Investigation of a dichlorodioxomolybdenum(vi)-pyrazolylpyridine complex and a hybrid derivative as catalysts in olefin epoxidation

Article abstract of DOI:10.1039/c3dt52981a

Treatment of the solvent adduct [MoO₂Cl₂(THF) ₂] with the ligand 2-(1-pentyl-3-pyrazolyl)pyridine (1, abbreviated as pent-pp) gave the dioxomolybdenum(vi) complex [MoO₂Cl ₂(pent-pp)] (2), which was characterised by elemental analysis, ¹H NMR, FT-IR spectroscopy and single crystal X-ray diffraction (XRD). Reaction of 2 with water in a Teflon-lined stainless steel autoclave at 100 °C led to the isolation of a molybdenum oxide/pyrazolylpyridine hybrid material with the composition [Mo₂O₆(pent-pp)] (3), which was characterised by variable temperature powder XRD, scanning electron microscopy, thermogravimetric analysis, FT-IR and ¹³C{¹H} CP MAS NMR spectroscopies. Compounds 2 and 3 display high activity and selectivity when used as (pre)catalysts for the epoxidation of cis-cyclooctene at 55 °C with tert-butylhydroperoxide as an oxidant. Further catalytic experiments with 3 as a (pre)catalyst were performed using the bio-derived olefins dl-limonene (Lim) and methyl oleate (Ole). The reaction of Lim gave mainly 1,2-epoxy-p-menth-8-ene and 1,2;8,9-diepoxy-p-menthane in a combined yield of 93% at 97% conversion (6 h), while the reaction of Ole led to 9,10-epoxystearate as the main product in 78% yield at 82% conversion (6 h). The catalytic reactions are homogeneous in nature. Starting with material 3, the oxodiperoxo complex [MoO(O₂)₂(pent-pp)] (4) was isolated from solution after a catalytic run, suggesting that it is formed from 3 and plays a catalytic role. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt52981a

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Investigation of a dichlorodioxomolybdenum(VI)-
pyrazolylpyridine complex and a hybrid derivative
as catalysts in olefin epoxidation†
Cite this: Dalton Trans., 2014, 43,
6059
Tatiana R. Amarante, Patrícia Neves, Filipe A. Almeida Paz, Anabela A. Valente,
Martyn Pillinger and Isabel S. Gonçalves*
Treatment of the solvent adduct [MoO2Cl2(THF)2] with the ligand 2-(1-pentyl-3-pyrazolyl)pyridine
(1, abbreviated as pent-pp) gave the dioxomolybdenum(VI) complex [MoO2Cl2(pent-pp)] (2), which was
characterised by elemental analysis, 1H NMR, FT-IR spectroscopy and single crystal X-ray diffraction
(XRD). Reaction of 2 with water in a Teflon-lined stainless steel autoclave at 100 °C led to the isolation of
a molybdenum oxide/pyrazolylpyridine hybrid material with the composition [Mo2O6(pent-pp)] (3), which
was characterised by variable temperature powder XRD, scanning electron microscopy, thermogravi-
metric analysis, FT-IR and 13C{1H} CP MAS NMR spectroscopies. Compounds 2 and 3 display high activity
and selectivity when used as (pre)catalysts for the epoxidation of cis-cyclooctene at 55 °C with tert-butyl-
hydroperoxide as an oxidant. Further catalytic experiments with 3 as a (pre)catalyst were performed using
the bio-derived olefins DL-limonene (Lim) and methyl oleate (Ole). The reaction of Lim gave mainly 1,2-
epoxy-p-menth-8-ene and 1,2;8,9-diepoxy-p-menthane in a combined yield of 93% at 97% conversion
(6 h), while the reaction of Ole led to 9,10-epoxystearate as the main product in 78% yield at 82% conver-
sion (6 h). The catalytic reactions are homogeneous in nature. Starting with material 3, the oxodiperoxo
complex [MoO(O2)2(pent-pp)] (4) was isolated from solution after a catalytic run, suggesting that it is
formed from 3 and plays a catalytic role.
Received 22nd October 2013,
Accepted 19th December 2013
DOI: 10.1039/c3dt52981a
www.rsc.org/dalton
composition [MoO2Cl2L], with a cis-oxo, trans-Cl, cis-L ligand
arrangement around the metal, and a distorted octahedral geo-
Introduction
The chemistry of metal oxides is important in numerous cata- metry.6 Of the numerous ligand types investigated, bipyridines
lytic processes.1 The desire for models of the metal oxide cata- and pyrazolylpyridines appear to be optimal since they are
lytic species has motivated research on the downsizing of resistant to oxidative degradation or exchange reactions under
metal oxides to small molecular fragments.2 Smaller metal the catalytic conditions.5 The efficacy of substituted pyrazolyl-
oxide assemblies can be better understood in terms of struc- pyridines as supporting ligands for oxomolybdenum catalysts
ture and reactivity.3 The first metal oxo molecular fragment had already been established in earlier work with oxodiperoxo
was probably molybdenum(VI) dichloride dioxide, MoO2Cl2, complexes of the type [MoO(O2)2L].7 More recently, a few com-
described by J. J. Berzelius in 1826.4
plexes of the type [MoO2Cl2L] bearing these ligands have been
The MoO2Cl2 fragment can be stabilised by adduct for- prepared and studied, and display interesting results in cataly-
mation with relatively strong basic donor ligands, such as 2,2′- tic olefin epoxidation.5g,h
bipyridine and derivatives.2,5 Many papers have been pub-
In assessing the potential of [MoO2Cl2L] complexes to be
lished on the effect of nitrogen donor ligands on the solubility, used as catalysts, the stability of the complexes under the reac-
stability and catalytic activity of the complexes. The vast tion conditions used is of critical importance. Previous studies
majority of the adducts are mononuclear and have the have shown that the Mo–Cl bond is susceptible to cleavage
(especially if no special measures are taken to exclude water
from the reaction system), frequently leading to oxo-bridged
dinuclear complexes.5h,8 In our group we have recently studied
the reaction of dichlorodioxomolybdenum(VI) complexes with
Department of Chemistry, CICECO, University of Aveiro, Campus Universitário de
Santiago, 3810-193 Aveiro, Portugal. E-mail: igoncalves@ua.pt
†Electronic supplementary information (ESI) available: Crystal packing diagrams
excess water, which has led to the isolation of several higher
and geometrical details of supramolecular contacts for 1HCl and 2. CCDC
nuclearity oxomolybdenum(VI) compounds, namely the
964290 for 1HCl and 964291 for 2. For ESI and crystallographic data in CIF or
octanuclear
complex
[Mo8O22(OH)4(4,4′-di-tert-butyl-2,2′-
other electronic format see DOI: 10.1039/c3dt52981a
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bipyridine)4],9 and the polymeric materials {[MoO3(2,2′-bipyri- X’Pert MPD diffractometer (Cu-Kα1,2 radiation, λ1 = 1.540598 Å
10
dine)][MoO3(H2O)]}n
and [Mo2O6(HpypzA)] (HpypzA
=
and λ2 = 1.544426 Å) equipped with an X’Celerator detector
and a flat-plate sample holder in a Bragg-Brentano para-focus-
[3-(pyridinium-2-yl)-1H-pyrazol-1-yl]acetate).11
In the present work we set out to further explore the chem- ing optics configuration (40 kV, 50 mA). The intensity data
istry and catalytic potential of molybdenum(VI)-pyrazolylpyri- were collected by the step-counting method (step 0.04°) in con-
dine compounds by employing the ligand 2-(1-pentyl-3- tinuous mode in the ca. 3.5 ≤ 2θ ≤ 70° range. In situ variable-
pyrazolyl)pyridine (1, abbreviated as pent-pp). The treatment temperature PXRD data were collected using the same diffract-
of the dichloro complex [MoO2Cl2(pent-pp)] (2) with water ometer equipped with a high-temperature Anton Paar HKL 16
under hydrothermal conditions led to the isolation of an chamber controlled by an Anton Paar 100 TCU unit. Heating
organic–inorganic hybrid material with the composition rates of 10 °C min−1 were used. At a given temperature,
[Mo2O6(pent-pp)] (3). The molybdenum-catalysed epoxidation samples were step-scanned in 0.03° 2θ steps with a counting
of cis-cyclooctene was chosen as a model reaction to assess the time of 2 s per step. Data were collected between 30 and
catalytic properties of 2 and 3, and material 3 was further 500 °C. Thermogravimetric analysis (TGA) was performed
examined in the epoxidation of the bio-derived olefins DL-limo- using a Shimadzu TGA-50 system at a heating rate of 5 °C
nene and methyl oleate.
min−1 under air. Scanning electron microscopy (SEM) images
were collected using a Hitachi S4100 instrument at 25 kV.
Samples were prepared by deposition on aluminium sample
holders followed by carbon coating using an Emitech K950X
carbon evaporator.
Experimental section
Materials and methods
Synthesis of compounds 1–4
MoO2Cl2 (Sigma-Aldrich), NaH (60% oil dispersion, Sigma-
Aldrich), 1-iodopentane (98%, Sigma-Aldrich), tetrahydrofuran
2-(1-Pentyl-3-pyrazolyl)pyridine (1). A solution of 2-(3-pyra-
(puriss p.a., Scharlau), hexane (puriss p.a. or anhydrous 95%, zolyl)pyridine (1.49 g, 10.26 mmol) in dry THF (20 mL) was
Sigma-Aldrich), pentane (99%, Sigma-Aldrich), acetone (puriss added to a suspension of NaH (0.49 g, 20.42 mmol) in THF
p.a., Fluka), trifluorotoluene (anhydrous, 99%, Sigma-Aldrich), (15 mL) and the liberation of hydrogen was observed. 1-Iodo-
cis-cyclooctene (95%, Sigma-Aldrich), methyl oleate (99%, pentane (1.6 mL, 12.40 mmol) was added and the mixture was
Sigma-Aldrich) and DL-limonene (97%, Sigma-Aldrich) were refluxed for 48 h. After cooling to ambient temperature, the
obtained from commercial sources and used as received. 1,2- solution was evaporated to dryness and a yellow solid was
Dichloroethane (99%, Sigma-Aldrich) was dried by stirring obtained. The solid was extracted with hexane (100 mL) and
over CaH2 overnight at ambient temperature, followed by dis- pentane (50 mL). The combined extracts were evaporated to
tillation and storage over activated 4 Å molecular sieves. A solu- dryness, giving the ligand 1 as an oil. Yield: 1.68 g, 76%. Anal.
tion of 5–6 M tert-butylhydroperoxide (TBHP) in decane calcd for C13H17N3 (215.29): C, 72.52; H, 7.95; N, 19.51. Found:
(Sigma-Aldrich) was dried prior to use by using activated 3 Å C, 72.28; H, 8.01; N, 19.32. FT-IR (KBr, cm−1): ν = 2956 (m),
molecular sieves. 2-(3-Pyrazolyl)pyridine was prepared by fol- 2930 (m), 2871 (m), 2861 (m), 1592 (s), 1565 (m), 1459 (s),
lowing a published procedure.12 The preparation of pent-pp (1) 1431 (m), 1401 (m), 1356 (m), 1329 (w), 1276 (m), 1229 (m),
and complex 2 was performed using standard Schlenk tech- 1146 (m), 1049 (w), 992 (w), 956 (w), 739 (w), 761 (s), 715 (w),
1
niques under an inert atmosphere.
695 (w), 668 (w), 620 (w), 403 (w). H NMR (300 MHz, CDCl3,
Microanalyses for CHN were carried out at the Department r.t.): δ = 0.89 (t, 3H, CH3), 1.31 (m, 4H, CH2), 1.92 (m, 2H,
of Chemistry, University of Aveiro, with a Truspec Micro CHNS CH2), 4.17 (t, 2H, NCH2), 6.86 (d, 1H, H4), 7.18 (m, 1H, H10),
630-200-200 elemental analyser. Mo was determined by 7.43 (d, 1H, H5), 7.70 (dt, 1H, H9), 7.92 (d, 1H, H8), 8.62 (dd,
ICP-OES at C.A.C.T.I., University of Vigo, Spain. Fourier trans- 1H, H11). 13C NMR (101 MHz, CDCl3, r.t.): δ = 13.19 (CH3),
form infrared (FT-IR) spectra were recorded in the range 22.19 (CH2), 28.74 (CH2), 30.15 (CH2), 52.56 (NCH2), 103.98
3500–350 cm−1 as KBr pellets using a Unican Mattson 7000 (C4), 119.95 (C10), 122.13 (C8), 130.42 (C5), 136.51 (C9), 149.31
spectrophotometer equipped with a DTGS CsI detector. Atte- (C3), 151.24 (C11), 152.36 (C7).
nuated total reflectance (ATR) FT-IR spectra were measured on
[MoO2Cl2(2-(1-pentyl-3-pyrazolyl)pyridine)]
(2). MoO2Cl2
the same instrument equipped with a Specac Golden Gate Mk (0.51 g, 2.55 mmol) was dissolved in THF (15 mL) and the
II ATR accessory having a diamond top-plate and KRS-5 focus- solution was stirred for 20 min. A solution of the ligand 1
ing lenses. 1H and 13C solution NMR spectra were obtained (0.55 g, 2.55 mmol) in THF (10 mL) was then added, and the
using a Bruker Avance 300 spectrometer. Solid state 13C{1H} mixture stirred for 2 h. The solution was evaporated to dryness
CP MAS NMR spectra were recorded using a Bruker Avance 500 and the resulting solid was washed with hexane (15 mL), and
(narrow bore) spectrometer with an ultrashielded static mag- finally vacuum-dried. Yield: 0.99 g, 94%. Anal. calcd for
netic field of 125.76 MHz. The spectra were recorded with C13H17Cl2MoN3O2 (414.13): C, 37.70; H, 4.14; N, 10.15. Found:
3.5 μs 1H 90° pulses, 1.5 ms contact time, spinning rates of C, 37.60; H, 4.25; N, 9.70. FT-IR (KBr, cm−1): ν = 3155 (w),
9 kHz and 5 s recycle delays. Chemical shifts are quoted in 3129 (m), 3094 (w), 2957 (m), 2929 (m), 2871 (m), 2861 (m),
ppm from TMS. Conventional powder X-ray diffraction (PXRD) 1611 (m), 1568 (w), 1534 (w), 1503 (w), 1440 (s), 1368 (m),
data were collected at ambient temperature with a Philips 1348 (w), 1294 (w), 1236 (m), 1226 (m), 1158 (w), 1147 (w),
6060 | Dalton Trans., 2014, 43, 6059–6069
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1098 (m), 1082 (m), 1053 (w), 1094 (m), 962 (w), 942 (vs), on Hampton Research CryoLoops13 with the help of a Stemi
908 (vs), 876 (w), 781 (vs), 716 (w), 693 (w), 646 (m), 622 (m), 2000 stereomicroscope equipped with Carl Zeiss lenses. Data
1
425 (w), 403 (m), 336 (vs). H NMR (300 MHz, DMSO-d6, r.t.): were collected at 150(2) K on a Bruker X8 Kappa APEX II CCD
δ = 0.86 (t, 3H, CH3), 1.18–1.35 (m, 4H, CH2), 1.78–1.88 (m, area-detector diffractometer (Mo Kα graphite-monochromated
2H, CH2), 4.21 (t, 2H, NCH2), 7.04 (d, 1H, H4), 7.55 (dt, 1H, radiation, λ = 0.71073 Å) controlled by the APEX2 software
H10), 7.93 (d, 1H, H5), 8.14 (overlapping signals, 2H, H9, H8), package,14 and equipped with an Oxford Cryosystems Series
8.62 (dd, 1H, H11).
Crystals of 2 suitable for XRD were obtained by slow were processed using SAINT+,16 and data were corrected for
diffusion of diethyl ether into a solution of 2 in CH2Cl2. absorption by the multi-scan semi-empirical method
[Mo2O6(2-(1-pentyl-3-pyrazolyl)pyridine)]n (3). A Teflon-lined implemented in SADABS.17 The structures were solved by
stainless steel autoclave was charged with
(0.25 g, direct methods of SHELXS-9718 and refined by full-matrix least
700 cryostream monitored remotely using Cryopad.15 Images
2
0.60 mmol) and water (15 mL), and heated in an oven at squares refinement cycles on F2 using SHELXL-97.18b,19 All
100 °C for 19 h. The resulting pale green microcrystalline solid non-hydrogen atoms were directly located from difference
was separated from the aqueous mother liquor (pH ≈ 2) by fil- Fourier maps and refined with anisotropic displacement
tration, washed with water (20 mL), acetone (20 mL), and parameters.
vacuum-dried. Yield: 0.123 g, 81%. Anal. calcd for
In 1HCl, the pentyl substituent was found to be affected by
C13H17Mo2N3O6 (503.17): C, 31.03; H, 3.40; N, 8.35; Mo, 38.1. an extensive structural disorder, affecting mainly the middle
Found: C, 31.00; H, 3.50; N, 8.18; Mo, 38.5. FT-IR (KBr, cm−1): part of the chain. While the first two carbon atoms and the
ν = 3449 (m), 3132 (w), 2956 (m), 2930 (m), 2869 (m), 1627 (m), terminal methyl group were refined in a single crystallographic
1611 (s), 1570 (m), 1558 (m), 1533 (m), 1504 (m), 1465 (m), location, the middle –CH2–CH2– portion (atoms C11 and C12)
1441 (s), 1371 (m), 1339 (m), 1290 (m), 1237 (m), 1193 (m), was modelled over two distinct crystallographic locations with
1159 (m), 1148 (m), 1099 (m), 1079 (m), 1053 (m), 1023 (m), the final rates of occupancy refining to 70.4(7)% and 29.6(7)%.
957 (vs), 910 (s), 804 (vs), 776 (s), 699 (m), 657 (vs), 505 (m),
The oxidation state of the crystallographically independent
427 (m), 400 (m), 354 (m), 323 (m). 13C{1H} CP MAS NMR: δ = molybdenum centre present in 2 was inspected by employing
14.3 (CH3), 22.4 (CH2), 29.2 (CH2), 52.2 (NCH2), 105.2 (C4), bond valence calculations following the theoretical models of
122.8 (C8, C10), 134.8 (C5), 140.3 (C9), 146.5, 148.0 (C3, C7, Brese & O’Keeffe20 and Brown & Altermatt,21 and based on the
C11).
refined MovO, Mo–Cl and Mo–N interactions. All calculations
Crystals of 2-(1-pentyl-3-pyrazolyl)pyridinium chloride were performed using PLATON,22 with the sum of the bond
(1HCl) suitable for XRD were isolated from the above aqueous valence (considering a +6 oxidation state) for Mo1 adding up
mother liquor by concentrating the solution and allowing it to to +5.96.
stand at ambient temperature for several days.
Hydrogen atoms bound to carbon were placed at their ideal-
[MoO(O2)2(2-(1-pentyl-3-pyrazolyl)pyridine)] (4). A mixture ised positions using appropriate HFIX instructions in SHELXL:
comprising 3 (9.0 mg, 0.018 mmol), 5–6 M TBHP in decane 43 for the aromatic, 23 for the –CH2– groups, and 33 or 137 for
(2.75 mmol) and 1,2-dichloroethane (1 mL) was heated at the terminal –CH3 groups (in compounds 1HCl and 2, respect-
55 °C for 6 h. After this time the reaction mixture was centri- ively). All these atoms were included in subsequent refinement
fuged and pentane was added to the liquid phase, resulting in cycles in riding-motion approximation with isotropic thermal
the precipitation of a pale yellow solid, which was filtered, displacement parameters (Uiso) fixed at 1.2 (aromatic and
washed with pentane, vacuum-dried at 55 °C for 1 h, and –CH2–) or 1.5 (–CH3 groups) times Ueq of the attached
identified as the oxodiperoxo complex 4 by elemental analysis, carbon atom. The hydrogen atom attached to the nitrogen
FT-IR and NMR spectroscopy. Yield: 2.5 mg, 18% (based on atom in 1HCl was directly found from difference Fourier
Mo). Anal. calcd for C13H17MoN3O5 (391.23): C, 39.91; H, 4.38; maps and was included in the structural model with the N–H
N, 10.74%. Found: C, 40.04; H, 4.35; N, 10.65%. FT-IR (ATR, distance restrained to 0.95(1) Å in such a way as to promote
cm−1): ν = 3118 (br), 2956 (w), 2926 (m), 2859 (w), 1613 (m), the best hydrogen bonding interaction with the closest
1571 (w), 1530 (vw), 1503 (w), 1463 (sh), 1440 (s), 1364 (m), chloride anion.
1291 (vw), 1227 (m), 1203 (vw), 1099 (w), 947 (s), 907 (m),
The last difference Fourier map synthesis showed: for 1HCl,
861 (s), 842 (sh), 769 (vs), 659 (s), 583 (vs), 534 (m), 426 (vw), the highest peak (0.539 e Å−3) and the deepest hole (−0.500 e
404 (w), 356 (w). 1H NMR (300 MHz, DMSO-d6, r.t.): δ = 0.86 (t, Å−3) located at 0.41 Å and 0.78 Å from H10A and C10′, respect-
3H, CH3), 1.31 (m, 4H, CH2), 1.82 (m, 2H, CH2), 4.20 (t, 2H, ively; for 2, the highest peak (0.541 e Å−3) and the deepest hole
NCH2), 6.79 (d, 1H, H4), 7.35 (m, 1H, H10), 7.81 (d, 1H, H5), (−0.600 e Å−3) located at 1.42 Å and 0.73 Å from Cl1 and Mo1,
8.12 (d, 1H, H8), 8.29 (m, 1H, H9), 8.56 (d, 1H, H11).
respectively. Crystal data and structure refinement details are
summarised in Table 1. Geometrical details of the various
supramolecular contacts present in the crystal structures of
Single-crystal X-ray diffraction studies
Crystals of 1HCl and complex 2 were directly harvested from 1HCl and 2, namely, weak and strong D–H⋯A hydrogen
the crystallisation vials and immediately immersed in highly bonding interactions, Cl⋯π and π⋯π contacts, are summar-
viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC ised in Tables S1 and S2 in the ESI.† Structural drawings have
140/13, Sigma-Aldrich). Suitable single-crystals were mounted been created using Crystal Impact Diamond.23
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the resulting mixture at 4 °C overnight. The precipitated solid
was separated by centrifugation, washed with pentane and
vacuum-dried (ca. 0.1 bar) at 55 °C for 1 h.
Table 1 Crystal data and structure refinement details
Compound
1HCl
2
The contact tests (ConTs) were carried out as follows: the
catalyst, the cosolvent and the oxidant were charged to a
reactor, and heated at 55 °C for 6 h with stirring at 1000 rpm.
Afterwards, the mixture was cooled to ambient temperature,
centrifuged (3500 rpm), and the liquid phase was separated
using a filter containing a 0.2 µm PTFE membrane. The liquid
phase (denoted as ConTs-solv, where solv is the cosolvent
used, i.e. TFT, DCE or hex) was transferred to a separate
reactor, and preheated at 55 °C for 10 min. Finally, the olefin
(also preheated) was added in an appropriate amount to
obtain a similar initial concentration to that used for the
TypCs.
Formula
M
Temperature/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
C
13H18ClN3
C13H17Cl2MoN3O2
414.14
150(2)
Orthorhombic
Pbca
8.4532(5)
11.4651(8)
32.826(3)
—
—
—
3181.4(4)
8
1.729
1.167
1664
251.75
150(2)
Monoclinic
P21/c
11.7600(7)
11.1632(6)
10.6280(6)
—
98.648(3)
—
1379.37(14)
4
γ/°
Volume/Å3
Z
Dc/g cm−3
μ(Mo-Kα)/mm−1
F(000)
1.212
0.260
536
The catalytic studies were extended to methyl oleate (Ole)
and DL-limonene (Lim) using a ConTs-DCE solution obtained
as described above. The initial concentration of olefin was the
same as that for the TypCs.
The evolution of the catalytic reaction was monitored using
a Varian 3900 GC equipped with a FID detector and a capillary
column (J&W Scientific DB-5, 30 m × 0.25 mm × 0.25 µm), and
calibration curves were measured using commercial standards.
The reaction products were identified by GC-MS (Trace GC
2000 Series Thermo Quest CE Instruments GC; Thermo Scien-
tific DSQ II), using He as the carrier gas.
Crystal size/mm
Crystal type
θ Range (°)
Index ranges
0.13 × 0.10 × 0.10
Colourless plates
3.65 to 29.13
−16 ≤ h ≤ 16
−15 ≤ k ≤ 15
−14 ≤ l ≤ 14
33 917
0.09 × 0.04 × 0.01
Colourless plates
3.53 to 29.13
−11 ≤ h ≤ 9
−15 ≤ k ≤ 15
−34 ≤ l ≤ 44
18 097
Reflections collected
Independent reflections
Data completeness
3697 (Rint = 0.0507) 4252 (Rint = 0.0717)
99.6%
99.3%
Data/restraints/parameters 3697/1/176
Final R indices [I > 2σ(I)]a,b R1 = 0.0415
wR2 = 0.1009
4252/0/191
R1 = 0.0448
wR2 = 0.0800
R1 = 0.0898
wR2 = 0.0927
m = 0.0361
n = 0.0
Final R indices (all data)a,b R1 = 0.0640
wR2 = 0.1139
Weighting schemec
m = 0.0560
n = 0.6778
Largest diff. peak and hole 0.539, −0.500 e Å−3 0.541, −0.600 e Å−3
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
P
2
2
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2
¼
½wðFo ꢀ Fc Þ= ½wðFo Þ ꢁ.
2
2
c w = 1/[σ2(Fo2) + (mP)2 + nP] where P = (Fo2 + 2Fc2)/3.
Results and discussion
Synthesis of [MoO2Cl2(2-(1-pentyl-3-pyrazolyl)pyridine)] (2)
The ligand 2-(1-pentyl-3-pyrazolyl)pyridine (1) (hereafter desig-
nated as pent-pp) was prepared in good yield by deprotonation
of 2-(3-pyrazolyl)pyridine followed by addition of 1-iodo-
pentane and reflux for 48 h (eqn (1)).5f,12 Characterisation data
(FT-IR, 1H and 13C solution NMR) were in line with those
reported by Thiel et al. for other 2-(1-alkyl-3-pyrazolyl)pyridine
derivatives.7b
Catalytic epoxidation
The catalytic epoxidation of cis-cyclooctene (Cy) with TBHP
was performed in 5 mL borosilicate batch reactors (equipped
with a PTFE-lined magnetic stirring bar and a valve for
sampling), under autogenous pressure, using an initial
molybdenum : olefin : oxidant molar ratio of 1 : 113 : 172
(16 µmol Mo), and 1 mL of cosolvent [1,2-dichloroethane
(DCE), trifluorotoluene (TFT) or hexane (hex)]. The procedure
for starting the catalytic reaction involved charging the reactor
with a catalyst, olefin and a cosolvent, and then immersing the
reactor into a thermostated oil bath at 55 °C, where it was kept
under stirring (1000 rpm) for 10 min to attain isothermal
initial conditions. Subsequently TBHP, also preheated at
ð1Þ
The complex [MoO2Cl2(pent-pp)] (2) was obtained in very
55 °C, was added to the reactor, and this was taken to be the good yield by simple ligand exchange with the solvent adduct
initial instant of the reaction. These typical reaction conditions [MoO2Cl2(THF)2] at ambient temperature. Complex
is
are denoted as TypCs.
sufficiently soluble in polar organic solvents to obtain solution
2
After a batch run, the reaction mixture was cooled to NMR spectra of good quality. A comparison of the 1H NMR
ambient temperature and the undissolved solid was separated spectra of the ligand 1 and complex 2 (recorded in DMSO-d6)
by centrifugation, washed with pentane and vacuum-dried shows that the resonances for the pyrazolylpyridine protons
(ca. 0.1 bar) at 55 °C for 1 h. In some cases, a solid (corres- are shifted to lower field after complexation, similar to that
ponding to the dissolved compound) was isolated from the reported previously for [MoO2Cl2(2-(1-butyl-3-pyrazolyl)pyridi-
liquid phase after adding pentane to the solution, and storing ne)]5f and [MoO2Cl2{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}].5g
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from the aqueous mother liquor obtained after reaction of
complex 2 with water to prepare material 3.
1HCl crystallises in the centrosymmetric monoclinic space
group P21/c (Table 1), with the asymmetric unit being com-
posed of a whole protonated molecular unit plus a charge-bal-
ancing chloride anion (Fig. 2). The pentyl substituent was
found to be affected by positional disorder, most likely due to
the high conformational flexibility allied to the inexistence of
structurally relevant supramolecular contacts associated with
this portion of the molecule (see below for further details).
Nevertheless, one conformation is predominant in the crystal
structure, with a refined rate of occupancy of 70.4(7)%. As
found in related structures, the crystal structure of the un-
coordinated molecule has the two central rings rotated along
the central C5–C6 bond so as to minimise steric repulsion
between the lone pairs of the nitrogen atoms. The pyridine
and the 2-(3-pyrazole) rings in 1HCl are almost coplanar, with
the average planes subtending a dihedral angle of only 4.40(9)°
(Fig. 2).
The crystal packing is essentially driven by the need to
effectively fill the available space, while optimising a number
of weak N–H⋯Cl and C–H⋯Cl hydrogen bonds (Fig. S1 in the
ESI†). The chloride anion arises as a pivotal structure-directing
moiety in the crystal structure of 1HCl: on the one hand, it is
engaged in a strong and highly directional N–H⋯Cl hydrogen
bond with the protonated pyridine ring (Fig. 1 and Table S1 in
the ESI†), and, on the other, it establishes various weak hydro-
gen bonds with neighbouring 2-(1-pentyl-3-pyrazolyl)pyridi-
nium molecules as shown in Fig. S1.† Other contacts are also
possible but have been omitted from the discussion due to the
long internuclear distances. The orientation of the various
2-(1-pentyl-3-pyrazolyl)pyridinium cations in the crystal struc-
ture is such that, while maximising the various weak hydrogen
bonds, the occurrence of π–π contacts is not possible. Instead,
the crystal structure exhibits a rather weak and poorly direc-
tional C–H⋯π contact between the end part of a pentyl chain
and a neighbouring 2-(3-pyrazole) ring (Table S1 in the ESI†).
Structure of 2. Compound 2 crystallises in the orthorhom-
bic centrosymmetric space group Pbca (Table 1) with the
Further proof of ligand coordination in 2 is provided by the
FT-IR spectrum (Fig. 1). Upon complexation the ligand under-
goes structural changes which affect the 1550–1650 cm−1
region. Specifically, the bands of 1 at 1565 cm−1 (C–C inter-
ring stretching mode) and 1592 cm−1 (pyridyl C–N stretching
mode) are shifted to 1568 and 1611 cm−1, respectively, for
complex 2, in agreement with results for other molybdenum(VI)-
pyrazolylpyridine complexes.5f,h Additionally, the C–H out-of-
plane bending mode shifts from 761 cm−1 for 1 to 781 cm−1
for 2. The symmetric and asymmetric MovO stretching
modes of 2 are observed as intense bands at 942 and
908 cm−1, respectively, which are typical values for a dioxo-
molybdenum(VI) complex containing the cis-dioxo group.5
strong band at 336 cm−1 is assigned to νasym(Mo–Cl).5h
A
Single crystal XRD studies
Structure of 1HCl. The ligand 1 is isolated as an oil and so
no single crystal XRD study was possible. However, crystals of
the hydrochloride salt (1HCl) suitable for XRD were isolated
Fig. 2 Schematic representation of the molecular moieties composing
the asymmetric unit of 1HCl. Non-hydrogen atoms are represented as
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are represented as small spheres with arbitrary radii. For geometrical
details of the most structurally relevant supramolecular contacts present
in the crystal structure see Table S1 in the ESI.†
Fig. 1 FT-IR KBr (a–c) and ATR (d–f) spectra of the ligand 1 (a),
complex 2 (b), material 3 (c), and the solids 3-S (d), 3-S-run2 (e) and 3-L
(f) recovered from catalytic reactions.
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Table 2 Selected bond lengths (Å) and angles (°) for the crystallogra-
phically independent Mo6+ coordination environment present in
[MoO2Cl2(pent-pp)] (2)
Mo1–O1
Mo1–O2
Mo1–N2
1.700(2)
1.685(3)
2.326(3)
Mo1–N3
Mo1–Cl1
Mo1–Cl2
2.344(3)
2.3769(10)
2.3694(11)
O1–Mo1–Cl1
O1–Mo1–Cl2
O1–Mo1–N2
O1–Mo1–N3
O2–Mo1–O1
O2–Mo1–Cl1
O2–Mo1–Cl2
O2–Mo1–N2
95.38(10)
96.54(10)
159.71(11)
90.42(11)
105.54(12)
96.08(9)
O2–Mo1–N3
N2–Mo1–Cl1
N2–Mo1–Cl2
N2–Mo1–N3
N3–Mo1–Cl1
N3–Mo1–Cl2
Cl2–Mo1–Cl1
164.00(11)
82.24(8)
80.79(8)
69.29(10)
81.01(8)
82.26(8)
159.48(4)
96.73(10)
94.75(11)
Fig. 3 Schematic representation of the [MoO2Cl2(pent-pp)] molecular
unit present in 2, showing the labelling scheme for all non-hydrogen
atoms, which are represented as thermal ellipsoids drawn at the 50%
probability level. Hydrogen atoms are represented as small spheres with
arbitrary radii. Table 2 lists selected bond lengths and angles.
(Table S2 in the ESI† gives geometrical details of the various
supramolecular contacts).
Reaction of complex 2 with water
The reaction of complex 2 with water was carried out hydro-
thermally in a sealed Teflon-lined stainless steel digestion
bomb (autogenous pressure, 100 °C, 19 h). A pale green solid
(3) was obtained, which was recovered by filtration, washed
with water and acetone, and vacuum-dried. With the aim of
obtaining a better understanding of the nature of material 3, it
was characterised by elemental analysis, powder XRD, SEM,
TGA, FT-IR and 13C{1H} MAS NMR spectroscopies.
Elemental analysis of 3 (C, H, N and Mo) was consistent
with the empirical formula C13H17Mo2N3O6, i.e., [Mo2O6(pent-
pp)]. Compound 3 is (poorly) microcrystalline and displays
several low angle (5–15° 2θ) reflections in the powder XRD
pattern (see below). However, structure solution and refine-
ment from the data was not possible due to the poor quality of
the high angle data (which could not be improved by using
either long acquisition times or synchrotron radiation
sources). SEM examinations for 3 revealed irregular grain
shapes consisting of sheet-like lamellar structures (Fig. 4). EDS
analyses confirmed the absence of Cl in the product.
The thermal decomposition behaviour of compound 3 was
studied by thermogravimetry and variable temperature powder
XRD (performed under air). Both techniques confirm that 3 is
stable up to 200 °C. A weight loss of 43.1% then takes place
between 200 and 440 °C (Fig. 5), which is attributed to the
decomposition of the organic component. In this temperature
range the powder XRD pattern transforms into a pattern
characteristic of MoO3 (Fig. 6). The observed weight loss for
the transformation of 3 into MoO3 is in very good agreement
with the theoretical value of 42.8% based on the empirical
formula given above. No further weight loss takes place until
700 °C. Towards higher temperatures, the sublimation of
MoO3 takes place, as indicated by an abrupt mass loss up to
800 °C.25
asymmetric unit being composed of a whole molecular unit as
depicted in Fig. 3. In contrast to that observed for the hydro-
chloride salt of the ligand 1 (see above), the pendant pentyl
chain is not disordered, even though the structural differences
with 1HCl clearly demonstrate the high conformational flexi-
bility of this chain (compare, for example, the distinct confor-
mations shown in Fig. 2 and 3). Upon coordination to Mo6+
the two average planes of the organic ligand (corresponding to
the pyridine and the 2-(3-pyrazole) rings) remain, as observed
in 1HCl, almost coplanar with the mutual dihedral angle
increasing only slightly to 5.41(19)°.
The crystallographically independent Mo6+ metal centre is
coordinated to one N,N-chelated 2-(1-pentyl-3-pyrazolyl)pyri-
dine molecule, two cis-oxo groups and two chloride anions,
with the overall coordination geometry resembling a highly
distorted octahedron, {MoCl2N2O2}. While the cis-oxo groups
are trans coordinated to the organic ligand, forming the equa-
torial plane of the octahedron and markedly exerting their
trans effect (as clearly evidenced in Fig. 3), the chloride anions
occupy the apical positions of the octahedron. This multitude
of coordinated species leads, simultaneously, to a wide range
of bond lengths and angles as evidenced by the refined values
summarised in Table 2. Nevertheless, a search in the literature
and in the Cambridge Structural Database24 (Version 5.34,
November 2012 with 3 updates) shows that these values fall
within the typical ranges found in related compounds.
The crystal packing of individual [MoO2Cl2(pent-pp)] com-
plexes seems to be essentially driven by geometrical factors
mostly related to chemical affinity as depicted in Fig. S2 in the
ESI.† Indeed, in the crystal structure there is a segregation of
the hydrophobic part (mainly composed of the pentyl substitu-
ent) and the hydrophilic portion of the complexes (the metal
coordination sphere). The latter portion is rather rich in a
number of weak supramolecular interactions, namely C–H⋯O
and C–H⋯Cl hydrogen bonds and π–π contacts between co-
ordinated pent-pp molecules belonging to adjacent complexes
The 13C{1H} CP MAS NMR spectrum of 3 displays several
signals between 100 and 150 ppm that can be readily assigned
to the pyrazolylpyridine ring carbon atoms (Fig. 7). Additional
signals between 14 and 52 ppm are due to the pentyl group.
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Fig. 5 TGA curve of material 3.
Fig. 4 SEM images of material 3.
As shown in Fig. 1, the FT-IR spectrum of material 3 in the
range 1000–1650 cm−1 is very similar to that for complex 2,
which is further proof that in 3 the ligand pent-pp is intact
and coordinated to Mo6+ centres in a bidentate fashion similar
to that found for 2. In the region of Mo–O stretching vibrations
(500–1000 cm−1) compound 3 exhibits strong bands at 657,
804, 910, ca. 935 (sh) and 957 cm−1. A shoulder at 776 cm−1 on
the band at 804 cm−1 is assigned to the C–H out-of-plane
bending mode. The bands above 900 cm−1 may be assigned to
ν(MovO) vibrations, while the very intense and broad bands
at 657 and 804 cm−1 may be attributed to Mo–O stretching
vibrations of bridging Mo–O–Mo units. The former band is
Fig. 6 Variable temperature powder XRD study of compound 3.
1D chains of corner-sharing distorted {MoO4N2} octahedra,
exhibits a band at 622 cm−1 attributed to ν(Mo–O–Mo).26
Although at this stage the structure of material 3 remains
unknown, the characterisation data strongly point towards a
pyrazolylpyridine-molybdenum oxide hybrid with a polymeric
structure.
Catalytic epoxidation of olefins
Reactions with cis-cyclooctene. The catalytic performance of
typical of molybdenum oxide-based organic–inorganic hybrid compounds 2 and 3 was first evaluated by using cis-cyclo-
materials with polymeric structures.10,26 For example, the octene (Cy) as a model substrate, TBHP as the oxidant, and
material [MoO3(2,2′-bipy)], which has a structure comprising typical reaction conditions referred to as TypCs (please see the
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Fig. 8 Dependence of cyclooctene oxide yield on reaction time of the
epoxidation of cis-cyclooctene with TBHP at 55 °C for the catalyst/
Fig. 7 13C{1H} CP MAS NMR spectrum of 3. Spinning sidebands are
□
○
△
cosolvent systems 2/DCE (×), 3/DCE ( ), 3/TFT ( ), 3/hex ( ) and 4/DCE
indicated by asterisks.
◊
( ). Reaction conditions as indicated for TypCs in the Experimental
section. The dotted lines are visual guides.
Experimental section for details; 1,2-dichloroethane (DCE) was
used as a cosolvent). The two catalysts exhibited excellent
selectivity (100%) towards the epoxide product (CyO) until
100% conversion. Without adding a catalyst and/or the oxidant
TBHP, the reaction of Cy was negligible, indicating the essen-
tial roles of the transition metal and the hydroperoxide in the
formation of the active oxidising species. Iodometric titration
carried out for 3 without olefin, at 55 °C for 4 h, indicated neg-
ligible TBHP decomposition by the catalyst, and thus excellent
efficiency of TBHP consumption for the olefin epoxidation.
When compared with other mononuclear complexes of the
type [MoO2Cl2L], the results obtained with complex 2 (80%
CyO yield at 10 min) are comparable with those reported for
L = ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (ppEA, 80%) and 2-(1-
butyl-3-pyrazolyl)pyridine (92%),5f and superior to that for L =
2-[3(5)-pyrazolyl]pyridine (ppH, 32%).5h The catalytic reaction
was faster for 2 than for 3 (Fig. 8), which somewhat parallels
published data for molybdenum compounds possessing pyra-
zolylpyridine or bipyridine ligands in that the epoxidation rate
of Cy tends to be lower for polynuclear molybdenum com-
pounds than for the respective mononuclear precursor com-
plexes.5f,10,11 The lower activity exhibited by the polynuclear
compounds may be partly due to poor catalyst solubility and/
or inaccessibility of the reagent molecules to some of the
active species. Nevertheless, the catalytic activity of 3 (based on
60% CyO yield at 10 min) is higher than that reported for the
avoid competition between the solvent and reagent molecules
for coordination to the metal centre. The epoxidation rate
(conversion at 10 min/6 h reaction, Fig. 8) decreased in the
order DCE (60%/99%) > TFT (23%/98%) > hex (6%/91%);
product selectivity was always 100% towards CyO. Differences
in catalyst solubility may be responsible for these different
reaction rates. Underlying this hypothesis is the fact that the
contact tests (ConTs, described in the Experimental section)
revealed that the reactions were homogeneous in nature. The
conversions at 10 min for the ConTs decreased in the order
DCE (52%) > TFT (40%) > hex (32%), and CyO selectivity was
always 100%. This trend is consistent with both a visual evalu-
ation of the colour of the respective liquid phases (intense
yellow for DCE, pale yellow for TFT, and almost colourless for
hex) and the amount of undissolved solid which was separated
in the procedure for the ConTs (44 wt% for DCE, 80 wt% for
TFT and 89 wt% for hex). Hence, these results are consistent
with the catalyst being homogeneous, and suggest that its
solubility increases in the order hex < TFT < DCE. For each
solvent system, the initial amount of active species for the
ConTs is expectedly smaller than or approximately equal to
that for the TypCs (without filtration). For DCE, the initial reac-
tion rates were similar for the ConTs and the TypCs, whereas
for TFT and hex the initial reaction rates were higher for the
ConTs. These results are consistent with the enhanced catalyst
solubility in DCE.
polynuclear
compounds
[Mo8O24(ppEA)4]
(∼18%),27
[Mo2O4(µ2-O)Cl2(ppEA)2] (∼48%),8c [Mo4O12(ppH)4] (∼4%),27
Reactions with bio-derived olefins. The versatility of 3 was
explored for the catalytic epoxidation of bio-derived olefins,
namely DL-limonene (Lim), a cyclic monoterpene present in
citrus peel oil, and methyl oleate (Ole), present in, for
example, soybean oil. The epoxidation of Lim gives 1,2-epoxy-
p-menth-8-ene (LimOx) and 1,2;8,9-diepoxy-p-menthane
and
[Mo2O6([(3-pyridinium-2-yl)-1H-pyrazolyl-1-yl]acetate)]n
(74% CyO yield at 24 h reaction).11
The catalytic performance of 3 was further investigated
using trifluorotoluene (TFT) or hexane (hex) as a cosolvent.
Like DCE, the noncoordinating nature of these solvents should
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(LimDiox), which have applications as reactive diluents in cat- spectrum identical to that for 3-L. Further characterisation of 4
ionic and UV cure applications, and as intermediates in the by elemental analysis and 1H NMR spectroscopy led to its
production of flavours, fragrances, varnishes, metal coatings identification as the oxodiperoxo complex [MoO(O2)2(pent-
and biodegradable polycarbonates.28,29 With respect to Ole, pp)]. The FT-IR spectrum of 4 shows bands at 947 cm−1
vegetable oils may be converted to useful epoxide products (ν(MovO)), 861 cm−1 (ν(O–O)), 656, 582 and 535 cm−1 (ν(Mo-
with applications in the polymer industry (e.g., for producing (O2)2), which are all characteristic of [MoO(O2)2L] complexes
polyurethanes, and as PVC-plasticisers and stabilisers), or bio- bearing one bidentate ligand (L).34 The spectrum of 4 in the
degradable lubricants, coatings, biofuel additives, detergents, range 900–1650 cm−1 (comprising the organic ligand modes)
etc.28,30
is similar to that for 2. Oxodiperoxo complexes like 4 are well
Considering the homogeneous nature of the catalytic reac- known as effective homogeneous catalysts for the epoxidation
tion, there is no practical advantage in using a higher amount of olefins.34 Hence, the oxodiperoxo complex clearly plays a
of catalyst than that corresponding to its saturation concen- catalytic role in the system 3/TBHP/DCE. The formation of a
tration. Hence, a ConTs-DCE solution was used for the cataly- complex of the type [MoO2(O2)2L] from a polynuclear com-
tic reactions of the bio-based olefins at 55 °C (the catalytic pound during a catalytic epoxidation reaction has only been
reaction was initiated with the addition of the olefin to the previously reported for [Mo2O4(µ2-O)Cl2(ppEA)2].8c On the
ConTs-DCE solution).
other hand, the formation of active monoxodiperoxo species
The catalytic reaction of Lim gave mainly LimOx and from the reaction of monomeric oxomolybdenum complexes
LimDiox in a combined yield of 57%/93% at 62%/97% conver- with TBHP has been reported in several cases, e.g. [MoO2Cl2-
sion, reached at 10 min/6 h reaction. The byproducts included (tris(3,5-dimethyl-1-pyrazolyl)methane)] gave [Mo2O3(O2)2-
p-mentha-dienols, which are of interest as fragrances.31 (µ2-O)(H2O){tris(3,5-dimethyl-1-pyrazolyl)methane}],35 [MoO2(NCS)2-
Regioselectivity was in favour of the epoxidation of the endo- (di-tBu-bipy)] gave [MoO(O2)2(di-tBu-bipy)],36 and [CpMoO2X]
cyclic double bond, leading to a LimOx/LimDiox molar ratio of (Cp = cyclopentadienyl; X = CF3 or CH3) gave [CpMo(O2)OX].37
5.8 at 93% total epoxide yield (6 h). The LimOx yield of Mechanistic proposals of the reaction of [CpMoO2X] (X = CF3
ca. 79% at 6 h reaction compares favourably with that reported and CH3) with alkyl hydroperoxides (ROOH) to give
for polynuclear molybdenum compounds tested as catalysts in [CpMo(O2)OX] contemplate the formation of ROH as a co-
the same reaction at 55 °C: ca. 71–74% yield at 6 h reaction for product.37,38 Based on these studies, we may postulate that the
[Mo8O22(OH)4(di-tBu-bipy)4] and [Mo2O4(µ2-O)2(di-tBu-bipy)2] conversion of 3 to 4 involves the transformation of MovO
(di-tBu-bipy = 4,4′-di-tert-butyl-2,2′-bipyridine),9,32 and 61% for groups by reaction with TBHP to give Mo(O2) groups and tert-
(DMA)[MoO3(Hbpdc)]·nH2O (DMA
=
dimethylammonium; butanol as a co-product.
For comparison with the system 3/TBHP/DCE, 4 was tested
Hbpdc = deprotonated 2,2′-bipyridine-5,5′-dicarboxylic acid).33
The catalytic reaction of Ole (chosen as a model substrate as a catalyst, and used in an amount equivalent to the number
for fatty acid methyl esters) led to 9,10-epoxystearate (OleOx) as of moles of organic ligand added with 3 under the typical reac-
the main product in 30%/78% yield at 30%/82% conversion, tion conditions. The reaction was slower for 4 than for 3, and
reached at 10 min/6 h. In parallel with that observed for the the respective kinetic curves after 4 h reaction tended to
reaction of Lim, the catalytic results for the reaction of Ole in similar conversions (Fig. 8). Based on these results it seems
the presence of 3 (OleOx yield at 6 h) compare favorably with that 3 is converted to 4 plus different active species.
those reported for polynuclear molybdenum compounds
When 3-S was used in a second 6 h batch run (TypCs), the
tested as catalysts in the same reaction at 55 °C: 39% yield for initial reaction rate decreased (conversion at 10 min was 60%
[Mo8O22(OH)4(di-tBu-bipy)4]9 and 43% yield for (DMA) and 17% for runs 1 and 2, respectively), although the Cy con-
[MoO3(Hbpdc)]·nH2O.33 The initial Mo : olefin molar ratios versions tended to similar values (99%) after 6 h for the two
used in the previous studies were greater than that used in the runs. The drop in the initial reaction rate from run 1 to run
present work.
2 may be partly due to a lower initial amount of active species
Identification of the active species. For a batch reaction in run 2. The FT-IR spectrum of the solid recovered from run 2
carried out with material 3, TypCs, DCE as a cosolvent and Cy (denoted as 3-S-run2) was notably different from those of 3-S
as a substrate, a biphasic reaction mixture consisting of a pale and 3; the relative intensities of the bands assigned to the
green solid and a yellow solution was formed. The FT-IR spec- organic ligand decreased considerably (Fig. 1). These results
trum of the undissolved solid (denoted as 3-S, recovered at the are consistent with the formation of coproduct(s) during the
end of the reaction as described in the Experimental section) conversion of 3 to 4, which in this case seem to be poorly
was fairly similar to that for 3 (Fig. 1). Similar results were soluble.
obtained when using TFT or hex as cosolvents. A yellow solid
(denoted as 3-L) was isolated from the liquid phase of the reac-
tion and found to exhibit an FT-IR spectrum very different
from that for 3 (Fig. 1). In a separate experiment, a mixture
Conclusions
comprising 3, TBHP and DCE was heated at 55 °C for 6 h. The dioxomolybdenum(VI) complex [MoO2Cl2(pent-pp)] (2)
Addition of pentane to the resulting reaction solution led to (pent-pp = 2-(1-pentyl-3-pyrazolyl)pyridine) is a highly active
the precipitation of a yellow solid (4) that displayed a FT-IR and selective catalyst for the epoxidation of the benchmark
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substrate cis-cyclooctene. Hydrolysis of 2 under hydrothermal
conditions reproducibly gives a microcrystalline material with
the composition [Mo2O6(pent-pp)] (3). Although the structure
of 3 is presently unknown, characterisation data point towards
a polymeric molybdenum oxide hybrid in which the organic
ligands are coordinated to oxomolybdenum(VI) centres. When
material 3 is used in catalytic olefin epoxidation, it acts as a
source of soluble active species, which include but may not be
limited to the oxodiperoxo complex [MoO(O2)2(pent-pp)].
While most of the molybdenum oxide/organic hybrid
materials studied recently by us in catalytic olefin epoxidation
behave in a similar manner to 3, i.e., the materials act as a
source of soluble active species, some materials act as hetero-
geneous catalysts. By modifying the synthesis procedures and/
or the organic ligands employed it may be possible to obtain
porous materials that function effectively as heterogeneous cat-
alysts. This would be a major breakthrough and is currently a
focus of study in our laboratory.
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Acknowledgements
We are grateful to the Fundação para a Ciência e a Tecnologia
(FCT, project ref. PTDC/EQU-EQU/121677/2010), QREN,
FEDER, COMPETE, the European Union (EU), and the Associ-
ate Laboratory CICECO (PEst-C/CTM/LA0011/2013) for contin-
ued support and funding. The NMR spectrometers are part of
the National NMR Network (REDE/1517/RMN/2005), supported
by POCI 2010 and the FCT. The FCT is acknowledged for finan-
cial support towards the purchase of the single-crystal diffracto-
meter. We thank the FCT and the EU for a post-doctoral grant
to P.N. (SFRH/BPD/73540/2010) cofunded by MCTES and the
European Social Fund through the program POPH of QREN,
and for a PhD grant to T.R.A. (SFRH/BD/64224/2009).
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