Heterometallic CuII/LnIII polymers active in the catalytic aerobic oxidation of cycloalkenes under solvent-free conditions

Article abstract of DOI:10.1039/c8dt01913d

Heterometallic 3d-4f inorganic polymers were prepared using 3,5 pyridinedicarboxylic acid (H₂PDC), {[CuLn₂(PDC)₂(SO₄)₂(H₂O)₆]·H₂O}_n (Ln: Sm^III, CuSmPDC, Eu^III, CuEuPDC, Gd^III, and CuGdPDC). These catalysts are active in the aerobic oxidation of cycloalkenes under solvent-free conditions, with a conversion for the oxidation of cyclohexene of 71% after one hour of the reaction, and a TOF value of 1438 h^-1 for CuSmPDC. On the other hand, the oxidation of cycloheptene and cyclooctene exhibited slightly lower conversions of 52% and 47%, and TOF values of 1053 and 159 h^-1 after 1 and 6 hours of the reaction, respectively. The radical mechanism for the oxidation reaction of cyclohexene was assessed by Raman and EPR spectroscopy. The first evidenced the formation of Cu-O₂ adducts and the second permitted is to observe the presence of the oxygen centered radical species, which act as initiators of the reaction chain to generate the products. An increase in the temperature of the reaction correlates with the adduct formation, and with the enhancement of the oxidation reaction.

Full text of DOI:10.1039/c8dt01913d

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II
III
Heterometallic Cu /Ln polymers active in the
catalytic aerobic oxidation of cycloalkenes under
solvent-free conditions†
Cite this: DOI: 10.1039/c8dt01913d
a,b
a
a,b
a
b,c
P. Cancino, L. Santibañez, P. Fuentealba, C. Olea, A. Vega
and
a,b
E. Spodine
*
Heterometallic 3d–4f inorganic polymers were prepared using 3,5 pyridinedicarboxylic acid (H
2
PDC),
III
III
III
{
2 2 4 2 2 6 2 n
[CuLn (PDC) (SO ) (H O) ]·H O} (Ln: Sm , CuSmPDC, Eu , CuEuPDC, Gd , and CuGdPDC). These cat-
alysts are active in the aerobic oxidation of cycloalkenes under solvent-free conditions, with a conversion
for the oxidation of cyclohexene of 71% after one hour of the reaction, and a TOF value of 1438 h− for
CuSmPDC. On the other hand, the oxidation of cycloheptene and cyclooctene exhibited slightly lower
conversions of 52% and 47%, and TOF values of 1053 and 159 h− after 1 and 6 hours of the reaction,
respectively. The radical mechanism for the oxidation reaction of cyclohexene was assessed by Raman
1
1
Received 11th May 2018,
Accepted 23rd July 2018
and EPR spectroscopy. The first evidenced the formation of Cu–O adducts and the second permitted is
2
to observe the presence of the oxygen centered radical species, which act as initiators of the reaction
DOI: 10.1039/c8dt01913d
chain to generate the products. An increase in the temperature of the reaction correlates with the adduct
formation, and with the enhancement of the oxidation reaction.
rsc.li/dalton
Inorganic polymers offer a good choice to be used as cata-
lysts in this type of reaction, since they are insoluble and
Introduction
The oxidation of olefins, especially the reaction of cycloalk- stable under different reaction conditions, such as the temp-
8
enes, has been studied by many research groups due to the erature used in the oxidation reaction of cycloalkenes. Several
fact that it is possible to obtain oxidation products (epoxides, reviews can be found in the literature about inorganic poly-
9
–17
alcohols, ketones or aldehydes), which are used in the manu- mers as reusable catalysts in heterogeneous catalysis,
and
facture of perfumes, agrochemicals, and glues, and in the examples related to catalytic systems based on homonuclear
1
–4
18,19
20,21
22,23
pharmaceutical industry.
produce new efficient routes to obtain these products, to manganese(II,III) or iron(III) ions.
reduce the cost of the processes, and to reduce the formation cycloalkenes, copper-based MOFs can catalyze the oxidation of
Thus, the focus has been to MOFs with copper(II),
cobalt(II),
vanadium(IV),
2
4–26
Besides olefins and
5
27,28
of environmentally harmful byproducts.
The classical way to obtain the mentioned oxidation pro-
ducts has been by using homogeneous or supported cata- reported as catalysts in reactions such as hydrogenation of
lysts. The problem with this type of catalyst is well known to small olefins,
all researchers: the inability to recover the homogeneous cata-
other substrates, such as alkanes and cycloalkanes.
On the other hand, heterometallic MOFs have been
6,7
29,30
degradation of methyl blue and rhodamine
and the Michael reaction. Moreover, both mixed-metal
3
1,32
33
B
lysts, and for the supported materials, the high level of leach- and mixed-linker metal organic frameworks as heterogeneous
3
4
ing or decomposition of the catalyst, which prevents its catalysts have been studied. In the field of oxidation, hetero-
reusability.
metallic MOFs also show catalytic activity for different kinds of
substrates. For example, Cu based MOFs with adsorbed Pd
II
and Au nanoparticles have been used in the oxidation of
3
5,36
a
benzylic alcohols,
and a bimetallic catalyst of Pd–Au nano-
Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Santiago,
III
Chile. E-mail: espodine@uchile.cl
particles supported on an Al MOF was used in the aerobic
oxidative reaction of carbonylation of amines.
Besides the choice of the correct catalyst, which is related to
the activity and the selectivity it originates, variables such as
the reaction conditions related to temperature, solvents, and
oxidizing agents are very important as well. Among the
b
37
Centro para el Desarrollo de la Nanociencia y la Nanotecnología, CEDENNA,
Santiago, Chile
c
Facultad de Ciencias Exactas, Dpto. de Ciencias Químicas, Universidad Andrés
Bello, Viña del Mar, Chile
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8dt01913d
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mentioned factors, commonly used oxidants are TEMPO, iso- Cu: 6.23 (6.25); C14
Dalton Transactions
3
8
H
18CuGd
2
N
2
O
22
S
2
H
2
O; CuGdPDC. Exp.
3
9
40,41
butyraldehyde, and tert-butylhydroperoxide,
which have (calc %): C: 16.60 (16.38); H: 2.23 (1.96); N: 2.85 (2.72); S: 6.18
the problem of generating unwanted byproducts such as tert- (6.25); Cu: 6.15 (6.19).
4
2
43
butanol and hydrogen peroxide. Therefore, a better alterna-
tive is the use of molecular oxygen as the oxidizing agent,
Elemental analysis
since it does not generate any unwanted byproducts. Some The C, N, S and H contents of the bulk compounds were deter-
reports where molecular oxygen is used to oxidize olefins or mined using a Carlo-Erba EA-1108 microanalyzer and Cu ana-
cycloalkenes, with MOFs as catalysts, can be found in the lit- lysis was done on an atomic absorption spectrophotometer,
erature. For example, the oxidation of cyclohexene by mole- Shimadzu model AA-6200.
II
cular oxygen was reported, using MOF catalysts based on Ni
II
Thermogravimetric analysis
or Co [M (DOBDC)(H O) ]·8H O (DOBDC = 2,5-dihydroxy-
2
2
2
2
terephthalic acid). However, the obtained conversion was only The thermogravimetric analyses were done on NETZCH Iris
4
4
II
of 13% at 80 °C. Better results were reported using a Co
equipment. The TGA curves were recorded under a nitrogen
−
1
catalyst, [Co(L-RR)(H O)]·H O (L-RR = (R,R)-thiazolidine-2,4- atmosphere (20 mL min ) in the 20–800 °C range, using a
2
2
dicarboxylate) at a slightly lower temperature of 70 °C, increasing 10 °C min− heating rate (Fig. S1†).
1
the oxygen pressure from 1 to 5 bar. The obtained conversion
4
5
Single crystal X-ray diffraction
was of 37% with a selectivity of 49% to 2-cyclohexe-1-one.
III
III
Similarly, M-MIL-101 and M-MIL-100 (M: Cr or Fe ) were The data were collected on a Bruker Smart Apex diffractometer
used in the oxidation of cyclohexene at 60 °C, using molecular at 293 K, using separations of 0.3° between frames, and 10 s by
3
2
oxygen and a small amount of TBHP as the initiator of the frame. Data integration was made using SAINTPLUS. The
reaction. The conversion was not superior to 44% when Fe- structure of {[CuLn (PDC) (SO (H O) ]·H O} was solved by
2
2
4
)
2
2
6
2
n
3
3
MIL-100 was used. Moreover, at this temperature, the reaction direct methods using XL SHELXTL and completed (non-H
presented a competition between homogeneous and hetero- atoms) by Fourier difference synthesis. Refinement until con-
3
4
geneous catalysis. The reaction was genuinely heterogeneous vergence was obtained using XL SHELXTL and SHELXL97.
4
6
at 40 °C, but the conversion dropped to only 10%.
Recently, our group developed new CuLa-MOF, geometric basis and refined with restrictions. Table S1† sum-
based on 3,5-pyridinedicarboxylic acid (H PDC), marizes crystallographic data while the main geometric para-
[Cu0.5La (HPDC)(PDC) (SO )(H O) ]H O} (CuLaPDC), meters obtained for {[CuLn (PDC) (SO (H O) ]·H O} (CCDC
which showed good catalytic performance in the aerobic 1585350, 1585351, and 1585352) are given in Table S2.†
All hydrogen atoms were calculated in idealized positions on
a
2
{
2
2
4
2
2
2
n
2
2
4
)
2
2
6
2
n
47
oxidation of cycloalkenes and benzylic substrates. In the
present work, we report a new family of heterogeneous cata-
Powder X-ray diffraction (PXRD)
III
III
lysts, {[CuLn (PDC) (SO ) (H O) ]·H O} (Ln: Sm , Gd , A Siemens D5000 diffractometer was used to record all powder
Eu ), which have the same ligands as the previously reported diffractograms. The obtained diffractograms for CuSmPDC,
2
2
4 2
2
6
2
n
III
4
7
CuLaPDC, but are not isostructural with this polymer.
CuEuPDC, and CuGdPDC were compared with the calculated
patterns generated from the single crystal X-ray data
(
Fig. S2–S4†). Besides, the powder diffractograms of the reused
catalyst were compared with those of the pristine catalysts, in
order to assess the integrity of the frameworks (Fig. S5†).
Experimental
III
Synthesis of {[CuLn (PDC) (SO ) (H O) ]·H O} (Ln: Sm ,
2
2
4 2
III
2
6
2
n
III
CuSmPDC; Eu , CuEuPDC; Gd , CuGdPDC)
Nitrogen sorptometry
{
[CuLn (PDC) (SO ) (H O) ]·H O} (CuLnPDC) were obtained The surface area of the studied catalysts was measured
2 2 4 2 2 6 2 n
by hydrothermal synthesis. 1 mmol of 3,5-pyridinedicarboxylic using the Brunauer–Emmett–Teller (BET) method, on a
acid (H PDC), 0.25 mmol of lanthanide oxide (Ln ) and Quantachrome instrument, with a previous thermal treatment
.5 mmol of copper sulfate pentahydrate (CuSO ·5H O) were of the catalysts at 120 °C. The results of the physical character-
2
2 3
O
0
4
2
2
mixed with 10 mL of water in a Teflon vessel. The reaction ization of the catalyst by N sorptometry show that the specific
mixture was heated at 170 °C for 72 h in a Parr autoclave. area is <1 m gr−1. See the ESI (Fig. S6†).
2
Then, the reaction mixture was cooled for 10 h until it reached
room temperature. The products were filtered and dried at
Raman measurements
room temperature. Light blue crystals, together with a light Raman spectra were recorded with a 514 nm laser excitation
blue microcrystalline solid, were obtained; the crystals were on a Renishaw Raman Spectrometer (“Reflex”) equipped with a
separated manually from the bulk product, being suitable for CCD detector. The laser power on the sample was 25 mW and
single crystal X-ray diffraction. Yield: ca. 30%. Elemental ana- a total of 20 acquisitions were taken for each spectrum. A com-
lysis: C H CuSm N O S H O; CuSmPDC. Exp. (calc %): C: mercial Linkam cell (THMS600) was used for collecting spectra
1
4
18
2
2
22
2
2
1
6
6.81 (16.60); H: 1.98 (1.99); N: 2.60 (2.77); S: 6.32 (6.33); Cu: under controlled conditions. Thus, the sample was first sub-
2 2 22 2 2 2
.24 (6.27); C14H18CuEu N O S H O; CuEuPDC. Exp. (calc %): jected to a N
flow (20 ml min− ) at 120 °C for ∼3 h. Afterward,
1
−
1
C: 16.64 (16.55); H: 2.23 (1.98); N: 2.82 (2.75); S: 6.22 (6.31); the gas was changed to O (20 ml min ) at the same tempera-
2
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ture. Finally, the temperature was lowered to 25 °C under an
O2 flow and changed to N . Spectra were collected at 25 °C
2
2
under a N flow.
Electron paramagnetic resonance (EPR)
The EPR spectra were recorded on Bruker ESP 3220 equip-
ment. The used parameters were modulation frequency, 50
4
kHz; modulation amplitude, 2 G; receiver gain: 7 × 10 ; conver-
sion time, 40.96 ms; time constant, 81.92 ms.
Catalytic reactions
In a typical reaction, 10 mmol of cycloalkene (cyclooctene,
cycloheptene or cyclohexene) were introduced in closed
reinforced glass vessels (bottom flask of 25 mL capacity, which
can endure a pressure of 5 bar), together with 0.005 mmol of
the catalyst. These mixtures were stirred at variable tempera-
tures (50–120 °C). After the reaction was quenched, the catalyst
was filtered through a Nylon filter (0.2 μm) and the reaction
mixture was analyzed by GC. Nitrobenzene was used as an
external standard. GC and GC-MS, using available commercial
samples as standards, identified the products.
The reusability of the catalyst was tested using cyclohexene
as a substrate. After the reaction was quenched, the reaction
mixture was filtered and the catalyst was dried and reused
directly without further treatment for a subsequent run with
fresh cyclohexene. The absence of active species leached from
the solid to the solution was quantified using ICP. The
amount of copper in solution was less than 0.001% for the
three studied catalysts. Homogeneous catalytic reactions were
carried out using soluble salts in cyclohexene, Cu(acac)
2
and
Ln(acac) , in order to probe the influence of each metal ion on
3
the oxidation reaction.
II
Fig. 1 (a) Coordination sphere of Cu ions, (b) coordination sphere of
III
Ln ions, and (c) propagation of the framework; view along the [100]
direction. Legend: = Cu; = Ln; = S; = O; = C; = N; ○ = H.
Results and discussion
Structural description
The inorganic polymers based on copper(II) and lanthanide(III) Fig. 1c shows the way the inorganic polymer propagates along
III
III
2 2 4 2 2 6 2 n
metal ions {[CuLn (PDC) (SO ) (H O) ]·H O}
(Ln: Sm , Eu , the [100] direction. Table S2† summarizes the interatomic dis-
III
and Gd ; PDC: 3,5-pyridinedicarboxylate) are isostructural tances and angles, corresponding to the first coordination
and crystallize in the triclinic system with space group P ˉ1 . sphere of both metal centres.
Fig. 1 shows the first coordination sphere of the copper(II) and
The framework grows along the a, b and c axes, generating
lanthanide(III) ions. The geometry around the Cu(II) is an a covalent 3D non-porous structure. The sulphate groups act
axially elongated octahedron, being completed by two oxygen as bridges between two lanthanide(III) atoms, and one
2
−
atoms from two equivalent sulphate groups (Cu2–O2; Cu2– copper(II) atom, while the PDC bridges three lanthanide(III)
i
O2 ), two oxygen atoms from two ligand water molecules (Cu2– atoms and one copper(II) atom.
i
2−
O1; Cu2–O1 ), and two nitrogen atoms from two PDC groups
The thermal stability for the different catalysts was studied
i
II
(
Cu2–N1; Cu2–N1 ) (Fig. 1a). Besides, the Cu ion is a c type using thermogravimetric analysis. Fig. S1† shows the first step
crystallographic inversion centre. The Ln centres are octa- where the compound loses the crystallization water, and from
coordinated; the coordination sphere around Ln is com- 120 °C to 250 °C the solid loses the coordinated water mole-
pleted by three oxygen atoms from three different PDC
III
III
2
−
II
III
cules of the Cu and Ln centres. The next step corresponds
groups (Ln1–O6; Ln1–O7; Gd1–O8), three oxygen atoms from to the decomposition of the organic material and sulphate
two sulphate groups, and two oxygen atoms from two water groups in different processes until 800 °C. The thermal
molecules (Ln1–O10; Ln1–O11). The sulphate ligand chelates process did not reach an asymptotic value near this tem-
one lanthanide(III) centre (O3–Ln1–O4), and binds in a mono- perature, indicating that the decomposition of the sample had
dentate way a second lanthanide atom (Ln1–O5) (Fig. 1b). not ended. The thermal analysis permits us to conclude that
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the catalyst is stable at the temperature used in the catalytic radical path as the main reaction pathway in the catalyzed
study.
system (Fig. S8†).
The reaction conditions were optimized using CuSmPDC in
the oxidation of cyclohexene at different temperatures from 50
to 120 °C (Fig. 2). The reaction was carried out with an initial
pressure of molecular oxygen of 5 bar, which was used as the
oxidizing agent.
The conversion increased as the temperature was increased,
the reaction time decreasing considerably for the attainment
of the same conversion values. Moreover, the maximum con-
version of 71% was reached after 1 hour of the reaction when
the reaction was carried out at 120 °C.
The effect of the oxygen concentration was tested using
pressures from 1 to 5 bar (Fig. 3). These results show a direct
dependence of the conversion on the oxygen pressure; the best
result was 71% after 1 hour of the reaction using 5 bar of the
initial pressure. In addition, an experiment performed with an
initial 5 bar pressure was recharged with 5 more bar of oxygen,
when the first amount of oxygen was consumed. For this reac-
tion, the conversion reached 90% after one additional hour.
The selectivity was also pressure dependent; Fig. S10† shows
the selectivity for all experiments. The main product at 1 bar
was cyclohexenol (56%) and cyclohexenone (34%), while the
main product at 5 bar was cyclohexenone (51%) and cyclohexe-
nol dropped to 44%. These results can be easily explained
since cyclohexenol is an intermediate product of oxidation,
which with the increase of pressure is transformed into the
ketone. Furthermore, when the reaction mixture was recharged
Catalytic results
The bulk compounds were first used as catalysts in the aerobic
oxidation of cyclohexene. The powder diffractograms of the
bulk of the three catalysts were compared with the respective
simulated patterns obtained from single crystal data. The
results confirmed that the bulk samples correspond to the
diffracted single crystals (Fig. S2–S4†). Scheme S1† presents
the products obtained in the presence and absence of the
different catalysts. We assumed that the catalytic reactions
occur on the surface of the inorganic polymers, because the
studied compounds do not present pores in their structure
(Fig. S6†). These systems are similar to that reported by
II
Cancino et al. for a heterometallic catalyst based on Cu and
III 47
La .
The catalytic results showed a similar conversion for the
three catalysts, around 70% after 1 hour of reaction (Fig. S7
and S8†). Besides, the main product was 2-cyclohexenone
(50%) for all the studied catalysts. Table 1 summarizes the
catalytic results obtained for this family of catalysts; the results
do not present an appreciable difference for the three studied
catalysts.
A blank experiment in the absence of a catalyst gave a much
lower conversion, thus corroborating that the heterometallic
catalyst promotes the main path for the oxidation of the sub-
strate. Besides, the blank experiment produced a higher
amount of epoxide compared with the catalytic experiments.
This increase of epoxide in the blank experiment suggests that
the catalyst enhances the formation of radical species, which
produce cyclohexenone as the main product. Moreover, when
the reaction was carried out in the presence of a catalyst and
hydroquinone, the conversion dropped to ca. 30%, compared
with 70% in the catalytic reactions (Table 1), corroborating the
III
Table 1 Oxidation of cyclohexene at 1 hour using CuLnPDC (Ln: Sm ,
III
III
Eu , and Gd ) as catalysts
Conv. (%) TOF (h−
1
)
Products
Selectivity (%)
Catalyst
CuSmPDC 71
1438
1401
1396
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
55
42
3
51
44
5
51
43
6
46
43
Fig. 2 Conversion at different temperatures for the oxidation of cyclo-
hexene, using CuSmPDC as catalyst.
CuEuPDC 69
CuGdPDC 68
Blank
30
Cyclohexene oxide 11
Reaction conditions: Catalyst (0.005 mmol, based on copper
(
0.00031 mmol)), substrate (10 mmol), 120 °C. Ratio S/C = 32 000/1 or
0
.003 mol% of copper per mole of substrate. TON is calculated as
II
moles of products per mole of the active site (Cu ions); TOF is calcu-
II
lated as moles of products per mole of the active site (Cu ions) per Fig. 3 Dependence of the conversion for the oxidation of cyclohexene
hour (TOF values were calculated using the time reported in the table).
on oxygen pressure, using CuSmPDC as a catalyst.
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with 5 bar after the initial oxygen was consumed, the main Table 2 Catalytic results for the oxidation of cycloalkenes using
CuSmPDC
product continued to be the ketone (64%), while one extra
product was detected, 1,2-cyclohexanediol (11%) (Fig. S10†).
Consumption of
Time
(h)
Conv.
(%)
TOF
Selectivity
(%)
These results can be explained by assuming that the new
amount of oxygen allowed the oxidation reaction to continue
with the generation of the diol.
Under the used optimal conditions, the catalyst was
active for at least four cycles (Fig. 4), with the loss of its activity
being less than 8%; the main product for all the cycles was
always 2-cyclohexenone. The stability of the catalyst was veri- (0.00031 mmol)), substrate (10 mmol), 120 °C. Ratio S/C = 32 000/1 or
fied using PXRD, which showed that the framework of the cata-
lyst remained invariable after four catalytic cycles (Fig. S5†). All
−1
Subs.
O
2
(bar)
(h
)
C6
C7
C8
5
5
3.5
1
1
6
71
52
47
1438
1053
159
55 (ketone)
40 (ketone)
99
(epoxide)
Reaction conditions: Catalyst (0.005 mmol, based on copper
0
.003 mol% of copper per mole of substrate. Initial pressure was 5 bar.
TOF is calculated as moles of products per mole of the active site (Cu
ions) per hour (TOF values were calculated using the time reported in
II
the diffractograms of the re-used catalysts showed the main the table).
peaks of the pristine catalysts. However, additional peaks
appeared which indicated the presence of some kind of un-
identified impurity (Fig. S5†). Moreover, the amount of copper
in solution was quantified using plasma-ICP, and the detected
Finally, the optimized conditions were used to test the cata-
amount was less than 0.001% after each cycle. Thus, the lytic performance of CuSmPDC in the oxidation of two other
heterogeneous nature of the catalyst was demonstrated.
cycloalkenes: cycloheptene (C7) and cyclooctene (C8), as a way
Control experiments in homogeneous media were carried of testing the scope of other substrates. Scheme S1† shows the
II
III
out in order to study the influence of Cu or Ln on the cata- detected oxidation products: epoxide (b), conjugated alcohol
lytic oxidation reaction, using the corresponding amount of (c) and conjugated ketone (b).
Cu(acac)2 or Ln(acac) (H O) , equivalent to the copper(II) or
The results showed that the conversion decreased with the
3
2
3
lanthanide(III) concentration present in the catalysts. It is increase of the number of carbon atoms in the cyclic substrate
important to emphasize that the environment around the ions since lower conversions were obtained for cycloheptene and
in the used salts is not exactly the same as that of the ions in cyclooctene (Table 2). The selectivity of the main product also
the studied inorganic polymers, since the copper(II) ion is tet- varied for the different cycloalkenes. The main oxidation
racoordinated square planar, while the lanthanide ions are product for cyclohexene and cycloheptene was the corres-
non-coordinated, with three labile water molecules in the first ponding conjugated ketone. In contrast, the oxidation of
coordination sphere. Nevertheless, interesting results were cyclooctene produced almost exclusively cyclooctene epoxide
obtained. Cu(acac)
2
, as the homogeneous catalyst, permitted as the main product (Table 2). Cyclooctene epoxide is pro-
us to obtain similar selectivity to the corresponding heterome- duced due to the lower reactivity of the allylic position in
tallic catalyst (51, 43 and 6% for cyclohexenone, cyclohexenol cyclooctene, compared to that of the other studied alkenes.
and cyclohexene oxide, respectively). On the other hand, Moreover, this substrate showed a null conversion in the
Gd(acac) (H O) and Eu(acac) (H O) gave similar selectivities absence of the heterometallic catalyst (blank experiment,
3
2
3
3
2
3
to those of the blank experiment (46, 43 and 11% for cyclohex- Table S4†). Besides, the addition of hydroquinone completely
enone, cyclohexenol and cyclohexene oxide, respectively, inhibited the catalyzed reaction in the presence of CuSmPDC
Table S3†). Thus, the selectivities permit us to assess that the (Table 2 and Fig. S9†). In this way, it is possible to assess that
copper(II) centres of the heterometallic polymers guide the for- the geometry and the conformation of a small ring favor the
mation of the products.
allylic oxidation to cyclohexenone and cycloheptenone, while
for the medium size ring, such as in cyclooctene, epoxidation
is favored.
4
8,49
Reaction mechanism
In order to obtain some insight into the mechanism of the
aerobic oxidation of cyclohexene, catalyzed by the CuLnPDC
frameworks, combined analyses by means of Raman and EPR
spectroscopy were performed. The Raman spectra of the cata-
lyst were recorded at different temperatures under an oxygen
or nitrogen flow. The spectrum recorded in the presence of
molecular oxygen at 120 °C presented two bands at 996 and
1
1
043 cm− , reported in the literature as the O–O vibrations of
5
0–52
the superoxide species.
Two more bands at 796 and
1
8
34 cm− were also present in the spectrum; these are typically
Fig. 4 Percentages of conversion and selectivity (ketone) after 1 hour
of the reaction for the four cycles.
5
0,53
observed for metal-peroxide vibrations
(Fig. 5).
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Fig. 5 Raman spectra evidencing the interaction between the catalyst
and the oxidant.
Fig. 7 EPR spectrum of the reaction in the presence of DMPO as a spin
trap.
These results confirm the presence of Cu–O
2
adducts
involved in the reaction mechanism, these species being
responsible for the onset of the oxidation reaction. It is poss-
ible to correlate the intensity of the band at 1043 cm at three these results. While the Raman spectra showed the interaction
different temperatures with the conversion values of the oxi- between the catalyst and the oxidant, generating the Cu–O
Thus, the initial stage of the reaction could be explained by
−1
2
dation reaction. Thus, as the temperature was increased, the adducts, the hyperfine constant of the EPR indicated that an
absorption band became more intense, evidencing a higher oxygen centred radical was trapped. These radicals are the
concentration of the Cu–O
2
adduct, leading to higher conver- initiators of the chain reactions which generate the different
sions (Fig. 2 and 6).
The Raman spectra showed the formation of Cu–O
adducts, while in order to detect the radical species, EPR spec-
products, reactions that are well described in the literature
(Scheme S2†).
To the best of our knowledge, homonuclear Cu-MOFs have
5
6–58
2
troscopy was used. The reaction was carried out with the same also been used as catalysts in the aerobic oxidation of cycloalk-
procedure described above, but adding 5,5-dimethyl-1-pyrro- enes. The results permit us to conclude that the reaction con-
line N-oxide (DMPO) as a spin trap.
ditions and the coordination sphere around the copper centre
The obtained reaction mixture was filtered in order to sep- are very important in the catalytic process. Under mild con-
arate the catalyst, and the EPR spectrum of the reacting solu- ditions, it is necessary to use a solvent and a co-catalyst or an
tion was recorded. This showed the classical pattern of the activator, as reported for [Cu
DMPO with oxygen centred radical adducts. Furthermore, the catalyst in acetonitrile at 40 °C in the aerobic oxidation of
hyperfine coupling constant a
= 13.0 G was also typical of the cyclooctene, assisted by trimethylacetaldehyde. The results
interaction of DMPO with oxygen centred radical species gave a conversion of 99% and a selectivity of 99% for cyclo-
3 2
(BTC) ], which was used as a
N
5
4,55
59
(Fig. 7).
octene oxide.
This is an excellent catalytic system, but the use of solvent
and the formation of byproducts remain a problem for the
environment. An example of a catalytic system under solvent-
free conditions and oxygen as the sole oxidant is the use of
2 2 4 2
[Cu(bpy)(H O) (BF ) (bpy)] (bpy: 4,4′-bipyridine) in the oxi-
dation of cyclohexene (45 °C and 1 atm of molecular oxygen).
However, this system shows a conversion of less than 10% and
a high selectivity to the conjugated hydroperoxide (close to
6
0
9
0%). Another catalyst used under similar conditions (80 °C,
1
bar and 10 h of reaction) was [Cu (OH)(BTC)(H O)] ·2nH O
2
2
n
2
(
BTC = 1,3,5-benzenetricarboxylic acid), which gave a conver-
sion of only 11.7%, being now the main product 2-cyclohexe-
4
4
none (36%). In order to increase the conversion under
solvent-free conditions, the temperature has to be increased.
This fact was proved by using copper oxide as a catalyst in the
oxidation of cyclohexene at 100 °C with 4 bar of oxygen as the
sole oxidant. This catalytic system allowed conversion of 76%
after 6 h of the reaction and a selectivity of 60% for the
2
Fig. 6 Temperature dependence of the formation of the Cu–O adduct
6
1
in the presence of molecular oxygen.
ketone.
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Paper
An example of a heterometallic catalyst is {[Cu0.5La
2
(HPDC) acknowledge the group of Prof. García; Universitat Politécnica
(
PDC) (SO )(H O) ]·H O} , also active in the oxidation of cyclo- de Valencia, Spain, for recording the Raman spectra.
2 4 2 2 2 n
hexene at 120 °C, under a pressure of 5 bar of oxygen. The con-
version after 6 h was 70% with 60% of selectivity to ketone,
−
1 47
with a TOF value of 61 h
.
These results are very interesting
since it was the first heterometallic 3d–4f MOF used for this
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