Copper (II) complexes with novel Schiff-based ligands: synthesis, crystal structure, thermal (TGA–DSC/FT-IR), spectroscopic (FT-IR, UV-Vis) and theoretical studies

Article abstract of DOI:10.1007/s10973-021-10803-5

This study aimed to synthesize two novel Schiff-base ligands through the condensation between N-(2-aminoethyl)pyrazoles and 2-hydroxy-1-naphthaldehyde, which are: NaphPz ((E)-1-(((2-(1H-pyrazol-1-yl)ethyl)imino)methyl)naphthalen-2-ol)) and NaphDPz ((E)-1-(((2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)imino)methyl)naphthalen-2-ol). These novel pyrazole-imines were synthesized, characterized and used as copper (II) ion complexing agents. Different synthetic routes have been adapted to obtain the [Cu(NaphPz)Cl], [Cu(NaphDPz)Cl] and [Cu(NaphPz)₂] complexes in the solid state, the first two in the crystalline form and the latter as a powder. The minimum metal–ligand stoichiometry for the three complexes was defined by TGA–DSC thermoanalytical data and by single-crystal X-ray diffraction for the crystalline samples which belong to the P2₁/n space group. The products of the thermal decomposition of the material were also monitored by TGA–DSC/FT-IR in air and N₂ atmospheres in order to suggest how thermal decomposition of the organic portion of the complex occurs. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations compared to experimental results (UV-Vis and FT-IR) show a high degree of correlation. From HOMO/LUMO orbitals, the main major charge distributions, responsible for the absorption bands of the complexes, were determined.

Full text of DOI:10.1007/s10973-021-10803-5

Journal of Thermal Analysis and Calorimetry
https://doi.org/10.1007/s10973-021-10803-5
Copper (II) complexes with novel Schif‑based ligands: synthesis,
crystal structure, thermal (TGA–DSC/FT‑IR), spectroscopic (FT‑IR,
UV‑Vis) and theoretical studies
Jenifer Meyer Moreira1 · Guilherme Fava Campos1 · Leandro Moreira de Campos Pinto2 ·
Gabriel Rodrigues Martins2 · Bárbara Tirloni3 · Cristiane Storck Schwalm1 · Cláudio Teodoro de Carvalho1
Received: 29 January 2021 / Accepted: 29 March 2021
© Akadémiai Kiadó, Budapest, Hungary 2021
Abstract
This study aimed to synthesize two novel Schif-base ligands through the condensation between N-(2-aminoethyl)pyrazoles
and 2-hydroxy-1-naphthaldehyde, which are: NaphPz ((E)-1-(((2-(1H-pyrazol-1-yl)ethyl)imino)methyl)naphthalen-2-ol))
and NaphDPz ((E)-1-(((2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)imino)methyl)naphthalen-2-ol). These novel pyrazole-imines
were synthesized, characterized and used as copper (II) ion complexing agents. Diferent synthetic routes have been adapted
to obtain the [Cu(NaphPz)Cl], [Cu(NaphDPz)Cl] and [Cu(NaphPz)2] complexes in the solid state, the frst two in the crys-
talline form and the latter as a powder. The minimum metal–ligand stoichiometry for the three complexes was defned by
TGA–DSC thermoanalytical data and by single-crystal X-ray difraction for the crystalline samples which belong to the
P21/n space group. The products of the thermal decomposition of the material were also monitored by TGA–DSC/FT-IR in
air and N2 atmospheres in order to suggest how thermal decomposition of the organic portion of the complex occurs. Density
functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations compared to experimental
results (UV-Vis and FT-IR) show a high degree of correlation. From HOMO/LUMO orbitals, the main major charge distri-
butions, responsible for the absorption bands of the complexes, were determined.
Keywords Synthesis · Schif bases · TGA–DSC/FT–IR · Crystal structure · Theoretical calculation
Introduction
groups leads to mono-, di-, tri- or multidentate ligands,
which can be strategically designed to stabilize diferent
metal centers with varying oxidation numbers [5]. Imines
with RR’C=NR’’ structure, where R’’ can be an H sub-
stituent or an organic group are known as Schif’s bases,
and these molecules containing a C=N bond, derived from
the condensation between primary amines with a ketone or
aldehyde, give rise to a family of compounds known to pre-
sent interesting biological activities and a very extensive
coordination chemistry; in some cases, the complexation of
these molecules to a metallic center enhances the intrinsic
biological efects of these compounds [6]. In the area of
coordination chemistry, Schif bases are considered quite
versatile ligands, since their synthesis is simple and modu-
lar, allowing to modify the steric and electronic properties
around the coordination site with some ease, just varying
the aldehyde or ketone used in the condensation reaction.
In this context, the synthesis of polydentate pyrazole-
imine hybrid derivatives presents itself as an interesting and
promising proposal, since it can lead to compounds with
Pyrazole derivatives have a wide range of important bio-
logical activities, which give them a large number of phar-
maceutical applications [1–3]. Due to such functionalities,
the synthesis and characterization of pyrazole derivatives is
an area of research that has attracted much attention from
scientifc community [4]. Concomitantly, pyrazole-imine
derivatives have also been used as complexing agents and
the combination of the imine moiety with other coordinating
* Cláudio Teodoro de Carvalho
claudiocarvalho@ufgd.edu.br
1
Federal University of Grande Dourados, UFGD, CP 364,
Dourados, MS 79804–970, Brazil
2
Federal University of Mato Grosso do Sul, Campo Grande,
MS 79074-460, Brazil
3
Federal University of Santa Maria, UFSM, Santa Maria,
RS 97105-900, Brazil
Vol.:(0123456789)
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J. M. Moreira et al.
potential biological activities and applications in catalysis.
Silva et al. [7] reported the synthesis of three pyrazole-
imines derived from salicylaldehydes, in which the pyra-
zole ring is joined to the imine function by a bis-methylene
bridge. The ligands were used to complex the gallium(III)
ions in the presence of triethylamine and NH4PF6, using
methanol as a solvent. In this work, three gallium complexes
with a general formula equal to [GaL2]PF6, where pyrazole-
imine (L) acts as a monoanionic tridentate ligand, were
obtained and evaluated for cytotoxicity against human car-
cinoma cells. FeII, CoII, NiII, ZnII and CrIII complexes with
pyrazole-imine ligands containing three coordination sites
have also been obtained and successfully used as catalysts
in ethylene oligomerization and polymerization reactions
[8, 9]. Finally, Gama et al. [10] reported a study involving
pyrazole-imine complexes with copper(II) ions, obtained
by equimolar reactions of tridentate pyrazole-imines and
CuCl2.H2O. All isolated complexes presented a minimum
formula equal to CuLCl, where L represents a pyrazol-imine
ligand. Analyzes by single-crystal X-ray difraction allowed
to observe that, in the solid state, the complexes derived
from a dichlorinated pyrazole-imine and its analog diiodi-
nated have distinct crystalline structures. According to the
data obtained, one complex has a dinuclear structure with
distorted pyramidal quadratic geometry, while the other has
a quadratic geometry.
Experimental
Synthesis of the ligands (adapted from Hamann
and Tuczek [11])
Preparation of free 2‑chloroethanamine solution
from the corresponding hydrochloride
A suspension containing 2.784 g of 2-chloroethanamine
hydrochloride (24 mmol) in 8 mL of acetonitrile was pre-
pared in a 25-mL round bottom fask, which was cooled
to 0o C (controlled by thermometer) using an ice bath. At
this temperature, 3.35 mL of triethylamine (24 mmol) were
dropwise added to the suspension under magnetic stirring
and, after the end of the addition, the reaction mixture
was kept under stirring at this low temperature for 1 h.
The crude reaction mixture was then fltered, employing
minimal amounts of acetonitrile for washing of the inor-
ganic residue, and the fltrate containing the desired free
2-chloroethanamine was reserved for direct use in the next
reaction step without further manipulation.
N‑alkylation of pyrazoles
To a 50-mL round bottom fask were added 16 mmol of the
corresponding pyrazole (1.09 g of 1H-pyrazole or 1.538 g
of 3,5-dimethyl-1H-pyrazole), 8 mL of acetonitrile and
1.92 g of NaOH (48 mmol), and the resulting suspension
was kept under magnetic stirring at room temperature for
30 min. After this period, the temperature was raised to
75 °C using an oil bath and the fltrate from the previous
step was added dropwise into the reaction vessel; after the
end of the addition, the reaction mixture was kept under
heating and stirring for 5 h, then cooled down to room tem-
perature, fltered and the fltrate was concentrated under
reduced pressure (rotatory evaporator) to remove the solvent
and excess volatile reagents. Through this procedure, the
N-alkylated pyrazoles were obtained as yellowish viscous
oils and used in the next step without further purifcation.
As reported, pyrazole-imine compounds and their deriva-
tives can be versatile candidates with potential for several
applications, ranging from biological activities to use in
important industrial processes, such as oligomerization and
polymerization. Therefore, considering this perspective, this
study had as its primary objective the synthesis and charac-
terization of two novel Schif-base ligands and their com-
plexation with copper (II) ions for development of further
studies related to their applications.
The three coordination complexes obtained:
[Cu(NaphPz)], [Cu(NaphPz)Cl] and [Cu(NaphDPz)Cl],
were characterized using thermoanalytical data from
TGA–DSC/DTG, TGA/FT-IR and single-crystal X-ray dif-
fraction data for the chlorinated complexes obtained as a
single crystal. Theoretical calculations were also performed
for all complexes using time-dependent density-functional
theory (TDDFT), supported by experimental data from UV-
Vis and FT-IR and, show consequently how charge transfers
occur in systems through HOMO/LUMO molecular orbitals.
2-(1H-pyrazol-1-yl)ethan-1-amine: 1.39 g,
1
78% yield. H NMR: δ 7.50 (d, J = 1.5 Hz, 1H); 7.41 (d,
J = 2.1 Hz, 1H); 6.23 (t, J = 2.1 Hz, 1H); 4.20–4.11 (m,
2H); 3.18–3.07 (m, 2H). 13C NMR: δ 139.7; 129.7; 105.5;
55.1; 42.3. FT-IR: 3364, 3297 (ν N–H).
2-(3,5-dimethyl-1H-pyrazol-1-yl)ethan-
1
1-amine: 2.2 g, quantitative yield. H NMR: δ 5.73 (s,
1H); 3.94 (t, J = 6.0 Hz, 2H); 3.02 (t, J = 6.0 Hz, 2H); 2.17
1 3

Copper (II) complexes with novel Schif‑based ligands: synthesis, crystal structure, thermal…
Fig.1 Schematic diagram
CI
NH2.HCI
for the synthesis of ligands
HO
NaphHPz and NaphHDPz
(a)
NEt3,MeCN
O
R
R
R
CI
(b)
(c)
NaOH
+
N
N
HO
NH2
R
N
R
N
R
N
MeCN,∆
absolute EtOH
3 h, reflux
N
H
N
NH2
R = H
R = H (1a)
R = H (2a)
R = Me
R = Me (1b)
R = Me (2b)
(s, 3H); 2.14 (s, 3H). 13C NMR: δ 147.5; 139.1; 105.0;
51.2; 42.1; 13.4; 11.1. FT-IR: 3361, 3297 (ν N–H).
48.6; 13.7; 11.0. FT-IR: 1616, 1544, 1355, 1206, 760.
Melting point: 124 °C (Fig. 1).
Synthesis of the complexes
Schif base formation (condensation
with 2‑hydroxy‑1‑naphtaldehyde)
Synthesis of the [(Cu(NaphPz)2] complex
To a 10-mL round bottom fask were added 3 mmol of the
appropriate N-alkylated pyrazole, 542.4 mg of 2-hydroxy-
1-naphtaldehyde (3.15 mmol) and 3 mL of absolute etha-
nol. The resulting reaction mixture was then subjected to
magnetic stirring, under refux conditions for 3 h. After
this period, the crude reaction mixture was cooled down,
either at room temperature or at approximately 20 °C to
furnish the desired products as solids after vacuum fltra-
tion and washing with cold absolute ethanol.
In this synthetic route, frstly it was obtained the copper (II)
carbonate, prepared by electrolysis process using a 12 V 2A
switching power supply and two copper electrodes (copper
wire). The electrodes were immersed in 50 mL of 5% sodium
hydrogen carbonate suspension for two hours until complete
formation of blue precipitate or the reaction stops. The pre-
cipitate obtained was washed with distilled water on What-
man nº 40 flter paper, until elimination of sodium ions and
drying. In complexation reaction, 91 mg of (E)-1-(((2-(1H-
pyrazol-1-yl)ethyl)imino)methyl)naphthalen-2-ol ligand
(NaphHPz), in excess, were solubilized in a small portion
of ethanol and added to a 25 mL reaction fask containing
8 mL of a 1: 1 mixture (ethanol and water) and 23 mg of
CuCO3. The fask was then adapted to a refux condenser and
heated in oil bath, maintained at 60 °C for 12 h. At the end
of this period, the brown precipitate formed was collected,
washed with distilled water, dried, weighed (41% yield) and
kept stored in a desiccator until the moment of the analyses.
(E)-1-(((2-(1H-pyrazol-1-yl)
ethyl)imino)methyl)naphthalen-2-ol (NaphHPz): pre-
cipitated as a microcrystalline yellow solid after 2 h at
room temperature; 560 mg, 70% yield. 1H NMR: δ 14.56
(s, 1H); 8.46 (s, 1H); 7.82–7.67 (m, 2H); 7.67–7.55 (m,
2H); 7.47–7.39 (m, 1H); 7.36 (d, J = 2.1 Hz, 1H);
7.32–7.19 (m, 1H); 6.98 (d, J = 9.2 Hz, 1H); 6.16 (t,
J=2.1 Hz, 1H); 4.46 (t, J=5.4 Hz, 2H); 4.10 (t, J=5.4 Hz,
2H). 13C NMR: δ 172.1; 160.7; 140.6; 136.6; 133.4;
130.8; 129.3; 128.0; 126.8; 123.2; 123.1; 118.5; 107.6;
105.9; 54.7; 52.6. FT-IR: 1626, 1614, 1541, 1530, 1354,
Synthesis of the [Cu(NaphPz)Cl] complex
To a 25-mL round bottom fask containing a solution of
(E)-1-(((2-(1H-pyrazol-1-yl)ethyl)imino)methyl)naphthalen-
2-ol (NaphHPz) (133 mg, 0.5 mmol) in methanol (MeOH)
(2.7 mL, HPLC grade) under magnetic stirring, it was added
1180, 754. Melting point: 140 °C.
.
(E)-1-(((2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)imino)
methyl)naphthalen-2-ol (NaphHDPz): precipitated as an
amorphous brownish solid after overnight refrigeration of
the crude reaction at approximately 20 °C (freezer);
a solution of 85 mg of CuCl2 2H2O in 1.1 mL of MeOH
(HPLC grade), followed by further 3.5 mL of MeOH. Right
after the addition of the copper salt solution, a signifcant
color change was observed, from yellowish orange (ligand
solution) to dark green. The reaction mixture was then kept
under magnetic stirring at room temperature for 2 h, fltered,
and the fltrate allowed to slowly concentrate over a week
(kept on an undisturbed bench, at room temperature, in a vial
with a needle serving as the opening on the lid). After this
time, both block and needle-shaped dark green crystals were
1
700 mg, 80% yield. H NMR: δ 8.26 (s, 1H); 7.64 (d,
J = 8,4 Hz, 1H); 7.61 (dd, J = 8.0, 1.1 Hz, 1H); 7.40 (ddd,
J = 8.4, 7.0, 1.4 Hz, 1H); 7.24 (ddd, J = 7.9, 7.1, 1.0 Hz,
1H); 6.93 (d, J = 9,3 Hz, 1H); 5.66 (s, 1H); 4.30–4.21 (m,
2H); 4.08–4.00 (m, 2H); 2.26 (s, 3H); 2.09 (s, 3H). 13C
NMR: δ 174.0; 160.3; 148.7; 140.8; 137.1; 133.6; 129.3;
128.0; 126.6; 123.9; 123.1; 118.3; 107.3; 105.5; 53.7;
1 3

J. M. Moreira et al.
observed on the bottom and walls of the vial, respectively.
After careful decantation and drying of the solid material,
62 mg of the block-shaped crystals were isolated, corre-
sponding to 34% yield of the desired complex.
was kept at 250 °C and the transfer line of 120 cm at 225 °C.
The masses of the samples for the TGA–DSC curves record-
ing were about 10 mg with heating rate of 10 ºC min−1 in
alumina crucibles. The online FT-IR spectra were recorded
with a resolution of 4 cm−1.
Density functional theory (DFT) was carried out using
Gaussian 16 (Revision B.01) [13] employing the HSE06
hybrid functional [14, 15] with 6-311G(d,p) basis set for C,
H, N, O and Cl atoms, and LanL2DZ [16] basis set for Cu
atom associated with the efective core potential (ECP). The
geometry optimization was computed using Berny’s optimi-
zation algorithm and the calculations of harmonic vibrational
frequencies were also performed for all molecular structures.
For each optimized ground state, the frequency analysis
showed the absence of imaginary frequencies, confrming that
the global minimum has been reached. The vertical excitation
energies were calculated using time-dependent DFT (TD-
DFT) methodology. Orbital molecular (MO) diagrams were
reproduced using GaussView (version 6) [17]. The Polarizable
Continuum Model (PCM) [18] approach was used for single
point calculations in acetone.
Synthesis of the [Cu(NaphDPz)Cl] complex
To a 25-mL round bottom fask containing a solution of (E)-
1-(((2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)imino)methyl)
naphthalen-2-ol (NaphHDPz) (147 mg, 0.5 mmol) in MeOH
(2.7 mL, HPLC grade) under magnetic stirring, it was added
.
a solution of 85 mg of CuCl2 2H2O in 1.1 mL of MeOH
(HPLC grade), followed by further 3.5 mL of MeOH. Right
after the addition of the copper salt solution, a signifcant
color change was observed, from brownish (ligand solution)
to deep purple. The reaction mixture was then kept under
magnetic stirring at room temperature for 2 h, fltered, and
the fltrate allowed to slowly concentrate over a week (kept
on an undisturbed bench, at room temperature, in a vial with
a needle serving as the opening on the lid). After this time,
block-shaped deep purple crystals were observed and after
careful decantation and drying, 60 mg of the solid material
were isolated, corresponding to 31% yield of the desired
complex.
Crystal structure
Single-crystal data were collected with a Bruker D8 Venture
difractometer operating with an Incoatec X-ray source with
Montel two-dimensional optics, Ag-Kα radiation (λ=0.56086
Ǻ) and a Photon 100 detector. The structures were solved by
dual space methods using Bruker XT [19] and refned with
Bruker XL [20] on F2, using anisotropic temperature param-
eters for all non-hydrogen atoms. The hydrogen atoms posi-
tions were calculated starting from the idealized positions.
Crystal data and further details regarding the data collection
and refnement for [Cu(NaphPz)Cl] and [Cu(NaphDPz)Cl] are
shown in Table S1 (supplementary material). CCDC 2,018,666
and 2,018,665 contain the supplementary crystallographic
data for the complexes [Cu(NaphPz)Cl] and [Cu(NaphDPz)
Cl]. These data can be obtained free of charge on http://www.
ccdc.cam.ac.uk; from the Cambridge Crystallographic Data
Centre at 12 Union Road, Cambridge CB2 1EZ, UK; via fax:
(+44) 1223–336-033; or via e-mail: deposit@ccdc.cam.ac.uk.
Graphical representations [21] of the structures were prepared
for each compound.
Instrumental and classical analysis
1H and 13C NMR analyses were performed using CDCl3
as the solvent on a Bruker Avance III HD-400 MHz spec-
trometer, operating at 400 MHz for the 1H and 100 MHz for
13C nuclei, respectively. Chemical shifts (δ) are expressed in
parts per million (ppm) and were determined using the resid-
ual solvent signals as the internal references. The Attached
Proton Test (APT) experiment was employed for 13C NMR
analysis.
FT-IR/ATR spectra of the complexes were performed
using a Nicolet iS10 FT-IR spectrophotometer using an ATR
accessory with Ge window.
The TGA–DSC curves were obtained using a thermal
analytical system, model STA 449 F3 Jupiter® and the
experimental data were obtained by Proteus® Software.
For the analysis of the samples, masses near to 5.0 mg and
alpha-alumina crucibles were used under air purge gas fow
of 50 mL min−1 with heating rate optimized to 10° C min−1.
The copper ion in the residue of the [Cu(NaphDPz)Cl] com-
plex was also determined by complexometric titration with
a standard EDTA solution, according to the method used by
Oliveira and collaborators [12].
Results and discussion
TGA–DSC
Initially, thermogravimetric analysis (TGA) in air atmos-
phere was performed to establish the stoichiometry of
the copper complexes. The theoretical and experimental
analytical data from the TGA curves of the compounds
The gaseous fragments from the thermal decomposition of
the complexes were monitored using a Mettler® TGA–DSC
system coupled to a Nicolet® FT-IR spectrophotometer
equipped with gas cell and DTGS KBr detector. The gas cell
1 3

Copper (II) complexes with novel Schif‑based ligands: synthesis, crystal structure, thermal…
are in agreement with the formulas: [Cu(NaphPz)
4
100
Exo↑
Cl], [Cu(NaphDPz)Cl] and [Cu(NaphPz)2], wherein
2
NaphPz = (E)-1-(((2-(1H-pyrazol-1-yl)ethyl)imino)
80
methyl)naphthalen-2-olate, NaphDPz = (E)-1-(((2-(3,5-
0
(a)
dimethyl-1H-pyrazol-1-yl)ethyl)imino)methyl)naphthalen-
60
– 2
– 4
– 6
– 8
2-olate and Cl corresponds to a chlorine atom.
The [Cu(NaphPz)Cl] complex, Fig. 2a, obtained in
anhydrous form, is thermally stable up to 100 °C (TGA),
from which temperature a small mass loss is observed,
more pronounced around 200 °C with an endothermic
peak at 216 °C in DSC curve. In order to assess this frst
mass loss step, both the peak temperature in DSC (maxi-
mum reaction) and the end of the reaction in DTG, shown
in Fig. S1 (TGA–DSC/DTG) as a complement to Fig. 2a,
were adopted as parameters to infer the percentage of mass
loss referred to this step, which was determined at 4.0%.
The correlation of these thermal events in TGA–DSC/DTG
curves (supported by TGA/FT-IR section data) shows us
that the mass loss in this step is due to evaporation of the
solvent used in the synthesis of this compound. In addi-
tion, it was not possible to establish a defned proportion
for the methanol molecule (CH3OH) from calculation of
the minimum formula and therefore, a possible coordina-
tion of methanol molecules with the metallic center can
be disregarded, but is likely that part of CH3OH exerts a
strong interaction with the organic ligand, once the peak
at 216 °C (DSC) shows considerable heat absorption in
a small temperature range in TGA curve. Thus, the per-
centage of methanol is excluded, as shown in Table 1, to
compose the stoichiometric ratio defned as [Cu(NaphPz)
Cl]. A solvent-free material is desirable for the compound
in powder form, but the complete elimination of the sol-
vent by heating, for instance, is difcult because whenever
an operating temperature limit was established, it culmi-
nated in the start of decomposition. As a consequence, we
present the results as obtained in the original synthesis,
since the total elimination of all the solvent through this
method was not possible. In addition, the rare microcrys-
tals obtained in the synthesis were used for the analysis
of single-crystal X-ray difraction as a supplement to the
TGA–DSC data.
40
20
[Cu(NaphPz)CI]
[Cu(NaphDPz)CI]
[Cu(NaphPz)2]
– 10
6
0
Exo↑
100
4
2
0
80
60
40
20
(b)
– 2
– 4
– 6
– 8
0
30
25
20
15
10
5
100
80
60
40
20
0
Exo↑
(c)
0
– 5
200
400
600
800
1000
Temperature/°C
Fig. 2 (a–c) TGA–DSC curves in air atmosphere using a heating
ratio of 10 ºC/min and initial mass near 5.0 mg. Peaks up (exo) and
peaks down (endo)
C–O (353,5 kJmol−1) and C-N (305 kJmol−1), C=N (600
kJmol−1), N–N (163 kJmol−1), N=N (409 kJmol−1) it is
possible to theoretically insert, based on the binding ener-
gies and typical interatomic distances [22, 23], that the
fragmentation of the organic ligand must begin through
lower energy chemical bonds. The third step, with a slow
mass loss of 13.48%, between 320 and 475 °C (TGA),
produces two small exothermic peaks at 368 and 427 °C
probably related to the fragmentation of the open chain
(–C–C-N=C–) and formation of remaining stable mate-
rial from the cyclical part of the ligand. Finally, the last
step, with mass loss of 40.30% between 475 and 690 °C
(TGA) associated with an intense exothermic peak at
571 °C (DSC), occurs due to oxidation/combustion of the
organic matter, as well as fragmentation of Cu-Cl bond
Considering the second step, between 231 °C and
320 °C with mass loss of 23.52% (TGA) associated with a
small exothermic peak at 278 °C in DSC curve, it can be
related to the thermal decomposition of the organic ligand.
This thermal event, which is not very pronounced in the
DSC, shows that the decomposition products formed and
consequentially released in this step react weakly with the
oxidizing atmosphere used in the analysis, but with a ther-
mal balance of released heat greater than that absorbed,
i.e., delta H less than zero. In addition, analyzing the
types of bonds in the organic ligand such as C–C (346,8
kJmol−1), C=C (614,2 kJmol−1), C–H (413,4 kJmol−1),
1 3

J. M. Moreira et al.
Table 1 Analytical and
thermoanalytical (TGA) data
of the compounds; in which
Cu=copper; NaphPz and
NaphDPz=ligands
Compounds
Ligand/%
Calc
Residue (CuO)/%
Methanol* /%
TGA
Calc
TGA
Calc
TGA
[Cu(NaphPZ)CI]
[Cu(NaphDPZ)CI]
[Cu(NaphPZ)2]
78.10
79.67
86.57
78.30
79.05
86.28
21.90
20.33
13.43
21.70
2.95
13.72
–
–
–
4.0*
2.8*
–
Calc.=calculated, Excluding the methanol solvent (%) for the stoichiometric calculus
[24], leading to formation of an apparently stable residue,
but which slowly gains mass (1.00%) up to around 950
ºC. This small mass gain is probably due to oxidation of
metallic copper in the Cu and CuO mixture, data which at
950 ºC (TGA) were used to calculate the stoichiometry.
For [Cu(NaphDPz)Cl] complex, Fig. 2b, the synthesis
was performed in the same experimental conditions as those
of [Cu(NaphPz)Cl]. In the TGA–DSC curves, between 192
and 250 °C, it is possible to highlight a small constant mass
loss that becomes more accentuated after 242 °C, with an
endothermic peak at 249 °C. To determine the fnal tem-
perature of the reaction, the peak temperature in the DTA
(maximum reaction ratio) and fnal temperature onset were
adopted. However, for this compound, as the frst mass loss
is constant, the TGA curve does not present a maximum
infection point and as a consequence, no defned peak in
the DTG curve. Thus, similarly to what was previously
described for [Cu(NaphPz)Cl], this mass loss is also due to
evaporation of the solvent methanol used in the synthesis
of this complex, so that it is possible to assume that part of
the methanol solvent molecules also exerts a strong interac-
tion with the organic ligand. The inferred data for the frst
mass loss step (2.8%) of [Cu(NaphDPz)Cl] compound can
be better supported by Fig. S2 (TGA–DSC/DTG), of the
supplementary material.
to the oxidation/combustion of the cyclical and/or aromatic
part of the ligand and rupture of the Cu-Cl bond, leading to
formation of an apparently stable residue, but which slowly
gains mass (2.39%) up to around 950 °C. As a result, other
TGA–DSC analyses were carried out, which reproduced the
same result, thus discarding possible problems or equipment
calibration errors. The residue was then evaluated visually
and using classical techniques at two diferent temperatures,
at 650 °C and at the fnal temperature of analysis (950 °C).
The visual evaluations showed a completely black residue
at both temperatures, which excludes the formation of a
mixture of oxides or copper (I) oxide. A qualitative test
at 650 °C for chloride showed no evidence of this ion in
the residue and fnally, complexometric analysis showed a
copper content of 99.3%. Therefore, from these results, we
concluded that this small mass gain is due to the oxidation
of metallic copper particles, probably produced from cop-
per reduction by the carbonaceous material (reducing agent)
and/or by the chloride ions [25].
The last synthesized complex [Cu(NaphPz)2], Fig. 2c,
was obtained in the absence of chloride and through the
reaction of copper carbonate with the ligand in its acid form.
This compound is stable up to 215 °C and presents a thermal
behavior slightly diferent from the other two compounds
studied. The melting peak temperature in DSC curve, in air
atmosphere, is shown at 227 °C.
For the second step, considering the mass loss interval
between 250 and 360 °C, with mass loss of 30.2% in TGA
curve, it is necessary to carefully evaluate the DSC curve
after the end of frst step because an exothermic event occurs
superimposed immediately after the endothermic peak in
the frst stage at 249 °C, giving the impression that an endo-
thermic peak at 310 °C is present. Therefore, it is correct to
afrm that the valley at 310 °C is due to the return of the
baseline to the beginning of a new exothermic event with
a peak temperature at 335 °C. Furthermore, the fragments
released in this step are more stable, reacting less intensely
with the oxidized atmosphere. In the third step, between
370 °C and 470 °C (TGA), a relatively small mass loss of
5.64% is observed, which is signaled by an exothermic peak
in the DSC curve at 370 °C. Thus, based on the binding
energies it is possible that this mass loss is due to thermal
decomposition of part of the acyclic chain (–C–C–N=C–),
besides fragments of dimethylpyrazole. The last mass loss of
45.6%, precisely between 470 and 620 °C (TGA), is related
For [Cu(NaphPz)2] complex, the frst thermal decom-
position step in air atmosphere starts at around 260 °C
and ends at 326 °C, with a mass loss of 40.85% (TGA)
associated with a small exothermic peak in the DSC curve
at 310 °C. This marked mass loss step, which coincides
with pyrazole fragments and the open chain (40.85%) of
the two ligands coordinated to the metal center, shows that
the fragmentation way is highly dependent on chemical
environment. Thus, it is possible that the second mass loss
step is inherent to the thermal decomposition of the more
stable fraction of the ligand, part of the cyclic/aromatic
portion. In this step, the 12.00% mass loss (TGA), between
326 and 435 °C is signaled in DSC curve by an exothermic
event (shoulder) starting at 336 °C. The last step, between
435 and 607 °C, with mass loss of 33.43% (TGA) asso-
ciated with an intense exothermic peak at 535 °C is due
to oxidation/combustion of the organic matter, leading to
formation of copper oxide (CuO) as fnal residue.
1 3

Copper (II) complexes with novel Schif‑based ligands: synthesis, crystal structure, thermal…
Complementing the thermal analysis study, the gaseous
products released in each step of the TGA-DTA curves
were monitored on-line and identifed by TGA/FT-IR sys-
tem; such study will be better discussed further.
overlap the release of most of the organic fragments. Thus,
the gaseous products evolved for copper complexes from the
thermal decomposition were monitored in inert atmosphere
(N2), and the corresponding data is shown in Fig. 3a–h. The
FT-IR spectra obtained on-line were evaluated based on data
available in the literature [26, 27] and by digital spectral
library-based, provided by OMNIC®.
Evolved gas analysis
In addition to the results obtained by TGA–DSC on stoi-
chiometry and thermal behavior of the compounds, the
gaseous products were also monitored step by step through
a TGA–DSC/FT-IR system under air and nitrogen atmos-
pheres, and the same products were detected under both
atmospheres. However, in air atmospheres the release of gas-
eous products like carbon dioxide (CO2) and carbon monox-
ide (CO), produced from the organic matter decomposition,
The frst gaseous product released and identifed from
the thermal decomposition of [Cu(NaphPz)Cl] and
[Cu(NaphDPz)Cl] complexes was only methanol, according
to Fig. 3a. The FT-IR spectra obtained around 15 min or at
180 °C (TGA) corroborate the data discussed in the section
of TGA–DSC analysis, wherein the frst endothermic event
in each DSC curve is due to evaporation of the methanol
solvent used in the synthesis of the compounds. The gaseous
0,006
Fig. 3 Representative FT-IR
0.008
0.006
spectra for the main gaseous
0.004
0.002
(18 min)
(e)
(a)
(38 min)
products monitored, in nitrogen
atmosphere, during the thermal
decomposition of the complexes
0.004
0.002
0.000
– 0.002
– 0.004
– 0.006
0.000
– 0.002
– 0.004
– 0.006
– 0.008
– 0.010
– 0.008
0.004
0.05
0.04
(b)
(29 min)
(25 min)
(f)
0.002
0.000
0.03
0.02
0.01
– 0.002
– 0.004
– 0.006
– 0.008
0.00
– 0.01
0.025
0.004
(g)
(26 min)
(31 min)
(c)
0.020
0.015
0.002
0.000
0.010
– 0.002
– 0.004
– 0.006
0.005
0.000
– 0.005
– 0.008
0.025
0.015
0.010
0.020
0.015
0.010
0.005
(h) (53 min)
(d)
(30 min)
0.005
0.000
0.000
– 0.005
– 0.010
– 0.005
3000
2500
2000
1500
1000
3000
2000
1500
1000
2500
Wavenumber/cm– 1
1 3

J. M. Moreira et al.
methanol (CH3OH) produces a set of characteristic peaks at
1008, 1033 and 1062 cm−1, in addition to a band between
3000 and 2800 cm−1 related to C–H stretches.
pattern that the thermal degradation follows. Thus, in this
process the appearance of pyrazole or methylpyrazole, fol-
lowed by the gaseous product ammonia is observed at frst,
which shows that fragmentation begins with lower energy
bonds such as N–N and C–N. As the temperature increases,
peaks can be seen in the FT-IR fngerprint region, which
may be related to fragments from the breaking of C–C and/
or C–O bonds. Furthermore, although the degradation does
not occur in a homogeneous and sequential manner, it is pos-
sible that the formation of products arising from the break-
ing of stronger bonds C=C or C=N of the aromatic part
will occur last, due to the stability that the ring provides to
the system.
For the [Cu(NaphDPz)Cl] complex, above 245 °C (TGA),
one peak highlighted at 783 cm−1 (FT-IR), related to frag-
ments of dimethylpyrazole, can be observed in Fig. 3b; sub-
sequently, at 248 ºC, it is also possible to observe peaks
at 992, 965 and 930 cm−1 related to ammonia, according
Fig. 3c. Regarding the fragments released in form of ammo-
nia, Fig. 3d, these are more pronounced around 300 °C
(TGA) and then decreases up to 450 °C. At the same time,
when there is a decrease in gaseous products, new bands in
the region 1100–1200 cm−1, above 450 °C, are now also
observed in Fig. 3e. These bands are probably related to
fragments (–C–C–)n of the aromatic part of the ligand, which
is more stable and decomposes at higher temperature.
For [Cu(NaphPz)2] complex, the frst fragment observed
at 245 °C is pyrazole at 744 cm−1 (Fig. 3f), as well as for
[Cu(NaphPz)Cl] complex around 260 °C, after elimina-
tion of methanol solvent, as previously discussed. In these
compounds, similarly to [Cu(NaphDPz)Cl] complex, it is
observed the release of ammonia shortly after pyrazole
fragment, as shown in Fig. 3g, so that these two fragments
are observed in the FT-IR spectra up to close to 500 °C,
that is, for [Cu(NaphPz)2] and [Cu(NaphPz)Cl] complexes.
Above this temperature, Fig. 3h, ammonia-free fragments
are observed, with emphasis on pyrazole fragment that con-
tinues to be released in a lower proportion, which is prob-
ably accompanied by fragments such as (–C–C–N–C–)n or
even for fragments involving C-O bond due to an intense
peak at 1182 cm−1. According to the literature, this peak
can be characteristic of secondary aliphatic amines [26, 27],
while for C-O bond, the probable possibility is the fragmen-
tation of the aromatic part to produce gaseous products like
(–C–O–C–)n, which is plausible for high temperatures.
In summary, taking the majority of fragments monitored
through TGA/FT-IR and the theoretical binding energies
that involve the organic ligand into account, it is possible to
make some kind of logical association of the approximate
Crystalline structure
The complexes were previously characterized by thermo-
gravimetry and simultaneous diferential scanning calo-
rimetry (TGA–DSC) to determine the stoichiometric ratio,
while the structural arrangement for [Cu(NaphPz)Cl] and
[Cu(NaphDPz)Cl] complexes was determined using single-
crystal X-ray difraction. The structure for [(Cu(NaphPz)2]
complex has not been determined due to the absence of suit-
able single-crystals for this material.
The molecular structures for the mononuclear
[Cu(NaphPz)Cl] and [Cu(NaphDPz)Cl] complexes, designed
with DIAMOND [20], are shown in Fig. 4.
In each complex, the CuII atom is chelated by a (N,N,O)-
tridentate Schif-base and a chloride ligand. For complex
[Cu(NaphPz)Cl], the CuII atom adopts a distorted square
planar environment, in agreement with the T param-
eter. The geometry can be confirmed from the geomet-
ric parameter τ by applying the following calculation:
T4 = 360°–(α − β)/141°, for structures with coordination
number four. The values of T4 will range from 1.00—for a
perfect tetrahedral geometry—to zero—for a perfect square
planar geometry. Intermediate structures, including trigonal
pyramidal and seesaw, are in the range of 0 to 1.00 [28]. For
the [Cu(NaphPz)Cl] complex, T4 =0.07, and as the value is
[Cu(NaphDPz)CI]
[Cu(NaphPz)CI]
C7
C8
C9
C7
C4
C8
C6
C12
C6
C12
C13 C14
N3
C11
C17
C15
C9
C11
O1
N1
C10
C1
C13
C15
C5
C10
C14
C5
N2
N1
N3
N2
C4
C16
Cu1
C16
C1
Cu1
C3
C3
O1
C2
C2
C18
Cl1
Cl1
Fig. 4 Molecular structure of the [Cu(NaphPz)Cl] and [Cu(NaphDPz)Cl] complexes. The thermal ellipsoids indicate the 50% probability level
1 3

Copper (II) complexes with novel Schif‑based ligands: synthesis, crystal structure, thermal…
close to zero, it confrms the distorted square-planar geom-
etry; for the [Cu(NaphDPz)Cl] complex, T4 =0.41, confrm-
ing the seesaw geometry, respectively, as shown in Fig. 5.
It has been seen that other copper complexes show seesaw
coordination geometry [27]. The angles α and β used for the
calculations are described in Table S2 of the supplementary
material. The diference in the coordination geometry of
[Cu(NaphPz)Cl] and [Cu(NaphDPz)Cl] is due to the spatial
orientation of the methyl groups in the ligand of the latter
complex.
the theoretical* data and (a–c) the experimental ones. The
optimization of the molecular geometry structures of the
compounds studied was performed using Berny’s optimiza-
tion algorithm, and the calculations of harmonic vibrational
frequencies were also performed for all molecular structures.
However, in the correlation between theoretical and exper-
imental spectra, it is observed small diferences in shape
and number of bands, given that theoretical calculation is
performed on the basis of a single molecule without taking
the interference of molecular interaction into account for
calculating FT-IR. However, the main bands can be observed
in the spectra without any prejudice to the interpretation.
For [Cu(NaphPz)Cl] and [Cu(NaphDPz)Cl] complexes
between 3500 and 2900 cm−1 (Fig. S3), a band was observed
with the maximum at around 3400 cm−1 and relatively high
peaks around 3000 cm−1, when this region is compared to
that one of the [Cu(NaphPz)] compound, which presents
in the same region only stretches related to the C-H bond.
This observation corroborates the experimental data deter-
mined by TGA–DSC and TGA/FT-IR, related to the release
of the methanol solvent, used in the synthesis of the two
complexes.
For the (N,N,O)-tridentate Schiff-base and chloride
ligand, the bond lengths to the metal center in [Cu(NaphPz)
Cl] complex are Cu(1)–O(1) (1.9051(15) Å, Cu(1)–N(1)
(1.9756(16) Å, Cu(1)–N(2) (2.0026(17) Å and Cu(1)–Cl(1)
(2.2765(6) Å. For [Cu(NaphDPz)Cl]) complex, these bond
lengths are Cu(1)–O(1) 1.876(2) Å, Cu(1)–N(1) 1.950(3) Å,
Cu(1)–N(2) 1.972(3) Å and Cu(1)–Cl(1) 2.2783(11). Both
sets of bond lengths are in agreement with those reported for
other copper(II) complexes containing (N,N,O)-tridentate
Schif-base and chloride ligand [10, 29].
The angles (in degrees) for the bonding in [Cu(NaphPz)
Cl] complex are O(1)–Cu(1)–N(1) (90.84(7),
O(1)–Cu(1)–N(2) (175.62(7), N(1)–Cu(1)–N(2) (92.27(7),
O(1)–Cu(1)–Cl(1) (88.15(5), N(1)–Cu(1)–Cl(1) (173.85(5)
and N(2)–Cu(1)–Cl(1) (89.07(5); for [Cu(NaphDPz)
Cl]) complex, the bonding angles are O(1)−Cu(1)−N(1)
93.18(11), O(1)−Cu(1)−N(2) 154.38(12), N(1)−Cu(1)−N(2)
93.29(12), O(1)−Cu(1)−Cl(1) 92.07(8), N(1)−Cu(1)−Cl(1)
148.34(9) and N(2)−Cu(1)−Cl(1) 95.31(9). The changes
observed, from one complex to the other, confrm the dis-
tortion in the coordination geometry caused by the spatial
orientation of the methyl group.
In relation to the spectra fngerprint region, the theoreti-
cal* and experimental data, Fig. 6 (a*–c* and a–c) lead to a
convergent correlation, with displacements around 20 cm−1
for the main bands. In this sense, the following correlations
can be made, so that in the 1600 cm−1 region there is an
overlap of bands attributed to C=C and C=N stretching, as
well as in 1400–1000 cm−1 region, assigned to the bend-
ing in the plane mode. Around 800 cm−1 it is observed the
out-of plane bending for OCC bonds, while the bands near
750 cm−1 can be assigned to torsional mode for HCCC and
HCCN bonds. Therefore, from these correlations it is pos-
sible to suggest that the optimized structures through theo-
retical calculation are in line with the experimental results
presented.
Infrared spectroscopy: theoretical and experimental
On the basis on experimental TGA–DSC and FT-IR data
obtained from the complexes, a theoretical calculation
employing quantum chemical approach was performed to
obtain theoretical FT-IR spectra, in addition to the experi-
mental data, as shown in Fig. 6, wherein (a*–c*) represents
UV–Vis absorption study
The [Cu(NaphPz)Cl], [Cu(NaphDPz)Cl] and [Cu(NaphPz)2]
complexes, solubilized in acetone, were analyzed by UV–Vis
spectrometry and the experimental results obtained com-
pared to the DFT calculation, as shown in Fig. 7. Although
the acetone solvent has maximum absorption at 210 and
280 nm, close to the wavelengths under study (327 nm and
450 nm), there was no interference of the solvent in the
absorption of the complexes, using them at a concentration
of 1.0 × 10–5 molL−1. Optimized molecular structures of
the complexes were evaluated using time-dependent DFT
(TD-DFT) methodology to show the spatial distribution of
charges in highest occupied molecular orbitals (HOMO) and
lowest unoccupied molecular orbitals (LUMO), in order to
indicate how the electron transport in the systems occurs.
[Cu](NaphPz)Cl]
N1
[Cu](NaphDPz)Cl]
N2
N1
N2
O1
Cu1
Cu1
O1
Cl1
Cl1
Fig. 5 Coordination polyhedron of CuII center in [Cu(NaphPz)Cl]
and [Cu(NaphDPz)Cl] complexes
1 3

J. M. Moreira et al.
110
100
90
100
90
80
70
60
50
40
30
100
90
80
70
60
50
80
(b)
(a)
(c)
70
60
50
40
[Cu(NaphPz) ]
[Cu(NaphPz)Cl]
[Cu(NaphDPz)Cl]
1500 1000
30
2
2000
2000
1500
1000
2000
1500
1000
110
100
90
110
100
90
100
80
60
40
20
80
80
70
70
60
(c*)
(b*)
(a*)
50
60
40
50
30
40
[Cu(NaphPz)2]
1500 1000
[Cu(NaphDPz)Cl]
1500 1000
20
[Cu(NaphPz)Cl]
30
2000
2000
2000
1500
1000
Wavenumber/cm–1
Fig. 6 FT-IR Spectra (2000–600 cm-1): experimental (a, b and c) and theoretical (a*, b*and c*) for the synthetized complexes
1.2
0.8
0.6
[Cu(NaphPz)Cl]
0.8
0.6
[Cu(NaphDPz)Cl]
[Cu(NaphPz) )
2
1.0
0.8
0.6
0.4
0.4
0.2
0.0
0.4
0.2
0.0
0.2
0.0
– 0.2
1.0
– 0.2
1.0
– 0.2
1.0
0.8
0.6
0.4
0.8
0.6
0.4
0.8
0.6
0.4
0.2
0.0
0.2
0.0
0.2
0.0
200 300 400 500 600 700 800
Wavenumber/nm–1
200 300 400 500 600 700 800
200 300 400 500 600 700 800
Fig. 7 Experimental (—) and theoretical (….) UV-Vis spectra for [Cu(NaphPz)Cl], [Cu(NaphDPz)Cl] and [Cu(NaphPz)2] complexes
To defne how the electrons are distributed among the
orbitals of the complexes, we use as a base the ligand
donor–acceptor character, defned by the spectrochemical
series, as well as single-crystal geometry obtained by X-ray
difraction data for [Cu(NaphPz)Cl] and [Cu(NaphDPz)
Cl] complexes. In these mononuclear Cu(II) complexes,
the metal atom is chelated by a tridentate (N,N,O)-Schif
base and a chloride ligand; therefore, it is possible to defne
that the [Cu(NaphPz)2] complex, obtained in powder form
with two Schif base units, has a coordination number equal
to 6. In addition, the copper (II) ion with d9 confguration,
regardless of the ligand, sufers geometry distortion defned
as Jahn–Teller efect [30], which compresses the structure
in the z-axis direction followed by a decrease in band gap
1 3

Copper (II) complexes with novel Schif‑based ligands: synthesis, crystal structure, thermal…
energy between HOMO/LUMO orbitals. From the main
characteristics of the metal–ligand binding presented, we
can then state that the charge distribution in these complexes
is carried out through bindings with π character orbitals
(pπ–dπ), which are favored by the chloride ligand and the
Schif base oxygen atom. Another bonding character that
can be identifed is that of (dπ–pπ*) type, which are electron
donations from the flled d orbitals to the π* orbitals, involv-
ing Schif base nitrogen atoms.
Conclusions
Copper (II) mononuclear complexes in anhydrous form,
using an unprecedented Schif-base ligand were synthe-
sized and characterized in the solid state or solution by
thermoanalytical methods (TGA–DSC, TGA/FT-IR),
single crystal X-ray difraction, FT-IR, UV–Vis and DFT
calculations.
Based on the TGA–DSC thermoanalytical data, it was
possible to defne the stoichiometric ratio of the synthe-
sized complexes, thermal stability, decomposition tem-
peratures, as well as the temperature of formation of the
fnal residue used in the calculation of stoichiometry. The
online monitoring of the main volatile products related to
thermal decomposition, performed by TGA/FT-IR, shows
that the complexes obtained in methanol solvent medium
adsorbed a small amount of the solvent, while the complex
obtained in ethanol/water was solvent free.
From the charge distributions, obtained by DFT cal-
culations, for the frontier molecular orbitals (FMO)
HOMO → LUMO of major contributions, according Fig.
S4, in the [Cu(NaphPz)Cl] and [Cu(NaphDPz)Cl] com-
plexes with similar structures, the charge distribution
between them is slightly diferentiated, probably due to
methyl groups. This information can be better corrobo-
rated remembering that [Cu(NaphDPz)Cl] presents a very
distorted tetrahedral geometry described as a seesaw, as
shown in Fig. 4. Concomitantly, the description of charge
transfer (CT) on the complexes studied, as shown in Fig.
S4, can be summarized by metal (M), Schiff base (L)
and halide (X) interactions, as well as that of intraligand
transfer (IL). Thus, for the [Cu(NaphPz)Cl] complex, the
HOMO/LUMO orbitals with major contribution, Fig. S4,
and oscillator strength (f) are shown as follow: H-1β→Lβ
(69%, f=0.0021), Hβ→Lβ (46%, f=0.0000), Hα→L+1α
(47%, f=0.0000) and H-3β→Lβ (43%, f=0.0255); for the
[Cu(NaphDPz)Cl] complex Hα → Lα (44%, f = 0.0004),
Hβ→L+1β (46%, f=0.0004), H-4β→Lβ (45%, f=0.0003)
and H-5β → Lβ (37%, f = 0.0564) and, for Cu(NaphPz)2
complex, Hα→L+1α.
From single-crystal X-ray diffraction data, it was
possible to establish that the chloride and Schif-base
ligands are coordinated to the central ion with coordina-
tion number four, forming structures of distorted square-
planar geometry for [Cu(NaphPz)Cl] complex and seesaw
geometry for [Cu(NaphDPz)Cl] complex. The geometry
could be confrmed from the geometric parameter τ4. For
[Cu(NaphPz)] obtained in powder form with stoichiometry
defned by TGA–DSC, it is possible to state that it is a
hexa-coordinated complex.
Regarding UV-Vis and FT-IR experimental data, they
were supported by DFT calculations and showed high cor-
relation for the optimized geometries, from which it was
possible to demonstrate how HOMO → LUMO charge
transfer occurs in the studied systems.
(24%, f = 0.0003), H-1β → L + 1β (27%, f = 0.0003),
H-1β→Lβ (88%, f=0.0040) and Hβ→Lβ (36%, f=0.0040).
For a general approach, evaluating the CT by
HOMO → LUMO orbitals for the complexes, a larger
charge displacement is observed on the naphthalene con-
jugate system for [Cu(NaphDPz)Cl] complex than for the
[Cu(NaphPz)Cl] complex, since there is a charge transfer
from metal and halide orbitals mixture (M+ X) to Schif’s
base ligand (L). This efect, clearly seems to be caused by
the addition of two alkyl groups to the pyrazole aromatic
ring that acts as an electron "pusher" for the system. On
the other hand, for the hexa-coordinated [Cu(NaphPz)2]
complex, it is observed that the absence of the chloride
ligand favors a preferential charge distribution, probably
directed by the nitrogen atoms that are part of the naphtha-
lene ring conjugate system, as well as by the Schif’s base
oxygen atoms linked to the metal center. In this case, the
nitrogen atoms in the pyrazole aromatic ring act as elec-
tron donors, as it can be verifed by the charge distribution
along the LUMO orbitals.
Supplementary Information The online version contains supplemen-
tary material available at https://doi.org/10.1007/s10973-021-10803-5.
Acknowledgements The authors thank Brazilian Foundations CNPq,
Fundect, Capes and Finep for fnancial support, and are grateful to
Professor Massao Ionashiro for allowing us to carry out measurements
in the Ivo Giolito Thermal Analysis Laboratory (LATIG) – UNESP,
and Professional Noemi Marques de Carvalho for English reviewing.
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