A mononuclear tantalum catalyst with a peroxocarbonate ligand for olefin epoxidation in compressed CO2

Article abstract of DOI:10.1039/c9cy00056a

A new class of tantalum-based peroxocarbonate ionic liquid ([P_4,4,4,4]₃[Ta(η²-O₂)₃(CO₄)]) has been generated through the reaction of pressurized CO₂ with [P_4,4,4,4]₃[Ta(O)₃(η²-O₂)] in the presence of H₂O₂ during the reaction process. The newly formed species has been verified by NMR, FT-IR, HRMS and density functional theory (DFT) calculations. The CO₂-induced monomeric peroxocarbonate anion-based ionic liquid is more advantageous than the monomeric peroxotantalate analogue for the epoxidation of olefins under very mild conditions. Interestingly, the transformation between peroxotantalate and peroxocarbonate species is completely reversible, and CO₂ can actually act as a trigger agent for epoxidation reaction. The further mechanism studies by DFT calculation reveal that peroxo η²-O₂ (site a) affords higher reactivity towards the CC bond than that of peroxocarbonate-CO₄ (site b). These quantitative illustrations of the relationship between structural properties and kinetic consequences enable rational design for an efficient and environmental IL catalyst for the epoxidation of olefins.

Full text of DOI:10.1039/c9cy00056a

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Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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DOI: 10.1039/C9CY00056A
Journal Name
ARTICLE
A mononuclear tantalum catalyst with a peroxocarbonate ligand
for olefin epoxidation in compressed CO2
†
‡
‡
†
†
†
Wenbao Ma, Yunxiang Qiao, Nils Theyssen, Qingqing Zhou, Difan Li, Bingjie Ding,
Received 00th January 20xx,
Accepted 00th January 20xx
,§
,†
Dongqi Wang,* and Zhenshan Hou*
DOI: 10.1039/x0xx00000x
2
A new class of tantalum based-peroxocarbonate ionic liquid ([P4,4,4,4
]
3
[Ta( -O
2
)
3
(CO
4
)]) has been generated through reaction
www.rsc.org/
2
of the pressured CO
2
with [P4,4,4,4
]
3 3 2
[Ta(O) ( -O )] in the presence of H
2
O
2
during the reaction process. The newly formed
-induced
species has been verified by NMR, FT-IR, HRMS and density functional theory (DFT) calculations. The CO
2
monomeric peroxocarbonate anion-based ionic liquid is more advantageous than that of the monomeric peroxotantalate
analogue for the epoxidation of olefins under very mild condition. Interestingly, the transformation between
peroxotantalate and peroxocarbonate species is completely reversible, and CO
2
can actually act as a trigger agent for
2
epoxidation reaction. The further mechanism studies by DFT calculation reveal that the peroxo  -O
2
(site a) affords higher
reactivity towards C=C bond than that of the peroxocarbonate-CO (site b). These quantitative illustrations of relationship
4
between structural properties and kinetic consequences enable rational design for an efficient and environmental IL catalyst
for the epoxidation of olefins.
hydrogenation of olefins and arenes,5 hydroaminoalkylation of
alkenes6 and metathesis of alkanes.7 However, their catalytic
performances for the epoxidation of olefins have rarely been
explored.8
Introduction
Transition metal peroxocarbonates have been studied for some time.
These transition metals mainly include Rh, Fe, Mn, Pt, etc. They
could be prepared through either the reaction between metal
peroxo complexes ([L
dioxide, or the reaction between carbon dioxide complexes of
transition metal and dioxygen. It has been demonstrated that these
compounds are potent candidates for oxygen transfer to oxophiles
like phosphines, olefins and ethers. This unique property provides a
prospective application in catalytic oxidation. However, these
transition metal peroxocarbonates have almost been reported to act
as stoichiometric oxidants rather than as highly active catalysts for
the oxidation reactions.
Nowadays tantalum has a wide range of applications in the
design of catalytic materials. Ta-containing catalysts are increasingly
applied in oxidation reactions, most often in the form of solid
catalysts based on tantalum oxide or nitride. As for homogeneous
systems, the complexes of tantalum have been designed for
promoting some important catalytic transformations including
1
2
Epoxidation of olefins is a very important organic reaction to
n
M( -O
2
)], L=different ligands) and carbon
9
produce glycols, glycol ethers, alkanolamines and polymers. The use
1
c
of H
2
O
2
as an oxidant in the epoxidation process is much more
reduction,
promising because water is the only product of H
2 2
O
2
which is more advantageous than application of environmentally-
harmful oxidants, such as peracids and alkylhydroperoxides.10 The
activation of hydrogen peroxide normally requires a metallic catalyst.
Microporous titanium silicate zeolite (TS-1) has been widely used to
the epoxidation of small olefins with hydrogen peroxide. However,
the limited substrate tolerance might be observed.11 Some
supported Ta/Nb oxide catalyst systems also afforded a moderate to
good catalytic activity for epoxidation.12 Additionally, common
drawbacks of organometallic complexes, such as their structure
instability and the poor recycling performance, largely restrict their
application.13 Thus there still remained a huge challenge of acquiring
high catalytic activity and excellent recyclability under mild condition.
Ionic liquids (ILs) are organic salts which have low melting point.
ILs have many unusual properties, such as low vapor pressure, wide
liquid range, strong polarity, inherent designer nature and
recyclability. ILs have realized not only a wide popularity as “green”
solvents for synthesis chemistry, but also as powerful catalysts,
3
4
†
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis,
East China University of Science and Technology, Shanghai 200237, People’s
Republic of China.
‡
Max-Planck-Institut fꢀr Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mꢀlheim
an der Ruhr, Germany.
§
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049,
electrolytes, precursors or additives in some important areas of
research as well as in industrial applications.14 Functionalized ILs are
generally named “task-specific ionic liquids” (TSILs), which are
China
*Z.H.: tel, +86-21-64251686; e-mail, houzhenshan@ecust.edu.cn.
The Supporting Information is available free of charge on the website. Experimental
section, Characterization of the ionic liquid catalysts (HRMS), additional
experimental results, and calculated structures, Mulliken atomic charges, bond
length and Mayer bond order (PDF).
prepared through the incorporation of functional groups into the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or lgo i ng ys
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ARTICLE
Journal Name
cation or anion of ILs. The functionalized ILs can impart tailored These results indicated clearly that the peroxotantaVileawteA-rtbicaleseOdnlinIeL
2
DOI: 10.1039/C9CY00056A
2
capabilities to ILs for a specific application, which have attracted a [P4,4,4,4
]
3
[Ta( -O
2 4
) ] indeed could react with CO to form a new Ta-
considerable attention.15
peroxocarbonate species.
Recently, we have developed the monomeric peroxotantalate
Next, FT-IR spectra of the new formed IL were presented
2
anion-functionalized IL catalysts. The resultant IL catalysts according to following procedures. 0.5 mmol of [P4,4,4,4]3[Ta(O) ( -
3
2
(
[P4,4,4,n
]
3
[Ta(O)
3
( -O
2
)], P4,4,4,n=quaternary phosphonium cation,
2
O )] was mixed with 1 mL methanol and 5 mmol aqueous hydrogen
n=4, 8 and 14) have shown an excellent catalytic activity and peroxide. After stirring for 10 min at 0 C, diethyl ether was poured
selectivity for the epoxidation of allylic alcohols.16 However, they into the solution rapidly to precipitate the oxidizing species
showed a very poor catalytic activity for the epoxidation of olefins. ([P4 ] [Ta(
2
-O ) ]).16 This was then carefully washed with 5 mL
,4,4,4 3
2 4
In this work, we reported that a new tantalum complex containing a cold diethyl ether in a sequential manner. After that, the oxidizing
peroxocarbonate ligand can be formed in-situ by reaction of species was characterized by in-situ FT-IR spectra in an atmosphere
2
[
P
4,4,4,n
]
3
[Ta(O)
3
( -O
2
)] with compressed CO
2
in the presence of H
2
O
2
.
2
of compressed CO for the sake of comparison. The spectra are
-1
The resulting Ta-peroxocarbonate ILs have been characterized in shown in Fig. 2. The band at about 1655 cm in KHCO3 was
detail by experimental methods including NMR, IR and HRMS as well characteristic for the ν(C=O) of HCO3- (Fig. 2a). No obvious
as DFT calculations. The Ta-peroxocarbonate ILs showed an adsorption band around 1500 and 2000 cm was observed for gas
outstanding activity for the epoxidation of olefins, and CO seemed CO2 (Fig. 2b), which was coincident with that reported in the
to act as a trigger reagent here. It was found that the transformation literature.21 Moreover, the homoleptic complex ([P4,4,4,4]3[Ta(
-O ) ])
20
-1
2
2
2 4
between Ta-peroxocarbonate and peroxotantalate anions was showed a band at 1644 cm-1, which could be attributed to the
reversible completely, leading to an excellent regeneration of IL adsorption of water molecules on the IL catalyst (Fig. 2c).22 When the
catalysts. To the best of our knowledge, this is the first report about
Ta-peroxocarbonate complexes and their high catalytic activities
towards epoxidation.
2
3 2 4 2
oxidizing species ([P4,4,4,4] [Ta( -O ) ]) was treated with 2.0 MPa CO
for 1 min in-situ, two new bands at 1683 cm-1 and 1282 cm-1
appeared and increased with time. These could be attributed to the
ν(C=O) and ν(C-O), respectively which resulted from the vibration of
peroxocarbonate species (Fig. 2c-2f).1f,1i
Results and Discussion
4
9.0 (CD OD)
Herein we reported the preparation of in-situ generated tantalum-
based peroxocarbonate ILs derived from peroxotantalate based ILs.
They were prepared in two steps according to the previously
3
a
2
00 180 160 140 120 100 80
60
60
40
40
20
20
0
0
16
reported method. Firstly, [P4,4,4,n]OH were obtained from [P4,4,4,n] X
125.9
-
-
(
X=Cl or Br ) and KOH by ionic exchange, where [P4,4,4,n] represents
b
quaternary phosphonium cations. Then peroxotantalate-based ILs
2
00 180 160 140 120 100 80
169.8
2
(
[
[P4,4,4,n
P
]
3
[Ta(O)
3
( -O
2
)]) were prepared via the reaction of
[Ta(O ] in H O. After that, tantalum-based
160.9
-
4,4,4,n]OH with (NH
4
)
3
2
)
4
2
c
peroxocarbonate IL catalysts were obtained in situ by reaction of the
homoleptic peroxocompound with compressed CO
In order to identify the structure of newly formed species in a
compressed carbon dioxide atmosphere, the resulting species has
been characterized in-situ by high-pressured 13C NMR spectroscopy
2
2
2
00 180 160 140 120 100 80
60
60
60
40
40
40
20
20
20
0
0
0
2
.
169.8
160.9
d
00 180 160 140 120 100 80
169.8
5
7.2
(
0
CO
Fig. 1). The 13C NMR spectrum of [P4,4,4,4
.05 mL 30% H O (molecular ratio H O
2 2 2 2
]
3
[Ta(O)
/ IL = 6) in the absence of
2
was shown in Fig. 1a. The peaks between 10 and 30 ppm were
3 2
( -O )] containing
2
e
00 180 160 140 120 100 80
Chemical shift
attributed to quaternary phosphonium cations.17 In addition, the
Fig. 1 13C NMR spectra of peroxotantalate-based ILs in CD OD (49.0
3
2
0
.05 mL 30% H
showed only a signal of dissolved carbon dioxide molecules at about the absence of CO
25.9 ppm (Fig. 1b). Nevertheless, the introduction of the presence of 1.0 MPa CO
2
O
2
in CD
3
OD solution in the presence of 1 MPa CO
2
ppm): (a) [P4,4,4,4
]
3
[Ta(O)
3
( -O
2
)] containing 0.05 mL 30% H
2
O
2
in
in
2
. (b) Sole CD
3
OD containing 0.05 mL 30% H O
2 2
2
1
2
. (c) [P4,4,4,4
]
3
[Ta(O)
in the presence of 1.0 MPa CO
)] containing 0.05 mL 30% H in the
. (e) Cyclooctene was added to the CD OD
-,18 while the latter one is assigned to the peroxocarbonate solution (d) untill the epoxidation reaction was completed. The
3
( -O
2
)] containing
2
[
P
4,4,4,4
]
3
[Ta(O)
3
( -O
2
)] into CD
3
OD solution containing 0.05 mL 30% 0.05 mL 30% H
2
O
2
2
. (d)
2
H
2
O
2
in the presence of 1.0 MPa CO
2
led to the appearance of two [P4,4,4,4
] [Ta(O)
3
3
( -O
2
2 2
O
new peaks at 160.9 and 169.8 ppm. The former one is attributed to presence of 2.0 MPa CO
HCO
species (Fig. 1c). Moreover, when the pressure of carbon dioxide signal at 169.8 ppm arisen from Ta-peroxocarbonate species was
was increased to 2 MPa, the peak of dissolved carbon dioxide could still observed clearly and peak at 57.2 ppm was contributed to the
2
3
3
1i
again be observed at 125.9 ppm (Fig. 1d). It was noting that the peak resonance signal of cyclooctene oxide.
-
at about 158.4 ppm ascribed to HCO
4
species was not observed
under the present conditions, likely due to low H
2
O
2
concentration.19
2
| J. Name., 2012, 00, 1-3
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or lgo i ng ys
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ARTICLE
tabulated in Table 1. The geometries of these species are collected in
1655
View Article Online
)3- by H
DOI: 10.1039/C9CY 02 0056 A3 -
3 2 2 2 2 4
(η -O O to produce Ta(η -O )
a
b
2
Fig. S2. The oxidation of TaO
was calculated to be a strongly exothermic process. In the CO
atmosphere, the tantalum tetraperoxide may accommodate a
molecule of CO
to afford the Ta-peroxocarbonate anion (Ta(η2-
2
1
683 1644 1282
2
3-
c
d
O
2
)
3
(CO
4
) ) with a loss of free energy of 21.04 kJ/mol (Table 1), which
2
)3-, TaO
(η2-
may further react with H
)(CO (CO
)3- and TaO
2
O
2
to give TaO(η -O
2
)
2
(CO
4
2
O
2
4
3
4
)3- in a stepwise manner. According to
e
f
calculations, it is also possible for a second molecule of CO to insert
2
into the Ta-peroxocarbonate anion, but with much less
exothermicity than the above-mentioned channels. This agrees with
the experimental measurements that the Ta-biperoxocarbonate
species were not detected. As a result, according to the above
3500
3000
2000
1500
-
1
Wavenumber (cm )
3
Fig. 2 In situ high-pressured FT-IR spectra of (a) KHCO in the
experimental and theoretical results, the new tantalum-containing
absence of CO
2
. (b) 2.0 MPa CO
2
in the absence of catalyst. (c)
2
species produced from the oxidation of [P4,4,4,4
]
3
[Ta(O)
3
( -O
2
)] by
2
2
[P
4,4,4,4
]
3
[Ta( -O
2
)
4
] in the absence of CO
after 1 min. (e) [P4,4,4,4
2
. (d) [P4,4,4,4
]
3
[Ta( -O
2
)
4
]
H
2
O
2
in our experiment under high pressure CO
2
atmosphere is
2
under 2.0 MPa CO
MPa CO
after 5 min.
2
]
3
[Ta( -O
] under 2.0 MPa CO
2
)
4
] under 2.0
2
3 2 3 4
proposed as [P4,4,4,4] [Ta( -O ) (CO )] (Scheme 1).
2
2
after 3 min. (f) [P4,4,4,4
3
] [Ta( -O
2
)
4
2
1
0
,0
,8
1,0
run 1
run 2
0,8
2
Then the IL catalyst ([P4,4,4,4
]
3
[Ta(O)
3
( -O
2
)]) was stirred for 20
0,6
0,4
0,6
0,4
0,2
0,0
min in methanol containing hydrogen peroxide under an CO
atmosphere of 2.0 MPa at 0 C. After that, the solution was analyzed
2
0
,2
2
0,0
by HRMS (Fig. S1). The ESI-MS of [P4,4,4,4
with 2 MPa CO
corresponding to [TaO
]
3
[Ta( -O
2
)
4
] in CH
3
OH dealt
0
10
20
30
40
0
10
20 30
t (min)
40
t (min)
1,0
1,0
0,8
0,6
0,4
0,2
0,0
2
afforded an ion centered at 306.9283 m/z
run 3
run 4
]- species, which was probably due to
0,8
(CO )H
3 4 2
0,6
0,4
0,2
0,0
the decomposition of the labile peroxo bonds of intermediate during
HRMS experiments.
Based on the above result, we can conclude that the active
0
10
20
t (min)
30
40
0
10
20
30
40
2
intermediate [P4,4,4,4
]
3
[Ta( -O
2
)
4
] could react with carbon dioxide to
t (min)
2
afford a new species. In order to identify further how much CO
2
can Fig. 3 CO
2
absorption recycles of [P4,4,4,4
]
3
[Ta( -O
2
)
4
]. In each run,
2
be incorporated into the Ta-peroxocarbonate IL, [P4,4,4,4
]
3
[Ta( -O
2
)
4
]
2
CO was absorbed at 0 C and 3.0 MPa, and desorbed at 50 C under
(
2.752 g, 2.5 mmol) was purged continuously with 3 MPa CO
2
at 0 C. the vacuum.
The amount of CO
2
absorbed into the IL was determined at regular
intervals by the electronic balance with an accuracy of 0.1 mg. The
results are shown in Fig. 3 (run 1). It was found that the weight of
ionic liquid increased gradually with time until the absorption
capacity of CO reached up to 2.3 mmol. After that, the resulting Ta-
2
peroxocarbonate IL was dealt under reduced pressure condition at
Catalytic Performance
As reported previously, the peroxocarbonate complexes are
potential oxidants in general, which could cause oxygen transfer
from the peroxocarbonate moiety to the oxophiles. Therefore, the
o
[Ta(2- catalytic activity of the new Ta-peroxocarbonate species has been
5
0 C for 2 h, and then was reactivated with H
2
O
2
. The [P4,4,4,4
]
3
O
2
)
4
] was purged continuously with 3 MPa CO
2
again and the results examined under a variety of reaction conditions. The addition of CO2
are shown in Fig. 3 (run 2). Similar steps were repeated another two has a tremendous effect on the reaction. As shown in Fig. 4a, no
times. It can be seen that the molar ratio of the absorbed CO
2
to cyclooctene conversion was observed in the absence of CO with
2
2
2
[
P
4,4,4,4
]
3
[Ta( -O
2
)
4
] was equivalent to ca. 1, indicating that the in- [P4,4,4,4]3[Ta(O) ( -O )] catalyst. However, the conversion of
3
2
situ prepared Ta-peroxocarbonate anionic species only contained cyclooctene increased sharply with pressure and achieved a
one peroxocarbonate ligand, which was also in good agreement with maximum value of 87.2% around 2.0 MPa CO , which was about
2
that of HRMS. The amount of CO absorption by IL did not decreased twice as much than that the yield at atmospheric pressure (40.2%).
2
obviously in the whole recycles (Fig. 3), indicating that the structural A further pressure increase over 2.0 MPa caused a lower conversion
transformation between Ta-peroxocarbonate and peroxotantalate of cyclooctene. It’s noting that there was no phase separation when
anions is completely reversible.
the pressure was below 4.0 MPa. Therefore a lower conversion could
The potential formation of Ta-peroxocarbonate and be explained by a substrate dilution effect in the CO -swollen liquid
2
peroxotantalate anions and their relative thermodynamic stabilities reaction phase.23 Furthermore, the effect of temperature on
were further evaluated by DFT calculations. Starting from TaO
3
(η2-
3-
2
O ) , these species were considered to be produced via the channels
This journal is © The Royal Society of Chemistry 20xx
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Page 4 of 10
ARTICLE
Journal Name
1
00
100
80
60
40
20
0
Table 1 The Reaction (free) energies (in kJ/mol) in the formation of
possible Ta-peroxocarbonate and peroxotantalate anionic species
a
b
View Article Online
DOI: 10.1039/C9CY00056A
80
via oxidation or CO
2
insertion reaction
6
4
2
0
0
0
0
Reactions
ΔEbond
ΔG
2
)3- + 3H
2
3-
TaO
3
(η -O
2
2
O
2
 Ta(η -O
2
)
4
+ 3H
)3-
(CO
2
O
-237.58 -233.19
-77.07 -21.04
)3-+ -14.30 -51.06
2
3-
2
Ta(η -O
2
)
)
4
3
+ CO
2
 Ta(η -O
2
)
3
(CO
4
2
)3-
2
Ta(η -O
2
(CO
4

TaO(η -O
2
)
2
4
0
1
2
3
4
20
30
40
50
o
60
P (MPa)
Temperature ( C)
1
/2O
2
Fig. 4 (a) Effect of CO
Reaction condition: 2.0 mmol cyclooctene, 2.0 mmol H
[Ta(O) (-O )], 2.5 mL CH OH, 40 C. (b) Effect of
temperature on epoxidation of cyclooctene. Reaction condition:
2.0 mmol cyclooctene, 2.0 mmol 0.05 mmol
[Ta(O) (-O )], 2.0 MPa CO , 2.5 ml CH
2
pressure on epoxidation of cyclooctene.
2
)3-
TaO(η2-
Ta(η -O
O
Ta(η -O
O
2
)
3
(CO
4
+
H
2
O
2


2 2
O , 0.05
3
-
2
)
2
(CO
4
) + H
2
O + O
2
-107.98 -167.03
-221.23 -330.25
-345.73 -493.43
2
)3-
(η2-
TaO
2
mmol [P4,4,4,4
]
3
3
2
3
2
)
3
(CO
4
+
2H
2
O
2
3
-
2
)(CO
4
) + 2H
2
O + 2O
2
2
)3- + 3H
)3-
2 2
H O ,
Ta(η -O
2
)
3
(CO
4
2
O
2
 TaO
3
(CO
4
+
[P
4,4,4,4
]
3
3
2
2
3
OH.
3
H
2
O + 3O
2
2
)3- + CO
2
3--I -72.64 -20.66
3-- -71.14 -18.78
Ta(η -O
Ta(η -O
2
)
)
3
(CO
(CO
4
4
2
 Ta(η -O
2
)
2
(CO
4
)
2
2
)3- + CO
2
TBHP under the similar reaction conditions or slightly higher reaction
temperatures (Table S2, entries 1-3). This indicated that the present
Ta-peroxocarbonate
hydroperoxide in the desired manner under the present conditions.
Moreover, the epoxidation of cyclooctene was investigated
further by variation of the catalyst structure (Table 2). In the absence
of catalysts (entry 1), almost no conversion of cyclooctene was
2
3
2
 Ta(η -O
2
)
2
(CO
4
)
2
II
TaO
TaO
2
)3- + CO
)3- + CO
3--I
3--II
species
cannot
activate
tert-butyl
2
(η -O
2
)(CO
)(CO
4
4
2
2
 TaO
 TaO
2
(CO
(CO
4
)
)
2
2
-46.04
7.61
2
2
(η -O
2
2
4
-71.14 -18.78
observed. The same holds for [P4,4,4,4]Br (entry 2) and KHCO
), indicating that the catalytic activity originates from the tantalum-
containing anionic part. It was worth noting that the epoxidation did
3
(entry
3
not proceed at all without CO
2
or under 2.0 MPa N
2
atmosphere in
2
2
Scheme 1 The structure evolution of [P4,4,4,4
]
3
[Ta(O)
3
( -O
2
)] in the the presence of [P4,4,4,4]3[Ta(O) ( -O )] (entries 4 and 5). In contrast,
3
2
presence of hydrogen peroxide under compressed CO
2
.
the introduction of 2.0 MPa CO to [P
2
2
] [Ta(O) ( -O )] solution
2
4,4,4,4 3
3
resulted in a fantastic increase of conversion up to 87.2% (entry 6).
the reaction has also been investigated and the result was shown in (NH ) [Ta(O ) ] showed a poor solubility in methanol, resulting in a
4
3
2 4
Fig. 4b. It can be seen that a maximum conversion of cyclooctene was liquid-solid biphasic system, which could account for a poor
achieved at 40 C. At a higher temperature, the conversion of cyclooctene conversion (entry 7). A
cyclooctene decreased, probably due to decomposition of H
and/or active Ta-peroxocarbonate species. This reveals that both the Table 2 Epoxidation of cyclooctene with different catalysts in the
presence of CO
and appropriate reaction temperature are very presence of the pressured CO2a
2 2
O
2
crucial for the improvement of activity.
Con.
Sel./%
entry
catalysts
None
[P4,4,4,4]Br
/%b epoxide othersc
The epoxidation of cyclooctene was performed in different
reaction media in order to examine the influence of different
solvents. A poor conversion has been obtained in dichloromethane
and ethyl acetate (Table S1, entries 1 and 2), which was probably due
to the biphasic state during the course of reaction. The reaction also
displayed a low activity in acetonitrile (Table S1, entry 3). However,
the conversion of cyclooctene increased significantly up to 42.3% and
1
2
3
4
5
6
7
8
9
1.1
2.3
3.4
2.3
2.0
≥ 99
≥ 99
≥ 99
≥ 99
≥ 99
≥ 99
≥ 99
≥ 99
≥ 99
< 1
< 1
< 1
< 1
< 1
< 1
< 1
< 1
< 1
KHCO
[Ta(O)
[Ta(O)
3
3
3
2
)]d
[P4,4,4,4
[P4,4,4,4
[P4,4,4,4
]
]
3
3
( -O
2
2
)]e
( -O
2
2
]
3
[Ta(O)
3
( -O
2
)] 87.2
4.5
)] 78.0
8
7.2%, in ethanol and methanol, respectively (Table S1, entries 3-5),
(NH
4
)
3
[Ta(O
2
)
4
]
showing that the present epoxidation reaction is favored in strong
polar protic solvents. Previous reports from our group and from
other authors have demonstrated that the hydrogen bonding
interaction between polar protic solvents and the peroxometalate
specie activates the peroxo bond, resulting in a lower barrier for
epoxidation.24 Futhermore, tert-butyl hydroperoxide (TBHP) has also
been tested as an oxidant. The results showed that the epoxidation
of olefins did not proceed with
2
[P4,4,4,8
]
3
[Ta(O)
3
( -O
2
2
[P4,4,4,14
]
3
[Ta(O)
3
( -O
2
)] 64.9
aReaction conditions: substrate (2 mmol), H
O (2.0 mmol, 30%
2 2
aqueous solution), catalyst (0.05 mmol), 40 C, 12 h, 2.5 mL
. GC conversion with dodecane as an
0 0 0
internal standard. Conversion=([R ]-[R])/[R ], [R ] refer to the
initial concentration of cyclooctene, [R] refer to the
concentration after the reaction. cRefer to cyclooctanediol.
b
CH
3
OH, 2.0 MPa CO
2
dwithout CO . e2 MPa N
2 2 2
instead of 2 MPa CO .
4
| J. Name., 2012, 00, 1-3
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or lgo i ng ys
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ARTICLE
2
moderate to good activity could be achieved using Table 3 Epoxidation of different substrates with [P4,4,4,4
]
3
[Ta(O)
3
( -
View Article Online
2
2
atmosphere of 2.0 MPaa
DOI: 10.1039/C9CY00056A
[
P
4,4,4,8
]
3
[Ta(O)
the pressured CO
structure of the phosphonium cation has an additional influence on
the catalytic activity. The addition of CO may affect the acidity of the
3
( -O
2
)] and [P4,4,4,14
]
3
[Ta(O)
3
( -O
2
)] catalysts under
O
2
)] under a CO
2
2
(entries 8 and 9), which showed that the decent
t
T
Con.
Sel./%
entries
substrates
/C /h
/%b epoxide others
2
1
40 12 87.2
40 12 98.6
≥99
≥99
<1
<1
reaction mixture. It has been reported that acidity could exert an
impact on the catalytic performance of transition metal peroxo
complexes.25 Nevertheless, the control experiments indicated that
almost no conversion of cyclooctene was observed even if 0 to 0.03
2c
3
3
mmol HNO was added to reaction system. This revealed that the
40 12 88.7
40 12 96.8
95.0
5.0
<1
acidity wasn’t a decisive factor in the present epoxidation reaction.
Sequentially, the reaction kinetics was investigated in order to
gain an insight into the pathways of hydrogen peroxide activation
and olefin epoxidation. The effects of stirring speed and catalyst
amount on the reaction rate have been screened to exclude the
presence of diffusion limitations prior to kinetic studies. The results
are shown in Fig. S3. A stirring of no less than 800 rpm was needed.
If the molar ratio of substrate and IL catalyst (nsub./ncat.) was less than
4
≥99
5d
40 12 67.5
40 12 65.4
40 12 50.2
67.0
64.2
59.6
33.0
35.8
40.4
d
6
4
0, the conversion of cyclooctene remained almost constant.
With regard to the results of other groups and our previous
7d
8
9
40 12
9.7
≥99
≥99
<1
<1
research, the rate of epoxidation of olefins showed a linear
dependence on the concentration of substrate and independence on
the concentration of hydrogen peroxide at low concentrations.16,26
Next, the reaction kinetics of the catalyst were investigated and the
activation energy was attained. The results are shown in Fig. 5, where
the reaction rate constant was determined from the experimental
data by assuming pseudo-first-order reaction kinetics. The activation
energy (Ea) of the catalyst under the present condition is about 43.4
40 12 28.1
1
0e
0
0
1.5 98.9
3.5 96.7
≥99
≥99
<1
<1
OH
OH
1e
1
1
1
2e
3e
0
0
3.5 94.2
5.5 72.5
≥99
≥99
<1
<1
OH
OH
-1
a
2 2
Reaction conditions: substrates (2.0 mmol), H O (2.0 mmol, 30%
kJmol , which is indeed lower than previously reported values for
metal-based catalysts where activation energies are in the range of
aqueous solution), catalyst (0.05 mmol), 40 C, 2.5 mL of CH
3
OH, 2
b
9-100 kJmol- .
1 27
MPa CO . GC conversion with dodecane as an internal standard.
4
2
cH (2.4 mmol, 30% aqueous solution). dThe corresponding
In the next step, a series of olefins were tested in order to
2 2
O
aromatic aldehydes (benzaldehyde, p-tolualdehyde, and p-
chlorobenzaldehyde) have been detected as the main by-products.
eSolvent-free condition.
examine the substrate scope of the present IL catalyst. The results
were summarized in Table 3. Conversions between 87.2% and 96.8%
were obtained in the epoxidation of cyclic olefins (Table 3, entries 1-
4
). An even higher conversion of about 98.6% could be
2 2
reached if slightly more H O was used (entry 1 vs 2). In addition, the
9
7
6
4
3
1
0
5
0
5
0
5
0
3.2
catalyst also transformed vinyl arenes into epoxides, although a small
amount of benzylic aldehydes (benzaldehyde, p-olualdehyde, and p-
chlorobenzaldehyde) were detected as the main by-products (entries
5-7). The catalyst also afforded an excellent selectivity for
epoxidation of linear terminal alkenes in spite of a low reactivity
(entry 8). A slightly higher reactivity can be observed for linear
internal alkenes (entry 9), which is consistent with the mechanism
involving facile transfer of electrophilic oxygen from the active
peroxo species of the IL catalyst to electron-rich olefins to afford
epoxides. Notably, the IL catalyst system also displayed an excellent
catalytic activity and selectivity for the epoxidation of allylic alcohols
a
b
2
2
2
1
.8
.4
.0
.6
3
3
3
2
23K
13K
03K
93K
1.2
0
2
4
6
8
10 12 14 16 18
3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45
t (h)
1/T (1/K)
Fig. 5 (a) Conversion/time profiles of epoxidation reactions
catalyzed by [P4,4,4,4
Arrhenius plots for the epoxidation reaction of cyclooctene and
. The observed rate constants (k) were calculated with the
initial rates by Fig. 5a at different temperatures. Reaction
2
]
3
[Ta(O)
3
( -O
2
)] at different temperatures. (b)
even at 0 C (entries 10-13). The amount of residual H
measured. For example, a residue of 2%-5% (entries 1-4 and 10-13),
7%-21% (entries 5-7) and 38%-45% (entries 8 and 9) initial H can
be found after reaction, respectively. These observations indicated
that those electron-rich olefins indeed owned high H utilization
2 2
O was also
2 2
H O
1
2 2
O
conditions: cyclooctene (2.0 mmol),
H
2
O
2
(2.0 mmol),
OH.
2 2
O
2
[P
4,4,4,4
]
3
[Ta(O)
3
( -O
2
)] (0.05 mmol), 2.0 MPa CO2, 2.5 mL CH
3
efficiency. Compared to the reported Mn/bicarbonate catalyst
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or lgo i ng ys
Page 6 of 10
ARTICLE
Journal Name
system,13g the present system afforded a higher TOF and oxidant formation of C-O bonds and afford the epoxide prVoiedwuAcrtti.cleInOntlhinee
utilization efficiency.
reaction at the site a, the π orbital of the C=CDbOoI:n1d0.i1n0t3e9r/aCc9tsCwY0it0h05th6Ae
2
2
The recyclability of [P4,4,4,4
]
3
[Ta(O)
or N and the results are shown in Fig.
. It was found that the IL catalyst maintained high catalytic activity
for at least five cycles in the compressed CO . However, once CO
was replaced by N in an alternating manner, the conversion
3
( -O
2
)] has been examined σ* orbital of η -O
2
ligand to demolish the O-O bond (Table 4).
alternately in pressured CO
2
2
6
100
2
2
8
6
4
0
0
0
2
decreased by at least one order of magnitude each time under
otherwise identical reaction conditions. This interesting observation
revealed that CO
2
could indeed act as a trigger reagent for such
2
epoxidations. The conversion between [P4,4,4,4
]
3
[Ta( -O
2
)
4
] and
With CO2
With N2
2
20
0
[
3
P
4,4,4,4
]
3
[Ta( -O
2
)
3
(CO
4
)] is fully reversible as shown previously in Fig.
)]3- anion is believed to be the catalytically
2
, and the [Ta( -O
2
)
3
(CO
4
active species here.
1
2
3
4
5
6
7
8
9
10 11
Run
Fig. 6 Recyclability of the [P4,4,4,4 [Ta(O)
presence of the pressured CO or N
2 mmol), H (2 mmol, 30% aqueous solution), [P4,4,4,4
)] (0.05 mmol), 2.5 mL CH OH, 12 h, 40 C, 2.0 MPa CO
, 5, 7 and 9; 2.0 MPa N at run 2, 4, 6, 8 and 10.
2
]
3
3
( -O
2
)] catalyst in the
Reaction Mechanism
2
2
. Reaction conditions: substrate
[Ta(O)
(2-
at run 1,
On the basis of the above research, it can be concluded that the
(
O
3
2
O
2
]
3
3
2
)]- is easily converted into [Ta(-
and compressed CO . In principle,
there are two types of sites that potentially insert one O atom into
the C=C bond to give the epoxide product: the peroxo -O (site a)
and the peroxocarbonate-CO (sites b), i.e. the epoxidation may
proceed along the two pathways as shown in (1) and (2):
parent IL anion [Ta(O)
3
( -O
2
2
3
2
3-
O
2
)
3
(CO
4
)] in the presence of H
2
O
2
2
2
2
Table 4 Frontier orbitals (isovalue 0.05a.u.) and relative (free)
4
-1
energies (in kJ mol ) for the epoxidation at sites a and b. Data in
parentheses are Gibbs Free Energy changesa
Site a
Site b
2
)3- + C
)3- + C
2
)3- + C
(
(
site a) Ta(η -O
2
)
)
3
(CO
(CO
4
8
H
H
14  TaO(η -O
2
)
2
(CO
4
8
H
14
O
(1)
(2)
HOMO
LUMO
HOMO
LUMO
2
2
)3- + C
site b) Ta(η -O
2
3
4
8
14  Ta(η -O
2
)
3
(CO
3
8
H
14
O
In order to elucidate this, density functional theory was used to
RCb
locate the stationary points along the pathways for the two sites. As
can be seen in Fig. S4 for both pathways, the C=C bond of the
cyclooctene substrate does not interact with the active sites directly.
When approaching the transition states (TSa and TSb), the length of
the peroxo bond that deliver one of its O atom to the cyclooctene
increases by 0.336 (TSa) and 0.305 (TSb) Å. The migrating O atom (O6
in TSa, and O1 in TSb) attaches to the C=C bond more symmetrically
in TSa than in TSb, suggesting a higher propensity for the reaction at
site a to happen synchronously than that at site b.
TSb
PCb
Energetically, the activation energy of pathway a was calculated
-1
-1
to be 75.4 kJ mol , which is 39.1 kJ mol lower than that to the
pathway b (Table 4). We note that these values are larger than the
experimentally measure value (43.4 kJ/mol). This deviation is
understandable for two reasons: One might be an insufficient
description of the solvent effect and the other is believed to appear
on the sole calculation of this single elementary reaction step only.
Mulliken population analysis was carried out to further
understand the energetic difference between the two pathways. As
can be seen in Table 4, in the reactant state, the HOMO for site a is
constituted by the π* orbitals of peroxide ligands that coordinated
with Ta in a η-manner. The LUMO is determined to be the σ* orbital
of the peroxo group of the carbonate peroxide ligand. When the
≠
≠
ΔG (ΔE )
75.4 (86.6)
-148.3 (-135.9)
114.5 (108.0)
-130.3 (-140.9)
ΔG (ΔE)
a
ΔE = ETS - ERC; ΔE = EPC - ERC; ΔG = GTS - GRC, ΔG = GPC - GRC. bThe
≠
≠
atoms of the cyclooctene substrate in RC, TS and the corresponding
epoxide product in PC which are far away from the reaction center
and have no contribution to the HOMO and LUMO orbitals and
therefore not shown for simplicity.
Table 5 Bond energy decomposition (kJ/mol) of TSa and TSb
substrate accesses site b, the π orbital of the C=C bond interacts with
2
)3-. The reorganization of the
the LUMO orbital of Ta(η -O
2
)
3
(CO
4
frontier orbitals cleaves the σO-O and πC=C bonds to facilitate the
6
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View Article Online
DOI: 10.1039/C9CY00056A
Steric interaction
Pauli Elec.
Orbital interaction
Solvation
screening
Sum
Kinetic
Coulomb XC
Sum
c+da
TSa
TSb
Δ(a-b)
6.26E+4 -1.46E+4 4.80E+4 -1.90E+5 1.17E+5 5.34E+3 -6.71E+4 -1.76E+3
6.25E+4 -1.46E+4 4.79E+04 -1.88E+5 1.16E+5 5.25E+3 -6.71E+4 -1.71E+3
8.24E+0
8.15E+0
0.08
124.67 -41.57
83.10
-1034.29 878.60
92.09
-63.61
-49.51
a
The sum of contributions of cavitation and dispersion.
2
]3- in the excess of hydrogen
Bond energy decomposition of the two transition states tetraperoxotantalate species [Ta( -O
2
)
4
according to Morokuma28 and Ziegler , as shown in Table 5, peroxide, which could accommodate one molecule of CO
29
2
indicates that the reaction at site a benefits more from orbital to generate the tantalum-based peroxocarbonate species [Ta(2-
interaction and solvation while feels more impedance from the steric O ) (CO )]3- under compressed CO2. The Ta-peroxocarbonate
2
3
4
interaction, mainly by the Pauli repulsion. This is consistent with the complexes were identified as the active species for olefin
orbital analysis, as seen in Table 4, where there is more overlap epoxidation. There are two types of sites (e.g., 
2
-O22- and the CO4
2
)
between the σ* orbital of the peroxo moiety and the π orbital of
cyclooctene in the TS along the reaction at site a than at site b. The
composite effect of these interactions moderately favor the
epoxidation of cyclooctene to happen at the site a.
3-
2 3 4
in the tantalum-based peroxocarbonate anion ([Ta(-O ) (CO )] )
that are potentially able to insert one O atom into the C=C bond to
give the epoxide product. However, according to the DFT calculations
2
2
and experiments, the peroxo site  -O afforded a higher reactivity
towards the epoxidation of the C=C bond. As a result, the epoxide is
We have also analyzed the atomic charges of key atoms and bond
2
)]3-. As shown in Fig. S5 and
properties of key bonds in [Ta( -O
2
)
3
(CO
4
produced via a three-membered ring transition state. [TaO(2-
2
S6, upon the insertion of CO into the Ta-O bond, the atomic
charge of Operoxo becomes more positive, and the covalency of Ta-
Operoxo bond is enhanced. These features leads to a higher reactivity
)]3- is formed and then is reconverted to replenish the
2 2 4
O ) (CO
oxidizing species with hydrogen peroxide and complets the catalytic
cycle (Scheme 2). As we have indicated in the previous work, the
quaternary phosphonium cations could not only play a crucial role in
of Ta-Operoxo bond in [Ta( -O
2
)] than in [Ta( -O
3-
2
]3-. We also
2
)
3
(CO
4
2
)
4
2
)]3-, the Operoxo atoms (site a) of the η2- balancing the negatively charged Ta-peroxocarbonate anion, but also
note that in [Ta( -O
2
)
3
(CO
4
peroxide ligand carry similar atomic charge as the Ocarboperox atom ensure a homogeneous phase in the course of the reaction.
site b), suggesting the two sites have close eletrophilicity, which may
Our previous research illustrated that the coordination between
thus not be the determining factor responsible for their distinct the α-hydroxy acid and the peroxoniobate anion causes the Operoxo
reactivity.
The analysis of the key bonds indicates that the O-O distance in than that of [Nb(O-O) (OOH) ]
(
-
atom in [Nb(OO)
2
LA] (LA=lactic acid) to bear a less negative charge
24b The lower electron density of the
the η -peroxide ligand (1.51 Å) is marginal longer than that in the reactive center (O atom) exhibits a higher reactivity towards to C=C
-
2
2
.
2
peroxocarbonate group (1.48 Å) (Fig. S6). This shows that it is easier bond owing to the electrophilic addition mechanism of the
to break the former bond than the latter one. As both O atoms are epoxidation process. Actually, we considered that CO played a quite
2
involved in a peroxo bond and appear in the first coordination shell
of Ta, it is conceivable that the direct contact with Ta may influence
their reactivity towards the epoxidation of olefin significantly. The
examination of the bond nature indicates that Ta-Operox has a bigger
MBO (Mayer bond order) of 0.70 than Ta-Ocarboperox (0.55) (Fig. S6),
indicating more significant covalency of the former than the latter,
and stronger ionic feature of the latter. In organometallic chemistry,
a highly reactive M-E (M: metal atom, E: non-metallic atom) is often
displaying a relative weak polarity. This suggests that the Ta-Operox
site may have stronger reactivity than the Ta-Ocarboperox site, which is
consistent with the calculated energy barriers of the epoxidation at
the two sites. In another paralleling experiment, as shown in Fig. 1e,
it can be seen that the peak at 169.8 ppm attributed to Ta-
similar role to that of -hydroxy carboxylic acid, the insertion of CO
2
makes the atomic charge of Operoxo atoms in [Ta( -O )]3- more
2
2 3 4
) (CO
2
3-
positive, which hence gives a higher reactivity than [Ta( -O
However, CO can in-situ generate or remove peroxocarbonate
ligand reversibly. The use of CO is more flexible and eco-friendly, as
compared with that of organic acids.
2 4
) ] .
2
2
3
peroxocarbonate species was still observed clearly in the CD OD
solution after the reaction was completed. This demonstrated that
2
the reaction proceeded mostly at the peroxo  -O
2
(site a) instead of Scheme 2 Proposed mechanism for epoxidation of cyclooctene with
the peroxocarbonate-CO
4
(sites b).
H O catalyzed by peroxotantalate-based ionic liquids in
2
2
Based on the catalyst characterization, the activity test of the compressed CO2.
epoxidation reaction and DFT calculations, a reaction mechanism is
2
)]3- is oxidized to the
proposed accordingly. The anion [Ta(O)
3
( -O
2
Conclusion
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or lgo i ng ys
Page 8 of 10
ARTICLE
Journal Name
In summary, we have demonstrated that the tantalum-based
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2
DOI: 10.1039/C9CY00056A
peroxocarbonate species [P4,4,4,4
]
3
[Ta( -O
2
)
3
(CO
[Ta(O)
in the presence of H
peroxocarbonate species have been verified experimentally by NMR,
4
)] can be in-situ
2
generated from the reaction of [P4,4,4,4
]
3
3
( -O
2
)] with the
compressed CO
2
2 2
O . The structure of
FT-IR, HRMS and DFT calculations. The newly formed tantalum- 2 (a) M. Aresta, E. Quaranta and A. Ciccarese, J. Mol. Catal., 1987,
based peroxocarbonate ILs displayed a high reactivity and selectivity
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acts 3 D. Bayot and M. Devillers, Coordin. Chem. Rev., 2006, 250, 2610.
feature allows this IL catalyst to be recyclable facilely. The DFT
calculation indicated further that due to the insertion of CO into the
Ta-O bond, the atomic charge of Operoxo becomes more positive, and
the covalency of Ta-Operoxo bond is enhanced. These features leads to
a higher reactivity of Ta-Operoxo
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)]3- than that in [Ta( -O ]3- anions. Two
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bond in [Ta( -O ) (CO )
2 3 4 2 4
possible reaction pathways were investigated and the results
2
indicated that the epoxidation at the η -peroxo site was observed to
be more competitive than at the peroxocarbonate site. Mononuclear
tantalum complexes have received significantly much less attention
in the literature as epoxidation catalysts, especially than that of these
reported tungsten peroxo complex catalysts.30 This work is the first
example of CO
2
-induced metal-based peroxocarbonate ILs. We 5 D. R. Mulford, J. R. Clark, S. W. Schweiger, P. E. Fanwick and I. P.
consider that this catalytically active metal peroxocarbonate ILs
remained a huge challenge for mechanic studies and the
investigation will inspire the rational design of new ILs for more
efficient catalysis from the viewpoint of fundamental science and
practical application.
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Conflicts of interest
There are no conflicts to declare.
8
9
1
1
(a) H. Egami and T. Katsuki, Angew. Chem. Int. Ed., 2008, 47, 5171.
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Acknowledgements
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(a) Y. Zhu, Q. Wang, R. G. Cornwall and Y. Shi, Chem. Rev., 2014,
The authors are grateful for financial support from the National
Natural Science Foundation of China (21373082, 21773061), the
Innovation Program of the Shanghai Municipal Education
Commission (15ZZ031).
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Catalysis Science & Technology
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DOI: 10.1039/C9CY00056A
Table of Contents
Mononuclear tantalum complex tethering a peroxocarbonate ligand has been proved to
be particularly important in the epoxidation reactions.