Copper(II) complexes as catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine

Article abstract of DOI:10.1016/j.saa.2015.01.115

Two new mononuclear copper(II) complexes [Cu (L) (NO₃)₂] (1) and [Cu (L) Br₂] (2) where (L = bis(1-(pyridin-2-ylmethyl)-benzimidazol-2-ylmethyl)ether) are synthesized and characterized by single-crystal X-ray diffraction analysis, elemental analysis, UV-Visible, IR spectroscopy, EPR and cyclic voltammetry. The complexes exhibit different coordination structures; the E_1/2 value of the complex (1) is found to be relatively more cathodic than that of complex (2). X-band EPR spectra at low temperature in DMF supports a tetragonally distorted complex (1) while complex (2) shows three different g values suggesting a rhombic geometry. These complexes were utilized as a catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine assisted by molecular oxygen. The initial rate of reaction is dependent on the concentration of Cu(II) complex as well as substrate, and was found to be higher for the nitrate bound complex, while presence of acetate anion acts as a mild inhibitor of the reaction, as it is likely to pick up protons generated during the course of reaction. The inhibition suggests that the generated protons are further required in another important catalytic step.

Full text of DOI:10.1016/j.saa.2015.01.115

Accepted Manuscript
Copper(II) Complexes as Catalyst for the Aerobic oxidation of o-phenylenedi-
amine to 2,3-diaminophenazine
Raghvi Khattar, Anjana Yadav, Pavan Mathur
PII:
S1386-1425(15)00136-5
http://dx.doi.org/10.1016/j.saa.2015.01.115
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Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
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21 January 2015
30 January 2015
Please cite this article as: R. Khattar, A. Yadav, P. Mathur, Copper(II) Complexes as Catalyst for the Aerobic
oxidation of o-phenylenediamine to 2,3-diaminophenazine, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.01.115
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Copper(II) Complexes as Catalyst for the Aerobic oxidation of o-
phenylenediamine to 2,3-diaminophenazine
Raghvi Khattar, Anjana Yadav and Pavan Mathur*
Department of Chemistry, University of Delhi, Delhi, India
Abstract
Two new mononuclear Copper(II) complexes [Cu (L) (NO3)2] (1) and [Cu (L) Br2](2) where
(L=bis(1-(pyridin-2-ylmethyl)-benzimidazol-2-ylmethyl)ether) are synthesized and
characterized by single-crystal X-ray diffraction analysis, elemental analysis, UV-Visible, IR
spectroscopy, EPR and Cyclic voltammetry. The complexes exhibit different coordination
structures; The E1/2 value of the complex (1) is found to be relatively more cathodic than that
of complex (2). X-Band EPR spectra at low temperature in DMF supports a tetragonally
distorted complex (1) while complex (2) shows three different g values suggesting a rhombic
geometry. These complexes were utilized as a catalyst for the aerobic oxidation of o-
phenylenediamine to 2,3-diaminophenazine assisted by molecular oxygen. The initial rate of
reaction is dependent on the concentration of Cu(II) complex as well as substrate, and was
found to be higher for the nitrate bound complex, while presence of acetate anion acts as a
mild inhibitor of the reaction, as it is likely to pick up protons generated during the course of
reaction. The inhibition suggests that the generated protons are further required in another
important catalytic step.
Keywords: Bis-benzimidazoles, Copper(II) complex, aerobic oxidation, o-phenylenediamine,
2,3-diaminophenazine.
Corresponding Author: Tel.No:91-1127667725(1381), Fax: 91-1127666605
E-mail address: pavanmat@yahoo.co.in (Pavan Mathur)
1

1. Introduction
Aerobic oxidation of aromatic amines catalyzed by copper(II) metal ion complexes is important
as it develops an understanding into the mechanism involved in copper oxygenase/oxidases, as
well as for synthetic applications. The oxidation of chromogenic o-phenylenediamine has been
employed for assaying the activity of multi-copper containing oxidase (Laccase) [1] and also
horseradish peroxidase [2].
Earlier oxidation of o-phenylenediamine has been reported to utilize molecular oxygen in the
form of µ-peroxo-cobalt(III) and oxocobalt(IV) complexes [3] while cobaloxime(II) complexes
have been shown to oxidize the diamine to 2,2-disubstituted-2H-benzimidazoles[4]. A bis-
imidazole copper(II) complex is reported to oxidize the diamine to 2,3-diaminophenazine via two
one electron steps [5], while Co(III), Mn(III) and Cu(II) oxalate complexes are reported to
oxidize the phenylenediamine via an inner sphere mechanism [6]. Oxidation of 2-aminophenol a
close analougue of o-phenylenediamine to form 2-aminophenoxazin-3-one are also reported [7].
Benzimidazole based ligands have been extensively used for forming copper(II) complexes
as they mimic the imidazole functionality found in native systems. Due to their bulky nature
they cause steric crowding, generate unusual coordination geometries and affect redox
properties of metal ion bound to them [8-11]. The present paper reports the synthesis, spectral
and structural studies on two copper(II) complexes with N-picolyl substituted benzimidazolyl
ligand having two imines nitrogen moieties and one ether oxygen as donor groups. Since
reports of aerobic oxidation of o-phenylenediamine using copper(II) complexes are scarce,
therefore the present complexes are used to study the kinetics of aerobic oxidation of o-
phenylenediamine to 2,3-diaminophenazine. Earlier we have reported the oxidation of o-
phenylenediamine assisted by H2O2 using a Fe(III) bis-benzimidazolyl diamide complex [12].
2. Experimental
2.1.Analysis and physical measurements
Elemental analysis were obtained using Elemental Analyzer at USIC, University of Delhi,
Delhi. X-ray diffraction data were collected on an Oxford Diffraction Xcalibur CCD
diffractometer with graphite monochromated (Mo-Kα radiation, λ=0.71073 Aº), temperature
of 298(2) K at IIT Bombay. The electronic spectra were recorded in HPLC grade DMF on a
Shimadzu 1601 spectrophotometer in the region of 200-1100 nm. IR Spectra were recorded
2

in solid state as KBr Pellets on a Perkin Elmer FTIR-2000 spectrometer in the range 400-
4000 cm-1 in the department of chemistry, university of Delhi, Delhi. Cyclic Voltammetric
measurements were carried out using a BAS CV 50W electrochemical analysis system. The
cyclic voltammograms of the complexes were recorded in 3:2 DMSO: CH3CN with tetra-n-
butylammonium perchlorate (TBAP) 0.1M as supporting electrolyte. A three electrode
configuration composed of Pt working electrode 3.1 mm2 area, a Pt wire counter electrode,
and an Ag/AgNO3 electrode reference electrode were used for the measurement. The
reversible one electron Fc+/Fc couple in the above solvent system has an E1/2 of 59.5mV vs.
Ag/AgNO3 electrode. Solution state EPR spectra were recorded on an X-Band Bruker-
spectrometer and X-band E-112 ESR Spectrometer with a variable temperature liquid
Nitrogen cryostat at 120K at IIT-Kanpur, Kanpur, India and IIT- Bombay, India.
2.2.Materials
All chemicals diglycolic acid (Aldrich), benzene-1, 2-diamine, 2-(chloromethyl)
pyridine and spectroscopic solvents were obtained from commercial sources and used
without further purification. Bis (2-benzimidazolyl methyl) ether (DGB) was prepared
as described earlier [13].
2.2.1. Synthesis of ligand (L)
The ligand (L) bis(1-(pyridin-2-ylmethyl)-benzimidazol-2-ylmethyl)ether) was synthesized
by N-picolylation of bis(2-benzimidazolyl methyl) ether as described earlier [14]. The
ESI/M+1, M+2 peak is found to be at 461 and 462, respectively.
2.3.Preparation of Copper (II) complexes
2.3.1. Synthesis of complex [Cu (L) (NO3)2] (1)
A methanolic solution (10 mL) of Cu (NO3)2 .3H2O (52.4mg, 0.22 mmol) was added to a
methanolic solution (15 mL) of the ligand (100mg, 0.22 mmol). The resulting bluish colored
solution was stirred for 1 h. The reaction mixture was then concentrated on a rotatory
evaporator to 2 mL. A sky blue colored product separated on cooling. The product was
centrifuged and washed with small amounts of cold methanol. The product obtained was then
recrystallized from water: methanol (1: 2) mixture, bluish colored compound crystallized on
cooling and was dried over P2O5. Yield: 42%, Anal. found for C28H24N8O7Cu (648.09): C,
3

51.4; H, 3.8; N, 17.1%. Calcd: C, 51.8; H, 3.7; N, 17.3%. UV/Vis spectrum (DMF) λmax, nm
(log ε, M-1 cm-1) = 269 (4.48), 274 (4.48), 281 (4.43), 722 (1.90). IR (KBr Pellets, cm-1):
ν= 1594 (C=N benzimidazole and pyridine), 1458(C=N-C=C (benzimidazole)), 754 (C=C
benzene), 1312 (C-H benzene),1002, 1312(O-N-O (sym)) and1478 (O-N-O(asym)).
2.3.2. Synthesis of complex [Cu (L) Br2] (2)
The ligand (100mg, 0.22mmol) was dissolved in methanol (15 mL). A methanolic solution of CuBr2
(48.5mg, 0.22 mmols) (10 mL) was added to the ligand solution. A yellow colored solution was
formed and was stirred for half hrs and the product was obtained upon reducing the volume to nearly
half. The product so obtained was then recrystallized from water: methanol (1: 2) mixture, yellow
colored compound crystallized on cooling and was dried over P2O5. Yield: 70%, Anal. found for
C28H24N6OBr2Cu (683.89): C, 49.4; H, 3.6; N, 11.9%. Calcd: C, 49.1; H, 3.5; N, 12.2%.
UV/Vis spectrum (DMF) λmax, nm (log ε, M-1 cm-1) = 269 (4.26), 273 (4.24), 280 (4.20),
917(2.40) IR (KBr Pellets, cm-1): ν= 1595 (C=N benzimidazole and pyridine), 1478(C=N-
C=C (benzimidazole)), 751 (C=C benzene), 1323 (C-H benzene).
3. Results and Discussion
Characterization of Copper(II) complexes
3.1. Crystal structure description of [Cu (L) (NO3)2] (1)
The complex 20mg was dissolved in 25 ml in HPLC grade methanol. Slow evaporation at
room temperature, resulted in the formation of thin plates of bluish crystal. The ORTEP
diagram and atom numbering scheme is shown in (Fig.1). Crystal data collection, refinement
parameters and selected bond lengths and bond angles are given in (Table S1 and Table S2).
A total of 9919 reflections were measured, of which 4831were unique and 2145 were
considered as observed (I > 2σ (I)]. It crystallizes in the Triclinic with space group P-1.
Cu(II) ion is coordinated in a distorted octahedral environment by two N atoms of
benzimidazole (N1, N3), two nitrate groups and O1 atom of ether linkage in the ligand. One
nitrate group is bidentate, O5 and O7 are involved in coordination and the other is
monodentate, O2 is coordinated to Copper(II). The Cu-N bond distances of 1.959(4) Å (Cu–
N1) and 1.974(5) Å (Cu–N3) are in the range found for similar benzimidazole ligated
compounds [15-18]. (Cu –O5), (Cu –O2) and (Cu –O1) bond distances are 1.981(5), 1.980(4)
and 2.342(4) Å. Bond angles C(7)-N(1)- Cu(1) ,C(10)-N(3)-Cu(1), N(1)-Cu(1)-N(3) and
N(1)-Cu(1)-O(1) are 117.9(4) º,118.1(4) º, 150.47(18)º and 75.71(15)º respectively. These are
4

comparable to bond angle found with similar type of ligated complexes, as in [Cu(OBB)2]-
picrate-DMF [18] and [Cu(Meobb)2](NO3)2.2CH3OH [15].
3.2. Crystal structure description of [Cu (L) Br2] (2)
The complex 20 mg was dissolved in warm 25 mL HPLC grade methanol. Slow
evaporation at room temperature, resulted in the formation of orange crystal. The ORTEP
diagram and atom numbering scheme is shown in (Fig.2). Crystal data collection,
refinement parameters and selected bond lengths and bond angles are given in (Table S1 and
Table S3). A total of 25414 reflections were measured, of which 4978 were unique and
3098 were considered as observed (I > 2σ (I)]. It crystallizes in the Monoclinic with space
group P 21/c. Cu(II) ion is penta-coordinated by two N atoms of benzimidazole (N1, N4) ,
two bromide groups and O1 atom of ether linkage in the ligand. The Cu-N bond distances
of 1.953(6) Å (Cu–N1) and 1.947(6) Å (Cu–N4) are in the range found for similar
benzimidazole ligated compounds [16(a)]. (Cu –O1) bond distance is 2.253(5) Å. Bond
angles C(7)-N(1)- Cu(1) and C(10)-N(4)-Cu(1) are 118.9(5)º and 118.7(5)º.
Present complex has τ = 0.359, where α is the angle between N1-Cu-N4 (151.2(3) º) and β
is the angle between O1-Cu-Br2 (129.64(14) º) indicating a highly distorted square
pyramidal geometry [19]. The equatorial bond angles deviates from the expected value of
120º (O1-Cu (1) - Br1 = 116.00(14) º, Br1-Cu (1) - Br2 =114.36(5) º and O1-Cu-Br2 =
129.64(14) º. The deviation in equatorial bond angles suggests remarkable asymmetry
within the trigonal plane. Sum of the equatorial bond angles is 360º indicating that Cu
centre mainly resides in the equatorial plane.
3.3.Electronic Spectroscopy
Electronic spectra of the Cu(II) complexes were recorded in HPLC grade DMF. Three peaks
in the range 265–285 nm are observed in the complexes and are assigned to intraligand π–π*
transition of benzimidazole and pyridyl moiety present in the ligating system [20]. The [Cu
(L) (NO3)2] complex has a d-d band at λmax 722 nm. This falls in the range of tetragonally
distorted six coordinate complexes [21] while the [Cu (L) Br2] complex has a d-d band with
λmax917nm, the relatively low energy of this band is in conformity with five coordinate
copper(II) complexes.
5

3.4. IR spectral studies
Strong bands at 1616 and 1463 cm-1 for free ligand are assignable to the pyridyl (C=N) and
benzimidazole (C=N-C=C) stretching mode [22]. These are shifted upon complexation by 5-
15 cm-1 indicating the binding of imine nitrogen to Cu(II) as found for other metal complexes
with benzimidazoles. In the nitrate complex bands appear at 1594 and 1458 cm-1 and in
bromide complex bands appear at 1595 and 1478 cm-1. In the nitrate complex (1) three bands
appear at 1002, 1312 and 1478 cm-1 the separation of two highest peak is around 165 cm-1
,
[21] indicating bidentate nitrate bound to copper(II). Presence of benzene ring is confirmed by
a very strong peak in the region 750-755cm-1.
3.5.Cyclic voltammetry
The complex [Cu (L) (NO3)2](1) and [Cu (L) Br2](2)in (3:2) DMSO : MeCN displays a quasi
reversible redox waves due to the Cu(II)/Cu(I) reduction process with E1/2 value at -193.8 and
-124.0 mV, while the value of IPc/IPa ratio found to be 0.85 and 1.0 respectively (Fig.S1 and
Fig.S2). The present E1/2 values of these complexes are relatively negative when compared to
other benzimidazole bound Cu(II) complexes [23]. E1/2 value of nitrate bound complex (1) is
more cathodic than the bromide bound complex (2), this indicates that the Cu(II) state is
stabilized due to a six coordinate nitrate bound complex, relative to a bromide bound
complex. The E1/2 value with N-pyridyl substituted ligand(L) is much more cathodic than the
E1/2 values with un-substituted ligand (OBB) in [Cu(OBB)2](pic)2.4(DMF) [21(b)] and mixed
ligand complex [Cu(L1)(L2)](ClO4) .mEt2O.nH2O [22(a)] and [Cu(Meobb)2](NO3)2.2CH3OH
[20(a)].
3.6.EPR data
The X-band EPR spectra of the Copper(II) complexes have been recorded at liquid nitrogen
temperature in DMF, and EPR spectral parameters of the complex (1) are given in (Table
S4). Complex (1) shows four g|| components and a broadening of g┴ component (Fig.S3).
Such a broadening of the g┴ component is indicative of a lowered symmetry, and is reflected
in a small value of A|| leading to large g||/A|| [24]. Nitrogen super hyperfine (N-SHF) structure
is observed in the perpendicular region with a coupling constant of AN = 16 2G, typical of N-
SHF interaction [24(b)]. Whereas in complex (2) no four line hyperfine pattern is observed, and
EPR spectra have been analyzed as given by Kneubuhl et al [25]. This shows three different g
6

values implying a rhombic geometry (Fig.S4). The values g1, g2 and g3 are reported in (Table
S4).
4. Aerobic oxidation of o-phenylenediamine
4.1. General procedure:
(a) A 0.19 mM (2mL) methanolic solution of Cu(II) complexes [Cu(NO3)2L] or [CuBr2L]
was added to a methanolic solution 5.7 mM (2 mL) of o-phenylenediamine and
molecular oxygen was passed for 5min. The ratio of catalyst to substrate is 1:30. One
ml of this reaction mixture is diluted by one ml of methanol and the absorption
spectra of this solution were recorded with time in the range 300-1100 nm at room
temperature, for a period of 40 minutes. A new band at 425-430 nm increases which
confirms the formation of 2,3-diaminophenazine [12]. The concentration of product
formed was calculated using the reported extinction coefficient of 2,3-
diaminophenazine [12]. Absorbance versus wavelength plot is shown in fig.3(a). The
initial rates of reaction are tabulated in table.1 and table.3.
(b) A blank experiment was also carried out wherein a methanolic solution of o-
phenylenediamine (5.7mM) (4mL) was bubbled with molecular oxygen for 5 minutes.
One ml of this reaction mixture is diluted to make two ml with methanol and
absorption spectra were recorded in the range of 300-1100 nm at room temperature,
for a period of 60 min. A band at 425-430 nm was found to slowly increase during 60
minutes. Absorbance versus wavelength plot is shown in fig.3(b). The rate of aerial
oxidation in the absence of catalyst is found to be tenfold lower than in the presence
of catalyst, confirming the role of copper(II) complexes in the aerobic oxidation of o-
phenylenediamine.
(c) An experiment was carried out to check the effect of acetate ion on the rate of
formation of 2,3-diaminophenazine. For this experiment the concentration of o-
phenylenediamine and Cu(II) complex was kept at 0.19mM(2mL) and 5.7mM (2mL),
0.01 mL of 2M acetate anion was added so as to keep the ratio of catalyst: substrate:
anion as 1:30:10.5. The oxidation reaction in presence of acetate anion shows a
different behavior. Two new intermediate bands at 581 nm and 927 nm are generated
simultaneously which are found to decay. After 4-5 minutes a band at 430 nm starts to
appear and the two intermediate bands start to decrease in intensity. Absorption and
concentration versus time plots are shown in fig.4 and fig.5.
7

4.2. Kinetic studies
The kinetics of the oxidation reaction between o-phenylenediamine (OPD) in presence of
catalytic amount of copper(II) complexes was performed at room temperature under the
following conditions:
4.2.1. Substrate variation
The amount of substrate (0.96, 1.93, 3.8, 5.7 mM) was varied while keeping the amount of
catalyst fixed at 0.19mM at least two sets of data were taken. Plot of concentration of 2,3-
diaminophenazine(λmax ~ 430 nm) formed versus time is given in fig.6. The initial rates are
given in table 1. The rate of oxidation reaction with varying substrate concentration is plotted
in fig.S5.This shows that the initial rate of reaction increases as the concentration of the
substrate increases at a fixed catalyst concentration. The logarithm of initial rate of reaction
was plotted against the ln[substrate] while keeping the concentration of catalyst constant.
This plot is shown in Fig.7 from where the slope is found to be 0.62 suggesting a pseudo first
order dependence, on the substrate concentration.
4.2.2. Catalyst variation
The amount of copper(II) catalyst (0.09, 0.19, 0.28, 0.38 mM) was varied while keeping the
amount of o-phenylenediamine (substrate) fixed at 5.7 mM at least two sets of data were
taken. Plot of concentration of 2,3-diaminophenazine formed (λmax~430 nm) versus time is
given in fig.8. The initial rates are given in table 2. The initial rate of oxidation reaction with
varying catalyst concentration is given in fig.S6. This shows that the rate of reaction increases
as the concentration of the substrate increases at a fixed catalyst concentration, up to ratio of
substrate: complex (30:1). The logarithm of initial rate of reaction was plotted against the
ln[catalyst], while keeping the concentration of substrate constant. This plot is shown in Fig.9
from where the slope is found to be 0.61 suggesting a pseudo first order dependence on the
catalyst concentration.
The initial rate of reaction with [Cu(NO3)2(L)] complex is higher than that found for
[CuBr2(L)] complex (ratio of catalyst: substrate being 1:30); the relative lowering could be
-
due to a greater stability of the Cu(I) state for the Br- bound complex rather than the NO3
bound as suggested by their E1/2. This may make a reoxidation step in the catalytic cycle
relatively difficult.
8

The intermediate bands generated at λmax~581 nm and 930nm start to decay as soon as they
are formed in the presence of acetate anion, while the 2,3-diaminophenazine band starts to
grow (λmax~430 nm). An attempt was made to obtain the relative velocity of decay of the
intermediate bands and there were compared with the velocity of formation of 2,3-
diaminophenazine band, using the absorption versus time plots (fig.4). It is found that the
band at λmax~581 nm and 930 nm decays by almost three fold and tenfold slowly than the
growth of the band at λmax~430 nm. This suggests that species formed at λmax~581 nm and
930 nm may not have any role to play in the formation of phenazine. The drop in rate of
reaction in the presence of OAc- ions for both the complexes, suggests that protons generated
during the course of reaction are further required in another catalytic step, OAc- anions, pick
up these released H+ ions, causing their concentration to deplete and thus impeding an
intermediate step.
Based on above discussion a possible scheme of catalytic oxidation is given below:
The first step involves the deprotonation of o-phenylenediamine in the presence of copper(II)
catalyst, thus forming a weak coordination bond between deprotonated o-phenylenediamine
and copper(II) complex. The Cu(II) complex than oxidizing o-phenylenediamine by one
electron thereby reducing itself to Cu(I) species. The reduced catalyst Cu(I) is then reoxidised
by molecular oxygen and the generated superoxide picks up a released proton to give the
-.
hydro superoxide radical (HO2 ) and Cu(II). The hydro superoxide further oxidizes the amine
to o-quinonediimine and forming H2O2 (Catalytic step I). The presence of superoxide in the
reaction was confirmed by carrying the oxidation reaction in the presence of ascorbic acid,
which is known to be a superoxide quencher [26]. The final ratio were catalyst: substrate:
quencher :: 1:30:1. It was found that in the presence of ascorbic acid the rate of reaction
diminishes to nearly half the original value (table.1 fig. S7), confirming the role of
superoxide in the oxidation reaction(scheme I) .
The o-quinonediimine formed condenses with another molecule of o-phenylenediamine.
Cyclization and oxidation by two molecules of H2O2generated in the catalytic cycle are used
to further give the 2,3-diaminophenazine (Catalytic step II)
9

Catalytic step –I
NH2
NH2
HN
CuII
+
H2O2
.
HN
-
HO2
-H+
H2N
+
Cu(II)
HN
CuII
H2N
.
-
HO2
N
H
CuI
O2
H2N
HN
Catalytic step- II
H
N
NH
NH2
OPDA
N
NH2
+
-2H+
-2e-
H2O2
+
NH
2H2O
NH
NH2
NH
NH2
Cyclization
N
NH2
N
N
NH2
NH2
-2H+, -2e-
+
NH2
2H2O
+
H2O2
N
H
2,3-diaminophenazine
Scheme.1 Reaction scheme for the oxidation of o-phenylenediamine to 2,3-
diaminophenazine.
10

Conclusion
Two new mononuclear copper(II) complexes [Cu (L) (NO3)2] and [Cu (L) Br2] are
synthesized and characterized. These complexes carry out the aerobic oxidation of o-
phenylenediamine to 2,3-diaminophenazine. Rate of oxidation in the absence of the catalyst
shows a tenfold drop confirming the rate of Cu(II) complex as a catalyst in this reaction.
Kinetic study shows that the reaction follows first order with respect to catalyst and substrate.
Initial rate of reaction was found to be higher for nitrate bound complex. The presence of
acetate anion causes a mild inhibition of reaction, suggesting the role as a proton sponge in
the catalytic cycle. The catalytic oxidation proceeds via a copper(II)/ copper(I) cycle.
Acknowledgment
We gratefully acknowledge the financial support from the University of Delhi, Delhi, India
for the special grant. Anjana Yadav gratefully acknowledges JRF from UGC, India, and one
of the authors, Pavan Mathur gratefully acknowledges the grant of sabbatical leave from the
University of Delhi, Delhi, India.
Supplementary material
Crystallographic data for the structural analysis have been deposited with the Cambridge
Crystallographic Data Centre [Cu (L) (NO3)2] (1) and [Cu (L) Br2] (2) having CCDC
1010841 and 1010840 respectively. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif
supplementary material contain EPR, Cyclic voltammetry, Rate versus concentration plots,
tables for crystal structure and refinement, table of bond angles and bond lengths.
11

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Fig.1.ORTEP diagram of copper(II) complex(C28 H24 Cu N8 O7)(1) drawn in 20% thermal probability
ellipsoids showing atomic numbering scheme. Hydrogens are omitted for the sake of clarity.
13

Fig.2.ORTEP diagram of copper(II) complex(C28 H24 Br2 Cu N6 O)(2) drawn in 20% thermal probability
ellipsoids showing atomic numbering scheme. Hydrogens are omitted for the sake of clarity.
Fig.3. (a)Time dependant UV-Visible spectral changes for the oxidation of o-
phenylenediamine catalysed by the copper(II)complex[CuL(NO3)2] in dioxygen at room
14

temperature for 40 min. (b) Blank experiment for the oxidation of o-phenylenediamine in
dioxygen at room temperature in the absence of the catalyst for 60 min.
Fig.4.Time dependant UV-Visible spectral changes for the oxidation of o-phenylenediamine
catalysed by the copper(II)complex[CuL(NO3)2] in dioxygen at room in presence of acetate
anion.
Fig.5. Plot of formation of 2,3–diaminophenazine vs time while keeping:
(A) Substrate concentration [0.19mM], catalyst concentration [CuL(NO3)2][5.79mM]
and acetate anion[0.01ml of 2M] giving a ratio 1:30:10.5.
(B) Substrate concentration [0.18mM], catalyst concentration [Cu(L)(Br)2][5.49mM]
and acetate anion[0.007ml of 2M] giving a ratio 1:30:10.5.
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Fig.6. Plot(A-D): Formation of 2,3-diaminophenazine vs time while keeping catalyst
[Cu(L)(NO3)2] concentration fixed[0.19M] and varying the substrate concentration.
Plot(E): Formation of 2,3-diaminophenazine vs time while keeping
catalyst[Cu(L)(Br)2]
concentration [0.18M] and the substrate concentration [5.49M], giving a ratio of catalyst :
substrate as (1:30).
16

Fig.7. Plot of Ln[Initial Rate] of 2,3-diaminophenazine formed vs Ln[concentration of
substrate].
Fig.8. Plot of formation of 2,3-diaminophenazine vs time while keeping substrate
concentration fixed and varying the catalyst concentration.
17

Fig.9. Plot of Ln[Initial Rate] of 2,3-diaminophenazine formed vs Ln[concentration of
catalyst].
Table1. Rate of oxidation of o-phenylenediamine with varying concentration of substrate
S.No. Concentration
Concentration of
Ratio
Initial rate of reaction
of catalyst(mM) substrate(mM)
*10-5mM(initial rates)
1.
2.
3.
4.
5.
0.19
0.19
0.19
0.19
0.19
0.96
1.93
3.86
5.79
1:5
237
324
450
612
355
1:10
1:20
1:30
5.79 + 0.19mM (ascorbic 1:30:1
acid)
6.
7.
-
5.79
-
50.2
0.19
5.79+0.01ml of 2M, OAc- 1:30:10.5 510
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Table2. Rate of oxidation of orthophenylenediamine with varying concentration of catalyst
[Cu(NO3)2(L)]
S.No.
Concentration of
catalyst(mM)
Concentration of
substrate(mM)
Ratio
Initial rate of reaction
*10-5mM(initial rates)
1.
2.
3.
4.
5.
0.09
0.19
0.28
0.38
5.79
5.79
5.79
5.79
0.5:30
1:30
1.5:30
2:30
340
612
650
857
Table3. Rate of oxidation of o-phenylenediamine in presence of a catalyst [CuBr2L]
S.No.
Concentration of
catalyst(mM)
0.18
Concentration of
substrate(mM)
5.49
Ratio
1:30
Initial rate of reaction
*10-5mM(initial rates)
1.
2.
400
189
0.18
5.49+ 0.007ml of 2M 1:30:10.5
OAc-
19

GRAPHICAL ABSTRACT
Copper(II) Complexes as Catalyst for the Aerobic oxidation of o-
phenylenediamine to 2,3-diaminophenazine
Raghvi Khattar, Anjana Yadav and Pavan Mathur*
Department of Chemistry, University of Delhi, Delhi, India
Copper(II) complexes have been utilized as a catalyst for the aerobic oxidation of o-
phenylenediamine to 2,3-diaminophenazine. Rate of oxidation in the absence of the catalyst
shows a tenfold drop confirming the role of Cu(II) complex as a catalyst in this reaction.
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Highlights
ꢀ Copper(II) complexes of a bisbenzimidazolyl ligand are synthesised.
ꢀ Complexes catalyse the aerobic oxidation of o-phenylenediamine.
ꢀ Reaction follows first order kinetics with respect to substrate and catalyst.
ꢀ Rate of oxidation shows tenfold drop in absence of catalyst.
ꢀ Addition of acetate anion causes a mild inhibition of rate of reaction.
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