Aerobic oxidation of 2,4,6-tri-tert-butylphenol to quinones catalyzed by copper(II) complexes of an N-octylated bis-benzimidazolyl ligand

Article abstract of DOI:10.1016/j.ica.2015.06.026

Abstract Copper(II) complexes of a N-octylated bisbenzimidazolyl ligand are synthesized and characterized. These complexes carry out the oxidative dealkylation of 2,4,6-tri-tertbutylphenol (TTBP) using molecular oxygen to 2,6-ditertbutylbenzoquinone and 4,6-di-tertbutylbenzoquinone. The oxidation proceeds via a phenoxyl radical species detected spectrophotometrically, and by EPR. A reactive copper(II)-dioxygen species is involved that carries out the oxidation reaction. A comparison of the rates of formation of the 4,6-di-tert-butylbenzoquinone versus 2,6-di-tert-butylbenzoquinone suggests that alternative pathway may exist for the formation of the para-quinone derivative. Isolation of an intermediate 4,4′-peroxybis(2,4,6-tri-tert-butylcyclohexa-2,5-dienone species and its structural characterization supports the above contention.

Full text of DOI:10.1016/j.ica.2015.06.026

Accepted Manuscript
Aerobic oxidation of 2,4,6-tri-tert-butylphenol to quinones catalyzed by cop-
per(II) complexes of an N-octylated bis-benzimidazolyl ligand
Anjana Yadav, Pavan Mathur
PII:
S0020-1693(15)00324-2
http://dx.doi.org/10.1016/j.ica.2015.06.026
ICA 16592
DOI:
Reference:
Inorganica Chimica Acta
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Accepted Date:
12 March 2015
25 June 2015
26 June 2015
Please cite this article as: A. Yadav, P. Mathur, Aerobic oxidation of 2,4,6-tri-tert-butylphenol to quinones catalyzed
by copper(II) complexes of an N-octylated bis-benzimidazolyl ligand, Inorganica Chimica Acta (2015), doi: http://
dx.doi.org/10.1016/j.ica.2015.06.026
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Aerobic oxidation of 2,4,6-tri-tert-butylphenol to quinones catalyzed by
copper(II) complexes of an N-octylated bis-benzimidazolyl ligand
Anjana Yadav and Pavan Mathur*
Department of Chemistry, University of Delhi, Delhi-110007, India
Abstract
Copper(II) complexes of a N-octylated bisbenzimidazolyl ligand are synthesized and
characterized. These complexes carry out the oxidative dealkylation of 2,4,6-tri-
tertbutylphenol (TTBP) using molecular oxygen to 2,6-ditertbutylbenzoquinone and 4,6-di-
tertbutylbenzoquinone. The oxidation proceeds via a phenoxyl radical species detected
spectrophotometrically, and by EPR. A reactive copper(II)- dioxygen species is involved that
carries out the oxidation reaction. A comparison of the rates of formation of the 4,6-di-tert-
butylbenzoquinone versus 2,6-di-tert-butylbenzoquinone suggests that alternative pathway
may exist for the formation of the para-quinone derivative. Isolation of an intermediate 4,4’-
peroxybis(2,4,6-tri-tert-butylcyclohexa-2,5-dienone species and its structural characterization
supports the above contention.
Keywords:
Homogeneous catalysis, kinetic study, oxidative dealkylation, 2,4,6-tri-tert-butylphenol,
phenoxyl radical, reactive copper(II)/O2 species.
Corresponding Author
Tel. +91 1127667725X1381; Fax: +91 1127666605
*E-mail: pavanmat@yahoo.co.in (P. Mathur)
1

1. Introduction
Oxidation of hindered phenols, has been explored intensively as these phenols form relatively
stable free radicals which play an important role in the inhibition of auto-oxidation of organic
substrates [1]. Oxidation of various substituted phenols by cobalt(II) complexes and kinetics
for the formation of di-tert-butyl-p-quinone are reported. The reaction has been found to be
first order in dioxygen, substrate and the cobalt complex; mononuclear dioxygen cobalt,
peroxydicobalt and phenolato cobalt (III) complexes have been proposed as active catalysts
[2-5]. Substituted biphenols, diphenoquinone and para-quinone have been reported as
oxidation products of hindered phenols using copper(I) and copper(II) complexes [6-9]. A
mechanism involving tert-butyl migration and a 6-hydroperoxy-2,4-cyclohexadienone
intermediate is also postulated, involving a copper(I) mediated oxidation [10]. A recent study
at low temperature employs a cupric-superoxo complex for the study of oxidation of
substituted di-tertbutylphenols and proposes a cupric superoxo-phenoxyl radical as
intermediate forming 2,6-di-tert-butyl-1,4-benzoquinone [11]. Another study proposed a
saturation kinetics in phenolic O-H oxidation using mononuclear Mn(III)-OH complex [12].
Earlier we have reported the oxidation of hindered phenol by utilizing Cu(II) and Mn(III)
complexes with bisbenzimidazolyldiamide ligand [13(a,c),14]. The present paper reports the
kinetics of the formation of the substituted ditert-butyl-1,4-benzoquinone and 1,2-
benzoquinone using molecular oxygen and monomeric copper(II) bis-benzimidazolyl
complexes. To the best of our information the paper reports for the first time detection of the
formation of phenoxyl radical by absorption spectrscopy under aerobic conditions, at room
temperature, using a copper(II) bis-benzimidazolyl complex along with the isolation of a
4,4’-peroxybis(2,4,6-tri-tert-butylcyclohexa-2,5-dienone species from oxidation reaction
solution.
2. Experimental
2.1 Materials
All solvents used were of HPLC grade. 2,4,6-tri-tert-butylphenol were obtained from Sigma-
Aldrich.
Authentic
compounds
2,6-di-tert-butylbenzoquinone
and
3,5-di-tert-
butylbenzoquinone were obtained from Alfa Aesar and were used as such.
2

2.2. Physical measurements
Elemental analysis copper(II) complex was obtained on VARIO EL III instrument from
USIC, University of Delhi, Delhi, India. Electronic spectra were recorded in HPLC-grade on
a Shimadzu 1601 spectrometer from 200-1000 nm at the Department of Chemistry,
University of Delhi, Delhi. IR spectra were recorded in the solid state as KBr pellets on a
Perkin-Elmer FTIR-2000 Spectrometer in the region of 400-4000 cm-1. 1H NMR were
obtained on a 400 MHz JEOL instrument at the Department of Chemistry, University of
Delhi, Delhi. Conductance values were measured on a digital conductivity bridge from
popular traders, India (Model: PT: 825) at Department of chemistry, University of Delhi.
Electrochemistry was studied on a BAS-CV 50W electrochemical analysis system at the
Department of Chemistry, University of Delhi, Delhi. Tetra butyl ammonium perchlorate
(0.1M) was used as a supporting electrolyte. A three electrode configuration composed of Pt-
disk working electrode (3.1 mm2 area), a Pt wire counter electrode, and an Ag/AgNO3
reference electrode was used for measurements. The reversible one electron Fc+/Fc
(ferrocenium/ferrocene) couple has an E1/2 of 108 mV vs. Ag/AgNO3. The EPR
measurements were made on X-Band ESR- varian, USA system, equipped with a E-112 ESR
spectrometer with X and Q band at IIT Bombay, India. Single- crystals of the complex
suitable for X-ray diffraction data were collected on an X’calibur single crystal X-ray
diffractometer having a CCD camera at the University of Delhi, Delhi, India.
3. Synthesis and analysis
3.1 Synthesis of ligand and its copper(II) complexes
The ligand [1-octyl-2-((2-((1-octyl-1H-benzo[d]imidazol-2-yl)methoxy)phenoxy)methyl)-
1H-benzo[d]imidazole] and its copper(II) complex [Cu(L)(NO3)2] were synthesized as
reported earlier[15].
3.2. Synthesis of [Cu(L)(Cl)2] complex
The chloride complex was prepared by a procedure similar to that reported for the preparation
of [Cu(L)(NO3)2] [15] except that CuCl2 was used instead of Cu(NO3)2 for complexation, the
brief procedure consisted of the following (scheme1):
A solution of Cu(Cl)2.2H2O (0.13 mmol) in methanol (1ml) was added to a solution of the
ligand (100 mg, 0.13 mmol) in methanol (5 mL). The solution turns green in color and after
3

10 minutes of stirring a greenish product is obtained. This was centrifuged and washed with
small amount of cold methanol and dried over P2O5. Yield : 80%. Anal. Found for Cu
C38H50N4O2Cl2.H2O (747.26) : Calcd: C, 61.0; H, 6.9; N, 7.4. Found: C, 60.9; H, 6.9; N,
7.7%
λMAX (nm),(DMF)[log ε] :272[4.51], 279[4.50], 288[4.38], 334[4.35], 700[1.96]
λMAX (nm),(MeOH)[log ε] :269[4.27], 276[4.35], 283[4.34], 365[2.84], 645[1.98]
λMAX (nm),(EtOH)[log ε] : 650[1.97]
λMAX (nm),(EtOH-H2O)[log ε] : 640[1.83]
IR(KBr pellets, cm-1): 3448 ν (O-H), 1478 ν (C=N-C=C-benzim), 758 ν (c=c benz)
Electronic Absorption data for [Cu(L)(NO3)2] complex
λMAX (nm),(MeOH)[log ε] :269[4.24], 276[4.33], 283[4.32], 350[2.58], 655[1.83].
λMAX (nm),(EtOH)[log ε] : 650[1.89]
λMAX (nm),(EtOH-H2O)[log ε] : 620[1.86]
Scheme I. Synthesis of ligand and Copper(II) complex
3.3. X-ray crystallography
Single crystal suitable for X-ray diffraction studies were grown by slow evaporation in
methanol. The intensity data were collected at 150(2) K temperature on Xcalibur CCD
diffractometer with graphite monochromatized Mo/Kα radiation. A total of 13965 reflections
were measured of which 3408 were unique and observed (I> 2σ (I)].The data were corrected
for Lorentz and polarization effects. Multi-scan absorption correction was applied. The
structure was solved by direct methods using SHELXS-97 [16(a)] and refined by full-matrix
4

least-squares refinement techniques on F2, using SHELXL-97 [16(a)]. All calculations were
done with the help of WinGX package of crystallographic programs [16(b)]. For the
molecular graphics, the program Diamond2 and Mercury were used. All non-hydrogen atoms
were refined anisotropically. All hydrogen atoms were fixed geometrically with Uiso values of
1.2 times the Uiso values of their respective carrier. Thermal ellipsoid plot (ORTEP) for
[Cu(L)(Cl)2] is shown in figure 1.Crystal and structure refinement data along with selected
geometrical parameters are summarized in table 1 and 2 respectively.
3.4. Oxidation of 2,4,6-tri-tert-butylphenol and isolation of reaction products
1mL of 2,4,6-tri-tert-butylphenol solution (85mg, 324 mM) in methanol was mixed with 2mL
of copper complex solution (6mg, 4 mM). The solution was exposed to a slow stream of
bubbled with molecular oxygen for twenty minutes and contents stoppered. The solution was
stirred at room temperature for 5 hrs. The formation of oxidation products was monitored by
TLC by using 100% toluene solvent system. A TLC of the reaction mixture on silica gel
gives two spots with 2,4-Dinitrophenylhydrazine. Two products were found at Rf =0.20(A)
and 0.69(B). These were isolated using preparative TLC. The yield of the products was 23%
(A) and 14% (B).
1H NMR spectrum of product A in CDCl3 (fig.S1) displays signals at δ(ppm): 1.20(s, 9H),
1.24(s, 9H), 6.19(d, 1H), 6.90(d,1H), confirming the formation of 4,6-di-tert-butyl-1,2-
benzoquinone [13(a,b)].
1H NMR spectrum of product B in CDCl3 (fig.S2) displays signals at δ(ppm): 1.26(s, 18H),
6.49(s, 2H), confirming the formation of 2,6-di-tert-butyl-1,4-benzoquinone [13(a,b)].
Product A shows UV bands at 255 nm (logε=3.05), 400 nm (logε=2.9), and 557 nm (logε=
1.48). Product B shows UV band at 255 nm (logε= 3.25), 318 nm (logε= 2.77), and 450 nm
(logε=1.65). These bands were confirmed by comparision with authentic samples fig. S3 and
our earlier work [13(a)].
4. Result and discussion
4.1 Crystal structure description
The ORTEP diagram of [Cu(L)(Cl)2] with atomic numbering scheme is shown in figure 1.
The complex crystallizes in triclinic P-1 space group. The copper atom in [Cu(L)(Cl)2] forms
a square planar coordination geometry. The ligand provides two coordination sites through
two imine nitrogens of two benzimidazole rings, N2 and N3 (Cu(1)-N(3)= 1.959(12)and
5

Cu(1)-N(3)= 1.949(13) Å), and two chloro ligand, Cl(1) and Cl(2) (Cu(1)-Cl(1)= 2.265(4)
and Cu(1)-Cl(2)= 2.297(5) Å). A highly disordered water molecule is also present in the
lattice which was refined only isotropically. Atoms of one of the octyl chain showed high
thermal parameters with reflects in larger shaped ellipsoids. Although many attempts had
been made to model the disorder but it could not be done due to insufficient electron density
in difference map due to high thermal motion of the octyl chain. Only some of the atoms
were refined anisotropically and remaining were refined isotropically and the refinement was
done with fixed carbon carbon distances and angles which stabilized the refinement to a
somehow reasonable model. Table.2 gives the selected bond lengths and bond angles.
4.2. Characterizations
4.2.1. Electronic spectroscopy. Since the kinetic studies were carried out in methanolic
solution thus the electronic spectra of the complexes were also recorded in methanol. Three
peaks in the range of 268-288 nm were observed for the complexes which are assigned to
intra ligand п-п* transition in C=N-C=C system of benzimidazol moiety. The Cu(II)
complexes exhibit a LMCT band in the region of 350 nm which shows slight solvent
dependence. A symmetrical but broad d-d band is also observed in region of 650 nm,
characteristic of distorted square planar geometry [17]. This band also shows weak solvent
dependence.
4.2.2. Cyclic voltammetry. Cyclic voltammograms of the copper(II) complexes were
recorded in methanolic and ethanolic solution with 0.1M TBAP as a supporting electrolyte. A
three electrode configuration composed of Pt-disk working electrode, a Pt wire counter
electrode, and an Ag/AgNO3 reference electrode was used for measurements. In these solvent
systems the complexes display a quasi-reversible redox wave in their cyclic voltammograms
due to the Cu(II)/Cu(I) reduction process. The change in solvent from methanol to ethanol
causes the E1/2 of [Cu(L)(NO3)2] complex to become relatively cathodic, while there is almost
no change for the corresponding [Cu(L)(Cl)2] complex (Table 3). The E1/2 values follows the
order NO3>Cl-.
4.2.3. EPR Data. EPR spectra of the copper (II) complex [Cu(L)(Cl)2] was recorded in DMF
at liquid nitrogen temperature (fig S4). The observed EPR parameters are: gII=2.28, g┴ =2.06,
2
2
AII = 148, and (gII/AII )*104 = 154. The spectra typically indicate a dx -
ground state
y
(gII>g┴>2.00232) with low hyperfine coupling constant and high (gII/AII ) ratio suggesting
deviation from planarity [18]. The EPR spectra of complex [Cu(L)(Cl)2] shows nitrogen
6

superhyperfine structure in the perpendicular region, arising from the interaction of the
copper(II) odd electron with nitrogen atom. A clear five line pattern is found in the
perpendicular region for the complex with coupling constant aN=14 3guass, indicating that
the imine nitrogen from the two benzimidazole are coordinated to Cu atom in the basal plane.
4.2.4. Molar conductance. The molar conductivity values were measured in HPLC grade
DMF. The values are found to be 110.0 Ω-1cm2M-1,91.1 Ω-1cm2M-1for [Cu(L)(NO3)2],
[Cu(L)(Cl)2], Implying partial dissociation of anion in DMF solution. Data shows that they
could be regarded as 1:1 electrolytes [19].
To confirm whether the Cu(II) complex in the final catalytic step contains chloride as a
ligand or not, a conductance measurement was carried out for the complex in the methanolic
medium. The value of molar conductance obtained is 30 Ω-1cm2M-1, this suggests much
lower anion dissociation in comparison to when the conductance was measured taken in
DMF. Further, the conductance measurement was also taken on the methanolic oxidation
reaction solution containing the Cu(II) complex, TTBP and molecular oxygen after one hour.
It was found that conductance due to Cu(II) complex (30 Ω-1cm2M-1) remains practically
unchanged after a 1 hr reaction. This confirms that the chloride anion remains in the
coordination sphere of Cu(II) during the entire oxidation cycle.
5. Kinetic Studies
Oxidation of 2,4,6-tri-tert-butylphenol was carried out at room temperature using the
copper(II) complexes [Cu(L)(Cl)2], and [CuL(NO3)2] in the presence of molecular oxygen
and monitored spectrophotometrically. The detailed procedure is as follows:
(a) Experimental procedure
Molecular oxygen was bubbled for ten minutes in a septum sealed 2 mL methanolic solution of
the copper complex (2.0 mM). 1 mL methanolic solution of the 2,4,6-tri-tertbutylphenol(TTBP)
(160.5mM) was transferred via air tight syringe to the septum sealed solution of Cu(II) complex
(catalyst). A 0.5 mL of this reaction mixture was syringed out and diluted to make 2.5 ml by
MeOH (final concentration of Cu(II) complex [Cu(L)(Cl)2] (0.26mM), substrate(10.7mM)). This
solution was placed in a 1 cm path length optical cell, sealed and placed in a spectrophotometer.
Spectra were recorded with time interval of five minutes in the range of 300-900 nm at room
temperature for a period of 60 min. The formation of oxidation products was followed by
observing the increase in intensity of new absorption bands in the region 300-500 nm.
7

Generation of bands at 312 nm and 400 nm (fig.2.) indicate the formation of di-tert–butyl-p-
quinone and di-tert-butyl-o-quinone respectively in the reaction mixture [13(a)].The assignment
of these bands to the respective quinones was matched with the spectra of authentic samples
(fig.S3). Simultaneously a new band grows in intensity at a 629 nm (fig.2(inset)), this is assigned
to the phenoxyl radical species [20] (Scheme II) and confirmed by a EPR experiment( fig.S5
and 3), wherein the growth of a new radical signal is observed at g~2.0005. Phenoxyl radical is
reported to absorb at 382 nm, 400 nm, and 634 nm [21] and another report shows that the
phenoxyl radical absorbs at 312 nm and 400 nm and 629 nm [22]. This complicates the situation
and thus it becomes imperative that the contribution of the phenoxyl radical is factored out from
observed data at 312 nm and 400 nm. This was done for each time interval by calculating the
concentration of phenoxyl radical using λ=629 nm and its extinction coefficient. The corrected
data for 312 nm and 400 nm was therefore used to obtain the concentration of the o/p-quinone
formed. The rate of oxidation for the 2,4,6-tri-tert-butylphenol to form the o-quinone and p-
quinone were obtained by plotting the concentration of di-tert-butyl-o-quinone(λ=400nm,
ε=851.13) and di-tert-butyl-p-quinone(312 nm, ε=575.43) formed versus time (fig.4a,b). The
above experiment was repeated in 2:1 ethanol: water medium with a catalyst: substrate ratio of
1:15 depicted in fig.5. Surprisingly there is a large enhancement in the rate of formation of the p-
quinone in the case of [Cu(L)(NO3)2] complex (fig.6). Rate data are collected in table 4.
It is found that the rate of formation of the oxidised products is larger for the [Cu(L)(Cl)2]
complex than [Cu(L)(NO3)2] complex in methanolic medium, while the order is reversed in
2:1 EtOH: H2O medium, thus indicating that solvent polarity and hydration of anions do play
a role in the oxidation reaction [23]. The rate of formation of di-tert-butyl p-quinone is found
to be nearly two to three fold higher than the rate of formation di-tert-butyl-o-quinone for
both the medium. Further the relative difference in rates is smaller for the o-quinone product
when comparing the two copper(II) complexes as catalysts; while the rates of formation of p-
quinone derivative have a larger spread. This indicates that there may be different pathways
for the formation of the p-quinone derivative.
(b) Formation of p-quinonoid dimer
Confirmation of this is found by obtaining single crystals of a peroxy bound p-quinonoid
dimer from the oxidation reaction solution. The reaction was carried out as described in
section (a). The reaction was monitored spectrally for an hour and after the generation of the
characteristic bands for the quinones (312 & 400 nm) the reaction was allowed to stand at
room temperature for 24 hrs. It has not been possible to ascertain its yield since only a few
8

crystals could be isolated from the reaction mixture after 24 hrs. This species is likely to form
by a direct interaction of O2 moiety with the phenoxyl radical (Scheme III, step-I);
subsequent decomposition and rearrangement would results in the formation of p-quinone
derivative (Scheme III), as the dimer is consumed only a minor quantity would be available
for isolation.
Crystals were obtained that were found to be suitable for X-ray diffraction analysis, an
ORTEP plot is shown in figS6. The structure of this quinonoid dimer is reported earlier [24],
however in the present work some parameters were found to be different from the earlier
XRD study. These are given below:
The compound crystallizes in orthorhombic space group P b c a. The C(1)-O(1) and C(22)-
O(4) distance were found to be 1.223(3)Å and 1.220(3)Å respectively. The C(19)-O(3) and
C(4)-O(2) were found to be 1.450(3)Å and 1.447(3)Å respectively. The O(2)-O(3) bond
distance was found to be 1.487(3)Å .
(c) Oxidation reaction under d4-MeOD
To check if dissociation of a proton from the tri-substituted phenol plays a role in the rate
determining step, the oxidation was carried out in deuterated methanol. The rates of oxidation
reaction were found 7.0*10-7 M min-1 and 18*10-7 M min-1 for [Cu(L)(Cl)2] and [Cu(L)(NO3)2]
in MeOD medium for the formation of o-quinone derivative; while the rates found for the p-
quinone derivative are 28*10-7 M min-1 and 52*10-7 M min-1 for [Cu(L)(Cl)2] and [Cu(L)(NO3)2]
in MeOD medium respectively. It is found that for o-quinone the ratio rate(MeOH)/rate(MeOD) is
8:1 for [Cu(L)(Cl)2], while it is 2.5:1 for [Cu(L)(NO3)2]. Similar effect is observed on the rate
of oxidation to form the p-quinone species. The above implies that a large KIE is operative
confirming that proton dissociation from phenol is important in the rate determining step
[18].
(d) Oxidation reaction under Oxygen Depleted Conditions( ODC)
The oxidation reaction was also monitored spectrophotometrically under oxygen depleted
condition (ODC) (details given in S1.6(a)). The rates are approximately eight times lower for
[Cu(L)(Cl)2]. A similar lowering in the rate of oxidation was observed for the [Cu(L)(NO3)2]
(Fig. S7 and S8) (table 4). This relative lowering of oxygen content causes an inhibition of
the rate of oxidation of tri-substituted phenol. This confirms the role of molecular oxygen in
the mechanistic pathway.
9

(e) Effect of tert-butanol on the oxidation reaction
We have carried out a new experiment wherein all experimental conditions were
maintained as in section (a), but tert-butanol (15µL) was added externally to the reaction
mixture. The ratio kept was 1:40:40 (catalyst : substrate: tert-butanol). It is found that the
rate of reaction in the presence of tert-butanol is two times lower when compared to the
reaction, where no tert-butanol is present (fig.S9) rates are given in table 4. This
retardation of the rate of reaction suggests the formation of tert-butanol in the oxidation
reaction rather than isobutylene (Scheme II/III). Further isobutylene is not expected to be
a final product in the present case as it has reported to be found as a byproduct when the
reaction is carried out at temperatures of 100 0C or above [25].
(f) Oxidation reaction in presence of copper(II) chloride salt
An experiment was carried out to check whether the tetradentate ligand is required for this
chemistry by replacing the Cu(II) complex with Cu(II) chloride (salt). Procedure similar to
that given in section (a) was adopted, but CuCl2.2H2O(salt) was used instead of Cu(II)
complex. It is found that rate of reaction is about four to five times lower for the formation of
p/o-quinones (Fig. S10). Rates are given in table 4. Thus Cu(II) cation with the tetradentate
ligand bound to it, is required in the reaction.
(g) Oxidation reaction in absence of copper(II) complex
Another blank experiment in absence of catalyst was also carried out. A solution of 2,4,6-tri-
tert-butylphenol in 3 mL MeOH (53.7mM ) was exposed to molecular oxygen for 10
minutes. A 0.5 mL of this reaction mixture was syringed out and diluted to make 2.5 mL by
MeOH in a quartz cuvette and sealed (final concentration of TTBP is 10.7mM). Absorption
spectra were recorded in the range of 200 to 900 nm at room temperature for a period of 30
minute. A very weak generation of the quinone products are observed in the UV- Visible
spectra suggesting that molecular oxygen on its own is hardly capable of initiating this
reaction, (fig.7(a-o) ). The rate of oxidation is found to be nearly 30 times and 20 times lower
for the formation of the ortho/ para quinone respectively. After 30 minutes copper(II)
complex 0.05 mL (8.0mM) (final concentration of Cu(II) complex is 0.15mM in 2.5 mL)
is syringed in to the above solution. It is found that there is an immediate rise in
intensity of band in the region of 600-700 nm and a slow shift of bands towards 600 nm
(fig.7 (inset)). This clearly suggests that as soon as Cu(II) complex is added to the
10

substrate there is immediate formation of phenoxyl radical, along with the formation of
the o/p-quinones. This suggests that copper in its +2 oxidation state is required for the
initiation of this oxidation reaction, possibly generating a reactive [copper(II)—O2]
species that carries out the oxidation and oxygenation of phenol (Scheme-II). For further
clarity, absorption intensity of pure copper (II) complex (0.15 mM) in 2.5 mL of MeOH is
also recorded (fig.7 inset curve(a)). It is clear that once the oxidation reaction is initiated the
phenoxyl radical generated completely masks the parent d-d band, as the phenoxyl radical
absorbs in the same region with a much higher extinction coefficient compared to the d-d
band of the copper (II) complex.
(h) Oxidation reactions under ascorbic acid
(i)To a solution of copper (II) complex (1.34mM) in 3 mL of methanol, showing the d-d
band, in the region of 650 nm, was added a solution ascorbic acid 26 µL(77mM) under
nitrogen so as to make a ratio of 1:0.5 [complex : ascorbic acid]. The solution immediately
changed from color light bluish to colourless. Visible range was again scanned and it was
found that the solution did not have a d-d band (fig.8 inset). This confirmed that Cu(II)
complex has been reduced to Cu(I) species. To this Cu(I) species formed under ODC , TTBP
(42.3mg) was added (final concentration of TTBP is 53.7 mM in 3 mL). 0.5 mL of this
solution was diluted to make 2.5 mL and scanned with time for a period of 30 minutes,
[complex: ascorbic acid: TTBP :: 1:0.5: 40]. It is found that under oxygen depleted
condition (ODC), the Cu(I) complex is nearly incapable of interacting with the substrate
as the solution only weakly absorbs at 400 nm and there is no growth of band intensity
at 312 nm, 400 nm and 629 nm (fig.8 a-h).
(ii)Molecular oxygen was bubbled (nearly 15 minutes) to the above solution (i) containing
Cu(I) species. Spectral scanning after exposure to O2 was continued for 40 minutes. This
clearly shows the formation of phenoxyl radical band at 629 nm(fig.8(inset)) and also
growth of bands in the region of 300-400 nm. Suggesting that once the Cu(I) is oxidized
to Cu(II) using mol. O2, the reactive Cu(II)/ O2 species interacts with the substrate. The
rate of reaction was found to be approximately two times lower for the formation of o/p-
quinone (fig 8 curve 3-13) (Rates are given in table 4).
(iii)Experiment (ii) was repeated but with an aim to pin point the changes taking place in the
d-d band region. The complex was reduced by ascorbic acid (fig.9 curve2) and 42.3mg TTBP
added followed by extended exposure with molecular O2 (fig.9 curve3). This solution was not
diluted and scanning was carried out in the 600 -700 nm (fig.9). The solution becomes
11

greenish at this point. It is found that upon exposer to O2 a d-d band in the region of 650
nm is formed confirming that reoxidation of Cu(I) complex regenerates back the parent
Cu(II) complex (fig.9 curve 3). Formation of a phenoxyl radical is again confirmed (fig.9
curve 4-8).
(i) Effect of acetate on the oxidation reaction
An experiment was also carried out to check the effect of acetate anion on the rate of
oxidation reaction. The experimental details were as given in procedure section (a), the ratio
of TTBP and the Cu(II) catalysts was kept at 40:1. Sodium acetate was dissolved in doubled
distilled water and 10µL (2 M) were added to the catalyst + substrate solution. The ratio of
catalyst: substrate: anion was 1:40:5.2; 0.5 ml of the reaction mixture was diluted with
methanol to make a 2.5 ml solution and this was studied spectrophotometrically. Plot of
concentration of o-quinone formed versus time in presence of sodium acetate for
complex:substrate(1:40) in methanol for different copper(II) complexes.
The spectrum show bands at λmax 312nm, 400 nm and 629 nm,(fig.10) however there is
significant increase in the rate of reaction in the presence of acetate anion. As the reaction
progresses the band at λmax at 629 nm starts to decay and another band start to appear at
around 557 nm which grows in intensity with time. The 557 nm band is assigned to a low
energy band of the 4,6-di-tert-butylbenzoquinone species [13(a)]. An isobestic point at λmax
592 nm observed. The changes for the [Cu(L)(Cl)2] complex are shown in fig 6. Similar
changes were found for the [Cu(L)(NO3)2] complex. The rate of formation of the oxidised
product is nearly 22 times faster in comparison to reactions in absence of acetate. The rates
were found to level off for the two catalyst and both the catalyst show a rate of 1137 10*10-7
M min-1 (fig.11). Higher rate of reaction for formation of 4,6-di-tert-butylbenzoquinone in the
presence of acetate anion can be associated with larger concentration of phenoxyl radical
formed.
A cyclic voltammogram of the copper(II) complex, under nitrogen in methanol shows a redox
couple with a E1/2 of +0.165 volts. However when acetate anion is added to the parent
copper(II) complex, it is found that the parent Cu(II)/Cu(I) redox couple shifts to a relatively
cathodic E1/2 of +0.100 volts (fig S11 ). This suggests that the added acetate stabilises the
copper(II) state making it relatively more difficult to reduce. Thus to explain the rate
accleration we envisage that the role of acetate anion is in facilitating the deprotonation of the
trisubstituted phenol. This deprotonated phenol is more susceptible to oxidation since in this
12

case a much larger concentration of the phenoxyl is observed (fig.10) causing the rate of
reaction to be 22 times higher.
(j) Substrate and catalyst dependence on the oxidation reaction
Substrate and catalyst dependence was also studied for the [Cu(L)(NO3)2] complex and rate
of reactions obtained. From the Ln plot, the slope is found to be 1.09 for p-quinone and 1.00
for o-quinone, suggesting that the reaction obeys first order kinetics [Details are given in
S1.6(e)]. Increasing the catalyst concentration of [Cu(L)(NO3)2] resulted in an enhanced
formation of the phenoxyl radical, while the phenoxyl radical formation was independent of
changes in 2,4,6-tri-tert-butylphenol concentration, fig.S13(d)and fig.S15(d), suggesting that
the generation of phenoxyl radical is dependent on the copper(II) complex(catalyst).
Conclusion
Thus in summary of the experiments under (e), (f), (g) and (h), it can be concluded that
neither copper(I) under limited oxygen is capable of initiating the reaction nor molecular
oxygen alone interacts with 2,4,6-tri-tert-butyl phenol to a significant extent. The reaction
works with copper in its +2 oxidation and is the key component possibly forming a [Cu(II)-
O2] reactive intermediate. This reactive intermediate oxidizes two phenol molecules to form
the phenoxyl radicals (confirmed by EPR), and a Cu(II) hydroperoxo reactive [Cu(II)—O2H
] species. Such a reactive species has been reported earlier [11]. The existence of a
copper(II)-hydroperoxo species in the present case is strengthened by the isolation of a
peroxydimer in the present study. The hydroperoxo species attacks at ortho and para
positions of the phenoxyl to form hydroperoxo intermediates [26], that dealkylate to give the
respective ortho/para quinone and tert-butanol.
13

Scheme II. Generation of phenoxyl radical and formation of 2,6-di-tert-butylbenzoquinone
and 4,6-di-tert-butylbenzoquinone in the presence of Cu(II) complex and molecular oxygen.
Scheme III. Formation of 2,6-di-tert-butylbenzoquinone via an alternate pathway of peroxy
bound p-quininoid dimer.
14

6. Summary
Copper(II) complexes of a N-octylated bisbenzimidazolyl ligand are synthesized and utilized
to carry out the Oxidative dealkylation of 2,4,6-tri-tertbutylphenol (TTBP) using molecular
oxygen.
Two products are formed via a phenoxyl radical species detected
spectrophotometrically and by EPR. The reaction is pseudo first order with respect to the
catalyst and substrate concentration. It is interesting to note that the rates of formation of the
p-quinone product are three fold higher when compared to the o-quinone product; further
single crystals of a peroxy bound p-quinonoid dimer were obtained from the oxidation
reaction solution strengthening our proposal of an alternate pathway for the formation of p-
quinone derivative.Effect of presence of externally added acetate anion increases the rate of
reaction by 22 times. Higher rate of reaction for formation in the presence of acetate anion
can be associated with larger concentration of phenoxyl radical formed.
Acknowledgement
We gratefully acknowledge financial support from the UGC for providing the JRF to one of
the author (A.Y) and University of Delhi, Delhi, India for special research grant. The authors
are highly thankful to Dr. Kuldeep Mahiya for helping in solving crystal structure. One of the
author (Pavan Mathur) gratefully acknowledges sabbatical leave from University of Delhi,
Delhi India.
Supporting Material
CCDC 1046210 contains the supporting crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge crystallographic data centre via
www.ccdc.cam.ac.uk/data_request.cif. EPR Spectroscopy, Detail Kinetic Studies and rate
plots, are given in supporting Information.
15

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16

E. I. Solomon, S. Fukuzumi, K. D. Karlin, J. Am. Chem. Soc. 136(2014) 9925-9937.
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[15]A. Yadav, P. Mathur, Inorg. Chim.Acta 427 (2015) 62-71.
[16](a) G.M. Sheldrick. SHELXS-97 and SHELXL-97, Program for Crystal Structure Solution and
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[17] G. Batra, P. Mathur, Inorg. Chem, 31 (1992) 1575.
[18] A. Yadav, P. Mathur, Catal. Commun 55(2014) 1-5.
[19] W.J. Geary, Coord. Chem. Rev. 7(1971) 81-22.
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1825.
17

Figures
Fig.1. ORTEP diagram of the copper(II) complex [C38H50Cl2CuN4O3] drawn in 30% thermal
probability ellipsoids showing atomic numbering scheme. Only selected atoms are shown as
ellipsoids for a better visualization of the structure.
18

1.5
1.0
0.5
0.0
Substrate- 2,4,6-tri-tert-butylphenol
Catalyst- [Cu(L)(Cl)2 ]
0.031
0.030
0.029
0.028
0.027
0.026
0
10 20 30 40 50
Wavelength(nm)
300
400
500
600
700
Wavelength(nm)
Fig.2. Time dependant UV-Visible spectral changes for the oxidation of 2,4,6-
tritertbutylphenol in methanolic solvent system catalysed by the copper(II) complex
[LCu(Cl)2] in dioxygen at room temperature. Inset: Showing growth of new band attributed
to the phenoxyl radical at 629nm.
Figure.3. EPR spectrum of catalyst : substrate (1:10)After 60 minutes in methanol at LNT..
Condition: microwave power 5 mW; microwave frequency 9.1GHz; field set 3000G, at X-
band, showing the growth of a new radical signal at g~ 2.0005.
19

Fig.4(a). Plot of concentration of 4,6-di-tert-butylbenzoquinone(λ=400nm) formed by the
oxidation of 2,4,6-tri-tert-butylphenol(TTBP) using the ratio copper(II) complex: TTBP
(1:40) in methanolic medium.
Fig.4(b). Plot of concentration of 2,6-di-tertbutylbenzoquinone(λ=312nm) formed by the
oxidation of 2,4,6-tri-tertbutylphenol(TTBP), using the ratio copper(II) complex: TTBP
(1:40) in methanolic medium.
20

1.0
0.5
0.0
Catalyst - [CuL(Cl) ]
2
EtOH: H O (2:1)
2
0.0260
0.0255
0.0250
0.0245
0.0240
0.0235
0.0230
0
10 20 30 40 50
Time(min)
300 350 400 450 500 550 600 650 700
Wavelength(nm)
Fig.5.Time dependant UV-Visible spectral changes for the oxidation of 2,4,6-
tritertbutylphenol in ethanol+water(2:1) solvent system catalysed by the copper(II) complex
[LCu(Cl)2] in dioxygen at room temperature. Inset: Showing growth of new band attributed
to the phenoxyl radical at 629nm.
Fig.6(a).Plot of concentration of 4,6-di-tert-butylbenzoquinone formed versus time using
Cu(II) complexes in 2:1 ethanol: water solvent at 400 nm.
21

Fig.6(b). Plot of concentration of 2,6-di-tert-butyl-benzoquinone formed versus time using Cu(II)
complexes in 2:1 ethanol: water solvent at 312 nm.
1.0
0.05
0.04
0.03
0.02
0.01
0.00
0.8
0.6
0.4
0.2
0.0
8
Cu(II) complex added
(a)
600
1
500
700
800
o
Wavelength(nm)
a
300
400
500
600
700
800
Wavelength(nm)
Fig. 7 Time dependant UV-Visible spectral changes for the oxidation of 2,4,6-tritertbutylphenol in
methanolic solvent system (i) a-o shows TTBP bubbled with molecular oxygen upto 30 minutes (ii)
(spectra1-8) addition of copper(II) complex [LCu(Cl)2] at room temperature. Inset: Showing growth
of new band attributed to the phenoxyl radical at 629nm.
22

Fig. 8. Time dependant UV-Visible spectral changes for the oxidation of 2,4,6-tritertbutylphenol in
methanolic solvent system using (i) curve a-h shows Cu(I) complex species + TTBP scan upto 30
minutes (ii) after 30 minutes molecular oxygen bubbled to Cu(I) containing solution (spectra 3-13) at
room temperature. Inset: Showing growth of new band attributed to the phenoxyl radical at 629nm.
3.0
0.3
2.5
2.0
1.5
1.0
8
0.2
0.5
7
0.0
400
500
600
700
800
6
Wavelength(nm)
5
0.1
0.0
4
3
1
2
400
500
600
700
800
900
Wavelength(nm)
Fig.9. curve (1) d-d band of [Cu(L)(Cl)2] complex (2) Vanishing of d-d band after addition
of 26µL ascorbic acid to the copper(II) complex (3) after passing molecular oxygen to
23

reaction mixture showing regeneration of d-d band at 650 nm. Curve(4-8) growth of band
that shifts to 629 nm (phenoxyl radical). Inset: UV-Visible spectra of reaction mixture
containing catalyst (1.34mM) and TTBP (53.7mM) in oxygen saturated methanol showing
growth of band at 400 nm.
Catalyst- [Cu(L)(Cl )]
2
2.5
0.14
0.12
2.0
0.10
0.08
1.5
0.06
0.04
1.0
0.02
0.00
0.5
500
600
700
800
Wavelength(nm)
0.0
300
400
500
600
700
800
Wavelength(nm)
Fig.10. Time dependant UV-Visible spectral changes for the oxidation of 2,4,6-
tritertbutylphenol in methanolic solution catalysed by the [LCu(Cl)2]in dioxygen at room
temperature, in presence of acetate anion.
Fig.11. Plot of concentration of 4,6-di-tert-butylbenzoquinone formed versus time in
presence of sodium acetate using different copper(II) complex: substrate(1:40) in methanolic
medium.
24

Table.1 Crystallographic Data and Structure Refinement of complex.
Copper (II) complex
Empirical formula
C38H50CuCl2N4O3
Formula weight
Temperature
745.26
150(2) K
0.71073
Triclinic
Wavelength/ Å
Crystal system
Space group
a/ Å
P-1
12.145
14.038
b/ Å
c/ Å
14.095
71.23o
α, deg
β , deg
γ , deg
69.62o
85.84o
3
2130.7
2
Volume/Å
Z
25

3
1.162
Dc/ Mg/m
µ/mm-1
0.674
F(000)
786
R(int)
0.3593
Crystal size/mm3
Reflections collected
Independent reflections
0.18 x 0.14 x 0.10
13965
3408
2
1.109
Goodness-of-fit on F
R indices R1 & wR2(all data) 0.3273, 0.3480
Largest diff. peak and hole/
-3
e.Å
0.815 and -1.117 e.Å-3
Table 2. Bond lengths [Å] and angles [°] for [Cu(L)(Cl)2 ]
N(2)-Cu(1) 1.949(13)
26

N(3)-Cu(1)
1.959(12)
2.265(4)
2.297(5)
1.538(18)
1.430(19)
Cl(1)-Cu(1)
Cl(2)-Cu(1)
C(31)-N(4)
C(8)-N(1)
C(15)-N(2)-Cu(1)
C(14)-N(2)-Cu(1)
C(24)-N(3)-Cu(1)
C(25)-N(3)-Cu(1)
131.3(9)
126.1(10)
129.7(12)
123.2(9)
Table.3.Cyclic voltammetry data of the complexes in methanol and ethanol solution.
Complexes
[Cu(L)(NO3)2]
(MeOH)
EA
EC
E1/2
IC/IA
0.80
+341
+109
+225
[Cu(L)(Cl)2]
(MeOH)
+275
+291
+280
+65
+99
+49
+170
0.70
0.72
0.70
[Cu(L)(NO3)2]
(EtOH)
+195
[Cu(L)(Cl)2]
(EtOH)
+164.5
• Error in E1/2 =
5
Table.4. Rate of reaction with the respective copper(II) complexes
27

S.no.
Complexes
Ratio
Rate of reaction[*10-7 M min-1]
For o-quinone(400 nm)
Rate of reaction[*10-7 M min-1]
for
(MeOH)(d4-MeOD)(ODC)
p-quinone(312 nm)
(MeOH)(d4-MeOD)(ODC)
1.
2.
3.
4.
1:40
1:40
1:40
(44)
(53)
(10)
(17.5)
(6.9)
(7.4)
(6.1)
(96)
(145)
(42)
(54)
(52.0)
(28.0)
(19.5)
(19.9)
[LCu(NO3)2]
[LCu(Cl)2]
CuCl2.2H2O
(salt)
[LCu(Cl)2]
1:40:40(t (27)
ert-
butanol)
5.
1:40:0.5( (20)
ascorbic
(85)
[LCu(Cl)2]
acid)
Oxidation of TTBP with Cu(II) complexes using 2:1 ethanol:water solvent system
6.
7.
1:15
1:15
54
28
252
135
[LCu(NO3)2]
[LCu(Cl)2]
28

Graphical abstracts
Aerobic oxidation of 2,4,6-tri-tert-butylphenol to quinones catalyzed by
copper(II) complexes of an N-octylated bis-benzimidazolyl ligand
Anjana Yadav and Pavan Mathur*
Department of Chemistry, University of Delhi, Delhi-110007, India
30