Synthesis, Characterization, and Catalytic Activities of A Nickel(II) Monoamido-Tetradentate Complex: Evidence For NiIII–Oxo and NiIV–Oxo Species

Article abstract of DOI:10.1002/chem.201605157

A new mononuclear nickel(II) complex, [Ni^II(dpaq)Cl] (1), containing a tetradentate monoamido ligand, dpaq (dpaq=2-[bis(pyridin-2-ylmethyl)amino]-N-(quinolin-8-yl)acetamide), has been synthesized and characterized by IR spectroscopy, elemental analysis, and UV/Vis spectroscopy. The structure of the nickel complex has been determined by X-ray crystallography. This nonheme Ni^II complex 1 catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Olefin epoxidation using this catalytic system has been proposed to involve a new reactive Ni^IV–oxo (4) species, based on the evidence from a PPAA (peroxyphenylacetic acid) probe, Hammett studies, H₂¹⁸O exchange experiments, and ESI mass spectroscopic analysis. Moreover, the nature of solvent significantly influenced partitioning between heterolytic and homolytic O?O bond cleavage of the Ni–acylperoxo intermediate (2). The O?O bond of 2 proceeded predominantly through heterolytic cleavage in a protic solvent, such as CH₃OH. These results suggest that possibly a Ni^IV–oxo species is a common reactive intermediate in protic solvents. The two active oxidants, namely Ni^IV–oxo (3) and Ni^III–oxo (4), which are responsible for stereospecific olefin epoxidation and radical-type oxidations, respectively, operate in aprotic solvents.

Full text of DOI:10.1002/chem.201605157

A Journal of
Accepted Article
Title: Synthesis, characterization, and catalytic activities of nickel(II)
monoamido-tetradentate complex: Evidence for NiIII-Oxo and
NiIV-Oxo species
Authors: Cheal Kim, Kwon Hee Bok, Myoung Mi Lee, Ga Rim You, Hye
Mi Ahn, Ka Young Ryu, Sung-Jin Kim, and Youngmee Kim
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To be cited as: Chem. Eur. J. 10.1002/chem.201605157
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Synthesis, characterization, and catalytic activities of nickel(II)
monoamido-tetradentate complex: Evidence for NiIII-Oxo and NiIV-
Oxo species
Kwon Hee Bok,[a] Myoung Mi Lee,[a] Ga Rim You,[a] Hye Mi Ahn,[a] Ka Young Ryu,[a] Sung-Jin Kim,[b]
Youngmee Kim,[b] Cheal Kim*[a]
Abstract: A new mononuclear nickel(II) complex, [Ni(II)(dpaq)Cl] (1),
containing tetradentate monoamido ligand, dpaq (dpaq=2-
metal-based catalysts. Specifically, considerable interest is
currently being paid to the chemistry of nonheme high-valent
metal-oxo species (MIV=O or MV=O: M=Fe or Mn) as they have
been identified and/or implicated in the catalytic cycles of a
number of heme and nonheme metal-containing enzymes that
activate or evolve dioxygen.[6-12] For example, several high-valent
iron-oxo intermediates have been investigated as biomimetic
chemical models of cytochrome P450[13-15] and as catalysts in
alkane hydroxylation, alkene epoxidation and sulfoxidation
reactions in various oxygenation reactions.[16-30] Manganese-oxo
porphyrins [(Porp)MnV(O)]+ and (Porp)MnIV(O) have also been
broadly explored.
Although iron and manganese complexes are considered as
the most promising catalysts for various organic reactions and
biomimetic oxidation reactions, various metal-based catalysts for
oxidation reactions have been recently reported by replacing iron
and manganese metals with different transition metals.[31-52]
Among them, nickel complexes have come up as an attractive
transition metal catalysts for oxidation reaction because they have
been involved in enzymatic catalysis, have shown efficient
oxidation reactions and have been relatively less explored
compared to the corresponding Fe- and Mn-oxo intermediates.
High-valent nickel-oxo (NiIII-Oxo and NiIV-Oxo) species is believed
to be possible intermediates in a number of nickel based oxidation
reactions.[53,54] In this regard, although a few high-valent NiIII-Oxo
species has been demonstrated through product analyses and
spectroscopic studies, no clear evidence for NiIV-Oxo species has
been reported to-date. Many nickel complexes are inert toward
oxidants, such as dioxygen, H2O2, tert-butyl hydroperoxide (t-
BuOOH) and meta-chloroperoxybenzoic acid (MCPBA).[55]
However, Ni(II) complexes with highly electron-rich ligands are
susceptible to Ni3+/2+ redox potential rending the oxidation
reaction thermodynamically favorable. For example, Sadler et. al
reported that nickel-peptide complexes, in which the main chain
amides ligate the metal, were readily oxidized to Ni3+.[56] This
report prompted us to develop a new nickel(II) complex with
electron-rich ligand for an efficient catalytic reactivity and
investigate the physical and chemical properties of its reactive
high-valent Ni-oxo species. Therefore, we synthesized and
characterized a new nickel complex containing amide group as
an electron-rich moiety.
a
(bis(pyridin-2-ylmethyl)amino)-N-(quinolin-8-yl)acetamide), has been
synthesized and characterized by IR, elemental analysis, and UV-
visible spectroscopy. The structure of the nickel complex has been
determined by X-ray crystallography. This nonheme NiII complex 1
catalyzed epoxidation reaction of a wide range of olefins with meta-
chloroperoxybenzoic acid (MCPBA) under mild conditions. Olefin
epoxidation by this catalytic system has been proposed to involve a
new reactive NiIV-Oxo (4) species, based on the evidence from PPAA
(peroxyphenylacetic acid) probe, Hammett studies, H218O-exchange
experiments, and ESI-mass spectroscopic analysis. Moreover, the
nature of solvent significantly influenced partitioning between
heterolytic and homolytic O-O bond cleavage of Ni-acylperoxo
intermediate (2). The O-O bond of the Ni-acylperoxo intermediate 2
proceeded predominantly via heterolytic cleavage in a protic solvent
such as CH3OH. These results suggested that possibly NiIV-Oxo
species was a common reactive intermediate in protic solvents. The
two active oxidants, viz. NiIV-Oxo (3) and NiIII-Oxo (4), which are
responsible for stereospecific olefin epoxidation and radical-type
oxidations, respectively, operate in aprotic solvents.
Introduction
Development of efficient methods for selective and effective olefin
epoxidation is a topic of immense importance in chemical
synthesis,[1,2] because epoxides are well known as polyvalent
building blocks in a wide range of chemical reactions.[3,4] In
particular, oxygenation of organic substrates using metal-based
catalysts is widely studied in the field of bio-inorganic chemistry[5]
because it is selective, rapid, inexpensive, and closely related to
the biomimetic oxidation reactions of metalloenzymes.
High-valent metal-oxo species have been reported as
intermediates in the oxygenation of organic substrates using
[a]
[b]
K. H. Bok, M. M. Lee, G. R. You, H. M. Ahn, K. Y. Ryu, Prof. C.
Kim
Department of Fine Chemistry, Seoul National University of
Science and Technology
Seoul 139-743, Korea
E-mail: chealkim@seoultech.ac.kr
Prof. S.-J. Kim, Prof. Y. Kim
Herein, we report a new mononuclear nickel(II) complex,
[Ni(II)(dpaq)Cl] (1, see Scheme 1) complex, containing
a
tetradentate monoamido ligand, dpaq (dpaq=2-(bis(pyridin-2-
ylmethyl)amino)-N-(quinolin-8-yl)acetamide), which catalyzed
efficiently epoxidation reaction of a wide range of olefins in
presence of meta-chloroperoxybenzoic acid (MCPBA) under mild
Department of Chemistry and Nano Science, Ewha Womans
University
Seoul 120-750, Korea
Supporting information for this article is given via a link at the end
of the document.
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Scheme 1. Synthesis of [Ni(dpaq)Cl] complex 1.
Olefin epoxidation reaction catalyzed by nickel complex 1
conditions. Olefin epoxidation by this catalytic system is proposed
to involve a reactive NiIV-Oxo species, based on reactivity and
Hammett study, H218O-exchange experiment, ESI-mass
spectroscopic analysis and use of PPAA (peroxyphenylacetic
acid) as a mechanistic probe.
Catalytic epoxidation of various olefins by using m-
chloroperbenzoic acid (MCPBA) as a terminal oxidant and 1 as
catalyst was conducted. The oxidant MCPBA was added to a
mixture of substrate, nickel complex 1, and solvent
(CH3CN/CH2Cl2, 1:1 v/v; 1 mL). The reaction was complete within
1 min at room temperature. Control experiment suggested that
direct oxidation of substrate by MCPBA was negligible. Moreover,
UV-visible spectrophotometry showed that the complex 1 was
robust enough during the catalytic reactions. The results of the
epoxidation reaction are summarized in Table 1. Cyclic olefins
such as cyclopentene, cycloheptene, and cyclooctene were
oxidized to the corresponding epoxides with moderate good
yields (38.8-71.7%, entries 1-3). Meanwhile, cyclohexene (entry
4) afforded cyclohexene oxide (40.2%) as a major product and
small amounts of 2-cyclohexen-1-one (6.4%) and 1-cyclohexen-
1-ol (6.4%) as by-products. This result indicated that the free
radical oxidation reactions were a little involved in this epoxidation
reaction.[57] Terminal olefins such as 1-hexene and 1-octene
Results and Discussion
Synthesis and structure description of 1
We synthesized nickel complex [Ni(dpaq)Cl] 1 by reacting NiCl2
and the ligand (dpaq) in absolute ethanol. The formation of the
nickel complex was confirmed by ESI-mass spectrometry,
elemental analysis and X-ray crystallography. Two independent
Ni(L)Cl complexes were formed in which each NiII ion is
coordinated by one ligand and one chloride forming an octahedral
complex (Fig. S1). An asymmetric unit contains two Ni(L)Cl and
two methanol solvent molecules. The structures of both Ni(L)Cl
have been shown in Figs. 1 and S2.
(entries
5
and 6), well-known as difficult-to-oxidize
substrates,[58,59] were oxidized to their corresponding epoxides
with low yields (24.2% and 21.0%, respectively). Internal olefins,
viz. cis-2-hexene and cis-2-octene were both oxidized to cis-2-
hexene oxide and cis-2-octene oxide (21.3 % and 19.3%; entries
7 and 9) in addition to similar amounts of trans-2-hexene oxide
(26.4%) and trans-2-octene oxide (21.6%). Formation of these
trans-epoxides in quite a large amount is a very rare case in
metal-catalyzed olefin epoxidation to the best of our knowledge,
and suggested that Ni-oxo-olefin-radical intermediate might have
a long lifetime, which is enough to rotate cis-olefin radical to trans-
olefin one (Scheme 2).[60]
Scheme 2. Possible trans-oxide formation mechanism from cis-olefin.
Figure 1. Structure of one Ni(L)Cl with labelling scheme. Displacement
ellipsoids are shown at the 50% probability level. The selected bond lengths (Å):
Ni1-N12 2.025(8), Ni1-N11 2.079(8), Ni1-N14 2.103(8), Ni1-N13 2.117(7), Ni1-
N15 2.124(7), Ni1-Cl11 2.414(3).
Styrene was oxidized to styrene oxide (30.8%; entry 11) with
some amounts of benzaldehyde (4.0%) and phenylacetaldehyde
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(8.8%). While cis-stilbene was oxidized to cis-stilbene oxide
(2.1%; entry 12) and trans-stilbene oxide (23.9%) along with some
benzaldehyde (8.0%) and 2-phenylacetophenone (5.9%), trans-
stilbene was converted to trans-stilbene oxide (43.2%; entry 13)
as a major product and minor amounts of benzaldehyde (10.0%)
and 2-phenylacetophenone (2.7%). The aromatic olefins showed
the formation of aldehyde and ketone in moderate yields and a
low stereochemical retention. These results indicated that to
some extent either nickel(III)-oxo or peroxyl radical might be
involved as epoxidizing agent in this catalytic reaction. Based on
prior reports, these species would be expected to form non-
stereospecific or radical-induced rearranged products from the
epoxidation of olefins having aromatic rings.[61,62] On the other
hand, the formation (1-5%) of chlorobenzene derived from
decarboxylation of m-chlorobenzoate radical was observed for
most olefins, indicating that the intermediate NiII–OOC(O)R (2)
generated from the reaction of 1 with MCPBA undergoes a small
portion of homolysis to afford NiIII-Oxo (4) species and m-
chlorobenzoate radical.[63] Therefore, these combined results
suggested that two different oxidants, NiIV-Oxo (3) and NiIII-Oxo
(4), might have been generated in these catalytic reactions and
the NiIII-Oxo complex would be responsible for the non-
stereoretentive portion of the epoxidation reaction.
benzaldehyde
4.0±0.1
8.8±0.3
phenylacetaldehyde
cis-stilbene oxide
trans-stilbene oxide
benzaldehyde
12
cis-stilbene
2.1±0.3
23.9±2.8
8.0±0.6
5.9±0.3
0.5±0.0
2-
phenylacetophenone
13
trans-
stilbene
trans-stilbene oxide
43.2±0.9
-
benzaldehyde
10.0±1.3
2.7±2.7
2-
phenylacetophenone
[a] Reaction conditions: olefins (0.20 mmol), nickel complex 1 (0.002 mmol),
MCPBA (0.02 mmol), and solvent (1 mL, CH2Cl2/CH3CN =1:1). [b] Based on
oxidant.
Competition experiments of styrene and para-substituted
styrenes for Hammett plot
Table 1. Olefin epoxidations by MCPBA with nickel complex
1 in
In order to further understand the electronic nature of the reactive
species that are responsible for the olefin epoxidation reaction,
we studied the substituent electronic effects on the reaction rate
using styrene and four para-substituted styrenes. The Hammett
plot showed ρ value of -0.72, which suggested that the active
species was electrophilic (Figure 2). The value was lower than
that reported for [Ni(TpCF3Me)(κ2-mCPBA)] (TpCF3Me = hydrotris(3-
trifluoromethyl-5-methylpyrazolyl)borate) (ρ = -1.35).[64]
CH2Cl2/CH3CN (1:1) at room temperature.[a]
Yield
[%][b]
Chlorobenzene
Yield [%][b]
Entry
Substrate
Product
1
2
3
4
cyclopentene
cycloheptene
cyclooctene
cyclohexene
epoxide
38.8±2.6
71.7±1.7
65.0±0.4
40.2±2.6
6.4±0.3
0.9±0.1
2.0±0.0
1.5±0.2
1.7±0.4
epoxide
epoxide
epoxide
2-cyclohexene-1-ol
-OCH3
ρ = -0.72
0,8
0,3
2-cyclohexene-1-
one
6.4±0.4
-CH3
5
6
7
1-hexene
epoxide
24.2±1.1
21.0±1.8
21.3±2.5
26.4±2.9
36.2±0.1
5.5±0.3
4.5±0.2
2.1±0.3
-Cl
-CN
-H
1-octene
epoxide
-0,2
-0,7
-1,2
cis-2-hexene
cis-oxide
trans-oxide
trans-oxide
8
9
trans-2-
hexene
3.7±0.1
2.2±0.4
-0,8
-0,4
0
0,4
0,8
σ+
cis-2-octene
cis-oxide
19.3±0.1
21.6±0.9
1.49
Figure 2. Hammett plot for selective reactivity of styrene to para-substituted
styrenes by nickel complex 1 with MCPBA.
trans-oxide
cis/trans -oxide
cis/trans-2-
octene
H218O-exchange experiments
10
11
trans-2-
octene
trans-oxide
34.0±0.8
30.8±0.8
6.0±0.5
-
Isotope labeling experiments were carried out because 18O-
labeling experiments are commonly used to study reaction
mechanisms of understanding the source of oxygen atoms found
styrene
epoxide
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in oxidation products.[32] Epoxidation of cyclohexene by nickel
complex 1 and MCPBA was conducted in presence of excess
H218O in a mixture of distilled CH3CN and anhydrous CH2Cl2 (v/v,
1:1).
generated from the O-O bond cleavage of Ni(II)-OOC(O)R
species in the olefin epoxidation.
Product distribution of the O−O bond cleavage of PPAA with
catalyst 1
Table 2. Percentage of 18O-incorporated H218O into cyclohexene oxide
formed during cyclohexene oxidation by MCPBA and nickel complex 1.[a]
To gain insight into the O-O bond activation mechanism, we used
peroxyphenylacetic acid (PPAA) as a mechanistic probe. This is
because the heterolytic and homolytic O–O bond cleavage mode
of Ni–OOC(O)R (2) species could be easily determined by
quantitative analysis of its degradation products.[37,57a,58,59,65]
While heterolytic cleavage of the O–O bond of the Ni-acylperoxo
species 2 generates phenylacetic acid (PAA, 5) (pathway (a) in
Scheme 3), O–O bond is cleaved homolytically to form an acyloxyl
radical, which affords benzaldehyde (6), benzyl alcohol (7), and
toluene (8) via a rapid β-scission of the acyloxyl radical (pathway
(c) of Scheme 3). On the other hand, the direct oxidation reaction
of the Ni-acylperoxo intermediate 2 with substrate affords PAA,
and apparently affects the O–O bond cleavage mode (pathway
(b) in Scheme 3). First, an experiment with PPAA as oxidant under
the same reaction conditions as used for MCPBA was carried out
in presence of cyclopentene (Table 3, entry 1). The heterolytic
cleavage product, phenylacetic acid (67.9%), and the homolytic
cleavage product, benzaldehyde (20.6%), were formed,
demonstrating that the NiII-OOC(O)R species generated from the
reaction of 1 and PPAA underwent a partitioning of 74%
heterolysis and 26% homolysis to produce NiIV-Oxo (3) and NiIII-
Oxo (4) species, respectively. These results showed a strong
evidence that reactive NiIV-Oxo species could be generated from
the heterolysis of NiII-acylperoxo intermediate 2, while Ray et. al
Amount
of
Amount Amount
of of
Amount of
cyclohexene
(M)
18O in cyclohexene
oxide (%)
Entry
MeOH
(%)
MCPBA H218O
(M)
(M)
1.111
1.111
1.111
1
2
3
0.05
0.05
0.05
0
3
0.005
0.005
0.005
7.4±0.2
12.5±0.5
14.7±0.5
10
[a] Reaction conditions: nickel complex 1 (0.002 mmol), cyclohexene (0.05
mmol), MCPBA (0.005 mmol), and solvent (1 mL, distilled CH3CN/
anhydrous CH2Cl2=1:1 )
As shown in Table 2, the cyclohexene oxide product contained
7.4% of 18O derived from H218O (entry 1). These results suggested
that a high-valent Ni-Oxo species might act as the reactive
intermediate in olefin epoxidation. Next, we increased the amount
of CH3OH up to 10 % under the same condition in order to study
the protic solvent effects. 18O incorporation in the epoxide product
gradually increased from 7.4 % to 14.7 % (entries 1-3), indicating
that more portion of the high-valent Ni-Oxo species was
Table 3. Yield of products derived from PPAA mediated by the nickel complex 1 in the presence of various olefins in a mixture of CH3CN/CH2Cl2 (1/1) at room
temperature.[a]
Heterolysis[b]
Homolysis[b]
Hetero/Homo
[5/(6+7+8)][b]
Entry
Substrates
Oxidation products[b]
5
6
7
-
8
-
1
2
3
4
cyclopentene
cycloheptene
cyclooctene
cyclohexene
61.8±0.2
66.7±0.3
64.0±2.1
67.9±1.7
21.8±0.8
20.3±1.4
19.7±0.7
20.6±1.2
73.9/26.1
76.7/23.3
76.5/23.5
76.7/23.3
oxide
23.4±1.5
63.1±1.0
47.5±1.3
33.8±1.1
4.0±0.1
4.7±0.0
3.1±0.2
5.0±0.0
24.2±1.5
6.6±0.6
22.4±0.5
23.8±0.5
6.0±0.1
24.8±0.9
-
-
oxide
oxide
-
-
-
-
oxide
ol
one
5
6
7
1-hexene
1-octene
64.8±1.6
67.1±2.9
64.1±0.0
24.6±0.9
18.3±8.9
19.0±0.8
-
-
-
-
-
-
72.5/27.5
78.6/21.4
75.5/24.5
oxide
oxide
cis-2-hexene
cis-oxide
trans-oxide
oxide
8
9
trans-2-hexene
cis-2-octene
59.3±3.1
63.9±2.2
23.5±1.1
20.6±0.9
-
-
-
-
71.7/28.3
75.7/24.3
cis-oxide
trans-oxide
oxide
10
trans-2-octene
63.6±1.7
25.7±0.1
71.2/28.8
[a] Reaction conditions: substrate (0.20 mmol), catalyst (0.002 mmol), PPAA (0.02 mmol), and solvent (1 mL, CH3CN/CH2Cl2=1:1). [b] Based on PPAA; 5-8
indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively.
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reported that the reactive NiIV-Oxo species was observed under
the conditions of ESI-MS experiment.[66]
of Mn(III)-acylperoxo species might become a predominant
reactive intermediate in protic solvents, such as CH3OH.[12b] We
gradually increased the amount of methanol used as a protic
solvent source in the aprotic solvent system (CH2Cl2/CH3CN=1:1),
ranging from 0% to 50%. With the gradual increase of CH3OH,
the percentage of heterolysis increased and displayed saturation
at about 30% methanol (Figure 3 and Table 4). These results
indicated that in Ni complex-catalyzed olefin epoxidation
reactions, the protic solvent pushes the O-O bond cleavage of
NiII–OOC(O)R species to heterolysis, maybe, by general-acid
catalysis.[65,67]
Mechanism study
Scheme 3. Plausible degradation of PPAA for the formation of the high-valent
nickel-oxo active oxidants.
The most plausible mechanism for the formation of the reactive
species responsible for olefin epoxidation reaction with Ni
complex has been proposed in Scheme 4. Peracid reacts with
nickel(II) complex 1 to form a Ni-acylperoxo species (NiII-
OOC(O)R (2)), which then undergoes either a heterolysis (~75 %)
or homolysis (~25 %) to afford NiIV-Oxo or NiIII-Oxo species in
aprotic solvent (CH2Cl2/CH3CN=1:1). The resulting NiIV-Oxo
intermediate generated from the heterolysis might be responsible
for the olefin epoxidation, whereas a NiIII-Oxo complex generated
from homolysis might be ascribed to a radical-type oxidation with
a loss of stereospecificity. Little substrate effect on the ratio of
In order to further investigate variation in the ratio of
heterolysis to homolysis of the O–O bond cleavage according to
olefin types, various olefins such as cyclic, terminal, and internal
aliphatic olefins were used. Regardless of the type of substrates,
similar ratios (72/28~78/22) of heterolysis to homolysis were
observed (entries 2-10). These results demonstrated that the Ni-
acylperoxo species 2 underwent the partition of ~75 % heterolysis
and ~25 % homolysis toward most olefins, and it might not act as
a reactive oxidant. By contrast, most Mn- and some Fe- and Co-
acylperoxo species are involved in olefin epoxidation
reactions.[37,65a-c]
Furthermore, we also studied the solvent effects on the O-
Table 4. Yield of products derived from PPAA mediated by the nickel complex 1 in the presence of cyclohexene in the various amounts of methanol as protic
solvent at room temperature.[a]
Heterolysis[b]
Homolysis[b]
Oxidation products[b,c]
Solvent
(MeOH :
Hetero/Homo
[5/(6+7+8)][b]
Entry
5
6
7
8
oxide
33.8
34.0
34.5
34.3
14.9
7.5
one
4.7
3.7
3.2
2.5
-
ol
4.0
3.4
3.0
2.9
-
MeCN : MC)
1
2
3
4
5
6
7
0 : 50 : 50
1 : 49.5 : 49.5
3 : 48.5 : 48.5
5 : 47.5 : 47.5
10 : 45 : 45
67.9±1.7
70.6±1.4
72.1±4.4
70.0±2.3
74.7±6.6
85.8±4.6
81.4±0.6
20.6±1.2
19.1±1.1
14.7±0.1
11.3±0.3
10.0±0.3
9.1±0.2
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3.30 (76.7/23.3)
3.70 (78.6/21.4)
4.90 (83.1/16.9)
6.19 (86.2/13.8)
7.47 (88.2/11.8)
9.43 (90.4/9.6)
10.71 (91.5/8.5)
30 : 35 : 35
-
-
50 : 25 : 25
7.6±0.4
8.7
-
-
[a] Reaction conditions: cyclohexene (0.20 mmol), nickel complex 1 (0.002 mmol), PPAA (0.02 mmol), and solvent (1 mL, CH3OH/CH3CN/CH2Cl2). [b] Based on
PPAA; 5-8 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively. [c] oxide, one and ol indicate cyclohexene oxide, cyclohexenone
and cyclohexenol respectively.
O bond cleavage of Ni-acylperoxo species 2 (Table 4). In this
context, it has been previously reported that in Mn(saloph)
complex, high-valent Mn(V)-oxo species derived from heterolysis
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heterolysis vs homolysis indicated that the intermediate NiII-
OOC(O)R might not act as a reactive oxidant.
be an active NiIV-Oxo species, as obtained from product analyses,
Hammett study, H218O-exchange experiments, ESI-MS and use
of PPAA as a mechanistic probe. Importantly, this is the strong
evidence that reactive NiIV-Oxo species can be generated from
the heterolysis of NiII-acylperoxo intermediate. Moreover,
partitioning between the heterolytic and homolytic O-O bond
cleavage of Ni-acylperoxo intermediate was significantly
affected by the nature of solvent, and the O-O bond of the Ni-
acylperoxo intermediate proceeded predominantly via
heterolytic cleavage in a protic solvent such as CH3OH. These
results suggested that possibly NiIV-Oxo species was the common
reactive intermediate in protic solvents, whereas the two active
oxidants, NiIV-Oxo and NiIII-Oxo, operated in aprotic solvents. Our
future work will focus on further studying the biologically relevant
reactivity of nickel(II) complex/H2O2 systems and obtaining
spectroscopic evidence for the NiIV-Oxo species.
Scheme 4. Plausible mechanism for the formation of the reactive species from
the reaction of peracids with the nickel complex 1.
In protic solvent, Ni(II)-OOC(O)R (2) undergoes heterolytic
cleavage predominantly to produce
a
discrete NiIV-Oxo
intermediate as a dominant reactive species.
In order to observe the proposed reactive intermediates,
NiII-OOC(O)R, NiIV-Oxo, and NiIII-Oxo, we used spectroscopic
instruments, such as EPR and ESI-mass. The EPR spectrum
obtained from the reaction of 1 with 6 equiv of MCPBA in a solvent
mixture of CH2Cl2 and CH3CN at 0 oC showed an axial signal with
g values of 2.18 and 2.10, which is a typical for Ni(III) electron
configuration (Fig. S3). In addition, ESI-mass studies indicated
the formation of a possible NiIV-Oxo species, as shown in Fig. 4.
The positive-ion mass spectrum indicated that the peak at m/z =
534.20 was assignable to [(dpaq)NiIV-Oxo + 1 solvent]+ [calcd,
534.11] intermediate.
Figure 4. Positive-ion electrospray ionization mass spectrum obtained 1 min
after MCPBA (0.02 mmol) was added into a mixture of 1 (0.002 mmol) and
cyclohexene (0.20 mmol) in CH3CN/CH2Cl2 (1/1).
Experimental Section
General: Olefins, epoxides, 2-cyclohexen-1-ol, 2-cyclohexen-1-one,
ethanol, methanol, acetone, 3-chloropropionyl chloride, 8-aminoquinoline,
2,2’-dipicolyamine, N,N-diisopropylamine, potassium iodide, pyridine,
NiCl2, chloroform, diethyl ether, ethanol, methanol, acetonitrile,
dichloromethane, dodecane, MCPBA (65%), and H218O (97% 18O
enrichment) were purchased from Aldrich Chemical Co. and were used
without further purification. Peroxyphenylacetic acid (PPAA) was
synthesized according to literature methods.[65a,d]
Figure 3. Percentage of heterolysis derived from PPAA mediated by the nickel
complex 1 in the various ratios of methanol as protic solvent.
Instruments: Product analysis for olefin epoxidation, PPAA experiment,
and 18O incorporation reactions of cyclohexene oxide were conducted by
using Perkin Elmer gas chromatograph equipped with a FID detector and
a 30 m capillary column (Hewlett-Packard, DB-5 or HP-FFAP). 1H NMR
spectra were recorded on a Varian 400 MHz spectrometer and chemical
shifts were recorded in ppm. Electro spray ionization mass spectra (ESI-
MS) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM
Advantage MAX quadrupole ion trap instrument by infusing samples
directly into the source using a manual method. Spray voltage was set at
4.2 kV, and the capillary temperature was at 80 oC. Absorption spectra
Conclusions
To conclude, a new mononuclear nickel(II) complex was designed
and synthesized which contained a tetradentate monoamido
ligand and displayed efficient catalytic activity toward numerous
olefin epoxidation reactions using MCPBA as oxidant under mild
conditions. The active oxidant in these systems was proposed to
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10.1002/chem.201605157
Chemistry - A European Journal
FULL PAPER
were recorded at room temperature using a Perkin Elmer model Lambda
2S UV/Vis spectrometer. Elemental analysis for carbon, nitrogen and
hydrogen was carried out using a Flash EA 1112 elemental analyzer
(thermo) at the Organic Chemistry Research Center of Sogang University,
Korea. IR spectra were measured on a BIO RAD FTS 135 spectrometer
as KBr pellets.
periodically from the reaction mixture. Dodecane as an internal standard
was used 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.
Competitive reactions of styrene and para-substituted styrenes by
the [Ni(dpaq)Cl] complex 1 with MCPBA: MCPBA (0.01 mmol) was
added to a mixture of styrene (0.01 mmol) and para(X)-substituted styrene
(0.01 mmol, X = -OCH3, -CH3, -Cl, and -CN), nickel complex 1 (0.002
mmol), and solvent (CH3CN/CH2Cl2 1:1 v/v; 1 mL). The mixture was stirred
for 10 min at room temperature. The amounts of styrenes before and after
reactions were measured by GC. The relative reactivities were determined
using the following equation: kx/ky = log(Xf/Xi)/log(Yf/Yi) where Xi and Xf are
the initial and final concentration of para-substituted styrenes and Yi and
Yf are the initial and final concentration of styrene.[65b]
Preparation of the ligand dpaq: 2-Chloro-N-(quinolin-8-yl)acetamide
(0.46 g, 2.1 mmol), bis((pyridin-2-yl)methyl)amine (0.39 g, 2 mmol), N,N-
diisopropylethylamine (0.39 mL, 2.2 mmol) and potassium iodide (0.37 g,
2.2 mmol) were dissolved in acetonitrile (30 mL), stirred and refluxed for 1
day under a nitrogen atmosphere. The mixture was cooled to room
temperature and the solvent was removed under reduced pressure to
obtain a yellow oil, which was purified by silica gel column chromatography
(45 : 1 v/v CHCl3–CH3OH). Yield: 1.61 g (89.6%). 1H NMR (400 MHz,
DMSO-d6, 25 °C): δ = 11.43 (s, 1H), 9.05 (d, 1H), 8.61 (d, 1H), 8.48 (d,
2H), 8.42 (d, 1H), 7.87 (d, 2H), 7.78 (t, 2H), 7.67 (m, 2H), 7.56 (t, 1H), 7.24
(t, 2H), 3.96 (s, 4H), 3.54 (s, 2H).
18O-labeled H218O experiments: To a mixture of cyclohexene (0.05 mmol),
nickel catalyst 1 (0.002 mmol), and H218O (20 L, 1.11 mmol; 97% 18O
enriched, Aldrich Chemical Co.) in a dried solvent mixture(CH3CN/CH2Cl2
1:1; 1 mL) was added MCPBA (0.005 mmol). The reaction mixture was
stirred for 10 min at room temperature and then directly analyzed by
GC/Mass. The 16O and 18O composition in cyclohexene oxide were
Preparation of [Ni(dpaq)Cl] complex 1: A solution of 2-(bis(pyridin-2-
ylmethyl)amino)-N-(quinolin-8-yl)acetamide (0.39 g, 1 mmol) in absolute
ethanol was added to a solution containing NiCl2 (0.19 g, 1.5 mmol) in
absolute ethanol. It was stirred for 12 h at room temperature. A right-brown
precipitate was filtered, washed several times with ethanol and dried in
vacuum to obtain the pure solid. LRMS (ESI): m/z calcd for C23H20N5NiO :
440.10; found 440.13. IR(KBr) : ν(cm–1) = 3309(w), 2360(w), 1648(m),
1596(s), 1461(w), 1396(m), 1306(w), 963(m), 767(s). Anal. calcd for
C24H24ClN5NiO2 : C, 56.67; H, 4.76; N, 13.77. Found: C, 56.62; H, 4.57; N,
13.88.Crystals of 1 were obtained by slow diffusion of hexane into
methanol solution.
determined by the relative abundance of mass peaks at m/z = 97 for 16
and m/z = 99 for 18O. All reactions were run at least twice.
O
Analysis of the O−O bond cleavage products from the oxidation
reactions of substrates by PPAA in the presence of nickel complex
1: All of the reaction conditions are the same as the MCPBA experiment
except for using PPAA as an oxidant.
X-ray
crystallography:
A light
yellow plate-like specimen
Acknowledgements
of C24H24ClN5NiO2, approximate dimensions 0.020 mm x 0.080 mm
x 0.320 mm, was used for the X-ray crystallographic analysis. The X-ray
intensity data were measured. A total of 2041 frames were collected. The
total exposure time was 11.35 h. The frames were integrated with the
Bruker SAINT software package using a narrow-frame algorithm. The
integration of the data using an orthorhombic unit cell yielded a total
of 135742 reflections to a maximum θ angle of 28.40° (0.75 Å resolution),
of which 11162 were independent (average redundancy 12.161,
completeness = 99.8%, Rint = 18.41%, Rsig = 11.49%) and 7407 (66.36%)
Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education,
Science and Technology (NRF-2014R1A2A1A11051794, NRF-
2016M3D3A1A01913239 and NRF-2016M3D3A1A01913239 for
the Korea C1 Gas Refinery R&D Center) are gratefully
acknowledged.
were
greater
than
2σ(F2). The
final
cell
constants
volume
of a = 14.8374(9) Å, b = 8.7974(5) Å, c = 34.332(2) Å,
= 4481.4(5) Å3, are based upon the refinement of the XYZ-centroids
of 9106 reflections above 20 σ(I) with 4.745° < 2θ < 52.05°. Data were
corrected for absorption effects using the multi-scan method (SADABS).
The ratio of minimum to maximum apparent transmission was 0.882. The
calculated minimum and maximum transmission coefficients (based on
crystal size) are 0.7370 and 0.9800. The final anisotropic full-matrix least-
squares refinement on F2 with 595 variables converged at R1 = 8.08%, for
the observed data and wR2 = 20.07% for all data. The goodness-of-fit
was 1.014. The largest peak in the final difference electron density
synthesis was 3.697 e-/Å3 and the largest hole was -0.592 e-/Å3 with an
RMS deviation of 0.171 e-/Å3. On the basis of the final model, the
Supplementary data
Supplementary data related to this article can be found at http: //
Keywords: nickel complex • olefin epoxidation • multiple reactive
oxidants • nickel-oxo species
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10.1002/chem.201605157
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Kwon Hee Bok, Myoung Mi Lee, Ga Rim
You, Hye Mi Ahn, Ka Young Ryu, Sung-
Jin Kim, Youngmee Kim, Cheal Kim*
Page No. – Page No.
Synthesis,
catalytic
monoamido-tetradentate
Evidence for NiIII-Oxo and NiIV-Oxo
species
characterization,
and
activities
of
nickel(II)
complex:
Text for Table of Contents
NiIV-Oxo vs NiIII-Oxo: A new mononuclear nickel(II) complex [Ni(II)(dpaq)Cl] displayed efficient catalytic activity toward
numerous olefin epoxidation reactions. Partitioning between the heterolytic and homolytic O-O bond cleavage of Ni-acylperoxo
intermediate was significantly affected by the nature of solvent. NiIV-Oxo species was the common reactive intermediate in protic
solvents, whereas the two active oxidants, NiIV-Oxo and NiIII-Oxo, operated in aprotic solvents.
This article is protected by copyright. All rights reserved.