P-Quinoneimine as an intermediate in the oxidative coupling of 2-amino-5-methylphenol to 4a,7-dimethyldihydro-2-aminophenoxazinone catalyzed by a monomeric copper(II) complex

Article abstract of DOI:10.1016/j.catcom.2014.06.001

Copper(II) complex of a tetradentate N-picolatedbisbenzimidazolyl ligand is synthesized and characterized. XRD study reveals that copper(II) is in a distorted square plane comprising of two imine nitrogen, one oxygen atom from nitrate and another from a water molecule. This complex carries out the oxidative coupling of aminomethylphenol to phenoxazinone through an intermediate p-quinoneimine. The reaction is pseudo first order with respect to the catalyst and is self-inhibiting with increasing substrate concentration. Anion like acetate is found to increase the rate of reaction by nine times. EPR monitoring indicates the formation of a Cu(I) species which is the likely cause of inhibition of the catalytic reaction.

Full text of DOI:10.1016/j.catcom.2014.06.001

Catalysis Communications 55 (2014) 1–5
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
p-Quinoneimine as an intermediate in the oxidative coupling of
2
4
-amino-5-methylphenol to
a,7-dimethyldihydro-2-aminophenoxazinone catalyzed by a
monomeric copper(II) complex
Anjana Yadav, Pavan Mathur ⁎
Department of Chemistry, University of Delhi, Delhi 110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper(II) complex of a tetradentate N-picolatedbisbenzimidazolyl ligand is synthesized and characterized. XRD
study reveals that copper(II) is in a distorted square plane comprising of two imine nitrogen, one oxygen atom
from nitrate and another from a water molecule. This complex carries out the oxidative coupling of
aminomethylphenol to phenoxazinone through an intermediate p-quinoneimine. The reaction is pseudo first
order with respect to the catalyst and is self-inhibiting with increasing substrate concentration. Anion like acetate
is found to increase the rate of reaction by nine times. EPR monitoring indicates the formation of a Cu(I) species
which is the likely cause of inhibition of the catalytic reaction.
Received 6 March 2014
Received in revised form 2 June 2014
Accepted 3 June 2014
Available online 17 June 2014
Keywords:
Homogeneous catalysis
2-Amino-5-methylphenol
© 2014 Elsevier B.V. All rights reserved.
p-Quinoneimine
Phenoxazinone synthase
EPR
1
. Introduction
Copper and its metalloproteins are utilized in a variety of biological
intermediate species p-quinoneimine, and finally to 4a,7-
dimethyldihydro-2-aminophenoxazinone (Scheme 1). To the best
of our information this is the first report where a clear intermediate
has been detected spectrophotometrically and its decay kinetics
studied.
functions [1–3], including electron transfer, oxygen transport and sub-
strate oxidation [4–11]. Phenoxazinone synthase is a copper(II) con-
taining enzyme that catalyzes the oxidative coupling of two molecules
of aminophenol to phenoxazinone chromophore [12]. The oxidative
coupling reaction has been the center of various studies using transition
metal ion complexes as catalyst [13–21].
2. Experimental
Instrumentations and synthesis of ligand and its copper(II) complex
Kinetics of the oxidation of 2-aminophenol to 2-aminophenoxazine-
are provided in supplementary content (S2–S3).
3
-one shows a first order dependence on copper catalyst, dioxygen and
substrate concentration using CuCl(Phen) catalyzed reaction [21], while
another report shows saturation kinetics on the concentration of 2-
aminophenol and dioxygen pressure [22]. First order rate of reaction
at low concentration of 2-aminophenol and zero order at its higher con-
centration have also been reported [23]. A free radical pathway has been
proposed for this coupling reaction [24–27], and a 2-aminophenoxyl
radical has been found to be the key step in the activation process [28].
Earlier we have reported the oxidation of 2-aminophenol to
phenoxazinone using iron(III) and a copper(II) complex [29,30].
In continuation of our earlier studies the present work reports, a
copper(II) complex of a tetradentate bisbenzimidazolyl [L1] ligand,
and the catalytic oxidation of 2-amino-5-methylphenol, to an
2.1. Kinetics of oxidation of 2-amino-5-methylphenol (OAMPH)
The copper(II) complex [Cu(L
1 2 3 3
)(H O)(NO )]·NO was employed as
a catalyst for the oxidation of 2-amino-5-methylphenol in a mixed
methanol:acetonitrile solution, in the presence of molecular oxygen.
The experiment was followed spectrophotometrically. The detailed pro-
cedure is as follows:
Molecular oxygen was passed for 10 min in a septum sealed 2 ml
methanolic solution of the copper complex (2.15 mM). This was
then transferred to 2 ml acetonitrile solution of 2-amino-5-
methylphenol(14.33 mM). A 0.2 ml of this reaction mixture was
diluted to 2 ml with acetonitrile and was placed in a 1 cm path length
optical cell in a spectrophotometer. The final concentration of the copper
complex is 0.098 mM and 0.65 mM for 2-amino-5-methylphenol. The
ratio of copper(II) catalyst:2-amino-5-methylphenol is 1:6.6. Spectra
⁎
Corresponding author. Tel.: +91 1127667725x1381; fax: +91 1127666605.
E-mail address: pavanmat@yahoo.co.in (P. Mathur).
http://dx.doi.org/10.1016/j.catcom.2014.06.001
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

2
A. Yadav, P. Mathur / Catalysis Communications 55 (2014) 1–5
OH
NH
2
N
NH
2
N
Cu(II)Catalyst
NH
2
Oxidation
O2
-
O H
+
O
O
O
CH
3
2-Amino-5-methylphenol
p-quinoneimine
4a,7-Dimethyldihydro-2-
aminophenoxazinone
Scheme 1. Reaction for oxidation of 2-amino-5-methylphenol.
were recorded with time interval of five minute's in the range of 300–900
nm at room temperature for a period of 135 min and are depicted in Fig. 1.
A band at 520 nm appears and is assigned to the formation of p-
quinoneimine[(z)-2-amino-4-(2-hydroxy-4-methylphenylimino)-5-
methylcyclohexa-2,5-dienone] (dark red solution) [18]. This band decays
and could be observed clearly up to 100 min. Within 5–10 min of the ini-
tiation of the reaction another band starts to appear around 415 nm. The
solution slowly turns yellow after 25–30 min. The presence of the band
at a 415 nm confirms the formation of 4a,7-dimethyldihydro-2-
aminophenoxazinone (APX) [18]. This band increases in intensity with
time over a period of 135 min of serial scanning. An isobestic point is ob-
tained at approximately 470 nm, between the decaying band at 520 nm
and the generation of the new band at 415 nm. The concentration of
the phenoxazinone formed was obtained from absorbance at time t and
extinction coefficient (ε) [18]. The product of the reaction was isolated
using preparative TLC. The yield of the product was found to be 57.5%.
28 6 2 2 3 3 2 3
the complex [Cu(C34H N O )(H O)(NO )]·NO ·2H O·CH OH is
shown in Fig. 2. The complex crystallizes in a triclinic crystal system
with P-1 space group. The Cu(II) atom is coordinated by two imine ni-
trogen of two benzimidazole moieties, one oxygen atom of one of the
nitrate anion and one oxygen from the water molecule, affording a
square planar coordination geometry. Table. S2 gives the selected
bond lengths and bond angles. The ligand and its copper(II) complex
were characterized by electronic spectroscopy, IR and EPR spectral tech-
nique (details are provided in supplementary content (S6,S7)).
3.2. Kinetic studies
The oxidation reaction between 2-amino-5-methylphenol (AMPH)
and dioxygen in the presence of catalytic amount of [Cu(L1)(H2-
3 3
O)(NO )]·NO was performed at room temperature under the follow-
ing conditions:
1
IR and H NMR spectra confirm the formation of 4a,7-dimethyldihydro-
2
-aminophenoxazinone [detailed procedure in supplementary content
3.2.1. Catalyst variation
(
S13)]. This oxidation reaction is carried out in a mixed MeOH-MeCN
(i)The amount of copper(II) catalyst (0.032, 0.065, 0.081, 0.098,
0.114, 0.130 mM) was varied while keeping the amount of 2-amino-
5-methylphenol (substrate) fixed at 0.65 mM. A plot of concentration
of aminophenoxazinone formed (λ = 415 nm) against time, was ob-
tained. Experimental data points were best fit using linear fit software
available in origin 8. This is shown in Fig. S5 (a). The average rate of
the formation of aminophenoxazinone was calculated from the slope
of the above best fit plot and the rates are given in Table 1. Average
rate of reaction versus concentration of catalyst is shown in Fig. 3; this
depicts a linear increase in the rate of reaction, up to an optimum ratio
of substrate:complex (6.6:1). The logarithm of average rate of reaction
was plotted against the ln[catalyst] while keeping the concentration of
substrate constant. This plot is shown in Fig. S5(b) from where the
slope is found to be 0.75 suggesting a pseudo first order dependence
of the catalytic reaction in a limited range.
solution, as it was found that in pure MeOH, the p-quinoneimine
band(520 nm) decay was very quick, while its rate of decay was slower
in mixed solvent system. It was found that the addition of external hydro-
gen peroxide to the above reaction caused a lowering in the rate of reac-
tion suggesting that during the reaction there could be a buildup of
hydrogen peroxide [details in S14]. Although the p-quinoneimine is spec-
trally stable all efforts to isolate it failed as it rapidly oxidized to the yellow
dimethyldihydroaminophenoxazinone as has been reported earlier [18].
3
. Result and discussion
3
.1. Crystal structure description
The ORTEP diagram of the ligand bis(1-(pyridin-2-ylmethyl)-1,2-
(
ii) Fig. 1 also depicts the decay of the 520 nm band attributed to p-
bis(2-benzimidazolyloxamethyl)benzene is shown in Fig. S3. While
quinoneimine [18]. The decay reaction was also studied under similar
conditions as reported above.
Plot of decay of concentration of p-quinoneimine versus time is shown
in Fig. S5(c) and the rate of decomposition is tabulated in Table S3. The
plot of rate of decay of p-quinoneimine versus catalyst concentration is
shown in Fig. 3(inset). The behavior of decay of p-quinoneimine is parallel
to the formation of dimethyldihydroaminophenoxazinone, as is apparent
from a comparison in Fig. 3, confirming that p-quinoneimine is the
key intermediate in the conversion of 2-amino-5-methylphenol to
dimethyldihydroaminophenoxazinone.
3.2.2. Substrate variation
(
i) The amount of substrate (0.65, 0.86, 0.97, 1.3, 1.9 mM) was
varied while keeping the amount of catalyst fixed at 0.098 mM.
The plot of concentration of dimethyldihydroamino-
phenoxazinone formed against time is shown in Fig. S6(a) and
the rates are given in Table 2. The rate of oxidation reaction
with varying substrate concentration is given in Fig. 4. This
shows that the rate of formation reaction decreases as the con-
centration of the substrate increases at a fixed catalyst concen-
tration. An inverse saturation behavior is observed and may be
considered as a case of self-inhibition.
Fig. 1. Time dependant UV–visible spectral changes for the oxidation of 2-amino-5-
methylphenol in 1:19 MeOH:MeCN solvent system catalyzed by the copper(II) complex
in dioxygen at room temperature up to 135 min.

A. Yadav, P. Mathur / Catalysis Communications 55 (2014) 1–5
3
Fig 2. ORTEP diagram of the copper(II) complex drawn in 30% thermal probability ellipsoids showing atomic numbering scheme. Hydrogen atoms have been omitted for clarity.
(
ii) Fig. S6(b) shows the concentration versus time plot for the decay
of p-quinoneimine at 520 nm and the average decay rate is given
in Table. S4. The decomposition of p-quinoneimine with sub-
strate variation follows the same pattern as for the formation of
dimethyldihydroaminophenoxazinone [Fig. 4(inset)].
this, the rates of oxidation reaction were compared in pure MeOH and
MeOD medium. It is found that the rate of MeOH/rateMeOD is 3:1. Im-
plying that a KIE is operative (Figs. S11 and S12), thus confirming the
above details.
To understand the effect of solvent and added acetate anion on the
blue shift of the product band, the oxidation reaction was carried out
in 1:19 MeOH:MeCN medium as reported in the experimental section
and the product band is found at λmax 415 nm shown in Fig. 1. While
when the oxidation reaction was carried out in pure methanolic medi-
um in the absence of acetate anion it was found that the product band
blue shifts to λmax 400 nm shown in Fig. S11. The solvent dependence
is understood in terms of H-bonding of hydroxylic solvents causing
blue shifts and vice versa [31,32]. When the oxidation reaction is carried
out in the presence of acetate anion in 1:19 MeOH:MeCN solvent medi-
um the product band blue shifts back at λmax 400 nm as shown in
Fig. S10. This suggests that the addition of acetate anion, to a mainly
non-hydroxylic solvent system restores H-bonding of the product caus-
ing blue shift. The acetate anion facilitates deprotonation and rate en-
hancement and also restores H-bonding of the formed product.
The scope of the reaction has been tested on two more substrate's
e.g. o-phenylenediamine and 2-amino-4-tertbutylphenol. (Detailed
procedure and rate of formation of product are given in SI).
3
.3. Effect of the presence of anions
An experiment was carried out to check the effect of anions like ac-
etate 2 M (10 μl, 5 μl), oxalate 0.02 M(10 μl), and phosphate
.02 M(10 μl). The procedure adopted was similar to that reported
under experimental section the anions were added as sodium salts.
For this experiment the ratio of 2-amino-5-methylphenol and the cata-
3 2 3
lyst [Cu(L)NO ·H O]·NO was kept at 10:1. Different anions (10 μl)
were added to the catalyst; 0.2 ml of this reaction mixture was diluted
with acetonitrile to make a 2 ml solution (MeOH:MeCN, 1:19 solvent
system) and was studied spectrophotometrically.
0
The spectra show that there is in general a blue shift in the 415 nm
3
−
2−
−
and 520 nm bands upon the addition of PO
4
, C
2 4
O and OAc anions.
3
−
2−
While PO
4
and C O anions tend to retard the reaction, the rate of re-
2 4
action is found to be nine times higher when the reaction is carried out
in the presence of acetate anion; with marked spectral changes (details
are provided in supplementary content (S15)). This suggests that de-
protonation may be an important step in the catalytic cycle. To confirm
Table. 1
Rate of oxidation of 2-amino-5-methylphenol with varying catalyst concentration.
−
5
S. No.
Concentration(mM)
substrate:catalyst)
Ratio
(substrate:catalyst)
Rate of reaction 10
−
1
(
(mM min
)
1
2
3
4
5
6
.
.
.
.
.
.
0.65:0.032
0.65:0.065
0.65:0.082
0.65:0.098
0.65:0.114
0.65:0.130
6.6: 0.32
6.6: 0.66
6.6:0.83
6.6:1.0
6.6:1.16
6.6:1.30
9.4
14.1
18.3
21.48
16.25
14.09
Fig. 3. Plot of rate of formation reaction for the oxidation of 2-amino-5-methylphenol vs.
the concentration of catalyst at fixed substrate concentration. Inset: Rate of decomposition
reaction for the intermediate p-quinoneimine (λ = 520 nm).

4
A. Yadav, P. Mathur / Catalysis Communications 55 (2014) 1–5
Table. 2
diminishing the g signal intensity. It is apparent from Fig. S13 that the
larger the concentration of substrate the greater the formation of
copper(I) species, or loss of copper(II) species. It is also likely that the
fraction of copper(I) species converting back to copper(II) may be low,
causing lowered availability of the active copper(II)species for the cata-
lytic reaction and thus resulting in the diminishing of the rate of reac-
tion, with increasing substrate concentration. To confirm that Cu(I) is
the catalytically inactive species an experiment where in Cu(II) catalyst
was initially reduced by adding an excess of quinol was done (Fig. S15).
This reduced Cu(I) species is then utilized for the oxidation of 2-amino-
Rate of oxidation of 2-amino-5-methylphenol with varying concentration of substrate.
S. No.
Concentration(mM)
catalyst:substrate)
Catalyst:
substrate
Rate of reaction
10 (mM min−1)
−5
(
1
2
3
4
5
.
.
.
.
.
0.098:0.65
0.098:0.86
0.098:0.97
0.098:1.3
0.098:1.9
1:6.6
1:8.7
1:9.8
1:13.2
1:19.3
21.4
17.5
13.7
12.5
12.0
5
-methylphenol. The experimental figure and the rate of reaction are
3
.4. EPR monitoring of the oxidation reaction with varying substrate
given in SI. It is clearly seen that there is hardly any formation of
dimethyldihydro-2-aminophenoxazinone [Fig. S15 (a)]. This clearly
suggests that our catalyst in +1 oxidation state at best is only weakly
capable of carrying out the oxidation of 2-amino-5-methylphenol.
concentration
An EPR titration experiment was carried out by taking a fixed
amount of catalyst 0.0028 mmol and varying the amount of substrate
and keeping the volume of solution same in all cases (5 ml). It was
found that a mixture of the copper(II) catalyst plus four fold excess of
4. Conclusion
2
g
-amino-5-methylphenol results in an EPR spectrum where the fourth
II hyperfine line becomes part of the g line while gII and AII values
We have synthesized a new copper(II) complex with a tetradentate
N-picolyl-bisbenzimidazolyl ligand. The copper(II) ion forms a square
planar complex and is capable of oxidizing 2-amino-5-methylphenol
to 4a,7-dimethyldihydro-2-aminophenoxazinone. This reaction is mon-
itored spectrophotometrically and a clear p-quinoneimine intermediate
is also observed, that decays to form phenoxazinone. The rate of decom-
position reaction of p-quinoneimine is much faster than the rate of for-
mation of phenoxazinone. The effect of the presence of externally added
anions shows that in the presence of acetate anion the rate of reaction is
nine times higher, while phosphate and oxalate anion tend to inhibit the
rate of reaction. The reaction also forms a Cu(I) species, as found from
EPR studies. This is not reoxidized reversibly back to copper(II) and
causes the inhibition of the catalytic reaction.
change to 2.21 and 181 G in comparison to 2.37 and 161.6 G found for
the parent copper (II) catalyst in the absence of substrate [Fig. S13].
The gII/AII ratio is an index of distortion of the square plane, for a tetrag-
onal copper (II) complex. In the present case this changes to 122 from
1
46.5, indicating that the catalyst changes to a relatively more planar
copper(II) species, during the interaction with the substrate. It is inter-
esting to note that upon further increase in substrate concentration to
six fold, eight fold there is a minor change in the gII and AII values for
the intermediate copper(II) species. However the relative intensity of
the overall spectrum drops. The g peak intensity drops by three times
as the substrate concentration changes from four fold to eight fold rela-
tive to copper(II) complex [Fig. S13]. During the reaction there is no pre-
cipitation of a Cu(II) species, ruling out the possibility of a formation of
less soluble Cu(II) species during reaction. The drop in overall intensity
of the g peak is not due to the formation of strongly antiferromagneti-
cally coupled polynuclear Cu(II) species, since such a species would
still give a d–d band in the visible spectrum [27,33,34]. In contrast it is
found that the d–d band of the present catalyst nearly vanishes upon
the addition of a substrate (Fig. S14). This suggests that the active
copper(II) complex, changes its redox state to copper(I), thereby
Acknowledgment
We gratefully acknowledge financial support from the UGC (17-06/
2012(i)EU-V) for providing the JRF to one of the author (A.Y.) and
University of Delhi, Delhi, India (DRCH/R&D/2013-14/4155) for a
special research grant. The authors are highly thankful to Dr. Kuldeep
Mahiya for helping in solving the crystal structure.
Fig. 4. Plot of rate of formation reaction for the oxidation of 2-amino-5-methylphenol vs. the concentration of substrate, at fixed catalyst concentration. Inset: Rate of decomposition re-
action, for the intermediate p-quinoneimine (λ = 520 nm).

A. Yadav, P. Mathur / Catalysis Communications 55 (2014) 1–5
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