Synthesis, characterization, crystal structure, and electrochemical properties of three copper(II) complexes with 3,5-dihalosalicylaldehyde Schiff bases derived from amantadine

Article abstract of DOI:10.1080/00958972.2019.1655643

Complexes 1-3, C₃₄H₃₆X₄CuN₂O₂ (X = Cl, Br, I), were synthesized with copper chloride dihydrate and three new Schiff base ligands derived from amantadine and 3,5-dihalosalicylaldehydes. They were characterized by IR, UV–VIS, elemental analysis, molar conductance, and single-crystal X-ray diffraction. Single-crystal X-ray diffraction analysis reveals that 1 and 2 crystallize in the triclinic system, Pī space group. Each asymmetric unit consists of one copper(II) ion, two corresponding deprotonated Schiff base ligands and one lattice dichloromethane molecule. 3 crystallizes in the monoclinic system, P2₁/n space group. Each asymmetric unit consists of one copper(II) ion and two deprotonated iodo- Schiff base ligands. The tetra-coordination of the central copper(II) ion in 1-3 is constructed by two nitrogen atoms and two oxygen atoms from the corresponding Schiff base ligands, forming a distorted tetrahedral geometry. Electrochemical properties of the complexes were determined by cyclic voltammetry. (Figure presented.).

Full text of DOI:10.1080/00958972.2019.1655643

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Synthesis, characterization, crystal structure, and
electrochemical properties of three copper(II)
complexes with 3,5-dihalosalicylaldehyde Schiff
bases derived from amantadine
Xu-Dong Jin, Han Wang, Xiao-Kang Xie, Jia-Yue Sun & He-Ming Liang
To cite this article: Xu-Dong Jin, Han Wang, Xiao-Kang Xie, Jia-Yue Sun & He-Ming Liang (2019):
Synthesis, characterization, crystal structure, and electrochemical properties of three copper(II)
complexes with 3,5-dihalosalicylaldehyde Schiff bases derived from amantadine, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2019.1655643
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1655643
Synthesis, characterization, crystal structure, and
electrochemical properties of three copper(II) complexes
with 3,5-dihalosalicylaldehyde Schiff bases derived
from amantadine
Xu-Dong Jin, Han Wang, Xiao-Kang Xie, Jia-Yue Sun and He-Ming Liang
College of Chemistry, Liaoning University, Shenyang, People’s Republic of China
ABSTRACT
ARTICLE HISTORY
Received 2 April 2019
Revised 23 July 2019
Accepted 26 July 2019
Complexes 1-3, C34H36X4CuN2O2 (X ¼ Cl, Br, I), were synthesized
with copper chloride dihydrate and three new Schiff base ligands
derived from amantadine and 3,5-dihalosalicylaldehydes. They
were characterized by IR, UV–VIS, elemental analysis, molar con-
ductance, and single-crystal X-ray diffraction. Single-crystal X-ray
diffraction analysis reveals that 1 and 2 crystallize in the triclinic
KEYWORDS
Copper(II) complex;
amantadine; 3,5-
dihalosalicylaldehyde Schiff
base; synthesis; crystal
structure; electrochemical
property
ꢀ
system, Pi space group. Each asymmetric unit consists of one
copper(II) ion, two corresponding deprotonated Schiff base
ligands and one lattice dichloromethane molecule. 3 crystallizes
in the monoclinic system, P21/n space group. Each asymmetric
unit consists of one copper(II) ion and two deprotonated iodo-
Schiff base ligands. The tetra-coordination of the central copper(II)
ion in 1-3 is constructed by two nitrogen atoms and two oxygen
atoms from the corresponding Schiff base ligands, forming a dis-
torted tetrahedral geometry. Electrochemical properties of the
complexes were determined by cyclic voltammetry.
CONTACT Xu-Dong Jin
386237385@qq.com
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2019.1655643.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
X.-D. JIN ET AL.
1. Introduction
Transition metal complexes with Schiff base ligands have been extensively investi-
gated due to their biological activities, catalytic activities, electronic properties, and
photochemical reactions [1–3] and are commonly used as antiviral, antitumor, and
antibacterial agents [4, 5]. They can also be applied as electrode modifiers in electroa-
nalysis [6–9]. Copper is one of the integral trace elements in human beings, playing a
significant role in enzyme active sites [10, 11]. As a transition metal ion, copper(II) may
have a wide range of coordination geometries [12]. Copper complexes with Schiff
bases can be used to develop surface modified electrodes for sensor applications [13].
In many countries, amantadine (SymmetrelTM) and rimantadine (FlumadineTM) have
been widely used to treat or prevent seasonal influenza as efficacious remedies [14–17]. It
can also alleviate Parkinson symptoms [18, 19]. Salicylaldehyde and substituted salicylalde-
hydes, especially halogenated salicylaldehydes, were used to produce herbicides, insecti-
cides and fungicides [20–22]. As an extension of our previous studies on the
electrochemical properties of copper complexes with Schiff bases derived from amanta-
dine or rimantadine [23, 24], we designed and synthesized three copper(II) complexes
with the Schiff bases derived from amantadine and 3,5-dihalosalicylaldehydes.
The coordination behaviors of the ligands towards copper(II) were investigated and
their absolute structures were determined by single-crystal X-ray diffraction analysis.
Moreover, electrochemical properties of the complexes were determined by cyclic vol-
tammetry in dimethylformamide (DMF) and dichloromethane.
2. Experimental
2.1. Materials and methods
All chemicals and solvents were purchased from Sinopharm Chemical Reagent. They
were of analytical grade and used without purification. Elemental analyses were car-
ried out on a Perkin Elmer Flash EA 1112. Infrared spectra (IR) were scanned from
4000 to 400 cmꢀ1 via KBr pellets on a Nicolet NEXUS FT-IR 5700 spectrophotometer.
UV–VIS spectra were measured on a Perkin Elmer Lambda 25 spectrophotometer.
Melting points were measured on a WRS-1B micro melting point apparatus and are
1
uncorrected. H NMR chemical shifts (d) for the Schiff base ligands were recorded at
300 MHz on a Varian Mercury-Vx300 spectrometer in CDCl3 solvent containing TMS as
an internal standard. The molar conductance for the complexes in DMF (1.0 ꢁ 10ꢀ3
mo1 Lꢀ1) were measured using a DDS-11A conductometer.
2.2. Synthesis of the ligands
Three Schiff base ligands, 2-((adamantan-1-ylimino)methyl)-4,6-dichlorophenol (HL1),
2-((adamantan-1-ylimino)methyl)-4,6-dibromophenol (HL2) and 2-((adamantan-1ylimino)-
methyl)-4,6-diiodophenol (HL3), were prepared analogously to the literature [25, 26]. The
general synthetic route for ligands in this work is shown in Figure 1.
Amantadine hydrochloride (563 mg, 3.0 mmol) and potassium hydroxide (168 mg,
3.0 mmol) in 50 mL anhydrous ethanol were stirred for 10 min and white precipitates

JOURNAL OF COORDINATION CHEMISTRY
3
Figure 1. The formation of the Schiff base ligands.
(KCl) were filtered out. Then the transparent liquid was added dropwise to a solution
of 3,5-dihalosalicylaldehydes (halo ¼ Cl, Br, I; 382, 560, 748 mg; 2.0 mmol) in 30 mL
anhydrous ethanol. The mixture was refluxed for 1 h and then cooled to room tem-
perature. The yellow Schiff base precipitates were collected after 1 week of slow solv-
ent evaporation.
HL1: 390 mg, yield 61%. Orange block. M.p. 167.5 ꢂ 168.5 ꢃC. UV–VIS (dichlorome-
thane, c ¼ 1 ꢁ 10ꢀ5 g mLꢀ1): kmax ¼ 235 (0.9470), 333 (0.1174), 429 (0.1464); kmin
¼
1
3
305 (0.0497), 369 (0.0481). H NMR (CDCl3, 300 MHz): d 15.67 (d, J ¼ 4.2, 1 H, Ar-OH);
3
4
4
8.12 (d, J ¼ 5.1, 1 H, CH ¼ N); 7.41 (dd, J ¼ 2.1/2.4, 1 H, Ar-H); 7.09 (d, J ¼ 2.4, 1 H, Ar-
H); 2.22 (s, 3 H, CH, adam.H); 1.89 (s, 6 H, CH2, adam.H); 1.80 - 1.67 (m, 6 H, CH2,
adam.H). Anal. Calc. for C17H19Cl2NO (324.24): C, 62.97; H, 5.91; N, 4.32. Found: C,
62.92; H, 5.86; N, 4.35.
HL2: 690 mg, yield 83%. Yellow powder. M.p. 175.7 ꢂ 176.5 ꢃC. UV–VIS (dichlorome-
thane, c ¼ 1 ꢁ 10ꢀ5 g mLꢀ1): kmax ¼ 235 (1.2255), 339 (0.1289), 430 (0.2004); kmin
¼
1
4
313 (0.0730), 371 (0.0573). H NMR (CDCl3, 300 MHz): d 8.08 (s, 1 H); 7.70 (d, J ¼ 2.24,
4
1 H); 7.29 (d, J ¼ 2.21, 1 H); 2.22 (s, 3 H, CH); 1.90 (s, 6 H, CH2); 1.67-1.81 (m, 6 H, CH2).
Anal. Calc. for C17H19Br2NO (413.15): C, 49.42; H, 4.64; N, 3.39. Found: C, 49.47; H, 4.61;
N, 3.35.
HL3: 620 mg, yield 61%. Yellow floc. M.p. 178.5 ꢂ 179.5 ꢃC. UV–VIS (dichloromethane,
c ¼ 1 ꢁ 10ꢀ5 g/mL): kmax ¼ 242 (1.1883), 346 (0.1094), 434 (0.2266); kmin ¼ 330
(0.1001), 403 (0.1379). 1H NMR (CDCl3, 300 MHz): d 15.68 (s, 1 H, Ar-OH); 8.05 (d,
4J ¼ 2.1, 1 H, Ar-H); 7.97 (s, 1 H, CH¼N); 7.45 (d, 4J ¼ 2.1, 1 H, Ar-H); 2.34 (s, 3 H, CH,
adam.H); 1.90 (d, J ¼ 2.4, 6 H, CH2, adam.H); 1.80–1.67 (m, 6 H, CH2, adam.H). Anal. Calc.
for C17H19I2NO(507.15): C, 40.26; H, 3.78; N, 2.76. Found: C, 40.34; H, 3.82; N, 2.73.
2.3. Synthesis of the complexes
Complexes bis(2-(1-adamantyliminomethyl)-4,6-dichlorophenolato-N,O)copper(II) (1),
bis(2-(1-adamantyliminomethyl)-4.6-dibromophenolato-N,O)copper(II) (2) and bis(2-(1-
adamantyliminomethyl)-4,6-diiodophenolato-N,O)copper(II) (3) were prepared in a simi-
lar procedure with a corresponding Schiff base ligand, NaOH, and copper(II) chloride
dihydrate in anhydrous methanol, respectively (Figure 2).
NaOH (80 mg, 2.0 mmol) in 20 mL of anhydrous methanol was added to a stirring
solution of a ligand (HL1, HL2, HL3; 648, 826, 1014 mg; 2.0 mmol) in 20 mL anhyd-
rous methanol. The mixture was stirred for 20 min and then copper(II) chloride

4
X.-D. JIN ET AL.
Figure 2. Proposed structures of the complexes.
dihydrate (170 mg, 1.0 mmol) in 10 mL of anhydrous methanol was added dropwise.
The mixed solution was refluxed for 2 h and then kept at room temperature until a
large amount of precipitate was observed. The solid product was isolated by suc-
tion filtration, washed with a few drops of anhydrous methanol and dried.
1: 660 mg, yield 93%. Dark taupe powder. M.p. > 300 ꢃC. UV–VIS (dichloromethane,
c ¼ 1.0 ꢁ 10ꢀ5 g mLꢀ1): kmax ¼ 245 (0.9851), 327 (0.1674), 384 (0.2476); kmin ¼ 305
(0.1301), 348 (0.1411). Anal. Calc. for C34H36Cl4CuN2O2 (710.02): C, 57.51; H, 5.11; N,
3.94. Found: C, 57.45; H, 5.15; N, 3.98.
2: 830 mg, yield 93%. Dark taupe powder. M.p. > 300 ꢃC. UV–VIS (dichloromethane,
c ¼ 1.0 ꢁ 10ꢀ5 g mLꢀ1): kmax ¼ 246 (0.9555), 330 (0.1609), 385 (0.2438); kmin ¼ 306
(0.1246), 348 (0.1401). Anal. Calc. for C34H36Br4CuN2O2 (887.72): C, 46.01; H, 4.09; N,
3.16. Found: C, 46.04; H, 4.13; N, 3.12.
3: 980 mg, yield 91%. Taupe powder. M.p. > 300 ꢃC. UV–VIS (dichloromethane,
c ¼ 1.0 ꢁ 10ꢀ5 g mLꢀ1): kmax ¼ 246 (0.9670), 329 (0.1853), 389 (0.2348); kmin ¼ 354
(0.1530). Anal. Calc. for C34H36I4CuN2O2 (1075.83): C, 37.96; H, 3.73; N, 2.60. Found: C,
37.93; H, 3.75; N, 2.62.
2.4. X-ray crystallography
Single crystals of 1–3 suitable for X-ray analysis were developed from a solution of
CH3OH/CH2Cl2 (1:2 v/v) through slow evaporation at room temperature. The crystallo-
graphic data collections were conducted on a Bruker Smart Apex II CCD with graphite
monochromated Mo Ka radiation (k ¼ 0.71073 Å) using the x-scan technique. The data
were integrated by using the SAINT program, which also corrected the intensities for
Lorentz and polarization effects [27]. An empirical absorption correction was applied
using SADABS [28]. The molecular structures were solved by SIR2004 [29] for 1 and
SHELXS [30] for 2 and 3. All non-hydrogen atoms were refined anisotropically on F2
by the full-matrix least-squares technique using the SHELXL crystallographic software
package [31]. Hydrogens were generated geometrically. All calculations were per-
formed on a personal computer with the SHELXL crystallographic software package.
Details of the crystal parameters, data collection, and refinement are summarized in
Table 1. Selected bond lengths and angles with their estimated standard deviations
are given in Table 2. The absolute structures for complexes, which were visualized by
Diamond [32], are shown in Figures 3–5. The weak CHꢄꢄꢄX and halogen bond interac-
tions of 1–3 are shown in Figures 6–8.

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Crystallographic data for 1–3.
C
34H36Cl4CuN2O2ꢄCH2Cl2
C
34H36Br4CuN2O2ꢄCH2Cl2
C
34H36I4CuN2O2
Empirical formula
(1)
(2)
(3)
Formula weight (g molꢀ1
)
794.91
0.25 ꢁ 0.22 ꢁ 0.21
Triclinic
972.75
0.15 ꢁ 0.10 ꢁ 0.10
Triclinic
1075.79
Crystal size (mm)
Crystal system
Space group
a (Å)
0.25 ꢁ 0.14 ꢁ 0.09
Monoclinic
P21/n
Pꢀi
9.9535(6)
Pꢀi
9.9860(4)
14.202(7)
b (Å)
11.8309(7)
16.8277(10)
110.347(2)
92.772(2)
110.812(2)
1703.00(18)
2
3.1 – 28.31
ꢀ13 ꢅ h ꢅ 13
ꢀ15 ꢅ k ꢅ 15,
ꢀ22 ꢅ l ꢅ 22
1.550
11.9005(5)
16.9848(7)
109.9409(18)
92.801(2)
110.7636(19)
1740.78(13)
2
3.1 – 28.4
ꢀ13 ꢅ h ꢅ 13
ꢀ15 ꢅ k ꢅ 15
ꢀ22 ꢅ l ꢅ 22
1.856
9.972(5)
25.097(12)
90.00
95.677(10)
90.00
3537(3)
4
2.9 – 25.02
ꢀ16 ꢅ h ꢅ 15
ꢀ11 ꢅ k ꢅ 11
ꢀ29 ꢅ l ꢅ 29
2.020
c (Å)
a (ꢃ)
b (ꢃ)
c (ꢃ)
V (Å3)
Z
h range (ꢃ)
Index ranges
q (g cm-3)
m (mm-1)
1.148
5.407
4.141
Reflections collected / Unique
81637 / 8507
[R(int) ¼ 0.0692]
8507 / 0 / 415
1.104
89924 / 8683
[R(int) ¼ 0.0486]
8683 / 0 / 415
1.119
32970 / 6237
[R(int) ¼ 0.1200]
6237 / 6 / 388
1.106
Data / restraints / parameters
Goodness-of-fit (GoF)
F(000)
T/K
818.0
296(2)
0.0364 / 0.0927
962.0
223(2)
0.0301 / 0.0692
2044.0
150(2)
0.0602 / 0.1257
b
R1a/wR2 (I > 2r(I))
R1/wR2 (all data)
0.0414 / 0.0973
0.0368 / 0.0721
0.1042 / 0.1407
aR1 ¼ RjjFoj–jFcjj/jFoj.
2 2
bwR2 ¼ [Rw(Fo2–Fc ) /Rw(Fo2)2]1/2
2.5. Electrochemical studies
The solvent, copper(II) chloride dihydrate, Schiff base ligands, and copper(II) complexes
were investigated by cyclic voltammetry. They were prepared with a concentration of
1.0ꢁ 10ꢀ3mol Lꢀ1 in DMF and CH2Cl2, respectively. Electrochemical data were collected
using a CHI 660E Electrochemical Workstation (Shanghai Chenhua). A three-electrode cell
was used for all electrochemical experiments. This system consisted of glassy carbon (work-
ing), platinum wire (counter) and Ag/AgCl (reference) electrodes. Test solutions contained
1.0ꢁ 10ꢀ3mol compounds and 0.15 mol tetra-n-butylammonium hexafluorophosphate in
DMF and CH2Cl2 solution. During the cyclic voltammetry measurement a constant flux of
N2 was kept over the solution surface in order to check the diffusion of atmospheric oxy-
gen into the solution. All measurements were carried out at room temperature. The elec-
troactivity range for this system was 3.0 V (from ꢀ1.5 to 1.5 V vs. SCE) at a scan rate of
100 mV sꢀ1. The data were integrated by using the OriginPro 8 program [33].
3. Results and discussion
3.1. Elemental analysis and molar conductance
Based on the elemental analysis, the C, H, N contents suggested that 1–3 consist of
one copper(II) ion and two deprotonated ligands. Although all complexes are soluble
in dichloromethane and chloroform, they are less soluble than their corresponding
ligands in other solvents such as methanol, ethanol, acetone and dimethyl sulfoxide.

6
X.-D. JIN ET AL.
Table 2. Selected bond lengths (Å) and angles (ꢃ) for 1–3.
1
2
3
Cu1-O1
Cu1-N1
Cu1-O2
Cu1-N2
N1-C7
N1-C8
N2-C24
N2-C25
O1-Cu1-O2
O2-Cu1-N2
O1-Cu1-N2
O2-Cu1-N1
O1-Cu1-N1
N2-Cu1-N1
1.8962(12)
1.9993(14)
1.8917(13)
2.0077(14)
1.283(2)
1.503(2)
1.289(2)
1.500(2)
137.26(6)
95.90(6)
Cu1-O1
Cu1-N1
Cu1-O2
Cu1-N2
N1-C7
N1-C8
N2-C24
N2-C25
O1-Cu1-O2
O2-Cu1-N2
O1-Cu1-N2
O2-Cu1-N1
O1-Cu1-N1
N2-Cu1-N1
1.8918(17)
2.008(2)
1.8952(17)
1.998(2)
1.284(3)
1.500(3)
1.285(3)
1.503(3)
137.26(8)
93.82(8)
97.40(8)
100.65(8)
95.63(8)
141.59(8)
Cu1-O1
Cu1-N1
Cu1-O2
Cu1-N2
N1-C7
N1-C8
N2-C24
N2-C25
O2-Cu1-O1
O2-Cu1-N2
O1-Cu1-N2
O2-Cu1-N1
O1-Cu1-N1
N2-Cu1-N1
1.906(7)
2.001(8)
1.906(7)
1.976(9)
1.284(13)
1.511(14)
1.282(13)
1.505(13)
133.4(3)
96.2(3)
97.3(4)
99.5(3)
96.2(4)
142.7(4)
101.15(5)
97.30(6)
93.96(6)
140.39(6)
The molar conductance values (KM) are 4.1, 2.18 and 11.32 S cmꢀ2 molꢀ1 for 1–3,
respectively, indicating that the complexes are non-electrolytic in nature [34].
3.2. IR spectra
IR spectra of the ligands and the complexes are provided in the Supplementary Material
and the main IR data are collected in Table 3. In general, free O–H stretching vibration
bands should be at 3500–3600 cmꢀ1, however, they display red-shifted values for HL1-HL3
at 3440, 3416 and 3411 cmꢀ1 due mainly to an effect of intramolecular hydrogen bond-
ing. These vibrations are absent in the complexes, illustrating that phenolic OH groups
are deprotonated as O ! Cu coordination occurs. The strongest absorptions for the
ligands at 1628, 1626 and 1627 cmꢀ1 are assigned to the C¼N stretch, which red-shifted
to 1609, 1607 and 1610 cmꢀ1 in 1–3. In the complexes, these vibration bands undergo
downward shift by 19, 19 and 17 cmꢀ1, implying coordination of the azomethine nitrogen
atom to copper(II) [35, 36]. Bands at 1450–1475 cmꢀ1 for ligands and 1431–1449 cmꢀ1 for
complexes are assigned to C–N stretching vibrations. Meanwhile, bands at 1213, 1212 and
1225 cmꢀ1 for the ligands as well as 1169, 1154 and 1147 cmꢀ1 for the complexes are
C–O stretching vibrations [35]. In low frequency regions, the remarkable appearance of
absorptions at 555–558 and 446–452 cmꢀ1 for the complexes can be assigned to Cu–O
and Cu–N vibrations, which indicate that oxygen and nitrogen atoms of the Schiff bases
are coordinated to copper(II) [35, 36].
3.3. UV–VIS spectra
UV–VIS spectra of the ligands and the complexes are provided in the Supplementary
Material. All the complexes show significant changes compared to their corresponding
ligands from 200–600 nm. The absorption bands at 235–242 nm for the ligands and
ꢆ
245–246 nm for the complexes are clearly assigned to the p-p transition of C¼N and
benzenoid conjugated systems. Bands at 333–346 nm for the ligands should be
ꢆ
assigned to the p-p transitions of the benzene ring, which were greatly influenced by
the substituent groups. The corresponding complexes’ absorptions are at 327–330 nm.
1
3
ꢆ
Bands at 429 nm, 430 nm and 434 nm for HL -HL are attributed to n-p transition of
the C¼N group. Besides the above, 1–3 have obvious absorption bands at 384, 385,

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 3. The molecular structure of 1. Thermal ellipsoids are drawn at 50% of probability.
Figure 4. The molecular structure of 2. Thermal ellipsoids are drawn at 50% of probability.
and 389 nm, respectively, which are assigned to charge transfer transitions from the
ligands to copper(II) of N ! Cu and O ! Cu.
3.4. Crystal structures of complexes
ꢀ
The crystallographic analysis reveals that 1 and 2 crystallize in the triclinic system, Pi
space group; each asymmetric unit comprises one mononuclear complex molecule
and one lattice dichloromethane molecule. Complex 3 crystallizes in the monoclinic

8
X.-D. JIN ET AL.
Figure 5. The molecular structure of 3. Thermal ellipsoids are drawn at 50% of probability.
Figure 6. The short C-HꢄꢄꢄCl contacts in the packing of 1. View toward plane for hkl ¼
2.25, ꢀ11.68.
system, P21/n space group; each asymmetric unit comprises one mononuclear complex
molecule. The central copper(II) ion in 1–3 is bonded to the oxygen and nitrogen

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 7. The short C-HꢄꢄꢄCl and C-HꢄꢄꢄBr contacts in the packing of 2. View toward plane for hkl
¼ 3.38, ꢀ11.83.
Figure 8. The short contacts in the packing of 3. View toward plane for hkl ¼ 10.25, ꢀ6.22, 5.68.

10
X.-D. JIN ET AL.
Table 3. Main IR data for HL1-HL3 and 1–3 (cmꢀ1).
Compound
tO-H
tCH2
tC¼N
tC-N
tC-O
tM-O
tM-N
HL1
1
3440(w)
2849(s)
1628(s)
1610(s)
1626(s)
1608(s)
1627(s)
1610(s)
1458(s)
1449(s)
1450(s)
1440(s)
1475(m)
1431(s)
1213(s)
1168(s)
1211(m)
1154(m)
1225(m)
1147(m)
–
–
–
3433(w)
–
3411(w)
–
2850(m)
2847(m)
2849(m)
2850(m)
2848(w)
558(w)
–
556(w)
–
452(w)
–
446(w)
–
HL2
2
HL3
3
555(w)
451(w)
donors of the two bidentate Schiff base ligands. The dihedral angle between the two
coordination planes of 1–3 defined by O1-Cu1-N1 and O2-Cu1-N2 is 56.95ꢃ for 1, by
O1-Cu1-N1 and O2-Cu1-N2 is 56.25ꢃ for 2, and by O1-Cu1-N1 and O2-Cu1-N2 is 57.77ꢃ
for 3. The two phenyl rings are in the intersecting planes with a dihedral angle
defined by C1-C3-C6 and C18-C20-C22 of 55.02ꢃ for 1, by C6-C1-C2 and C18-C23-C22
of 54.71ꢃ for 2, and by C2-C1-C6 and C18-C23-C22 of 66.89ꢃ for 3. Moreover, bond
angles also reveal that the coordination geometry of copper in 1–3 is distorted tetra-
hedral; the parameters s4 ¼ [360-(a þ b)]/141ꢃ [37] for 1–3 are 0.58, 0.59 and 0.59,
respectively, of which the angles are N2-Cu1-N1 (a) and O2-Cu1-O1 (b) (see Table 2).
Complexes 1–3 are expected for typical Schiff base ligands (containing a short C¼N
bond distance of 1.283(2)/1.289(2) Å for 1, 1.284(3)/1.285(3) Å for 2 and 1.282(13)/
1.284(13) Å for 3) coordinated to a metal center, where the imine form is predomin-
ant. The distinctly shortened Cu1-O1 and elongated Cu1-N1 bonds are 1.896(12) and
1.999(14) Å for 1, 1.892(17) and 2.008(2) Å for 2, and 1.906(7) and 2.001(8) Å for 3
(normal bond lengths are ca. 1.940 and 1.960 Å). Compared to similar CuN2O2 struc-
tures, it can be found that the C¼N, Cu–O and Cu–N bond lengths are very close to
those of compounds reported [23, 24], although they have different substituent
groups on the benzene ring. However, the angles for the coordination geometry of
central copper ion are distinct. For example, the geometry around copper(II) is dis-
torted square planar with a small dihedral angle between the two coordination planes
in salicylaldehyde and 4-methoxyl-salicylaldehyde Schiff bases copper(II) complexes.
Another coincident case was found in 3-methoxy-salicylaldehyde Schiff bases
copper(II) complex with the dihedral angle of 0ꢃ. In addition, the coordination geom-
etry and the s4 parameter value of dihalogenated complexes 1-3 are very similar to
the copper(II) complexes derived from mono-halogenated salicylaldehyde Schiff bases.
The coordination geometry around the copper(II) ion reflects the Jahn-Teller effect
[38]. The shortest distance between two adamantine carbons from two ligands is
4.129 Å for C10-C26 in 1, 4.175 Å for C9-C26 in 2, and 4.402 Å for C11-C34 in 3,
respectively. The distance of two neighboring copper ions along the y axis is 9.953 Å
in 1, 7.124 Å in 2, and 8.631 Å in 3. The bidentate coordination mode of 1–3 does not
form intramolecular hydrogen bonds for the deprotonated ligands, but there still exist
two six-membered rings through a copper(II), two oxygen and two nitrogen
atoms [39].
The Mercury software has been used for obtaining the packing mode and data of
intermolecular interactions. Though the angles of 1–3 between the two adjacent ben-
zene rings in the stacking complexes are 1.85ꢃ, 0ꢃ and 0.87ꢃ, <10ꢃ, the distance
between the centroid of benzene ring to the plane of another benzene ring is 6.721,
3.569 and 4.345 Å, >3.5 Å [40]. Thus, we can conclude that 1–3 do not possess p–p

JOURNAL OF COORDINATION CHEMISTRY
11
stacking interactions. Complexes 1–3 are not found to possess weak C–Hꢄꢄꢄp interac-
tions, but rather several weak C–HꢄꢄꢄX (X ¼ Cl, Br, I) interactions (Figures 6–8). Complex
1 has three C–HꢄꢄꢄCl interactions, which can be distinguished as two situations. The
first is about C11–H11AꢄꢄꢄCl5 between the complex molecule and dichloromethane
molecule in the asymmetric unit. The distance of C11ꢄꢄꢄCl5 is 3.776 Å. In another situ-
ation, C12–H12AꢄꢄꢄCl2 and C26–H26BꢄꢄꢄCl2 are observed, with distances of 3.828 and
3.550 Å, between adjacent complex molecules. Complex 2 has one weak C–HꢄꢄꢄCl inter-
action and seven weak C–HꢄꢄꢄBr interactions. They are summarized also in two situa-
tions, one in which C28–H28AꢄꢄꢄCl1, C35–H35AꢄꢄꢄBr1 and C35–H35BꢄꢄꢄBr2 are formed
through the complex molecule and a dichloromethane molecule. The distances of
CꢄꢄꢄX are 3.803, 3.820 and 3.902 Å, respectively. The other five weak C–HꢄꢄꢄBr interac-
tions
(C11–H11AꢄꢄꢄBr1,
C31–H31ꢄꢄꢄBr3,
C9–H9BꢄꢄꢄBr4,
C13–H13ꢄꢄꢄBr4,
and
C29–H29AꢄꢄꢄBr4) are formed between adjacent copper complex molecules. The CꢄꢄꢄBr
distances are 3.824, 3.702, 3.584, 3.621 and 3.902 Å in turn. Complex 3 possesses three
weak interactions. C11–H11BꢄꢄꢄI2, C15–H15BꢄꢄꢄI2 and C14–H14BꢄꢄꢄI2 are formed from
adjacent complex molecules. The CꢄꢄꢄI distances are 4.028, 4.006 and 3.866 Å,
respectively.
Complexes 1 and 2 are not found to possess halogen bonding interactions, but 3
possesses two kinds of weak intermolecular IꢄꢄꢄI interactions in the packing structure.
Both I1ꢄꢄꢄI3 and I2ꢄꢄꢄI3 interactions are formed through adjacent copper complex mole-
cules. I1ꢄꢄꢄI3 interaction belongs to the type II motifs with the characteristic of h1
86.57ꢃ and h2 172.89ꢃ, respectively, where h1 and h2 are the C-IꢄꢄꢄI angles [41]. The
I1ꢄꢄꢄI3 distance is 3.79 Å close to the Danis value 3.89 Å for type II. The I2ꢄꢄꢄI3 distance is
3.95 Å and the value of h1 and h2 are 89.29ꢃ and 139.52ꢃ, respectively. The weak
C–HꢄꢄꢄX interactions and halogen bonds help stabilize the crystal structure and form
an infinitive three-dimensional structure.
3.5. Electrochemistry
The electrochemical behavior was investigated in DMF and dichloromethane contain-
ing tetra-n-butylammonium hexafluorophosphate as a supporting electrolyte. As
shown in Figure 9(a), the solvent was tested under the addition of 0.15 mol supporting
electrolyte; no redox peak was found in the scanning region. In Figure 9(b), copper
chloride dihydrate exhibited a quasi-reversible oxidation and reduction peak at 0.343
(Epc) and 0.473 V vs. Ag/AgCl (Epa) in DMF solution. The DEp ¼ (Epa ꢀ Epc) value is
130 mV and the ipa/ipc ratio is 1.107. At the potentials of 0.850 (Epc) and 1.200 V vs.
Ag/AgCl (Epa) also exists a redox peak. Because copper chloride dihydrate is insoluble
in dichloromethane, its electrochemical behaviors in dichloromethane are
not available.
The cyclic voltammograms of the ligands are shown in Figures 10 and 11. In DMF
solution, HL1-HL3 exhibit very similar oxidation peaks at 1.151, 1.159 and 1.130 V vs.
Ag/AgCl; the corresponding reduction peaks are not observed. In dichloromethane
solution, in anodic potential regions, HL1-HL3 exhibit oxidation peaks at 1.350, 1.372
and 1.347 V vs. Ag/AgCl, respectively. HL2 and HL3 have the reverse weak reduction
peaks at 1.135 and 1.102 V vs. Ag/AgCl, but the peaks are not observed for HL1. In the

12
X.-D. JIN ET AL.
Figure 9. The cyclic voltammogram for solvent (a) and CuCl2ꢄ2H2O (b) in DMF solution.
Figure 10. The cyclic voltammogram for HL1-HL3 in DMF solution.
cathodic potential region, HL1-HL3 exhibit reduction peaks at ꢀ0.794, ꢀ0.777 and
ꢀ0.769 V vs. Ag/AgCl, which can be assigned to the reduction of the imine group [42].
The cyclic voltammograms of the complexes are shown in Figures 12 and 13. The
data for the complexes are collected in Table 4. Compared with the cyclic voltammo-
gram of the solvent, copper chloride dihydrate and ligands, all complexes perform
reversible reductive responses in cathodic potential regions. In DMF solution, the
reduction potentials for 1–3 are observed at ꢀ0.765, ꢀ0.750 and ꢀ0.752 V vs. Ag/AgCl
(Epc), respectively, the characteristic peak B for Cu(II) ! Cu(I). The corresponding Epa
values are observed at -0.688, -0.671 and -0.674 V vs. Ag/AgCl, the peak A for Cu(I) !
Cu(II). The DEp ¼ (Epa ꢀ Epc) values of 1–3 are 77, 80 and 78 mV, respectively.
Furthermore, the ipa/ipc ratios are 0.92, 0.83 and 0.81, respectively. These results indi-
cate the quasi-reversibility character of the redox process.
DMF is a commonly used solvent for measuring electrochemical properties, but it is
a highly coordinating solvent that the Cu center might be interacting with in a non-
negligible manner. Therefore, the tests were also carried out in dichloromethane.
There have exhibited similar oxidation and reduction potential peaks at E1/2 ꢀ0.454,
ꢀ0.434, and ꢀ0.459 V vs. Ag/AgCl in dichloromethane solution. Peak D is characteristic
of the Cu(II) ! Cu(I) couple, while C is assigned to Cu(I) ! Cu(II) couple. The peak
potential is shifted to more positive potential compared to that in DMF. Furthermore,
the midpoint potentials of 1ꢀ3 are very similar in both DMF and dichloromethane,
which indicate that the halogen substituent has no obvious effect on the electrochem-
ical behavior. This conclusion is different from previous literature [24]. The DEp value
and ipa/ipc ratios of the complexes reveal that the redox process is quasi-reversible in

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 11. The cyclic voltammogram for HL1-HL3 in dichloromethane solution.
Figure 12. The cyclic voltammogram for 1–3 in DMF solution.
Figure 13. The cyclic voltammogram for 1–3 in dichloromethane solution.
Table 4. Redox potential data of 1–3 in DMF and dichloromethane solution.
Compound
Solvent
Epa/V
Epc/V
DEp/mV
E1/2/V
ipa/ipc
1
DMF
Dichloromethane
DMF
Dichloromethane
DMF
ꢀ0.688
ꢀ0.397
ꢀ0.671
ꢀ0.380
ꢀ0.674
ꢀ0.402
ꢀ0.765
ꢀ0.510
ꢀ0.750
ꢀ0.488
ꢀ0.752
ꢀ0.515
77
113
80
108
78
ꢀ0.727
ꢀ0.454
ꢀ0.711
ꢀ0.434
ꢀ0.713
ꢀ0.459
0.92
0.93
0.83
0.77
0.81
0.97
2
3
Dichloromethane
113
dichloromethane. For copper(II) complexes with Schiff base ligands, a peak current
ratio less than unity [43] can be commonly observed [44, 45]. This behavior can be
attributed to the existence of trace water in the solvent or supporting electrolyte
according to the literature [46]. In addition, it is reported that the Cu(II)L2/[Cu(I)L2]
reaction in most CuN2O2 coordination complexes is quasi-reversible because the
[Cu(I)L2] species are chemically decomposed to a copper ion, which is subsequently is
reduced to copper metal [47].

14
X.-D. JIN ET AL.
4. Conclusion
Three Schiff base ligands and their corresponding copper(II) complexes have been pre-
pared and their structures were characterized. Single-crystal X-ray diffraction analyses
revealed that each complex molecule consists of one copper(II) ion and two deprotonated
Schiff base ligands. The Schiff base ligands serve as bidentate ligands coordinating
through an oxygen atom and a nitrogen atom to the copper(II) ion. The geometry around
the metal in the complexes is distorted tetrahedral. The electrochemical properties of the
complexes revealed the quasi-reversible one electron transfer redox processes.
Funding
This work was financially supported by Foundation of Liaoning Provincial Department of
Education Innovation Team Projects (grant number LT2017010); the Cause of Public Welfare
Scientific Research Fund (grant number 2015005008); Shenyang Science and Technology Plan
Project (grant number F13-289-1-00); Liaoning University’s College Students Innovation and
Entrepreneurship Training Program (grant number x201610140158).
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