Benzene Hydroxylation by Bioinspired Copper(II) Complexes: Coordination Geometry versus Reactivity

Coordination Geometry versus Reactivity

Article

Benzene Hydroxylation by Bioinspired Copper(II) Complexes:

Sethuraman Muthuramalingam, Karunanithi Anandababu, Marappan Velusamy,

and Ramasamy Mayilmurugan*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b03676

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sı Supporting Information

ABSTRACT: A series of bioinspired copper(II) complexes of N -tripodal

and sterically crowded diazepane-based ligands have been investigated as

catalysts for functionalization of the aromatic C−H bond. The tripodal-

ligand-based complexes exhibited distorted trigonal-bipyramidal (TBP)

geometry (τ, 0.70) around the copper(II) center; however, diazepane-

ligand-based complexes adopted square-pyramidal (SP) geometry (τ,

.037). The Cu−N bonds (2.003−2.096 Å) are almost identical and

shorter than Cu−N

bonds (2.01−2.148 Å). Also, their Cu−O (Cu−

amine

, 1.988 Å; Cu−O

water

, 2.33 Å) bond distances are slightly varied. All of

triflate

the complexes exhibited Cu → Cu redox couples in acetonitrile, where

the redox potentials of TBP-based complexes (−0.251 to −0.383 V) are

higher than those of SP-based complexes (−0.450 to −0.527 V). The d−d

bands around 582−757 nm and axial patterns of electron paramagnetic

resonance spectra [g , 2.200−2.251; A , (146−166) × 10 cm ] of the

complexes suggest the existence of ﬁve-coordination geometry. The bonding

parameters showed K > K for all complexes, corresponding to out-of-

−

−1

∥

⊥

plane π bonding. The complexes catalyzed direct hydroxylation of benzene

using 30% H O and aﬀorded phenol exclusively. The complexes with TBP geometry exhibited the highest amount of phenol

formation (37%) with selectivity (98%) superior to that of diazepane-based complexes (29%), which preferred to adopt SP-based

geometry. Hydroxylation of benzene likely proceeded via a Cu -OOH key intermediate, and its formation has been established by

electrospray ionization mass spectrometry, vibrational, and electronic spectra. Their formation constants have been calculated as

−

2 −1

(

2.54−11.85) × 10

from the appearance of an O (π* ) → Cu ligand-to-metal charge-transfer transition around 370−390 nm.

The kinetic isotope eﬀect (KIE) experiments showed values of 0.97−1.12 for all complexes, which further supports the crucial role of

Cu-OOH in catalysis. The O-labeling studies using H O showed a 92% incorporation of O into phenol, which conﬁrms H O

as the key oxygen supplier. Overall, the coordination geometry of the complexes strongly inﬂuenced the catalytic eﬃciencies. The

geometry of one of the Cu -OOH intermediates has been optimized by the density functional theory method, and its calculated

electronic and vibrational spectra are almost similar to the experimentally observed values.

INTRODUCTION

atom economy. This process mostly occurs via the activation

■

of one of the C−H bonds; however, its higher bond energy

Phenol is an industrially valuable precursor material used to

synthesize several chemical products.

manufactured only by a three-step cumene process from

−

7−9

−3

(460 kJ mol ) makes that diﬃcult.

In recent years, many

Currently, it is

researchers are thoughtfully exploring the possibility of an

eﬃcient methodology by designing new catalysts that are able

to perform selective phenol synthesis in a single step from

benzene and dioxygen (O ) as oxidants, where the overall yield

−6

of phenol is ∼5%.

Phenol is also produced by the

benzene and greener oxidants like O or activated oxygen

sulfonation and Dow processes. The main disadvantages

of these processes are poor atom economy and high reaction

temperature and pressure. Right now, the Dow process is no

longer under practice because of its adverse chlorine economy

and alkali production. Therefore, it is extremely essential to

adopt suitable methods for the industrial manufacture of

phenol, which should preferably defeat the shortcomings of the

sources. The eﬀorts through conventional approaches like

Received: December 18, 2019

−6

cumene process. Therefore, direct benzene hydroxylation is

one of the most eﬃcient ways to produce phenol with a better

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

electrochemical oxidation, photochemical oxidation, and

Article

12a

12b

eﬃciency and selectivity. The present tripodal ligand designs

and their electronics provide noteworthy variances from

sterically constrained diazepane-based ligands and other

previously reported copper(II) complexes and thus enhanced

catalytic performances.

12c

Fenton-type reaction are reported to aﬀord lower yields of

phenol with poor selectivity. On the other hand, heteroge-

neous catalysts were also employed to produce phenol using

3−15

H O ,

wherein leaching caused low catalytic eﬃciency

and product loss. Hence, bioinspired transition-metal-based

homogeneous catalysts may oﬀer better advantages and can

perform catalysis under mild conditions. The eﬃciency of

these catalysts can be regulated by changing the ligand

RESULTS AND DISCUSSION

■

Synthesis and Characterization of Complexes. The

ligands N,N-bis(2-pyridylmethyl)-N′,N′-dimethylethane-1,2-

diamine (L1), N,N-bis(2-pyridylmethyl)-N′,N′-dimethylpro-

pane-1,3-diamine (L2), N,N-bis(4-methoxy-3,5-dimethylpyri-

din-2-ylmethyl)-N′,N′-dimethylpropane-1,3-diamine (L3), 1,4-

bis(2-pyridinylmethyl)-1,4-diazepane (L4), 1,4-bis(2-quinolyl-

methyl)-1,4-diazepane (L5), and 1,4-bis(2-pyridinylethyl)-1,4-

diazepane (L6) were synthesized as reported previously by

electronics and thus redox potentials.

In nature, copper monooxygenases such as dopamine β-

monooxygenase, tyramine β-monooxygenase, peptidylglycine-

α-hydroxylating monooxygenase, polysaccharide monooxyge-

7−21

22,23

nases,

methane monooxygenase,

and tyrosinase are

well-known for activating challenging C−H bonds elegantly at

2−34

ambient condition using O . Over the past few decades,

us.

The copper(II) complexes [Cu(L)(ClO )]ClO (1−

bioinorganic chemists successfully reported numerous bio-

inspired model complexes to activate the C−H bond. Itoh and

co-workers reported several model copper(II) complexes for

quantitative intramolecular hydroxylation at the benzylic

6) have been isolated by the reactions of Cu(ClO ) ·6H O

and the respective tetradentate N ligands in methanol

(Scheme 1). Additionally, SO CF -ion-containing com-

−

position. Karlin and co-workers showed intramolecular

hydroxylation on electron-rich aromatic systems using

bioinspired models of tyrosinase, where various copper

complexes and their reactive intermediates were extensively

Scheme 1. Structures of Copper(II) Complexes 1−6

studied. All of these intramolecular C−H functionalizations

are stoichiometric and are known to occur via copper

5,26

superoxide species.

Recently, Gagnon and Tolman have

shown [CuOH] as the reactive core to activate the C−H

bond of polysaccharide using Cu -OH complexes of

III

pyridinedicarboxamide-based ligands under stoichiometric

condition.

On the other hand, in 2011, Perez and co-workers reported

[

Tp Cu(NCMe)] (Tp = hydrotrispyrazolylborate ligand) as a

catalyst for benzene hydroxylation and attained a phenol yield

of 21% but with poor selectivity (phenol:benzoquinone =

0:30) at 80 °C. Later in 2017, the same group proposed two

possible mechanisms, such as electrophilic aromatic sub-

stitution via copper oxyl species and a rebound mechanism by

plexes [Cu(L)(SO CF )]SO CF (1a−6a) were also synthe-

sized to understand the inﬂuence of counterions in catalysis.

The complexes have been characterized by various analytical

methods (cf. below). Complexes 4 and 6 were crystallized, and

their molecular structures were reported as distorted SP

experimental and density functional theory (DFT) studies. In

015, Kodera and co-workers showed benzene hydroxylation

using a dicopper(II) complex of the hpa ligand [Cu (μ-OH)6-

hpa] , where hpa is 1,2-bis[2-[bis(2-pyridylmethyl)-

geometry. Suitable single crystals of 2a and 4a were isolated

by diethyl ether diﬀusion into complexes in acetonitrile

aminomethyl]-6-pyridyl]ethane. It aﬀorded 22% of phenol

conversion and 4.8% of p-benzoquinone over 40 h. Liu and

(

CH CN) and used for X-ray diﬀraction studies.

Molecular Structures of Copper(II) Complexes. Single

co-workers reported the catalyst [CuL(HOCH )ClO ] (L =

-[[[(1-methyl-1H-imidazol-2-yl)methyl](pyridin-2-ylmethyl)-

crystals of 2a have been crystallized as [Cu(L)(H O)]-

amino]methyl]-2-tert-butylphenol) for benzene hydroxylation

SO CF ) in a monoclinic P 21/c space group (Figure 1 and

3 3 2

and displayed a yield of 14.6% phenol with a turnover number

(

TON) of 48.7 at 80 °C. Therefore, the strategy and

synthesis of bioinspired catalysts for benzene hydroxylation

remain a challenging and evergreen interest to date. This

article reports bioinspired copper(II) complexes of N -tripodal

and sterically constrained diazepane-based ligands, which

adopted distorted trigonal-bipyramidal (TBP)- and square-

pyramidal (SP)-based geometries around copper centers,

respectively. The complexes have been employed as catalysts

for benzene hydroxylation to produce phenol by utilizing the

H O oxygen source. The TBP-based complexes exhibited

selective phenol formation yields of up to 37% at 60 °C and

6% at 25 °C, which are relatively higher than those of SP-

Figure 1. Molecular structures of [Cu(L2)(H O)](CF SO ) (2a)

3 2

28−31

based complexes and other reported copper complexes.

These are novel examples of Cu -N complexes that illustrate

and [Cu(L4)(CF SO )](CF SO ) (4a). Thermal ellipsoids are drawn

−

from 50% probability. Hydrogen atoms and CF SO₃at the outer

the importance of coordination geometry versus catalytic

coordination are excluded for lucidness.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Tables 1 and S6 ). It adopted a ﬁve-coordinate distorted TBP

geometry. The distortion is predicted by the Addison

bond angles of 4a are lower than those expected for normal

SP geometry. The other bond angles O(1)−Cu−N(1)

[

91.63(3)°], O(1)−Cu−N(2) [91(3)°], N(4)−Cu−O(1)

Table 1. Bond Distances (Å) and Bond Angles (deg)

[103(3)°], and N(3)−Cu−O(1) [95.8(14)°] are larger than

the value expected for normal SP geometry (90°); however,

the N(1)−Cu−N(2) [83.5(5)°] and N(4)−Cu−N(3)

Cu(1)−N(1)

Cu(1)−N(2)

Cu(1)−N(3)

Cu(1)−N(4)

Cu(1)−O(1)

2.096(5)

2.033(5)

2.148(5)

2.084(5)

1.998(5)

2.003(3)

2.01(8)

2.02(4)

2.015(17)

2.33(5)

[83.6(17)°] bond angles are lower than 90° because of the

steric crowding presented by the 1,4-diazepane backbone.

Electronic and EPR Spectral Studies. The absorption

spectra of 1−6 exhibited broad bands around 583−730 nm in

the solid state, which correspond to the d−d transitions

(

Figure S1 and Table S1 ). In an CH CN solution, they

N(1)−Cu(1)−N(2)

N(1)−Cu(1)−N(4)

N(2)−Cu(1)−N(4)

N(1)−Cu(1)−N(3)

N(2)−Cu(1)−N(3)

N(4)−Cu(1)−N(3)

N(1)−Cu(1)−O(1)

N(2)−Cu(1)−O(1)

N(4)−Cu(1)−O(1)

N(3)−Cu(1)−O(1)

81.05(18)

113.74(19)

80.51(19)

125.2(2)

96.3(2)

119.3(2)

94.7(2)

167.2(2)

92.3(2)

83.5(5)

appeared around 582 −757 nm with smaller ε values (171−280

116.6(7)

158.87(18)

161.11(10)

79.2(16)

83.6(17)

91.63(3)

91(3)

−1

cm ; Figure S2 and Table S1). The shoulders around

96−309 nm and transitions around 237−264 nm correspond

35,39−41

to π → π* transitions of the ligands.

based molecular orbitals at higher energy may be excited into

These ﬁlled ligand-

the partially ﬁlled e orbital set and caused the ligand-to-metal

42,43

charge-transfer (LMCT) transitions.

The positions of the

d−d bands suggest the existence of ﬁ ve-coordination geometry

103(3)

95.8(14)

around the copper(II) center (Figures S2 and S3 and Table

96.5(2)

35,41

S1 ).

They appeared to be a combination of two transitions,

Standard deviations in parentheses.

→ d , d and d

→ d . This may have originated

x −y

from the interconversion of TBP and SP coordination

geometries, which is known to occur in solution via changes

in the ligand ﬁeld (LF) at the z direction and by changing the

parameter τ = 0.70 [τ = (β − α)/60; β = 167.2° and α =

25.2°], which is 0 and 1 for the SP and TBP geometries,

42,43

equatorial-plane LF.

being a combination of d

Thus, it resulted in the ground state

respectively. The TBP geometry is constituted by nitrogen

and d and consequencely in a

atoms of the ligand L2 and one H O molecule at axial

x −y

change of the d−d band positions from the value expected for

coordination. The observed Cu−N (2.033−2.148 Å) and Cu−

42,43

a perfect TBP/SP geometry.

(1.998 Å) bond lengths are analogous to the previously

water

The geometry of the copper(II) center is directly associated

with the energy of the d−d transitions. The d−d bands around

586−620 nm for 4−6 possibly correspond to more SP

reported copper(II) complexes with TBP geometry. One

nitrogen atom [N(2)] and H O molecule [O(1)] occupied the

axial sites with a N(2)−Cu−O(1) bond angle of 167.2(2)°,

which is higher than the bond angle N(1)−Cu−N(3)

35,39,40

geometry.

Interestingly, the addition of Et N caused the

[125.2(2)°] at the equatorial site and the bond angle N(4)−

shift of d−d bands to low energy (734−745 nm) along with

Cu−N(3) [119.3(2)°] constituted by pyridine nitrogen

donors. The distortion in the TBP geometry is revealed by

variation of the N(1)−Cu−N(3) [125.2(2)°], N(3)−Cu−

N(4) [119.3(2)°], and N(1)−Cu−N(4) [113.74(19)°] bond

angles at the equatorial site. The axial bond angle N(2)−Cu−

O(1) deviated from 180°, which additionally conﬁrmed the

existence of signiﬁcant distortion in the coordination

formation of the shoulder around 590−630 nm (Figures S4

and S5 ). Therefore, the addition of Et N appears to alter the

coordination geometry and is believed to exhibit mixed SP and

5,42

TBP geometries in solution.

All four nitrogen donors of

CN; however, the

L4−L6 support SP geometry in CH

addition of a base may result in the decoordination of one

of the pyridyl arms. This likely facilitated the interconversion

35,42,43

geometry. The complexes of the other tripodal ligands L1

and L3 are expected to display similar TBP-based coordination

geometries.

of TBP or less SP geometry,

which is predominantly

noticed for 6 than 4 and 5 and correlates to the catalytic

activity diﬀerences (cf. below). This geometrical interconver-

sion may be fast, and no clear isosbestic points were noticed

for these complexes. Solomon et al. rationalized that the ﬁve-

coordination geometry normally undergoes an interconversion

of SP to TBP geometry and vice versa via either a C2v

The molecular structure of 4a exhibited the Cu−N O

coordination sphere (Figure 1) with a more SP geometry,

which is revealed from the τ value of 0.037 (Table 1). The four

nitrogen atoms of L4 occupied equatorial locations, whereas an

−

oxygen atom of the CF SO₃ion was placed at the axial

distortion (Berry pseudorotation) or a C distortion. The

position. The observed Cu−O

bond distance of 4a

second conversion is associated with ligand displacement

reactions in the square plane, wherein an axial ligand switches

to the plane for displacement of a ligand equatorial site.

triflate

[

2.33(5) Å] is lengthier than the Cu−O

bond length of

water

35,43

a but shorter than those of 4 [2.46(2) Å] and 6 [2.52(6) Å],

which adopted identical distorted SP geometries. This axial

is possibly caused by the pseudo-

There was no signiﬁcant shift of d−d transitions caused by 1−

elongation of Cu−O

3 with Et N due to the absence of major structural

triflate

Jahn−Teller eﬀect and positioned to be highly labile. This can

rearrangements. These complexes adopted more TBP even

without the addition of a base (Figure S4 ).

be easily replaced by a solvent molecule or H O during

catalysis. The bond lengths Cu−N (2.00 and 2.01 Å) and

Electron paramagnetic resonance (EPR) spectra were

measured at 70 K using a mixture (8:2) of solvents methanol

and N,N-dimethylformamide (DMF). The hyperﬁne splitting

Cu−N

(2.01 and 2.02 Å) of 4a are indistinguishable from

amine

those of 2a but somewhat lower than those of 4 (Cu−N , 1.95

and 1.99 Å; Cu−N

, 1.98 and 2.01 Å) and 6 (Cu−N , 2.01

amine

was discerned in the parallel region (g ) for all complexes

amine

∥

and 2.02 Å; Cu−N

, 2.03 and 2.032 Å). The N(1)−Cu−

(Figure 2 and Table S1). Complexes 1−3 displayed g values

∥

−

N(3) [161.11(10)°] and N(2)−Cu−N(4) [158.87(18)°]

of 2.245−2.250. The f values were calculated as 151−154 cm

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

The covalencies of in-plane σ bonds (α ), in-plane π bonds

(

β ), and out-of-plane π bonds (γ ) were estimated from the

45,47

EPR data and d−d transitions.

Complexes 1−6 exhibited

α (0.690−0.731), β (0.992−1.110), and γ (0.264−0.956)

covalencies (Table S1 ), where ionic bonding has α values near

to 1, and this value has a reciprocal correlation with covalent

character. The orbital reduction parameters K = α β and K

∥

⊥

α γ were calculated as 0.699−0.800 and 0.193−0.688,

respectively. Complexes 1−6 exhibited K > K , which reveals

∥

⊥

the existence of a remarkable quantity of π bonding at the

outer plane. The pure σ-bonding K = K and the π-bonding

∥

⊥

K < K in the plane have been described in the literature.

Figure 2. EPR spectra of 1−6 at 70 K in methanol/DMF (8:2) and g

values provided on each spectra. Frequencies: 1, 9.00903 GHz; 2,

∥

⊥

Redox Properties. The redox potentials of the complexes

.00743 GHz; 3, 9.00851 GHz; 4, 9.01365 GHz; 5, 9.01277 GHz; 6,

.01523 GHz. Power = 0.99 mW; modulation frequency = 100 kHz;

were studied by cyclic voltammetry using a three-electrode

setup and the supporting electrolyte NBu PF in CH CN. The

modulation amplitude = 10 G; T = 70 K.

redox couple Cu /Cu has been observed at a negative

potential range for 1−6 (Figure S6 and Tables S1 and S2).

The redox potential of −0.251 V for 1 is more positive

compared to those of complexes 2 (−0.351 V) and 3 (−0.383

V). The more negative redox potential of 3 may be imposed by

the electron-releasing −Me and −OMe groups. These redox

potentials are higher than those of 4 (−0.513 V), 5 (−0.450

−

and are higher than 105−135 cm , which is expected for

43−45

geometries with a square base.

It indicates that the

geometry is strongly distorted and deviated from planarity to

3−45

form a TBP geometry.

Thus, the ground state appears to

V), and 6 (−0.527 V) versus Ag/Ag . The ΔE values of 1 (86

be the mixing of d

and d orbitals. It is further supported

x −y

by the lower hyperﬁne coupling constant values (A ), (146−

mV) and 3 (83 mV) are slightly away from reversibility;

however, 2 showed a redox couple far away from reversibility

(ΔE, 134 mV). The redox couple of 4 is close to reversibility

(ΔE, 77 mV), but 5 (ΔE, 158 mV) and 6 (ΔE, 150 mV)

showed redox couples far away from reversibility. Thus, the

structural and electronic variations of the ligands appear to

∥

−

−1

49) × 10 cm , which are closer to the A values reported

∥

for the TBP geometry. Conversely, complexes 4−6 showed

g (2.200−2.251) > g (1.990−2.013) and f values of 133−142

∥

⊥

−

cm and are comparable to the values reported for geometries

43−45

with a square-planar base (cf. above).

The larger A value

∥

−

−1

−

of (159−166) × 10

The minor g components 2.556 for 1, 2.613,

.504, and 2.425 for 4 and 2.532 and 2.415 for 6 are possibly

ascribed to the minor species that may have been generated

further supports the SP

inﬂuence the redox potential. Further, the SO CF derivatives

4,45

geometry.

1a−6a exhibited redox behavior almost identical with that of

their respective perchlorate complexes (Figure S7 and Tables

S1 and S2 ). The order of the redox potential is 1 > 2 > 3 > 5 >

4 > 6.

during solvation or geometrical interconversions (cf.

above). However, these minor species values are signiﬁcantly

diﬀerent from values obtained for Cu(ClO ) ·6H O. The

hyperﬁne coupling of a d electron with copper(II) nuclear spin

Benzene Hydroxylation Reactions. Hydroxylation of

benzene was performed using copper(II) complexes as

catalysts and H O (30%) as the oxygen source at 60 °C.

has perhaps contributed via lower Fermi contact, spin dipolar

The reaction condition was optimized using 3 (see details in

Table S3). Under optimized conditions, 5 mmol of benzene

(0.45 mL) and 0.05 mol % copper(II) complex (2.5 μmol)

were mixed in CH CN (2.0 mL). Further, the base Et N (2.5

−

and orbital dipolar. The SO CF₃derivatives of the

complexes displayed EPR spectral parameters almost identical

with those of their respective perchlorate complexes.

Table 2. Catalytic Benzene Hydroxylation

TOF (h⁻¹)

k_o_b_s(×10⁻²_S⁻¹)ⁱ

7.51 ± 0.004

cat.

[C] (%)

[Y] (%)

[S] (%)

TON

KIE

0.97

32 ± 0.5

33 ± 0.3

34 ± 0.8

35 ± 1.0

37 ± 0.5

38 ± 1.3

25 ± 0.5

27 ± 0.2

17 ± 0.8

18 ± 1.0

29 ± 0.3

32 ± 0.6

640 ± 10

660 ± 6

128

132

136

140

148

152

100

108

680 ± 16

700 ± 20

740 ± 16

760 ± 26

500 ± 10

540 ± 4

340 ± 16

360 ± 20

580 ± 6

0.99

9.29 ± 0.002

11.85 ± 0.002

0.98

1.08

1.04

6.26 ± 0.004

0.98

1.12

116

128

2.54 ± 0.01

640 ± 12

Reaction conditions: substrate (5 mmol), catalyst (2.5 μmol, 0.05%), Et N (2.5 μmol), and 30% H O (25 mmol) in CH CN for 5 h.

Conversion. Yield (reported values represent the mean value of three reactions). Selectivity. Turnover number. Turnover frequency. Catalyst

(

2.5 μmol), a 1:1 mixture of C D (0.25 mmol) and C H (0.25 mmol), and 30% H O (2.5 mmol) at 60 °C for 5 h. KIE observed for the

6 6 6 6 2 2

−4

reaction of a 1:1 mixture of C D and C H with [CuL3(OOH)] (1 × 10 M in CH CN). The spectral changes observed for 1−6 (1 × 10 M)

−4

with 10 equiv of H O and Et N (1 equiv) in CH CN at −40 °C. The spectral changes for the reaction of [CuL3(OOH)] (1 × 10 M) and

benzene (50 equiv) in CH CN at −40 °C.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

μmol) and aqueous 30% H O₂(25 mmol) were added

those observed at elevated temperature (60 °C) for all

complexes, but the trends of the reactivity are found to be

identical. These yields are higher than those of the other

gradually, and the reactions were performed for 5 h. Finally,

the solution was passed via a silica column for removal of the

catalyst and subsequently subjected to gas chromatography

reported complexes at room temperature. The stoichiometric

GC)−mass spectrometry (MS)/GC analyses (Figures S8 and

S9 and Table 2). Complex 1 exhibited phenol formation of

2% and a TON of 640, wherein 76% of selectivity was

reaction of 3, benzene, and H O aﬀorded 21% yield of phenol

with a selectivity of 52%, and a signiﬁcant amount of p-

benzoquinone (20%) was also observed (Figure S11 ). The

achieved. Replacement of the ethylene chain in 1 by a

propylene chain led to 2, which showed a slightly enhanced

phenol yield of 34%, a TON of 680, and a much better

selectivity of 96%. Further introducing electron-donor groups

on pyridine arms to obtain complex 3 showed a better catalytic

eﬃciency than 1 and 2. It aﬀorded a phenol yield of 37% with a

TON of 740 and an improved selectivity of 98%. The spacer

catalytic reactions using more than 1 equiv of Et N showed

neither inhibition nor acceleration of the product yield.

However, less than 1 equiv of Et N showed a decreased

yield of phenol formation (26%). Interestingly, the bulk-scale

catalytic reaction using the most eﬃcient catalyst 3 exhibited

phenol formation of up to 0.441 g (4.7 mmol) at 60 °C over

48 h. Besides, the catalytic reaction using O instead of H O

2 2 2

connected to the −NMe group and electron-donating groups

displayed a decreased phenol yield (9%) with a selectivity

>99% at 60 °C for 24 h, while employing tert-butyl

hydroperoxide as an oxygen source showed 16% yield of

phenol under identical reaction conditions. Further, the

reaction of phenol (5 mmol) as the substrate and 3 as the

catalyst exhibited no hydroxylated product at 60 °C, and the

substrate was recovered almost quantitatively. This is strongly

revealing that the catalysts are proﬁcient in executing

monohydroxylation of the aromatic C−H bond selectively,

without any overoxidation. Also, toluene and ethylbenzene

were used as the substrates to study the C−H functionalization

ability of alkyl versus aromatic bonds. Toluene showed the

formation of o- and p-cresol (yield of 8%; selectivity of 83%)

and benzaldehyde (yield of <1%; selectivity of 17%) without

quinone formation at 60 °C (Scheme 2). It suggests that the

on pyridines inﬂuences signiﬁcantly for the variation of the

catalytic eﬃciency. On the other hand, the catalyst 4 showed a

signiﬁcant decrease in phenol formation (25%) and selectivity

(

56%) (Table 2) over those of 1−3. This catalytic eﬃciency

further declines by replacing pyridines in 4 by a quinoline arm

to attain 5, which displayed the lowest phenol yield (17%),

TON (340), and selectivity (39%) among the series. The

increase of the chelate ring size as in 6 showed moderately

increased phenol yield (29%), TON (580), and selectivity

(

60%) over those of 4 and 5. The decreasing order of the

catalytic power of the complexes is observed as 3 > 2 > 1 > 6 >

> 5. The replacement of the tripodal backbone by a sterically

crowded cyclic diazepane unit results in a decrease of the

catalytic eﬃciency.

Benzene hydroxylation reactions have been performed for

−

SO CF -containing complexes (1a−6a) under identical

Scheme 2. Catalytic Hydroxylation of Toluene by 3

reaction conditions to understand the role of the counterion.

Complex 1a showed 33% of phenol formation and a TON of

60, wherein 78% of selectivity was achieved. Similarly,

complexes 2a−6a showed the slightly diﬀerent phenol yields

at 60 °C: 2a, 35%; 3a, 38%; 4a, 27%; 5a, 18%; 6a, 32%. The

results advocate that a change of the counterion is not

signiﬁcantly aﬀected by the phenol yield and catalytic power

(

Table 2). The currently observed catalytic eﬃciency is

aromatic sp carbon preferably functionalized, and similar

signiﬁcantly higher than those in the previous report by

Kodera et al., where 22% conversion of benzene to a mixture of

products (phenol and p-benzoquinone) has been attained at 50

observations were reported in the literature, whereas the use

of the substrate ethylbenzene displayed selective functionaliza-

tion of sp carbon and aﬀorded 2.1% of ethylphenol formation

C over 40 h. The copper(II) complexes of tripodal ligand

but with the huge declined selectivity (48%) and without

quinone formation. Cumene showed the formation of a

mixture of substituted isopropyl phenol (yield, 6%) and <1%

2-phenyl-2-propanol (Scheme S1 ). Anisole aﬀorded o-, m-, and

p -methoxyphenols as products with a collective yield of 4%

(Scheme S2 ). 1-Chlorobenzene showed the formation of a

mixture of chlorophenols (4%; Scheme S3). The substrate

nitrobenzene exhibited 5% formation of a mixture of

nitrophenols (Scheme S4 ).

Mechanism of Benzene Hydroxylation. Kinetic isotope

eﬀect (KIE) has been investigated using complexes (2.5 μmol),

a mixture of C D (0.25 mmol) and C H (0.25 mmol), as

substrates under the conditions speciﬁed in Table 2. The KIE

values were determined from the phenol to phenol-d ratio by

the GC−MS/GC method (Figure S12 and Table 2). The

calculated KIE values of 0.97−1.12 for 1−6 are within the

conﬁnes of values described for intramolecular aromatic

hydroxylation (0.99−1.10). The metal-bound oxygen key

intermediates are known to display a KIE value of ∼1.0 for

aromatic hydroxylation via an electrophilic aromatic sub-

scaﬀolds are found to be more eﬃcient than the cyclic 1,4-

diazepane support. This diﬀerence likely originated from the

geometries of complexes and their catalytically active

intermediates [(L)CuOOH] (cf. below). Complexes 1−3

adopted more TBP coordination geometry and are expected to

stabilize almost identical geometry for [(L)CuOOH] ,

whereas diazepane-based ligands L4 and L5 support more SP

geometry in 4 and 5. The additional methylene group

connecting pyridine and diazepane units in 6 possibly enforces

geometry between TBP and SP and is likely responsible for a

better yield of phenol than complexes 4 and 5 (cf. above). The

ligand L3 showed a more steric nature with the highest %V_b_u_r

of 75.2% than L1 and L2 (Figure S10 ). This steric crowding is

also directly associated with the highest catalytic eﬃciency of

L3 among tripodal ligand complexes.

The benzene hydroxylation reactions were also carried out at

5 °C (Table 2), where the catalyst 1 displayed 12% yield of

phenol and a TON of 240. Similarly, other complexes were

also aﬀorded phenol formation: 2, 13%; 3, 16%; 4, 9%; 5, 5%

and 6, 11% (Table S4 ). These yields are relatively smaller than

stitution reaction. Further, these observations suggest that

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

the copper(II) center. The rate of formation of the species was

Article

breakage of the C−H bond is unlikely to be the rate-

calculated as (11.85 ± 0.002) × 10⁻s⁻¹(t₁_/₂, 58 s) from the

growth of this LMCT band (Figure S16 and Table 2). The

time-dependent DFT (TD-DFT) calculation showed the

energy of the oxo-to-copper(II) LMCT transition at 366 nm

and supports the experimental observation. The solution

Fourier transform infrared (FT-IR) spectral studies further

supported our predictions, and the stretching frequencies of

determining factor. The observed KIE values support that

Fenton (1.7) or copper-insertion-type (2.9−4.8) reactions

may not be followed. In addition, KIE values were

determined over various time intervals for 3 and showed

KIE values of ∼1.0 throughout the reaction time (Figure S13),

which are almost similar to values obtained after 5 h. The use

of excess substrate (1 mmol, 4 equiv to oxidant) with a limited

amount of H O (0.25 mmol) displayed KIE of 1.12, which is

−

the Cu−O and O−O bonds appeared at 705 and 878 cm ,

respectively. They were shifted to 673 and 846 cm when

−

comparable to the above observations (Figure S14). The

isotope-labeling experiments were executed for the most active

using H O (90% atom purity) instead of H O (Figure 3).

2 2 2 2

−1

The observed isotopic shift Δ − Δ = 32 cm is

comparable to those of other structurally and spectroscopically

well-characterized Cu -OOH species. The calculated vibra-

tion spectra of Cu -OOH species showed stretching

catalyst 3 by utilizing 90% atom purity H₂¹⁸O in water and

keeping the other reaction conditions the same. It showed that

2% of O insertion into phenol (Figure S15). However, the

−

frequencies at 625 cm corresponding to the Cu−O bond

use of H₂O and H O₂showed almost no O-atom-

−

and 847 cm for the O−O bond, which look very near to the

experimentally observed values (Figure 3). Notably, these

vibrational frequency values are lower than those of copper

incorporated phenol product. These results suggest that the

utilization of H O is crucial for the transformation of benzene

to phenol.

Intriguingly, the reaction of 3 with 30% H O (10 equiv)

•

superoxo (Cu-OO ) intermediates, wherein the values for side-

on Cu-OO and end-on Cu-OO species are reported at 1043

and 1122 cm , respectively, with ν(Cu−O) around 422−474

cm . Further, the addition of 50 equiv of benzene to

(L3)Cu -OOH] species showed that the disappearance of

vibrational frequencies at 705 and 878 cm corresponds to the

Cu−O and O−O bonds, respectively, and supports involve-

ment of the Cu -OOH putative intermediate. On the other

hand, warming up the solution to room temperature showed

the disappearance of these vibrational frequencies. The mass

spectral (ESI-MS) analysis of the species displayed m/z 496.00

•

and 1 equiv of Et N exhibited O (π* ) → Cu LMCT

−

transition at 390 nm in CH CN at −40 °C, which may

correspond to the Cu -OOH intermediate (Figure 3). The

−1

[

base Et N has seemingly facilitated the binding of H O with

−

(

[

calcd m/z 496.21) in CH CN, which corresponds to

(L3)Cu -OOH] (Figure S17). Remarkably, the addition of

benzene (50 equiv) to [(L3)Cu -OOH] at −40 °C in

CH CN leads to the instant collapse of species with the

pseudo-ﬁrst-order reaction proﬁle. This decay has coexisted

with the vanishing of the LMCT band (Figure 4 ). The decay

−4

Figure 4. Reaction of [CuL3(OOH)] (1 × 10 M) with benzene

50 equiv) in CH CN at −40 °C and its electronic spectral changes.

(

Time interval: 1 s. Inset: Plot of time versus absorbance at 390 nm.

−

constant has been determined as k = (6.26 ± 0.004) × 10

−

(Figure S18 ). The plot of the ﬁrst-order rate constants

Figure 3. (a) Electronic spectral changes for the reaction of 3 (1 ×

versus various amounts of benzene showed a linear correlation

−

M) with 10 equiv of H O and Et N (1 equiv) in CH CN at

2 2 3 3

to obtain the second-order rate constant k = 2.7 ± 0.009 ×

−

40 °C. Inset: Plot of time versus absorbance at 390 nm. (b) Solution

−

−1

II 16 16 II 18 18

(Figure S19 ). Product analysis for this reaction

FT-IR spectra of [Cu - O OH] species (blue) and [Cu - O OH]

−

showed the formation of phenol selectively. Also, KIE values

were determined from the kinetics by adding a mixture (1:1)

species (red) obtained by the reaction of 3 (1 × 10 M) and 10

equiv of H₂¹⁶O and H O , respectively, in the presence of E N (1

equiv) in CH CN. The calculated vibrational spectra of [CuL3-

of benzene-d and benzene to [(L3)Cu -OOH] species at

(

OOH)] by TD-DFT (black).

−40 °C in CH CN. The KIE was calculated as 1.08 by GC-

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

MS/GC investigations after complete decay of the LMCT

band (Figure S20 and Table 2), which is consistent with the

earlier observations.

Benzene hydroxylation performed using the catalyst 3 and

the radical trapping agent 2,2,6,6-tetramethylpiperidin-1-yl-

oxyl exhibited no distinguishable change in the phenol yield at

[(L3)Cu -OOH] has been optimized by DFT calculations.

The calculations were performed using split valence basic sets

(def2-SVP and def2-TZVP for all atoms). The calculated

Cu−O (1.8981 Å) and O−O (1.4310 Å) bond distances are

closer to the previously reported values determined from

crystal structures of copper(II) hydroperoxo complexes (Cu−

0 °C. This result eradicates the possibility of the radical-type

O, 1.888 Å; O−O, 1.460 Å). The optimized structure of

II +

reaction mechanism. The use of another radical trapping

agent, dimethylpyrroline N-oxide (DMPO), was not suggested

helpful information, where more or less equal quantities of

DMPO−OH radicals have been determined by EPR with or

without the use of a catalyst. The oxidation of the substrate cis-

[(L3)Cu -OOH] revealed a distorted TBP geometry (τ,

0.64), where the −OOH group occupied an axial site, which is

comparable to the previous reports (Figure 5 and Table

62−64

S5 ).

,2-dimethylcyclohexane has been carried out under an

optimized reaction condition using 3 as the catalyst, which

showed 12.6% of the cis-hydroxylated product with a trace of

trans product (<1%; Figure S21 ). This stereoretention of C−H

oxidation reactions indicates that Fenton-type species are

unlikely involved, which supports our earlier prediction by

KIE (cf. above). Besides, the use of CCl as a trapping agent

aﬀorded 96% and 4% selectivity of phenol (including the

benzoquinone byproduct) and chlorobenzene, respectively

(Scheme 3 and Figure S22 ). This suggests that the use of CCl₄

Scheme 3. Benzene Hydroxylation by 3 in the Presence of

CCl₄

Figure 5. (a) Catalytic benzene hydroxylation by 3 and H O . (b)

Optimized structure of [(L3)Cu -OOH] by DFT using B3LYP/

def2-SVP and def2-TZVP basis sets.

does not signiﬁcantly aﬀect the selectivity of phenol and

eliminates the possibility of the Fenton-type reaction path-

way. The reaction without catalysts has not aﬀorded any

amount of chlorobenzene formation. In another experiment,

cyclohexane was used as the substrate, 3 as the catalyst, and

CONCLUSIONS

■

CCl as the radical trapping agent, where 95% selectivity of

Bioinspired copper(II) complexes of N ligands have been

cyclohexanol and cyclohexanone formation with 5% of

cyclohexyl chloride was observed (Figure S23), which again

supports exclusion of the possibility of the radical-type reaction

reported as catalysts for direct benzene hydroxylation. The

molecular structures of tripodal ligand-based complexes

exhibited more TBP geometry; however, diazepane-based

complexes adopted more SP geometry. All catalysts showed

benzene hydroxylation to form phenol selectively by utilizing

H O as an oxidant at 25/60 °C, and no overoxidized products

pathway.

_S_i_m_i_l_a_r_l_y_,_t_h_e_s_p_e_c_i_e_s_[₍_L₎_C_u_-_O_O_H_]+ of 1, 2, and 4 and 5

have been obtained by the reaction of respective complexes

with 30% H O (10 equiv) and 1 equiv of Et N at −40 °C in

were observed. The complexes with more TBP geometry

showed the highest catalytic eﬃciency (37% yield of phenol)

and selectivity (98%). However, the diazepane-based com-

plexes with more SP displayed comparatively reduced yields

and poor selectivity. The hydroxylation reaction mechanism

likely proceeds via Cu -OOH species, and they were

characterized by various spectroscopies and supported by

DFT and TD-DFT calculations. The KIE investigations

support the involvement of Cu−O

isotope-labeling studies indicate that H O

2 2

oxygen atom supplier to form phenol. The coordination

geometry of the copper(II) center directly impacts the

catalysis, as evidenced by the fact that more TBP-based

CH CN. They showed almost similar electronic spectral

signatures and kinetic behaviors: 1, λ , 380 nm; k , (7.51

max

obs

−

2 −1

0.004) × 10 s , and t₁_/₂, 91 s; 2, λ_m_a_x, 370 nm; k , (9.29

obs

−

−1

0.002) × 10 s , and t₁_/₂, 74 s; 6 , λ _m _a _x , 375 nm; k , (2.54

obs

−

2 −1

0.01) × 10 s , and t₁, 27 s (Figures S24 −S26). Further,

the formation of these [(L)Cu -OOH] species was conﬁrmed

by ESI-MS spectra: m/z 366.20 (calcd m/z 366.11)

correspond to [(L1)Cu -OOH] ; m/z 379.80 (calcd m/z

80.12) for [(L2)Cu -OOH] ; m/z 378.40 (calcd m/z

78.11) for [(L4)Cu -OOH] ; m /z 405.8 (calcd m/z

species. The oxygen

is the crucial

06.14) for [(L6)Cu -OOH] (Figures S27 −S30). However,

characterization of the intermediate of 5 has been unproduc-

tive in spite of constant eﬀorts because it likely to have a very

short lifetime.

geometry of complexes better supports the generation of Cu -

Repeated attempts to isolate single crystals of species

(L)Cu -OOH] were unsuccessful for all complexes under

OOH species and higher yield of phenol and selectivity. The

coordination geometry of Cu -OOH species was optimized as

[

various conditions. However, the geometry of intermediate

distorted TBP.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

which can be found at https://www.molnac.unisa.it/OMtools/

Article

Radical Trapping Experiments Using CCl . Catalytic amounts

EXPERIMENTAL SECTION

Synthesis of Ligands. The present ligands L1−L6 have been

synthesized as reported previously.

perchlorate salt with ligands should be handled carefully due to

possible explosion.

Synthesis of Copper(II) Complexes (1−6). The copper(II)

complexes were synthesized by a known procedure. The solution

■

of 3 (2.5 μmol) and benzene (5.0 mmol) were dissolved in CH CN

2−34

(2.0 mL). Then, 30% H O (2.5 mL, 25 mmol), Et N (2.5 μmol), and

2 2 3

Caution! Copper(II)

CCl (2.4 mL, 25 mmol) were slowly added to the reaction mixture.

The solution was stirred at 60 °C for 5 h. After completion of the

reaction, the usual workup and product analysis were followed, which

was similar to that of benzene hydroxylation without CCl .

DFT and TD-DFT Calculations. Optimization of the geometries

was carried out by a DFT method. The electronic and vibrational

spectra of key intermediates were calculated by TD-DFT. All of these

calculations were performed by using double-ζ-quality split-valence

basic sets with polarization functions (def2-SVP) and larger triple-ζ

basis sets (def2-TZVP) for all atoms; TD-DFT calculations were also

performed using B3LYP and def2-TZVP basis sets for all atoms, and

of Cu(ClO ) ·6H O (0.185 g, 0.5 mmol) in methanol (5 mL) was

added dropwise to the ligands (0.5 mmol) in methanol (20 mL). The

reaction mixtures were stirred for 3 h at 25 °C. The color of the

solutions turned blue, and removal of the solvent aﬀorded blue

residues, which were washed twice with hexane (2 × 10 mL) to

remove the excess ligand. The complexes were dried under high

vacuum over fused CaCl₂.

calculations were carried out by ORCA 4.0. The steric map

The complexes [Cu(L)(SO CF )]SO CF (1a−6a) were isolated

calculations were performed by the SambVca 2.1A web application,

as blue solids using Cu(CF SO ) as the starting material, as reported

3 2

previously. Suitable blue crystals of 2a and 4a for single-crystals X-

ray crystallographic studies were isolated by diethyl ether diﬀusion

sambvca2.1/index.html .

into a CH CN solution of the respective complexes.

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03676.

[

Cu(L1)(ClO )]ClO (1). Yield: 0.27 g (86%). Analytically calculated

■

elements for C H CuN O Cl : C, 36.07; H, 4.16; N, 10.52. Found:

C, 36.04; H, 4.12; N, 10.48. ESI-MS (m/z): 432.06 ([M − ClO ] ).

[Cu(L2)(ClO )]ClO (2). Yield: 0.23 g (81%). Analytically calculated

4 4

elements for C H CuN O Cl : C, 37.34; H, 4.42; N, 10.25. Found:

Chemical details, physical methods (instrumentation),

single-crystal X-ray diﬀraction measurement details,

structural solution and reﬁnements, other additional

experimental methods, and additional spectral, redox,

and kinetic data (PDF)

C, 37.28; H, 4.37; N, 10.21. ESI-MS (m/z): 446.07 ([M − ClO ] ).

[Cu(L3)(ClO )]ClO (3). Yield: 0.19 g (74%). Analytically calculated

4 4

elements for C H CuN O Cl : C, 41.67; H, 5.47; N, 8.45. Found:

C, 41.63; H, 5.45; N, 8.42. ESI-MS (m/z): 562.16 ([M − ClO ] ).

[Cu(L4)(ClO )]ClO (4). Yield: 0.28 g (91%). Analytically calculated

4 4

elements for C H CuN O Cl : C, 37.48; H, 4.07; N, 10.28. Found:

Accession Codes

C, 37.53; H, 3.85; N, 10.25. ESI-MS (m/z): 444.03 ([M − ClO ] ).

CCDC 1959083 and 1959084 contain the supplementary

crystallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

[Cu(L5)(ClO )]ClO (5). Yield: 0.17 g (65%). Analytically calculated

4 4

elements for C H CuN O Cl : C, 46.56; H, 4.06; N, 8.69. Found:

C, 46.51; H, 4.02; N, 8.65. ESI-MS (m/z): 544.09 ([M − ClO ] ).

[Cu(L6)(ClO )]ClO (6). Yield: 0.26 g (89%). Analytically calculated

4 4

elements for C H CuN O Cl : C, 39.84; H, 4.57; N, 9.78. Found:

C, 40.03; H, 4.88; N, 9.63. ESI-MS (m/z): 472.67 ([M − ClO ] ).

Procedure for Benzene Hydroxylation. The complex [Cu(L)-

(

ClO )]ClO (2.5 μmol) and benzene (5.0 mmol) were mixed in

4 4

AUTHOR INFORMATION

Corresponding Author

■

CH CN (2.0 mL). Then, 30% H O (2.5 mL, 25 mmol) and Et N

(2.5 μmol) were added very slowly to the reaction mixture. The

stirring was continued at 60/25 °C for 5 h. After cooling to room

temperature, the catalyst was removed by passing through a solution

on silica gel. A known amount of nitrobenzene was added as an

internal standard. The conversion and yield of the reaction were

determined with GC−MS/GC.

Ramasamy Mayilmurugan − Bioinorganic Chemistry

Laboratory/Physical Chemistry, School of Chemistry, Madurai

Kamaraj University, Madurai 625021, India; orcid.org/

000-0002-5370-594X; Email: mayilmurugan.chem@

mkuniversity.org

Determination of the Kinetic Deuterium Isotope Eﬀect

(

KIE). A mixture of the substrate benzene (0.25 mmol) and benzene-

Authors

d₆(0.25 mmol) was added to a solution of the copper(II) complex

Sethuraman Muthuramalingam − Bioinorganic Chemistry

Laboratory/Physical Chemistry, School of Chemistry, Madurai

Kamaraj University, Madurai 625021, India

Karunanithi Anandababu − Bioinorganic Chemistry

Laboratory/Physical Chemistry, School of Chemistry, Madurai

Kamaraj University, Madurai 625021, India

(

2.5 μmol) in CH CN (2 mL). Then, 30% H O (0.50 mL) and Et N

2.5 μmol) were added slowly to the reaction mixture under constant

3 2 2 3

stirring, which was continued for an additional 5 h at 60 °C. Finally,

the reaction mixture passed through a solution on silica gel to remove

the complex. The product distribution was calculated by GC−MS/

GC analysis. The KIE value was calculated from the ratio of phenol

and phenol-d₅.

Marappan Velusamy − Department of Chemistry, North

Eastern Hill University, Shillong 793022, India

Isotopic Labeling Studies Using H ¹⁸^O2 ^aⁿ^d^H2¹⁸O and

Kinetic Measurements. All of these experiments were performed as

reported previously by us with suitable modi ﬁ cations. Details of the

experiments are provided in the Supporting Information.

11c

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.9b03676

Solution FT-IR Measurements. Solution-state FT-IR experi-

ments were performed on a Thermo Nicolet 6700 spectrometer. The

Notes

II 16 16

II 18 18

species of 3 [Cu - O OH] and [Cu - O OH] were freshly

The authors declare no competing ﬁnancial interest.

generated from 30% H O and H₂O₂(90% atom purity),

respectively, in CH CN. Finally, solutions were carefully transferred

ACKNOWLEDGMENTS

■

to the CaF cell via a microsyringe to measure the spectra.

We acknowledge the Science and Engineering Research Board,

New Delhi, India, and Board of Research in Nuclear Science,

Mumbai, India, for funding. S.M. sincerely acknowledges the

Council of Scientiﬁc and Industrial Research, New Delhi,

Catalytic Hydroxylation of Other Substrates. The most active

catalyst 3 (2.5 μmol) and appropriate substrates (5.0 mmol) were

used and the reactions performed by keeping other procedures similar

to that followed for benzene hydroxylation.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(14) Parida, K. M.; Rath, D. Structural Properties and Catalytic

Article

India, for a Senior Research Fellowship [ref no. 09/

Oxidation of Benzene to Phenol over CuO-impregnated Mesoporous

Silica. Appl. Catal., A 2007, 321, 101−108.

114(0224)/19-EMR-I].

(15) Song, S. Q.; Jiang, S. J.; Rao, R. C.; Yang, H. X.; Zhang, A. M.

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