Catalytic aspects of a copper(II) complex: biological oxidase to oxygenase activity

Article abstract of DOI:10.1007/s12039-017-1379-y

Abstract: A coper(II) complex, [Cu(dpa) ₂(OAc)](ClO ₄) (1) [dpa =2 , 2 ^′-dipyridylamine; OAc = acetate], has been synthesized and crystallographically characterized. X-ray structure analysis revealed that this mononuclear Cu(II) complex crystallizes as a rare class of hexa coordination geometry named bicapped square pyramidal geometry with P2 ₁/ c space group. This copper complex displays excellent catalytic efficiency, k _cat/ K _M(h ^{- 1}) = 6.17 × 10 ⁵ towards the oxidative coupling of 2-aminophenol (2-AP) to aminophenoxazin-3-one. Further, upon stoichiometric addition of copper(II) complex to 3,5-DTBC in presence of molecular oxygen in ethanol medium, the copper complex affords predominantly extradiol cleavage products along with a small amount of benzoquinone and a trace amount of intradiol cleavage products at a rate, k _obs= 1.09 × 10 ^{- 3}min ^{- 1}, which provide substantial evidence for the oxygen activation mechanism. This paper presents a novel addition of a copper(II) complex having the potential to mimic the active site of phenoxazinone synthase and catechol dioxygenase enzymes with significant catalytic efficiency. Graphical Abstract : SYNOPSIS The mononuclear copper complex having unusual hexa coordination geometry exhibits significant catalytic efficiency, k _cat/ K _M(h ^{- 1}) = 6.17 × 10 ⁵ towards oxidation of 2-aminophenol which predominantly produced extradiol cleavage products at a rate, k _obs= 1.09 × 10 ^{- 3}min ^{- 1} upon addition of 3,5-DTBC in presence of molecular oxygen. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-017-1379-y

J. Chem. Sci.
© Indian Academy of Sciences
DOI 10.1007/s12039-017-1379-y
REGULAR ARTICLE
Catalytic aspects of a copper(II) complex: biological oxidase
to oxygenase activity
a
c
a,b,∗
BISWAJIT CHOWDHURY , MILAN MAJI and BHASKAR BISWAS
a
Department of Chemistry, Raghunathpur College, Purulia, West Bengal 723 133, India
Department of Chemistry, Surendranath College, 24/2 M G Road, Kolkata, West Bengal 700 009, India
Department of Chemistry, National Institute of Technology, Mahatma Gandhi Avenue, Durgapur, West Bengal
b
c
713 209, India
E-mail: mr.bbiswas@rediffmail.com
MS received 19 February 2017; revised 30 August 2017; accepted 7 September 2017
ꢀ
Abstract. A coper(II) complex, [Cu(dpa) (OAc)](ClO ) (1) [dpa =2, 2 -dipyridylamine; OAc = acetate], has
2
4
been synthesized and crystallographically characterized. X-ray structure analysis revealed that this mononuclear
Cu(II) complex crystallizes as a rare class of hexa coordination geometry named bicapped square pyramidal
−1
geometry with P21/c space group. This copper complex displays excellent catalytic efficiency, kcat/KM(h ) =
5
6
.17 × 10 towards the oxidative coupling of 2-aminophenol (2-AP) to aminophenoxazin-3-one. Further,
upon stoichiometric addition of copper(II) complex to 3,5-DTBC in presence of molecular oxygen in ethanol
medium, the copper complex affords predominantly extradiol cleavage products along with a small amount of
−3
−1
benzoquinone and a trace amount of intradiol cleavage products at a rate, kobs = 1.09 × 10 min , which
provide substantial evidence for the oxygen activation mechanism. This paper presents a novel addition of
a copper(II) complex having the potential to mimic the active site of phenoxazinone synthase and catechol
dioxygenase enzymes with significant catalytic efficiency.
Keywords. Copper(II); crystal structure; phenoxazinone synthase activity; catechol dioxygenase; bio-mimetic
chemistry.
1
. Introduction
these copper proteins is phenoxazinone synthase which
is a copper-containing oxidase that catalyzes the cou-
pling of 2-aminophenols to form the 2-aminophenoxaz-
inone chromophore. This reaction constitutes the final
stepinthebiosynthesisofthepotentantineoplasticagent
Over the past few decades, biochemical oxidation
reactions have significantly stimulated the coordination
chemists to design various metal complexes as struc-
tural and functional mimic of the active site of different
metalloproteins and metalloenzymes.1 Bio-inspired
catalysis mediated by different copper proteins and
enzymes include primarily dioxygen transport (hemo-
cyanin, Hc), aromatic ring oxidations (tyrosinase, Tyr,
catechol oxidase, and quercetin-2,3-dioxygenase,
the biogenesis of neurotransmitters and peptide hor-
mones(dopamine-β-monooxygenase, D β M, andpep-
tidylglycine α-amidating monooxygenase, PHM,
hydrogen peroxide generation (galactose and glyoxal
oxidases,
Fet3p,
26–29
actinomycin.
On the other hand, catechol dioxy-
–3
genases of intradiol class utilizes a non-heme iron(III)
cofactor in catalyzing the cleavage of the carbon–
carbon bond between the two catechol oxygens; and
the extradiol dioxygenases utilize a nonheme iron(II)
cofactor in catalyzing the cleavage of the carbon–carbon
4
5
6
7–9
30,31
bond adjacent to the catechol oxygens.
In the last
10
decade, Youngme et al., and Choudhury et al., produced
1
1,12
same Cu(II)-dipyridyl complex using different reac-
32
tion methodology. Here we introduce a copper(II)-
1
3–18
iron homeostasis (ceruloplasmin3 and
and methane oxidation (particulate methane
dipyridylamine complex [Cu(dpa) (OAc)](ClO ) (1)
2
4
1
9–21
ꢀ
[
dpa = 2, 2 -dipyridylamine; OAc = acetate], with
22–25
monoxygenase, pMMO).
A prominent member of
potential ability to mimic the functional sites of phenox-
−1
3
azinone synthase enzyme with k (h ) = 1.83 × 10
cat
*
For correspondence
and catechol dioxygenase enzyme with the
Electronic supplementary material: The online version of this article (doi:10.1007/s12039-017-1379-y) contains supplementary material,
which is available to authorized users.

Biswajit Chowdhury et al.
decomposition rate, 1.09 × 10−3 min , respectively. CHNmicroanalyser. Electrosprayionization(ESI)massspec-
−1
Coordinative unsaturation of the Cu(II) centre in ethanol trum was recorded on a Q-TOF MicroTM Mass Spectrometer.
The electrochemical studies were carried out using Cyclic
medium facilitates the formation of substrate-enzyme
voltammograms were recorded in CH3CN solutions contain-
adduct and account in favour of such oxidase to oxyge-
◦
ing 0.1 M TBAP at 25 C using a three-electrode configuration
nase activity.
(Pt working electrode, Pt counter electrode, Ag/AgCl refer-
ence) and a PC-controlled PAR model 273A electrochemistry
system. All the experimental solutions were degassed for 30
min with high-purity argon gas before any cyclic voltammetry
of a sample was done. The Electron Paramagnetic Resonance
2
. Experimental
2.1 Materials
(EPR) spectrum was recorded on a Bruker EMX-X band spec-
trometer.
ꢀ
High purity 2, 2 -dipyridylamine (Aldrich, UK), copper(II)
perchlorate hexahydrate (Fluka, Germany), sodium acetate
2.4 Crystal structure determination and refinement
(E. Merck, India), 2-aminophenol (E. Merck, India), 3,5-
di-tert-butylcatechol (Sigma Aldrich Corporation, St. Louis,
MO, USA) were obtained from commercial sources and
used as purchased. All other chemicals and solvents were
of analytical grade and were used as received without further
purification.
Single crystal X-ray diffraction data of the copper(II) complex
were collected using a Rigaku XtaLABmini diffractometer
equipped with Mercury CCD detector. The data were col-
lected with graphite monochromated Mo-K α radiation (λ =
0
.71073 Å) at 293 K using ω scans. The data were reduced
3
3
Caution! Perchlorate salts of metal ions are potentially explo- using Crystal Clear suite 2.0 and the space group determi-
sive, especially in the presence of organic ligands. Only a nation was done using Olex2. The structure was resolved by
small amount of material should be prepared and it should be direct method and refined by full-matrix least-squares proce-
3
4
handled with care.
dures using the SHELXL-2014/7 software package through
the OLEX2 suite.35 The crystallographic bond distance and
bond angle are given in Table 1 and Table S1 (in Supplemen-
tary Information).
2
.2 Synthesis of [Cu(dpa) (O Ac)](ClO )(1)
2
4
The copper(II) complex was synthesized by addition of aque-
ous solution of dipyridylamine (0.3420 g, 2 mmol) into a
2
.5 Catalytic oxidation of 2-aminophenol
solution of Cu(ClO )2 · 6H2O (0.3650 g, 1 mmol)] in the
4
same solvent (20 mL) keeping the solution on a magnetic In order to examine the penoxazinone synthase activity,
−3
stirrer with stirring. Then solid sodium acetate (0.0820 g, 1 1 × 10 M solution of 1 in EtOH was treated with 10 equiv.
mmol) was added in solid into the blue solution and stirring
continued to 30 min more. The blue solutions were turned
Table 1. Crystallographic refinement parameters
into green and the supernatant liquids were kept in air for
slow evaporation. After 7–10 days the fine microcrystalline
compound was separated out and washed with hexane and
dried in vacuo over silica gel indicator. The spectroscopic
measurements and elemental analyses confirm the struc-
tural formation of the complex. Yield = ∼ 0.28g, (∼ 77%
based on metal salt). Anal. calc. (%) C22H22N6ClO6Cu: C,
of [Cu(dpa) (OAc)](ClO4) (1).
2
Parameters
Cu(II) compound
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C22H21N6O6ClCu
564.44
293
Monoclinic
P21/c
13.885(12)
7.899(7)
22.202(18)
2582.7(2)
4
4
1
1
1
7
6.73; H, 3.92; N, 14.86; Found(%): C, 46.69; H, 3.88; N,
−1
4.89. Selected IR bands (KBr pellet, cm ): 3321 (m),
637(s), 1604(s),1423(m), 1378(s), 1095(s). UV-Vis (λ, nm;
−4
0
M, 1 cm cell length, abs, ethanol): 298(1.86), 725–
b (Å)
45(0.00180) (broad band); ESI-MS (MeCN): m/z, 406.10
c (Å)
Volume (Å )
+
3
(
[Cu(dpa) ]-H ); (Calc. 406.09).
2
Z
−
3
ρ (gcm
μ (mm
F (000)
)
1.541
1.058
2.3 Physical measurements
−
1
)
1156
3.1 to 27.50
Infrared spectrum (KBr) was recorded with a FTIR-8400S
◦
◦
◦
−
1
θ ranges ( )
SHIMADZUspectrophotometerintherange400–3600cm
.
Rint
0.038
15670
5578
1
6
H NMR spectrum in DMSO-d was obtained on a Bruker
Avance 300 MHz spectrometer at 25 C and was recorded at
R (reflections)
wR2 (reflections)
Final R indices
Largest peak and hole (eA
◦
299.948 MHz. Ground-state absorption measurements were
0.0634, 0.1929
0.77, −0.44
made with a Jasco model V-730 UV-Vis spectrophotometer.
Elemental analyses were performed on a Perkin Elmer 2400
◦−3
)

Catalytic aspects of a copper(II) complex
of 2-aminophenol (2-AP) under aerobic conditions at room MHz, CDCl3): δ = 3, 5-di-tert-butyl-2-pyrone: 6.09 (m, 2H);
temperature. Absorbance vs. wavelength (wavelength scans) 4,6- di-tert-butyl-2-pyrone: 7.11 (d,1H), 7.24 (d, 1H); 3,5-
ofthesolutionwasrecordedataregulartimeintervalof15min di-tert-butylbenzoquinone: 6.19 (d, 1H), 6.74 (d, 1H); 3,5-di-
for aminophenol oxidation in the wavelength range 300–800 tert-butyl-5-(carboxymethyl)-2-furanone: 6.84 (s, 1H).
nm. Kinetic experiments were performed spectrophotometri-
cally2
6–28
with Cu(II) complex and 2-AP in EtOH at 25 C
◦
for aminophenol oxidation activity. 0.04 mL of the complex
−3
3. Results and Discussion
solution, with a constant concentration of 1 × 10 M, was
added to 2 mL of 2-AP of a particular concentration (varying
−
3
−2
3.1 Synthesis and formulation of the Cu(II)-dpa
complex (1)
its concentration from 1×10 M to 1×10 M) to achieve
−3
the ultimate concentration of the complex as 1×10 M. The
conversion of 2-aminophenol to 2-aminophenoxazine-3-one
was monitored with time at 433 nm (time scan)28 in EtOH.
The copper(II) complex was synthesized by addition of
ꢀ
2
, 2 -dipyridylamine to a solution of Cu(ClO ) · 6H O
To determine the dependence of rate on substrate concentra-
tion, kinetic analyses were performed in triplicate. Effect of
4
2
2
in water followed by the addition of sodium acetate
catalyst concentration on the reaction rate for the catalytic (NaOAc) at room temperature (Scheme 1). The structure
aminophenol oxidation in EtOH medium was determined by was determined by routine spectroscopic techniques
−
3
varying the concentration of copper(II) complex (5×10
M
including single crystal X-ray diffraction analysis.
.2 Description of the crystal structure
−4
−2
to 5×10 M) with a constant concentration (1×10 M) of
each substrate. Here, the kinetic analyses were also performed
in triplicate.
3
The X-ray crystal structure analysis of copper(II) com-
plex reveals that it crystallizes in monoclinic crystal
2
.6 Catechol dioxygenase activity of the
coper(II)-dipyridylamine complex
system with P2 /c space group. An ORTEP of the
1
copper(II) complex is shown in Figure 1. The crys-
tallographic structural parameters are given in Table 1
To investigate the catechol dioxygenase activity of the com-
−3
plex, a 10 M solution of 1 in ethanol (EtOH) solvent was
−3
treated with a 10 M solution of 3,5-di-tert-butylcatechol and bond angles, and bond distances are given in Table
+
(
DTBC) in oxygen saturated ethanol medium at room tem- S1. Monocationic [Cu(dpa) (OAc)] unit exists in a
2
perature. Absorbance vs. wavelength (wavelength scans) of rare class of six coordination geometry named bicapped
the solution was recorded at regular time intervals for 6 h in
square pyramidal geometry and the residual cationic
the wavelength range 200–900 nm.36 It may be noted here
charge is counterbalanced by a perchlorate ion. This
that a blank experiment without catalyst did not show the
copper(II) complex resembles to previously reported
formation of any cleavage products up to 12 h in EtOH.
bicapped square pyramidal chromophore, [Cu(bpy)
2
The solvent was equilibrated at the atmospheric pressure
38
(
O NO)](NO ) and this structure is known as a rare
◦
2
3
of O2 at 25 C using a known procedure with modifica-
tions.37 Investigation of dioxygen reactivity of the in situ
class of coordination geometry in the scientific litera-
ture. The square plane contains N1,N2 (dpa), N4(other
dpa) atoms and O1 atom of acetate ligand [Cu1-N1,
generated Cu(II)-catecholate adduct was carried out in oxy-
◦
36
gen saturated ethanol medium at 25 C. Kinetic analyses
of the catechol cleavage reactions were carried out by time-
dependent measurement of the disappearance of the lower while N2 (other dpa) and O2(acetate) atoms occupy
1
.99Å;Cu1-N2, 2.02Å;Cu1-N4, 2.01Å;Cu1-O1, 2.0Å]
energy DBC2 -to-Cu(II) LMCT band at 824 nm by exposing the apical position [Cu1-N3, 2.15Å; Cu1-O2, 2.70Å].
−
to molecular oxygen.
Though there are two Cu-O bonds (axial and equato-
rial) around the Cu centre but Cu1-O2 bond gets axially
39
2.7 Determination of 3,5-di-tert-butyl catechol
elongated due to Jahn-Teller distortion. In searching
for the origin of the existence of this unusual geom-
etry of the copper(II) complex in the solid state, we
closely observe the role of H-bonding interaction. In the
crystalline state of Cu(II) complex, perchlorate anion
has a substantial role to rotate the pyridine rings of
cleavage products
0.2260 g (0.04 mmol) of the Cu(II) complex was reacted
with 0.088 g (0.04 mmol) of DTBC in an oxygen saturated
ethanol (100 mL) at ambient condition and then allowed
to stir for 12 h. The reddish brown solution slowly turned
to green. The residue was then treated with 15 mL of 3M
HCl and the catechol cleavage products were extracted with
diethylether (3 × 10 mL) and dried over sodium sulfate.
The catechol cleavage products were analyzed by ESI-MS
and were quantified by H NMR spectroscopy. H NMR C(17)-H(17)···O4, 2.601Å; C(8)-H(8)···O3, 2.635Å;
data for 3,5-di-tert-butyl catechol cleavage products (500 C(14)-H(14)···O3, 2.652Å; N(6)-H(6)···O3, 2.028Å;
−
the dipyridyl amine. The oxygen atoms of the ClO4
ion act as donor centres and H-atoms of the pyridine
rings as well as secondary amines behave as acceptors
[C(10)-H(10)···O5, 2.68Å; C(17)-H(17)···O6, 2.677Å;
1
1

Biswajit Chowdhury et al.
H
O
N
Water mdium
O
N
N
Cu(ClO4)
2
NaOAc
NH
ClO
N
N
Cu
4
N
N
HN
Scheme 1. Preparative procedure for the Cu(II) complex.
3.3 Electronic, EPR and electrochemical
characterization
The solution integrity of the copper complex has been
performed through UV-Vis and EPR spectral analysis.
The UV-Vis spectrum in ethanol medium at room tem-
perature shows a series of high intensity transitions in
the range of 225 to 238 and 295 nm which are origi-
nated from the intraligand electronic transitions (Figure
S1). A very broad low energy transition centered at 741
nm (Figure S1 in Supplementary Information), which is
assigned to the ligand field transition for the copper(II)
complex. The X-band EPR spectrum of the copper com-
plex (Figure S2 in SI) at low temperature (LT, 77K) was
recorded in frozen MeCN solution (Figure S2 (Left))
and in powder state at 77 K (Figure S2 (Right) in SI)
to examine the geometry of copper center in 1. The
bicapped square pyramidal environment was also con-
firmed from the presence of four lines in the LT spectrum
and suggested the existence of a single species (Figure
S2).
Figure 1. An ORTEP diagram of Cu(II)-dpa complex (30%
ellipsoid probability).
We have also examined the electrochemical behaviour
of this copper(II) complex in ethanol at RT. The
reduction and oxidation potentials for Cu(II)-dpa com-
plex was observed, respectively, at -773 and +42
mV (Figure S3 in SI) at the scan rate of 20 mV
−1
s . The high value of reduction potential suggests
that the reduction is quite difficult for central Cu(II)
ion.
3.4 Catalytic oxidation of 2-aminophenol
The phenoxazinone synthase activity of the copper(II)
complex was studied using 2-aminophenol (2-AP) as
a convenient model substrate, in air saturated ethanol
◦
solvent at room temperature (25 C). For this purpose,
−
3
a 1 × 10 M solution of the copper(II)-dipyridylamine
Figure 2. Perchlorate ion mediated 3D crystalline
architechture through C/N-H · · · O interaction in solid
state.
−2
complex was treated with 1×10 M (10 equiv.) of 2-AP
and the course of the reaction was followed by recording
the UV–Vis spectra of the mixture at an interval of 15
min for 3 h.
Spectral bands at 225–238, 295 and 741 nm appeared
Figure 2, Table S2 in Supplementary Information] to in the electronic spectrum of the copper complex in
form a 3D architecture through C-H · · · O/N-H · · · O methanol, whereas 2-AP showed a single band at 267
interactions along b axis.
nm. As the reaction proceeded, there was a gradual

Catalytic aspects of a copper(II) complex
oxidation. The reused copper(II)-dipyridylamine cata-
lyst exhibited no change in reactivity after four cycles
also. In order to make a comparison of the phenoxazi-
none activity between our copper-dpa complex and the
reported copper(II) complex, [Cu(bpy) (O NO)](NO )
2
2
3
of the same class, we performed phenoxazinone syn-
thase activity for the both complexes under similar
reaction conditions. We find that Cu(II)-dpa complex
exhibits better catalytic efficiency towards the oxida-
tive coupling of 2-aminophenol than the reported copper
38
complex.
Thoughthisclassofcatalyticoxidationreactionunder
aerobic condition bears special attention, we could
not find enough literature reports to make a compar-
ison between phenoxazinone synthase activity of the
Cu(II)-dipyridylamine in the present case and previ-
Figure 3. Increase of aminophenoxazinone band at 433 nm
after addition of 10 equivalents of 2-AP to a Cu(II) solution in
EtOH medium. The spectra were recorded after every 9 min.
Inset: Plot of Abs versus time.
40a
ously reported works. Chaudhury et al.,
modelled
a tetracopper complex for the catalytic aerial oxida-
tion of 2-aminophenol to 2-amino-phenoxazine-3-one,
decrease in intensity of the band at 267 nm26 and an and proposed an “on-off” mechanism of the radicals
initial new broad band centered at 433 nm with increas- together with redox participation of the metal center
ing intensity was formed (Figure 3), which indicated the behind the mimics of six-electron oxidative coupling in
2
6–29
formation of the respective phenoxazinone species.
the catalytic function of the coppercontaining enzyme
The phenoxazinone was purified by column chromatog- phenoxazinone synthase. Begley et al.,40b suggested
raphy and isolated in high yield (81.6% for 1) by slow that 2-aminophenoxazinone synthesis proceeds via a
evaporation of the eluant. The product was identified by sequence of three consecutive 2-electron aminophe-
1
1
H NMR spectroscopy. H NMR (CDCl , 300 MHz,) nol oxidations and that the aminophenol moiety is
3
δ
H
: 7.62 (m, 1H), 7.48 (m, 3H), 6.41 (s, 1H), 6.30 (s, regenerated during the reaction sequence by facile tau-
1H).
tomerization reactions.
Kinetic studies were performed to understand the
In order to evaluate the mechanistic aspects of the
extent of the efficiency. The kinetics of oxidation of catalytic cycle by copper(II)-dipyridylamine complex
-AP were determined by the method of initial rates (Scheme 2) for the oxidative coupling of phenoxazi-
2
and involved monitoring the growth of the phenoxazi- none product, mass spectral analysis of the reaction
none band at 433 nm as a function of time (Figure S4 in mixture in ethanol medium provides valuable informa-
28,29
SI).
The plot of rate constants versus concentration tion regarding the reactive species and product formed
of the substrate were also analyzed on the basis of the in the reaction. The mass spectrum of the reaction mix-
Michaelis-Menten approach of enzymatic kinetics to get ture (Figure S5 in SI) exhibits mainly the characteristic
−
4
the values of the kinetics parameters, V 5.11 × 10
peaks at m/z 213.57 and 557.99 with isotope distribution
max
−5
−3
(
Sd. Error. 7.78 × 10 ), K = 2.98 × 10 (Sd. Error patterns which further consolidated the corroboration
M
−3
3
+
9
.14 × 10 ) and k = 1.83 × 10 . The observed respectively [(2-amino-3H-phenoxazine-3-ones)+H ]
cat
+
rate constant versus substrate concentration plot for and [[Cu(dpa) (2-AP)]+H ]. Further, the characteris-
2
copper(II)-dipyridylamine complex in EtOH is shown tic peak at m/z ∼ 557.99 nm indicates the presence
in Figure S4 (in SI).
of coordinated iminobenzosemiquinonato radical dur-
ing the generating apx product.
The high catalytic efficiency value (k /K
=
cat
M
5
2
.09 × 10 ) for copper(II) complex also indicates its
high reactivity towards aminophenol oxidation. We
also investigate the effect of catalyst concentration
on reaction rate and the rate of catalytic oxidation
3.5 Catechol dioxygenase activity of the
copper(II)-dipyridylamine complex
increase with increasing the concentration of copper(II)- The oxygenation reactions for the copper(II)-dipyrid-
dipyridylamine complex in a linear manner. Reuse of ylamine complex was carried out using 3,5-di-tert-
this copper(II)-dipyridylamine catalyst (Figure S9) for butylcatechol (DTBC) as the model substrate in bio-
aminophenol oxidation was also examined under sim- friendly ethanol, and the advantages of using the latter
ilar reaction conditions after completion of catalytic as substrate are the relatively high stability of the main

Biswajit Chowdhury et al.
Scheme 2. Proposed mechanistic routes for aminophenol oxidation by copper(II) complex.
Scheme 3. Oxygenation products of DTBC for Cu(II) com-
plex in ethanol.
cleavage product and the fast reaction of the cate-
cholate complex with dioxygen at room temperature
(
Scheme 3).
2
−
The DTBC adducts of Cu(II) complex was gen-
erated in situ in ethanol solution, and their reactivity
toward O was investigated by monitoring the decay
2
2
−
of the low energy DTBC to Cu(II) LMCT band Figure 4. Absorption spectral changes during the reaction
Figure 4). The green solution of the in situ Cu(II)-
catecholate adduct reacts with dioxygen in bio-friendly
ethanol medium at ambient conditions over a period of
of the in situ generated adduct [Cu(dpa) (2-AP)] with O2 (The
(
2
spectra were recorded after every 12 min). Inset: Plot of Abs
versus Time.
4
h and two new visible bands with maximum absorp-
tion at 516 and 824 nm (Figure 4), were observed for
the catecholate adduct of Cu(II) complex. The lower and Lewis acidity of the metal ion in the complex.41,42
energy visible bands with decreasing in absorbance are The disappearance of the lower-energy catecholate-
2
−
attributed to DTBC to-copper(II) LMCT transitions to-copper(II) LMCT band (Figure 4) on oxygenation
30,31
involving two different catecholate ligand orbitals.
Commonly, it is seen that the energy of the LMCT tran- linearity of the plot [1+ log(Absorbance)] versus time
sition strongly depends on the nature of the ligands (Figure 5), and the value of k was obtained from the
exhibits pseudo first-order kinetics, as judged from the
43
obs

Catalytic aspects of a copper(II) complex
slope of the plot. The pseudo-first order rate constant
−
3
−1
was determined as k : 1.09 × 10 min .
obs
Further, the cleavage products from catechol bound
Cu(II) species were identified and quantified by H
1
NMR spectroscopy (Figure S6 in SI). The distribu-
tion of catechol-derived products was found to be
principally 4,6-di-tert-butyl-2-pyrone and 3,5- di-tert-
butyl-2-pyrone, as extradiol cleavage products, in major
amount and 3,5-di-tert-butylbenzoquinone as minor
product, while 3,5-di-tert-butyl-5-(carboxymethyl)-2-
furanone as intradiol cleavage product was found in
trace quantity. In the reaction of (Cu complex+DTBC)
with O in ethanol, 81% of extradiol cleavage products
2
(
42.6%+38.4%) were obtained along with the forma-
tion of a trace amount of intradiol cleavage product as
a side product (Scheme 4). The percentage of minor
oxidation product, 3,5-di-tert-butylbenzoquinone, was
found to be a small one (16.8%) also. The amount of
Figure 5. Plot of [1+log(Absorbance)] versus time for the
◦
reaction of [Cu(dpa) (DBC)] with O2 at 25 C in EtOH
2
solution.
H
N
H
N
0
H
N
+
0
OH
OH
N
N
N
N
N
N
N
N
O
O
O
O
O
O
CuII
CuII
CuI
N
N
N
N
N
N
H
N
H
H
O
H
O
+
O
N
O
N
O
N
+
O
O
CuII
O
O
O
N
O
N
O
CuII
N
N
H
NH
N
N
O
N
+ꢀ
O2
HN
Criegee
O
Rearrangement
H
N
2
+
Alkenyl
migration
N
O
N
O
O
Solvent
Solvent
CuII
O
O
N
N
Extradiol products
N
H
Scheme 4. Proposed mechanistic pathway for the formation of cleavage products by 1.

Biswajit Chowdhury et al.
organic product from catechol cleavage accounts for to afford predominantly extradiol cleavage products
∼
95% of 3,5-di-tert-butyl catechol. The remaining 5% along with a small amount of benzoquinone, and a trace
was accounted as unreacted substrate. amount of intradiol cleavage products. The yield of
The formation of extradiol cleavage products for the cleavageproductsisstronglyinfavourofmolecularoxy-
in situ catecholate adduct of the Cu(II) complex is gen activation mechanism. This paper presents a novel
expected for having a vacant coordination site on the cat- addition of a copper(II) complex having the potential
echolate adduct for oxygen coordination, which favors to mimic the active site of phenoxazinone synthase and
dioxygen-activation pathway.4
4–46
From ESI mass spec- catechol dioxygenase enzyme with significant catalytic
trometric analysis of the reaction mixture, it is revealed efficiency.
that at the primary stage 3,5-DTBC forms in situ adduct
Supplementary Information (SI)
but the presence of semiquinone band ∼ 516 nm helps
to detect DTBSQ radical. The existence of the DTBQ
radical in solution during the investigation of dioxy-
genase activity was confirmed by the EPR signal at
CCDC 1513638 contains the supplementary crystallographic
data for 1. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the
g = 2.075 (Cu complex with 3,5-DTBC in EtOH) Cambridge Crystallographic Data Centre, 12 Union Road,
(
Figure S7 in SI). But existence of Cu(I)-semiquinone Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-
species in solution facilitates dioxygen activation mech- mail: deposit@ccdc.cam.ac.uk. Experimental data such as IR
anism and provides one electron from Cu(I) to the
anti-bonding orbital of dioxygen which generates oxo-
andUV-Visspectra, cyclicvoltagram, EPRspectraatlowtem-
1
perature, ESI mass spectra, H NMR of cleavage products,
rate vs. substrate plot, Lineweaver-Burk plot, bond distance,
bond angle & H-bonded interaction parameters are available
at www.ias.ac.in/chemsci.
species in solution and produces extradiol cleavage
products in a major amount.43 A small amount of
benzoquinone indicates the natural oxidation of the sub-
strate in solution. Probably, trace amount of intradiol
catechol cleavage reaction proceeded by peroxo inter-
Acknowledgements
4
BB sincerely thanks Science & Engineering Research Board
mediate that underwent 1,2-Criegee rearrangement to
(SERB), a statutory body of Department of Science & Tech-
yield the intradiol catechol cleaved products analogous
to the native enzyme. Earlier models have shown that
the dioxygenase activity strongly depends on the nature
of the ligand set and the coordination mode of the cat-
nology (DST), New Delhi for the financial support under
the START UP GRANT for YOUNG SCIENTIST (No.
SB/FT/CS-088/2013 dtd. 21/05/2014). BB is greatly indebted
to Prof. T.K. Paine, Department of Inorganic Chemistry,
Indian Association for the Cultivation of Science, Kolkata,
West Bengal, India for his valuable help in recording solid
state EPR spectrum at 77 K. BB thanks Dr. Angshuman Roy
Choudhury of IISER Mohali, Mohali 140 306, India for help-
47–51
echolate ligand.
Speier et al., reported an aerobic
oxidation of a (phenanthrenediolato)copper(II) complex
of tmeda, which is in resonance with two valence tau-
tomers of the (phenanthrenediolato) copper(II) and the
(
(
52
phenanthrenesemiquinonato) copper(I) states. The ing to collect crystallographic data of the copper complex.
phenanthrenesemiquinonato) copper(I) tautomer can
bind a dioxygen molecule to form a dioxygen complex,
which is decomposed to ring-cleavage products.
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