Syntheses, X-ray crystal structures of two new Zn(II)-dicyanamide complexes derived from H2vanen-type compartmental ligands: Investigation of thermal, photoluminescence, in vitro cytotoxic effect and DFT-TDDFT studies

Article abstract of DOI:10.1016/j.ica.2019.04.041

Two new dicyanamide modulated zinc metal complexes [Zn₄(L^OMe)₂(μ₁-dca)₂(μ_1,5-dca)₂](1)and [Zn₃(L^OEt)₂(H₂O)(μ₁-dca)(μ_1,

Full text of DOI:10.1016/j.ica.2019.04.041

Accepted Manuscript
Research paper
Syntheses, X-ray crystal structures of two new Zn(II)-dicyanamide complexes
derived from H2vanen-type compartmental ligands: Investigation of thermal,
photoluminescence, in vitro cytotoxic effect and DFT-TDDFT studies
Dhrubajyoti Majumdar, Sourav Das, Renjith Thomas, Zakir Ullah, S.S. Sreejith,
Dhiraj Das, Pooja Shukla, Kalipada Bankura, Dipankar Mishra
PII:
S0020-1693(19)30295-6
https://doi.org/10.1016/j.ica.2019.04.041
ICA 18886
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
4 March 2019
19 April 2019
19 April 2019
Please cite this article as: D. Majumdar, S. Das, R. Thomas, Z. Ullah, S.S. Sreejith, D. Das, P. Shukla, K. Bankura,
D. Mishra, Syntheses, X-ray crystal structures of two new Zn(II)-dicyanamide complexes derived from H2vanen-
type compartmental ligands: Investigation of thermal, photoluminescence, in vitro cytotoxic effect and DFT-TDDFT
studies, Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.04.041
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1
Syntheses, X-ray crystal structures of two new Zn(II)-dicyanamide complexes derived from
H2vanen-type compartmental ligands: Investigation of thermal, photoluminescence, in vitro
cytotoxic effect and DFT-TDDFT studies
Dhrubajyoti Majumdar a,b, Sourav Das c, Renjith Thomas d, Zakir Ullah e, S. S. Sreejith f, Dhiraj
Das g, Pooja Shukla c, Kalipada Bankura a, Dipankar Mishra a,*
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 Chemistry; Institute of Infrastructure Technology Research and Management;
Near Khokhara Circle, Maninagar East, Ahmedabad-380026, Gujarat, India
d
Department of Chemistry, St Berchmans College (Autonomous), Changanassery, Kerala
686101, India
e
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST),
Daejeon 34141, Republic of Korea & Center for Catalytic Hydrocarbon Functionalization,
Institute for Basic Science (IBS), Daejeon 34141, Republic of Korea,
f
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi,
682022, Kerala, India
g Department of Chemical Sciences, IISER, Mohali, Manauli, PO 140306, India
AUTHOR EMAIL ADDRESS: dmishra.ic@gmail.com

2
ABSTRACT
Two new dicyanamide modulated zinc metal complexes [Zn4(LOMe)2(µ1-dca)2(µ1,5-dca)2] (1) and
[Zn3(LOEt)2(H2O)(µ1-dca)(µ1,5-dca)](2) have been synthesized using H2vanen-type
compartmental ligands. Schiff base ligands and the complexes were characterized by means of
elemental analyses, FT-IR, FT-Raman, UV-Visible, powder X-ray diffraction, TGA and
fluorescence spectroscopy. Dicyanamide modulated Zn4/Zn3-nuclear metal complexes were
structurally characterized by single crystal X-ray diffraction studies. In 1, the asymmetric Zn2-
nuclear unit was ensembled with one fully deprotonated Schiff base ligand [LOMe]2- along with
two dicyanamide ions where two structurally independent Zn(II) metal centers are found in the
X-ray crystal structure. Single X-ray crystal structure confirmed the environment of Zn1 is
distorted square pyramidal whereas Zn2 acquires distorted tetrahedral geometry. Unlike 1, in 2
three independent zinc metal centers have been identified as square pyramidal (Zn1), distorted
trigonal bipyramidal (Zn2) and distorted tetrahedral (Zn3). 1 and 2 geometry were optimized
using hybrid B3LYP functional with DGDZVP basis set to explain frontier molecular orbitals,
molecular electrostatic potential and Hirshfeld surface (dnorm surfaces and 2D fingerprint plots).
The electronic UV-Vis properties were determined by TD-DFT approach. The steady state and
time-resolved fluorescence properties have been explored in DMSO solution. 1 and 2 exhibit bi-
exponential decay and intra-ligand (π→π*) fluorescence behaviors with lifetimes in the range
(2.45-5.71 ns). In addition, complexes solid-state and different solvent-dependent absorption and
fluorescence spectra have been reported. Finally, the cytotoxic effect of the investigated
dicyanamide complexes against breast cancer cell line (MCF7) shows promising results which
makes them prospective complexes for anticancer medicament studies.

3
Keywords: Schiff base, Zn(II), Dicyanamide, DFT, Cytotoxic, Photoluminescence, TGA
1. Introduction
Ω-shape H2vanen-type compartmental ligands [1] on account of its tunable design [2] are
capable of forming complexes with different transition metal ions, which can exhibit high
thermodynamic stability, good photoluminescence and biological properties [3-8]. In this
juncture, coordination chemistry of d10 metal ions under the influence of identical ligands is
always flourishing and well-studied research area since such ligands have preparative
accessibilities, varied denticity, forming complexes with interesting molecular architectures and
topologies [9-19]. However, systematic investigation of common counter anion effect involving
-
pseudo-halide like dicyanamide anion [N(CN)2 ], coordination chemistry of group 12 metal ions
with salen-type motifs is still unveiled of research. Salen-type complexes are employed as
catalysts for organic reactions, supramolecular building blocks, nonlinear optical materials,
interesting magnetic properties etc. [20-32]. Unlike other metals, zinc function as innumerable
important role in chemical and biological field e.g. constructive active site of many hydrolytic
enzymes, act as Lewis acid catalysts [33a-b], immune system, as a vital cofactor in many
biological processes [33c-d]. Pseudo-halides and conjugated Schiff base platforms having
interesting binding properties with zinc or other transition metal complexes always exhibit
significant bioactivity owing to the presence of azomethine linkage, heteroatoms and presence of
active metal centers [33e]. Moreover, research of cytotoxicity against breast cancer cell lines of
Zn(II)-Schiff base complexes have irrefutably been growing exponentially because of their
proven effectiveness as attractive frontrunner complex structures [34, 35]. This enhanced
bioactivity may be explained with the help of ‘Overton’s concept’ or ‘Chelation theory’ [36].
During the last few years, we have been exploring the photoluminescence, DFT, thermal, in vitro

4
antibacterial and anti-biofilm properties of Zn(II)/Cd(II) metal complexes derived from N,O-rich
ortho vanillin Schiff base platform especially in presence of pseudo-halide anions [SCN]-/[N3]-
-
/[N(CN)2 ].Therefore, the only outcome of previous works is that common pseudo-halides linked
metal complexes exhibit strong photoluminescence, antimicrobial and anti-biofilm properties
[37a-h].To search further photoluminescence properties in solid-state as well as different solvent-
dependent absorption and fluorescence spectra, complexation was carried out with Zn(II) metal
ion and H2vanen-type ligands in presence of dicyanamide anion.
The unique d10 candid nature, zero CFSE value always favor zinc metal ion to attain different
molecular architectures [38a-f]. Hence the key factors for syntheses of zinc metal complexes are
always careful selection of organic ligands along with effect of pseudo-halide anions such as
dicyanamide or other common pseudo-halides [39]. Dicyanamide, a larger pseudo-halide ion-rod
is under active consideration of complexation since it exhibits versatile coordination motifs with
different metal ions [40-44]. Till date different dimensional molecular architectures and
topologies of dicyanamide pseudo-halide involving complexes have been emerged in literature
[45-60]. The µ1,5-bridging mode of dicyanamide is always expected but terminal coordination
mode is rare due to its long chain length and negative charge delocalization over the symmetrical
molecule [61]. In our dicyanamide complexes, a rare combination of terminal as well as µ1,5-
bridging mode have been observed.
Herein, we report syntheses, X-ray crystal structures, DFT/TDDFT, cytotoxic effect, different
solvent-dependent absorption and fluorescence spectra, steady state and time-resolved
fluorescence properties of Zn4/Zn3-nuclear metal complexes viz., [Zn4(LOMe)2(µ1-dca)2(µ1,5-
dca)2] (1) and [Zn3(LOEt)2(H2O)(µ1-dca)(µ1,5-dca)] (2) respectively.

5
2. Experimental section
2.1. Materials and physical measurements
All the research chemicals were of analytical grade and used as purchased without further
purification. All synthetic reactions were done in open air. Elemental analyses were performed
on a Perkin-Elmer 2400 elemental analyzer. IR and Raman spectra were recorded as KBr pellets
(4000–400 cm-1) by using Perkin–Elmer spectrum RX 1 and BRUKER RFS 27 (4000-50 cm-1).
UV-Visible spectra (200-1100 nm) were determined by using Hitachi model U-3501
spectrophotometer. Fluorescence spectra in DMSO solvent (spectroscopic grade) were measured
by using Perkin-Elmer LS50B Spectrofluorometer model at room temperature (298K).
Fluorescence lifetime measurements were recorded by using JOBIN-VYON M/S Fluorimeter.
Thermo-gravimetric analyses (TGA) 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
environment of nitrogen atmosphere using a platinum cell. Powder diffraction measurements
were carried out using BRUKER AXS, GERMANY X-ray diffractometer model using radiation
Cu Kα-1. Using equation (1), quantum yield (Φ) for dicyanamide bridged Zn(II)-complexes have
been determined where quinine sulfate is preferentially used as the secondary standard (Φ = 0.57
in water) [62]
S AS (Abs)R nS2



-----------------------------------------------------------------------------------(1)
R AR (Abs)S nR2
According to equation (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 parameters for the studied sample and reference respectively.

6
2.2. X-ray crystallography
Crystal data of dicyanamide complexes were collected on a Bruker SMART CCD [63]
diffractometer using Mo Kα radiation at λ = 0.71073 Å. For collection of crystal data different
common popular programs were operated e.g. SMART program used for collecting frames of
data, indexing reflections, and determining lattice parameters, SAINT [64] for integration of the
intensity of reflections and scaling, SADAB [65] for absorption correction, and popular
SHELXTL for space group and structure determination and least-squares refinements on F2. The
crystal structure of 1-2 were fully solved and refined by full-matrix least-squares methods
against F2 by using the common program SHELXL-2014 [66] and Olex-2 software [67]. It is
worth mentioning that the Alert Level A in the checkCIF of 2 is related only to the Rint value is
greater than 0.25 since examined single crystal was a small-sized, brittle and weakly diffracting.
All the non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen
positions were fixed at calculated positions and refined isotropically. Different crystallographic
diagrams were constructed for two complexes by using latest Diamond software [68]. The
crystallographic data and full crystal structure refinement parameters for 1-2 are submitted in
Table 1. Crystallographic data (excluding structure factors) of both complexes have been
deposited with the Cambridge Crystallographic Data Centre as supplementary publication
numbers CCDC 1887056-1887057. Copies of the data can be obtained, free of charge, on
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.cam.ac.uk, or fax: +44 1223 336033.

7
Table 1 Crystal data and full structure refinement parameters of 1 and 2
Formula
M/g
C44H36N16O8Zn4
1178.45
C88H88N20O19Zn6
2122.00
Crystal system
Triclinic
Monoclinic
Space group
P-1
C2/c
a/Å
9.626(4)
27.467(3)
b/Å
c/Å
α (°)
β (°)
γ (°)
V/Å3
11.345(4)
108.821(4)
108.821(4)
94.207(5)
101.057(5)
1294.7(9)
17.906(2)
19.190(2)
90.00
95.518(7)
90.00
9394.4(18)
Z
2
4
ρc/g cm-3
1.511
1.500
μ/mm-1
1.896
1.585
F(000)
Cryst size (mm3)
596
4352
0.2× 0.2 × 0.1
0.280× 0.220 × 0.180
θ range (deg)
0.963
0.988
Limiting indices
-12 ≤ h ≤ 13
-15 ≤ k ≤ 15
-17≤ l ≤ 17
-32≤ h ≤ 32
-21≤ k ≤ 19
-22≤ l ≤ 22
Reflns collected
Ind reflns
39885
6706 [Rint = 0.0756, Rsigma
0.0594]
48010
8002 [Rint = 0.5605, Rsigma
0.3608]
=
=
Completeness to θ (%)
0.963
0.998
Refinement method
Full-matrix-block least-
Full-matrix-block least-squares on
squares on F2
F2
Data/restraints/
parameters
6706/657/327
8002/0/604
Goodness-of-fit on F2
Final R indices
[I > 2θ(I)]
1.002
R1 = 0.0738
wR2 = 0.2110
0.926
R1 = 0.0995
wR2 = 0.1127
R indices (all data)
R1 = 0.1516
R1 = 0.2674
wR2 = 0.2666
wR2 = 0.1435
Largest diff. peak and
hole(e·Å-3)
1.324 and -0.590
0.091 and -1.002

8
2.3. Computational methodologies
All DFT calculations are performed on the optimized geometry using hybrid B3LYP functional
with DGDZVP basis set. Frequency calculations are performed for the absence of imaginary
frequencies, such that a geometry minimum is obtained. All calculations were performed using
Gaussian 09W software [69] and visualized using Gauss View 5.0 [70]. Hirshfeld surface
analysis and 2D fingerprint plots were generated using Crystal Explorer 17 via cif files obtained
as a result of single crystal X-ray diffraction. The Hirshfeld surface were calculated with DFT
using B3LYP functional and 6-31G(d) basis set using the TONTO Abinitio and DFT code fixed
within the Crystal Explorer 17 [71]. The TD-DFT calculations for (1-2) are performed using
CAM-B3LYP functional using DGDZVP basis set with methanol as solvent using the PCM
model to incorporate the solvent effects.
2.4. Cell culture and growth inhibition
Breast cancer cell line MCF7 was grown in DMEM (Dulbecco’s Modified Eagle Medium) as
monolayer supplemented with 10 % FBS (Foetal Bovine Serum), 100 U/ml penicillin and 100
µg/ml streptomycin at 37°C in a humid, 5% CO2 atmosphere. The cytotoxic effect of Zn(II)-
dicyanamide complexes are evaluated by MTT (3-(4, 5 dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide) assay. Cells (5 X 103 per well of 96 well plate) were seeded 24
hours prior to the treatment. The cells were treated with different concentrations of the
compounds (0 – 100 μM) for 48 hours in triplicate. After 48 hours of drug treatment, MTT
solution (20 μM of 5 mg/ml stock for each well) was added to the media and cells were further
allowed to be incubated for 4 hours in humid 5% CO2 incubator. Then, media containing MTT
solution was replaced by MTT solvent (isopropanol, 4 mM HCl and 0.01% Triton X-100) and

9
incubated for 15 min at room temperature with gentle rocking to ensure the complete dissolution
of Formazan. Finally, the absorbance was measured at 590 nm using a Thermo Pierce Elisa plate
reader. All experiments were carried out at least three biological triplicates. The percentage of
viable cells was calculated in comparison with the growth of vehicle treated cells and it was
taking as 100%.
2.5. Syntheses of Schiff base ligands
Schiff base ligands were synthesized by the common condensation process of 1 mmol (0.152 g)
3-methoxysalicylaldehyde or (0.1662 g) 3-ethoxysalicylaldehyde with 0.5 mmol (0.0301 g)
ethylenediamine (en) in (30 mL) of methanol for ca. 2 h (Scheme 1). The orange yellow solution
was used for dicyanamide complex formation.
Scheme 1. Synthetic Scheme for H2vanen-type ligands

10
2.6. Synthesis of [Zn4(LOMe)2(µ1-dca)2(µ1,5-dca)2] (1)
To the methanolic solution (20 mL) of zinc acetate dihydrate (0.2195 g, 1 mmol), yellow colored
solution of Schiff base ligand (H2LOMe) was added directly followed by mixing an aqueous
methanolic solution (5 mL) of sodium dicyanamide (0.0891 g, 1 mmol). The overall reaction
mixture was then refluxed for 30 min at 60 0C. Then few drops of Dichloromethane (DCM) were
added. Solution was cooled at room temperature and constant stirring continued for 3 h. Finally,
the light yellow filtrate was kept for crystallization by slow evaporation at room temperature.
After 15 days yellow colored single crystal suitable for X-ray crystallography was obtained.
Crystals were isolated by filtration and air dried. Yield: 0.467g, Anal. Calc. for C44H36N16O8Zn4:
C, 44.85; H, 3.08; N, 19.02. Found: C, 44.78; H, 3.00; N, 19.00 %. IR (KBr cm-1) selected bands:
ν(C≡N), 2286 m, 2220 m, 2170 vs, ν(C꞊N), 1643 vs, FT-Raman (cm-1) selected bands: ν(C꞊N),
1640 vs, ν(C≡N), 2276 m, 2208 s, 2128 s, UV-Vis λmax (DMSO): 285 nm, 363 nm.
2.7. Synthesis of [Zn3(LOEt)2(H2O)(µ1-dca)(µ1,5-dca)] (2)
Complex 2 was prepared by adopting the similar procedure as 1 except Schiff base solution used
is (H2LOEt). Yield: 0.459g, Anal. Calc. for C88H88N20O19Zn6: C, 49.81; H, 4.18; N, 13.20 Found:
C, 49.75; H, 4.13; N, 13.14 %. IR (KBr cm-1) selected bands: ν(C≡N), 2280 m, 2217 m, 2180 vs,
ν(C꞊N), 1638 vs, FT-Raman (cm-1) selected bands: ν(C꞊N), 1654 vs, ν(C≡N), 2281 s, 2227 m,
2172 w, UV-Vis λmax (DMSO): 279 nm, 368 nm.
3. Results and discussion
3.1. Syntheses
Schiff base ligands were synthesized by the condensation of ethylenediamine (en) with 3-
methoxysalicyldehyde or 3-ethoxysalicyldehyde in MeOH at 1:2 molar ratio [72]. Complexes (1-
2), derived from two identical ligands were prepared in good yields by taking the following

11
procedure where 1:1:1 molar ratio of zinc acetate dihydrate, Schiff base ligands (H2LOMe/H2LOEt
)
and sodium dicyanamide in methanolic solution under stirred condition (Scheme 2). The
versatile binding ability of H2vanen-type ligands and dicyanamide anions with metal ions (M2+)
are presented in Scheme S1-S2. The crystal complexes were isolated only from slow evaporation
of mixed solvent (CH3OH-DCM) medium at room temperature. H2vanen-type ligands comprises
two imines, two phenols and methoxy or ethoxy groups. After de-protonation action of [LOMe 2-
]
/[LOEt]2- the N2O2 imine-based chelating site of ligand has the novel property to bind perfectly
with several transition metal ions whereas pseudo-halide preferentially dicyanamide anions
assisted complexation with zinc metal ion is still now unveiled of research. Such compartmental
ligands have additional coordination moiety (N2O2) to a transition metal ion that reorganizes the
ligand in a Ω-shape generating a second potential recognition site O2O2 which are further able to
accept several metal ions [1] (Scheme 3). Air stable yellow coloured neutral zinc metal
complexes are successfully characterized by elemental analyses, UV-Vis, FT-IR, FT-Raman,
powder X-ray diffraction, TGA, X-ray crystallography and fluorescence spectroscopy.
According to Scheme 2, two Schiff base ligands are behaving as ‘pendant’ character on account
of its additional donor groups attached to its periphery. In modern coordination chemistry, such
pendant ligands are used in a variety of different chemical application. Till date Mitra et al.
reported different salen-type ligands involving Zn(II)-dicyanamide complexes [73]. To the best
of our knowledge H2vanen-type ligands have never been utilized to synthesize neutral
dicyanamide modulated Zn4/Zn3-nuclear metal complexes (1 or 2).

12
Scheme 2: Synthetic scheme for 1 and 2
Scheme 3: H2vanen-type ligand showing Ω-shape of O2O2 potential site

13
3.2. Spectral characterizations
H2vanen-type Schiff base formations are well characterized by IR, and UV-Vis spectroscopic
studies (Fig.S1, Fig.S2). The characteristic imines ν(C=N) of two symmetric ligands are found to
be 1633, 1631 cm-1 respectively [74a-b] (Fig.S1). In complexes (1-2), these stretching vibration
bands are shifted to 1643, 1638 cm-1 [74c] (Fig.S1). Pseudo-halide dicyanamide anion [N(CN)2]-
showed three sharp strong characteristic band preferably in the region 2300-2170 cm-1 [75]. Two
medium intensity bands (at 2286, 2220 cm-1 in 1 and 2280, 2217 cm-1 in 2) and effective strong
intensity band (at 2170 cm-1 in 1 and 2180 cm-1 in 2) are indicative of the presence of
dicyanamide in these two complexes. In this context, IR spectra with other reported Zn(II)-
dicyanamide complexes were compared in Table 2 [76]. FT-Raman spectral data of some
reported Zn(II)-dicyanamide complexes were further consider to established the bridging
propensity of pseudo-halide [N(CN)2]- anions (Table 3) [77]. Aliphatic C-H stretching bands for
dicyanamide complexes were observed within the range 2926-2828 cm-1and 2901-2870 cm-1
[37g]. In dicyanamide complexes, Ar-O stretching frequencies observed near at 1243-1217 cm-1
and 1242-1217 cm-1 which is similar to the other reported salen-type ligands [78].
Table 2 Comparison of asymmetric stretching frequency [FT-IR], νas(dca), of the dicyanamide
anion with other reported dicyanamide complexes
Complexes
ν(dca) cm-1
Ref
νas+νs(C≡N)
νas(C≡N)
νs(C≡N)
[Zn4(LOMe)2(µ1-dca)2(µ1,5-dca)2]
2286
2220
2170
This work
This work
[Zn3(LOEt)2(H2O)(µ1-dca)(µ1,5-dca)]
2280
2217
2232
2180
2179
Free [N(CN)2]- anion
[Zn2(L)(µ1,5-dca)2]n
2300-2170
2370
a
b
2345-2323 2253-2190

14
{Zn2(L1)(µ1,5-dca)2dca.CH3OH}2
[Zn2(L2)(µ1,5-dca)dca]n
2357-2345
2371-2345
2379-2360
2292
2295
2244
2277
2226
-
2223
2134
2213
2173
2193
2188
2134
964
73
73
73
c
[Zn3(L3 )(µ1,5-dca)2]n
2
[(HL2)Zn2(L2)(dca)2]ClO4
{Zn2(L1)(µ1,5-dca)2dca](H2O)}n
{[Zn2(L)(µ1,5-dca)](ClO4)}n
{[Na(CuIIL3)2](μ1,5-dca)}n
[Zn(dca)2]
2345
2377
2244
39
-
2189
1417-1408
2232
d
e
f
2298-2209
2286
[Cu2(µ1,5-dca)(µ-L)2ClO4]n
2179
[Zn(LH)(OAc)(dca)]
[Cd(LH)(OAc)(dca)]
[CuL1(µ1,5-dca)]n
2170
2157
2343-2123
-
-
-
-
-
-
g
h
i
[CuL2(µ1,5-dca)]n
[CuL(µ1,5-dca)n].nH2O
2293
2240
2174
[ZnL(dca)]n
[BN(CN)2]
[(Cu(II)L)2M(II)( µ1,5-dca)2]n
2380-2230
-
-
j
k
l
-
-
2287
2288
2170
2165
Table 3 Raman spectra of different stretching vibration ν(CN) bands for M(II)-dicyanamide
complexes
M(II)-
Different Raman spectral bands [cm-1]
Ref
dicyanamide
complexes
νs(C≡N)
νas(C≡ νs(CNC) δ(CNC)/ ν(MN)amide ν(MNCN)
N)
δ(NCN)
KN(CN)2
2219
2209
2260
2195
982
963
972
669
546
682
523
746
683
690
533
665
533
670
-
-
a
a
a
a
a
a
Fe{[N(CN)2]2}
Co{[N(CN)2]2}
Ni{[N(CN)2]2}
Cu{[N(CN)2]2}
Zn{[N(CN)2]2}
2194
2176
2198
291
300
317
318
333
256
213
266
236
2265
2276
2207
2185
2174
1045
980
950
288
2294
2272
2289
310
278
238
206
2217
2190
980
ν(dca) cm-1
νas(C≡N
νas+νs(C≡N)
νs(C≡N)
-

15
[Cu(dca)2]
2272
2174
2168
2294
2105
-
-
b
c
Li{N(CN)2]
2236
2246
2288
2276
{LiCs2[dca]3}
[Zn4(LOMe)2(µ1-dca)2(µ1,5-dca)2]
2133
2208
-
-
c
2128
This
wor
k
This
wor
k
[Zn3(LOEt)2(H2O)(µ1-dca)(µ1,5-
2281
2227
2172
dca)]
3.3. UV-Vis spectra
The UV-Vis spectroscopic study of Schiff base ligands along with the dicyanamide complexes
was done in the wavelength region (200-1100 nm) using spectroscopic grade DMSO as solvent
(Fig.S2). Free Schiff base ligands exhibit bands at 332 nm and 336 nm which are assigned to
π→π* and n→π* transitions. Dicyanamide complexes exhibit ligand–based transition at 285 nm,
363 nm (for 1) and 279 nm, 368 nm (for 2) respectively due to π→π* or n→π* type of transitions
[79, 80]. No characteristic broad d-d absorption band was assigned in the UV-Vis spectra of
either complex 1 or 2.
3.4. X-ray powder diffraction studies
The crystalline phase purity of two dicyanamide complexes was confirmed by X-ray powder
diffraction pattern. The X-ray powder diffraction patterns for both dicyanamide complexes were
recorded experimentally after considering 2θ scan from 40 to 500. As clearly observed in the
experimental PXRD patterns (Fig.S4), most of the major peak positions of the bulk solids of both
dicyanamide complexes agree very well with the patterns simulated from single-crystal
diffraction data (CIF for 1 & 2) obtained from CCDC Mercury software consisting that single
crystals and bulk material are the same. It also further confirms phase purity of the bulk crystal
sample.

16
4. Description of X-ray crystal structure
4.1. Crystal structure of [Zn4(LOMe)2(µ1-dca)2(µ1,5-dca) 2] (1)
Single crystal X-ray analysis reveals that complex 1 crystallizes in triclinic space group P-1 (Z =
2). As depicted in Fig.1, the asymmetric unit comprises one half the overall molecule, i.e
[Zn2(LOMe)(dca)2]. The selected important bond angles (⁰) and bond length (Å) are given in Table
S1.
Fig.1. Perspective view of asymmetric unit of 1 where hydrogen atoms are omitted for clarity
Asymmetric di-nuclear unit was ensembled with one fully deprotonated ligand [LOMe]2- along
with two cyanamide ions. In the formation of di-nuclear assembly a fully deprotonated ligand
grasps two Zn(II) ions in µ2-η2:η1:η1:η2 mode (Fig.2). In this binding mode, the fully

17
deprotonated hexadentate ligand utilizes only four coordination sites i.e. two phenolate oxygen
atoms (O1, O2) and two imine nitrogen atoms (N1, N2). Interestingly, tetradentate pocket (O2N2)
has been used to accommodate Zn(1) metal center. On contrary, tetradentate pocket (O4)
accommodate Zn(2) metal center where only two phenolate µ2-oxygen atoms have been utilized
in binding of Zn(2) metal center. Coordination number of Zn(2) metal center is fulfilled by
binding of two dca anions. One of the dca anion shows µ1 coordination mode acts as a terminal
ligand. On the other hand, another dca anion shows µ1,5 coordination mode (Fig.2) to connect
Zn(2) and Zn(1) centers of subunits. So, this bridging coordination mode has connected the two
di-nuclear asymmetric units to form overall tetranuclear assembly (Fig.3). The tetranuclear unit
possesses a 16-member macrocycle ring (Zn4O4N8C2) between two connected di-nuclear motifs
through µ1,5 coordination mode of dicyanamide anion (Fig.4).
N
N
N
N
C
C
Zn
Zn
C
C
N
N
O
O
N
N
O
O
Zn
Zn
Zn
μ2 – η2: η1: η1: η2
μ1
μ1, 5
Fig.2. Binding mode of the ligand [LOMe]2– and anionic dca in 1

18
Fig.3. Tetranuclear assembly of complex 1
Fig.4. 16-member macrocycle ring (Zn4O4N8C2) between two connected di-nuclear motifs

19
Two structurally independent Zn(II) centers are found in the single X-ray crystal structure. Zn(1)
displays 5-coordination geometry with distorted square pyramidal shape with a ґ value 0.16 [81]
(ideal value of tau (ґ) for square pyramidal geometry is 0) which is consists of one apical N atom
from bridging dicyanamide anion and basal N2O2 donors from fully deprotonated ligand (Fig.5).
Zn(2) shows four coordination in a distorted tetrahedral geometry (Fig.5) which is composed by
two N atoms from dca anion, two phenolate oxygen atoms from deprotonated coordinating
ligand [LOMe]2-.
Fig.5. Coordination geometry around Zn centers (a) Zn1 acquires a distorted square pyramidal
shape (b) Zn2 acquires distorted tetrahedral shape
The bond lengths of metal coordinated phenoxy atom are of range 1.984- 2.040 Å while the
metal coordinated nitrogen atoms are of range 1.935- 2.062Å. The bond angle Zn1-O2-Zn2 and
Zn1-O1-Zn2 are 101.8° and 99.66° respectively (Fig.3). The distance between Zn 1 and Zn 2
metal centers is 3.100 Å that is totally comparable to other Zn—Zn separation of double
phenoxo-bridged Zn2 complexes [82a-b]. The bond distances in 1 are totally comparable to other
reported Zn(II)-dicyanamide complexes (Table 4) [76]. Complex 1 shows 2D supramolecular
assembly formation owing to the presence of N--H secondary interactions in the solid-state

20
structure (Fig.S5). The formation of 2-D polymeric structure is felicitated by N4 and N5 of
terminal dca anion bonding with C-H groups (Fig.S5).
4.2. Crystal structure of [Zn3(LOEt)2(H2O)(µ1-dca)(µ1,5-dca)] (2)
Single X-ray data analysis reveals that the compound crystallizes in the monoclinic system in the
C2/c space group (Z=4). The structure of complex 2 is detailed in Fig.6. The selected bond
distances (Å) and angles (⁰) are given in Table S1.
Fig.6. Molecular structure of 2 where hydrogen atoms are omitted for clarity
The crystal structure of complex 2 consists of a neutral and trinuclear unit [Zn3(LOEt)2(H2O)(µ1-
dca)(µ1,5-dca)] with a water of crystallization. The homometallic trinuclear complex 2 was
assembled with the help of two fully deprotonated ligands [LOEt]2-. As discussed in previous
structure, the fully deprotonated hexadentate ligand utilizes tetra cordination to accommodate the

21
Zn(II) ions. Two ligands has been utilzed in the complex formation which has shown different
coordination behaviour. One of the deprotonated ligand shows coordination behaviour in µ2-
η2:η1:η1:η2 mode (Fig.2) which accommodate two Zn(II) centers where phenolate oxygen atom
functions as a bridging ligand to form a four-membered Zn2O2 motif. On the other hand, other
deprotonated ligand display µ1-η1:η1:η1:η1 coordination mode (Fig.7) to accommodate only one
Zn(II) center. Futhermore, one of the dca anion shows µ1,5 coordination mode (Fig.2) to connect
Zn(3) and Zn(1) centers of different subunits to construct the trinuclear assembly. Another dca
anion displays µ1 coordination mode to coordinate the Zn(3) metal center in order to fulfil charge
and coordination number. Besides, coordination number of Zn(2) metal center is fulfilled by
coordination of water molecule.
N
O
N
N
N
O
Zn
Zn
Zn
O
O
O
O
O
O
µ2-η2: η1: η1: η2
µ2-η1: η1: η1: η1
Fig.7. Binding mode of the ligand [LOEt]2– in 2
The zinc atom in mono-nuclear moiety is five-coordinated with square pyramidal geometry. The
Zn2 atom in di-nuclear moiety is also five-coordinated with distorted trigonal bipyramidal
geometry. Here, Zn3 atom present shows distorted tetrahedral structure being four-coordinated
(Fig.8). In the vicinity of all the Zn atoms both oxygen and nitrogen coordination geometry are
found. The bond length of metal coordinated phenolate oxygen is in the range of 1.978 - 2.732
Å. The Zn-N bond lengths are of range 1.922 – 2.090 Å. The bond angle of Zn2-O5phenoxy-Zn3

22
and Zn2-O7phenoxy-Zn3 are 99.4 and 99.2º respectively. The distance between Zn 2 and Zn 3
metal centers is 3.082 Å that is comparable to other Zn—Zn separation of double phenoxo-
bridged Zn2 complexes [82a-b]. The bond distances of 1 and 2 are totally comparable to other
reported Zn(II)-dicyanamide Schiff base complexes (Table 4) [76]. The complex 2 displays rich
supramolecular interaction through C-H…N and C-H…Π interaction to generate a 2D assembly
(Figure S6). The structural aspect, architectures and bridging propensity of dicyanamide anion
involving complex 1 & 2 has been compared with different dicyanamide complexes coordinated
to closely identical salen-type ligands (Table 5) [83]. Complex 1 & 2, some selected important
bond distances (Å) and angles (°) values are shown in Table 6.
Fig. 8 Local coordination assembly of (a) Zn1 having square pyramidal (b) Zn2 having distorted
trigonal bipyramidal (c) Zn3 having distorted tetrahedral

23
Table 4 Bond distances (Å) comparison of Zn(II)-dicyanamide complexes with other reported
dicyanamide complexes
Complexes
Bond distances (Å)
1.943-2.043
1.922-2.080
1.220-1.280
1.986-2.074
2.036-2.073
1.988-2.080
2.005-2.119
1.954-2.094
1.984-2.068
2.102-2.180
1.980-2.644
1.980-2450
1.970-1.988
1.907-1.987
1.987-2.346
1.948-2.463
1.958-2.373
1.973-2.258
2.046-2.154
2.002-2.207
2.017-2.064
Ref
[Zn4(LOMe)2(µ1-dca)2(µ1,5-dca)2]
[Zn3(LOEt)2(H2O)(µ1-dca)(µ1,5-dca)]
Free [N(CN)2]- anion
This work
This work
a
[Zn2(L)(µ1,5-dca)2]n
b
{Zn2(L1)(µ1,5-dca)2dca.CH3OH}
[Zn2(L2)(µ1,5-dca)2dca]n
73
73
73
c
[Zn3(L3 )(µ1,5-dca)2]n
2
[(HL2)Zn2(L2)(dca)2]ClO4
{Zn2(L1)(µ1,5-dca)2dca](H2O)}n
{[Zn2(L)(µ1,5-dca)](ClO4)}n
{[Na(Cu(II)(L3)2](μ1,5-dca)}n
[Zn(dca)2]
39
d
e
f
[Cu2(µ1,5-dca)(µ-L)2ClO4]n
[Zn(LH)(OAc)(dca)]
[Cd(LH)(OAc)(dca)]
[CuL1(µ1,5-dca)]n
g
h
[CuL2(µ1,5-dca)]n
[CuL(µ1,5-dca)n].nH2O
[ZnL(dca)]n
i
j
[BN(CN)2]
k
l
[(Cu(II)L)2M(II)( µ1,5-dca)2]n

24
Table 5 Structurally characterization of M(II)-dicyanamide Schiff base complexes of the anionic
forms of present investigated ligand (H2LOMe/H2LOEt) with closely resemblance ligand (H2LOMe1
)
Complexes
Closely identical
salen-type ligands
Present
investigated nature of
salen-type
ligands
Bridging
Complex
nature
Ref
(H2LOMe1/H2LOEt
)
[N(CN)2]-
anion
anionic donor forms
[Zn4(LOMe)2(µ1-dca)2(µ1,5-
dca)2]
-
In 1,
diphenoxido
µ1,5 as
well as
Zn4/Zn3-
nuclear
This
work
N(iminne)2O( terminal
metal dca
phenoxo)2
Zn(II) metal
occupation.
-OMe also
bridging complexes
of dca
observed.
Two
coordinated complexes
[Zn3(LOEt)2(H2O)(µ1-dca)(µ1,5-
-
with Zn(II)
are neutral
This
dca)]
work
For 2, water
is
additionally
coordinated
with zinc
metal. -OEt
also
coordinated
with Zn(II)
-
1. (i) [Cu(II)Zn(II)(dca)2(µ1,5
dca)2]
(i)Diphenoxido
nuclear
N(iminne)2O(phenoxo)2
di-
moiety.
µ1,5-dca
µ1,5-dca
Hetero-
metallic
di-nuclear
a
(ii)[{Cu(II)LCd(II)(dca)}{Cu(I Cu(II)
I)LCd(II)}{Cu(II)(MeCN)LCd occupation.
(II)( µ1,5-dca)]n
metal
1D
polymer
(ii) Diphenoxido poly-
nuclear moiety,
O(phenoxo)2O(ethoxy)
2 Zn(II) or Cd(II) metal
occupation.
2. [Zn2(L)(µ1,5-dca)2]n
Diphenoxido
-
-
µ1,5-dca
µ1,5-dca
2D
hexagonal
helical
76b
73
moiety.
N(iminne)2O(phenoxo)2
Zn(II) metal
chains
occupation.
3.{Zn2(L1)(µ1,5dca)2dca.CH3O
H}
Diphenoxido
moiety.
Discrete,
1D

25
N(iminne)2O(phenoxo)2
Zn(II) metal
polymer
and 1D
[Zn2(L2)(µ1,5-dca)2dca]n
occupation.
loop like
polymer
[Zn3(L3 )(µ1,5-dca)2]n
2
4. [(HL2)Zn2(L2)(dca)2]ClO4
Diphenoxido
moiety.
-
µ1,1-dca
Di-nuclear
76c
N(iminne)2O(phenoxo)2
along with O(Methoxy)
Zn(II) metal
occupation.
Phenoxido
5.{Zn2(L1)(µ1,5dca)2dca](H2O)
-
-
µ1,5-dca
µ1,5-dca
1D
polymer
39
}
moiety.
n
N(iminne)2O(phenoxo)
N(Py) Zn(II) metal
occupation.
{[Zn2(L)(µ1,5-dca)](ClO4)}n
6.{[Na(Cu(II)(L3)2](μ1,5-
dca)}n
Diphenoxido
1D
Polymer
76d
moiety.
N(iminne)2O(phenoxo)2
Cu(II) metal
occupation. Na+ metal
also occupied in the
O(methoxy)
compartment
7. [(Cu(II)L)2M(II)( µ1,5-
dca)2]n
Phenoxido
moiety.
-
µ1,5-dca
2D
polymer
76 (l)
N(iminne)2O(phenoxo)
of M(II) metal
occupation
8.[Cu2(µ1,5-dca)(µ-L)2ClO4]n
N(iminne)2O(phenoxo)2
Cu(II) metal
-
-
-
-
-
µ1,5-dca
µ1,5-dca
µ1,5-dca
µ1,5-dca
µ1,5-dca
1D
Polymer
76f
76g
76h
76i
b
occupation.
9.[Zn(LH)(OAc)(dca)]
[Cd(LH)(OAc)(dca)]
N(iminne)O(phenoxo)
of Zn(II)/Cd(II) metal
occupation.
N(iminne)O(phenoxo)/
NMe2/NEt2, Cu(II)
metal coordinated.
N(iminne)O(phenoxo)/
NMe2, Cu(II) metal
coordinated.
Mono-
nuclear
10.[CuL1(µ1,5-dca)]n
[CuL2(µ1,5-dca)]n
Poly-
nuclear
11.[CuL(µ1,5-dca)n].nH2O
1D Poly-
nuclear
12.[Cu2(L3)(dca)2]n.H2O.1.5Me N(iminne)O(phenoxo)/
Polymer
OH
N-morpholine of Cu(II)
metal coordinated.

26
Table 6 Selected some important bond distances (Å) and angles (°) for 1 and 2
Bond lengths (Å)
Value (Å)
Bond angles (°)
Value (°)
1
Zn1-N1
Zn1-N2
Zn1-N6
Zn1-O1
Zn1-O2
Zn2-N3
Zn2-O2
Zn2-O1
2.662(6)
2.049(9)
1.943(6)
2..040(5)
2.010(6)
1.935(7)
1.984(6)
2.016(5)
N1-Zn1-N2
O1-Zn1-O2
O2-Zn2-O1
N3-Zn2-N6
N3-Zn2-O1
O2-Zn1-N8
O1-Zn1-N8
N1-Zn1-O1
81.4(3)
78.4(2)
79.4(2)
116.00(3)
110.5(2)
105.1(2)
104.2(2)
89.00(3)
N1-Zn1-O2
N1-Zn1-N8
145.8(3)
108.8(3)
N2-Zn1-N8
112.9(3)
2
Zn1-N1
Zn1-N2
Zn1-O3
Zn1-O1
Zn1-N5
Zn2-N4
2.080(9)
2.09(9)
N1-Zn1-N2
O1-Zn1-O3
O3-Zn1-N5
N5-Zn1-N1
N5-Zn1-N2
N3-Zn2-N4
78.9(3)
91.8(2)
2.010(6)
1.978(6)
2.014(8)
2.043(8)
103.4(3)
99.3(3)
104.2(3)
81.10(3)
Zn2-N3
Zn2-O5
Zn2-O7
Zn3-O5
Zn3-O7
Zn3-N7
2.010(8)
2.019(6)
2.012(6)
2.022(6)
2.034(6)
1.922(8)
O5-Zn2-O7
N7-Zn3-N8
O5-Zn3-O7
N7-Zn3-O5
78.5(2)
123.6(3)
77.9(2)
111.00(30
Zn3-N8
1.968(6)
4.3. Hirshfeld surface
Hirshfeld surface analysis is generally used to study the intermolecular between different
molecules and intra molecular interaction within a molecule in a crystal lattice. It's size and

27
shape of the complex is used as tool for qualitative and quantitative interpretation of close
contacts within the crystal system of the complex. Hirshfeld surface of a particular complex is
explained as a set of points in the 3D region where the contribution to the density of electron is
same to the total contribution from all other molecules. MESP mapping on the Hirshfeld surface
acts as a tool to study electrostatic complementarily between nearby molecules, which provides
basic knowledge about the crystal packing. Total electrostatic interactions are determined as a
sum of electrostatic interaction, polarization, dispersion and exchange repulsion which all are
calibrated against dispersion corrected DFT [84]. HS summarizes the complex information
present in a molecule into a color plot, which is a finger print of the various types of
intermolecular interactions in the unit cell. Details of dnorm, shape Index and curved Index are
provided for 1 and 2 in Fig. 9 and Fig. 10.
For 1, the globularity value is 0.568 and a sphericity value if 0.042. This shows that 1 deviate
from perfect spherical shape. For 2, the globularity value is 0.695 and a sphericity value is 0.02.
These values suggest that 1 is more deviate from the sphericity than 2. 2D finger plots provides
information about different types of interactions between these complexes and other atom of the
surround molecules. The HS study shows that for 1, the maximum interaction is between the
complex and hydrogen atoms from the surrounding molecules (67.3%). 15.2 % of interaction is
between the molecule and surrounding carbon atoms, 8.5% between the nearby nitrogen atoms,
8.8% between the oxygen atoms and 0.2% between zinc atoms. In case of 2, the maximum
interaction is observed between the complex and outside hydrogen atoms, from the surrounding
complex molecules (57.3%). It was followed by interaction with carbon (19.2%), with nitrogen
(18.9%), with oxygen (2.7%) and finally least with zinc (1.7%). Unlike 1, we can see that
interaction with zinc, nitrogen and carbon is more for complex 2.

28
Fig.9. Hirshfeld surface analysis of 1: dnorm, Shape index, curvedness and different 2D plots

29
Fig.10. Hirshfeld surface analysis of 2: dnorm, Shape index, curvedness and different 2D plots
4.4. FMO analysis
Mapping of the frontier molecular orbitals released interesting results for 1 and 2. HOMO of the
complex is found to lie primarily on the zinc atoms, while LUMO is found to be present more on
the ligand atoms of one fragment. The bridging system is neither having HOMO or LUMO,
which makes the bridging very strong and stable. This is having effect on the overall stability of
the complex. The frontier molecular orbitals of 1 and 2 are represented in Fig.S7 and Fig.S8. The
energy of HOMO for 1 is -7.2608 eV, while that of LUMO is -1.1058 eV. The HOMO-LUMO
gap is -6.1550 eV, which is very high which makes the complex stable and unreactive. These
data can be used to find other frontier orbital data as follows, ionization potential: I=-EHOMO
,
electron affinity A=-ELUMO, hardness η= (I-A)/2, and electrophilicity index ω = µ2/2η. For this
complex, the ionization energy is 7.2608 eV and electron affinity are 1.1058 eV. The hardness is