Cr (BTC) ,
works derived from the MFU-4 family show exceptional
robustness against thermal and hydrolytic decomposition
and are therefore attractive for technical applications, such as
3
II
[14]
containing dinuclear Cr paddle-wheel units, was shown to
[
7]
bind oxygen at room temperature. However, after the first
adsorption step, only approximately 82% of the total amount
of oxygen could be desorbed, thus indicating that 18% of the
O2 binds irreversibly. CPO-27 (also termed MOF-74) is
[
*] D. Denysenko, Dr. M. Grzywa, Prof. Dr. D. Volkmer
Universitꢁt Augsburg, Institut fꢂr Physik
Lehrstuhl fꢂr Festkçrperchemie
Universitꢁtsstrasse 1, 86159 Augsburg (Germany)
E-mail: dirk.volkmer@physik.uni-augsburg.de
Dr. J. Jelic, Prof. Dr. K. Reuter
Technische Universitꢁt Mꢂnchen
Chair of Theoretical Chemistry and Catalysis Research Center
Lichtenbergstrasse 4, 85747 Garching (Germany)
[**] We gratefully acknowledge funding by the priority program 1362
“
Porous Metal–Organic Frameworks (MOFs)” of the Deutsche
Figure 1. Solvothermal synthesis of MFU-4l. Atom colors: Zn (octahe-
dral) pink, Zn (tetrahedral) yellow, Cl green, O red, N blue, C gray, H
white. The black square outlines the cubic unit cell of MFU-4l (space
Forschungsgemeinschaft (DFG).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310004.
ꢀ
group: Fm3m, a=31.057 ꢀ; see Ref.[13]).
5
832
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Angew. Chem. Int. Ed. 2014, 53, 5832 –5836

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Chemie
[
15]
[16]
gas separation and catalysis. Their fundamental building
Thermally activated decomposition of bulk metal for-
[
17]
units, so-called Kuratowski units, feature coordination sites
mates has been reported previously. The decomposition of
zinc formate, for example, leads to ZnO, whereas copper(II)
[
18]
reminiscent of scorpionate-type complexes. In contrast to
the latter, however, the coordination sites in MFU-4 frame-
works do not have to be protected from unfavorable side
reactions by bulky substituents R (Figure 2a). As a result,
frameworks can be generated that contain free metal centers
[
19]
formate yields the reduced products Cu O and Cu. Our
2
studies show that the thermally induced decomposition of
coordinated formate ligands is a versatile approach for
generating reactive metal sites in MFU-4l-type porous
materials. Thus, the decomposition of 1 under flowing nitro-
gen gas took place in the temperature range of 190–2808C, as
shown by the curve derived from thermogravimetric analysis
(TGA; see Figure S1 in the Supporting Information),
whereby the crystalline order of the framework remained
unchanged (see Figure S27). FTIR spectra showed clearly the
formation of ZnÀH species (i.e. 2; Figure 3). Thus, a character-
istic ZnÀformate stretching vibration, which appeared as
À1
a shoulder in the FTIR spectrum at 1610 cm , disappeared
completely when compound 1 was heated at 3008C (Figure 4).
Figure 2. a) Scorpionate complex. b) Part of the fundamental building
unit (Kuratowski unit) of MFU-4-type frameworks comprising coordina-
tively unsaturated, substrate-accessible coordination sites (M=(tran-
sition) metal, L=ligand, R=bulky substituent, for example, tBu).
with (almost) unconstrained substrate accessibility (Fig-
ure 2b). With the aim of constructing catalytically active
frameworks, suitable MFU-4l derivatives can be used as
starting materials, within which the required coordinatively
highly unsaturated metal centers can be formed. The first
example, as shown below, establishes the formation of
a highly reactive zinc(II) hydride species 2, generated from
MFU-4l-formate (1), which was readily obtained from MFU-
Figure 4. FTIR spectra of 1 and 2.
4l through chloride–formate ligand exchange (Figure 3,
path a). Alternatively, postsynthetic metal metathesis, that
II
I
is, the replacement of Zn ÀCl units with Cu ions (Figure 3,
Three new bands appeared upon heat treatment: at 1793, 482,
I
À1
path b), led to threefold-coordinated unsaturated Cu sites
and 453 cm . On the basis of DFT calculations (using the
[
20]
[21]
that showed reversible and remarkably strong chemisorption
of small molecules (i.e. H , O , N , and C H ) under ambient
PBE functional), corrected for dispersive interactions,
we assign these bands to a ZnÀH stretch mode and two ZnÀH
bending modes, respectively (see Figures S29 and S30).
Molecular hydride complexes of zinc scorpionates, pre-
2
2
2
2
4
conditions.
pared in a different way (either starting from ZnH or by
2
treatment of a ZnÀF precursor with Et SiH), have been
3
reported previously. These zinc hydride scorpionates were
shown to react with various electrophiles, such as CO , CS ,
2
2
[22]
RNCS, RI, and CH COCl. To demonstrate a comparable
3
reactivity of the MFU-4l hydride derivative 2, we added
PhCOCl to a suspension of 2 in benzene at room temperature
and obtained PhCHO as the main product, as determined by
GC/MS analysis (see Figure S23). This test case indicates the
potential of the hydride-containing MFU-4l-type framework
2
as a hydride-transfer and reducing agent.
We previously showed that all four peripheral Zn ions of
II
II
each Kuratowski unit in MFU-4l could be substituted by Co
upon heating with CoCl in DMF. An equivalent approach
with CuCl led to Cu -MFU-4l (3), in which, however, on
[16]
2
II
2
Figure 3. Reaction paths leading to MFU-4l derivatives 2 and 5 with
active metal sites. Reaction conditions: i) HCOOLi/MeOH, room
average only two out of four tetrahedrally coordinated Zn
II
II
centers per Kuratowski unit are replaced with Cu ions. Cu -
MFU-4l-formate (4) could then be readily prepared by
a subsequent ligand-exchange reaction, in analogy to the
temperature; ii) 3008C, 30 min, under vacuum; iii) CuCl , DMA, 608C;
2
iv) HCOOLi/MeOH, room temperature; v) 1808C, 1 h, under vacuum.
DMA=N,N-dimethylacetamide.
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5833

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synthesis of 1 (Figure 3). The TGA curve (see Figure S1)
adsorption, as calculated by using the Clausius–Clapeyron
À1
shows that 4 decomposes in two steps: The first weight loss at
equation, is 32 kJmol , again in very good agreement with
II
À1
1
20–1808C corresponds to the decomposition of Cu -formate
the DFT-calculated value of 25 kJmol (Table 1). The CuÀH
2
I
units, and the second weight loss at 190–2808C corresponds to
the transformation of Zn-formate units. Sorption and spec-
troscopic studies showed that thermal activation of 4 led
bonding energy in Cu -MFU-4l (5) is thus significantly higher
I
Table 1: Experimental and DFT-calculated isosteric heats of adsorption
finally to a MFU-4l-type framework with free Cu centers,
À1
I
in kJmol in Cu -MFU-4l (5).
I
termed Cu -MFU-4l (5; Figure 3). As proven by gas-sorption
H2
N2
O2
C2H
4
CH4
measurements, 5 formed very stable complexes with CO and
C H .
experiment 32.3Æ0.4 41.6Æ0.6 52.6Æ0.6 88Æ4 14.9Æ0.4
DFT-B3LYP 25 44 46 84 15
2
4
3
À1
The quantity of bound species (ca. 38 cm g ; see
Figures S13 and S14) was nearly identical for both gases and
very close to the calculated value (37.8 cm g ) based on the
analytically determined Cu content, thus suggesting the
3
À1
À1 [24]
than typical values for H physisorption (5–10 kJmol ),
2
I
I
formation of stable 1:1 Cu ÀCO and Cu ÀC H coordination
but also significantly lower than in ionic metal hydrides or
Kubas-type metal dihydrogen complexes, which normally
2
4
units. The corresponding stretch modes of coordinatively
bound CO and C H molecules were observed in the FTIR
[
25]
require high desorption temperatures of 150–4008C. For
2
4
À1
spectra at 2081 and 1541 cm , respectively (Figure 6b), in
good agreement with the DFT-calculated values (2056 and
example, an activation energy for hydrogen desorption of
À1
[26]
156 kJmol has been measured for Mg NiH . Intermetallic
2 4
À1
1
517 cm , respectively). Both vibrational frequencies are
compounds, such as LaNi , show significantly lower desorp-
5
shifted to lower wavenumbers, as compared to those of the
tion temperatures (as low as room temperature); however,
À1
À1
[25]
free gas molecules (2143 cm for CO and 1623 cm for
they feature only low hydrogen-storage capacities.
[
23]
I
C H ). One of the most interesting properties of 5 is the
In fact, the hydrogen binding at the Cu centers matches
2
4
unprecedented strong and reversible chemisorption of H .
well a suggested optimal binding energy of approximately
25 kJmol for hydrogen storage, thus enabling the accumu-
2
3
À1
À1
The amount of chemisorbed H (38 cm g ; Figure 5a; see
2
also Figure S13) was the same as for CO and C H , which
lation and release of hydrogen at close to ambient temper-
ature. Although the amount of chemisorbed hydrogen in 5
2
4
I
[24]
indicates a 1:1 stoichiometry of Cu ÀH coordination units.
2
The adsorption/desorption isotherm measured at 273 K
demonstrates full reversibility of hydrogen binding. Under
is not high (0.34 wt%), owing to the low volumetric density of
I
Cu centers, this degree of chemisorption is possible at close to
equilibrium conditions (1 bar H2 pressure), approximately
ambient temperature and pressure, thus indicating the great
potential of copper(I)-containing materials with a higher
density of active metal sites for hydrogen storage. Higher
storage capacities have only been observed either at low
I
8
0% of all Cu centers formed a 1:1 hydrogen complex at this
temperature (Figure 5b). The isosteric heat of hydrogen
[
27]
temperature or at high pressure. For example, CPO-27-Ni
[28]
showed less than 0.3 wt% H uptake at 298 K and 65 bar.
2
Compound 5 also showed reversible chemisorption of N and
2
O2 with the same 1:1 stoichiometry (see Figure S13) and
À1
isosteric heats of adsorption of 42 and 53 kJmol , respec-
tively (Figure 6a). In contrast, methane showed only physi-
À1
sorption, as indicated by the low value of 15 kJmol
determined for the isosteric heat of adsorption (Figure 6a;
see also Figure S14). This value was also confirmed by DFT
calculations (Table 1), which showed only dispersive inter-
I
actions between Cu centers and CH (see Figure S31). The
4
I
fact that methane does not bind to Cu centers suggests 5 as
a potential candidate for CH /N2 separation with highly
4
selective N uptake. In the case of normal physisorption, most
2
frequently observed in porous materials, CH is adsorbed
4
[29]
preferentially over N . The exceptionally high difference of
3 kJmol between the heats of adsorption of C H and CH
2
À1
7
2
4
4
indicates that 5 is a very promising material for alkene/alkane
separation or alkene capture. The isosteric heat of ethylene
adsorption on CPO-27-Fe, for comparison, is about
À1
4
5 kJmol and is the highest within the series of CPO-27
2
+
2+
frameworks containing different metal ions (Mg , Mn ,
2
+
2+
2+
2+
Fe , Co , Ni , and Zn ), whereas the heat of CH
4
À1 [30]
adsorption on CPO-27-Fe is 20 kJmol . As summarized
in Table 1, all isosteric heats of adsorption were very well
reproduced in DFT calculations, thus confirming the picture
Figure 5. a) Hydrogen-adsorption isotherms on 5 at 163–193 K.
b) Hydrogen-adsorption/desorption isotherm on 5 at 273 K.
5
834
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Figure 7. Binding geometries for a) H , b) N , c) O , and d) C H at the
2
2
2
2
4
I
Cu sites within the Kuratowski unit of MFU-4l, as determined by DFT
calculations (interatomic distances in ꢀ).
In summary, we have provided sound experimental and
theoretical evidence for the synthesis, structure, and reactivity
of robust metal–organic frameworks featuring open metal
sites. The reactivity of these frameworks, which are derived
from the MFU-4l structural family, towards small molecules
was demonstrated by hydride transfer to electrophiles and by
strong binding of small gas molecules. Since both processes
are fundamental steps of catalytic transformation processes,
further investigations in this direction should be highly
Figure 6. a) Dependence of isosteric heat of adsorption on loading for
different gases on 5. b) FTIR spectra of CO and C H complexes of 5.
2
4
I
I
of highly reactive and accessible Cu centers in Cu -MFU-4l
5).
(
We further note that 5 is air-stable (meaning that it is not
oxidized irreversibly or destroyed in air like many copper(I)-
containing compounds) and changes its color reversibly from
I
slightly yellow (in a vacuum) to dark-gray (in air), which
rewarding. Selective chemisorption properties of Cu centers
are of great interest for the storage, separation, and sensing of
small gas molecules.
I
indicates the reversible binding of O molecules at the Cu
2
centers. Quantitative information about this process was
gleaned by UV/Vis spectroscopy (see Figure S3). The binding
of nitrogen was followed by diffuse reflectance infrared
Fourier transform (DRIFT) spectroscopy. The experimental Experimental Section
À1
frequency for the IR-active NÀN bond stretch (2242 cm ; see
Detailed synthetic procedures and characterization, TGA, XRPD,
I
and GC/MS data, FTIR and UV/Vis/NIR spectra, gas-sorption
isotherms, crystallographic data, and details of the DFT calculations
are provided in the Supporting Information. CCDC 971404 (1),
Figure S10) is typical for Cu ÀN adducts, as previously
2
[
11]
described,
(
and close to the DFT-calculated value
À1
2215 cm ). As observed for CO and C H , the NÀN
2
4
971402 (3), and 971403 (4) contain the supplementary crystallo-
vibrational frequency was shifted to lower wavenumbers as
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
À1 [23]
compared to that of the free N molecule (2331 cm ).
2
I
The DFT-calculated geometries of the formed Cu –H ,
2
I
I
I
Cu –N , Cu –O , and Cu –C H complexes in the Kuratowski
unit are presented in Figure 7. H binds in an h (side-on)
2
2
2
4
2
Received: November 18, 2013
Revised: March 11, 2014
Published online: May 20, 2014
2
mode with a HÀH bond distance of 0.83 ꢀ and a CuÀH bond
1
distance of 1.66 ꢀ (Figure 7a). N binds in an h (end-on)
2
mode with a NÀN bond distance of 1.12 ꢀ and a CuÀN bond
Keywords: chemisorption · copper · hydrides · hydrogen ·
metal–organic frameworks
distance of 1.83 ꢀ; the Cu-N-N angle is 1808 (Figure 7b). O
.
2
1
also binds in an h (end-on) mode with an OÀO bond distance
of 1.29 ꢀ, a CuÀO bond distance of 1.92 ꢀ, and a Cu-O-O
2
angle of 105.78 (Figure 7c). C H forms an h (side-on)
2
4
complex with a CÀC bond distance of 1.39 ꢀ and a CuÀC
bond distance of 2.03 ꢀ (Figure 7d). All atomic distances
within the bound species are longer than in the free molecules
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