[Cu(H2btec)(bipy)∞: A novel metal organic framework (MOF) as heterogeneous catalyst for the oxidation of olefins

Article abstract of DOI:10.1039/b810414j

A new extended metal-organic framework [Cu(H₂btec)(bipy)] _∞ (1) (H₄btec= 1,2,4,5-benzenetetracarboxylic acid; bipy= 2,2′-bipyridine) has been hydrothermally synthesized. Violet crystals are formed in a monoclinic system wi

Full text of DOI:10.1039/b810414j

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PAPER
www.rsc.org/dalton | Dalton Transactions
[Cu(H2btec)(bipy)]•: a novel metal organic framework (MOF) as
heterogeneous catalyst for the oxidation of olefins†
Kareen Brown,a Santiago Zolezzi,a Pedro Aguirre,a Diego Venegas-Yazigi,b,c Vero´nica Paredes-Garc´ıa,b,d
Ricardo Baggio,e Miguel A. Novak f and Evgenia Spodine*a,b
Received 19th June 2008, Accepted 28th November 2008
First published as an Advance Article on the web 15th January 2009
DOI: 10.1039/b810414j
A new extended metal-organic framework [Cu(H2btec)(bipy)]• (1) (H4btec=
1,2,4,5-benzenetetracarboxylic acid; bipy= 2,2¢-bipyridine) has been hydrothermally synthesized.
˚
Violet crystals are formed in a monoclinic system with a space group C2/c; a = 10.1810(18) A,
◦
˚
˚
b = 14.4360(18) A, c= 12.894(3) A, b= 112.94(3) . In the title compound 1 each Cu(II) centre has a
distorted square planar environment, completed by two N atoms from one bipy ligand and two O atoms
belonging to two dihydrogen benzene-1,2,4,5-tetracarboxylate anions (H2btec2-). The {Cu(bipy)}
2+
moieties are bridged by H2btec2- anions to form an infinite one-dimensional coordination polymer with
a zig-zag chain structure along the c axis. A double-chain structure is formed by hydrogen bonds
between adjacent zig-zag chains. There are also p–p stacking interactions between the bipy ligands, with
˚
an average distance of 3.62 A resulting in a two-dimensional network structure. Compound 1 was tested
as a catalyst for the oxidation of cyclohexene and styrene, with tert-butyl hydroperoxide (TBHP) as
oxidant. The catalytic activity (24 h and 75 ◦C) found for [Cu(H2btec)(bipy)]• shows a high value for
the conversion of cyclohexene (64.5%), and a lower one for styrene (23.7%). High turnover frequency
(TOF) values for the epoxide products were observed, indicating that the catalyst synthesized in this
work, not only has a high activity and selectivity for epoxidation reactions but is also very efficient.
The aromatic polycarboxylate transition metal complexes are
Introduction
interesting due to the variety of bridging abilities of the polycar-
boxylate ligand in the formation of extended porous frameworks.
1,2,4,5-Benzenetetracarboxylic acid (H4btec) presents interesting
characteristics such as four carboxylate groups, which may be
completely or only partially deprotonated. This fact allows
interesting structures with high dimensionalities. Depending upon
the number of deprotonated carboxylate groups, this ligand can act
not only as hydrogen-bond acceptor, but also as hydrogen-bond
donor. Hence, H4btec may be a good choice for the construction of
polymeric structures. Numerous complexes with the H4btec ligand
have been reported.5–14 although the construction of complexes
from H4btec, 2,2¢-bipyridine or 1,10-phenantroline, and Cu(II)
building blocks is still limited.
In the catalysis area, the epoxidation of olefins using metal
catalysts plays a central role in the selective and partial oxidation
of both saturated and unsaturated hydrocarbons.15–22 Selective
oxygen transfer to olefins remains an important research area in
industrial and synthetic chemistry, because epoxides are widely
used as epoxy resins, paints, surfactants, and intermediates in
various organic syntheses.23
The design and synthesis of supramolecular coordination poly-
meric networks, especially those constructed by hydrogen bonding
and p–p stacking interactions, have been a field of rapid growth
due to their special physical properties and potential application
in functional materials, for molecular adsorption, ion exchange,
and heterogeneous catalysis.1–4
aDepartamento de Qu´ımica Inorga´nica y Anal´ıtica, Facultad de Ciencias
Qu´ımicas y Farmace´uticas. Universidad de Chile, Olivos, 1007, Santiago,
Chile. E-mail: espodine@uchile.cl; Fax: 562-9782868; Tel: 562-9782862
bCentro para la Investigacio´n Interdisciplinaria Avanzada en Ciencias de los
Materiales (CIMAT), Universidad de Chile, Santiago, Chile
cFacultad de Qu´ımica y Biolog´ıa, Universidad de Santiago de Chile, Santiago,
Chile
dDepartamento de Qu´ımica, Universidad Tecnolo´gica Metropolitana,
Santiago, Chile
eDepartamento de F´ısica, Comisio´n Nacional de Energ´ıa Ato´mica (CNEA),
Buenos Aires, Argentina
f
Instituto de F´ısica-UFRJ, Rio de Janeiro, Brazil
† Electronic supplementary information (ESI) available: Fig. S1: simulated
(black) and experimental (red) X-ray powder diffraction patterns of
[Cu(H2btec)(bipy)]•, at room temperature. Fig. S2: TGA and calculated
DTA curves of [Cu(H2btec)(bipy)]•. Fig. S3: the field dependence of
magnetization at two different temperatures. Fig. S4: catalytic activity of
[Cu(H2btec)(bipy)]• for the cyclohexene oxidation. (a) Substrate conver-
sion (%). (b) Chemoselectivity for cyclohexene epoxide (%) (white bars =
30 ◦C; gray bars = 50 ◦C; black bars = 75 ◦C). Fig. S5: catalytic activity
of [Cu(H2btec)(bipy)]• for the styrene oxidation. (a) Substrate conversion
(%). (b) Chemoselectivity for styrene epoxide (%) (white bars = 30 ◦C; gray
bars = 50 ◦C; black bars = 75 ◦C)]. CCDC reference number 682181 for
complex 1. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b810414j
The activation of hydroperoxides by transition metals has
received much attention, since Group IV–VII metals have been
employed for industrial olefin epoxidation with organic hydroper-
oxides (the Halcon–Arco process).24 Besides, heterogenized ho-
mogeneous catalysts, mixed metal exchanged zeolites and resins,25
polymerized chiral ligand26 complexes have been used in liquid
phase epoxidation of olefins.27
However, epoxidations with copper are still rare. Exam-
ples, such as iodosylbenzene with copper(II) nitrate and
1422 | Dalton Trans., 2009, 1422–1427
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triflate with stilbenes,28 silylalkyl peroxybenzoate with Cu(II)
trifluoroacetate,29 and binuclear trisamino Cu(II) complexes have
been reported.30 Recently the heterogeneous copper(II) complex
with the MCM-41 polymer has shown a good activity in epoxi-
dation reactions, with a variety of olefinic compounds, using tert-
BuOOH (TBHP) as oxidant.31
◦
˚
Table 1 Selected bond distances (A) and angles ( ) for (1)
Bond distances
Cu1-N1
1.973(2)
Cu1-O1
1.934(19)
Bond angles
Herein we report the hydrothermal synthesis, structural and
magnetic characterization, and catalytic behavior of a new metal-
organic framework [Cu(H2btec)(bipy)]• (1), in the epoxidation of
cyclohexene and styrene using TBHP as an oxidant.
O1-Cu1-O1
O1-Cu1-N1
89.69(12)
94.62(9)
N1-Cu1-N1 #
O1-Cu1-N1#
81.86(13)
172.24(8)
of the copper(II) centre. The symmetric H2btec ligand presents two
independent carboxylate groups, one of which is unprotonated,
and thus accounts for charge balance of the copper cation, which
lies at a special position. The ligand acts as a bridge between
neighboring copper centres, by way of atom O1 in two symmetry
related carboxylate groups binding in a monodentate fashion.
The remaining three carboxylate oxygens (O1, O2, O3) enter in
stabilizing H-bonding interactions, to be discussed below. This
bridging action of the H2btec ligand results in the formation of
zig-zag chains running along c, with acute turning points which
Results and discussion
Crystal structure of [Cu(H2btec)(bipy)]• (1)
X-Ray crystallographic analysis shows that compound 1 exhibits
an infinite 1D chain-like structure. Fig. 1 presents a diagram of
the structure showing the labelling scheme used.
˚
make them to look like broad strips, some 20 A wide, along the
b direction. The planar elements in the strip (bipy and H2btec)
are almost coplanar to each other and parallel to (100). Thus
interdigitation of lateral strips promotes p–p interactions between
adjacent aromatic rings, and the result is the interconnection of
strips into layers parallel to (100) (Fig. 2). The C4–H4 ◊ ◊ ◊ O3#3
H-bond further contributes to the interplanar cohesion. These
layers, in turn, are linked to each other through the strong
O4–H4A ◊ ◊ ◊ O2#2 H-bond. A summary of the most relevant non-
bonding interactions is presented in Table 2 (H-bonds and p–p
contacts).
Fig. 1 Molecular diagram of 1 extended to see the way in which strips
build up. The intra-chain C1–H1 ◊ ◊ ◊ O3 H-bond is shown in broken lines.
Displacement ellipsoids drawn at a 50% level [symmetry codes: #1: 1 - x,
y, 0.5 - z; #5: 1 - x, 1 - y, 1 - z].
Fig. 2 Packing scheme of 1 shown down the c axis [symmetry code: #2:
0.5 + x, 0.5 - y, 0.5 + z].
With respect to the previous description, the title metal-organic
The Cu(II) center is coordinated into an almost square planar
arrangement by two nitrogen atoms from one chelating 2,2¢-
bipyridine ligand, and two O atoms from two carboxylate groups
from two bridging H2btec groups. The copper coordination
polyhedron presents crystallographic symmetry, being bisected by
a two-fold axis which goes through the metal and the central
C–C bond of the bipy unit, while the H2btec ligand evolves
around a symmetry center. As a result only half of a formula unit
is independent (z¢ = 0.5). Table 1 shows selected bond angles
and distances. The single (but doubled by symmetry) Cu–O and
hybrid compound 1 is the first polymeric complex that contains
2+
the mixed ligands (bipy and H2btec2-), where the {Cu(bipy)}
moieties are bonded by H2btec2-, displaying one m2-bridging mode
and a monodentate coordination mode.
Additionally, other similar MOF containing 1,2,4,5-benzene
tetracarboxylic acid and diimine ligands have been previously
reported.5–8 Structural parameters of (1) and the above related
compounds are summarized in Table 3.
XRPD and TG-DTA analyses of the polycrystalline material
˚
Cu–N bond lengths are 1.934(19) and 1.973(2) A respectively;
cis angles in the coordination plane have values of 81.86(13)◦
89.69(12) and 94.62(9)◦, while trans ones have values of 172.24(8)◦,
confirming the distorted square-planar coordination environment
The simulated diffraction pattern of the reflections of the single
crystal diffraction agrees with the experimental XRPD pattern of
the violet microcrystalline powder as shown in Fig. S1.†
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Table 2 Hydrogen bonds and p–p contacts for 1
d(D–H)/ d(H ◊ ◊ ◊ A)/ d(D ◊ ◊ ◊ A)/
D-H ◊ ◊ ◊ A
<(DHA)/◦
˚
A
˚
A
˚
A
O(4)–H(4a) ◊ ◊ ◊ O(2)#2 0.82(3)
C(1)–H(1) ◊ ◊ ◊ O(3) 0.93
C(4)–H(4) ◊ ◊ ◊ O(3)#3 0.93
1.78(4)
2.48
2.52
2.585(3)
3.214(4)
3.142(4)
167(4)
136
125
Symmetry codes: #2 x + 1/2, -y + 1/2, z + 1/2; #3 -x + 1, -y, -z + 1.
saa/◦
a
a
˚
˚
Group1-Group2
ccd /A
ipd /A
[Cg1 ◊ ◊ ◊ Cg1]#4
[Cg1 ◊ ◊ ◊ Cg2]#5
3.629(2)
3.656(2)
3.36(1)
3.37(2)
22.3(1)
22.9(7)
Cg1: N(1), C(1), C(2), C(3), C(4), C(5); Cg2: C(6), C(7), C(8), C(6), C(7),
C(8).
Fig. 3 Temperature dependence of molar magnetic susceptibility for
[Cu(H2btec)(bipy)]•.
Symmetry codes: #4 = 1 - x, -y, 1 - z; #5 = 1/2 + x,-1/2 + y, z.
a ipd, interplanar distance (distance from one plane to the neighbouring
centroid); ccd, center-to-center distance (distance between ring centroids);
sa, slippage angle (angle subtended by the intercentroid vector to the plane
normal). For details, see C. Janiak, J. Chem. Soc., Dalton Trans. 2000, 3885.
In order to determine the magnitude of the exchange interac-
tion, the magnetic susceptibility data were fitted using the magnetic
chain model.32
N b2g2
kT
0.25 + 0.074975x + 0.75235x2
Ï
Ì
¸
˝
cM
=
1+ 0.9931x + 0.172135x2 + 0.757825x3
(1)
Ô
Ó
Ô
˛
Thermogravimetric analysis (TGA) is shown in Fig. S2.† This
shows that compound 1 possesses good thermal stability up to
250 ◦C. The TGA curve exhibits three continuous weight loss
stages in the temperature range 250–445 ◦C. The total weight loss
(85.1%) is in good agreement with the calculated value (Dm%
86.9), corresponding to the concomitant release of bipy and
H2btec ligands. The DTA curve also confirms that the network
of compound 1 is stable below 250 ◦C, and shows that the weight
loss corresponds to an exothermic process.
x = J / kT
The best fit to eqn (1) gave g = 2.05, J = -1.4 cm-1.
The very small value of J is in the order of those found
for other copper (II) complexes with carboxylate bridg-
ing ligands, in which there is no coplanarity between the
CuNO moiety and tetracarboxylate ligand. A similar small
J value was reported by Kopel et al., who found a J =
-0.56 cm-1 for [Cu3(mdpta)3 (btc)](ClO4)3·4H2O, where mdpta
corresponds to N,N-bis-(3-aminopropyl)methylamine and H3btc
to 1,3,5-benzenetricarboxylic acid.33 Copper complexes with the
1,2,4,5-tetracarboxylic acid (H4btec) having different degrees
of deprotonation, [{Cu(btec)(4-apy)2}·(4-Hapy)2·(H2O)4]n, and
[{Cu(H3btec)2(pyz)·(H2O)2}·H2O]n, were reported by Majunder
et al., with an isotropic interaction parameter of J = -0.50 and
-0.79 cm-1 respectively.34
Magnetic susceptibility
Variable-temperature magnetic susceptibility measurements were
performed at 0.1 T on powdered crystals of [Cu(H2btec)(bipy)]•
in a temperature range 2.5–292 K. The data for 1 are plotted
in Fig. 3 as cM versus T and cMT versus T, together with the
fit of the experimental data. For 1, the cMT value at 292 K is
0.38 cm3 mol-1 K, which is very close to that of the spin only value
of 0.375 cm3 mol-1 K, calculated for non interacting Cu(II) centers
with a g value equal 2.0. The value of cMT decreases smoothly
from 292 to 30 K; below this temperature the value of cMT
shows a sharper decrease reaching a value of 0.25 cm3 mol-1 K at
2.5 K, suggesting a very weak antiferromagnetic interaction at low
temperatures between the copper(II) ions. This antiferromagnetic
interaction can also be observed in the plots of M versus H at
different temperatures (see Fig. S3†).
Catalytic results
The catalytic activity of [Cu(H2btec)(bipy)]• (1) was investigated
using cyclohexene and styrene as substrates, and TBHP as the
oxidant. The catalytic oxidation results were analyzed in terms of
conversion and chemoselectivity. The reaction of epoxidation of
olefins using 1 as an heterogeneous catalyst were monitored under
nitrogen, and the conversion and chemoselectivity were estimated
at different time-intervals and temperatures. The informed TOF
(moles of epoxide per mole of copper center per hour) values
are informed for 30 ◦C, 50 ◦C, 75 ◦C and 24 h of reaction
-1
The plot of cM versus T obeys the Curie-Weiss law, in the
range of 40–292 K with cM = C/(T - q), giving a Curie constant
of 0.378 cm3 K mol-1 with a negative Weiss constant q of -0.8 K.
Table 3 Structural parameters of metal-organic hybrid compounds
Crystal system,
space group
˚ ˚
Cavity size/A Dimensionality Ipd/A Ref.
Hybrid compound
Geometry
Synthesis
(1) [Cu(H2btec)(2,2¢-bipy)]•
(2) [Cu(H2btec)(1,10-phen)]•
(3) [Cu(btec)(2,2¢-bipy)2]•
(4) [Cu(btec)(2,2¢-bipy)2]•
Monoclinic, C2/c Dist. square planar Hydrothermal
Monoclinic, C2/c Dist. square planar Hydrothermal
—
—
2D
2D
3D
3D
3D
3.63
3.64
3.56
3.41
3.26
This work
5
6
7
8
Monoclinic, P21/n Dist. square planar Conventional 4.5 ¥ 6.1
Monoclinic, P21/n Dist. tetrahedral
Hydrothermal 8.0 ¥ 4.5
Solvothermal 6.5 ¥ 6.6
(5) [Cu(btec)(1,10-phen)2]•·(H2O)• Monoclinic, P21/c Dist. tetrahedral
1424 | Dalton Trans., 2009, 1422–1427
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Table 4 Effect of the catalytic activity of 1 on the epoxidation of cyclohexenea at different temperatures and 24 h of reaction time
Chemoselectivity (%)
T/◦C
TON
TOF/h-1
Conversion (%)
Epoxide
Cyclohexanone
2-Cyclohexenone
30
50
75
738
1550
1886
31
65
79
27.4
54.5
64.5
67.3
71.1
73.1
16.8
8.9
7.7
15.8
18.3
19.2
a Reaction conditions: catalyst [Cu(2,2-bipy)(H2btec)]• 0.01 mmol; cyclohexene, 40 mmol; TBHP 40 mmol; cyclohexene/TBHP/catalyst 400:400:1,
1,2-dichloroethane 10 mL.
Table 5 Effect of the catalytic activity of 1 on the epoxidation of styrenea
at different temperatures and 24 h of reaction time
depend on the catalyst. However, with increasing reaction time
the production of benzaldehyde decreases while the concentration
of styrene epoxide increases.
Scheme 1 shows the proposed mechanism for the formation
Chemoselectivity (%)
T/◦C TON TOF/h-1 Conversion (%) Epoxide Benzaldehyde
of the epoxide based on previous literature.35–37 The mechanism
begins with the generation of the oxygenated intermediary species
I, which is produced by the coordination of the TBHP to the
Cu(II) center, with an expansion of the coordination number. In
the second step, a nucleophilic attack of the olefin substrate on
species I occurs, followed by a concerted oxygen transfer, leading
to the formation of species II and t-butanol. Finally, in the last
step the rupture of the Cu–O bond is produced, giving epoxide
(P1) and regenerating the catalyst.
30
50
75
65
314
673
3
13
28
6.2
14.5
23.7
26.0
54.2
71.0
74.0
45.8
29.0
a Reaction conditions: catalyst [Cu(2,2-bipy)(H2btec)]• 0.01 mmol; styrene,
40 mmol; TBHP 40 mmol; styrene/TBHP/catalyst 400:400:1, 1,2-
dichloroethane 10 mL.
in Tables 4 and 5. With these results it is possible to observe
that the [Cu(H2btec)(bipy)]• system presents a good efficiency as
heterogeneous catalyst for the epoxidation of cyclohexene, and a
moderate efficiency for the epoxidation of styrene.
Fig. S4† presents the conversion and chemoselectivity graphs
obtained for the cyclohexene oxidation. The cyclohexene oxidation
catalyzed by 1 yielded mainly the following products: cyclohexene
epoxide, 2-cyclohexenone and cyclohexanone, where the epoxide
corresponds to the principal product.
The conversion of cyclohexene at 75 ◦C and 24 h of reaction
time is doubled when the catalyst is used as compared with the
control reaction in the absence of 1. These results show the great
efficiency of the [Cu(H2btec)(bipy)]• catalyst on the oxidation.
The catalyst also presents a high chemoselectivity toward the
epoxide formation, reaching a value of 73.1% at 24 hours of
reaction time and 75 ◦C. For the side products such as
2-cyclohexenone and cyclohexanone, the chemoselectivity is only
19.2% and 7.7%, respectively (Table 4). The values obtained in
the absence of the catalyst for the same experimental conditions
(control experiment: 24 h, 75 ◦C) were of 0.2%, 40.0% and
43.2% for cyclohexene epoxide, 2-cyclohexenone and cyclohex-
anone respectively. Therefore, it is important to remark that
the [Cu(H2btec)(bipy)]• catalyst presents a high chemoselectivity
towards the epoxide formation.
The epoxidation of styrene was done in similar conditions
as those used for cyclohexene. The results of conversion and
chemoselectivity for the epoxidation reaction of styrene using
1 are summarized in Table 5 and Fig. S5.† The conversion of
styrene is three times lower as compared to the conversion of
cyclohexene. The oxidation reaction of styrene with TBHP yielded
only benzaldehyde and styrene epoxide as products.
Scheme 1
The difference in conversion observed for cyclohexene and
styrene is related with the mechanism involved in the oxidation
reactions. When a peroxide like as TBHP is used as oxidant,
two mechanisms for oxygen activation have been reported. One
of them involves the homolytic cleavage of the O–O bond,
producing a low chemoselectivity in the obtained products,
while the other mechanism involves the heterolytic cleavage of
the O–O bond.38 For the oxidation of cyclohexene the allylic
hydrogen is as reactive as the olefinic double bond, in the presence
of the following radical species: tert-butilperoxide C(CH3)3OO∑
and alcoxide C(CH3)3O∑ generating the secondary products,
2-cyclohexenone and cyclohexanone.31b,c However in the het-
erolytic mechanism proposed in Scheme 1, the main obtained
product is cyclohexene epoxide beside a greater total conversion in
products is obtained. In the case of styrene, only two products are
generated. These also correspond to a competitive mechanism in
which the t-butylperoxide radical attacks the double bond to give
the benzaldehyde and the heterolytic mechanism similar to the one
proposed for cyclohexene generates the cyclohexene epoxyde.
For short times of reaction (30 min) both in the presence
and absence of the catalyst, only benzaldehyde was observed as
product, indicating that the benzaldehyde production does not
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Table 6 Crystal and structure refinement data for 1
Experimental
Empirical formula
Formula weight
Temperature (K)
C20H12CuN2O8
Synthesis of [Cu(H2btec)(bipy)]• (1)
471.86
294(2)
0.71073
Monoclinic
C2/c
10.1810(18)
14.4360(18)
12.894(3)
112.94(3)
1745.2(7)
4
1.796
956
The reaction mixture for the synthesis of 1 consists of V2O5
(0.091 g), Cu(NO3)2·3H2O (0.2416 g), 2,2¢-bipyridine (0.078 g),
1,2,4,5-benzenetetracarboxylic acid (0.127 g) and water (10 mL)
in a molar ratio of 1:2:0.5:0.5:550. The mixture is loaded in a
Teflon-lined stainless steel autoclave (23 mL) and heated at 160 ◦C
for 48 h, and then the temperature is slowly lowered to 80 ◦C
(10 ◦C/h). After, the reaction vessel is slowly cooled down to
room temperature and the resulting product filtered, washed with
distilled water and dried at 60 ◦C. The solid phase consisted
of violet crystals and powder corresponding to 1. The violet
product was collected in ca. 90% yield. Crystals, suitable for X-ray
diffraction studies, were also collected by filtration several weeks
later. Microcrystalline solid 1: Anal. found (calcd.) C: 50.75%
(50.86%); N: 5.99% (5.93%); H: 2.56% (2.54%); Cu: 13.55%
(13.47%). IR spectrum: uncoordinated protonated carboxylate
group, 1718 cm-1; monodentate mode of carboxylate group, nas (COO)
1573 cm-1, ns (COO) 1380 cm-1 (Dn = 193 cm-1). If V2O5 is not used a
mixture of different products is obtained, with 1 in a lower yield.
˚
Wavelength (A)
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
b (◦)
3
˚
Volume (A)
Z
Dc (gcm-3)
F(000)
Crystal size (mm3)
0.60 ¥ 0.25 ¥ 0.10
2.59 to 27.89
-13 ≤ h ≤ 12, -18 ≤ k ≤ 18,
-16 ≤ l ≤ 16
7220, 1972(0.0360), 1664
q range for data collection (◦)
Index ranges
Reflections: collected, indep. (Rint),
observed [I > 2s(I)]
Absorption correction, m (mm-1)
Semi-empirical from
equivalents, 1.310
0.88 and 0.68
Max. and min. transmission
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
1.054
R1 = 0.0420, wR2 = 0.1121
R1 = 0.0515, wR2 = 0.1179
0.541, -0.278
-3
˚
Largest diff. peak and hole (e A )
Materials and measurements
All reagents were purchased commercially and used without
further purification. Infrared spectra were recorded in KBr pellets
using a Bruker Vector 22 FTIR instrument in the range 400–
4000 cm-1. The differential thermal analysis (DTA) and ther-
mogravimetric analysis (TGA) were performed on a NETZSCH
equipment. The TG/DTA curves were registered in the 293–
773 K range, under nitrogen atmosphere (20 mL/min) using a
5 ◦C/min heating rate. The magnetic susceptibility measurements
at variable temperature between 2.5–292 K were performed
on a polycrystalline sample, using a Cryogenics S600 SQUID
susceptometer-magnetometer at 0.1 T. The diamagnetic correc-
tions were calculated using Pascal constants.39
autoclave reactor together with 1,2-dichloroethane (10 mL) as
solvent; when the temperature reaction was reached the substrate
(cyclohexene, 40 mmol) and the oxidant (TBHP 40 mmol) were
added. Aliquots of the solution (10 mL) were removed at intervals
of 0; 0.5; 1; 2; 4; 6 and 24 h and analyzed by gas chromatography
(GC). The epoxidation reaction was also tested for styrene as sub-
strate, using the same procedure and molar ratio. Gas chromato-
graphic analyses were carried out with a Perkin Elmer 5890 GC,
equipped with a flame ionization detector (FID). Styrene was
analyzed with a capillary column Carbowax 20M (25 m ¥ 0.2 mm ¥
0.2 mm), and cyclohexene with EquityTM-1 (30 m ¥ 0.25 mm ¥
1.0 mm) column and nitrogen as carrier gas. The oxidation
products were identified by spiking, using standard compounds
and the measurements of retention times were performed using
the following conditions: temperature program 180 ◦C, initial time
5 min; 10 ◦C/min; final time 15 min, the retention times registered
X-Ray diffraction
A violet 0.60 ¥ 0.25 ¥ 0.10 crystal of compound 1 was taken directly
from the synthesis vessel. Data collection was made on a Bruker
SMART APEX diffractometer, using 0.3◦ of separation between
frames and monochromated Mo Ka radiation (l = 0.71073). Data
integration was made using SAINT. Absorptions corrections have
been made using SADABS. The structure was solved by means of
direct methods using XS in SHELXTL and completed by Fourier
Difference Synthesis. Refinement until convergence using XL
SHELXTL. Table 6 presents the crystal and structure refinement
data.40–45 Phase identification was carried out with X-ray powder
diffraction technique on a D5000 Siemens diffractometer, using
were: cyclohexene, TR = 10.3 min; cyclohexene epoxide, TR
=
15.2 min; 2-cyclohexen-1-one, TR = 17.2 min; cyclohexanone,
TR = 16.0 min, and the retention times for styrene, TR = 9.9 min;
styrene oxide, TR = 16.1 min; benzaldehyde, TR = 15.6 min.
Conclusions
The metal organic framework [Cu(H2btec)(bipy)]• was prepared
by hydrothermal synthesis and characterized by X-ray diffraction,
thermal gravimetric analysis and magnetic susceptibility. This
material was tested as catalyst for the oxidation of olefins, and
to the best of our knowledge the first MOF copper(II) catalyst,
with H2btec and bipy as ligands, for the oxidation of olefins is
being reported.
The use of 1 as catalyst for the conversion of cyclohexene and
styrene to the corresponding epoxide products, at 75 ◦C and 24 h
of reaction, doubles the percentage of the product with respect to
the control experiment. It is important to remark that the activity
˚
Cu Ka1 radiation (l = 1.54056 A).
Catalytic studies
The epoxidation reactions were performed using a reactor (25 mL)
provided with a mechanical stirrer and an oil bath. All the
reactions were carried out un◦der nitrogen atmosphere. Experi-
◦
◦
ments were done at 30 C, 50 C and 75 C, using the following
reaction condition: the catalyst (0.01 mmol) was added in the
1426 | Dalton Trans., 2009, 1422–1427
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of the catalyst with cyclohexene is almost three times higher than
with styrene.
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The authors thank FONDAP 11980002 grant for financial
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