Anion-reliant structural versatility of novel cadmium(II) complexes: Synthesis, crystal structures, photoluminescence properties and exploration of unusual O···S chalcogen bonding involving thiocyanate coligand

Article abstract of DOI:10.1016/j.ica.2017.09.005

In this work we have reported two new Cd(II) complexes viz. [Cd₂(L)₂(NO₃)₂] (1) and [Cd₂(L)₂(NCS)₂] (2) where HL = (E)-2-methoxy-6-((quinolin-8-ylimino)methyl)phenol. Both of the

Full text of DOI:10.1016/j.ica.2017.09.005

Accepted Manuscript
Research paper
Anion-reliant structural versatility of novel cadmium(II) complexes: synthesis,
crystal structures, photoluminescence properties and exploration of unusual O·
· ·S chalcogen bonding involving thiocyanate coligand
Pravat Ghorai, Paula Brandão, Antonio Bauzá, Antonio Frontera, Amrita Saha
PII:
S0020-1693(17)30965-9
http://dx.doi.org/10.1016/j.ica.2017.09.005
ICA 17861
DOI:
Reference:
Inorganica Chimica Acta
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19 June 2017
1 September 2017
2 September 2017
Please cite this article as: P. Ghorai, P. Brandão, A. Bauzá, A. Frontera, A. Saha, Anion-reliant structural versatility
of novel cadmium(II) complexes: synthesis, crystal structures, photoluminescence properties and exploration of
unusual O· · ·S chalcogen bonding involving thiocyanate coligand, Inorganica Chimica Acta (2017), doi: http://
dx.doi.org/10.1016/j.ica.2017.09.005
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Anion-reliant structural versatility of novel cadmium(II) complexes:
synthesis, crystal structures, photoluminescence properties and exploration of
unusual O···S chalcogen bonding involving thiocyanate coligand
Pravat Ghorai,a Paula Brandão,b Antonio Bauzá,c Antonio Frontera,*c and Amrita Saha*a
aDepartment of Chemistry, Jadavpur University, Kolkata- 700032, India.
E-mail: asaha@chemistry.jdvu.ac.in;amritasahachemju@gmail.com; Tel. +91-33-24572941
bDepartment of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-
193 Aveiro, Portugal
cDepartament de Química, Universitat de les IllesBalears, Crta. De Valldemossa km 7.5, 07122
Palma (Baleares), Spain. E-mail:toni.frontera@uib.es.
Abstract
In this work we have reported two new Cd(II) complexes viz. [Cd2(L)2(NO3)2] (1) and
[Cd2(L)2(NCS)2] (2) where HL= (E)-2-methoxy-6-((quinolin-8-ylimino)methyl)phenol. All two
complexes have been characterized using different spectroscopic techniques along with single
crystal X-ray crystallography. Complexes 1 and 2 are di-(phenoxido)-bridged Cd(II) complexes
where NO3¯ and NCS¯ act as terminal coligand. Complex 1 and 2 feature uncommon distorted
monocapped trigonal prismatic geometry and distorted octahedral geometry around Cd(II) centre
respectively. Different non covalent interactions are present in the crystal packing of complexes
1 and 2. Complex 2 reveals chalcogen bonding and unconventional π–π interactions. The
1

energetic features of such types of interactions have been studied by means of DFT calculations
and characterized energetically using the NBO computational tool.
Keywords: Cd(II) complexes; 8-Aminoquinoline based Schiff base ligand;
Supramolecular interactions; Chalcogen bonding; DFT calculations.
2

1. Introduction
In past few decades, crystal engineering aspires to understand the nature and roles of
noncovalent weak interactions. Noncovalent interactions play a major role in determining the
structure and macroscopic properties such as density, solubility, thermal and mechanical
behavior, electronic and optical properties of larger molecules, which would further help to
fabricate materials possessing desired parameters. These weak forces are ubiquitous in macro
molecular compounds like proteins, [1-3] polysaccharides [4,5] and further govern their
secondary to quaternary structures. Among various noncovalent forces, the most important one is
conventional hydrogen bonding (such as NH···O and OH···O), [1-6] which has various utility in
the field of chemical, biological and material science. Another class of important interactions
which has gained significant attention is the π···π stacking interaction, extensively found in
biomolecules like DNA and proteins, [7] plays a major role in several organic functional
materials. [8] Moreover, halogen bonds [9] and chalcogen bonds [10,11] were also well
recognized in crystal engineering and supramolecular assemblies. The well-known chalcogen
bond also known as noncovalent chalcogen–chalcogen interaction, namely Y–Ch···D, where the
donor (D) atom is also a chalcogen, like the halogen–halogen interaction. The electron-deficient
chalcogens (Ch) [12] (the group VI elements: S, Se and Te) have a localized electropositive
region at its outermost end (namely σ-hole) [10] and it can interact with an electron
donor/nucleophilic moiety (D), where Y is generally an electron withdrawing group. This type of
interaction has various applications in different field such as crystal engineering, [13−16]
molecular recognition, [17,18] bio molecular systems, [19,20] solid state ordering of materials,
[18] and organic reactivity [21]. In this regard two works need special mention. Iwaoka and
coworkers [22] have established presence of non bonded S···O interactions for stabilizing folded
3

protein structures. In another work Bleiholder et al. [13] have analyzed intermolecular
interactions between chalcogen centres utilizing large set of model systems.
On the other hand Schiff base coordinated metal complexes played a major role in the
development of coordination chemistry. Such complexes are extensively used in the field of
synthetic chemistry, supramolecular chemistry, material science, homogenous catalysis,
biological chemistry as well as molecular magnetism, transport and separation phenomena [23].
Unsymmetrical tetradentate Schiff base ligands with N, O donors have been widely studied as
blocking ligands in coordination chemistry mainly due to their capabilities of forming stable
complexes of different coordination number. Among different synthetic techniques, one common
strategy to build up di- and polynuclear transition metal complexes is to the use of blocking
ligands with lesser donor sites in combination with bridging ligands. Therefore, unsymmetrical
Schiff base ligands with N, O or N donors along with various polydentate bridging anions have
been able to produce diverse possibilities in structure as well as supramolecular architectures.
Schiff bases ligands based on salicylaldehyde and 8-aminoquinoline used as blocking
ligands are scare in literature [24, 25]. Our group has engaged to prepare different mono and
polynuclear complexes involving this Schiff base ligand for last few years. Such metal
complexes exhibit interesting catalytic, magnetic and metal ion sensing properties. In this present
work we are reporting synthesis, crystal structure, supramolecular interactions of two different
dinuclear Cd(II) complexes 1 and 2, involving (E)-2-methoxy-6-((quinolin-8-
ylimino)methyl)phenol Schiff base ligand (HL) and two different anionic ligands nitrate and
thiocyanate respectively. Complexes 1 and 2 are di-phenoxido bridged dinuclear Cd(II) complex
where nitrate and thiocyanate act as terminal coligand. It is interesting to mention that in
complex 1 nitrate ions are oriented in opposite direction, whereas in complex 2 two thiocyanate
4

coligands are present in same direction. DFT calculation has been deployed to establish such
type of structural preferences by anionic coligands. Both the complexes exhibit remarkable
supramolecular interactions. In complex 2 the thiocyanate establishes two unconventional
interactions, a π–π stacking where the π-system of the thiocyanate interacts with the π-system of
the iminoquinoline ring and O···S chalcogen bond. Formation mechanisms of all these
interactions are supported by means of density functional theory (DFT) studies. Molecular
electrostatic potential (MEP) surface analysis of compound 2 is further performed for evaluation
of different nonbonding interactions and the extent to which these weak interactions stabilize the
complex.
2. Experimental Section
2.1 Materials and Physical measurements
All reagents or analytical grade chemicals and solvents were purchased from commercial
sources and used without further purification. Elemental analysis for C, H and N were carried out
using a Perkin–Elmer 240C elemental analyser. Infrared spectra (400–4000 cm–1) were recorded
from KBr pellets on a Nicolet Magna IR 750 series-II FTIR spectrophotometer. Absorption
spectra were measured using a UV-2450 spectrophotometer (Shimadzu) with a 1-cm-path-length
quartz cell. Emission was examined by LS 55 Perkin–Elmer spectrofluorimeter at room
temperature (298 K) in DMSO solution under degassed condition. Measurements of 1H NMR
spectra were conducted using a Bruker 300 spectrometer in D6-DMSO solvent.
2.2 X-ray crystallography
Single crystal X-ray data of complexes 1 and 2 were collected on a Bruker SMART APEX-
II CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at
5

150(2) K. Data processing, structure solution, and refinement were performed using Bruker
Apex-II suite program. All available reflections in 2θmax range were harvested and corrected for
Lorentz and polarization factors with Bruker SAINT plus [26]. Reflections were then corrected
for absorption, inter-frame scaling and other systematic errors with SADABS [27]. The
structures were solved by the direct methods and refined by means of full matrix least-square
technique based on F2 with SHELX-2013 [28]. All the non-hydrogen atoms were refined with
anisotropic thermal parameters. The C-H atoms were included at calculated positions and refined
with isotropic thermal parameters equivalent to 1.2 times those of the atom to which they are
attached (Table S1). Complex 1 contains some residual peaks around both cadmium atoms.
Firstly we refine the model considering the cadmium atoms disorder over two positions, however
the second atom for each cadmium exhibited thermal ellipsoids very large, because of that we
considered that the residual peaks are result of absorption peaks from cadmium atoms. Crystal
data and details of data collection and refinement for 1 and 2 are summarized in Table 1.
Molecular diagrams were drawn with Diamond 4.0.
2.3 Theoretical methods
The calculations of the noncovalent interactions were carried out using the
TURBOMOLE version 7.0 [29] using the M06-2X/def2-TZVP level of theory. To evaluate the
interactions in the solid state, we have used the crystallographic coordinates. This procedure and
level of theory have been successfully used to evaluate similar interactions [30]. The interaction
energies were computed by calculating the difference between the energies of isolated monomers
and their assembly. The interaction energies were corrected for the Basis Set Superposition Error
(BSSE) using the counterpoise method [31].
2.4 Synthesis of Schiff base ligand [HL= (E)-2-methoxy-6-((quinolin-8-ylimino)methyl)phenol]
6

The tridentate Schiff base ligand (HL) was prepared by the standard method [25]. Briefly,
4.0 mmol (0.576 g) of 8-aminoquinoline was mixed with 4.0 mmol (0.608 g) of o-vanillin in 30
ml of methanol. The resulting solution was heated to reflux for ca. 1h and allowed to cool. The
dark orange methanol solution was used directly for complex formation.
2.5 Preparation of [Cd2(L)2(NO3)2] (1)
A 10 ml methanolic solution of Cd(NO3)2.4H2O (2.0 mmol, 0.616g) was added to a methanolic
solution of HL (2.0 mmol) and the resultant reaction mixture was stirred at room temperature
for 4 h. Deep red colored crystals resulted from the slow evaporation of methanolic solution of
the complex at room temperature. Yield: 0.677 g (75%). Anal.Calc. for C34H26Cd2N6O10: C
45.20%; H 2.90 %; N 9.30%.Found: C 44.87%; H 2.24%; N 8.76%. IR (cm-1, KBr): ν(C=N)
1610 m; ν(C-N) 1210 s; ν(C-H) 735 s. UV-Vis, λmax (nm), (ε (dm3mol-1cm-1)) in DMSO: 340
(25583), 450 (22568). 1H NMR (D6-DMSO, 300 MHz) δ ppm: 3.73 (-OCH3) (s, 3H), 6.35-8.65 (ArH)
(m, 9H), 8.86 (-CH=N) (s, 1H).
2.6 Preparation of [Cd2(L)2(NCS)2] (2)
A 10 ml methanolic solution of Cd(NO3)2.4H2O (2.0 mmol, 0.616g) was added to a methanolic
solution of HL (2.0 mmol) followed by addition of NaSCN (2.0 mmol, 0.162g) in 10 ml
methanolic solution and the resultant reaction mixture was stirred at room temperature for 4 h.
Deep red colored crystals resulted from the slow evaporation of methanolic solution of the
complex at room temperature. Yield: 0.734 g (82%). Anal.Calc. for C36H26Cd2N6O4S2: C
48.28%; H 2.93%; N 9.38%. Found: C 47.98%; H 2.15%; N 9.02%.IR (cm-1, KBr): ν(C=N)
1605 m; ν(C-N) 1210 s; ν(C-H) 735 s; ν(SCN) 2070s. UV-Vis, λmax (nm), (ε (dm3mol-1cm-1)) in
7

DMSO: 340 (34956), 450 (31460). 1H NMR (D6-DMSO, 300 MHz) δ ppm: 3.72 (-OCH3) (s, 3H),
6.38-8.82 (ArH) (m, 9H), 8.93 (-CH=N) (s, 1H).
3. Results and discussion
3.1 Syntheses, IR, UV/Vis spectra, photoluminescence properties of the complexes 1and 2
The Schiff base ligand (HL) is prepared by following a reported procedure [25]. Briefly,
8-aminoquinoline and o-vanillin are mixed in 1:1 molar ratio in methanol solvent and reflux for
one hour. The ligand is directly used for synthesis of metal complexes without further
purification. Complex 2 is prepared by reaction between Cd(NO3)2.4H2O: HL : NaX (where X=
SCN¯ ) in 1:1:1 molar ratio in methanol solvent under stirring condition. Complex 1 is obtained
by mixing Cd(NO3)2.4H2O and HL in 1:1 molar ratio in methanol solvent under stirring
condition. Complexes 1 and 2 are di-phenoxido bridged dinuclear of Cd(II). In both cases NO3¯
and SCN¯ act as terminal co-ligands (Scheme 1). Both the complexes are dark red in colour and
isolated in very high yield. They are characterized by elemental analysis and different
spectroscopic techniques. The complexes exhibit strong and sharp stretching frequency at
around 1600 cm-1 in their IR spectra, due to presence of azomethine group, ν(C=N). Complex 1
exhibits multiple stretching frequencies at 1470 and 1380 cm-1 respectively which are
characteristic of chelating nitrate group within the molecule (Fig.S1) [32]. Complex 2 exhibits a
sharp stretching frequency at 2070 cm-1 which is characteristic of stretching frequency for
terminal thiocyanate ion (Fig.S2) [32]. Bulk purity of both the complexes is examined by
performing powdered XRD study (Fig.S3 and S4 for 1 and 2, respectively).
The UV-vis spectra of ligand and both the complexes (10µM) are recorded in DMSO
solvent (Fig. S5). Both complexes exhibit two absorption bands around 350 and 450 nm. All
8

these transitions are mainly ligand centered due to presence of d10 Cd(II) ion. Therefore,
absorption peaks are assigned as ligand based n→π* and π→π* types of transition.
Photoluminescence properties of HL, 1 and 2 are studied in DMSO solvent at concentration
10µM. The free Schiff base ligand exhibits weak emission peak at around 570 nm upon
excitation at 470 nm. Whereas complexes 1 and 2 exhibit much stronger photoluminescence
(Fig. 1) upon excitation at 450 nm, they exhibit much intense emission peak at 590 nm. Weak
fluorescence intensity of the Schiff base ligand arises from rotation of phenyl ring and quinoline
ring around single C-C or C-N bond [33(a)]. Metal centre upon coordination with ligand HL
increases rigidity within ligand system following CHEF mechanism. Coordination of HL with
Cd(II) also restricts CH=N isomerization indicating enhanced emission intensity [33b, 33c].
1H NMR spectra of both complexes has been recorded in D6-DMSO solvent. For
complex 1, imine protons appear at δ = 8.86 ppm, aromatic protons appear at δ = 6.35-8.65 ppm
and methoxy protons appear at δ = 3.73ppm (Fig.S6). Whereas for complex 2, imine protons
appear at δ = 8.93 ppm, aromatic protons appear at δ = 6.38-8.82 ppm and methoxy protons
appear at δ = 3.72ppm (Fig.S7).
Scheme 1: The route to the syntheses of complexes 1 and 2.
9

Fig.1. Fluorescence properties of HL and Complexes 1 and 2 in DMSO at concentration 10µM.
3.2 Description of the crystal structure of [Cd2(L)2(NO3)2] (1)
Single crystals of complex 1 are obtained from slow evaporation of its methanolic
solution. It crystallizes with
space group in the triclinic unit cell. Crystal structure of complex
P1
1 is depicted in Fig. 2. Selected bond distances and bond angles are listed in Table S2. The
asymmetric unit consists of two independent cadmium complexes, [Cd(L)(NO3)] sitting in an
inversion centre. Complex 1 is a di-phenoxido-bridged Cd(II) complex of formula
[Cd2(L)2(NO3)2]. Upon close inspection it has been observed that coordination environment
around each Cd(II) centre (CdA and CdB) is very similar. For simplicity we have discussed
coordination environment of one neutral species labeled as ‘B’. Cd(II) centre is hepta-
coordinated, where the metal centre is coordinated with one imine nitrogen atom (N1B), one
quinoline nitrogen atom (N2B), two phenoxido oxygen atoms (O1B and O1Bb) and two oxygen
¯
atoms (O3B and O4B) of a bidentate NO3 anion (Fig. S8). Coordination geometry for a hepta
10

coordinated metal centre will either be a monocapped-trigonal prismatic geometry (MCTP) or a
monocapped-octahedral geometry (MCO). (Scheme 1S). Scheme 1S clearly suggests that in case
of MCO geometry the value of two angles N1B–CdB–O1Bb and N2B–CdB– O1Bb are 180° and
~125° respectively (for ideal case) whereas in MCTP geometry both angles will be equal.
Therefore, the difference in the two angles will vary from 0 to 55° depending on if it is MCTP or
MCO. For complex 1 the values of both angles are 138.9° and 133.53° respectively. The
difference between the two angles is 13.06° which strongly suggests distorted monocapped-
trigonal prismatic geometry around the metal centre [34]. Here O1B, O2B, O3B and O4B form
the square plane and N1B is present in the capping position. The Cd(II)–unique nitrogen (N1B)
distance is 2.305Å and is shorter than that of Cd(II)–quinoline-N distance (2.310 Å) as can be
seen in Fig S8. The Cd(II) ion is displaced towards the imine nitrogen from the square plane. In
the double phenoxido-bridged Cd(II) complex, the Cd2O2 core is slightly asymmetric, where
CdB–O1B and CdB–O1Bb distances are 2.276Å and 2.194Å respectively (Table S2). The two
Cd atoms are separated by 3.630Å, and the CdB–O1Bb–CdBb angle is 108.61°. The crystal
packing of complex 1 is presented in Fig. 3. The solid state structure is stabilized by means of
hydrogen bonding interactions established between the aromatic H atom and the coordinated
nitrate. Moreover, antiparallel π˖˖˖π stacking interactions between the quinoline rings are also
established governing the formation of a 2D network in the bc plane. In Fig. 3 we have also
indicated the intramolecular C–H···π interactions that are established between the methyl group
and the aromatic ring.
11

Fig. 2. Crystal structure of 1. Atoms are shown as 30% thermal ellipsoids. H atoms are omitted
for clarity. a= 1-X,1-Y,1-Z; b= -X,1-Y,-Z.
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Fig.3. 2D supramolecular architecture of complex 1 propagating along the bc plane showing H-
bonding (black dashed lines), π-π interaction (red dashed lines) and intramolecular CH-π
interactions (blue dashed lines). Distances in Å.
3.3 Description of the crystal structure of [Cd2(L)2(NCS)2] (2)
Complex 2 crystallizes from its methanolic solution with the monoclinic P21/n space group. The
perspective view of 2 along with atom numbering scheme at metal coordination sphere is
depicted in Fig. 4, while important bond distances and angles are given in Table S2. The
asymmetric unit contains the whole molecule, [Cd2(L)2(NCS)2]. Complex 2 is a di-phenoxido
bridged Cd(II) complex in which each of the Cd(II) centre is hexa coordinated. It is well known
that a hexa coordinated metal centre may adopt either octahedral or trigonal prismatic geometry.
For an ideal octahedron there are two exactly parallel, staggered equilateral triangles of side s
while in a Trigonal Prism, the equilateral triangles are eclipsed. The inter-triangular distance and
twist angle in both the cases are represented by h and ф, respectively (Fig. S9). Therefore, exact
13

geometry around Cd(II) centres in complex 2 could be measured by evaluating s/h ratio and twist
angle (ф) values [35]. In case of complex 2 the s/h ratio and the average twist angle are 1.18,
20.13°, for Cd1 centre and 1.18, 30.46°, for Cd2 centre respectively, which are significantly
differing from ideal trigonal prismatic geometry (for an ideal trigonal prismatic geometry, s/h =
1.00, ф=0° and for regular octahedron with square faces s = h, s/h = 1.22, ф=60°). Therefore the
geometry around the cadmium centres is distorted octahedron.
Each Cd(II) centre is coordinated with one quinoline nitrogen (N1 for Cd1 and N4 for
Cd2), one imine nitrogen (N2 for Cd1 and N5 for Cd2) and phenoxido oxygen (O1 for Cd1 and
O3 for Cd2) of same uni-negative Schiff base ligand together with methoxy oxygen atom (O4 for
Cd1 and O2 for Cd2) of another bridging mono-negative Schiff base ligand. Remaining
coordination no. is satisfied by a terminal (N-bound) thiocyanate group. Both Cd1 and Cd2
atoms lie significantly above the mean N2O2 plane by 0.690 Å and 0.768 Å respectively. The
dihedral angle between two CdN2O2 planes is 61.77°. The Cd-N (imine) and Cd-N (pyridyl)
bond distances are vary within the range 2.318-2.336 Å and 2.288-2.304 Å respectively,
whereas, Cd-O (phonolic), Cd-O (methoxy) bond distances are found within range 2.190-2.313Å
and 2.597-2.643Å respectively. Separation between two Cd centres is 3.592 Å. The values of
Cd1–O1–Cd2 and Cd1––O3––Cd2 angles are 107.59° and 105.59° respectively. All these values
are comparable with those found in other Schiff base complexes of Cd [36].
14

Fig.4. Asymmetric unit of 2. Atoms are shown as 30% thermal ellipsoids. H atoms are omitted
for clarity.
3.4 Computational study
The theoretical study is devoted to analyze some unconventional noncovalent interactions
that are present in the crystal packing of compound 2 (Fig.4) focusing our attention to the role of
the pseudo halide ligand influencing the crystal packing. We have studied the striking O···S
15

chalcogen bonding and π–π stacking interactions involving the π-system of the pseudo halide
ligand in compound 2.
As a first approximation to rationalize the different role of the pseudo halide ligand in
compound 2, we have optimized mononuclear Cd complex of formulae CdLX, X = NCS (Fig.5)
and computed the Molecular Electrostatic Potential (MEP) surfaces in order to analyze the
electron donor/acceptor properties of the pseudo halide. The MEP value at the extension of the
C=S bond is slightly negative for the NCS ligand. The S atom presents an anisotropic
distribution of the electron density with a belt of negative electrostatic potential around the
central part of the S atom, leaving the outermost region depleted. In compound 2 the more
positive region (apart from the metal centre) corresponds to the aromatic H-atoms of
iminoquinoline moiety due to its coordination to Cd.
Fig.5. MEP surface of compound 2. Energies at selected points of the surfaces are given in
kcal/mol.
16

In Fig.6a we represent a partial view of the crystal packing of this compound.
Interestingly, a supramolecular 1D chain is formed due to an intriguing combination of
interactions. Apart from conventional C–H···N interactions where the pseudohalide does not
participate, the thiocyanate establishes two unconventional interactions. First, a π–π stacking
where the π-system of the thiocyanate interacts with the π-system of the iminoquinoline ring and
second one is O···S chalcogen bond. The former can be explained by considering the MEP
surface shown in Fig.5 since the π-system of thiocyanate presents negative value of MEP and the
π-system of iminoquinoline is positive (due to the coordination to the Cd). Therefore, the
stacking interaction is electrostatically enhanced. The chalcogen bonding (the O···S distance is
shorter than the sum of van der Waals radii, 3.32 Å) cannot be explained simply in terms of
electrostatic effects since the MEP value at the S atom is slightly negative (–5 kcal/mol) and the
MEP value at the O-atom is large and negative. Therefore, this interaction is likely dominated by
induction or dispersion effects that compensate the electrostatic repulsion anticipated by the
MEP analysis. Moreover, the concurrent participation of the thiocyanate in the π-stacking
interaction likely makes more positive the MEP value at the σ-hole of the S atom as a
consequence of the charge donation from the SCN to the π-system of the iminoquinoline moiety.
We have analyzed energetically a dimer extracted from this infinite 1D assembly. The interaction
energy is shown in Fig.5 and it is large and negative, indicating a strong binding (∆E2 = –23.6
kcal/mol). In an effort to evaluate the contribution of the interactions involving the pseudohalide,
we have computed an additional theoretical model where the thiocyanate has been replaced by a
hydride. As a result the interaction energy is reduced to ∆E3 = –14.9 kcal/mol, that is the
contribution of the H-bonding and other long range weak interactions. The difference (∆E2 –∆E3
17

= –8.7 kcal/mol) corresponds to the interactions involving the SCN (π-stacking and chalcogen
bond).
Related unconventional π-stacking interactions (similar to SCN···π observed in
compound 2) have been previously reported in the literature. For instance, unconventional aryl–
carbonyl interactions have been studied in substituted pyridine carboxamides [37] and
Ni(II)/Cu(II) malonate complexes [38] with comparable π-stacking distances. Moreover, π-
interactions involving pseudohalides have been experimentally demonstrated in flavoproteins
and studied theoretically. The parallel arrangement of the pseudohalide and the flavine rings is
defined as a π-staking interaction [39]. Finally, the interactions of alkene and cyano with
aromatic rings have been studied and demonstrated using NMR spectroscopy [40]
Fig.6. (a) X-ray fragment of 2, H-atoms and cations omitted for clarity. (b,c) Theoretical models
used to evaluate the π–π, and O···S interactions. Distances in Å.
18

Finally, in an effort to further analyze the O···S interaction, we have carried out the
natural bond orbital (NBO) analysis to study orbital effects. We have analyzed the second order
perturbation analysis, [41] since it is very useful to study donor acceptor interactions.
Remarkably, we have found a donor-acceptor orbital contribution (E(2) = 1.91 kcal/mol) due to
an electron donation from the lone pair (LP) of O to the antibonding C–S (σ*) orbital of the
thiocyanate ligand, thus confirming the σ-hole O···S interaction and that the S atom is acting as a
Lewis acid in spite of its slightly negative MEP value. Distances of above mentioned
supramolecular interactions are provided in Table 2.
3.5 Rationalization of the syn/anti structural difference in complexes 1 and 2
A significant structural difference between compounds 1 and 2 is the orientation of the
anionic co-ligands. That is, in compound 1 the nitrate ligands are pointing to opposite direction
(anti) and, conversely, the pseudohalides are pointing to the same direction in compound 2 (syn).
In order to evaluate energetically the stability of both orientations, we have fully optimized the
geometries of complexes 1 and 2 starting from the crystallographic coordinates (see Fig. 7).
Moreover, we have also optimized two hypothetical complexes, denoted as 1’ and 2’ where the
syn arrangement of the nitrate coligands is used in the optimization of 1’ and the anti orientation
of the SCN coligands is used optimization of 2’. In complex 1, the isomer observed
experimentally is more stable (5.8 kcal mol−1 in solution) than the hypothetical one (1’) thus
explaining the experimental formation of complex 1 with anti arrangement of nitrate ligands. In
the case of compound 2, both isomers (2 and 2’) are basically isoenergetic (difference 0.7 kcal
mol−1). Therefore, the observation of complex 2 (syn) in the solid state is likely due to packing
forces, which can easily compensate/reinforce the 0.7 kcal/mol difference between both isomers.
A likely explanation for the larger energetic difference between both isomers in 1 is the shorter
19

distance between the anionic ligands in 1 (see green arrows in Fig. 7). That is, in case of nitrate
its coordination using two oxygen atom causes a shorter distance between the anions compared
to the pseudohalide (monocoordination). Therefore, the electrostatic repulsion between the
anions contributes to the destabilization of the syn arrangement in 1.
Fig. 7. DFT-optimized structures of the experimentally observed complexes 1 and 2 and their
hypothetical complexes varying syn/anti arrangement of the anionic co-ligands (1’, 2’).
Distances in Å
20

4. Conclusions
In this present study, we have shown that new Cd(II) complexes with interesting
supramolecular architecture can be synthesized by using 8-aminoquinoline based classic Schiff
ligand. The results reported in this paper clearly show that the Schiff base ligand HL is a
versatile ligand, which is capable of exhibiting various co-ordination modes. Here we present
two new coordination compounds with structural diversity in which HL ligand can act as a
phexoxido bridged tetradentate ligand (N2O2) generating dinuclear compounds 1 and 2,
respectively. Complexes 1 and 2 bear the unique example of anion (nitrate and thiocyanate)
dependent structural diversity. In complex 1 nitrate ions are present in anti orientation and in
complex 2 thiocyanate ions are present in syn orientation. Both the complexes exhibit
interesting non covalent interactions in their crystal packing, including chalcogen bonding and
unconventional π–π interactions. The energetic features of these interactions have been studied
by means of DFT calculations and characterized energetically by using the NBO computational
tool. These results open the door toward the synthesis of new compounds of different transition
metals involving this ligand showing novel physical properties, such as photoluminescence,
magnetism, combined with a biological interest.
21

Acknowledgments
A. S. gratefully acknowledges the financial support of this work by the DST, India (Sanction No.
SB/FT/CS-102/2014, dated- 18.07.2015). The authors are thankful to Mr. Saikat Banerjee,
Research Scholar, Jadavpur University, for scientific help.
Appendix A. Supplementary data
CCDC 1519900-1519901 contain the supplementary crystallographic data for complexes 1 and
2. This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or email:deposit@ccdc.cam.ac.uk.
References
[1] L. Pauling, R.B. Corey, Proc. Natl. Acad. Sci. U.S.A. 37 (1951) 251-256.
[2] L. Pauling, R.B. Corey, Proc. Natl. Acad. Sci. U.S.A. 37 (1951) 729-740.
[3] L. Pauling, R.B. Corey, H.R. Branson, Proc. Natl. Acad. Sci. U.S.A. 37 (1951) 205-211.
[4] (a) B. Casu, M. Reggiani, G.G. Gallo, A. Vigevani, Tetrahedron 22 (1966) 3061-3083; (b)
J. W. Ellis, J. Bath, J. Am. Chem. Soc. 62 (1940) 2859-2861.
[5] W. Banks, C.T. Greenwood, Biopolymers 11 (1972) 315-318.
[6] L. Pauling, The Nature of the Chemical, Cornell University Press: Ithaca, NY, 1960.
[7] K.E. Riley, P. Hobza, On the Importance and Origin of Aromatic Interactions in Chemistry
and Biodisciplines. Acc. Chem. Res. 46 (2013) 927-936.
22

[8] C.D. Sherrill, Energy Component Analysis of Pi Interactions., Acc. Chem. Res. 46 (2013)
1020-1028.
[9] A. Bauza, D. Quinonero, P.M. Deya, A. Frontera, CrystEngComm 15 (2013) 3137-3144.
[10] W.Z. Wang, B.M. Ji, Y.J. Zhang, Phys. Chem. A 113 (2009) 8132-8135.
[11] G. Cavallo, P. Metrangolo, T. Pilati, G. Resnati, G.Terraneo, Cryst. Growth Des. 14 (2014)
2697-2702.
[12] P. Sanz, M. Yáñez, O. Mó, J. Phys. Chem. A 106 (2002) 4661-4668.
[13] (a) C. Bleiholder, D.B. Werz, H. Köppel, R. Gleiter, J. Am. Chem. Soc. 128 (2006) 2666-
2674; (b) C. Bleiholder, R. Gleiter, D.B. Werz, H. Köppel, Inorg. Chem. 46 (2007) 2249-2260.
[14] M. E. Brezgunova, J. Lieffrig, E. Aubert, S. Dahaoui, P. vFertey, S. Lebègue, J.G. Ángyán,
M. Fourmigué, E. Espinosa, Cryst. Growth Des. 13 (2013) 3283-3289.
[15] J. Fanfrlik, A. Přáda, Z. Padĕlková, A. Pecina, J. Macháček, M. Lepšik, J. Holub, A.
Ružička, D. Hnyk, P. Hobza, Angew. Chem., Int. Ed. 53 (2014) 10139-10142.
[16] K. Suresh, V.S. Minkov, K.K. Namila, , E. Derevyannikova , E. Losev, A. Nangia, E.V.
Boldyreva, Cryst. Growth Des. 15 (2015) 3498-3510.
[17] D.B. Werz, R. Gleiter, F. Rominger, J. Am. Chem. Soc. 124 (2002) 10638-10639.
[18] T. Kusamoto, H.M. Yamamoto, R. Kato, Cryst. Growth Des. 13 (2013) 4533-4541.
[19] Y. Nagao, T. Hirata, S. Goto, S. Sano, A. Kakehi, K. Iizuka, M. Shiro, J. Am. Chem. Soc.
120 (1998) 3104-3110.
[20] J.C. Taylor, G.D. Markham, J. Biol. Chem. 274 (1999) 32909- 32914.
[21] K.-i. Fujita, K. Murata, M. Iwaoka, S. Tomoda, Tetrahedron 53 (1997) 2029-2048.
[22] M. Iwaoka, S. Takemoto, S. Tomoda, J. Am. Chem. Soc. 124 (2002) 10613- 10620.
23

[23] (a) D. Chatterjee, E. Ember, U. Pal, S. Ghosh and R.V. Eldik, Dalton Trans. 40 (2011)
10473-10480; (b) L. F. Lindoy, Coord. Chem. Rev. 4 (1969) 41; (c) P.A. Vigato, S. Tamburini,
Coord. Chem. Rev. 248 (2004) 1717-2128.
[24] (a) H. Xu, C. He, Y.X. Sui, X.M. Ren, L.M. Guo, Y.G. Zhang, S. Nishihara, Y. Hosokoshi,
Polyhedron , 26 (2007) 4463-4469; (b) S. Nayak, P. Gamez, B. Kozlevˇcar, A. Pevec, O.
Roubeau, S. Dehnen, J. Reedijk, Polyhedron 29 (2010) 2291-2296.
[25] (a) M. Shyamal, T.K. Mandal, A. Panja, A. Saha, RSC Adv. 4 (2014) 53520-53530; (b)
A.B. Pradhan, S.K. Mandal, S. Banerjee, A. Mukherjee, S. Das, A.R.K. Bukhsh, A. Saha,
Polyhedron 94 (2015) 75-82; (c) P. Ghorai, A. Chakraborty, A. Panja, T.K. Mondal, A. Saha,
RSC Adv. 6 (2016) 36020-36030.
[26] G.M. Sheldrick, SAINT, Version 6.02, SADABS, Version 2.03, Bruker AXS Inc., Madison,
Wisconsin, 2002.
[27] G.M. Sheldrick, SADABS: software for empirical absorption correction, University of
Gottingen, Institute fur Anorganische Chemieder Universitat, Gottingen, Germany, (1999-2003).
[28] G.M. Sheldrick, Acta Cryst. C71 (2015) 3-8.
[29] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett. 162 (1989) 165-169.
[30] (a) A. Bauzá, A. Terrón, M. Barceló-Oliver, A. García-Raso, A. Frontera, Inorg. Chim.
Acta. 452 (2016) 244-250; (b) D. Sadhukhan, M. Maiti, G. Pilet, A. Bauzá, A. Frontera, S. Mitra,
Eur. J. Inorg. Chem. 11 (2015) 1958-1972; (c) M. Mirzaei, H. Eshtiagh-Hosseini, Z. Bolouri, Z.
Rahmati, A. Esmaeilzadeh, A. Hassanpoor, A. Bauza, P. Ballester, M. Barceló-Oliver, J.T
Mague, B. Notash, A. Frontera, Cryst. Growth Des. 15 (2015) 1351-1361; (d) P. Chakraborty, S.
Purkait, S. Mondal, A. Bauzá, A. Frontera, C. Massera, D. Das, CrystEngComm 17 (2015)
4680-4690.
24

[31] S.F. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553-566.
[32] K. Nakamoto, Infrared Spectra of Inorganic Compounds, Wiley, New York, 1970.
[33] (a) M.I. Knyazhansky, A.V. Metelitsa, A.Ja. Bushkov, S.M. Aldoshin, Journal of
Photochemistry and Photobiology A: Chemistry 97 (1996) 121-126; (b) S. Banerjee, A. Saha,
Journal of Coordination Chemistry 69 (2016) 3092-3106; (c) S. Das, M. Dutta, D. Das, Anal.
Methods, 5 (2013) 6262-6285.
[34] (a) J. Amoroso, P.G. Edwards, S.T. Howard, B.M. Kariuki, J.C. Knight, L. Ooi, K.M.A.
Malik,. L. Stratford, A.-R.H. Al-Sudani, Dalton Trans. (2009) 8356–8362; (b) H. Keypour, S.
Salehzadeh , R.G. Pritchard , R.V. Parish, Polyhedron 19 (2000) 1633–1637.
[35] N. Barman, S. Banerjee, P. Brandao, A. Bauza, A. Frontera, A. Saha, Journal of
Coordination Chemistry 69 (2016) 1188–1205.
[36] S. Banerjee, A. Bauzá, A. Frontera, A. Saha, RSC Adv., 6 (2016) 39376-39386.
[37] C.R. Wilson, O.Q. Munro, Acta Cryst. C 66 (2010) o513-o516.
[38] S.R. Choudhury, P. Gamez, A. Robertazzi, C.-Y. Chen, H.M. Lee, S. Mukhopadhyay,
Cryst. Growth. Des. 8 (2008) 3773.
[39] C. Estarellas, A. Frontera, D. Quinonero, P.M. Deya, Chem. Asian J. 6 (2011) 2316-2318.
[40] A.E. Aliev, J.R.T. Arendorf, I. Pavlakos, R.B. Moreno, M.J. Porter, H.S. Rzepa, W.B.
Motherwell, Angew. Chem. Int. Ed. 54 (2015) 551-555.
[41] F. Weinhold, C.R. Landis, Valency and Bonding: A Natural Bond Orbital Donor–Acceptor
Perspective, Cambridge University Press, Cambridge UK, 2005.
25

Table 1. Crystal parameters and selected refinement details for 1 and 2.
Compound
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
1
2
C34H26Cd2N6O10 C36H26Cd2N6O4S2
903.41
150(2)
895.55
150(2)
Triclinic
Monoclinic
P21/n
P1
11.1334(12)
11.5708(12)
13.3872(15)
101.606(4)
91.994(4)
106.082(4)
1615.7(3)
2
12.2466(13)
15.1116(18)
18.714(2)
90
b (Å)
c (Å)
α(°)
102.886(4)
90
β(°)
γ(°)
Volume (Å3)
3376.0(7)
4
Z
Dcalc (g cm–3)
1.857
1.762
Absorption coefficient (mm–1)
1.388
1.434
26

F(000)
896
1776
2.258- 25.855
39732
2.86- 26.20
41661
θ Range for data collection (°)
Reflections collected
Data / restraints / parameters
Goodness-of-fit on F2
6190/ 0/471
1.112
7457/0/ 453
1.055
R1 = 0.0552
wR2 = 0.1435
R1 = 0.0789
wR2 = 0.1681
2.280/-0.873
R1 = 0.0301
wR2 = 0.0606
R1 = 0.0462
wR2 = 0.0656
1.496/-0.746
Final indices[I>2σ(I)]
R indices (all data)
Largest diff. peak / hole (e Å-3)
Table 2. Geometric features of the supramolecular interactions highlighted in Fig. 3 and Fig 6 for
1 and 2, respectively
Complex 1
Labels of interaction
C17B–H17E···Cg(A)
C10B–H10B···O5B
Cg(A)a···Cg(A)a
Complex 2
Distance (Å)
2.810(0)
2.580(0)
Angle (º)
128.0(0)
110.0(0)
-
3.566(4)
Labels of Interaction
O1···S2C19
C···Cg(B)
Distance (Å)
3.213(2)
3.443(0)
Angle (º)
158.6(1)
-
aCg(A): centroid: N2BC12BC13BC14BC15BC16B. bCg(B): C31C32
27

Graphical abstract (Pictogram)
Anion-reliant structural versatility of novel cadmium(II) complexes:
synthesis, crystal structures, photoluminescence properties and exploration of
unusual O···S chalcogen bonding involving thiocyanate coligand
Pravat Ghorai,a Paula Brandão, b Antonio Bauzá,c Antonio Frontera,*c and Amrita Saha*a
aDepartment of Chemistry, Jadavpur University, Kolkata- 700032, India.
E-mail: asaha@chemistry.jdvu.ac.in;amritasahachemju@gmail.com; Tel. +91-33-24572941
bDepartment of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-
193 Aveiro, Portugal.
cDepartament de Química, Universitat de les IllesBalears, Crta. De Valldemossa km 7.5, 07122
Palma (Baleares), Spain.E-mail:toni.frontera@uib.es.
28

Graphical abstract (Synopsis)
Anion-reliant structural versatility of novel cadmium(II) complexes:
synthesis, crystal structures, photoluminescence properties and exploration of
unusual O···S chalcogen bonding involving thiocyanate coligand
Pravat Ghorai,a Paula Brandão, b Antonio Bauzá,c Antonio Frontera,*c and Amrita Saha*a
aDepartment of Chemistry, Jadavpur University, Kolkata- 700032, India.
E-mail: asaha@chemistry.jdvu.ac.in;amritasahachemju@gmail.com; Tel. +91-33-24572941
bDepartment of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-
193 Aveiro, Portugal.
cDepartament de Química, Universitat de les IllesBalears, Crta. De Valldemossa km 7.5, 07122
Palma (Baleares), Spain.E-mail:toni.frontera@uib.es.
In this work we have reported two new Cd(II) complexes viz. [Cd2(L)2(NO3)2] (1)
and[Cd2(L)2(NCS)2] (2) where HL= (E)-2-methoxy-6-((quinolin-8-ylimino)methyl)phenol.
Different non covalent interactions are present in the crystal packing of complexes 1 and 2.
Complex 2 reveals chalcogen bonding and unconventional π–π interactions.
29