Synthesis, characterization, and crystal structure analysis of Zn(II) and Cd(II) coordination compounds containing 4-((pyridin-4-ylmethylene)amino)phenol Schiff-base ligand

Article abstract of DOI:10.1016/j.molstruc.2020.128846

Zn(II) and Cd(II) coordination compounds containing 4-((pyridin-4-ylmethylene)amino)phenol ligands were synthesized and then characterized using spectroscopic techniques and single crystal X-ray crystallography. The molecular structure and crystal packing of these complexes, as well as the crystal packing of coordination complexes based on 4-substituted-1-(pyridin-4-yl)methanimine ligand, retrieved from Cambridge Structural Database (CSD), were investigated using structural, Hirshfeld surface analyses, and theoretical calculations. The crystal structure of these coordination complexes is governed by a combination of π···π stacking, C–H···π, O–H?N, and C–H···X hydrogen bonding interactions. This study provides further insight into the understanding of the role of weak non-covalent interactions in the crystal structure of group IIB containing Schiff-base ligand coordination compounds.

Full text of DOI:10.1016/j.molstruc.2020.128846

Journal Pre-proof
Synthesis, characterization, and crystal structure analysis of Zn(II) and Cd(II)
coordination compounds containing 4-((pyridin-4-ylmethylene)amino)phenol Schiff-
base ligands
Taraneh Hajiashrafi, Shiva Salehi, Maciej Kubicki, Keith J. Flanagan, Mathias O.
Senge
PII:
S0022-2860(20)31171-6
DOI:
https://doi.org/10.1016/j.molstruc.2020.128846
MOLSTR 128846
Reference:
To appear in: Journal of Molecular Structure
Received Date: 28 May 2020
Revised Date: 23 June 2020
Accepted Date: 6 July 2020
Please cite this article as: T. Hajiashrafi, S. Salehi, M. Kubicki, K.J. Flanagan, M.O. Senge, Synthesis,
characterization, and crystal structure analysis of Zn(II) and Cd(II) coordination compounds containing
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1
CRediT authorship contribution statement
Taraneh Hajiashrafi: Writing - original draft, review & editing, Software
Shiva Salehi: acquisition of data, analysis and/or interpretation of data
Maciej Kubicki: acquisition of data
Keith J. Flanagan: acquisition of data
Mathias O. Senge: revising the manuscript

2
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1. Conflict of Interest
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We wish to confirm that there are no known conflicts of interest associated with this
publication and there has been no significant financial support for this work that could
have influenced its outcome.
2. Funding
Funding was received for this work.
All of the sources of funding for the work described in this publication are acknowledged
below:
We gratefully acknowledge the financial support from the Research Council of
Alzahra University- Science Foundation Ireland
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Synthesis, characterization, and crystal structure analysis of
Zn(II) and Cd(II) coordination compounds containing 4-
((pyridin-4-ylmethylene)amino)phenol Schiff-base ligands
Taraneh Hajiashrafi,a* Shiva Salehi,a Maciej Kubicki,b Keith J. Flanagan,c Mathias O. Senge,c
a Department of Chemistry, Faculty of Physics and Chemistry, Alzahra University, PO Box 1993891176,
Tehran, Iran
b Department of Chemistry, Adam Mickiewicz University, ul. Grunwaldzka 6, 60-780 Poznan, Poland
c
School of Chemistry, Trinity Biomedical Science Institute, Trinity College Dublin, The University of
Dublin, 152-160 Pearse Street, Dublin 2, Ireland
Abstract
Zn(II) and Cd(II) coordination compounds containing 4-((pyridin-4-ylmethylene)amino)phenol ligands
were synthesized and then characterized using spectroscopic techniques and single crystal X-ray
crystallography. The molecular structure and crystal packing of these complexes, as well as the crystal
packing of coordination complexes based on 4-substituted-1-(pyridin-4-yl)methanimine ligand,
retrieved from Cambridge Structural Database (CSD), were investigated using structural, Hirshfeld
surface analyses, and theoretical calculations. The crystal structure of these coordination complexes is
governed by a combination of π···π stacking, C-H···π, O-H···N, and C-H···X hydrogen bonding
interactions. This study provides further insight into the understanding of the role of weak non-covalent
interactions in the crystal structure of group IIB containing Schiff-base ligand coordination compounds.
Keywords:
Coordination compounds of group IIB; Schiff-base ligand; π-interactions, metallosupramolecular
chemistry

1. Introduction
In the last two decades, the supramolecular chemistry of inorganic compounds has attracted increasing
interest because of their fascinating structures and their potential applications in various fields such as
catalysis[1], gas storage [2], and molecular recognition [3, 4]. It is now well-established that the manner
in which molecular building blocks are arranged within supramolecular structures has a tremendous
effect on their physical and chemical properties [5, 6]. In this regard, supramolecular chemists have
attempted to understand the rules of supramolecular assembly of metal-containing compounds by
studying the solid-state structures of these systems using X-ray crystallography data to provide a
detailed picture of the supramolecular structure[7, 8]. Crystallographic studies on d-block metal
complexes revealed fascinating information about the impact of the ligand structures and denticities [9,
10], nature of the metal center [11, 12], counter-ions [13, 14] and reaction conditions [15, 16] on the
crystal structure of metal complexes [17, 18]. Among these, rational selection of the organic ligands
plays a decisive role in designing well-defined architectures not only because of their role in directing
the dimensionality and topology of the final structure but also due to providing potential interaction sites
to generate pre-designed patterns of intermolecular interactions [19-21]. A variety of supramolecular
synthons based on highly directional intermolecular interactions such as hydrogen bonding, halogen
bonding, and even weak directional π-stacking interactions have been identified and employed as
synthetic vectors for the assembly of metal-containing materials [17, 22-25]. It should be noted that
although weak interactions such as π-stacking [26] and non-classical hydrogen bonds [26, 27] are
usually regarded as weak intermolecular contacts compared to the classical hydrogen bonds [28], there
is experimental evidence showing these interactions can even affect the coordination geometry of metal
complexes [29-33]. Therefore, supramolecular chemists have a large toolbox of functionalities capable
of non-covalent interactions at their disposal, which allow them to design the metal-containing building
blocks into well-defined crystal structure according to the chemical information encoded in their

molecular structure [34, 35]. In this respect, Hirshfeld surface analysis [36] and the theoretical
calculations can be of great help in understanding the role of intermolecular interactions governing the
solid-state structure of coordination compounds. Hirshfeld surface analysis is a three-dimensional
graphical tool that helps one understands all the intermolecular interactions present in the crystal lattice.
As a continuation of our research interest toward the understanding of the supramolecular organization
of group IIB coordination compounds [37-39], three new crystal structures, namely [Zn(L4-OH)2Cl2] (1),
[Cd(L4-OH)2I2]
(2)
and
[Cd(L4-OH)2I2]•MeOH
(3),
containing
the
4-((pyridin-4-
ylmethylene)amino)phenol (L4-OH) ligand were synthesized and then characterized using spectroscopic
techniques and single crystal X-ray crystallography. The crystal structures of these complexes, as well
as the crystal packing of coordination complexes based on 4-substituted-1-(pyridin-4-yl)methanimine
ligand, retrieved from Cambridge Structural Database (CSD), were investigated using structural,
Hirshfeld surface analyses and theoretical calculations. This study reveals the role of π-interactions and
hydrogen bonds in the organization and stabilization of these metal-containing crystal structures.
2. Experimental Section
2.1 Apparatus and reagents
Starting materials for the synthesis of compounds 1 and 2 were purchased from Aldrich Chemical
Company and used without further purification. The Fourier transform infrared spectra were recorded at
room temperature on a Nicolet Fourier Transform IR (Nicolet 100 spectrometer) in the range 500-4000
cm-1 using KBr pellets. Elemental analyses (carbon, hydrogen, and nitrogen) were determined using an
ECS 4010 CHN-O made in Costech, Italy. Melting points were measured by an Electrothermal 9100
melting point apparatus and were corrected.

2.2 Synthesis of [Zn(L
4-OH)2Cl2] (1), [Cd(L4-OH)2I2] (2) and [Cd(L4-OH)2I2].MeOH (3)
The synthesis of L4-OH ligand was reported previously by some of us [38]. [Zn(L4-OH)2Cl2] (1),
4-OH
[Cd(L
) I ] (2), and [Cd(L4-OH)2I2]•MeOH (3) were synthesized according to the reported procedure
2 2
by some of us [38]. Scheme 1 shows the L4-OH ligand and 1-3 complexes syntheses.
To a solution of 0.1 mmol MX2 (M=Zn or Cd; X=Cl or I) in 5 mL methanol, a solution of L4-OH (0.1
mmol) in 5 mL of methanol was added with stirring. The mixture was heated at 50 ˚C for about 15
minutes, followed by a reduction of the solvent volume, resulting in the immediate formation of bright
yellow solid. Stirring was continued for 10 min and then the mixture was filtered with a filter paper. The
solid residue was washed with a little cold methanol, dried in vacuo and then dissolved in 20 mL of hot
methanol. Upon slow evaporation of the filtrate at room temperature, colorless crystals of 1 and 2
suitable for X-ray diffraction were collected within a week. Notably, two different crystals were
obtained for compound 2 including the yellow prism crystals (2) and colorless block crystals (3). The
crystal structure analysis revealed that compound 3 was the methanol solvate of compound 2. Also, it
should be noted that other zinc and cadmium halide complexes with L4-OH ligand that would have been
of interest here could not be studied because no suitable crystals were obtained.
L4-OH FT-IR (KBr pellet, cm-1): 3272(w), 3065(w), 2998(w), 2947(w), 2880(w),2804(w), 2737(w),
2677(w), 2603(w), 1603(vs), 1575(vs), 1506(m), 1463(s), 1414(m), 1251(vs), 1155(vs), 1002(m),
840(s). Melting Point: 201-203°C [38].
Compound 1 (yield 75%). Anal. calcd for C24H20Cl2N4O2Zn: C, 54.11; H, 3.78; N, 10.52. Found: C,
54.18 H, 3.86; N, 10.46. IR (KBr pellet, cm-1): 3759(w), 3351(vs), 1619 (vs), 1585(vs), 1503(s),
1429(s), 1373(w), 1334(s), 1267(s), 1206(vs), 1156(w), 1027(m), 837(vs), Figure S2a. Melting Point:
172-173 °C. Photoluminescence properties, λexcitation(nm): 400 and λemission(nm):580

Compounds 2 (yield 60% (2+3)*). Anal. calcd for C24H20Cd I2N4O2 (2): C, 37.80; H, 2.64; N, 7.35.
Found: C, 37.78 H, 2.61; N, 7.35. FT-IR (KBr pellet, cm-1): 3444(w), 3423(br), 3139(br), 1614 (s),
1574(vs), 1540(s), 1509(w), 1458(m), 1423(m), 1376(w), 1284(s), 1238(s), 1160(s), 1010(s), 835(s),
544(w), Figure S2(b) and (c). Melting Point: 177–179 °C. Photoluminescence properties, λexcitation(nm):
400 and λemission(nm):530
Compounds 3. Anal. calcd for C26H28Cd I2N4O4 (3): C, 37.77; H, 3.41; N, 6.78. Found: C, 37.75 H,
3.45; N, 6.75. FT-IR (KBr pellet, cm-1): 3423(br), 3127(br), 1613 (s), 1573 (vs), 1541(s), 1509(w),
1457(m), 1418(m), 1374(w), 1284(s), 1237(s), 1158(s), 1008(s), 834(s), 541(w). Melting Point: 175–
177 °C. Photoluminescence properties, λexcitation(nm): 405 and λemission(nm):542
* We were more interested in the crystal than the synthesis so they weren’t completely separated and that
further studies will be conducted to investigate and control this formation as it would be interesting for
other complexes too. The lower yields are due to removing the mother liquor which would contain some
of the compounds but in a mixture of impurities and solvents that can’t crash out due to the
concentration of the liquid. “(yield 60% (2+3) Compounds 2 and 3 were formed in 60:40 ratio)”
2.3 Single-Crystal Diffraction Studies
X-ray diffraction data were collected for 1 at 130(1) K on a Rigaku four-circle SuperNova
diffractometer with Atlas CCD detector and mirror-monochromated CuKα radiation (λ = 1.54178 Å); for
2 at 100(1) K on a Rigaku four-circle Xcalibur diffractometer, with Eos CCD detector and graphite-
monochromated MoKα radiation (λ = 0.71069 Å), and for 3 at 100(1) K on a Bruker APEX-II four-circle
diffractometer, with graphite-monochromated MoKα radiation (λ = 0.71069 Å). The data were corrected
for Lorentz-polarization as well as for absorption effects [40]. Precise unit-cell parameters were
determined by a least-squares fit of 3293 (1), 11830 (2), and 9793 (3) reflections of the highest intensity,
chosen from the whole experiment. The structures were solved with SHELXT-2013 and refined with the

full-matrix least-squares procedure on |F2| by SHELXL-2013 [41]. All non-hydrogen atoms were refined
with anisotropic thermal parameters, hydrogen atoms were placed in idealized positions and refined as
‘riding model’ with isotropic displacement parameters set at 1.2 (1.5 for methyl groups) times Ueq of
appropriate carrier atoms. In 1 the central C-CH=N-C group was found disordered over two alternative
positions, site occupation factors of 0.5 were ascribed for both sites, Figure S1. The crystallographic
information is listed in Table 1 and further experimental and refinement detail can be found in the SI.
CCDC-1993550 (for 1), 1993551 (for 2), and 1993553 (for 3) contains the supplementary
crystallographic data for this paper.
Hirshfeld surface analysis and two-dimensional fingerprint plots were generated by using Crystal
Explorer 3.1 software [36]. These surfaces visually summarize the spatial arrangement and relevance of
intermolecular interactions by their color code.
2.4 Theoretical methods
DFT calculations of the non-covalent interactions were carried out using the ORCA quantum chemistry
suite [42]. The BLYP exchange-correlation functional [43] with the recent D3 empirical dispersion
correction (BLYP-D3) was used to evaluate the binding energies[44]. The basis set superposition error
(BSSE) is not taken into consideration because small BSSE effects are assumed to be absorbed by the
D3 empirical potential [45]. The decomposition of the interaction binding energy was also computed at
BLYP-D3/TZ2P. To evaluate the interactions in the solid-state, we have used the crystallographic
coordinates. This procedure and level of the theory have been successfully used to evaluate similar
interactions [46-48]. An all-electron triple-zeta basis-set with two polarization functions, TZ2P, has
been used to describe all the atoms. The core electrons were treated by using frozen core approximation.
The zeroth-order regular approximation (ZORA) was applied for treating the relativistic effect.

3. Results and Discussion
3.1 Synthesis and crystal structure analysis
Schiff-base ligands are easily prepared ligands that can form stable complexes with metal ions [49, 50].
The substituted-1-(pyridin-4-yl)methanimine ligands are suitable building blocks for crystal engineering
studies of coordination compounds, since they are capable of coordinating to the metal ion in a
predictable manner while providing potential interaction sites to generate self-assembled structures.
Compounds (1) and (2) were synthesized by reacting 4-((pyridin-4-ylmethylene)amino)phenol ligand
with the corresponding metal halide salts. Two different crystals were obtained for compound 2 from the
same growth solution, the 3 entity is referred as 2 solvate, which is further supported by single crystal
X-ray diffraction data. The FT-IR characteristic frequencies of compounds 1-3 as well as the free L4-OH
ligand are summarized in Sections 2.2 and are shown in Figure S2. The FT-IR spectra of complexes 1, 2
and 3 show ν(C=N) stretching frequencies at 1619, 1614 and 1613 cm-1, respectively. The C-O
stretching frequencies are assigned at 1267, 1284 and 1284 cm-1 for 1-3, respectively. The O-H
stretching frequencies of compounds 1-3 have appeared at 3351, 3423 and 3423 cm-1, respectively,
which are completely different from the free L4-OH ligand O-H frequency. The pyridine in-plane ring
deformation frequencies of complexes 1-3 are assigned at 642, 639 and 635 cm-1, respectively. Terminal
M-X and M-Py stretching frequencies as well as pyridine out of plane ring deformation frequencies are
located in the far IR-spectrum, at lower frequencies than could be observed here. The
photoluminescence properties of complexes 1-3 are listed in section 2.2. Interestingly, the complexes 2
and 3 have different luminescence properties as for excitation and emission wavelengths. ORTEP
drawings of compounds 1-3 which are drawn at 50% ellipsoid probability level are shown in Figure 1
[51]. Selected bond distances and angles are listed in Table 2. In 1 and 2 the complex molecules are
unsymmetrical (however some degree of pseudo-symmetry can be found), so the asymmetric part
contains one molecule; in 3 the complex is C2-symmetrical, central Cd ion lies across crystallographic

two-fold axis in the space group Pba2, and only one ligand molecule and one solvent – methanol
molecule is symmetry-independent. The molecular structure of all three compounds shows discrete
mononuclear four-coordinate zinc(II) or cadmium(II) units, coordinated by two pyridyl nitrogen atoms
of two different L4-OH ligands and two terminal halide ions. According to the four-coordinate geometry
index (τ4) defined by Houser and his co-workers, within the continuum between square planar (τ4=0)
and tetrahedral (τ4=1), the coordination geometry around M(II) can be described as tetrahedral (1) and
trigonal pyramidal (2 and 3), with τ4 values of 0.98, 0.88, 0.73 for compounds 1, 2, and 3, respectively
[52]. The dihedral angle between the planes of phenyl and pyridyl rings of free L4-OH ligand has been
reported as 8.84˚[53]. For compounds 1-3, the defined dihedral angle between the aromatic rings of L4-
OH
ligand lies between 5.50-52.81º and thus the ligand can adjust itself to meet the requirement of
coordination geometry of the metal atom and the optimal inter-atomic distances in the crystal structure,
Table S1. The molecular overlay of compounds 2 and 3 and the structural overlay of L4-OH ligands,
which adopt trans conformation, in compounds 1-3 and the free L4-OH ligand [53] are shown in Figure
S3. In this regard, Vittal and his co-workers report the crystal structure of [ZnBr2(4-styrylpyridine)2] in
which Zn(II) is in perfect tetrahedral geometry and the dihedral angle between the two aromatic rings of
4-styrylpyridine ligand is similar to that for compound 1 [54]. In Hg(II) complexes containing
substituted styrylpyridine ligand, the two aromatic rings of the corresponding ligand no longer show the
large dihedral angle, but instead the two aromatic rings adopt a nearly coplanar geometry [55-57].
In the crystal structure of 1, adjacent mononuclear units are linked to each other through a combination
of π-interactions, namely pyridylπ-πphenyl stacking (ring centroid to ring centroid distance =3.753 Å),
pyridylC-H···πphenyl, phemylC-H···πpyridyl, as well as pyridylC-H···O non-classical hydrogen bonds in the
crystallographic b-direction. The overall supramolecular structure results from linking the [Zn(L4-
OH)2Cl2] units via imineC-H···Cl, O-H···Cl and C-H···πphenyl interactions in the ac-plane, Figure 2. For
compounds 2 and 3, while 3 is a solvate form of 2, they exhibiting both striking similarities as well as

some significant differences in their supramolecular structure. In the crystal structure of 2, discrete
neutral [Cd(L)2I2] units are linked to each other, along the crystallographic a-direction, through pyridylπ-
πpyridyl (ring centroid to ring centroid distance =3.975 Å), pyridylC-H···πphenyl and I···πpyridyl interactions,
ꢁꢂ ꢃ
Table 4 and Figure 3(a). The O-H···N, phenylC-H···O [ꢀꢁ 8 graph set notation] and phenylC-H···I
hydrogen bonds link these units in the bc-plane to form the three-dimensional (3D) network, Figure
3(b). Analysis of the crystal structure of 3 reveals that the [Cd(L4-OH)2I2] units are stacked on one
another via imineπ-πphenyl, imineπ-πpyridyl (bond centroid to ring centroid distances of 3.503 and 3.555 Å,
respectively), and O(lone pair)···πphenyl interactions along the crystallographic c-direction, Figure 4(a).
The methanol molecules of crystallization link discrete molecular units, in the ab-plane, via MeOHO–
H···Nimine and MeOHC-H···OOH hydrogen bonds, which is further assisted by imineC-H···I interaction to
generate the 3D crystal packing, Table 3, Figure 4(b).
A search in the Cambridge Structural Database (CSD) on the coordination compounds based on (4-
substituted)-1-(pyridin-4-yl)methanimine ligand returned 13 hits [58]. All of these coordination
compounds contain Hg(II) as metal centers that have different molecular structures and supramolecular
features, which are listed in Table 5. These results reveal that the variation of L ligand para-
substituents can have a profound effect on the coordination geometry of metal atom as well as
supramolecular arrangement of these coordination compounds. In this regard, the [Cd(L4-F)I2] is a 1D
coordination polymer with the cadmium atom in a distorted octahedral configuration, while both [Cd(L4-
OH)I2] polymorphs are discrete coordination complexes in which the cadmium atoms are found in a
trigonal pyramidal environment [59, 60]. Also, the [Hg(L4-OH)Br2] and [Hg(L4-Br)2Br2] are discrete
complexes with four-coordinate seesaw geometries for Hg(II) ions, while [Hg(L4-Cl)Br2] and [Hg(L4-
F)Br2], [Hg(L4-I)Br2] are all 1D coordination polymers with a four-coordinate seesaw and five-
coordinate square pyramid geometries [38, 59]. These differences are also observed for the [Hg(L4-
OH/F/Cl/Br/I
) I ] series which the data are presented in Table 5.
2 2

3.2 Hirshfeld surface analysis and theoretical calculation
Analysis and calculations of the Hirshfeld surface analysis were carried out to analyze the
intermolecular contacts present in the crystal packing of coordination compounds 1-3. The Hirshfeld
surface analysis is a powerful graphical tool for obtaining additional intuition into the intermolecular
contacts in crystalline systems [36]. A bar chart of percentage contributions of the different types of
intermolecular contacts and the (2D) fingerprint plots of the coordination compounds are shown in
Figure 5 and Table S2. The Hirshfeld surface analysis and associated fingerprint plots reveal that the
crystal packing of compounds 1-3 is supported by H···H, C-H···π, C-H···X, C-H···O, and π···π
stacking interactions, Figures 5 and S5. The H···H and C-H···π intermolecular contacts are the most
abundant contacts and play important roles in governing the crystal packing and C-H···X (X=Cl, I), C-
H···O/O-H···O, and π···π interactions are the next priorities, respectively. The small contribution from
the other interatomic contacts, namely N··· π to the Hirshfeld surface of compounds 1-3 listed in Table
S2, show negligible influence upon the packing. The large contribution for the van der Waals H···H
interactions is typical of organic molecules and is consistent with the presence of CH groups along the
ligand backbone [61]. The comparison of the Hirshfeld surfaces of compounds 2 and 3 reveals that in
the crystal structure of 2 the C-H···π interaction is dominant to the C-H···I interaction, while this is vice
versa for 3. Hirshfeld surfaces of complexes 1 (a), 2 (b) and 3 (c) are also mapped with shape index
(-1.0 (red) and +1.0 (blue)) and curvedness (between -4.0 (red) and +0.4 (blue), Figure S4. Distinctive
features can be identified on the left and right-handed sides of shape index and curvedness plots of
compounds 1 and 2, leading to patterns of intermolecular interactions that are different on each side of
these two complexes. The pattern of red and blue triangles on the same region of the shape index
surface and the presence of a large flat green surface on the curvedness surface indicate the π⋯π
stacking interactions. This feature is observed in all three compounds plots, whereas in the case of
compound 3 it is more significant. The C-H···π interactions manifests itself on the shape index plots of

compounds 1-3 as large red depression in the surface above the aromatic ring of concave curvature,
while C-H donor regions have the opposite curvature. The hydrogen bond dominates the appearance of
all these plots, appearing as a pair of sharp spikes with the upper spike (where de >di) corresponds to the
hydrogen-bond donor and the lower spike (where de <di) corresponds to the hydrogen-bond acceptor.
The theoretical study is devoted to analyzing the non-covalent interactions that direct the self-assembly
of coordination compounds 1-3 in the solid-state, focusing our attention to the C-H···X hydrogen
bonding, C-H···π, and π-π stacking interactions. The binding energies obtained from the BLYP-D3
calculation on two relative fragments provide us with an opportunity to evaluate the intermolecular
contacts between [ML4-OHX2] molecular units, Tables 3 and 4. The selected fragments were cut out
directly from the CIF file and are shown in Figure S6. The energies of intermolecular interactions are in
the range of those found for a series of neutral complexes [32, 46, 62]. The non-classical C-H···X and
C-H···O and classical O-H···N hydrogen-bonding energies in compound 2 calculated to be in the range
of 3.20-10.20 kcal/mol which is in agreement with the reported values for hydrogen bonding in the
crystal structure of molecular solids, Table 3 [63, 64]. Also, the CH···π interaction energy in compound
1 is calculated to be -15.32 kcal/mol, while that of π···π and I···π interactions in compound 2 are -25.50
and -17.47 kcal/mol. Not surprisingly, the interaction energies of dimeric fragments containing several
π-interactions and hydrogen bonds in 1 and 3 are larger than that of the fragments in which only one
aromatic ring is involved in π-interactions, Tables 3 and 4, Figure S6.
4. Conclusion
Herein, the crystal structures of three new group IIB coordination compounds containing the 4-
((pyridin-4-ylmethylene)amino)phenol ligand are reported. The crystal structures of these compounds,
as well as the crystal packing of coordination complexes based on (4-substituted)-1-(pyridin-4-
yl)methanimine ligand, retrieved from Cambridge Structural Database (CSD), were investigated.

Structural analysis show the tetrahedral (1) and trigonal pyramidal (2 and 3) coordination geometry
around the metal centers in these structures. It has been revealed that the L4-OH adjusts its conformation
to adopt the coordination geometry requirement of the metal atom and the optimal inter-atomic distances
in the supramolecular structure. The crystal structure analyses revealed that the main interactions
governing the crystal structure are π-π, C-H···π, as well as O-H···N, C-H···X hydrogen bonding
interactions. The results reported herein are useful for understanding the coordination chemistry and
solid-state structure of group IIB containing Schiff-base ligands.
AUTHOR INFORMATION
Corresponding Authors
Phone: +98-21-85692619 and +98-21-82884416.
E-mail: t.hajiashrafi@alzahra.ac.ir
ACKNOWLEDGMENT.
We gratefully acknowledge the financial support from the Research Council of Alzahra University and
Science Foundation Ireland (SFI IvP 13/IA/1894)
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N
NH2
50°C
O
N
+
EtOH
N
HO
HO
MX2
N
N
N
50°C
MeOH
MX
+
2
N
HO
2
M=Zn, Cd
X=Cl, I
HO
Scheme 1. L4-OH ligand and coordination complex syntheses

(a)
(b)
(c)
Figure 1. ORTEP diagram of compounds 1 (a), 2 (b), and 3 (c), drawn at 50% probability ellipsoids

Figure 2. Representation of the crystal packing of compound 1 showing the association of molecular units through a
combination of π-π stacking C-H•••π, as well as C-H•••O non-classical hydrogen bonds in the crystallographic b-direction.
The overall supramolecular structure results from linking the [Zn(L)2Cl2] units via C-H•••Cl, O-H•••Cl and C-H•••π
interactions in the ac-plane.

(a)
(b)
Figure 3. Representation of the crystal packing of compound 2 showing the supramolecular association of discrete
molecular units along the crystallographic a-direction through π-π stacking, C-H···π and I···π interactions (a) View showing
2
the three-dimensional crystal packing; the O-H···N, R2 (8) C-H···O and C-H···I hydrogen bonds link these units in the bc-
plane (b).

(a)
(b)
Figure 4. Representation of the crystal packing of compound 3 showing the supramolecular association of discrete molecular
units imineπ-πphenyl, imineπ-πpyridyl, O(lone pair)···πphenyl interactions along the crystallographic c-direction (a) View showing the
three-dimensional crystal packing; the methanol molecules of crystallization link discrete molecular units, in the ab-plane,
via MeOHO–H•••Nimine and MeOHC-H•••OOH hydrogen bonds, which is further assisted by imineC-H•••I interaction (b)

Figure 5. Hirshfeld surface area and two-dimensional fingerprint plots of complexes 1 (a), 2 (b) and 3 (c).

Figure 6. Horizontal bar chart showing the relative contributions of different intermolecular interactions to the
Hirshfeld surface areas in compounds 1–3.

Table 1. Structural data and refinement parameters for compounds 1-3.
1
2
3
formula
fw
λ/Å
C24H20Zn Cl2N4O2 C24H20CdI2N4O2 C26H28CdI2N4 O4
532.71
0.71073
130(1)
762.6
826.72
0.71073
100(1)
Orthorhombic
Pba2
0.71073
100(1)
Monoclinic
I2/a
T/K
crystal system Monoclinic
space group
a/Å
b/Å
c/Å
P21/n
9.3689(6)
12.9152(7)
19.1424(13)
90
10.5408(3)
19.3211(4)
25.6456(7)
90
9.1610(13)
32.257(4)
4.7290(7)
90
α/˚
β/˚
93.373(6)
90
2312.2(2)
1.530
91.511(2)
90
5221.2(2)
1.940
90
90
1397.4(3)
1.965
2
3.029
796
55.1
0.0604
1.131
γ/˚
V/Å3
Dcalc/Mg.m-3
Z
4
8
μ (mm-1)
F(000)
2θ (˚)
R (int)
GOOF
3.847
1088
65.5
0.2152
0.968
0.0641
0.1249
1993550
3.229
2896
50.0
0.0264
1.052
0.0174
0.0389
1993551
a
R1 (I>2σ(I))
0.0284
0.0678
1993552
b
wR2 (I>2σ(I))
CCDC No.
a R1 =Σ||Fo| - |Fc||/Σ|Fo|. b wR2 = [Σ(w(Fo2 - Fc2)2)/Σw(Fo )2] .
2
½

Table 2. Selected bond distances (Å) and angles (°) for complexes 1-3.
Complex
1
2
3
M1-N1A
M1-N1B
M1-X1
2.032(4)
2.019(4)
2.234(2)
2.228(2)
107.5(2)
114.75(7)
107.1(1)
108.1(1)
106.1(1)
112.9(1)
2.293(2)
2.283(2)
2.6932(5)
2.7117(6)
107.19(6)
129.33(2)
105.77(5)
104.00(5)
105.18(5)
103.87(5)
2.290(1)
2.290(1)
2.7304(2)
2.7304(2)
119.8(1)
137.48(1)
99.4(2)
101.5(1)
99.4(2)
101.5(1)
Bond
distance
M1-X2
N1A-M1-N1B
X1-M1-X2
N1A-M1-X1
N1A-M1-X2
N1B-M1-X1
N1B-M1-X2
Bond
angle

Table 3. Selected hydrogen bond geometries for coordination compounds 1-3
Compound
D-H⋯A
d(D-H)/Å d(H⋯A)/Å d(D⋯A)/Å <D-H⋯A/° Binding Energy (kcal/mol)
Sym. Code
-x,1-y,1-z
-29.75I
O12A-H12A···Cl1
O12B-H12B···Cl2
C7B-H7B···Cl1
0.840
0.840
0.950
0.95
2.331(1)
2.312(1)
2.916(1)
3.145(3)
2.618(2)
2.453(2)
1.990(1)
1.964(2)
3.127(5)
3.135(4)
3.85(2)
158.1(1)
167.2(1)
169.0(1)
154.5(2)
162.9(1)
123.7(2)
175.0(1)
177.7(2)
-33.78II
2-x,1-y,-z
1-x,1-y,-z
-20.25
1
C10B-H10B···Cl1
C10A-H10A···O12B
C6A-H6A···O12A
O12A-H12A···N8B
O12B-H12B···N8A
4.022(7)
3.538(8)
3.083(8)
2.828(2)
2.803(2)
1-x,1-y,-z
0.950
0.950
0.840
0.840
1.5-x, -½+y,½-z
½+x, ½-y,-½+z
½+x, ½+y,-½+z
½+x,-½+y,½+z
-10.20
-7.32
C13A-H13A···N8B
C13A-H13A···I1
0.950
0.950
0.950
0.950
0.950
0.950
0.950
2.651(2)
3.1065(1)
2.618(3)
2.716(2)
2.696(3)
2.511(1)
2.712(3)
3.363(3)
3.837(2)
3.522(3)
3.327(3)
3.286(3)
3.404(3)
3.571(3)
132.1(1)
135.0(2)
159.1(2)
122.6(2)
120.9(1)
156.7(1)
150.8(1)
½+x, ½+y,-½+z
1-x, ½+y,½-z
2-x,1-y,-z
-½+x,½+y,-½+z
2-x,-½+y,½-z
-½+x,½+y,-½+z
-½+x,-½+y,½+z
2
3
C11A-H11A···O12A
C14A-H14A···O12B
C14B-H14B···O12A
C11A-H11A···O15A
C3B-H3B···O12A
-3.20
-5.34
O1S-H1S···N8
O12-H12···O1S
0.840
0.840
2.012(1)
1.841(3)
2.848(6)
2.677(6)
172.5(1)
172.9(2)
½+x, ½-y,½+z
x,y,z
C11-H11···O12
C7-H7···I1
C5-H5···O1S
C10-H10···O1S
0.950
0.950
0.950
0.950
2.579(3)
3.1568(2)
2.423(1)
2.585(1)
3.314(8)
4.064(6)
3.341(7)
3.354(8)
134.4(1)
160.3(1)
162.5(1)
138.2(1)
-½+x,½-y,z
1-x,1-y,1-z
½+x,½-y,1+z
½+x,½-y,1+z
I The dimer binding energy connected to each other via two C11A-H11A···πpyridyl(N1A-C6A) and two O112A-H12A···Cl1···Cl1 interactions.
II The dimer binding energy connected to each other via two (N1B-C6B)pyridylπ···πphenyl(C9B-C14B) and two O12B-H12B···Cl2 interactions.
Table 4. π-interactions for compounds 1-3
π···π
Binding
plane
energy
Offset
angle
(˚)
to
C-H/atom···π
Compound
Interaction
Sym. code
dcg-cg
(Å)
(kcal/mol)
plane
angle
(˚)
doffset (Å)
C11A-H11A···πpyridyl(N1A-C6A)
C2B-H2B···πphenyl(C9A-C14A)
C3A-H3A···πphenyl(C9B-C14B)
C13A-H13A···πpyridyl(N1B-C6B)
(N1B-C6B)pyridylπ···πphenyl(C9B-C14B)
(N1B-C6B)pyridylπ···πpyridyl(N1B-C6B)
C3B-H3B···πphenyl(C9B-C14B)
I···πpyridyl(N1B-C6B)
2.834(1)
2.638(1)
2.817(2)
2.922(3)
-
-
-
-
-
-29.75I
-15.32
-x,1-y,1-z
-x,1-y,1-z
2-x,1-y,-z
½+x,½-y,-½+z
2-x,1-y,-z
½-x,y,1-z
1.5-x,y,1-z
-½+x,1-y,z
x,y,-1+z
1
3.753(2)
3.975(2)
19.58
18.39
5.88
24.45
1.25
1.28
-33.78II
-25.50
-
3.161(1)
3.875(2)
3.488(2)
3.497(2)
3.355(1)
2
3
-17.47
O(lone pair)···πphenyl
imineπ···πphenyl(N1-C6)
imineπ···πphenyl(C9-C14)
-
-
-
-
-
-
-
-
-
-
-
-
-36.32III
x,y,-1+z
x,y,-1+z
I The dimer binding energy connected to each other via two (N3-C18)pyridylπ···πphenyl(C20-C25) and two C15-H13···Cl3 interactions.
II The dimer binding energy connected to each other via two (N2-C13)pyridylπ···πphenyl(C2-C7) and two C8-H8···I1 interactions.
III The dimer binding energy connected to each other via two sets of imineπ···πphenyl(N1-C6)
, imineπ···πphenyl(C9-C14) and O(lone pair)···π interactions.

Table 5. Molecular features of coordination compounds containing (4-substituted)-1-(pyridin-4-yl)methanimine ligand
Ref Code
Space group Metal atom coordination geometry
Molecular structure
Reference
-
I2/a
4-coordinate tetrahedral
Discrete coordination complex This work
P21/a
Pba2
I2/a
4-coordinate trigonal pyramidal
4-coordinate trigonal pyramidal
6-coordnate octahedral
4-coordinate seesaw
Discrete coordination complex This work
Discrete coordination complex This work
-
-
1D coordination polymer
Discrete coordination complex
Discrete coordination complex
1D coordination polymer
1D coordination polymer
1D coordination polymer
Discrete coordination complex
1D coordination polymer
1D coordination polymer
1D coordination polymer
1D coordination polymer
1D coordination polymer
1D coordination polymer
[38]
[38]
[38]
[60]
[59]
[59]
[59]
[59]
[59]
[59]
[59]
[59]
[59]
KOWWEB
KOWWIF
KOWWOL
ZUNSUY
HOYLIS
HOYLOY
HOYLUE
HOYMAL
HOYMEP
HOYMIT
HOYMOZ
HOYMUF
ZUNTAF
Pba2
Pba2
P21/c
Pī
4-coordinate seesaw
6-coordnate octahedral
5-coordinate square pyramidal
4-coordinate seesaw
P212121
Fdd2
P21/c
Fdd2
P212121
Pna21
P21/c
C2/c
4-coordinate seesaw
5-coordinate square pyramidal
4-coordinate seesaw
4-coordinate seesaw
4-coordinate seesaw
4-coordinate seesaw
6-coordnate octahedral

•
•
Group IIB complexes containing Schiff-base ligands were synthesized.
The molecular structure and crystal packing of these compounds were
investigated.
•
The crystal packing was governed by π-stacking and non-classical hydrogen
bonds.