Synthesis, characterization and biological activity of two Schiff base ligands and their nickel(II), copper(II), zinc(II) and cadmium(II) complexes derived from S-4-picolyldithiocarbazate and X-ray crystal structure of cadmium(II) complex derived from pyridine-2-carboxaldehyde

Article abstract of DOI:10.1016/j.ica.2014.01.001

A substituted dithiocarbazate moiety has been prepared using 4-picolylchloride hydrochloride to obtain S-4-picolyldithiocarbazate (S4PDTC) and its by-product (S4diPDTC). Two Schiff bases were synthesized by reacting S-4-picolyldithiocarbazate with pyridine-2-carboxaldehyde (Pc4PDTC) and 4-carboxybenzaldehyde (Cb4PDTC). Ni(II), Cu(II), Zn(II) and Cd(II) complexes of the Schiff bases were characterized by various physico-chemical and spectroscopic methods viz. CHNS elemental analyses, molar conductivity, IR, UV-Vis, ¹H and ¹³C NMR, mass spectrometry and magnetic susceptibility. X-ray crystallographic analysis has been carried out on the by-product and the Cd complex of the ligand derived from (Pc4PDTC). The ligands and their metal complexes were assayed in vitro for cytotoxic, antimicrobial and antioxidant activities. The Pc4PDTC Schiff base showed moderate cytotoxicity against human promyolicytic leukemic (HL60) cell line with a CD₅₀ value of 9 μg/cm³ while Cb4PDTC was inactive. Complexing Pc4PDTC with cadmium(II) and copper(II) enhanced its cytotoxicity against the human promyolicytic leukemic cell line (HL60) from moderately to highly active. S4PDTC showed marked antimicrobial activity against bacteria but was inactive towards fungi. Copper(II) and cadmium(II) complexes of the two Schiff bases showed clear inhibition zones for most of the fungi and bacteria tested. Antioxidant properties of Pc4PDTC and Cb4PDTC Schiff bases were comparable to the commercially available buthylatedhydroxytoluene (BHT).

Full text of DOI:10.1016/j.ica.2014.01.001

Inorganica Chimica Acta 413 (2014) 68–76
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization and biological activity of two Schiff
base ligands and their nickel(II), copper(II), zinc(II) and cadmium(II)
complexes derived from S-4-picolyldithiocarbazate and X-ray
crystal structure of cadmium(II) complex derived from
pyridine-2-carboxaldehyde
b
c
c
Teng-Jin Khoo a, , Mohammed Khaled bin Break , K.A. Crouse , M. Ibrahim M. Tahir ,
⇑
A.M. Ali d, A.R. Cowley e, D.J. Watkin e, M.T.H. Tarafder f
a School of Pharmacy, Faculty of Science, University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia
b School of Chemical Sciences and Food Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia
c Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, Malaysia
d Laboratory of Molecular and Cell Biology, Institute of BioScience, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
e Chemical Crystallography, Chemistry Research Laboratory, 12 Mansfield Road, Oxford University, OX1 3TA England, United Kingdom
f Department of Chemistry, Rajshahi University, Rajshahi 6205, Bangladesh
a r t i c l e i n f o
a b s t r a c t
Article history:
A substituted dithiocarbazate moiety has been prepared using 4-picolylchloride hydrochloride to
obtain S-4-picolyldithiocarbazate (S4PDTC) and its by-product (S4diPDTC). Two Schiff bases were
synthesized by reacting S-4-picolyldithiocarbazate with pyridine-2-carboxaldehyde (Pc4PDTC) and
4-carboxybenzaldehyde (Cb4PDTC). Ni(II), Cu(II), Zn(II) and Cd(II) complexes of the Schiff bases were
characterized by various physico-chemical and spectroscopic methods viz. CHNS elemental analyses,
molar conductivity, IR, UV–Vis, 1H and 13C NMR, mass spectrometry and magnetic susceptibility. X-ray
crystallographic analysis has been carried out on the by-product and the Cd complex of the ligand derived
from (Pc4PDTC). The ligands and their metal complexes were assayed in vitro for cytotoxic, antimicrobial
and antioxidant activities. The Pc4PDTC Schiff base showed moderate cytotoxicity against human
Received 1 November 2013
Received in revised form 23 December 2013
Accepted 3 January 2014
Available online 24 January 2014
Keywords:
S-4-Picolyldithiocarbazate
Synthesis
Characterization
Bioactivity of Schiff base ligands and
complexes
promyolicytic leukemic (HL60) cell line with a CD50 value of 9 l
g/cm3 while Cb4PDTC was inactive.
Complexing Pc4PDTC with cadmium(II) and copper(II) enhanced its cytotoxicity against the human
promyolicytic leukemic cell line (HL60) from moderately to highly active. S4PDTC showed marked anti-
microbial activity against bacteria but was inactive towards fungi. Copper(II) and cadmium(II) complexes
of the two Schiff bases showed clear inhibition zones for most of the fungi and bacteria tested. Antioxi-
dant properties of Pc4PDTC and Cb4PDTC Schiff bases were comparable to the commercially available
buthylatedhydroxytoluene (BHT).
X-ray crystal structure
Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved
1. Introduction
dithiocarbazate Schiff bases and complexes, while S-picolyldithioc-
arbazate derivatives have started to be studied recently and have
The chemistry and the biological potential of Schiff base ligands
derived from dithiocarbazates and their metal complexes have
been investigated extensively [1–13]. Interest is still high as few
have found their way into application as therapeutic drugs, health,
skin care products and in paint dye manufacturing [1–3,14]. Most
of the work previously has focused upon S-methyl and S-benzyl
shown promising biological activities when investigated [15].
Therefore, this has led us to anticipate that a study of derivatives
possessing pyridine will provide interesting results and may help
develop an understanding of the biological activity of this class
of compounds. We report herein the synthesis, separation and
characterization of S-4-picolyldithiocarbazate (S4PDTC) (1) and
the single crystal structure of its by-product (S4diPDTC) (2)
together with two multidentate Schiff bases and their transition
metal complexes. Moreover, their anti-microbial, cytotoxic and
⇑
Corresponding author. Tel.: +60 389248213; fax: +60 389248018.
E-mail address: TengJin.Khoo@nottingham.edu.my (T.-J. Khoo).
http://dx.doi.org/10.1016/j.ica.2014.01.001
0020-1693/Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved

T.-J. Khoo et al. / Inorganica Chimica Acta 413 (2014) 68–76
69
characterized. The dark-orange solution filtrate was rotavaporated.
The crude formed was rinsed and washed with cold MeOH to give
an orange solid which was dried and characterized. The white pre-
cipitate that did not dissolve in MeOH was characterized to be (1)
and the latter was (2).
Schiff bases were prepared by adding a solution of (1) (0.1 mol)
in absolute ethanol (40 cm3) to an equimolar amount of the
aldehyde dissolved in 50 cm3 of the same solvent in the presence
of 1–2 drops of sulfuric acid. The mixture was heated on a steam
bath for 10 min and then cooled to room temperature until the
Schiff bases precipitated. Products were washed with cold ethanol
and dried in vacuo over silica gel. The yield was determined and the
melting point recorded. Schemes 1 and 2 illustrate the synthesis
procedures for the compounds.
Scheme 1. Reaction scheme for the synthesis of (1).
antioxidant activities have been evaluated and correlated with the
newly elucidated structures. The expected structures for the two
Schiff bases synthesized by reacting S-4-picolyldithiocarbazate
with 4-carboxybenzaldehyde (Cb4PDTC) (3) and pyridine-
2-carboxaldehyde (Pc4PDTC) (4) are shown in Scheme 1.
2.2. 2.Preparation of metal complexes
2. Experimental
Schiff bases (7.6 mmol) prepared as outlined above were dis-
solved in ethanol (50 cm3). These solutions were combined with
solutions containing a stoichiometric amount of metal salt in the
same solvent (ꢁ50 cm3). The mixtures were heated to 100 °C for
5 min and then the volume of each was reduced by ꢁ40%. Products
precipitated were isolated, and dried in a vacuum desiccator over
P2O5. Attempts to synthesize the Ni(II) complex of (4) were
unsuccessful.
All chemicals and solvents were of analytical grade, and were
used as supplied. 100% Hydrazine hydrate, carbon disulphide and
potassium hydroxide were obtained from MERCK while
Cd(CH3COO)2ꢀ2H2O and Cu(NO3)2ꢀ2.5H2O were obtained from
R&M MARKETING. 4-picolylchloride hydrochloride, pyridine-2-
carboxaldehyde and 4-carboxybenzaldehyde were obtained from
SIGMA–ALDRICH while absolute ethanol, NiCl2ꢀ6H2O, and
ZnCl2were obtained from FRIENDEMANN SCHMIDT.
2.3. 3.Physical measurements
2.1. 1Preparation of (1) and its Schiff bases
The analyses for carbon, hydrogen, nitrogen and sulfur were
carried out using a LECO CHNS-932 analyzer. The IR spectra (KBr
pellets) were recorded (4000–400 cmꢂ1) using a Perkin Elmer FTIR
100% Hydrazine hydrate (10 g, 0.31 mol) was added to KOH
(11.4 g, 0.20 mol) in 90% EtOH (70 ml) and the mixture cooled to
0 °C in an ice bath. CS2 (15.2 g, 0.20 mol) was added dropwise with
constant stirring (880 rpm) for 1 h. During this time two layers
formed and the lower layer (light brown) was separated then dis-
solved in cold 40% EtOH (60 ml). The ethanolic solution was kept in
an ice bath while 4-picolylchloride hydrochloride (32 g, 0.2 mol)
dissolved in 80% MeOH was added dropwise with vigorous stirring
(1100 rpm). The yellowish precipitate which formed after ꢁ2 h
was filtered and dried over anhydrous silica gel and was later
dissolved in MeOH to give a white precipitate and dark-orange
solution. The white precipitate was filtered off, dried and
1750X spectrophotometer. The molar conductance of a 10ꢂ3
M
solution of each metal complex in DMSO was measured at 29 °C
using a Jenway 4310 conductivity meter and a dip-type cell with
a platinized electrode. Melting points were determined using an
Electrothermal digital melting point apparatus while the UV–Vis
spectra were recorded on a Shimadzu UV-2501 PC spectrophotom-
eter (800–200 nm) in DMSO solution. Magnetic susceptibility was
determined using a Sherwood Scientific MSB-AUTO Magnetic Sus-
ceptibility Balance at room temperature. 1H and 13C spectra were
obtained using a 400 MHz JEOL JNM-ECA instrument. Mass spectra
Scheme 2. Reaction scheme for synthesis of the Schiff base ligands (3) and (4).

70
T.-J. Khoo et al. / Inorganica Chimica Acta 413 (2014) 68–76
were obtained using a SHIMADZU GCMS QP5050A DIMS with Elec-
tron Ionization technique.
2.5.2. Antimicrobial assay
Antimicrobial activity of each sample was qualitatively deter-
mined by a modified disc diffusion method as previously reported
[12]. Compounds that showed positive (>15 mm) antimicrobial
activities with the disc diffusion assay were subjected to the broth
dilution method for the quantitative measurement of microbio-
static (inhibitory) activity as described by Hufford and Clark [20].
The lowest concentration that completely inhibited visible micro-
bial growth was recorded as the minimum inhibitory concentra-
2.4. X-ray crystal structure determination
Suitable quality crystal were mounted on glass fibers using per-
fluoropolyether oil and cooled rapidly to 150 K in a stream of cold
N2 using an Oxford Cryosystems CRYOSTREAM unit. Diffraction
data were measured using an Enraf–Nonius KappaCCD diffractom-
tion (MIC,
l
g/cm3). The pathogenic microbials used were
Methicillin resistant sthaphylococcus (MRSA), Bacillus subtilis wild
type (B29), Subtilis mutant (mutant defective DNA repair-B28),
Pseudomonas aeruginosa (60690), Candida albicans (C.A.), Aspergillus
ochraceous (398), Saccaromyces ceciricaee (20341) and Candida
lypolytica (2075).
eter (graphite monochromated Mo K
a radiation, k = 0.71073 Å).
Intensity data were processed using the DENZO-SMN package
[16]. The structures were solved using the direct-methods program
SIR92 [17], which located all non-hydrogen atoms. Subsequent full-
matrix least-squares refinement on F was carried out using the
CRYSTALS program suite [18].
2.5.3. 3.Antioxidant assay
Coordinates and anisotropic thermal parameters of all non-
hydrogen atoms were refined. All other hydrogen atoms were posi-
tioned geometrically after each cycle of refinement. Important
crystallographic data for the elucidated structures are presented
in Table 1.
Antioxidant assays were carried out using the ferric thiocyanate
(FTC) method as previously reported by Kikuzaki and Nakatami
[21]. A mixture of 0.002 g of sample in 99.5% ethanol (4 cm3),
4.1 cm3 of 2.51% linoleic acid in 99.5% ethanol, 8 cm3 of 0.05 M
phosphate buffer (pH 7) and 3.9 cm3 of H2O in
a vial
(diam. = 38 mm, h = 75 mm) with screw cap was placed in a dark
oven at 40 °C. To 0.1 cm3 of this solution was added 9.7 cm3 etha-
nol (75%) and 0.1 cm3 NH4CNS (30%). Precisely 3 min. after the
initial addition of 0.1 cm3 of 2 ꢃ 10ꢂ2 M FeCl2 in 3.5% HCl to the
reaction mixture, the red absorbance was measured at 500 nm.
Subsequently, readings were taken at 24 h intervals until the
absorbance of the control reached its maximum.
2.5. Biological activity
2.5.1. 1.Cytotoxicity
The CEM-SS (Human T-lymphoblastic leukemic) and HL60
(Human promyolicytic leukemic) cell lines were obtained from
the National Cancer Institute, USA. The cells were cultured in
RPMI-1640 (Sigma) medium supplemented with 10% fetal calf
serum. Cytotoxicity was determined using the microtitration of
3. Results and discussion
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide
3.1. 1.Physical properties and analytical data of Schiff base ligands and
their complexes
(MTT) assay (Sigma, USA) as reported by Mosmann [19]. Cytotox-
icity was expressed as CD50, i.e. the concentration that reduced
the absorbance of treated cells by 50% with reference to the control
(untreated cells).
The physical properties and analytical data results have been
summarized in Table 2. Results of the CHNS elemental analyses
Table 1
Crystallographic data and structure refinement details.
(2)
(5)
Emperical weight
Formula weight
T
C
15H19N4O1S2
C20H18.67Cd1.33Cl1.33N5.33O0.67S2.67
627.05
150
335.47
150
Crystal class
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
triclinic
P1
monoclinic
P21/n
ꢀ
4.5627(2)
10.4137(3)
17.2731(7)
83.9493(13)
84.1499(14)
78.1442(15)
796.02(5)
2
8.1716(2)
20.8073(4)
10.7152(3)
90
101.7141(12)
90
1783.95(8)
3
1.751
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
qcalc (g cmꢂ3
)
1.40
Radiation type
Wavelength (Å)
Crystal size (mm)
h (°)
Mo K
0.71073
a
Mo Ka
0.71073
0.01 ꢃ 0.01 ꢃ 0.01
0.10 ꢃ 0.10 ꢃ 0.10
5–27
5–27
Reflections measured/independent (Rint
)
6467/3554 (0.0002)
7254/4003 (0.017)
hmax
27.42
27.5
Limiting indices
Refinement on
R-factor
ꢂ5 6 h 6 5, 13 6 k 6 13, ꢂ22 6 l 6 22
ꢂ10 6 h 6 10, ꢂ27 6 k 6 26, ꢂ13 6 l 6 13
F
F
0.057
0.032
Rw-factor
0.058
0.039
Goodness of fit (GOF) on F
1.038
1.00
Minimum and maximum residual electron density (e Åꢂ3
Reflections used
)
ꢂ0.65 and 0.75
ꢂ0.68 and 1.15
2687
3180
r
(I) limit
3.00
3.00
Number of parameters
200
226

T.-J. Khoo et al. / Inorganica Chimica Acta 413 (2014) 68–76
71
Table 2
Analytical data and physical properties of the Schiff bases and their complexes.
Compound
Color
m.p (°C)
Found (Calcd.) (%)
Molar conductance (
X )
ꢂ1 cm2 molꢂ1
C
H
N
S
(1)
beige
184–86
57.5 (42.2)
5.2 (4.5)
11.4 (21.1)
26.3 (32.2)
–
(2)
orange
(3)
(4)
pale orange
pale yellow
dark yellow
dark green
green
320
189
55.5 (54.4)
53.9 (54.2)
39.4 (39.0)
37.4 (38.3)
37.5 (37.5)
42.7 (43.4)
41.9 (40.6)
41.1 (41.9)
45.0 (45.5)
43.7 (44.8)
4.4 (4.0)
4.5 (4.2)
3.2 (3.2)
2.8 (3.1)
2.7 (2.9)
3.6 (3.6)
3.4 (3.2)
3.4 (3.0)
3.9 (3.6)
3.4 (3.5)
12.4 (12.7)
19.2 (19.4)
12.6 (12.1)
17.1 (14.0)
17.1 (16.8)
13.6 (13.5)
9.5 (8.4)
9.5 (9.8)
10.5 (9.4)
9.1 (9.2)
19.9 (19.3)
22.1 (22.2)
13.8 (13.9)
14.9 (15.7)
14.9 (15.4)
15.1 (15.4)
12.9 (12.7)
16.1 (14.9)
16.9 (14.3)
15.5 (14.1)
–
–
[Cd(Pc4PDTC)2] (5)
[Cu(Pc4PDTC)2Cl] (6)
[Cu(Pc4PDTC)2NO3] (7)
[Zn(Pc4PDTC)2] (8)
[Cd(Cb4PDTC)2] (9)
[Cu(Cb4PDTC)2Cl] (10)
[Ni(Cb4PDTC)2] (11)
[Zn(Cb4PDTC)2] (12)
ꢁ250d
non-electrolyte
59
63
non-electrolyte
non-electrolyte
41
182
174
orange
ꢁ260d
>270d
278
>235d
>300d
pale yellow
light green
dark brown
light yellow
10
non-electrolyte
Notes: d = decompose.
for both Schiff base ligands were in agreement with calculated val-
ues and the case was similar with their complexes. Molar conduc-
tance studies have revealed that (7), (6) and (10) are 1:1
and v(N–N) but do not exhibit v
(S–H) at ca. 2700 cmꢂ1. All these
are characteristic of an unprotonated Schiff base in its thione form,
as it is well documented [6–13] that this type of dithiocarbazate
moiety can display thione-thiol tautomerism in solution (Fig. 1).
The band indicating ortho-substitution of pyridine appears at
628 cmꢂ1 and is as expected for (4). (3) also displays the character-
electrolytes having molar conductivities of 63, 59 and 41
X
ꢂ1 cm2 -
molꢂ1 in DMSO, presumably due to solvation according to the fol-
lowing equations:
istic carboxylic acid functionality with carboxylic
v(O–H) band in
½CuðPc4PDTCÞ NO3ꢄ þ DMSO ! ½CuðPc4PDTCÞDMSOꢄþ þ NO3ꢂ
½CuðPc4PDTCÞ22Clꢄ þ DMSO ! ½CuðPc4PDTCÞDMSOꢄþ þ Clꢂand
½CuðCb4PDTCÞ2Clꢄ þ DMSO ! ½CuðCb4PDTCÞDMSOꢄþ þ Clꢂ
the 3100–3550 cmꢂ1 range and (C@O) at 1688 cmꢂ1 while the
v
para-substitution of the benzene ring has been confirmed by an
absorption peak at 768 cmꢂ1. As for the metal complexes, the
strong bands assigned to v
(NH) at 3234 and 2920–2972 cmꢂ1 for
All the complexes dissolve readily only in donor solvents such
as DMSO and DMF. The chloride ion that has been included in
the formulas for Cu complexes might have come from 4-picolyl-
chloride hydrochloride. The molar conductivity value for (11) has
been found to be very low to account for any dissociation of the
complexes in DMSO indicating its non-electrolytic nature. Elemen-
tal analyses data suggest a 2:1 ligand to metal ratio for the com-
plexes, and all of these data have been used to suggest formulas
for the complexes.
(1) and its Schiff bases were lost in all the metal complexes. This
deprotonation of the nitrogen atom is believed to be due to the
tautomerism phenomenon whereby the ligands exist in their thiol
forms within the complex. The red shift of azomethine
v(C@N)
stretching band and the loss of (C@S) bands in all the complexes,
v
indicate coordination of the ligands with metal ions via the
azomethine nitrogen and thiolato group. The formation of a
metal-nitrogen (b-N) bond results in lowering of C@N bond order
thus decreasing the v(C@N) band frequencies, however the band
frequencies were found to increase after coordination and this
might be due to decrease in repulsion between the nitrogen lone
pair of electrons and the nearby electrons as a result of participa-
tion of the nitrogen electrons in coordination with the metal ion
[12]. Complexes of (4) showed no major shift of the para substi-
tuted pyridine band at 775 cmꢂ1, however the ortho substituted
out-of-plane and in-plane pyridine bands showed generally posi-
tive shifts which might indicate that the ligand undergoes coordi-
nation with the metal ions via the nitrogen atom of the pyridine
ring. Furthermore, complexes of (3) showed positive shifts in the
bands for para pyridine indicating their involvement in chelation,
3.2. 2.IR spectral analysis
The IR spectra of the compounds have been tabulated in Table 3.
IR spectrum for (1) displays bands at 3234, 1608, 1142 and
1072 cmꢂ1 assigned to
v(NH), v(NH2), v(C@S) and v(N–N), respec-
tively. The absorption peaks at 666 and 756 cmꢂ1 indicate mono-
substitution of the pyridine ring.
Characterization via IR indicated that the Schiff bases formed
were indeed obtained as proposed. The solid state IR spectra of
both free Schiff bases display bands for
v(C@S), v(C@N), v(NH)
Table 3
Infrared spectral data (band maxima).
Compound
Infrared Absorption Bands (frequency, cmꢂ1
)
Pyridine bands
v
(C@S)
v
(C@N)
v
(O–H)
v
(NH)
v
(N–N)
v
(NH2)
v
(Ar–COOH)
⁄
⁄
⁄
(1)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
1142
1116
1148
3234
2972
2920
1072
1070
1044
1064
1070
1070
1068
1068
1042
1071
1069
1608
756 (para)
768 (para)
1608
1606
1616
1610
1612
1616
1608
1636
1610
1608
3438
–
–
–
–
–
–
–
–
–
–
1688
⁄
⁄
406(ip), 628(op) (ortho)
410(ip), 618(op) (ortho)
421(ip), 648(op) (ortho)
421(ip), 649(op) (ortho)
414(ip), 634(op) (ortho)
782
764
778
780
⁄
⁄
⁄
⁄
⁄
⁄
⁄
⁄
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
3424
3434
3425
3436
1688
1694
1690
1689
(10)
(11)
(12)
– = Lost, ‘‘ip’’ = in-plane, ⁄ = not present, ‘‘op’’ = out-of-plane.

72
T.-J. Khoo et al. / Inorganica Chimica Acta 413 (2014) 68–76
N
N
S
S
N
NH
N
N
thioneS
N thiol
N
SH
Fig. 1. Thione and thiol tautomers of pyridine-2-carboxaldehyde Schiff base (4).
while the lack of any shifts in the bands representing the carboxyl-
ate functionality present in the complexes indicates that the
carboxylate group is not involved in any coordination with the
metal ions.
caused by 3A2g (F) ? 3T1g (P) transitions and might indicate an octa-
hedral arrangement [22,23].
Magnetic susceptibility data have revealed that (8) was found to
be diamagnetic at room temperature in accordance with a d10 con-
figuration while (11) is paramagnetic with a leff value of 1.52 B.M
which appears to deviate from spin only values as it falls within the
range of (0–2.8) [24]. Previous studies on the magnetic susceptibil-
ities of Ni complexes have shown that some of these complexes
might exhibit such values, and the reason has been attributed to
spin state isomerism, presence of more than one stereochemistry
for the complex in the unit cell or due to interactions between sol-
vent molecules and complex [24]. The leff values of Cu complexes
seem to be higher than the normal values of (1.7–1.8) and that
could be due to the presence of interactions between ethanol
solvent molecules and the Cu complexes or due to the presence
of ferromagnetic interactions [25–27].
3.3. 3.Mass spectrometry, UV–Vis and magnetic susceptibility
Mass spectral data revealed that the synthesized products were
obtained as expected. MS characterization confirmed the presence
of (1) and its by-product (2) through the highest m/z at 199 for (1)
and highest m/z at 288 for (2). Fragment of (1) at m/z 92 points to
the presence of methylpyridine while m/z 107 represents the dith-
iocarbazate moiety. In (2), the fragment m/z 105 indicates the pres-
ence of aminomethylpyridine, a clear indication of the formation of
the ligand.
Furthermore, the mass spectral data for the Schiff base ligands
confirmed the presence of these compounds as expected, whereby
the mass spectra obtained for (3) and (4) showed the highest m/z at
329 for the former and m/z 288 for the latter. Characteristic frag-
ments appearing in the spectrum of each Schiff base are, m/z 105
representing the dithiocarbazate moiety and m/z 92 indicating
methyl pyridine. The peak at m/z 130 (methylbenzylcarboxyl)
appearing in the spectrum of (3) and m/z 78 (pyridine) in that of
(4), indicate the presence of the expected functionality in each
molecule.
3.4. 1H and 13C NMR
The clearest characterization of the synthesized compounds can
be seen from the 1H NMR and 13C NMR spectrums in DMSO-d6 at
18 °C, where the 1H NMR spectrum of (1) showed the following sig-
nals: the pyridine ring protons appear as doublets at 8.48 ppm
(J = 4.6 Hz) and 7.38 ppm (J = 6.0 Hz) while the singlet at
4.33 ppm is assigned to –SCH2- protons. Spectrum of 13C NMR
showed the presence of single pyridine ring at 150.3 and
124.4 ppm for the four carbons.
The electronic spectral data recorded in DMSO and magnetic
moments of the complexes are given in Table 4. The absorptions
with wavelengths less than 350 nm are mostly due to ligand cen-
In both the 1H NMR and 13C NMR of (2), the presence of two
pyridine rings is very clear as indicated by chemical shift at 7.28
and 8.60 ppm (shift for ortho-pyridine protons) for the hydrogens
in the pyridine attached to thiolato group while the shift at 8.02
and 8.83 ppm (ortho-pyridine protons) points to hydrogen in the
other pyridine. Carbon shifts are indicated at 124.4, 150.3, 124.1
and 149.9 ppm corresponding to the similar positions.
tered transitions of aromatic electrons (p ?
p⁄) in addition to
non-bonding electrons (n ? p⁄) present in the nitrogen atom of
the azomethine group in the Schiff base complexes. The bands at
around 400 nm might be assigned to electron transitions from
the pp orbitals of the donor atoms to the d-orbitals of the metal,
1H NMR spectra of both Schiff bases were determined in DMSO-
d6. (3): d 7.25(2H, d), d 7.08(1H, s), d 6.78(2H, d), d 6.57(2H, d), d
6.20(2H, d) and d 3.30(2H, s) while (4): d 7.70(2H, d), d 7.60(2H,
d), d 7.40(1H, s), d 7.00(2H, d), d 6.60(2H, d) and d 3.70(2H, s). These
data are in line with expectations in each case.
while the bands that are around 650 nm can be assigned to d–d
transitions in the complexes [22]. The band referring to d–d tran-
sitions for Cu complexes can be assigned to 2Eg ? 2T2g transitions
and indicates a Jahn–Teller distorted octahedral arrangement
[23]. It is possible that the band at 400 nm for the Ni complex is
3.5. Crystal structure of (2)
Table 4
UV–Vis and magnetic susceptibility.
ORTEP diagram of (2) molecule with atom numbering scheme is
shown in Fig. 2, while significant bond lengths and angles have
been summarized in Table 5. The molecule is L-shaped and is
mostly planar except for the pyridine ring attached to the thiolato
group which is twisted around 79.08° relative to the rest of the
molecule. The fact that (C7–S2) is shorter than the single bond
(C7–S1), with bond lengths of 1.658 and 1.752 Å respectively,
proves that the former contains a double bond and that the mole-
cule exists in its thione tautomer. Moreover, the bond C7—N2 has a
length of 1.351 Å whereas C8—N3 has a bond length of 1.284 Å
Compounds
UV–Vis Spectraa (kmax, nm)
leff (B.M.)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
259 (4.1), 322 (4.3), 381 (4.2)
333 (4.5), 409 (4.2), 654 (2.5)
331 (4.5), 408 (4.1), 650 (4.0)
326 (2.7), 393 (2.7)
300 (4.4), 350 (4.7)
332 (3.9), 347 (3.7)
diamag.
2.78
2.93
diamag.
diamag.
3.0
346 (4.4), 364 (4.3), 400 (3.8)
304 (4.2), 320 (4.2), 333 (4.2) 3.92
1.52
3.92
Loge(L cmꢂ1 moleꢂ1) are given in parentheses.
a

T.-J. Khoo et al. / Inorganica Chimica Acta 413 (2014) 68–76
73
sulfur is in a trans position with the pyridine ring attached to the
imine group while the pyridine rings are cis to each other, across
the C7—N2 bond.
Analysis of any interactions between the molecules in the
structure revealed the presence of hydrogen bond interactions
(Table 6).The nitrogen at the two pyridine rings, lay good
2-dimensional hydrogen bond linkages in the molecular packing
(Fig. 3). The distance between layers in the molecular packing
varies from 4.563 Å (maximum) in some areas to a minumum va-
lue of 3.363 Å in other regions and such distances are enough to
accommodate an ethanolic solvent molecule, which in this case,
is an ethanolic molecule that is disordered.
Overall, the structure seems very similar to (4) except that both
pyridine rings in this by-product are at the para position while (4)
has one of the pyridine rings substitution in ortho position.
Fig. 2. ORTEP diagram of (2) (at 50% probability displacement ellipsoids) with
atomic numbering scheme. Ethanol solvent molecule has been obscured for clarity.
3.6. Crystal structure of (5)
Table 5
The molecular structure of (5) along with the atomic numbering
scheme is shown in Fig. 4, while significant bond lengths and an-
gles have been summarized in Table 7. The compound crystallizes
with a distorted ethanolic molecule and is a dinuclear complex
containing two cadmium ions bridged by two chlorine atoms.
The chloride ions are thought to have come from trace amounts
of 4-picolylchloride hydrochloride that was initially used to syn-
thesize the complex. The assymetric unit comprises of half the
molecule while the other half is related via a centre of inversion sit-
uated in the Cd1–Cl1–Cd1–Cl1 ring. The distance between the two
cadmium ions is 3.906 Å while each cadmium ion is coordinated to
six atoms (N1, N4, N3, S2, Cl1, Cl1) in a distorted octahedral geom-
etry. This form of geometry was further proven by the following
bond angles of (N1–Cd1–Cl1 = 163.93°), (S2–Cd1–N4 = 144.01°)
and (N3–Cd1–Cl1 = 170.78°) which indicate that the ligands are
nearly orthogonal to each other thus forming a distorted octahe-
dral geometry. Coordination polymer is formed by N1–Cd coordi-
nation, moreover, the ligand coordinates in a facial fashion.
Selected geometry parameters of (2) (Å, °).
C1—N1
C14—O6
C7—S1
1.339 (5)
1.615 (19)
1.752 (3)
C8—N3
N2—N3
C7—S2
1.284 (4)
1.362 (4)
1.658 (3)
C2—C1—N1
S1—C7—N2
C15—C14—O6
122.8 (4)
112.9 (2)
169.1 (16)
C16—S1—C7
C7—N2—N3
C14—O6—H5
101.94 (15)
118.6 (2)
155.4
Table 6
Hydrogen bond interactions of (2) (Å, °).
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
N2—H7ꢀ ꢀ ꢀN1i
0.86
0.83
0.93
0.96
0.97
1.96
1.87
2.87
1.88
2.74
2.800 (4)
2.703 (5)
3.782 (4)
2.804 (16)
3.127 (3)
164
178
165
159
105
N4—H20ꢀ ꢀ ꢀN4ii
C4—H4ꢀ ꢀ ꢀS2iii
C15—H18ꢀ ꢀ ꢀO6iv
C16—H21ꢀ ꢀ ꢀS2 (Intramolecular)
The coordination bonds between each of the Cd ions with the
Schiff base ligands result in the formation of fused rings around
each Cd ion; (Cd1–S2–C7–N2–N3) and (N3–C8–C10–N4–Cd1), fur-
thermore, another cyclic structure is formed from the coordination
of Cd with Cl. The 5-membered ring (Cd1–S2–C7–N2–N3)
possesses an envelope conformation on Cd 1 with puckering
Symmetry codes: (i) x, y + 1, z; (ii) ꢂx + 4, ꢂyꢂ1, ꢂz + 1; (iii) ꢂx + 2, ꢂyꢂ1, ꢂz; (iv)
x + 1, y, z.
which is shorter than the former indicating that the latter pos-
sesses a double-bond character and belongs to the imine group.
The molecule crystallizes in the conformer in which the thione
Fig. 3. 2-Dimensional hydrogen bond network of (2) molecules.

74
T.-J. Khoo et al. / Inorganica Chimica Acta 413 (2014) 68–76
Fig. 4. ORTEP diagram of (5) (at 50% probability displacement ellipsoids) with atomic numbering scheme. Ethanol solvent molecule has been obscured for clarity.
parameters; Q = 0.0988 (18) Å,
puckering amplitude and assesses the extent of deviation of a cer-
tain ring from planarity while stands for the pseudorotation
u = 174.6 (16)°. Q stands for the
Table 7
Selected geometry parameters of [Cd(Pc4PDTC)CH3COO] (Å, °).
u
Cl1—Cd1
C413—N1
C7—S1
C8—N3
Cd1—Cl1—Cd1
C412—C411—N1
C5—C6—S1
S1—C7—S2
S1—C7—N2
2.7055 (7)
1.348 (4)
1.758 (3)
1.292 (4)
95.99 (2)
123.6 (3)
111.3 (2)
111.22 (19)
117.6 (2)
S2—Cd1
N1—Cd1
N4—Cd1
2.5742 (8)
2.441 (3)
2.365 (3)
phase angle. The puckered (Cd1–S2–C7–N2–N3) atoms possess
deviations from planarity of ꢂ0.0620(10), 0.0467(12), ꢂ0.014(3),
ꢂ0.025(3), 0.054(2) Å, respectively.
C7—S2—Cd1
Cd1—N1—C413
N2—N3—Cd1
N1—Cd1—Cl1
Cl1—Cd1—N4
96.02 (11)
124.7 (2)
121.76 (19)
163.93 (7)
82.96 (7)
The other fused 5-membered ring (Cd1–N3–C8–C10–N4)
exhibits a twisted conformation on Cd1–N3 bond with puckering
parameters; Q = 0.121(3) Å,
u = 204.9(15)°. The puckered
(Cd1–N3–C8–C10–N4) atoms show deviations from planarity of
ꢂ0.0693(12),
0.075(3),
ꢂ0.052(3),
0.009(3),
0.037(3) Å,
respectively.
The 4-membered ring formed from the coordination of Cd ions
with chlorine atoms is a perfect square planar with an r.m.s devi-
ation of 0.000 Å.
Table 8
Hydrogen bond interactions of (5) (Å, °).
The Schiff base ligand is essentially planar with an r.m.s devia-
tion of 0.093 Å, except for the 4-pyridine ring of the dithiocarbaz-
ate moiety which is nearly perpendicular to the rest of the Schiff
base ligand with a torsion angle (C5–C6–S1–C7) of 91.67°. The sim-
ilarity of the (C7–S2) bond length of 1.739 Å to that of the single
bond (C7–S1) which has a bond length of 1.758 Å, shows that the
ligand coordinates with the metal ion via its thiol tautomer. More-
over, the shorter (C8–N3) bond length of 1.292 Å relative to that of
the (C7–N2) indicates the presence of a double-bonded imine
group. The (C–N) and (C–S) bond lengths are similar to that of
Cg is the centroid of the ring (Cd1–N3–C8–C10–N4)
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
C4—H4ꢀ ꢀ ꢀN2 (Intramolecular)
C6—H6ꢀ ꢀ ꢀN2 (Intramolecular)
C9—H8ꢀ ꢀ ꢀN2 (Intramolecular)
C6—H5ꢀ ꢀ ꢀCl1i
0.93
0.97
0.96
0.97
0.94
2.62
2.40
2.33
2.68
2.65
3.316 (4)
2.872 (4)
2.766 (5)
3.639 (3)
3.177 (3)
132
110
106
169
116
C413—H3ꢀ ꢀ ꢀCgii
Symmetry codes: (i) ꢂx + 1/2, y + 1/2, ꢂz + 1/2; (ii) xꢂ1/2, ꢂy + 1/2, z + 1/2.
Fig. 5. 1-dimensional hydrogen bond network of (5) molecules.

T.-J. Khoo et al. / Inorganica Chimica Acta 413 (2014) 68–76
75
previously reported related structures [28,29], which might indi-
3.7. Cytotoxic activities
cate that such bond lengths are typical of Schiff base ligands and
complexes derived from dithiocarbazates. The ring formed by Cd
and Cl form an angle of 97.91(4)° with the plane defined by the
chelate system (S2, Cd1, N4, C10, C8, N3, N2, N7).
The results of cytotoxicity screening are given in Table 9. (4) is
moderately active against HL 60 cell lines while (3) is inactive in its
free ligand form. However, the cytotoxicity of (4) towards human
promyolicytic leukemic cells (HL 60) increases upon formation of
the complexes (5), (6) and (7) to the highly active region with
Analysis of interactions between molecules of the complex
revealed the absence of significant
p–p intercations. However,
inter- and intramolecular hydrogen bonding interactions exist
CD50 values of 1.20, 1.50 and 1.70 l
g/cm3 respectively. Such con-
between the molecules of the complex, as well as C–Hꢀ ꢀ ꢀ
p
centrations are generally considered to be in the therapeutically
useful range and are therefore worthy of further study. The Schiff
base containing two pyridine rings, (4), was highly active against
interactions (Table 8). The C6–H5ꢀ ꢀ ꢀCl1 intermolecular hydrogen
bond links the molecules of the crystal to form one-dimensional
polymeric chains (Fig. 5).
HT-29 with CD50 of 0.7 lg/ml while (3) was found to be inactive to-
wards both cell lines assayed. This difference can possibly be
attributed to the presence of the lone pair donor electrons on the
nitrogen in (4) as these electrons can attack the DNA of tumour
cells. In contrast, the comparative stability of the carboxylic func-
tionality in (3) might have given rise to its inactivity against both
cell lines. The cadmium(II) and two copper(II) complexes of (4)
are also highly active against HT-29 and HL 60 while it is interest-
ing to note that among the complexes of (3) studied in this work,
only (10) is active against HT-29. Our previous findings have also
shown [12,13,29,15] that chelation of Schiff bases with cad-
mium(II) metal ion enhanced the activity significantly, however
it can be observed that Zn(II) complexes are not active although
Zn(II) is too a d10 metal.
Table 9
Results for screening against colon cancer cells (HT-29) and human promyolicytic
leukemic cells (HL60).
Compounds
CD50 (l
g/cm3)
HT-29
HL 60
(1)
(4)
(5)
(6)
(7)
(8)
(3)
(9)
10.5
0.7
0.8
1.0
0.4
16.8
22.0
23.4
5.5
13.2
9.0
1.20
1.50
1.70
ict
ict
6.40
ict
3.8. Antimicrobial activity
(10)
(11)
ict
ict
(12)
N.A
ict
Qualitative and quantitative antimicrobial test results are
presented in Tables 10 and 11. The entire range of compounds
was evaluated against four types of fungi and bacteria. The results
indicated that only copper(II) and cadmium(II) complexes are
active and were thus the only ones reported. (9) and (10) are anti-
microbial but are not active against cancer cell lines. (9) is selec-
tively active against candida albicans (C.A.) with an MIC value of
Standard:
Doxorubicin
Tamoxifen
6.0
36.00
N.A
N.A
Notes: N.A = not available, ict = inactive.
CD50 (l
g/cm3) = the concentration to reduce growth of malignant cells by 50%.
CD50 < 5 l l lg/
g/cm3 are very active, between 5 and 10 g/cm3 are moderate, 10–20
cm3 are weak while >20 g/cm3 are inactive.
l
781 lg/ml, making it potentially useful as a specific drug against
Table 10
Qualitative antimicrobial assay resultsa/mm.
Compounds
2075
CA
398
20341
B28
B29
MRSA
60690
60691
(1)
(4)
(5)
(6)
(7)
(3)
(9)
(10)
–
–
14
–
11
15
17
–
14
–
–
20
25
–
14
–
15
12
15
–
ꢁ
ꢁ
27
10
–
14
14
12
7
11
30
ꢁ
28
7
–
16
17
10
–
16
31
ꢁ
23
–
ꢁ
ꢁ
ꢁ
17
17
ꢁ
–
ꢁ
ꢁ
ꢁ
–
ꢁ
ꢁ
ꢁ
27
10
ꢁ
14
15
–
ꢁ
–
15
14
ꢁ
–
12
11
ꢁ
8
ꢁ
32
ꢁ
15
30
ꢁ
ꢁ
9
Streptomysin (antifungal control)
Nystatin (antifungal control)
30
ꢁ
ꢁ
21
25
24
ꢁ
Notes: – = Tested compound inactive, ꢁ = not tested.
a
Inhibition diameters of 15 mm and above indicate that the compound is active.
Table 11
Quantitative antimicrobial assay results/MIC (
lg/ml).
Compound
CA
398
20341
B28
B29
MRSA
60690/60691
(1)
(5)
(6)
(7)
–
–
–
–
–
ꢁ
12500
–
12500
3125
12500
––
6250
3125
–
50000
12
–
25000
50000
12500
6250
–
100000
–
3125
781
781
6250
–
12500
6250
–
12500
–
–
6250
100000
–
100000
–
25000
49
–
(9)
(10)
Streptomysin
Nystatin
–
–
–
–
–
49
–
12
3125
3125
6250
–
Notes: – = Tested compound inactive, ꢁ = not tested.

76
T.-J. Khoo et al. / Inorganica Chimica Acta 413 (2014) 68–76
study. Khoo Teng Jin would like to extend his sincere gratitude to
A. R. Cowley and D. J. Watkin in Crystallographic Research Labora-
tory, University of Oxford for their professional guidance and MOS-
TI for the National Science Foundation (NSF) Scholarship and
attachment grant.
Appendix A. Supplementary material
CCDC 294222 and 786894 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2014.01.001.
References
[1] N. Bharti, R. Maurya, F. Naqvi, A. Bhattacharya, S. Bhattacharya, A. Amir, Eur. J.
Med. Chem. 35 (2000) 481.
Fig. 6. Absorbance values of the compounds using FTC method.
[2] J. Li, L. Zheng, I. King, W. Terrence, S. Chen, Curr. Med. Chem. 8 (2001) 121.
[3] M. Ali, A. Mirza, R. Butcher, M. Tarafder, B. Tan, A. Ali, J. Inorg. Biochem. 92
(2002) 141.
[4] M. Ali, R. Bose, J. Inorg. Nucl. Chem. 39 (1977) 265.
[5] M. Ali, S. Livingstone, Coord. Chem. Rev. 13 (1974) 101.
[6] M. Ali, M. Tarafder, J. Inorg. Nucl. Chem. 39 (1977) 1785.
[7] M. Ali, C. Haroon, M. Nazimuddin, S. Majumder, M. Tarafder, M. Khair,
Transition Met. Chem. 17 (1992) 133.
C.A. Chelation by copper(II) ion enhances activity but chelation by
nickel(II) ion generally reduces activity as was also observed by Ak-
bar Ali et al. [9]. Complexation with cadmium(II) ion resulted in
great enhancement of biological activity.
[8] M. Tarafder, M. Ali, N. Saravanan, W. Weng, S. Kumar, N. Umar-Tsafe, K. Crouse,
Transition Met. Chem. 25 (2000) 295.
[9] M. Ali, K. Fernando, D. Palit, M. Nazimuddin, Transition Met. Chem. 20 (1995)
19.
3.9. Antioxidant activities
The daily absorbance readings for (1), (3), (4), Vitamin E, BHT
and the control solution are plotted in Fig. 6. (1), (3) and (4) show
high antioxidant activity compared to Vitamin E, comparable to
the commercially available BHT. The high antioxidant properties
can be related to the presence of nitrogen or oxygen, which are ac-
tive electron pair donors that can form coordination bonds but do
not undergo ionization. Metal complexes were not tested using
antioxidant assay because of their insolubility in the ethanolic
medium.
[10] S. Raj, B. Yamin, Y. Yussof, M. Tarafder, H.-K. Fun, K. Crouse, Acta Crystallogr.,
Sect. C C56 (2000) 1236.
[11] M. Tarafder, A. Kasbollah, K. Crouse, A. Ali, B. Yamin, H.-K. Fun, Polyhedron 20
(2001) 2363.
[12] M. Tarafder, T.-J. Khoo, K. Crouse, A. Ali, B. Yamin, H.-K. Fun, Polyhedron 21
(2002) 2547.
[13] M. Tarafder, T.-J. Khoo, K. Crouse, A. Ali, B. Yamin, H.-K. Fun, Polyhedron 21
(2002) 2691.
[14] M. Hossain, M. Alam, J. Begum, M. Ali, M. Nazimuddin, F. Smith, R. Hynes,
Inorg. Chim. Acta 249 (1996) 207.
[15] K. Crouse, K.-B. Chew, M. Tarafder, A. Kasbollah, A. Ali, B. Yamin, H.-K. Fun,
Polyhedron 23 (2004) 161.
[16] Z. Otwinowski, W. Minor, in: C. Carter, R.M. Sweet (Eds.), Processing of X-ray
Diffraction Data Collected in Oscillation Mode, Methods Enzymology,
Academic Press, 1997, 276.
4. Conclusion
[17] A. Altomare, G. Cascarano, G. Giacovazzo, A. Guagliardi, M. Burla, G. Polidori,
M. Camalli, J. Appl. Crystallogr. 27 (1994) 435.
[18] D. Watkin, C. Prout, J. Carruthers, P. Betteridge, R. Cooper, J. Appl. Crystallogr.
36 (2003) 1487.
In summary, this research has shown, relative to (1), the
enhancement of cytotoxic activity upon the introduction of a
pyridine ring, while the introduction of a benzoic acid group re-
sulted in lower cytotoxic activity which might indicate that the
pyridine ring acts as a pharmacophore. However, antimicrobial
bioassay revealed different results whereby, relative to (1), activity
was lowered by both Schiff base ligands. Results have highlighted
the enhancement of bioactivity by chelation, specifically the high
antifungal activity of (9) which would require further testing to
ensure its high activity. Finally, this research emphasizes the
interesting potential role of derivatives of (1) as antioxidants.
[19] T. Mosmann, J. Immunol. Meth. 65 (1983) 55.
[20] C. Hufford, A. Clark, Discovery and Development of New Drugs for Systemic
Opportunistic Infections, Studies in Natural Product Chemistry, vol. 2, Elsevier,
Amsterdam, 1988. p. 421.
[21] H. Kikuzaki, N. Nakatani, J. Food Sci. 58 (1993) 1407.
[22] S. Karaböcek, N. Karaböcek, Polyhedron 16 (1997) 1771.
[23] N. Raman, S. Ravichandran, C. Thangaraja, J. Chem. Sci. 116 (2004) 215.
[24] E. Barefield, D. Busch, S. Nelson, Q. Rev. Chem. Soc. 22 (1968) 457.
[25] M. Chapman, P. Ayyappan, B. Foxman, G. Yee, W. Lin, Cryst. Growth Des. 1
(2001) 159.
[26] M. Kato, H. Jonassen, J. Fanning, Chem. Rev. 64 (1964) 99.
[27] N. Halim, K. Kassim, A. Fadzil, B. Yamin, Malays. J. Anal. Sci. 16 (2012) 56.
[28] T.-J. Khoo, M. Break, M. Tahir, K. Krouse, A. Cowley, D. Watkin, Acta
Crystallogr., Sect. E 69 (2013) m323.
Acknowledgements
[29] M. Break, S. Mehta, M. Tahir, K. Crouse, T.-J. Khoo, Acta Crystallogr. Sect. E 69
(2013) 178.
The authors thank the Ministry of Higher Education Malaysia
(MOHE) under FRGS (F0010.54.02) for providing a grant for this