Square planar versus square pyramidal copper(II) complexes containing N3O moiety: Synthesis, structural characterization, kinetic and catalytic mimicking activity

Article abstract of DOI:10.1016/j.ica.2018.11.024

In quest of copper complexes having [CuN₃O] cores, [Cu(phen)(L–Val)(H₂O)]NO₃ (1) and [Cu(bpy)(L–Val)]ClO₄ (2) complexes have been synthesized and structurally characterized (phen = 1,10–phenanthroline; bpy = 2,2′–bipyridine). Complex 1 possesses a distorted square-pyramidal, whereas 2 has a distorted square-planar coordination geometry. Structures of 1 and 2 have supramolecular networks formed via inter- and intra-molecular hydrogen bonding interactions. The kinetics and mechanism of ligand substitution of 1 and 2 by thiourea (TU) were studied in detail and showed a biphasic process in which an initial fast reaction is followed by a slower one. The activation parameters for the fast reaction: ΔH^# = 68 ± 4 and 73 ± 5 kJ mol^?1, ΔS^# = 43 ± 10 and 54 ± 9 J K^?1 mol^?1 for 1 and 2, respectively, supports a dissociative substitution mechanism. Whereas for the slow reaction: ΔH^# = 33 ± 6 and 43 ± 3 kJ mol^?1, ΔS^# = ?77 ± 10 and ?56 ± 9 J K^?1 mol^?1 for 1 and 2, respectively, support an associative substitution mechanism. It is concluded from the activation parameters that the difference in structure does not affect the mechanism. Complexes 1 and 2 have also been evaluated as functional models for the catechol oxidase enzyme and phenoxazinone synthase. The model complexes 1 and 2 show catecholase activity of K_cat = 10.9 × 10³ and 11.4 × 10³ h^?1 and phenoxazinone synthase activity of K_cat = 2.1 × 10³ and 4.3 × 10³ h^?1, respectively. Compared to the enzyme itself (K_cat = 8.3 h^?1), the model complexes 1 and 2 are promising candidates as functional mimics for catechol oxidase and phenoxazinone synthase.

Full text of DOI:10.1016/j.ica.2018.11.024

Accepted Manuscript
Research paper
Square Planar versus Square Pyramidal Copper(II) Complexes Containing N3O
moiety: Synthesis, Structural Characterization, Kinetic and Catalytic Mimicking
Activity
Shaban Y. Shaban, Abd El-Motaleb M. Ramadan, Mohamed M. Ibrahim,
Fawzya I. Elshami, Rudi van Eldik
PII:
S0020-1693(18)31697-9
https://doi.org/10.1016/j.ica.2018.11.024
ICA 18639
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
3 November 2018
17 November 2018
19 November 2018
Please cite this article as: S.Y. Shaban, A.E-M. Ramadan, M.M. Ibrahim, F.I. Elshami, R.v. Eldik, Square Planar
versus Square Pyramidal Copper(II) Complexes Containing N3O moiety: Synthesis, Structural Characterization,
Kinetic and Catalytic Mimicking Activity, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.
2018.11.024
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Square Planar versus Square Pyramidal Copper(II) Complexes Containing N3O
moiety: Synthesis, Structural Characterization, Kinetic and Catalytic Mimicking
Activity
Shaban Y. Shabana,b*, Abd El-Motaleb M. Ramadana, Mohamed M. Ibrahima,
Fawzya I. Elshamia, Rudi van Eldikb,c,*
a
Chemistry Department, Faculty of Science, Kafrelsheikh University, Kafrelsheikh
Egypt e-mail: shaban.shaban@sci.kfs.edu.eg
b
Department of Chemistry and Pharmacy, University of Erlangen-Nuremberg,
Egerlandstr. 1, 91058 Erlangen, Germany e-mail: rudi.vaneldik@fau.de
c
Faculty of Chemistry, Jagiellonian Unversity, Gronostajowa 2, 30-387 Krakow,
Poland
Abstract
In
quest
of
copper
complexes
having
[CuN3O]
cores,
[Cu(phen)(LVal)(H2O)]NO3 (1) and [Cu(bpy)(LVal)]ClO4 (2) complexes have
been synthesized and structurally characterized (phen = 1,10phenanthroline; bpy =
2,2'bipyridine). Complex 1 possesses a distorted square-pyramidal, whereas 2 has a
distorted square-planar coordination geometry. Structures of 1 and 2 have
supramolecular networks formed via inter- and intra-molecular hydrogen bonding
interactions. The kinetics and mechanism of ligand substitution of 1 and 2 by thiourea
(TU) were studied in detail and showed a biphasic process in which an initial fast
reaction is followed by a slower one. The activation parameters for the fast reaction:
H# = 68 ± 4 and 73 ± 5 kJ mol-1, S# = 43 ± 10 and 54 ± 9 J K-1 mol-1 for 1 and 2,
respectively, supports a dissociative substitution mechanism. Whereas for the slow
reaction: H# = 33 ± 6 and 43 ± 3 kJ mol-1, S# = -77 ± 10 and -56 ± 9 J K-1 mol-1 for
1 and 2, respectively, support an associative substitution mechanism. It is concluded
from the activation parameters that the difference in structure does not affect the
mechanism. Complexes 1 and 2 have also been evaluated as functional models for the
catechol oxidase enzyme and phenoxazinone synthase. The model complexes 1 and 2
show catecholase activity of Kcat =10.9 x 103 and 11.4 x 103 h-1 and phenoxazinone
synthase activity of Kcat = 2.1 x 103 and 4.3 x 103 h-1, respectively. Compared to the

enzyme itself (Kcat = 8.3 h-1), the model complexes 1 and 2 are promising candidates
as functional mimics for catechol oxidase and phenoxazinone synthase.
Keywords: 1,10phenanthroline; 2,2'bipyridine; LValinate; Copper(II) complex;
Crystal structure; Catechol oxidase; Phenoxazinone synthase, Catalytic promiscuity
1. Introduction
Complexes containing N,Odonor moieties play an important role in the
active sites of a large number of metalloproteins in biological systems and application
for numerous catalytic processes [1]. Amino acids which act as N,Odonor ligands
are the building blocks of proteins and take part in all major processes in organisms
[2]. L-valine occupies importance as a ligand in many metallo-enzymatic reactions in
human life and are particularly involved in stress, energy and muscle metabolism and
promotes muscle growth and tissue repair [3]. Copper complexes containing
LValine, as N,Odonor ligands are used as models to study the pharmacological
effects of drugs [4]. Furthermore, copper complexes containing phenanthroline and
bipyridine moieties are known for their chelating mode of coordination and for
developing new diagnostic and therapeutic agents [5].
Catechol oxidase (CO) is a dinuclear Cu(II) containing enzyme with a type-3
active site. In many biological systems, CO oxidizes organic substrates such as
substituted catechol to substituted oquinone through fourelectron reduction of
dioxygen to water under mild conditions [6]. On the other hand, phenoxazinone
synthase (PHS), a penta-nuclear copper containing enzyme, catalyzes two molecules
of substituted 2-aminophenol to the phenoxazinone chromophore during synthesis of
Actinomycin D [7]. Catechol oxidase and phenoxazinone synthase (PHS) must
contain the Cu(II) ion in the required geometry with a suitable redox potential to
switch between Cu(II)/Cu(I) species. Therefore, the geometry as well as the redox
potential of the catechol oxidase and phenoxazinone synthase (PHS) models are very
important. Furthermore, accessible labile site(s) on the models for possible
substrate/oxygen binding should be present [8].
In our previous works, we have reported a series of mixed ligand copper(II)
complexes of 2,2'bipyridine and glycine and studied their antioxidant (superoxide
dismutase and catalase) biomimetic catalytic activities and found to be promising
candidates [9]. These results have stimulated us to investigate copper(II) complexes,
2 | P a g e

which, in addition to phenanthroline and bipyridine ligands, contain L-alanine instead
of glycine, as the chemical and biological activities of complexes can be affected by a
change of the substituent of the ligand. As an extension of these previous works and
with the goal to design copper(II) mimics of catechol oxidase and phenoxazinone
synthase [10], we report herein the synthesis and structural characterization of two
copper(II) complexes, [Cu(Phen)(LVal)(H2O)]NO3 (1) and [Cu(bpy)(LVal)]ClO4
(2). Furthermore, the ligand substitution properties as well as the catechol oxidase and
phenoxazinone synthase activities are reported and correlated with the structure of the
complexes.
2. Experimental
2.1. Materials and instrumentation
Caution! Perchlorate salts are potentially explosive and only small quantities should
be handled with care. IR spectra were recorded using a FTIR Bruker
spectrophotometer in the range of 4004000 cm-1. UVVis spectra were measured
using an UVLambda 25 Perkin Elmer spectrophotometer. Magnetic moments were
measured by Gouy's method at room temperature. The electrochemical measurements
were recorded using an Auto Lab Potentiostat PGSTAT 302 (Eco Chemie, Utrecht,
The Netherlands) driven by the GPES data processing software (version 4.9, Eco
Chemie). Kinetic studies were performed on a Kinet Asyst SF-61DX2 stoppedflow
instrument (thermostated at 23 °C) with an optical path length of 1 cm, at 300 nm for
thiourea, 400 nm for 3,5di(tertbutyl)catechol (3,5DTBC) and 420 nm for o-
aminophenol (OAP).
2.2. Detection of hydrogen peroxide during catalysis:
A mixture of 10 ml of 3,5DTBC (10−3M) and 10 ml of complex 1 (2.5 × 10−5M)
was stirred at room temperature for 1 h and then the pH of the solution was adjusted
to 2 to stop further oxidation. Extraction with CH2Cl2 followed by the addition of 1 ml
of potassium iodide solution (10%) and 4 drops of ammonium molybdate solution
(3%) to the aqueous layer leads to the formation of I3- which was monitored
spectrophotometrically by measuring the intensity of the band at around 355 nm.
2.3. Synthesis of complexes
3 | P a g e

General method: To a mixture of LValine (1 mmol) and NaOH (1 mmol) in water, a
methanolic solution of CuX2·nH2O (1.0 mmol, X = NO3, ClO4) was added. Slow
addition of a methanolic solution of 1,10phenanthroline or 2,2'bipyridine (1.0
mmol) with stirring at room temperature led to the formation of fine colored crystals.
The microcrystalline product formed was filtered off, washed several times with cold
methanol and ether, and finally dried under vacuum.
2.4. Data Collection and Structure Refinement of complexes 1 and 2
Blue single crystals of the complexes were grown from a saturated CH3OH/H2O
solution at room temperature for four days. A suitable single crystal was mounted on
top of a glass capillary using protective perfluoropolyalkyl ether oil. Crystallographic
data were collected on a Smart CCD diffractometer of Bruker AXS using Mo-Ka
radiation (λ = 0.71073 Å, graphite monochromator). Lorentz-Polarization corrections
were performed by SAINT [11]. Absorption corrections were made by using
SADABS [12]. The structures were solved by direct method using SHELXS-2013 to
find the position of heavy atoms. Other non-hydrogen atoms were located by Fourier
syntheses and refined using SHELXS-2013 [13]. The least-squares refinements were
performed using all independent reflections by the full matrix on F2. All non-hydrogen
atoms were refined anisotropically. Selected crystallographic data, data collection and
structure refinement details are summarized in Table 1.
3. Results and Discussion
3.1. Synthesis and characterization of complexes 1 and 2
Copper(II) complexes 1 and 2 were synthesized from equimolar amounts of L–Valine,
1,10–phenanthrolene or 2,2'bipyridine and the appropriate Cu(II) salt in methanol
(Scheme 1). The complexes are soluble in water and polar organic solvents such as
methanol, ethanol, DMF and acetonitrile, but not in less polar organic solvents.
Structures of both complexes were determined using single crystal Xray structural
analysis and supported by analytical and spectroscopic methods.
4 | P a g e

Scheme 1. Syntheses of the copper (II) complexes 1 and 2.
characteristic bands at 3310 and 3268 cm1 that
are attributed to the stretching frequency of the coordinated amino group for 1 and 2,
respectively. The coordination of the COO group of LVal to the central Cu(II) was
supported by: i) the absence of any band in the region 17001750 cm1and ii) the
bands around 1621–1656 and 14321477 cm1 attributed to the υasym(COO–) and
υsym(COO–) stretching vibrations of the coordinated carboxylate, respectively [14].
The mono-dentate coordination of the carboxylate group has been confirmed by the
value of Δυ ≈ 200 cm1 {Δυ = υasym(COO–) – υsym(COO–)}[15]. Characteristic bands
due to the coordinated C=N moieties of the phenanthroline moiety at 1603 and 1583
cm1 are observed for 1 and 2, respectively [16]. Furthermore, the formation of
complexes was also deduced from the presence of new medium intensity bands
around 520550 and 410 cm1, corresponding to CuN and CuO, respectively. The
absorptions due to the presence of water molecules in 1 and 2 are evidenced by the
broad absorptions around 3400 cm1. The absorption attributed to the perchlorate
anion in 2 was found at 1116 cm1 and the absence of splitting of this band supports
that the perchlorate anions are not involved in the coordination.
Electronic spectral analysis, as well as electrochemical properties, were
studied to obtain further structural and solution properties of both complexes. Uv-vis
5 | P a g e

absorption spectra of both complexes in methanol show intense absorption bands at
223 and 228 nm for 1 and 2, respectively assigned to intra-ligand transitions.
Bands at 272293 nm region are due to n transitions of (C=N) of ligand origin
[17] while the broad absorption bands in the visible region at 625 and 670 nm with
molar absorption coefficients,  = 52 and 33 M1cm1 are attributed to dd transitions
[18] of Cu(II) centers for 1 and 2, respectively (Figure S1, Supporting Information).
The electrical molar conductivity measurements of 1 and 2 in 104 M DMF solution
show an electrolytic behavior with molar conductance values of 73 and 87
Ω−1mol−1cm2, respectively [19]. These values fall in the range expected for 1:1
electrolyte behavior, suggesting the nonparticipation of the nitrate and perchlorate
anions in the coordination sphere. The room temperature solid state magnetic
moments of both complexes, close to 1.95 BM, are expected for the spin only (S = ½)
behavior of 1.73 B.M. for the d9 configuration and shows the absence of any copper-
copper interaction.
The electrochemical behavior of 1 and 2 was studied in MeOH under
experimental conditions that were employed for the kinetic and catalytic studies.
Cyclic voltammograms of 1 and 2 are shown in Figure 1. The voltammograms show
reversible peaks at 183 and 194 mV for 1 and 2, respectively, indicating a
oneelectron reduction followed by the corresponding oxidation, which can be
assigned to the CuII/CuI species. The ratio of the peak currents ipa/ipc ≈ 1.0, suggesting
a chemically reversible CuII/CuI oneelectron transfer process. Futhermore, the value
of the limiting peak-to-peak separation (ΔEp, 80 mV) lies within the normal values for
a reversible oneelectron redox process (ΔEp, 60 mV for a reversible oneelectron
redox process) [20]. An additional one-electron oxidation at 394 and 412 mV was also
observed for 1 and 2, respectively. The ipa/ipc values are far from unity suggesting that
both complexes undergo structural reorganization on electron transfer.
6 | P a g e

5
4
1
2
3
2
1
0
-1
-2
-3
0.0
0.1
0.2
0.3
0.4
0.5
0.6
E[V]
Figure 1: Cyclic voltammograms of 1 and 2 in methanol in the presence 0.10 M
NBu4PF6, at a scan rate = 50 mV/s.
In addition, the molecular and crystal structures of 1 and 2 were determined by
a single-crystal Xray diffraction study. Thermal ellipsoid plots of the molecular
structures including the atom-numbering scheme are shown in Figure 2 and 3. The
crystallographic data of 1 and 2 are listed in Table 1, whereas selected bond angles
and bond distances are collected in Tables 2 and 3. Complex 1 and 2 crystallize in the
monoclinic space group P 121/c 1. The molecular structure of 1 involves a five-
coordinate [Cu(Phen)(LVal)(H2O)]+ cation in which the Cu(II) ion coordinates both
phenanthroline N donors, the amino N donor and one of the O donor atoms from the
Lvalinate ligand, while a water oxygen atom is coordinated in the axial position. The
coordination geometry around the Cu(II) center can be determined by calculating the
geometry parameter τ (τ = (β−α)/60, α and β are the smallest and largest angles
around central atom, respectively). For a perfect squarepyramidal geometry, τ =
zero, whereas for a perfect trigonalbipyramidal geometry, τ = 1.0 [21]. τ is 0.11,
indicating a coordination geometry in a distorted squarepyramidal orientation
(Figure 2). The five atoms, N1, N2, N3, O2 and Cu1 are in the equatorial positions
and are approximately coplanar. The bond angles around the Cu(II) center range from
82.70(6) to 101.64(7)° in the equatorial positions and from 93.30(7) to 102.06(10)°
for the apical positions. This is in agreement with those reported for related Cu(II)
complexes, viz. [Cu(Phen)(LVal)(H2O)]Cl [22], [Cu(Phen)(LVal)(H2O)]ClO4 [23]
and [Cu(Phen)(Val)(H2O)]NO3·2H2O [24]. The carboxylate group of Lvaline
coordinates to Cu(II) via one oxygen atom as an unidentate ligand. The nitrate ion is
7 | P a g e

not involved in the coordination sphere of the Cu ion but is involved in the crystal
lattice. The molecular structure of 2 is a four-coordinate [Cu(bpy)(LVal)]+ cation and
the Cu(II) ion in this case adopts a distorted square-planar via the coordination of both
bipyridyl nitrogens, the amino N donor and one of the O donor atoms from the
Lvalinate ligand. The Cu(1)N(3) distance of the amino nitrogen donor and the
Cu(1)N distances to the bipyridyl nitrogen atoms (N1) and N(2) (1.988(3) and
1.997(2) Å) are within the normal CuN bond distances [25]. The square planar
geometry in 2 is slightly distorted and the pyridine rings are slightly twisted.
In the crystal packing of 1, both hydrogen atoms of lattice and coordinated
water molecules are involved in hydrogen bonding interactions which are given in
Table 4. The hydrogen atoms, H(3W) and H(4W), undergo hydrogen bonding with
the carboxylate oxygen atoms, O(1) and O(2) (Figure 3). On the other hand, the
hydrogen atoms H(1W) undergo hydrogen bonding with the nitrate oxygen O(5) and
nitrogen N(4) atoms. Also the hydrogen atom H(2W) undergo hydrogen bonding with
the lattice water oxygen O(4) atoms, nitrate oxygen O(5) and nitrogen N(4) atoms,
and carboxylate oxygen atoms, O(1) and O(2) (Figure 3 and Table 4). Complex 2 also
shows a hydrogen bonding network in which the perchlorate anions are involved in
hydrogen bonding interaction between H(3) and O of the perchlorate anion. The
N(3)H(3)...O distances lie within the acceptable limits of hydrogen bond interactions.
A CH...O type intermolecular hydrogen bonding is also present, with CH acting as
the hydrogen-bond donor and the perchlorate oxygen atoms as the acceptors. (Figure
4 and Table 5).
Figure 2. Presentation of the single crystal Xray structure of 1 (left) and 2 (right),
showing the labeling scheme of non-hydrogen atoms.
8 | P a g e

Fig. 3. Inter- and intra-molecular hydrogen bonding interactions in complex 1.
Fig. 4. Inter- and intra-molecular hydrogen bonding interactions in complex 2.
Table 1: Crystallographic data collections and structure refinement of 1 and 2.
1
2
C15H20ClCuN3O7 (453.33)
Emp. formula (Formula wt.)
C17H22CuN4O7(457.92)
Temperature [K]
Wavelength [Å]
Crystal system
Crystal color
100(2)
100(2) K
0.71073
0.71073
Monoclinic
Blue
Monoclinic
Blue
9 | P a g e

Crystal size [mm]
Space group
0.080 x 0.170 x 0.230
P 1 21/c 1
970.2(5), 2
0.060 x 0.070 x 0.150
P 1 21/c 1
1778.6(7), 4
Volume [Å3], Z
Unit cell dimensions [Å; °]
a = 11.837(3); α = 90
b = 6.6647(19); β = 108.391(6)
c = 12.960(4); γ = 90
1.567
1.174
474
a = 10.025(2), α = 90
b = 8.0550(18), β =101.1(5)
c = 22.444(5), γ = 90
1.693
1.423
932
Density(calc.) [g/cm3]
 [mm-1]
F(000)
Ө-range [°]
Index ranges of h, k, l
Refinement method
1.66 to 28.73°
1.85 to 28.80°
-11≤h≤13,-10≤k≤10, -30≤l≤30
Full-matrix least-squares on F2
SHELXL-
2013(Sheldrick,2014)
Σ w(Fo2 - Fc2)2
18749
8748 [R(int) = 0.0236]
0.9190 and 0.8150
8748 / 7 / 523
-11≤h≤ 16, -9≤ k≤ 6, -17≤l≤17
Full-matrix least-squares on F2
Refinement program
SHELXL-2013(Sheldrick,2014)
Σ w(Fo2 - Fc2)2
9956
4447 [R(int) = 0.0196]
0.9120 and 0.7740
4447 / 7 / 296
1.037
Function minimized
Reflections collected
Independent reflections
Max. and min. Transmission
Data /restraints / parameters
Goodness-of-fit on F2
Δ/σmax
0.986
0.001
0.002
4117 data; I>2σ(I): R1=0.0231, 7903 data; I>2σ(I): R1=
Final R indices
wR2 = 0.0536
R1 = 0.0269, wR2 = 0.0550
0.0(0)
0.0271, wR2 = 0.0603
R1 = 0.0322, wR2 = 0.0619
0.0(0)
R indices (all data)
Absolute structure parameter
Largest diff. peak [e.Å-3] and hole 0.338 and -0.267
R.M.S. deviation from mean [eÅ-3] 0.053
0.793 and -0.348
0.062
Table 2: Selected bond lengths (Å), bond angles (°) of 1.
1.947(16)
2.005(19)
2.186(2)
2.000(18)
2.015(16)
1.233(3)
1.274(2)
1.252(3)
90.6(7)
115.8(16)
103.2(18)
123.7(18)
112.6(15)
128.6(14)
112.5(13)
107.0(13)
132.6(18)
130.2(13)
111.5(12)
Cu1O1
Cu1N3
Cu1O3
Cu1N2
Cu1N1
N4O6
N4O5
N4O7
O1Cu1N2
N2Cu1N3
N2Cu1N1
O1Cu1O3
N3Cu1O3
O1Cu1N3
O1Cu1N1
N3Cu1N1
N2Cu1O3
N1Cu1O3
Cu1N3H3A
Cu1N3H3B
Cu1O3H1W
C13O1Cu1
C12N2Cu1
C6N2Cu1
C14N3Cu1
Cu1O3H2W
C1N1Cu1
C5N1Cu1
170.1(11)
82.7(6)
93.3(7)
91.7(9)
82.8(7)
163.8(10)
101.6(7)
96.2(9)
102.1(10)
Table 3: Selected bond lengths (Å), bond angles (°) of 2.
1.933(2)
1.985(2)
2.336(2)
93.27(11)
176.56(15)
81.51(12)
84.66(12)
Cu1O9
Cu1N3
Cu1O10
O9Cu1N3
N3Cu1N1
N3Cu1N2
O9Cu1N1
O9Cu1N2
N1Cu1N2
172.41(11)
100.28(12)
1.997(3)
1.988(3)
91.68(9)
91.18(11)
108.8(2)
91.62(10)
93.96(10)
Cu1N2
Cu1N1
O9Cu1O10
N1Cu1O10
C1N1Cu1
N3Cu1O10
N2Cu1O10
10 | P a g e

Table 4: Inter- and intra-molecular hydrogen bond distances (Å) and angles (°) of 1.
A…H
D…A
DH…A
DH
< DH…A
C1H1…O7
0.95
2.37
3.27(3)
3.02(3)
3.47(3)
2.98(3)
2.97(3)
3.43(3)
2.75(3)
2.72(3)
2.81(3)
3.06(2)
2.96(3)
158.9
C12H12…O1
C12H12…O4
N3H3A…O7
N3H3B…O5
O3H1W…N4
O3H1W…O5
O3H2W…O4
O4H3W…O1
O4H4W…O1
O4H4W…O2
0.95
2.55
111.1
0.95
2.53
170.8
0.89(3)
0.92(3)
0.90(12)
0.90(12)
0.91(12)
0.90(13)
0.89(13)
0.89(13)
2.11(3)
2.08(3)
2.60(16)
1.85(13)
1.82(14)
1.95(16)
2.48(3)
2.07(13)
167.0(3)
163.0(2)
152.0(2)
174.0(3)
167.0(3)
159.0(3)
123.0(3)
174.0(4)
D = donor
A = acceptor H = hydrogen
11 | P a g e

Table 5: Inter- and intra-molecular hydrogen bond distances (Å) and angles (°) of 2.
A…H
D…A
DH…A
DH
< DH…A
C6H6…O14
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
2.47
2.60
2.36
2.60
2.47
2.40
2.64
2.59
2.62
2.41
2.58
2.55
3.288(4)
3.513(4)
3.285(5)
3.388(5)
2.992(4)
3.311(4)
3.320(5)
3.389(4)
3.471(4)
3.287(5)
3.356(5)
3.061(4)
3.824(3)
3.187(4)
3.286(4)
2.865(3)
2.977(5)
3.127(4)
3.042(4)
3.020(4)
3.318(4)
143.7
C9H9…O5
160.7
C12H12…O5
C14H14…O5
C15H15…O9
C21H21…O13
C22H22…O6
C24H24…O1
C24H24…O4
C27H27…O1
C29H29…O1
C30H30…O11
O13H2W…Cl1
O13H2W…O7
O13H2W…O8
O13H1W…O12
N1H1A…O14
N1H1B…O3
N4H4D…O13
N4H4E…O7
O14H4W…O2
164.0
140.8
114.7
160.8
129.4
141.7
148.6
154.1
138.8
114.1
0.890(13) 2.970(18)
0.890(13) 2.304(16)
0.890(13) 2.61(3)
0.890(13) 2.009(18)
161.0(4)
171.0(4)
133.0(3)
161.0(4)
152.0(4)
145.0(4)
154.0(3)
152.0(4)
149.0(3)
0.80(5)
0.81(4)
0.88(4)
0.79(4)
2.25(5)
2.43(4)
2.23(4)
2.30(4)
0.897(13) 2.52(2)
3.3. Kinetic and catalytic studies
Both complexes were investigated for the catalytic oxidation of 3,5di-
tertbutyl catechol (3,5DTBC) and o-aminophenol (OAP). The kinetic and catalytic
activity was investigated in methanol as a solvent because of the good solubility of the
complexes, substrates and their oxidation products. To minimize the possibility of
auto-oxidation in air, no additional bases were used during the catalytic reactions. As
previously reported [10], the reaction between the catecholase anion with model
complexes containing a good leaving group initiates the catalytic oxidation cycle and
if the leaving groups are stronger ligand than the catecholate anion, no oxidation will
be observed. Therefore, a detailed kinetic study of both complexes was also
performed.
3.3.1. Kinetic investigation on ligand substitution reactions:
12 | P a g e

The reactions of 1 and 2 with thiourea (TU) were studied kinetically in the spectral
range 300400 nm as a function of TU concentration and temperature in methanol.
Figure 5 shows UV/Vis spectra before and after the reaction together with kinetic
traces for complex 1 at 350 nm using the stoppedflow instrument. The kinetic traces
at the studied wavelength can be fitted to a two-exponential function. The kinetic
behavior can be discussed as two subsequent substitution reactions in which an initial
fast reaction (rate constant kobsd1) is followed by a slower one (rate constant kobsd2).
Both kobs1 and kobs2 increase linearly with the TU concentration in the absence of a
back reaction as shown in Figure 6, such that kobs1 = k1[TU] and kobs2 = k2[TU].
Activation parameters such as H# and S# can provide information about the
transition state of a given reaction and therefore be used as a mechanistic tool in
assigning the mechanism of the substitution reaction. The temperature dependencies
of the substitution reactions were studied by stopped-flow technique in the
temperature range 22 to 45 °C. The observed temperature dependencies are presented
in Figure 7 and show linear plots over the temperature ranges studied. From these
plots the activation parameters for the substitution by TU were estimated and are
summarized in Table 6. The values of ∆S# for both complexes are positive for the first
step and negative for the second one. This supports the operation of a dissociative
interchange (Id) mechanism for the first step and an associative interchange (Ia)
mechanism for the second reaction step.
0.8
0.20
0.6
0.19
0.18
0.4
0.17
0
10
20
30
40
50
Time (s)
0.2
b)
a)
350
0.0
300
400
450
 (nm)
500
550
600
Figure 5. UV/Vis spectral changes recorded for the reaction of 1 with thiourea (250
mM) in methanol at 296 K: a) spectrum before the reaction; b) spectrum obtained 100
13 | P a g e

seconds after mixing of the reactants. [1] = 6 x 104 M; Inset: the kinetic trace at 350
nm.
0.5
0.4
0.3
0.2
0.1
0.0
0.030
0.025
0.020
0.015
0.010
0.005
1
2
1
2
0.000
0.001
0.002
0.003
[TU] M
0.004
0.005
0.001
0.002
0.003
[TU] M
0.004
0.005
Figure 6. kobsd plots of the first (left) and second (right) reaction steps of complexes 1
and 2 vs TU concentration in methanol at 296 K. [comp] = 6 x 10-4 M.
3
2
1
0
3
2
Fast Step
Slow Step
Fast Step
Slow Step
1
0
-1
-2
-1
0.003150
0.003225
0.003300
0.003375
0.003225
0.003300
0.003375
0.003450
1/T [K-1]
1/T [K-1]
Figure 7. Eyring plots for the determination of the activation enthalpy and entropy for
the two reaction steps of complex 1 (left) and 2 (right) with thiourea in methanol
Experimental conditions: [complex] = 6 x 10-4 M, [TU] = 0.1 M at 325 nm.
[M1 s1] H# [kJ mol1] S# [J K1 mol1]
1st step
2nd step
1st step
2nd step
92 ± 7
68 ± 4
33 ± 6
73 ± 5
43 ± 3
43 ± 10
-77 ± 10
54 ± 9
1
2
6.1 ± 0.3
36 ± 3
4.5 ± 0.4
-56 ± 9
14 | P a g e

3.3.2. Catechol oxidase and phenoxazinone synthase mimicking activity
investigation:
The ability of both complexes to oxidize both substrates was studied by
mixing 0.1 mM solutions of the complexes with 100 mM of 3,5DTBC or OAP in
methanol in air. The reaction was followed by UV/Vis spectroscopy by observing the
increase of quinone and phenoxazinone chromospheres at 400 and 433 nm,
respectively. The time dependent spectra for both complexes show a gradual increase
of the intensities at the appropriate wavelengths indicating the catalytic oxidation.
blanck experiments, done under the same conditions but in the absence of complexes,
do not show a significant growth of the appropriate bands characteristic for the
oxidation products (quinone and phenoxazinone). Examples for the time dependent
spectral scans for the reaction of 1 with 3,5DTBC or OAP are shown in Figure 8.
0.5
0.6
0.5
0.4
0.3
0.2
0.1
2.0
1.5
1.5
1.0
0.4
0.3
0.2
0.1
0.5
0.0
1.0
-0.5
-1.0
0.5
0
100
200
300
400
500
600
700
Time (s)
0
1000
2000
3000
Spectrum of 1 before reaction
Time (s)
0.0
Spectrum of 1 before reaction
-0.5
-1.0
350
400
450
500
550
600
350
400
450
500
550
600
 (nm)
 (nm)
Figure 8: Growth of 3,5DTBQ at 400 nm (left) and 2-aminophenoxazine-3-one at
420 nm (right) after addition of 0.01 M 3,5DTBQ or 0.01 M OAP to a solution
containing complex 1 in methanol up to few minutes under aerobic condition at 25
°C.
Figure 8 shows that the addition of substrate to the complexes has an
immediate influence on the UV/Vis spectra of the complexes with respect to position
and intensity of the band at 400 nm for 3,5-DTBC and at 420 and 430 nm for OAP.
This suggests that the binding of substrate to the Cu(II) centers should be the first step
of the reaction. We have used stoppedflow techniques to study the reaction with both
substrates in more detail for which the results are shown in Figure 9.
15 | P a g e

0.12
0.09
0.06
0.03
0.00
0.2
0.1
Concentration
Concentration
0.0
-0.1
0
30
60
90
0
5
10
Time (s)
Time (s)
Figure 9: Representative kinetic traces obtained by stopped-flow spectrophotometry
monitored for the reaction of 2 (0.1 mM) with 3,5DTBC (left) and OAP (right) in
different concentrations in MeOH at 296 K.
Figure 9 shows that the kinetic traces for both substrates proceed in two steps.
The second step was found to be absent under an inert atmosphere, suggesting that the
growing band is for the oxidation product and the importance of molecular dioxygen
in the catalytic cycle. To determine the dependence of the observed reaction rates on
the substrate concentration, 104 M solutions of complexes were treated with variable
concentrations of both substrates in methanol, maintaining the pseudo-first order
condition. For the first step, the plot of kobs vs. concentration of substrate gave a linear
plot with a slope equal to k1 and an intercept equal to k-1, suggesting a pre-equilibrium
step for which the observed rate constant can be expressed by eq. (1) (see Figure 10).
]
(1)
1.4
1.2
1.0
0.8
0.6
0.4
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1
2
1
2
0.000
0.001
0.002
[OAP] M
0.003
0.001
0.002
0.003
0.004
[3,5-DTBC] M
Figure 10. Plot of kobs vs. substrate concentration for the first reaction step in MeOH
at 296 K. [complex] = 0.5 x 104 M, kinetic traces recorded at 400 nm.
16 | P a g e

The results presented in Tables 7 and 8 show that the coordination affinity of
3,5DTBC towards complexes 1 and 2 is lower than that of OAP. This can be traced
to the higher steric hindrance of 3,5DTBC compared to OAP. For the second
reaction step, the kinetic investigation was determined by following the method of
initial rate and monitoring the bands for quinone and phenoxazinone moieties. The
plots of rate constants versus substrate concentration show saturation kinetics (see
Figure 11) and were analyzed following Michaelis-Menten and LineweaverBurk
models for enzymatic kinetics using eq. (2) to obtain the values of the kinetic
parameters (Tables 7 and 8).
(2)
The data presented in Table 7 and 8 show that both complexes have high
catalytic efficiency (keff = kCat/KM) compared to the enzyme itself [26] as well as the
reported copper models [8]. The higher activity of both complexes may be accounted
for by the redox potential of both complexes in methanol. The redox potential values
of 1 (394 mV) and 2 (412 mV) are close to the reported E (360 mV) of the natural
enzyme. Furthermore, model 2 showed a little higher activity than 1 and this might be
due to: 1) the ability of such coordinately saturated complex 1 to dissociate and
consequently the energy of dissociation is one of the key factors in imparting catalytic
properties to this complex. The distorted square planar complex 2 already has a vacant
coordination site for substrate binding and thus needs no energy for ligand
dissociation as in the case of complex 1. Further, the distorted square planar structure
of 2 resembles the catechol oxidase active site Cu centers which are neither
tetrahedral nor square planar to minimize geometry change during the Cu(II)Cu(I)
transformation which makes the active site stable and efficient.
17 | P a g e

2.4
2.0
1.6
1.2
0.8
0.4
0.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
1
2
1
2
0.36
0.30
0.24
0.18
0.12
0.06
0.00
12
9
6
3
200
400
600
800 1000
200
400
600
800
1000
1/[OAP]

M
1/[3,5-DTBC]
1
2
0.000
0.001
0.002
0.003
0.004
0.000 0.001 0.002 0.003 0.004 0.005
[OAP] M
[3,5-DTBC] M
Figure 11. Initial rate vs. substrate concentration plots for the oxidation of both
substrates catalyzed by complexes 1 and 2 in methanol. Inset shows the
LineweaverBurk plots.
Table 7: Kinetic and catalytic parameters for catechol oxidase activity of 1 and 2 in
MeOH at 25 °C.
k1 (M1s1) k1 (s1) K1=k1/k-1(M1) kCat (h1)
kCat/KM (h1M1)
KM (M)
199
216
0.26
0.30
765
720
10.9 x 103 4.0 x 10-3 2.8 x 106
11.4 x 103 3.6 x 10-3 3.1 x 106
2.5 x 10-3
3.3 x 106
1
2
Catechol oxidase from I. batatas [26]
8.3 x 103
Table 8: Kinetic and catalytic parameters for phenoxazinone synthase activity of 1
and 2 in MeOH at 25 °C.
k1 (M1s1) k1 (s1) K1=k1/k-1(M1) kCat (h1)
kCat/KM (h1M1)
KM (M)
123
358
0.10
0.31
1230
1193
2.1 x 103
4.3 x 103
5.6 x 10-3 0.4 x 106
7.5 x 10-3 0.6 x 106
1
2
4. Conclusions
In conclusion, two copper (II) complexes, [Cu(phen)(LVal)(H2O)]NO3 (1) and
[Cu(bpy)(LVal)]ClO4 (2) (phen = 1,10phenanthroline; bpy = 2,2'bipyridine) have
been synthesized and structurally characterized. The crystal structure of
[Cu(phen)(LVal)(H2O)]NO3 possesses a distorted square-pyramidal coordination
18 | P a g e

geometry, whereas [Cu(bpy)(LVal)]ClO4 has a distorted square-planar geometry.
Structural features of both 1 and 2 reveal the formation of supramolecular networks
via hydrogen bonding interactions. The kinetics and mechanism of ligand substitution
of 1 and 2 by thiourea (TU) were studied and the substitution reaction of both
complexes is a biphasic process in which an initial fast and dissociative reaction is
followed by a slower and associative one. It is concluded from the activation
parameters that the difference in structure does not affect the mechanism. Complexes
1 and 2 have also been evaluated as functional models for the catechol oxidase
enzyme and phenoxazinone synthese. The model complexes 1 and 2 show
catecholase activity of Kcat =10.9 x 103 and 11.4 x 103 h1 and phenoxazinone
synthese activity of Kcat = 2.1 x 103 and 4.3 x 103 h1, respectively. Compared to the
enzyme itself (Kcat = 8.3 h1), the model complexes 1 and 2 are promising candidates
as functional mimics for catechol oxidase and phenoxazinone synthase.
5. Supplementary material
Crystallographic data (excluding structure factors) for the structures reported in this
paper have been deposited with the Cambridge Crystallographic Data Centre as
Supplementary Publication Nos. CCDC 1852551 and 1852552. These 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
We kindly acknowledge financial support from the Deutsche
Forschungsgemeinschaft (DFG), Germany and the Kafrelsheikh University,
Kafrelsheikh, Egypt.
References
[1]
a) A. Chakraborty, P. Kumar, K. Ghosh, P. Roy, Eur. J. Pharmacology, 2010,
647(1–3), 1; b) M. J. Clarke, Coord. Chem. Rev., 2003, 236, 209; c) C.G.
Hartinger, P. J. Dyson, Chem. Soc. Rev., 2009, 38, 391; d) K. Selmeczi, M.
Giorgi, G. Speier, E. Farkas, M. Reglier, Eur. J. Inorg. Chem., 2006, 1022; e)
N. H. Williams, B. Takasaki, M. Wall, J. Chin Acc. Chem. Res., 1999, 32,
19 | P a g e

485; f) G. Roelfes, M.E. Branun, L. Wang, L. Que, B.L. Feringa, J. Am. Chem.
Soc., 2000, 122, 11517.
[2]
[3]
K. Vassilev, M. Dimitrova, S. Turmanova, R. Milina. Syn React Inorg Met.
2013, 43, 243.
M. Chen, Y. Du, Y. Nomura, G. Zhu, B.S. Zhorov, K. Dong, Neurotoxicology.
2017, 60, 197.
[4]
a) I. Grecu, R. Sandulesu, M. Neamtu, Rev. Chim., 1986, 37(7), 589; b) M.
Markovic, N. Judas, J. Sabolovic, Inorg. Chem. 2012, 50, 3632; c) X. H. Zhou,
X. Y. Lee, S. Chen, J. Coord. Chem. 2005, 58, 993; d) B. Lou, X. Huang, X.
Lin. Z . Anorg. Allg. Chem. 2007, 633, 372; e) X. Le, S. Liao, X. Liu, X. Feng.
J. Coord. Chem. 2006, 59, 985; f) M. Markovic, N. Judas, J. Sabolovic, Inorg.
Chem. 2011, 50, 3632; g) J. Lakshmipraba, S. Arunachalam, D. A. Gandi, T.
Thirunalasundari, European Journal of Medicinal Chemistry, 2011, 46(7),
3013.
[5]
[6]
a) R. Pogni, M. C. Baratto, E. Busi, R. Basosi. J. Inorg. Biochem., 2002, 73,
157; b) Z. R. Liao, H. H. Fu, T. L. Tian, H. L. Cai, W. Q. Liu. Prog. Biochem.
Biophys., 1996, 23, 159; c) N. K. Fayad, T. H. Al-Noor, A. A. Mahmood, I. K.
Malih, Chemistry and Materials Research, 2013, 3, 2224.
a) E. I. Solomon, M. J. Baldwin, M. D. Lowery, Chem. Rev.1992, 92, 521; b)
E. I. Solomon, U. M. Sundaram, T. E. Machonkin, Chem. Rev. 1996, 96, 2563;
c) E. I. Solomon; R. K. Szilagyi, S. De Beer George, L. Basumallick, Chem.
Rev. 2014, 114, 419; d) N. Sarkar, K. Harms, A. Bauza´, A. Frontera, S.
Chattopadhyay, Chemistry Select 2017, 2, 2975.
[7]
A. W. Smith, A. C. Artigas, M. Wang, J. P. Allen, W. A. Francisco
Biochemistry 2006, 45, 4378; b) H. A. Choy, G. H. Jones, Arch. Biochem.
Biophys. 1981, 211, 55.
[8]
[9]
a) S. K. Dey, A. Mukherjee, Coord. Chem. Rev., 2016, 310, 80; b) E. I.
Solomon, U. M. Sundaram, T. E. Machonkin, Chem. Rev. 1996, 96, 2563.
M. M. Ibrahim, A. M. Ramadan, S. Y. Shaban, G. A. M. Mersal, S. A. El-
Shazly, S. Al-Juaid, Journal of Molecular Structure, 2017, 1134, 319.
[10] a) S. Y. Shaban, A. M. Ramadan, R. van Eldik, J. Coord. Chem., 2012, 65,
2415; b) S. Y. Shaban, A. M. Ramadan, M. M. Ibrahim, M. A. Mohamed, R.
van Eldik, Dalton Trans., 2015, 14110; c( A. M. Ramadan, S. Y. Shaban, M.
20 | P a g e

M. Ibrahim, J. Coord. Chem., 2011, 64, 3376; c) A. M. Ramadan, M. M.
Ibrahim, S. Y. Shaban, J. Mol. Struct. 2011, 1006, 348.
[11] SAINT, V6.02, Bruker AXS, Madison, WI, 1999.
[12] G.M. Sheldrick, SADABS, Area-detector Absorption Correction, Gottingen,
Germany, 1996.
[13] G.M. Sheldrick, SHELX 2013, University of Gottingen, Germany, 2013.
[14] J. Dehand, J. Jordanov, F. Keck, A. Mosset, J. J. Bonnet, J. Galy, Inorg.
Chem., 1979, 18, 1543.
[15] L. M. Marcotrigiano, L. Menabue, G. C. Fellacani, M. Saladini, M. Sola.
Inorg. Chem., 1985, 24, 3621.
[16] a) K. Nakamoto; Infrared and Raman spectra of inorganic and coordination
compounds, 3rd Edn, John Wiley and Sons Inc., New York, 1978; b) M. R.
Rosenthal, J. Chem. Educ. 1973, 50, 331.
[17] a) J. G. Sole, L. E. Bausa, D. Jaque, An Introduction to Optical Spectroscopy
of Inorganic solids, John Wiley & Sons Ltd. 2005; b) Y. Sikdar, R. Modak, D.
Bose, S. Banerjee, D. Bien´ ko, W. Zierkiewicz, A. Bien´ ko, K. Das Saha, S.
Goswami, Dalton Trans. 2015, 44, 8876; c) R. Gupta, D. Ghosh, R.
Mukherjee, J. Chem. Sci. 2000, 112, 179.
[18] a) W. B. Williamson, J. H. Lunsford, J. Phys. Chem. 1976, 80, 2664; b) B. J.
Hathaway, in G. Wilkinson, R. D. Gillard, J. A. McCleverty (Eds.)
Comprehensive Coordination Chemistry, Pergamon Press 1987, 533; c) M.
Garai, D. Dey, H. R. Yadav, A. R. Choudhury, M. Maji, B. Biswas, Chemistry
Select 2017, 2, 11040.
[19] W. J. Gear. Coord. Chem. Rev., 1971, 7, 81.
[20] a) E. R. Brown, R. F. Large, In Techniques of Chemistry: Physical Methods of
Chemistry; A. Weissberger, B. Rossiter, Eds.; Wiley: New York, 1971; Vol. 1,
Part IIA, p 475; b) S. Y. Shaban, F. W. Heinemann, R. van Eldik, Eur. J.
Inorg. Chem., 2009, 3111.
[21] a) A. W. Addison, T. N. Rao, J. Reedijk, J. Van Rijn, G. C. Verschoor, J.
Chem. Soc., Dalton Trans., 1984, 1349. b) S. Uhlenbrock, R. Wagner, B.
Krebs, J. Chem. Soc., Dalton Trans., 1996, 3731.
[22] X. Le, S. Liao, X. Liu, X. Feng, J. Coord. Chem., 2006, 59, 985.
[23] X.-H. Zhou, X.-Y. Le, L. Long, S. Chen, Chem. Res. Chinese U. 2005, 21,
119.
21 | P a g e

[24] A. Tovar-Tovar, J.-C. Garcı´a-Ramos, M. Flores-Alamo, L. Ruiz-Azuara, Acta
Cryst., 2011, E67, m1796.
[25] D. Xu, A. Xie, Y. Xu, C. Zhang, W. Chen, J. Coord. Chem. 1996, 39, 273.
[26] a) C. Eicken, F. Zippel, K. Buldt-Karentzopoulos, B. Krebs, FEBS Lett, 1998,
436, 293; b) J. Smith, C. J. Noble, R. C. Palmer, G. R. Hanson, G. Schenk, L.
R. Gahan, M. J. Riley, J. Bio Inorg Chem., 2008, 13, 499.
22 | P a g e

Graphical Abstract
Square Planar versus Square Pyramidal Copper(II) Complexes Containing N3O
moiety: Synthesis, Structural Characterization, Kinetic and Catalytic Mimicking
Activity
Shaban Y. Shaban, Abd El-Motaleb M. Ramadan, Mohamed M. Ibrahim, Fawzya I.
Elshami, Rudi van Eldik
In
quest
of
copper
complexes
having
[CuN3O]
cores,
[Cu(phen)(LVal)(H2O)]NO3 (1) and [Cu(bpy)(LVal)]ClO4 (2) complexes have
been synthesized and structurally characterized (phen = 1,10phenanthroline; bpy =
2,2'bipyridine). 1 possesses a distorted square-pyramidal, whereas 2 has a distorted
square-planar coordination geometry. Structural features of both 1 and 2 reveal the
formation of supramolecular networks via inter- and intra-molecular hydrogen
bonding interactions. The mechanism of ligand substitution of 1 and 2 by thiourea
(TU) is a biphasic process in which an initial fast and dissociative reaction is followed
by a slower and associative one. It is concluded from the activation parameters that
the difference in structure does not affect the mechanism. Complexes 1 and 2 have
also been evaluated as functional models for the catechol oxidase enzyme and
phenoxazinone synthese and they are promising candidates as functional mimics for
catechol oxidase and phenoxazinone synthase.
23 | P a g e

Research Highlights
 Biomemicking of copper enzymes using mixed NO-ligands.
 Effect of the structure on the biomimetic activity.
 Stabilization of supramolecular networks via inter- and intra-molecular
hydrogen bonding interactions in both complexes.
24 | P a g e