EPR, DFT and electrochemical interpretation of a Cu(II) derivative incorporating a Schiff base precursor

Article abstract of DOI:10.1016/j.poly.2018.11.058

The mononuclear Cu^II derivative, [Cu(L)(H₂O)₂] (1) [where H₂L = N,N′-bis(3-methoxysalicylidenimino)-1,3-diaminopropane] is afforded and systematically characterized. In 1, the central Cu^II atom is lin

Full text of DOI:10.1016/j.poly.2018.11.058

Accepted Manuscript
EPR, DFT and Electrochemical Interpretation of a Cu(II) Derivative Incorpo-
rating a Schiff Base Precursor
Kuheli Das, Sanchita Goswami, Belete B. Beyene, Amogne W. Yibeltal,
Eugenio Garribba, Antonio Frontera, Amitabha Datta
PII:
S0277-5387(18)30783-6
https://doi.org/10.1016/j.poly.2018.11.058
POLY 13605
DOI:
Reference:
Polyhedron
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Accepted Date:
13 November 2018
26 November 2018
Please cite this article as: K. Das, S. Goswami, B.B. Beyene, A.W. Yibeltal, E. Garribba, A. Frontera, A. Datta,
EPR, DFT and Electrochemical Interpretation of a Cu(II) Derivative Incorporating a Schiff Base Precursor,
Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.11.058
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EPR, DFT and Electrochemical Interpretation of a Cu(II) Derivative
Incorporating a Schiff Base Precursor
Kuheli Das,*a Sanchita Goswami,a Belete B. Beyene,b,d Amogne W. Yibeltal,b,c
Eugenio Garribba,e Antonio Frontera,f Amitabha Datta,*b
a Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Kolkata -
700009, India
b Institute of Chemistry, Academia Sinica, Nankang, Taipei – 115, Taiwan
c Department of Applied Chemistry, National Chiao-Tung University, Hsinchu – 300,
Taiwan
d Department of Chemistry, Bahir Dar University, P.O. Box – 79, Bahir Dar, Ethiopia
e Dipartimento di Chimica e Farmacia, Università di Sassari, Via Vienna 2, I-07100
Sassari, Italy
f Departament de Química, Universitat de les Illes Balears, Crta de Valldemossa km
7.5, 07122 Palma de Mallorca (Baleares), Spain
*Corresponding authors. kuheli.chemistry@gmail.com (K. Das),
amitd_ju@yahoo.co.in (A. Datta)

Abstract
The mononuclear CuII derivative, [Cu(L)(H2O)2] (1) [where H2L = N,N′-bis(3-
methoxysalicylidenimino)-1,3-diaminopropane] is afforded and systematically
characterized. In 1, the central CuII atom is linked to the NNOO donor atoms of the
di-compartmental Schiff base precursor and additionally coordinated with two water
molecules; thus achieves a distorted octahedral geometry. The EPR spectrum is
simulated with WinEPR software having g|| = 2.210 and g = 2.041. We also conduct
the DFT computational study which fits well with experimental affirmation. The room
temperature magnetic susceptibility of complex 1 confirms the effective magnetic
moment (eff) value as 1.99 B.M. The electrochemical measurement using cyclic
voltammetry showed redox potentials at +0.63, -1.25, and -1.82 V vs Ag/AgCl, which
are due to reversible and rapid Cu(III/II), Cu(II/I) and Cu(I/0) process.
_____________________________________________________________________
Keywords: Cu(II); Crystal structure; EPR; DFT; Redox
1. Introduction

Metal complexes incorporating different N,O donor ligands have received
considerable attention due to their ability to bind different cations [1], anions [2,3],
and neutral compounds [4,5]. The choice of ligands encoding some degree of
flexibility is an important factor in the design of metal-ligand complexes of different
nuclearity, dimensionality, redox ability, chirality, etc. which illustrates a range of
applications in selective host-guest recognition [6-9]. The coordination behaviour of
Schiff bases [10-15] has been drawing an immense interest since long back because of
their preparative accessibilities, structural variety, varied denticity and subtle steric
and/or electronic control on their frameworks leading to the formation of complexes
of not only different coordination numbers but also of different nuclearities those
possess interesting molecular and crystalline architectures [16-19] and related
properties [20,21]. Tetradentate Schiff bases with N2O2 donor set atoms provide
suitable coordination environments with metal ions; thus can readily form a wide
variety of metal derivatives complexes with copper(II) ions [22,23]. The hexa-
coordinated Cu(II) ion with d9 configuration prefers distorted octahedral geometry
which is a direct consequence of Jahn–Teller effect [24]. Thus, with a set of four
strongly and two weekly coordinating ligands, the later two always occupy the axial
positions. The distortion is usually seen as axial elongation, consistent with the
lability and geometric flexibility of the complex. The combined application of

theoretical methods and experimental techniques has highly interest for many years to
determine the structural and spectroscopic properties of compounds. Over the past ten
years, the computational methods based on DFT were found the favorite ones, and
much information can be obtained on the molecular geometry, vibrational frequencies,
atomic charges etc. [25,26].
Before some of us have reported few metal complexes incorporating tetradentate
Schiff base ligands [27-29]. Here, we utilize the potentially di-compartmental
hexadentate Schiff base precursor [30], N,N′-bis(3-methoxysalicylidenimino)-1,3-
diaminopropane (H2L) to afford the Cu(II) derivative, [Cu(L)(H2O)2] (1). This Schiff
baseligand has both the internal N2O2 cavity and the external O2O2 cavity. To utilize
only the internal N2O2 cavity, we accommodate one 3d metal ion, Cu(II) and to fulfill
the octahedral geometry of the metal ion, two water molecules are coordinated from
axial positions. Besides the structural elucidation, we report the EPR spectra, DFT
computation and electrochemical study of the Cu(II) derivative.
2. Results & discussion
2.1. Synthesis and characterization

The Schiff base precursor, H2L [where H2L = N,N′-bis(3-methoxysalicylidenimino)-
1,3-diaminopropane] is acquired from the condensation of o-vanillin and 1,3-
diaminopropane [30]. The stoichiometric reaction of H2L and anhydrous CuCl2 in
methanol affords compound 1 in moderate yield (see Scheme 1). The infrared
spectrum (KBr pellet, 400 – 4000 cm-1) of compound 1 is consistent with the
structural data given in this paper. A weak broad band in the region 3600-3400 cm-1
due to hydrogen bonds involving the OH group in free Schiff base is absent in the
metal derivative. This indicates that the phenolic oxygen is deprotonated and
coordinated with Cu(II) [31]. The strong C=N band appeared at 1634 cm-1 in 1,
indicates that the azomethine nitrogen atom is coordinated with Cu(II) [32-34].
N
N
N
N
(anhydrous)
CuCl2
RT, CH3OH
O
O
OH
HO
OMe
OMe
OMe
OMe
Cu
H2O
OH2
Scheme 1. Formation of compound 1.
Sharp bands appearing at nearly 496 and 421 cm-1 correspond to the Cu–N and Cu–O
stretching frequencies, respectively. In compound 1, a band at 358 nm may be
assigned to the nπ* transition of the imine group [35] and the band at higher energy
(269 nm) is associated with the aromatic ππ* intra-ligand charge-transfer transition.
A broad band in the range 620– 650 nm can be correlated to a d-d transition, which is

typical for a Cu(II) Schiff base complex [36]. The X-ray diffraction analysis confirms
that the resulting compound 1 is a mononuclear octahedral Cu(II) derivative.
2.2. Crystal structure
Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the
orthorhombic system having space group Pnma. The molecular structure of
compound 1 is depicted in Figure 1; selected bond distances and angles are listed in
Table 1. The title compound attributes a six-coordinated Cu(II) center with the
internal N2O2 cavity of the hexa-dentate Schiff base ligand and two coordinated water
molecules. The pair of Nimine atoms (N1 and N1′) and Ophenolic atoms (O1 and o1′) of
the Schiff base ligand define the equatorial plane. Two Owater molecules (O3 and O4)
belong at the axial position at distances, 2.089(2) and 2.103 (2) Å, respectively. The
bond angles around the Cu(II) ion are slightly distorted from those of a regular
octahedron and range from 86.03(6)° to 175.40(9)°. The two Owater molecules lie at
the trans position of the octahedron. The cis Ni-Ophenolic bond lengths [2.015(2) Å] is
considerably smaller than the trans Ni-Owater bond lengths [2.089(2) and 2.103(2) Å]
(Table 1). Deviation of atoms from the least-squares plane formed by O(1), N(1),
O(1′), and N(1′) is 0.22-0.24 Å. Thus the angles, O(1)-Cu(1)-N(1) and N(1)-Cu(1)-
N(1′) are either lesser or greater than the ideal value of 90. The trans angles O(1)-

Cu(1)-N(1) and N(1′)-Cu(1)-O(1′) deviate from the ideal bond angle of 180. The
N(1)–Cu(1)–N(1′) angle is 97.10(7)° and is typical of six-membered chelate ring [37-
40]. The two six-membered chelate-rings defined by the metal and corresponding o-
vanillin and amine moiety are not planar; deviated….The phenyl ring of the ligand
lies approximately on the same plane of the three chelate cycles. This is probably a
result of some conjugation between the aromatic system of phenyl group and
pseudoaromatic system of the six-membered chelate ring.
Figure 1. The molecular structure of complex 1. The hydrogen atoms are not
mentioned for clarity.
2.3. EPR spectra
EPR spectra were recorded on the polycrystalline powder of complex 1 at 298 and
100 K (Figure 2). As it is possible to note the resolution improves significantly with

lowering the temperature. At 298 K the dipolar interaction and intercenter exchange
between neighbouring units broaden the hyperfine lines, [41] and only one isotropic
absorption centred at g = 2.010 is revealed (Figure 2a). At 100 K, the spectrum
becomes axial with two transitions centred in the range 304-305 mT and around 330
mT were observed (Figure 2b). The spectrum was simulated with WinEPR software
with g|| = 2.210 and g = 2.041 (Figure 2c). The order g|| >> g > ge indicates a
2
2
ground state based on the Cu orbital dx −y and support the elongated octahedral
geometry determined by X-ray diffraction analysis [42-44]. Complex 1 was dissolved
in DMF, DMSO and CH3OH to examine the behavior in solution (Figure 3). In all
the three cases mononuclear Cu(II) species were detected with the spin Hamiltonian
parameters listed in Table 2. The values of gz are in the range 2.242-2.244, while
those of Az in the range 186-187 × 10–4 cm–1, with the rhombicity of x and y axes
being small. These parameters are characteristic of the Cu(II)–salpn complexes, where
salpn indicates N,N′-propylenediamine-bis(salicylideneiminate) anion; the
coordination mode is (O–, N, N, O–). The experimental parameters measured are
similar to those measured for similar ligands [45-50,27]. In DMSO and DMF it is

Figure 2. X-band EPR spectra recorded at 77 K on the polycrystalline powder of
complex 1: (a) 298 K and (b) 100 K. In the trace c the simulation of the spectrum at
100 K is also reported.
possible to observe, even if not perfectly resolved, the superhyperfine coupling
14
between the unpaired electron on copper with the two N nuclei of the ligand. If the
spin Hamiltonian parameters are compared with those of the analogous species
formed by salen, N,N′-ethylenediamine-bis(salicylideneiminate), a decrease of Az can
be noticed (the hyperfine coupling constants along the z axis for [Cu(salen)] is
reported in the range 194-209 × 10–4 cm–1) [51-55]. The reason can be assigned to the
weak axial coordination of solvent – as in the solid state – to give species with

composition [Cu(salpn)(Solvent)2] or – alternatively – to the different chelate ring
formation ((6,6,6) rings for salpn ligands and (6,5,6) for salen derivatives).
Figure 3. Anisotropic X-band EPR spectra recorded at 100 K on the polycrystalline
powder of complex 1 dissolved in: (a) DMSO; (b) DMF and (c) CH3OH. With the
asterisks the resonances due to the superhyperfine coupling between the unpaired
electron on copper with the two nuclei of 14N nuclei is shown.
Table 2. Spin Hamiltonian EPR parameters of complex 1 in organic solution.a
Solvent
gx
gy
gz
Ax
Ay
Az
DMSO
DMF
2.036
2.036
2.037
2.068
2.069
2.068
2.243
2.244
2.242
11
11
12
13
13
13
187
187
186
CH3OH
a A values reported in 104 cm1.

2.4. DFT computation
The calculated values of the g-tensor components at the B3LYP/def2-TZVP level of
theory for the X-ray structure of 1 are gz = 2.289, gx = 2.087, gy = 2.098, which
correlate well with the experimental values and indicate a ground state based on the
2
2
Cu-dx y orbital for which gz > gx ~ gy is expected. Indeed, the SOMO and spin
density plots shown in Figure 4 confirm the presence on the unpaired electron in the
2
2
dx y orbital.
Figure 4. SOMO (a) and spin density plot (b) of the X-ray of 1. At the B3LYP/def2-
TZVP level of theory.
We have also carried out DFT calculation using the [Cu(salpn)] (S = ½) complex in
solution, without the water molecules in apical positions. Since in the three solvents
(DMSO, DMF and MeOH) the EPR experiment gives almost identical results, we
have used only DMF to analyze the square planar complex. The spin Hamiltonian

parameters gz and Az are 2.24 and 172.5×104cm1, respectively, which are also in
good agreement with the experimental results gathered in Table 2. In case of the
inclusion of weakly coordinated solvent molecules in the apical positions, the values
slightly change to gz = 2.28 and Az = 177.6 ×104cm1,the latter is closer to the
experimental data, thus suggesting the presence of solvent molecules above and below
the CuO2N2 moiety.
2.5. Room temperature magnetic susceptibility study
The Cu(II) complexes usually have distorted octahedral geometry, but many
complexes of square-planar or approximately tetrahedral geometry are also known
[56]. However, stereochemistry has little effect on the magnetic moments of Cu(II)
complexes and values slightly above the spin-only value for one unpaired electron,
2
are expected [56]. A regular octahedral Cu(II) complex has the ground term Eg and
hence there is probability of no orbital contribution. The spin-only value
corresponding to one unpaired electron is 1.73 B.M., but the observed values belong
usually in the range 1.80-2.10 BM. The slightly higher value is due to the spin-orbit
coupling [56]. Complex 1 shows room temperature magnetic susceptibility value as
expected for an isolated d9 transition metal center. The effective magnetic moment

(eff) value is found to be 1.99 B.M. at 300 K, which is very consistent with expected
spin-only magnetic moment of a S = ½, d9 copper (II) system.
2.6. Electrochemical study
The electrochemical measurement was carried using cyclic voltammetry technique.
The voltammogram of 1 mM of complex 1 in DMSO using 0.1 M [Bu4N[PF6] as
supporting electrolyte, glassy carbon as working, Pt wire as counter and silver as
reference electrode at room temperature is shown in Figure 5. The solution purged
with N2 gas for 5 minutes prior to CV measurement in order to avoid the residue peak
due to dissolved oxygen. Up on oxidative scanning from 0 to +1.0 and reductive
scanning from 0 to -2.1 V vs Ag/AgCl, three reversible peaks were observed at E1/2
= +0.63, -1.25 and -1.82 V vs Ag/AgCl. The starting complex is Cu(II) and under
electrochemical condition, electron transfer occur between complex solution and
electrode. We presumed that the electrochemical redox behavior of the complex is
due to redox active central metal (copper) in the complex, as the ligand is
electrochemically silent in the potential window where the complex shows redox
potentials. Hence the reversible redox potential at E1/2 = +0.63 V vs Ag/AgCl (Figure
5a) is assigned to Cu(II/III) redox couple, while the redox potential peak at E1/2 = -
1.25 vs Ag/AgCl is due to Cu(II/I). The weak reversible redox potential peak at -1.82

V vs Ag/AgCl could be assigned to Cu(I/0) redox event (Figure 5b).
Figure 5. Cyclic voltammogram of 1 mM of complex 1 in dry DMSO solution
containing 0.1 M [Bu4N]PF6 using glassy carbon as working electrode, Pt wire as
counter and silver as reference electrode at room temperature; a) oxidative scan b)
reductive scan.
The scan rate dependent study showed that both the cathodic and anodic peaks of the
complex increase when the scan rate is increased (Figure 6). At a slow scan rate, the
diffusion layer grow much further from the electrode in comparison to a fast scan.
Consequently, the flux to the electrode surface (which is directly proportional to the
current) is considerably smaller at slow scan rates than it is at faster scan rates. Hence,
the cathodic or anodic current is lower at slow scan rates and higher at high rates
[57,58].

Figure 6. Linear fit of: (a) Square root of the scan rate vs cathodic peak current and (b)
the scan rate vs cathodic peak current.
Moreover, the scan rate study shows that the potential of current maximum peak does
not alter at different scan rates and this is a characteristic of electrode reaction, which
has rapid electron transfer kinetics, and is reversible. The information gathered from
scan rate dependent study is also important to predict whether the electrochemical
process is surface controlled (adsorption controlled) or diffusion controlled. For this
analysis, we choose the cathodic peak current at -1.25 V vs Ag/AgCl. As can be seen
from Figure 6, the magnitude of cathodic peak current (ic) is directly proportional to
square root of scan rate (Figure 6a) and the scan rate (Figure 6b). This strongly
suggest that the electrochemical process involving Cu(II/I) of the complex is a
mixture of diffusion controlled and adsorption (surface) controlled process [57].

However, the better linear relationship of cathodic current peak with square root of
scan rate shows that the electrochemical process is predominantly a diffusion-
controlled process. It means that the electroactive species is part of a solution (not
confined to the electrode surface) and the redox process depends on the rate at which
the molecule diffuse from the solution to electrode surface.
3. Conclusion
The mononuclear CuII derivative is afforded incorporating the hexadentate Schiff base
precursor,
[2-((E)-(2-(2-aminoethylamino)ethylimino)methyl)-6-methoxyphenol]
(H2L). The solid state structure of 1 shows that the central CuII atom possesses on an
octahedral environment. The compound is systematically characterized by IR and
UV-vis spectral techniques; EPR spectroscopy in different solvents (CH3OH, DMSO
and DMF) is measured and the values of gz are in the range 2.242-2.244, while those
of Az in the range 186-187 × 10–4 cm–1. DFT computation is governed to confirm the
geometry as well as the spectral evidence. On electrochemical study, compound 1
shows three reversible redox potentials when the CV scanned from +1.0 to -2.1 V vs
Ag/AgCl, owing to copper based redox events with performance of diffusion
controlled process. Further exploration on anti-mycobacterial, DNA binding and cell-
imaging studies subjecting this compound currently is in progress.

Acknowledgments
KD expresses her appreciation to the SERB (Science and Engineering Research
Board, India) Grant (PDF/2016/002832) for financial assistance.
Statement
We wish to assure that our manuscript does not contain any potential interest or "no
conflict of interest”.
4. Experimental Section
4.1. Materials
All the experiments were carried out under aerobic conditions. Cu(ClO4)2·6H2O was
purchased from Aldrich Chemicals. 1,3-diaminopropane and o-vanillin were
purchased from Merck, India. Solvents were of reagent grade and used without further
purification.
Caution! Perchlorate salts in presence of organic materials are potentially explosive.
They should be prepared in small amounts and handled with extreme care.
4.2. Physical measurements
Microanalytical data (C, H, and N) were collected on a Perkin–Elmer 2400 CHNS/O

elemental analyzer. FTIR spectra were recorded on a Perkin-Elmer RX-1
spectrophotometer in the range 4000–400 cm-1 as KBr pellets. Electronic spectra were
measured on a Perkin-Elmer Lambda 25 (U.V.–Vis.–N.I.R.) spectrophotometer. EPR
spectra were recorded from 0 to 8000 Gauss at liquid nitrogen temperature (100 K) or
room temperature (298 K) with an X-band Bruker EMX spectrometer equipped with a
HP 53150A microwave frequency counter. The microwave frequency was in the
range 9.40-9.41 GHz, microwave power was 20 mW (which is, with the ER4119 HS
resonator, below the saturation limit), time constant was 81.92 ms, modulation
frequency 100 kHz, modulation amplitude 0.4 mT, resolution 4096 points. EPR
spectrum of 1 solid at LT (Figure 2) was simulated with WinEPR SimFonia software
[59], with linewidths of 13, 13 and 18 mT for the x, y and z directions and a ratio
Lorentzian/Gaussian of 1. The cyclic voltammetry measurements were carried out on
CHI 621B electrochemical analyzer (CH Instruments, Austin, TX, USA) in DMSO
containing 0.1 M tetra n-butylammonium hexafluorophosphate [Bu4N(PF6)] as the
supporting electrolyte. The cell assembly consists of a glassy carbon as the working
electrode, Ag/AgCl as reference electrode, and Pt wire as the counter electrode.

Synthesis of
Bis(3,5-di-tert-
butylpyrazol-1-
yl)methane
(bdtbpzm) (2):
3,5-Di-tert-
butylpyrazole (3.00
g, 16.64 mmol),
KOH (3.60 g,

64.16 mmol), K
2
CO
3
(9.00 g, 65.12
mmol) and
benzyltriethylam-
monium chloride
(0.5 g) were

dissolved in
dichloromethane
(100 mL) and
heated under reflux
for 5 hours. Salts
were removed
by filtration and the
filtrate was

concentrated in
vacuo to dryness.
The white residue
was dissolved in
water and extracted
with pent-
ane (2 3 150 mL).
The organic layer
was dried (MgSO
4

) and the
solvent removed in
vacuo. The white
residue was
recrystallized
from pentane to
give bdtbpzm (2) as
a white powder,
which was

dried in vacuo.
Yield 2.40 g (77%)
Synthesis of
Bis(3,5-di-tert-
butylpyrazol-1-
yl)methane
(bdtbpzm) (2):
3,5-Di-tert-
butylpyrazole (3.00

g, 16.64 mmol),
KOH (3.60 g,
64.16 mmol), K
2
CO
3
(9.00 g, 65.12
mmol) and
benzyltriethylam-

monium chloride
(0.5 g) were
dissolved in
dichloromethane
(100 mL) and
heated under reflux
for 5 hours. Salts
were removed

by filtration and the
filtrate was
concentrated in
vacuo to dryness.
The white residue
was dissolved in
water and extracted
with pent-

ane (2 3 150 mL).
The organic layer
was dried (MgSO
4
) and the
solvent removed in
vacuo. The white
residue was
recrystallized

from pentane to
give bdtbpzm (2) as
a white powder,
which was
dried in vacuo.
Yield 2.40 g (77%)
Synthesis of
Bis(3,5-di-tert-
butylpyrazol-1-