Influence of the lanthanide(III) ion in {[Cu3Ln2(oda)6(H2O)6]?nH2O}n (LnIII: La, Gd, Yb) catalysts on the heterogeneous oxidation of olefins

Article abstract of DOI:10.1039/c6cy02115h

{[Cu₃Ln₂(oda)₆(H₂O)₆]?nH₂O}_n (Ln^III: La, Gd, Yb; odaH₂: oxydiacetic acid) are reported as reusable heterogeneous catalysts in the oxidation of olefins. An influence of the Ln^III ion on the catalytic performance of the series is observed, where the Yb^III based framework presents a larger activity. The mentioned heteronuclear species are catalytically more active than the corresponding homonuclear catalyst {[Cu(oda)₂]?0.5H₂O}_n. The use of t-butyl hydroperoxide (TBHP) as an oxidant gave conversions between 73-63% for styrene oxidation and between 57-48% for cyclohexene in dichloroethane/water (DCE/H₂O). In four cycles, the loss of catalytic activity was less than 10%. Experimental data permit the consideration of the redox active Cu^II centres as the initiators of radical species generated from TBHP, which are responsible for the oxidation process of the studied olefins. Electron paramagnetic resonance (EPR) spectra of the reaction solutions, obtained in the presence of the spin trap PBN (N-tert-butyl-α-phenylnitrone), corroborate radical species formation during the oxidation process. DFT calculations support an electronic influence of the Ln^III ions on the reactivity of the Cu^II centre, associated with changes in the stabilization of the empty Cu^II 3d_{x2 -y2} orbital, affecting the reduction of Cu^II with TBHP to Cu^I. Model calculations employing several density functionals support a higher electron affinity in the CuGdMOF system in comparison to CuLaMOF. In this way, electronic structure calculations agree with the interesting observed trend of the influence of the secondary metal centre (Ln^III ions) on the properties of the active centre (Cu^II).

Full text of DOI:10.1039/c6cy02115h

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PAPER
Influence of the lanthanideIJIII) ion in
III
{
[Cu Ln IJoda) IJH O) ]·nH O} (Ln : La, Gd, Yb)
3
2
6
2
6
2
n
Cite this: DOI: 10.1039/c6cy02115h
catalysts on the heterogeneous oxidation of
olefins†
P. Cancino, V. Paredes-García, C. Aliaga, P. Aguirre,*a
ab
bc
bd
bd
ab
D. Aravena and E. Spodine*
III
{
[Cu
3
Ln
2
IJoda)
6
IJH
O) ]·nH O}
2 6 2 n
(Ln : La, Gd, Yb; odaH
2
: oxydiacetic acid) are reported as reusable heteroge-
III
neous catalysts in the oxidation of olefins. An influence of the Ln ion on the catalytic performance of the
III
series is observed, where the Yb based framework presents a larger activity. The mentioned heteronuclear
species are catalytically more active than the corresponding homonuclear catalyst {[CuIJoda)
2 2 n
]·0.5H O} .
The use of t-butyl hydroperoxide (TBHP) as an oxidant gave conversions between 73–63% for styrene oxi-
dation and between 57–48% for cyclohexene in dichloroethane/water (DCE/H
2
O). In four cycles, the loss
II
of catalytic activity was less than 10%. Experimental data permit the consideration of the redox active Cu
centres as the initiators of radical species generated from TBHP, which are responsible for the oxidation
process of the studied olefins. Electron paramagnetic resonance (EPR) spectra of the reaction solutions,
obtained in the presence of the spin trap PBN (N-tert-butyl-α-phenylnitrone), corroborate radical species
III
formation during the oxidation process. DFT calculations support an electronic influence of the Ln ions
II
II
Received 6th October 2016,
Accepted 24th November 2016
on the reactivity of the Cu centre, associated with changes in the stabilization of the empty Cu 3d
−y
2
or-
x
2
II
I
bital, affecting the reduction of Cu with TBHP to Cu . Model calculations employing several density func-
tionals support a higher electron affinity in the CuGdMOF system in comparison to CuLaMOF. In this way,
electronic structure calculations agree with the interesting observed trend of the influence of the second-
DOI: 10.1039/c6cy02115h
III
II
www.rsc.org/catalysis
ary metal centre (Ln ions) on the properties of the active centre (Cu ).
tion mixtures are associated with these homogeneous cata-
Introduction
lysts. These disadvantages can be minimised when the
homogeneous catalysts are immobilised into insoluble sup-
ports or with the use of heterogeneous catalysts. In relation to
this last issue, metal–organic frameworks (MOFs) have all the
advantages of soluble metal complexes (selectivity, activities)
and due to their low solubility in organic solvents allow their
use in several catalytic cycles under heterogeneous
conditions.
The oxidative functionalization of olefins is an effective reac-
tion both for organic synthesis and in the production of fine
chemicals. Olefins can undergo several different modes of at-
tack by the oxidant, with the products being derived from the
epoxidation and oxidative cleavage of the double bond, as
well as the oxidation of the allylic C–H bond. Many soluble
metal complexes have been found to be active catalysts in the
oxidation reaction in the last few decades.
problems such as corrosion, deposition on reactor walls, diffi-
culty in recovery and separation of the catalyst from the reac-
1
2
–5
However, some
Metal–organic frameworks (MOFs) are a subfamily of coor-
6
dination polymers, which have been studied in the last
twenty years in many different fields of chemistry, such as
magnetism, non-linear optics, gas storage, luminescent de-
7
–13
vices and heterogeneous catalysis, among others.
The
a
Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Casilla
chemical characteristics of the organic linkers can generate
porous structures, thus increasing the active surface of the
catalyst. Besides, one of the advantages of MOFs, as com-
pared with the supported catalysts, is that they are in many
cases quite stable to leaching and therefore can be recycled.
For these reasons, MOFs are being widely studied as hetero-
2
33, Santiago, Chile. E-mail: paguirre@ciq.uchile, espodine@uchile.cl;
Fax: +56 2 29782868
b
Centro para el Desarrollo de Nanociencia y Nanotecnología (CEDENNA),
Santiago, Chile
c
Universidad Andrés Bello, Departamento de Ciencias Químicas, Santiago, Chile
d
Universidad de Santiago de Chile, Facultad de Química y Biología, Santiago,
Chile
1
4
geneous catalysts. Several authors report in different re-
views MOFs that have been used as catalysts in different
†
Electronic supplementary information (ESI) available: Structural and catalytic
data; DFT geometries and energies. See DOI: 10.1039/c6cy02115h
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2
9
reactions: the synthesis of MeOH, polymerization reactions,
pal product being epoxide with a yield of 100%.
ceriumIJIV) MOF, based on 2,5-dimethyl-1,4-benzenedicarboxy-
late, Ce-UiO-66-(CH , was shown by Dalapati et al. to be a re-
A
C–C coupling reactions (Heck or Suzuki), hydrogenation,
1
4–16
asymmetric reactions and oxidation reactions.
3 2
)
One of the reactions where MOFs have been used as
heterogeneous catalysts is the oxidation of olefins, since the
obtained products have great importance in the pharmaceuti-
usable catalyst for the oxidation of styrene and cyclohexene.
The principal oxidation product for the first cycle was benzal-
dehyde, with epoxide in 29% yield.
3
0
1
7–19
cal and agrochemical industries.
Homonuclear MOFs
Additionally, heterometallic 3d–4f MOFs have been less
studied as heterogeneous catalysts in olefin oxidation, and
within the scarce examples of 3d–4f MOFs, the activity of the
heterogeneous catalyst {[Cu0.5La IJHPDC)IJPDC) IJSO )IJH O) ]-
containing 3d or 4f metal ions have been reported as good
catalysts in the oxidation of olefins, and several 3d based
MOFs can be found in the literature as examples of this type
of heterogeneous catalyst. Metal–organic frameworks (MOFs)
based on copperIJII) with different linkers, such as trisIJ4′-
carboxybiphenyl)amine, 2,2′ or 4,4′-bipyridine, btec and H2-
btec (btec: 1,2,4,6-benzenetetracarboxylate anion), have been
2
2
4
2
2
H O} (PDCH : 3,5-pyridinedicarboxylic acid) in the oxidation
2
n
2
of olefins and benzylic substrates has been recently reported
3
1
by our group.
Another interesting family of 3d–4f MOFs is
{[Cu Ln IJoda) IJH O) ]·nH O} (Ln: lanthanideIJIII) ions and
odaH : oxydiacetic acid), which has been widely studied to be
2
0–23
studied as catalysts in the oxidation of olefins.
Recently,
3
2
6
2
6
2
n
our group reported a recyclable [Cu IJbipy) IJbtec)] catalyst for
2
2
n
2
32–39
the oxidation reaction of cyclohexene and styrene, showing
mostly characterised both structurally and magnetically.
2
3
high catalyst recovery after five cycles of reaction. Aerobic
olefin epoxidation using cyclooctene as a substrate and
However, the corresponding catalytic behaviour of these com-
pounds has not been reported to date. For this reason, we
III
{
(NH
4
)
2
ijCo
3
IJIna)IJBDC)
3
IJHCOO)]}
n
as
a
catalyst (Ina: iso-
3 2 6 2 6 2 n
considered {[Cu Ln IJoda) IJH O) ]·nH O} with Ln : La, Gd,
nicotinate and BDC: 1,4-benzenedicarboxylate) was reported
Yb as good candidates to be investigated as heterogeneous
catalysts, in order to have a preliminary understanding of the
possible role of the lanthanideIJIII) ions in the redox properties
of the copperIJII), which can be considered as the initiator of
the catalytic cycle in the oxidation of olefins.
The herein reported work presents the catalytic properties
of the above-mentioned isostructural heterometallic 3d–4f
frameworks in the oxidation of cyclohexene and styrene,
using tert-butylhydroperoxide as an oxidant, and DFT calcula-
tions which assess the importance of the lanthanide ions in
the redox properties of the copperIJII) centre which acts as the
initiator of the catalytic reactions.
by Sha et al. to produce cyclooctene oxide as the main prod-
2
4
uct. Moreover, Sen et al. reported high conversions for the
II
II
epoxidation of styrene, using Ni and Mn based MOFs with
2
5
malonate ligands. Thus, similar high conversions can be
obtained with catalytic systems that have different metal cen-
tres and ligands. Moreover, it becomes evident that the metal
centre and the coordination sphere of the metal ion in the
catalyst, together with the nature of the substrate and that of
the oxidant, determine the final products which are obtained
2
6
in the catalytic oxidation reaction of olefins.
Moreover, several mixed MOFs (two different metals or a
metal with different oxidation states) have been used as
heterogeneous catalysts in reactions such as aerobic oxida-
tion of 3,5-di-tert-butylcathecol, diastereoselective synthesis of Experimental
E-α,β-unsaturated ketones, cyanosilylation of aldehydes, cyclo-
III
Synthesis of {[Cu Ln IJoda) IJH O) ]·nH O} (Ln : La (1), Gd
3
2
6
2
6
2
n
hexene oxidation, and ring opening of epoxides. The catalytic
performance of these species has been reviewed by
Dhakshinamoorthy et al.
(
2), Yb (3))
2
7
The synthesis and structural characterization of the tested
heterometallic 3d–4f catalysts have been described else-
Although several efforts have been made to obtain new
rare earth based catalysts for the oxidation of olefins, only a
few examples using rare earth MOFs have been reported as
catalysts for this type of reaction. These catalysts have shown
different catalytic activities; conversion and yields are
strongly dependent on the metal centre of the catalyst and
the substrate. As an example, a homonuclear yttriumIJIII)
based MOF with nicotinate ligands as organic linkers can be
used as a catalyst in the oxidation of cyclooctene (86% con-
version; 100% selectivity for epoxide). When the used sub-
strate was styrene, the conversion increased to 92%, while
3
9
−3
−4
where. CuO (3.1 × 10 mol), Ln O (9.7 × 10 mol) and
2
2
3
−
oxydiacetic acid (odaH ) (7.5 × 10 mol) were mixed in H O
2
2
(
{
350 mL) and refluxed for 8 h. The formed crystalline solids,
[Cu La IJoda) IJH O) ]·nH O} , CuLaMOF (1), {[Cu Gd IJoda) -
3
2
6
2
6
2
n
3
2
6
IJH O) ]·nH O} , CuGdMOF (2), or {[Cu Yb IJoda) IJH O) ]
2
6
2
n
3
2
6
2
6
2 n
·nH O} , CuYbMOF (3), were filtered off and dried under
vacuum.
Synthesis of {[CuIJoda) ]·0.5H O} (4)
2
2
n
2
8
the epoxide selectivity decreased to 41%.
A
3D
The synthesis was performed following the technique de-
4
0
gadoliniumIJIII) based framework, {[Gd IJμ -CO ) -(NIC) IJμ -
scribed by Whitlow et al. Using an excess of oxydiacetic acid
2
6
6
3 9
32
3
−
2
OH)26]IJNO ) ·2H O}
3 2 2 n
(NIC = nicotinate anion), was also used
(9 × 10 mol) and adding this to basic copperIJII) carbonate (3
−
3
by Sen et al. as a heterogeneous catalyst in the epoxidation of
olefinic substrates, including α,β-unsaturated ketones. The
conversion for the oxidation of styrene was 57%, the princi-
× 10 mol), the reaction mixture was stirred for 30 minutes.
The obtained blue crystalline solid corresponds to {[CuIJoda)2]
2 n
·0.5H O} , CuMOF (4).
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Elemental analysis
obtained catalytic activity could be compared and analysed,
in relation to the role of the respective lanthanide cation
present in the studied catalysts.
The C, N, and H contents were determined using a Carlo-
Erba EA-1108 micro-analyser, and Cu analysis was performed
using an atomic absorption spectrophotometer, Shimadzu
model AA-6200.
For the oxidation of styrene and cyclohexene, reactions
were performed in a magnetically stirred two-necked round-
bottom 25 mL flask fitted with a condenser and placed in a
temperature controlled oil bath. All the reactions were carried
out under nitrogen atmosphere. Most of the experiments
were performed at 75 °C, using the following reaction condi-
tions: the catalyst (variable mass) was added to the reactor to-
gether with 10 mL of solvent ((i) 1,2-dichloroethane or (ii)
n-decane); when the reaction temperature was reached, the
substrate, styrene (4.6 mL, 40 mmol) or cyclohexene (4.1 mL,
{
[Cu La IJoda) IJH O) ]·12H O} (CuLaMOF); Calc: C: 17.8,
3 2 6 2 6 2 n
H: 3.7, Cu: 11.8; Exp: C: 17.6, H: 3.4, Cu: 11.6%;
[Cu Gd IJoda) IJH O) ]·12H O} (CuGdMOF); Calc: C: 18.2, H:
.9, Cu: 11.8%; Exp: C: 18.4, H: 3.7, Cu: 11.6%;
[Cu Yb IJoda) IJH O) ]} (CuYbMOF); Calc: C: 20.0, H: 4.2, Cu:
3.0%; Exp: C: 20.9, H: 3.9, Cu: 13.0%; {[CuIJoda) ]·0.5H O}
{
3
2
6
2
6
2
n
3
{
3
2
6
2
6 n
1
2
2
n
(CuMOF); Calc: C: 23.5, H: 2.4, Cu: 31.1; Exp: C: 23.9, H: 2.7,
Cu: 31.3%.
40 mmol), and the oxidant were added. The oxidant t-butyl
hydroperoxide (TBHP) was incorporated into the reaction
mixture, using TBHP 70% in water (3.9 mL, 40 mmol) or
TBHP 5 M in decane (7.3 mL, 40 mmol). The oxidant used in
the 1,2-dichloroethane catalytic system was (i) TBHP (70% in
water) and for the n-decane system (ii) TBHP (5.5 M in
n-decane). The studied molar ratios of the substrate/TBHP/
catalyst were 400/400/1, 800/800/1, 1200/1200/1 and 2400/
Thermogravimetric analysis
Thermogravimetric analyses were performed using the
NETZCH Iris equipment. The TGA curves were obtained in
the 20–700 °C temperature range, under a nitrogen atmo-
−
1
−1
sphere (20 mL min ), using a 10 °C min heating rate. The
catalysts were stable in the temperature range used for the
catalytic reactions. The thermograms are reported in the ESI†
2400/1 for the CuGdMOF catalyst in (i), while for the rest of
the experiments, including those using the CuLa and CuYb
MOFs, the molar ratio is 2400/2400/1 (ESI,† Fig. S3 and Table
S2). Aliquots of the solution (10 μL) were removed at different
reaction times and analysed by gas chromatography (GC).
Gas chromatographic analyses were carried out using a
Hewlett Packard 5890 GC, equipped with a flame ionization
detector (FID) and a Carbowax 20 M capillary column (25 m ×
(
Fig. S1a–d). In addition, the thermograms confirmed the dif-
ferent amounts of water present in the studied catalysts. The
number of solvate water molecules associated with CuLaMOF
and CuGdMOF is twelve, while CuYbMOF does not present
crystallization water molecules (Table S1†).
Powder X-ray diffraction
0.2 mm × 0.2 μm), using nitrogen as the carrier gas. The oxi-
A Siemens D5000 diffractometer was used to record all pow-
der diffractograms. The obtained diffractograms for
CuLaMOF (1), CuGdMOF (2), CuYbMOF (3), and CuMOF (4)
were compared with the calculated patterns generated from
the single crystal X-ray data reported in ref. 39 and 40 (ESI,†
Fig. S2). It was not possible to compare the experimental
powder diffraction data of CuLaMOF with that generated
from single crystal data, since the cif file for this compound
is not available from the CCDC. The attempts to obtain single
crystals for CuLaMOF were unsuccessful. The similarity of
both the experimental and the calculated diffractograms per-
mit to assess the identity of the bulk material with the single
crystals of the other studied catalysts.
dation products were identified by spiking, using standard
compounds, and by MS-GC.
The EPR spectrum of the solid catalyst was recorded on
a Bruker EMX-1572, operating at the X-band (9.0–9.9 GHz),
at 298 K. 5 mg of the catalyst were transferred to a quartz
capillary tube (1 mm diameter) before being inserted into a
regular tube (3 mm diameter) and then into the spectrome-
ter cavity (microwave power 0.638 mW; modulation ampli-
tude 10 G). The spectra showed a typical wide and intense
II
band associated with a Cu species (data not shown). Reac-
tion solutions using the catalytic conditions in the presence
of the spin trap N-tert-butyl-α-phenylnitrone (PBN), added to
form radical adducts (1 : 1 ratio between the spin trap and
the oxidant), were monitored. These solutions were filtered
before recording the spectra, in order to avoid the interfer-
Catalytic studies
II
ence of Cu . These spectra were also obtained at room tem-
The catalysts were not activated by thermal treatment and, af-
ter grinding, were added directly to the reaction mixture, tak-
ing into account that some of the catalytic runs were
performed using an aqueous solution of the oxidant (TBHP,
perature (298 K), using capillary tubes with the same di-
mensions as those used for the recording of the spectrum
of the catalysts.
7
0% in water). Thus, the preformed channels obtained by
ICP spectroscopy
thermal treatment would have been filled with water mole-
cules once the catalysts were exposed to the solvent. The
amount of used catalyst, both for the heterometallic ones
and for the homometallic copperIJII) species, was normalised
to contain an equal amount of copperIJII) ions, so that the
The solutions obtained after catalytic reactions were analysed
by optical ICP spectroscopy using a Perkin Elmer Optima
2000 DV model spectrometer. The free copper concentrations
were determined using standards of different concentrations.
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Continuous shape measurements
Structural trends in CuLnMOF systems were analysed in
terms of continuous shape measurements, employing the
41,42
SHAPE 2.1 program.
In short, SHAPE allows the determi-
nation of the deviation of an arbitrary geometry with respect
to ideal reference shapes, such as octahedra, tetrahedra, etc.
If the problem geometry exactly matches the reference shape,
it will present an S parameter of 0. The value of S will contin-
uously rise upon larger deviations of the problem geometry.
It is worth noting that S does not depend on the scale or the
orientation of the measured geometry.
Electronic structure calculations
Depending on the system size and DFT functional, calcula-
tions were performed using either the all-electron FHI-AIMS
4
3
44
II
II III
Fig. 1 Large (top, left), small Cu (top, right) and small Cu Ln
(
version 071914_7) or ORCA 3.0.3 codes. DFT calculations
III
4
5
(bottom) models used in DFT calculations. Color code: Ln (light blue),
Cu (blue), O (red), C (grey) and H (white).
performed with FHI-AIMS considered the PBE functional in
conjunction with the ‘tight’ basis set. ORCA calculations
tested density functionals of different types (i.e. PBE,
B3LYP, and TPSSh ) in conjunction with Def2-TZVP and
Def2-QZVPP basis sets. Lanthanide atoms were described
using the SARC2-QZVP basis set. Scalar relativistic correc-
tions were included in FHI_AIMS (atomic_zora keyword) and
ORCA (DKH2).
II
4
6
47
4
8
Truncation of the large model led to three smaller models:
(ii) small Cu , including only the central Cu and its immedi-
ate coordination environment, and (iii) small Cu Ln ,
designed to study the ligand mediated orbital interaction be-
tween metal ions. After the construction of the large model,
all hydrogens were optimised.
4
9
II
II
II III
To analyse the influence of the lanthanide ions, we stud-
ied the cases of CuLaMOF and CuGdMOF. Structural models
were constructed directly from the crystallographic structures
3
9
39
of CuPrMOF
and CuGdMOF.
We considered the
Results and discussion
CuPrMOF structure for the geometry of CuLaMOF (replacing
the Pr position by La) as the crystal structure of CuLaMOF is
not available and CuPrMOF is the compound of this family
with the earliest lanthanide presenting a resolved crystal
structure. Calculations for CuYbMOF were also attempted
but presented severe convergence issues. To converge, they
required large broadening parameters that resulted in several
orbitals presenting partial electron occupations.
This is expected since the weak ligand field splitting of f
orbitals leads to a manifold of low lying excited states associ-
ated with different f configurations, whose description is
troublesome in a single reference treatment such as DFT.
The results obtained by elemental analysis, powder X-ray dif-
fraction and TGA confirm that the prepared microcrystalline
compounds to be used as catalysts are the compounds
3
9,40
reported in the literature.
As shown in Fig. 2, the three
heterogeneous catalysts have micrometric size and crystallize
with different morphologies; (1) is mainly composed of irreg-
ular crystals with dimensions ranging from a length of 8 to
20 μm and a width of 5 to 10 μm, (2) appears as elongated
crystals with a length of 10 to 15 μm and a width of 3 to 5
μm. Finally, (3) crystallizes as “microroses” with diameters of
20 to 25 μm (Fig. 2).
III
III
La and Gd -high spin do not present this problem as they
The studied isostructural MOFs present different coordi-
0
7
II
III
have only one way to represent their f occupation (f and f ,
nation spheres for the Cu
and Ln
centres; the
III
II
respectively). To understand the role of Ln and Cu ions
and their interaction effect on the observed reactivity trends,
molecular models of different sizes were constructed: (i) the
large model, which considers a Cu centre surrounded by
four Ln ions (see Fig. 1). The two water molecules and four
hexacoordinated copperIJII) is surrounded by four oxygen
atoms belonging to carboxylate groups of different oda
II
III
oda groups belonging to the coordination environment of
II
Cu were kept intact from their crystallographic positions.
III
Ln positions were not modified, although the two oda
II
groups not connected to the central Cu were replaced by wa-
ter ligands, with respect to the position of the donor oxygen
atoms. After the construction of the model, hydrogen atoms
were relaxed using FHI-AIMS (PBE/tight, as described above).
Fig. 2 SEM micrographs of CuLaMOF (1), CuGdMOF (2) and CuMOF (3).
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molecules and two oxygen atoms from two water molecules,
III
while the Ln centre is surrounded by nine oxygen atoms as-
sociated with three chelating oda ligands, thus generating
the coordination polymers which extend in the three direc-
3
2,39
tions reaching a three-dimensional structure.
Catalytic activity
The oxidation of styrene and cyclohexene was tested by vary-
ing the nature of the lanthanide ions present in the catalysts,
solvent, and temperature.
Scheme 1 Detected oxidation products for styrene and cyclohexene.
Different molar ratios of substrate/TBHP/catalyst were
used in order to define the optimal reaction conditions. The
studied molar ratios of substrate/TBHP/catalyst were 400/400/
1
, 800/800/1, 1200/1200/1 and 2400/2400/1 for the CuGdMOF
ceed through a free radical pathway, with the initiator species
catalyst in dichloroethane. Table 1 shows the TON and TOF
values, which were calculated per copperIJII) centre. From the
data in Table 1, it is possible to infer that the most active
substrate/oxidant/catalyst molar ratio is 2400/2400/1, associ-
ated with the highest values as referred to catalytic perfor-
mance (TON) and reaction rate (TOF). Accordingly, the rest of
the experiments were performed using this molar ratio.
It is important to remark that independent of the experi-
mental conditions the products for the catalysed oxidations
remained unchanged. While the detected oxidation products
for styrene were benzaldehyde (A), 1-phenylacetaldehyde (B),
styrene epoxide (C), and 1,2-phenylethanediol (D), for cyclo-
hexene these were 2-cyclohexene-1-one (A), 2-cyclohexene-1-ol
3 3
being the same for the three catalysts, (CH ) COO˙. This or-
ganic radical would be formed by a redox reaction between
the oxidant TBHP and the copperIJII) centres (Scheme 2).
The above-mentioned reactions take into account the la-
bility of the water molecules of the first coordination sphere
of copperIJII), which enables the interaction of this metal cen-
tre with TBHP. On the other hand, the lanthanide ions were
considered as species that were not directly involved in the
catalytic reactions, since the corresponding coordination
sphere is formed by three chelating oda ligands that isolate
the lanthanide centres from the interaction with the oxidant
or substrate.
However, as evidenced by the conversion data, the
lanthanideIJIII) cation present in the framework affects the ac-
tivity of the catalytic process (Table 2). This fact may be
explained assuming that these ions are influencing the redox
properties of the copperIJII) centres, since the latter sites are
considered to activate the oxidant (TBHP).
(
(
B), cyclohexene epoxide (C) and 1,2-cyclohexanediol (D)
Scheme 1). Conversion and yields for different detected oxi-
dation products at 24 h for both substrates reacting in the bi-
phasic DCE/H O system in the presence of different
2
CuLnMOFs are given in Table 2.
The results were similar for the three catalysts under
study, showing that while the conversion increased in late
lanthanides (Yb) with respect to earlier ones (La and Gd), the
yield of the different oxidation products remained similar.
Besides, the TOF values show the same trend, clearly indi-
cating that the most active catalyst is CuYbMOF. The origin
of this trend will be discussed in detail later. If the catalyst
does not participate directly in the pathways to form the
products, the product formation can be considered to pro-
The catalytic activity of the studied 3d–4f catalysts,
3 2 6 2 6 2 n
{[Cu Ln IJoda) IJH O) ]·12H O} (CuLnMOF), was also com-
pared with that of the homonuclear catalyst, {[CuIJoda)
·0.5H O} (CuMOF). The corresponding catalytic studies in-
2 n
]
2
3
8
dicate that this homometallic catalyst is less efficient than
the heterometallic catalysts; the conversion increased to ca.
35% for CuLnMOF (Fig. 3). These data make evident the in-
fluence exerted by the lanthanideIJIII) ions present in the stud-
ied 3d–4f catalysts
Table 1 TOF and TON values obtained at 1 and 24 h for the oxidation of styrene and cyclohexene, catalysed by (2), using different S/C ratios
S/C ratio
Substrate
400 : 1
800 : 1
1200 : 1
2400 : 1
TON
TOF (h
957 (2568)
957 (107)
1957 (5347)
1957 (223)
3132 (7748)
3132 (323)
7202 (14950)
7202 (623)
−
1
1
)
TON
TOF (h
938 (2186)
938 (91)
1630 (4147)
1630 (173)
2532 (5914)
2532 (246)
4365 (11349)
4365 (473)
−
)
Reaction conditions: solvent: 1,2-dichloroethane; oxidant: tert-butylhydroperoxide in aqueous medium; temperature: 75 °C; substrate : oxidant
II
ratio = 1 : 1. TON is calculated as moles of products per mole of active site (Cu ions); TOF is calculated as moles of products per mole of active
II
site (Cu ions) per hour. Values in parentheses correspond to 24 h.
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Table 2 Conversion and yields of different oxidation products at 24 h for styrene and cyclohexene in DCE/water, using different CuLnMOFs
−
1
Substrate
Catalyst
% Conversion (TOF, h
)
(A) (%)
(B) (%)
(C) (%)
(D) (%)
CuLaMOF
CuGdMOF
CuYbMOF
64 (552)
69 (623)
73 (664)
20
24
23
47
48
43
33
28
34
0
0
0
CuLaMOF
CuGdMOF
CuYbMOF
48 (436)
52 (473)
57 (518)
56
58
58
31
30
29
5
4
4
8
8
9
Reaction conditions: solvent: 1,2-dichloroethane; oxidant: tert-butylhydroperoxide in aqueous medium; temperature: 75 °C. For styrene: (A)
benzaldehyde, (B) 1-phenylacetaldehyde, (C) styrene epoxide, and (D) 1,2-phenylethanediol; for cyclohexene: (A) 2-cyclohexene-1-one, (B)
2-cyclohexene-1-ol, (C) cyclohexene epoxide, and (D) 1,2-cyclohexanediol.
5
5,56
Reaction pathway
oxidation in particular, Silva et al. and Sebastian et al.
de-
scribed the formation of a radical intermediate (III, IV) when
In order to test the viability of a radical mechanism, the
CuGdMOF catalyst was used with TBHP (70% in water) as an
oxidant at 75 °C and cyclohexene or styrene as a substrate
with a 2400/2400/1 S/O/C ratio in 1,2-dichloroethane/water as
solvent, in the presence of 10 moles of hydroquinone (HQ)
as a free radical trapping agent.
styrene reacts with (CH ) COO˙ and reported that this inter-
3
3
mediate forms benzaldehyde and formaldehyde by internal
decomposition (V). Another possibility to complement this
5
6
−
2
mechanism is given by Sebastian et al., who proposed that
the intermediate radical species reacts with styrene to form
styrene oxide (VI). The following step is the isomerization of
styrene oxide to 1-phenylacetaldehyde (VII), or another possi-
The results showed that with the use of hydroquinone, the
conversion for the oxidation of styrene decreased from ca. 69
to 17%, while for cyclohexene the conversion decreased from
ca. 52 to 10%. These results permit to infer that the mecha-
nism has an important contribution from a radical path-
bility is that
a ring opening reaction produces 1,2-
phenylethanediol (VIII) (Scheme 3).
Assuming the radical pathway, the catalyst is supposed to
act as an activator, which decomposes the oxidant (TBHP),
5
0
way. In order to confirm this mechanism, EPR spectroscopy
was used to investigate the nature of the radical species pro-
duced by the heterogeneous catalyst. In order to facilitate the
detection of the reactive species, the measurements were run
using the spin trap N-tert-butyl-α-phenylnitrone (PBN), which
is known to form a long-lived adduct, useful to detect oxygen-
5
7
through a redox reaction (1), giving (CH
3 3
) COO˙. Then, the
oxidation reaction starts by the attack of (CH ) COO˙ on the
3
3
substrate (III), followed by the sequence of steps, which leads
to the final products. A new interaction between the reduced
catalyst and the oxidant regenerates, by a redox reaction, the
catalytic species and produces (CH ) CO˙ (2). Also, the radical
3 3
species formed in (1) and (2) (Scheme 2) can react with the
5
1,52
centred radicals.
Initially, control spectra were recorded,
all in aerated solutions for different substrates and oxidants,
in the presence of PBN.
The respective EPR spectra for the control samples did not
evidence the formation of any adduct species with the spin
trap. The PBN adduct was generated only in the presence of
TBHP, using the catalytic conditions (Fig. 4).
substrates, dissolved oxygen, or between them, as is reported
5
0,55,58
by different researchers.
It is important to highlight
that Liu et al. proposed a specific mechanism for the decom-
position of alkyl hydroperoxides in the presence of copperIJII)
5
9
ions. A similar mechanism for the oxidation of cyclohexene
can be formulated, that is, the reaction of the catalyst and
The spectra clearly indicate that only one species has been
trapped, specifically (CH
3 3
) CO˙. This is due to the high reac-
5
3,54
tivity of (CH CO˙ as compared to that of (CH ) COO˙.
3
)
3
3 3
These spectra suggest that the catalytic processes begin with
the formation of oxygen-centred radical species, as shown in
Schemes 3 and 4. Thus, the prevailing mechanism can be as-
sumed to be the radical route. Adopting this mechanism, it is
possible to give an explanation to the different products
detected during the catalytic processes.
5
0
Ghosh et al. proposed a general scheme for the oxida-
tion of olefins assuming a free radical pathway. For styrene
Fig. 3 Temporal evolution of the conversion, using styrene as a
substrate and TBHP as an oxidant; 1,2-DCE/water as reaction media;
CuLaMOF (■); CuGdMOF (●); CuYbMOF (▲); CuMOF (▼).
Scheme 2 Activation of TBHP using CuLnMOF as a catalyst.
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Fig. 4 EPR spectra corresponding to the formed adduct species, using
TBHP in the presence of PBN. Catalytic conditions: (a) cyclohexene
and (b) styrene as substrates. Measurement conditions: centre field,
3
495 gauss; time constant, 5.12 ms; conversion time, 20.48 ms; sweep
field, 100 gauss; sweep time, 20.97 s; MW power, 25 mW; gain, 2.0 ×
03; modulation amplitude, 1.0 gauss; modulation frequency, 100 kHz;
1
time constant, 64 ms; MW frequency, 9 877 176 GHz. EPR parameters
of the PBN radical, g = 2.012 gauss, line width = 1.49 gauss, AN =
1
5.06 gauss, AH = 3.03 gauss.
Scheme 4 Mechanism for the catalysed oxidation of cyclohexene
with CuLnMOFs, in the presence of TBHP.
tional information for CuGdMOF and CuYbMOF are given in
the ESI,† Tables S3 and S4). For styrene and cyclohexene oxi-
dation, the conversion increased ca. 25%, when the anhy-
drous non-polar solvent decane was used, as compared to the
biphasic DCE/water system. However, the selectivity of cyclo-
hexene oxidation appears not to be sensitive to solvent
changes. On the other hand, when styrene was used as a sub-
strate, the main product in DCE/water and n-decane was
1-phenylacetaldehyde (B), followed by benzaldehyde (A), and
remained with similar yields. The variation in yields was only
observed for the minor products.
Scheme 3 Mechanism for the catalysed oxidation of styrene with
With respect to the generation of epoxide, both systems
showed a marked difference in the yield. While styrene epox-
ide was obtained in ca. 30% in DCE/water, an almost null
yield of the same product was detected in n-decane. However,
cyclohexene epoxide was not detected in either of the two re-
action media. This last fact demonstrates the importance of
the allylic oxidation of cyclohexene in the studied systems,
which generates the corresponding ketone instead of the ep-
oxide. Water has been reported by Alfayate et al. to act as an
inhibitor of the radical route in the oxidation of cyclo-
CuLnMOFs, in the presence of TBHP.
the oxidizing agent (TBHP). Step (3) corresponds to the attack
of the radical species on the olefinic position of cyclohexene,
while steps (6), (7) and (8) correspond to the attack of the
radical species on the allylic hydrogen. The attack on the al-
lylic hydrogen is definitively responsible for the major prod-
ucts of the oxidation of cyclohexene (Scheme 4).
6
0,61
hexene.
ied systems.
However, this issue was not observed in the stud-
Reaction conditions
Two reaction media were tested for catalyst performance,
dichloromethane/water and n-decane, considering the stabili-
zation of the oxidant tert-butylhydroperoxide in n-decane as
compared to the same oxidant in aqueous solution. A com-
parison of the effect of a completely anhydrous medium
In order to have a better understanding of the catalytic
systems under study, several runs were carried out at room
temperature (25 °C), 60 and 75 °C. The conversion and prod-
uct yields for the oxidation of styrene are shown in Table 4,
S5 and S6.† For styrene oxidation, the product yields are dif-
ferent for the reaction at 75 and 25 °C, with the main prod-
ucts being 1-phenylacetaldehyde and benzaldehyde, respec-
tively. This effect can be explained by the proposed
(n-decane) on the performance of the heterometallic catalysts,
CuLaMOF (1), CuGdMOF (2) and CuYbMOF (3), with that of
the biphasic system is given for CuLaMOF in Table 3 (addi-
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Table 3 Conversion and yields at 24 h of the different products for the oxidation of styrene and cyclohexene, using different solvents and CuLaMOFs
Substrate
Temperature
% Conversion
(A) (%)
(B) (%)
(C) (%)
(D) (%)
DCE/Water
n-Decane
64
81
27
35
44
40
29
8
0
17
DCE/Water
n-Decane
48
56
56
54
31
25
5
6
8
15
Reaction conditions: S/O/C ratios: 2400/2400/1; temperature: 75 °C; oxidant: tert-butylhydroperoxide in aqueous medium. For styrene: (A)
benzaldehyde, (B) 1-phenylacetaldehyde, (C) styrene epoxide, and (D) 1,2-phenylethanediol; for cyclohexene: (A) 2-cyclohexene-1-one, (B)
2-cyclohexene-1-ol, (C) cyclohexene epoxide, and (D) 1,2-cyclohexanediol.
Table 4 Conversion at different temperatures and yields (24 h) of the different products for the oxidation of styrene and cyclohexene using CuYbMOF
Substrate
Temperature
% Conversion
(A) (%)
(B) (%)
(C) (%)
(D) (%)
7
6
2
5 °C
0 °C
5 °C
73
54
26
23
26
68
43
49
12
34
18
12
0
7
8
7
6
2
5 °C
0 °C
5 °C
57
42
18
58
56
69
29
29
29
4
5
0
9
10
2
Reaction conditions: S/O/C ratios: 2400/2400/1; solvent: 1,2-dichloroethane; oxidant: tert-butylhydroperoxide in aqueous medium. For styrene: (A)
benzaldehyde, (B) 1-phenylacetaldehyde , (C) styrene epoxide, and (D) 1,2-phenylethanediol; for cyclohexene: (A) 2-cyclohexene-1-one, (B)
2-cyclohexene-1-ol, (C) cyclohexene epoxide, and (D) 1,2-cyclohexanediol.
mechanism in Scheme 3, since benzaldehyde is formed by
internal decomposition of the intermediary species (Bz-
mixture by filtration, washed with water and 1,2-
dichloroethane and dried under vacuum. As the as-prepared
catalysts were used without thermal treatment, the re-used
catalysts were not activated by thermal treatment and were
added directly to the following catalytic cycle.
Fig. 5a and b show the conversion obtained for both sub-
strates after four catalytic cycles, using CuGdMOF as a cata-
lyst. The catalytic results obtained for CuLaMOF and
CuYbMOF, showing a similar trend, are given in the ESI†
(Fig. S4a–d). For both styrene and cyclohexene, the catalyst
remained active for four cycles (further catalytic cycles were
not studied), showing a small decrease of ca. 10%, after the
fourth catalytic run.
4
9
C˙CHOO˙) (Scheme 3, step IV). This process should require
less energy than the formation of styrene oxide, since the for-
mation of styrene oxide requires an interaction between the
intermediate species with a molecule of styrene (Scheme 3,
6
2
step V). This process is therefore favoured by a higher tem-
perature, which provides energy to the system enhancing this
reaction. When styrene oxide is formed, the next step is the
isomerization of styrene oxide to generate 1-phenylacet-
6
3
aldehyde (Scheme 3, step VI); this process is an internal
rearrangement and produces the main product whether the
reaction is carried out at 60 or 75 °C.
The thermal behaviour of the cyclohexene oxidation reac-
tion is similar to that of styrene when the temperature is var-
ied; as expected in both cases an increase in the conversion
is observed as the temperature is increased. However, the
yield of the different products varies when the temperature is
decreased from 75 to 25 °C. For this system, the main prod-
uct did not change and remained as 2-cyclohexene-1-one, but
its yield increased from 58 to 69% as the temperature de-
creased from 75 to 25 °C. When the reaction took place at 25
The proposed mechanism considers the initial formation
of radical species by the redox reaction of the catalyst with
°C, the yield of the products 2-cyclohexen-1-one and
2
-cyclohexen-1-ol increased, together with some by-products.
It is probable that a low temperature inhibits the route of for-
mation of cyclohexene oxide and therefore its hydrolysis to
1
,2-cyclohexendiol.
In order to study the reusability of the catalysts, after each
catalytic cycle the catalyst was separated from the reaction
Fig. 5 Reusability of CuGdMOF for styrene (a) or cyclohexene (b)
oxidation. Run 1 (■); run 2 (●); run 3 (▲); run 4 (▼).
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the oxidant, species that must be consumed in the reaction
solution without the catalyst (Scheme 2).
14.344 Å for CuPrMOF to CuYbMOF, while the opposite trend
is observed in c, rising from 14.584 Å to 15.470 Å (Fig. 7).
These changes must affect the orientation of the four equato-
During the first three catalytic runs, the re-used catalysts
basically maintained their structure, as shown by the princi-
pal peaks of the corresponding recorded diffractograms (Fig.
S5a–f†). After the fourth cycle, some new peaks became evi-
dent in the corresponding diffractograms of the reused cata-
lysts, which obviated further catalytic studies.
II
rial O donor atoms of the Cu coordination environment, as
III
they belong to oda groups, directly connected to the Ln
II
ions. The most notable trend is the change in the Oeq–Cu –
Oax angle, which monotonically increases from 82.69°
II
(CuPrMOF) to 86.09° (CuYbMOF). Interestingly, Cu –O
eq
Besides, using both substrates, the leaching test was car-
ried out for the three catalysts. The amount of copperIJII) in
the respective solution after the first catalytic cycle was less
than 0.2 wt% for CuLa-MOF, 0.1 wt% for CuGd-MOF and 0.1
wt% for CuYb-MOF of the initial amount of copper present
in the catalyst. After the second, third and fourth catalytic cy-
cles, the amount of copperIJII) in solution was even lower than
bond distances are not particularly sensitive to the change in
III
Ln ions, ranging in an interval between 1.943 Å and 1.957
II
Å. On the other hand, Cu –O distances tend to be shorter
ax
in systems with later lanthanides (2.49 Å for CuYbMOF) in
comparison with earlier systems (2.57 Å for CuPrMOF).
II
Continuous shape measurements describe the Cu coordi-
nation environment as nearly octahedral, with an S value of
2.29 for CuPrMOF that decreases towards late lanthanides
(1.57 for CuYbMOF).
0
.02 wt% for all the studied systems. These results confirmed
that the heterogeneous catalysts did not present significant
leaching in the catalytic studies. Fig. 6 displays the hot filtra-
tion tests for (a) cyclohexene and (b) styrene, using CuYbMOF
as a catalyst. The figures corresponding to the other catalysts
are shown in the ESI† (Fig. S6a and b and S7a and b). These
graphs show that when the reaction mixture is separated af-
ter one hour of reaction in two portions, the conversion of
the filtered sample remains practically unchanged, while the
unfiltered one that contains the heterogeneous catalyst in-
creases its conversion. However, Fig. 6 also shows that once
the reaction solution is filtered, the obtained solution shows
a small increase in the conversion; the conversion reached a
plateau after ca. 2 hours. It is possible to assume that the fil-
trate must have a remnant concentration of formed radical
species for both studied systems.
II
This is not unexpected since changes in Oeq–Cu –Oax an-
II
gles (approaching 90°) and Cu –O distances (shortening of
ax
elongated axial distances) tend to a more regular octahedral
shape.
To gain further information on the effect of these struc-
II
tural changes on the electronic structure of the Cu centre
III
and the electronic role (if any) of the change in Ln ion, we
performed DFT calculations for structural models of
CuLaMOF and CuGdMOF (see “Electronic structure calcula-
tions” section for further information on the construction of
the models and the computational methodology).
As expected, large and small models predict that the un-
II
paired electron of the Cu ion resides in the d
x
2
−y
orbital, in
2
line with the elongated octahedral environment of this ion
(
Fig. 8).
In the large models including the four surrounding Ln
III
II
Influence of the lanthanide ions
ions, we observe a significant stabilization of the empty Cu
d
II
−1
To better understand the effect on the reactivity of the Cu
x
2
−y
2
orbital (ca. 2 kcal mol ) in CuGdMOF in comparison
centre, it is convenient to disentangle structural and
III
electronic effects associated with each Ln ion. In this way,
we first analysed the structural trends in existing compounds
of the CuLnMOF family to subsequently investigate them by
DFT calculations. Crystal structures for Pr, Nd, Sm, Eu, Gd,
Tb, Dy, Ho, Er and Yb CuLnMOFs can be found in the litera-
3
3,34,38,39,64
ture.
A smooth periodic trend is evident in the a
and c cell parameters, as a diminishes from 15.199 Å to
Fig.
6 Hot test filtration using CuYbMOF as a catalyst for (a)
cyclohexene oxidation and (b) styrene oxidation with a catalyst (■) and
filtered after 1 hour (●).
Fig. 7 Periodic trend in the length of a (●) and c (■) lattice vectors for
CuLnMOF crystalline structures through the lanthanide series.
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HF exchange) the largest. This is clearly associated with the
overestimation of low spin energies typical from GGA func-
tionals, while B3LYP tends to be biased toward high spin
6
6
states. We expect TPSSh to be more balanced in this respect.
The mechanism for the lanthanide effect can be clarified
by comparing the molecular orbital corresponding to the
II
III
II
empty Cu
d
x2−y2 orbital in the small Ln Cu models of
III
CuLaMOF and CuGdMOF. The Gd ion is more strongly
attracting the electronic density of the linking oda ligand
than La . In this way, the electron density of the oxygen do-
III
Fig. 8 Molecular orbital representing the unpaired electron of large
II
nor atom bonded to Cu is slightly depleted (Fig. S8†) and
II III
CuLaPOM (left) and small Cu Ln (right).
II
the ligand field perceived by the
orbitals of Cu is weak-
III
ened. This is in line with the higher Lewis acidity of Gd in
III
comparison to La and could be rationalised in terms of a
larger polarizing effect of Gd , although the physical mecha-
nism behind the effect is not purely electrostatic but orbital.
to CuLaMOF, consistent with the enhanced reactivity of the
systems incorporating later lanthanides (Tables S7 and S8†),
which will present a more favourable orbital to accept one
III
II
I
electron from TBHP, promoting the reduction of Cu to Cu .
Furthermore, if we keep the CuGdMOF geometry and only
change the Ln position for La, we observe some destabiliza-
Conclusions
The heterometallic frameworks, {[Cu Ln IJoda) IJH O) ]·nH O}
n
II
−1
3
2
6
2
6
2
tion of the Cu d
x
2
−y
2
orbital (0.4 kcal mol ).
III
III
III
III
(
Ln : La , Gd , Yb ), have been shown to be active for sev-
The complementary calculations (i.e. using the CuLaMOF
eral cycles as catalysts in the oxidation of the olefinic sub-
strates, cyclohexene and styrene. All the obtained data for the
catalytic systems can be rationalised by assuming a radical
mechanism as the principal route for the oxidation
processes.
With respect to the generation of epoxide, both systems
showed a marked difference in the yield. While styrene epox-
ide was obtained in ca. 30% in DCE/water, an almost null
yield of the same product was detected in n-decane. Cyclo-
hexene epoxide was not detected in an appreciable percent-
age in either of the two solvents. This last fact demonstrates
the importance of the allylic oxidation of cyclohexene in the
studied systems, which generates the corresponding ketone
instead of the epoxide.
geometry and replacing the Ln for Gd) yield a marked stabili-
−
1
zation of 6 kcal mol . In this way, the lanthanide ion is
exerting a genuinely electronic effect on the regulation of the
II
stability of the Cu d
x
2
−y
orbital.
2
II
To further check this conclusion, truncated (small Cu )
III
models were constructed by removing the Ln ions, just leav-
II
ing the coordination environment of the Cu centre. It is
interesting to note that the stabilization observed in the large
−
1
models (larger than 2 kcal mol ) is drastically reduced in the
−
1
geometry excluding the lanthanides to 0.2 kcal mol . Then,
the structural changes in the coordination environment of
II
the Cu between CuLaMOF and CuGdMOF are not account-
II
ing for the Cu
d
x
2
−y
stabilization, in line with the impor-
2
III
tance of the Ln electronic effect.
Finally, “dimeric” model (small Cu Ln ) was
The lack of significant leaching, and the almost unaltered
structures of the catalysts after several cycles, permits the
conclusion that the CuLnMOFs can be considered as reusable
heterogeneous catalysts.
II
III
a
II
constructed to represent the interaction between Cu and
Ln . In this model, one of the lanthanide ions of the large
model is kept alongside with the Cu coordination environ-
III
II
The best catalytic results were obtained for CuYbMOF, as
ment (Fig. 1). To check the robustness of our conclusions,
electron affinity (EA) calculations were performed using sev-
eral combinations of density functionals (PBE, B3LYP and
TPSSh) and basis sets (Def2-TZVP and Def2-QZVPP), both in
the gas phase and using water as a continuous solvent
III
III
compared to the catalysts that contain La
or Gd .
Electronic structure calculations are consistent with this
trend and predict the stabilization of the empty d
of Cu in the late lanthanides, thus favouring the reduction
of the metal centre with TBHP. The mechanism of such effect
x2−y2 orbital
II
6
5
(
COSMO model) (see Table S10† for more details).
can be understood by orbital stabilization of the d
x2−y2 orbital
In all cases, the additional electron associated with the EA
II
of Cu , as influenced by the changes in Lewis acidity between
II
is accommodated in the Cu d
x
2
−y
2
orbital, supporting the des-
III
III
III
Gd and La . Concretely, the stronger Lewis acidity of Gd
II
ignation of Cu as the active redox centre for the catalytic pro-
III
in comparison to La results in a weaker ligand field around
cess (Fig. 8). Furthermore, EA of CuGdMOF was larger than
II
II
Cu , stabilising its
orbital.
CuLaMOF, in line with the stabilization of the Cu
d
x
2
−y
or-
2
bital in the case of CuGdMOF. It is interesting to observe how
the characteristics of each density functional are reflected in
the calculated EA values. PBE, being a pure GGA functional,
presents the largest EA; TPSSh (hybrid meta-GGA, 10% HF ex-
change) shows intermediate values and B3LYP (hybrid, 15%
Acknowledgements
The authors thank Proyecto Anillo ACT1404 and Centres of
Excellence
with
Basal/CONICYT
Financing,
FB0807
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(
CEDENNA) for financial support. PCR acknowledges finan-
23 P. Cancino, V. Paredes-García, P. Aguirre and E. Spodine,
Catal. Sci. Technol., 2014, 4, 2599.
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cial support from CONICYT, Programa de Formación de Capi-
tal Humano Avanzado, Beca Doctorado Nacional 21110228. D.
A. thanks CONICYT + PAI “Concurso nacional de apoyo al
retorno de investigadores/as desde el extranjero, convocatoria
2
014 – 82140014” for financial support. This research was par-
tially supported by the supercomputing infrastructure of the
NLHPC (ECM-02) (Powered@NLHPC).
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