Influence of substituents on the structure of Schiff bases Cu(II) complexes

Article abstract of DOI:10.1016/j.molstruc.2020.127916

The relationship between molecular conformation and substituent effects of salicylaldehyde Schiff-base Cu(II) complexes was explored. For this study, eight samples of the complexes Cu(Sal-X)₂ (X = OMe, Me, H, F, Cl, Br, CF₃ and CN) w

Full text of DOI:10.1016/j.molstruc.2020.127916

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Influence of substituents on the structure of Schiff bases Cu(II) complexes
Yan Xiao, Chenzhong Cao
PII:
S0022-2860(20)30240-4
DOI:
https://doi.org/10.1016/j.molstruc.2020.127916
MOLSTR 127916
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To appear in: Journal of Molecular Structure
Received Date: 10 June 2019
Revised Date: 16 January 2020
Accepted Date: 14 February 2020
Please cite this article as: Y. Xiao, C. Cao, Influence of substituents on the structure of Schiff
bases Cu(II) complexes, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
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In this paper, two correlation equations are established. Substituent electron effect
displays important effects on the dihedral angle τ1 τ1 is the value of the dihedral angle
(
between the Cu-O-N plane and O-C=N plane) and the coordinate bond length (LCu-N
)
in the crystal structure of the complexes.

Influence of substituents on the structure of Schiff bases Cu(II) Complexes
Yan Xiao, Chenzhong Cao *
School of Resource Environment and Safety Engineering, School of Chemistry and Chemical Engineering, Key
Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education, Hunan University
of Science and Technology, Xiangtan, 411201, China
* Corresponding author. Email: czcao@hnust.edu.cn.
Abstract: The relationship between molecular conformation and substituent effects of salicylaldehyde
Schiff-base Cu(II) complexes was explored. For this study, eight samples of the complexes Cu(Sal-X)2 (X= OMe,
Me, H, F, Cl, Br, CF3 and CN) were obtained by reaction of Cu(OAc)2 with Schiff base ligands (Sal-X) derived
from Salicydaldehyde, and their crystal structures were characterized by the X-ray diffraction technique. The
QSPR methods were employed to achieve deeper insight into geometry structure of Cu(Sal-X)2. The results
show that the main geometrical parameters of Cu(Sal-X)2 are dominated by the substituents on the aniline ring.
That is, the dihedral angle τ1 between the Cu-O-N plane and O-C=N plane increases with the increase of
electron-donating effect and electron-withdrawing effect of substituents, whereas the electronic effect of
substituents has the opposite effect on
τ
2, which is the dihedral angle between the aniline ring plane and C-N-Cu
ex
cc
plane. Moreover, the bond length
decreases with growing excited-state parameter
of substituents
σ
LCu - N
which enhances the energy of C-N bond.
Key words: Copper complexes crystal; substituent effect; dihedral angle; QSPR; bond length;
1. Introduction
Schiff bases are the popular chelating ligands, and are able to easily coordinate various metal ions [1-4]. Many
Schiff bases metal complexes have versatile applications[ 5 - 9 ] for their characteristic properties such as
photochromism, magnetism, catalysis properties and remarkable biomedical activities. Some of these complexes
have been reported as magnetic materials[10], anticancer agents[11], catalyst[12] or energy materials[13]. Also, a
variety of metal complexes have been designed for electron transport hosts in organic light-emitting diodes
(OLEDs)[14]
.
[15]
It is well known that the structure dominates the properties and use of materials. Recent research
demonstrated that the steric structure and electronegativity of substituents on aniline ring of ligands had a large
effect on the photophysical and electrochemical properties of the corresponding complexes. In addition, it has
been reported that the electronic effect of substituents on the aniline ring has an important influence on
[17]
properties of the complexes such as the chelating stability [16], oxidation potentials
and the reactivity [18]. On
the other hand, molecule configuration can be fine-tuned by the substituents [19]. Some copper complexes exhibit
[20-22]
the change of molecular configuration from square planar to tetrahedral coordination
by introduction of
different substituents. It was also reported [23-25] that the size and nature of the substituent on the imino nitrogen
of the Schiff base ligand affected the coordination geometry around copper(II) in complexes. Therefore, in order
to the design of new materials for a specific purpose, we may change configuration through the substituent
electronic effects.
Owing to the difficulty in cultivating single crystals of compounds, the correlation between molecular
1

configurations and substituents is rarely studied. Fang et al[26-27] explored the relationship between molecular
configuration and the longest wavelength maximum λmax of ultraviolet absorption for Schiff bases with different
substituents on the aromatic rings. A conclusion was drawn that the dihedral angle
τ influenced the electronic
effects of substituents on the λmax, and the effect of the sin( ) on spectroscopic properties of Schiff bases was
τ
quantified. However, up to now, the influence rule of substituents on molecular configuration of Schiff bases
complexes has not been reported.
In coordination chemistry, copper complexes of Schiff bases have received considerable interest from the
researchers[28-30]. Therefore, in this work, to clarify the influence rules of substituents on molecular configuration
of Schiff bases complexes, eight samples of copper complexes Cu(Sal-X)2 (X= OMe, Me, H, F, Cl, Br, CF3 and
CN) was synthesized. Their crystal structures were measured experimentally, and the influence of substituents on
the dihedral angle and the bond length in crystal structure was quantified.
2. Experimental methods
2.1. Sample preparation
The copper complexes Cu(Sal-X)2 (1-8) were all prepared by the same methods as described in the literature
[31,32], as shown in Fig. 1.
C
X
N
O
Cu
δ1
L5
L4
O
N
C
X
δ2
δ3
L1
L3
L2
1-8: X=OMe, Me, H, F, CI, Br, CF3, CN
Fig.1. Schematic representation of the Cu(Sal-X)2 (1-8) molecules with geometrical definitions.
The Schiff-base ligands Sal-X were prepared by stirring a mixture of salicylaldehyde and corresponding
substituted aniline in a 1:1 molar ratio for 20 min at room temperature. The precipitated ligands were filtered off,
purified by recrystallization treatment with absolute ethanol and dried in a vacuum desiccator.
The copper complexes Cu(Sal-X)2 were synthesized through a reaction between Cu(OAc)2 and corresponding
Schiff base ligands Sal-X. A solution of Cu(OAc)2 (5 mmol) in absolute ethanol was gradually added to a
solution of Schiff base ligand (10 mmol) in absolute ethanol. The mixture was heated at reflux over 4 h, and then
cooled to room temperature. All the products are light brown precipitates. The crude products were filtered,
washed with absolute ethanol, and finally dried in a vacuum desiccator. Suitable Crystals of the complexes 1-8
were obtained by slow evaporation from absolute ethanol. All the complexes were characterized by the single
crystal X-ray diffraction.
2.2. X-ray crystallography
The structure determination of Cu(Sal-X)2 was carried out by X-ray diffraction method[33].The diffraction data
of the complexes were collected on a Bruker Apex-II CCD diffractometer with monochromatic Mo Kα radiation
(λ =
2

0.71073 Å) at 296(2) K, which was integrated and reduced applying SAINT with absorption and scaling correction being undertaken with SADABS program. The
structure was resolved by the direct method and all non-hydrogen atoms were refined by the full-matrix least-square procedure on F2 using SHELXL-2014 in conjunction
with the Olex2 program[34]. Anisotropic thermal parameters were designated to all non-hydrogen atoms. H atoms linked to N, C and O atoms were geometrically added and
isotropically refined by the riding model. The CIF files of the crystals 1-8 were available in the Supporting Information. The partial crystallographic data for the complexes
Cu(Sal-X)2 (1-8) were summarized in the table 1.
Table 1 The partial crystallographic and experimental data for the complexes 1-8.
Complex
Formula
Temp[K]
Space group
Z(Z' )
1
2
3
4
5
6
7
8
C28H24CuN2O4
296
C28H24CuN2O2
296
C26H20CuN2O2
296
C26H18CuF2N2O2
296
C26H18CuCI2N2O2
296
C26H18CuBr2N2O2
296
C28H18CuF6N2O2
296
C28H18CuN4O2
296
P 21/c
2
P 21/c
P 21/n
P -1
P 21/c
P 21/c
P b c a
R -3
2
2
2
2
2
2
9
a(Å)
13.414(3)
11.167(2)
7.8922(19)
90
12.1444(8)
7.4760(5)
13.5037(9)
90
11.920(3)
7.9269(17)
12.226(3)
90
10.1622(14)
10.5001(14)
10.5911(13)
94.384(4)
106.284(5)
92.239(3)
1079.4(2)
1.514
13.571(4)
10.728(3)
8.188(2)
90
13.828(9)
10.711(7)
8.203(5)
90
10.6858(14)
8.2782(12)
27.768(4)
90
42.964(3)
42.964(3)
3.8200(3)
90
b(Å)
c(Å)
α(◦)
β(◦)
97.057
90
111.082(4)
90
112.154(3)
90
98.92(2)
90
99.260(18)
90
90
90
γ(◦)
90
120
V(Å3)
Dcalc(g.cm−3)
Mu(cm−1)
F(000)
Mr
1173.2(5)
1.461
1143.96(13)
1.405
1069.9(4)
1.415
1177.7(6)
1.480
1199.3(13)
1.700
2456.4(6)
1.601
6106.6(9)
1.238
0.969
0.983
1.046
1.056
1.180
4.268
0.965
0.834
534
502
470
502
534
606
1196
2331
516.04
0.0244
1.036
484.03
455.98
491.96
524.86
0.0702
1.102
613.78
591.98
0.0457
1.127
506
Rint
GOF on F2
0.0414
0.1306
0.0471
0.0455
0.0761
1.088
1.022
1.047
1.014
1.090
a
R1 , wR2b [I>2σ(I)] 0.0261,0.0700
0.0361,0.0843
0.0484,0.0922
0.267/-0.274
0.0482,0.1185
0.0437/0.1133
0.452/-0.707
0.0463,0.0976
0.0990,0.1419
0.393/-0.654
0.0716,0.1615
0.1168,0.2126
0.540/-1.083
0.0512,0.1201
0.0829,0.1550
0.636/-0.827
0.0661,0.1789
0.0962,0.2183
0.781/-0.626
0.0397,0.0958
0.0561,0.1156
0.297/-0.398
a
R1 , wR2b [all data] 0.0313,0.0739
ꢀρmax/ꢀρmin (e Å-3) 0.273/-0.224
3

3. Results and discussion
3.1. Structure description of the complexes
Single-crystal X-ray diffraction shows that the complexes are structurally similar mononuclear copper(II)
complexes (Fig. 2 for the complexes 1–8). Table 2 summarizes the selected dihedral angles, bond lengths and
bond angles for the complexes 1-8. Complexes 4, 7 and 8 crystallized in the triclinic space group P-1,
orthorhombic space group Pbca and rhombohedral space group R-3, respectively, while 1, 2, 3, 5 and 6 all
crystallized in the monoclinic space groups P21/c. The results are in good agreement with the corresponding
values observed in other similar copper (II) complexes [35-37]
.
As can be seen from Fig. 2, there are two different molecular conformations in the crystal structure of the
complex 4. For all complexes, the coordination sphere of Cu atom is found to be formed by two bidentate
ligands. Each Cu atom in the complexes is four-coordinated by two imino N and two phenolic O atoms of the
corresponding Schiff base ligand, forming a square-planar coordination.
1 (X=OCH3)
2 (X=CH3)
3 (X=H)
4 (X=F)
5 (X=CI)
6 (X=Br)
4

7ꢀX=CF3ꢁ
8 (X=CN)
Fig.2. Molecular structures of complexes Cu(Sal-X)2 (1-8) with thermal ellipsoids set at the 50% probability.
Table 2 Selected bond lengths (Å) and angles ( º) for the complexes 1-8
1
2
3
4
5
6
7
8
bond lengths
LCu-O (Å)
LCu-N (Å)
L1 (Å)
1.8829(14) 1.8793(18) 1.8773(19) 1.878(3)
2.0361(14) 2.013(2) 1.9952(17) 1.995(3)
1.881(4)
2.019(5)
1.322(8)
1.410(9)
1.419(8)
1.299(7)
1.439(7)
1.872(4)
2.025(4)
1.319(7)
1.411(7)
1.420(7)
1.291(7)
1.440(7)
1.886(4)
2.016(4)
1.317(6)
1.415(7)
1.423(7)
1.300(7)
1.430(6)
1.9139(19)
2.045(2)
1.315(3)
1.419(4)
1.428(4)
1.296(3)
1.437(3)
1.309(2)
1.420(3)
1.428(3)
1.295(2)
1.433(2)
1.300(3)
1.414(4)
1.434(3)
1.287(3)
1.441(3)
1.306(3)
1.422(3)
1.424(3)
1.291(3)
1.442(3)
1.305(5)
1.414(6)
1.436(6)
1.293(5)
1.442(6)
L2 (Å)
L3 (Å)
L4 (Å)
L5(Å)
bond angles
δ1 (º)
90.61(6)
88.58(8)
91.54(7)
88.91(14)
89.23(19)
126.1(4)
122.4(4)
89.15(18)
126.1(3)
121.6(4)
90.95(17)
125.3(3)
121.7(3)
89.22(8)
124.80(12) 128.69(16) 128.21(14) 131.0(3)
121.34(12) 122.17(17) 123.15(15) 123.6(3)
122.89(17)
120.57(17)
δ2(º)
δ3(º)
dihedral angle
τ1(º)
23.37
47.97
21.51
67.77
18.84
75.97
19.82
67.49
20.91
53.6
21.65
54.08
22.9
29.56
36.14
τ2(º)
52.34
The letters L1-L5 correspond to distances depicted in Fig. 1.
The letters δ1 δ3 correspond to bond angles in Fig. 1.
-
τ1 is the value of the dihedral angle between the Cu-O-N plane and O-C=N plane in Fig. 1.
τ2 is the value of the dihedral angle between the aniline ring plane and C-N-Cu plane in Fig. 1.
3.2. Effect of substituents on molecular conformation
To elucidate the influence of the substituent X on the aniline ring of the ligand on the peculiarities of the
molecular conformation of the copper complexes, we investigated the crystal structure of the complexes. The
ex
cc
Hammett constant
and excited-state parameters
of the substituent X were listed in Table 3.
σ p
σ
ex
cc
Table 3 The Hammett constant
and excited-state parameters
of substituent X
σ p
σ
X
OMe
-0.27
-0.5
Me
H
F
CI
Br
0.23
-0.33
CF3
0.54
-0.12
CN
0.66
-0.70
a
-0.17
-0.17
0
0
0.06
0.06
0.23
-0.22
σ p
b
ex
cc
σ
5

aThe values were taken from Reference[38]
bThe values were taken from Reference[39,40]
In crystal structure, the dihedral angle is an important parameter to estimate planarity of a molecule. As
shown in Table 2, the dihedral angle τ1 between the Cu-O-N plane and O-C=N plane varies in the range of
18.84º–29.56º, while the variation range of the dihedral angle τ2 between the aniline ring plane and C-N-Cu
plane is from 36.14º to 75.97º. It should be noted that the dihedral angle is clearly affected by the substituent of
the aniline ring, and that the influence regularity of substituent on them is opposite: the dihedral angle τ2
becomes smaller as the electronic effect of the substituent increases, whereas the dihedral angle
with the increase of electron-donating effect and electron-withdrawing effect of the substituent X.
τ1 increases
Then, we choose the excited-state parameter ( ex ) and Hammett parameter ( ) of substituent X attached to
σ cc
σ p
aniline ring to correlate the dihedral angle
τ
1. The equation (1) is obtained.
2
2 + 9.94σ p
ex
CC
(1)
τ
1 =19.83+11.19
σ
R=0.9857, S=0.6548, F=85.31, n=8
Rcv=0.9774, Scv=0.6765
Where R is the correlation coefficient, S is standard deviation, F is F-statistics and n indicates the number of
sample. The correlation result of equation (1) is good. As shown in equation (1), the dihedral angle τ1 between
2
2
ex
CC
the Cu-O-N plane and O-C1=N plane becomes bigger when the value of
or
is higher. The distortion
σ
σ p
of the Cu chelating ring increases with the increase of electron-donating effect and electron-withdrawing effect
of substituents. To test the predictive ability of equation (1), we carried out the leave-one-out cross validation
(LOO). Here Rcv and Scv were the correlation coefficient and standard deviation of LOO, respectively. Fig. 3
shows that the calculated values
τ1(cal.) and the predicted values τ1(pred.) are in highly agreement with the
experimental ones 1(exp.).The result of LOO demonstrates good performance of equation (1).
τ
Fig. 3 The plot of the calculated τ1(cal.) by equation (1) and the predicted τ1(pred.) by LOO versus the experimental τ1(exp.) for
the Cu(Sal-X)2
Bond angle is another important parameter in crystal structure. Here, we discuss the coordinate bond lengths
related to the Cu atoms only. Through the comparison of bond lengths for the complexes in Table 2, it clearly
shows that the substituent X mostly affects Cu-O and Cu–N bonds. The Cu-O bond length (LCu-O) of complexes
6

increases from 1.872Å to 1.914Å, while the Cu-N coordinate bond length (
L
Cu-N) of complexes varies from
Cu-N, and find the correlation
1.995Å up to 2.045Å. Thus, we attempt to carry out a correlation analysis for
L
ex
cc
result of
L
Cu-N and excited-state parameter
is good. The equation (2) is established, and leave-one-out cross
σ
validation (LOO) was carried out to test predictive ability for this equation.
2
ex
CC
LCu−N = 2.000−0.097σcecx −0.046
σ
(2)
R=0.9870, S=0.0034, F=94.21, n=8
Rcv=0.9733, Scv=0.0038
ex
cc
Therefore, we can conclude that the excited-state parameter
of substituent X has an important effect on
σ
ex
cc
the bond length (
L
Cu-N). Bigger
value shortens the bond length (
L
Cu-N), and enhances the energy of Cu–N
σ
bond. As shown in Fig.4, LCu-N(cal.) and
L
Cu-N(pred.) are in good agreement with Cu-N(exp.). What’s more, the
L
leave-one-out cross validation gave the similar results with the calculated values by equation (2). Thus, the
equation (2) is reliable and has good predictive ability.
Fig. 4 The plot of the calculated LCu-N(cal.) by equation (2) and the predicted LCu-N(pred.) by LOO versus the experimental
LCu-N(pred.) for the Cu(Sal-X)2
4. Conclusion
In this study, a series of the complexes Cu(Sal-X)2 (1-8) with different substituents X=OCH3, CH3, H, F, CI,
Br, CF3 and CN in para-position of aniline ring, were synthesized, and eight single crystals were obtained. The
influence of the substituents X on the crystal configuration of Cu(Sal-X)2 was investigated. The Conclusions are
drawn as follows:
1. The dihedral angle τ1 between the Cu-O-N plane and O-C1=N plane varies with the substituent X. τ1 is
2
2
ex
CC
bigger when the value of
or
is higher. The distortion of the Cu chelating ring increases with the
σ
σ p
increase of electron-donating effect and electron-withdrawing effect of substituents. However, the substituent
electronic effect has the opposite effect on the τ2
.
ex
cc
2. The excited-state parameter
of substituents X has an important effect on the bond length (LCu-N).
σ
ex
cc
Bigger
values shorten the bond length (
LCu-N), and enhance the energy of C-N bond.
σ
7

3. Two correlation equations between the geometrical parameters (τ1 and LCu-N ) and the substituent
parameters are obtained. It is helpful to understand the influence of the substituent effect on the molecular
conformation of the complex crystals, and also provides a new theoretical reference for the research of this field
and the molecular design of new materials with specific properties.
Acknowledgements
This work was financially supported by the National Science Foundation of China (Project No. 21672058).
Appendix A. Supplementary data
CCDC 1909136, 1909139, 1909123, 1909141, 1909142, 1909145, 1909144 and 1909143 contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.
ccdc.cam.ac.uk (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336033).
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11

Highlights
1. Eight crystals of copper complexes with different substituents were cultivated, and the main
geometrical parameters were obtained.
2. The quantitative correlation between dihedral angle τ1 and substituent effects was obtained.
3. The quantitative correlation between the coordinate bond length (LCu-N) and excited-state
substituent parameter ( ex ) was established.
σ cc

Author Contribution Statement
Yan Xiao: Conceptualization, Methodology, Software, Validation, Visualization,
Data curation, Writing- Original draft preparation.
Chenzhong Cao: Investigation, Supervision, Writing- Reviewing and Editing.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: