Synthesis, characterizations and single crystal structure of di-nuclear azido-bridged Cd(II) coordination polymer with Schiff base precursor (H2LpentOMe): DFT, fluorescence, solvatochromism and in vitro antimicrobial assay

Article abstract of DOI:10.1016/j.ica.2019.119069

A di-nuclear azido-bridged Cd(II) 1-D coordination polymer [Cd₂(H₂L_pent^OMe)(μ_1,1-N₃)₂]_n (1) has been successfully synthesized using less explored bi-compartmental Schi

Full text of DOI:10.1016/j.ica.2019.119069

InorganicaChimicaActa496(2019)119069
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Inorganica Chimica Acta
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Synthesis, characterizations and single crystal structure of di-nuclear azido-
bridged Cd(II) coordination polymer with Schiff base precursor
(H2LpentOMe): DFT, fluorescence, solvatochromism and in vitro antimicrobial
assay
Dhrubajyoti Majumdara,b, Dhiraj Dasc, S.S. Sreejithd, Sudip Nage, Swapan Deyb,⁎
,
Surajit Mondalb, Kalipada Bankuraa, Dipankar Mishraa,⁎
a 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 Chemical Sciences, IISER, Mohali, Manauli, PO 140306, India
d Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682022, Kerala, India
e Department of Biochemistry, University of Calcutta, 35 Ballygunge Circular Road, Kolkata 700019, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Azide
Antibacterial
Cd(II)
A di-nuclear azido-bridged Cd(II) 1-D coordination polymer [Cd2(H2LpentOMe)(µ1,1-N3)2]n (1) has been suc-
cessfully synthesized using less explored bi-compartmental Schiff base ligand (H2LpentOMe) and characterized by
elemental analysis, FT-IR, FT-Raman, UV–Visible, SEM-EDAX, powder X-ray diffraction and fluorescence spec-
troscopy. Solid-state X-ray single crystal study revealed two different geometrical environment of Cd metal
centres with distorted square pyramidal (Cd1) and distorted pentagonal bipyramid (Cd2). Overall, small-sized
azide ions in asymmetric unit act as µ1,1 bridging mode. Geometry is optimized in gas phase using ORCA 3.0.3
and B3LYP level TZVP basis set to explain frontier molecular orbitals (FMO), molecular electrostatic potential
(MEP) and global reactivity. The HOMO-LUMO energy gap (3.434 Ev) suggests that chemical reactivity of
complex 1 is low but fairly stable. Moreover, molecular electrostatic potential map was drawn to identify re-
active regions in terms of electrophilic and nucleophilic. The steady state and time-resolved fluorescence
properties have been explored in DCM and solid-state condition at room temperature. Complex 1 exhibit bi and
tri-exponential decay in DCM as well as solid-state. The fluorescence behaviours are predominantly intra-ligand
in nature (π → π*) with lifetimes in the range (1.16–1.11 ns). Solvatochromism has been reported to show
solvent dependent absorption and fluorescence spectral changes. Fascinatingly, red shifted solvatochromism was
observed upon increasing solvent polarity. Finally, concentration dependent antimicrobial activity was in-
vestigated against two standard bacterial strains (Staphylococcus aureus (ATCC 25923) and Methicillin-resistant
Staphylococcus aureus (MRSA) where 250 µg/ml concentration of complex 1 is sufficient to show the anti-
microbial effect.
DFT
Schiff base
Solvatochromism
Fluorescence
1. Introduction
active hybrid compounds composed of organic and inorganic materials.
Herein we observed Schiff bases in conjunction with pseudo-halide
Today in modern scientific society application-based research works
gaining much more importance due to invention of human helpful
materials and saving environment from accidental disaster. In this
juncture, branch of materials science claims to be an active participant
for scientific revolution after gatherings physics, chemistry and biology
under single umbrella. The coordination polymers (CPs)/MOFs belongs
one of the active members in material science [1]. CPs are referred as
anions donor centres act as spacer and metal ions as active metal nodes
[2]. Thus, in the last two decades, conception of CPs/MOFs and poly-
nuclear discrete complexes has been expanded dramatically in presence
−
of well-known pseudo-halide ions such as N3–, NCS−, OCN–, [N(CN)2
]
[3–24] along with aromatic benzene, heterocyclic-multicarboxylic
compounds, oxalate, squarate and croconate dianions [25–29]. These
pseudo-halide ions effectively bind different metal ions to isolate large
⁎ Corresponding authors.
E-mail addresses: deyswapan77@yahoo.com (S. Dey), dmishra.ic@gmail.com (D. Mishra).
https://doi.org/10.1016/j.ica.2019.119069
Received 10 June 2019; Received in revised form 24 July 2019; Accepted 8 August 2019
Availableonline10August2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

D. Majumdar, et al.
InorganicaChimicaActa496(2019)119069
number of coordination complexes which have interesting molecular
and crystalline architectures with different dimensionality [3–29].
Among these, small-sized pseudo-halide azide ions is a nice linker since
it can bridge metal ions in many different coordination bonding modes
[4–22,30]. Although several azido-bridged CPs/MOFs have been pre-
viously reported but formation of M(II)-azido Schiff base complexes
always deserves new insights of research in respect novel structure
formation [31] and have broad spectrum of properties such as gas ad-
sorption and separation [32], ion exchange [33], sensing [31], lumi-
nescence [34], antibacterial and anti-biofilm [35]. In fact, research of
azido-bridged complexes with paramagnetic centre metal ions have
been subjected to extensive studies in connection of magnetic exchange
interaction whereas Schiff base complexes derived from d10 metal ions
did not receive such attention [4–22,30].
Due to our long-standing interest in ortho vanillin derived bi-com-
partmental Schiff base ligands, till date we have explored such ligand
complexation with d10 metal ions and their associated photo-
luminescence, DFT, in vitro antibacterial and anti-biofilm properties
vividly [36]. The outcome of previous research works is that azido-
bridged complexes exhibit photophysical, antibacterial and anti-biofilm
properties [35] but no exiguous number of studies on the concentration
dependent in vitro antimicrobial activity against two standard bacterial
strains (Staphylococcus aureus (ATCC 25923) and Methicillin-resistant
Staphylococcus aureus (MRSA). Unlike other metals, Cd(II) is well-
known to exhibit flexible geometry owing to its d10 electronic config-
uration and zero crystal field stabilization energy (CFSE). The geometry
of cadmium metal ion is primarily controlled by the steric requirements
of the Schiff base ligands [37]. Schiff base ligands based on o-vanillin
condensed with 1,3-diaminopentane are scare in literature [37]. Fur-
ther, since Schiff base ligands gaining day to day popularity in co-
ordination chemistry domain, researchers around the world have been
recommended such unique ligands for synthetic aspect of M(II)-com-
plexes due to their preoperational accessibilities, structural varieties,
extreme stability, plasticity, varied denticities [38]. In fact, herein
tuning of bi-compartmental Schiff base (H2LpentOMe) is to search if any
important changes occur in the molecular and crystalline architectures,
complex dimensionality and biological interest.
used as secondary standard (Φ = 0.57 in water) [39]
nS2
nR2
ΦS
ΦR
AS
AR
(Abs)R
(Abs)S
=
×
×
(1)
From Eq. (1) 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 para-
meters for the studied sample and reference compound respectively.
2.2. X-ray crystallography
Good quality crystal data for cadmium complex was collected on a
Bruker SMART CCD [40] diffractometer using Mo Kα radiation at
λ = 0.71073 Å. For data collection purpose different well-known pro-
grams were operated e.g. SMART program used for collecting frames of
data, indexing reflections and determining lattice parameters, SAINT
[41] for integration of the intensity of reflections and scaling, SADAB
[42] for absorption correction and popular SHELXTL for space group,
structure determination and least-squares refinements on F2. Crystal
structure was fully solved after refining by full-matrix least-squares
methods against F2 using the common program SHELXL-2014 [43] and
Olex-2 software [44]. It is worth mentioning that the Alert level A in the
checkCIF1 is related only to the Rint value is greater than 0.25 and 0.12
since the examined single crystal was a small-sized, brittle and weakly
diffracting (despite using Mo Kα radiation). Multiple attempts were
made to grow better quality diffracting crystal. The data here reported
is the best one among the collected. All the non-hydrogen atoms were
refined with anisotropic displacement parameters and hydrogen posi-
tions were fixed at calculated positions which is refined isotropically.
All crystallographic figures for complex 1 were constructed using latest
Diamond software [45]. The crystallographic data and full structure
refinement parameters are shown in Table 1. Crystallographic data
(excluding structure factors) have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication number
CCDC 1915504. Copies of the data can be obtained, free of charge, on
Table 1
In this paper, we have reported synthetic details, characterisations,
SEM-EDAX, solid-state single crystal structure, DFT, solvatochromism,
fluorescence and in vitro concentration dependent antimicrobial activity
of one new Cd(II) CP viz., [Cd2(H2LpentOMe)(µ1,1-N3)2]n (1).
Crystal structure parameters for complex 1.
Formula
C21H24 Cd2N8O4
M/g
677.28
Crystal system
Space group
a/Å
Monoclinic
C2/c
21.811(2)
2. Experimental section
b/Å
c/Å
12.5480(13)
19.0299(19)
2.1. Materials and physical measurements
α (°)
90
The research chemicals were of analytical grade and used as pur-
chased without further purification. o-vanillin and 1,3-diaminopentane
was purchased from Sigma Aldrich Company, USA. Cd(OAc)2·2H2O and
sodium azide (NaN3) was purchased from E. Merck, SDFCL, India.
Elemental analysis was performed on a Perkin-Elmer 2400 elemental
analyzer. FT-IR and FT-Raman spectra were recorded as KBr pellets
(4000–400 cm−1) using Perkin–Elmer spectrum RX 1 and BRUKER RFS
27 (4000–50 cm−1). EDAX experiments for weight percentage (%)
analysis of different elements was performed on EDAX OXFORD XMX N
using Tungsten filament. SEM images analysed by JEOL Model JSM −
6390LV. UV–Visible spectra (200–1100 nm) were determined by using
Hitachi model U-3501 spectrophotometer. Fluorescence spectra (spec-
troscopic grade DCM solvent) were measured by using Perkin-Elmer
LS50B Spectrofluorometer model at room temperature. Fluorescence
lifetime measurements were recorded using JOBIN-VYON M/S
Fluorimeter. Fluorescence lifetime (RT) in DCM solvent and solid-state
is calculated using Equation S1. Powder X-ray diffraction measurements
were carried out using BRUKER AXS, GERMANY X-ray diffractometer
model using radiation Cu Kα-1. Quantum yield (Φ) for 1 was de-
termined by applying Eq. (1) where quinine sulphate is preferentially
β (°)
107.288(6)
90
4972.9(9)
8
1.809
1.754
2672
0.2 × 0.2 × 0.1
0.996
−25 ≤ h ≤ 25
−14 ≤ k ≤ 14
−21 ≤ l ≤ 22
23,745
γ (°)
V/Å3
Z
ρc/g cm−3
μ/mm−1
F(0 0 0)
Cryst size (mm3)
θ range (deg)
Limiting indices
Reflns collected
Ind reflns
4119[Rint = 0.4422, Rsigma = 0.3375]
0.996
Completeness to θ (%)
Refinement method
Data/restraints/ parameters
Goodness-of-fit on F2
Final R indices [I greater than 2θ(I)]
Full-matrix-block least-squares on F2
4119/0/313
0.941
R1 = 0.0805
wR2 = 0.0627
R1 = 0.2469
wR2 = 0.0804
0.575 and −0.505
R indices (all data)
Largest diff. peak and hole (e ·Å−3
)
2

D. Majumdar, et al.
InorganicaChimicaActa496(2019)119069
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.: http://
www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi, e-mail:data_request@ccdc.ca-
m.ac.uk, or fax: +44 1223 336033.
2.3. Computational methodologies
The initial structure for the DFT calculations were taken from the
single crystal structure (cif 1) of the crystallographic coordinates of the
compound. Gas phase optimization of the title complex was done using
ORCA 3.0.3 software [46]. Frequency calculations were done to con-
firm that the optimized structure is the global minima and calculations
returned no imaginary frequency. The Becke’s three-parameter hybrid
exchange functional including the Lee–Yang–Parr nonlocal correlation
functional (B3LYP) [47] and the triple zeta valence basis set with one
set of polarization function (TZVP) [48] was used. 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 [49]. The choice of the TZVP basis set was
made on the known fact that it reduces the basis set superposition error
(BSSE) to negligible in the calculation of systems with non-covalent
interactions. The optimized structure of complex 1 was used to find out
electrostatic potential regions. The visualization of the frontier orbitals
was done using Avogadro software [50]. Orbital contribution analysis
of the frontier orbitals is carried out using Mulliken method by em-
ploying Multiwfn software [51].
Scheme 1. Synthetic Scheme for Schiff base ligand.
C
21H24N8O4Cd2: C, 37.24; H, 3.57; N, 16.54. Found: C, 37.18; H, 3.48;
N, 16.34%. IR (KBr cm−1) selected bands: ν 2965 (w), 2852 (w), 2074
(vs), 2049 (vs), 1622 (vs), 1469 (s), 1213 (s), FT-Raman (cm−1) se-
lected bands: 2987 (w), 2929 (s), 2085 (s), 1632 (vs), 1457 (s), 1217
(w), UV–Vis λmax (DCM): 353 nm.
3. Results and discussion
3.1. Synthesis
(H2LpentOMe) was synthesized by the condensation of 1,3-diamino-
pentane with o-vanillin in MeOH at 1:2 M ratio [52]. Complex 1 derived
from o-vanillin condensed Schiff base (H2LpentOMe) was prepared in
moderate good yield by taking the following procedure where 1:1:1 M
ratio of cadmium acetate dihydrate, (H2LpentOMe) and sodium azide in
methanolic solution under stirred condition (Scheme 2). Di-nuclear
azido-bridged cadmium complex was isolated only from slow eva-
poration of (CH3OH-DCM). Salicylaldehyde derivatives are always
useful carbonyl precursor for syntheses of large variety of Schiff base
ligands since additional coordinating group (-OR, R = Me, Et) en-
hanced not only ligand denticity but accelerated the possibility of CPs/
polynuclear complex formation [53]. The versatile bridging nature of
(H2LpentOMe) and azide ions are nicely presented in Scheme S1-S2. Li-
gand structure comprises two imines, two phenols and one (-OR,
R = Me) group. After de-protonation, the N2O2 imine-based chelating
site has the effective binding property with M2+ metal ions (Scheme 3).
In fact, CPs formation with Cd(II)-node and azide ions are scare in lit-
erature. Furthermore, ligand structural framework opens up similarities
with other bi-compartmental ligands (Scheme 3) [35,36,54]. In this
context, our research team extended the coordination behaviour of
(H2LpentOMe) with Cd(II) and small-sized azide ions (Table 2). The
stoichiometry of 1 was corroborated by the microanalytical results and
successfully characterized by FT-IR, FT-Raman, UV–Vis, SEM-EDAX,
powder X-ray diffraction, X-ray crystallography and fluorescence
spectroscopy.
2.4. Experimental of antimicrobial assay
The antimicrobial efficiency of complex 1 was tested against the test
organism Staphylococcus aureus (ATCC 25923) and Methicillin-resistant
Staphylococcus aureus (MRSA). The quantitative analysis was studied by
cell viability assay [51]. Briefly, the bacterial cells were cultured in
Luria Bertini (LB) broth in 96 well plate. Different concentration (250,
500 and 1000 µg/ml) of complex 1 were added in well containing
bacterial culture and incubated for 24 hrs. Here, gentamicin solution
(10 µg/ml) was utilized as a positive control. The antimicrobial effi-
ciency was investigated by measuring optical density of cells using
microplate reader (Biorad, USA) at 600 nm compared to control. Cell
viability was determined using the given well-known formula:
OD
OD
− OD
Sample
Blank
CellViability(%) =
× 100
− OD
Control
The absorbance values ofBtlarnekated cells and untreated cells are de-
noted as ODSample and ODControl respectively. The whole assay was
performed with independent triplicate experimental setup to obtain the
standard mean values.
OMe
2.5. 2.5. Synthesis of ligand (H2Lpent
)
Schiff base ligand (H2LpentOMe) was synthesized using a reported
procedure [52]. Briefly, using the condensation process of 1 mmol o-
vanillin (3-methoxysalicylaldehyde) (0.152 g) with 0.5 mmol
(0.0511 g) 1,3-diaminopentane in (25 mL) of methanol for ca. 3 h
(Scheme 1). The yellow solution was used for complex 1 formation.
2.6. Synthesis of [Cd2(H2LpentOMe)(µ1,1-N3)2]n (1)
To a methanol solution (25 mL) of cadmium acetate dihydrate
(0.266 g, 1 mmol), a yellow solution of Schiff base ligand was slowly
added followed by sodium azide (0.065 g, 1 mmol) in minimum volume
of aqueous methanol with constant stirring for 2.5 h. 5–6 drops of DCM
were then added and the resulting mixture was additionally refluxed for
1 h at 75 °C. Finally, yellow filtrate was kept in refrigerator for crys-
tallization by slow evaporation. After 5 days light yellow coloured
single crystal suitable for X-ray crystallography was obtained. Crystals
were isolated by filtration and air-dried. Yield: 0.455 g, Anal. Calc. for
Scheme 2. Synthetic scheme for complex 1.
3

D. Majumdar, et al.
InorganicaChimicaActa496(2019)119069
Scheme 3. Similarities of ligand (H2LpentOMe) with other compartmental ligands.
3.2. Spectral characterizations
[58]. The particle size is in the diameter range of few microns. In terms
of Fig. 4 very good quality grain images for 1 are obtained until 1-2-
1 µm growth rate. SEM images grain size maximum at 1 µm but grains
of mixed complex structures are observed from 50 µm to 5 µm. In fact,
different and smallest grains complex structures are observed only SEM
image at 50 µm.
Di-nuclear complex formation was characterized by (ATR-FT-IR, FT-
Raman and UV–Vis) spectroscopic methods (Fig. S1, Fig. S2, Fig. S3).
The characteristic imines (C]N) stretching vibration Schiff base ligand
was found to be 1629 cm−1 [52]. The absence of the NeH stretching
band near at 3150 cm−1 positively confirmed the condensation of all
the primary amine groups [52]. Stretching band at 3449 cm−1 of Schiff
base is due to OeH stretching which completely disappeared in 1. In 1,
FT-IR and FT-Raman (C]N) stretching vibration bands are shifted to
1622 cm−1 (for IR) (Fig. S1) and 1632 cm−1 (for Raman) respectively
(Fig. S3). These spectroscopic data confirmed the coordination mode of
the imine nitrogen atom to the cadmium metal centre [55]. 1 exhibit
strong bands near at 2049, 2074 (for FT-IR) and 2085 cm−1 (for FT-
Raman) [56] attributable to ν(N3) binding (Fig. S1 and Fig. S3). Ad-
ditionally, aliphatic CeH stretching resonance for 1 observed near at
2965 cm−1 (for FT-IR) and 2987 cm−1 (for FT-Raman) respectively. Ar-
3.4. UV–Vis spectra
We examined at room temperature UV–Vis absorption spectra of
complex 1 in DCM solvent to get insights into the complexation of Cd
(II) on the absorption property of (H2LpentOMe). In DCM, 1 exhibits li-
gand–based transition at 353 nm presumably due to π → π* or n → π*
type of transitions [55,59]. Due to d10 electronic configuration of Cd(II)
metal ion, no metal centric d-d broad absorption band was expected in
the UV–Vis spectra (Fig. S2).
O
stretching frequency observed at 1213 cm−1 (for FT-IR) and
3.5. X-ray powder diffraction
1217 cm−1 (for FT-Raman) which is identical with previously reported
other salen ligands [57]. Therefore, IR and Raman spectroscopic in-
vestigations gives clear conclusive idea of azide ions bridging pro-
pensity. Moreover, IR spectral data of Cd2-nuclear metal complex were
thoroughly compared with previously reported Cd(II)-azido complexes
(Table S2).
The crystalline phase purity of azido-bridged Cd2-nuclear metal
complex was confirmed by X-ray powder diffraction pattern. X-ray
powder diffraction patterns for 1 was experimentally recorded after
considering (low theta scan 2θ) from 40 to 500. From Fig. S5, it was
clearly (experimental) observed that most of the major peak positions of
the bulk solid samples agree very well with the patterns simulated from
single X-ray crystal diffraction data (CIF for 1), generally obtained from
CCDC Mercury software consisting that single crystals and bulk mate-
rial for 1 are the same. It also further confirms phase purity of the bulk
crystal samples of Cd2-nuclear metal complex.
3.3. EDAX-SEM
The weight (%) contribution of the elements in Cd2-nuclear metal
complex was confirmed from EDAX analysis (Table S3) and the related
EDAX profile in Fig. S4. The calculated and EDAX values of complex 1
are nearly in good agreement. Therefore, the empirical formula of 1 is
formulated as C21H24N8O4Cd2. The EDAX spectrum shows the peaks of
highest for C and thereafter O, Cd which further supported the em-
pirical formula of 1. The important structural morphological features of
1 is also supported by SEM (scanning electron microscope) images
(Fig. 1). SEM images characterized size and morphological structure
[58].
4. X-ray crystal structure
4.1. Crystal structure of [Cd2(H2LpentOMe)(µ1,1-N3)2]n (1)
The solid-state structure of azido-bridged Cd(II) coordination
polymer has been confirmed by single X-ray crystal diffraction study.
Complex 1 crystal structure parameters are shown in Table 1 and some
important crystallometric parameters are summarized in Table S1. The
molecule crystallizes in monoclinic space group C2/c with Z = 8. The
SEM images are produced with different concentrations for complex
1 (Fig. 1) and images does not indicate the presence of well-defined
crystals free from any shadow of the metal ion on the surface. It is seen
clearly from SEM images that complex 1 exhibit platelet-like structures
asymmetric unit (Fig. 2) consists of one deprotonated Schiff base ligand
OMe
[Lpent
]
2−, two cadmium ions (Cd1 and Cd2) encapsulated in the two
Table 2
Compartmental ligands coordinating ability with Cd(II) metal ion.
Compartmental ligands
Ligand pocket used
N2O2 and O2O2
N2O2 and O2O2
Cd(II)-azido CP/complexes
µ1,3 & µ1,1-N3−1-D CP
Ref
1. [N, N’-bis(3-methoxysalicylidenimino)-1,3-diamino propane] [H2LOMe
]
35a
35c
35b
[N, N’-bis(3-ethoxysalicylidenimino)-1,3-damino propane] [H2LOEt
]
2. N, N’-bis(2-hydroxy-3-methoxybenzylidene)-propane-1,2-diamine [H2LOMe
N, N’-bis(2-hydroxy-3-methoxybenzylidene)-propane-1,2-diamine [H2LOEt
3. N, N’-bis(3-methoxysalicylaldimine)-1,3-diaminopentane [H2LpentOMe
]
µ1,3 & µ1,1-N3−1-D CP
µ1,1-N3−discrete
]
]
N2O2 and O2O2. Also O atom in -OMe group
µ1,1-N3−1-D CP
This work
4

D. Majumdar, et al.
InorganicaChimicaActa496(2019)119069
Fig. 1. SEM images of complex 1.
ligand pockets, and two small-sized azide ions coordinated to Cd1 and
Cd2 metal. Therefore, single crystal structure divulges two different
geometrical environment of cadmium metal centres. Structural en-
vironment of two cadmium metal centres were calculated from Addison
parameter Tau (ґ), (ґ =|β-α|/60° where β, α are the two largest angles
around cadmium metal atom; ґ =0 for a perfect square pyramidal and
1 for a perfect trigonal bipyramidal geometry) [60]. The Cd1 is co-
OMe 2−
ordinated to two deprotonated Schiff base ligand [Lpent
]
pheno-
late oxygens (O2 and O3), two imine nitrogens (N1 and N2), and one
azide nitrogen (N6). The N1, N2, O2, and O3 occupied equatorial po-
sitions and N6 occupied the apical position. As a result, the Cd1 adopted
distorted square pyramidal geometry [(ґ) value 0.31] (Fig. 3) [35]
where Cd1 is shifted towards the axial position by 0.825 Å. The Cd2 is
further coordinated to two phenolate oxygen (O2 and O3), two
methoxy oxygens (O1 and O4) in OMe group, three nitrogen atoms
from three azide co-ligands. The Cd2 adopted distorted pentagonal
bipyramidal (PBP) geometry [ (ґ) value 0.09] [60] (Fig. 3) where the
basal plane is occupied by O1, O2, O3, O4, N5 atoms, and axial posi-
tions are occupied by N5a and N6 atoms. Herein, Cd2 is shifted toward
the axial position by 0.337 Å. The displacement of Cd1 and Cd2 is anti
in nature from the basal plane (Fig. 3). The phenolate oxygens O2 and
O3 bridged Cd1 and Cd2 where distance is 3.531 Å and Cd1-O2/O3-
Cd2 angle is 104.2°. The Cd-Cd bond distance is totally comparable
with previously reported Cd(II)-CP [61]. The N6 atom of azide co-li-
gand coordinated to Cd1 is connecting Cd2 atom of another symmetry-
related molecule and N5 atom of another azide co-ligand coordinated to
Cd2 is connecting another Cd2 atom of another symmetry-related mo-
lecule. As a result, it generated a (one-dimensional)1-D chain (Fig. 4)
through coordination along the c axis. Herein, the azide ion act as µ2-
bridging nature and coordinated two cadmium metal centers. In the
solid-state, 1-D chains are hydrogen bonded to form three-dimension
(3-D) network (Fig. S6). The azide ion bridging modes in complex 1 has
been compared with previously reported Cd(II)-azido Schiff base com-
plexes (Table S5). Selected some important bond distances and angles
are shown in Table 3.
Fig. 2. Perspective view of asymmetric unit of complex 1.
4.2. FMO analysis
Complex 1 is optimized in the gas phase at B3LYP/TZVP level of
theory and the frontier orbital investigation reveal that HOMO has an
energy of around −5.227 eV and LUMO is around −1.793 eV in energy
Fig. 3. Complete geometry of complex 1.
5

D. Majumdar, et al.
InorganicaChimicaActa496(2019)119069
Fig. 4. 1-D chain through coordination in complex 1.
Table 3
Selected some important Bond Distances (Å) and Bond angles (⁰) for complex 1.
Complex Selected bond distances Value (Å) Selected bond angles Value (°)
(Å)
(°)
1
Cd1-N1
Cd1-N2
Cd1-O2
Cd1-O3
Cd1-N6
Cd2-N6
Cd2-O1
Cd2-O2
Cd2-N5
Cd2-O3
Cd2-O4
N5-Cd2
N6-Cd2
2.26(1)
2.27(1)
N1-Cd1-N2
O2-Cd1-O3
O2-Cd1-N6
O3-Cd1-N6
N1-Cd1-N6
N2-Cd1-N6
O1-Cd2-O2
O2-Cd2-O3
O3-Cd2-O4
O1-Cd2-O4
N5-Cd2-O4
N6-Cd2-N5
O1-Cd2-N6
O3-Cd2-N6
O4-Cd2-N6
O3-Cd2-N5
O4-Cd2-N5
O1-Cd2-N5
O2-Cd2-N5
89.5(4)
75.9(2)
119.9(3)
102.8(3)
125.5(4)
94.8(4)
62.3(2)
75.0(2)
64.9(2)
151.6(2)
103.8(3)
106.4(3)
76.3(3)
95.7(3)
84.1(3)
87.6(3)
83.0(3)
96.5(3)
86.9(3)
2.210(7)
2.241(8)
2.218(9)
2.290(8)
2.693(8)
2.265(7)
2.22(1)
2.234(7)
2.587(6)
2.367(9)
2.29(8)
(Fig. 5). The calculation of energy difference between the frontier or-
bitals shows an energy gap of 3.434 eV which suggests that the mole-
cule is low in chemical reactivity and is fairly stable. This is further
probed by calculating global reactivity descriptors which predict a si-
milar character for the molecule (Table S6). Further the frontier orbital
composition analysis is carried out using the Mulliken method to in-
vestigate the character of these orbitals. As it is evident from Fig. 5, the
HOMO of the cadmium complex is entirely concentrated on one of the
azide co-ligands with a total contribution of ca. 96.77% (π type). Fur-
ther a small contribution from one of the Cd metal centre is also seen
(2.367%, d type). In case of LUMO, there is sharp contrast and as can be
seen from the Fig. 5, it is entirely composed of the (H2LpentOMe) with the
metal centre and azide co-ligands having no share. The azomethine
group on either side of the metal centre has the highest contribution
with a total of 50.62% (π* type). This is followed by benzenoid type
rings on both the sides of the ligand with a total of 40.69% (π* type)
contribution.
Fig. 5. Contour plot of frontier molecular orbitals of 1 (along with the energy
gap).
(II) metal centres have a positive potential with the first one in the N2O2
cavity having a slightly less value (+11.7 kcal/mol) than the one in the
O4 cavity (+12.9 kcal/mol). This is well corroborated by the atomic
charge calculations carried out using the Mulliken method where the
former has a value of +0.941 while the later has +0.985. Both the
azide co-ligands possess the highest negative potential with a value of
−49.5 kcal/mol and −45.1 kcal/mol respectively. This along with the
position of HOMO of the molecule suggests that the azide moiety dic-
tates the chemical reactivity of the compound. When it comes to the
highest positive potential, it is found near the azomethine hydrogen
atom on both sides which is a common trait in these class of compounds
[62]. It is also worth mentioning that the entire spacer diimine group
4.3. Electrostatic potential analysis
exhibits
a
positive potential ranging from + 32.4 kcal/mol
to +38.8 kcal/mol.
MEP analysis is important in that respect that it is related to the
electron density and is a very useful descriptors to find out the sites of
electrophilic and nucleophilic [62]. The electrostatic potential plots
were drawn on electron density isosurface using the DFT optimised
structures (Fig. 6). These plots display electrophilic (blue colored) and
nucleophilic (red colored) areas in the molecular surface. Both the Cd
5. Solvatochromism
In order to evaluate solvent variation absorption and fluorescence
spectra, we have analysed ‘solvatochromism’ where Cd2-nuclear metal
6

D. Majumdar, et al.
InorganicaChimicaActa496(2019)119069
H2O(355 nm)
4
3
2
1
0
DMSO(369 nm)
CHCl3(357 nm)
DCM(357 nm)
ETHER(358 nm)
THF(359 nm)
CH3CN(353 nm)
CH3OH(353 nm)
300
325
350
375
400
425
Wavelength (nm)
Fig. 7. Solvent-dependent absorption spectra for 1.
Fig. 6. Electrostatic potential plot mapped over the electron density isosurface
of 1 (Computed at B3LYP/TZVP depicting the nucleophilic and electrophilic
regions).
complex was dissolved in different variety of solvents. Dielectric con-
stant and hydrogen bonding capacity are two important properties for
all solvents, hence solvent variation greatly affected ground and excited
of the complex so that size of energy gap between them changes as
polarity of solvent changes. Therefore, on passing from highly polar to
less polar solvent absorption and fluorescence spectrum position, in-
tensity and shape of the bands will be completely changed. It is worth to
mention here that recently solvent dependent fluorescence emission
observed for cadmium coordination polymer [63]. In addition, positive
and negative ‘solvatochromism’ reflected red (for polarity of solvent if
increased) and blue shift in the absorption and fluorescence emission
spectrum. In our present situation, the absorption and fluorescence
spectra for complex 1 was taken in solvents like DCM, DMSO, CHCl3,
H2O, Ether, THF, CH3CN, CH3OH etc. It is noteworthy to mention here
that on passing from less polar to highly polar solvent no remarkable
changes of absorption spectrum band position except in DCM solvent it
is more pronounced [63] whereas in highly protic solvent (H2O,
CH3CN, Ether, CH3OH) band positions are completely broad. In fact,
solvent effects exposed large differences on the electronic fluorescence
emission spectra between polar and non-polar solvents [63]. Herein we
observed presence of highly protic solvent (H2O) fluorescence emission
maxima at 503 nm (Fig. 8) and a red shift fluorescence emission in-
crease of about (λem 23 nm) but for other solvents these variations are
less important. Therefore, increasing solvent polarity fluorescence
emission maxima are largely red shifted compared to that of absorption
band (Fig. 7). Thus, in 1 effect of solvent polarity on the fluorescence
emission maxima was more pronounced than that on the absorption
maxima [64]. The above fact indicates an increase of dipole moment of
excited state compared to that of ground state. Further, on passing from
less polar to highly polar solvent fluorescence spectral bands are more
structurally organized compared to that of absorption spectra. There-
fore, complex 1 is solvatochromic in nature [64].
Fig. 8. Solvent-dependent fluorescence emission spectra for 1.
photo excitation at wavelength 367 nm shows fluorescence maxima
with the major emission peak near at 479 nm and 475 nm respectively
(Fig. 9). Hence in DCM solvent complex 1 is fluorescent active over free
Schiff base ligand [52]. The quenching of fluorescence behaviour under
solid-state upon L-M complexation can be well explained with the help
of magnetic perturbation, redox activity and electrons [66]. The higher
fluorescence intensity of 1 over Schiff base is probably due to effective
coordination of ligand donor centers to the cadmium metal ion, thereby
increased the conformational rigidity of molecular complex via chela-
tion enhanced fluorescence [CHEF] and subsequently loss of energy by
radiation less thermal vibration. The filled d10 configuration of Cd(II)
metal ion is difficult hard to oxidize or reduce. Such type fluorescence
behaviour may be assigned due to the intra-ligand (π → π*) transition
or L → M charge transfer [CHEF] [67–70] but for quenching ‘Chelation
enhancement of Quenching effect’ (CHEQ) [71]. The emission nature of
Schiff base and complex 1 is identical with previously reported salen-
type ligands [6,52] and Cd(II)-Schiff base complexes (Table S7). The
low quantum yield of ligand is due to fast Photoinduced electron
transfer (PET) from ligand side to the conjugated phenolic moiety. Thus
L-M complexation prevents PET process and thereby enhances quantum
yield (ɸ) of complex 1 [72]. Complex exhibit good photoluminescence
behaviour in DCM over solid-state since in DCM fluorescence beha-
viours are mainly ligand centered whereas in solid-state it is totally
packing dependent [73]. To get a better understanding on emitting
property, time-resolved fluorescence spectroscopic studies (excitation
at 367 nm) in DCM and solid-state have been undertaken and lifetime
decay profiles have been shown in Fig. S7-8. The decay profile in DCM
will be best fitted to bi-exponential nature (with acceptable comparable
6. Photoluminescence
d10 metal ion complexes fluorescence properties are extremely im-
portant due to their versatile application in chemical sensors, in pho-
tochemistry and also in electroluminescent displays [65]. At room
temperature photoluminescence property of 1 is monitored in DCM
solvent followed by solid-state (Table 4). In DCM and solid-state upon
7

D. Majumdar, et al.
InorganicaChimicaActa496(2019)119069
Table 4
Room temperature time-resolved photoluminescence decay.
Compounds
λex (nm)
λem (nm) (DCM)
λem (nm) (Solid-state)
Quantum Yield (ɸ)
Ref
1
367
356
479
472
475
–
τ1[ns]
0.82(98%)
0.00664
0.0047
τ2[ns]
This work
52
τ3[ns]
–
(H2LpentOMe)
Complex
1
λex[nm]
λ
em[nm]
< τ > [ns]
2.53
χ2
1.16
367
479
11.01(2.0%)
DCM solvent
Solid-state
367
472
4.51(24%)
10.6 (19%)
0.56(57%)
7.74
1.11
Fig. 9. Fluorescence emission spectra for 1 in DCM solvent and solid-state.
χ2 values 1.16) whereas solid-state it is tuned to tri-exponential nature
(with acceptable comparable χ2 values 1.11) with lifetimes in the range
(1.16–1.11 ns). However, in solid-state condition cadmium complex
exhibit smaller lifetime than DCM and strictly there is a well definite
trend observed in DCM solvent over solid-state (Table 4). Interestingly,
in DCM, a regular lifetimes trend observed having two different decay
times with maximum (98%) and minimum (2%) contribution in am-
plitude whereas in solid-state this trend is irregular maximum (57%),
(24%) and minimum (19%) contribution in amplitude. Therefore,
higher lifetime decay response one type of crystal packing and lower
lifetime decay prefers another type of packing [73]. The calculated
average lifetime value further divulges that importance of excited states
stabilities of complex 1 is maximum in DCM than solid-state condition.
7. Antimicrobial activity
We have analysed concentration dependent antimicrobial activity of
cadmium metal complex against two standard bacterial strains
Staphylococcus aureus (ATCC 25923) and Methicillin-resistant
Staphylococcus aureus (MRSA). Concentration dependent antimicrobial
assay (Fig. 10) directly confirms that complex 1 has excellent anti-
microbial efficiency against different microorganisms. 250 µg/ml con-
centration of cadmium complex is sufficient to show the antimicrobial
effect against Staphylococcus aureus (ATCC 25923) and Methicillin-
resistant Staphylococcus aureus (MRSA). The cellular growth was in-
hibited by more than 75% at 250 µg/ml concentration of complex 1
compared to control. Such antimicrobial efficiency of 1 was explained
by joint concepts of Overtone’s and Chelation theory [74]. Normally
chelation enhances the delocalization of sigma electrons over the entire
chelate ring of Schiff base ligand and reduces the polarity of cadmium
metal ion to a large extent. Thereby easy penetration of complex 1 is
facilitated into the lipid membrane and blocking the cadmium metal
Fig. 10. Concentration dependent antimicrobial activity of complex 1 against
Staphylococcus aureus (ATCC 25923) and Methicillin-resistant Staphylococcus
aureus (MRSA).
mechanism is already published by A. D. Hashmi et al. in favour of the
above mention discussion (Fig. 11) [74].
8. Conclusion
This paper carefully describes the synthesis and structural aspect of
OMe
one new azido-bridged 1-D coordination polymer [Cd2(H2Lpent
)
(µ1,1-N3)2]n (1) derived from less explored bi-compartmental Schiff
base ligand (H2LpentOMe). The successful synthesis of CP was only
achieved when Schiff base ligand properly utilizing their compart-
mental N2O2– and O4− donor sets. In the asymmetric unit, solid-state X-
binding sites in enzymes of microorganisms [74].
A plausible
8

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pentagonal bipyramid geometry around cadmium metal centres. ORCA
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explain FMO, MEP and global reactivity of the title complex. HOMO-
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microbial activity was investigated against two standard bacterial
strains (Staphylococcus aureus (ATCC 25923) and Methicillin-resistant
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This research work did not receive any specific grant from funding
agencies in the public, commercial or not-profit sectors. Dr. Dipankar
Mishra gratefully acknowledges the financial grant sanctioned by UGC,
New Delhi, in his favour vide Minor Research Project (F.PSW-232/15-
16(ERO). D. J. Majumdar is thankful to Prof. Dr. A. L. Spek (Emeritus),
Utrecht University, Bijvoet Center for Biomolecular Research,
Padualaan 8 3584 CH,Utrecht, The Netherlands, for providing fruitful
suggestions in connection of A-Level alert resolution under X-ray
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Appendix A. Supplementary data
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