High-yield synthesis and catalytic response of chainlike hybrid materials of the [(MoO3): M(2,2′-bipyridine)n] family

Article abstract of DOI:10.1039/c8nj02668h

The one-dimensional organic-inorganic hybrid material [MoO₃(2,2′-bipy)] (1) (2,2′-bipy = 2,2′-bipyridine) has been used as a starting material to prepare the bipy-deficient phases [Mo₂O₆(2,2′-bipy)] (2) and [Mo₃O₉(2,2′-bipy)₂] (3) in excellent yields. The hybrid 2 was obtained by a solid-state thermal treatment of 1 (300 °C, 10 min) while 3 was obtained by a hydrothermal treatment of 1 (160 °C, 6 d). A study was performed to compare the catalytic properties of 1-3 in the epoxidation of cis-cyclooctene at 55 °C with tert-butylhydroperoxide (TBHP) or aqueous H₂O₂ as oxidant. In all cases Cy was converted to cyclooctene oxide (CyO) with 100% selectivity, and Cy conversions increased in the order 1 < 3 < 2, which parallels an increase in the Mo/2,2′-bipy molar ratio of the hybrid (1 < 1.5 < 2). With compound 2, CyO yields at 24 h were 96% for TBHP (cosolvent α,α,α-trifluorotoluene) and 53% for H₂O₂ (cosolvent CH₃CN). The catalytic reactions occurred in homogeneous phase with active species formed in situ from 1-3. All three hybrids react with aqueous H₂O₂ to give the catalytically active oxodiperoxo complex [MoO(O₂)₂(2,2′-bipy)]. The 2:1 hybrid 2 was further examined for the epoxidation of other cyclic and linear non-functionalised olefins with TBHP, namely cyclododecene, 1-octene and trans-2-octene, and the biomass-derived olefins dl-limonene, α-pinene and methyl oleate.

Full text of DOI:10.1039/c8nj02668h

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High-yield synthesis and catalytic response
of chainlike hybrid materials of the
Cite this:DOI: 10.1039/c8nj02668h
[(MoO3)m(2,20-bipyridine)n] family†
Sofia M. Bruno,
Lucie S. Nogueira,
Ana C. Gomes,
Anabela A. Valente,
Isabel S. Gonçalves
and Martyn Pillinger
*
The one-dimensional organic–inorganic hybrid material [MoO3(2,20-bipy)] (1) (2,20-bipy = 2,20-bipyridine)
has been used as a starting material to prepare the bipy-deficient phases [Mo2O6(2,20-bipy)] (2) and
[Mo3O9(2,20-bipy)2] (3) in excellent yields. The hybrid 2 was obtained by a solid-state thermal treatment
of 1 (300 1C, 10 min) while 3 was obtained by a hydrothermal treatment of 1 (160 1C, 6 d). A study was
performed to compare the catalytic properties of 1–3 in the epoxidation of cis-cyclooctene at 55 1C
with tert-butylhydroperoxide (TBHP) or aqueous H2O2 as oxidant. In all cases Cy was converted to
cyclooctene oxide (CyO) with 100% selectivity, and Cy conversions increased in the order 1 o 3 o 2,
which parallels an increase in the Mo/2,20-bipy molar ratio of the hybrid (1 o 1.5 o 2). With compound
2, CyO yields at 24 h were 96% for TBHP (cosolvent a,a,a-trifluorotoluene) and 53% for H2O2 (cosolvent
CH3CN). The catalytic reactions occurred in homogeneous phase with active species formed in situ from
1–3. All three hybrids react with aqueous H2O2 to give the catalytically active oxodiperoxo complex
[MoO(O2)2(2,20-bipy)]. The 2 : 1 hybrid 2 was further examined for the epoxidation of other cyclic and
linear non-functionalised olefins with TBHP, namely cyclododecene, 1-octene and trans-2-octene, and
the biomass-derived olefins DL-limonene, a-pinene and methyl oleate.
Received 29th May 2018,
Accepted 16th July 2018
DOI: 10.1039/c8nj02668h
rsc.li/njc
lattice MoO3.3 HMB can react further in a topotactic fashion with
Lewis bases (L) such as pyridine (py) to give the layered inter-
Introduction
The oxides and oxide-hydrates of molybdenum display a rich calation compound py0.3H0.5MoO3.4 On the other hand, when
variety of structural types and have been extensively studied for excess pyridine is heated with a-MoO3 under anhydrous conditions,
applications in catalysis, solar energy conversion, batteries, the structure of the MoO3 layers completely changes and a different
semiconductors and electronics, amongst others.1 The thermo- phase is obtained ([MoO3(py)]) in which {MoO5N} octahedra share
dynamically stable form of molybdenum trioxide, a-MoO3, has an corners with four other octahedra to form infinite molybdenum
orthorhombic structure consisting of distorted {MoO6} octahedra oxide sheets with alternate apical arrangements of Mo–N(py) and
that share edges and corners to form two-dimensional (2D) MoO3 MoQO.5 Performing the intercalation reaction with 4,40-bipyridine
sheets separated in the lattice by van der Waals gaps.1 The unique led to the compound formulated as [MoO3(4,40-bipy)0.5] which was
layered structure of a-MoO3 permits topotactic redox chemistry proposed to possess an analogous structure with the difference that
involving the intercalation of small ions (e.g. alkali metal cations the organic ligand acts as a pillar cross-linking adjacent layers.5
and H+) and a partial reduction of the Mo oxidation state.2 Protons
The compounds [MoO3(py)] and [MoO3(4,40-bipy)0.5] were
intercalate to give hydrogen molybdenum bronzes (HMB) HxMoO3 the first members of a class of hybrid materials that have been
(0 o x r 2) with metallic features and molybdenum/oxygen termed metal oxide–organic frameworks or coordination polymers.6
frameworks that are almost unchanged as compared to the host These compounds are distinguished by having an extended
metal oxide component decorated by organic ligands (frequently
an organoamine or organocarboxylate). The organic ligands play
a critical role in dictating the inorganic oxide substructure. The
original syntheses of [MoO3(py)] and [MoO3(4,40-bipy)0.5] were
tedious, involving long reaction times of two to four weeks,
elevated temperatures (160–250 1C), and intermediate regrinding
of the solid phase.5 In later years the techniques of hydrothermal
synthesis were found to be much more effective for the isolation
Department of Chemistry, CICECO – Aveiro Institute of Materials, University of
´
Aveiro, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal.
E-mail: mpillinger@ua.pt
† Electronic supplementary information (ESI) available: PXRD patterns, ATR
FT-IR spectra and Raman spectra of 1–5 and recovered catalysts, variable
temperature PXRD data for 2 and 3, and miscellaneous catalytic data (contact
tests, cyclooctene epoxidation with H2O2, and ethanolysis of styrene oxide). See
DOI: 10.1039/c8nj02668h
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of novel hybrid materials based on metal oxides. Zubieta and and methyl oleate (99%) were purchased from Sigma-Aldrich
coworkers confirmed the structure of [MoO3(4,40-bipy)0.5] by unless otherwise indicated, and used as received. The 1D
isolating single-crystals of the compound through the hydro- polymeric material [MoO3(2,20-bipy)] (1) was obtained in 97%
thermal reaction of MoO3 with 4,40-bipy at 150 1C.7 The layered yield as described previously by oxidative decarbonylation of
perovskite-like motif found in this compound and in [MoO3(py)] cis-[Mo(CO)4(2,20-bipy)] with tert-butylhydroperoxide (TBHP).14a
has also been identified in the tungsten isostructures8 and in the The [MoO3(2,20-bipy)]/molybdenum oxide hybrid material
hybrid materials [MO3(pyrazine)0.5] (M = Mo, W),8,9 [WO3(1,2- {[MoO3(2,20-bipy)][MoO3(H2O)]}n (4) was prepared by treatment
bipyridylethane)],8a [MO3(trz)0.5] (M = Mo, W; trz = 1,2,3-triazole of [MoO2Cl2(2,20-bipy)]16 with water under reflux.14b
or 1,2,4-triazole),7,10 and [MoO3(3,8-phenanthroline)0.5].11
Microanalyses for C, H and N were carried out at the
Chelating ligands such as 2,20-bipy tend to restrict the oxide Department of Chemistry, University of Aveiro, with a Truspec
dimensionality to chains or ribbons since their coordination Micro CHNS 630-200-200 elemental analyser. Thermogravi-
blocks at least two condensation sites on the same metal metric analysis (TGA) was performed using a Shimadzu TGA-50
center.12,13 In the case of 2,20-bipy, a hydrothermal synthesis system at a heating rate of 5 1C minꢀ1 under air. Powder X-ray
led to the compound [MoO3(2,20-bipy)], which has a structure diffraction (PXRD) data were collected at ambient temperature
consisting of 1D chains of corner-sharing distorted {MoO4N2} using a PANalytical Empyrean instrument equipped with a PIXcel
octahedra.13a By variation of the synthesis conditions, derivatives 1D detector set at 240 mm from the sample. Cu-Ka1,2 X-radiation
of this 1D polymer have been obtained in which the repeat unit (l1 = 1.540598 Å; l2 = 1.544426 Å) filtered with a nickel foil was
consists of one {MoO4} tetrahedron and either one, two or three used along with a standard transmission sample holder. Working
{MoO4N2} octahedra.13a,c
operating conditions for the X-ray tube: 45 kV and 40 mA.
Hydrothermal methods for the preparation of molybdenum Intensity data were collected in continuous mode in the ca.
oxide–organoamine hybrid materials suffer from common 3.0 r 2y r 70.01 range. Variable temperature PXRD was carried
problems such as low yields and the formation of mixtures out using a PANalytical X’Pert PRO3 HTK 16N high temperature
that must be mechanically separated. We have been exploring chamber containing a Pt heating filament and a Pt–Pt/Rh (10%)
alternative approaches involving the use of metallo-organic thermocouple. The powdered sample was deposited on a Pt
complexes as synthesis precursors.14 The oxidative decarbony- sheet which was then placed over the heating element. Heating
lation of molybdenum carbonyl complexes under mild conditions rates of 5 1C minꢀ1 were used. At a given temperature, samples
has proved to be a particularly useful technique and, in the case were step-scanned in 0.021 2y steps with a counting time of
of the tetracarbonyl complex cis-[Mo(CO)4(2,20-bipy)], allowed the 200 s per step. Transmission FT-IR spectra were measured on a
isolation of [MoO3(2,20-bipy)] (1) in near-quantitative yield.14a Mattson 7000 spectrometer (128 scans and resolution of 4.0 cmꢀ1
)
Polymeric 1 has given interesting results as a catalyst for the in the 300–4000 cmꢀ1 range using KBr pellets. Attenuated total
epoxidation of olefins,14a the oxidation of secondary amines reflectance (ATR) FT-IR spectra were recorded on the same
to nitrones,15 and the oxidation of sulfides to sulfoxides or instrument equipped with a Specac Golden Gate Mk II ATR
sulfones.15 The existence of the bipy-deficient derivatives accessory having a diamond top-plate and KRS-5 focussing lenses
of 1 provides a potential avenue for systematically studying (256 scans and resolution of 4.0 cmꢀ1). FT-Raman spectra were
structure–performance relationships with respect to the use of recorded on a RFS-100 Bruker FT-Spectrometer equipped with
the compounds in catalysis. With this in mind, we sought a Nd:YAG laser with an excitation wavelength of 1064 nm.
improved syntheses of these hybrid materials to make them 13C{1H} CP MAS NMR spectra were recorded using a Bruker
readily available for catalytic investigations. The synthetic advances Avance 400 (narrow bore) spectrometer with an ultrashielded
reported herein comprise the use of polymeric 1 as a starting static magnetic field of 100.6 MHz. The spectra were recorded
material to prepare [Mo2O6(2,20-bipy)] (2) (by a solid-state thermal with 2.75 ms 1H 901 pulses, 2.0 ms contact time, spinning rate of
transformation) and [Mo3O9(2,20-bipy)2] (3) (by a hydrothermal 14 kHz, and 5 s recycle delays. Chemical shifts are quoted in
reaction). A comparative study has been performed on the catalytic ppm from TMS.
performances of 1–3 in the epoxidation of cis-cyclooctene.
Syntheses
[Mo2O6(2,20-bipy)] (2). [MoO3(2,20-bipy)] (1) (21 mg,
0.07 mmol) was heated under air for 10 min in a muffle furnace
preheated to 300 1C. Yield: 15 mg, 96%. Anal. calcd for
Experimental section
Materials and methods
C
10H8Mo2N2O6 (444.06): C, 27.05; H, 1.82; N, 6.30. Found: C,
MoO3 (99.5%, Analar, BDH Chemicals), 2,20-bipyridine, 5.0–6.0 M 26.80; H, 1.87; N, 6.00%. FT-IR (KBr, cmꢀ1): 308 (m), 324 (m),
tert-butylhydroperoxide in decane, 30 wt% hydrogen peroxide, 355 (m), 380 (m), 704 (vs), 771 (vs), 883 (vs), 903 (vs), 919 (sh),
dichloromethane (puriss grade), diethyl ether (99.8%), acetone 941 (s), 960 (m), 1003 (w), 1028 (m), 1062 (w), 1101 (w), 1157 (w),
(99.5%), hexane (Z95%, Carlo Erba), pentane (Z95%, Carlo 1180 (w), 1221 (w), 1243 (w), 1284 (w), 1320 (m), 1425 (m),
Erba), a,a,a-trifluorotoluene (499%), acetonitrile (Z99.9%), 1444 (s), 1473 (m), 1493 (m), 1574 (m), 1598 (s), 1606 (sh),
cis-cyclooctene (Alfa Aesar, 95%), 1-octene (97%), trans-2- 3058 (w), 3105 (w), 3420 (br). Selected FT-Raman (cmꢀ1):
octene (97%), cyclododecene (mixture of cis and trans 895 (w), 936 (m), 969 (w), 1025 (w), 1308 (w), 1564 (w),
13
isomers, 96%), DL-limonene (Z95%, Merck), a-pinene (98%), 1595 (m), 3073 (w). C{1H} CP MAS NMR: d = 152.3 (C2/C20),
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149.5 (C2/C20, C6/C60), 144.9, 142.0 (C4/C40), 128.4 (C5/C50), for 1 h at 60 1C. The recovered catalysts (i-Cat-ox, where i is the
126.1 (C5/C50, C3/C30), 124.4 (C3/C30).
compound number and ox is the oxidant) were characterised by
[Mo3O9(2,20-bipy)2] (3). A mixture of [MoO3(2,20-bipy)] (1) ATR FT-IR spectroscopy and PXRD.
(0.20 g, 0.67 mmol) and H2O (25 mL) was heated under
Contact tests (CTs) were carried out by mixing the original
autogenous pressure and dynamic conditions (25 rpm) for 6 d catalyst with oxidant and solvent (without substrate) and
at 160 1C in a Teflon-lined stainless-steel digestion bomb. The stirring for 24 h at 55 1C. Subsequently the solid phase was
resultant solid was filtered, washed with water (3 ꢁ 5 mL), separated by centrifugation, and the supernant (L) was filtered
acetone (3 ꢁ 5 mL) and diethyl ether (2 ꢁ 5 mL), and finally through a 0.22 mm PTFE membrane, giving the solution
vacuum-dried. Yield: 0.14 g, 85%. Anal. calcd for C20H16Mo3N4O9 denoted i-CT-ox-L. The solid (S) was washed with pentane and
(744.18): C, 32.28; H, 2.17; N, 7.53. Found: C, 32.10; H, 2.17; dried under reduced pressure for 1 h at 60 1C, giving i-CT-ox-S.
N, 7.30%. FT-IR (KBr, cmꢀ1): 301 (w), 316 (w), 332 (m), 386 (m), These solids were characterised by ATR FT-IR spectroscopy and
420 (w), 442 (w), 635 (sh), 649 (vs), 665 (vs), 742 (s), 758 (s), 775 PXRD, and tested for the Cy reaction under similar conditions
(vs), 798 (vs), 897 (vs), 914 (vs), 938 (s), 1026 (m), 1161 (w), 1176 to those used for the original catalysts. The liquid phase
(w), 1227 (w), 1248 (w), 1315 (m), 1442 (s), 1473 (m), 1493 (m), i-CT-ox-L was transferred to a new reactor, and pre-heated to
1574 (w), 1597 (s), 3035 (w), 3076 (w), 3105 (w), 3438 (br). 55 1C for 10 min. Subsequently, the pre-heated substrate was
FT-Raman (cmꢀ1): 106 (w), 124 (w), 154 (w), 188 (w), 198 (w), added to the reactor in an amount to give the same initial molar
222 (w), 236 (w), 249 (w), 259 (w), 301 (w), 315 (w), 338 (w), concentration of Cy as that used for a normal catalytic run.
355 (w), 383 (w), 634 (w), 652 (w), 767 (w), 779 (w), 800 (w),
The oxidant efficiency was determined by iodometric titra-
814 (w), 830 (w), 870 (w), 896 (m), 925 (vs), 948 (m), 1024 (s), tion of the reaction mixtures, after a 24 h batch run at 55 1C.
1061 (w), 1164 (w), 1269 (w), 1288 (vw), 1316 (vs), 1494 (m), The values indicated in the discussion are the average of three
1565 (s), 1597 (vs), 2984 (w), 3078 (m), 3131 (w). 13C{1H} CP MAS replicates (range of experimental error was ca. 10%). The
NMR: d = 153.1, 150.5 (C2/C20), 148.8, 147.8 (C6/C60), 143.5, catalyst acidity tests were carried out as described in the ESI†
141.6, 138.9 (C4/C40), 128.8 (sh), 128.4, 127.3, 125.5 (C5/C50), using styrene oxide as substrate.
125.0 (sh), 123.2, 122.8 (sh), 120.7 (C3/C30).
[MoO(O2)2(2,20-bipy)] (5). A mixture of MoO3 (0.25 g,
1.73 mmol) and 30% H2O2 (2.50 mL, 24.5 mmol) was stirred
Results and discussion
Thermal stability studies and polymer syntheses
for 3 h at 60 1C under N2. The resultant yellow solution was
cooled in an ice-bath and then 2,20-bipy (0.27 g, 1.73 mmol)
was added slowly. A yellow paste was obtained at this stage.
After decanting off the supernatant, the residue was washed
with H2O (3 ꢁ 10 mL), pentane (4 ꢁ 10 mL) and diethyl ether
(2 ꢁ 10 mL), obtaining a yellow solid, which was finally
vacuum-dried. Yield: 0.43 g, 75%. FT-IR and Raman spectral
data for 5 were in agreement with those reported previously.17
Thermogravimetric analysis (TGA) of [MoO3(2,20-bipy)] (1)
under air shows a mass loss of 48.8% in the interval 200–
425 1C, divided into three sequential steps, corresponding to
the decomposition of 2,20-bipy moieties (Fig. 1). No further
weight loss takes place until 625 1C. The residual mass of 48.3%
at 425 1C is in very good agreement with the calculated value of
48.0% assuming that full decomposition of the organic ligands
leads to MoO3. This was confirmed by performing a variable
temperature PXRD experiment (Fig. 2). The results show that
Catalytic tests
The catalytic reactions were carried out in magnetically stirred,
closed borosilicate reaction vessels (10 mL capacity) which were
immersed in an oil bath set to 55 1C. For the epoxidation
reaction, the reactor was loaded with molybdenum complex
(18 mmol), cis-cyclooctene (1.8 mmol), oxidant (TBHP or H2O2,
2.75 mmol) and solvent (1 mL a,a,a-trifluorotoluene or
CH3CN). The solvent and catalyst were pre-heated at the reac-
tion temperature for 10 min prior to addition of the reactants
(instant taken as zero time). The course of the reaction was
monitored using a Varian 3800 GC equipped with a Bruker BR-5
capillary column (30 m ꢁ 0.25 mm; 0.25 mm) and a flame
ionisation detector, using H2 as the carrier gas. Undecane was
used as an internal standard added after the reaction. The
products were identified by GC-MS (Trace GC 2000 Series
(Thermo Quest CE Instruments) – DSQ II (Thermo Scientific)),
equipped with an Agilent J&W capillary DB-5 column (30 m ꢁ
0.25 mm; 0.25 mm) and using He as carrier gas.
After the catalytic reaction, the catalysts were separated from
the reaction mixtures by centrifugation, thoroughly washed
with pentane and dried under reduced pressure (ca. 1 mbar)
Fig. 1 TGA curves for compounds 1 (———), 2 (ꢂꢂꢂꢂꢂꢂꢂ) and 3 (– – –).
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The thermal behaviour of 1 suggests that it could be used
as a starting material for the synthesis of 2 via a carefully
controlled thermal treatment in the solid-state. A new high-
yield synthesis of 2 is desirable since the hydrothermal method
reported by Zubieta and coworkers, consisting of heating
a mixture of Na2MoO4, MoO3, 2,20-bipy, MnCl2 and H2O for
4 days at 200 1C, only led to a 10% yield of 2 (based on
molybdenum) as single-crystals (colourless plates).13a In their
in situ Raman study, Twu et al. found that heating 1 to 330 1C
gave 2, but that cooling of this sample to ambient temperature
led to the appearance of additional phases identified as
MoO3 and [Mo3O9(2,20-bipy)2] (3).20 We optimised the thermal
treatment of 1 and found that pure 2 could be prepared in
quantitative yield by heating 1 at 300 1C for 10 min in a muffle
furnace, which constitutes a clean and efficient protocol
(Scheme 1). The PXRD pattern of the resultant solid cooled to
ambient temperature matched the simulated pattern calculated
using the crystal structure data published for 2,13a and did not
display reflections due to MoO3 and/or 3 (Fig. S1 in the ESI†).
The Raman spectrum of
2 displays bands at 895
[nasym(MoQO)] and 936 cmꢀ1 [nsym(MoQO)], which concord
with the bands reported by Twu et al. (Fig. S2 in the ESI†).20
Four n(MoQO) bands are observed in the FT-IR spectrum at
883, 903, 919 and 941 cmꢀ1 (Fig. 3c). A very strong, broad band
at 704 cmꢀ1 is assigned as a Mo–O–Mo asymmetric stretching
vibration. The corresponding band for 1 is centred around
640 cmꢀ1. Regarding the internal 2,20-bipy vibrational modes,
the strong band at 771 cmꢀ1 is due to a CH out-of-plane
bending vibration (cf. 756 cmꢀ1 for free 2,20-bipy in the solid-
state). The shift of the ring stretching bands from ca. 1556 and
1572 cmꢀ1 for the free ligand to 1574 and 1598/1606 cmꢀ1 for 2
is a direct consequence of bidentate coordination of 2,20-bipy to
the MoVI centres. It is noteworthy that the spectra of 1 (Fig. 3b)
and 2 (Fig. 3c) are very similar in the 1000–1620 cmꢀ1 region
containing ligand-centred modes. The 13C{1H} CP MAS NMR
Fig. 2 Variable temperature PXRD study of [MoO3(2,20-bipy)] (1):
(a) ambient temperature, (b) 320 1C, (c) 350 1C, (d) 420 1C, (e) 570 1C.
The computed patterns for 1 (f), [Mo2O6(2,20-bipy)] (2) (g) and a-MoO3 (h)
were generated using the program Mercury (copyright CCDC, ver. 3.9)18
with crystallographic data published for 1/213a and a-MoO3.19 The asterisk
in pattern (e) identifies an unassigned reflection; all other reflections are
due to a-MoO3.
reflections characteristic of microcrystalline 1 are present up to spectra of 1 and 2 display between seven and eight resolved
320 1C, although a slight loss of crystallinity is apparent when peaks, some of which clearly comprise overlapping resonances
comparing the pattern obtained at this temperature (Fig. 2b) (Fig. 4). These spectra are therefore consistent with the reported
with that obtained for 1 at ambient temperature (Fig. 2a). crystal structures which show that each polymer contains
Further heating to 350 1C (Fig. 2c) leads to complete loss of one crystallographically distinct 2,20-bipy moiety, with the
reflections due to 1 and the appearance of a new phase with carbon atoms of each Cn/Cn0 pair being crystallographically
reflections that match those present in the simulated PXRD inequivalent.
pattern for the 1D polymeric material [Mo2O6(2,20-bipy)] (2) (Fig. 2g).
TGA of the polymer 2 shows that decomposition starts above
Accordingly, the residual mass of 74.4% at 350 1C in the TGA profile 350 1C (DTGmax = 385 1C; DTG = derivative thermogravimetric)
of 1 (i.e., the mass registered at the well-defined plateau observed (Fig. 1). An in situ PXRD study confirmed that the compound is
after the second of three sequential weight loss steps) is in excellent stable at 285 1C, while the pattern recorded at 420 1C corre-
agreement with the calculated value of 74.0% assuming complete sponds to MoO3 (Fig. S3 in the ESI†). These data are consistent
conversion of 1 to 2 with the loss of 0.5 equiv. of 2,20-bipy. In line with the thermal behaviour of 1 above 300 1C, i.e. after partial
with the TGA profile, the in situ PXRD study shows that the loss of 2,20-bipy moieties to give the intermediate phase 2.
intermediate phase 2 only persists over a narrow temperature range
The polymer formulated as [Mo3O9(2,20-bipy)2] (3) was the
since the pattern recorded at 420 1C (Fig. 2d) contains peaks third member of the series [(MoO3)m(2,20-bipy)n] described by
characteristic of orthorhombic a-MoO3 (Fig. 2h). The PXRD pattern Zubieta and coworkers.13a The repeat unit of the 1D polymer
recorded at 570 1C (Fig. 2e) shows loss of all reflections due to 2. chain in 3 consists of one {MoO4} tetrahedron and two
Sublimation of molybdenum oxide occurs above 625 1C (Fig. 1). {MoO4N2} octahedra (Scheme 1), as opposed to one {MoO4}
These results for the thermal behaviour of 1 are in agreement with tetrahedron and one {MoO4N2} octahedron in 2. Our attempts
the in situ Raman study reported by Twu et al.20
to prepare 3 by the literature method,13a consisting of the
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Scheme 1 Synthesis of compounds 1–3.
Fig. 4 13C{1H} CP MAS NMR spectra for (a) 2,20-bipy, (b) 1, (c) 2, (d) 3
and (e) 4.
Fig. 3 FT-IR (KBr) spectra in the range of 300–1800 cmꢀ1 for (a) 2,20-bipy,
(b) 1, (c) 2, (d) 3, (e) 4 and (f) 5.
yield was subsequently achieved by the hydrothermal treatment
of [MoO3(2,20-bipy)] (1) in H2O at 160 1C for 6 days. Phase purity
hydrothermal treatment of a mixture of MoO3, 2,20-bipy and was confirmed by PXRD (Fig. S1 in the ESI†).
H2O at 160 1C for 6 days (under both static and dynamic
The vibrational spectra of 3 display several MoQO stretching
conditions), were unsuccessful, leading to mixtures of 3 and bands in the region 880–950 cmꢀ1. According to Twu et al., the
other unidentified phases. A successful synthesis of 3 in 85% Raman bands at 896, 925, and 948 (Fig. S2 in the ESI†) can be
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ascribed to nasym(MoQO) of MoO4N2 octahedra, nsym(MoQO) of
MoO4N2 octahedra, and nsym(MoQO) of MoO4 tetrahedra,
respectively.20 Three n(MoQO) bands (with additional shoulders)
are observed in the FT-IR spectrum at 897, 914 and 938 cmꢀ1
(Fig. 3d). Strong, overlapping bands for nasym(Mo–O–Mo) are
observed at 649 and 665 cmꢀ1, i.e. at intermediate frequencies
when compared with the corresponding bands for 1 and 2.
The spectrum in the region containing ligand-centred modes
(1000–1620 cmꢀ1) is very similar to those for 1 and 2. According
to the crystal structure reported for 3,13a the asymmetric unit
contains two nonequivalent 2,20-bipy moieties, with the carbon
atoms of each Cn/Cn0 pair being crystallographically inequivalent.
The 13C{1H} CP MAS NMR spectrum of 3 is consistent with this
structure since at least sixteen peaks/shoulders can be observed in
the aromatic region (114–155 ppm) (Fig. 4d).
The thermal decomposition behaviour of 3 was examined
by TGA (Fig. 1) and in situ PXRD (Fig. S4 in the ESI†). The
compound is stable up to 250 1C. The PXRD pattern recorded at
this temperature is similar to that for 3 at ambient temperature,
although a general shift of reflections towards lower angles
took place upon heating. A further shift of the high-angle peaks
towards lower angles is evident upon increasing the tempera-
Fig. 5 Kinetic profiles of the catalytic epoxidation of Cy with TBHP, in the
presence of 1 (J), 2 (D), 3 (&), 4 (
}
), 5 (+), and MoO3 (ꢁ), at 55 1C.
ture to 280 1C, while the relative intensity of low-angle peaks (Lewis acid) and the oxidant TBHP (Lewis base) leads to an
around 111 2y decreases significantly, which can be associated active oxidising species responsible for O-atom transfer to the
with the onset of thermal removal of the organic ligands. Loss olefin, giving the respective epoxide product.16,21 The Lewis
of 2,20-bipy moieties takes place in three sequential steps up to acidity of 1–4 was assessed via the model reaction of the
425 1C with DTGmax at 305, 348 and 402 1C. The PXRD recorded acid-catalysed ethanolysis of styrene oxide (ESI†). The four
at 315 1C, i.e. after the first weight loss step (3.7% mass loss compounds led to 2-ethoxy-2-phenylethanol as the only product,
between 280 and 315 1C), shows complete loss of reflections at 70–100% conversion, at 24 h, 55 1C (Table S1, ESI†).
below 151 2y, while higher-angle reflections are retained, albeit Compounds 5 and MoO3 were also effective for the acid-
with a further shift towards lower angles. These results suggest catalysed reaction (Table S1, ESI†), and thus may act as Lewis
that the 1D polymeric chain structure may be retained up to acids in a similar fashion to 1–4.
315 1C despite partial loss of organic ligands. Some of the high-
The Cy reaction mixtures in the presence of 1–5 and MoO3
angle reflections persist even after the second weight loss step were biphasic solid–liquid (S–L). To check for a possible
up to 360 1C. A residual mass of 58.8% is obtained at 450 1C homogeneous phase catalytic contribution, contact tests (CTs)
after complete 2,20-bipy decomposition, which is close to the were performed in which each compound was treated with
calculated value of 58.0% for MoO3. However, unlike 1 and 2, oxidant and cosolvent (but no Cy) for 24 h at 55 1C. The
PXRD did not show the formation of a crystalline a-MoO3 phase resultant liquid phases, denoted i-CT-TBHP-L (i = 1–5 or
between 450 and 700 1C.
MoO3), were tested for Cy reaction, and led to significant
conversions (Fig. S8 in the ESI†). Hence, the catalytic reactions
in the presence of the studied molybdenum compounds
Catalytic studies
The polymeric compounds 2 and 3 were investigated for the occurred in homogeneous phase. The kinetic differences
liquid-phase epoxidation of cis-cyclooctene (Cy) using TBHP as between the homogeneous catalytic reaction using i-CT-
oxidant, a,a,a-trifluorotoluene (TFT) as cosolvent, and a reaction TBHP-L and the original catalyst i may be partly due to
temperature of 55 1C. For comparison, the MoO3/bipyridine differences in the amount of active species (in contrast to
hybrid compounds 1 and {[MoO3(2,20-bipy)][MoO3(H2O)]}n (4) the CTs, in the normal catalytic tests the olefin is present from
were tested under identical Cy reaction conditions. Commercial the initial instant, which may influence the polarity of
MoO3 and the mononuclear complex [MoO(O2)2(2,20-bipy)] (5) the medium and catalyst solubility). The solids i-CT-TBHP-S
were chosen for catalyst benchmarking. In all cases Cy was (i = 1–4) recovered from the CTs were effective for Cy epoxida-
converted to cyclooctene oxide (CyO) with 100% selectivity tion, and the kinetic profiles were roughly coincident with
(Fig. 5). The reaction without catalyst gave 6% conversion at those for the respective as-synthesised catalysts (Fig. S8a–d,
24 h. Based on mechanistic studies reported in the literature for ESI†). For MoO3 and 5, differences in reaction kinetics were
Mo-catalysed olefin epoxidation with hydroperoxide oxidants, a verified in relation to the respective solids MoO3-CT-TBHP-S
Lewis acid–base reaction (heterolytic mechanism) is generally and 5-CT-TBHP-S (Fig. S8e and f, ESI†).
involved in the formation of an active oxidising species.
Based on the amount of dissolved compound, theꢀi1nitial
Specifically, the coordination reaction between the metal centre turnover frequencies for Cy epoxidation (TOF, mol molMo hꢀ1
)
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were calculated, and increased in the order 5 (42) o MoO3 (75) o
After the catalytic reactions with TBHP or H2O2, work-up was
3 (147) o 1 (200) o 4 (248) o 2 (288). On the other hand, Cy performed to separate the solid and liquid phases, and the
conversion (%) at 24 h increased in the order 4 (62) o 1 (71) o solids (i-Cat-oxidant) were characterised by ATR FT-IR spectro-
MoO3 (77) E 3 (78) o 5 (90) o 2 (96). Hence, 2 possessed the scopy and PXRD. The data indicated that the chemical and
highest initial activity, and led to the highest Cy conversion at 24 h. microstructural features of the undissolved solids i-Cat-TBHP
Based on these results and literature data for 1 and 4 tested for (i = 1–5 or MoO3) were unchanged relative to those for the
Cy/TBHP (with DCE as cosolvent),14a,b the two compounds per- original catalysts (Fig. S11 and S12 in the ESI†). On the other
formed worse than 2. Several other polynuclear or polymeric hand, for the undissolved solids i-Cat-H2O2 (i = 1–4), the data
oxomolybdenum compounds have been tested for this specific were consistent with the exclusive presence of the oxodiperoxo
reaction using comparable catalytic conditions (Table 1). complex 5 (Fig. S13 and S14, ESI†). Accordingly, the solid and
Higher Cy conversions were reported for [Mo2O6(di-tBu-bipy)2],23 liquid phases for (1–4)/Cy/H2O2 changed colour from white/
[Mo8O24(pzpyEA)4],24 and [Mo2O6(pent-pzpy)].25
colourless to yellow (complex 5 is yellow), while no comparable
The oxidant efficiency for the molybdenum catalysts is an colour change was observed for (1–4)/Cy/TBHP. The catalytic
important factor for sustainable catalytic epoxidation processes. activity of 5 for the Cy/H2O2 reaction (Fig. S10, ESI†) indicates
Iodometric titrations indicated very high or excellent TBHP that this species contributes at least partially to the reaction
efficiencies of 92–100% for the molybdenum catalysts tested kinetics of the systems (1–4)/Cy/H2O2. The catalytic perfor-
(Fig. S9 in the ESI†).
mance of 5 resembles more closely that of the (pre)catalyst 1
The catalytic performances of 1–4 were further studied using than 2 and 3 (Fig. S10, ESI†), which may be partly associated
H2O2 as oxidant instead of TBHP, and keeping the remaining with the fact that the 2,20-bipy/Mo ratio (R) in 5 and 1 is the
reaction conditions constant. The Cy/H2O2 reaction in the same (equal to 1). There seems to be a relationship between
presence of 1–4 led to poor results (o5% conversion at 24 h), catalytic activity and the ratio R in the (pre)catalyst, i.e. conver-
which may be partly due to mass transfer limitations since sions at 6/24 h approximately follow the sequence R = 1 (1, 5) o
these systems consisted of triphasic solid–liquid(aqueous)– R = 0.67 (3) o R = 0.5 (2, 4) o R = 0 (MoO3). One cannot exclude
liquid(organic) mixtures. Changing the cosolvent from TFT to the possibility that inorganic MoO3-derived oxoperoxo species
CH3CN led to biphasic solid–liquid mixtures, and the catalytic may be formed (together with 5) from the precatalyst, and the
results improved (Fig. S10 in the ESI†): conversion (%) at 24 h amount of such species (which could be more active than 5) may
increased in the order 1 (16) o 3 (31) o 4 (53) = 2 (53). The increase as the ratio R decreases. It is known that the reaction of
kinetic profiles for (1–4)/H2O2/CH3CN were intermediate MoO3 with excess H2O2 can lead to oxodiperoxo species of the type
between those for 5 (21% conversion at 24 h) and MoO3 (74% [MoO(O2)2(L)2] and [O{MoO(O2)2(L)}2]2ꢀ (L = H2O, CH3CN), which
conversion at 24 h). In general, the H2O2 efficiencies were high may contribute to the catalytic process.28
(97–100%), with MoO3 leading to the lowest efficiency (91%)
(Fig. S9, ESI†).
The catalytic performance of 2 was further investigated for
the epoxidation of other cyclic and linear non-functionalised
olefins, namely cyclododecene (Cy12), 1-octene (1C8) and trans-
2-octene (2C8), under similar reaction conditions (Table 2). For
these substrates the respective epoxides were always the only
reaction products. The epoxidation of Cy12 gave 87% yield of
1,2-epoxycyclododecane at 24 h. A comparison of the cyclic
monoenes Cy12 and Cy indicates that the former is less
reactive, which may be partly due to steric effects associated
with the larger Cy12 molecules. On the other hand, a compar-
ison of the linear monoenes 1C8 and 2C8 indicates that the
latter is more reactive, which may be partly due to electronic
effects favouring the epoxidation of internal CQC bonds in
relation to that of terminal ones. These results are consistent
with the literature in that the reaction mechanism of olefin
epoxidation in the presence of molybdenum compounds may
involve an O-atom transfer step where an electrophilic oxidising
species (formed via reaction of the oxidant with the molybdenum
catalyst) attacks the olefin.21b,29 Overall, these results suggest that 2
Table 1 Comparison of the catalytic results for 2 for Cy epoxidation
with TBHP with literature data for organo-oxomolybdenum compounds
containing a bidentate N,N-ligand coordinated Mo centersa
Compoundb
Solvent/Tc (1C) Time (h) Conv.d (%) Ref.
2
1
4
5
TFT/55
DCE/55
DCE/55
6/24
6/24
24
72/96
48/81
45
This work
14a
14b
22
23
14a
14c
[C8mim]PF6/60 12
10
[Mo2O6(di-tBu-bipy)2] ws/55
[Mo8O24(di-tBu-bipy)4] DCE/55
6/24
6/24
6/24
6/24
6
6/24
6/24
6/24
6/24
93/100
65/90
29/51
55/92
99
[Mo3O9(pzpy)]e
ws/55
[Mo4O12(pzpy)4]
[Mo2O6(pent-pzpy)]e
[MoO(O2)2(pzpyEA)]
[Mo8O24(pzpyEA)4]
[MoO3(pbim)]
ws/55
24
25
DCE/55 f
TFT/55g
ws/55
B10/B20 26
89/100
30/56
81/98
24
27
27
TFT/55
TFT/70
[MoO(O2)2(pbim)]
a
Reaction conditions (unless otherwise specified): Mo : Cy : TBHP = is active for the epoxidation of cyclic and linear olefins, and may
b
1 : 100 : 150. Di-tBu-bipy
2-[3(5)-pyrazolyl]pyridine, pent-pzpy = 2-(1-pentyl-3-pyrazolyl)pyridine,
= =
4,40-di-tert-butyl-2,20-bipyridine, pzpy
lead to high regioselectivity towards more substituted CQC bonds.
The hybrid 2 was further explored for the epoxidation of the
biomass-derived olefins DL-limonene (Lim), a-pinene (Pin) and
methyl oleate (Ole). The major sources of these compounds are
citrus peel for Lim, turpentine (a byproduct of the wood pulp
industry) for Pin, and vegetable oils for Ole. The corresponding
pzpyEA
=
ethyl[3-(pyridin-2-yl)-1H-pyrazol-1-yl]acetate, pbim
=
=
c
2-(2-pyridyl)-benzimidazole. DCE = 1,2-dichloroethane, [C8mim]PF6
1-octyl-3-methylimidazolium hexafluorophosphate, ws = without addi-
d
e
tional solvent. Cy conversion. Homogeneous catalyst formed in situ
f
g
identified as [MoO(O2)2(L)]. Mo : Cy : TBHP = 1 : 113 : 172. Mo : Cy :
TBHP = 0.1 : 100 : 152.
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epoxides, 1,2-epoxy-p-menth-8-ene (1,2-LimO), a-pinene oxide
The epoxidation of Ole gave mainly OleO, formed in 57%
(PinO) and methyl 9,10-epoxyoctadecanoate (OleO), have yield at 24 h (Table 2). This performance compares favourably
a broad range of potential uses, e.g. as intermediates for the with that reported in the literature for the polymeric compound
synthesis of perfumes, flavours, pharmaceuticals and bio- [MoO3(pbim)], which led to 42% OleO yield at 69% conversion
polymers.30
(70 1C, 24 h).27 Under comparable reaction conditions,
2,20-bipyridine-5,50-
The catalytic epoxidation of Lim gave 1,2-LimO as the main [(CH3)2NH2][MoO3(Hbpdc)] (H2bpdc
=
product in 79% yield at 24 h (Table 2), revealing a high dicarboxylic acid), [Mo2O6(pent-pzpy)] and [Mo8O22(OH)4(di-
regioselectivity towards epoxidation at the internal and more tBu-bipy)4] led to 77–92% OleO yield at 77–94% conversion
electron-rich endocyclic double bond. These results are (6–24 h).25,31
comparable or superior to those reported for other polymeric
oxomolybdenum compounds bearing pyridine-containing
ligands (71–86% yield).23,25,27,31 Besides 1,2-LimO, 1,2:8,9- Conclusions
diepoxy-p-menthane and limonene diol were formed in 7 and
The study of the (hydro)thermal stability of the chainlike hybrid
2% yield, respectively. Increasing the TBHP : Lim molar ratio
from 1.5 to 2 still led to 1,2-LimO as the main product (79%
yield at 24 h), further demonstrating the pronounced regio-
selectivity in favour of internal CQC bonds.
material [MoO3(2,20-bipy)] (1) has provided new syntheses of
the related compounds [Mo2O6(2,20-bipy)] (2) and [Mo3O9(2,20-
bipy)2] (3) with yields that are superior to those obtained by
hydrothermal methods using the free ligand 2,20-bipy and
The reaction of Pin gave PinO as the main product in 19%
simple molydate salts as starting materials. It is remarkable
yield at 24 h (Table 2). Other products included campholenic
that the thermal removal of 2,20-bipy from 1, in the presence or
aldehyde (CPA), trans-pinocarveol (PCV) and iso-pino-
absence of water, can be achieved selectively and in a stoichio-
camphone (IPC), formed in up to 9% total yield. The for-
metric fashion, with retention of the 1D molybdenum oxide
mation of PinO rearrangement products like CPA and IPC
chain structure, to give 2 and 3. Compounds 1–3 belong to
is favoured in the presence of Lewis acid catalysts.32 CPA is
the most industrially important, being useful as an aroma
the [(MoO3)m(2,20-bipyridine)n] family of hybrid materials and
provide an interesting opportunity to study the effect of the
chemical or as an intermediate in the synthesis other aromas
organic ligand ordering and distribution on the materials’
(e.g. sandalwood-like fragrances) and some drugs.33 IPC is
one of the main components of Hyssop essential oil, which
properties. In the present work it has been shown that the
ratio m/n influences catalytic performance for the epoxidation
of olefins using hydroperoxide oxidants, with compound 2
leading to the best results. Under the reaction conditions used,
stimulates digestion, has expectorate properties, and anti-
bacterial/fungal activity.34
the hybrid acts as a precatalyst for active species formed in situ,
which promote high selectivities in the epoxidation of a variety
of olefins. The hybrid phase 2 may therefore be a better
Table 2 Epoxidation of olefins with TBHP in the presence of [Mo2O6(2,20-
bipy)] (2)a
oxidation (pre)catalyst than 1, which has already demonstrated
interesting behaviour when applied for the oxidation of other
substrates such as sulfides and amines.
Substrate
Conv.b (%)
72/96
Epoxide
Sel.b (%)
100/100
TOFc
288
Cy
Conflicts of interest
1C8
2C8
9/32
100/100
100/100
0
There are no conflicts to declare.
30/71
30
Acknowledgements
Cy12
41/87
100/100
70
This work was carried out with the support of CICECO – Aveiro
Institute of Materials – POCI-01-0145-FEDER-007679 (FCT
Ref. UID/CTM/50011/2013) and the CENTRO 2020 Regional
Operational Programme (Project CENTRO-01-0145-FEDER-
028031), co-financed by national funds through the FCT/MEC
and the European Union through the European Regional
Development Fund under the Portugal 2020 Partnership
Agreement. The FCT and the European Union are acknowl-
edged for a PhD grant to L. S. N. (PD/BD/109666/2015), and
post-doctoral grants to S. M. B. (SFRH/BPD/108845/2015) and
A. C. G. (SFRH/BPD/108541/2015), co-funded by MCTES and the
European Social Fund through the program POPH of QREN.
Lim
Pin
63/88
29/37
25/57
96/90
130
80
54/50
Ole
100/100
50
a
b
Reaction conditions: Mo : TBHP : olefin = 1 : 100 : 150, 55 1C. Olefin
c
conversion and epoxide selectivity at 6 h/24 h. Turnover frequency
(mol molMo
h
ꢀ1).
ꢀ1
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S. Smeets, L. B. McCusker, M. Pillinger and I. S. Gonc¸alves,
Inorg. Chem., 2014, 53, 2652–2665.
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