Generation of reactive cobalt oxo oxamate radical species for biomimetic oxidation of contaminants

Article abstract of DOI:10.1039/c7ra08317c

The non-heme oxamate anionic cobalt(iii) complexes [Co^III(opbaX)]^- (opbaX = 4-X-o-phenylenebis(oxamate), X = H, NO₂, CH₃) with different substituents were synthesized and applied to targeted micropollutant degra

Full text of DOI:10.1039/c7ra08317c

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Generation of reactive cobalt oxo oxamate radical
species for biomimetic oxidation of contaminants†
Cite this: RSC Adv., 2017, 7, 42875
*
*
Yun Zheng, Xuemei Jiang, Ran Zhang and Wenxing Chen
Nan Li,
The non-heme oxamate anionic cobalt(III) complexes [CoIII(opbaX)]ꢀ (opbaX
¼
4-X-o-
phenylenebis(oxamate), X ¼ H, NO2, CH3) with different substituents were synthesized and applied to
targeted micropollutant degradation. Typical radical scavengers (isopropanol and chlorine anions) have
no negative effect on the catalytic oxidation of substrates, and no DMPO–cOH or DMPO–cOOH (DMPO
¼ 5,5-dimethyl-pyrroline-oxide) signal was detected by electron paramagnetic resonance spin-trap
technique in [CoIII(opbaX)]ꢀ/H2O2 system, suggesting that the non-hydroxyl radical biomimetic catalytic
mechanism was dominant in the oxidation process. The results of high-definition ESI-MS pronounced
the presence of cobalt-oxo intermediates which played a key role in the catalytic oxidation of substrates.
Furthermore, density functional theory calculations were used to evaluate the viability of such cobalt-
oxo species and it demonstrated an optimizing electromer with a formulation of [CoIV]Oc]ꢀ or [CoIII–
OH]c. The calculations explained that the catalytic activity of [CoIII(opbaX)]ꢀ was significantly enhanced
by introducing an electron-withdrawing substituent which could change the coordination environment
of cobalt to generate more electron-deficient cobalt-oxo species with stronger oxidizing power. This
paper made a progress in verifying bio-mimic high oxidation state species and applied it to water
purification, which provided deep insight into the properties of transition-metal centers in aqueous
catalysis.
Received 27th July 2017
Accepted 30th August 2017
DOI: 10.1039/c7ra08317c
rsc.li/rsc-advances
In nature, bioenzymes like cytochrome P450 and horse-
radish peroxidase (HRP) utilizes O2 or H2O2 to convert recalci-
Introduction
trant compounds under mild conditions with unexpected
catalytic activities.13–16 However, these enzymes generate only in
organisms and are perishable in practical application. So some
researchers begin to distract their focus onto establishing arti-
cial enzymes to imitate the essential and general principles of
natural enzymes using stable and low-cost materials for a wide
range of applications. It has been found that bioenzymes'
selective oxidation of substrates attributes to the limited
oxidation potential of high-valent metal-oxo active species.17 To
date, Fe, Mn, Co, etc., metal complexes have been synthesized
for bio-mimic targeted redoxes.18–20 For example, iron(III)
porphyrin complexes can epoxide cyclohexene in protic
solvent;21 and cobalt(III) complexes of bis-N,N0-disubstituted
oxamides are able to catalyze secondary alcohols to the corre-
sponding ketones in acetonitrile and to epoxide olen in uo-
robenzene.22,23 However, successful applications of these
selective catalysts to the global issue of serious water pollution
have been seldom reported.
The widespread sewage containing a mass of industrial and
natural chemical substances has been one of the most serious
international distribution problems afflicting human health
and aquatic ecosystems.1,2 Although most contaminants are
present at trace concentrations, a number of them pose
considerable toxicological hazards particularly when present as
components of complex mixtures.3,4 Hence, the development of
green and cost-effective processes aiming to eliminate critical
pollutants, coupled with the search for ecosystem benign
products, is a great challenge for environmental researchers.5 In
recent years, many advanced oxidation technologies such as
Fenton or Fenton-like systems,6,7 TiO2/H2O2 system,8 and pho-
tocatalytic processes,9,10 have been developed for this purpose.
However, these predominantly catalytic mechanisms are mainly
dominated by the hydroxyl radical (cOH) process, which is
inhibited due to its unselectivity in the presence of some inor-
ganic or/and organic background constituents.11,12 Hence, it's
necessary to look for efficient and sustainable catalysts with
specicity.
As known, P450 and HRP both work with the same FeIV]O
active center in aqueous, while their different oxidant activation
power may mainly attributed to the different electron nature of
center Fe which is directly induced by the electronegativity of
the h ligands and substituents on macrocyclic ligand.15,24–26
Generally, the generation of high-valent metal-oxo active
National Engineering Lab for Textile Fiber Materials
& Processing Technology
(Zhejiang), Zhejiang Sci-Tech University, Hangzhou 310018, China. E-mail: linan@
zstu.edu.cn; wxchen@zstu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra08317c
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Scheme 1 The [CoIII(opbaX)]ꢀ activators used in this work.
species is determined by two competitive processes, heterolytic
and homolytic cleavage of the O–O bond.21,27 Based on this
theory, the transformed O–O bond cleavage has been achieved
in the catalytic system with cobalt phthalocyanine through
introducing axial h ligands in our previous studies, and the
results indicated that the introduction of the h ligand
changed the local environment around the cobalt cation and
hence transformed the cleavage of peroxide O–O from homol-
ysis to heterolysis.27,28 Here, we investigated the cleavage
tendency of O–O bond to see if it was possible to change the
catalyst's oxidation power by introducing various substituents
on benzene ring of Co macrocyclic ligand. Besides, few
researchers have presented direct evidence proving the exis-
tence of cobalt-oxo active species, so it's necessary to focus on
the depiction of the functioning active intermediates.
In this study, we synthesized square-planar phenyl-
enebis(oxamate) (opba) cobalt(III) complexes with related ligand
derivatives (Scheme 1) and emphasized H2O2 activation mech-
anism through analysing the possible electron transport and
substituents' effects on the catalysis. The effect of environ-
mental factors such as pH, temperature, initial H2O2 carried out
under mild conditions. The results of high-denition ESI-MS
pronounced the presence of cobalt-oxo intermediates further-
more, the density functional theory calculations were used
to evaluate the viability of such cobalt-oxo species and it
demonstrated an optimizing electromer with a formulation of
[CoIV]Oc]ꢀ or [CoIII–OH]c. In conclusion, the biologically
inspired subject addressed in this work illuminated cobalt-oxo
active species in water and broadened the designing strategy
for synthesizing novel bioenzyme alternatives for wider
applications.
Fig. 1 Synapt G2-S HDMS (ESI) in negative mode of complex 1 in
MeCN aqueous solution for (a) Q-TOF MS and (b) MS/MS.
complex dianion ([CoII(opba)]2ꢀ, m/z 153.4701). Furthermore,
the typical fragment ion masses were observed (see Fig. 1b and
Table S1 for ESI†), which matched the theoretical masses within
0.5 mDa error. Likewise, the accurate masses of methyl and
nitro substituent cobalt complex anions and fragment ions
conformed to the expectation of calculated value (Tables S2, S3
and Fig. S1, S2 see ESI†).
Moreover, the oxidation state of Co in the complex is sup-
ported by XPS signals of the complex. The spin–orbit splitting of
the Co2p level (DE) of complex 1 is shown in Fig. S3,† in which
both the binding energy level splitting of Co2p1/2–Co2p3/2 (DE ¼
14.31 eV) and two weak satellite peaks demonstrated the genera-
tion of oxidation state 3+ of Co in the surface region.29,30 We guess
that the strong electron-donating property of the N2–O2 coordi-
Results and discussions
Characterization of cobalt complexes
The square-planar cobalt(III) complexes anions, [CoIII(opbaX)]ꢀ, nation environment contributed to the generation of CoIII.31
were synthesized from the reaction between a Co(II) salt and the To further prove the chemical structure of our catalysts, H
1
proligands in methanol solution by means of oxidizing the NMR (D2O) spectra was employed. As shown in Fig. S4,† the
corresponding cobalt(II) complexes with dioxygen in situ. The sharp singlet assigned at d 2.23 ppm was attributed to the Ar–
solution state structure [CoIII(opbaX)]ꢀ and the oxidation state methyl protons of complex 3.32,33 In the spectra of complex 1, 2
of Co was identied by Synapt G2-S HDMS (ESI) and XPS. The and 3, the multiplets observed in the low eld region were the
accurate masses of anions [CoIII(opbaX)]ꢀ and fragment ions characteristic of benzene ring protons, while chemical shi to
were shown in Q-TOF MS and MS/MS spectra. In Q-TOF MS, this higher and lower magnetic eld are ascribed to the contribution
complex showed an ion at m/z 306.9402 (Fig. 1a), the mass was of electron-withdrawing nitro and electron-donating methyl
within 0.1 mDa deviating the theoretical mass (m/z 306.9401) respectively. In conclusion, the result of LC-MS and 1H NMR
for cobaltic complex anion [CoIII(opba)]ꢀ rather than cobaltous clearly described the chemical structure of our catalysts.
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Therefore, it's evident that the synthesized compounds
correspond to cobaltic complex anions [CoIII(opbaX)]ꢀ.
Catalytic oxidation of C.I. acid red 1 (AR1) and p-chlorophenol
In order to comparatively evaluate the effect of substituent
ligands on the catalytic degradation of organic pollutants, the azo
dye AR1 and p-chlorophenol (two typical aromatic pollutants)
were selected as model substrates. As shown in Fig. 2, AR1 is
barely degraded in the presence of the cobalt complexes alone
(curve a–c, Fig. 2) and is the same when H2O2 is present alone
(curve d, Fig. 2). When both the cobalt complex and 2 mM H2O2
was presented, the concentration of AR1 declined rapidly with
increasing reaction time (curve e–g, Fig. 2) with an degradation
rate order of [CoIII(opbaNO2)]ꢀ (2) > [CoIII(opba)]ꢀ (1) >
[CoIII(opbaCH3)]ꢀ (3). In addition, the catalytic elimination of p-
chlorophenol was carried out, and the removal rate was around
48%, 57% and 2% in the rst 10 min in the presence of complex
1, 2 and 3 respectively (Fig. S5†). Hence, we hypothesize a corre-
lation between the catalytic activity and the electron donor
capacity of ligands to account for the phenomenon: the electron-
withdrawing nitro substituent promoted a higher effective posi-
tive charge at cobalt compared with electron-donating methyl
substituent, which was analogous to the ligand effect on Fe(III)–
TAML (tetraamido macrocycle ligand),34 which indicated that the
presence of electron-withdrawing substituent facilitated the
generation of active intermediates.21,35,36
Fig. 3 (a) Effect of the temperature on catalyst oxidation of AR1 (0.05
mM) at pH 9.0; (b) effect of pH on the apparent rate constant kobs for
the oxidation of AR1 by H2O2 catalyzed by 1 at different temperatures.
The inset shows pH profiles of the removal rates of AR1 in the presence
of 1 and H2O2 after reaction for 60 min.
Effect of pH and temperature
The investigation of pH and temperature effect on 1-catalyzed
deposition of AR1 activated by H2O2 is shown in Fig. 3. Complex
1 can efficiently activate H2O2 in the range of alkalinity (pH 8–
11) which is consistent with the environmental conditions.37
Moreover, AR1 removal in initial 10 min increased from 37.3%
kinetics of AR1 decomposed by 1/H2O2 followed pseudo rst-
order reaction kinetics well in the initial reaction stage where
the concentration of H2O2 was enough.38 So we concluded that
the decomposition of AR1 has less dependence on temperature
from 15 to 45 ꢁC because of the relatively small value of Ea which
could be easily achieved in comparison with ordinary thermal
reactions.39
ꢁ
to 83.6% as the temperature increased from 15 to 45 C at pH
9.0 (Fig. 3a). An Ea value of 34.61 KJ molꢀ1 was obtained by
plotting kobs versus 1/T (Fig. S6†). And the regression coefficient
(R2) was close to 0.95, which indicated that the degradation
Furthermore, the pseudo-rst-order rate constants (kobs
)
increased at the beginning and then decreased with the
increase of pH values, nally the maximum rates occur at pH
9.0–9.5 (Fig. 3b). Hence, the result showed that the catalytic
reaction kinetics were affected signicantly by pH, which is
interpreted as follows: the hydrolysis of the complex somewhat
occurs and the dissociation of H2O2 was slower when pH moved
toward neutral, which caused the limited coordination between
HOOꢀ and the central cobalt ions because the formation of
active species was restrained. And when the pH of the solution
was high, HOOꢀ, the conjugate anion of H2O2, reacted with
a non-dissociated molecule of H2O2 and ultimately resulted in
the generation of oxygen and water.40,41
Effect of H2O2 concentration
The relationship between the degradation rate of AR1 and
initial H2O2 concentrations were investigated and shown in
Fig. S7,† which indicates that the AR1 can be effectively
removed even at a very low concentration of H2O2 (0.5 mM), and
Fig. 2 Bleaching of AR1 (0.05 mM) under different conditions: (a)
complex 3, (b) complex 1 and (c) complex 2 without H2O2; (d) H2O2; (e)
complex 3, (f) complex 1 and (g) complex 2 with H2O2; [H2O2] ¼ 2 mM,
[complexes 1–3] ¼ 0.02 mM, T ¼ 25 ꢁC, pH 9.0.
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even more than 90% of AR1 can be removed at such a low
concentration of H2O2 in 1 hour. So this reaction system could
signicantly reduce the disproportionate amount of H2O2 that
presented in some cOH-involving reaction systems, because in
high backgrounds of complicated background, cOH inevitably
react with easily handled constituents or the free coordination
catalysts in homogeneous system.34,42
Sustaining catalytic ability of the complexes
One of the most signicant distinguishes between bio-enzymes
and articial catalyst is stability. To checkout if the catalysts
were durable in water purication, representative 1 was exam-
ined by subsequent additions of dye and H2O2 at pH 9.0. As seen
in Fig. 4, the removal of AR1 aer 60 min was about 96% by the
end of the rst cycle, aer eight cycles with repeated additions
of the same amount of original dye and 1/3 times as much as the
original amount of H2O2, the removal rate of AR1 was still
maintained at more than 90%, which suggested that there
existed no suicidal inactivation in our reaction system.
Fig. 5 Effect of NaCl on catalytic oxidation of AR1 (0.05 mM) in the
presence of 1 (0.02 mM) and H2O2 (2 mM), T ¼ 25 ꢁC, pH 9.0. Error bars
represent the standard error from three independent experiments.
peroxide, isopropanol (IPA), a typical cOH scavenger,45,46 was
employed to determine whether cOH dominated the reactions or
not. As a represent, the catalysis of 1 in the presence of IPA was
carried out. As shown in Fig. 6a, the degradation rate of 1 pre-
sented in IPA (its concentration was 5000 times as 1) was
signicantly in good agreement with the control experiment,
which favored the non-cOH mechanism. Moreover, EPR spin-trap
technique (using DMPO (DMPO ¼ 5,5-dimethyl-pyrroline-oxide)
as the trapping reagent) was employed to detect the possibility
that free radicals were produced in the reactions described
above.47 As a result shown in Fig. 6b, no DMPO–cOH signal or
other free radicals showed up, indicating that cOH did not form
in the catalytic reaction, which supported the positive results of
isopropanol and NaCl experiments. An additional experiment
conducted in methanol solution was conducted to probe whether
there were any superoxide radicals (cOOH) generated during the
reaction process. As shown in Fig. 6c, no DMPO–cOOH species
were observed in 1/H2O2 system either, suggesting the catalytic
mechanism belongs to non-hydroxyl radical pathway.
Detection of active species
As the oxidation mechanism of organic pollutants with H2O2
catalyzed by [CoIII(opbaX)]ꢀ in aqueous solution has not been
previously reported, it is necessary to present clear insights into
the function mechanisms of these catalysts. As we know, NaCl is
a prevailing accelerant widely used in the dye industry and plays
a vital role in the dyeing process for the transfer of dye stuff to
fabric. However, various studies reported the retardation effect
of NaCl on degradation of pollutants especially on the cOH
mechanism.43,44 Hence, it is important to evaluate the impact of
inorganic salts on the catalytic oxidation systems. As expected,
the degradation efficiency of 1 displayed no signicant change
in the presence of NaCl compared with the control experiment
(Fig. 5). This meant that this catalytic system has potential and
practical application value in treating salt-containing waste-
water and that there were no free cOH generated in this system.
In order to investigate the intermediate active species which
generate based on the cleavage of O–O bond in hydrogen
To obtain the evidence for the reactive intermediates, ESI-MS
with a so MS ionization technique, was employed for direct
studies of catalytic intermediates, especially for the transient
intermediates.48,49 The unsubstituted complex 1 was chosen for
investigation and reacting with
2
equivalent m-chlor-
operbenzoic acid (mCPBA) playing as oxidant in the presence of
trace NH3$H2O at ꢀ40 ꢁC in CH3CN. The prominent ions at
a mass/charge (m/z) ratio of 322.9379, as shown in Fig. 7, with
a
mass corresponding exactly to the formulation of
[Co(O)(opba)]ꢀ within 2.8 mDa mass error. Hence, the electro-
mer of high-valent cobalt-oxo ([CoIV]O]ꢀ or [CoV]O]ꢀ) and
cobalt(III)-oxo ([CoIII–OH]) was presumed considering dehydro-
genation of MS in the negative ion mode.
Illuminating the valence state of the activated species is
imperative for the study of electron transmission pathway. DFT
calculations of Gaussian 09 program with functional B3LYP and
basis set 6-31G method is potent.50 Since the related interme-
diate spin d6 square-planar cobalt(III) complexes have previously
been reported, the electron spin density distributions were
calculated with S ¼ 0, S ¼ 1, S ¼ 2.51
Fig. 4 Cyclic catalyst oxidation of AR1 (0.05 mM), [complex 1] ¼
0.02 mM, [H2O2] ¼ 2 mM, T ¼ 25 ꢁC, pH 9.0.
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structures (Table S4†) in which we calculated the intermediates
by placing an oxygen atom near the optimized [CoIII(opbaX)]ꢀ
(Tables S5 and S6†). According to the calculated results, we
concluded that high-valent cobalt-oxo species formed
with a Co]O bond length order of [CoN(O)(opbaCH3)]ꢀ
>
[CoN(O)(opba)]ꢀ > [CoN(O)(opbaNO2)]ꢀ (Fig. 8, Tables S7–S9,† N
means the valence state of Co). In addition, the electron density
on Co was decreased by electron-withdrawing –NO2 and the
electrophilicity of Co was increased,52 while the electron-
donating –CH3 functioned inversely.
Moreover, a large unpaired electron spin density (ꢂ1, neatly
50% of the total) located around the oxo moiety of Co]O, which
meant there existed an unpaired electron on the oxo moiety
(Tables S10–S12†). And the spin density on excited Co reduced
Fig. 6 (a) Concentration changes of AR1 (0.05 mM) in the presence of
complexes 1–3 with or without isopropanol. Error bars represent the
standard error from three independent experiments; (b) DMPO spin-
trapping EPR spectra in AR1 (0.05 mM) aqueous solution in the pres-
ence of complex 1 and H2O2 (2 mM) with DMPO (20 mM); (c) DMPO
spin-trapping EPR spectra in AR1 (0.05 mM) methanol solution in the
presence of complex 1 and H2O2 (2 mM) with DMPO (20 mM).
Fig. 7 (a) High-definition electrospray mass spectra of complex 1; (b)
in the presence of 2 equivalents of mCPBA.
Fig. 8 Unpaired electron spin distribution and geometrically opti-
mized structure of [CoIII(opba)]ꢀ (a, b), [Co(O)(opbaCH3)]ꢀ (c, d),
[Co(O)(opba)]ꢀ (e, f), [Co(O)(opbaNO2)]ꢀ (g, h), Co(OH)(opbaCH3) (i, j),
Co(OH)(opba) (k, l), Co(OH)(opbaNO2) (m, n) based on DFT calculation.
Red, oxygen; blue, nitrogen; purple, cobalt. Selected bond lengths (A˚)
Firstly we calculated high-valent models ([CoIV]O]ꢀ or
[CoV]O]ꢀ) and chose the triplet (S ¼ 1) spin ground state
because it possessed the lowest energy of the optimized of Co–O and the deviation height (A˚) of Co are given.
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to 1.24 compared with the data calculated for ground state enhanced, which indicates that 3 might be more susceptible to
[CoIII(opba)]ꢀ (ꢂ1.8), which meant Co lost a d-electron for the oxidants aer substituted by electron-withdrawing substituent
coordination to O aer the O–O bond cleaved heterolytically. As while the electron of SOMO in 1 and 3 is more difficult to be
a result, the CoIII was oxidized to CoIV. So the active interme- excited. Thus the electron vacancies make the Co]O unit
diate can be described as [CoIV]Oc]ꢀ species, like the radical possess signicant desire for electron and electrophilic attack
species of O]FeIV–AcOc reported by Yong Wang et al.53 to the azo bond or naphthalene structure of dyes.57 This prin-
Furthermore, the N2O2-plane deviation of Co empowered O to ciple manifested well in the relative degradation efficiency that
accept electron via oxidizing the substrates, especially the seemed to depend on the electron nature of the substituents
[cO]Co(opbaNO2)]ꢀ with a nitro substituent which has the (Fig. 2 and S5†).
Secondly, the HO–CoIII(opbaX) patterns were also calculated
˚
shortest N2O2-plane deviation of Co (0.4078 A).
To understand the origin of these properties, we have by putting OH near to the optimized [CoIII(opbaX)]ꢀ in the same
calculated the Frontier molecular orbitals (FMOs). According to Gaussian 09 program (Fig. 9b). According to the total energy, we
the FMO theory, electrophiles require lowest unoccupied chose HO–CoIII(opba) and HO–CoIII(opbaCH3) models with S ¼
orbitals (LUMO) with high molecular orbital compositions on 1, and HO–CoIII(opbaNO2) model with S ¼ 2 (Table S13†). The
a reacting atom to achieve segistration with the FMOs on the different number of spinning electrons should ascribe to the
nucleophilic substrate.54–56 In our calculations (Fig. 9a), LUMO extension effect of –NO2.58 The bond length between Co and O is
˚
and LUMO+1 of catalysts are mainly formed by the contribu- around 1.8 A, declaring the single bond between Co and O.
tions of the O and Co d orbitals, which means the Co–O anti- Compared with high-valent models, these patterns possess
bonding orbitals are the key FMOs; the O is the reacting atom, smaller electron spin density distributed around the O atom
which interacts with the FMOs of the nucleophilic reagent. (ꢂ0.5), leaving near 1 spinning electron delocalized at the bis-
Moreover, the catalyst with the electron-donating methyl give benzoamido moiety (Tables S14–S16†) which manifested with
highest energy levels of LUMO and singly occupied MO (SOMO), the LUMO and SOMO orbitals (Fig. 9b). So the active interme-
and that with electron-withdrawing cyano group has the lowest diate can be depicted as [CoIII–OH]c.59 Furthermore, the orders
energy levels. The electron-withdrawing effect decreases the of Co–O bond length and orbital energy levels are the same as
energy of LUMO intensively and the SOMO–LUMO gap is those of the high-valent models, which can also explain the
oxidation power disparity of the three catalysts in contaminant
removal. In conclusion, this catalytic system is proposed to
proceed with either [CoIV]Oc]ꢀ or [CoIII–OH]c reactive
intermediates.
Accordingly, we depicted two generation pathway of reac-
tive intermediates (Scheme 2). As known, the O–O bond of
hydroperoxides cleaved either heterolytically or homolytically
in aqueous, and the tendency of heterolysis was signicantly
affected by the electronic nature of oxamate complex.21,27
When coordinated with electron-withdrawing ligand, center
Co was more affine to nucleophilic OOH moiety because of
electron deciency, which resulted to the rate acceleration
in contaminant degradation.60,61 In pathway (a), electron-
rich cobalt complexes showed a tendency to cleave the
hydroperoxide O–O bond homolytically, whereas electron-
decient cobalt complexes show a tendency to cleave the
Fig. 9 Frontier molecular orbitals energy level and molecular orbitals Scheme 2 Proposed pathway and formation of cobalt-oxo electro-
of [CoIV]Oc]ꢀ and [CoIII–OH]c.
mer (L ¼ bis-benzoamido).
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heterolytically, in which the tendency was in the order of 2 >
1 > 3. Then the O–O cleavage followed due to the unstable
structure of –CoIIIOOH2ꢀ with formations of OHꢀ and CoIV]
Oc. Notably, this reactive radical was different from the results
Catalytic experiments and analysis
The typical experimental procedure for the catalytic oxidation of
AR1 and p-chlorophenol was conducted with complexes 1–3
respectively and carried out in a 40 mL glass beaker with
a magnetic stirrer. The reaction temperature was maintained at
25 ꢁC or 45 ꢁC by means of a constant temperature water bath.
The initial concentration of AR1 or p-chlorophenol in 0.01 M
borate buffer was 0.05 mM with 0.02 mM cobalt complex, and
then a certain amount of H2O2 was added to the reaction
solution. At given time intervals, a small amount of reaction
mixture of AR1 was frequently taken out for observing the
concentration changes of substrates, and the concentration of
dyes was determined using an ultraviolet-visible (UV-vis) spec-
trometer (Hitachi U-3010). The concentration changes of p-
chlorophenol were analyzed by ultra-performance liquid chro-
matography (UPLC, Acquity BEH C18 column (1.7 mm, 2.1 ꢃ 50
mm), Waters). The removal rate was calculated by eqn (1), where
C is the concentration of AR1 or p-chlorophenol at each reaction
time and C0 is the initial concentration of the substrate.
´
reported previously by Isabel Fernandez et al., who proposed
that oxamide cobalt complex played a key role in acetonitrile
as CoIV]O structure.22 Considering the Gray's “oxo-wall”,62
the pathway (b) is less exotic, where Co–O2H2 broke the O–O
bond heterolytically and lost OHꢀ, leaving electron decient
OH moiety.63 According to the electron spin density distri-
bution result of HO–CoIII(opbaX), O atom obtained additional
0.5 spinning electron from Co, while the spin of Co did not
essentially changed. This meant the bis-benzoamido moiety
transferred electron to Co and Co transferred it to O,
thus forming the ligand-based radical intermediate HO–
CoIII(opbaX)c. Electron-withdrawing substituent facilitated
the O–O bond cleavage, which speeded up the formation of
reactive intermediates and the degradation efficiency of
pollutants.
Residual rate ¼ C/C0
(1)
Experimental
Materials and reagents
Assuming that the initial organic degradation constant ts
the data to a pseudo-rst-order reaction kinetic model when
H2O2 is in considerable excess, we obtained the apparent rate
constant kobs calculated from the slop of the plot ln(C/C0) versus
to time.
The spin-trapping reagent, 5,5-dimethyl-pyrroline-oxide,
(DMPO) was obtained from Tokyo Chemical Industry Co., Ltd.
H2O2 (9.7 M), m-chloroperbenzoic acid (mCPBA, 85%), dyes
including AR1, p-chlorophenol, and other chemicals used in the
syntheses were of analytical grade from commercial suppliers
and used as received. Ultrapure water (Millipore Milli-Q
Advantage A10 Ultrapure water system) was used throughout
the experimental process. The diethyl ester derivatives, 4-nitro-
o-phenylenebis (oxamic acid) (H2Et2opbaNO2) and 4-methyl-o-
phenylenebis (oxamic acid) (H2Et2opbaCH3), were synthesized
by the corresponding commercially available o-phenylenedi-
amine derivatives and ethyloxalyl chloride in tetrahydrofuran by
standard condensation techniques in a similar preparation
process as the unsubstituted analogue o-phenylenebis (oxamic
acid) (H2Et2opba).
Characterizations
The structure and cobalt oxidation state of the cobalt complexes
1–3 in MeCN aqueous solution were characterized by high-
denition mass spectrometry in Q-TOF mode, which were per-
formed on a Synapt G2-S HDMS (Waters) in negative mode
equipped with electrospray ionization (ESI) source. The capillary
voltage and cone voltage were set at 2.5 kV and 40 V respectively,
and the desolvation gas was set to 900 L hꢀ1 at 80 ꢁC. The energies
of collision ramp were controlled at 20–40 V for the MS/MS frag-
mentation information. The MS data were acquired using the lock
spray to ensure mass accuracy and reproducibility with leucine-
enkephalin (m/z 554.2615) in negative electrospray ionization
Preparation of catalysts
The cobalt(III) complexes 1–3 were synthesized according to the mode. X-ray photoelectron spectroscopy (XPS) data were recorded
steps reported previously.22,64 The Et2H2opbaX (X ¼ H, NO2, on a Thermo Scientic K-Alpha spectrometer (monochromatic Al
CH3) (5 mmol) has been dissolved in a 1 : 9 mixture (100 mL) of Ka, 1486.6 eV), and the binding energy peaks of the XPS spectra
methanol and water, and 50 mL NaOH (21 mmol) aqueous were calibrated by placing the principal C1s binding energy peak
1
solution was added. This resulting mixture was stirred at 70 ꢁC at 284.7 eV. H NMR spectra were recorded on a Bruker Avance
for 30 min to facilitate the hydrolysis of the ethyl ester groups. 400 MHz spectrometer. D2O was used as solvent and internal
Aer the temperature of the reaction solution fell to room standard (dH ¼ 4.71). The EPR signals were detected on a Bruker
temperature, 5.0 mmol Co(NO3)2 (50 mL) was added dropwise A300 spectrometer for the free radicals trapped by DMPO at room
with vigorous stirring under an oxygen atmosphere for 1 h. The temperature. The parameter setting typically as follows: center
resulting mixture was ltered and concentrated by evaporation eld, 3520 G; microwave frequency, 9.77 GHz; modulation
of the solvent to about 10 mL, and then cooling in the presence frequency, 100 kHz. We attempted to detect the residual p-chlor-
of ethanol to obtain the dark-green product of complex 1 (ca. ophenol using UPLC chromatographic separation with a BEH C18
1.6 g, 80% yield) aer drying under vacuum. Similarly, the brick- column (1.7 mm, 2.1 ꢃ 100 mm) using 30% mobile phases A
red complex 2 (ca. 0.6 g, 30% yield) and dark-brown complex 3 (acetonitrile) and 70% B (formic acid/water solution with
(ca. 0.5 g, 25% yield) were synthesized respectively except for a volume ratio of 1/1000). The ow rate was 0.4 mL minꢀ1, and the
employing ether in place of ethanol.
column oven temperature was set at 30 ꢁC.
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ꢁ
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Conclusions
Nature has developed various metalloenzymes such as heme-
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are capable of stubborn substrate transformations. So there has
long been signicant interest in modelling such enzyme active
sites and developing biomimetic catalysts for various purposes.
Based on the same FeIV active species yet with different catalytic
ability, here we designed [CoIII(opbaX)]ꢀ/H2O2 system which
exhibited excellent catalytic performance in oxidizing targeted
pollutants such as dyes and p-chlorophenol via non-hydroxyl
radical process pathway under high backgrounds of constitu-
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[CoIII–OH]c and [CoIV]Oc]ꢀ in catalysis.
´
´
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There are no conicts of interest to declare.
Acknowledgements
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