Heterogeneous catalysis with an organic-inorganic hybrid based on MoO3chains decorated with 2,2′-biimidazole ligands

Article abstract of DOI:10.1039/d1cy00055a

The discovery of selective heterogeneous catalytic systems for industrial oxidation processes remains a challenge. Molybdenum oxide-based polymeric hybrid materials have been shown to be oxidation catalysts under mild reaction conditions, although difficulties remain with catalyst recovery/reuse since most perform as homogeneous catalysts or possess low activity. The present study shows that the hybrid material [MoO₃(2,2′-biimidazole)]·H₂O (1) is a superior catalyst regarding these issues. The structure of1was confirmed (by single crystal and synchrotron X-ray powder diffraction) to comprise one-dimensional chains of corner-sharing {MoO₄N₂} octahedra. Strong MoO?H-N hydrogen bonds separate adjacent chains to afford parallel channels that are occupied by disordered water molecules. Hybrid1was additionally characterised by FT-IR spectroscopy,¹H and¹³C MAS NMR, scanning electron microscopy and thermogravimetric analysis. The catalytic studies highlighted the versatility of1for oxidation reactions withtert-butylhydroperoxide as oxidant. By complementing with characterisation studies, it was verified that the reaction occurs in the heterogeneous phase, the catalyst has good stability and is recoverableviasimple procedures. The chemical reaction scope covered epoxidation and sulfoxidation, and the substrate scope included biomass-deriveddl-limonene and fatty acid methyl esters to give renewable bio-products, as well as thiophene and thioanisole substrates.

Full text of DOI:10.1039/d1cy00055a

Catalysis
Science &
Technology
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PAPER
Heterogeneous catalysis with an organic–
inorganic hybrid based on MoO chains decorated
with 2,2′-biimidazole ligands†
3
Cite this: Catal. Sci. Technol., 2021,
11, 2214
Tatiana R. Amarante,‡ Patrícia Neves,‡ Filipe A. Almeida Paz,
Ana C. Gomes,
Martyn Pillinger,
Anabela A. Valente * and Isabel S. Gonçalves
*
The discovery of selective heterogeneous catalytic systems for industrial oxidation processes remains a
challenge. Molybdenum oxide-based polymeric hybrid materials have been shown to be oxidation catalysts
under mild reaction conditions, although difficulties remain with catalyst recovery/reuse since most
perform as homogeneous catalysts or possess low activity. The present study shows that the hybrid
material [MoO
was confirmed (by single crystal and synchrotron X-ray powder diffraction) to comprise one-dimensional
chains of corner-sharing {MoO
} octahedra. Strong MoO⋯H–N hydrogen bonds separate adjacent
chains to afford parallel channels that are occupied by disordered water molecules. Hybrid 1 was
3 2
(2,2′-biimidazole)]·H O (1) is a superior catalyst regarding these issues. The structure of 1
4 2
N
1
13
additionally characterised by FT-IR spectroscopy, H and C MAS NMR, scanning electron microscopy and
thermogravimetric analysis. The catalytic studies highlighted the versatility of 1 for oxidation reactions with
tert-butylhydroperoxide as oxidant. By complementing with characterisation studies, it was verified that the
reaction occurs in the heterogeneous phase, the catalyst has good stability and is recoverable via simple
procedures. The chemical reaction scope covered epoxidation and sulfoxidation, and the substrate scope
included biomass-derived DL-limonene and fatty acid methyl esters to give renewable bio-products, as well
as thiophene and thioanisole substrates.
Received 11th January 2021,
Accepted 16th January 2021
DOI: 10.1039/d1cy00055a
rsc.li/catalysis
techniques have been developed to synthesize nanomaterials
Introduction
10,12
with well-tuned sizes and shapes.
Nanostructured MoO
3
wires,
have been fabricated by
3,5,12
12,13
Bulk and supported molybdenum trioxide (MoO
materials have distinguished record as heterogeneous
catalysts for a broad range of transformations such as the
3
)-based
with different morphologies such as thin sheets,
12
12,14
6,12,15,16
a
rods, ribbons
and belts
hydrothermal/solvothermal, sol–gel, template-assisted, and
vapor phase deposition methods, among others. The techniques
of hydrothermal synthesis can be exploited to tailor not only the
morphology of MoO3 nanostructures but also the oxide
microstructure by the inclusion of structure-directing organic
1
–3
4
oxidation of alcohols,
olefin metathesis, benzylation of
5
arenes,
compounds, hydrodeoxygenation of biomass-derived phenolic
condensation of 1,2-diamines with carbonyl
6
7,8
compounds, furfuryl alcohol dehydration and condensation
reactions, hydrogen and oxygen evolution reactions, and
9
10
17
components. When organic ligands are incorporated, such as
10
photocatalytic degradation of organic pollutants. The three
crystal phases of MoO are the layered, thermodynamically
stable orthorhombic phase (α-MoO ), the metastable ReO -type
organonitrogen compounds, hybrid materials are obtained in
which the ligand coordinates to a molybdenum site of the oxide
3
1
8
19
array. The monodentate ligand pyridine (and its related
3
3
20
21
monoclinic phase (β-MoO ), and the hexagonal phase (h-
bridging analogues like pyrazine, 4,4′-bipyridine (4,4′-bipy),
and 3,8-phenanthroline (3,8-phen) ) yield perovskite-like
3
11
22
MoO
3
). To improve the catalytic performance of α-MoO
3
,
layered structures consisting of corner-sharing {MoO N}
5
polyhedra, while chelating ligands such as 2,2′-bipy tend to
restrict the oxide dimensionality to chains or ribbons since their
coordination blocks at least two condensation sites on the same
metal centre. Hydrothermal syntheses with 2,2′-bipy yield
3 m n
inorganic–organic hybrid materials of the [(MoO ) (2,2′-bipy) ]
CICECO – Aveiro Institute of Materials, Department of Chemistry, University of
Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
E-mail: atav@ua.pt, igoncalves@ua.pt
23
†
CCDC 1967536 (1). For crystallographic data in CIF or other electronic format
family with structures consisting of one-dimensional (1D) oxo-
see DOI: 10.1039/d1cy00055a
‡
These authors contributed equally to the publication.
bridged chains of corner-sharing {MoO } tetrahedra and/or
4
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2
4–26
{
MoO N } octahedra.
Analogous structures have been
decarbonylation of the tetracarbonyl complex cis-[Mo(CO) (2,2′-
bipy)] with TBHP under mild conditions.
4
2
4
27
30
reported for 1,10-phen (m = 3, n = 2), 2,2′-bipyridine-5,5′-
28
dicarboxylic acid (H
2
9
bpdc; m = n = 1), and 2,2′-biimidazole
ICP-OES analysis for Mo was performed at the University of
Aveiro. Microanalyses for C, H and N were carried out with a
Truspec Micro CHNS 630-200-200 elemental analyser. Standard
powder XRD data were collected at ambient temperature on a
Philips Analytical Empyrean diffractometer equipped with a
PIXcel 1D detector, with automatic data acquisition (X'Pert Data
2
(H biim; m = n = 1).
2
We have previously studied the 1D polymeric hybrids
−
(cation)
x
[(MoO
3
)
m
(L)
n
] (L = 2,2′-bipy, x = 0; L = Hbpdc , x = 1) as
catalysts for the epoxidation of olefins and the oxidation of
26,28,30,31
amines and sulfides.
With olefins as substrates and
tert-butylhydroperoxide (TBHP) or hydrogen peroxide as
oxidant, the epoxidation reactions were at least partially
catalysed in the homogeneous phase, i.e., the polymers acted as
Collector software version 4.2) using monochromatised Cu-K
α
radiation (λ = 1.54178 Å). Intensity data were collected by the
step-counting method (step 0.02°), in continuous mode, in the
2θ range 5–65°. Scanning electron microscopy (SEM) images
and elemental (Mo) mappings were obtained on a Hitachi SU-
70 microscope equipped with a Bruker Quantax 400 detector at
15 kV. Samples were prepared by deposition on aluminium
sample holders followed by carbon coating using an Emitech K
950 carbon evaporator. TGA was performed using a Shimadzu
26,28,30
sources of soluble active species.
published on the catalytic properties of the hybrid material
MoO (H biim)]·nH O (1). This compound possesses, however,
No report has yet been
[
3
2
2
characteristics that could make it an interesting catalyst. Firstly,
29
as reported in a recent crystallographic study, the solid-state
structure of 1 contains a solvent accessible void volume of
−1
18.0% of the unit cell volume. In contrast, the chains of
TGA-50 system at a heating rate of 5 °C min under air. FT-IR
spectra were collected using KBr pellets and a Mattson-7000
infrared spectrophotometer. Attenuated total reflectance (ATR)
FT-IR spectra were measured on a Bruker Tensor 27 equipped
with a Specac® Golden Gate Mk II ATR accessory having a
[
MoO (2,2′-bipy)] are densely packed in the crystalline state and
3
24
there is no accessible space in the network for solvents.
Secondly, the ligand H biim and its neutral metal complexes
2
are typically very poorly soluble in organic solvents, due largely
to the presence of strong intermolecular hydrogen-bonds
13
diamond top plate and KRS-5 focusing lenses. Solid-state
C
32
involving the N–H groups. The propensity of H
2
biim to
cross-polarization (CP) magic-angle spinning (MAS) NMR
spectra were recorded using a wide-bore Bruker Avance 400
simultaneously bind a metal center (as a chelate) and to act as a
hydrogen bond donor makes the ligand an excellent candidate
for the fabrication of robust inorganic–organic hybrid
supramolecular materials that could succeed as solid
1
spectrometer (9.4 T) at 100.62 MHz with 3.7 μs H 90° pulses,
2000 ms contact time, spinning rates of 10–12 kHz, and 5 s
1
recycle delays. H MAS NMR spectra were acquired on the same
33–35
1
catalysts.
instrument with 3.7 μs H 90° pulses, 2000 ms contact time,
In the present paper we report a comprehensive structural
study of the hybrid 1, employing both single-crystal and
powder X-ray diffraction (XRD) methods, and an investigation
of its catalytic performance in the epoxidation of olefins and
the oxidation of sulfides using TBHP as oxidant. It is
demonstrated that for these transformations 1 is a chemically
stable and easily recoverable heterogeneous catalyst.
spinning rate of 12 kHz, and 5 s recycle delays. Chemical shifts
are quoted in parts per million (ppm) from tetramethylsilane.
Synthesis of [MoO
3
(H
2
biim)]·nH
2
O (n ≈ 1) (1)
The ligand H biim was prepared and purified by adapting the
2
40
procedures described in ref. 38 and 39, respectively. To
prepare compound 1, a mixture of MoO (70.0 mg, 0.48 mmol),
H biim (65.2 mg, 0.48 mmol) and H O (25 mL) was heated
3
2
2
Experimental
Materials and methods
under autogenous pressure and dynamic conditions (20 rpm)
for 3 d at 160 °C in a Teflon-lined stainless-steel digestion
bomb. The resultant product comprised thin colourless plates
of 1, some of which were selectively removed for subsequent
X-ray crystallographic examination. The remaining material was
collected by filtration, washed with water, rinsed with diethyl
ether, and finally air-dried (91 mg, 63%). Found: C, 24.15; H,
Ammonium chloride (99.9%, VWR), 40% aqueous glyoxal,
ethylene glycol (99.5%, Fluka), cis-cyclooctene (95%, Alfa Aesar),
methyl oleate (99%), methyl linoleate (95%, Alfa Aesar),
DL-limonene (95%, Merck), methylphenylsulfide (99%),
diphenylsulfide (98%), thiophene (≥99%), benzothiophene
6 6 4 3 2
2.00; N, 19.05%. Calc. for C H MoN O ·H O: C, 24.34; H, 2.72;
(95%), 5.5 M TBHP in decane, anhydrous α,α,α-trifluorotoluene
N, 18.92%. TGA showed a mass loss of 6.2% at 100 °C (calc. for
n = 1: 6.1%) and a residual mass of 47.0% at 480 °C (calc. for
(>99%), diethyl ether (99.8%), pentane (≥95%, Carlo Erba),
anhydrous absolute ethanol (Carlo-Erba), ethyl acetate (≥99.7%,
Honeywell), isobutyl acetate (99%), undecane (>99%), methyl
decanoate (99%), and mesitylene (98%) were acquired from
Sigma-Aldrich unless otherwise indicated, and used as received.
The titanosilicate TS-1 (Si/Ti ≅ 50), a well-known industrial
13
1
3 C
MoO : 48.6%). C{ H} CP MAS NMR: δ 120.3 (C-5/C-5′), 128.6
1
(
C-4/C-4′), 135.9 (C-2/C-2′). H MAS NMR: δH 6.97 (br s, CH),
12.79 (s, NH).
36
Single-crystal XRD studies
Single crystals of compound [MoO
epoxidation catalyst (herein used as a benchmark catalyst),
was synthesised according to the method described in ref. 37,
using SiO powder as Si source. The hybrid material [MoO (2,2′-
3 2 2
(H biim)]·nH O (n = 1) (1)
were manually harvested and immersed in highly viscous
FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13,
2
3
bipy)] was prepared as described previously by the oxidative
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Sigma-Aldrich) to avoid degradation caused by the evaporation
the high brilliance of the synchrotron beam in combination
with a Si(111) crystal multi-analyser.
41
of the solvent. Crystals were mounted on Hampton MiTeGen
MicroLoops, typically with the help of Stemi 2000
stereomicroscope equipped with Carl Zeiss lenses.
a
The monochromatic wavelength was fixed at 0.495958(7) Å
and calibrated against the Si standard NIST 640c (certified
cell parameter a = 5.4311946(92) Å). Hard X-rays were selected
for data collection to significantly reduce radiation damage,
an occurrence observed in previous investigations using
XRD data were collected at 150(2) K on a Bruker D8 QUEST
diffractometer equipped with a Mo-Kα sealed tube (λ = 0.71073
Å), a multilayer TRIUMPH X-ray mirror, a PHOTON 100 CMOS
detector, and an Oxford Instruments Cryostream 700+ Cooler.
related materials but under different experimental
42
28,48–50
Diffraction images were processed using SAINT+, and data
were corrected for absorption by the multiscan semi-empirical
conditions.
Nevertheless, even at low temperature, the
high brilliance of the synchrotron source led to visible
damage of the samples. To minimize such effects consecutive
data collections were performed on fresh portions of the
sample by translating the capillary by ca. 1.3 mm.
43
method implemented in SADABS 2016/2.
The structure was solved using the algorithm
4
4
implemented in SHELXT-2014/5,
which allowed the
immediate location of almost all of the heaviest atoms
composing the molecular unit. The remaining missing and
misplaced non-hydrogen atoms were located from difference
Fourier maps calculated from successive full-matrix least-
A finely powdered sample of 1 was placed inside a
Hilgenberg borosilicate glass capillary (diameter ca. 0.9 mm)
which was spun during data collection to improve powder
averaging over the individual crystallites, ultimately removing
textural effects such as preferred orientation. Data were
collected in continuous mode in the 2θ range 2 to 35°, with
accumulation times increasing with the scattering angle. The
counts of the six detectors (covering roughly 5.5° 2θ) were
rebinned and normalised to give the equivalent step scans
(0.002°) suitable for further structural analyses.
2
squares refinement cycles on F using the latest SHELXL
4
5
from the 2018/3 release. All structural refinements were
4
6
performed using the graphical interface ShelXle.
Hydrogen atoms bound to carbon and nitrogen were
placed at their idealised positions using the HFIX 43
instruction in SHELXL. These hydrogen atoms were included
in subsequent refinement cycles with isotropic thermal
displacement parameters (Uiso) fixed at 1.2 × Ueq of the parent
non-hydrogen atoms.
The collected high-resolution powder XRD pattern was
indexed using the LSI-index algorithm implemented in TOPAS-
51,52
Academic V5,
and a whole-powder-pattern Pawley fit
A total of one water molecule per metal centre was found
from difference Fourier maps. These water molecules were
located over two distinct crystallographic positions, each with
a fixed site occupancy of 25% (determined from a previous
unrestrained refinement for these locations). The atoms
could not be modelled using a typical anisotropic treatment,
clearly indicating a disordered nature. The atoms were
included in the final structural model by assuming
independent isotropic thermal displacement parameters
unequivocally confirmed the orthorhombic space group Cmc2
1
as being the most suitable. Pattern indexation uncovered the
presence of a small amount of an unidentified crystalline
impurity, with the corresponding reflection discounted during
the final stages of the refinement (see Fig. 1).
The crystal structure was determined in TOPAS-Academic
V5 (ref. 51) by using a simulated annealing approach. In the
first step, one molybdenum atom and two oxygen atoms were
allowed to converge to their optimal positions within the unit
cell while using a battery of anti-bump restraints to ensure
chemically reasonable coordination geometries for the
(Uiso). Although the hydrogen atoms associated with these
moieties could not be located from difference Fourier maps,
they have been included in the empirical formula of the
compound.
6
crystallographically independent {MoO } octahedra. Despite
the high signal-to-noise ratio of the collected pattern, the
location of the atoms composing the crystallographically
independent organic component was extremely difficult, even
when using distance restraints like those employed for the
inorganic backbone. The derivation of the most suitable
location for the N,N-chelated organic linker was performed in
a second step by using a Fenske–Hall Z-matrix for half of this
chemical entity and treating the ligand as a rigid body inside
the unit cell. This strategy greatly facilitates the mobility of
this chemical entity inside the unit cell boundaries during
the global optimisation processes. It does not, however, allow
for the conformational flexibility associated with the mutual
rotations of the two rings. This was considered in a later
stage during Rietveld refinement.
Selected single crystal data for 1: C H MoN O , M =
6
8
4 4
2
3
96.10, orthorhombic, space group Cmc2
.8300(9), c = 14.745(3) Å, V = 1037.0 Å , Z = 4, 5653
1
, a = 18.363(5), b =
3
reflections measured, 966 unique (Rint = 0.0972), R = 0.0574
1
and wR
2 1 2
= 0.1230 (I > 2σ(I)), R = 0.1075 and wR = 0.1405 (all
−
3
data), highest difference peak 0.999 e Å at 1.07 Å from O2,
deepest hole −0.746 e Å at 0.78 Å from O1.
−
3
Synchrotron X-ray powder diffraction studies
High-resolution synchrotron X-ray powder diffraction data for
3 2 2
[MoO (H biim)]·nH O (n = 1) (1) were collected at 100 K,
using an Oxford Instruments cooling device, on the powder
diffractometer (Debye–Scherrer geometry) assembled at the
undulator beamline ID22 (ref. 47) (6 GeV, ∼200 mA) of the
European Synchrotron Radiation Facility (ESRF), Grenoble,
France. The high signal-to-noise ratio of the data is due to
5
3
Rietveld structural refinement
was performed with
TOPAS-Academic V5 (ref. 51) using a Chebyshev polynomial
throughout the entire angular range to model the
background contribution. The peak shapes for the powder
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3 2 2
Fig. 1 Final Rietveld plot (synchrotron XRD data) of [MoO (H biim)]·H O (1). Observed data points are indicated as a blue line, the best fit profile (upper
trace) and the difference pattern (lower trace) are drawn as solid red and grey lines, respectively. Blue vertical bars indicate the angular positions of the
allowed Bragg reflections for 1.
pattern were described using the fundamental parameters
approach, with preferred orientation effects being modelled
with a 4th order spherical harmonics approach.
Reactions were monitored using a Varian 3900 GC
equipped with a DB-5 capillary column (30 m × 0.25 mm ×
5
4
0.25 μm) and a FID detector, with H as the carrier gas. The
2
Selected powder diffraction and refinement data for 1:
reactant/products were quantified by calibrations and
identified by GC-MS (Trace GC 2000 Series Thermo Quest CE
Instruments GC; Thermo Scientific DSQ II), with He as the
carrier gas. The product identifications were based on
commercial mass spectrometry databases (Wiley6, NIST2.0,
NIST Chemistry WebBook, MAINLIB) and mass spectral
matching to available literature data. Undecane (for Cy8 and
DL-limonene (DL-lim) reactions), methyl undecane (for Ole and
1
orthorhombic, space group Cmc2 , a = 18.2315(3), b =
3
3
0
.82916(3), c = 14.7377(2) Å, V = 1028.86(3) Å , zero shift (2θ°)
.003(1), 546 independent reflections, no. global refined
= 7.67, Rwp = 10.9, Rexp = 1.44, goodness-
parameters = 75, R
p
of-fit = 7.52, Rbragg = 3.91.
Catalytic tests
methyl
linoleate
(LinOle))
and
mesitylene
(for
Oxidation reactions were carried out using 10 mL borosilicate
reactors equipped with a Teflon valve for sampling and a
Teflon-lined magnetic stirrer. Initially, catalyst 1 (18 μmol of
Mo), α,α,α-trifluorotoluene (TFT) (1 mL) and substrate (1.8
mmol) were added to the reactor, which was then immersed
in a temperature-controlled oil bath at 35 °C (for thioanisoles
and thiophenes) or 70 °C (for olefins), under stirring (1000
rpm). After 10 min, pre-heated TBHP (2.75 mmol for the
reaction with cis-cyclooctene (Cy8); 4.07 mmol for the
reactions with all other substrates) was added to the reactor,
and reaction timing started at this instant. A higher amount
of oxidant was used for the substrates other than Cy8 since
the stoichiometry of the corresponding reactions to
diepoxides or sulfones is two moles of TBHP per mole of
substrate (to aid comparison, the same oxidant : substrate
ratio was used for the monoene methyl oleate (Ole)).
methylphenylsulfide (mps), diphenylsulfide (dps) and
benzothiophene (bt)) were used as internal standards. The
experimental range of error was less than 6%, based on
replicates carried out for selected experimental conditions.
Iodometric titration was carried out to check the oxidant
efficiency: the reactor containing the catalyst, cosolvent and
oxidant (1/TBHP/TFT), but no substrate, was heated at 70 °C
for 4 h (stirring rate of 1000 rpm). After cooling the reactor to
ambient temperature, a sample was withdrawn, filtered and
titrated.
The catalytic reaction systems consisted of biphasic solid–
liquid mixtures. A filtration test was performed to check the
homo/heterogeneous nature of the catalytic reaction (1/Cy8/
TBHP): the solid was separated from the liquid phase of the
reaction mixture, at 1 h/70 °C, using a 0.2 μm PTFE
membrane. The filtrate was transferred to a separate reactor
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preheated to 70 °C) and stirred for 5 h at 70 °C, after which
Catalysis Science & Technology
(
time it was analysed by GC. Additionally, catalyst recycling
tests were performed in which the hybrid catalyst was
recovered and reused: after each catalytic batch run, the solid
catalyst was collected by centrifugation (5000 rpm), washed
first with diethyl ether and then with pentane, air-dried
overnight at ambient temperature, and finally vacuum-dried
(ca. 4 mbar) for 2 h at 60 °C, giving the used/recovered
catalyst denoted 1*. Catalyst 1* was reused twice; the initial
mass ratios of catalyst : substrate : oxidant were the same for
all batch runs.
Results and discussion
Synthesis and characterisation of [MoO (H biim)]·nH O (n ≈
3
2
2
1
) (1)
Compound 1 was prepared by the hydrothermal reaction of
MoO , H biim, and H O in the mole ratio 1 : 1 : 50 at 160 °C
for 3 days. The resultant crystalline product was isolated in
7% yield. This procedure is similar to the one employed by
3
2
2
2
Fig. 3 FT-IR spectra (KBr pellets) of (a) H biim and (b) compound 1.
6
2
9
Morales et al. SEM micrographs revealed that 1 displays a
−
1
morphology consisting of aggregates of irregularly shaped
cm , while free H biim only displays a very weak absorption
2
−
1
−1
thin plates (Fig. 2a). The FT-IR (4000–300 cm ) spectra of H -
in this region at 999 cm , which was previously assigned to
a δ(C,N–H) mode. The strong bands at 779 cm (broad) and
2
−
1
biim and 1 were measured in the solid state as KBr pellets
and are shown in Fig. 3. Wavenumbers and assignments
−
1
894 cm for 1 are attributed to ν(Mo–O–Mo) and ν(MoO)
(
made in accordance with the interpretations proposed by
modes, respectively.
5
5
2+
2+
13
1
Gruia et al. for Cd and Zn complexes with H
2
biim) are
2
The solid-state C{ H} CP MAS NMR spectrum of H biim
provided in Table 1. A comparison of the spectra of H biim
displays five sharp resonances in the region 110–145 ppm
(Fig. 4A). Two pairs of resonances at 118.2/120.3 and 126.4/
128.7 are assigned to C5/C5′ and C4/C4′, respectively, i.e. the
2
and 1 with respect to the ligand-centered bands shows
various shifts, intensity changes and splitting. Several of the
most significant alterations mimic those described by Gruia
et al. and can be attributed to the coordination of H biim as
2
49,55
Table 1 FT-IR bands and possible assignments
compound 1
2
for H biim and
a bidentate chelating ligand with both N–H groups remaining
−
1
protonated. For example, in the 1200–1050 cm range, where
δ(C–H), δ(N–H) and ring vibrations occur, the bands at 1104
Assignment
2
H biim
1
−
1
(
1
vs) and 1143 cm (w) for H
2
biim are replaced by bands at
ν(C–H)
3142 w
3112 w
3126 m
−
1
113 (m), 1139 (m) and 1185 cm (w) for 1. The hybrid
3000 s, br
2998 m, br
2898 m
material displays a band with medium intensity at 1005
ν(N–H)
2895 s, br
2929 m
R
R
R
R
1545 s
1536 m
1431 m
1405 w
1331 w
1218 vw
1185 w
1139 m
1113 m
1005 m
941 w
1435 m
1405 vs
1333 s
1217 m
1143 w
1104 vs
ν(C2–C2′)
R
δ(C,N–H)
δ(C,N–H)
δ(C,N–H)
δ(C,N–H)
R
999 vw
939 s
915 w
920 sh
R
887 s, br
ν(MoO)
γ(N–H)
ν(Mo–O–Mo)asym
γ(C–H)
894 s
847 w
830 w
779 vs, br
(Overlap with
above band)
682 s
437 w, 395 m
339 m, 285 m
763 m, 747 s,
736 m
R
689 s
5
5
Fig. 2 Representative SEM images (a and c) and corresponding EDS
maps for Mo (b and d) of 1 (a and b) and the used/recovered catalyst
Chelate ring deformation(s),
δ(Mo–O–Mo), ν(Mo–O–Mo)
sym
and/or
4
9
1* from the Cy8 reaction (c and d).
2
γ(MoO ).
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Fig.
5
TGA
(blue
line)
and
corresponding
differential
thermogravimetric (red line) curves for compound 1.
Information about the thermal stability and composition
of compound 1 was obtained by performing elemental and
thermogravimetric analyses (TGA). CHN analyses indicated
the empirical formula [MoO (H biim)]·nH O (n ≈ 1). TGA of
3
2
2
1
under air showed an initial mass loss of 6.2% up to 100 °C,
corresponding to the removal of water, and a succession of
overlapping events between 200 and 480 °C, corresponding
to the protracted oxidative decomposition of the organic
ligand and the formation of MoO (Fig. 5). The residual
3
MoO3 mass of 47.0% at 480 °C is close to the theoretical
value of 48.6% calculated on the basis of the above formula.
As expected, the TGA curve shows that the residual MoO
3
starts to sublime above 700 °C.
X-ray diffraction studies
1
3
1
1
Fig. 4 Solid-state C{ H} CP MAS NMR (A) and H MAS NMR (B)
spectra of (a and c) H biim and (b and d) compound 1. Asterisks
indicate spinning sidebands.
Single crystal XRD studies confirmed the formula [MoO (H -
3 2
2
biim)]·H
2
O for the hybrid material 1. The very thin colourless
isolated from the hydrothermal synthesis
plates of
1
diffracted rather poorly, especially at high angles. To
unequivocally solve and refine the crystal structure, long
acquisition times were required per frame (typically 240
seconds), with frame slicing of 1° up to a resolution of just
0.83 Å. Nevertheless, the Rint was rather large (0.0972) with
many of the atoms being refined with large ellipsoids,
typically indicating the presence of some structural disorder
associated with each individual moiety. Bulk studies of the
crystal structure of 1 were performed by using high-
resolution powder synchrotron XRD data collected at the
ESRF, Grenoble. The structure was once again solved and
refined, confirming the structural features previously
unveiled from the single crystal studies. These investigations
also showed the presence of trace amounts of unidentified
crystalline impurities (Fig. 1).
chemically
equivalent
Cn
and
Cn′
atoms
are
crystallographically inequivalent in the solid-state. On the
other hand, the ring-connecting C2/C2′ atoms only give rise
1
3
1
to one (more intense) signal at 138.8 ppm. The C{ H} CP
MAS NMR spectrum of 1 differs from that for the free organic
ligand in that only one resonance is observed for each C5/C5′
and C4/C4′ pair (at 120.3 and 128.6 ppm, respectively). The
crystallographic equivalence of each of these pairs of carbon
atoms was subsequently confirmed by the X-ray diffraction
studies (see below). Coordination of H biim to Mo centres
in 1 resulted in a slight upfield shift of the C2/C2′ resonance
from 138.8 to 135.9 ppm. The H MAS NMR spectra of H
biim and 1 are similar (Fig. 4B), displaying two well-resolved
resonances in the region 0–15 ppm: one line at 14.72 ppm
2
for H biim and 12.79 ppm for 1, assigned to NH protons,
and a broader, more intense signal at 5.31 ppm for H biim
and 6.97 ppm for 1, assigned to CH protons.
VI
2
1
2
-
The crystal structure of 1 agrees to some extent with the
single crystal analysis reported by Morales et al. for their
material formulated as [MoO (H biim)] and prepared
2
3
2
29
hydrothermally from MoO3 and H biim.
The structure
2
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1
comprises
a
1D neutral and linear
[MoO (H biim)]
∞ 3 2
coordination polymer that runs parallel to the crystallographic
010] direction of the unit cell with an intermetallic Mo⋯Mo
distance along the Mo → μ -O → Mo direction of 3.8300(9) Å
[
2
(Fig. 6). Although a similar intermetallic distance of 3.838 Å was
found by Morales et al., they determined an almost linear kink
angle of 175.24° for this bridge (arising from alternating long
(
2.179 Å) and very short (1.662 Å) Mo–O bond lengths), while
b
our structural analysis revealed a kink angle of 161.3(12)°
(arising from alternating long (2.158 Å) and short (1.722 Å) Mo–
O bond lengths).
b
The 1D polymer in 1 is analogous to that found in the
inorganic–organic hybrid materials [MO (L)] with L = 2,2′-
3
2
4
28
bipy (M = Mo, W (ref. 56)) or H bpdc (M = Mo), and
The basic building unit is
composed of a six-coordinated Mo metal centre bonded to
2
5
0
{
[MoO
3
(2,2′-bipy)][MoO
3
(H
2
O)]}
n
.
VI
three oxo groups (two terminal and one μ -bridging) and
2
one N,N′-chelated H
geometry of the {MoN O } polyhedron resembles a highly
2
biim organic moiety. The coordination
Fig.
7
Crystal packing of [MoO
3
(H
2
biim)]·H
2
O
(1) viewed in
2
4
perspective along the [010] direction of the unit cell.
distorted octahedron (Fig. 6(c)), mostly because of the trans
effect of the terminal oxo groups: while the Mo–(N,O) bond
lengths range from 1.722(13) to 2.346(19) Å, the cis and
trans (N,O)–Mo–(N,O) octahedral internal angles lie in
the ranges 69.1(9)–104.0(10)° and 156.6(6)–161.3(12)°,
respectively.
disordered nature of the water molecules and the large isotropic
displacement parameters.
Compound 1 contains one water molecule of crystallisation
per Mo centre. As depicted in Fig. 7, this molecule appears
VI
Catalytic studies
disordered over small channels running parallel to the [010]
Epoxidation of cis-cyclooctene (Cy8). For experimental
benchmarking of epoxidation catalysts, the reaction rate of
Cy8 with tert-butylhydroperoxide (TBHP) is often used. TBHP
is a convenient source of active oxygen and displays great
versatility in organic reactions, with the largest commercial
1
direction of the unit cell. Because individual
∞
3 2
[MoO (H biim)]
coordination polymers are engaged in strong and rather
directional hydrogen bonds connecting the oxo groups of one
polymer to the N–H moieties of the adjacent one [dD⋯A
=
5
7
2.78(2) Å with <(DHA) = 162°], supramolecular contacts with
application being the epoxidation of propylene. The main
drawback of TBHP is the risk of explosive decomposition
the solvent molecules are much weaker, thus explaining the
1
∞
Fig. 6 Schematic representation of the (a) 1D neutral
[MoO
3
(H
2
biim)] coordination polymer present in the crystal structure of compound 1
running parallel to the [010] direction of the unit cell, emphasising the (b) linear nature of the polymer and the intermetallic distances. (c) Distorted
VI
octahedral coordination environment of the crystallographically unique Mo metal centre. Selected bond lengths (Å) and angles (°): Mo1–O1
i
ii
ii
ii
ii
2
6
.158(12), Mo1–O1 1.722(13), Mo1–O2 1.750(11), Mo1–O2 1.750(11), Mo1–N1 2.346(19), Mo1–N1 2.346(19), O2–Mo1–O2 104.0(10), N1 –Mo1–N1
i
i
i
9.1(9), O1 –Mo1–O1 161.3(12), O1 –Mo1–O2 103.5(6), O2–Mo1–O1 87.8(5), O1 –Mo1–N1 89.5(7), O1–Mo1–N1 75.2(6), O2–Mo1–N1 91.4(7). Symmetry
transformations used to generate equivalent atoms: (i) x, −1 + y, z; (ii) −x, y, z.
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caused by heat, mechanical shock, friction, or chemical
incompatibility. Competing against these safety concerns,
however, is the recognition that TBHP presents fewer
handling risks than 70% H O in water or CH C(O)OOH, is
Catalyst recyclability and stability were studied by
recovering the solid material after a catalytic run by simple
centrifugation/washing/drying procedures (giving 1*). No
drop in catalytic activity was observed when the recovered
solid was reused; in fact, activity increased in consecutive
batch runs, with conversions at 6 h of 87 and 99% in runs 2
and 3 (Cy8O selectivity was always 100%). The ATR FT-IR
spectra of 1 and 1* were practically identical, demonstrating
the good chemical stability of 1 (Fig. 9A). The crystalline
structure (based on PXRD, Fig. 9B) and morphology (based
on SEM, Fig. 2) of 1 changed upon contact with the reaction
medium, which seems to correlate with the faster reaction
kinetics of the reused catalyst. As mentioned above, the
2
2
3
not very corrosive, often leads to selective metal-catalysed
oxidations, is freely soluble in organic media, and is
5
7,58
obtainable from renewable resources.
Furthermore, the
co-product, tert-butanol, can be easily recovered by
distillation and either recycled (to resynthesise TBHP) or
used in other industrial processes, such as the synthesis of
the fuel additive methyl tert-butyl ether (MTBE). The
importance of TBHP as an oxidant for industrial reactions
has motivated research into the development of safer
processes, such as the continuous preparation of anhydrous
original
catalyst
possessed
water
molecules
in
5
9
TBHP solutions.
crystallographically different positions, some of which were
The Cy8 reaction with TBHP in the presence of 1 at 70 °C,
with TFT as cosolvent, gave 1,2-epoxycyclooctane (Cy8O) as
the sole product in 83%/99% yield at 6 h/24 h (Fig. 8),
whereas without oxidant and/or without catalyst, the reaction
was very sluggish. Iodometric titration of the reaction mixture
indicated 100% oxidant efficiency for the hybrid catalyst 1,
i.e., negligible non-productive decomposition of TBHP into
2
tert-BuOH and O .
Since the catalytic reaction system consisted of a biphasic
solid–liquid mixture, a hot filtration test was performed to
assess whether the epoxidation reaction took place
homogeneously and/or heterogeneously. The results
confirmed that 1 performed as a heterogeneous catalyst
since, as shown in Fig. 8, the liquid phase obtained by
filtration of the reaction mixture at 58% conversion (1 h
reaction time) did not present any further Cy8 conversion up
to 6 h, whereas for the normal catalytic run the conversion
rose to 83% in the same time interval. No molybdenum was
detected in the liquid phase by ICP-AES analysis (detection
limit: ca. 0.50 μg Mo per L) and hence metal leaching was
deemed to be insignificant.
Fig. 8 Epoxidation of Cy8 with TBHP at 70 °C, in the presence of 1:
normal catalytic test ( ) and filtration test ( ). Epoxide (Cy8O)
selectivity was always 100%. The dashed lines are visual guides.
2
Fig. 9 ATR FT-IR spectra (A) and PXRD patterns (B) of the ligand H -
biim (a), compound 1 (b) and the used/recovered catalyst (1*) from the
reactions of Cy8 (c), Ole (d) and dps (e).
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The structure of 1 possesses dioxomolybdenum(VI) sites.
Scheme 1 is a simplified representation of possible transition
states (based on literature data) involved in olefin
(
represented
as
CH CH )
epoxidation
over
2
2
dioxomolybdenum(VI) active sites. Based on mechanistic
studies reported in the literature for oxomolybdenum(VI)-
catalysed olefin epoxidation with TBHP, the active species
may be formed via the reaction of TBHP with the
molybdenum sites, whereby a H-atom is transferred from the
ROOH oxidant molecule (R
=
tert-butyl) to an
oxomolybdenum group (MoO), leading to moieties of the
type {Mo(O)(OH)(OOR)} (pathway (i) in Scheme 1). These
2
intermediates may display an η -asymmetric coordination
6
0–63
mode (Mo(O O )R)
or exhibit a five-membered ring
α
β
involving hydrogen bonding between a hydroxide group Mo–
6
4–67
OH and the Oβ atom of Mo–O O R.
Alternatively,
α
β
Scheme
1
Simplified representation of possible transition states
2
CH )
pathway (ii) gives moieties of the type {Mo(O) (ROOH)} that
2
(
based on literature data) of olefin (represented as CH
2
possess a very weak Mo⋯O
β
interaction and a (stabilising)
hydrogen bond between ROOH and an oxomolybdenum
epoxidation with TBHP, over dioxomolybdenum(VI) active sites.
6
8
group (MoO). The epoxidation then occurs via an O-atom
63–66
transfer from the active oxidising site to the olefin.
fixed occupied sites. These crystallographic features may be
flexible, changing in the multicomponent catalytic reaction
medium, e.g., via water desorption or partial exchange for
other molecules. Importantly, the chemical structure and the
uniform distribution of molybdenum were preserved (also
verified with other substrates as discussed below).
Table 2 compares the catalytic performance of 1 with
literature data for Mo-based catalysts possessing chelating N,
N-bidentate ligands where Mo species are embedded (hybrid
polymers) or supported. Furthermore, 1 was benchmarked
with the well-known titanosilicate TS-1 heterogeneous
catalyst and one of the best performing polymeric hybrids
Table 2 Comparison of the catalytic behaviour in the epoxidation of cis-cyclooctene of compound 1 with the titanosilicate TS-1 and other Mo-based
catalysts possessing chelating N,N-bidentate ligands, where Mo species are either embedded (hybrid polymers) or supported
a
b
c
Entry
Catalyst
Solvent
Mol% Mo
TBHP/Cy8
T/°C
Conv. c1,c2,c3 (%)
He/Ho
Ref.
Hybrid polymer-embedded Mo complexes
1
2
3
4
5
6
7
8
9
1
TFT
TFT
DCE
TFT
TFT
TFT
DCE
Hexane
Hexane
TFT
1
1
1
1
1
1
1
1
1
1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
70
70
55
55
55
55
55
55
75
55
83,87,99
88
48
41
72
54
18,25
4.5,10,4.7
45
He
This work
This work
[MoO
[MoO
3
(bipy)]
(bipy)]
He + Ho
He + Ho
Ho
Ho
Ho
He + Ho
He
He
d
3
30
26
26
26
50
50
50
26
[Mo
[Mo
{[MoO
2
O
O
6
(bipy)]
(bipy)
(bipy)][MoO
3
9
2
]
3
3
(H
2
O)]}
n
10
33
Ho
Supported Mo complexes
[O Mo⋯MoO ]@MCM-41-bipy
[MoO(O ]@MCM-41-pzpy
MoO Cl @MCM-41-pzpy
11
12
13
14
15
16
17
18
19
2
2
None
2
0.4
2
0.8
1
2.8
0.7
0.7
0.1
1.5
1.03
1.5
1.03
1.2
1.5
2
55
61
55
61
61
55
50
75
75
60,11
100,100
85,79,79
Ho
He
He + Ho
He
He
He
He
He
He
69
70, 71
72
73
74
75
76
77
78
2
2
2
2
)
2
CHCl
None
CHCl
CHCl
DCE
CHCl
TFT
Toluene
3
2
e
[MoO(O
[MoO(O
)
)
2
]@HSGM-pzpy
(pzpy)]@SMNP
3
3
95,95,95
2
96,96,95f
54,39,30f
34,25,30
63,44,17g
65,33,16
[O
2
Mo⋯MoO
2
]@LDH-bipy
MoO
MoO
MoO
2
2
2
Cl
Cl
Cl
2
2
2
@GaMOF-bipy
@ZrMOF-bipy
@PMO-bipy
3
1.5
1
Other
TS-1
h
20
TFT
—
1.5
70
5
He
This work
a
For simplicity, the abbreviation 2,2′-bipy is shortened to bipy. MCM-41 = Mobil Composition of Matter no. 41; pzpy = functionalised
pyrazolylpyridine ligand; HSGM = hybrid sol–gel material; SMNP = silica-coated magnetic nanoparticle; LDH = layered double hydroxide;
b
GaMOF = gallium-based metal–organic framework; ZrMOF = zirconium-based MOF; PMO = periodic mesoporous organosilica. Cy8
conversions after a reaction time of 6 h (unless otherwise indicated) for one (c1), two (c2) or three (c3) consecutive cycles. Selectivity to Cy8O
c
d
e
was 100% or close to 100% in all cases. Heterogeneous (He)/homogeneous (Ho) nature of the reaction. DCE = 1,2-dichloroethane. Reaction
f
g
h
time of 4.5 h. Reaction time of 7 h. Cy8O yields at 8 h. Reaction time of 24 h.
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with
a
heterogeneous catalytic contribution, namely
namely bio-ethanol and the bio-esters ethyl acetate and
isobutyl acetate (produced via fermentation processes ),
7
9
[MoO (2,2′-bipy)], tested under identical reaction conditions
3
to those used for 1. For simplicity, the former comparison is
limited to systems devised on the concept of strong binding
which have all been classified as recommended (or preferred)
solvents based on safety, health and environment criteria.
8
0
between
a
chelating
N,N-bidentate
ligand
and
Cy8 conversions at 6 h/24 h were 75%/93% for ethanol, 38%/
100% for ethyl acetate and 29%/70% for isobutyl acetate. The
catalytic results using ethanol resemble more closely those
using TFT (83%/99% conversions at 6 h/24 h). For these four
solvents, the dielectric constants at 20 °C decrease in the
order ethanol (25.3) > TFT (9.2) > ethyl acetate (6.0) >
isobutyl acetate (5.3), which does not correlate directly with
the catalytic results. These results may be due to a complex
interplay of factors including polarity and competitive effects
between the oxygenated solvents and the oxidant in the
coordination to the metal centres.
oxomolybdenum(VI) units. Specifically, the systems are
divided into two families: (i) hybrid materials such as 1
consisting of 1D chains of corner-sharing {MoO (μ -O) }
2
2
2
2 2 2
tetrahedral units and/or {MoO (μ -O) (2,2′-bipy)} octahedral
units; (ii) supported materials in which individual
molybdenum centres are heterogenised via binding with N,N-
ligands that are either an integral part of the support
framework (bipyridine-based MOFs and PMOs, entries 17–19)
or immobilised on the support via covalent tethering (silica-
based supports, entries 11–15) or electrostatic interactions
(
LDHs, entry 16). Compound 1 is clearly superior to most of
Scale-up consisting of a ten-fold increase in the volume of
the reaction mixture for 1 (stirring rate of 1000 rpm) led to
63%/95% Cy8 conversion at 4 h/24 h, compared to 83%/99%
at the microliter scale (TFT as solvent). Hence, similar
conversion was reached in a 24 h batch, although the initial
reaction kinetics was slower, which may be partly due to
mass transfer limitations. Using more efficient types of
mixers (a magnetic stirrer was used in this work) may
advantageously enhance mass transfer.
The hybrid material 1 was further explored as a catalyst
for broader chemical reaction (epoxidation, sulfoxidation)
and substrate (biomass-derived olefins, thioanisoles and
thiophenes) scopes.
the previously studied chainlike hybrid materials, both in
terms of catalytic activity and ability to catalyse the reaction
heterogeneously.
3
The previously reported compound [MoO (2,2′-bipy)],
which was relatively active and was partly heterogeneous, led
to much slower initial reaction kinetics than 1 (27%
conversion at 1 h compared to 58% for 1); at 6 h/24 h similar
conversions were reached for the two catalysts (83–88% at 6
h and 100% at 24 h; entries 1 and 2 of Table 2). However,
3
[MoO (2,2′-bipy)] exhibited a small homogeneous catalytic
contribution at 70 °C (as previously reported for this catalyst,
albeit at 55 °C; entry 3, Table 2). Specifically, the liquid phase
of the hot filtration test of [MoO
0% Cy8 conversion between 30 min (the instant of catalyst
filtration) and 24 h (compared to 73% for a normal catalytic
test). Only the material {[MoO (2,2′-bipy)][MoO (H O)]} could
be used as a truly heterogeneous catalyst (with hexane as
solvent, entries 8 and 9), but the catalytic activity was low
3
(2,2′-bipy)] at 70 °C led to
Bio-based olefin epoxidation. Fatty acid methyl esters
(FAME) are obtained by the transesterification of vegetable
1
8
1,82
oils.
FAME may be used directly (for biodiesel
3
3
2
n
production) or after further modification such as
8
2–84
epoxidation,
which is useful in industry for the synthesis
of chemicals and intermediates such as plasticisers and
stabilisers of PVC, reactive diluents, detergents, and
intermediates in the production of polyurethane polyols or
components for lubricants, fuel additives, cosmetics and
(
45% Cy8O yield after 6 h at 75 °C compared with 83% for 1
at 70 °C, entries 1 and 9). With regard to the supported
materials, catalyst displays competitive or similar
1
8
5–91
performance in terms of activity and recyclability, even in
relation to the best catalysts reported by Thiel and co-workers
consisting of oxodiperoxo complexes tethered to silica-based
pharmaceuticals.
In industry, the traditional, most
commonly used method for the synthesis of epoxidized FAME
is the Prilezhaev reaction using in situ generated peracetic or
8
3,84
supports
via
spacer
groups
functionalised
with
performic acids.
This process presents, however, several
pyrazolylpyridine ligands (entries 12, 14, 15). Other supported
materials, such as those comprising immobilised dinuclear
drawbacks, such as a low epoxide selectivity due to oxirane
ring-opening reactions under acidic conditions, and the
production of highly corrosive waste. It is, thus, of great
academic and industrial interest to search for alternative
epoxidation routes, especially those that employ easily
recoverable and reusable heterogeneous catalysts.
complexes (entries 11 and 16) or MoO
7–19) suffer from low activities, leaching phenomena and/or
loss of activity upon recycling. The benchmark catalyst TS-1
2 2
Cl species (entries 13,
1
3
6
(
a type of industrial epoxidation catalyst ) led to 5% Cy8
conversion at 24 h (entry 20, Table 2), possibly due to
detrimental steric hindrance imposed by its microporosity.
The high boiling point and hydrophobic character of TFT
makes it an attractive solvent for catalytic applications,
reducing VOC emissions and protecting moisture sensitive
catalysts/products. Nevertheless, TFT is classified as being
toxic to aquatic life with long-lasting effects (GHS H411), and
a highly flammable liquid and vapour (GHS H225). Hence,
catalyst 1 was further studied using bio-based solvents,
The hybrid material 1 was an effective catalyst for the
epoxidation of FAME, namely methyl oleate (Ole) and methyl
linoleate (LinOle). The reaction of Ole led to 34%/72%/97%
conversions at 1 h/6 h/24 h, 70 °C (TFT as solvent). The non-
catalysed reaction was very sluggish (5% conversion at 24 h).
Selectivity to methyl 9,10-epoxyoctadecanoate (OleO) was
nearly 100% at 72% conversion, with only a slight drop to
97% occurring at very high conversion (97%) due to the
concomitant formation of the polyol methyl 9,10-
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dihydroxyoctadecanoate (OleDiol) (Table 3). This catalytic
possessed lower activity in the initial stage of the reaction
(up to 6 h) than 1 (72% conversion at 6 h).
performance is on a par with that displayed by [MoO (2,2′-
3
bipy)], which led to 82% Ole conversion after 6 h at 75 °C (1
mol% Mo, TBHP/Ole = 1.5, DCE as cosolvent). Under the
same conditions, except with TFT as the cosolvent instead of
The catalytic reaction of LinOle in the presence of 1 gave
31%/66%/88% conversion at 1 h/6 h/24 h (Table 3). LinOle
was not reactive in the absence of a catalyst. The catalytic
reaction gave mono- (LinOleO) and diepoxide (LinOleDO)
products, specifically, methyl 9,10-epoxy-12Z-octadecenoate
and methyl 12,13-epoxy-9Z-octadecenoate (86% total
monoepoxide selectivity at 66% conversion), and methyl 9,10-
12,13-diepoxyoctadecanoate (14% diepoxide selectivity at 66%
conversion). The total monoepoxide selectivity decreased from
86% to 65% as conversion increased from 66% to 88% due to
the concomitant formation of the diepoxide (34% selectivity at
9
2
3 2 3 2
DCE, the hybrid material [(CH ) NH][MoO (Hbpdc)]·nH O led
2
8
to 93% Ole conversion after 6 h. The Ole epoxidation
−
reactions with the 2,2′-bipy and Hbpdc hybrids were,
however, entirely homogeneous in nature, i.e., the hybrids
acted as sources of soluble active species. When tested under
identical reaction conditions to those used for 1, catalyst
3
[MoO (2,2′-bipy)] led to 100% OleO selectivity at 62%/96%
Ole conversion (6 h/24 h), suggesting that the latter
Table 3 Catalytic performance of 1 in epoxidation and sulfoxidation reaction routes, from bio-based DL-lim and FAME, and thiophene and thioanisole
a
substrates
b
c
Reaction scope
Substrate scope
Cy8
Conv. (%)
Product selectivity (%)
Epoxidation (70 °C)
58/83/99
Cy8O (100/100/100)
73/97/100
DL-Lim
Ole
LimO (87/80/55)
OleO (99/99/97)
LimDO (3/12/35)
LimDiol (1/1/3)
3
3
4/72/97
1/66/88
OleDiol (0/0/3)
LinOleO
LinOle
LinOleO (100/86/65)
LinOleDO (0/14/34)
Sulfoxidation
98/100/100
(35 °C)
mps
Sulfoxide (99/93/67)
Sulfone (1/7/33)
8
2
1/100/—
dps
bt
(
(
100/80/—)
(0/20/—)
/21/35
100/39/13)
(0/61/75)
a
Reaction conditions: initial concentration of substrate = 0.76–1.04 M, initial Mo : substrate : TBHP molar ratio = 1 : 100 : 153 for Cy8, and 1 :
00 : 226 for the other substrates, TFT as cosolvent. Conversion of substrate at 1 h/6 h/24 h. Product selectivities at 1 h/6 h/24 h are
b
c
1
indicated in parentheses.
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8%, conversion, 24 h). These results suggest that 1 may
favour the formation of polyepoxides, which are interesting
intermediates to polyols. When tested under identical reaction
Paper
8
pharmaceuticals, and other valuable fine chemicals, as chiral
94–98
auxiliaries, and as ligands in transition metal catalysis.
Apart from synthetic chemistry, other applications of
sulfoxidation have emerged, such as in extractive processes
for the removal of refractory sulfur compounds from
conditions to those used for 1, catalyst [MoO (2,2′-bipy)] led to
3
8
8
7%/72% LinOleO and 13%/28% LinOleDO selectivity at 63%/
5% LinOle conversion (6 h/24 h), which is comparable to
9
9
transportation fuels.
Much of the recent and current
that for 1 (Table 3). TS-1 lacked catalytic activity for the
epoxidation of Ole and LinOle, likely due to the inaccessibility
of the bulky substrate molecules to the active sites.
research on sulfoxidation processes focuses on the use of
metal-based catalysts to achieve efficient chemoselective or
enantioselective transformations. As in other fields of
catalysis, the development of more environmentally
acceptable protocols that employ recoverable catalysts, such
as heterogeneous systems, is of pressing concern in
Naturally occurring cyclic monoterpenes, such as R-(+)-
limonene (R-(+)-lim or D-lim) extracted from citrus peel waste,
are another promising renewable olefin source. Their wide
availability, relatively low cost and sustainable environmental
profile make them attractive as raw material for further
functionalisation, with epoxidation being one of the key
transformations. The mono- and diepoxides of D-lim are key
intermediates in the upgrading of limonene since a broad
spectrum of oxygenated derivatives are accessible via epoxide
1
00–102
sulfoxidation chemistry.
The hybrid material 1 was tested as a catalyst for the
oxidation of thioanisoles and thiophenes with TBHP at 35
°C. The conversion of methylphenylsulfide (mps) was 98%
at 1 h, giving the corresponding sulfoxide with practically
100% selectivity. As the reaction progressed, complete
conversion was reached, and sulfoxide selectivity decreased
from 99 to 67% with the concomitant formation of the
sulfone with 33% selectivity. Hence, the operating
conditions of the catalytic process may be controlled to
adjust the reaction outcome according to the desire for
sulfoxide vs. sulfone products (Table 3). Without catalyst,
the mps reaction was much slower, giving 20% conversion
at 1 h. With diphenyl sulfide (dps), 1 led to 100% sulfoxide
selectivity at 81% conversion (no sulfone was formed);
increasing the reaction time to 6 h led to yields of 80% for
the sulfoxide and 20% for the sulfone. The substrate dps
was less reactive without catalyst (100% sulfoxide selectivity
9
3
ring-opening reactions.
Catalyst 1 was effective for the epoxidation of DL-lim
(racemic mixture of the two enantiomeric forms D-lim and
L-lim), leading to 73%/97%/100% conversion at 1 h/6 h/24 h,
0 °C (Table 3). Without catalyst, the Lim reaction was very
7
sluggish (5% at 24 h). Similar results (95%/100% conversion
at 6 h/24 h) were obtained for the homogeneously catalysed
reaction using the material [(CH
3 2 3 2
) NH][MoO (Hbpdc)]·nH O
as source of active species (1 mol% Mo, TBHP/Ole = 1.55,
2
8
TFT as cosolvent, 75 °C).
In the presence of 1, selectivity to the monoepoxide
,2-epoxy-p-menth-8-ene (LimO) peaked at 87% after 1 h
1
reaction, and then decreased to 55% at 100% conversion due
to the concomitant formation of the diepoxide LimDO (35%
selectivity at 100% conversion). These results tally with those
for LinOle concerning the apparent ability of 1 to favour the
formation of polyepoxides from bio-derived olefins. Minor
at 69% conversion). The benchmark catalyst [MoO (2,2′-
3
bipy)] led to 100%/70% sulfoxide selectivity at 78%/100%
dps conversion (1 h/6 h), a catalytic performance very
similar to that for 1, although the bipy hybrid acted as a
homogeneous catalyst. TS-1 was inactive, leading to similar
results to those without catalyst (100% sulfoxide selectivity
at 45% conversion, 6 h reaction).
The catalytic sulfoxidation screening tests showed that 1
was much more effective for the oxidation of thioanisoles
than thiophenes. Thus, in the presence of 1, thiophene did
not react, while benzothiophene reacted to give the
corresponding sulfoxide and sulfone products with 13 and
75% selectivity, respectively, at 35% conversion (24 h).
Without catalyst, no reaction occurred (Table 3).
products
cyclohexanediol and others resulting from allylic oxidation
e.g., p-mentha-2,8-dienol and p-mentha-6,8-dien-2-one). The
included
1-methyl-4-(1-methylethenyl)-1,2-
(
catalytic system presented high regioselectivity towards the
epoxidation of the endocyclic CC double bond, with the
LimO/LimDO molar ratio being greater than ca. 1.6 until
1
00% conversion. When tested under identical reaction
conditions to those used for 1, catalyst [MoO (2,2′-bipy)] led
3
to 100%/79% LimO selectivity at 75%/92% DL-lim conversion,
at 6 h/24 h. At 24 h, LimDO and LimDiol were formed with
The chemical and structural features of 1 after catalytic
reactions with sulfides were preserved (exemplified for the
catalyst used in the reaction of dps, in Fig. 9).
1
1% and 10% selectivity, respectively. Overall, [MoO (2,2′-
3
bipy)] seemed to be less active than 1 with bioderived olefins,
and TS-1 was inactive (with Lim, Ole and LinOle).
Characterization of the catalyst recovered after reactions
with bioderived olefins suggested that 1 is chemically and
structurally stable (exemplified for the catalyst used in the
reaction of Ole, in Fig. 9).
Conclusions
The hydrothermally synthesised organic–inorganic hybrid
material [MoO (H biim)]·H O (1) is an effective catalyst in
3
2
2
Sulfoxidation systems. The selective oxidation of sulfides
is an important process since organic sulfoxides and sulfones
serve as versatile intermediates/building blocks in the
synthesis of agrochemicals, biologically active compounds,
epoxidation and sulfoxidation of various substrates with TBHP
as oxidant. In the catalytic epoxidation systems (substrates:
cis-cyclooctene, methyl oleate, methyl linoleate, DL-limonene),
the epoxide products were formed in yields of at least 87% at 24
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h/70 °C. In the catalytic sulfoxidation systems, the conversions
of the thioanisoles (methylphenylsulfide, diphenyl sulfide) were
in the range 81–98% at 1 h/35 °C, giving the corresponding
sulfoxides with excellent selectivity (≥99%); the reaction time
may be optimised to tune the sulfoxide/sulfone product
distribution at 100% conversion. Thiophenes (thiophene,
benzothiophene) were more demanding than thioanisoles.
Hybrid 1 exhibited heterogeneous catalytic behaviour: it passed
the hot filtration test, could be easily separated from the
reaction mixture, and was stable in consecutive batch runs.
These features place 1 on a higher footing than most of the
previously studied oxomolybdenum(VI)-based polymeric hybrid
catalysts. The main significance of this development is that 1
can be used directly as a catalyst following its hydrothermal
synthesis – there is no need of immobilisation (e.g. of a
molecular catalyst) on a solid support or post-synthetic
modification. Challenges remain in developing polymeric
hybrids with enhanced catalytic activity for more demanding
reactions such as those of thiophenes, under mild conditions.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was carried out with the support of CICECO –
Aveiro Institute of Materials [FCT (Fundação para a Ciência e
a Tecnologia) Ref. UIDB/50011/2020 & UIDP/50011/2020] and
the COMPETE 2020 Operational Thematic Program for
Competitiveness and Internationalization (Project POCI-01-
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the FCT/MCTES and the European Union through the
European Regional Development Fund under the Portugal
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020 Partnership Agreement. The positions held by T. R. A.
and P. N. were funded by national funds (OE), through FCT, I.
P., in the scope of the framework contract foreseen in the
numbers 4, 5 and 6 of article 23 of the Decree-Law 57/2016 of
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of the single-crystal diffractometer. The European
Synchrotron Radiation Facility (ESRF, Grenoble, France) is
gratefully acknowledged for approving the experiment CH-
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