Synthesis, characterization and bio-activity of a bidentate NS Schiff base of S-allyldithiocarbazate and its divalent metal complexes: X-ray crystal structures of the free ligand and its nickel(II) complex

Article abstract of DOI:10.1007/s11243-013-9783-8

Transition metal complexes ML₂ (2-6) [where M = Ni(II), Cu(II), Zn(II), Cd(II), Pd(II) and HL = allyl-2-(4-benzyloxyphenylmethylene)hydrazine carbodithioate (1)] have been prepared by the reaction of the ligand with metal ions in 2:1 molar ratio and characterized by physicochemical techniques and spectroscopic methods. The crystal structures of the free ligand and its nickel(II) complex 2 have been determined by X-ray diffractometry. The ligand exists in its thione tautomeric form both in solution and in the solid state. In complex 2, square-planar coordination of nickel(II) was achieved by two chelating ligand moieties coordinating through the azomethine nitrogen and the deprotonated thione sulphur atom. Based on the crystal structures of analogous dithiocarbazate species, a square-planar geometry was assumed for the copper(II) and palladium(II) complexes, and a tetrahedral coordination sphere for the zinc(II) and cadmium(II) derivatives. The in vitro bactericidal activity suggests that the palladium(II) complex is strongly active against two bacteria. The cadmium(II) complex is moderately cytotoxic with an LC₅₀ value of 409 μg/ml, but less active than gallic acid, LC₅₀ = 78 μg/ml.

Full text of DOI:10.1007/s11243-013-9783-8

Transition Met Chem (2014) 39:141–149
DOI 10.1007/s11243-013-9783-8
Synthesis, characterization and bio-activity of a bidentate NS
Schiff base of S-allyldithiocarbazate and its divalent metal
complexes: X-ray crystal structures of the free ligand and its
nickel(II) complex
•
M. Al-Amin A. A. Islam M. C. Sheikh
•
•
M. S. Alam E. Zangrando M. A. Alam
•
•
•
M. T. H. Tarafder R. Miyatake
Received: 19 September 2013 / Accepted: 11 November 2013 / Published online: 27 November 2013
ꢀ Springer Science+Business Media Dordrecht 2013
Abstract Transition metal complexes ML2 (2–6) [where
M = Ni(II), Cu(II), Zn(II), Cd(II), Pd(II) and HL = allyl-2-
(4-benzyloxyphenylmethylene)hydrazine carbodithioate (1)]
have been prepared by the reaction of the ligand with metal
ions in 2:1 molar ratio and characterized by physicochemi-
cal techniques and spectroscopic methods. The crystal
structures of the free ligand and its nickel(II) complex 2 have
been determined by X-ray diffractometry. The ligand exists
in its thione tautomeric form both in solution and in the solid
state. In complex 2, square-planar coordination of nickel(II)
was achieved by two chelating ligand moieties coordinating
through the azomethine nitrogen and the deprotonated thi-
one sulphur atom. Based on the crystal structures of analo-
gous dithiocarbazate species, a square-planar geometry was
assumed for the copper(II) and palladium(II) complexes,
and a tetrahedral coordination sphere for the zinc(II) and
cadmium(II) derivatives. The in vitro bactericidal activity
suggests that the palladium(II) complex is strongly active
against two bacteria. The cadmium(II) complex is moder-
ately cytotoxic with an LC50 value of 409 lg/ml, but less
active than gallic acid, LC50 = 78 lg/ml.
Introduction
It is more than a hundred years from the first synthesis of
dithiocarbazic acid by Curtius [1]. Since then, a consider-
able number of S-alkyl/aryl-esters, the Schiff bases of such
compounds and of their corresponding complexes have
been prepared for potential applications such as antibac-
terial [2–5], anticancer [3, 4] and antifungal [3, 5] activi-
ties. Dithiocarbazates constitute one of the most important
classes of mixed hard–soft nitrogen-sulphur donor ligands
[6, 7], having four potential donor atoms, of which two are
sterically available at a time to chelate metal ions. Free
dithiocarbazic acid and its S-methyl/benzyl esters behave
typically through the NS donors with formation of a five-
membered chelate ring [8, 9], but SS complexes are also
feasible with a four-membered chelate ring [8]. However, a
change of suitable groups at the hydrazine moiety of these
compounds can modify the donor sequence of such ligands,
giving rise to different coordination geometries [10, 11].
Although the majority of previous studies on dithiocar-
bazates were based on S-methyl and S-benzyl dithiocar-
bazate chelating agents, Schiff base ligands derived from
S-allyldithiocarbazates have received less attention [12–17].
The allyl group in the molecular structure is relevant for
many interesting chemical reactions and can also be used
as ligand [18]. The present work reports the coordination
M. A.-A. A. A. Islam (&) Á M. A. Alam
Department of Chemistry, Rajshahi University of Engineering
and Technology, Rajshahi 6204, Bangladesh
e-mail: aamin_ruet@yahoo.com
M. C. Sheikh
Department of Applied Chemistry, Faculty of Engineering,
University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
M. S. Alam
Department of Chemistry, Jagannath University, Dhaka 1100,
Bangladesh
E. Zangrando
Department of Chemical and Pharmaceutical Sciences,
Via L. Giorgieri 1, 34127 Trieste, Italy
M. T. H. Tarafder
Department of Chemistry, Rajshahi University, Rajshahi 6205,
Bangladesh
R. Miyatake
Water Quality Management Center, University of Toyama,
3190 Gofuku, Toyama 930-8555, Japan
123

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Transition Met Chem (2014) 39:141–149
behaviour of S-allyl-b-N-(4-benzyloxyphenyl) methylene-
hydrazine carbodithioate with some transition metal ions
and a study of their biological activities.
RAPID diffractometer using filtered Cu-Ka radiation
˚
(k = 1.54187 A). Both the structures were solved by direct
methods [20] and successive Fourier syntheses and refined
by the full-matrix least-squares method based on F2 with
all observed reflections [21]. The methine carbon atom
C(17) of ligand 1 was found disordered over two positions
with occupancies 0.718(18)/0.282(18). In addition, the
benzoyl group of the ligand exhibits a high librational
motion, but attempts to resolve this disorder were rather
unsatisfactory. Hydrogen atoms were geometrically located
and refined using the riding model. All calculations were
done using the Crystal Structure [22] package except for
refinement, for which SHELXL-97 was used [21]. Crystal
data and details of data collections and refinements are
summarized in Table 1, while a selection of bond lengths
and angles is listed in Table 2.
Experimental section
Materials
All the chemicals and solvents were of reagent grade and
used without further purification. Chemicals 4-hydroxy-
benzaldehyde (BDH, England), hydrazine hydrate (99.99 %),
nickel(II) acetate tetrahydrate, copper(II) acetate monohy-
drate, zinc(II) acetate dihydrate and cadmium(II) acetate
dihydrate were purchased from Fluka Chemika (Switzer-
land), palladium(II) nitrate hydrate from Sigma-Aldrich
(USA), allyl bromide from SRL Pvt. Ltd. (India), and carbon
disulphide and potassium hydroxide from Merck (India).
Solvents chloroform and ethanol were obtained from Active
Fine (Bangladesh), while tetrahydrofuran and MgSO4 from
Merck (India). The ligand precursor, 4-benzyloxybenzalde-
hyde, was prepared following the literature report [19].
Table 1 Crystallographic data of the Schiff base (1) and of nickel(II)
complex (2)
1
2
Empirical formula
fw
C18H18N2OS2
342.47
C36H34N4NiO2S4
741.63
Physical measurements
Crystal colour, habit
Crystal system
Space group
Yellow, plate
Triclinic
Brown, plate
Triclinic
IR spectra (4,000–400 cm-1) were obtained as KBr pellets
using an FTIR-8400/8900 spectrophotometer, Shimadzu
(Japan) at BCSIR laboratories (Bangladesh). 1H NMR
(0–12 ppm) and 13C NMR (0–200 ppm) spectra were
recorded on a JNM 400 MHz and on a 100 MHz spec-
trometer, respectively, in CDCl3 (and in some cases, d6-
DMSO) using TMS as internal standard at the Department
of Applied Chemistry, Toyama University (Japan). Mass
spectra were obtained on a JEOL JMS-D300 mass spec-
trometer (scan range, mass-to-charge (m/z) ratio 0–850)
at Venture Business Laboratory, University of Toyama
(Japan), and C, H, N and S microanalyses with a Perkin-
Elmer 2400 II organic elemental analyser (USA) from the
Department of Chemistry, Okayama University, Tsushima,
Okayama (Japan). Magnetic susceptibility measurements
were obtained on a magnetic susceptibility balance (Sher-
wood Scientific, UK), molar conductance measurements
with a heavy-duty conductivity/temperature meter (Extech
Instruments, USA, model no. 407303). UV–visible spectra
were obtained on a T60 UV–vis spectrophotometer (PG
P 1
P 1
˚
a (A)
4.62872(9)
11.4807(2)
17.1138(3)
73.6004(10)
84.4221(10)
79.4286(10)
856.64(3)
2
10.51235(19)
13.6357(3)
13.8453(3)
116.1672(7)
93.6116(7)
103.8577(7)
1,696.69(6)
2
˚
b (A)
˚
c (A)
a (deg)
b (deg)
c (deg)
3
˚
V (A )
Z
Dcalcd (g cm-3
)
1.328
1.452
Crystal size (mm)
l(Cu-Ka) (mm-1
F(000)
0.53 9 0.33 9 0.08 0.43 9 0.40 9 0.17
)
3.852
360
3.440
772
h max (deg)
65.08
9,218
68.24
19,760
No. of reflections
collected
Rint
0.0600
0.0795
T
min, Tmax
0.485, 0.976
2,894
0.429, 0.557
6,103
No. of independent
reflections
Instruments, UK) between 200 and 800 nm using 10-5
M
No. of observed
reflections (I [ 2r(I))
2,301
213
4,850
424
solutions in chloroform at the Department of Chemistry,
RUET (Bangladesh).
No. of refined
parameters
X-ray crystal structure determination
Goodness-of-fit, S (F2) 1.097
1.044
R1, wR2 (I [ 2r(I))
0.0699, 0.1851
1.676, -0.714
0.0496, 0.1404
0.790, -0.590
Intensity measurements for the structures reported were
taken at a temperature of 173(1) K on a Rigaku R-AXIS
3
Residuals (e/A )
˚
123

Transition Met Chem (2014) 39:141–149
143
(M?, 5.59 %), 341 (M–H, 1.35 %), 340 (M–H2, 10.0 %),
330 (M–C, 1.89 %), 325 (M–NH3,8.04 %), 268 (M–C3H3S,
7.62 %), 228 (M–C4H6N2S, 7.16 %), 91 (M–C11H11ON2S2,
100 %). HRMS (EI?) Calcd for C18H18N2OS2 (M?):
342.0861; found (M?):342.0863.
˚
Table 2 Selected bond lengths (A) and angles (ꢁ) of LH (1) and of
complex 2
1
2
Ligand 1
2
Ligand 2a
S(1)–C(15)
1.677(4)
1.750(4)
1.818(5)
1.276(6)
1.329(6)
1.380(5)
115.0(4)
120.6(4)
121.5(4)
121.4(3)
114.3(3)
124.4(3)
107.8(4)
102.4(2)
1.722(3)
1.757(3)
1.712(2)
1.756(3)
S(2)–C(15)
Synthesis of complexes 2–6
S(2)–C(16)
1.810(3)
1.795(3)
N(1)–C(14)
1.293(4)
1.298(4)
A solution of 0.5 mmol of the metal salt Ni(OAc)2.4H2O,
Cu(OAc)2.H2O, Zn(OAc)2.2H2O, and Cd(OAc)2 .2H2O
dissolved in 30 ml ethanol/methanol (Pd(NO3)2.H2O dis-
solved in 50 ml acetonitrile) was added to a boiling solu-
tion of the Schiff base (0.342 g, 1 mmol) in 100 ml ethanol
in the presence of 2–5 drops of conc. H2SO4. The resulting
solution was refluxed from 0.5 to 12 h. The precipitate
formed in each case was separated, washed with hot eth-
anol and dried under vacuum over anhydrous CaCl2. The
nickel(II) complex (2) was crystallized by slow evapora-
tion method from boiling chloroform (70 ml) in the pre-
sence of toluene (10 ml) as brown platelet single crystals
suitable for X-ray crystallography. However, any attempt
to crystallize the other metal complexes was unsuccessful.
The physicochemical characterizations of the complexes
are listed below:
N(2)–C(15)
1.272(4)
1.276(4)
N(1)–N(2)
1.424(3)
1.420(3)
N(2)–N(1)–C(14)
N(1)–N(2)–C(15)
N(1)–C(14)–C(11)
N(2)–C(15)–S(1)
N(2)–C(15)–S(2)
S(1)–C(15)–S(2)
C(17)–C(16)–S(2)
C(15)–S(2)–C(16)
a
113.34(17)
111.87(17)
132.7(3)
113.46(17)
112.21(17)
133.0(2)
125.77(17)
120.26(15)
113.96(17)
113.46(19)
102.65(13)
125.49(17)
120.78(15)
113.72(17)
117.8(3)
103.70(14)
Atom labels of ligand 2 refer to Fig. 3
Synthesis of the Schiff base
The Schiff base was prepared as in a previous study [12]: to
a mixture of 5 ml (5.06 g, 0.1 mol) hydrazine hydrate and
5.6 g (0.1 mol) KOH in 30 ml of ethanol at 5 ꢁC, a solu-
tion of carbon disulphide 6.1 ml (7.61 g, 0.1 mol) and allyl
bromide 8.6 ml (12.10 g, 0.1 mol) was added successively
with continuous stirring in an ice bath. To this mixture, a
solution of 4-benzyloxybenzaldehyde 21.2 g (0.1 mol) in
ethanol (50 ml) was added and stirring was continued for
10 min; then, the pale yellow solid formed was separated,
washed with ethanol and dried under vacuum. The com-
pound was crystallized from chloroform and toluene (24:6;
v/v) as yellow platelet single crystals suitable for X-ray
diffraction after 15 days at *10–15 ꢁC (39.25 g, 76 %);
m.p. 144 145 ꢁC. Anal. Calc. for C18H18N2OS2 requires: C,
63.1; H, 5.3; N, 8.2; S, 18.7. Found: C, 63.2; H, 5.4; N, 8.2;
S, 18.7 %. Selected IR data (KBr pellet, cm-1): 3,107 s
t(N–H), 1,599 s t(C=N), 1,097 s t(C=S), 1,028 s t(N–N),
Nickel(II) complex (2) as brown solid, yield: 0.29 g
(62 %). m.p. 177–178 ꢁC. Anal. Calc. for C36H34N4O2S4-Ni
requires: C, 58.3; H, 4.6; N, 7.6; S, 17.3. Found: C, 58.4; H,
4.7; N, 7.5; S, 17.3 %. Selected IR data (KBr pellet, cm-1):
1,581 s t(C=N), 1,603 m t(C=C, ring), 1,259 s t(C–O–C),
1,024 s t(N–N), 960 m t(C–S–S). 1H NMR (500 MHz,
CDCl3, ppm) d: 3.81 (d, J = 6.5 Hz, 4H, SCH2), 5.12 (s, 4H,
CH2O), 5.32, 5.18 (dd, J = 17, 10 Hz, 4H, = CH2),
5.95–6.03 (m, 2H, -CH), 6.96 (d, J = 9.0 Hz, 4H, C9,13),
7.98 (d, J = 8.5 Hz, 4H, C10,12), 7.34–7.41 (m, 10H, ph),
7.62 (bs, 2H, CH=N); 13C NMR (CDCl3, 125 MHz) d:
135.91 (C=N), 70.16 (CH2O), 161.82, 135.57, 128.70,
128.29, 127.46, 124.99, 124.51, 114.78 (ring), 37.78, 132.62,
118.52 (allyl). UV–vis spectrum [kmax, CHCl3, nm (loge):
243(3.51), 280(3.66), 331(3.71), 385(3.47), 441(3.22).
LRMS (FAB?, 70 eV, %) m/z: 740 (M?, 0.25 %), 613 (M–
C5H7N2S, 1.13 %), 569 (M–C12H11O, 1.29 %), 524 (M–
C14H16O2, 1.38 %), 460 (M–C16H12ON2S, 2.61 %); HRMS
(FAB) Calcd for C36H34N4NiO2S4 (M ? 1): 740.0896;
found (M ? 1): 740.0918. K (CHCl3, 10-5M, ohm-1cm2 -
mol-1): 0.1; leff = diamagnetic.
1
983 m t(C–S–S). H NMR (500 MHz, CDCl3, ppm) d:
3.99 (d, J = 7.5 Hz, 2H, SCH2), 5.03 (s, 2H, CH2O), 5.36,
5.20 (dd, J = 17, 10 Hz, 2H, = CH2), 5.94–6.02 (m, 1H,
-CH), 6.99 (d, J = 9.0 Hz, 2H, C9,13), 7.66 (d,
J = 9.0 Hz, 2H, C10,12), 7.43 (dd, J = 2, 6.5 Hz, 2H,
C2,6), 7.34 (dd, J = 6.5 Hz, 2H, C3,5), 7.39 (t, 1H,
Copper(II) complex (3) as orange solid, yield: 0.32 g
(72 %). m.p. 163–165 ꢁC. Anal. Calc. for C36H34N4O2S4-
Cu requires: C, 57.9; H, 4.6; N, 7.5; S, 17.2. Found: C, 58.0;
H, 4.6; N, 7.5; S, 17.2 %. Selected IR data (KBr pellet,
cm-1): 1,622w t(C=C, allyl), 1,587 s t(C=N), 1,603 s,
1,560w, 1,508 s t(C=C), 1,256 s t(C–O–C), 1,024 m t(N–
N), 997 m t(C–S–S). UV–vis spectrum [kmax, CHCl3, nm
(loge): 246(3.94), 283(3.88), 341(3.94), 584(1.97). LRMS
J = 7.0 Hz, C4), 7.85 (s, 1H, C=N), 10.75 (s, 1H, NH); 13
C
NMR (CDCl3, 125 MHz, ppm) d: 197.77 (C=S), 145.49
(CH=N), 161.15, 136.26, 129.60, 128.64, 128.17, 127.47,
125.65, 115.23 (ring), 70.08 (CH2O), 37.44, 132.395,
118.85 (allyl); UV–vis spectrum [kmax, CHCl3, nm (loge):
240 (3.10), 347(3.59), LRMS (EI?, 70 eV, %) m/z: 342
123

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Transition Met Chem (2014) 39:141–149
1
(FAB?, 70 eV, %) m/z: 747 (M ? 2, 0.33 %), 612
(M–C9H9O, 0.74 %), 461 (M–C16H16ON2S, 4.30 %), 307
(M–C18H19ON2S3Cu, 43.45 %), 210 (M–C22H22ON3S4Cu,
28.41 %), 154 (M–C35H29O2NS3, 100 %). K (CHCl3,
10-5M, ohm-1cm2 mol-1): 3.1; leff = 1.86 B. M.
1,019 m t(N–N), 963 m t(C–S–S). H NMR (500 MHz,
CDCl3, ppm) d: 3.87 (d, J = 6.5 Hz, 4H, SCH2), 5.09
(s, 4H, CH2O), 5.31, 5.20 (dd, J = 16.5, 16.5 Hz,
4H, = CH2), 5.84–6.04 (m, 2H, -CH), 6.89 (d, J =
8.0 Hz, 4H, C9,13), 7.04 (d, J = 8.5 Hz, 4H, C10,12),
7.30–7.46 (m, 10H, ph), 7.57 (s, 2H, CH=N); 13C NMR
(CDCl3, 125 MHz) d: 133.23, 130.10, 128.71, 128.52,
127.64, 127.56, 127.47, 115.08 (ring), 99.86 (CH2O),
Zinc(II) complex (4) as yellow solid, yield: 0.26 g
(58 %). m.p. 210–212 ꢁC. Anal. Calc. for C36H34N4O2S4-
Zn requires: C, 57.8; H, 4.6; N, 7.5; S, 17.1. Found: C, 57.8;
H, 4.6; N, 7.5; S, 17.1 %. Selected IR data (KBr pellet,
cm-1): 1,620 s t(C=C, allyl), 1,603 s t(C=N), 1,572,
1,506 m t(C=C), 1,250vs t(C–O–C), 1,024 m t(N–N),
957 m t(C–S–S). 1H NMR (500 MHz, DMSO-d6 ?
CDCl3, ppm) d: 3.34 (d, merged with DMSO, 4H, SCH2),
5.12 (s, 4H, CH2O), 7.10 (d, J = 9.0 Hz, 4H, C9,13), 7.79
(d, J = 9.0 Hz, 4H, C10,12), 7.33–7.46 (m, 10H, ph), 8.60
(s, 2H, CH=N). UV–vis spectrum [kmax, CHCl3, nm (loge):
239(3.32), 332(3.91). LRMS (FAB?, 70 eV, %) m/z: 790
(M ? G, 1.07 %), 708 (M–C3H3, 5.32 %), 493 (M–C17H18O2,
3.43 %), 422 (M–C20H21O2S, 6.74 %), 386 (M–
C17H16ON2SZn, 13.48 %), 238 (M–C21H20ON2S4Zn,
15.06 %), 191 (M–C31H27O2N2SZn), 169 (M–C24H25ON4S4Zn,
96.63 %), 85 (M–C32H29O2N4S3Zn, 100 %). K (CHCl3,
10-5M, ohm-1cm2 mol-1): 00; leff = diamagnetic.
Cadmium(II) complex (5) as yellow solid, yield: 0.2 g
(42 %). m.p. 216 ꢁC. Anal. Calc. for C36H34N4O2S4-Cd
requires: C, 54.4; H, 4.3; N, 7.0; S, 16.1. Found: C, 54.1; H,
4.3; N, 6.9; S, 16.2 %. Selected IR data (KBr pellet, cm-1):
1,648 s t(C=C, allyl), 1,589 s t(C=N), 1,556 m, 1,506 s
(C=C), 1,258 s t(C–O–C), 1,022 m t(N–N), 939 m t(C–S–
S). 1H NMR (500 MHz, CDCl3, ppm) d: 3.70 (d,
J = 5.5 Hz, 4H, SCH2), 5.04 (s, 4H, CH2O), 5.17, 5.03 (dd,
J = 17, 12.5 Hz, 4H, = CH2), 5.84–5.97 (m, 2H, -CH),
6.86 (d, J = 8.0 Hz, 4H, C9,13), 6.96 (d, J = 8.0 Hz, 4H,
C9,13), 7.54 (d, J = 8.5 Hz, 4H, C10,12), 7.32–7.37 (m,
10H, ph), 7.74 (s, 2H, CH=N); 13C NMR (CDCl3, 125 MHz)
d: 136.05 (C=N), 70.16, 70.02 (CH2O), 162.01, 135.20,
132.88, 130.35, 128.65, 128.63, 128.58, 118.07, 117.80,
115.37, 114.47 (ring), 34.92, 133.62, 128.24,128.19, 127.51,
127.47 (allyl). UV–vis spectrum [kmax, CHCl3, nm (loge):
240(3.53), 349(3.84). LRMS (FAB?, 70 eV, %) m/z: 796
(M?, 2.02 %), 713 (M–C6H10, 2.29 %), 599 (M–C14H13O,
1.62 %), 580 (M–C14H16O2, 1.88 %), 522 (M–C20H18O,
4.93 %), 470 (M–C17H14N2OS2, 3.23 %), 455 (M–
C18H17N2OS2, 12.38 %), 309 (M–C28H27N2OS2, 12.21 %);
HRMS (FAB) Calcd for C36H34N4CdO2S4 (M ? 1):
796.0598; found 796.0500; K (CHCl3, 10-5M, ohm-1 cm2
mol-1): 00; leff = diamagnetic.
36.04, 128.62, 115.08 (allyl). UV–vis spectrum [kmax
,
CHCl3, nm (loge): 243(3.99), 273(3.96), 330(3.86),
658(2.04). LRMS (70 eV) m/z: 789 ([M ? 1]?, 2.13), 613
(M–C13H19, 0.55 %), 571 (M–C14H17O2, 1.10 %), 527
(M–C14H13OS2, 1.96 %), 438 (M–C26H22O, 4.43 %), 421
(M–C20H19N2OS2, 6.01 %), 395 (M–C28H25O2, 4.79 %),
307 (M–C18H19N2OS3, 3.74 %), 91 (M–C29H27N4O2S4Pd,
100 %); HRMS (FAB) Calcd for C36H34N4PdO2S4
(M ? 1): 788.0599; found 788.0672. K (CHCl3, 10-5 M,
ohm-1 cm2 mol-1): 0.0; leff = diamagnetic.
Qualitative antimicrobial assay
In vitro bactericidal activity of compounds 1–6 was
determined by the Kirby-Bauer disc diffusion method [23].
Briefly, Trypticase Soy Agar media (Sigma-Aldrich) were
used as basal media for testing bacteria. These agar media
were inoculated with 0.2 ml of the 24-h liquid cultures
containing microorganisms. Sample discs were placed
gently on pre-inoculated agar plates, and Staphylococcus
aureus (S. a.), Klebsiella pneumonia (K. p.), Escherichia
coli (E. c.), Cronobacter sakazakii (C. s.), Citrobacter
freundii (C. f.) and Salmonella enterica (S. e.) were incu-
bated aerobically at 37 ꢁC for 24 h. A disc with DMSO
only was used as a negative control and nalidixic acid was
used as a positive control. Inhibitory activity was measured
(in mm) as the diameter of the observed inhibition zones.
Antimicrobial and cytotoxic assays were carried out at the
Food analysis and Research laboratory, Centre for Advance
Research in Sciences, University of Dhaka, Dhaka-1000,
Bangladesh.
Cytotoxic assay
Brine shrimp nauplii (A. salina) were used in the in vivo
cytotoxicity assay, according to the method of Mayer et al.
[24] with some modifications. Brine shrimp nauplii were
hatched in a small tank divided by a net and containing
artificial sea water (3.8 % NaCl); the tank was partially
exposed to incandescent light in order to attract the nauplii.
The assay was performed 24 h after hatching, and no food
was added during the hatching and experimental periods.
Test samples (3 mg) were dissolved in 0.6 ml of DMSO to
obtain stock solutions of 5 mg/ml for each sample. From
Palladium(II) complex (6) as reddish brown solid, yield:
0.24 g (52 %), m. p. 195–200 ꢁC. Anal. Calc. for
C36H34N4O2S4-Pd requires: C, 54.8; H, 4.3; N, 7.1; S, 16.3.
Found: C, 54.7; H, 4.3; N, 6.9; S, 16.3 %. Selected IR data
(KBr pellet, cm-1): 1,653 mw t(C=C, allyl), 1,570 s
t(C=N), 1,601 m, 1,506 s t(C=C), 1,259 s t(C–O–C),
123

Transition Met Chem (2014) 39:141–149
145
the stock solutions, different concentrations of test samples
were placed in separate vials, and the volume of each vial
was made up to 5 ml with artificial sea water to obtain the
desired final concentrations. Twenty brine shrimp nauplii
were then placed in each vial. The negative control was
prepared in the same manner, omitting the sample. Gallic
acid was used as standard. After 24 h of incubation, the
vials were observed using a magnifying glass, and the
number of survivors in each vial was counted. Tests were
performed in triplicate, and the resulting data were trans-
formed to probit analysis software [25] for the determina-
tion of LC50 values.
ligands to the metals had taken place via the azomethine
nitrogen atom [16, 17]. Moreover, the t(N–N) and t(CSS)
bands at 1,028 and 983 cm-1 in the free ligand were
lowered in the complexes to 1,024–1,019 and
963–939 cm-1, respectively [15, 16], due to the delocal-
ization of bonding electrons upon complexation.
In its 1H NMR spectrum, the free ligand showed no band
at 4.0 ppm; however, it showed resonance at 10.75 ppm for
the NH proton [13, 26], indicating that in solution, it exists
in thione form (Scheme 1) and the absence of this band in
the complexes also suggested that it coordinated through
enethiolization and subsequent deprotonation of the thione
sulphur atom [12–17]. The azomethine (CH=N) proton
exhibited a singlet at 7.85 ppm for the free ligand [13],
which was observed in the complexes at 7.57–7.62 ppm,
indicating its coordination through the azomethine nitrogen
atom [13, 16, 17]. Moreover, the doublet for the SCH2
protons (3.99 ppm) in the ligand was also shifted to the
upfield (3.34–3.87 ppm) as a result of complexation [13,
17]. The 13C NMR spectrum of the Schiff base showed
signals at 197.77 and 145.49 ppm for the thioamide and
azomethine carbons, respectively [14]. Although the for-
mer was not observed in the complexes, the azomethine
carbon resonance was shifted upfield as a result of com-
plexation. In addition, the allylic SCH2, CH and CH2 car-
bon signals observed for the free ligand at 37.44, 132.40
and 118.85 ppm, respectively [14], were also affected in
the complexes.
Results and discussion
The ligand allyl 2-(4-benzyloxyphenyl)methylene hydra-
zine carbodithioate (HL) with N,S donor set was prepared
in situ by 1:1 condensation of S-allyldithiocarbazate with
4-benzyloxybenzaldehyde in ethanol. The reaction of HL
with divalent metal salts in 2:1 molar ratio afforded the
corresponding metal chelates. The yield, colour, melting
points, elemental analyses, solution molar conductivity,
magnetic susceptibility and spectroscopic characterizations
are presented in the experimental section. All analytical
and spectroscopic data are in good agreement with their
proposed structures. The ligand is less soluble in ethanol/
methanol, and solubility was enhanced by the addition of
2–5 drops of conc. sulphuric acid during reactions with
metal salts. However, the ligand and its complexes are
fairly soluble in other organic solvents. The molar con-
ductivities of solutions of the complexes indicated their
non-electrolytic nature. The ligand can exhibit thione–thiol
tautomerism (Scheme 1) and Z/E isomeric forms around
the thioamide functional group (–NH–C(=S)SR) [12].
Important spectroscopic properties of the metal complexes
are discussed below.
The solution UV–vis spectrum (Fig. 1) of the ligand
showed bands at 240 and 347 nm, assigned to the p ? p*
and n ? p* transitions for the aromatic ring and dithioc-
arbazate moiety, respectively [17]. In addition to these
intra-ligand transitions, another band at 273–283 nm was
also observed in the nickel(II), copper(II) and palladium(II)
complexes, probably arising from the p ? p* transition
for the azomethine group [17]. In particular, the square-
planar nickel(II) and palladium(II) complexes are expected
1
1
to exhibit three bands corresponding to the A1g ? A2g,
1
1
1
1A1g ? B1g and A1g ? Eg transitions, respectively.
However, in ligands containing sulphur atoms, the strong
inter-ligand and charge transfer transitions invariably
interfere with the observation of these bands [17]. The band
at 441 nm was assigned to the S ? Ni(II) charge transfer
transition [17] and a broad weak band at 658 nm to the d–d
transition of square-planar palladium(II). The copper(II)
complex showed a very broad weak band at 584 nm,
The solid-state IR spectrum of the Schiff base showed
bands at 3,107 and 1,097 cm-1, corresponding to the t(N–
H) and t(C=S) stretching, which were absent in the spectra
of the complexes, suggesting the coordination of 1 via the
deprotonated thiolate sulphur atom as shown in Scheme 1
[16, 17]. In addition, the lowering of the t(C=N) stretching
from 1,599 cm-1 in the ligand to 1,589–1,570 cm-1 in the
complexes revealed that coordination of the bis-chelated
Scheme 1 Thione (I) and thiol
(II) forms of the Schiff base 1
9
10
8
5
11
14
6
O
O
N N
HS
N NH
4
12
13
7
1
15
S
S
S
17
3
2
16
18
(I)
(II)
123

146
Transition Met Chem (2014) 39:141–149
Fig. 1 UV–visible spectra of compounds 1–6
Fig. 3 ORTEP diagram (thermal ellipsoids at 50 % probability level)
of 2 with atom numbering scheme
2
assigned to the 2B1 ? A1 transition diagnostic of a square-
planar geometry [16].
Crystal structure determination
The molecular structures of the Schiff base (1) and its
nickel(II) complex (2) were determined by single crystal X-
ray diffraction method, and ORTEP drawings are shown in
Figs. 2 and 3, respectively. A list of selected interatomic
distances and angles is given in Table 2. Both compounds 1
and 2 crystallize in triclinic system with space group P 1.
In the ligand (1), the dithiocarbazate group adopts an EE
configuration with respect to the C=N bond of the benzyl-
idene group [26]. The b-nitrogen and the thioketo sulphur
are in trans configuration (E) with respect to the C(15)–N(2)
bond. The ligand is in its thione tautomeric form with
Fig. 4 Crystal packing of the Schiff base 1 with indication of N(2)–
H…S(1) interactions
electron delocalization throughout the molecule. In the
crystal, the molecules are pairwise-connected by N(2)–
˚
C(15)=S(1) bond length of 1.677(4) A, and all bond lengths
are closely comparable to values of previously reported
derivatives [13, 16, 26]. The whole ligand has coplanar
atoms with the exception of the allyl fragment, indicating an
˚
H…S(1) hydrogen bonds (N…S = 3.361(4) A, N–H…S
angle of 160ꢁ) to form centrosymmetric dimers (Fig. 4).
The arrangement is similar to that observed for the methyl-
3-(4-benzyloxyphenyl)methylene dithiocarbazate as earlier
reported [26] and also in the allyl-2-[(1-pyrrolmethyl-
ene)hydrazine carbodithioate [13], a feature frequently
observed in solid state structures of these complexes. No
appreciable p–p interaction among the aromatic rings is
present in the crystal packing (Fig. 4).
With respect to the free ligand, a rotation about the
C(15)–N(2) by 180ꢁ is required to allow the N,S coordi-
nation to the metal. As confirmed by the spectral and
magnetic study, the complex is monomeric with a slightly
distorted square-planar geometry. The two Schiff bases, in
their deprotonated imino thiolate form, are coordinated to
the nickel(II) centre via the azomethine nitrogen N(1) and
thiolate sulphur S(1) atoms in a trans-planar configuration,
where the N2S2 donor set presents coplanar atoms with a
Fig. 2 ORTEP diagram (thermal ellipsoids at 50 % probability level)
of 1 with atom numbering scheme
123

Transition Met Chem (2014) 39:141–149
147
˚
Table 3 Coordination bond lengths (A) and angles (ꢁ) of complex 2
Ni–S(1)
2.1731(9)
Ni–N(1)
1.9275(18)
Ni–S(3)
2.1728(9)
86.31(7)
177.74(3)
94.25(7)
Ni–N(3)
1.9293(18)
86.17(7)
S(1)–Ni–N(1)
S(1)–Ni–S(3)
S(1)–Ni–N(3)
S(3)-Ni–N(3)
N(1)-Ni–N(3)
S(3)-Ni–N(1)
178.91(9)
93.31(7)
˚
maximum displacement of 0.02 A from the mean plane.
This coordination geometry is the most frequently
observed [13, 16] for this type of ligands, although Ni
complexes with cis configuration have also been reported
[27, 28]. In the cited complexes, the cis configuration
appears to be stabilized by p–p stacking interactions
occurring between the aromatic rings. In complex 2, the
two ligands present closely comparable bond lengths and
angles within their e.s.d.’s. In addition, within each ligand,
the benzyloxy ring forms an angle of 23.5 and 9.3ꢁ with the
adjacent semicarbazate fragment, indicating an electron
delocalization system in the latter, that could justify the
small differences in bond angles observed between the two
dithiocarbazate groups. On the other hand, the dihedral
angle formed by the benzyloxy and phenyl rings in the two
ligands is of 78.2 and 88.8ꢁ.
Fig. 5 Crystal packing of the nickel(II) complex (2)
and for the zinc(II) [33, 34] and cadmium(II) [35, 36]
derivatives (4, 5) a tetrahedral coordination sphere, based
on crystal structures of analogous dithiocarbazate species.
Antibacterial activity
˚
The Ni–S bond distances (of 2.1731(9), 2.1728(9) A)
˚
and the Ni–N ones (1.9275(18), 1.9293(18) A) are com-
The Schiff base (1) and its complexes were evaluated for
their in vitro antibacterial activities against two Gram-
positive bacteria, i.e. Staphylococcus aureus and Klebsiella
pneumonia, and four Gram-negative bacteria, E. Coli,
Cronobacter sakazakii, Citrobacter freundii and Salmo-
nella enterica, by disc diffusion methods. As presented in
Table 4, the Schiff base ligand 1 and its zinc(II) (4) and
cadmium(II) (5) complexes did not show any bactericidal
activity against selected bacterial strains used in the present
study. Inhibition zones of the test compounds were mea-
sured at a dose of 500 lg disc-1, and nalidixic acid, a
parable with those found in most square-planar nickel(II)
complexes of nitrogen-sulphur chelating agents [13, 16],
and the chelating angles (mean value 86.24(7)ꢁ) are
indicative of the deviations from the values of typical
square-planar geometry (Table 3).
Upon coordination, some salient features are observed
with respect to the free ligand, as the elongation of the
S(1)–C(15) and S(3)–C(33) bond distances (1.722(3),
˚
˚
1.712(2) A), compared to the value for 1 (1.677(4) A), that
validate the coordination with deprotonated thiolate sul-
phur atom. Correspondingly, the C=N bond distance, of
positive control, was evaluated at a dose of 50 lg disc-1
.
˚
1.329(6) A in the free ligand 1, is shortened to 1.272(4) and
˚
1.276(4) A in the complex. These geometrical differences
Among the metal complexes with Schiff base ligand 1,
between the free ligand and the Ni complex, involving also
the bond angles, are reported in Table 2 and agree with
similar observations reported in previous papers [13, 16,
17]. It is worth noting the different conformation assumed
by the allyl groups, being the torsion angles S(2)–C(16)–
C(17)–C(18) and S(4)–C(34)–C(35)–C(36) of 114.1(3) and
7.4(5)ꢁ, likely for packing requirements. No significant p–p
interaction was detected in the crystal packing of the
complex, being the shorter centroid-to-centroid distances
Table 4 Antibacterial profile of compounds 1–6 in terms of the
inhibition zone
Compa.
Conc.
(lg disc-1
Gram-positive
Gram-negative
)
S. a. K. p. E. c. C. s. C. f. S. e.
2
3
6
500
50
–
–
–
–
–
9
7
9
–
8
7
–
8
9
10
5
–
–
8
15
7
˚
between aromatic rings of ca. 5.0 A. (Fig. 5).
Nalidixic
acid
10
10
10
Since attempts to crystallize other metal complexes
failed, we assume for the copper(II) [29, 30] and palla-
dium(II) [31, 32] complexes (3, 6) a square-planar geometry
‘‘–’’ indicates no activity
a
Compounds 1, 4 and 5 showed no activity
123

148
Transition Met Chem (2014) 39:141–149
Table 5 Results of cytotoxic
effect of compounds 1–6 and
gallic acid
Compa.
LC50 (lg/ml)
95 % confidence
limit (lg/ml)
Lower
Regression equation
R2b
Upper
1
1,346
1,234
409
1,057
1,038
126
1,656
1,586
812
y = 1.566x ? 1.031
y = 2.0233x - 1.2728
y = 1.8941x ? 0.1396
y = 1.7267x - 0.4622
y = 2.5933x - 2.4888
0.79
0.93
0.90
0.91
0.88
4
a
Compounds 2 and 3 showed
no activity
5
6
1,429
78
1,142
65
1,959
94
b
Log (conc.) vs probit
correlation coefficient
Gallic Acid
three compounds inhibited some Gram-negative bacteria,
and one compound resisted the growth of only one Gram-
negative bacteria strain. In detail, nickel(II) complex 2 was
found to be weakly active against E. Coli, C. freundii and
S. enterica. The copper(II) complex 3 was also weakly
active against C. sakazaki but showed moderate activity
against E. coli, and S. enterica. Palladium(II) complex 6
showed good bactericidal activity against S. aureus and S.
enterica, while the positive control, nalidixic acid, showed
less activity at a concentration of 50 lg disc-1. In this
study, the complexes with Schiff base ligand 1 appear to
have greater activity against Gram-negative bacteria com-
pared to Gram-positive bacteria. However, further studies
are needed in order to clarify this point.
Conclusion
The ligand afforded bis-chelated neutral ML2 complexes
with divalent metal ions, acting in a uninegative bidentate
fashion with its NS donor set. Although the Schiff base
showed no antibacterial activity against the test organisms,
its metal chelates exhibited weak-to-moderate action. In
particular, the complexes showed a significant activity
against Gram-negative bacteria in comparison with Gram-
positive bacteria. Although the palladium(II) complex is a
strong antibacterial agent and the cadmium(II) complex
showed potential cytotoxicity, they are generally toxic to
the environment. Our continuing interest in this area will
be the subject of forthcoming papers.
Cytotoxic activity
Supplementary data
The brine shrimp bioassay is an excellent tool to be used
for monitoring the biological activity in order to predict the
ability to kill cancer cells [37], and LC50 values obtained in
this assay can be used in correlation of a more specific
activity [38, 39]. A summary of the cytotoxic assay results
of the Schiff base ligand 1 and its complexes (2–6) is
presented in Table 5. Among all these compounds, cad-
mium(II) complex 5 showed the highest cytotoxicity with
LC50 values of 409 ppm, followed by zinc(II) complex 4,
Schiff base ligand 1 and palladium(II) complex 6 with
LC50 values of 1,234, 1,346 and 1,429 ppm, respectively.
On the other hand, nickel(II) complex 2 and copper(II)
complex 3 were found to be inactive against A. salina. It is
worth noting that the complex of cadmium(II) resulted in a
[3 times increase in the toxicity against A. salina with
respect to that of ligand 1, while in the case of complexes 4
and 6, the activity remains almost the same compared to
that of Schiff base ligand. However, formation of com-
plexes of nickel(II) and copper(II) resulted in the loss of
cytotoxicity with respect to that of ligand 1 against brine
shrimp nauplii (A. salina). However, all the compounds
exhibited lower toxicity against A. Salina compared with
that of the standard cytotoxic agent, gallic acid (LC50
78 ppm).
CCDC 940562 and 936064 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (?44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.1007/
s11243-013-9783-8.
Acknowledgments The authors MAAAI and MAA thank the
Department of Chemistry, RUET, for laboratory facilities, and MCS
thanks the Department of Applied Chemistry, Faculty of Engineering,
University of Toyama, Japan, for analytical facilities. MSA
acknowledges Food analysis and Research Laboratory, Centre for
Advance Research in Sciences, University of Dhaka, Dhaka-1000,
Bangladesh, for providing the facilities to perform antibacterial
screening and cytotoxicity assay.
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