A hydrogen-bonded assembly of cucurbit[6]uril and [MoO2Cl2(H2O)2] with catalytic efficacy for the one-pot conversion of olefins to alkoxy products

Article abstract of DOI:10.1039/c9dt02127b

The reaction of the macrocyclic cavitand cucurbit[6]uril (CB[6]) and the diaqua complex [MoO₂Cl₂(H₂O)₂] in hydrochloric acid solution gave a water insoluble supramolecular compound with the general composition 2[MoO₂Cl₂(H₂O)₂]·CB[6]·xH₂O·yHCl·z(CH₃COCH₃) (2). Single crystal X-ray diffraction (XRD) analysis revealed the presence of barrel-shape supramolecular entities, {CB[6]·10(H₂O)}, aligned in layers which are shifted relative to adjacent layers to form a brick-like pattern. The CB[6]/water hydrogen-bonded entities further engage in intermolecular interactions with water, HCl and [MoO₂Cl₂(H₂O)₂] molecules to form a three-dimensional (3D) framework. Compound 2 was characterised by thermogravimetric analysis (TGA), IR and Raman vibrational spectroscopy, and ¹³C{¹H} CP MAS NMR. The reference complex [MoO₂Cl₂(H₂O)₂]·(diglyme)₂ (1) and compound 2 were studied for the oxidative catalytic conversion of olefins (cis-cyclooctene, cyclohexene and styrene) with aqueous H₂O₂ as oxidant. Using alcohols as solvents, 2 was employed in a one-pot two-stage strategy for converting olefins to alkoxy products, which involves oxidation (with H₂O₂) and acid chemistry. Mechanistic studies were carried out using different intermediates as substrates, and the type of solvent and substrate scope were investigated. The results demonstrated the ability of the CB[6]/Mo^VI supramolecular adduct to function as an acid-oxidation multifunctional catalyst, and its recovery and reuse via relatively simple procedures.

Full text of DOI:10.1039/c9dt02127b

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A hydrogen-bonded assembly of cucurbit[6]uril
and [MoO2Cl2(H2O)2] with catalytic efficacy for the
one-pot conversion of olefins to alkoxy products†
Cite this: DOI: 10.1039/c9dt02127b
a
Lucie S. Nogueira, a Margarida M. Antunes, a Ana C. Gomes,
c
Luís Cunha-Silva, b Martyn Pillinger, *a André D. Lopes,
Anabela A. Valente *a and Isabel S. Gonçalves
*
a
The reaction of the macrocyclic cavitand cucurbit[6]uril (CB[6]) and the diaqua complex [MoO2Cl2(H2O)2]
in hydrochloric acid solution gave a water insoluble supramolecular compound with the general compo-
sition 2[MoO2Cl2(H2O)2]·CB[6]·xH2O·yHCl·z(CH3COCH3) (2). Single crystal X-ray diffraction (XRD) analysis
revealed the presence of barrel-shape supramolecular entities, {CB[6]·10(H2O)}, aligned in layers which
are shifted relative to adjacent layers to form a brick-like pattern. The CB[6]/water hydrogen-bonded enti-
ties further engage in intermolecular interactions with water, HCl and [MoO2Cl2(H2O)2] molecules to form
a three-dimensional (3D) framework. Compound 2 was characterised by thermogravimetric analysis
(TGA), IR and Raman vibrational spectroscopy, and 13C{1H} CP MAS NMR. The reference complex
[MoO2Cl2(H2O)2]·(diglyme)2 (1) and compound 2 were studied for the oxidative catalytic conversion of
olefins (cis-cyclooctene, cyclohexene and styrene) with aqueous H2O2 as oxidant. Using alcohols as sol-
vents, 2 was employed in a one-pot two-stage strategy for converting olefins to alkoxy products, which
involves oxidation (with H2O2) and acid chemistry. Mechanistic studies were carried out using different
intermediates as substrates, and the type of solvent and substrate scope were investigated. The results
demonstrated the ability of the CB[6]/MoVI supramolecular adduct to function as an acid-oxidation multi-
functional catalyst, and its recovery and reuse via relatively simple procedures.
Received 20th May 2019,
Accepted 1st July 2019
DOI: 10.1039/c9dt02127b
rsc.li/dalton
(⋯OvMo⋯OvMo⋯) in two directions, with the Cl atoms
occupying the apical positions of distorted {MoO4Cl2} octahe-
Introduction
Molybdenum(VI) dichloride dioxide, MoO2Cl2, was first dra.3 Although MoO2Cl2 is moisture sensitive, monohydrate
reported by Berzelius in 1826, and is generally believed to be and dihydrate derivatives are known. In MoO2Cl2·H2O, the
the first molybdenum oxo compound.1 MoO2Cl2 is a very introduction of a water molecule into the coordination sphere
useful starting material for synthesising a variety of molyb- of Mo atoms breaks down the layers into chains in which one
denum compounds and has gained importance as a catalyst in of the oxygen atoms is terminal and the other is bridging.4
organic transformations.2 Anhydrous MoO2Cl2 has a layered When two H2O molecules are coordinated to the Mo atom, the
structure in the solid-state consisting of infinite chains structure consists of isolated [MoO2Cl2(H2O)2] molecules.
The neutral species [MoO2Cl2(H2O)2] exists in solutions of
alkali molybdates in concentrated hydrochloric acid and can
aDepartment of Chemistry, CICECO – Aveiro Institute of Materials,
be extracted with oxygenated organic solvents such as diethyl
ether.5 Two main avenues have been pursued to isolate air-
stable crystalline solids containing the diaqua complex: (1)
adduct formation with polyethers such as diglyme6 or crown
ethers such as 18-crown-6;7 (2) co-crystallisation with
ammonium ions such as pyridinium,8 triethylammonium,9
1,3-bis(4-pyridinium)propane (H2dipy-pra)10 and 2,2′-dipyridy-
lammonium (Hdpa)11 (all with chloride as the counterion).
These compounds display intricate hydrogen-bonding net-
works that give rise to supramolecular frameworks. The 2D
network material [MoO2Cl2(H2O)2]·(H2dipy-pra)Cl2 was found
University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
E-mail: mpillinger@ua.pt, atav@ua.pt, igoncalves@ua.pt
bREQUIMTE/LAQV & Department of Chemistry and Biochemistry, Faculty of Sciences,
University of Porto, 4169-007 Porto, Portugal
cCCMar, and Department of Chemistry and Pharmacy, FCT,
University of the Algarve, P-8005-039 Faro, Portugal
†Electronic supplementary information (ESI) available: Materials and methods
experimental details, synthesis of CB[6], XRD experimental details and geometri-
cal data for H-bonding interactions in 2, experimental and simulated PXRD pat-
terns for 2, TGA curves, ATR FT-IR, PXRD and SEM results for recovered cata-
lysts, mass spectral data for selected products. CCDC 1900402. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c9dt02127b
This journal is © The Royal Society of Chemistry 2019
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to be an efficient catalyst, with H2O2 as the oxidant and
According to the XRD study, compound 2 is a supramolecu-
NaHCO3 as the cocatalyst, in the epoxidation of olefins.10 lar adduct of a CB[6] molecule and two [MoO2Cl2(H2O)2]
Later, Costa et al. reported that the 3D framework material complexes (Fig. 1), with a high number of water molecules of
[MoO2Cl2(H2O)2]·Hdpa·Cl·H2O was an effective catalyst crystallisation and HCl, ultimately formulated as
2
(without any cocatalyst added such as NaHCO3) for the H2O2- [MoO2Cl2(H2O)2]·(C36H36N24O12)·17H2O·2HCl. The crystal
mediated epoxidation of cyclooctene.11
structure was solved in the centrosymmetric triclinic space
ˉ
In the present paper, a new supramolecular approach to the group P1 with the asymmetric unit (asu) containing one
solid-state isolation of [MoO2Cl2(H2O)2] is reported, using the neutral complex [MoO2Cl2(H2O)2], one half of a CB[6] mole-
macrocyclic cavitand cucurbit[6]uril (CB[6]). Cucurbit[n]urils cule, and several solvent molecules (8.5 H2O molecules of crys-
(CB[n = 5–8, 10, 14]) are composed of n glycoluril units lined tallisation and one HCl molecule; the CB[6] cavity is occupied
by 2n methylene bridges to form a hydrophobic cavity accessi- by H2O molecules, with the crystal refined model pointing to
ble through two carbonyl-lined portals.12 CBs can interact with 2.5 molecules occupying six potential sites). The most impor-
metal complexes to form either host–guest inclusion com- tant structural features of the {MoO4Cl2} centre of the in-
pounds, in which the CB acts as a molecular container,12 or organic complex in 2 are consistent with those observed in the
outer-sphere exclusion complexes, in which the CO groups previous crystal structures containing this complex, reported
coordinate either directly or indirectly with metal ions, com- in the Cambridge Structural Database (version 5.39-2018):15,16
plexes or clusters.13 Fedin and co-workers have made the CSD codes YEYWOO,6 FEKCED,7a FEKCED01,7b KEQBUE,7b
largest contribution to CB[n]-based coordination chemistry XMOPYC10,8 LAPPOH,9 KICNIU,10 MUPGIO,11 and VONXUS.17
with metal cations.14 In aqueous solution, transition-metal The MoVI coordination centre displays a distorted octahedral
aqua complexes generally interact with CBs indirectly via geometry, with the four O-atoms (two oxido groups, MovO,
hydrogen bonds between the coordinated water molecules and and two water molecules, Mo–Ow) in the basal plane, and two
the portal oxygen atoms of the CB. This gives rise to supramo- chlorine atoms, Mo–Cl, occupying the axial positions (Fig. 1;
lecular adducts which crystallise with complex network struc- Table 1 gives details about the coordination bond lengths and
tures, such as that found with CB[6] and the trinuclear molyb- internal angles).
denum oxo cluster [Mo3O4(H2O)6Cl3]+.14c
The extended crystal structure of the adduct 2 reveals a
Although CB[6] is virtually insoluble in water, it is soluble supramolecular assembly of metal complexes with CB[6]. Since
in acidic aqueous solution. Fedin and co-workers were success- the Mo complex is not incorporated into the hydrophobic
ful in isolating poorly soluble CB[6] coordination complexes by cavity of CB[6], it can be envisaged as an exclusion complex or,
pairing the cavitand with metal complexes that were stable in alternatively, as a lattice exclusion compound (Fig. 2).18 The
HCl solution, such as the trinuclear molybdenum complex structural cohesion of this novel adduct is guaranteed by an
mentioned above. We have now used an analogous strategy to extensive network of intermolecular interactions, mainly
isolate a supramolecular adduct from a HCl solution contain- strong O–H⋯O and C–H⋯Cl hydrogen bonds, intermediated
ing CB[6] and [MoO2Cl2(H2O)2]. The crystal structure of the by numerous H2O molecules of crystallisation and HCl mole-
adduct consists of a 3D hydrogen-bonding framework built
from assemblies of CB[6] and the diaqua complex in a net
ratio of 1 : 2. Encouraged by the catalytic results reported pre-
viously for adducts containing the diaqua complex, the utility
of the CB[6] adduct in catalysed organic reactions, namely oxi-
dation (the epoxidation of olefins) and oxidation/acid catalysis
(the one-pot conversion of olefins with aqueous H2O2 to alkoxy
ketone and alcohol products) has been investigated. The cata-
lyst stability was studied and mechanistic insights were
obtained by using different types of intermediates as
substrates.
Results and discussion
Synthesis and characterisation of supramolecular adduct 2
[MoO2Cl2(H2O)2]·(C36H36N24O12)·xH2O·yHCl·z(CH3COCH3) (2)
The supramolecular adduct 2 precipitated after mixing an
aqueous solution of [MoO2Cl2(H2O)2] (prepared by dissolving
MoO3 in 6 M HCl) with a hydrochloric acid solution of CB[6].
Prolonged standing of the mother liquor at ambient tempera-
Fig. 1 Crystal structure of the supramolecular adduct of
[MoO2Cl2(H2O)2] and CB[6] (2). Non-H atoms of the asu are represented
as ellipsoids with thermal displacement at the 50% probability level,
while those generated by symmetry are drawn in ball-and-stick mode,
ture led to the formation of pale yellow single crystals suitable
for structure determination by XRD.
and most of the H-atoms are omitted for clarity (symmetry transform-
ation used to generate equivalent atoms: (i) −x, 2 − y, 1 − z).
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Table 1 Selected bond lengths (Å) and angles (°) for the MoVI coordi-
nation centre of the neutral complex [MoO2Cl2(H2O)2] in the crystal
structure of 2
cules. Interestingly, five carbonyl groups of each CB[6] rim
interact with five H2O molecules by O–H⋯O hydrogen bonds,
leading to the formation of barrel-shape supramolecular enti-
ties, {CB[6]·10(H2O)} (Fig. 2a; geometric information concern-
ing all the strong hydrogen bonds are detailed in Table S2 in
the ESI†). These barrel-shape supramolecular entities are
involved in further contacts with the remaining H2O molecules
of crystallisation and HCl molecules, as well as with the
complex [MoO2Cl2(H2O)2], leading to the formation of a 3D
hydrogen-bonded framework (Fig. 2b and c). The occurrence
of a high number of H2O molecules of crystallisation and, con-
sequently, the extensive network of strong intermolecular
interactions is crucial in reinforcing the cohesion of the crys-
talline structure.
Mo1–O1
Mo1–O2
Mo1–O1W
1.6779(15)
1.6859(14)
2.2231(14)
Mo1–O2W
Mo1–Cl2
Mo1–Cl1
2.2750(14)
2.3672(5)
2.3804(5)
O1–Mo1–O2
103.55(8)
95.01(7)
172.23(7)
96.12(6)
95.48(6)
161.42(7)
84.21(6)
97.61(5)
O2–Mo1–Cl1
97.11(5)
77.22(6)
79.56(4)
81.52(4)
82.95(4)
83.12(4)
158.563(19)
O1–Mo1–O1W
O1–Mo1–O2W
O1–Mo1–Cl2
O1–Mo1–Cl1
O2–Mo1–O1W
O2–Mo1–O2W
O2–Mo1–Cl2
O1W–Mo1–O2W
O1W–Mo1–Cl2
O1W–Mo1–Cl1
O2W–Mo1–Cl2
O2W–Mo1–Cl1
Cl2–Mo1–Cl1
Fig. 2 Selected features in the crystal structure of the supramolecular adduct of [MoO2Cl2(H2O)2] and CB[6] (2): (a) barrel-shape supramolecular
entity {CB[6]·10(H2O)}; (b) O–H⋯O and O–H⋯Cl interactions between adjacent neutral Mo complexes and CB[6] molecules, intermediated by water
and HCl molecules; (c) overall crystalline arrangement viewed along the [1 0 0] direction of the unit cell. O–H⋯O and O–H⋯Cl interactions are rep-
resented as blue and pink dashed lines, respectively.
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The bulk powder obtained in the reaction for 2 was further
characterised by elemental analysis, powder XRD (PXRD), N2
adsorption at −196 °C, TGA, and the spectroscopic techniques
FT-IR, FT-Raman and 13C{1H} CP MAS NMR. Fig. S1 in the
ESI† compares the observed PXRD pattern of the bulk micro-
crystalline sample of 2 with a simulated pattern based on the
single crystal structure data. The good match between the two
patterns proves that the powder had the same crystalline
structure with a high degree of phase purity. Elemental analysis
of the powder indicated the composition 2[MoO2Cl2(H2O)2]·
(C36H36N24O12)·15H2O·1.5HCl·0.6(CH3COCH3), which is similar
to that determined by X-ray crystallography except for minor
differences in the water and HCl contents, and the presence of
acetone. The presence of acetone (originating from the CB[6]
starting material) was confirmed by 13C{1H} CP MAS NMR,
while the water content was supported by TGA measurements
which revealed a mass loss step of 20.6% up to 150 °C attribu-
ted to the removal of water and acetone (Fig. S2 in the ESI†).
Acetone was not detected by X-ray crystallography, possibly
because of disorder. The TGA profiles of the starting material
CB[6] and compound 2 show that the organic cavitand starts
to decompose around 290 °C. Two overlapping steps are
clearly visible for 2, a slightly prolonged one with a derivative
thermogravimetric (DTG) maximum at 330 °C, followed by a
more abrupt loss with DTGmax = 390 °C. For comparison, the
DTGmax value for CB[6] is 400 °C. Decomposition of the
organic matter in 2 is complete at 500 °C, leaving residual
MoO3 (12.0%), which starts to sublime above 700 °C.
The N2 adsorption–desorption isotherm for 2 at −196 °C
was of type II, which may be associated with monolayer–multi-
layer adsorption on an essentially non-porous material. The
specific surface area was 9 m2 g−1. The inner cavity diameter of
CB[6] is ca. 0.39 nm which is slightly greater than that of the
N2 molecule (ca. 0.36 nm), but the accessibility of the adsor-
bate molecules to the cavities seems to be hindered in the
crystal structure of 2. The pore size distribution was centred at
ca. 2.8 nm which may be associated with intercrystallite void
spaces. One cannot exclude the possibility of the surface area
changing upon dispersion of the solid in a liquid phase (in
this work, the catalytic tests are biphasic solid–liquid) partly
due to interfacial interactions between the two phases.
Fig. 3 FT-IR (A) and Raman (B) spectra of complex 1, CB[6] and com-
pound 2.
The FT-IR and FT-Raman spectra of 2 are dominated by the
characteristic bands of CB[6] (Fig. 3). Nevertheless, a few
additional bands are present that can be assigned to the incor-
porated molybdenum complex: (IR, cm−1) 334 (νasym(Mo–Cl)),
two and three resolved lines. This can be attributed to the fact
that the crystallographically inequivalent carbons for each type
of carbon functionality (i.e., six CO groups, six C–H and six
CH2) may have slightly different environments and therefore
small differences in chemical shift, which can result in the
observation of multiple sharp resonances for one chemically
distinct carbon type.
915 (νasym(MovO)); (Raman, cm−1
) 252 (ρ(MoO2)), 310
(νsym(Mo–Cl)), 920 (νasym(MovO)), 960 (νsym(MovO)).7a The
reference complex [MoO2Cl2(H2O)2]·(diglyme)2 (1)6 displays
bands at near-identical frequencies: (IR) 334, 914 and
953 cm−1; (Raman) 252, 315, 920 and 951 cm−1
.
Catalytic studies
13C{1H} CP MAS NMR spectra for CB[6] and 2 are shown in
Fig. 4. The spectrum for CB[6] shows three somewhat broad Olefins, which may derive from fossil or renewable sources,
and featureless signals at 155.5, 70.5 and 52.0 ppm for the can be converted to many useful products such as epoxides,
three chemically distinct carbon types (CvO, C–H and brid- 1,2-alkoxy alcohols and 1,2-alkoxy ketones.19 Hydrogen per-
ging CH2, respectively). The spectrum for 2 is slightly different oxide is an attractive oxidant for catalytic olefin epoxidation
in that each of the three resonances is split, showing between due to its relatively low cost and eco-friendliness, giving water
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Fig. 4 13C{1H} CP MAS NMR spectra for CB[6] and the supramolecular
adduct 2.
Scheme 1 Products obtained in the oxidation of cis-cyclooctene,
styrene and cyclohexene with H2O2 in the presence of 2, using MeCN or
EtOH as solvent.
as the coproduct from its consumption.20 Epoxides are versa-
tile intermediates for various organic reactions with, for
example, nucleophiles, acids, bases, reducing and oxidizing
agents.19 In particular, epoxides may be converted in alcohol
media to 1,2-disubstituted products (which usually have a trans
configuration), such as 1,2-alkoxy alcohols and 1,2-alkoxy
ketones, which have important applications as intermediates in
the pharmaceutical sector.21 For example, trans-2-methoxycyclo-
hexanol is a key intermediate for the synthesis of tricyclic beta-
lactam antibiotics.21c On the other hand, the conversion of
olefins in alcohol medium to alkoxy products is a useful tool in
organic synthesis for improving product selectivity by enabling
protective group formation.22 With these considerations in
mind, we studied the catalytic ability of 2 in two stages, firstly to
assess its performance for the epoxidation of olefins with H2O2,
and secondly to apply this catalytic efficacy in the one-pot con-
version of olefins to alkoxy ketone and alcohol products.
with 2 involved molybdenum-containing active species. The
results for the model complex [MoO2Cl2(H2O)2]·(diglyme)2 (1)
(100% Cy8O selectivity at 93% conversion) are comparable with
those for 2. An important difference between compounds 1
and 2 is that the former was completely soluble in the reaction
medium and therefore difficult to separate, whereas 2 could be
recovered and reused in consecutive batch runs (discussed
below). Decreasing the reaction temperature from 70 to 55 °C
led to slower kinetics in the presence of 2 (100% Cy8O selectivity
at 73% conversion), as did the use of N,N′-dimethylformamide
(DMF) as solvent instead of MeCN (100% Cy8O selectivity at
28% conversion). These results may be partly due to the stron-
ger basicity of DMF than MeCN, which may lead to competitive
effects between solvent and reactant molecules in the coordi-
nation to the (Lewis acid) metal centre.23 Without a solvent or
using water, the reaction was very sluggish (≤2% Cy8 conversion
at 24 h). Water is less basic than MeCN, suggesting that non-
Olefin epoxidation
The catalytic properties of 2 were explored starting with cis- protic polar solvents of moderate basicity may be favourable.
cyclooctene (Cy8) as a model substrate, which is often used in
There are relatively few reports on catalytic olefin epoxi-
the literature for catalyst benchmarking of molybdenum com- dation with H2O2 as the oxygen source and complexes of the
pounds bearing the {MoO2Cl2} moiety in olefin epoxidation type [MoO2Cl2(L)n] as the catalyst source. With the 2D network
with hydroperoxide oxidants. The reaction of Cy8 with H2O2 at material [MoO2Cl2(H2O)2]·(H2dipy-pra)Cl2 as the catalyst
70 °C in the presence of 2, using acetonitrile as solvent, led to (1 mol%), NaHCO3 as cocatalyst (25 mol%) and MeCN as
100% epoxide (Cy8O) selectivity at 85% conversion (Scheme 1, solvent, Wang and co-workers obtained a high Cy8O yield of
Table 2). Pristine CB[6] led to a sluggish reaction of Cy8 99% after 1 h at 25 °C.10 Under conditions more similar to
(10% conversion), suggesting that the epoxidation reaction those used in the present work (no NaHCO3 cocatalyst), the 3D
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Table 2 Oxidation of olefins with H2O2 in the presence of compounds
1, 2 and CB[6]a
Catalyst Substrate Product
T (°C) Solvent Conversion (%)
1
2
2
2
2
2
Cy8
Cy8O
70
55
70
70
70
70
70
MeCN
MeCN
MeCN
DMF
None
H2O
MeCN
MeCN
MeCN
93
73
85
28
2
1
CB[6]
2
2
10
42
47
Cy6
Sty
Cy6(OH)2 70
BA 70
a Reaction conditions: Molar ratio Mo : substrate : H2O2 = 1 : 100 : 152,
24 h reaction. The selectivity to the indicated product was 100% in all
cases.
framework material [MoO2Cl2(H2O)2]·Hdpa·Cl·H2O led to a
Cy8O yield of 80% after 24 h at 50 °C,11 which is comparable
with the catalytic performance measured for 2.
Scheme 2 Possible products of the oxidation reaction of styrene with
H2O2.26,27,29
Compound 2 was effective for the catalytic conversion of
other industrially produced alkenes, namely styrene (Sty) and
cyclohexene (Cy6), which were chosen as representative aro-
matic and aliphatic substrates (Table 2, Scheme 1). There are
several reports of the oxidation of styrene or cyclohexene cata-
lysed by dioxomolybdenum(VI) complexes, where H2O2 has
been used as an oxidant.24 In many cases, especially for
styrene, aqueous 30% H2O2 as the sole oxidant gives poor cata-
lytic efficiency, with low styrene oxide (StyO) selectivity,
whereas the addition of the cocatalyst NaHCO3 activates the
catalytic oxidation process and leads to much higher StyO
selectivity.20b,25 The results obtained in the present work
(without a cocatalyst) are consistent with these observations.
Thus, the oxidation of Sty at 70 °C gave benzaldehyde (BA)
with 100% selectivity at 47% conversion (24 h). According to
literature studies on the mechanism of Sty oxidation, StyO is
an intermediate to BA, and the latter may be formed via
nucleophilic attack of H2O2 on StyO giving a hydroxyl–hydro-
peroxy intermediate followed by oxidative cleavage,26–28 or BA
may be formed via the direct oxidative cleavage of the side
chain CvC double bond of Sty via a radical mechanism
(Scheme 2).26,27,29 The initial addition of a radical scavenger
(2,6-di-tert-butyl-4-methyl phenol) to the catalytic reaction
mixture, in an equimolar amount relative to the substrate,
strongly affected the reaction kinetics (2% conversion com-
pared to 47% conversion without radical scavenger). These
results suggest the predominance of a radical mechanism. The
reaction of Cy6 with H2O2 in the presence of 2 gave cyclohexa-
nediol (Cy6(OH)2) in 100% selectivity at 42% conversion
(Scheme 1). The diol may be formed via the intermediate
formation of cyclohexene oxide (Cy6O),30 as suggested in
Scheme 2 for StyO.
media. These reaction systems involve oxidation and acid
chemistry. The one-pot conversion was carried out in two
stages (24 h reaction at 70 °C followed by 24 h reaction at
90 °C) with the objective of enhancing selectivity to the inter-
mediates in the first stage (involving oxidation catalysis), fol-
lowed by conversion of the intermediates to the alkoxy pro-
ducts in the second stage (involving acid catalysis). Catalyst 2
was tested first with Cy8 as model substrate.
The one-pot first-stage process of 2/Cy8/H2O2/ethanol led to
high epoxide selectivity of 90% at 89% conversion, and the
products 2-ethoxycyclooctanol [Cy8(OH)(OEt)] and 2-ethoxy-
cyclooctanone [Cy8(vO)(OEt)] were formed in low yields of 3%
and 5%, respectively (Table 3, entry 5). Using methanol (MeOH)
or 1-butanol (BuOH) as solvent led to high epoxide selectivity
(93–99%) at high conversion (80–90%) (entries 3 and 7). Low
yields of the alkoxy products were formed for BuOH. BuOH and
EtOH led to higher Cy8 conversions than MeOH.
In the one-pot two-stage process in the different alcohol
media, 2 led to epoxide yields of 42–58% and relatively high
yields of alkoxy products (33–40% total yield at 48 h) (Table 3).
The reaction in MeOH was slowest. The epoxide yield
decreased and the total yield of alkoxy products increased in
the order: BuOH → EtOH → MeOH (entries 8, 6 and 4, respect-
ively). The differences in kinetics may be partly due to differ-
ences in nucleophilicity of the alcohols. For example, since
EtOH is a stronger nucleophile than MeOH, the formation of
the respective alkoxy products (via alcoholysis) may be faster
(8% (entry 5) compared to <1% (entry 3) at 24 h), driving the
epoxidation step forward and thus increasing Cy8 conversion
(compared to the reaction using MeOH).
One-pot conversion of cis-cyclooctene to alkoxy products
In the one-pot two-stage process with ethanol as solvent,
Compound 2 was tested as a multifunctional catalyst for the pristine CB[6] gave a sluggish reaction (ca. 25% conversion at
integrated conversion of olefins to alkoxy products in alcohol 48 h, Table 3, entry 18). The model complex 1 led to 92% con-
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Table 3 Catalytic results for the oxidation of cyclooctyl substrates with H2O2 in the presence of 2 using an alcohol as solventa
Entry
1
Catalyst
Substrate
Cy8
Product
T (°C)
t (h)
Solvent
EtOH
Conversion (%)
91
Selectivity (%)
1
Cy8O
70
24
83
7
10
32
31
33
4
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8O
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8(OH)2
2
Cy8
90b
48
EtOH
92
3
4
2
Cy8
Cy8
Cy8O
Cy8(OH)(OMe)
Cy8O
Cy8(OH)(OMe)
Cy8(OMe)2
Cy8O
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8O
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8O
Cy8(OH)(OBu)
Cy8(vO)(OBu)
Cy8O
Cy8(OH)(OBu)
Cy8(vO)(OBu)
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8(vO)
70
24
48
MeOH
MeOH
80
82
>99
<1
51
46
3
90
3
90b
5
6
7
8
Cy8
Cy8
Cy8
Cy8
70
24
48
24
48
EtOH
EtOH
BuOH
BuOH
89
90
90
90
6
90b
70
57
18
26
93
2
5
90b
64
10
27
55
45
43
57
52
48
0
9
1
Cy8O
Cy8O
Cy8O
Cy8O
70
90
70
70
24
24
24
24
EtOH
EtOH
EtOH
EtOH
22
71
12
0
10
11
12
2
None
13
14
15
2
Cy8(OH)
Cy8(OH)
Cy8(OH)2
70
70
70
24
24
24
EtOH
EtOH
EtOH
50
14
100
100
100
5
52
28
15
8
91
0
0
None
2
Cy8(vO)
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8(vO)OH
Cy8(vO)2
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8(vO)OH
Cy8(vO)2
Cy8O
Cy8O
Cy8(OH)(OEt)
Cy8(vO)(OEt)
Cy8(OH)(OEt)
Cy8(vO)(OEt)
16
None
CB[6]
Cy8(OH)2
70
24
EtOH
99
17
18
19
Cy8
Cy8
Cy8O
70
24
48
24
EtOH
EtOH
EtOH
6
25
0
100
100
0
90b
70
20
Cy8O
90
24
EtOH
2
78
22
a Reaction conditions: Molar ratio Mo : substrate : H2O2 = 1 : 100 : 152. b First 24 h at 70 °C.
version and a total yield of 59% for Cy8(OH)(OEt) and Cy8(vO) or to check the ability of the catalyst to perform specific types
(OEt) (entry 2), which represents similar performance to that of chemical transformations. Based on these studies, an
obtained with 2 (90% conversion, 40% yield of alkoxy pro- overall mechanistic proposal is presented in Scheme 3.
ducts, entry 6). However, as mentioned above, complex 1 has Specifically, catalyst 2 with the epoxide Cy8O as substrate led
different solubility characteristics compared to 2, which has a to Cy8(OH)(OEt) and Cy8(vO)(OEt), formed in ca. 6% yield
major impact on catalyst recovery processes.
each at 12% conversion (24 h) (Table 3, entry 11). These
results, taken together with those for the one-pot two-stage
process, indicate that the alkoxy products are formed via the
Mechanistic insights
To gain mechanistic insights into these reaction systems, 2 intermediate formation of the epoxide (path A in Scheme 3),
was tested with cyclooctyl substrates at 70 °C using H2O2/EtOH with this consecutive step being favoured at higher tempera-
(Table 3). The choices of the substrates were based on the ture (in the second stage at 90 °C, entry 6 in Table 3). The
possibility of them being intermediates in the overall reaction, epoxide ring-opening step may be promoted by Lewis acidity
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formed, may undergo etherification with EtOH to give the
alkoxy products. The ketones Cy8(vO)2 and Cy8(vO)OH may
be formed via oxidative dehydrogenation of the diol with
H2O2. From these results one cannot fully exclude the possi-
bility of the diol being an intermediate in the reaction of the
olefin to the alkoxy products in H2O2/EtOH. Nevertheless,
since Cy8(vO)2 and Cy8(vO)OH were not present in measur-
able amounts in the catalytic system 2/Cy8/H2O2/EtOH
(Table 3, entries 5–6), the epoxide alcoholysis route (path A)
seems to be more important.
One-pot conversion of other olefins to alkoxy products
The reaction of Cy6 gave 2-ethoxycyclohexanol [Cy6(OH)(OEt)]
as the main product in 60% yield at 64% conversion (24 h),
while the diol Cy6(OH)2 was formed in 4% yield (Table 4).
These results are interesting in that the alkoxy product was the
main product in a one-pot strategy at the lower temperature of
70 °C. The absence of the alkoxy ketone product suggests that
the oxidative dehydrogenation of Cy6(OH)(OEt) is demanding.
In support of this supposition, the catalytic reaction of cyclo-
hexanol [Cy6(OH)] with H2O2/EtOH in the presence of 2 only
resulted in 12% conversion after 24 h. Moreover, a comparison
of the results for the substrates Cy8(OH) (Table 3, entry 13) and
Cy6(OH) (Table 4) indicates that the latter is less prone to cata-
lytic oxidative dehydrogenation. Although the epoxide Cy6O
was not present in measurable amounts with Cy6 as substrate,
it may be an intermediate in the formation of the diol and
alkoxy products, as proposed in Scheme 3 for Cy8. Accordingly,
Cy6O was found to be a very reactive substrate with 2/H2O2/
EtOH, giving the diol and alkoxy alcohol in 8% and 92% yield,
respectively, after 24 h (Table 4).
There are very few reports describing the reaction Cy6/H2O2
in alcohol medium in the presence of molybdenum(VI) cata-
lysts without a cocatalyst, where alkoxy products were formed.
For the reaction Cy6/H2O2/MeOH at 60 °C in the presence of
the homogeneous catalyst [Mo(O)(O2)2(H2O)n], Carrasco et al.
found that the epoxide Cy6O was formed in trace amounts,
while the main products were the diol and alkoxy alcohol (12
and 87% selectivity at 65% conversion, 18 h), somewhat in par-
allel with that observed for 2.31
Scheme 3 One-pot conversion of cyclooctene in H2O2/EtOH to alkoxy
alcohol and alkoxy ketone products; a mechanistic proposal involving
hydrolysis and alcoholysis.
associated with the molybdenum centre or Brønsted acidity
associated with the cucurbituril component of 2. Control tests
with Cy8O as substrate in the presence of CB[6] or 1 (70 °C,
24 h) indicated that the ring-opening reaction did not occur in
the presence of the cucurbituril component (CB[6] led to 0%
conversion, entry 19 in Table 3), whereas the molybdenum
compound 1 presented some activity (22% conversion, 100%
selectivity to alkoxy products, entry 9). For a better understand-
ing, the reaction of Cy8O was further tested at 90 °C for 24 h.
Under these conditions, 1 led to 71% conversion (entry 10)
and CB[6] led to a sluggish Cy8O reaction (2% conversion,
entry 20). Hence, the active species involved in the formation
of the alkoxy products may essentially be of Lewis acid type.
The alkoxy alcohol may be converted to the corresponding
alkoxy ketone via oxidative dehydrogenation with H2O2. The
oxidative dehydrogenation activity of 2 for converting alcohols
to carbonyls was confirmed by the finding that using cyclo-
octanol [Cy8(OH)] as substrate led to cyclooctanone [Cy8(vO)]
as the sole product in 50% yield at 24 h (Table 3, entry 13). cis-
1,2-Cyclooctanediol [Cy8(OH)2] was not present in measurable
amounts in the reaction of Cy8 in the presence of 2/H2O2/
EtOH, although its formation via acid-catalysed hydrolysis of
the epoxide cannot be completely excluded since water was
added together with H2O2. With Cy8(OH)2 as substrate, the
catalytic reaction was very fast (100% conversion at 24 h, entry
15), indicating that the diol is a very reactive intermediate
when compared to Cy8O; without catalyst, the diol conversion
was 99% (entry 16). Catalyst 2 with the diol (entry 15) led to
Cy8(OH)(OEt) and Cy8(vO)(OEt) in 5% and 52% yield, respect-
ively, together with a 28% yield of 2-hydroxycyclooctanone
Table 4 Catalytic results for the oxidation of cyclohexyl substrates and
styrene with H2O2 in the presence of 2 using ethanol as solventa
T
(°C)
t
(h)
Conversion
(%)
Selectivity
(%)
Substrate
Cy6
Product
Cy6(OH)2
Cy6(OH)(OEt)
Cy6(OH)2
Cy6(OH)(OEt)
Cy6(vO)
BA
70
70
24
24
64
7
93
8
92
100
26
9
Cy6O
100
Cy6(OH)
Sty
70
70
24
24
12
57
StyO
Ph(COOEt)
Sty(OH)2
5
58
[Cy8(vO)OH] and
a 15% yield of 1,2-cyclooctanedione
a Reaction conditions: Molar ratio Mo : substrate : H2O2 = 1 : 100 : 152.
[Cy8(vO)2] (path B in Scheme 3). Hence, the diol, once
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With Sty as substrate, 2 led mainly to styrene glycol results suggest that the catalytic reaction occurred essentially
[Sty(OH)2] (58% selectivity at 57% conversion); other products in heterogeneous phase.
included StyO, BA and ethyl benzoate (Ph(COOEt)) formed in
The solids recovered after each batch run were characterised
5, 15 and 3% yield, respectively (Table 4). The corresponding by attenuated total reflectance (ATR) FT-IR spectroscopy, PXRD
alkoxy alcohol product was not present in measurable and SEM (with EDS elemental mapping). Representative
amounts. The results somewhat parallel those reported by results are shown in the ESI† for the reactions 2/Cy8/H2O2/
Biradar et al. for Sty/H2O2/BuOH in the presence of the MeCN, 2/Sty/H2O2/MeCN and 2/Sty/H2O2/EtOH performed at
complex [CpMo(CO)3(CuCPh)] (0.2 mol%) in that the main 70 °C, and 2/Cy8/H2O2/EtOH performed at 90 °C. The ATR
product was the diol (82% selectivity at 95% conversion, 65 °C/ FT-IR spectra (Fig. S3†), PXRD patterns (Fig. S4†) and SEM
10 h), and other reaction products included BA.32 The diol images (Fig. S5†) of the solids recovered after the first run of
may be formed via acid-catalysed hydrolysis of the epoxide in a each reaction system were similar, but displayed differences
manner similar to that represented in Scheme 3 for Cy8.
when compared with those for 2. No further significant altera-
tions were observed for the solids recovered after the runs 2
and 3. Hence, structural features of 2 changed during the first
contact with the reaction medium and then remained similar,
which correlates with the catalytic activity in consecutive batch
runs for the reaction 2/Cy8/H2O2/MeCN. Regarding the FT-IR
spectra, the major change between 2 and the recovered solids
was the weakening (or complete loss) of the νasym(MovO)
absorption band at 915 cm−1 (Fig. S3†). This was accompanied
by the appearance of some very weak bands between 850 and
900 cm−1, especially one at 888 cm−1, which may be due to per-
oxide ligands [ν(O–O)] formed via the coordination of H2O2 to
molybdenum centres. The catalytic role of peroxomolybdenum
(VI) species that may be formed in (dioxomolybdenum/H2O2)-
catalysed olefin epoxidation systems is well-established, based
on studies of different research groups worldwide.33 Several
mechanistic hypotheses have been put forward (mainly based
on gas-phase density functional theory) for olefin epoxidation
with hydroperoxides in the presence of oxoperoxomolybdenum
species; reaction mechanisms may involve O-atom transfer from
a η2-O2 peroxo ligand33c,34 or a hydroperoxo ligand33d,34a,35
to the olefin (which may or may not be coordinated to the
metal centre).
Catalyst reuse
With MeCN or EtOH as solvent, the reactions of Cy8 with H2O2
in the presence of 2 always led to biphasic solid–liquid mix-
tures after 24 h batch runs. Solids were recovered for reuse by
centrifugation, washing with acetone, and drying. Compound
2 was reused for four consecutive batch runs of the reaction
Cy8/H2O2/MeCN and two consecutive batch runs of the reac-
tion Cy8/H2O2/EtOH at 70 °C for 24 h. For MeCN, Cy8 conver-
sion decreased from run 1 to run 2, and to a lower extent from
run 2 up to run 4 (Fig. 5). In the EtOH medium, partial loss of
activity was observed from the first to the second run (Fig. 5).
A contact test (CT) was carried out for 2 by mixing it
with H2O2/MeCN (without substrate) at 70 °C for 24 h. The
resultant liquid and solid phases (denoted LP-CT-MeCN and
SP-CT-MeCN, respectively) were separated and tested for the
reaction Cy8/H2O2/MeCN at 70 °C for 24 h. The liquid phase
LP-CT-MeCN did not lead to significant olefin conversion
(10% at 24 h), while the solid phase SP-CT-MeCN led to
similar catalytic results to those for run 2 (Fig. 5). These
The PXRD patterns of all the recovered solids indicated that
the crystalline structure of 2 underwent a reorganisation under
the different reaction conditions to give a CB[6] crystal packing
arrangement equivalent to that present in as-synthesized CB[6]
(Fig. S4†). No marked change in morphology occurred accord-
ing to SEM (Fig. S5†). EDS elemental mappings of 2 and all
the recovered solids revealed uniform distributions of Mo, N
and O (Fig. S5†). The EDS-determined N/Mo atomic ratio was
9.4 for 2, and 12.7 for the solid recovered after run 1 of the
reaction 2/Cy8/H2O2/MeCN performed at 70 °C. This may point
to slight metal leaching during the first catalytic run using
MeCN as solvent. However, after run 1, the N/Mo ratio
remained roughly constant (less than 5% variance), which cor-
relates with the steady catalytic activity after run 1. These
results, together with those for the contact test, suggest that
any metal species leached into solution had negligible catalytic
contribution. With ethanol as the solvent (2/Cy8/H2O2/EtOH),
the N/Mo ratio for the used catalyst was 9.2, similar to that for
the original catalyst, suggesting good stability towards metal
leaching in the alcohol media used for one-pot conversions.
The understanding of the behaviour of these types of crystal-
line supramolecular compounds for solid–liquid catalysis is
Fig. 5 Reaction of Cy8 at 70 °C for 24 h using 2/H2O2/MeCN (catalyst
reuse for four runs), the liquid (LP) or solid (SP) phases from the contact
test (CT) for 2 in MeCN medium, and 2/H2O2/EtOH (catalyst reuse for
two runs).
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important, being a subject of investigation in our laboratories, (CH3COCH3) (1826.3): C, 24.86; H, 4.37; N, 18.41. Found: C,
with special focus on the structural and morphological 25.07; H, 4.11; N 18.52. TGA revealed a weight loss of 20.6% up
stability.
to 150 °C [calcd for loss of 19H2O and 0.6(CH3COCH3):
20.7%]. After isolating the precipitate, the mother liquor was
left to stand, which led to the formation of yellow single crys-
tals suitable for XRD.
Conclusions
Catalytic oxidation experiments
In conclusion, we have demonstrated that the macrocyclic cavi-
tand cucurbit[6]uril can be used for the solid-state isolation of The catalytic experiments were performed in tubular glass
the complex [MoO2Cl2(H2O)2] from hydrochloric acid solution. pear-shaped reactors, equipped with a PTFE-coated magnetic
In the resultant crystalline supramolecular compound (2), the stirring bar and a valve, under batch operation mode.
diaqua complexes are embedded between barrel-shaped hydro- Typically, for the oxidation reactions, the reagents were
gen-bonded entities, {CB[6]·10(H2O)}, which are arranged in loaded into the reactor in the molar proportion 1 : 100 : 152
layers. Although the activity of compound 2 for the catalytic (Mo : substrate : oxidant); specifically, 18 μmol Mo, 1.8 mmol
oxidation of cis-cyclooctene is moderate, it has the advantage substrate, 2.75 mmol H2O2 and 2 mL of a solvent. The loaded
of using H2O2 as oxidant, and the selectivity towards the reactor was immersed in a thermostatically controlled oil bath
epoxide is very high (90–100%) for reaction temperatures up to heated at 70 °C (stirring rate of 1000 rpm to avoid mass trans-
70 °C and different solvents. Another advantage is that the fer limitations) which was taken as the instant that the reac-
reaction outcome with an alcohol as solvent can be shifted tion began (zero time).
towards the formation of alkoxy ketone and alkoxy alcohol pro-
Catalyst stability studies involved the reuse of the recovered
ducts by adopting a one-pot strategy comprising an oxidation catalyst, a contact test, and characterisation of the used cata-
stage at 70 °C (with H2O2) followed by an acid-catalysed stage lysts. Compound 2 was used for four consecutive catalytic
at 90 °C. To the best of our knowledge, this is the first report batch runs, under similar reaction conditions (Cy8/H2O2/
of the one-pot Mo-catalysed oxidation of an alkene with H2O2 MeCN, 70 °C, 24 h). Catalyst recovery involved separation of
in alcohol medium to give alkoxy ketone products. While the the solid from the reaction mixture by centrifugation (ca. 3500
reaction of cyclohexene in MeCN gave the diol with 100% rpm for 15 min), thoroughly washing with acetone, drying at
selectivity, the reaction in ethanol gave 2-ethoxycyclohexanol room temperature overnight and then under vacuum at 50 °C
as the main product (93% selectivity) even at the lower temp- for 1 h.
erature of 70 °C. Mechanistic studies using the probable
The CT was carried out to check for the presence of leached
cyclooctyl and cyclohexyl intermediates as substrates indicate active metal species in the liquid phase. First, the original cata-
that epoxide alcoholysis is the most important pathway for the lyst was contacted with MeCN/H2O2 at 70 °C/24 h under
formation of the alkoxy products. The CB[6]/MoVI catalyst pre- similar conditions to those used for a normal catalytic test, but
sented steady performance after run 1, which correlated with without substrate. Subsequently, the mixture was cooled to
the similar characterisation results of the catalyst recovered ambient temperature, centrifuged (3500 rpm), and filtered
from run 2 onwards. Considering the known effectiveness of through a 0.2 μm PTFE membrane to separate the solid and
molybdenum(VI) catalysts for olefin epoxidation, the data pre- liquid phases. The liquid phase (LP) obtained from the CT
sented here encourage a more comprehensive study of MoVI- (LP-CT-MeCN) was transferred to a separate clean reactor and
containing supramolecular compounds as multifunctional cat- the substrate Cy8 was then added to give the same initial con-
alysts for the target integrated catalytic reaction systems.
centration (0.79 M) as that used for a normal catalytic test.
Afterwards, the homogeneous reaction mixture was heated at
70 °C for 24 h (1000 rpm). The undissolved solid obtained
from the CT was washed with acetone, dried overnight under
atmospheric conditions and then under vacuum for 1 h at
50 °C, giving SP-CT-MeCN.
Experimental
Synthesis of adduct 2
[MoO2Cl2(H2O)2]·(C36H36N24O12)·xH2O·yHCl·z(CH3COCH3) (2)
The reactors were cooled to ambient temperature in cold
A suspension of MoO3 (1.20 g, 8.30 mmol) and 6 M HCl water before opening and suitable work-up procedures. The
(10 mL) was heated near the boiling point for 2 h until most of analyses were always carried out for freshly prepared samples.
the MoO3 was dissolved, and then cooled to room temperature Individual experiments were performed for each reaction of
and filtered. A portion (2 mL) of the solution containing the 24 h (experimental error of ca. 5%). The evolution of the cata-
complex [MoO2Cl2(H2O)2] was slowly added to a solution of lytic reactions was monitored by GC using a Varian 3800
CB[6] (0.4 g, 0.32 mmol) in 3.8 M HCl (17.2 mL). The mixture instrument equipped with an Agilent J&W VF-5 ms capillary
was stirred for 2 h at room temperature during which time a column (30 m × 0.25 mm × 0.25 μm), a flame ionisation detec-
colourless or very pale yellow solid precipitated. After allowing tor, H2 as the carrier gas and undecane as internal standard.
the mixture to stand for one day, the precipitate was filtered The reaction products were identified by GC-MS with a Trace
and vacuum-dried. Yield: 0.42 g, 72% (based on CB[6]). Anal. GC Ultra 2000 instrument (Thermo Quest CE instruments)
Calcd for 2[MoO2Cl2(H2O)2]·(C36H36N24O12)·15H2O·1.5HCl·0.6 equipped with a DSQ II (Thermo Scientific) mass selective
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We acknowledge the support of CICECO – Aveiro Institute of
Materials [FCT (Fundação para a Ciência e a Tecnologia) Ref.
UID/CTM/50011/2019], REQUIMTE-LAQV (UID/QUI/50006/
2019), Centre of Marine Sciences – CCMAR (UID/Multi/04326/
2019), and the CENTRO 2020 Regional Operational
Programme (Project CENTRO-01-0145-FEDER-028031; PTDC/
QUI-QOR/28031/2017), co-financed by national funds through
the FCT/MEC and the European Union (EU) through the
European Regional Development Fund under the Portugal
2020 Partnership Agreement. The FCT and the EU are acknowl-
edged for a Ph.D. grant to L. S. N. (PD/BD/109666/2015). The
positions held by M. M. A. and A. C. G. 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 29 August, changed by
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