A cobalt-substituted Keggin-Type polyoxometalate for catalysis of oxidative aromatic cracking reactions in water

Article abstract of DOI:10.1039/d0cy01758b

Efficient detoxification of harmful benzene rings into useful carboxylic acids in water is indispensable for achieving a clean water environment. We report herein that oxidative aromatic cracking (OAC) reactions in water were achieved using a catalytic system with a cobalt-substituted Keggin-Type polyoxometalate (Co-POM) as a catalyst, an Oxone monopersulfate compound as a sacrificial oxidant and sodium bicarbonate as an additive under mild conditions. Sodium bicarbonate plays a crucial role in the selective OAC reactions by Co-POM using ethylbenzenesulfonate as a model substrate. The reactive species was characterized to be a cobalt(iii)-oxyl species based on 31P NMR, UV-vis spectroscopic, kinetic, and theoretical analyses. The electrophilicity of the cobalt(iii)-oxyl species was demonstrated by a linear relationship with a negative slope in the Hammett plots of initial rates obtained from the OAC reactions of m-xylenesulfonate derivatives. Besides, we have verified the degradation pathway of the OAC reactions using benzene as a model substrate in the catalytic system. The degradation was initiated by an electrophilic attack of the cobalt(iii)-oxyl species on benzene to yield phenol followed by producing catechol, muconic acid, maleic/fumaric acid, tartaric acid derivatives and formic acid on the basis of 1H NMR spectroscopic analysis.

Full text of DOI:10.1039/d0cy01758b

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PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
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DOI: 10.1039/D0CY01758B
ARTICLE
A cobalt-substituted Keggin-type polyoxometalate for catalysis of
oxidative aromatic cracking reactions in water
Yoshihiro Shimoyama,a Satoru Tamura,b Yasutaka Kitagawa,c Dachao Hong,*a,d and Yoshihiro Kona,d
Received 00th January 20xx,
Accepted 00th January 20xx
Efficient detoxification of harmful benzene rings into useful carboxylic acids in water is indispensable for achieving a clean
water environment. We report herein that oxidative aromatic cracking (OAC) reactions in water were archived by the
catalytic system using a cobalt-substituted Keggin-type polyoxometalate (Co-POM) as a catalyst, Oxone® monopersulfate
compound as a sacrificial oxidant and sodium bicarbonate as an additive under mild conditions. Sodium bicarbonate plays a
crucial role in the selective OAC reactions by Co-POM using ethylbenzenesulfonate as a model substrate. The reactive
species was characterized to be a cobalt(III)-oxyl species based on 31P NMR, UV-vis spectroscopic, kinetic, and theoretical
analyses. The electrophilicity of the cobalt(III)-oxyl species was demonstrated by a linear relationship with a negative slope
in the Hammett plots of initial rates obtained from the OAC reactions of m-xylenesulfonate derivatives. Besides, we have
verified the degradation pathway of the OAC reactions using benzene as a model substrate in the catalytic system. The
degradation was initiated by an electrophilic attack of the cobalt(III)-oxyl species on benzene to yield phenol followed by
producing catechol, muconic acid, maleic/fumaric acid, tartaric acid derivatives and formic acid on the basis of 1H NMR
spectroscopic analysis.
DOI: 10.1039/x0xx00000x
However, these reactions require toxic halogen-containing
oxidants and solvents such as NaIO4 and CCl4. It remains
challenging to crack harmful aromatics into useful carboxylic
acids in environmentally benign systems under mild
conditions.9
Introduction
Aromatic compounds such as benzene have been widely used
as primary raw materials for industrial production of their
derivatives and synthetic resins.1 In these compounds, benzene,
polychlorinated biphenyls (PCBs) and polyaromatic
hydrocarbons (PAHs) have been strictly restricted for their
waste management due to their carcinogenicity, noxiousness
and teratogenicity.2 As these compounds are highly stable by
virtue of their resonance stabilization and strong chemical
bonding,3 oxidative aromatic cracking (OAC) reactions by
dioxygen requires high temperatures and results in massive
emission of CO2,4,5 which has caused greenhouse effect
impacting on environmental issues and industrial benefits.6
Therefore, conversing the aromatics into valuable compounds
under mild conditions has been exploited. For example, in-situ
formed ruthenium(VIII) tetraoxide (RuVIIIO4) by sodium
periodate (NaIO4) as an oxidant has been reported so far in
catalytic oxidation of aromatics under mild conditions.7,8
Recently, Kojima et al. have reported an oxidative cracking of
benzene rings to afford carboxylic acids in water using a
ruthenium(II) complex having a N-heterocyclic carbene (NHC)
ligand as a catalyst (Scheme 1).10 In this reaction, a cerium(IV)
oxidant with excess amounts was used to generate a
ruthenium(III)-oxyl (RuIII-O•) species11 that exhibits selective
catalytic hydroxylation of aromatic compounds. However, a
reactive species such as metal-bound oxyl-radical or high-valent
metal-oxo species formed in the catalytic OAC reactions can
also oxidize organic aromatic ligands themselves, resulting in
the degradation of metal complexes with organic ligands.10,12
Thus, oxidative-tolerant ligands such as inorganic metal oxide
anions are required to develop efficient catalysts for OAC
reactions. Furthermore, metal complex catalysts using earth-
abundant metal ions are desired to apply in practical reactions.
a. Interdisciplinary Research Center for Catalytic Chemistry, National Institute of
Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba,
Ibaraki 305-8565, Japan. E-mail: hong-d@asit.go.jp (D.H)
b. Institute for Energy and Material/Food Resources, Technology Innovation
Division, Panasonic Corporation, 1006 Kadoma, Kadoma City, Osaka 571-8508,
Japan.
c.
Department of Materials Engineering Science, Graduate School of Engineering
Scheme 1 Catalytic cracking of benzene rings by Ru(II)-NHC catalyst.10
Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-8531,
Japan
d. Global Zero Emission Research Center, National Institute of Advanced Industrial
Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
Electronic Supplementary Information (ESI) available: [NMR and UV-vis spectra,
electrochemical measurements, table and time profiles of products and Cartesian
coordinates of Co(III) oxyl species]. See DOI: 10.1039/x0xx00000x
Polyoxometalates (POMs) composed of metal oxide anions
are known as highly durable materials toward oxidation.13
Several types of polyoxometalates were reported to be efficient
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catalysts for oxidation reactions.14,15 Foreign metal ions are
introduced into metal oxide anions of POMs as M-substituted
POMs in which M acts as a reactive centre. For example, cobalt-
substituted Keggin-type POMs have been studied as water
oxidation catalysts in water.16-18 The reactive species of cobalt-
substituted POMs have been proposed as cobalt-oxyl or cobalt-
oxo species in the water oxidation. In this context, cobalt-
substituted POMs can be promising catalysts toward OAC
reactions. However, there has been no report so far on cobalt-
substituted POMs as catalysts for OAC reactions, and the
reaction mechanism of OAC reactions by cobalt-substituted
POMs has yet to be clarified.
We report herein that a monocobalt-substituted Keggin-type
polyoxometalate (K5[PCo(OH2)W11O39], Co-POM) acts an
efficient catalyst for OAC reactions using Oxone® as an
environmentally benign oxidant and bicarbonate salts as
additives in water. The mechanistic insights into the OAC
reactions by Co-POM in water were revealed based on NMR,
UV-vis spectroscopic, kinetic, and theoretical analyses.
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DOI: 10.1039/D0CY01758B
Scheme 2 Schematic representation of catalytic oxidation of EtBnS in this work.
Table 1 TONs and A/B ratio of the product obtained from the catalytic OAC reactions a
TON (Yield, %) b
A (= [5])
Entry
Catalyst
A/B ratio c
B (= [1 + 2 + 3])
1
2
3
4
5
Co-POM
Co-POM d
–
31 (6.1)
29 (5.7)
1.08
0.18
0.12
0.06
0.18
20 (4.0)
– (0.3)
4 (0.9)
2 (0.4)
113 (22.5)
– (2.5)
CoII(ClO4)2
79 (15.6)
12 (2.2)
Results and Discussion
H3[PW12O40
]
The catalyst of Co-POM was prepared according to a literature
procedure19 and characterised by 31P NMR, UV-vis and FT-IR
measurements (Fig. S1).17,20 A solo peak at 465 ppm derived
from Co-POM was observed in the 31P NMR spectra, indicating
the single product without any impurities. Additionally, UV-vis
and FT-IR spectra were also exhibited a characteristic band at
530 nm and 800–1100 cm–1, respectively, which are consistent
with the reported spectra.17,20 The electrochemical
measurements of Co-POM in water were also performed, as
showed in Fig. S2, in which an irreversible oxidation wave at
+1.45 V (vs. Ag/AgCl) is assigned to the oxidation potential of
Co(II) centre, and reversible redox waves at –0.85 and –1.04 V
can be derived from redox of W ions.20b We have employed the
prepared Co-POM as a catalyst for OAC reactions as below.
The catalytic OAC reactions were performed in D2O
CoII(ClO4)2 +
6
7 (1.4)
58 (11.6)
0.12
H3[PW12O40
]
CoIICl2 +
7
Na2WO4 +
Na2HPO4
8 (1.5)
6 (1.3)
60 (11.9)
73 (15.1)
0.13
0.09
8 e
Ru(II)-NHC f
a Conditions: [Catalyst] = 0.20 mM, [NaHCO3] = 1.0 M, [EtBnS] = 0.10 M, [Oxone®]
= 0.30 M, Temp. = 30 ℃, Solvent: D2O (1.0 mL), reaction time: 24 h. TON =
[product]/[catalyst], Yield
[5]/([1]+[2]+[3]). d No additive. e [Ru(II)-NHC] = 0.20 mM, [(NH4)2CeIV(NO3)6] = 0.10
M, [EtBnS] = 0.10 M, Temp.: 30 ℃, Solvent: D2O, reaction time: 24 h. f Ru(II)-NHC
catalyst was synthesized according to the previous literature.10
b
c
=
([product]/[EtBnS])
×
100.
A/B ratio =
Villiger oxidation22 of
2
by Oxone® to yield 4-
acetoxybenzenesulfonate followed by its hydrolysis rather than
the OAC reactions by an electrophilic attack of Co-oxyl species
to the aromatic ring of 2 having electron-withdrawing groups
(sulfonate and acetyl) (Fig. S4). Thus, we categorized acetic acid
as B in the reactions. The catalytic oxidation of EtBnS by Co-
POM has afforded propionic acid as a major product with A/B
ratio of 1.08, indicating the aromatic oxidation was primarily
catalysed by Co-POM with a TON of 31 (entry 1). The TON for
Co-POM has been achieved to 498 under optimized conditions
with 5.0 µM Co-POM. In the absence of sodium bicarbonate,
benzylic oxidation predominantly took place in the catalytic
EtBnS oxidation by Co-POM, resulting in the decrease of A/B
ratio to 0.18 (entry 2). Sodium bicarbonate plays an essential
role in the selective aromatic oxidation of EtBnS by Co-POM.
Sodium bicarbonate is used to stabilize Co-POM by buffering pH
within 5–8 in water because the reaction solutions acidified by
Oxone® result in the hydrolysis of Co-POM in the absence of 1.0
M sodium bicarbonate (vide infra).
containing sodium 4-ethylbenzenesulfonate (EtBnS) as
a
substrate, Co-POM as a catalyst, Oxone® monopersulfate
compound (KHSO4·K2SO4·2KHSO5) as a sacrificial oxidant and
sodium bicarbonate as an additive. The products obtained in
the reactions were quantified by 1H NMR spectroscopic
analyses. We have observed phenylethanol derivative (1),
acetophenone derivative (2), acetic acid (3), formic acid (4), and
propionic acid (5) as the products in the 1H NMR spectra of the
reaction solution after 24 h (Scheme 2, Fig. S3).21 The turnover
number (TON) and product concentrations for these products
are shown in Table 1 (see also Table S1 for the TONs of all
products). To distinguish between benzylic oxidation and
aromatic oxidation (OAC reactions), we utilize A/B ratios that
are defined as [5]/([1]+[2]+[3]) in the catalytic EtBnS oxidation
reactions. As the source of formic acid can be derived from both
benzylic oxidation and aromatic oxidation, it was excluded from
the calculation of A/B ratios to avoid overestimation. Acetic acid
can be produced in the benzylic oxidation through Baeyer-
Several control experiments were performed to support the
superiority of Co-POM as an effective catalyst for the selective
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aromatic oxidation in the catalytic oxidation of EtBnS. (entries Oxone® concentration showed a pseudo-volcano feature and a
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DOI: 10.1039/D0CY01758B
3–7). Only trace amounts of products were observed without a plateau region in the range from 0.15 M to 0.30 M was observed
Co-POM catalyst in the reaction solution (entry 3). When Co- (Fig. 1c). Therefore, the optimized conditions in this work were
POM was replaced with cobalt(II) perchlorate salt (entry 4)23 or determined to be 1.0 M NaHCO3 and 0.30 M Oxone® (pH 5.9)
well-known Keggin-type phosphotungstic acid (H3[PW12O40]) operating at 30 ℃ for the catalytic reactions.
(entry 5), almost no aromatic oxidation was observed in the
To reveal the mechanistic insights into the OAC reactions by
catalytic oxidation of EtBnS. The result denied the possibility Co-POM, we investigated UV-vis absorption changes in sub-
that dissociated Co(II) ions from Co-POM act as catalysts for the stoichiometric or catalytic oxidation of EtBnS. UV-vis spectral
–
aromatic oxidation. Besides, the use of both cobalt(II) titration upon addition of HSO5 to Co-POM was attempted to
perchlorate and H3[PW12O40], or the use of all of cobalt(II) reveal intermediates of Co-POM. Highly broad absorption band
chloride, NaWO4 and Na2HPO4, which are the components of around 400 – 900 nm has increased upon the addition of 3.0
–
–
Co-POM, as catalysts afforded products oxidized at the benzylic mol equiv of HSO5 in the UV-vis spectral titration by HSO5 in
position as a major product with low A/B ratios in the catalytic water with sodium bicarbonate (Fig. 2a). The apparent colour of
oxidation of EtBnS (entries 6–7). These results demonstrate that the solution turned brown during the rise of the broad
Co-POM in combination with sodium carbonate is indispensable absorption band. The absorption band with a characteristic
for the aromatic oxidation in the catalytic oxidation of EtBnS peak at 495 nm was continuously raised by the addition of
–
–
and a sulfate or a persulfate radical derived from the oxidant HSO5 up to 13 mol equiv of HSO5 (Fig. 2b). Meanwhile, the
should be ruled out as a reactive species.24 Furthermore, the solution colour was changed to orange-brown. Besides, the
reactivity of Co-POM was compared to that of a reported similar absorption band was also observed in the catalytic
catalytic system by a Ru(II)-NHC complex.10 A low A/B ratio reaction solutions without EtBnS at the initial reaction state and
(0.09) for Ru(II)-NHC complex was observed in the catalytic it was rapidly despaired by the addition of EtBnS (Fig. S6),
oxidation of EtBnS (entry 8). It would be ascribed to a poor indicating a plausible reactive species of Co-POM has been
–
reactivity of Ru(III)-oxyl species, derived from the Ru(II)-NHC generated by HSO5 . When 10 mol equiv of EtBnS was added to
complex, to an electrophilic attack toward the aromatic ring of the orange-brown solution (Figs. 2c and 2d), the characteristic
EtBnS. The reactivity of Co-POM is superior to that of the absorption at 495 nm immediately decayed within 300 seconds
reported Ru(II)-NHC complex (TON = 6), although the TON to produce a pink species that was observed for the solution
–
comparison would not be fair because the reaction conditions before the addition of HSO5 . The pink species suggest the
in this work differ from the reported literature.
regeneration of Co-POM as confirmed by 31P NMR
We have also examined the dependence of A/B ratios on measurements together with the production of hydroxylated
reaction temperatures and concentrations of sodium EtBnS (4-ethyl-3-hydroxy-benzene- sulfonate and 4-(1’-
1
bicarbonate and Oxone® in the catalytic oxidation of EtBnS by hydroxyethyl)-benzenesulfonate) as characterized by H NMR
Co-POM. The A/B ratios were depressed in the increase of and LC-MS measurements (Fig. S7).
reaction temperatures (Fig. 1a). The A/B ratio was maximized at
(a)
(b)
the concentration of 1.0 M sodium bicarbonate (Fig. 1b). The
result can be attributed to the pH differences that strongly
affect the stability of Co-POM in the reaction solutions. Co-POM
is demonstrated to be hydrolyzed at a pH lower than 5 and
higher than 8 (Fig. S5). In the absence of NaHCO3, the pH with
0.30 M Oxone® remains to be 2 due to the acidification by
Oxone®, resulting in the hydrolysis of Co-POM. On the other
hand, the pH with 1.0 M NaHCO3 has shifted to 8.8 in the
absence of Oxone®. In addition, the plots of A/B ratios against
495
|
–
3 mol equiv HSO5
–
—
—
Co-POM
3 mol equiv HSO5
—
—
–
13 mol equiv HSO5
Brown
species
Brown
species
Orange-brown
species
Co-POM
Wavelength / nm
Wavelength / nm
(c)
(d)
Orange-brown
species
495
|
(a)
(b)
(c)
Pink species
(Co-POM)
–
—
—
14 mol equiv HSO5 (0 s)
14 mol equiv HSO5
–
+10 mol equiv EtBnS (506 s)
—
Co-POM
Wavelength / nm
Time / s
Reaction Temp. / ºC
[NaHCO3] / M
[Oxone®] / M
Fig. 2 UV-vis spectral titration upon addition of (a) 0 – 3.0 mol equiv. and (b) 3 – 13 mol
equiv of Oxone® (HSO5–) into water containing NaHCO3 (10 mM), Na2SO4 (50 mM) and
Co-POM (1.0 mM) at RT. (c) UV-Vis absorption changes after an addition of EtBnS (10
mM) to the titration solution. (d) Time-profile of the absorbance at 495 nm for the
solution of (c).
F
ig. 1 Dependence of A/B ratio on (a) reaction temperatures and concentrations of (b)
NaHCO3 and (c) Oxone® in the OAC reactions of EtBnS by Co-POM. The reactions were
performed in D2O (1.0 mL) containing Co-POM (0.20 mM), NaHCO3 (1.0 M), EtBnS (0.10
M) and Oxone® (0.30 M) at (a) 30–80 ℃, (b) the concentrations of NaHCO3 were varied
from 0 to 2.0 M at 30 ℃, and (c) the concentrations of Oxone® were varied from 0.050
to 0.40 M at 30 ℃.
We attempted to characterize the reactive species (brown
and orange-brown) based on paramagnetic 31P NMR spectral
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measurements in D2O. A new diamagnetic 31P NMR signal at most stable among these optimized structures (Table S2). TD-
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DOI: 10.1039/D0CY01758B
+12.5 ppm was observed for the Co-POM solution upon DFT calculations show that the optimized structure with the
–
addition of 3.0 mol equiv of HSO5 (Fig. S8a), supporting the sextet spin multiplicity gives good consistency in the observed
formation of brown species in the UV-vis spectra (Fig. 2a). This absorption band of the orange-brown species with the peak at
signal can be assigned to
a
diamagnetic CoIII species, 495 nm, which is assigned to the ligand-to-metal charge transfer
–
([PCoIII(HSO5 )W11O39]5–), that formed from 1e–-oxidation of (LMCT) band of electron transition from 2p orbitals of bridging
[PCoII(OH2)W11O39]5– followed by coordination of HSO5 to CoIII oxygen atoms (258B) to an anti-bonding orbital of Co-O bond
–
centre in a low-spin state (Scheme 3). It is reasonable to oxidase (261B) (Fig. 3b). The oscillator strength at 554.19 nm is also
–
Co(II) of Co-POM to Co(III) by HSO5 based on the oxidation attributed to the LMCT band of electron transition from 258B to
potential of Co(II) centre (E = +1.45 V vs Ag/AgCl) and the Co d orbital (259B). Also, the cobalt(III)-oxyl species have been
–
reduction potential of HSO5 (E > +1.6 V vs Ag/AgCl).25 Excess proposed as a reactive intermediate in catalytic oxidation
HSO5– required for the experiments suggests that the formation reactions so far.26-28 Thus, we conclude that the reactive orange-
of the diamagnetic CoIII species has an equilibrium between brown species is primarily derived from the CoIII-oxyl
–
[PCoIII(OH2)W11O39]5– and [PCoIII(HSO5 )W11O39]5–. When further intermediate ([PCoIII(O•)W11O39]5–).
–
addition of HSO5 (14 mol equiv) into the solution, two
Based on the results of UV-vis spectral, 31P NMR spectral
paramagnetic 31P NMR signals appeared at +54.7 ppm and measurements and theoretical calculations, we propose the
+95.7 ppm (Fig. S8b). These paramagnetic species at +54.7 ppm reaction mechanism of sub-stoichiometric oxidation of EtBnS by
and +95.7 ppm could be assigned to [PCoIII(O•)(W11O39)]5– (CoIII- Co-POM (Scheme 3) at the initial step. [PCoII(OH2)W11O39]5– is
–
oxyl) or [PCoIV(O)(W11O39)]5– (CoIV-oxo) that represented as oxidized by HSO5 to generate [PCoIII(OH2)W11O39]4– that is
–
–
equivalent electronic structures.
coordinated to HSO5 in the presence of excess HSO5 to form
Theoretical calculations on [PCo(O)W11O39]5– under vacuum [PCoIII(HSO5 )W11O39]5–.
The
CoIII-oxyl
species,
–
– 29
have been conducted to distinguish whether CoIII-oxyl or CoIV- [PCoIII(O•)(W11O39)]5–, formed from O–O cleavage of HSO5 ,
oxo species are the dominant reactive species in the catalytic reacts with EtBnS to produce hydroxylated EtBnS and
reaction by Co-POM. The optimized structures under doublet, regenerate [PCoII(OH2)W11O39]5–.
quartet and sextet spin multiplicity are displayed in Fig. 3a.
Scheme 3 Plausible reaction mechanism of sub-stoichiometric oxidation of EtBnS by Co-
POM.
Kinetic analysis on the absorption decay of the CoIII-oxyl
species was investigated to scrutinize a detailed reaction
mechanism in the sub-stoichiometric oxidation by Co-POM
using sodium m-xylenesulfonate (mXyS) as
a standard
substrate. We have employed mXyS as a substrate in order to
synthesize its derivatives with facile methods. In the sub-
stoichiometric oxidation of mXyS by Co-POM, the apparent rate
constants (kobsH) for mXyS was determined to be 8.9  10–2 s–1
by fitting the absorption change at 495 nm based on pseudo-
first-order kinetics (Fig. 4a). The rate constant (kobsD) for sodium
trideuterated m-xylenesulfonate (mXyS-d3) was obtained to be
6.3  10–2 s–1 (Fig. S9). A kinetic isotope effect (KIE = kH/kD) was
calculated to be 1.4 in the sub-stoichiometric oxidation of mXyS
by the CoIII-oxyl species. A KIE value of 1.7 was reported in
oxidation reactions of benzene by hydroxyl radicals generated
from H2O2, which was rationalized as the electrophilic attack of
hydroxyl radicals on the aromatic ring of benzene.30 Compared
to the reported KIE value, the obtained KIE (1.4) demonstrates
an electrophilic addition on the benzene ring of mXyS by the
Fig. 3 (a) DFT-optimized structures, spin distributions and total energies of
5–
[PCo(O)W11O39
]
lying on doublet (left), quartet (centre) and sextet (right) spin
multiplicity. (b) TD-DFT calculations for the optimized structures overlaid with an
UV-vis absorption obtained from addition of 13 mol equiv. of HSO5– to Co-POM in
water (see Fig. 2b). Inset shows the corresponding molecular orbitals (258B, 259B
and 261B). All these calculations were conducted at the UB3LYP/LanL2TZ (W), 6-
31G* (Co), 6-31+G* (O, P) level of theory.
Judging from the spin distribution of these optimized structures,
the terminal oxygen coordinated to the Co centre with a spin
density of approximately 1.0 indicates the oxyl-radical character
of [PCo(O)W11O39]5– drawn as [PCoIII(O•)W11O39]5–. The
structure with the sextet spin state was calculated to be the
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CoIII-oxyl species. The results also suggest that the OAC ppm, 7.8–6.3 ppm, 5.6 ppm, 5.5 ppm and 4.3 ppm,VrieewspAreticclteiOvenllinye,
reactions of EtBnS by Co-POM may undergo the step of determined based on a reported literaturDeO.3I4: 1W0.h10e3n9/cDa0tCeYc0h1o75l 8oBr
electrophilic addition on the benzene ring by the CoIII-oxyl hydroquinone was employed as substrates in the OAC
species. The sub-stoichiometric oxidation of monosubstituted reactions, these characteristic peaks obtained using phenol as a
m-xylenesulfonate (YmXyS, Y = Br, NO2) by Co-POM provides an substrate were also observed in the 1H NMR spectra (Figs. S11b
apparent rate constant of 7.1  10–2 for BrXyS (kobsBr) and 6.2  and S11c). The results suggest the primary degradation pathway
10–2 s–1 for NO2mXyS (kobsNO2) (Fig. 4b and 4c). The Hammett in benzene oxidation should include phenol, catechol as
plots of the rate constants showed a linear relationship against oxidative intermediates. The production of formic acid was
Hammett parameters of substituents31 with a negative slope ( confirmed by the oxidation reactions using trans,trans-muconic
= –0.5) in sub-stoichiometric oxidation of YmXyS (Fig. 4d). The acid (ttMA), maleic acid (Mal) and tartaric acid (Tar) as
result also demonstrates that the OAC reactions by Co-POM substrates (Fig. S12). Therefore, the plausible degradation
took place via electrophilic addition on the aromatic ring of pathway of benzene by Co-POM in Scheme 4 is corroborated by
substrates by a CoIII-oxyl species, as mentioned above (Fig. 4e). the 1H NMR spectral measurements.
Our results are consistent with reported catalytic OAC reactions
of benzene by a Ru(II)-NHC complex10 and a Cu complex32 in
which the reactive species are proposed as a RuII-oxyl (RuIII-O•)
and a CuII-oxyl (CuII-O•) species, respectively.
Scheme 4 plausible degradation pathway of the catalytic OAC reaction of benzene by
Co-POM.
Conclusions
In summary, we have successfully constructed a catalytic
system using Co-POM as a catalyst and Oxone® as an oxidant
for environmentally benign OAC reactions of aromatic rings,
Fig. 4 (a) Time profiles of absorption decay of CoIII-oxyl species generated by the reaction
of Oxone® ([HSO5–]: 14 mol eq.) and Co-POM in the oxidation of YmXyS (Y = H, Br, NO2).
[Co-POM]: 1.0 mM, [Oxone®]: 7.0 mM ([HSO5–]: 14 mM), [YmXyS]: 10 mM, [Na2SO4]: 50
mM, [NaHCO3]: 10 mM, temp.: 25 ℃. (b) Hammett plots on the apparent rate constant
(kobs) in the sub-stoichiometric oxidation of YmXyS by Co-POM. (b) Schematic
representation of benzene hydroxylation by the CoIII-oxyl (CoIII-O•) species.
which allows us to obtain useful carboxylic acids under mild
conditions. In particular, oxidative cleavage of aromatic rings
was executed dominantly in the presence of sodium
bicarbonate as an additive in the catalytic system. The reactive
species was characterized to be a cobalt(III)-oxyl species on the
basis of control experiments, kinetic analysis, theoretical
calculations, and UV-vis spectroscopic analysis. We
demonstrate that aromatic substrates were oxidized by the
cobalt(III)-oxyl species through an electrophilic addition on
aromatic rings in the catalytic system. The degradation pathway
of benzene in the catalytic system was verified by 1H NMR
spectral measurements in which phenol, catechol, and muconic
acid derivatives were identified as oxidative intermediates. The
catalytic oxidation using Co-POM as a catalyst and Oxone® as an
oxidant can provide a fundamental understanding of the nature
of cobalt(III)-oxyl species derived from Co-POM for selective
oxidation reactions. Finally, this work offers valuable insights
into OAC reactions by the Co-POM catalytic system toward the
development of efficient catalysts for degradations of
persistent organic pollutants into useful carboxylic acids in an
environmentally benign manner.
We have also performed catalytic OAC reactions of benzene
by Co-POM to reveal benzene degradation pathways by 1H NMR
spectra. While degradation pathways of benzene have been
widely studied so far,33 the degradation pathways by Co-POM
has yet to be validated. In the catalytic OAC reactions of
benzene by Co-POM, we observed several product peaks that
are assigned to formic acid (8.4 ppm), phenol (around 7.0 ppm),
1
maleic acid (6.5 ppm) and fumaric acid (6.0 ppm) by H NMR
spectra (Fig. S10 and Scheme 4). In the benzene oxidation,
formic acid was observed as the primary oxidation product with
a TON of 12 for Co-POM and a concentration of 2.4 mM based
on 1H NMR spectroscopic analysis. As phenol should be the first
2-electron oxidized intermediate among these products, we use
phenol as a substrate to investigate the subsequent oxidation
steps. Fig. S11a shows the 1H NMR spectrum of the OAC
reaction of phenol by Co-POM: formic acid, cis,cis-Muconic acid
(ccMA), muconolactone (Mlac), tetrahydrofuro[3,2-b]furan-
2,5-dione (Lac2), and tartaric acid (Tar) were observed at 8.4
Conflicts of interest
There are no conflicts to declare.
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6-31G* basis sets40,41 for Co atom, and 6-31+G* basis sets40,41
for O and P atoms. The Gaussian 09 (Revision C.01) program
package42 was used for all DFT calculations.
Acknowledgements
View Article Online
DOI: 10.1039/D0CY01758B
D.H. gratefully acknowledges support from the Japan Society for
the Promotion of Science (JSPS) by the Leading Initiative for
Excellent Young Researchers (LEADER). Y.S also gratefully
acknowledges support from Grants-in-Aid (20K15328) from
JSPS.
Notes and references
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