One step phenol synthesis from benzene catalysed by nickel(ii) complexes

Article abstract of DOI:10.1039/c9cy01471c

Nickel(ii)complexes of N₄-ligands have been synthesized and characterized as efficient catalysts for the hydroxylation of benzene using H₂O₂. All the complexes exhibited Ni²⁺ → Ni³⁺ oxidation potentials of around 0.966-1.051 V vs. Ag/Ag⁺ in acetonitrile. One of the complexes has been structurally characterized and adopted an octahedral coordination geometry around the nickel(ii) center. The complexes catalysed direct benzene hydroxylation using H₂O₂ as an oxygen source and afforded phenol up to 41% with a turnover number (TON) of 820. This is unprecedentedly the highest catalytic efficiency achieved to date for benzene hydroxylation using 0.05 mol% catalyst loading and five equivalents of H₂O₂. The benzene hydroxylation reaction possibly proceeds via the key intermediate bis(μ-oxo)dinickel(iii) species, which was characterized by HR-MS, vibrational and electronic spectral methods, for almost all complexes. The formation constant of the key intermediate was calculated to be 5.61-9.41 × 10^-2 s^-1 by following the appearance of an oxo-to-Ni(iii) LMCT band at around 406-413 nm. The intermediates are found to be very short-lived (t_1/2, 73-123 s). The geometry of one of the catalytically active intermediates was optimized by DFT and its spectral properties were calculated by TD-DFT calculations, which are comparable to experimental spectral data. The kinetic isotope effect (KIE) values (0.98-1.05) support the involvement of nickel-bound oxygen species as an intermediate. The isotope-labeling experiments using H₂¹⁸O₂ showed 92.46% incorporation of ¹⁸O, revealing that H₂O₂ is the key oxygen supplier to form phenol. The catalytic efficiencies of complexes are strongly influenced by the geometrical configuration of intermediates, and stereoelectronic and steric properties, which are fine-tuned by the ligand architecture.

Full text of DOI:10.1039/c9cy01471c

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One step phenol synthesis from benzene
catalysed by nickelIJII) complexes†
Cite this: Catal. Sci. Technol., 2019,
9, 5991
Sethuraman Muthuramalingam, a Karunanithi Anandababu,
a
Marappan Velusamyb and Ramasamy Mayilmurugan
a
*
NickelIJII)complexes of N4-ligands have been synthesized and characterized as efficient catalysts for the
hydroxylation of benzene using H2O2. All the complexes exhibited Ni2+ → Ni3+ oxidation potentials of
around 0.966–1.051 V vs. Ag/Ag+ in acetonitrile. One of the complexes has been structurally characterized
and adopted an octahedral coordination geometry around the nickelIJII) center. The complexes catalysed
direct benzene hydroxylation using H2O2 as an oxygen source and afforded phenol up to 41% with a
turnover number (TON) of 820. This is unprecedentedly the highest catalytic efficiency achieved to date
for benzene hydroxylation using 0.05 mol% catalyst loading and five equivalents of H2O2. The benzene
hydroxylation reaction possibly proceeds via the key intermediate bis(μ-oxo)dinickelIJIII) species, which was
characterized by HR-MS, vibrational and electronic spectral methods, for almost all complexes. The
formation constant of the key intermediate was calculated to be 5.61–9.41 × 10−2 s−1 by following the
appearance of an oxo-to-NiIJIII) LMCT band at around 406–413 nm. The intermediates are found to be very
short-lived (t1/2, 73–123 s). The geometry of one of the catalytically active intermediates was optimized by
DFT and its spectral properties were calculated by TD-DFT calculations, which are comparable to
experimental spectral data. The kinetic isotope effect (KIE) values (0.98–1.05) support the involvement of
nickel-bound oxygen species as an intermediate. The isotope-labeling experiments using H218O2 showed
92.46% incorporation of 18O, revealing that H2O2 is the key oxygen supplier to form phenol. The catalytic
efficiencies of complexes are strongly influenced by the geometrical configuration of intermediates, and
stereoelectronic and steric properties, which are fine-tuned by the ligand architecture.
Received 24th July 2019,
Accepted 24th September 2019
DOI: 10.1039/c9cy01471c
rsc.li/catalysis
the low value-added by-product acetone. The major drawbacks
Introduction
of this method are
a three-step process, high energy
Phenol is one of the valuable chemical precursors for the
synthesis of various industrial products such as dyes, phenol–
consumption and excess formation of by-products such as
acetone, catechol/hydroquinone, and benzoquinone.3b The
sulfonation process is another process to synthesise phenol at
high temperature, but its main disadvantage is the poor atom
economy (36%).4a Also, phenol synthesized from
chlorobenzene by the Dow process at high temperature and
pressure. This process is no longer practiced because of the
adverse economy of chlorine and alkali production.4b Phenol
can also be produced by the oxidation of toluene in a two-
step process with a relatively lower yield.4c In fact, the poor
selectivity in most of these processes is due to phenol
oxidation, which is more reactive than benzene as it lacks
kinetic control.5 Conversely, aromatic hydroxylation is
challenging as it requires a higher C–H bond dissociation
energy (112 kcal mol−1).6 In the near future, the development
of an alternative and attractive one-step method is crucial to
produce phenol using a cheap and green oxidant, like O2,7 or
activated oxygen sources.8 Conventional methods, such as
electrochemical oxidation,9a photochemical oxidation,9b and
Fenton-type reactions,9c are known to exhibit lower yields and
formaldehyde
resins,
bisphenol
A,
caprolactam,
pharmaceuticals, medicines and raw materials for a large
number of chemicals.1 In recent years, many researchers have
been exploring new methodologies to synthesize phenol in a
single step.2 In industry, phenol is produced by the cumene
process, which is carried out at high temperature and
pressure under strong acidic conditions.3a This method
employs the propylation of benzene to form cumene, which
undergoes autoxidation to afford cumene hydrogen peroxide
(CHP) via cumene peroxide radicals. CHP undergoes the Hock
rearrangement to produce phenol with a yield of ∼5% and
a Bioinorganic Chemistry Laboratory/Physical Chemistry, School of Chemistry,
Madurai Kamaraj University, Madurai-625021, India.
E-mail: mayilmurugan.chem@mkuniversity.org
b Department of Chemistry, North Eastern Hill University, Shillong-793022, India
† Electronic supplementary information (ESI) available. CCDC: 1863321. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9cy01471c
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poor selectivity. Heterogeneous catalysts, such as TS-1,10a
SBA-15,10b molecular sieves, etc.,11 were also employed, which
are loaded with redox-active metal ions to obtain better
efficiency, wherein, leaching caused a loss of product and low
catalytic efficiency. Hence, homogeneous catalysts using
transition metals may offer better advantages as they are
capable of performing catalysis under mild conditions. Their
catalytic efficiency can be tuned by varying the ligand
architecture and redox potentials.12 Several metal complexes
with vanadium,13 iron,14 and copper15 were reported as
catalysts for benzene hydroxylation. Also, the metalloenzyme
dimethylpropane-1,3-diamine (L3), 1,4-bisij(2-pyridinylmethyl)]-
1,4-diazepane (L4) and 1,4-bisIJ2-quinolylmethyl)-1,4-diazepane
(L5) involved a simple substitution reaction18b and that of
1,4-bisIJ2-pyridinylethyl)-1,4-diazepane
(L6)
involved
a
condensation reaction as reported previously.18c The nickelIJII)
complexes [NiIJL)IJCH3CN)2]IJBPh4)2 1–6 were isolated in good
yields (78–96%) by the reaction of NiIJClO4)2·6H2O with the
respective ligands and NaBPh4 in an acetonitrile–methanol
mixture. All the complexes were characterized by elemental
analysis, ESI-MS, and electronic spectral and redox methods.
Diffusion of diethyl ether into an acetonitrile solution of 2
afforded violet colored crystals suitable for X-ray
cytochrome-P450
is
reported
to
catalyse
benzene
hydroxylation under mild conditions.16 The first nickelIJII)
complexes for benzene hydroxylation were reported in 2015
by Itoh and co-workers,17 which is the only one report
available in the literature to the best of our knowledge, so far.
They used nickelIJII) complexes of trisij2-(pyridine-2-
yl)ethyl]amine (TPEA) based ligands and H2O2 as an oxygen
source to produce 21% phenol at 60 °C with a turnover
number (TON) of 749 after 219 hours. A lower amount of
benzoquinone was also reported as a by-product.17 We,
herein, report the synthesis and characterization of nickelIJII)
complexes of N4 ligands (Scheme 1) as efficient catalysts for
selective benzene hydroxylation using H2O2 at 60 °C and 25
°C. The present ligand architectures and electronic variation
provide significant differences from those of previously
reported catalysts and thus improved catalytic performance.
To the best of our knowledge, our report is about novel
examples of NiIJII)–N4 catalysts for benzene hydroxylation,
which showed selective phenol formation with up to 41%
yield, a TON of 820 and turnover frequency (TOF) of up to
164 h−1 using a relatively lower catalyst loading (0.05 mol%)
and five equivalents of H2O2 at 60 °C. Interestingly, 21%
phenol production was achieved at 25 °C with a TON of 420
under identical conditions. These are the highest yields and
catalytic efficiencies reported so far in the literature.
crystallography
studies.
Broad
and
unassignable
1
paramagnetic H NMR spectral peaks were observed for 1–3,
however, complexes 4–6 showed reasonably sharp
paramagnetic peaks (Fig. S3†) in CD3CN. The effective
magnetic moment (μeff) was calculated to be 2.6–2.9 for the
spin-only value by the Evans method at room temperature.
The molecular structure of 2 exhibits an octahedral
coordination geometry (Fig. 1, Table S1 and S2†). Four
nitrogen atoms of the ligand and two acetonitrile molecules
complete the octahedral coordination geometry of NiIJII) and
two Ph4B− anions are present in the outer coordination
sphere. The Ni–Npy bonds (2.105 and 2.109 Å) are slightly
shorter than Ni–Namine (2.151 and 2.132 Å) due to the strong
coordination of pyridyl nitrogen atoms compared to that of
tertiary amine nitrogen atoms. These fall in the range of
bond lengths of previously reported nickelIJII) complexes.19
The nickelIJII) complexes of 1,4-diazepane based ligands (L4–
L6) are reported to display an octahedral geometry.20 All the
complexes exhibit two broad absorption bands in the ranges
of 468–594 (3A2g to T1g (F) (ν2)) and 843–934 nm (3A2g to T2g
(F) (ν1)) together with a very weak shoulder in the range of
3
3
3
1
783–792 nm corresponding to the spin-forbidden A2g to E1g
(D) transition in acetonitrile (Fig. S4 and Table S3†).20 The
redox behavior of the complexes was measured by cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
in acetonitrile using a Pt-disc and Ag/Ag+ as the working and
reference electrodes, respectively (Fig. 1, S5, S6 and Table
S3†). All of the complexes exhibit well-defined Ni2+ → Ni3+
oxidation potentials (Eox) but their corresponding reduction
peaks are not discerned (Fig. 1). The Eox of 1 (1.008 V), 2
Results and discussion
Synthesis and characterization
The ligands N,N-bisIJ2-pyridylmethyl)-N′,N′-dimethylethane-1,2-
diamine
(L1)
and
N,N-bisIJ2-pyridylmethyl)-N′,N′-
dimethylpropane-1,3-diamine (L2) have been synthesized by
the reductive amination reaction.18a The synthesis of N,N-
bisIJ4-methoxy-3,5-dimethylpyridin-2-ylmethyl)-N′,N′-
Fig. 1 X-ray crystal structure of 2 (left) and cyclic voltammograms of
1–6 (5 × 10−3 M) using a Ag/Ag+ reference electrode in acetonitrile at
Scheme 1 The N4-ligands used in the present study.
5992 | Catal. Sci. Technol., 2019, 9, 5991–6001
25 °C (right). Scan rate, 100 mV s−1
.
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(1.024 V) and 3 (1.051 V) are slightly higher than those of 4–6
(0.966–0.997 V) vs. Ag/Ag+. However, the peak potentials
observed by DPV slightly deviated from those by CV due to
irreversibility, which is consistent for all the complexes.
(58%) compared to 4 and 5, which are still lower than those
of 1–3 (Fig. 3).
The order of catalytic efficiency is 3 > 2 > 1 > 6 > 4 > 5.
In general, the complexes of tripodal ligands showed the
highest catalytic efficiencies compared to those of the cyclic
diazepane backbone, which are possibly influenced by the
coordination geometry of the proposed key intermediate
bis(μ-oxo)dinickelIJIII) species (cf. below). The complexes 1–3
are expected to stabilize cis-α coordination geometry for
intermediates by tripodal ligands. However, the diazepane
based ligands L4 and L5 are more likely to support a trans
configuration for the intermediate of 4 and 5 rather than a
cis-α or cis-β coordination geometry.18a,19b,20 On the other
hand, due to the additional methylene group, ligand L6 more
Catalytic benzene hydroxylation
The benzene hydroxylation reactions were investigated using
the nickelIJII)-complexes and H2O2 (30%) as catalysts and an
oxygen source, respectively. The catalytic reactions were
performed under optimized reaction conditions, where
benzene (0.45 mL, 5 mmol) and a catalytic amount of the
complex (2.5 μmol, 0.05 mol%) were dissolved in acetonitrile
(3.0 mL). Then, triethylamine (5.0 μmol) and 30% aqueous
H2O2 (25 mmol) were added very slowly into the reaction
mixture under constant stirring at 60 °C and the stirring was
continued for 5 hours (Scheme 2). Afterward, the reaction
mixture was passed through a silica column to remove the
catalyst. The reaction mixture was analyzed and quantified by
GC-MS/GC (Fig. S7, S8† and Table 1). Complex 1 showed
phenol formation with a yield of 31% and TON of 620,
wherein 60% selectivity was achieved. Introducing an
additional methylene group in the alkyl chain attached to the
–NMe2 group in 1 to obtain 2 showed an increased phenol
yield of up to 37% and a TON of 740 with an enhanced
selectivity of 82% under identical conditions. Further,
electron releasing groups were introduced on the pyridine
likely adopts
a
cis-α or cis-β configuration for the
bis(μ-oxo)dinickelIJIII) species, which resulted in a relatively
higher yield of phenol than 4 and 5. In addition, steric maps
and percent buried volumes (% Vbur) were calculated for all
the complexes to investigate steric impact vs. catalytic
efficiency by using the SambVca 2.1 A web application.21
A
steric map provides a graphical representation of the steric
profile of a ligand using colour-coded contour maps (Fig.
S9†). The greater steric nature of ligand L3 is revealed by a
higher % Vbur (72.7%), which showed the highest catalytic
efficiency among the tripodal ligands (L1–L3).21 Similarly, L6
displayed a % Vbur of 73.1, which is correlated to the highest
catalytic efficiency among 4–6. Overall, the highest yield of
phenol (up to 41%), TON (820) and improved selectivity
(97%) were achieved using lower catalyst (3) and H2O2
loadings at 60 °C (Table 1). This yield of phenol is
interesting, which is the highest catalytic efficiency reported
so far using a first-row transition metal catalyst and H2O2 as
an oxygen source. The catalytic efficiency of 3 is likely
increased by its electron releasing groups and the cis-α-bisIJμ-
oxo)dinickelIJIII) species (cf. below). In fact, this catalytic
efficiency is concomitantly higher than that of previously
reported nickelIJII) complexes by Itoh and co-workers, where
21% phenol formation was reported at 60 °C and 14% at 25
°C.17 A large excess of H2O2 was used by them and the
electronic nature of the ligand may have caused the partial
conversion of the catalytically active bis(μ-oxo)dinickelIJIII)
intermediate into an inactive species.22 Also, iron14 and
copper15 complexes were reported to exhibit lower yields of
phenol, 19.9% and 14.6%, respectively, with poor selectivity.
Additionally, the catalytic reactions were performed at 25
°C by maintaining identical catalyst and H2O2 loadings. The
reaction was monitored over different time intervals. The
highest yield of phenol was observed over 5 hours and then
the yield was found to be saturated for all complexes (Fig.
S10†). Under these conditions, complex 1 showed 19%
phenol formation with a TON of 380 and 68% selectivity
(Table 1 and Fig. 2), whereas, catalyst 2 exhibited an
increased phenol yield of up to 19%, a TON of 380 and a
much-improved selectivity (91%). The most efficient catalyst
3 afforded the relative highest yield of phenol, 21%, TON,
420, and selectivity, 98%, at this temperature. As expected,
rings to obtain
3 which exhibited the best catalytic
performance. The phenol yield was increased up to 41% with
a TON of 820 and a much improved selectivity of 97%
compared to those of 1 and 2. In fact, complexes 1–3 are
structurally similar but they exhibited enormously varied
catalytic efficiencies, which are tuned by the length of the
alkyl chain attached to the –NMe2 group, electron releasing
substituents on the pyridine arms and steric nature. On the
other hand, catalyst 4 afforded a significantly lower phenol
yield (18%), selectivity (43%) and TON (360) (Table 1 and
Fig. 2) than 1–3. Thus, the replacement of the acyclic
backbone with a sterically hindered cyclic diazepane unit
leads to a decrease in catalytic efficiency. The catalytic
efficiency further decreased by replacing the pyridyl arms in
4
with quinolyl arms to obtain 5, which exhibited a
substantially lower yield of phenol (16%), TON (320) and
selectivity (44%). The quinolyl moieties may hinder the
binding of H2O2 with the nickelIJII) center to form the oxygen
bound key reactive intermediate, resulting in a huge decrease
in the catalytic efficiency. Complex 6 afforded a significantly
increased yield of phenol (27%), TON (540) and selectivity
Scheme 2 Catalytic benzene hydroxylation with 3 and H2O2.
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Table 1 Catalytic benzene hydroxylation using 1–6
Cat.a
Eox (V)b
[T]c (°C)
[C]d (%)
[Y]e (%)
[S]f (%)
TONg
TOFh (h−1
)
KIEi
0.96
1
1.008
60
25
60
25
60
25
60
25
60
25
60
25
53
27
45
21
42
22
43
21
36
20
47
23
31
19
37
19
41
21
18
14
16
13
27
17
60
68
82
91
97
98
43
68
44
65
58
75
620
380
740
380
820
420
360
280
320
260
540
340
124
76
148
76
164
84
72
56
64
52
108
68
2
3
4
5
6
1.024
1.051
0.990
0.966
0.997
1.02
1.05
0.97
0.98
1.03
a
Reaction conditions: benzene (5 mmol), complex (2.5 μmol, 0.05%), triethylamine (5.0 μmol), hydrogen peroxide (30%) (25 mmol) in
acetonitrile over 5 hours. Concentration: 5 × 10−3 M in acetonitrile at 25 °C. Supporting electrolyte: 0.5 M TBAP; reference electrode: Ag/Ag+;
b
working electrode: Pt-sphere; counter electrode: Pt wire; scan rate, 100 mV s−1
.
Temperature. Conversion. Yield. Selectivity to phenol was
h i
c
d
e
f
g
calculated as [(phenol yield)/(yield of all products)] × 100%. Turnover number. Turnover frequency. Kinetic isotopic effect (kH/kD); reaction
conditions: catalyst (2.5 μmol), 1 : 1 mixture of C6D6 (0.25 mmol) and C6H6 (0.25 mmol), hydrogen peroxide (30%) (2.5 mmol) at 60 °C over 24
hours.
catalyst 4 showed a lower yield of phenol (14%), TON (280)
and selectivity (68%) than 1–3. The quinolyl based catalyst 5
showed the relative lowest yield of phenol (13%), TON (260)
and selectivity (65%) among the series. Further, catalyst 6
exhibited a significantly higher yield (17%), TON (340) and
selectivity (75%) than 4 and 5, but these are lower than those
of 1–3. Overall, the observed phenol yields are much lower
than those at 60 °C (Fig. S11†), but higher than those of
other previously reported complexes at 25 °C.17 The catalytic
efficiency trend at 25 °C is almost identical to that observed
at 60 °C.
The most efficient catalyst 3 was used to examine the role
of H2O2 loading, wherein the yield of phenol increases
linearly while increasing the amount H2O2 used, but the
selectivity decreases inversely (Fig. S12 and Table S4†).
Therefore, the catalytic efficiency of the complexes is also
strongly influenced by the amount of H2O2 used at a constant
catalyst loading (Table S4†). On the other hand, the reaction
was performed with an equal amount of benzene (0.45 mL, 5
mmol) and H2O2 (5 mmol) using 1 as the catalyst, which
showed a relatively lower yield of phenol, 25%, and TON,
500, at 60 °C, which are further decreased to 12% yield and a
TON of 240 at 25 °C under identical conditions (Fig. S13†).
Similarly, complexes 2–6 showed lower amounts of phenol
formed at this relatively lower amount of H2O2 loading: 2,
27%; 3, 30%; 4, 15%; 5, 12%; 6, 18% at 60 °C; 2, 14%; 3,
15%; 4, 12%; 5, 7%; 6, 14% at 25 °C (Fig. S13 and Table S5†).
Thus, the use of a lower amount of H2O2 (one equivalent)
showed relatively improved selectivities with decreases in the
yields. A higher amount of H2O2 loading (five equivalents)
resulted in the best yield of phenol of up to 41% but with a
relatively lower selectivity. The poor selectivity is due to C–C
coupling side reactions and not because of the over-oxidation
of the product phenol. In fact, a minor amount of biphenyl
(≈1%) was obtained as a by-product for all these reactions,
which possibly limits the selectivity to phenol formation at
60° C. A bulk-scale reaction was performed for the most
efficient catalyst
3 which showed an exclusive phenol
formation of up to 0.481 g (46%, 5.1 mmol) at 60 °C over 96
Fig. 2 Temperature-dependence of phenol formation catalyzed by
1–6 (2.5 μmol) using H2O2 (25 mmol) and triethylamine (5 μmol) in
acetonitrile.
Fig. 3 Time-dependence of phenol formation catalyzed by 1–6. The
reaction conditions: benzene (5 mmol), H2O2 (25 mmol), catalyst (2.5
μmol) and triethylamine (5 μmol) in acetonitrile (3.0 mL) at 60 °C.
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hours (Fig. S14†). The product was characterized by GC-MS
and 1H-NMR. This is the highest yield achieved using
nickelIJII) catalysts so far in the literature.
Further, the catalytic reaction was performed using phenol
(5 mmol) and 3 as the substrate and catalyst respectively at
60 °C. The hydroxylation of phenol did not occur and only
the substrate was recovered quantitatively under identical
reaction conditions (Scheme 3 and Fig. S15†). This strongly
reveals that the catalysts are capable of selectively executing
the monohydroxylation of an aromatic C–H bond.
In the case of toluene as a substrate (5 mmol), by
employing catalyst 3 and H2O2 (five equivalents), o- and
m-cresol and benzaldehyde were produced as products
without the formation of quinone derivatives at 60 °C
(Scheme 4). This exhibited a combined yield of cresols of up
to 10% with a reasonably good selectivity (85%) and <1%
benzaldehyde. This reveals that the aromatic sp2 carbon was
selectively oxygenated and such observations were also
reported previously.6,17 However, by using ethylbenzene as a
substrate, the selective activation of sp2 carbon showed 3%
yield of ethylphenol with a huge decrease in the selectivity
(57%) and without the formation of quinone derivatives.
Scheme 4 The catalytic hydroxylation of toluene with 3.
trisij2-(pyridin-2-yl)ethyl]amine by Itoh and co-workers.17
Values in this range is reported for intramolecular aromatic
hydroxylation (0.9–1.0).23 In fact, a KIE value of ∼1.0 is
reported for aromatic hydroxylation using metal-bound
oxygen species as key intermediates via an electrophilic
aromatic substitution reaction.23 On the other hand, the
results showed that C–H bond cleavage of benzene is not
involved in the rate-limiting step. The KIE value suggests that
Fenton-type (1.7)24 or metal insertion reactions (2.9–4.8) are
unlikely involved.25 Isotope-labeling experiments were carried
out to verify the origin of the oxygen atom. This reaction was
performed only for the most active catalyst 3 using H218O2
with a 90% atom purity in H216O, keeping other reaction
conditions identical. 92.46% 18O was incorporated into the
phenol product, which is almost quantitative (Fig. S17†),
whereas, the reaction carried out using H218O and H2O2
showed no 18O atom incorporation into the product. The
results unequivocally reveal that H2O2 is the key oxygen
supplier for the conversion of benzene to phenol.26
While using cumene as
a substrate, o-, m- and
p-isopropylphenol were formed as the major products
(combined yield of 8.3%) and <1% 2-phenyl-2-propanol was
also formed as
a minor product (Scheme S1†). Using
nitrobenzene as a substrate afforded o-, m- and p-nitrophenol
as products with a combined yield of 5% (selectivity: 94%,
4%, and 2% for o-, m-, and p-substitutions, respectively)
(Scheme S2†). Using chlorobenzene, o-chlorophenol as the
major product (84%) and m-chlorophenol as the minor
product (16%) were formed, wherein no formation of
p-chlorophenol was noticed (Scheme S3†). In the case of
anisole as a substrate, o-methoxyphenol was observed as the
major product (71%) with the minor products m- and
p-methoxyphenol (29%) (Scheme S4†).
Interestingly, the treatment of 2 with 10 equivalents of
H2O2 (30%) and two equivalents of Et3N in acetone at −80 °C
exhibited the formation of brown colored species. It showed
Catalytic mechanism
Kinetic isotope effect (KIE) measurements were performed
for all the complexes (2.5 μmol) using a 1 : 1 mixture of C6D6
(0.25 mmol) and C6H6 (0.25 mmol) as a substrate at 60 °C
over 24 hours. Other reaction conditions were identical to
those in Scheme 2. The KIE values were determined to be
0.98–1.05 for 1–6 from the ratio of phenol to phenol-d5 using
GC-MS/GC analysis (Fig. S16† and Table 1). Such smaller
values of KIE are also reported for nickelIJII) complexes of
Fig.
4 (a) The catalytic cycle of benzene hydroxylation. (b) The
electronic spectral changes for the reaction of 2 (1 × 10−4 M) with 10
equivalents of H2O2 and 2 equivalents of Et3N in acetone at −80 °C.
spectra were measured in one second time intervals. The inset shows
the time course of the absorbance at 410 nm.
Scheme 3 The reaction of phenol with H2O2 using 3.
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a characteristic oxo-to nickelIJIII) charge transfer (LMCT)
transition27 at 410 nm (ε, 4230 cm−1), possibly corresponding
to the key intermediate [(L2NiIII)2IJμ-O)2]2+ (Fig. 4). The
formation constant (kobs) and t1/2 values were calculated to be
7.40 × 10−2 s−1 and 93 seconds, respectively (Fig. S18†). The
intermediate was found to be EPR silent (Fig. S19†). Further,
the species [(L2NiIII)2IJμ-O)2]2+ was characterized by solution
FT-IR spectroscopy and exhibited Ni–O stretching vibration at
673 cm−1, which is shifted to 626 cm−1 while using H218O2
(90% atom purity) instead of H216O2 (Fig. 5). The observed
isotopic shift 16Δ − 18Δ = 47 cm−1 is comparable to those
previously reported for metal–oxo complexes, which were
structurally and spectroscopically characterized.22,28 The HR-
ESI mass spectrum showed a prominent peak at m/z =
359.1365 in acetonitrile corresponding to [(L2NiIII)2IJμ-O)2]2+
(Fig. S20a†). The 18O labeling studies using H218O2 showed a
prominent shift in the m/z value, 361.1420, in acetonitrile
(Fig. S20b†) attributed to [(L2NiIII)2IJμ-18O)2]2+. Interestingly,
the addition of 50 equivalents of benzene to freshly generated
[(L2NiIII)2IJμ-O)2]2+ in acetone at −80 °C resulted in immediate
decay of species, which is accompanied by the disappearance
of the oxo-to-nickelIJIII) LMCT transition (Fig. 6). The decay
constant kd was calculated to be 7.53 × 10−2 s−1 (Fig. S21†).
After a while, the GC-MS analysis of the reaction mixture
showed the exclusive formation of phenol. Further, treating 2
with H2O2 (10 equivalents) and Et3N showed an irreversible
oxidation peak at 1.067 V in the cyclic voltammogram (Fig.
S22†), which is well shifted from the peak potential of 2 and
supports the change of the oxidation state of the nickel
center. Similarly, the species [(LNiIII)2IJμ-O)2]2+ for other
complexes were generated using 10 equivalents of H2O2
Fig.
6 ×
The reaction of [(L2NiIII)2IJμ-O)2]2+ (1 10−4 M) with 50
equivalents of benzene in acetone at −80 °C. The inset shows the time
course of the absorbance at 410 nm.
(30%) and two equivalents of Et3N in a CH3CN : MeOH (8 : 2)
mixture at −40 °C. Almost identical electronic spectral
signatures and kinetic behaviours were observed for 3 (λmax
,
413 nm; ε, 2100 cm−1; kobs, 9.41 × 10−2 s−1; t1/2, 73 seconds), 4
(λmax, 406 nm; ε, 1830 cm−1; kobs, 5.61 × 10−2 s−1; t1/2, 123
seconds) and 6 (λmax, 409 nm; ε, 2640 cm−1; kobs, 5.89 × 10−2
s−1; t1/2, 117 s) (Fig. S23–S25†). The formation of species was
confirmed by HR-ESI mass spectrometry, m/z: 474.1992
corresponding to [(L3NiIII)2IJμ-O)2]2+; m/z: 355.1036 for
[(L4NiIII)2IJμ-O)2]2+; m/z: 383.1431 for [(L6NiIII)2IJμ-O)2]2+ (Fig.
S26–S28†). However, the characterization of these
intermediates for 1 and 5 were unsuccessful despite our great
efforts as these are possibly very short-lived intermediate
species.
Repeated attempts to isolate single crystals of the
bis(μ-oxo)dinickelIJIII) complexes were unsuccessful under
various conditions. However, the geometry of the
intermediate bis(μ-oxo)dinickelIJIII) of
2
was optimized
(Fig. 7a) and the HOMO–LUMO energy gap was calculated by
DFT (Fig. S29†).29 The calculations were performed using the
B3LYP 631-G (d,p) (for C, H, N and O) and LanL2DZ (for Ni)
basis sets. The optimized structure showed almost identical
bond distances, bond angles and Ni–Ni distances (2.742 Å)
(Table S6 and S7†) to those of previously reported similar
intermediates.30 In particular, the Ni–O bond distance (1.805
Å) converged into a typical reported value, 1.8 Å.30 The
HOMO–LUMO energy gaps for the possible intermediates
such as [(L2NiIII)2IJμ-O)2]2+, [NiIIIIJOOH)IJL2)]2+ and [NiIIIIJO2)-
IJL2)]+ were calculated to be 2.2773 eV, 5.6613 eV, and 3.5153
eV, respectively (Fig. 7b). Among these intermediates, only
[(L2NiIII)2IJμ-O)2]2+ showed a low energy gap. The molecular
orbital analysis reveals that the HOMO electrons localized on
the nickel center and bridged oxygen atoms. The LUMO
electrons are localized around O–Ni–O and four nitrogen
atoms of the ligand. The TD-DFT results are comparable with
the experimental electronic spectra of [(L2NiIII)2IJμ-O)2]2+,
where the energy of the oxo-to-nickelIJIII) LMCT is close to that
in the experimentally observed spectra at 410 nm. This
Fig.
5
The vibrational spectra of [(L2NiIII)2IJμ-16O)2]2+ (blue) and
[(L2NiIII)2IJμ-18O)2]2+ (red), which were generated from 2 (1 × 10−4 M) by
adding 10 equivalents of H216O2 and H218O2, respectively, in the
presence of Et3N (2 equivalents) in acetone. The calculated vibrational
spectra of [(L2NiIII)2IJμ-O)2]2+ by TD-DFT (black).
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Conclusions
NickelIJII) complexes of N4 ligands have been synthesized and
characterized as catalysts for aromatic C–H activation. The
complexes exhibited an octahedral coordination geometry.
The catalysts are efficient and selective to the hydroxylation
of benzene into phenol using H2O2. The coordination
geometry of intermediates and stereoelectronic and steric
factors appear to strongly influence the catalytic efficiency of
the complexes. In particular, the complex with electron
releasing groups and a cis-coordination arrangement afforded
a higher phenol yield of up to 41% and a TON of 820 without
any over-oxidized products. In fact, this is the highest
catalytic efficiency ever reported for benzene hydroxylation
using nickelIJII) catalysts and other first-row transition metals.
The KIE experiment results support the involvement of
nickel-bound oxygen species as a key intermediate. Therefore,
the reaction mechanism likely proceeded via highly valent
bis(μ-oxo)dinickelIJIII) species, which is characterized by
spectral methods and further supported by DFT and TD-DFT
calculations. The isotope-labeling experiments indicate that
H2O2 is the key oxygen supplier for phenol formation.
Experimental section
Materials
Fig. 7 (a) Optimized structure of [(L2NiIII)2IJμ-O)2]2+ using the B3LYP
631-G/LANL2DZ basis set. (b) Energy profile diagram of [(L2NiIII)2IJμ-
O)2]2+, [NiIIIIJOOH)IJL2)]2+, and [NiIIIIJO2)IJL2)]+. HOMO and LUMO are
calculated by DFT using the B3LYP 6-31-G(d,p)/LANL2DZ basis set.
Commercially available chemicals were used without further
purification.
2-pyridinecarboxaldehyde, 2-picolylchloride hydrochloride,
3-(dimethylamino)-1-propylamine, nickelIJII) perchlorate
Homopiperazine,
2-vinylpyridine,
hexahydrate, 2-chloromethyl-4-methoxy-3,5-dimethylpyridine
hydrochloride, 2-chloromethyl quinoline hydrochloride,
sodium tetraphenylborate, benzene, toluene, nitrobenzene,
tetrabutylammonium perchlorate (TBAP), chloroform-d,
acetonitrile-d3, and benzene-d6 were purchased from Sigma-
Aldrich. Aqueous H218O2 (2.0%) in H216O (90% 18O atom
purity) was purchased from Iconisotopes, USA. N,N-
Dimethylethylenediamine, sodium triacetoxyborohydride, and
ethylbenzene were purchased from Alfa Aesar. Sodium
bicarbonate and magnesium sulfate were purchased from
SRL chemicals, India. Anhydrous acetonitrile, sodium
hydroxide, acetic acid, silica gel and a 30% aqueous solution
of hydrogen peroxide were purchased from Merck, India. Dry
methanol, triethylamine, and phenol were purchased from S.
D fine chemicals, India.
corresponds to the HOMO to LUMO+1 transition (Fig.
S30†).31 Further, the calculated vibrational spectrum showed
a vibrational mode at 650 cm−1, which is close to the
experimental value (673 cm−1) (Fig. 5). The calculated
electronic transitions of other intermediates [NiIIIIJOOH)IJL2)]2+
and [NiIIIIJO2)IJL2)]+ do not correspond to the experimental
electronic spectral data.
Therefore, we expected that the overall structure of the
bis(μ-oxo)dinickelIJIII) complex is similar to those in previous
reports.22,32,33 Its characteristic absorption band at around
410 nm and its ε values are almost identical to those of the
structurally characterized bis(μ-oxo)dinickelIJIII) complexes,
which
were
generated
from
well-defined
very similar
bis(μ-hydroxido)dinickelIJII) complexes.32
A
spectral signature at around 400 nm was reported for the
bis(μ-oxo)dicopperIJIII) complex.27 The binding of H2O2 with
the NiIJII) center is seemingly one of the essential steps for
generating the key bis(μ-oxo)dinickelIJIII) intermediate22,32 and
benzene hydroxylation. This step is likely influenced by the
electron density around the nickelIJII) center and steric and
geometrical arrangements. The catalytic efficiency trend is
found to be influenced by the ligand architecture and the
stereoelectronic and steric properties of the ligand likely
facilitates the buildup of greater electron density around the
nickelIJII) center via an appropriate coordination geometry
required for higher catalytic efficiency.
Instrumentation
NMR spectra were recorded on Bruker 300 MHz and Varian
500 MHz spectrometers. Electron ionization mass spectra
(ESI-MS) were measured on a Thermo LC-MS instrument.
HR-MS spectra were recorded on a Bruker-maXis mass
spectrometer. UV-vis spectra were recorded on an Agilent
8453 diode-array spectrophotometer with a cooling unit from
Unisoku (Osaka, Japan). FT-IR spectra were recorded using a
Thermo Nicolet 6700. Electron paramagnetic resonance (EPR)
measurements were carried out on a JEOL JES-TE100 ESR
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spectrometer operating at X-band frequencies. Elemental
analyses were carried out using a Heraeus Vario Elementar
automatic analyser. Cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) were performed using a three-
electrode cell configuration (CHI, model 440). A platinum
sphere, platinum wire and Ag/Ag+ (non-aqueous) were used
as the working, auxiliary and reference electrodes,
respectively. Bu4NClO4 was used as a supporting electrolyte in
acetonitrile. GC-MS and GC analysis were performed on an
Agilent 5977E GC/MSD using an HP-5 MS ultra-inert (30 m ×
250 μm × 0.25 μm) capillary column.
room temperature. The color of the solution turned into blue
and then NaBPh4 (0.684 g, 2 mmol) was added into the
acetonitrile solution. Pink coloured precipitates were filtered
off, washed with small quantities of ice-cold methanol and
then dried. Purple colored single crystals of 2 suitable for
X-ray
crystallographic
analysis
were
obtained
by
recrystallization in acetonitrile followed by diethyl ether
diffusion.
[NiIJL1)IJCH3CN)2]IJPh4B)2 1. Yield, 0.28 g (86%). Analytically
calculated elements for C68H68B2N6Ni: C, 77.81; H, 6.53; N,
8.01%. Found: C, 77.78; H, 6.54; N, 7.98%. ESI-MS (m/z):
164.12 [(M − 2CH3CN − 2BPh4)2+]. IR (KBr) (ν, cm−1): 3441,
3051, 2985, 1951, 1606, 1477, 1442, 1294, 1263, 1103, 1028,
985, 844, 705, 611.
X-ray crystal structure analysis
The experiment was performed on an Agilent Technologies
Supernova-E CCD diffractometer. Suitable single crystals of 2
with a suitable size were selected from the mother liquor and
immersed in paraffin oil, then mounted on the tip of a glass
fiber. The structure was solved by direct methods using the
program SHELXS-2013. Refinements and all further
calculations were carried out using SHELXL-2013. The
H-atoms were included in calculated positions and treated as
riding atoms using the SHELXL default parameters. The non-
H atoms were refined anisotropically, using weighted full-
matrix least-squares on F2. CCDC 1863321 contains the
supplementary crystallographic data for this paper.
[NiIJL2)IJCH3CN)2]IJPh4B)2 2. Yield, 0.31 g (92%). Analytically
calculated elements for C69H70B2N6Ni: C, 77.91; H, 6.63; N,
7.90%. Found: C, 77.90; H, 6.60; N, 7.88%. ESI-MS (m/z):
171.17 [(M − 2CH3CN − 2BPh4)2+]. IR (KBr) (ν, cm−1): 3523,
3051, 2914, 2274, 1944, 1606, 1575, 1477, 1259, 1153, 1024,
989, 844, 704, 607.
[NiIJL3)IJCH3CN)2]IJPh4B)2 3. Yield, 0.28 g (86%). Analytically
calculated elements for C75H82B2N6Ni O2: C, 76.35; H, 7.01;
N, 7.12%. Found: C, 76.32; H, 7.02; N, 7.10%. ESI-MS (m/z):
229.12 [(M − 2CH3CN − 2BPh4)2+]. IR (KBr) (ν, cm−1): 3442,
3055, 2936, 1641, 1579, 1477, 1425, 1267, 1097, 1111, 734,
705, 624.
[NiIJL4)IJCH3CN)2]IJPh4B)2 4. Yield, 0.35 g (96%). Analytically
calculated elements for C69H68B2N6Ni: C, 78.06; H, 6.46; N,
7.92%. Found: C, 78.04; H, 6.43; N, 7.90%. ESI-MS (m/z):
170.15 [(M − 2CH3CN − 2BPh4)2+]. IR (KBr) (ν, cm−1): 3444,
3076, 2966, 2007, 1610, 1483, 1438, 1369, 1294, 1242, 1087,
Synthesis of the ligands
The present ligands L1, L2, and L4–L6 were synthesized by a
previously reported procedure.18
Synthesis of N,N-bisIJ4-methoxy-3,5-dimethylpyridin-2-
ylmethyl)-N′,N′-dimethylpropane-1,3-diamine
1
(L3).
769, 619. H NMR (500 MHz, CD3CN), δ, −15.82, 12.36, 17.44,
34.67, 45.82, 74.79, 98.53, 122.00, 185.89 ppm.
2-Chloromethyl-4-methoxy-3,5-dimethylpyridine hydrochloride
(2.22 g, 10 mmol), 3-(dimethylamino)-1-propylamine (0.58 mL,
4.63 mmol) and K2CO3 (3.22 g, 23.30 mmol) in 80 mL of CH3-
CN were placed in a 100 mL round bottom flask. The mixture
was stirred for 2 days under an Ar atmosphere. After removal
of the solvent, the crude oil was dissolved in 100 mL of
dichloromethane and washed with water. After drying over
Na2SO4, the solution was filtered and removed by rotary
evaporation. The yellow oil was purified by column
chromatography over alumina (ethyl acetate : hexane = 2 : 1, Rf
[NiIJL5)IJCH3CN)2]IJPh4B)2 5. Yield, 0.21 g (78%). Analytically
calculated elements for C77H72B2N6Ni: C, 79.61; H, 6.25; N,
7.23%. Found: C, 79.59; H, 6.22; N, 7.20%. ESI-MS (m/z):
220.27 [(M − 2CH3CN − 2BPh4)2+]. IR (KBr) (ν, cm−1): 3444,
3061, 2937, 2036, 1637, 1485, 1373, 1211, 1105, 835, 750, 690,
623. 1H NMR (500 MHz, CD3CN), δ, −17.37, 7.92, 13.82,
14.87, 27.32, 42.25, 89.66, 108.37, 142.61, 146.24, 172.10,
196.41 ppm.
NiIJL6)IJCH3CN)2]IJPh4B)2 6. Yield, 0.325
g
(92%).
1
= 0.54) yielding a yellow oil (65% yield). H NMR (300 MHz,
Analytically calculated elements for C71H72B2N6 Ni: C, 78.26;
H, 6.66; N, 7.71%. Found: C, 78.24; H, 6.63; N, 7.69%. ESI-MS
(m/z): 184.18 [(M − 2CH3CN − 2BPh4)2+]. IR (KBr) (ν, cm−1):
3525, 3091, 2962, 2189, 1610, 1566, 1490, 1317, 1259, 1087,
779, 619, 545. 1H NMR (500 MHz, CD3CN), δ, −17.72, 8.12,
12.72, 16.84, 35.67, 41.40, 66.79, 102.37, 177.47, 210.57 ppm.
CDCl3), δ, 8.14 (s, 2H), 3.70 (s, 6H), 3.68 (s, 4H), 2.52–2.47 (t,
2H), 2.25 (m, 2H), 2.21 (s, 6H), 2.13 (s, 6H), 2.07 (s, 6H),
1.68–1.58 (q, 2H) ppm. 13C NMR (300 MHz, CDCl3), δ, 164.37,
157.76, 148.64, 126.53, 125.22, 60.11, 59.99, 58.35, 53.22,
45.72, 25.11, 13.48, 10.94 ppm. ESI-MS (m/z): calculated for
C23H36N4O2 [M + H]+, 401.2917; found, 401.2960. IR (KBr) (ν,
cm−1): 3441, 2926, 2360, 1645, 1566, 1471, 1398, 1257, 1097,
1001, 763, 663, 623.
General procedure for catalytic benzene hydroxylation
Synthesis of the NiIJII) complexes (1–6). The NiIJII)
complexes are prepared by a known procedure with slight
modifications.19,20 A methanol solution (5 mL) of NiIJClO4)2
·6H2O (0.365 g, 1 mmol) was added to the respective ligands
(1 mmol) in methanol (5 mL) and stirred for 30 minutes at
A catalytic amount of the complex [NiIJL)IJCH3CN)2]IJPh4B)2 (2.5
μmol) was dissolved in acetonitrile (3.0 mL). Triethylamine
(5.0 μmol), benzene (5.0 mmol) and aqueous hydrogen
peroxide (2.5 mL, 25 mmol) were introduced slowly into the
solution. The reaction mixture was stirred at 60/25 °C for 5
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hours. After cooling to room temperature, the catalyst was
removed by passing the solution through silica-gel. Then, a
known amount of nitrobenzene was added as an internal
standard. Conversion and yield of the reaction were
determined with GC-MS/GC. The control experiment without
a catalyst showed no benzene oxidation products under
identical experimental conditions.
microsyringe into the solution. The reaction was carefully
monitored at 410 nm. After a while, the reaction mixture was
passed through a silica column and the formation of phenol
was confirmed by GC-MS/GC.
Solution FT-IR measurements
Solution FT-IR measurements were carried out for the freshly
generated [(L2NiIII)2IJμ-O)2]2+] species transferred into a CaF2
cell using a microsyringe and then spectra were measured.
Determination of kinetic deuterium isotope effect
Benzene (0.25 mmol), benzene-d6 (0.25 mmol), and
triethylamine (5.0 μmol) were added into a solution of
[NiIJL)IJCH3CN)2]IJBPh4)2 (1–6) (2.5 μmol) in acetonitrile (2 mL).
An aqueous solution of hydrogen peroxide (30%) (0.50 mL)
was added slowly under constant stirring. The mixture was
stirred for 24 hours at 60 °C. After cooling to room
temperature, the complex was removed by passing the
solution through silica-gel. The product distribution was
determined by GC-MS/GC. The kinetic isotope effect (KIE)
value for phenol production was calculated as (moles of
phenol)/(moles of phenol-d5).
Catalytic hydroxylation of toluene
The most active catalyst 3 (2.5 μmol) was dissolved in
acetonitrile (3.0 mL). Triethylamine (5.0 μmol), toluene (5.0
mmol) and aqueous hydrogen peroxide (2.5 mL, 25 mmol)
were slowly added to the solution. The reaction mixture was
stirred at 60 °C for
5 hours. After cooling to room
temperature, after the usual workup, the products were
analyzed by GC-MS/GC.
DFT methods
The geometry optimizations were performed using density
functional theory (DFT) and the HOMO–LUMO energy gap
was calculated using TD-DFT. The above calculations were
performed using the B3LYP 6-31-G(d,p) (for C, H, N and
O) and LanL2DZ (for Ni) basis sets All DFT calculations
were carried out by using the Gaussian 09 program
package.29
Catalytic benzene hydroxylation reaction with H218O2
Catalyst 3 (2.5 μmol) was dissolved in acetonitrile and
benzene (0.12 mmol). Then, triethylamine (5.0 μmol) and
H218O2 (5 equivalents) were slowly added into the reaction
mixture. The mixture was stirred for 10 h at 60 °C. After
cooling to room temperature, the catalyst was removed by
passing the solution through silica-gel. The reaction mixture
was analyzed by GC-MS.
Calculation of steric maps
The steric maps were obtained using the SambVca 2.1 A web
application, which can be found at https://www.molnac.
unisa.it/OMtools/sambvca2.1/index.html.21
Catalytic benzene hydroxylation reaction with H218O
Complex 3 (2.5 μmol) was dissolved in H218O/acetonitrile (0.1
mL/1 mL). Triethylamine (5.0 μmol), benzene (0.5 mmol) and
aqueous hydrogen peroxide (0.5 mL, 5 mmol) were slowly
introduced into the solution. The reaction mixture was
stirred at 60 °C for 5 h. After the usual workup, the products
were analyzed by GC-MS.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Kinetic measurements
We acknowledge the Science and Engineering Research Board
(SERB), New Delhi and the Board of Research in Nuclear
Science (BRNS), Mumbai for funding.
Kinetic
experiments
for
the
formation
of
the
bis(μ-oxo)dinickelIJIII) complexes were carried out in a 1 cm
path length UV-vis cell on a Unisoku thermostated cell holder
designed for low-temperature measurements. The deaerated
solution of the nickel complex (1 × 10−4 M) in the cell was
kept at the desired temperature for several minutes and
References
1 K. Weissermel and H. J. Arpe, Industrial Organic Chemistry,
John Wiley & Sons, 3rd edn, 2008.
aqueous H2O2 (10 equivalents) was added using
a
microsyringe in the presence of two equivalents of Et3N. The
rate constants for the formation of the bis(μ-oxo)dinickelIJIII)
intermediates were determined by monitoring the increase in
the new absorbance band at around 406–413 nm.
2 G. I. Panov, Advances in oxidation catalysis, Kluwer
Academic/Plenum Publishers, 2000.
3 (a) H. Hock and S. Lang, Ber. Dtsch. Chem. Ges. B, 1944, 77,
257–264; (b) P. Borah, X. Ma, K. T. Nguyen and Y. Zhao,
Angew. Chem., Int. Ed., 2012, 51, 7756–7761.
4 (a) G. I. Panov, A. K. Uriate, M. A. Rodkin and V. I. Sobolev,
Catal. Today, 1988, 41, 365–385; (b) R. Morrison and R. T.
Boyd, Organic Chemistry, Prentice, Hall, Englewood Cliffs,
The species [(L2NiIII)2IJμ-O)2]2+] was freshly generated from
2, H2O2 (10 equivalents) and two equivalents of Et3N at −80
°C in acetone. After reaching the maximum intensity at 410
nm, the substrate benzene was then added using
a
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View Article Online
Paper
New Jersery, 1992, 6th edn; (c) B. Lee, H. Naito, M. Nagao
and T. Hibino, Angew. Chem., Int. Ed., 2012, 51, 6961–6965.
5 J. C. Lewis, P. S. Coelho and F. H. Arnold, Chem. Soc. Rev.,
2011, 40, 2003–2021.
Catalysis Science & Technology
19 (a) T. Nagataki, Y. Tachi and S. Itoh, Chem. Commun.,
2006, 4016–4018; (b) M. Sankaralingam, M. Balamurugan, P.
Vadivelu, C. H. Suresh and M. Palaniandavar, Chem. – Eur.
J., 2014, 20, 11346–11361.
6 Y. R. Luo, Comprehensive Handbook of Chemical Bond
Energies, CRC Press, Boca Raton, FL, 2007.
7 N. Herron and C. A. Tolman, J. Am. Chem. Soc., 1987, 109,
2837–2839.
20 (a) M. Sankaralingam, P. Vadivelu and M. Palaniandavar,
Dalton Trans., 2017, 46, 7181–7193; (b) M. Balamurugan, R.
Mayilmurugan, E. Suresh and M. Palaniandavar, Dalton
Trans., 2011, 40, 9413–9424.
8 (a) D. Bianchi, R. Daloisio, R. Bortolo and M. Ricci, Appl.
Catal., A, 2007, 327, 295–299; (b) L. V. Pirutko, V. S.
Chernyavsky, A. K. Uriarte and G. I. Panov, Appl. Catal., A,
2002, 227, 143–157.
9 (a) B. Lee, H. Naito and T. Hibino, Angew. Chem., Int. Ed.,
2012, 51, 440–444; (b) J. W. Han, J. Jung, Y. M. Lee, W. Nam
and S. Fukuzumi, Chem. Sci., 2017, 8, 7119–7125; (c) C. Walling
and R. A. Johnson, J. Am. Chem. Soc., 1975, 97, 363–367.
10 (a) L. Balducci, D. Bianchi, R. Bortolo, R. D. Aloisio, M.
Ricci, R. Tassinari and R. Ungarelli, Angew. Chem., Int. Ed.,
2003, 42, 4937–4940; (b) A. N. Kharat, S. Moosavikia, B. T.
Jahromi and A. Badiei, J. Mol. Catal. A: Chem., 2011, 348,
14–19.
11 (a) K. M. Parida and D. Rath, Appl. Catal., A, 2007, 321,
101–108; (b) S. Q. Song, S. J. Jiang, R. C. Rao, H. X. Yang and
A. M. Zhang, Appl. Catal., A, 2011, 401, 215–219.
12 (a) S. Itoh and Y. Tachi, Dalton Trans., 2006, 4531–4538; (b)
T. Nagataki, K. Ishii, Y. Tachi and S. Itoh, Dalton Trans.,
2007, 1120–1128.
21 (a) A. Poater, F. Ragone, R. Mariz, R. Dorta and L. Cavallo,
Chem. – Eur. J., 2010, 16, 14348–14353; (b) A. Poater, F.
Ragone, S. Giudice, C. Costabile, R. Dorta, S. P. Nolan and L.
Cavallo, Organometallics, 2008, 27, 2679–2681; (c) L.
Falivene, Z. Cao, A. Petta, L. Serra, A. Poater, R. Oliva, V.
Scarano and L. Cavallo, Nat. Chem., 2019, 11, 872–879; (d) A.
Poater, B. Cosenza, A. Correa, S. Giudice, F. Ragone, V.
Scarano and L. Cavallo, Eur. J. Inorg. Chem., 2009, 13,
1759–1766.
22 K. Shiren, S. Ogo, S. Fujinami, H. Hayashi, M. Suzuki, A.
Uehara, Y. Watanabe and Y. Moro-oka, J. Am. Chem. Soc.,
2000, 122, 254–262.
23 (a) P. L. Holland, K. R. Rodgers and W. B. Tolman, Angew.
Chem., Int. Ed., 1999, 38, 1139–1142; (b) K. Honda, J. Cho, T.
Matsumoto, J. Roh, H. Furutachi, T. Tosha, M. Kubo, S.
Fujinami, T. Ogura, T. Kitagawa and M. Suzuki, Angew.
Chem., Int. Ed., 2009, 48, 3304–3307.
24 R. Augusti, A. O. Dias, L. L. Rocha and R. M. Lago, J. Phys.
Chem. A, 1998, 102, 10723–10727.
13 K. Kamata, T. Yamaura and N. Mizuno, Angew. Chem., Int.
Ed., 2012, 51, 7275–7278.
25 M. H. Emmert, A. K. Cook, Y. J. Xie and M. S. Sanford,
Angew. Chem., Int. Ed., 2011, 50, 9409–9412.
14 (a) E. V. Kudrik and A. B. Sorokin, Chem. – Eur. J., 2008, 14,
7123–7126; (b) A. Raba, M. Cokoja, W. A. Herrmann and
F. E. Kuhn, Chem. Commun., 2014, 50, 11454–11457; (c) B.
Xu, W. Zhong, Z. Wei, H. Wang, J. Liu, L. Wu, Y. Fengb and
X. Liu, Dalton Trans., 2014, 43, 15337–15345.
26 K. Ohkubo, T. Kobayashi and S. Fukuzumi, Angew. Chem.,
Int. Ed., 2011, 50, 8652–8655.
27 M. J. Henson, P. Mukherjee, D. E. Root, T. D. P. Stack
and E. I. Solomon, J. Am. Chem. Soc., 1999, 121,
10332–10345.
15 (a) A. Conde, M. M. Dıaz-Requejo and P. J. Perez, Chem.
Commun., 2011, 47, 8154–8156; (b) T. Tsuji, A. A. Zaoputra,
Y. Hitomi, K. Mieda, T. Ogura, Y. Shiota, K. Yoshizawa, H.
Sato and M. Kodera, Angew. Chem., Int. Ed., 2017, 56,
7779–7782.
16 (a) O. Shoji, S. Yanagisawa, J. K. Stanfield, K. Suzuki, Z.
Cong, H. Sugimoto, Y. Shiro and Y. Watanabe, Angew.
Chem., Int. Ed., 2017, 56, 1–6; (b) O. Shoji, T. Kunimatsu, N.
Kawakami and Y. Watanabe, Angew. Chem., Int. Ed.,
2013, 52, 6606–6610.
28 (a) B. Shin, K. D. Sutherlin, T. Ohta, T. Ogura, E. I.
Solomon and J. Cho, Inorg. Chem., 2016, 55, 12391–12399;
(b) M. K. Coggins, V. Martin-Diaconescu, S. DeBeer and
J. A. Kovacs, J. Am. Chem. Soc., 2013, 135, 4260–4272; (c)
M. K. Coggins and J. A. Kovacs, J. Am. Chem. Soc.,
2011, 133, 12470–12473.
29 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F.
Ogliaro, M. J. Bearpark, J. Heyd, E. N. Brothers, K. N.
Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K.
Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J.
Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E.
Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K.
Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J.
Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B.
17 Y. Morimoto, S. Bunno, N. Fujieda, H. Sugimoto and S. Itoh,
J. Am. Chem. Soc., 2015, 137, 5867–5870.
18 (a) R. Mayilmurugan, H. Stoeckli-Evans and M.
Palaniandavar, Inorg. Chem., 2008, 47, 6645–6658; (b) T.
Dhanalakshmi, E. Suresh, H. Stoeckli-Evans and M.
Palaniandavar, Eur. J. Inorg. Chem., 2006, 22, 4687–4695; (c)
M. Schmidt, D. Wiedemann, B. Moubaraki, N. F. Chilton,
K. S. Murray, K. R. Vignesh, G. Rajaraman and A.
Grohmann, Eur. J. Inorg. Chem., 2013, 5-6, 958–967; (d) S.
Kim, J. W. Ginsbach, A. Imtiaz Billah, M. A. Siegler, C. D.
Moore, E. I. Solomon and K. D. Karlin, J. Am. Chem. Soc.,
2014, 136, 8063–8071.
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Paper
Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian
09, 2009.
32 S. Hikichi, M. Yoshizawa, Y. Sasakura, M. Akita and Y. Moro-
ka, J. Am. Chem. Soc., 1998, 120, 10567–10568.
30 Y. Morimoto, Y. Takagi, T. Saito, T. Ohta, T. Ogura, N.
Tohnai, M. Nakano and S. Itoh, Angew. Chem., Int. Ed.,
2018, 57, 7640–7643.
31 J. D. Parham, G. B. Wijeratne, D. B. Rice and T. A. Jackson,
Inorg. Chem., 2018, 57, 2489–2502.
33 (a) S. Itoh, H. Bandoh, S. Nagatomo, T. Kitagawa and S.
Fukuzumi, J. Am. Chem. Soc., 1999, 121, 8945–8946; (b) S.
Itoh, H. Bandoh, M. Nakagawa, S. Nagatomo, T. Kitagawa,
K. D. Karlin and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123,
11168–11178.
This journal is © The Royal Society of Chemistry 2019
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