Single-step benzene hydroxylation by cobalt(ii) catalysts: Via a cobalt(iii)-hydroperoxo intermediate

Article abstract of DOI:10.1039/c9cy02601k

The cobalt(ii) complexes of 4N tetradentate ligands have been synthesized and characterized as the catalysts for phenol synthesis in a single step. The molecular structure of the complexes showed a geometry in between square pyramidal and trigonal bipyramidal (τ, 0.49-0.88) with Co-N_amine and Co-N_Py bond distances of 2.104-2.254 ? and 2.043-2.099 ?, respectively. The complexes exhibited a Co²⁺/Co³⁺ redox potential around 0.489-0.500 V vs. Ag/Ag⁺ in acetonitrile. The complexes catalyzed hydroxylation of benzene using H₂O₂ (30%) and afforded phenol selectively as the major product. A maximum yield of phenol up to 29% and turnover number (TON) of 286 at 60 °C, and a yield of 19% and TON of 191 at 25 °C are achieved. This is the highest catalytic performance reported using cobalt(ii) complexes as catalysts to date. This aromatic hydroxylation presumably proceeded via a cobalt(iii)-hydroperoxo species, which was characterized by ESI-MS, and vibrational and electronic spectral methods. The formation of key intermediate [(L)Co^III(OOH)]²⁺ was accompanied by the appearance of the characteristic O → Co(iii) ligand to metal charge transfer (LMCT) transition around 488-686 nm and vibration modes at 832 cm^-1 (O-OH) and 564 cm^-1 (Co-O). The geometry of one of the catalytically active intermediates was optimized by DFT and its spectral properties were calculated by TD-DFT calculations. These data are comparable to the experimental observations. The kinetic isotope effect (KIE) values (0.98-1.07) support the involvement of cobalt-bound oxygen species as a key intermediate. Isotope-labeling experiments using H₂¹⁸O₂ showed an 89% incorporation of ¹⁸O, revealing that H₂O₂ is the main oxygen supplier for phenol formation from benzene. The catalytic efficiencies of cobalt complexes are tuned by ligand architectures via their geometrical configurations and steric properties.

Full text of DOI:10.1039/c9cy02601k

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DOI: 10.1039/C9CY02601K
Journal Name
ARTICLE
Single-Step Benzene Hydroxylation by Cobalt(II) Catalysts via
Cobalt(III)-Hydroperoxo Intermediate
a
a
b
Karunanithi Anandababu, Sethuraman Muthuramalingam, Marappan Velusamy and Ramasamy
Mayilmurugan*
Received 00th January 20xx,
Accepted 00th January 20xx
a
DOI: 10.1039/x0xx00000x
The cobalt(II) complexes of tetradentate ligands (4N) have been synthesized and characterized as the catalysts for phenol
synthesis in single-step. The molecular structure of complexes showed geometry in between square pyramidal and trigonal
bipyramidal (τ, 0.49 – 0.88) with Co-Namine (2.104 – 2.254 Å) and Co-NPy bond distances (2.043 – 2.099 Å). The complexes
www.rsc.org/
2
+
3+
+
exhibited Co /Co redox potential around 0.489 - 0.500 V vs Ag/Ag in acetonitrile. The complexes catalyzed hydroxylation
of benzene using H (30%) and afforded phenol selectively as a major product. The maximum yield of phenol up to 29%
2 2
O
and turnover number (TON) of 286 at 60 °C and yield of 19% and TON of 191 at 25 °C are achieved. This is the highest
catalytic performance reported using cobalt(II) complexes as catalysts to date. This aromatic hydroxylation presumably
proceeded via cobalt(III)-hydroperoxo species, which was characterized by ESI-MS, vibrational and electronic spectral
III
2+
methods. The formation of key intermediate [(L)Co (OOH)] was accompanied by the appearance of the characteristics O
-1

Co(III) ligand to metal charge transfer (LMCT) transition around 488-686 nm and vibration modes at 832 cm (O-OH) and
5
64 cm-1 (Co-O). The geometry of one of the catalytically active intermediates was optimized by DFT and its spectral
properties were calculated by TD-DFT calculations. These data are comparable to the experimental observations. The kinetic
isotope effect (KIE) values (0.98 - 1.07) support the involvement of cobalt-bound oxygen species as a key intermediate. The
18O showed an 89% incorporation of 18O, reveals that H
isotope-labeling experiments using H O is the main oxygen supplier
2 2 2 2
for phenol formation from benzene. The catalytic efficiencies of cobalt complexes are tuned by ligand architecture via their
geometrical configurations and steric properties.
as O
2 2 2 2
, N O, and H O were employed. The transition metal-
5
a
5b
Introduction
containing heterogeneous catalysts such as TS-1, SBA-15,
5
c-d
and molecular sieves were reported as less efficient catalysts
due to leaching. Hence, the search for new catalysts for the
production of phenol in a single-step is evergreen interest. In
this context, the Co(III)-peroxo species are known to play a
crucial role as active intermediates in electrophilic and
Phenol is an important raw material and chemical
intermediate for the production of drugs, resins, dyes,
1
caprolactam and etc. Currently, phenol is produced in the
2
industry via a three-step cumene process. However, the
process has disadvantages such as high energy consumption,
poor yield of phenol and an equal amount of by-product
acetone formation. To overcome these limitations, several
researchers are focusing attention to design the catalyst that
affords phenol selectively in a single-step without or with a
minor amount of by-products. In fact, one of the main
challenges in benzene hydroxylation is the activation of C-H
6
a,b
nucleophilic reactions, nonetheless, which are less exploited
6
a
as catalysts for aromatic C-H activation/hydroxylation. In
016, Silva and co-workers have firstly reported cobalt(II)-N
type of complexes as a catalyst for benzene hydroxylation with
2
2 2
O
6
c
low catalytic efficiency. In 2017, Fukuzumi and co-workers
have reported photocatalytic hydroxylation of benzene using
III
2+
5
[
Co (Cp*)(bpy)(H
and bpy = 2, 2-bipyridine) and photocatalyst [Ru (Me
in presences of O , where TON of 500 and benzoquinone as a
2
O)] (Cp*= η -pentamethylcyclopentadienyl
-1
bond (bond energy = 460 kJ mol ), often transition metal
II
2+
2 3
Phen) ]
3
catalysts were employed. However, only limited progress has
2
been made using transition metal complexes (V, Fe, Ni, Cu, Os,
4
g
by-product were observed. In this article, we report the
synthesis and characterization of cobalt(II) complexes of
tetradentate ligands (Scheme 1) as the efficient catalysts for
4
a-g
4h
etc.) and photocatalysts, where eco-friendly oxidants such
2 2
selective hydroxylation of benzene using H O as an oxygen
aBioinorganic Chemistry Laboratory/Physical Chemistry, School of Chemistry,
Madurai Kamaraj University, Madurai- 625021, India.
mayilmurugan.chem@mkuniversity.org.
Department of Chemistry, North Eastern Hill University, Shillong-793022, India.
Footnotes relating to the title and/or authors should appear here.
supplier. The ligand architecture, steric and electronic nature of
ligands provide a significant difference from the previously
reported catalysts and thus better catalytic performances. It is
noteworthy that 29% of phenol formation is achieved by direct
hydroxylation of benzene, which showed TON of 286 and
b
†
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
-1
turnover frequency (TOF) up to 57 h . This is the highest yield
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or gl oi ng sy
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All the cobalt(II) complexes exhibited two absorption bands
and catalytic efficiency reported so far in the literature using
cobalt(II) complexes as catalysts for hydroxylation of benzene.
This conversion likely proceeds via Co(III)-peroxo species, which
was studied by spectral and DFT methods.
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4
4
4
in the range of 472 - 528 nm ( A
to E) in acetonitrile. Also, the ligand-based π-π* transitions
2
to E(P))
D
a
O
n
I
d
: 1
5
0
9
.1
7
03
-
9
6
/C
2
9
8
C
n
Y
m
026
(
01K
A
2
4
8
1
0
appeared around 271 - 284 nm. Interestingly, a very similar
absorption bands were observed in solid-state (Figure S5),
which clearly reveals that all the complexes retain their
geometry in solution as well. The complexes showed a well-
Results and discussion
Synthesis and Characterization. The ligands L1 - L5 have been
synthesized by the previously reported procedures. The
cobalt(II) complexes [Co(L)Cl]X, 1 - 5 (X = ClO
synthesized by the reaction of respective ligands and
2
+
3+
defined Co /Co redox couple around 0.489 - 0.500 V vs.
7
+
Ag/Ag in acetonitrile and are far away from the reversibility
4 4
/BPh ) were
2
+
3+
(
0
Figure S11-S12). The Co /Co redox potential of 1 - 3 (0.492 -
.500 V) and 4 - 5 (0.489 - 0.496 V) are almost identical.
CoCl
NaClO
2
·6H
2
O, followed by the addition of an equivalent of
8
4
/NaBPh
4
in methanol. All the complexes were Catalytic benzene hydroxylation. Catalytic hydroxylation of
characterized by HR-ESI, electronic spectra, redox methods, and benzene investigated using Co(II)-complexes as catalysts and
their molecular structures were determined by single-crystal X- H O (30%) as an oxygen source. The catalytic reaction was
2
2
ray crystallography studies. Four nitrogen atoms of ligand and a optimized using benzene (5 mmol), a catalytic amount of
II
+
chloride ion constitute the [Co (4N)Cl] coordination sphere and complex (5 µmol, 0.1 mol%) in acetonitrile (5.0 mL). Finally,
a ClO /BPh occupied at the outer coordination sphere (Figure H O (25 mmol) was very slowly added into the reaction mixture
4
4
2
2
1
). The complexes exhibited geometry in between square under constant stirring at 60 °C and then the reaction was
pyramidal and trigonal bipyramidal as predicted by Addison continued for 5 hours (Scheme 2). Afterward, the reaction
structural parameter, τ = 0.49 - 0.88. The complexes displayed mixture was passed through the silica column to remove the
9
almost identical Co-Namine bond distances (2.104 - 2.254 Å) and catalyst. The reaction mixture was analysed and quantified by
are longer than Co-NPy bonds (2.043 - 2.099 Å) due to relatively GC-MS/GC (Figure S14).
stronger overlapping of p-orbital of pyridine nitrogens with d-
II
orbital of Co -center. The Co-Cl bond distances are in the range
of 2.2659 - 2.2817 Å (Table S1 - S2), apparently, they are labile
and possibly replaced by solvent molecule followed by H
2 2
O
during the catalysis (cf. below).
O
Scheme 2. Catalytic benzene hydroxylation by Co(II) Complexes.
Table 1. Catalytic benzene hydroxylation using 1 – 5.
N
N
N
N
N
O
cata
Tb
[C]c
[Y]d
[S]e
TONf TOF
g
KIEh
ν
N
N
N
N
N
N
(
o
C) (%) (%) (%)
(h-1
)
(cm-1
)
i
N
L1
L2
1
2
3
4
5
60
28
16
29
18
30
20
24
16
26
16
24
14
27
17
29
19
18
13
20
14
86
87
93
94
97
95
74
81
77
87
237
142
268
171
286
191
181
134
198
162
47
28
55
34
57
38
36
27
40
32
0.99 844.93
1.03 844.97
1.07 844.29
0.98 843.13
0.98 844.99
L3
2
5
60
N
N
N
N
N
N
N
N
2
5
60
2
5
L4
L5
60
25
60
Scheme 1. The 4N-ligands used in the present study
Cl
(
a)
(b)
Cl
2
5
N4
N3
a
Co
Co
Reaction condition: Benzene (5 mmol), complex (5 µmol, 0.1 mol%) and hydrogen
N3
N2
N4
b
peroxide (30%) (25 mmol) in 5 mL acetonitrile at 25 °C/60 °C for 5 hours.
c
d
e
f
g
Temperature. Conversion. Yield. selectivity. Turnover number. Turnover
N1
h
frequency. Catalyst (5 µmol), 1:1 mixture of C D (2.5 mmol) and C H (2.5 mmol),
hydrogen peroxide (30%) (2.5 mmol) at 60 °C for 5 hours. Solid-state IR data of
a-5a.
6
6
6 6
N2
i
N1
1
Complex 1 showed a lower yield of phenol (24%) and
selectivity than 2 (27%). Interestingly, introducing electron
releasing groups on pyridine arms of 2 to obtain 3, resulted in a
slight acceleration of phenol yield (29%) and much-improved
selectivity (97%) than 1 and 2. In fact, 1 - 3 are containing almost
similar structural skeleton but they varied by electronic and
steric substituents. The higher catalytic efficiency of 3 is possibly
facilitated by the electron releasing groups on the ligand L3. On
the other hand, the complex 4 containing sterically constrained
diazepane backbone showed a lower yield of phenol, 18%, and
selectivity (74%) than those of 1 - 3 (Table 1). While using 5 as
catalyst showed a slight increase in phenol yield (20%) and
(
c)
(d)
N2
Cl
N3
N3
N4
Co
Co
N4
Cl
N1
N1
Figure 1. The molecular structures of 1 (a), 2 (b), 4 (c) and 5 (d). The
hydrogen atoms and counter ions are omitted for clarity.
2
| J. Name., 2012, 00, 1-3
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selectivity (77%) than 4 (Figure S15, S16A). Generally, the employed as substrates using catalyst 3 and five equivalents of
View Article Online
DOI: 10.1039/C9CY02601K
H O at 60°C over 24 hours. The toluene afforded combined
2 2
complexes containing the tripodal backbone showed the
highest catalytic efficiency than the sterically constrained cyclic yield of 17% (selectivity: o-, p-cresol, and benzaldehyde as 33%,
diazepane backbone. The catalytic efficiency is strongly 34%, and 33%) and without the formation of any quinone
influenced by electronic and steric factors and decreasing order derivatives (Scheme 3, Figure S20). This is enlightening that the
of catalytic efficiency is found as 3 > 2 > 1 > 5 > 4. The calculated catalysts are capable of executing selective hydroxylation at
2
4e
steric maps by SambVca 2.1A showed %Vbur of 70.7 - 71.6% for aromatic sp carbon. Anisole showed the formation of o-, m-
- 3 are lower than that of 5 (74.6%) (Figure S18). The phenol and p-methoxyphenol as products with a combined yield of 22%
1
formation exhibited by 3 is the maximum catalytic efficiency (selectivity: 70%, 26%, 4% for o-, m-, p-substitutions
achieved so far in the literature using cobalt(II) complexes as the respectively) (Figure S21). Whereas, chlorobenzene afforded o-
catalyst. In fact, it is a concomitantly higher yield than the and p-chlorophenol as the products with the combined yield of
previous reported Co(II) complex by Silva and co-workers, 15% (Figure S22). The nitrobenzene poorly converted into o-
where 23% of phenol was reported using relatively higher and p-nitrophenol (8%) (Figure S23). Reactivity of substituted
6
c
12a
12b
catalyst loading (1 mol%). The copper and iron complexes
benzene is correlated with Hammett constants (Figure S24),
were reported to catalyse lower yield of phenol, 14.6% and which shows electron-donating groups such as -Me and -OMe
1
9.9% respectively. However, the nickel complexes reported by showed better hydroxylation than the electron-withdrawing
3
b, 4f
us afforded 41% phenol yield, which is the highest yield known group.
in the literature.13
However, all these yields are relatively lower than
those exhibited by benzene (Table S7), which clearly suggests
The catalytic reactions were performed at 25 °C without that the catalyst 3 is efficient and selective towards mono-
altering other conditions specified in table 1. The complex 1 hydroxylation of benzene.
showed 14% of phenol formation with the selectivity of 87%. As
OH
O
expected, the complex 2 afforded a slightly enhanced yield of
phenol (17%) and selectivity (94%). The yield of phenol further
increased to 19% with the selectivity of 95% while using 3 as a
catalyst. Similarly, the catalytic reaction performed using 4,
showed 13% of phenol formation with the selectivity of 81%,
which is lower than those of 1 - 3. The catalyst 5 showed slightly
enhanced phenol yield (14%) and selectivity (87%) than 4 (Table
+
[
Co(L3)Cl] , CH CN
OH
3
+
+
H O , 60 C
2
2
Scheme 3. Catalytic hydroxylation of toluene by 3.
Mechanism of benzene hydroxylation. The kinetic isotopic
effects (KIE) were determined by employing 1:1 ratio of C
2.5 mmol) and C (2.5 mmol) as substrates at 60 °C over 5
hours. The KIE values were calculated as 0.98 - 1.07 for 1 - 5
from the ratio of products C OH and C OH using GC-MS/GC
data (Figure S25, S26 and Table 1). Very similar KIE values (0.9 −
.0) were reported for intramolecular aromatic hydroxylation.
In fact, the KIE value 1.0 is reported for aromatic hydroxylation
using metal-bound oxygen species as key intermediate via
electrophilic aromatic substitution reaction. The results show
that benzene C-H bond cleavage is not involving in the rate-
limiting step. Also, KIE values suggest that benzene
hydroxylation is not proceeded via a radical type reaction,
whereas the KIE value of Fenton’s type reaction is reported in
6 6
H
1
, Figure S15, S16B), but it is still lower yield than those of 1 - 3.
(
6 6
D
The catalytic efficiency trend is almost identical to that of
observed at 60 °C, but their corresponding phenol yields are
relatively lower. The influence of H O loading was examined
2 2
using the most efficient catalyst 3 at 60 °C and keeping other
reaction conditions identical. Wherein, the yield of phenol was
linearly increased on increasing the amount of H O used,
2 2
however, the selectivity was reciprocally declined (Figure S19,
Table S5). Thus, the catalytic efficiency is strongly influenced by
6
H
5
6 5
D
1
14
the amount of H
amounts of benzene (5 mmol) and H
2
O
2
used. Whereas, the reaction of equal
(5 mmol) showed
2 2
O
relatively lower yield, 17%, and TON, 172 at 60 °C for 1, which is
further lowered to 9%, with TON of 91 at 25 °C under identical
condition (Figure S17). Similarly, complexes 2 - 5 showed the
relatively lower amount of phenol conversion under this
condition, such as 2, 18%; 3, 20%; 4, 12% and 5, 13% at 60 °C
and 2, 11%; 3, 14%; 4, 10% and 5, 12% at 25 °C (Table S6, Figure
S17). Nevertheless, the selectivity is improved for all the
15a
the range of 1.7 − 1.8. The KIE values were determined over
various time intervals and showed values of 1.0 throughout
the reaction time (Figure S27). Catalysis using TEMPO as a
radical trapping agent and catalyst 3, showed almost no
changes in the phenol yield and ruled out the possibility of
involvement of radical type mechanism. Further, benzene
2 2
catalysts as compared 5 equivalents of H O . In contrast, the
hydroxylation was carried out using mixed CH
solvents, showed selectivity of 95% for phenol formation and
% of chlorobenzene as a minor product at 60 °C (Figure S28).
This revealing that the use of a radical trapping agent (CCl ) did
3 4
CN: CCl (4:1)
higher yields of phenol are achieved for 5 equivalents of H
2 2
O
(
(
Figure S19). Interestingly, the bulk-scale reaction of benzene
30 mmol), catalyst 3 (50 µmol) and H (50 mmol) showed the
5
2 2
O
4
exclusive formation of phenol up to 0.27 g at 60 °C over 120
hours. Use of CoCl 6H O as catalyst did not show benzene
not significantly affect the selectivity of phenol and supports
2
2
4f
further to exclude the radical type reaction pathway. The
hydroxylation under our experimental conditions.
oxidation of cis-1,2-dimethyl cyclohexane under similar
reaction conditions using 3 as catalyst exhibited 2% of cis
hydroxylated product formation with the trace of trans product.
In another experiment, cyclohexane was used as a substrate
While using phenol as a substrate and 3 as catalyst displayed
no hydroxylated product formation under identical condition,
and merely substrate was recovered quantitatively. In addition,
toluene, anisole, chlorobenzene, and nitrobenzene were also
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-
1
and CCl
4
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as a radical trapping agent, which showed a combined symmetric stretching of 2a was observed at 540 cm , which is
-1
18
DOI: 10.1039/C9CY02601K
, observed isotopic shift is
(cal.) = 21 cm ). The calculated
yield of 3% (selectivity = 93% and 7% of cyclohexanol and shifted to 523 cm while using H
cyclohexanone respectively) and without the formation of
2
O
2
1
6
18
-1 16
18
-1
 -  = 17 cm (  -

-1
-1
cyclohexyl chloride (Figure S29). All these observations strongly vibrational frequency by DFT at 840 cm (O-O) and 511 cm
support to exclude the possibility of radical type reaction in (Co-O) are closer to experimental values (Figure 2b). These
benzene hydroxylation. Further, the metal insertion reaction value are comparable to those of previously reported
type may not be involved in the mechanism, which is known to structurally and spectroscopically well-characterized side-on
1
5b
III
+
display KIE values of 2.0 − 4.8. Isotope labelling studies were Co(III)-peroxo complexes such as [Co (Me
3
-TPADP)(O
2
)] (888
-1
performed to authenticate the origin of oxygen atom, where we cm ) (Me
3
-TPADP
=
3,6,9-trimethyl-3,6,9-triaza-1(2,6)-
with 90% atom pyridinacyclo decaphane); [Co (12-TMC)(O
1
8
III
+
-1
employed the most active catalyst 3 and H
purity in H O at 60 °C. The result showed nearly 89% of
incorporation into the product as O labelled phenol [Co (13-TMC)(O
determined by GC-MS (Figure S30). Under the identical reaction 1,4,7,10-tetraazacyclotridecane).
2
O
2
2
)] (902 cm ) (12-
1
8
2
O
TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclodecane) and
1
8
III
+
-1
2
)] (902 cm ) (13-TMC = 1,4,7,10-tetramethyl-
6
a-b
The isolation of single
crystals of the cobalt(III)-peroxo species was unsuccessful with
where no 18O atom incorporation was determined by GC-MS. our great efforts. The direct reaction of 1a - 5a with benzene
This is clearly revealing that the H is the key oxygen supplier exhibited very lower yields phenol, 1a: 0.6%; 2a: 0.8%; 3a: 0.9;
for the formation of phenol from benzene.
1
8
conditions, the reaction was carried out using H
2 2 2
O and H O ,
2 2
O
1
5c
4a: 0.7% and 5a: 0.3%. The observed phenol yields are
The aromatic hydroxylation reaction likely proceeds via extremely poor as compared to those in Table 1, which suggests
cobalt(III)-peroxo species, which were characterized that side-on Co(III)-peroxo type intermediate is unlikely to be
spectroscopically and possible geometries were optimized by involved in the benzene hydroxylation mechanism.
DFT calculations. The cobalt(III)-peroxo species of 1 − 5 were
generated by adding 10 equivalents of H
of Et N in acetonitrile at -40 ˚C, which exhibited the formation
of thermally stable species. In particular, the treatment of 10
equivalents of H (30%) to 3 and 2 equivalents of Et
2
O
2
and 2 equivalents
(a)
(b)
3
O
2 2
3
N
exhibited a color change from purple to yellow at -40 °C in
acetonitrile. Where the new absorption bands were appeared
-
1
-1
-2 -1
at 385 nm (, 2280 M cm ; kobs, 1.7 ± 0.0003 × 10 s , t1/2, 40
-1
-1
-1
-1
s), 553 nm (, 315 M cm ) and 576 nm (, 275 M cm ) (Figure
S31). The HR-ESI mass spectrum showed a prominent peak at
III
+
(c)
(d)
2
m/z, 491.0321 corresponds to [Co (L3)(O )] 3a (calculated m/z,
DFT
4
91.2068). Further, the same species was generated using
1
8
18
isotopically labelled H
2 2
O (90% O atom), which resulted in a
prominent shift in m/z value, 495.2139 and is ascribed to
III
18
+
[
Co (L3)( O
substitution of
species has two oxygen atoms. In solid, the FT-IR spectrum
2
)] (Figure S32). The four-mass unit shift upon the
1
6
18
2 2
O with O indicates that the intermediate
-1
showed the vibrational band at 840 cm corresponds to O-O
stretching frequency (Table 1). The intermediate showed a
1
6
-4
Figure 2. Electronic spectral changes for the reaction of 2 (4 × 10 M)
well-shifted oxidation potential (Epa, 0.640 V) from 3 and with 2 equivalents NEt and 10 equivalents of H O at -40 ˚C in CH CN
3
2
2
3
suggests that there is a change in the oxidation state of cobalt (a); ESI-MS of 2a (insert). Vibrational spectra of 2a in CH
3
CN (b). The
-4
III
+
electronic spectral changes for the reaction of 2a (4 × 10 M) with 3
(
2
Figure S33). Similarly, the key intermediate [Co (L)(O )] of
equivalents HClO
4
at -25 ˚C in CH
3
CN and ESI-MS (insert) (c).
other complexes 1a, 2a, 4a, and 5a were generated under
identical conditions. They showed similar electronic spectral
changes and kinetics (Figure S34 – S36; Table S8). Further, their
formations were confirmed by ESI-MS spectra, m/z: 361.1 for 1a
Vibrational spectra of 2b in CH
3
CN (d).
Further, the reaction of 3a with other substrates such as
triphenylphosphine and thioanisole showed no oxygen atom
transfer (OAT). This implies that species 3a is incapable of
performing electrophilic OAT reactions as similar to the
(calculated m/z, 361.1); m/z: 375.2 for 2a (calculated m/z,
3
75.3) (Figure 2a (insert), S38); m/z: 373.1 for 4a (calculated
m/z, 373.1) and m/z: 401.2for 5a (calculated m/z, 401.1) (Figure
S37, S39-S40). Also, they showed completely distinct cyclic
voltammograms from their parent complexes (Figure S41). The
solution state vibration (FT-IR) spectrum was measured for
6b,17b
previous reports.
propionaldehyde was converted into acetophenone (71%)
scheme 4) via deformylation pathway as similar to the
previously reported side-on Co(III)-peroxo complexes
Whereas, substrate 2-phenyl
(
III
+
-
[
1
Co (L2)(O
,
2
)] 2a, exhibited O-O stretching frequency at 858 cm
III
+
III
+ 17b-c
(
[Co (bbphc)(O
2
)]
,
[Co (13-TMC)(O
2
)] ).
These results
-1
18
which is shifted to 816 cm while using H
2
O
2
(90% atom
The observed isotopic
clearly indicate that the intermediates 1a - 5a likely side-on
1
6
17a
purity) instead of H
shift 16 -  = 42 cm is almost closer to the calculated value
2
O
2
(Figure 2b).
Co(III)-peroxo species, which are known to display nucleophilic
18
-1
6b,17b-c
character
and do not facilitate benzene hydroxylation
-1
of 51 cm for the O-O harmonic oscillator. In addition, Co-O
2
reactions under these conditions.
4
| J. Name., 2012, 00, 1-3
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+
The addition of 3 equivalents of H (HClO
4 3
View Article Online
) to 3a in CH CN at intermediate and facilitates the OAT reactions (Scheme
6
b,19b,19d
DOI: 10.1039/C9CY02601K
-
25 ˚C showed the spontaneous colour change from yellow to 4).
III
2+
green, which possibly corresponds to [(L3)Co (OOH)] , 3b. The
formation was accompanied by the appearance of the
characteristic LMCT (Ligand to Metal Charge Transfer)
transitions at, 588 (ε, 350 M cm ) and 697 nm (ε, 437 M cm
(Figure S42). ESI-MS spectra of the solution showed a peak
266.8 accounts for [(L3)Co (OOH)(CH
calculated m/z = 266.62) (Figure S43), which is a prominent
shift and distinct from 3a. This result indicates that 3a possibly
converted into hydroperoxo species by H and this chemistry is
O
L
CoII
Cl
O
-1
-1
-1
-
O
O
1
18
)
O2
H2
CH CN
3
III
2+
L
CoIII
at m/z
=
3
CN)]
N,
Et3
S
(
O
S
L
CoII
Base Acid
+
NCCH3
well explored and its spectral signatures are comparable to the
OOH
CoIII
6
b,19
III
previous reports.
Similarly, hydroperoxo species [LCo (O-
OH
L
OH)]2+ of other complexes were generated by the reaction of
OPPh3
+
NCCH3
respective side-on species with 3 equivalents of H at -25 ˚C in
PPh3
acetonitrile, which showed the identical characteristic spectral
changes (Figure S44, S46, Table S8). Further they were
confirmed by the observation of ESI-MS spectra of adducts
Scheme 4. Reaction pathway for Benzene hydroxylation.
The optimized geometry of 2 and its possible peroxo
III
2+
[
(L)Co (OOH)(CH
3
CN)] ; 2b: the calculated m/z = 208.58, Found
21
intermediates was optimized by DFT using LanL2DZ level
Figure S53) and TD-DFT calculations were performed using
B3LYP 631-G (for C H N O) and LANL2DZ (for Co) basis sets. The
m/z = 208.2) (Figure 2c, S47); 4b: the calculated m/z = 207.57,
Found m/z = 207.1 and 5b: the calculated m/z = 221.59, Found
m/z = 221.6 (Figure S48 – S49), which are completely distinct
mass pattern from their respective side-on species. Further, the
addition of benzene to 2b and 3b at -25 ˚C in acetonitrile
exhibited disappearance of bands at 587, 629 and 697 nm for
(
III
+
III
2+
total energy of [(L2)Co (O
2
)] , [(L2)Co (OOH)]
] were calculated as 32016 eV, 32024 eV, and
9939 eV respectively. The TD-DFT results are comparable to
and
III
+
[
2 2
(Co (L2)) (µ-O)
5
III
+
the experimental electronic spectra of [(L2)Co (O
(L2)Co (OOH)] , wherein the energy of peroxo to cobalt(III)
2
)] and
2
b (Figure S50) and 588 and 697 nm for 3b (Figure S51). GC-MS
III
2+
[
analysis of the reaction mixture showed the formation of
phenol. The solution state vibration (FT-IR) spectrum of the
LMCT (Ligand to Metal Charge Transfer) is closer to the
experimentally observed values (415, 475, 505 nm and 390, 495
and 810 nm). (Figure S54).
III
2+
[
L2Co (OOH)] 2b is completely different from that of 2a,
-1
which exhibited two vibration modes at 832 cm (O-OH) and
5
while using H
vibrational frequency at 832 cm is comparable to that of a low-
spin bleomycin-Co(III)-hydroperoxo complex (828 cm ). This
is good agreement with the calculated vibrational spectrum
64 cm-1 (Co-O), which are shifted to 781 cm and 544 cm
-1
-1
Conclusion
1
8
16
2
O
2
instead of H
2
O
2
(Figure 2d). Observed O-O
The cobalt(II) complexes of 4N ligands were synthesized
and structurally characterized as efficient catalysts for benzene
hydroxylation. The molecular structures of complexes showed
geometry in between square pyramidal and trigonal
bipyramidal. The catalysts are found to be efficient and
selective for hydroxylation of benzene to form phenol using
-1
-1 20
-
1
-1
[
784 cm (O-OH) and 536 cm (Co-O)] (Figure 2d). The isotopic
1
6
18
-1
-1
shift of  -  = 51 cm (O-OH) and 20 cm (Co-O) are in closer
agreement with calculated values of 49 cm and 26 cm
-1
-1
2 2
H O . The 29% yield of phenol with TON of 286 is attained
respectively. These values are comparable to those reported
for structurally and spectroscopically characterized Co(III)-OOH
complexes such as low-spin bleomycin-Co(III)-hydroperoxo
without any over oxidized products, which is the highest
catalytic efficiency reported for cobalt-based catalysts so far in
the literature. The hydroxylation reaction apparently proceeds
via cobalt(III)-OOH intermediates, which is established by the
spectral methods and TD-DFT calculations. The KIE experiment
results support the involvement of cobalt-bound oxygen species
as a key intermediate. The isotope-labeling experiments
2
0
complex. The isolated species 1a - 5a (5 µM) were reacted with
+
benzene in presence of three equivalents of H and showed
phenol formation exclusively with yields of 1a (7%), 2a (9%), 3a
(11), 4a (5%) and 5a (6%). However, the yields were enhanced
while increasing the catalyst loading to 50 µM, where yields of
phenol, 1a: 22%; 2a: 25%; 3a: 27%; 4a: 15% and 5a: 17%. The
observed phenol yields are almost identical to values in Table 1.
The results are suggesting that benzene hydroxylation likely
2 2
indicate the H O is the key oxygen supplier for phenol
formation.
Experimental section
III
2+
III
+
proceeds via [LCo (O
2
H)] species rather [LCo (O
2
)] species.
III
2+
The involvement of [LCo (O
2
H)] species was further
Materials
supported by the reaction of 3b with triphenylphosphine and
thioanisole. Wherein products were analyzed by GC-MS,
showed formation of triphenylphosphine oxide (35%) and hydrochloride, 2-vinylpyridine, cobalt(II)chloride hexahydrate,
The chemicals homopiperazine, 3-(dimethylamino)-1-
propylamine, 2-chloromethyl-4-methoxy-3,5-dimethylpyridine
methyl phenyl sulfoxide (66%) respectively at 25°C over 3 h. So, sodium
tetraphenylborate,
nitrobenzene,
benzene,
tetrabutyl
toluene,
it clearly suggests that the electrophilic nature of Co(III)-OOH ammoniumhexafluorophosphate,
chloroform-d, Benzene-d
6
, anisole, chlorobenzene, 2-phenyl
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Page 6 of 9
ARTICLE
Journal Name
propionaldehyde(2-PPA) purchased from sigma Aldrich and stirred for an additional hour. The complexes were precipitated
View Article Online
DOI: 10.1039/C9CY02601K
1
8
18
H
2
O
2
– 90% O atom were purchased from icon isotope. 2-
as solid and then filtered and washed with cold methanol.
Co(L1)Cl](Ph B) 1. Isolated as green solid, yield = 61%.
Analytically calculated elements for C40 CoCl: C, 70.24; H,
.19; N, 8.19 %. Found: C, 70.21; H, 6.17; N, 8.17 %. HR-ESI
pyridinecarboxaldehyde, N, N-dimethylethylene diamine,
sodium triacetoxyborohydride, Thianisole, triphenylphosphine
was purchased from Alfa Aesar. Anhydrous acetonitrile,
Dichloromethane, Hydrogen peroxide (30%) from Merck, India.
Dry methanol, triethylamine, phenol were purchased from SD
fine chemicals.
[
4
H
42BN
4
6
+
(m/z): Calculated for C16
H22ClCoN
4
[M] 364.0865; Found.
64.0911. Diffusion of diethylether into acetonitrile solution of
complex with perchlorate counter ion afforded single crystals
suitable for X-ray analysis.
3
Physical Methods
All workup procedures were carried out under ambient
conditions. Electrospray Ionization Mass Spectra (ESI-MS)
were measured on a Thermo LC-MS instrument. UV-Vis
spectra were recorded on an Agilent 8453 spectrometer with
a cooling unit by Unisoku (Osaka, Japan). FT-IR measurements
were recorded using Thermo Nicolet 6700. Cyclic Voltammetry
[
Co(L2)Cl](Ph
Analytically calculated elements for C41
.35; N, 8.03 %. Found: C, 70.51; H, 6.32; N, 8.01 %. HR-ESI
4
B) 2. Isolated as purple solid, Yield = 77%.
4
H44BN CoCl: C, 70.55; H,
6
+
(m/z): Calculated for C17
H24ClCoN
4
[M] 378.1021; Found
3
78.1012. Diffusion of diethylether into acetonitrile solution of
complex afforded single crystals suitable for X-ray analysis.
[Co(L3)Cl](Ph B) 3. Isolated as purple solid, Yield = 57%.
Analytically calculated elements for C H BN CoO Cl: C, 69.34;
(CV) was performed using a three-electrode cell configuration.
A platinum sphere (acetonitrile medium), platinum wire and
4
Ag(s)/Ag+ were used as working, auxiliary and reference
4
7
56
4
2
electrodes
respectively.
The
tetrabutyl
H, 6.93; N, 6.88 %. Found: C, 69.31; H, 6.91 N, 6.85 %. HR-ESI
ammoniumhexafluorophosphate was used as a supporting
electrolyte in acetonitrile. The E1/2 values were observed under
identical conditions for various scan rates. GC-MS and GC
analysis performed on Agilent 5977E GCMSD using HP-5 MS
ultra-inert (30 m × 250 µm × 0.25 µm) capillary column.
+
(
m/z): Calculated for C23
94.1856.
Co(L4)Cl](Ph
Analytically calculated elements for C41
.08; N, 8.05 %. Found: C, 70.71; H, 6.05; N, 8.03 %. HR-ESI
(m/z): Calculated for C H ClCoN [M] 376.0865; Found
4 2
H36ClCoN O [M] 494.1859; Found
4
[
4
B) 4. Isolated as purple solid, Yield = 49%.
CoCl: C, 70.75; H,
H
42BN
4
6
X-ray Crystal Structure Analysis
+
1
7
22
4
The experiment was performed on the Agilent Technologies 376.0925. Diffusion of diethylether into acetonitrile solution of
Supernova-E CCD diffractometer. The suitable single crystals of complex with perchlorate counter ion afforded single crystals
1
, 2, 4, and 5 of suitable size were selected from the mother suitable for X-ray analysis.
liquor and immersed in paraffin oil, then mounted on the tip of [Co(L5)Cl](Ph B) 5. Isolated as light blue solid Yield = 71%.
4
glass fiber. The structures were solved by direct methods using Analytically calculated elements for C H BN CoCl: C, 71.33; H,
4
3
46
4
the program SHELXS-2013. Refinement and all further 6.40; N, 7.74 %. Found: C, 71.31; H, 6.37; N, 7.71 %. HR-ESI
calculations were carried out using SHELXL-2013. The H-atoms (m/z): Calculated for C H ClCoN [M]
404.1178; Found
+
1
9
26
4
were included in calculated positions and treated as riding 404.1229. Diffusion of diethylether into acetonitrile solution of
atoms using the SHELXL default parameters. The non-H atoms complex afforded single crystals suitable for X-ray analysis.
were refined anisotropically, using weighted full-matrix least-
General Procedure for Catalytic Hydroxylation Reaction
2
squares on F . CCDC 1954523, 1954536, 1954537 and 1954539
A catalytic amount of the [Co(L)Cl](Ph
4
B) complex (5 µmol,
are containing the supplementary crystallographic data for this
paper. This data can be obtained free of charge from The
0
.1 mol%) was dissolved in acetonitrile (5.0 mL), benzene (5.0
mmol) and aqueous hydrogen peroxide (30%) (2.5 mL, 25
mmol) were introduced dropwise into the reaction solution
over a period of 30 minutes. The mixture was stirred at 25 °C/60
Cambridge
Crystallographic
Data
Centre
via
ww.ccdc.cam.ac.uk/data request/cif.
Synthesis of Ligands
°
C for 5 hours. After cooling to room temperature, the catalyst
Ligands
,2-diamine
N,N-Bis(2-pyridylmethyl)-N’,N’-dimethylethane-
(L1), N,N-Bis(2-pyridylmethyl)-N’,N’-
was removed by passing through the silica-gel column. Then, a
known amount of nitrobenzene was added as an internal
standard. Conversion and yield of the reaction were determined
with GC-MS/GC. No benzene oxidation products were detected
in control experiments using the same experimental conditions
but without the addition of a catalyst.
1
dimethylpropane-1,3-diamine (L2), N1,N1-bis((4-methoxy -3,5-
dimethylpyridin-2-yl)methyl)-N3,N3-dimethylpropane-1,3-
diamine (L3), 1,4-Bis[(pyridin-2-yl-methyl)]-1,4-diazepane (L4),
,4-Bis[2-(pyridin-2-yl)ethyl]-1,4-diazepane (L5) synthesized
using previously reported.
1
7
Determination of Kinetic Deuterium Isotope Effect
Synthesis of Co(II) Complexes (1 - 5)
The Co(II) Complexes are prepared by known procedure . To
the solution of ligands (0.18 mmol) in methanol (10 mL),
CoCl 6H O (0.18 mmol) in methanol was added drop wise
2 2
under stirring at room temperature. It was refluxed for 3 hours,
subsequently the mixture was filtered and cooled to room
Benzene (2.5 mmol), benzene-d
6
(2.5 mmol), were added
(1 - 5) (5.0 µmol) in acetonitrile
5 mL). An aqueous solution of hydrogen peroxide (30%) (25
8
II
into a solution of [Co (L)Cl]BPh
4
(
mmol) was added directly under constant stirring over a period
of 30 minutes. The mixture was stirred for 5 h at 60 °C. After
cooling to room temperature, the complex was removed by
passing through the solution on silica-gel column. The product
distribution was determined by GC-MS. The kinetic isotope
temperature.
Then,
one
equivalent
of
sodium
tetraphenylborate/ sodium perchlorate in methanol was added,
6
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
effect (KIE) value for phenol production was calculated by https://www.molnac.unisa.it/OMtools/sambvca2.1/index.html
View Article Online
1
1
DOI: 10.1039/C9CY02601K
(
moles of phenol)/(moles of phenol-d
5
).
.
Catalytic Hydroxylation reaction of benzene with H
2
18O
2
DFT Methods
The geometry optimizations were performed using density
(10 functional theory (DFT) methods. For the metal ion involving
equivalent) into the reaction solution. The mixture was stirred systems, the LANL2DZ basis sets with the Becke3–Lee–Yang–
2 h at 60 °C. After cooling to room temperature, the cobalt Parr hybrid functional (B3LYP) were used, while the 6-311G (d)
complex was removed by passing through the silica-gel column. basis sets were used for elements other than metals. All DFT
II
[
Co (L3)Cl]BPh
4
(5 µmol) (3) was dissolved into 3 mL
1
8
acetonitrile, and benzene (0.12 mmol) and add H
2 2
O
1
Then the reaction mixture was analyzed by GC-MS.
Catalytic Benzene Hydroxylation Reaction with H
18O
calculations were carried out by using the Gaussian 09 program
package.
20
2
1
8
2
The complex 3 (5 µmol) was dissolved in H O/acetonitrile
(0.1 mL/1 mL). Benzene (0.5 mmol) and aqueous hydrogen Conflicts of interest
peroxide (0.5 mL, 5 mmol) were slowly introduced into the
solution. The reaction mixture was stirred at 60 °C for 5 h. After
cooling to room temperature, the cobalt complex was removed
by passing through the silica-gel column, the products were
analyzed by GC-MS.
“
There are no conflicts to declare”.
Acknowledgments
We acknowledge Science and Engineering Research Board
(
(
SERB), New Delhi and Board of Research in Nuclear Science
BRNS), Mumbai for funding.
Catalytic Hydroxylation of substituted benzene
The most active catalyst 3 (5 µmol) was dissolved in
acetonitrile (5.0 mL). Substrate (5.0 mmol) and 30% of hydrogen
peroxide (2.5 mL, 25 mmol) were slowly added into the solution
over a period of 30 minutes. The reaction mixture was stirred at
References
1
K. Weissermel, H. J. Arpe, Industrial Organic Chemistry, John
Wiley & Sons, 3rd edn, 2008.
2
3
H. Hock, S. Lang, Ber. Desch. Chem. Ges., 1944, B77, 257–264.
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2
017, 7, 5677–5686.
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a) B. Lee, H. Naito, T. Hibino, Angew. Chem. Int. Ed., 2012, 51,
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III
+
Kinetic experiments for the formation of [Co (L)(O
2
)] ,
III
2+
[
Co (L)(OOH)(CH CN)] complex was carried out in a 1 cm path
3
length UV-visible cell on a Unisoku thermostated cell holder
designed for low-temperature measurements. The deaerated
-4
solution of the cobalt complexes 1 - 5 (4 × 10 M) in the cell was
kept at the desired temperature for several minutes and
2 2
aqueous 30% H O (10 equivalent) was added from a
2
017, 8, 8373–8383. (g) B. B. Sarma, R. Carmieli, A. Collauto,
microsyringe in the presence of two equivalent of triethylamine
I. Efremenko, J. M. L. Martin, R. Neumann, ACS Catal., 2016,
III
+
in acetonitrile at -40°C /25 °C. The formation of the [Co (L)(O
2
)]
6
, 6403-6407. (h) J. W. Han, J. Jung, Y. -M. Lee, W. Nam, S.
intermediates was monitored by UV-visible, FT-IR and ESI-MS.
Fukuzumi, Chem. Sci., 2017, 8, 7119–7125.
III
2+
Further, [Co (L)(O
HClO to generate [Co (L)(OOH)]
monitored.
2
)] species were treated with 3 equivalent
5
(a) L. Balducci, D. Bianchi, R. Bortolo, R. D. Aloisio, M. Ricci, R.
Tassinari, R. Ungarelli, Angew. Chem. Int. Ed., 2003, 42, 4937–
III
2+
4
intermediate and
4
940. (b) A. N. Kharat, S. Moosavikia, B. T. Jahromi, A. Badiei,
J. Mol. Catal. A: Chem., 2011, 348, 14–19. (c) T. Jiang, W.
Wang, B. Han, New J. Chem., 2013, 37, 1654–1664. (d) A.
Dubey, S. Kannan, Catal. Commun., 2005, 6, 394–398.
III
CN)]2+ was freshly generated
at -25 °C in
The species [Co (L)(OOH)(CH
from [Co (L)(O
3
III
)]2+ and 3 equivalents of HClO
2
4
acetonitrile. After, the substrate benzene was added via
microsyringe into the solution. The reaction was carefully
monitored and the reaction mixture was passed through a silica
column and the formation of phenol was confirmed by GC-
MS/GC.
6
7
a) J. Cho, S. Sarangi, W. Nam, Acc. Chem. Res., 2012, 45, 1321–
1
330. (b) B. Shin, K. D. Sutherlin, T. Ohta, T. Ogura, E. I.
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Inorg. Chem., 2008, 47, 6645–6658. (b) T. Dhanalakshmi, E.
Suresh, H. Stoeckli-Evans, M. Palaniandavar, Eur. J. Inorg.
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B. Moubaraki, N. F. Chilton, K. S. Murray, K. R. Vignesh, G.
Rajaraman, A. Grohmann, Eur. J. Inorg. Chem., 2013, 5-6, 958–
Solution FT-IR Measurement
Solution FT-IR measurements were recorded for freshly
III
2+
III
2+
generated [Co (L)(O
2
)] and [Co (L)(OOH)(CH
3
CN)] species,
9
67. (d) S. Kim, J. W. Ginsbach, A. Imtiaz Billah, M. A. Siegler,
C. D. Moore, E. I. Solomon, K. D. Karlin, J. Am. Chem. Soc.,
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which were transferred into the CaF
then spectra were measured.
2
cell via microsyringe and
2
8
9
S. S. Massoud, R. C. Fischer, F. A. Mautner, M. M. Parfait, R.
Herchel, Z. Trávníček, Inorg. Chem. Acta., 2018, 471, 630–639.
A. W. Addison, T. Nageswara Rao, J. Chem. Soc. Dalton Trans.,
Calculation of Steric Maps
The steric maps were obtained using the SambVca 2.1A web
application, which can be found at
1
984, 1349–1356.
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
1
1
0 Y.-M. Guo, M. Du, X.-H. Bu, J. Mol. Struct., 2002, 610, 27–31.
View Article Online
DOI: 10.1039/C9CY02601K
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Table of Content
DOI: 10.1039/C9CY02601K
Single-Step Benzene Hydroxylation by Cobalt(II) Catalysts
via Cobalt(III)-Hydroperoxo Intermediate
a
a
b
Karunanithi Anandababu, Sethuraman Muthuramalingam, Marappan Velusamy, and
a
Ramasamy Mayilmurugan*
Cobalt(II) complexes are reported as the efficient and selective catalysts for single-step phenol
formation from benzene using H O . The catalysis proceeds likely via the cobalt(III)-
2
2
hydroperoxo species.