A discrete {Co4(μ3-OH)4}4+ cluster with an oxygen-rich coordination environment as a catalyst for the epoxidation of various olefins

Article abstract of DOI:10.1039/c5dt03422a

Using the sterically hindered terphenyl-based carboxylate, the tetrameric Co(ii) complex [Co₄(μ₃-OH)₄(μ-O₂CAr^4F-Ph)₂(μ-OTf)₂(Py)₄] (1) with an asymmetric cubane-type core has been synthesized and fully characterized by X-ray diffraction, UV-vis spectroscopy, and electron paramagnetic resonance spectroscopy. Interestingly, the cubane-type cobalt cluster 1 with 3-chloroperoxybenzoic acid as the oxidant was found to be very effective in the epoxidation of a variety of olefins, including terminal olefins which are more challenging targeting substrates. Moreover, this catalytic system showed a fast reaction rate and high epoxide yields under mild conditions. Based on product analysis and Hammett studies, the use of peroxyphenylacetic acid as a mechanistic probe, H₂¹⁸O-exchange experiments, and EPR studies, it has been proposed that multiple reactive cobalt-oxo species Co^VO and Co^IVO were involved in the olefin epoxidation.

Full text of DOI:10.1039/c5dt03422a

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PAPER
4
+
A discrete {Co (μ -OH) } cluster with an oxygen-
4
3
4
rich coordination environment as a catalyst for the
Cite this: DOI: 10.1039/c5dt03422a
epoxidation of various olefins†
a
b
a
a
a
Sun Young Lee, Namseok Kim, Myoung Mi Lee,* Young Dan Jo, Jeong Mi Bae,
a
b
a
Min Young Hyun, Sungho Yoon* and Cheal Kim*
Using the sterically hindered terphenyl-based carboxylate, the tetrameric Co(II) complex
4
F-Ph
[Co
4
(μ -OH)
3
4
(μ-O
2
CAr
)
2 2 4
(μ-OTf) (Py) ] (1) with an asymmetric cubane-type core has been syn-
thesized and fully characterized by X-ray diffraction, UV-vis spectroscopy, and electron paramagnetic reso-
nance spectroscopy. Interestingly, the cubane-type cobalt cluster 1 with 3-chloroperoxybenzoic acid as
the oxidant was found to be very effective in the epoxidation of a variety of olefins, including terminal
olefins which are more challenging targeting substrates. Moreover, this catalytic system showed a fast
Received 3rd September 2015,
Accepted 14th December 2015
reaction rate and high epoxide yields under mild conditions. Based on product analysis and Hammett
18
studies, the use of peroxyphenylacetic acid as a mechanistic probe, H
2
O-exchange experiments, and
DOI: 10.1039/c5dt03422a
V
IV
EPR studies, it has been proposed that multiple reactive cobalt-oxo species Co vO and Co vO were
involved in the olefin epoxidation.
www.rsc.org/dalton
5
–7
properties. A few cubane-like cobalt(III) complexes have been
known to catalyze the oxidation of some commercially impor-
1
. Introduction
Metalloenzymes are well known to catalyze biologically impor- tant organic substrates. For example, the cobalt(III)-oxo cubane
tant oxygenation reactions efficiently under mild conditions. clusters, Co O (O CMe) (py) , have shown efficient catalytic
4
4
2
4
4
To mimic such enzymatic actions, several transition metal- activities for the aerobic oxidation of ethylbenzene and
5
a
based catalysts have been recently synthesized and used to p-xylene.
Also, the tetrameric cubane-like complex
1
achieve O–O bond activation and subsequent oxygenations. In [Co O (O CC H -X) (py) ] was examined as a catalyst for the
4
4
2
6
4
4
4
this regard, iron and manganese porphyrins have been con- homogeneous t-BuOOH oxidation of benzyl alcohols under
6
sidered as the most successful catalysts for the oxidation reac- mild conditions. Chakrabarty et al. demonstrated the effec-
1
,2
III
4
tions of various organic substrates. On the other hand, the tiveness of
a
cobalt(III) complex, Co (μ -O) (μ-O CCH )
3
4
2
3 4
oxidation reaction studies of hydrocarbons by using cobalt (4-CNpy) , for the catalytic oxidation of α-pinene by dioxygen
4
3
7
complexes have been relatively less explored. In these studies, in aqueous media. They proposed an oxidation mechanism
3
4
cobalt(IV)-oxo and a few cobalt(V)-oxo species have been where both cobalt(III) and cobalt(IV) could be involved in the
suggested to participate actively in the transfer of oxygen catalytic cycle. Although a few cubane-like cobalt(III) complexes
atoms with an oxidant, such as 3-chloroperoxybenzoic acid have been reported to catalyze the oxidation of some organic
(MCPBA) and iodosylbenzene (PhIO).
substrates, they have only shown reactivity for easy-to-oxidize
Furthermore, oxo-bridged transition metal complexes with substrates such as benzyl alcohols, styrene, ethylbenzene and
3
–7
ancillary N- and O-donor ligands are of considerable interest α-pinene.
In order to further develop a new cubane-type
to synthetic inorganic chemists because of their interesting tetracobalt cluster capable of being applied to the catalytic
structures as well as their magnetic, optical, and catalytic hydrocarbon oxidation of a wide range of substrates such as
terminal and internal olefins, we synthesized the title complex
and examined the catalytic epoxidation of various olefins
under mild conditions.
a
Department of Fine Chemistry, Seoul National University of Science and Technology,
Seoul 139-743, Korea. E-mail: ohno801126@nate.com, chealkim@seoultech.ac.kr;
Herein, we report the synthesis and full characterization
of a tetranuclear Co(II) complex with the general formula
Fax: +82-2-973-9149; Tel: +82-2-970-6693
b
Department of Chemistry, College of Natural Sciences, Kookmin University,
8
61-1 Jeoungnung-dong, Seongbuk-gu, Seoul 136-702, Korea.
E-mail: yoona@kookmin.ac.kr
Electronic supplementary information (ESI) available. CCDC 1408240. For ESI
[
Co (μ -OH) (μ-O CR) (μ-OTf) (Py) ] (1), which comprises an
4
3
4
2
2
2
4
asymmetric cubane-type core housed within two sterically
bulky carboxylate ligands. In addition, 1 was found to be
robust, readily available and active in epoxidation reactions of
†
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt03422a
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various olefins. Moreover, 1 is the first cubane-type cobalt cata- from 2.112(3) Å to 2.127(3) Å, as summarized in Table 3.
lyst to afford high conversions and epoxide yields in the epoxi- Hormillosa et al. reported so-called Bond Valence Sum (BVS),
1
8
dation of terminal olefins. Based on product analysis, H
2
O
in which the average distance between metal cations and
experiments, Hammett studies, and the use of peroxyphenyl- ligands can be used to estimate the oxidation state of the metal
V
9
acetic acid (PPAA) as a mechanistic probe, Co vO and cation. The calculated BVS values in the present case were dis-
IV
Co vO species were proposed to be possible reactive inter- tributed between 2.035 and 2.065, indicating that the oxidation
mediates responsible for the olefin epoxidation.
states of Co cations in the cubane were all 2+ (Table 3). All of
the Co(II) sites had slightly distorted octahedral geometries
with a NO5 donor atom set, composed of pyridine, triflate, and
carboxylate, as well as three hydroxo ligands, each bridging
three Co atoms. The assignment of hydroxide to oxo was con-
firmed by the observed Co–O distances, ranging from 2.050(2)
to 2.265(3) Å, and also by the refinement of the associated
hydrogen atoms from the difference electron density maps.
Comparable Co–Ohydroxo distances occur in other high-spin
2
. Results and discussion
Synthesis
An intriguing report by Lee et al. described the successful
preparation of hydroxo-bridged tetranuclear iron complexes
with combined ligand sets consisting of pyridine derivatives
and bulky carboxylates, which led us to prepare the hydroxo-
bridged cobalt cubane-type clusters via a similar synthetic
1
0
cobalt(II) complexes. Organized variations in the Co⋯Co dis-
tances were observed, depending on the nature of the bridging
8
route. To prepare Co(II) analogues of the reported tetranuclear
ligands. There were three different types of {Co
2
2
(μ-OH) } units
4
F-Ph
iron complexes, the reaction was examined using stoichio-
metric reagents (Scheme 1). The reaction between
in the cubane: triply bridged {Co
bridged {Co (μ-OH) (μ-OTf)}
Co (μ-OH) (Py)}, thus the six Co⋯Co separations can be
2 2 2
(μ-OH) (μ-O CAr
)}, triply
bridged
and
doubly
2
2
4
F-Ph
NaO
2
CAr
2 3
, Co(OTf) ·2CH CN, pyridine, triethylamine and
{
2
2
H
2
O in THF under argon afforded red block crystals of the
grouped into three different lengths – distributed around
3.0 Å, 3.1 Å and 3.2 Å – resulting in a distorted cubic core com-
4
F-Ph
neutral [Co (μ -OH) (μ-O CAr
) (μ-OTf) (Py) ] complex (1)
4
3
4
2
2
2
4
in moderate yield (60%) (Scheme 1).
posed of {Co
4
(μ
3 4
-OH) }. Intermetallic Co⋯Co distances
decreased with increasing numbers of bridging ligands; the
Characterization
4F-Ph
intermetallic distances in the {Co (μ-OH) (μ-O CAr
)} units
(μ-OTf)} units and
(Py)} units. The
2
2
2
The structure of complex 1 was determined by X-ray crystallo- were 0.1 Å shorter than the {Co
graphy (Fig. 1), and the crystallographic data and the selected 0.2 Å shorter than doubly bridged {Co
2
(μ-OH)
2
2
(μ-OH)
2
bond lengths and angles have been listed in Tables 1–3, Co–O–Co angles were closely distributed, ranging between
respectively. The compound had a slightly distorted cubic-type 77.79(9)° and 85.58 (8)° (Table S1†). As a trend, a decrease in
{
Co (μ -OH) } unit with interpenetrating Co and (OH)4 dis- the Co⋯Co distances was accompanied by a decrease in the
4
3
4
4
phenoids. The Co–N distances from each Co atom ranged Co–O–Co bond angles in the {Co
(μ-OH) } motifs. The elec-
2
2
4
+
Scheme 1 Synthesis of {Co
4
(μ
3
-OH)
4
}
cluster 1.
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Table 1 X-ray
crystallographic
data
CH
for
O·0.5CH
the
Cl
[Co
complex
4 3 4
(μ -OH)
4
F-Ph
(
μ-O
2
CAr
)
2
(μ-OTf)
2
(Py)
4
]·0.5(CH
3
2
)
2
2
2
Formula
62.5 4 10 4 14.5 2
C H52ClCo F N O S
1616.37
293(2)
Formula weight, g mol−
1
T, K
Radiation, Å
Crystal system
Space group
a, Å
b, Å.
c, Å
α, °
β, °
0.71073
Triclinic
Pˉ1
12.8536(6)
14.3535(8)
20.0526(11)
95.450(2)
105.838(2)
93.734(2)
3527.0(3)
2
1.522
1.111
0.0519
0.1607
γ, °
V, Å
3
Z
−3
ρcalc, g cm
−
1
μ, mm
R
wR
a
1
b
2
2
Goodness-of-fit on F
1.063
Reflections collected
17 602
a
b
2
o
2 2
2 2 1/2
R
1
o
= ∑||F | − |F
c
||/∑|F
o
|. wR
2
= {∑[w(F
− F
c
) ]/∑[w(F
o
) ]}
.
Table 2 Selected interatomic distances (Å) and angles (°) for the
4F-Ph
[Co
4
(μ
3
-OH)
4
(μ-O
2
CAr
2 2 4 2 2
) (μ-OTf) (Py) ]·0.25CH Cl complex
Co(1)–O(1)
Co(1)–O(2)
Co(1)–O(3)
Co(1)–O(5)
Co(1)–O(9)
Co(1)–N(1)
Co(2)–O(1)
Co(2)–O(2)
Co(2)–O(4)
Co(2)–O(6)
Co(2)–O(12)
Co(2)–N(2)
2.117(2)
2.076(2)
2.062(2)
2.063(2)
2.209(3)
2.118(3)
2.092(2)
2.107(2)
2.056(2)
2.052(2)
2.246(3)
2.112(3)
Co(3)–O(1)
Co(3)–O(3)
Co(3)–O(4)
Co(3)–O(7)
Co(3)–O(1)
Co(3)–N(3)
Co(4)–O(2)
Co(4)–O(3)
Co(4)–O(4)
Co(4)–O(8)
2.069(2)
2.084(2)
2.075(2)
2.087(2)
2.265(3)
2.115(3)
2.050(2)
2.083(2)
2.103(2)
2.058(2)
2.216(3)
2.127(3)
Co(4)–O(13)
Co(4)–N(4)
Co(1)⋯Co(2)
Co(1)⋯Co(3)
Co(1)⋯Co(4)
Co(2)⋯Co(3)
Co(2)⋯Co(4)
Co(3)⋯Co(4)
3.0283(6)
3.1348(6)
3.1946(6)
3.2253(6)
3.1199(6)
3.0164(6)
O(1)–Co(1)–O(2)
O(1)–Co(1)–O(3)
O(2)–Co(1)–O(3)
O(1)–Co(2)–O(2)
O(4)–Co(2)–O(1)
O(4)–Co(2)–O(2)
O(1)–Co(3)–O(3)
O(1)–Co(3)–O(4)
O(3)–Co(3)–O(4)
O(2)–Co(4)–O(3)
O(2)–Co(4)–O(4)
O(3)–Co(4)–O(4)
85.55(8)
81.62(8)
78.77(9)
85.41(8)
77.79(9)
82.09(8)
82.25(8)
77.88(9)
85.58(8)
78.90(9)
82.33(8)
84.90(8)
Fig. 1 (top)
ORTEP
(μ-OTf) (Py)
ellipsoids. (top) The non-hydroxo hydrogen atoms are omitted for
drawings
of
the
4 3 4
[Co (μ -OH)
4F-Ph
(
μ-O
2
CAr
)
2
2
4
] cluster, showing 50% probability thermal
4
F-Ph
−
clarity. (bottom) The aromatic rings of the Ar
non-hydroxo hydrogen atoms are omitted for clarity.
CO
2
ligands and the
tronic spectrum of the cobalt complex recorded in CH
2 2
Cl
solution is shown in Fig. S1.† The central peak at 531 nm
ε = 150 M− cm ) is a characteristic feature of high-spin Co(II)
1
−1
(
ions. To understand the magnetic properties of the distorted Table 3 Average distances of Co–O bonds and Co–N bonds of each
7
cube (1) with 4 octahedral d ions located within the steric metal site and calculated BVS values
bulk of the terphenyl-based ligands, magnetic susceptibility by
the Evans method was determined at 297 K (see the ESI†),
Average value of
Co–O bond distances
Co–N bond
distance
BVS calc.
values
resulting in 4.61 (B.M. per Co atom). It reiterates that Co(II)
7
Co(1)
Co(2)
Co(3)
Co(4)
2.105
2.111
2.116
2.084
2.118
2.112
2.115
2.216
2.065
2.059
2.035
2.054
ions are high-spin d , considering that experimental magnetic
moments for octahedral Co(II) high-spin complexes are distrib-
1
1
uted over 4.3–5.2 B.M.
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a
Table 4 Olefin epoxidations by MCPBA with the cubane-type cobalt cluster 1 in CH
2
Cl
2
/CH
3
CN (1 : 1) at room temperature
b
b
Entry
Substrate
Product
Conversion (%)
Yield (%)
1
2
3
4
Cyclopentene
Cycloheptene
Cyclooctene
Cyclohexene
Epoxide
Epoxide
Epoxide
Epoxide
82.0 ± 1.4
98.5 ± 1.3
99.2 ± 0.5
88.2 ± 0.6
74.5 ± 3.1
97.9 ± 2.2
92.0 ± 2.1
61.5 ± 2.4
4.1 ± 0.1
1.5 ± 0.1
67.8 ± 0.6
50.7 ± 4.0
62.4 ± 0.5
17.1 ± 1.0
73.7 ± 4.2
63.6 ± 1.5
16.1 ± 0.1
64.0 ± 0.7
42.8 ± 1.4
5.5 ± 0.1
—
10.3 ± 0.4
33.5 ± 1.6
15.9 ± 0.1
8.7 ± 0.5
40.6 ± 4.1
20.6 ± 1.4
6.3 ± 0.4
2
2
-Cyclohexene-1-one
-Cyclohexene-1-ol
5
6
7
1-Hexene
1-Octene
cis-2-Hexene
Epoxide
Epoxide
cis-Oxide
72.6 ± 5.1
59.7 ± 7.4
79.5 ± 2.8
trans-Oxide
trans-Oxide
cis-Oxide
trans-Oxide
trans-Oxide
Epoxide
Benzaldehyde
Phenylacetaldehyde
cis-Stilbene oxide
trans-Stilbene oxide
Benzaldehyde
8
9
trans-2-Hexene
cis-2-Octene
78.1 ± 0.4
86.6 ± 1.5
1
1
0
1
trans-2-Octene
Styrene
79.3 ± 0.7
82.0 ± 1.6
12
cis-Stilbene
93.2 ± 1.1
2-Phenylacetophenone
13
trans-Stilbene
trans-Stilbene oxide
Benzaldehyde
93.4 ± 0.1
2-Phenylacetophenone
a
b
Reaction conditions: olefin (0.035 mmol), catalyst (0.001 mmol), MCPBA (0.10 mmol), and solvent (1 mL, CH
2
Cl
2
/CH
3
CN = 1 : 1). Based on the
substrate.
Olefin epoxidation reaction catalyzed by 1
(42.8%; entry 11) with a small amount of benzaldehyde (5.5%).
While cis-stilbene was oxidized to cis-stilbene oxide (10.3%;
We conducted the catalytic epoxidation of various olefins with entry 12) and trans-stilbene oxide (33.5%) along with some
MCPBA in the presence of the cubane-type cobalt cluster 1 benzaldehyde (15.9%) and 2-phenylacetophenone (8.7%),
under mild conditions. MCPBA (0.1 mmol) was added to a trans-stilbene was converted to trans-stilbene oxide (40.6%;
mixture of substrate (0.035 mmol), catalyst 1 (0.001 mmol), entry 13) as a major product with minor amounts of benz-
3 2 2
and solvent (CH CN/CH Cl , 1 : 1 v/v; 1 mL). The mixture was aldehyde (20.6%) and 2-phenylacetophenone (6.3%). In the
stirred for 10 min at room temperature. Control experiments case of olefins having aromatic rings, the substrates gave by-
suggested that direct oxidation of the substrate by MCPBA was products of aldehyde and ketone in moderate yields and
negligible and that 1 was stable enough during the catalytic showed very low stereochemical retention. These results
reactions according to the UV-vis spectroscopic study. The demonstrated that either the peroxyl radical or oxocobalt(IV) as
results of the epoxidation reaction are summarized in Table 4. the epoxidizing agent might be partially involved in this cata-
Cyclic olefins such as cyclopentene, cycloheptene, and cyclo- lytic reaction. These species were expected to generate non-
octene were oxidized to the corresponding epoxides in good stereospecific or radical-induced rearranged products from the
yields (74.5–97.9%, entries 1–3), with conversions ranging epoxidation reaction of aromatic olefins, based on previous
1
2b,15–17
•
from 82.0 to 99.2%. Meanwhile, cyclohexene (entry 4) afforded reports.
2 2
Another possibility is that the Co(IV)-OCR CR
cyclohexene oxide (61.5%) as a major product and small intermediate with a long lifetime generated from the reaction
V
amounts of 2-cyclohexen-1-one (4.1%) and 1-cyclohexen-1-ol of cis-olefin with Co vO species may rotate to produce trans-
1
2
(
1
1.5%). Although terminal olefins such as 1-hexene and epoxide. Therefore, all the combined results suggested that
V
IV
-octene (entries 5 and 6) are well known as difficult-to-oxidize two different oxidants, viz. Co vO and Co vO, might be gen-
substrates,
1
3,14
IV
their corresponding epoxides were obtained in erated in these catalytic reactions and the Co vO complex
good yields (67.8% and 50.7%, respectively). cis-2-Hexene and would be responsible for the non-stereoretentive portion of the
cis-2-octene were used to probe the epoxidation stereo- epoxidation reaction.
chemistry, which were oxidized mainly to cis-2-hexene oxide
and cis-2-octene oxide (62.4% and 63.6%; entries 7 and 9) in oxidize olefins because of their electron-deficient nature,
Terminal olefins have been considered as difficult-to-
1
3,14
addition to some amounts of trans-2-hexene oxide (17.1%) and while their corresponding products 1,2-epoxides are known as
trans-2-octene oxide (16.1%), respectively. These results versatile starting materials for the synthesis of more compli-
demonstrated that a stereochemical retention of 57.0% for cis- cated compounds. Therefore, we investigated the terminal
2
-hexene and 60.0% for cis-2-octene was retained in this epoxi- olefin epoxidation by using 1, because 1-hexene and 1-octene
dation reaction. Styrene was oxidized to styrene epoxide were reasonably oxidized to their corresponding epoxides. The
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a
Table 5 Terminal olefin epoxidation by MCPBA with the cubane-type cobalt cluster 1 in CH
2
Cl
2
/CH
3
CN (1 : 1) at room temperature
b
b
Entry
Substrate
Product
Conversion (%)
Yield (%)
1
2
3
4
5
6
7
8
9
1-Hexene
1-Octene
1-Nonene
1-Decene
1-Undecene
1-Dodecene
1-Tridecene
1-Tetradecene
1-Pentadecene
1-Hexadecene
1-Octadecene
Vinylcyclohexane
1,2-Epoxyhexane
1,2-Epoxyoctane
1,2-Epoxynonane
1,2-Epoxydecane
1,2-Epoxyundecane
1,2-Epoxydodecane
1,2-Epoxytridecane
1,2-Epoxytetradecane
1,2-Epoxypentadecane
1,2-Epoxypentadecane
1,2-Epoxypentadecane
2-Cyclohexyloxirane
72.6 ± 5.1
59.7 ± 7.4
52.2 ± 4.4
52.4 ± 0.9
77.0 ± 2.0
77.6 ± 0.1
75.6 ± 0.1
76.4 ± 0.2
79.1 ± 0.4
48.4 ± 0.5
46.5 ± 2.7
57.1 ± 1.0
67.8 ± 0.6
50.7 ± 4.0
37.3 ± 0.8
35.9 ± 1.2
45.4 ± 1.2
50.3 ± 0.8
59.5 ± 0.3
41.6 ± 0.6
51.5 ± 1.7
43.0 ± 8.9
40.8 ± 3.6
31.5 ± 0.5
1
1
1
0
1
2
a
b
2 2 3
Reaction conditions: olefin (0.035 mmol), catalyst (0.001 mmol), MCPBA (0.10 mmol), and solvent (1 mL, CH Cl /CH CN = 1 : 1). Based on the
substrate.
1
8
results are summarized in Table 5. All of the terminal olefins
were oxidized to the corresponding epoxides in moderate
yields (31.5–67.8%), with conversions in the range of
H
2
O experiments
To further investigate the mechanism of olefin epoxidation, we
carried out isotope labeling experiments using H
O-labeling experiments are commonly used in the studies of
reaction mechanisms to test the involvement of a high-valent
metal-oxo intermediate in metal-mediated oxygen atom trans-
18
2
O, because
4
6.5–79.1%, indicating that our catalytic system could be 18
useful for the epoxidation reaction of a variety of terminal
olefins. To the best of our knowledge, 1 is the first cubane-type
cobalt catalyst to afford high conversions and epoxide yields in
20–22
fer reactions.
Epoxidation of cyclohexene by 1 and MCPBA
5
–7
the epoxidation of terminal olefins by MCPBA.
was conducted in the presence of a large excess (100–400
equiv.) of H
1
8
2 3
O to the substrate in a mixture of CH CN and
CH Cl (v/v, 1 : 1). GC-MS analysis of the products showed no
Competition experiments of styrene and para-substituted
styrenes for the Hammett plot
2
2
1
8
18
O-incorporation into cyclohexene oxide from H
2
O. This
result suggested two possibilities: one is that the rate of
oxygen atom transfer from Co-oxo species to olefin might be
faster than the oxygen exchange rate between Co-oxo species
In order to examine the nature of the reactive species respon-
sible for the epoxidation reaction, we studied the influence of
substituent electronic effects on the reaction rate using styrene
and four para-substituted styrenes. The analysis of a Hammett
plot showed a ρ value of −0.49, indicating that the active
oxidant was electrophilic (Fig. 2). This value is comparable to
18
and H2 O, and the other is that the direct olefin epoxidation
III
by Co -OOC(O)R species may occur before its O–O bond
cleavage.
1
7c
III
those reported for Mn(salen) (ρ = −0.3),
Mn tetraphenyl-
1
8
M,Me
porphyrin (ρ = −0.3), and Mn(
PyTACN)(CF SO (ρ =
3
3 2
)
1
9
−0.37).
EPR studies
In order to figure out the oxidation state of cobalt ions after
the epoxidation reaction, we carried out the electron paramag-
netic resonance (EPR) study. The EPR spectrum of the starting
complex was silent (Fig. S2†), indicating that there is a mag-
netic exchange between the four Co(II) ions. Moreover,
complex 1 obtained 5 s after the catalytic epoxidation reaction
also showed a silent EPR spectrum. This result assumed that
II
III
the Co complex might be first oxidized to a Co complex by
III
MCPBA, and then the Co complex acted as a catalyst.
Furthermore, we carried out EPR studies at low temperature in
IV
order to trap any possible intermediates, such as Co vO
species. An intermediate state frozen
5 seconds after
complex 1 was mixed with MCPBA at −40 °C in a mixture of
CH CN and CH Cl (v/v, 1 : 1) showed silent EPR signals,
3
2
2
which indicated that it was considerably difficult to capture
the possible reactive species with the EPR spectroscopy
method.
Fig. 2 Hammett plot for selective reactivities of styrene to para-substi-
tuted styrenes by the cubane-type cobalt cluster 1 with MCPBA.
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Product distribution of the O–O bond cleavage of PPAA with
catalyst 1
catalyst 1 with PPAA was carried out in the absence of a sub-
strate (entry 1, Table 6). The results showed that the heterolytic
cleavage product, phenylacetic acid (73.3% based on PPAA),
PPAA has often been used as a mechanistic probe to identify and the homolytic cleavage products, benzaldehyde (17.0%)
the cleavage mode of the peracid O–O bond through quantitat- and benzyl alcohol (1.2%), were formed. This result demon-
III
ive determination of the degradation products derived from strated that the intermediate Co -OOC(O)R species underwent
1
5,16,21,22
the reaction of the catalyst and PPAA.
While hetero- the partition of the heterolysis (80.1%) and the homolysis
V
IV
lytic cleavage of the O–O bond of the Co-acylperoxo species 3 (19.9%) to give high-valent Co vO (4) and Co vO (5) species.
generated phenylacetic acid (PAA, 6; pathway (a) of Scheme 2), Next, in order to investigate the concentration effects of an
the O–O bond is cleaved homolytically to form an acyloxyl easy-to-oxidize substrate, cyclohexene, we carried out the
1
2a,15,16
radical, which afforded benzaldehyde (7), benzyl alcohol (8), epoxidation reaction of it by 1 with PPAA.
When the
and toluene (9) via a rapid β-scission of the acyloxyl radical concentration of cyclohexene increased gradually from 0 to
1
6
(
pathway (b) of Scheme 2). The direct oxidation of the sub- 160 mM, heterolysis increased slightly from 80 to 87 (entries
III
strate by the Co-acylperoxo intermediate 3 gave PAA (6; 1–5). This result implied that the Co -OOC(O)R species 3
pathway (c) of Scheme 2). For a control test, the reaction of might be a little involved in the olefin epoxidation reaction
Scheme 2 Possible degradation pathway of PPAA by the Co(III) complex.
Table 6 Yield of products derived from PPAA mediated by the cubane-type cobalt cluster 1 in the absence and presence of cyclohexene in a
a
3 2 2
mixture of CH CN/CH Cl (1 : 1) at room temperature
b
d
Homolysis
Oxidation products
b
Cyclohexene
[mM]
Heterolysis
6
Hetero/Homo
[6/(7 + 8 + 9)]
c
Entry
7
8
9
Oxide
Ol
One
1
2
3
4
5
0
20
40
80
160
73.3 ± 0.4
76.2 ± 0.7
81.5 ± 0.3
80.3 ± 1.1
86.7 ± 2.3
17.0 ± 0.4
14.1 ± 0.1
13.8 ± 0.0
11.4 ± 0.1
10.8 ± 0.1
1.2 ± 0.4
1.3 ± 0.0
1.4 ± 0.1
1.4 ± 0.2
2.4 ± 1.1
—
—
—
—
—
4.02 (80.1/19.9)
4.95 (83.2/16.8)
5.36 (84.3/15.7)
6.27 (86.2/13.8)
6.57 (86.8/13.2)
—
—
—
e
e
b
b
e
e
b
b
e
28.4 ± 0.7
20.2 ± 0.6
23.4 ± 0.4
36.2 ± 3.0
6.8 ± 0.2
5.0 ± 0.0
7.2 ± 0.1
11.8 ± 0.1
3.4 ± 0.0e
2.7 ± 0.1
b
4.0 ± 0.1b
7.5 ± 0.6
a
b
3 2 2
Reaction conditions: substrate (0–0.16 mmol), catalyst (0.001 mmol), PPAA (0.04 mmol), and solvent (1 mL, CH CN/CH Cl = 1 : 1). Based on
c
d
PPAA. 6–9 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively. Ol, one, and oxide indicate 2-cyclohexen-1-ol,
2
e
-cyclohexen-1-one, and cyclohexene oxide, respectively. Based on cyclohexene.
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Table 7 Yield of products derived from PPAA mediated by the cubane-type cobalt cluster 1 in the absence and presence of 1-octene in a mixture
a
3 2 2
of CH CN/CH Cl (1 : 1) at room temperature
b
Homolysis
b
d
1
-Octene
Heterolysis
6
Hetero/Homo
[6/(7 + 8 + 9)]
Oxidation products
1-Octene oxide
c
Entry
[mM]
7
8
9
1
2
3
4
5
0
20
40
80
160
73.3 ± 0.4
80.3 ± 5.0
78.9 ± 0.5
77.0 ± 0.4
86.0 ± 1.3
17.0 ± 0.4
20.3 ± 0.9
19.9 ± 0.3
19.1 ± 0.6
19.3 ± 0.7
1.2 ± 0.4
1.4 ± 0.2
1.2 ± 0.1
1.3 ± 0.3
3.3 ± 2.1
—
—
—
—
—
4.02 (80.1/19.9)
3.70 (78.7/21.3)
3.74 (78.9/21.1)
3.77 (79.0/21.0)
3.80 (79.2/20.8)
—
7.1 ± 0.3
4.8 ± 0.2
6.1 ± 0.2
10.1 ± 1.4
a
b
Reaction conditions: substrate (0–0.16 mmol), catalyst (0.001 mmol), PPAA (0.04 mmol), and solvent (1 mL, CH
CN/CH Cl
3 2 2
= 1 : 1). Based on
c
d
PPAA. 6–9 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively. Based on 1-octene.
Scheme 3 Plausible mechanism for the olefin epoxidation reaction by 1 with peracids.
when the concentration of cyclohexene was very high. We heterolysis might be responsible for the olefin epoxidation
changed the substrate from cyclohexene to a difficult-to- that showed a high yield of epoxide and a high stereochemical
IV
oxidize olefin, 1-octene, to examine the cleavage mode of PPAA retention, whereas a Co vO complex generated via homolysis
2
2
on the type of substrate. The ratio (80 : 20) of the heterolysis was ascribed to a radical-type oxidation that showed the for-
and the homolysis was fairly constant, depending on the mation of allylic oxidation products and a loss of stereospecifi-
change of the 1-octene concentration (entries 1–5, Table 7). city. The direct epoxidation of olefin by Co-OOR species
III
These results suggested that the Co -OOC(O)R species would occurred to a lesser extent with only an easy-to-oxidize sub-
V
not be involved in the epoxidation of 1-octene, even at its high strate, such as cyclohexene. Therefore, the oxidants Co vO
IV
concentration.
and Co vO might act simultaneously as key active intermedi-
It can also be mentioned that we have observed similar ates in the olefin epoxidation reaction catalyzed by the cubane-
reactivity patterns between this cubane-type cobalt-cluster type cobalt cluster 1.
system and a mononuclear nonheme cobalt(III) complex pre-
1
5a
viously reported. The similar reactivity patterns suggest that
V
the simultaneous operation of the two active oxidants, Co vO
IV
or Co vO species, might prevail in all cobalt-catalyzed olefin 3. Conclusions
epoxidations.
We have synthesized a tetranuclear Co(II) complex 1 with an
asymmetric cubane-type core using the bulky carboxylate
ligand, for the first time, and studied the oxidation of a wide
Mechanistic comments
Based on our mechanistic studies, the most plausible mechan- range of olefins using the cluster 1. In particular, the catalyst 1
ism for the formation of the reactive species responsible for was effective for the epoxidation of terminal olefins, the so-
olefin epoxidation could be sketched as shown in Scheme 3. called difficult-to-oxidize substrate. Reactivity and Hammett
1
8
First, the cobalt(II) cluster 1 was oxidized to a cobalt(III) cluster studies, EPR, H
2
O exchange experiments, and the use of
by a peracid molecule, which further reacted with another PPAA as a mechanistic probe suggested that Co(V)vO and
peracid molecule to form an initial cobalt-acylperoxo inter- Co(IV)vO operated simultaneously as active oxidant species in
III
mediate (Co -OOC(O)R, 3). Then, 3 underwent either a hetero- the epoxidation reactions. Under extreme conditions, in which
V
IV
lysis or a homolysis to afford Co vO or Co vO species, the concentration of an active substrate was very high, the
V
respectively. The resulting Co vO intermediate formed from Co(III)-OOC(O)R species might also be a possible reactive
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species for epoxidation. Moreover, similar reactivity patterns 2818 (w), 2741 (w), 1606 (m), 1579 (m), 1560 (m), 1519 (w),
between this cubane-type cobalt-cluster system and the pre- 1488 (w), 1447 (m), 1412 (w), 1378 (w), 1291 (s), 1244 (s),
viously reported mononuclear nonheme cobalt(III) complex 1223 (s), 1163 (m), 1074 (w), 1032 (s), 1011 (m), 848 (w),
V
IV
suggested that Co vO or Co vO species might operate sim- 837 (w), 821 (w), 803 (m), 790 (w), 776 (w), 750 (w), 743 (w),
ultaneously as key reactive oxidants in all the cobalt-catalyzed 719 (w), 701 (m), 689 (m), 638 (s), 612 (m), 586 (w), 575 (w).
olefin epoxidations. Our future work will focus on observing Anal. Calcd for 1 or Co
spectroscopic evidence for the high-valent cobalt-oxo species N, 3.65. Found: C, 46.78; H, 3.10; N, 3.62.
Co(V)vO and Co(IV)vO) described herein.
4 60 46 10 4 14 2
C H F N O S : C, 46.89; H, 3.02;
(
X-Ray single crystal structural analysis
A single crystal was mounted at room temperature on the tips
of quartz fibers, coated with Paratone-N oil, and cooled under
a stream of cold nitrogen. Intensity data were collected on a
Bruker CCDC area diffractometer running the SMART software
4
General
. Experimental section
Olefins, epoxides, 2-cyclohexen-1-ol, 2-cyclohexen-1-one, benz-
aldehyde, acetonitrile, dichloromethane, dodecane, MCPBA
2
4
package, with Mo Kα radiation (λ = 0.71073 Å). The structure
2
1
8
18
was solved by direct methods and refined on F by using the
2
(65%), and H O (97% O enrichment) were purchased from
2
5
SHELXTL software package. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms of the bridged
hydroxo O–H groups in the core were assigned from the elec-
tron density map; all other hydrogen atoms were assigned at
idealized positions and given thermal parameters equivalent
to 1.2 times the thermal parameter of the carbon atom to
which they were attached. Data collection and experimental
details of the complex are summarized in Table 1, and relevant
interatomic distances and angles are listed in Tables 2 and 3.
Structural information was deposited at the Cambridge
Crystallographic Data Center (CCDC reference number for 1 is
Aldrich Chemical Co. and were used without further purifi-
cation. PPAA was synthesized according to the literature
methods. All manipulations for the synthesis of the cubane
1
2a
cluster 1 were performed under anaerobic conditions. All the
solvents were distilled and dried prior to use: CH
tilled over CaH , and tetrahydrofuran (THF) and diethyl ether
Et O) were distilled over a mixture of sodium and benzo-
2 2
Cl was dis-
2
(
2
phenone under a nitrogen atmosphere. The m-terphenyl-based
carboxylate
carboxylic acid (HO2CAr
ture procedures. The sodium salt (NaO
ligand,
4,4″-fluoro-[1,1′:3′,1″-terphenyl]-2′-
4
F-Ph
), was prepared according to litera-
2
3
4F-Ph
2
CAr
) of this
1408240).
carboxylic acid was prepared by treating a MeOH solution of
the free acid with 1 equiv. NaOH and removing the volatile
fractions under reduced pressure.
Catalytic olefin epoxidations by MCPBA in the presence of the
cobalt cubane cluster 1
Instrumentation
MCPBA (0.1 mmol) was added to a mixture of substrate
(
0.035 mmol), cubane-type cobalt cluster 1 (0.001 mmol), and
Product analysis for olefin epoxidation, PPAA experiments, and
O incorporation reactions of cyclohexene oxide were con-
1
8
solvent (CH CN/CH Cl , 1 : 1 v/v; 1 mL). The mixture was
3
2
2
stirred for 10 min at room temperature. Each reaction was
monitored by GC/mass analysis of 20 μL aliquots withdrawn
periodically from the reaction mixture. Dodecane was used as
an internal standard to quantify the yields of the products and
conversions of the substrates. All reactions were run at least in
triplicate, and the average conversions and product yields are
presented. Conversions and product yields were calculated
with respect to substrates.
ducted by using a PerkinElmer gas chromatograph equipped
with a FID detector and a 30 m capillary column (Hewlett-
Packard, DB-5 or HP-FFAP). IR spectra were recorded on a Bio-
Rad FTS 135 spectrometer as KBr pellets. UV-vis spectra were
recorded with a PerkinElmer model Lambda 2S UV/Vis
spectrometer. SW-EPR spectra were recorded at 5 K using an
X-band Bruker EMX-plus spectrometer equipped with a dual
mode cavity (ER 4116DM).
4
F-Ph
Synthesis of [Co
4
(μ
3
-OH)
4
(μ-O
2
CAr
)
2
(μ-OTf)
2
(Py)
4
] (1)
·2MeCN
Competitive reactions of styrene and para-substituted styrenes
for the Hammett plot
To a rapidly stirred THF (5 mL) solution of Co(OTf)
2
4
F-Ph
(
0
0
271 mg, 0.617 mmol) was added NaO
2
CAr
.308 mmol). After 1 h of stirring, pyridine (Py, 68.3 mg, (0.02 mmol) and para(X)-substituted styrene (0.02 mmol, X =
.863 mmol) and triethylamine (62.4 mg, 0.617 mmol) were –OCH , –CH , –Cl, and –CN), cubane-type cobalt cluster 1
(11.1 µL, (0.001 mmol), and solvent (CH CN/CH Cl , 1 : 1 v/v; 1 mL).
.617 mmol) was added after 30 min. After 1 h of stirring, the The mixture was stirred for 10 min at room temperature. The
volatile portion of the solution was removed under reduced amounts of styrene before and after the reactions were
pressure. The solid portion was dissolved in CH Cl (3 mL), measured by GC. The relative reactivities were determined
and the resulting inhomogeneous dispersion was filtered using the following equation: k /k = log(X /X )/log(Y /Y ) where
(0.100 g, MCPBA (0.03 mmol) was added to a mixture of styrene
3
3
sequentially added. Argon-saturated
H
2
O
3
2
2
0
2
2
x
y
f
i
f
i
through Celite. Red color block crystals of 1 were isolated upon
X
i
and X
f
are the initial and final concentrations of para-substi-
and Y are the initial and final concen-
vapor diffusion of Et O into the filtrate after 4 days. FT-IR tuted styrenes and Y
2
i
f
(
KBr, cm− ) 3651 (w), 3637 (w), 3446 (b), 3067 (w), 3001 (w), trations of styrene.
1
12
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1
8
18
O-labeled H
2
O experiments
N. S. Finney, P. J. Pospisil, M. L. Guler, T. Ishida and
E. N. Jacobsen, J. Am. Chem. Soc., 1998, 120, 948;
(h) R. Zhang and M. Newcomb, Acc. Chem. Res., 2008, 41,
468; (i) M. E. Crestoni, S. Fornarini and F. Lanucara, Chem.
To a mixture of cyclohexene (0.005 mmol), cobalt catalyst
1
8
(
0.001 mmol), and H
2
O (10–40 μL, 0.556–2.22 mmol; 97%
1
8
O enriched, Aldrich Chemical Co.) in a dried solvent CH CN/
Cl (1 : 1 v/v; 1 mL) was added MCPBA (0.01 mmol). The
2 2
3
–
Eur. J., 2009, 15, 7863; ( j) C. Mukherjee, A. Stammler,
CH
H. Bogge and T. Glaser, Inorg. Chem., 2009, 48,
9476.
reaction mixture was stirred for 3 min at room temperature
1
6
and then directly analyzed by GC/mass analysis. The O and
1
8
3 (a) J. Rosenthal and D. G. Nocera, Acc. Chem. Res., 2007, 40,
543; (b) X. Zhang, K. Sasaki and C. L. Hill, J. Am. Chem.
Soc., 1996, 118, 4809; (c) T. Punniyamurthy, S. S. J. Kalra
and J. Iqbal, Tetrahedron Lett., 1995, 36, 8497; (d) D. Dhar,
Y. Koltypin, A. Gedanken and S. Chandrasekaran, Catal.
Lett., 2003, 86, 197; (e) I. Fernández, J. R. Pedro,
A. L. Rosello, R. Ruiz, I. Castro, X. Ottenwaelder and
Y. Journaux, Eur. J. Org. Chem., 2001, 1235; (f) J. D. Koola
and J. K. Kochi, J. Org. Chem., 1987, 52, 4545;
O compositions in cyclohexene oxide were determined by the
1
6
relative abundance of mass peaks at m/z = 97 for O and m/z =
9
average values are presented.
1
8
9 for O. All reactions were run at least three times and the
Analysis of the O–O bond cleavage products from the
oxidation reactions of substrates by PPAA in the presence of
the cubane-type cobalt(II) cluster 1
PPAA (0.04 mmol) was added to a mixture of substrate
(
g) R. V. Patel, J. G. Panchal and S. K. Menon, J. Inclusion
(0–0.16 mmol), cubane-type cobalt cluster 1 (0.001 mmol), and
Phenom. Macrocyclic Chem., 2010, 67, 63; (h) W. Nam,
I. Kim, Y. Kim and C. Kim, Chem. Commun., 2001, 1262;
solvent (distilled CH CN/CH Cl , 1 : 1 v/v; 1 mL). The mixture
3
2
2
was stirred for 10 min at room temperature. Each reaction was
monitored by GC/mass analysis of 20 μL aliquots withdrawn
periodically from the reaction mixture. Dodecane was used as
an internal standard to quantify the yields of products and
conversions of substrates. All reactions were run at least in
triplicate, and the average conversions and product yields are
presented. Conversions and product yields were calculated
with respect to the substrate or PPAA.
(
i) F. F. Pfaff, S. Kundu, M. Risch, S. Pandian, F. Heims,
I. Pryjomska-Ray, P. Haack, R. Metzinger, E. Bill, H. Dau,
P. Comba and K. Ray, Angew. Chem., 2011, 123, 1749,
(
Angew. Chem., Int. Ed., 2011, 50, 1711); ( j) J. G. McAlpin,
Y. Surendranath, M. Dinca, T. A. Stich, S. A. Stoian,
W. H. Casey, D. G. Nocera and R. D. Britt, J. Am. Chem. Soc.,
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Commun., 2000, 605; (l) P. Sarmah and B. K. Das, Indian
J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem.,
2
014, 53, 41; (m) A. Sartorel, M. Bonchio, S. Campagna and
Acknowledgements
F. Scandola, Chem. Soc. Rev., 2013, 42, 2262; (n) V. Artero
and M. Fontecave, Chem. Soc. Rev., 2013, 42, 2338.
The Korea CCS R&D Center and the Basic Science Research
Program of the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
4
5
(a) T. J. Collins, S. Ozaki and T. G. Richmond, J. Chem. Soc.,
Chem. Commun., 1987, 803; (b) W. Nam, J. Y. Ryu, I. Kim
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K. S. Min, Dalton Trans., 2011, 40, 5762.
(
NRF-2014R1A2A1A11051794 and NRF-2015R1A2A2A09001301)
are gratefully acknowledged.
(a) R. Chakrabarty, S. J. Bora and B. K. Das, Inorg. Chem.,
2
007, 46, 9450; (b) F. Evangelisti, R. Guttinger, R. More,
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