Catecholase activity of mononuclear copper(II) complexes of tridentate 3N ligands in aqueous and aqueous micellar media: Influence of stereoelectronic factors on catalytic activity

Article abstract of DOI:10.1016/j.ica.2018.09.069

A series of mononuclear copper(II) complexes of the type [Cu(L)Cl₂] 1–6, where L is a tridentate 3 N ligand such as N-methyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L1), N-ethyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L2), N-phenyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L3), N,N-dimethyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L4), N,N-diethyl-N′-(pyrid-2-ylmethyl)-ethylenediamine (L5) and N-methyl-N′-(pyridin-2-ylmethyl)benzene-1,2-diamine (L6), has been isolated and studied as functional models for catechol oxidase enzymes in different aqueous micellar media. The X-ray crystal structures of 2 and 5 contain a CuN₃Cl₂ chromophore with a trigonal bipyramidal based coordination geometry. Interestingly, all the complexes form cationic [Cu(L)(DBC)(H₂O)]⁺ adduct species in which DBC^2? acts as a monodentate ligand, and the adduct species interacts strongly with anionic SDS micelles. The rates of the oxygenase reaction follow the trend SDS > TX-100 ～ water > CTAB, revealing that the rate of the reaction in micellar media are higher than those in aqueous solution and that the nature of biomimetic microenvironments of the micellized copper(II) catalyst and also the ligand stereoelectronic factors like donor atom basicity and steric bulk of ligand N-alkyl substituents, determine the catecholase-like activity.

Full text of DOI:10.1016/j.ica.2018.09.069

Accepted Manuscript
Catecholase Activity of Mononuclear Copper(II) Complexes of Tridentate 3N
Ligands in Aqueous and Aqueous Micellar Media: Influence of Stereoelectronic
Factors on Catalytic Activity
Natarajan Anitha, Natarajan Saravanan, Tamilarasan Ajaykamal, Eringathodi
Suresh, Mallayan Palaniandavar
PII:
S0020-1693(18)31008-9
https://doi.org/10.1016/j.ica.2018.09.069
ICA 18527
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 July 2018
16 September 2018
23 September 2018
Please cite this article as: N. Anitha, N. Saravanan, T. Ajaykamal, E. Suresh, M. Palaniandavar, Catecholase Activity
of Mononuclear Copper(II) Complexes of Tridentate 3N Ligands in Aqueous and Aqueous Micellar Media:
Influence of Stereoelectronic Factors on Catalytic Activity, Inorganica Chimica Acta (2018), doi: https://doi.org/
10.1016/j.ica.2018.09.069
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Catecholase Activity of Mononuclear Copper(II) Complexes of Tridentate
3N Ligands in Aqueous and Aqueous Micellar Media: Influence of
Stereoelectronic Factors on Catalytic Activity
Natarajan Anitha,a Natarajan Saravanana Tamilarasan Ajaykamala, Eringathodi Sureshb
,
and Mallayan Palaniandavar*a
a School of Chemistry, Bharathidasan University, Tiruchirapalli 620 024, India.
bAnalytical Science Discipline, Central Salt and Marine Chemicals Research Institute,
Bhavnagar - 364 002, India.
_________________________________________________________________
*Author to whom correspondence is to be addressed at palanim51@yahoo.com,
palaniandavarm@gmail.com

Abstract
A series of mononuclear copper(II) complexes of the type [Cu(L)Cl2] 1-6, where L is a
tridentate 3N ligand such as N-methyl-N’-(pyrid-2-ylmethyl)ethylenediamine (L1), N-ethyl-N’-
(pyrid-2-ylmethyl)ethylenediamine (L2), N-phenyl-N’-(pyrid-2-ylmethyl)ethylenediamine
(L3), N,N-dimethyl-N’-(pyrid-2-ylmethyl)ethylenediamine (L4), N,N-diethyl-N’-(pyrid-2-
ylmethyl)-ethylenediamine (L5) and N-methyl-N’-(pyridin-2-ylmethyl)benzene-1,2-diamine
(L6), has been isolated and studied as functional models for catechol oxidase enzymes in
different aqueous micellar media. The X-ray crystal structures of 2 and 5 contain a CuN3Cl2
chromophore with a trigonal bipyramidal based coordination geometry. Interestingly, all the
complexes form cationic [Cu(L)(DBC)(H2O)]+ adduct species in which DBC2- acts as a
monodentate ligand, and the adduct species interacts strongly with anionic SDS micelles. The
rates of the oxygenase reaction follow the trend SDS > TX-100 ~ water > CTAB, revealing that
the rate of the reaction in micellar media are higher than those in aqueous solution and that the
nature of biomimetic microenvironments of the micellized copper(II) catalyst and also the
ligand stereoelectronic factors like donor atom basicity and steric bulk of ligand N-alkyl
substituents, determine the catecholase-like activity.
2

Introduction
Metalloenzymes that activate molecular oxygen possess a great potential as catalysts for
specific oxidation reactions and as guide for development of efficient small molecule catalysts
[1-6]. Oxidation of organic substrates with molecular oxygen under mild conditions is of great
interest for industrial and synthetic processes from both economical and environmental point of
view [7-10]. There are many copper and iron containing enzymes which serve as highly
efficient oxidation catalysts in biological systems [11,12]. The well-known copper containing
catechol oxidase (CO) enzyme, which catalyses the oxidation of a wide range of o-diphenols
(catechols) to the corresponding o-quinones without acting on monophenols, has attracted
much attention [2b-18, 11-14]. Also, it plays a vital role in medical diagnosis of the hormonally
active catecholamines adrenaline and noradrenaline [15]. The active site of the enzyme contains
a catalytic dicopper (type-3) centre with each copper coordinated to three histidine nitrogens.
Many copper(II) complexes that mimic the structural and electronic properties of the enzyme
have been isolated and the effect of several factors such as metal–metal distance,
electrochemical properties, exogenous bridging ligands, ligand structure and pH of the medium
has been investigated [19, 20-26]. Very recently, studies on the oxidation of phenolic substrates
with biomimetic copper complexes have led to significant progress in understanding the
enzyme activity [27-36]. Tolman and co-workers [34] have reported that the oxidation of 3,5-
di-tert-butylcatechol (H2DBC) and tetrachlorocatechol (H2TCC) by -2:2 peroxo-
dicopper(II) and -oxo-dicopper(III) complexes result in dissociation of the dinuclear core and
formation of mononuclear copper(II)-semiquinonate adducts. Thomson and Calabrese have
interacted a bis-methanolate-bridged copper(II) dimer with H2DBC (vide supra) to obtain a
CuII-semiquinonate complex, and a reduced CuI mononuclear species [35].
Aqueous micelles could be regarded as submicroscopic reaction media and reactants are
distributed rapidly between water and micellar pseudo phases, which are regarded as distinct
reaction regions. The micellar phase can take up reactants and provide a medium different from
that of the bulk solvent and hence act as biomimetic or membrane mimicking media to carry
out enzymatic reactions. Several reports are available to demonstrate the enhancement in
reaction rates by several orders of magnitude and also changes in product selectivity in aqueous
micelles [37]. Recent literature reveals that copper complexes are active towards the auto-
3

oxidation of catechols in micellar media [39–40]. The very low solubility of H2DBC (3,5-
DTBC) in aqueous medium can be considerably enhanced in the presence of micelles and the
catalytic potential of amphiphilic copper complexes towards autooxidation of H2DBC (3,5-
DTBC) has been examined in aqueous micellar media. Such studies provide further
understanding of the structure-spectra correlation and structure-function activity of the enzyme
active sites [38-40, 41-50]. In our laboratory, we have recently made a reasonable insight into
the nature of the redox active site and catechol dioxygenase activity of iron(III) complexes of
several linear 3N ligands in aqueous SDS micellar medium [51].
As a part of our ongoing studies on functional models for various oxidase
metalloenzymes [51-52], we have now investigated the catecholase activity of Cu(II)
complexes in aggregated systems, which have the potential to organize reactants at a molecular
level, thereby promote catalysis and hence duplicate the native enzymes. We have isolated the
1:1 mononuclear Cu(II) complexes 1 – 6 of systematically modified tridentate 3N ligands with
varying steric and electronic properties (L1 – L6, Scheme 1) and investigated them as models
for the catecholase activity in both aqueous and aqueous SDS, CTAB, and TX100 micellar
media. We intend to study the spectral and redox properties of the Cu(II) complexes and their
catecholate adducts in the organized assemblies and understand the nature of the species
formed. A study of catecholase activity in these media would be undertaken to obtain an insight
into the structure-activity relationship. We have studied the kinetics of catalytic oxidation of
H2DBC to 3,5-di-tert-1,2-benzoquinone (DBQ) with dioxygen with an aim to gain an insight
into the influence of CuII/CuI redox on catecholase activity in aqueous micellar media. The
ligand L5 is expected to offer steric hindrance to the binding of catechols and thereby simulate
the five-coordinate geometry found for the enzyme-substrate complex [13]. The X-ray crystal
structures of [Cu(L2)Cl2] 2 and [Cu(L5)Cl2] 5 have been now determined to demonstrate that
the copper(II) complexes make available cis-coordination sites for adduct formation with
DBC2-. The structure of [Cu(L4)Cl2] has been already determined [53]. The complex 5 exhibits
the highest rate of quinone formation in aqueous SDS micellar media, illustrating the
attainment of enzyme-like activity in micellar media. Also, the rates and turn over numbers for
the catecholase-like activity observed in aqueous CTAB and aqueous TX-100 micelles are
higher than those observed for similar complexes [54,55] in non-aqueous solvents.
4

H
H
H
N
N
N
H
H
N
N
N
N
N
N
N
N
N
N
H
L2
L1
L3
H
H
H
N
N
N
H
N
N
L4
L5
L6
Scheme 1 Structures of Tridentate 3N Ligands
Experimental
Syntheses of Ligands
N-Methyl-N’-(pyrid-2-ylmethyl)ethylenediamine (L1)
The ligand was synthesized using the reported procedure [52]. To N-
methylethylenediamine (0.74 g, 10 mmol) in methanol (20 mL) was added dropwise pyridine-
2-carboxaldehyde (1.10 g, 10 mmol) in methanol (20 mL). The mixture was stirred overnight to
get a bright yellow solution. To this was added NaBH4 (0.57 g, 15 mmol), the solution stirred
for another day and then rotaevaporated to dryness. The resulting solid was dissolved in water
and the organic layer was extracted with CH2Cl2 and dried with anhydrous sodium sulphate.
The CH2Cl2 layer was rotaevaporated to get N-methyl-N’-(pyrid-2-ylmethyl) ethylenediamine
1
as an yellow oil. The yield was 1.15 g (70%). H NMR (200 MHz, CDCl3):  2.0 (s, 2H,
NHCH2), 2.5 (d, 3H, CH3), 2.7 (t, 4H, CH2NH), 4.14 (d, 2H, CH2py), 7.3-8.6 (m, py).
N-Ethyl-N’-(pyrid-2-ylmethyl)ethylenediamine (L2)
The ligand L2 was prepared by the same method [53] as that used for L1, except that N-
ethylethylenediamine (0.88 g, 10 mmol) was used instead of N-methylethylenediamine. Yield:
1
1.22 g (72%); H NMR (200 MHz, CDCl3):  2.0 (s, 2H, NHCH2), 2.6 (m, 2H, CH2CH3), 1.0
(t, 3H, CH3), 2.7 (t, 4H, CH2NH), 4.14 (d, 2H, CH2py), 7.3-8.6 (m, py).
5

N-Phenyl-N’-(pyrid-2-ylmethyl)ethylenediamine (L3)
The ligand L3 was prepared by the method used for preparing L1, except that N-
phenylethylenediamine (1.36 g, 10 mmol) was used instead of N-methylethylenediamine.
1
Yield: 1.22 g (74%); H NMR (200 MHz, CDCl3):  4.0 (s, 2H, NH), 4.2 (s, 3H, CH3), 4.7 (s,
CH2), 6.4-7.0 (m, aryl), 7.3-8.6 (m, py).
N,N-Dimethyl-N’-(pyrid-2-ylmethyl)ethylenediamine (L4)
The ligand was prepared as reported elsewhere [52]. In methanol (50 mL) N,N-
dimethylethylenediamine (0.88 g, 10 mmol) and pyridine-2-carboxaldehyde (1.10 g, 10 mmol)
were stirred together. After 24 h NaBH4 (0.38 g, 10 mmol) was added with stirring at 0 C. The
resulting yellow solution was stirred overnight and then refluxed for 1 h. This solution was
concentrated under reduced pressure and then hydrochloric acid (6M, 5 mL) was added. After a
few minutes the solution was made alkaline with sodium hydroxide solution (2 M). The sodium
borate obtained was filtered off and the filtrate was rotaevaporated under reduced pressure. The
resulting crude product was extracted with CH2Cl2 and then dried over sodium sulfate. The
ligand L3 was obtained as a yellow oil after removal of solvent under reduced pressure of the
organic layer. The yield was 1.45 g (80%). The electronic spectral data of the ligand field and
ligand based transitions of L4 [646 nm (150 M-1 cm-1) 262 nm (17950 M-1 cm-1)] coincide well
with those reported.31 1H NMR (200 MHz, CDCl3):  2.0 (s, H, NH), 2.2 (s, 6H, CH3), 2.5 (t,
2H, CH2N(CH3)2), 2.7 (t, 2H, CH2NH), 4.14 (d, 2H, CH2py), 7.3-8.6 (m, py).
N,N-Diethyl-N’-(pyrid-2-ylmethyl)ethylenediamine (L5)
The ligand L5 was prepared by using the method employed for preparing L4, except
that N,N’-diethylethylenediamine (1.16 g, 10 mmol) was used instead of N,N’-
1
dimethylethylenediamine.Yield: 1.22 g (77%); H NMR (200 MHz, CDCl3):  2.0 (s, H, NH),
2.4 (m, 10H, N-(C2H5)2), 2.5 (t, CH2), 2.7 (t, CH2NH), 3.05 (s, NH), 3.9 (s, CH2), 7.14-8.58 (m,
py).
N-Methyl-N’-(pyridin-2-ylmethyl)benzene-1,2-diamine (L6)
The ligand L6 was prepared by the method used for L1, except that N-methyl-1,2-
phenylenediamine (1.36 g, 10 mmol) was used instead of N-methylethylenediamine. Yield:
6

1.22 g (79%); 1H NMR (200 MHz, CDCl3):  2.0 (s, 1H, NHCH2), 2.0 (s, 1H, NHCH2), 4.0 (s,
NH), 2.9-3.2 (t, CH2), 4.1 (s, CH2), 6.4-7.0 (m, aryl), 7.3-8.6 (m, py).
Preparation of Copper(II) Complexes
[Cu(L1)Cl2] 1
The complex 1 was prepared by the reported procedure [53]. To a solution of L1 (0.17
g, 1 mmol) in methanol (10 mL) was added CuCl2·2H2O (0.170 g, 1 mmol) in methanol (5
mL), stirred well and cooled. The pale blue crystalline precipitate obtained was filtered off,
washed with small amounts of cold methanol and dried under vacuum. Yield: 0.22 g (75%).
The electronic spectral data of 1 (645 nm (120 M-1cm-1) and 255 nm (18275 M-1cm-1)) coincide
well with those reported already [50]. ESI-MS, m/z = 263.07 [Cu(L1)Cl]+.
[Cu(L2)Cl2] 2
This complex was prepared in a manner analogous to that described for 1 using L2
instead of L1. A blue colored precipitate obtained was filtered off, washed with small amounts
of cold methanol and dried under vacuum. An ethanolic solution of the complex on ether
diffusion yielded pale blue coloured crystals suitable for X-ray diffraction studies was obtained
after a day. Yield: 0.24 g (72%). Anal. Calcd for C10H17N3Cu1Cl2: C, 38.29; H, 5.46; N, 13.39.
Found: C, 38.32; H, 5.42; N, 13.41%. ESI-MS, m/z = 277.13 [Cu(L2)Cl]+.
[Cu(L3)Cl2] 3
This complex was obtained by employing the method used for that described for 1 using
L3 instead of L1. The pale blue colored microcrystalline product was filtered off, washed with
small amounts of cold methanol and dried under vacuum. Yield: 0.28 g (76%). Anal. Calcd for
C14H17N3Cu1Cl2: C, 46.44; H, 4.77; N, 11.67 Found: C, 46.48; H, 4.74; N, 11.62%. ESI-MS,
m/z = 325.07 [Cu(L3)Cl]+
[Cu(L4)Cl2] 4
The complex 4 was prepared by the reported procedure [53]. The deep blue colored
microcrystalline product was filtered off, washed with small amounts of cold methanol and
dried under vacuum. Yield: 0.20 g (51%). The electronic spectral data of 4 (645 nm (150 M-
7

1cm-1) and 260 nm (18540 M-1cm-1)) are in agreement with the reported values [53]. Anal.
Calcd for C10H17N3Cu1N3: C, 38.29; H, 5.46; N, 13.39 Found: C, 38.02; H, 5.07; N, 13.72%.
ESI-MS, m/z = 313.07 [Cu(L4)Cl]+
[Cu(L5)Cl2] 5
This complex was synthesized by using the procedure similar to that described for 1
using L5 instead of L1. The pale blue colored microcrystalline product was filtered off, washed
with small amounts of cold methanol and dried under vacuum. Yield: 0.20 g (51%). An
ethanolic solution of the complex on ether diffusion yielded pale blue coloured crystals suitable
for X-ray diffraction studies was obtained after a day. Anal. Calcd for C12H21N3Cu1Cl2: C,
43.19; H, 3.93; N, 12.59 Found: C, 43.24; H, 4.07; N, 12.72%. ESI-MS, m/z = 340.68
[Cu(L5)Cl]+
[Cu(L6)Cl2] 6
This complex was isolated by employing the procedure used for isolating 1 using L6
instead of L1. The pale blue colored microcrystalline product was filtered off, washed with
small amounts of cold methanol and dried under vacuum. Yield: 0.236 g (68 %). Anal. Calcd
for C13H15N3Cu1Cl2: C, 38.29; H, 5.46; N, 13.39 Found: C, 38.34; H, 5.53; N, 13.47%. ESI-
MS, m/z = 333.07 [Cu(L6)Cl]+.
Physical measurements
Elemental analyses were performed on a Perkin Elmer Series II CHNS/O Analyzer 2400. 1H
NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. The electronic spectra were
obtained on an Agilent diode array-8453 spectrophotometer. The EPR spectra were recorded on a
JEOL JES-TE 100 X-band spectrometer. ESI-mass spectrometry was performed on a Thermo
Finnigan LCQ 6000 Advantage Max instrument. Cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) were performed using a three electrode cell configuration. A platinum sphere, a
platinum plate and Ag(s)/Ag+ were used as working, auxiliary and reference electrodes respectively.
The supporting electrolyte used was NBu4ClO4 (TBAP). The temperature of the electrochemical cell
was maintained at 25.0 ± 0.2 °C by a cryocirculator (HAAKE D8 G). By bubbling research grade
dinitrogen the solutions were deoxygenated and an atmosphere of nitrogen was maintained over the
8

solutions during measurements. The E1/2 values were observed under identical conditions for various
scan rates. The instruments utilized included an EG & G PAR 273 Potentiostat/Galvanostat and
Pentium-IV computer along with EG & G M270 software to carry out the experiments and to
acquire the data. The product analyses were performed using a HP 6890 GC series Gas
Chromatograph equipped with an FID detector and an HP-5 capillary column (30 m × 0.32 mm ×
2.5 μm) and GC-MS analysis was performed on a Perkin Elmer Clarus 500 GC-MS instrument
using a PE-5 with the previously reported temperature program.
Crystallographic Data Collection and Structure Analysis
A crystal of suitable size selected from the mother liquor was immersed in paraffin oil,
then mounted on the tip of a glass fiber and cemented using epoxy resin. Intensity data for the
crystals were collected using MoK ( = 0.71073 Å) radiation on a Bruker SMART APEX
diffractometer equipped with CCD area detector at 273 and 293 K. The crystallographic data
were collected in Table 1. The SMART [56] program was used for collecting frames of data,
indexing the reflections, and determination of lattice parameters; SAINT [56] program for
integration of the intensity of reflections and scaling; SADABS[57] program for absorption
correction, and the SHELXTL[58] program for space group and structure determination, and
least-squares refinements on F2. The structure was solved by heavy atom method. Other non-
hydrogen atoms were located in successive difference Fourier syntheses. The final refinement
was performed by full-matrix least-squares analysis. Hydrogen atoms attached to the ligand
moiety were located from the difference Fourier map and refined isotropically. The selected
bond lengths and bond angles are given in Tables 1 and 2 respectively.
4.3 Results and Discussion
Synthesis of Ligands and Copper(II) Complexes
A series of copper(II) complexes of systematically varied linear 3N ligands have been
synthesized with an aim to understand the effect of varying ligand electronic and steric factors
upon the structures, spectra and catecholase activity of the complexes in aqueous and aqueous
micellar media. The ligands L1-L6 were synthesised by condensation of pyridine-2-
carboxaldehyde with the corresponding sterically hindered diamines to form the Schiff base
followed by reduction with sodium borohydride. The ligands were purified by washing the crude
9

product repeatedly with water, extracting with diethyl ether:hexane mixture and passing the extract
through basic alumina column after drying it with Na2SO4. The ligands L1 and L4 were
synthesized by using the procedures reported earlier [52-54]. The new ligands L2, L3, L5 and
L6 were synthesized by using procedures reported for similar ligands with slight modifications.
The results of elemental analyses of L1-L6 were satisfactory and the 1H NMR spectra confirm
the identity of the ligands. All the ligands, except L6, readily form complexes on treating them
with one equivalent of copper(II) chloride in methanol solution at room temperature. The
stoichiometries of all the complexes were derived from elemental analysis and those of 2, 4
[53] and 5 were consistent with their X-ray crystal structures. They are confirmed by the ESI-
MS studies for 1 – 6 in acetonitrile solution. The study reveals that apart from the mononuclear
[Cu(L)Cl2] species, dinuclear -dichloro-bridged [Cu(L)Cl2Cu(L)]Cl2 species are also present
in high abundance. The terminal amine nitrogen of the ligands are differently substituted with
one (L1, L2) or two alkyl (L4, L5) or one phenyl substituent (L3) in order to cause steric and
electronic effects on the Cu(II) coordination geometry and thus tune the reactivity of complexes
towards H2DBC.
Description of Structures [Cu(L2)Cl2] 2 and [Cu(L5)Cl2] 5
The asymmetric unit cell of [Cu(L2)Cl2] 2 contains two crystallographically
independent molecules. In both the molecules copper(II) possesses a five-coordinate square-
based pyramidal geometry which is very similar to that of [Cu(L4)Cl2] [53]. The chloride ion
Cl2 is located at a distance (Cu-Cl2, 2.2887 Å) shorter than the other chloride ion Cl1 (Cu-Cl1,
2.2595 Å), obviously because of steric hindrance from the bulky -NEt group. This is consistent
with the relatively high Cl2-Cu-N3 bond angle of 137. As the R value of the structure (Figure
1) is higher than the maximum limit allowed, its geometrical parameters are not either
discussed further or compared individually with its analogues.
The ORTEP representation of the structure of [Cu(L5)Cl2] 5 including atom numbering
Scheme is shown in Figure 2. The crystallographic data are given in Table 1 while the bond
lengths and bond angles are collected in Table 2. The X-ray structure of the complex molecule
contains a five-coordinate CuN3Cl2 chromophore constituted by two imine (N1, N3) and one
pyridine (N2) nitrogen of the tridentate ligand L5 and two chloride ions. The geometry around
copper(II) is best described as square pyramidal distorted trigonal bipyramidal (SPDTBP) [38],
10

as seen from the value of the trigonal index[59]  of 0.61 [ = ()/60, where  = N(1)-
Cu(1)-Cl(1) = 174.22 and  = N(3)-Cu(1)-Cl(2) = 137.50; for perfect square pyramidal and
trigonal bipyramidal geometries the  values are zero and unity respectively]. The corners of
the trigonal plane of the TBP geometry are occupied by N2 and N3 atoms of the L5 ligand and
one of the chloride ions (Cl2) and the axial positions are occupied by the N1 nitrogen atom of
L5 and the other chloride ion (Cl1). The axial chloride ion is coordinated at an average angle of
109.89(3) to the CuN2Cl plane. The Cu-Npy (2.1657 Å) and Cu-Namine (Cu-N3, 2.1352, Cu-N1,
2.0328 Å) bond distances fall in the range of values found for similar complexes [60,61]. The
axial N1 nitrogen atom is located at a distance shorter than the equatorially located N2 and N3
nitrogen atoms.
It is interesting that to the trigonal index of 5 is much higher than that ( = 0.09) of the
analogous complex [Cu(L4)Cl2] 4 with trigonal bipyramidal distorted square based pyramidal
(TBDSBP) coordination geometry[53], clearly indicating that the higher steric bulk of the N,N-
diethyl group in 5 confers steric constraints at Cu(II) much higher than that of N,N-dimethyl
group in 4. The coordination of pyridine nitrogen to Cu(II) in [Cu(L5)Cl2] leads to a Cu-Npy
(2.166, 2.135 Å) and the terminal Cu-Namine (2.090 Å) bond distance longer by 0.135 Å and
0.045 Å respectively than that in [Cu(L4)Cl2]. On replacing the two methyl groups on the
terminal nitrogen atom in 4 by two ethyl groups as to obtain 5, the Cu-Namine distance (2.032 Å)
of the central amine nitrogen atom in the former remains almost unaffected, while the Cu-Npy
(2.031 Å) distances increase due to the higher steric bulk of the ethyl groups. As steric bulk of
the NEt2 group renders the nitrogen lone pair orbital to deviate further from its orientation
towards the dx2–y2 orbital of Cu(II), making the Cu-NEt2 bond in 5 weaker than the Cu-NMe2
bond in 4, which is compensated by the concomitant shortening of both the axial Cu-Cl(16)
(2.532 Å), and equatorial Cu-Cl(15) bonds (2.267 Å) in comparison with those in 4.
Electronic Absorption Spectra
The electronic absorption spectra of all the six complexes in methanol solution are very
similar to each other and show a low energy ligand field (LF) band in the range 645 - 690 nm
(Table 3) and a ligand-based band around 265 nm. The LF band energy of the complexes with
–NH(R) donors follow the order 1 > 2 > 3; on replacing the N-Me group in 1 by the bulky N-Et
group to obtain 2, the lone pair orbital on the terminal amine nitrogen in 2 towards the d-orbital
11

of copper(II) deviates more due to the steric bulk of the N-ethyl groups rendering the terminal
Cu-Namine bond weaker (cf. above) and hence the lower LF band energy observed for 2.
Similarly, on replacing the methyl group in 2 by the electron-withdrawing (electron sink)
property of the N-phenyl group as in 3, decreases the donor ability of the amine nitrogen donor
atom decreases leading to a lower LF band energy for 3. A similar decrease in LF band energy
is observed upon incorporating the benzene ring in 2, as in 6. As expected, a decrease in LF
band energy is observed on replacing the -NMe2 group in 4 by the bulky -NEt2 group as in 5
(cf. above). In contrast to the observation of only one LF band in methanol solution, all the
complexes exhibit two LF bands in aqueous solution, suggesting changes in coordination
geometry like increase in planarity of the square pyramidal coordination geometry and
substitution of one or more chloride ions by solvent water molecules.
N
N
N
N
Cl
Cu
Cu
N
Cl
N
Cl
Cl
2,4
5
DBC2-
DBC2-
N
N
O
N
N
t-Bu
t-Bu
Cu
Cu
N
O
N
O
O
t-Bu
t-Bu
N
N
Cu
O
N
O
H2O
t-Bu
t-Bu
Scheme 2 Possible structures of catecholate adducts of copper(II) complexes of tridentate 3N
ligands
12

Upon adding one equivalent of 3,5-di-tert-butylcatecholate anion (DBC2-) to one equivalent of
1–6 in N2 saturated aqueous solution, both the LF bands undergo a blue shift (12–54 nm),
which is expected of the strong bidentate coordination of catecholate anion by substituting the
coordinated chloride ions (Scheme 2). Upon micellization of the DBC2- adducts in aqueous
SDS micellar solution, the LF bands are shifted to longer wavelengths (74–174 nm) with
slightly or highly enhanced molar absorptivity. As the weakly bound axial catecholate oxygen
is substituted by a water molecule to form [Cu(L)(DBC)(H2O)] results in decrease in LF
strength. However, the increase in positive charge on copper(II) leads to stronger interaction of
the monopositive copper(II) complex with the anionic SDS micellar head group followed by
micellization with incorporation of the coordinated DBC2- and pyridine ring in the hydrophobic
region of the micelles. The LF bands in CTAB micellar solution also undergo a red-shift (2 –
31 nm) with increase in molar absorptivity for all the complexes but lower than those in SDS
and Triton X-100 micellar solutions, revealing that most of the complex adduct species are
located in the aqueous phase rather than in the micellar pseudophase.
Electron Paramagnetic Resonance Spectra
The EPR spectral data of the complexes and their DBC2- adducts are collected in Table
4 and a few typical spectra are presented in Figures 3 and 4. The frozen solution EPR spectra
2
2
of all the six complexes are axial with g > g > 2.0, suggesting the presence of a dx -y ground
state in copper(II) located in square-based geometries[59,61], which is consistent with the
distorted square-based copper(II) geometries of 2, 4 and 5 evident from their X-ray crystal
structures (cf. above). The g (2.211 - 2.267) and A (166-181  10-4 cm-1) values observed for
the complexes fall in the range expected for CuN3O chromophore [59-62] in the g vs. A plot,
illustrating that the weak axial chloride ion and even the strongly bound equatorial chloride ion
are possibly replaced by solvent molecules[60] to form the complex species [Cu(L)(H2O)Cl]+
and [Cu(L)(H2O)2]2+ with respectively CuN3O and CuN3O2 chromophores in aqueous solution
(cf. above). For 6, the value of g is higher and that of A is lower than those for 4, suggesting
that the distortion in Cu(II) coordination geometry in the former is higher, which is consistent
with the ligand field spectral results (cf. above). The g/A values fall in the range 111 - 133 cm,
13

which is typical of square planar coordination geometry around Cu(II) with very low
tetrahedral geometric distortion [53].
The increase in g and A and decrease in g/A values of almost all the [Cu(L)Cl2]
complexes upon adding DBC2- to yield the [Cu(L)(DBC)] adducts support the formation of
catecholate adducts, which is conformity with the ligand field spectral studies. However, the
decrease in value of g from 2.267–2.211 to 2.239–2.219 and the large increase in A from 166–
181 to 182–190  10-4 cm-1 observed suggests enhanced planarity of Cu(II) coordination
geometry. This is supported by the decrease in values of the g/A index as well as the
appearance of nitrogen super hyperfine (N-shf) signals in both the perpendicular and parallel
regions. The g and A values for [Cu(L2)(DBC)] are respectively higher and lower than those
for the [Cu(L3)(DBC)] adduct, suggesting the presence of enhanced distortion of the
copper(II) coordination geometry in the former, which is consistent with the ligand field
spectral results. Such electronic and EPR spectroscopic results are useful in diagnosing the
formation of solubilized catecholate adducts in aqueous micellar environment and in
illustrating their reactivity in solution. (cf. below)
Electrochemical Behaviour of Complexes and DBC2- Adducts in Aqueous Micellar
Solution
The electrochemical features of 1 - 6 and their 1:1 DBC2- adducts were investigated in
water and in aqueous SDS, CTAB and TX-100 micellar solutions by employing cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) on a stationary platinum sphere
electrode [63,64]. All the complexes exhibit a coupled pair of redox waves at negative
potentials (Tables 5), which are assigned to Cu(II)/Cu(I) redox couple and the observed DPV
responses for them in the presence and absence of DBC2- in water and aqueous micellar
solutions are shown in Figures 5-9. Additional waves corresponding to dimeric Cu(II) (cf. ESI-
MS results above) and/or aquated monomeric species are observed at more positive potentials
in DPV responses. The E1/2 values of the Cu(II)/Cu(I) redox couple of the complexes with the
same chelating ring systems with varying N-methyl substituents follow the trends, 1 < 2 < 3 >
4 > 5 < 6 (Table 5) in aqueous SDS micellar media. Upon replacement of the terminal -NMe
group in 1 by the more sterically demanding -NEt group to obtain 2, the nitrogen lone pair
2
2
orbital deviates from being oriented towards dx -y orbital of copper(II) more, weakens the Cu-
14

Namine bond and hence destabilizes the Cu(II) oxidation state rendering the Cu(II)/Cu(I) redox
potential more positive [63,65]. Similarly, the incorporation of the electron-withdrawing N-
phenyl substituent on the terminal amine nitrogen as in 3 tends to weaken the Cu-Namine bond,
destabilizes the Cu(II) oxidation state and renders the Cu(II)/Cu(I) redox potential more
positive (Figures 7, 9, 10). The incorporation of one more methyl (4) or ethyl (5) group on the
terminal amine nitrogen in 1 or 2 leads to enhance the ligand steric bulk and hence raises the
Cu(II)/Cu(I) redox potential to a more positive value (E1/2: 4, -0.232; 5, -0.188 V), as illustrated
above. Also, the replacement of two methyl groups on the terminal amine nitrogen (4) by two
bulkier ethyl groups (5) leads to weakening of the Cu-Namine bond, which is evidenced by the
X-ray structures of 4 and 5 (Cu-Namine: 4, 2.090; 5, 2.135 Å), and confers a more positive value
of Cu(II)/Cu(I) redox potential. The incorporation of a fused benzene ring in 1 to give 6 leads
to decrease in the donor abilities of both the amine nitrogen donor atoms, weakens the Cu-
Namine bond and destabilizes the Cu(II) oxidation state, leading to a higher Cu(II)/Cu(I) redox
potential for 6. It is interesting that the stereoelectronic properties of the donor atom determines
the Cu(II)/Cu(I) redox potential, which is in line with the ligand field band
positions (cf. above).
The Cu(II)/Cu(I) redox potentials of all the complexes in aqueous SDS micellar solution
are more positive (18-49 mV) than those in aqueous solution. This suggests the formation of
monocationic [Cu(L)(H2O)Cl]+ aquo species or dicationic [Cu(L)(H2O)2]2+ species in
equilibrium with [Cu(L)Cl2] in solution, which interacts preferentially with the negatively
charged head groups of the SDS micelles leading to facile reduction of Cu(II). Similarly, the
redox potentials of the complexes, except 1, are more positive (5-39 mV) in aqueous cationic
CTAB micellar solution than in aqueous solution implying formation of [Cu(L)Br2] rather than
[Cu(L)Cl2] or the monocationic [Cu(L)(H2O)Cl]+ or dicationic [Cu(L)(H2O)2]2+ species,
leading to stabilization of Cu(I) rather than Cu(II) oxidation state. Interestingly, the
Cu(II)/Cu(I) redox potentials of 3, 5 and 6 in aqueous Triton X-100 are more positive than
those in aqueous solution, illustrating that the neutral Cu(I) complex species [Cu(L)Cl] formed
upon reduction of [Cu(L)Cl2] or the monocationic [Cu(L)(H2O)Cl]+ species, interact with the
neutral micelles more strongly via hydrophobic interaction of the benzene rings in them
followed by micellar solubilization, rendering the reduction of Cu(II) more facile. However, the
redox potentials of 1 and 4 in aqueous Triton X-100 are slightly more negative than, and that of
15

2 is the same as, that in aqueous solution, suggesting that the Cu(II) species are located mostly
in aqueous solution. All these observations clearly suggest that the Cu(II) complexes [Cu(L)Cl]
exist as the aquated species [Cu(L)(H2O)Cl]+ and [Cu(L)(H2O)2]2+ in aqueous solutions and
interact with the anionic, cationic and neutral micelles to different extents and undergo
micellization solublization depending on the nature of the micelles and stereoelectronic
properties of the 3N ligands.
The redox behavior of DBC2- adducts of all the complexes generated in situ in aqueous
and aqueous micellar solutions (cf. spectral studies) have been also studied. The cyclic
voltammogram of [Cu(L1)(DBC)] species in aqueous solution, features a quasireversible (E1/2,
+0.082 to -0.086 V, Ep, 102 mV) DBSQ/DBC2- redox wave. The observed Cu(II)/Cu(I) redox
potentials (Table 5) reflect considerable decrease in Lewis acidity of the copper(II) center[36]
upon coordination to DBC2-, supporting the suggestion that the copper(II) center in catecholase
enzyme takes part not only in activation of the substrate molecules but also in the latter stages
of enzyme reaction. The redox potentials of coordinated DBSQ/DBC2- couple for all the
complexes in aqueous solution fall in the range +0.004 – -0.088 V, which is considerably more
positive than that of free DBSQ/DBC2- couple (-1.434 V), reflecting the significant stabilization
of coordinated DBC2- towards oxidation and the role of primary ligands in determining the
extent of stabilization. The redox potential of DBSQ/DBC2- couple of the adducts
[Cu(L)(DBC)] in aqueous solution becomes more positive for 2 and 5 upon micellization in
SDS micelles. It is evident that the cationic aquo species [Cu(L2)(DBC)] and
[Cu(L5)(DBC)(H2O)] with a positive charge on copper(II) (cf. above) interact strongly with the
anionic SDS micelles. The adduct species [Cu(L2)(DBC)(H2O)] interacts strongly with the
cationic CTAB due to hydrophobic interaction of the coordinated solvent molecules. The redox
potentials of DBSQ/DBC2- couple of the adduct of 3 in SDS, CTAB and Triton X-100 micelles
are almost the same and are less positive than those in aqueous solution. The redox potential of
the adduct of 4 in SDS micelles is lower than, and that in CTAB and Triton X-100 are almost
the same as and are less positive than those in aqueous solution, suggesting that the nature of
the species is similar to those present in aqueous phase. This clearly suggests that the electron
transfer from the bound catecholate adduct to dioxygen is determined by the nature of the
micelles. The redox potentials are largely a function of hydrophobicity, local environment of
the donor atoms and the ligand field strength as determined by the donor atom type.
16

Kinetics and Reactivity Studies
In this study, the kinetics of oxidation of H2DBC was determined by monitoring the
development of the 400 nm (max, 1900 M-1 cm-1) band of DBQ[65,66] (Figure 9) and the
results are collected in Tables 6 and 7. The reactivity studies were performed in aqueous and
aqueous micellar solutions because of the good solubility of 1-6 as well as the substrate H2DBC
and its quinone product DBQ. A solution of the complexes (3.010-4 M) in aqueous and
aqueous micellar solutions were treated with an equivalent amount of H2DBC in the presence
of molecular oxygen (saturated) at 25 C. The course of the reaction was followed over the first
60 - 120 sec duration of the reaction by monitoring the development of the quinone band as a
function of time and the pseudo-first order rate constant kobs was determined (Table 6). For all
the complexes, a linear relationship between the initial rate and complex concentration is
obtained with a large initial excess of them, revealing a first-order dependence on the catalyst
concentration. The reactivity and kinetics of 1-6 performed in aqueous and aqueous micellar
solutions towards oxidative dehydrogenation of catechol vary significantly in all the reaction
media (Tables 6,7). No significant decrease in absorption of the 400 nm band corresponding to
auto-oxidation of catechol was observed in blank experiments performed without a catalyst. At
low concentrations of H2DBC (stoichiometric) a first-order dependence of the substrate
concentration is observed. The data obtained for all the three micelles were analysed by
applying the Michaelis-Menten approach (Figures 10 – 12) and the kinetic parameters
evaluated, such as maximum velocity (Vmax), the rate constant for the dissociation of the
complex-substrate intermediate (Kcat), the Michaelis binding constant (KM) and the turn-over
efficiency (Kcat/KM) are collected in Table 7. It is remarkable that the rates and turn over
numbers for the catecholase-like activity observed in aqueous CTAB and aqueous TX-100
micelles for the same complex are also higher than those observed for similar complexes in
non-aqueous solvents [54,55].
Effect of Ligand Donors on Catalytic Activity
All the complexes show a significant catecholase activity with 5 showing the highest
rate of reaction in SDS medium and 6 showing the highest rate in other media. Interestingly, for
all the Cu(II) complexes, the catecholase activity depends upon the nature of micelles with the
17

rate generally varying as SDS > TX-100 > water > CTAB. Thus, the reaction rate in aqueous
SDS micelles is 1.7 to 5.4 times higher, and that in aqueous Triton X-100 micelles also 1.4 to
6.3 times higher, than that in aqueous solution. Interestingly, the rate observed in aqueous
CTAB micelles is 1.5 - 4.0 times lower than that in aqueous solution. In aqueous SDS micelles
showing the highest rate for the oxidative dehydrogenation of catechol, the observed rate of
reaction depends upon the ligand donor atom type. The trend in reactivity 5 (2.6) > 4 (2.2) > 2
(2.1) > 1 (2.0) > 6 (1.7) > 3 (1.5) × 101M-1s-1 is illustrated by invoking the catecholase reaction
mechanism [46] shown in Schemes 3a and 3b. As formation of Cu(I) species facilitates
catecholase activity, a ligand system that stabilizes Cu(I) oxidation state would be expected to
enhance the catecholase activity. The observed trend in rate is in line with the variation in
Cu(II)/Cu(I) redox potential of complexes and thus the rate of reaction of 5 with a higher
Cu(II)/Cu(I) redox potential (E1/2, -0.208 V vs Ag/Ag+) on account of constraints at Cu(II) (,
0.61, cf. above) higher than 4. So, it is inferred that the catecholase activity increases with
increase in bulkiness of ligand substituents. In fact, it has been assumed [68] that the
coordination geometry around copper in dinuclear Cu(II) complexes, in addition to the
Cu····Cu distance, is the key factor that determines the catalytic activity of complexes (cf.
above). Also, it has been argued [69] that the square planar coordination geometry of a Cu(II)
complex is not ideal for displaying catecholase activity [41,70]. The decrease in absorbance of
700 nm the LF band observed for all the present complexes (Figure 9b) reveals that the
complexes are reduced to Cu(I) species during catalysis.
N
N
N
N
N
O
O
H2O
H2O
Cl
Cl
II
II
II
Water
DBC2-
Cu
N
Cu
N
Cu
N
N
N
N
N
N
O
O
O
O
I
I
I
Water
°
Cu
N
+
Cu
N
H2O Cu
N
O
O
N
N
Scheme 3a Non-catalytic Pathway of Quinone Formation
18

Similar observations have been made previously [41] for other copper(II)
complexes exhibiting catecholase activity. Thus, it is interesting that the catalytic
activity increases as the steric crowding at the terminal nitrogen donor atom is
increased. Interestingly, in aqueous SDS micelles, 5 shows the highest rate of
catecholase activity while 6 shows the lowest rate. In contrast, in all other micellar
media, 6 shows the highest activity while 5 shows a lower activity. It is obvious that the
actual species involved in catalysis, namely, [Cu(L)Cl2] and [Cu(L)Br2] in aqueous TX
100 and aqueous CTAB respectively as discussed above, are different from that in SDS
micellar media. In this regard, it is interesting to note that the rate of reaction observed
in CTAB is almost the same as that observed in aqueous solution as the active species
are located mainly in the aqueous phase (cf. above). Also, it is obvious that the cationic
CTAB micelles would bind to the deprotonated catechol and stabilize it and that the
formation of the dinuclear intermediate species [(DBSQ)(L)CuI(O2)CuI(L)(DBSQ)],
rather than a mononuclear copper(I) intermediate species would be facilitated in
cationic CTAB micelles. Also, the conversion of CuII to CuI species would be facilitated
more in CTAB than in aqueous solution, and in anionic SDS and neutral Triton X-100
micelles. The labile sites oriented in cis positions enhance the rate of catechol cleavage
reaction by enhancing the probability of catecholate adduct formation and by decreasing
the free energies of activation for oxygen insertion, demonstrating higher catecholase-
like activity. The dinuclear µ-peroxo complex formation has been invoked previously to
illustrate the catecholase activity of mononuclear Cu(II) complexes [71]. In the case of
Cu(II) complexes valence isomerism of [CuII(L)(H2DBC)] to [CuI(L)(DBQ)] is
significant. The latter reacts with dioxygen first to form a µ-peroxo Cu(II) complex,
which reacts with the CuI species to form the dinuclear copper dioxygen complexes in
the second step.
19

Step I
N
N
N
N
N
N
O
O
DBC2-
II
Water
H2O
H2O
Cl
II
II
Cu
N
Cu
N
Cu
N
Cl
O
N
N
N
N
O
O
O
I
O2 +
Cu
N
Cu
N
II
O
O
Step II
HO
N
N
N
N
N
O
O
O
N
I
CuII
+
N
Cu
Cu
N
II
O
O
O
O
O
O
O
O
II
Cu
N
N
N
N
N
N
N
CuI
N
O
O
II
H2O
H2O
H2O
O
O
O
O
O
O
2
2
Cu
N
I
+
Cu
N
N
N
N
+H2O2
Overall reaction
OH
OH
O
O
+
+
O2
H2O2
Scheme 3b Catalytic Pathway of Quinone Formation
20

The copper system provides a reaction pathway for bound peroxidic species. This is in
agreement with the previous report by Taylor and Williams [48] that copper ions are good
scavengers for the dangerous peroxidic species and superoxide ion. Thus, the present copper
complexes, which catalyze catecholase reactions efficiently in micelles, may be regarded as
functional models for catechol oxidase enzymes and the study demonstrates the key role of the
redox-active copper in the oxidative dehydrogenation.
Conclusions and Relevance to Copper Oxygenases
The mononuclear Cu(II) complexes 2 and 5 contain CuN3Cl2 chromophore with
trigonal bipyramidal coordination geometry. In 5, the axial chloride is coordinated at a distance
longer than that coordinated in the trigonal plane. The electronic absorption and EPR spectral
studies reveal that one of the coordinated chloride ions in the complexes is substituted by water
in aqueous solution and that the complexes form catecholate adducts upon adding DBC2-. In
aqueous SDS micellar solution, the adduct species present are [Cu(L)(DBC)(H2O)] in which
DBC2- acts as a monodentate ligand and the adducts interact strongly with the anionic micelles.
In contrast, the neutral adduct species [Cu(L)(DBC)] with bidentatively coordinated DBC2- are
located mostly in aqueous phase of aqueous CTAB and Triton X-100 micellar systems.
Electrochemical studies reveal that the cationic species [Cu(L)Cl]+/ [Cu(L)(H2O)Cl]+ interact
strongly with the anionic head groups of SDS micelles and show less reversible Cu(II)/Cu(I)
redox changes. In contrast, the neutral [Cu(L)Cl2] and [Cu(L)Br2] species formed in TX-100
and CTAB micelles respectively show more reversible Cu(II)/Cu(I) redox changes. The
spectacular observation that the rates of oxygenase reaction in organized assemblies varies as
SDS > TX-100 > water > CTAB reveals that the nature of the micellized copper(II) catalyst in
biomimetic microenvironments varies and plays a very important role in determining the
oxidation of catechol to o-benzoquinone. The ligand electronic and steric factors are found to
be important in determining the efficiency of the catalysts towards the oxidation. Interestingly,
the labile sites oriented in cis positions alter the reaction pathways and enhance the rate of
catechol cleavage reaction by increasing the probability of oxygen insertion and by decreasing
the free energies of activation.
21

Acknowledgement
This work was supported by the Science and Engineering Research Board (SERB Scheme
No. EMR/2015/002222). We thank the Department of Science and Technology (FIST Program),
New Delhi for the use of the X-ray diffractometer at School of Chemistry. We thank Dr.
Balachandran Unni Nair, Central Leather Research Institute, Chennai for providing access to the
ESI-MS facility.
†Electronic supplementary information (ESI) available. CCDC 1046097 for complex
[Cu(L5)Cl2] 5.
22

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27

N2
Cl2
Cu1
N1
N3
Cl1
Figure 1 Molecular structure of [Cu(L2)Cl2] 2 (50% probability factor for the thermal
ellipsoid). Hydrogen atoms have been omitted for clarity.
N1
N2
N3
Cu1
Cl2
Cl1
Figure 2 Molecular structure of [Cu(L5)Cl2] 5 (50% probability factor for the thermal
ellipsoid). Hydrogen atoms have been omitted for clarity.
28

g||
(A)
g
160 G
g||
g
(B)
Figure 3 X-band EPR spectra of [Cu(L3)Cl2] 3 complex (A) and its adduct with DBC2- (B) in
frozen aqueous methanol:acetone (4:1 V/V) glass at 77 K
29