Coordination frameworks assembled from CuII ions and H2-1,3-bdpb ligands: X-ray and magneto structural investigations, and catalytic activity in the aerobic oxidation of tetralin

Article abstract of DOI:10.1039/c4dt01880j

The syntheses and crystal structures of H₂-1,3-bdpb·MeOH, [Cu^II₂(1,3-bdpb)(OCH₃)₂] (CFA-5) and [Cu^ICl(H₂-1,3-bdpb)] (H₂-1,3-bdpb = 1,3-bis(3,5-dimethyl-1H-pyrazol-4-yl)benzene) are described. The copper(ii) containing metal-organic framework (termed Coordination Framework Augsburg University-5, CFA-5) crystallizes in the trigonal crystal system, within the space group R3 (no. 148) and the unit cell parameters are as follows: a = 26.839(3), c = 15.8317(16) ?, V = 9876.2(19) ?³. CFA-5 features a two-fold interpenetrated 3-D microporous framework structure of cross-linked wheel-shaped {Cu^II(pz)(OMe)}₁₂ fundamental building units, each containing twelve copper(ii) ions, μ₂-bridging MeO^- groups and pyrazolate (pz^-) ligands. Replacing copper(ii) acetate by copper(ii) chloride in the synthesis leads to compound [Cu^ICl(H₂-1,3-bdpb)], which crystallizes in the orthorhombic crystal system, within the space group Pnma (no. 62) and the unit cell parameters are as follows: a = 6.1784(8), b = 6.1784(8), c = 6.1784(8) ?, V = 1583.8(4) ?³. In contrast to the former compound, CuCl(H₂-1,3-bdpb) is a non-porous compound consisting of Cu^I-Cl zigzag chains expanding in the direction [100] and H₂-1,3-bdpb ligands. CFA-5 is characterized by elemental and thermogravimetric analyses, variable temperature powder X-ray diffraction and IR-spectroscopy; and its porosity and magnetic properties are described in detail. CFA-5 shows a promising catalytic activity in the heterogeneously catalyzed aerobic oxidation of tetralin, which is compared with other catalytically active metal-organic frameworks.

Full text of DOI:10.1039/c4dt01880j

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PAPER
II
Coordination frameworks assembled from Cu
ions and H -1,3-bdpb ligands: X-ray and magneto
structural investigations, and catalytic activity in
2
Cite this: Dalton Trans., 2014, 43,
16846
the aerobic oxidation of tetralin†
a
b
a
a
Maciej Grzywa, Christof Geßner, Björn Bredenkötter, Dmytro Denysenko,
c
c
b
a
Jan van Leusen, Paul Kögerler, Elias Klemm and Dirk Volkmer*
II
I
The syntheses and crystal structures of H
-1,3-bdpb)] (H
2
-1,3-bdpb·MeOH, [Cu
2
(1,3-bdpb)(OCH
)
3 2
] (CFA-5) and [Cu Cl-
(H
2
2
-1,3-bdpb = 1,3-bis(3,5-dimethyl-1H-pyrazol-4-yl)benzene) are described. The copper(II)
containing metal–organic framework (termed Coordination Framework Augsburg University-5, CFA-5)
crystallizes in the trigonal crystal system, within the space group R 3¯ (no. 148) and the unit cell parameters
3
are as follows: a = 26.839(3), c = 15.8317(16) Å, V = 9876.2(19) Å . CFA-5 features a two-fold inter-
II
penetrated 3-D microporous framework structure of cross-linked wheel-shaped {Cu (pz)(OMe)}12 funda-
mental building units, each containing twelve copper(II) ions, μ
2
-bridging MeO− groups and pyrazolate
–
(
[
(
pz ) ligands. Replacing copper(II) acetate by copper(II) chloride in the synthesis leads to compound
I
2
Cu Cl(H -1,3-bdpb)], which crystallizes in the orthorhombic crystal system, within the space group Pnma
no. 62) and the unit cell parameters are as follows: a = 6.1784(8), b = 6.1784(8), c = 6.1784(8) Å,
3
V = 1583.8(4) Å . In contrast to the former compound, CuCl(H
2
-1,3-bdpb) is a non-porous compound
I
consisting of Cu –Cl zigzag chains expanding in the direction [100] and H
2
-1,3-bdpb ligands. CFA-5 is
Received 23rd June 2014,
Accepted 29th August 2014
characterized by elemental and thermogravimetric analyses, variable temperature powder X-ray diffrac-
tion and IR-spectroscopy; and its porosity and magnetic properties are described in detail. CFA-5 shows a
promising catalytic activity in the heterogeneously catalyzed aerobic oxidation of tetralin, which is
compared with other catalytically active metal–organic frameworks.
DOI: 10.1039/c4dt01880j
www.rsc.org/dalton
within the framework structures suggest a host of applications
in the field of heterogeneous catalysis in particular.
On searching for suitable MOF candidates for catalytic oxi-
Introduction
1
a,7
In the last decade the major driving force for the continued
interest in metal–organic frameworks (MOFs) has been the dation processes, copper-containing frameworks seem to be
promising technical applications of these materials ranging particularly interesting owing to the fact that this element is
1
2
3
from gas storage and separation, drug delivery, sensing to present in the active site of many metalloenzymes, such as
4
8
electrochemistry. The large specific surface areas of up to copper-containing oxidases or oxygenases. Biologically inspired
140 m g , open pores with apertures of up to 98 Å, the MOF catalysts trying to mimic structural and functional aspects
2
−1 5
6
7
high thermal stability and a high volume density of active sites of these enzymes could be applied for wide-ranging synthetic
applications in the oxidation of organic intermediates contain-
ing non-activated C–H-bonds. However, until now, only a few
reports have appeared in the literature where Cu-MOFs were
a
Chair of Solid State and Materials Chemistry, Institute of Physics,
successfully applied as oxidation catalysts. In particular, the Cu-
Augsburg University, Universitätsstr. 1, 86159 Augsburg, Germany.
E-mail: dirk.volkmer@physik.uni-augsburg.de
9
catalyzed hydroxylation of phenol, oxidation of trimethylsilyl
1
0
b
enolates to α-hydroxy ketones, allylic oxidation of cyclohex-
Institute of Chemical Technology, Faculty of Chemistry, University of Stuttgart,
1
1
Pfaffenwaldring 55, 70569 Stuttgart, Germany
enes, cross-dehydrogenative coupling reaction of ethers with
c
12
Institute of Inorganic Chemistry RWTH Aachen University, Landoltweg 1,
2-carbonyl-substituted phenols, and oxidation of benzene
5
†
2074 Aachen, Germany
Electronic supplementary information (ESI) available: Atomic coordinates,
13
derivatives and benzylic compounds have been described.
Some years ago we had started the development of Cu-con-
taining MOF catalysts containing pyrazole ligands for the
purpose of introducing oxygen functions into the C–H bonds
bond lengths and angles, packing diagrams, IR and UV/Vis spectra. CCDC
006742–1006744. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt01880j
1
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1
4
of organic substrates. Our previous studies on the reactivity
of CFA-2 (a Cu(I)-containing MOF) toward molecular oxygen
for instance have shown that this compound is stable during
oxidation and reduction of the Cu ions, suggesting its usage in
liquid-phase oxidation reactions. The exploration of pyrazo-
late-based Cu-MOFs follows the simple rational and experi-
mental observation that this ligand forms strong coordinative
bonds with Cu(I) and Cu(II), thus leading to robust coordi-
nation polymers. Similar to carboxylate-type ligands, pyrazo-
II
2 2
Scheme 1 Synthesis routes to CFA-5 [Cu (1,3-bdpb)(OMe) ] and
2
lates are known to form μ -bridges between metal ions. Unlike
I
2 22 4
[Cu Cl(H -1,3-bdpb)]; L = C18H N .
the former, the hydrolytic stability of N-heterocyclic ligand-
based frameworks containing the latter, i.e. electron-rich tran-
sition metal ions, is exceptionally high which has been convin-
1
5
Replacing copper(II) acetate by copper(II) chloride in the syn-
thesis leads to the compound [Cu Cl(H -1,3-bdpb)] (Scheme 1).
cingly demonstrated. More speculatively, we might anticipate
that pyrazolate heterocycles could mimic the coordination of
Cu ions by histidine (= imidazolate) ligands, a coordination
motif which is frequently observed in (multi-)copper oxidase
I
2
Single crystal structure analyses
1
6
enzymes, thus opening routes towards biomimetic catalysis.
2 22 4 3 2
H -1,3-bdpb·MeOH (C16H N ·CH OH). H -1,3-bdpb·MeOH
μ
2
-Bridging pyrazolate ligands have been frequently used to crystallizes in the monoclinic crystal system within the space
1
7
generate trinuclear copper (I/II) [Cu (μ -pz) ] metallocycles,
group P2 /m (no. 11). Detailed description of the crystal struc-
1
3
2
3
which later on were introduced as secondary building units ture of H
2
-1,3-bdpb·MeOH is presented in ESI.†
(1,3-bdpb)(OCH ] (CFA-5). CFA-5 crystallizes in the tri-
locyclic coordination units of the type [{Cu (μ-OH)(μ -pz)} ], gonal crystal system within the space group R ˉ3 (no. 148). The
1
4,18
(SBUs) into metal–organic frameworks.
Polynuclear metal-
[Cu
2
3 2
)
II
2
n
1
9
n = 6, 8, 9, 12 and 14, have been described, which might asymmetric unit consists of eighteen carbon, four nitrogen,
become SBUs of the as yet undiscovered MOFs. One such two oxygen and two copper atoms. An Ortep style plot of the
example, CFA-5, a novel MOF which contains a dodecanuclear asymmetric unit of CFA-5 with atom labels is shown in the
II
{
Cu (pz)(OMe)}12 SBU is presented in this article. CFA-5 was ESI, Fig. S3.† CFA-5 features a 3-D two-fold interpenetrated
characterized by elemental and thermogravimetric analyses, microporous structure. The twelve metal atoms are connected
−
X-ray diffraction and IR-spectroscopy and its porosity and mag- by twelve μ -bridging MeO and by twelve pyrazolate moieties
2
2
−
netic properties have been studied in detail. Additionally, the belonging to 1,3-bdpb ligands, respectively, creating dodeca-
results of catalytic tests of CFA-5 in the aerobic oxidation of nuclear copper(II) rings, as shown in Fig. 2a. In the literature
tetralin are presented and compared to other Cu-MOFs.
several examples of copper pyrazolate structures created from
1
9
four up to tetradecanuclear copper rings are known. To the
best of our knowledge, CFA-5 represents the first example of a
copper-containing MOF in which a dodecanuclear copper(II)
SBU was structurally characterized. In these SBUs, each Cu(II)
center exhibits an unusual distorted tetrahedral coordination
geometry (as shown in Fig. 2b). The coordination geometry of
each Cu(II) (N,N′,O,O′) subset lies between that of the tetra-
hedrally coordinated Cu(I) ions and that of Cu(II) complexes
featuring a square planar coordination environment. The Cu–
N distances range between 1.924(5) and 1.963(5) Å, whereas
the Cu–O distances range between 1.904(4) and 1.929(5) Å (see
Table 1). These values are in good agreement with those found
Results and discussion
Syntheses and characterization
2
H -1,3-bdpb (1,3-bis(3,5-dimethyl-1H-pyrazol-4-yl)benzene) was
synthesized according to the published procedure. The com-
pound CFA-5 was obtained as green hexagonal prism crystals
20
2
(Fig. 1) after heating a MeOH solution of H -1,3-bdpb and
copper(II) acetate hydrate in the presence of 2,6-lutidine. By
applying microwave irradiation instead of conventional heating
the reaction time was reduced drastically, from 3 d to 25 min.
1
9
in structurally related copper compounds.
Placing a least-squares plane through the twelve Cu(II) ions
2
−
of each SBU, six 1,3-bdpb ligands (blue and purple ligands
in Fig. 2a) are found above and the six remaining ones (black
and green) are placed below this plane. Among the twelve co-
ordinating pyrazolate moieties six are almost perpendicular to
the plane passing through the Cu centers (blue and green
ligands; the angle between the Cu12 plane and a least-squares
plane passing through the pyrazole ring is equal to 79.8(1)°).
2
−
Pyrazole moieties of the other six 1,3-bdpb ligands (black
and purple) each include a more flatter angle of 46.71(9)° com-
−
Fig. 1 SEM image of CFA-5 (left) and optical micrograph (right).
pared with the {Cu } plane. The μ -bridging MeO ligand is
1
2
2
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II
Fig. 2 (a) Wheel-shaped SBU of CFA-5, a framework featuring dodecanuclear Cu(II) coordination units showing cross-sectional non-bonding Cu ⋯-
II
Cu distances. (Symmetry transformations used to generate equivalent Cu atoms: (i) –x + y, −x, z; (ii) −y, x − y, z; (iii) −y + 2/3, x − y + 1, 3z + 1/3.) (b)
2
Selected part of the dodecanuclear SBU in CFA-5 showing the μ -bridging modes of the coordinated pyrazolate and methoxide ligands within a
II
dodecanuclear {Cu (pz)(OMe)}12 ring. (Symmetry transformations used to generate equivalent Cu atoms: y − 1/3,−x + y + 1/3,−z + 1/3.) (c) Crystal
packing diagram of CFA-5 including the positions of open channels running along the c direction of the crystal lattice. (d) View perpendicular to the
channel axis onto a packing model of CFA-5, including subsets of the two symmetry-related interpenetrating frameworks which are shown in black
2−
and red color. (e) Packing diagram of CFA-5 showing the two-fold interpenetrated porous 3D framework (methyl groups from 1,3-bdpb ligands
were omitted for clarity).
2
−
always found opposite to the 1,3-bdpb ligand that bridges expanding in the c-direction of the crystal lattice (see Fig. 2c
the same pair of Cu(II) ions. Of the twelve methoxides, six are and d). Taking the van der Waals radii of hydrogen atoms
pointing their methyl groups towards the center of the dodeca- (1.2 Å) into account, the narrowest channel diameter calcu-
nuclear SBU, thus leading to a hydrophobic internal channel lated between the hydrogen atoms of the MeO− groups is
surface.
4.5 Å, whereas the pore width, calculated between carbon
II
Defining the wheel-shaped {Cu (pz)(OMe)} fragment as atoms of the methyl groups and pyrazolate rings, is 5.86 Å (see
1
2
SBU of CFA-5, these SBUs are connected by the meta-substi- Fig. S4, ESI†). An estimation with the program SQUEEZE
2
1
tuted benzene ring of the 1,3-bis(3,5-dimethyl-1H-pyrazol-4-yl)- reveals that the initial solvent’s accessible void volume is
3
benzene ligand, thus creating one-dimensional channels 1213.3 Å , which is 12.3% of the unit cell volume (9876.2(19)
16848 | Dalton Trans., 2014, 43, 16846–16856
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a
Table 1 Selected bond lengths of CFA-5
2
very close to the value found for H -1,3-bdpb·MeOH (41.24(5)°).
The structure is created by Cu(I)–Cl zigzag chains expanding
in the [100] direction and H -1,3-bdpb ligands. The coordi-
2
Atoms
Bond length (Å)
2−
nation geometries of the Cu(I) ions are tetrahedral, being com-
posed of two chlorine and two nitrogen atoms from two H2-
Cu(1)–O(1)
Cu(1)–N(4)
Cu(1)–N(1)
Cu(1)–O(2)
Cu(2)–O(1)
Cu(2)–O(2)#1
Cu(2)–N(3)
Cu(2)–N(2)
1.923(4)
1.924(5)
1.925(5)
1.929(5)
1.904(4)
1.921(4)
1.942(5)
1.963(5)
1,3-bdpb ligands, as depicted in Fig. S6a.† The Cu–N distances
range from 1.969(3) to 1.970(3) Å, while Cu–Cl distances range
from 2.41(2) to 2.53(2) Å. These values are in good agreement
26
with those found in structurally related copper compounds.
2
The compound CuCl(H -1,3-bdpb) exhibits a layered structure
in which the layers are shifted from each other by half of the b
period (6.45 Å) (see Fig. S6b†). The van der Waals interactions
a
Symmetry transformations used to generate equivalent atoms: #1 x −
y + 2/3,x + 1/3,−z + 1/3.
3
between CH groups and weak hydrogen bonds are responsible
for the observed crystal packing motif. The closest non-
Å ) for a probe radius of 1.68 Å, corresponding to the approxi- bonding distance between hydrogen atoms of the methyl
mate van der Waals radius of argon. For MeOH molecules groups from closely-packed molecules is equal to 2.67(1) Å,
3
2
2
with an approximate van der Waals radius of 1.97 Å, the value while N⋯Cl distance is equal to 3.25(2) Å. The atomic coordi-
3
of 1091.8 Å (11.1% of the unit cell volume) is calculated. The nates and isotropic thermal parameters, selected bond lengths
CFA-5 coordination network can be described as a twofold and angles are presented in Tables S5 and S6 (ESI†),
interpenetrated 3D (see Fig. 2e) uninodal six-connected net of respectively.
the pcu type (alpha-Po primitive cubic), with a point (Schläfli)
1
2
3
symbol (4 × 6 ), with the Cu rings as six-connected nodes
1
2
TGA and XRPD studies
2
−
and the 1,3-bdpb ligands as spacers expanding 8.78(7) Å in
2
3
The phase purity of CFA-5 was confirmed by XRPD measure-
ment under ambient conditions. The experimental XRPD
pattern is consistent with the simulated one as gleaned from
the single crystal X-ray diffraction data, as shown in Fig. 3.
Differences in peak intensities are due to occluded solvent
molecules.
length. The atomic coordinates and isotropic thermal para-
meters, selected bond lengths and angles are presented in
Tables S3 and S4 (ESI†), respectively.
II
In the dodecanuclear coordination units found in [{Cu -
+
−
(
μ -OH)(μ -pz)} ] compounds, [PPN ][Cl {[Cu(μ-OH)(μ -pz)] } ],
2
2
12
2
6+12 2
+
−
[Bu
4
N ][Cl {[Cu(μ-OH)(μ
2 2
-pz)]6+12} ], six pyrazoles are in-plane
In addition, the thermal stability of CFA-5 was determined
by thermogravimetric (TG) and VTXRPD measurements. Prior
to TG measurement, the sample was heated at 100 °C under
II vacuum for 2 h in order to remove the occluded solvent mole-
and six out-of-plane created by Cu(II) ions. In comparison, in
II
the dodecanuclear {Cu (pz)(OMe)}
CFA-5 six pyrazoles of 1,3-bdpb ligands are above and six
coordination unit of
2
1
2
−
II
below the plane. The cross-sectional non-bonding Cu ⋯Cu
cules (MeOH). As shown in Fig. 4, the thermogravimetric
profile of CFA-5 under nitrogen exhibits a weight loss of 6.8%
between 200 and 300 °C corresponding to the loss of one
MeOH molecule per formula unit. The next degradation step
(−37.5%), in which a fraction of the organic ligand is lost, is
observed between 300 and 600 °C. According to the XRPD data
presented in Fig. 5, the sample is stable up to ca. 150 °C.
distances in the dodecanuclear ring of CFA-5 (see Fig. 2a) are
slightly smaller (12.2 Å) than in the above-mentioned com-
pounds (12.7 Å). The Cu–O distances from the OH groups in
II
the molecular {Cu (OH)(pz)} compounds are slightly larger
1
2
1.909(6)–1.957(6)) than the Cu–O distances of the OMe−
(
groups (CFA-5) 1.904(4)–1.929(5). Apart from the {Cu(μ-pz)}12
compounds, two additional literature reports on dodeca-
nuclear metallomacrocycles appeared, containing Zn or
Co(III) ions, namely ([(μ
2
-5-methyl-3-phenylpyrazole)
2 2
Zn -
2
4
(
μ -OCH CH S)] )
and
-pz)12](NO
-1,3-bdpb)]. The compound crystallizes in the
orthorhombic crystal system within the space group Pnma (no.
2). The asymmetric unit consists of one copper, one chlorine,
([PPN] [Co (μ -OH) (μ -4-NO -
3
2
2
6
2
12
2
12
2
2
2
5
pz)12(μ
2
-3,5-Me
2
2 2
) ).
I
[Cu Cl(H
2
6
two nitrogen and nine carbon atoms. An Ortep style plot of the
asymmetric unit of CuCl(H -1,3-bdpb) with atom labels is
2
shown in the ESI, Fig. S5.† The copper, chlorine and two
carbon atoms of the phenyl ring are lying on the crystallo-
graphic mirror plane (4c in Wyckoff notation). The two Me-pz
rings of each H -1,3-bdpb ligand are inclined with respect to
2
the central benzene ring. The least-squares planes running
through the atoms of the Me-pz ring and the atoms of the
benzene ring, respectively, have an angle of 40.96(2)°, which is Fig. 3 Calculated and measured X-ray powder patterns for CFA-5.
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Fig. 4 Temperature dependent weight loss of CFA-5 under flowing
nitrogen gas.
Fig. 6 Argon adsorption (black squares, triangles) and desorption (red
squares, triangles) isotherms measured for samples of CFA-5, desolvated
at 100 °C (top) and 250 °C (bottom curves), respectively.
Fig. 5 VTXRPD plots of CFA-5 kept in air, sampled in a temperature
range of 50–250 °C.
Fig. 7 Pore size distribution for CFA-5 desolvated at 100 °C (black line)
and 250 °C (red line) calculated by fitting the NLDFT model to the argon
At 200 °C characteristic peaks at 6.60–6.76° 2θ (corresponding sorption data.
to the (2 ˉ1 0) and (101) planes), 9.44 (20 ˉ1 ), 11.42 ((300),(3 ˉ2 ˉ1 )),
1
1
1.80 (10 ˉ2 ), 13.54 (202), 14.82 (4 ˉ3 1), 15.05 (3 ˉ2 2), 17.47 (5 ˉ1 0),
7.58 (5 ˉ3 ˉ1 ) become weaker and the peaks >20° in 2θ disappear,
which indicates framework damage. Subsequent heating of
the sample leads to amorphization of the material.
transition kernel for argon adsorption at 77 K based on a slit-pore
2
9
model. The distribution calculated by fitting the sorption
data reveals micropores with a diameter of ca. 6.6 Å (Fig. 7,
which is slightly higher than the data from the crystal structure
Physisorption results
(
5.86 Å)).
CFA-5 becomes amorphous after prolonged heating at T >
CFA-5 exhibits permanent porosity, which is confirmed by
argon gas sorption. Prior to sorption measurement, the crystal- 250 °C, as indicated by VTXRPD data. However, the Ar gas
line sample was heated at 100 °C under vacuum for 1 h. The sorption measurement, performed for the sample heated at
argon adsorption isotherm follows type I behaviour, typical for 250 °C under vacuum for 1 h, shows that the material remains
microporous solids. The maximum uptake achieved at 77 K and porous. The maximum uptake achieved at 77 K and p/p
= 0.99
0
3
−1
3
−1
p/p
0
= 0.99 is 252 cm g
(Fig. 6). The micropore volume is 192 cm g (Fig. 6). The micropore volume obtained from
3
−1
3
−1
obtained from the sorption isotherm is 0.2 cm g on applying the sorption isotherm is 0.1 cm
g
applying the NLDFT
2
7
27
the NLDFT method. The same value was obtained using the method. The adsorption data were fitted to the BET equation
28
2
−1
de Boer t-method micropore analysis. The adsorption data to give a surface area of 241 m g , the Langmuir surface area
2
2
−1
were fitted to the BET equation to give a surface area of 460 m
is equal to 278 m g . The pore size distribution calculated by
, the Langmuir surface area is equal to 510 m g . To evalu- fitting the sorption data shows that the volume of small pores,
−1
2
−1
g
ate the pore size distribution, the argon sorption isotherms corresponding to the channels as described in the crystallo-
sampled at 77 K were analyzed using non-local density func- graphic section (<10 Å), is strongly reduced and the maximum
tional theory (NLDFT) implementing a carbon equilibrium is shifted to a lower value (5.5 Å), whereas the volume of larger
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Scheme 2 Reaction scheme of the oxidation of tetralin: (I) mono oxy-
genation according to nature (s-MMO, CP 450), (II) radical autoxidation
mechanism.
energy of the C–H bond of methane is quite high (421 kJ
−
1
mol ), a catalyst is indispensable to open reaction pathways
with low activation energies comparable to the enzymatic oxi-
dation with pMMO. Currently, solid catalysts for selective oxi-
dation of methane with molecular oxygen in the gas phase
need high reaction temperatures of more than 200 °C. These
high temperatures favour total oxidation of methanol, which
can be oxidised more easily than methane. Thus, for prelimi-
Fig. 8 Temperature dependence of the effective magnetic moment μeff
per asymmetric unit of CFA-5 (cf. Fig. 3b) at 0.1 and 1.0 tesla; inset: field
dependence of the molar magnetization M at 2.0 K.
m
nary catalytic studies it is advisable to choose a substrate with
pores remains nearly the same (Fig. 7). These larger pores are
probably related to crystal defects or intercrystalline cavities.
13c
an activated C–H bond like benzylic compounds,
e.g. oxi-
dation of tetralin gives tetralol and tetralon as products (see
Scheme 2). As shown in Scheme 2, there are two major mecha-
nisms. One is the radical autoxidation mechanism proceeding
via tetralin hydroperoxide as an intermediate which is decom-
posed into the products. The latter step is by far not as simple
as shown in Scheme 2, because two different propagation
cycles consuming tetralin hydroperoxide in bimolecular
Magnetic studies
The magnetic measurements of CFA-5 are presented in Fig. 8
as the temperature dependence of the effective magnetic
moment μeff per asymmetric unit at 0.1 and 1.0 tesla. At 290 K,
the effective moment of 1.37μ (0.1 T; 1.35μ at 1.0 T) reveals
B
B
32
dominant strong antiferromagnetic exchange interactions
within the compound since these moments are considerably
smaller than the values expected for the two uncoupled Cu(II)
elementary steps are postulated. The radical autoxidation
can be enhanced by a catalyst through radical formation
(
initiation) and hydroperoxide decomposition (Haber–Weiss
3
0
3
3
centers (2.40–3.11μB). Upon cooling, the effective moment
steadily decreases to 0.34μ (0.1 T; 0.32μ at 1.0 T) at 2.0 K,
cycle). The other mechanism is analogous to the enzymatic
mono oxygenation through a proton driven formation of a
high-valent metal-oxo species from molecular oxygen which
B
B
further indicating antiferromagnetic exchange interactions.
The μeff curve exhibits a change in the curvature at approxi-
mately 45 K with a less steep slope at further decreasing temp-
eratures indicating paramagnetic impurities or/and minor
ferromagnetic exchange interactions within the compound. A
3
4
oxygenates C–H bonds forming the corresponding alcohols.
In order to benchmark catalytic results, not only CFA-5,
but also [Cu(2-pymo) ] and [Cu (BTC) ] (HKUST-1) were tested.
2
3
2
[
Cu(2-pymo)
2
] had already been used in the aerobic oxidation
1
3b
rapid increase of χ
m
(not shown) by decreasing the tempera-
of tetralin by Xamena et al.
They observed that [Cu(2-
ture in this interval is in agreement with this observation.
Because of the high symmetry of the compound and the
known tendency of compounds consisting of multiple coupled
Cu(II) centers to contain paramagnetic impurities, the afore-
mentioned explanation of this behaviour due to ferromagnetic
interactions seems to be less probable.
The dependence of the molar magnetization Mm on the
magnetic field B up to 5 T at 2.0 K is depicted in the inset of
Fig. 8. In this region, the magnetization is almost independent
of the applied field and retains a value close to zero. Therefore,
the ground state of the compound corresponds to an S = 0
state, i.e. singlet spin ground state.
pymo) ] significantly enhances the tetralin hydroperoxide for-
mation according to the radical autoxidation mechanism.
2
Ryan et al. investigated both [Cu(2-pymo) ] and [Cu (BTC) ] by
2
3
2
3
1
DFT calculations and came to the conclusion that on both Cu-
MOFs the decomposition of the tetralin hydroperoxide can
3
5
only occur at the outer surface. In the case of [Cu(2-pymo)2]
the pore window size is 5.04 Å and, thus, is smaller than the
kinetic diameter of tetralin which is expected to be larger than
3
5
5
.85 Å. Since the narrowest channel diameter of CFA-5 is
4.5 Å, tetralin and tetralin hydroperoxide are also not expected
to access the inner surface of the MOF. In the case of
Cu (BTC) ] the pore size is large enough (10.7 Å according to
[
3
2
3
6
Schlichte et al.) and the coordination at the free coordination
Catalytic studies
site of the Cu paddlewheel structural motif indeed occur, but
Cu-MOFs mimicking copper-containing enzymes like pMMO the calculated activation energy was not lower than the one for
3
5
should be able to catalyse direct oxidation of methane to the gas phase. Therefore, the inner surface of [Cu
3 2
(BTC) ]
methanol with molecular oxygen. However, since the binding should not act as a catalyst. However, the DFT calculations
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Fig. 9 Progress of tetralin conversion over time for CFA-5, [Cu(2-
Fig. 10 Progress of THP selectivity over time for CFA-5, [Cu(2-pymo)
2
],
pymo)
2 3 2
], and [Cu (BTC) ]. T = 90 °C; 20 g tetralin, tetralin/Cu molar
and [Cu (BTC) ]. T = 90 °C; 20 g tetralin, tetralin/Cu molar ratio: 2000/1.
3
2
ratio: 2000/1.
showed that undercoordinated Cu clusters at the outer surface
3
5
of Cu-MOFs should decompose tetralin hydroperoxide.
In Fig. 9, the measured time dependence of the conversion
of tetralin over time is shown for the three catalysts CFA-5,
[
Cu(2-pymo) ] and [Cu (BTC) ]. All the three catalysts exhibit
2 3 2
an induction period which is typical for the radical autoxida-
tion mechanism (see Scheme 2, mechanism II). CFA-5 shows
an activity very similar to that of [Cu(2-pymo) ], but both cata-
2
lysts display a significantly shorter induction period than
3 2
[Cu (BTC) ]. Since we verified experimentally that no uncata-
lysed initiation of the reaction occurred, Cu at the outer
surface of the MOFs seems to be an active site in the initial for-
mation of tetralin radicals. Xamena et al. excluded leaching of
Fig. 11 Progress of THP selectivity over time with hot filtration of the
catalyst after 6 h for CFA-5, [Cu(2-pymo) ], and [Cu (BTC) ]. T = 90 °C;
0 g tetralin, tetralin/Cu molar ratio: 2000/1.
1
3b
2
3
2
copper.
By an optimized ICP-MS measurement procedure
2
we could detect only 3.5 ppm Cu in the solution after 24 hours
which corresponds to leaching of approx. 1.5% of Cu from the
original Cu-MOF. Since Xamena et al. have shown that copper
acetate corresponding to an assumed copper leaching of 10%
same, irrespective of whether or not the catalyst is present (not
shown), this allows the conclusion that all three catalysts do
not catalyse the THP formation, if the initiation has been com-
pleted (which is evidently the case after 6 h).
1
3b
gives a much slower increase of conversion over time,
we
also assume that copper leaching is negligible. However, this
will be investigated more deeply in future work.
The observed induction period (see Fig. 9) is an indication
for the radical autoxidation mechanism proceeding via tetralin
hydroperoxide (THP) as an intermediate. As expected, the
measured selectivities of THP are significant and reach more
To sum up, based on our catalytic studies and on the pub-
13b,35
lished knowledge
a reaction mechanism can be proposed
as shown in Fig. 12:
•
Initiation of radical THP formation by all the investigated
Cu-MOFs.
• Heterogeneously catalyzed THP decomposition by CFA-5
and [Cu(2-pymo) ].
than 80% in the case of [Cu (BTC) ] (see Fig. 10). Surprisingly
3
2
2
CFA-5 and [Cu(2-pymo) ] show a different course of THP
selectivity, because for these Cu-MOFs THP selectivity is not
increasing, but decreasing with progressing time. Thus, the
2
Finally, due to the proposed mechanism as shown in
Fig. 12, it can be assumed that CFA-5 will be also an active
catalyst for the oxidants H O or TBHO since it catalyzes the
2 2
most likely explanation might be that CFA-5 and [Cu(2-pymo)
2
]
catalyse the decomposition of THP, whereas [Cu (BTC) ] is not
3
2
decomposition of hydroperoxide species. This is already
able to do this. For further verification of this conclusion, a
hot filtration test was performed after 6 h reaction time. As
can be seen from Fig. 11 the reaction proceeds even after
removal of the catalyst due to the radical autoxidation mechan-
ism with an increasing selectivity to THP. This is a clear evi-
13a,c
known from other Cu-MOFs.
Conclusions
dence that CFA-5 and [Cu(2-pymo)2] catalyse the The work reported here focuses on the synthesis and charac-
decomposition of THP whereas [Cu (BTC) ] does not. Since terization of a novel metal–organic framework constructed
3
2
II
the course of conversion over time after 6 h is almost the from wheel-shaped dodecanuclear {Cu (pz)(OMe)} coordi-
1
2
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2960 w, 2919 w, 1609 w, 1581 m, 1525 w, 1456 m, 1436 s, 1415 w,
1401 w, 1380 w, 1365 w, 1314 w, 1288 m, 1275 w, 1239 w, 1148 w,
1071 w, 1035 m, 1010 s, 981 w, 891 w, 836 m, 800 s, 784 w,
760 s, 710 s, 700 m, 640 m, 586 w, 506 m, 447 w, 406 w. The
IR spectrum of H -1,3-bdpb is shown in Fig. S7 (ESI†). For the
2
2
single-crystal X-ray diffraction experiment H -1,3-bdpb was
recrystallized from MeOH.
Synthesis of [Cu (1,3-bdpb)(OCH ) ] (CFA-5)
2
3 2
Solvothermal method. A mixture of Cu(OAc) ·H O (11 mg,
2
2
0
.055 mmol) and H
solved in MeOH (4 mL). 2,6-Dimethylpyridine (2,6-lutidine)
0.05 mL) was added and the solution was placed in a glass
tube (10 mL). The tube was closed with a cap and heated at
20 °C for 3 d and subsequently cooled to room temperature.
2
-1,3-bdpb (15 mg, 0.056 mmol) was dis-
(
1
The green crystals were filtered off by suction and washed
thoroughly with MeOH. The synthesis can be similarly per-
formed at larger quantities (upscale factor: 50). Yield: 12 mg,
9
6% (based on Cu(OAc) ·H O). [Cu (C H N )(OCH ) ] calcd:
2 2 2 16 16 4 3 2
C: 47.67, H: 4.89, N: 12.35; found C: 47.44, H: 4.83, N: 12.28.
IR: (cm− ) 1605 w, 1584 w, 1539 w, 1492 w, 1424 m, 1377 w,
1
1331 m, 1301 w, 1244 m, 1167 w, 1130 w, 1063 w, 1034 w, 900
Fig. 12 Proposed reaction mechanism as a combination of homo-
geneous radical propagation cycles and heterogeneously catalyzed w, 816 m, 776 s, 709 s, 700 m, 662 w, 547 s, 447 s. The IR and
35
steps (initiation, THP decomposition).
UV/Vis spectra of CFA-5 are shown in Figs S7 and S8,†
respectively.
2 2
Microwave irradiation method. A mixture of Cu(OAc) ·H O
nation units, created by twelve copper(II) ions connected by
twelve μ -bridging MeO groups and pyrazolate (pz ) ligands.
2
The compound remains porous after solvent removal, as con-
firmed by TG, VTXRPD and gas sorption. The magnetic
measurements reveal dominant strong antiferromagnetic
exchange interactions within the compound. Catalytic tests of
CFA-5 in the aerobic oxidation of tetralin show a similar per-
−
−
(11 mg, 0.055 mmol) and H -1,3-bdpb (15 mg, 0.056 mmol)
2
was dissolved in MeOH (3 mL). 2,6-Dimethylpyridine (2,6-luti-
dine) (0.05 mL) was added and the solution was placed in a
Pyrex sample tube (10 mL). The tube was closed with a cap and
placed in a microwave synthesizer (CEM, Discover S). The
resulting mixture was heated to 150 °C at 300 W for 25 min
and cooled to room temperature. The green microcrystalline
material was filtered off and washed with MeOH. Yield: 12 mg,
6% (based on Cu(OAc) ·H O). This material exhibited the
2 2
same analytical results as the ones obtained by the solvother-
mal method.
2
formance as with [Cu(2-pymo) ]. Both Cu-MOFs catalyse the
decomposition of tetralin hydroperoxide after 6 h reaction
time and initiate its formation within the first 6 h reaction
time. For both Cu-MOFs it can be suggested that low co-
ordinated Cu centers at the outer surface are responsible for
the observed activity since in both cases the substrate cannot
access the pore system. Both Cu MOFs contain diazahetero-
cyclic linkers which might be relevant for their catalytic
activity. Under the applied test conditions [Cu (BTC) ], with its
9
Synthesis of CuCl(H
.067 mmol) and H -1,3-bdpb (15 mg, 0.056 mmol) was dis-
solved in MeOH (4 mL). 2,6-Dimethylpyridine (2,6-lutidine)
0.05 mL) was added and the solution was placed in a glass
tube (10 mL). The tube was closed with a cap and heated at
20 °C for 3 d and cooled to room temperature. The colorless
crystals were filtered off by suction and washed thoroughly
with MeOH. Yield: 9 mg, 44% (based on H -1,3-bdpb). CuCl
C H N ) calcd: C: 52.60, H: 4.97, N: 15.34; found C: 52.40,
2 2
-1,3-bdpb). A mixture of CuCl (9 mg,
0
2
(
3
2
carboxylate linkers, does not seem to form catalytically active
Cu centers, neither at the inner nor at the outer surface.
1
2
(
1
6
18 4
Experimental
Materials and general methods
−
1
H: 5.04, N: 14.94. IR: (cm ) 3228 m (→NH), 1604 w, 1591 m,
552 w, 1507 m, 1477 w, 1452 m, 1431 m, 1410 w, 1396 w,
1
Commercially available reagents of analytical grade were used 1378 w, 1362 w, 1300 w, 1239 s, 1161 m, 1099 w, 1068 w,
as received without further purification. [Cu(2-pymo) ] was syn- 1042 m, 1025 m, 1012 m, 980 w, 889 m, 808 s, 789 m, 779 m,
2
3
7
thesized according to the published procedure, [Cu
was purchased from Sigma-Aldrich (Basolite C-300).
3
(BTC)
2
]
761 s, 713 s, 675 s, 658 s, 610 s, 585 s, 517 s, 454 w. The IR
spectrum of 3 is shown in Fig. S7.†
Synthesis of H -1,3-bdpb. H -1,3-bdpb was synthesized
Microwave irradiation method. A mixture of CuCl
(9 mg,
2
2
2
2
0
according to the published procedure.
H
2
1
-1,3-bdpb is soluble 0.067 mmol) and H -1,3-bdpb (15 mg, 0.056 mmol) was dis-
2
in DMF, DMAc, DMSO, EtOH, and MeOH. H-NMR (400 MHz). solved in MeOH (2 mL). 2,6-Dimethylpyridine (2,6-lutidine)
C H N calcd: C: 72.15, H: 6.81, N: 21.04; found C: 71.93, H: (0.05 mL) was added and the solution was placed in a Pyrex
.80, N: 20.49. IR: (cm ) 3162 w, 3121 w, 3068 w, 3006 w, sample tube (10 mL). The tube was closed with a cap and
1
6
18 4
−1
6
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placed in a microwave synthesizer (CEM, Discover S). The Cambridge CB21EZ, UK as supplementary publication no.
resulting mixture was heated to 140 °C at 100 W for 30 min CCDC 1006742-1006744.
and cooled to room temperature. The colorless microcrystal-
Physical methods
line material was filtered off and washed with MeOH. Yield:
1
1 mg, 54% (based on H -1,3-bdpb). This material exhibited Fourier transform infrared (FTIR) spectra were recorded with
2
the same analytical results as the ones obtained by the solvot- ATR in the range 4000–400 cm− on a Bruker Equinox 55 FT-IR
1
hermal method.
spectrometer. The following indications are used to character-
ize absorption bands: very strong (vs), strong (s), medium (m),
weak (w). Elemental analyses (C, H, N) were carried out on a
Perkin-Elmer 2400 Elemental Analyzer. Thermogravimetric
analysis (TGA) was performed with a TGA Q500 analyser in the
temperature range of 25–800 °C under flowing nitrogen at a
heating rate of 10 K min− . The argon sorption isotherms at
Single-crystal X-ray diffraction
X-ray data for the single crystal structure determinations of H2-
1
2
,3-bdpb·MeOH, CFA-5 and CuCl(H -1,3-bdpb) were collected
on a Bruker D8 Venture diffractometer. Intensity measure-
ments were performed using monochromated (doubly curved
silicon crystal) MoKα radiation (0.71073 Å) from a sealed
microfocus tube. Generator settings were 50 kV, 1 mA. Data
collection temperature was −173 °C. APEX2 software was used
for preliminary determination of the unit cell. Determination
of integrated intensities and unit cell refinement were per-
formed using SAINT. The structures were solved and refined
using the Bruker SHELXTL Software Package.
1
−
196 °C up to p/p = 1 were measured using a Quantachrome
0
Autosorb-I ASI-CP-8 instrument. High purity gas was used for
the adsorption experiments (argon 99.999%). Ambient temp-
erature X-ray powder diffraction (XRPD) patterns were
measured with a Seifert XRD 3003 TT diffractometer equipped
3
8
39
with a Meteor 1D detector operated at 40 kV, 40 mA, CuK
1
0
α
(λ =
.54247 Å) with a scan speed of 10 s per step and a step size of
.02° in 2θ. The variable temperature XRPD experiment data
4
0
Selected
-1,3-
crystal data and details of structure refinements for H
2
were collected in the 2θ range of 5–60° with 0.02° steps, with a
Bruker D8 Advance diffractometer equipped with a Lynxeye
linear position-sensitive detector, an MRI TCPU1 oven, in
transmission geometry. The sample was loaded into a capillary
bdpb·MeOH, CFA-5 and CuCl(H -1,3-bdpb) are provided in
Table 2. Complete crystallographic data for the structures
H -1,3-bdpb·MeOH, CFA-5 and CuCl(H -1,3-bdpb) reported in
2 2
this paper have been deposited in the CIF format with the
Cambridge Crystallographic Data Center, 12 Union Road,
2
(
Hilgenberg) made from special glass no. 10, 0.5 mm diameter
and 0.01 mm wall thickness. The patterns were recorded in a
temperature range from 30 to 250 °C, in the 5–60° 2θ range,
with one step per 1 s, and an angular step width of 0.02° in 2θ.
2
Table 2 Crystal data and structure refinement for compounds H -1,3-
bdpb·MeOH, CFA-5 and CuCl(H
2
-1,3-bdpb)
Temperature program between measurements: heating rate
1
(
0.5 °C s− ), then 10 min isothermal. Magnetic susceptibility
2
H -1,3-
2
CuCl(H -1,3-
Compound
bdpb·MeOH
CFA-5
bdpb)
data of CFA-5 were recorded using a Quantum Design
MPMS-5XL SQUID magnetometer. The polycrystalline samples
were compacted and immobilized into cylindrical PTFE cap-
sules. The susceptibility data were acquired as a function of
the field (0.1–5.0 T) and temperature (2.0–290 K). All data were
corrected for the contribution of the sample holder and the
diamagnetic contributions of CFA-5 calculated from tabulated
Empirical
formula
Formula
C
17
16
H
22
18
N
4
O
,
18
C H22Cu
2
N
O
4 2
C
C
16
H
18
N
4
ClCu
ClCu
C
H
N
4
2 16 4
[Cu (C16H N )
H
16 18
N
4
3
CH OH
(OCH ) ]
3 2
1
M
r
/g mol−
298.39
100(2)
453.48
100(2)
365.33
100(2)
0.71073
Orthorhombic
Pnma (no. 62)
6.1784(8)
12.9041(17)
19.866(3)
90
90
90
1583.8(4)
4
1.532
1.549
752
3.16 to 25.00
12 815
1460
0.0391
1.140
T/K
Wavelength/Å
Crystal system
Space group
0.71073
0.71073
Trigonal
/m (no. 11) R ˉ3 (no. 148)
1
Monoclinic
P2
−9
3
−1
values (χdia(CFA-5) = –2.8 × 10 m mol per asymmetric unit).
a/Å
b/Å
c/Å
α/°
β/°
γ/°
4.7416(3)
12.1325(7)
14.0001(8)
90
95.107(2)
90
802.19(8)
2
1.235
0.080
320
2.92 to 26.36
16 984
1705
0.0634
1.129
26.839(3)
26.839(3)
15.8317(16)
90
90
120
9876.2(19)
18
1.372
1.955
4176
2.65 to 25.00
19 551
3874
0.1439
1.036
0.0679
0.1515
Catalytic investigations
The method used to determine the catalytic activity is based
13b
on the work of Xamena et al. Catalytic tests were carried out
in a three-necked flask equipped with a reflux condenser and a
gas inlet tube. Before each experiment, the glassware was pas-
sivated for a period of 12 h with 1 M aqueous sodium hydrox-
ide solution, subsequently neutralized by rinsing with 1 M
hydrochloric acid for 30 min and finally washed with deminer-
alized water and dried at 80 °C in an oven. With this passiva-
tion procedure, uncatalysed initiation of the reaction could be
avoided. The reaction temperature was set to 90 °C and the
applied molar ratio of tetralin/metal was always 2000 : 1. 20 g
of tetralin were used both as a solvent and a reactant. The
required amount of catalyst was calculated from the previously
3
V/Å
Z
D
−
3
c
/g cm
−
1
μ/mm
F(000)
θ Range/°
Refls. collected
Refls. unique
R(int)
GooF
a
R
1
[I > 2σ(I)]
0.0521
0.1049
0.0440
0.1243
b
2
wR (all data)
Largest diff. peak 0.244 and
0.763 and −0.925 0.448 and
−
3
and hole/Å
−0.219
−0.441
−
1
mentioned ratio. Synthetic air at a flow rate of 0.5 mL s was
adjusted by a needle valve. A HP 6890N GC with AutoSampler
a
b
2
2
c
2
0
1/2
R
1
= ∑||F
0
| − |F
c
||/∑|F
0
|. wR
2
= ∑[w(F
0
− F
)2]/∑[w(F
)2]
.
16854 | Dalton Trans., 2014, 43, 16846–16856
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Paper
was used to analyze the reaction mixture. All samples were
measured five times and the results were averaged. Peroxide
concentration was determined by the TPP method. In time-
resolved experiments, small samples of the reaction solution
were taken after 6, 24 and 30 hours.
Conversion (X) of tetralin and selectivity (S) to tetralin
hydroperoxide were calculated according to the following for-
mulas:
Z. Liu, S. Asahina, H. Kazumori, M. O’Keeffe, O. Terasaki,
J. F. Stoddart and O. M. Yaghi, Science, 2012, 336, 1018.
7 (a) Y. Liu, W. Xuan and Y. Cui, Adv. Mater., 2010, 22, 4112;
(b) L. Ma and W. Lin, Top. Curr. Chem., 2010, 293, 175;
(c) M. Tonigold, Y. Lu, A. Mavrandonakis, A. Puls,
R. Staudt, J. Möllmer, J. Sauer and D. Volkmer, Chem. –
Eur. J., 2011, 17, 8671; (d) A. Dhakshinamoorthy, M. Alvaro
and H. García, Catal. Sci. Technol., 2011, 1, 856; (e) A. Corma,
H. García and F. X. Llabrés i Xamena, Chem. Rev., 2010,
4
1
nTetralin;0 ꢀ nTetralin
XTetralin ¼
110, 4606.
nTetralin;0
8
L. Que and W. B. Tolman, Nature, 2008, 455, 333.
nTHP
STHP ¼
9 L. Jian, C. Chen, F. Lan, S. Deng, W. Xiao and N. Zhang,
Solid State Sci., 2011, 13, 1127.
nTetralin;0 ꢀ nTetralin
1
1
1
1
0 D. Jiang, T. Mallat, F. Krumeich and A. Baiker, J. Catal.,
2008, 275, 390.
The sum of selectivities to THP, tetralol and tetralone was
always larger than 95%. Thus, the consecutive oxidation of
tetralol and tetralone was negligible and the carbon balance
fulfilled.
1 D. Jiang, T. Mallat, D. M. Meier, A. Urakawa and A. Baiker,
J. Catal., 2010, 270, 26.
2 N. T. S. Phan, P. H. L. Vu and T. T. Nguyen, J. Catal., 2013,
3
06, 38.
3 (a) S. Marx, W. Kleist and A. Baiker, J. Catal., 2011, 281, 76;
b) F. X. Llabres i Xamena, O. Casanova, R. Galiasso
Tailleur, H. Garcia and A. Corma, J. Catal., 2008, 255, 220;
c) S. Wang, L. Li, J. Zhang, X. Yuan and C.-Y. Su, J. Mater.
(
Acknowledgements
(
This work was supported by a grant from the Ministry of
Science, Research and Art Baden-Württemberg and by the
German Research Foundation (DFG) within Priority Program
Chem., 2011, 21, 7098.
1
4 M. Grzywa, C. Geßner, D. Denysenko, B. Bredenkötter,
F. Gschwind, K. Fromm, W. Nitek, E. Klemm and
D. Volkmer, Dalton Trans., 2013, 42, 6909.
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15 (a) M. Tonigold, Y. Lu, A. Mavrandonakis, A. Puls, R. Staudt,
J. Möllmer, J. Sauer and D. Volkmer, Chem. – Eur. J., 2011,
1
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