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Olefin epoxidation by hydrogen peroxide catalysed by molybdenum
complexes in ionic liquids and structural characterisation of the proposed
intermediate dioxoperoxomolybdenum speciesw
Published on 02 July 2010. Downloaded by Gral Universidad Sevilla on 27/08/2015 11:20:28.
Matthew Herbert,a Eleuterio Álvarez,b David J. Cole-Hamilton,c Francisco Montilla*a and
Agustı́n Galindo*a
Received 18th March 2010, Accepted 15th June 2010
DOI: 10.1039/c0cc00462f
The complex [Mo4O16(dmpz)6] (1) has been isolated as part of a
study of oxodiperoxomolybdenum catalysed epoxidation of
olefin substrates with hydrogen peroxide in ionic liquids. Notably,
1 is the first dioxoperoxomolybdenum species to be structurally
characterised.
Over the past few decades important advances have been made
in understanding the mechanisms of peroxomolybdenum
catalysed epoxidation reactions, a topic which has given rise
to some controversy.1 Different mechanisms, particularly
those described by Mimoun2 and Sharpless,3 have been
proposed, though more recent studies have highlighted the
fine relationship between the mechanism and the specific
experimental conditions.4 The chemical nature of the oxidant
reagent is a particularly important determinant. Industrially,
molybdenum catalysed epoxidations used in fine chemical
synthesis typically employ tert-butylhydroperoxide (TBHP).5
The substitution of the organoperoxo oxidants used in such
processes for more environmentally friendly oxidants such as
molecular oxygen6 or hydrogen peroxide7 would be beneficial
for obvious reasons.
We have recently conducted several investigations of olefin
epoxidation by hydrogen peroxide catalysed by simple
molybdenum compounds in ionic liquids (ILs).8 As part
of this we reported on the oxidation of cis-cyclooctene and
other olefin substrates to their corresponding epoxides using
the oxidant-catalyst precursor combination UHP-MoO3
(UHP = urea-hydrogen peroxide adduct) in the ionic liquid
C4mim-PF6.9 Developing upon this preliminary system, we
have subsequently investigated means of controlling the
undesired hydrolytic side reaction, allowing the use of aqueous
hydrogen peroxide as the oxidant, and the influence of
coordinating species on the catalysis.10 Table 1 displays some
preliminary results illustrating the effect of some N-donor
ligand additives.
The epoxidation reaction clearly proceeded more quickly in
the IL media than in chloroform (entries 1, 4 and 7).
Additionally, in the IL media, pyrazoles were found to induce
a
Departamento de Quı´mica Inorgánica, Facultad de Quı´mica,
Universidad de Sevilla, Aptdo 1203, 41071 Sevilla, Spain.
E-mail: [email protected], [email protected]; Fax: + 34 954 557153
b
Instituto de Investigaciones Quı´micas, CSIC-Universidad de Sevilla,
Avda. Américo Vespucio 49, 41092 Sevilla, Spain
c
School of Chemistry, University of St Andrews, North Haugh,
St Andrews, UK
w Electronic supplementary information (ESI) available: Experimental
and computational details, selected structural parameters and
coordinates of computed compounds. See DOI: 10.1039/c0cc00462f
This journal is
c
The Royal Society of Chemistry 2010
complete selectivity for the epoxide and significantly enhance
catalytic activity with respect to both our previously reported
system with UHP9 and systems testing other N-donor species.10
Whilst unsubstituted pyrazole (pz) gave enhanced conversions,
3,5-dimethylpyrazole (dmpz) gave an even more markedly
activated system. With the addition of dmpz, complete
conversion to the epoxide was observed in C12mim-PF6
within the two hours, the optimum result encountered in
the study.10 Furthermore, the system was easily recyclable
and ten subsequent catalytic cycles were run with no decline
in catalytic activity observable if the base species was
replenished between cycles (see ESIw for details). The use of
oxodiperoxopyrazolylmolybdenum complexes as catalysts for
oxidation reactions has a limited number of precedents,
though apparently not in olefin epoxidation. Supported
[Mo(O)(O2)2(H2O)(pz)] has been used to catalyse the heterogeneous oxidation of sulfides,11 and [Mo(O)(O2)2(dmpz)2] has
been previously described as a homogeneous alkylbenzene
oxidation catalyst.12
To further understand the mechanism of the oxygen atom
transfer to the olefin substrate, [Mo(O)(O2)2(pz)2] (2) (X-ray
characterised, Fig. 1z) and [Mo(O)(O2)2(dmpz)2]12 (3) were
investigated as stoichiometric epoxidising reagents, reacting
the complexes with cis-cyclooctene in C8mim-PF6 at a 1 : 2
complex : olefin ratio in the absence of oxidant. Yields
of ca. 50% epoxide were obtained, suggesting that these
oxodiperoxomolybdenum complexes are capable of epoxidising
only 1 equivalent of olefin despite possessing two peroxo
ligands.13,14 After the stoichiometric oxidation of the
cis-cyclooctene, completion of the catalytic cycle can be
demonstrated by reoxidising the metal complex with a small
Table 1 Molybdenum catalysed epoxidations of cis-cyclooctene in
the presence of selected N-donor base additivesa
Entry
Base additive
Solvent
Conversion
Yield
1
2
3
4
5
6
7
8
9
None
Cl3CH
C8mim-PF6
C12mim-PF6
Cl3CH
C8mim-PF6
C12mim-PF6
Cl3CH
C8mim-PF6
C12mim-PF6
17
38
40
16
63
73
23
84
99
1
25
40
3
63
73
8
84
99
Pyrazole
3,5-Dimethylpyrazole
a
Aqueous [Mo(O)(O2)2(H2O)n] 0.025 mmol, base additive 0.10 mmol,
30% H2O2 (aq) 3.0 mmol, cis-cyclooctene 1.0 mmol, solvent 2.0 mL,
T = 60 1C, t = 2 h. Extraction with pentane (3 3 mL), yields and
conversions calculated by GC.
Chem. Commun., 2010, 46, 5933–5935 | 5933
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Fig. 1 Molecular structure of [Mo(O)(O2)2(pz)2] (2) (30% probability
thermal ellipsoids; H atoms have been omitted for clarity). For
selected structural data see ESI.w
excess (1.2 equiv.) of aqueous H2O2 under appropriate reaction
conditions. The oxodiperoxo complex is regenerated and after
eliminating any excess H2O2 further addition of cis-cyclooctene
yields the same result (ca. 50% conversion). Visually, clear
colour changes were apparent both when the complex reacted
with the olefin and when it was regenerated with H2O2 (ESIw).
The reaction profile for the stoichiometric epoxidation of
the model olefin substrate ethylene by 3 calculated through
DFT calculations carried out at the B3LYP level is shown by
Fig. 2. The reaction is exergonic, in common with related
studies.15 The transition structure TS1 adopts the spirocyclic
form proposed by Sharpless3 and theoretically analysed for
[Mo(O)(O2)2(L)] models by several groups.16 y The structural
parameters of TS1 agree well with those of similar species as
previously reported (ESIw). The first intermediate after oxygen
abstraction by the olefin, A, is a dioxoperoxo complex with the
two dmpz ligands occupying approximately cis positions, as
calculated for the parent oxodiperoxo complex 3. Complex A
then isomerises to the second intermediate B, where the two
dmpz ligands are in mutually trans positions, via an energetically
favourable transition.
All efforts to isolate the metal complex remaining following
stoichiometric epoxidation, presumably the dioxoperoxo B,
have so far been unsuccessful. However, in attempting to
crystallise the oxodiperoxo complex 3 a small quantity of
yellow crystals of compound 1 were isolated and characterised
by X-ray diffraction (Fig. 3z). Complex 1, [Mo4O16(dmpz)6],
is a tetranuclear complex consisting of a symmetrical
Fig. 2 Energy profile of the model epoxidation reaction with
[Mo(O)(O2)2(dmpz)2] (3) showing the formation of the intermediate
dioxoperoxo B, with zero point corrected electronic energies and
Gibbs free energies (italics) (kcal mol1).
5934 | Chem. Commun., 2010, 46, 5933–5935
Fig. 3 Actual molecular structure (left, 30% probability thermal
ellipsoids) and optimised structure (right) of [Mo4O16(dmpz)6] (1).
H atoms have been omitted for clarity. For selected structural data see
ESI.w
{Mo(O)(m-O)(O2)(dmpz)}2 binuclear core to which two
[Mo(O)2(O2)(dmpz)2] units are coordinated. The latter are
dioxoperoxomolybdenum species which are stabilised by their
coordination to the neighbouring Mo1 atom via one of the oxo
ligands. The two Mo–Ooxo bonds of the dioxoperoxo species,
Mo2–O5 and Mo2–O8, are found to be similar (1.763(2) and
1.707(2) Å, respectively) whilst being considerably shorter
than the Mo1–O5 bond (2.346(2) Å). The dioxoperoxo species
has been proposed as intermediate in several oxidation reactions
catalysed by molybdenum and has been theoretically analysed
in several studies,15,17 y, but 1 is, to our knowledge, the first
dioxoperoxomolybdenum molecular species to be isolated in
the solid state and experimentally characterised.18,19 Interestingly,
the structural parameters of the computed complex B fit
reasonably well with the experimentally determined values of
the [Mo(O)2(O2)(dmpz)2] units in 1 (ESIw), strongly supporting
the dioxoperoxo formation hypothesis.
The possible mechanism by which 1 had formed was also
investigated by DFT (Fig. 4). The first step involves the
reduction of one of the peroxo ligands of [Mo(O)(O2)2(dmpz)2]
(3) to an oxo, as in complexes A and B. This reaction is
energetically favourable for 3, but not for the mono-dmpz
complex, [Mo(O)(O2)2(dmpz)]. Subsequently, dissociation of a
dmpz ligand from A or B affords the intermediate dioxoperoxo
C. Due to the unsaturation of species C, formation of the
dimer species D is consequently favourable, thus forming the
symmetrical core observed in the experimentally synthesised
complex 1. The structural parameters computed for D agree
well with the experimental values of 1, with the exception of
the pyramidalisation observed at the Mo centre which results
from the absence of any ligand trans to the oxo group. In the
final step compound B coordinates to Mo atom of D, forming
complex 1.
In summary, the [Mo4O16(dmpz)6] (1) complex, which
possesses coordinated dioxoperoxomolybdenum units, was
isolated and structurally characterised during an investigation
into the use of coordinating N-heterocyclic bases to enhance
the oxodiperoxomolybdenum catalysed epoxidation of olefins
with H2O2 in ILs. Dioxoperoxomolybdenum complexes have
been proposed as reduced intermediate species in several Mo
catalysed oxidation processes, but this is the first time that a
complex of this type has been isolated in solid state and
characterised.
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Fig. 4 Energy profiles in the formation of the binuclear intermediate {Mo(O)(m-O)(O2)(dmpz)}2 D (left) and the final complex 1 (right), with zero
point corrected electronic energies and Gibbs free energies (italics) (kcal mol1).
This research was supported by the European Commission
(Marie Curie Training Network MRTN-CT-2004-504005), the
Spanish Ministerio de Ciencia e Innovacion (CTQ2007-61037)
and Junta de Andalucia (Proyecto de Excelencia, FQM-02474).
We would like to thank Professor Ernesto Carmona for
offering fruitful discussion and also CICA and UGRGrid
(University of Granada) for permitting the use of their
computational resources.
Notes and references
z Crystal data for 1: C30H48Mo4N12O162CH2Cl2, M = 1386.42,
monoclinic, a = 14.0998(9) Å, b = 10.0755(6) Å, c = 18.6382(12) Å,
a = 90.001, b = 103.831(2)1, g = 90.001, V = 2571.0(3) Å3, T =
173(2) K, space group P21/c, Z = 2, 57 206 reflections measured, 7927
independent reflections (Rint = 0.0352). The final R1 values were
0.0297 (I > 2s(I)). The final wR(F2) values were 0.0708 (I > 2s(I)).
The final R1 values were 0.0354 (all data). The final wR(F2) values were
0.0735 (all data). Crystal data for 2: C6H8MoN4O5, M = 312.10,
monoclinic, a = 7.5552(3) Å, b = 12.3729(5) Å, c = 11.5778(5) Å,
a = 90.001, b = 104.3810(10)1, g = 90.001, V = 1048.38(7) Å3, T =
173(2) K, space group P21/c, Z = 4, 25 643 reflections measured, 3194
independent reflections (Rint = 0.0281). The final R1 values were
0.0222 (I > 2s(I)). The final wR(F2) values were 0.0576 (I > 2s(I)).
The final R1 values were 0.0231 (all data). The final wR(F2) values were
0.0582 (all data).
y A similar reaction profile to that shown in Fig. 2 was computed
starting instead from the [Mo(O)(O2)2(dmpz)] complex, which has
only one dmpz ligand (see ESIw).
1 D. V. Deubel, G. Frenking, P. Gisdakis, W. A. Herrmann,
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7 Selected examples: (a) K. Schröder, S. Enthaler, B. Bitterlich,
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14 Monoperoxo complexes exhibit higher barriers for direct oxygen
transfer to ethylene. See: (a) C. di Valentin, P. Gisdakis,
I. V. Yudanov and N. Rösch, J. Org. Chem., 2000, 65, 2996;
(b) I. V. Yudanov, C. di Valentin, P. Gisdakis and N. Rösch,
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18 Dioxoperoxo Mo structures have previously been proposed but
X-ray analysis of such a complex has not yet been reported:
(a) C. L. Bianchi and F. Porta, Vacuum, 1996, 47, 179;
(b) S. Tollari, S. Bruni, C. L. Bianchi, M. Rainoni and F. Porta,
J. Mol. Catal., 1993, 83, 311.
19 For related tungsten species see: B. Pecquenard, S. Garcı́a-Castro,
J. Livage, P. Y. Zavalij, M. S. Whittingham and R. Thouvenot,
Chem. Mater., 1998, 10, 1882.
Chem. Commun., 2010, 46, 5933–5935 | 5935