Syntheses, crystal structures and photo physical aspects of azido-bridged tetranuclear cadmium (II) complexes: DFT/TD-DFT, thermal, antibacterial and anti-biofilm properties

Article abstract of DOI:10.1016/j.molstruc.2018.11.010

In this work we have reported two novel tetranuclear Cd(II) complexes viz. [Cd₄ (L^OMe)₂ (μ_1,1-N₃)₃ (μ_1,3-N₃)]_n (1) and [Cd₄ (L^OEt)

Full text of DOI:10.1016/j.molstruc.2018.11.010

Accepted Manuscript
Syntheses, crystal structures and photo physical aspects of azido-bridged
tetranuclear cadmium (II) complexes: DFT/TD-DFT, thermal, antibacterial and anti-
biofilm properties
Dhrubajyoti Majumdar, Swapan Dey, S.S. Sreejith, Jayanta Kumar Biswas, Monojit
Mondal, Pooja Shukla, Sourav Das, Tapan Pal, Dhiraj Das, Kalipada Bankura,
Dipankar Mishra
PII:
S0022-2860(18)31314-0
DOI:
https://doi.org/10.1016/j.molstruc.2018.11.010
MOLSTR 25839
Reference:
To appear in: Journal of Molecular Structure
Received Date: 5 August 2018
Revised Date: 2 November 2018
Accepted Date: 5 November 2018
Please cite this article as: D. Majumdar, S. Dey, S.S. Sreejith, J.K. Biswas, M. Mondal, P. Shukla, S.
Das, T. Pal, D. Das, K. Bankura, D. Mishra, Syntheses, crystal structures and photo physical aspects of
azido-bridged tetranuclear cadmium (II) complexes: DFT/TD-DFT, thermal, antibacterial and anti-biofilm
properties, Journal of Molecular Structure (2018), doi: https://doi.org/10.1016/j.molstruc.2018.11.010.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
GRAPHICAL ABSTRACT

ACCEPTED MANUSCRIPT
Syntheses, crystal structures and photo physical aspects of azido-bridged tetranuclear
cadmium (II) complexes: DFT/TD-DFT, thermal, antibacterial and anti-biofilm properties
a,b
b,*
c
d
Dhrubajyoti Majumdar , Swapan Dey , S. S. Sreejith , Jayanta Kumar Biswas , Monojit
d
e
e,*
f
g
a,
Mondal , Pooja Shukla , Sourav Das , Tapan Pal , Dhiraj Das , Kalipada Bankura ,
a,*
Dipankar Mishra
a,b
Department of Chemistry, Tamralipta Mahavidyalaya, Tamluk-721636, West Bengal, India
b
Department of Applied Chemistry, Indian Institute of Technology(Indian School of Mines),
Dhanbad, Jharkhand 826004, India
c
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi,
6
82022, Kerala, India
d
Department of Ecological Studies & International Centre for Ecological Engineering, University
of Kalyani, Kalyani 741235, West Bengal, India
e
Department of Chemistry; Institute of Infrastructure Technology Research and Management;
Near Khokhara Circle, Maninagar East, Ahmedabad-380026, Gujarat, India
f
Department of Chemistry, BIT Sindri, Dhanbad-828123, Jharkhand, India
g
Department of Chemical Sciences, IISER, Mohali, Manauli, PO 140306, India
AUTHOR
EMAIL
ADDRESS:
dmishra.ic@gmail.com,
deyswapan77@yahoo.com,
d.sourav245@gmail.com

ACCEPTED MANUSCRIPT
ABSTRACT
In this work we have reported two novel tetranuclear Cd(II) complexes viz. [Cd (L ) (µ -
OMe
4
2
1,1
OEt
OMe
OEt
N ) (µ -N )] (1) and [Cd (L ) (µ -N ) (OAc)] (2) where (H L ) and (H L ) are two
3
3
1,3
3
n
4
2
1,1
3 3
2
2
2
important less explored salen-type Schiff base ligands. Both of the complexes have been
characterized by using common spectroscopic techniques, elemental analyses (C, H and N), X-
ray powder diffraction pattern (PXRD) and thermal analysis by TGA along with single x-ray
crystallography. The complete structural study discloses that in both cases the fully deprotonated
OMe 2-
OEt 2-
ligand [L ] or [L ] utilized all potential coordination sites to accomodate four Cd(II) ions.
Complex 1 is a one-dimensional polymer with azide (N ) linkage having both (ꢀ1,1 end on) and
ꢀ1,3 end-to-end) azido bridging but complex 2 is a discrete octanuclear ensamble where two
3
(
[
2
+
Cd (O) (N) ] units bridged to each other showing ꢀ1,1 end on end on azide bridging.
4
4
2
Exploration of photo physical properties in DMSO solvent reveals that Cd(II) complexes
OMe
enhance appreciably the fluorescence behavior over free Schiff base ligands (H2L ) and
OEt
(
H L ). DFT calculations performed at B3LYP/def2-TZVP level of theory reveal both the
2
energetics and composition of FMOs in these complexes and also show electrophilic and
nucleophilic areas via molecular electrostatic maps [ESP]. The antibacterial, membrane damage
assay and anti-biofilm properties of complexes 1 and 2 were investigated very carefully against
some important Gram-positive and Gram-negative bacterial strains.
Keywords: Antimicrobial, azide, Cd(II) coordination compounds, DFT, photoluminescence,
TGA

ACCEPTED MANUSCRIPT
1
. Introduction
In the past few decades, the design and construction of polymeric and discrete metal-organic
1
0
frameworks (MOFs) of d group 12 metal ions like zinc, cadmium or Hg are of great interest
and booming field of investigation due to the prosthetic beauty and potential applications as
functional materials [1-3]. Thus, a major focused have drawn towards N,O-donor salen-type
Schiff base complexes due to their preoperational accessibilities, structural varieties, extreme
stability, plasticity, varied denticities [4a] and have broad spectrum of bioactivities such as
antibacterial, antimicrobial, anti-allergic, antiviral, anti-oxidative, anticonvulsant, antitumor and
anticancer[4b-d]. There are some novel researches which potentially supported that microbicidal
and anticancerous activities of Schiff base ligands are appreciably enhanced after complex
formation with transition metal ions. Thus synthetic chemists will have a target to discover
always new complexes that have medicinal effects or less toxic. In nature biofilms are the most
important common path of bacterial growth which is responsible to various clinical infections
[4e]. Due to this adverse effect scientists have found Cd(II) Schiff base complexes antibiofilm
properties promoted significantly by different types of bacteria.This antibiofilm activity
effectively explored by salen-type (N O ) Schiff bases and their associated complexes which is
2
2
potentially significant in biomedical field [4f].Thus scientists around the world are tried to find
out new antibiotics related to new Schiff-base complexes which have significant antimicrobial
potency. On the other hand utilizing salen-type (N O ) Schiff base ligands, inorganic synthetic
2
2
chemists have devised zero to three-dimensional coordination polymers (CPs) in presence of
multifunctional organic or inorganic linkers as auxiliary moieties [5]. In this context, azide is
widely used as a worthy coordinating ancillary co-ligand due to their smaller length, rigidity and
versatile coordination modes of donor atoms [6-30]. So, group 12 metal ion such as Cd(II)

ACCEPTED MANUSCRIPT
polymeric or discrete complexes which are derived from azide ancillary co-ligands are of current
interest because of their unique pale color, higher thermal stability and possible strong
luminescence properties [31-42]. Till date several azido bridged discrete and coordination
polymers (CPs) have been reported but exploration of Cd(II)-azido bridged complexes are
always deserves attention due to the uncertainty of the product formation and versatility of
getting new different dimensional structural products and their associated novel photo physical,
antimicrobial and anti-biofilm properties which are of profound interest to both structural and
synthetic chemists.
In our previous publications, our highly motivated research group have successfully explored the
luminescent and antimicrobial, antibiofilm properties of halides and pseudohalides metal
1
0
complexes of 3d
metal ions where use of Schiff base ligand [N, N'-Bis(3-
OMe
methoxysalicylidenimino)-1,3-diaminopropane] (H2L ) allowed us to isolate a homometallic
OMe
-
trinuclear linear complex [Cd (L ) (Cl) ] in presence of NaCl as Cl spacer [43].Utilizing the
3
2
2
identical ligand in presence of thiocyanate anion, an 1D chain of Zn(II) coordination polymer
(
OMe)
1
[Zn
2
L
n
(µ1,3-SCN)(η SCN)] has been reported [44]. Apart from these usual works, further
analyzed two Cd(II) Azide/thiocyanate linked 1D and 2D coordination polymers
OMe
OMe
{
[Cd (N ) (L ) ].H O} , [Cd (L ) (SCN) ] , three Zn(II) dinuclear complexes
4
3 4
2
2
n
4
2
4 n
OEt
OEt
[
Zn (L )(CH OH) (SCN)(OAc)],
[Zn (L )(CH OH) (N ) ],
2
3
2
2
3
2
3 2
OEt
OEt
[
Zn (L )(Cl) (CH OH)].CH OH, Cd(II) polynuclear-discrete complexes viz,[Cd L (µ -
2
2
3
3
2
1,3
1
OEt
1
OEt
SCN)(η -SCN)] , {Cd (L ) (µ -N ) (η -N )] , [Cd (L )(Cl) .CH OH].CH OH] and their
n
4
2
1,1
3 3
3
2
2
2
3
3
novel luminescent and antimicrobial properties vividly [45-47].Taking cues from such novel
research works, we were prompted to explore the reactions of nonessential toxic Cd(II) metal ion
OMe
OEt
in presence of two less explored salen-type (N
2
O
2
) ligands (H
2
L
2
)/(H L ) using 1,2-

ACCEPTED MANUSCRIPT
diamnopropane (Pn) instead of 1,3-propane diamine in assisting the formation of complexes
OMe
OEt
[
Cd (L ) (µ -N ) (µ -N )] (1) and [Cd (L ) (µ -N ) (OAc)] (2).
4 2 1,1 3 3 1,3 3 n 4 2 1,1 3 3 2
The present article primarily focuses on the syntheses of complex 1 and 2, X-ray crystal
structure, DFT/TD-DFT calculations, photo physical, thermal behavior, antibacterial efficacy,
membrane damage assay and anti-biofilm properties.
2
2
2
. Experimental
.1. General remarks and physical measurements
.1.1. Materials
All chemicals were of reagent grade, purchased from commercial sources and used as received
without further purification. High purity 3-methoxysalicylaldehyde, 3-ethoxysalicylaldehyde and
1
,2-diaminopropane (Pn) was purchased Sigma Aldrich Company, USA. Cd(OAc) .2H O was
2 2
directly purchased from E. Merck, India. High purity NaN was purchased from SDFCL, India.
3
Solvent methanol was used of AR grade.
2
.1.2. Instrumentation
Elemental analyses (C, H, and N) of salen-type Schiff base ligands (H
cadmium(II) complexes were determined with a Perkin–Elmer CHN analyzer 2400. FT-IR
OMe
OEt
2
L
2
)/(H L ) and
-1
spectra of Cd(II) complexes were recorded as KBr pellets within the range 4000–400 cm
-1
having 16 scans at a wave number resolution of 4 cm on a Perkin–Elmer spectrum RX 1 using
detector DTGS (Deuterated triglycine sulfate). The abbreviations commonly used in this
manuscript stands for vs=very strong, s=strong, m=medium, w=weak. The electronic UV-Vis
OMe
OEt
spectra of both Cd(II) complexes and free salen-type ligands (H L )/(H L ) in DMSO
2
2
solvent were recorded on a Hitachi model U-3501 spectrophotometer. Perkin-Elmer LS50B
spectrofluorimeter model was used directly for the fluorescence measurements of complex 1 and

ACCEPTED MANUSCRIPT
2
as well as free salen-type ligands at room temperature (298K). Thermo-gravimetric analyses
(TGA) of both Cd(II) complexes were carried out on a TGA-5OH analyzer from ambient
°
°
temperature to 700 C at a temperature rate of 10 C/min in a flowing 30ml/min under nitrogen
atmosphere using a platinum cell. X-ray powder diffraction measurements for tetranuclear Cd(II)
complexes were carried out using BRUKER AXS, GERMANY X-ray diffractometer model after
0
0
scanning 2 theta from 4 to 50 .The radiation used for PXRD measurement purpose is Cu K-
alpha-1.The fluorescence quantum yields (Φ) of Cd(II) complexes have been determined using
the following well known mathematical equation (1) associated with fluorescence spectroscopy
where Quinine sulfate is used as the secondary standard (Φ = 0.57 in water) [48] .
2
ΦS AS (Abs)R nS
=
×
×
(1)
2
R
ΦR AR (Abs)S
n
According to equation (1) abbreviations used stands for: A terms denote the fluorescence area
under the curve; Abs denotes absorbance; n is the refractive index of the medium; Φ is the
fluorescence quantum yield; and subscripts S and R denote parameters for the studied sample
and reference, respectively.
2
.2. X-ray crystallography
The crystal data of azido-bridged tetranuclear Cd(II) complexes (1-2) have been collected on a
Bruker SMART CCD [49] diffractometer (Mo K radiation, λ = 0.71073 Å). The program
α
SMART was used for collecting frames of data, indexing reflections, and determining lattice
parameters, SAINT [50] for integration of the intensity of reflections and scaling, SADAB[51] for
absorption correction, and SHELXTL for space group and structure determination and least-
2
squares refinements on F . The crystal structures of cadmium (II) complexes (1-2) were fully

ACCEPTED MANUSCRIPT
2
solved and refined by full-matrix least-squares methods against F by using the program
SHELXL-2014[52] and Olex-2 software [53]. All the non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen positions were fixed at calculated positions and
refined isotropically. Different types of crystallographic figures for cadmium(II) complexes (1-2)
were generated using latest Diamond software[54].A summary of the crystallographic data and
complete structural refinement parameters of cadmium(II) complexes (1-2) are clearly presented
in Table S1[see supplementary material]. Crystallographic data (excluding structure factors) of
tetranuclear cadmium (II) complexes (1-2) have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos. CCDC 1811084-1811085.
Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge
mail:data_request@ccdc.cam.ac.uk, or fax: +44 1223 336033.
.2.1. Computational DFT methods
CB2
1EZ,
U.K.:
http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi,
e-
2
Frontier orbital analyses and global reactivity indices of both tetranuclear Cd(II) complexes were
explored using DFT calculations employing ORCA 3.0.3 software package [55] using the
Becke’s three-parameter hybrid exchange functional including the Lee–Yang–Parr nonlocal
correlation functional (B3LYP) [56] and the triple zeta valence basis set with one set of
polarization function (TZVP) [57]. To accelerate the calculations, we utilized the resolution of
identity (RI) approximation with the decontracted auxiliary def2–TZV/J Coulomb fitting basis
sets and the chain–of–spheres (RIJCOSX) approximation to exact exchange as implemented in
ORCA [57]. The Initial geometries of the complexes were taken from their crystal coordinates
and the optimized geometries were confirmed to be global energy minima by performing the
frequency calculations which had no imaginary frequency. The choice of the TZVP basis set was

ACCEPTED MANUSCRIPT
made since it reduces the basis set superposition error (BSSE) to negligible in the calculation of
systems with noncovalent interactions. The visualization of orbitals was done using Avogadro
software [58]. For calculating the composition of Frontier orbitals and distribution of ESP
(Electrostatic potential), Multiwfn program [59, 60] was used and .molden type file generated
using orca_2mkl functionality was employed as the input for the program. The Time Dependent-
Density Functional Theory (TD-DFT) calculation of both tetranuclear Cd(II) complexes were
performed using Gaussian 09 [60(b)] at DFT/B3LYP level using 6-31+G* basis set for C, H, O
and N atoms and SDD basis set and an effective core potential was used for Cd atom. The effect
of the solvent was modeled with the polarized continuum model (PCM) [60(b)].The
crystallographic coordinates have been considered for the TD-DFT calculation. All generated
molecular orbitals (MOs) during TD-DFT calculations were visualized using common software
Gauss view 5.0.
2
.2.2. Experimental of antibacterial and anti-biofilm properties of tetranuclear Cd(II) complexes
Two Gram positive bacterial strains (Bacillus subtilis MTCC 441 and Enterococcus gallinarum
MTCC 7049) and two Gram negative bacterial strain (Enterobacter aerogenes MTCC 111 and
Proteus vulgaris MTCC 744) was selected for in vitro evaluation of antibacterial potentials of
OMe
OEt
two complexes 1 and 2 and two of their respective ligands (H2L /H L ). The bacterial strains
2
were obtained from Microbial Type Culture Collection and Gene Bank (MTCC), Institute of
Microbial Technology, Chandigarh, India. For the antimicrobial assay, two Cd(II) complexes as
OMe
OEt
well as the corresponding ligands (H
2
L
2
/H L ) were dissolved in dimethyl sulfoxide
(DMSO) solvent. Selected concentrations of complexes (1-2) were prepared in sterilized Luria-
5
Bertani medium (modified). The bacterial culture (~5×10 colony forming units, CFU)/mL) was
inoculated to the respective medium maintaining the final concentration of DMSO set within 1%.

ACCEPTED MANUSCRIPT
All the bacterial strains were cultured in an incubator at 37°C and under mechanical shaking
(150 rpm) for 24 h. Antibacterial potential and minimum inhibitory concentrations (MIC) of the
OMe
OEt
complexes (1-2) and the Schiff base ligands (H2L /H L ) were probed against the selected
2
bacterial strains belonging to different categories following standard protocol prescribed by
Clinical and Laboratory Standards Institute (CLSI) (CLSI, 2012) [61]. The MICs were
determined measuring the turbidity in the broth using UV-Vis spectrophotometer at 600nm. The
threshold concentrations of the respective Cd(II) complexes (1-2) and their Schiff base ligands
OMe
OEt
(H
2
L
/H
2
L ) at which the bacterial growth (OD600< 0.05) was inhibited under 24 h
incubation were considered as individual MICs [62]. Individual susceptibilities of the bacterial
OMe
OEt
strains to the complexes(1-2) and their ligands (H2L /H L ) were tested following agar well
2
diffusion method and the antibacterial efficiencies of those chemical entities were defined by
measuring the diameter (mm) of the growth inhibition zone developed in the process [63].
The time-kill curves of the bacterial strains were obtained based on the growth pattern following
chemical treatment of the bacterial strains remaining in their mid log phase with the said
OMe
OEt
complexes (1-2) and ligands (H2L /H L ) at their respective MICs [62].The bacterial
2
cultures without any chemical treatment served as control. Optical density of the cultures was
determined with a 30 min interval frequency using UV-Vis spectrophotometer at 600nm. The
viable cell numbers were enumerated adopting the spread plate method and counting the
bacterial colony forming units (CFU/mL) developed on the agar plate.
Biofilm biomass was determined following the standard method [64].The bacterial cells of each
representative belonging to Gram positive (E. gallinarum) and Gram negative (P. vulgaris) were
cultured for 16 h in brain-heart infusion medium. The inoculums of the bacterial cultures (1 mL)
were added to the fresh media (100 mL) and incubated at 37 °C. The bacterial cell suspension (2

ACCEPTED MANUSCRIPT
mL; OD600= 0.1) derived from the prepared culture passing through the mid-log phase was then
transferred to a sterile Petri plates (35 mm) and kept under incubation at 37 °C to allow biofilm
formation. The used media was disposed of the plates and media containing respective
OMe
OEt
concentrations (MIC) of the Cd(II) complexes (1-2) and ligands (H2L /H L ) were added
2
afresh to the plates, maintaining triplicates for each treatment. A positive control was set with
biofilm without any chemical treatment, and a negative control was maintained only with sterile
broth. The plate was subsequently incubated at 37 °C for 24 h under static condition. Thereafter
aspiration was done for the contents of each plate. The plates were subjected to vigorous shaking
with 2 mL of phosphate buffer saline (PBS) to ward of all the non-adhering bacteria. Fixation of
the biofilm containing attached bacteria was performed with methanol (2 mL) for 15 min. It was
followed by staining of the biofilm with 2 mL of 2% crystal violet for 5 min at room
temperature. Differentiation of the biofilm matrices adhered to the plate surface was
accomplished by slowly ringing the plate in sterile Mili-Q water. Drying of the plates was done
leaving them in the open air at room temperature. Resolubilization of the dye adhering the
biofilm was performed with 2 mL of 33% (v/v) glacial acetic acid and absorbance was measured
at 570 nm using UV-Vis spectrophotometer.
Both bacterial strains E. gallinarum and P. vulgaris were grown overnight in a sterile plate (35
mm) to develop biofilm [65].The selected complexes (1-2) and corresponding ligands
OMe
OEt
(
H2L /H L ) were added following the method as described in connection with antibiofilm
2
assay. Positive and negative control sets were maintained as defined before. The spent media was
dispensed from the plates and washed with PBS to remove planktonic bacteria. The plates were
dried in the air. Fresh media (2 mL) containing tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide (MTT; 0.5 mg/mL) was added to each plate and incubated in the

ACCEPTED MANUSCRIPT
dark for 2 h at 37 °C. The MTT solution was discarded and DMSO (2 mL) was added
immediately to the plate to solubilize the formazan crystals developed in the plate and mixed
thoroughly. It was left for incubation for 15 min at 25 °C [66]. The absorbance of the solution
was measured using a UV-Vis spectrophotometer at 570 nm.
Membrane damage assay was performed to test the efficiency of the Cd(II) complexes (1-2) and
their Schiff base ligands in inflicting damage to bacterial cell membrane integrity. Membrane
[
62]
damage was evaluated by quantification of nucleic acids released from the cell. This assay was
performed on one representative each from Gram positive (E. gallinarum) and Gram negative (P.
vulgaris) bacterial category [67]. Overnight cultures of the both bacterial strains were subjected
to centrifugation at 8000 rpm for 5 min. The cell pellet was washed three times by phosphate
buffer saline (PBS, pH 7.4) and resuspended in the liquid media. The bacterial suspension with a
defined optical density (0.7 at 600 nm) was distributed into different conical flasks. The
concentrations of the complexes (1-2) and ligands were set at their respective MICs against the
bacterial strains. Control set was maintained without any chemical treatment. Samples (2 mL)
were collected at 30 min interval and filtered using syringe filter (0.2 µm), and the filtrate was
measured using UV-Vis spectrophotometer at 260 nm to estimate the nucleic acids released out.
2
.3. Synthetic Methodologies
OMe
OEt
2
.3.1. Syntheses of salen-type Schiff base ligands (H
2
L
2
/H L )
OMe
OEt
The Schiff base ligands (H2L
and H L ) was synthesized by the condensation of 1 mmol
2
(
0.152g) 3-methoxysalicylaldehyde or (0.1662g) 3-ethoxysalicylaldehyde with 0.5 mmol
0.0371g) 1,2-propanediamine (Pn) in (50 mL) of methanol for ca. 1 h (see Scheme 1). The dark
(
yellow colored solution was then used directly for tetranucear azido-bridged Cd(II) complex
formation.

ACCEPTED MANUSCRIPT
OMe
OEt
Scheme1. Synthetic representation of salen-type ligands (H2L /H L )
2
OMe
2
.3.2. Synthesis of complex [Cd (L ) (µ -N ) (µ -N )] (1)
4
2
1,1
3 3 1,3
3
n
To the methanolic solution (30 mL) of cadmium acetate dihydrate (0.266g, 1 mmol), dark orange
OMe
yellow solution of Schiff base (H2L ) was added directly followed by mixing a solution of
sodium azide (0.065g, 1 mmol) in minimum volume of aqueous methanol with constant stirring
for 2 h. (10 mL). Few drops of CH
3
CN were then added and the resulting mixture was
additionally refluxed for 1 hour at 70 C. Finally yellow colored filtrate was kept in refrigerator
̊
for crystallization by slow evaporation. After 10 days yellow colored single crystal suitable for
X-ray crystallography was obtained. Deep yellow crystals was isolated by filtration and air dried.
Yield: 0.358 g (68.75%), Anal. Calc. for C H N O Cd : C, 35.15; H, 3.11; N, 17.26. Found:
3
8
40 16
8
4
-1
C, 35.08; H, 3.05; N, 17.30 %. IR (KBr cm ) selected bands: ν 2986 (w), 2837 (w), 2070 (vs),
1
2
608 (vs), 1473 (s), 1217 (w) 1008 (w), 881 (w), UV-vis (λmax/nm): 370 nm.
OEt
.3.3. Synthesis of complex [Cd (L ) (µ -N ) (OAc)] (2)
4
2
1,1
3 3
2
To synthesize 2 similar procedure has been adopted as described for complex 1 except Schiff
OEt
2
base ligand (H L ) was used. The deep yellow solution was allowed to stand under freezing
condition where yellow crystals were separated by filtration and was air dried. Yield: 0.236 g
63.5 %), Anal. Calc. for C88 : C, 38.53; H, 3.75; N, 13.28. Found: C, 38.47; H,
(
102 26 8
H N O20Cd

ACCEPTED MANUSCRIPT
-
1
3
1
.69; N, 13.21%. IR (KBr cm ) selected bands: ν 3792 (b), 2913 (w), 2300 (vs), 2069 (vs),
803(vs), 1590 (vs), 1526 (w), 1329 (s), 1163 (w), 934 (w), UV-vis (λmax/nm): 371 nm.
3
. Results and discussion
3
.1. Syntheses of Cd(II) complexes and characterizations
OMe
OEt
Less explored compartmental salen-type (N O ) Schiff base ligands (H L /H L ) are
2
2
2
2
employed during complex 1 and 2 formations. Salen-type ligands formation commonly (1:2
molar ratio) belongs to condensed nature of Schiff base involving between 1,2-diaminopropane
(
Pn) and o-vaniline/ (OMe / OEt substituted) in methanol solvent ( Generally H L type where H
2
1
is a dissociable kind of proton).Schiff base ligands were successfully characterized by FT-IR, H
1
3
NMR and C spectroscopic studies shown in Fig.S13 (see supplementary material).Both
cadmium (II) complexes were derived by utilizing two close resemblance Schiff base ligands
OMe
OEt
(
H2L /H L ). Complex 1 and 2 were successfully synthesized by using cadmium (II) acetate
2
dihydrate as metal precursor in methanol solution and minimum volume of water-methanol
solution of sodium azide (NaN ) were added to methanolic solution of ligands (see Scheme
3
OMe
OEt
2
).Ligands (H2L /H L ) potentially acts as a strong compartmental hexadentate (N O )
2 2 4
nature and thus have strong coordinating ability with Cd(II) metal centers during tetranuclear
complex formation. Complex 1 is a one-dimensional polymer having azide (N ) linkage exhibit
both (ꢀ1,1 end on) and (ꢀ1,3 end-to-end) bridging fashion but complex 2 is a discrete octanuclear
3
2
+
ensemble where two [Cd (O) (N) ] units bridged to each other (ꢀ end on end on) by ancillary
4
4
2
1,1
azide co-ligands. Both complexes were successfully isolated only from CH OH-CH CN mixed
3
3
solvent medium at room temperature. Complex 1 and 2 is deep yellow colored, highly stable in
air and soluble in most of the common organic solvents. Complexes were successfully
characterized by different well known analytical tools like elemental analysis; UV-visible, TGA,

ACCEPTED MANUSCRIPT
X-ray powder diffraction pattern, single-crystal X-ray diffraction and Fourier transform Infrared
spectroscopic studies. A complete systematic characterizations approach of both Cd(II)
complexes using TGA and X-ray powder diffraction pattern (PXRD) will be clearly submitted
in Fig.S14 & Fig.S15(see supplementary material).
Scheme2. Synthetic representations of tetranuclear Cd(II) complexes
Fourier transforms Infrared spectroscopy of Cd(II) complexes were analyzed and compared
OMe
OEt
systematically with those of the corresponding free salen-type ligands (H2L /H L ).The FT-
2
IR spectrum of both tetranuclear Cd(II) complexes and in related some other reported Cd(II)
azide complexes were analyzed very carefully in order to predict the bridging propensity of
ancillary azide co-ligand (Table S4, see Supplementary material) [68].The appearance of strong
-1
C=N stretching value at 1630 cm indicate positively the well-known salen-type Schiff base
-1
-1
formation. An intense strong band at 1608 cm and 1620 cm for both cadmium(II) complexes
OMe
OEt
is shifted considerably towards lower frequencies as compared to free ligands (H2L /H L ),
2
that further supported coordination of binding mode of the imino nitrogen atoms of concerned
ligands with the Cd(II) metal centers. Both Cd(II) complexes (1-2) exhibits bands near at 2070
-1
and 2069 cm strongly attributable to ν(N ) binding shown in Fig.S1-S2 (see supplementary
3
material). Additionally aliphatic C-H stretching resonances for both tetranuclear Cd(II)

ACCEPTED MANUSCRIPT
1
complexes observed in the range 2918-2897 cm . Finally Ar-O stretching frequencies in both
1
Cd(II) complexes observed near at 1204-1212 cm which is identical to the other reported salen-
type ligands [69]. Thus a careful investigation of different characteristic FT-IR stretching bands
for tetranuclear complexes gives an positive idea about the versatile bridging propensity of azide
co-linker. The UV–Visible spectra of tetranuclear Cd(II) complexes were recorded in DMSO
solvent 200-1100 nm at ambient room temperature. In DMSO solvent at room temperature
*
Cd(II) complexes exhibits ligand–based transition at 370 & 371 nm presumably due to π→π or
*
10
n→π transitions [70,71]. Since Cd(II) metal ion have filled d configuration hence no metal
centric d-d transition was observed in complex spectrum 1 or 2. The UV–Visible absorption
spectrum of tetranuclear Cd(II) complexes (1-2) are clearly depicted in Fig.S3 (see
Supplementary material).
4
. X-ray crystal structure explanation
4
.1. Crystal growth description
X-ray diffraction quality deep yellow colored single crystals have grown for both Cd(II)
complexes in mixed CH OH-CH CN solvent medium by slow evaporation at normal room
3
3
temperature. Full Crystallographic data and detailed structural refinement parameters for
tetranuclear cadmium(II) complexes (1-2) are clearly presented in Table S1[see supplementary
material].
OEt
4
.2. Crystal structure of complex [Cd (L ) (µ -N ) (OAc)] (2)
4 2 1,1 3 3 2
Single crystal X-ray diffraction study of 2 divulges that it is a neutral octanuclear cadmium
OEt
4 2 3 3 2 3
complex [Cd (L ) (Ac)(N ) ] where, Ac=Acetate anion and N =Azide ion. The complex 2
crystallized in the triclinic space group P-1 (Z=1). The perspective view of the asymmetric unit
of 2 is given in the Fig.1 and selected bond parameters and bond angle are summarized in Table

ACCEPTED MANUSCRIPT
OEt 2-
S3 (see Supplementary material). From the perspective of design, the ligand [L ] having two
types of pockets which are tetradentate (O2N2) and (O4) respectively. The formation of
tetranuclear assembly of asymmetric unit of 2 involves by the coordination action of the two
OEt 2-
OEt 2-
doubly deprotonated ligands [L ] . In 2 each of the two [L ] binds to four cadmium metal
ions. Interestingly utilizing tetradentate pocket (O2N2) accommodate only one Cd(II) ion. On
contrary, other tetradentate pocket (O4) housing three Cd(II) ions at a time utilizing bridging µ3-
oxygen atom and bridging µ2-oxygen atom. Apart from the binding action provide by the
OEt 2-
-
[
3
L ] the tetranuclear framework is further strengthened by the two bridging µ1,1-N groups.
2
Besides this, one acetate ion attached to one of the terminal Cd(II) ion in η -OAc fashion to
neutralize the overall charge of the complex in Fig.S5 (see supplementary material). A close
inspection reveals that the asymmetric units of 2 further grow at the azide bridging (N8-N9-N10)
to give the overall a homometallic octanuclear ensemble. The coordination modes of the ligands
involved in the assembly of 2 are given in Fig.2. Further investigation of the crystal structure of
OEt 2-
2
reveals that the µ3-oxygen, µ2-oxygen of the ligand [L ] and µ2 azide leads to the di
-
2
+
cationic tetranuclear core [Cd
4
(O)
4
(N)
2
]
featuring a butterfly shaped topology (see Fig.3)
[72,73]. The butterfly shaped Cd4 core is consist of four cadmium metal ions; Cd2 and Cd3
represents the body where as Cd1 and Cd4 represents the wing. Among the four cadmium metal
ions the metal ions Cd1 and Cd4 are six coordinated and contain a 5O, 1N and 3O, 3N
coordination environment respectively with a trigonal prismatic structure where as Cd2 and Cd3
are seven coordinated and both contain a 4O, 3N coordination environment with square face
monocapped in Fig.S6 (see supplementary material). The Cd-O bond distance fall in the
following range, the average Cd-OEt=2.465Å and Cd-ꢀ
3
O =2.379Å and Cd-ꢀ O =2.256Å and
2
Cd-N bond distance fall in the range 2.329-2.342(9) Å with the small bond distance for Cd-µ1,1
-

ACCEPTED MANUSCRIPT
N (average ~ 2.289). The Cd-O-Cd and Cd-N-Cd bond angles in compound 2 are in the range
3
9
8.108 (2) -103.11 (2). All the bond distances and bond angles are similar and are comparable to
found in the literature (Table S5, see supplementary material) [74].
Fig.1. The perspective view of the asymmetric unit of complex 2
OEt 2-
Fig.2. Bindings mode of ligand [L ] , azido and acetate anion

ACCEPTED MANUSCRIPT
Fig.3. Butterfly shaped tetrameric core in complex 2
OMe
4
.3. Crystal structure of complex [Cd
4
(L
2 3 3 3 n
) (µ1,1-N ) (µ1,3-N )] (1)
Crystallographic investigation of complex 1 reveals that the asymmetric unit contains four
crystallographic independent cadmium metal ions and it is a neutral tetranuclear complex
OMe
[
Cd (L ) (N ) ] where, N =Azide. Compound 1 crystallizes in the monoclinic space group
4 2 3 4 3
P2 /n (Z=4). The asymmetric unit 1 is given in the Fig.S7 (see supplementary material) and
1
selected bond parameters are summarized in Table S2 (see Supplementary material). Like
OEt
OMe
H
2
L , the ligand (H
2
L
) having two unsymmetrical tetradentate pockets comprised of O2N2
and O4 coordination sphere. The formation of tetranuclear assembly of asymmetric unit of 1
OMe 2-
involves by the cumulative coordination action of the two doubly deprotonated ligands [L
]
.Two of such doubly deprotonated ligands binds to the four cadmium metal ions by using their
above-mentioned pockets. Besides this the asymmetric unit is further strengthened by the two

ACCEPTED MANUSCRIPT
-
bridging µ1,1-N
3
groups. Two tetranuclear asymmetric unit joins to its adjacent unit by two
-
equivalent end-on [N ] coligands which are bridging between two symmetrically generated Cd4
3
OMe
atoms forming a octanuclear ensemble [Cd (L ) (N ) ] (see Fig.3). It is exactly showing
4
2
3 4 2
resemblance to the structure of 2. Further investigation on 1 reveals that each octanuclear
-
building unit are linked to each other by means of two trans end-to-end (EE ꢀ-1,3)[N3]
coligands which actually bridging between two symmetrically generated Cd1 atoms forming a
one-dimensional polymeric chain. Here it is interesting to mention that there is an existence of
eight membered ring (Cd2N6) which is unlikely stabilized in chair conformation due to trans
end-to-end (EE ꢀ-1,3) binding of linear azido ligands in Fig.S8 (see supplementary material).
Perhaps due to presence of more flexible –CH -CH moiety in 1 creates steric effect and oppose
2
3
to the formation of such chair conformation arrangement. It is interesting to note that unlike 2 the
is a 1D polymeric structure. Perhaps due to presence of more flexible –CH -CH moiety in 1
1
2
3
creates steric effect and oppose to the formation of such chair conformation arrangement. It is
interesting to note that unlike 2 the 1 is a 1D polymeric structure. Although both the crystal
structure of 2 and 1 contain C-H·····π but the H atom of the different group involving in C-
H·····π are different in Fig.S4 (see supplementary material). The different coordination modes of
ligands present in 1 are given in Fig.4.Out of four cadmium metal ions the Cd1 and Cd4 are six
coordinate both having 3O, 3N coordination environment with an octahedral arrangement for the
Cd1 and trigonal prismatic for Cd4 metal ion. The metal ions Cd2 and Cd3 are seven coordinated
and both contain 4O, 3N coordination environment with square face monocapped geometry (see
Fig.5).The Cd-O bond distance fall in the range 2.235(4)-2.503(4) and Cd-N bond distance
2
.242(5)- 2.364(6) Å. All the bond distance and bond angles are similar and are comparable to
found in the literature TableS5 (see supplementary material) [74].To enhance the broad

ACCEPTED MANUSCRIPT
coordination chemistry knowledge of structurally characterized tetranuclear Cd(II)-azido bridged
OMe
OEt
complexes in presence of current investigated ligands (H2L /H L ), we have also explored
2
the coordination chemistry in details involving very close resemblance ligands in the same field
as a concise comparative manner. Apart from, in Table S6 (see supplementary material) [75] we
II
II
have also clearly focused on the importance of M chemistry [M =bivalent metal other than Cd
metal ions] involving identical salen-type ligands with reference to ligand potential donor
centers, azide bridging versality etc.
2
-
Fig.4. Bindings mode of ligand [L ] and azide ion
1

ACCEPTED MANUSCRIPT
Fig.5. Metal coordination geometry in complex 1
5
. Computational DFT approach
.1. Frontier molecular orbital analysis
5
The frontier molecular orbitals, HOMO and LUMO and their energy values gives an indication
of the approximate chemical reactivity and kinetic stability of tetranuclear Cd(II) complexes. The
HOMO-LUMO analysis was carried out on the DFT optimized structures and on analysis it was
found that the energy gap is comparable but highest on 2 with a difference of about 0.286 eV in
Fig.S9 (see supplementary material). From the HOMO-LUMO energy values the global
reactivity descriptors can be predicted in Table S7 [see supplementary material]. Orbital
compositional analysis was carried out on the frontier orbitals which returned interesting results.

ACCEPTED MANUSCRIPT
On performing orbital compositional analysis using Mulliken method it was found that for 1,
HOMO is mainly centered on two benzenoid rings with a contribution of 21.79% and 35.22% (π
type) (one from each ligand unit) and also from the associated phenoxo and methoxy oxygen
atoms 25.29% (p type) and azide coligands (14.49%, p type) with metal having no contribution
whatsoever. In the case of LUMO both the metal centres and azide coligands have no
contribution and the azomethine groups contribute heavily with a cumulative contribution of
about 48.729 % followed by the benzenoid rings of the ligand units with 39.88% (π* type). In the
case of 2, similar to the case of 1 no contribution from the metal centres were found and here
only one of the ligand contributes with the benzenoid rings with a share of 58.35% (π type). This
is followed by the associated phenoxo and ethoxy oxygen atoms 25.21%, azomethine nitrogen
atom 3.22% (p type) from the same ligand. All the azide coligands have some minor share with a
cumulative value of about 4.408%. The LUMO contribution comes from the ligand unit which
had no share in HOMO. The azomethine groups in the ligand have the highest share (78.14%)
followed by the benzenoid rings which gives a cumulative contribution of about 18.63% (π*
type). Due to the similarity in solid state structures similar results were obtained for both the
complexes on performing Mulliken charge analysis. It was found that the highest positive charge
is concentrated on the four Cd metal centers in both the complexes ranging from +0.92 to +1.5.
Oxygen donor atoms have the highest negative charge with phenoxo type in the range -0.71 to -
0
.74 for both complexes while that of the methoxy type falls in the range -0.55 to -0.57 (-0.58 to
-
0.45 in 2). The azide co-ligands closely follow with the charges ranging from -0.65 to -0.28
highest for the bonded and least for the middle one) (-0.67 to -0.38 in 2).The coordinated donor
(
azomethine nitrogen atoms have a consistent value ranging from -0.44 to -0.41 (-0.428 to -0.435
in 2).

ACCEPTED MANUSCRIPT
5
.2. Electrostatic potential maps (ESP) analysis
The electrostatic potential maps (ESP) where generated on DFT optimized structures at
B3LYP/def2-TZVP level of theory to visualize electrophilic and nucleophilic regions in the
molecule (see Fig 6). The red regions reveal the areas prone to nucleophilic attack while blue
electrophilic ones undergo attacks. Owing to the similarity in their structures both the complexes
1
and 2 have similar solid state architectures and exhibit the same type of hydrogen bonding
interactions. This is also reflected in their molecular electrostatic potential maps whereby both
the complexes show comparable minimum and maximum values. From the crystal structure
analysis it is clear that both the complexes exhibit non-classical intramolecular hydrogen
bonding interactions with methoxy carbon as the donor and azide nitrogen as the acceptor. This
is further evidenced from ESPs where both the complexes showed a positive potential (+9
kcal/mol for complex 1 and +11 kcal/mol for complex 2) in the proximity of the methoxy carbon
bound hydrogen (C35-H35B in complex 1 and C39-H39A in complex 2) and a negative potential
near azide nitrogen (N12 in complex 1 and N6 in complex 2) (-38 kcal/mol in complex 11 and -
4
2 kcal/mol in complex 2). Also in complex 1, the centroid Cg(5) comprising of atoms C1 to C6
is at a negative potential (π-basic ring) of about -11 kcal/mol which justifies its CH···π
interaction with methoxy carbon bound hydrogen H35A (+25 kcal/mol). Similarly in complex 2,
Cg(4) comprising of atoms C12 to C17 which is at a negative potential of about –15 kcal/mol
interacts with ethoxy bound hydrogen (H39C (+23 kcal/mol)) of adjacent molecule. The trend
for the highest positive and negative potentials is again similar on both the complexes and in line
with the reported complexes with the highest positive potentials seen near the azomethine bound
hydrogen (NH), spacer diimine group (+30 to +47 kcal/mol) [76] and the highest negative

ACCEPTED MANUSCRIPT
potentials are seen near azide coligands (with regions near terminal N atoms reaching up to -48
kcal/mol).
Fig.6. Electrostatic potential map of complex 1 and complex 2 on electron density isosurface
computed at the B3LYP/def2-TZVP level of theory.
6
. Computational TD-DFT analysis
The TD-DFT study of tetranuclear azido-bridged Cd(II) complexes exhibits electronic transitions
which are mainly due to intra-ligand charge transfer. The calculated absorption maxima in Figure
S10 black color (see supplementary material) arises at 354.13 nm for complex 1 (Experimental
~370 nm) with oscillator strength, f = 0.0945. HOMO-1 mainly consists of ligand π-orbitals and
very small contribution from metal d orbital and HOMO-2 is consists of ligand π-orbitals only.
Whereas the LUMO and LUMO+1 are consist of ligand π*-orbitals only. In case of complex 2, it
shows calculated peak in Figure S10 red color (see supplementary material) at 359.22 nm

ACCEPTED MANUSCRIPT
(Experimental ~371 nm) arises mainly due to intra-ligand transitions with oscillator strength, f =
0
.1586. TD-DFT related all orbital electronic transitions are clearly shown in Table S8 [see
supplementary material]. All the MOs generated in connection of TD-DFT calculations of Cd(II)
complexes are lucidly presented in Figure S10 [see supplementary material].
7
. Photo physical properties of Cd(II) complexes
OMe
OEt
Fluorescence behavior of salen-type Schiff base ligands (H2L /H L ) and tetranuclear
2
complexes (1-2) were recorded in DMSO solvent at room temperature in Table 1 and the ligand
centered emission spectra are compared in Fig.7 revealing that ligands are practically non-
emissive but the Cd(II) complexes exhibit strong fluorescent behavior. Upon photo excitation at
OMe
OEt
3
33 nm, Schiff base Ligands (H2L /H L ) exhibits a fluorescent emission centered at 500
2
and 501 nm.Cd(II) complexes (1-2) upon photo excitation at wavelength 372 & 373 nm shows
photoluminescence with the main emission peak at 519 & 516 nm. The emission bands for
complex 1 and 2 are nearly similar to those of the other reported Cd(II) complexes (Table S9, see
supplementary material) [77]. For complexes (1-2), no emission emanating from metal-centered
1
0
MLCT/LMCT excited states are possible since it is hard to oxidize or reduce d configuration
Cd(II) metal ion. The photo luminescent emission nature of Cd(II) metal complexes (1-2) may
therefore be assigned mainly due to the L→M charge transfer. As a common popular knowledge
of fluorescence chemistry, the enhancement of the emission intensities of (1-2) may be due to the
metal ligand chelation effect [78-81] or most probably the increase in conformational rigidity of
OMe
OEt
salen-type ligands (H
78,79]. Now at the time of metal binding process with N,O-donor ligand, non-radiative channels
and flexible bonds are inactive due to strong binding. This well known phenomenon popularly
2
L
2
/H L ) chelation imparted via N, O-donor center with Cd(II) metal
[
1
0
referred as ‘CHEF EFFECT’ which has been mainly reported in d configuration metal ions viz.

ACCEPTED MANUSCRIPT
Cd(II)/Zn(II)/Hg(II) especially those containing organic backbone fused ring structures involving
nitrogen or oxygen potential donor centers. Thus the enhancement of fluorescence phenomenon
through stable chelate complex formation in presence of salen-type ligands is more interesting
and attractive as it opens up the new avenue of opportunity for different photochemical
OMe
OEt
applications of these complexes [81]. A closer inspection of salen-type ligands (H2L /H L )
2
divulges that identical absorption and emission maxima spectra have been observed for ligands
containing a methoxy/ethoxy group in ortho position of the phenol moiety [82]. Further, reported
OMe
OEt
Schiff base ligands (H
2
L
2
/H L ) quantum yield (ɸ) values are very low which are nearly
OEt
'
identical to other reported Schiff base ligands (H
2
L ) [N,N-bis(3-ethoxysalicylidenimino)-1,3-
-3
0
-3
daminopropane] (4.6x10 ), (H L ) [N,N'-bis(salicylidene)-1,3-diaminopentane] (9.3x10 ) as
2
OMe
'
well as Schiff base (H2L ) [N,N-bis(3-methoxysalicylidenimino)-1,3-daminopropane]
-3
(
8.9x10 )[83,84]. The quantum yield (Φ) values comparison of synthesized Cd(II) complexes (1-
2
) with other reported cadmium complexes are nearly identical and shown in Table 2 as a
concise supporting evidence [85].
OMe
OEt
Table 1 Photo-physical parameters of Schiff base ligands (H
2
L
2
/H L ) and tetranuclear
Cd(II) complexes in DMSO at room temperature (298 K)
Compound
Absorption (λmax), nm Excitation (λmax), nm Emission (λmax), nm
OMe
Ligand(H2L
)
264, 332,427
264, 333, 427
285, 372
333
333
372
373
500
501
519
516
OEt
Ligand(H L )
2
II
Cd complex (1)
II
Cd complex (2)
285, 373

ACCEPTED MANUSCRIPT
OMe
OEt
Fig. 7. Ligand centered emission spectra of ligands (H2L /H L ) and tetranuclear Cd(II)
2
complexes in DMSO at 298K

ACCEPTED MANUSCRIPT
Table 2 Quantum yield (Ф) values comparison of salen-type ligands, tetranuclear Cd(II)
complexes and other reported important cadmium(II) complexes
Compounds
Quantum yield (ɸ)
References
OMe
Schiff base(HL
)
0.00353,
0.01308
This work
OEt
Schiff base(HL )
This work
a
OEt
-3
Identical Schiff base(H L )
4.6x10
2
OMe
-3
Identical Schiff base (H2L
)
8.9x10
b
0
-3
Identical Schiff base (H L )
9.3x10
c
2
OMe
[
Cd
2
L
(µ1,1-N
3
)(µ1,3-N
3
)]
n
(1)
0.02742
0.02872
0.03767
0.03410
This work
OEt
[
Cd (L ) (µ -N ) (OAc)]
2
This work
4
2
1,1
3 3
OEt
1
[
Cd (L ) (µ -N ) (η -N )]
d
e
4
2
1,1
3 3
3 2
1
[Cd (N ) (L ).H O]
4 3 4 2 n
[
Cd(N )(NO )(terpy)(H O)]
<0.1
<0.1
f
3
3
2
f
[
Cd (µ -N ) (N ) (terpy) ]
2
1,1
3 2
3 2
2
II
2
[
Cd (N ) (L ) ]
0.0236
0.0067
0.0370
0.0330
0.03219
g
3
3 4
2 n
II
1
[
Cd (N ) (L )]
2 3 3 n
1
h
[
Cd (SCN) (L ) ]
4 4 2 n
2
[
Cd (L ) (SCN) (CH OH)]
i
2
2
2
3
3
j
[
Cd (L ) (NO ) ]
2 2 3 2