Synthesis and structural characterization of a dinuclear copper(II) complex with a (N,S) donor ligand: Catecholase and phenoxazinone synthase activities

Article abstract of DOI:10.1016/j.ica.2018.11.046

[Cu^II₂(L)₂Cl₄] (1) [L = 2-(benzylthio)ethanamine] has been synthesized and characterized using spectroscopic and X-ray crystallographic results. Structural analyses reveal bidentate chelation behaviour of the neutral L having a distorted square pyramidal coordination environment around each copper(II) with one L, one chloride and two bridging chloro. 1 behaves as an effective catalyst towards oxidation of 3,5-di-tert-butylcatechol to its corresponding quinone with molecular oxygen and also the coupling of o-aminophenol to its phenoxazinone chromophore in methanolic solutions with turnover numbers 2.18 × 10² h^?1 (catecholase activity, MeOH), and 3.60 × 10² h^?1 (phenoxazinone synthase activity, MeOH) and 2.36 × 10² h^?1 (Phenoxazinone synthase activity, MeCN).

Full text of DOI:10.1016/j.ica.2018.11.046

Accepted Manuscript
Research paper
Synthesis and structural characterization of a dinuclear copper(II) complex with
a (N,S) donor ligand: catecholase and phenoxazinone synthase activities
Debalina Mondal, Ayon Kanti Ghosh, Arnab Chatterjee, Rajarshi Ghosh
PII:
S0020-1693(18)31788-2
https://doi.org/10.1016/j.ica.2018.11.046
ICA 18661
DOI:
Reference:
Inorganica Chimica Acta
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26 October 2018
29 November 2018
Please cite this article as: D. Mondal, A.K. Ghosh, A. Chatterjee, R. Ghosh, Synthesis and structural characterization
of a dinuclear copper(II) complex with a (N,S) donor ligand: catecholase and phenoxazinone synthase activities,
Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.11.046
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1
Synthesis and structural characterization of a dinuclear copper(II)
complex with a (N,S) donor ligand: catecholase and phenoxazinone
synthase activities
Debalina Mondal†, Ayon Kanti Ghosh, Arnab Chatterjee, Rajarshi Ghosh*
Department of Chemistry, The University of Burdwan, Burdwan 713 104, India
Abstract
[CuII (L)2Cl4] (1) [L = 2-(benzylthio)ethanamine] has been synthesized and characterized
2
using spectroscopic and X-ray crystallographic results. Structural analyses reveal bidentate
chelation behaviour of the neutral L having a distorted square pyramidal coordination
environment around each copper(II) with one L, one chloride and two bridging chloro. 1 behaves
as an effective catalyst towards oxidation of 3,5-di-tert-butylcatechol to its corresponding
quinone with molecular oxygen and also the coupling of o-aminophenol to its phenoxazinone
chromophore in methanolic solutions with turnover numbers 2.18 × 102 h-1 (catecholase activity,
MeOH), and 3.60 × 102 h-1 (phenoxazinone synthase activity, MeOH) and 2.36 × 102 h-1
(Phenoxazinone synthase activity, MeCN).
Keywords: Copper(II), (N,S) donor ligand, catecholase activity, phenoxazinone synthase activity
_____________
†Present address: Department of Chemistry, Visva Bharati, Shantiniketan 731 235, India
*Corresponding author: rghosh@chem.buruniv.ac.in

2
Design and synthesis of small molecules following the active sites of different
metalloenzymes is a trend of current day research [1-5]. In these studies, synthesized molecules
are allowed to catalyse the respective substrates to their products. New molecules are being
synthesized to give better yield and greater catalytic efficiency. Catecholase and phenoxazionone
synthase are two copper containing enzymes which catalyse oxidation of catechol (1,2-diphenol)
to o-quinone and oxidative coupling of two molecules of 2-aminophenol derivative to
phenoxazinone chromophore, respectively. In the former, the process is common in bacteria,
fungi and plants. The highly reactive o-quinones formed get polymerized to melanins which are
thought to protect the plants from different environmental stresses (attack by pathogens, insects,
etc and different adverse climatic conditions) [6]. The latter enzymatic process is found in the
antineoplastic agent actinomycin D which is clinically used for the treatment of Wilm’s tumor,
gestational choriocarcinoma and other tumors [7]. After the publication of the active site
structures of the catecholase [8] and phenoxazinone synthase [7] as multicopper oxidases a
number of groups around the world are active in synthesizing different copper and non-copper
metal complexes to mimic structural and/or functional behaviour of the enzymes [2, 3, 9, 10]. In
continuation to our continued interest in this very field [11, 12] here we shall report synthesis, X-
ray structural characteization, and catecholase and phenoxazinone synthase activities of a newly
synthesized dinuclear Cu(II) complex [CuII (L)2Cl4] (1) [L = 2-(benzylthio)ethanamine].
2
The ligand L was synthesized [13] by stirring benzyl mercaptan with 2-chloroethylamine in
dry ethanol in presence of in situ formed sodium ethoxide (Scheme 1). The organic product was
extracted and solvent was evaporated as yellowish brown solid. The ligand was characterized by
IR and electronic spectroscopy. IR spectrum was recorded in the range 4000 to 400 cm-1. Some
relevant frequencies are at 3068 (ῡN-H), 756 (ῡC-S bending) cm-1 [14]. Absorption bands of the

3
1
ligand are found at 373 (n-π*), 215, 261 (π-π*) nm in MeOH solution at 298K. The HNMR
spectrum of L in CDCl3 showed the characteristic peaks of the protons (Fig. S2; Supporting
information). The signals for the amine protons of 2-(benzylthio)ethanamine appeared at  1.75
(t, J = 7.6 Hz, 2H). The two triplet signals for four protons of the two methylene groups were
found at  2.81 (t, J = 6.8 Hz, 2H) for one methylene group  to amine group (C2) and  3.14 (t,
J = 6.8 Hz, 2H) for another methylene group α to the NH2 group (C1). The signal appeared at 
3.60 (s, 2H) due to the methylene group which is α to the C-S bond (C3). The signals of the five
aromatic protons appeared at the range between δ 7.25 ppm to δ 7.33 ppm (m, 5H). The
molecular ion peak of L in HRMS appeared at m/z = 168.09 which is the molecular weight of the
ligand (Fig. S3; Supporting information).
Copper(II) chloride dihydrate was mixed with L at room temperature in methanol [13].
The mixture was stirred for 3 hours. Then the blue coloured solution was kept in covered beaker
for crystallization. After about a month bluish green crystals of 1 were obtained. The IR spectral
bands of 1 at 1452 cm-1, 3335 cm-1 and 760 cm-1 were attributed to the asymmetric ῡ aromatic(C-H)
,
ῡamine(N-H) and ῡC-S, respectively. The absorption bands of 1 in MeOH at 205 and 246 nm were
due to the (π - π*) transition of the ligand and at 650 nm for d-d transition of the copper(II)
center [15].
Single crystal X-ray diffraction analysis of 1 (Fig. 1) [16] reveals that the geometry
around each Cu(II) is distorted square pyramidal. Calculation of  (= 0.13) [17] also suggests the
same. Here each copper(II) center is coordinated with an amine nitrogen, a thiol sulfur, two
bridging chloride and one terminal chloride anion. The bond lengths [18] in the equatorial plane

4
vary between 1.989 (3) to 2.2897 (12) Å and the Cu-Cl bond distance of 2.893(12)Å corresponds
to axial bond.
1 was found to catalyse the aerial oxidation of the substrate 3,5-di-tert-butyl catechol (3,5-
DTBC) (catecholase activity) having two bulky t-butyl substituents on the ring and low quinone-
catechol reduction potential to corresponding ortho-quinone derivative 3,5-di-tert-butyl quinone
(3,5-DTBQ) which is highly stable and shows a maximum absorption at ~390 nm in methanol.
Optical spectral bands of 1 (at 205, 246 and 650 nm) and 3,5-DTBC (single band at 281 nm) do
not overlap. The repetitive UV-Vis spectral scan was recorded (Fig. 2) with the solution having a
mixture of 1 into 100 equivalents of 3,5-DTBC in methanol under aerobic conditions. The
spectral scan shows very smooth increase of a quinone band at around ~390 nm which ultimately
shifts to 398 nm as reported by Krebs et al [19]. To prove the biomimetic behaviour of 1 enzyme
kinetic experiments were performed UV-Vis spectrophotometrically thermostated at 25°C in
MeOH. 0.04 ml of complex solution, with a constant concentration of 1 × 10-4 M, was added to 2
ml of 3,5-DTBC of a particular concentration (varying its concentration from 1 × 10-3 M to 1 ×
10-2 M) to make the ultimate concentration of 1 as 1 × 10-4 M. The conversion of 3,5-DTBC to
3,5-DTBQ was monitored with time at a wavelength of 398 nm for solutions. The rate for each
concentration of the substrate was determined by the initial rate method. The rate versus
concentration of substrate data were analyzed on the basis of Michaelis-Menten approach (Fig.
3) of the enzyme kinetics to get the various kinetic parameters Vmax, KM and Kcat (turn over
number) which are listed in Table 1.
The mechanism for catecholase activity of 1 was explained by mass spectral analysis of a
1:100 mixture of 1 and 3,5-DTBC. 1 was supposed to be dissociated and form a mononuclear

5
complex which is characterized by m/z at 302.7782 (Fig. S5; Supporting information). The m/z
at 487.1883 indicated (Scheme 2) the formation of the adduct of 1 and 3,5-DTBC (Fig. S6;
Supporting information). m/z peak at 243.1242 was found to be sodium aggregate of 3,5-DTBQ
(Fig. S7; Supporting information). With these and some other mass spectral data the catalytic
cycle (Scheme 3) of the catecholase activity was proposed and the probable intermediates in the
cycle were characterized by mass spectral analysis (Figs. S5, S6, S8(a-c), Supporting
information). In this process molecular oxygen of air oxidized 3,5-DTBC to the corresponding
quinone, 3,5-DTBQ and reduced itself to hydrogen peroxide which was identified and
characterized spectrophotometrically (S1; Supporting information) [20].
The phenoxazinone synthase activity of 1 had been studied by using the substrate, o-
aminophenol (OAPH) which was oxidised by aerial oxygen and catalysed by 1 to the
corresponding 2-aminophenoxazinone-3-one (APX). The spectral band of OAPH appears at 230
nm and 283 nm in methanol (inset to Fig. 4), and 232 nm and 290 nm in acetonitrile (inset to Fig.
5). The solutions of 1 equivalent of 1 in methanol and acetonitrile were treated separately with
100 equivalent of OAPH and the repetitive UV-Vis scans were recorded. Each of the
representative scans are shown in Figs. 4 and 5. The characteristic peak of APX grows smoothly
at 420 nm in MeOH and 425 nm in MeCN [21]. The APX was identified by mass spectral data at
m/z 213.0585 (Fig. S9; Supporting information). Enzymatic kinetic experiments were performed
UV-Vis spectrophotometrically thermostated at 25°C similarly as did in catecholase activity.
0.04 ml of complex solution, with a constant concentration of 1 × 10-4 M, was added to 2 ml of
OAPH of a particular concentration (varying its concentration from 1 × 10-3 M to 1 × 10-2 M) to
achieve the final concentration of 1 as 1 × 10-4 M. The conversion of OAPH to APX was
monitored with time at a wavelength of 420 nm and 425 nm for solutions in methanol and

6
acetonitrile, respectively. The rate for each concentration of the substrate was determined by the
initial rate method. The rate versus concentration of substrate data (Figs. 6 and 7) were analyzed
on the basis of Michaelis-Menten approach of enzyme kinetics of the various kinetic parameters
Vmax, KM and Kcat (turn over number) in MeOH and MeCN (Table 2).
To predict the mechanism of phenoxazinone synthase activity by 1, it is found that initially
OAPH forms adduct with 1 which is identified by mass spectrometry at m/z = 737.7379 (OAPH-
1) (Fig. S10; Supporting information). This adduct generates 2-amoinophenoxazine-3-one
through some intermediate with molecular oxygen. Reduced molecular oxygen in the form of
H2O2 was detected spectrophotometrically (S1; Supporting information) [20].
One new dinuclear copper(II) complex was synthesized and crystallographically
characterized. It was found to catalyze separately the aerial oxidation of 3,5-ditertiarybutyl
catechol to its corresponding quione and o-aminophenol to phenoxazinone chromophore. The
turn over number for the catecholase activity by 1 in methanol was better than that of some other
reported complexes [9(a), 10(c), 22a, 22c] though highest turn over number was reported by Das
et al [23] and Chattopadhyay et al [24] separately. Similarly, the turn over number for the
phenoxazinone synthase activity is better than few reported ones [21].
Supplementary data
CCDC 1874950 contains the supplementary crystallographic data for 1. This data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk
Acknowledgement

7
AKG and AC thank CSIR, New Delhi and PURSE program, DST, Govt, of India,
respectively for their fellowships. Different infrastructural facilities at our university provided by
PURSE program, DST, Govt of India is gratefully acknowledged.
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9
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[13] Synthesis of L: Benzyl mercaptan (8 mmol, 0.99 g) was added to sodium ethoxide solution
(Sodium ethoxide was prepared in situ by addition of pinch of metallic sodium in dry ethanol) by
stirring. Then 2-chloroethylamine (8 mmol, 0.63 g) was added slowly to the reaction mixture and
stirred for about 1 hour. The precipitation of sodium chloride confirms the formation of the 2-
1
(benzylthio)ethanamine. This product was characterized by H NMR (δ (ppm): 1.75 (t, J = 7.6
Hz, 2H), 2.81 (t, J = 6.8 Hz, 2H), 3.14 (t, J = 6.8 Hz, 2H), 3.60 (s, 2H), 7.25-7.33 (m, 5H)) and
high resolution mass spectrometry (m/z= 168.0927). Sodium chloride was separated by filtration.
IR (Cm-1): 3068, 756; UV-Vis (nm): 215, 261, 373
Synthesis of 1: Ethanolic solution of copper(II) chloride dihydrate (0.1 mmol, 0.02 g) was added
dropwise to the ligand L (0.1 mmol, 0.017 g) in ethanol with gentle stirring. The brown coloured
ligand solution changed to light blue. The reaction mixture was kept in open air. After 30 days
green coloured block shaped crystals of 1 appeared. Yield, 0.0373 g (62%). IR (Cm-1): 1452 (ῡ
aromatic(C-H)), 3335 (ῡamine(N-H)),760 (ῡC-S); UV-Vis (nm): 205, 246, 650
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group P12/n; a = 5.8489(2), b = 24.5460(6), c = 8.8292(2) Å;  = 101.849(2)˚; V = 1240.57(6)
Å3; Z = 2; Dcalc = 1.615 mg/m3; F(000) = 672; Goodness-of fit (GOF) F2 = 1.101; R1 = 0.0751,
wR2 = 0.1807.
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Bond distances (Å): Cu(1)-N(1), 1.989(5); Cu(1)-S(1), 2.3359(12); Cu(1)-Cl(1), 2.2897(12);
Cu(1)-Cl(1ʹ), 2.2402(14); Cu(1)-Cl(1ʹʹ), 2.893(12)
Band angles (°): N(1)-Cu(1)-S(1), 85.49(15); S(1)-Cu(1)-Cl(1), 92.91(5); N(1)-Cu(1)-Cl(1′),
93.87(6);
N(1)-Cu(1)-Cl(1′), 87.91(15); S(1)-Cu(1)-Cl(1′), 169.94(5); Cl(1)-Cu(1)-N(1),
177.82(16); N(1)-Cu(1)-Cl(1ʹʹ), 83.85(15); Cl(1ʹʹ)-Cu(1)-Cl(1ʹ), 92.74(5); Cl(1ʹʹ)-Cu(1ʹ)- Cl(1),
94.80(15); Cl(1ʹʹ)-Cu(1ʹ)-S(1), 94.09(5); Cu(1)-Cl(1ʹʹ)-Cu(1ʹ), 87.26 (6); Cu(1)-Cl(1ʹ)-Cu(1ʹ),
87.26(5)
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Table 1 ─ Kinetic parameters for the oxidation of 3,5-DTBC catalyzed by 1
Solvent Vmax (M s-1) Std. error Kcat (h-1)
MeOH 6.06 × 10-6 6.64× 10-6 6.20 × 10-4 7.04 × 10-5 2.18 × 102
KM (M)
Std. error
Table 2 ─ Kinetic parameters for the oxidation of OAPH catalyzed by 1
Solvent Vmax (M s-1)
MeOH 1.00 × 10-5
MeCN 6.57 × 10-6
Std. error
KM (M)
Std. error
Kcat (h-1)
9.17 × 10-6 6.70 × 10-4
8.34 × 10-6 3.70 × 10-4
9.06 × 10-4 3.60 × 102
8.62 × 10-4 2.36 × 102

13
Scheme 1. Synthesis of L
Scheme 2. Reaction intermediate

14
Scheme 3. Catalytic cycle for the catecholase activity

15
Fig. 1. ORTEP of 1 with 40% ellipsoid probability
Fig. 2. Change in spectral pattern of 3,5-DTBC in presence of 1 in MeOH (reaction for 2 h)

16
Fig. 3. Plot of rate vs. [Substrate] in presence of 1 in MeOH for catecholase activity.
Fig. 4. Change in spectral pattern of OAPH in presence of 1 in MeOH (reaction for 2 h)

17
Fig. 5. Change in spectral pattern of OAPH in presence of 1 in MeCN (reaction for 2 h)

18
Fig. 6. Plot of rate vs. [substrate] in presence of 1 in MeOH for phenoxazinone synthase activity
Fig. 7. Plot of rate vs. [substrate] in presence of 1 in MeCN for phenoxazinone synthase activity

19

20
 Dinuclear copper(II) molecule
 X-ray diffraction study
 Catecholase activity
 Phenoxazinone synthase activity