Coordination behavior of three geometric isomers of indole-based S-benzyldithiocarbazone ligands towards nickel, zinc and cadmium divalent ions

Article abstract of DOI:10.1016/j.ica.2010.11.008

Three indolyl-imine ligands have been synthesized through the condensation of S-benzyldithiocarbazate with indole-2-carbaldehyde, indole-3-carbaldehyde and indole-7-carbaldehyde. Treatment of these Schiff bases with acetate salts of Ni(II), Zn(II) and Cd(

Full text of DOI:10.1016/j.ica.2010.11.008

Inorganica Chimica Acta 366 (2011) 233–240
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Coordination behavior of three geometric isomers of indole-based
S-benzyldithiocarbazone ligands towards nickel, zinc and cadmium divalent ions
a
b
Hamid Khaledi a, , Hapipah Mohd Ali , Marilyn M. Olmstead
⇑
a Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
b Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 August 2010
Received in revised form 28 October 2010
Accepted 4 November 2010
Available online 17 November 2010
Three indolyl-imine ligands have been synthesized through the condensation of S-benzyldithiocarbazate
with indole-2-carbaldehyde, indole-3-carbaldehyde and indole-7-carbaldehyde. Treatment of these
Schiff bases with acetate salts of Ni(II), Zn(II) and Cd(II) in ethanol yielded a series of complexes of 2:1
type (ligand/metal ratio) in which the ligands coordinated to the metal ions as monoanionic NS bidentate
chelates. While the 2-imineindole and 3-imineindole formed the expected five-membered chelate rings,
the X-ray crystal structure of [Cd(HL3)(py)2], (HL3 = the mono-deprotonated 7-imineindole), revealed an
unusual mode of coordination, namely formation of four-membered rings with the metal atom. Reaction
of the 7-imineindole with the metal ions in the presence of potassium hydroxide produced complexes of
the type [M(L3)(H2O)] in which the Schiff base acts as a dianionic NNS tridentate ligand.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Schiff base
Indolecarbaldehyde
S-benzyldithiocarbazate
Divalent metal complexes
X-ray crystal structure
1. Introduction
2. Experimental
Schiff bases derived from the reaction of S-alkyl- and S-aryl-
dithiocarbazate with heterocyclic aldehydes and ketones have
been extensively studied over the last decades [1–7]. The attention
on these compounds arose mainly due to their capability of chelat-
ing of metal ions in a wide variety of coordination modes. In most
of the cases, complexations involve the imine nitrogen and the thi-
oamide sulfur atoms to form five-membered rings with the metal
atoms. The resulting metal complexes have been shown to exhibit
interesting biological and physico-chemical properties. Among the
large number of the applied heterocyclic carbonyl compounds, in-
dole-3-carbaldehyde has been used for synthesizing the corre-
sponding S-benzyldithiocarbazate Schiff base which upon
coordination to diphenyltin(IV) gave an antibacterial and anti-
fungal active compound [8]. Recently, we reported the crystal
structure of this Schiff base (H2L2, Fig. 1) and its copper(II) and
nickel(II) complexes [9–11]. In order to expand our view about
the coordination behavior of this type of chelating agents, we stud-
ied the metal complexation of three different Schiff bases formed
by condensation of S-benzyldithiocarbazate with indole-2-carbal-
dehyde, indole-3-carbaldehyde and indole-7-carbaldehyde. Fig. 1
shows the chemical diagram of the Schiff bases.
2.1. Materials and measurements
Indole carbaldehydes were purchased from the Aldrich–Sigma
Company. S-benzyldithiocarbazate was prepared as reported pre-
viously [7]. Ethanol was distilled prior to use. Magnetic susceptibil-
ities were measured with
a Sherwood Scientific MSB-AUTO
magnetic susceptibility balance at 298 K. Diamagnetic corrections
were applied using Pascal’s constants. The IR spectra were re-
corded on a Perkin–Elmer RX1 FT-IR spectrometer with samples
prepared as KBr pellets. The NMR spectra were recorded on a JEOL
Lambda 400 MHz FT-NMR spectrometer. The electronic spectra
were measured by means of a Shimadzu UV-3600 UV–VIS–NIR
Spectrophotometer in the region 200–1100 nm.
2.2. Preparation of the Schiff bases
2.2.1. General procedure for the preparation of the Schiff bases
A mixture of S-benzyldithiocarbazate (1.98 g, 10 mmol) and the
appropriate indole carbaldehyde (1.45 g, 10 mmol) in ethanol
(100 mL) was refluxed for 3 h. The mixture was then cooled to
room temperature and the precipitated Schiff base was filtered
off, washed with cold ethanol and dried over silica gel.
⇑
Corresponding author. Tel.: +60 3 79674246; fax: +60 3 79674193.
2.2.2. H2L1
E-mail
addresses:
khaledi@siswa.um.edu.my,
hamid.khaledi@gmail.com
2.88 g, 89%. Anal. Calc. for C20H19N3O2S: C, 65.73; H, 5.24;
(H. Khaledi), hapipah@um.edu.my (H. Mohd Ali), mmolmstead@ucdavis.edu
(M.M. Olmstead).
N, 11.50. Found: C, 65.60; H, 5.24; N, 11.29%. 1H NMR (DMSO-d6):
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.11.008

234
H. Khaledi et al. / Inorganica Chimica Acta 366 (2011) 233–240
Fig. 1. Chemical diagram of the neutral ligands.
d 4.49 (s, 2H, SCH2); 6.91 (d, 1H, Ar-H); 7.00 (t, 1H, Ar-H); 7.17 (t,
1H, Ar-H); 7.26 (m, 1H, Ar-H); 7.33 (t, 2H, Ar-H); 7.43 (d, 3H, Ar-
H); 7.55 (d, 1H, Ar-H); 8.27 (s, 1H, CHN); 11.30 (s, 1H, indole
NH); 13.31 (s, 1H, NHCS). 13C NMR (DMSO-d6): d 37.99 (SCH2);
108.91, 112.19, 119.88, 121.09, 124.10, 127.35, 127.71, 128.58,
129.40, 131.99, 136.71, 138.18 (Ar); 139.39 (CHN); 195.56 (NHCS).
2.3.2. [Zn(HL1)2]
0.636 g, 89%. Anal. Calc. for C34H28N6S4Zn: C, 57.17; H, 3.95;
N, 11.77. Found: C, 56.89; H, 4.11; N, 11.42%. 1H NMR (DMSO-d6):
d 4.66 (s, 2H, SCH2); 7.06 (t, 1H, Ar-H); 7.22–7.42 (m, 6H, Ar-H);
7.49 (d, 2H, Ar-H) 7.62 (d, 1H, Ar-H); 7.86 (s, 1H, CHN); 11.36
(s, 1H, indole NH). 13C NMR (DMSO-d6): d 35.69 (SCH2); 112.31,
112.50, 120.54, 122.12, 125.22, 127.31, 127.33, 128.68, 128.99,
129.95, 136.94, 137.53 (Ar); 143.71 (CHN); 182.14 (NCS). IR (KBr,
IR (KBr, cmꢀ1): 3443 s (
1027 s ( CSS).
tNH); 3119 s (tNH); 1600 s (tCN); 1332 br;
t
cmꢀ1): 3384 m (
s; 1037 m, 940 m (
Suitable crystals for crystallographic analysis were obtained by
slow evaporation from a DMF solution of the complex at room
temperature.
t
NH); 1597 s (
tCN); 1444 s; 1334 m; 1128
2.2.3. H2L2
tCSS).
2.96 g, 91%. Anal. Calc. for C20H19N3O2S: C, 65.73; H, 5.24; N,
11.50. Found: C, 65.33; H, 5.67; N, 11.29%. 1H NMR (DMSO-d6):
d 4.55 (s, 2H, SCH2); 7.10 (t, 1H, Ar-H); 7.17 (t, 1H, Ar-H); 7.25
(m, 1H, Ar-H); 7.30 (t, 2H, Ar-H); 7.42 (dd, 3H, Ar-H); 7.91 (d, 1H,
Ar-H, J = 2.7); 8.15 (d, 1H, Ar-H); 8.42 (s, 1H, CHN); 11.74 (s, 1H, in-
dole NH); 13.17 (s, 1H, NHCS). 13C NMR (DMSO-d6): d 37.12 (SCH2);
108.86, 112.07, 121.09, 121.72, 122.96, 123.93, 127.08, 128.45,
129.13, 132.85, 137.13, 137.71 (Ar); 144.22 (CHN); 193.22 (NHCS).
2.3.3. [Ni(HL1)2]ꢁH2O
0.624 g, 86%. Anal. Calc. for C34H30N6NiOS4: C, 56.28; H, 4.17;
N, 11.58. Found: C, 56.08; H, 4.18; N, 11.39%. 1H NMR (DMSO-d6):
d 4,54 (s, 2H, SCH2); 7.10 (t, 1H, Ar-H); 7.21 (t, 1H, Ar-H); 7.30
(t, 1H, Ar-H); 7.37 (t, 2H, Ar-H); 7.49 (m, 3H, Ar-H); 7.62 (d, 1H,
Ar-H); 8.20 (s, 1H, CHN); 8.41 (d, 1H, Ar-H); 12.28 (s, 1H, indole
IR (KBr, cmꢀ1): 3405 s (
tNH); 3272 s (tNH); 1610 s (tCN); 1490 m;
1314 s; 1022 s (tCSS).
NH). IR (KBr, cmꢀ1): 3405 br (
br; 1340 s; 1128 s; 1028 m, 1011 m (
(nm) DMSO, (
, molꢀ1 dm3 cmꢀ1)] 413 (24 540); 271 (21 940).
eff = 0.00 B.M.
tNH,
tOH); 1582 m; 1560 s; 1451
2.2.4. H2L3
tCSS). UV–Vis spectrum [kmax
2.66 g, 82%. Anal. Calc. for C20H19N3O2S: C, 65.73; H, 5.24;
N, 11.50. Found: C, 65.21; H, 5.73; N, 11.78%. 1H NMR (DMSO-d6):
d 4.65 (s, 2H, SCH2); 6.59 (dd, 1H, Ar-H); 7.15 (t, 1H, Ar-H); 7.26
(m, 1H, Ar-H); 7.33 (t, 2H, Ar-H); 7.41 (d, 1H, Ar-H); 7.46 (d, 2H,
Ar-H); 7.53 (t, 1H, Ar-H); 7.75 (d, 1H, Ar-H); 8.53 (s, 1H, CHN);
10.25 (s, 1H, indole NH); 13.57 (s, 1H, NHCS). 1H NMR (THF-d8):
d 4.67 (s, 2H, SCH2); 6.53 (m, 1H, Ar-H); 7.10 (t, 1H, Ar-H); 7.20–
7.34 (m, 5H, Ar-H); 7.45 (d, 2H, Ar-H); 7.69 (d, 1H, Ar-H); 8.31
(s, 1H, CHN); 10.16 (s, 1H, indole NH); 12.37 (s, 1H, NHCS). 13C
NMR (DMSO-d6): d 37.21 (SCH2); 102.37, 116.56, 119.50, 123.96,
125.47, 126.53, 127.16, 128.34, 128.43, 129.08, 131.74, 137.22
(Ar); 147.93 (CHN); 194.88 (NHCS). 13C NMR (THF-d8): d 37.74
(SCH2); 101.64, 116.23, 118.51, 123.14, 124.75, 124.82, 126.37,
127.60, 128.13, 128.48, 131.73, 136.58 (Ar); 145.98 (CHN);
e
l
2.3.4. [Cd(HL2)2]
0.72 g, 93%. Anal. Calc. for C34H28CdN6S4: C, 53.64; H, 3.71;
N, 11.04. Found: C, 53.21; H, 3.37; N, 11.34%. 1H NMR (DMSO-d6):
d 4.53 (s, 2H, SCH2); 7.11–732 (m, 5H, Ar-H); 7.43–7.49 (m, 3H,
Ar-H); 7.68 (d, 1H, Ar-H); 8.17 (s, 1H, Ar-H); 8.45 (s, 1H, CHN);
11.92 (s, 1H, indole NH). 13C NMR (DMSO-d6): d 35.64 (SCH2);
107.93, 112.34, 117.31, 120.93, 122.50, 126.84, 126.95, 128.43,
128.98, 132.94, 135.32, 137.37 (Ar); 145.35(CHN); 178.59 (NCS).
IR (KBr, cmꢀ1): 3394 m (
932 s ( CSS).
tNH); 1591 s (tCN); 1230 m; 1025 m,
t
The X-ray quality crystal of [Cd(HL2)2(py)2]ꢁpy2 was obtained by
slow evaporation of a pyridine solution of the complex at room
temperature.
195.23 (NHCS). IR (KBr, cmꢀ1): 3426 s (
t
NH); 3107 m (
tNH);
1593 m; 1578 m; 1523 s; 1322 s; 1028 s (
tCSS).
2.3. Synthesis of the complexes
2.3.5. [Zn(HL2)2]
2.3.1. General method of preparation of [M(HL)2]
0.648 g, 91%. Anal. Calc. for C34H28N6S4Zn: C, 57.17; H, 3.95;
N, 11.77. Found: C, 57.65; H, 3.68; N, 11.49%. 1H NMR (DMSO-d6):
d 4.58 (s, 2H, SCH2); 7.19–7.29 (m, 3H, Ar-H); 7.32–7.37 (m, 2H,
Ar-H); 7.47–7.52 (m, 3H, Ar-H); 7.85 (d, 1H, Ar-H); 8.08 (s, 1H,
CHN); 8.56 (d, 1H, J = 2.8 Hz, Ar-H,); 12.12 (s, 1H, indole NH). 13C
NMR (DMSO-d6): d 36.44 (SCH2); 108.31, 113.07, 118.29, 121.95,
123.40, 127.36, 127.76, 129.11, 129.60, 134.91, 136.00, 137.39
A solution of the appropriate hydrated metal acetate salt
(1.0 mmol) in ethanol was added to a hot ethanolic solution of
the Schiff base (0.65 g, 2.0 mmol). The mixture was refluxed for
30 min and then cooled to room temperature. The complex that
separated out was filtered off, washed with ethanol and dried over
silica gel.

H. Khaledi et al. / Inorganica Chimica Acta 366 (2011) 233–240
235
(Ar); 146.69 (CHN); 179.04 (NCS). IR (KBr, cmꢀ1): 3399 br (
1600 s ( CN); 1231 m; 1209 w, 945 m ( CSS).
tNH);
(t, 1H, Ar-H); 7.31 (t, 3H, Ar-H); 7.44 (d, 2H, Ar-H); 7.54 (d, 1H,
Ar-H); 7.70 (d, 1H, Ar-H); 8.86 (s, 1H, CHN). 13C NMR (DMSO-d6):
d 34.39 (SCH2); 100.75, 116.23, 116.98, 123.98, 125.76, 126.82,
128.34, 129.18, 131.39, 137.55, 138.39, 140.65 (Ar); 157.96
t
t
2.3.6. [Cd(HL3)2]
(CHN); 175.28 (NCS). IR (KBr, cmꢀ1): 3409 br (
tOH); 1600 m;
0.542 g, 71%. Anal. Calc. for C34H28CdN6S4: C, 53.64; H, 3.71;
N, 11.04. Found: C, 53.17; H, 3.44; N, 10.67%. 1H NMR (THF-d8):
d 4.70 (s, 2H, SCH2); 6.43 (t, 1H, J = 2.3 Hz, Ar-H); 7.00 (t, 1H,
Ar-H); 7.16 (d, 1H, Ar-H); 7.19–7.28 (m, 3H, Ar-H); 7.49 (d, 2H,
Ar-H); 7.59 (d, 1H, Ar-H); 8.62 (s, 1H, CHN); 10.26 (s, 1H, indole
NH). 13C NMR (THF-d8): d 39.14 (SCH2); 101.99, 117.79, 119.03,
123.18, 125.16, 125.48, 126.95, 128.27, 128.52, 129.12, 132.93,
137.96 (Ar); 152.46 (CHN); 190.00 (NCS). IR (KBr, cmꢀ1): 3407 s
1577 m; 1552 m; 1453 br; 1335 s; 1228 m; 1021 m, 957 s (tCSS).
2.3.9. [Cd(L3)(H2O)]
0.222 g, 49%. Anal. Calc. for C17H15CdN3OS2: C, 44.99; H, 3.33;
N, 9.26. Found: C, 45.22; H, 3.06; N, 9.20%. 1H NMR (DMSO-d6): d
4.39 (s, 2H, SCH2); 6.47 (d, 1H, Ar-H); 6.92 (t, 1H, Ar-H); 7.21 (t,
1H, Ar-H); 7.27–7.31 (m, 3H, Ar-H); 7.40–7.43 (m, 3H, Ar-H);
7.68 (d, 1H, Ar-H); 8.86 (s, 1H, CHN). 13C NMR (DMSO-d6): d
34.88 (SCH2); 100.99, 116.32, 117.86, 124.24, 127.28, 127.37,
128.84, 128.96, 129.71, 132.05, 138.08, 139.15 (Ar); 160.76
(
t
NH); 1590 m; 1573 s; 1440 s; 1344 s; 1234 m; 1040 m, 982 m
CSS).
(t
The X-ray quality crystal of [Cd(HL3)2(py)2] was obtained by
(CHN); 173.67 (NCS). IR (KBr, cmꢀ1): 3402 br (
tOH); 1595 m;
slow evaporation of a pyridine solution of the complex at room
temperature.
1576 m; 1452 br; 1344 s; 1227 m; 1041 w, 945 m (
tCSS).
2.3.7. General method of preparation of [M(L3)(H2O)]
2.3.10. [Ni(L3)(H2O)]
An ethanolic solution (20 mL) of H2L3 (0.325 g, 1 mmol) and
KOH (0.14 g, 2.5 mmol) was added dropwise to a solution of the
metal(II) acetate (1.2 mmol) in the same solvent (10 mL) with con-
stant stirring. The resulting solution was refluxed for 30 min and
then cooled to room temperature. The brown nickel(II) complex
precipitated during the reaction. The zinc(II) and cadmium(II) com-
plexes were separated out on addition of water to the cooled solu-
tion. The solids were filtered off, washed with water several times
and dried over silica gel.
0.366 g, 91%. Anal. Calc. for C17H15N3NiOS2: C, 51.03; H, 3.78; N,
10.50. Found: C, 50.70; H, 3.68; N, 10.75%. IR (KBr, cmꢀ1): 3409 br
(
t
OH); 1601 m; 1585 w; 1533 s; 1460 m; 1232 s; 1049 m, 986 w
CSS). UV–Vis [kmax (nm) solid]: 274; 381; 464. eff = 1.50 B.M.
(t
l
2.3.11. [Ni(L3)(Et3N)(H2O)]
The complex was prepared following the general procedure for
preparation of [M(L3)(H2O)], except for using a few drops of trieth-
ylamine instead of KOH.
2.3.8. [Zn(L3)(H2O)]
0.328 g, 81%. Anal. Calc. for C17H15N3OS2Zn: C, 50.19; H, 3.72;
N, 10.33. Found: C, 50.02; H, 3.90; N, 10.04%. 1H NMR (DMSO-d6):
d 4.41 (s, 2H, SCH2); 6.47 (d, 1H, Ar-H); 6.95 (t, 1H, Ar-H); 7.23
0.351 g, 70%. Anal. Calc. for C23H30N4NiOS2: C, 55.10; H, 6.03; N,
11.18. Found: C, 55.59; H, 4.34; N, 11.53%. IR (KBr, cmꢀ1): 3392 br
(tOH); 1602 m; 1538 s; 1455 m; 1233 s; 1051 m, 982 w (
tCSS).
leff = 1.13 B.M.
Table 1
Crystal data and structure refinement.
[Zn(HL1)2]
34H28N6S4Zn
714.23
orthorhombic
P c a 21
[Cd(HL2)2(py)2]ꢁpy2
C54H48CdN10S4
1077.66
[Cd(HL3)2(py)2]
[Ni(L3)(py)]
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C
C44H38CdN8S4
919.46
orthorhombic
P c c n
C22H18N4NiS2
461.23
monoclinic
P c
triclinic
ꢀ
P1
29.2662(4)
5.46790(10)
20.0663(3)
10.2268(13)
13.0466(17)
19.321(3)
9.8897(18)
20.836(4)
20.652(4)
5.9660(4)
15.0622(12)
11.2428(9)
b (Å)
c (Å)
a
b (°)
(°)
100.495(2)
91.444(2)
94.441(6)
c
(°)
101.803(2)
2476.0(6)
Volume (Å3)
3211.10(9)
4255.6(14)
1007.26(13)
Z
4
2
4
2
Density (calculated) (g cmꢀ3
)
1.477
1.061
1472
1.445
0.659
1108
1.435
0.751
1880
1.521
1.187
476
Absorption coefficient (mmꢀ1
F(0 0 0)
)
Crystal size (mm3)
h range for data collection (°)
Index ranges
0.49 ꢂ 0.18 ꢂ 0.15
0.39 ꢂ 0.32 ꢂ 0.06
1.07–25.01
ꢀ12 6 h 6 8, ꢀ15 6 k 6 15,
ꢀ22 6 l 6 22
11 675
8504 [Rint = 0.0289]
To h = 25.01°: 97.4%
0.9615 and 0.7832
0.35 ꢂ 0.19 ꢂ 0.06
0.32 ꢂ 0.13 ꢂ 0.04
1.72–27.49
1.97–25.25
2.26–25.99
ꢀ37 6 h 6 37, ꢀ7 6 k 6 7,
ꢀ24 6 l 6 26
29 190
7277 [Rint = 0.0631]
To h = 25.25°: 100.0%
0.8571 and 0.6245
ꢀ11 6 h 6 11, ꢀ24 6 k 6 25,
ꢀ16 6 l 6 24
15 959
3842 [Rint = 0.0686]
To h = 25.25°: 99.6%
0.9563 and 0.7789
ꢀ7 6 h 6 7, ꢀ18 6 k 6 18,
ꢀ13 6 l 6 13
Reflections collected
Independent reflections
Completeness
Maximum and minimum
transmission
8122
3902 [Rint = 0.1155]
To h = 25.00°: 99.9%
0.8571 and 0.6245
Data/restraints/parameters
7277/3/413
1.009
8504/416/628
1.030
3842/169/261
1.016
3902/2/263
1.018
Goodness-of-fit (GoF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0463, wR2 = 0.1025
R1 = 0.0640, wR2 = 0.1122
0.506(13)
2.063 and ꢀ0.338
R1 = 0.0564, wR2 = 0.1308
R1 = 0.0966, wR2 = 0.1585
R1 = 0.0377, wR2 = 0.0705
R1 = 0.0760, wR2 = 0.0830
R1 = 0.0668, wR2 = 0.1307
R1 = 0.1076, wR2 = 0.1488
0.18(3)
0.859 and ꢀ0.617
Absolute structure parameter
Largest difference in peak and
hole (e Åꢀ3
0.902 and ꢀ0.553
0.397 and ꢀ0.417
)

236
H. Khaledi et al. / Inorganica Chimica Acta 366 (2011) 233–240
Fig. 2. The molecular structure of [Zn(HL1)2] showing the atom labeling scheme (50% probability ellipsoids).
Table 2
a Bruker SMART APEX II CCD area-detector diffractometer (graphite-
monochromated Mo K radiation, k = 0.71073 Å). The orientation
Selected bond lengths (Å) and bond angles (°) for [Zn(HL1)2].
a
matrix, unit cell refinement and data reduction were all handled
by the APEX2 software (SAINT integration, SADABS absorption correc-
tion) [12]. The structures were solved using direct or Patterson
methods in the program SHELXS-97 [13] and were refined by full ma-
trix least-squares methods on F2 with SHELXL-97. All the non-hydro-
gen atoms were refined anisotropically and all the C-bound
hydrogen atoms were placed at calculated positions and refined
isotropically. N-bound hydrogen atoms were located in difference
Fourier maps and refined with distance restraint of N–H 0.88 Å.
Drawings of the molecules were produced with XSEED [14]. Crystal
data and refinement are summarized in Table 1.
Bond lengths
Zn(1)–N(2)
Zn(1)–N(5)
Zn(1)–S(1)
Zn(1)–S(3)
2.034(3)
2.044(3)
2.2661(13)
2.2698(14)
S(1)–C(10)
S(3)–C(27)
N(3)–C(10)
N(6)–C(27)
1.744(4)
1.732(4)
1.310(5)
1.297(5)
Bond angles
N(2)–Zn(1)–N(5)
N(2)–Zn(1)–S(1)
N(5)–Zn(1)–S(1)
102.37(11)
86.80(10)
123.30(11)
N(2)–Zn(1)–S(3)
N(5)–Zn(1)–S(3)
S(1)–Zn(1)–S(3)
118.82(11)
87.23(10)
136.44(4)
The X-ray quality crystal of [Ni(L3)(py)] was obtained by slow
evaporation of a solution of the complex in a mixture of pyridine
and DMSO.
3. Results and discussion
2.4. Crystallography
3.1. General characterizations
Suitable crystals for crystallographic analysis were mounted on
a glass fiber using perfluoropolyether oil and cooled rapidly to
100 K in a stream of cold N2. Diffraction data were measured using
Condensation of indole-2-carbaldehyde, indole-3-carbaldehyde
and indole-7-carbaldehyde with S-benzyldithiocarbazate gave the
expected Schiff bases, H2L1, H2L2 and H2L3, respectively. Like most
Fig. 3. The molecular structure of [Cd(HL2)2(py)2]ꢁpy2. Ellipsoids correspond to 30% probability.

H. Khaledi et al. / Inorganica Chimica Acta 366 (2011) 233–240
237
thiosemicabazones and esters of dithiocabazones they can undergo
band at ca. 2700 cmꢀ1
.
1H NMR spectra of the compounds exhibit
thione-thiol tautomerization. However, spectroscopic data show
the existence of the Schiff bases as their thione tautomers in solid
state and solution. The solid state IR spectra show thioamide N–H
bands at 3119, 3272 and 3107 cmꢀ1, but do not reveal any S-H
the thioamide NH, but no signal attributable to a thiol proton.
Treatment of the ligands with the acetate salts of metal(II) ions
in ethanol, accompanied by mono-deprotonation, led to the forma-
tion of the metal diligand complexes. This is reflected in omission
of the thioamide NH resonance of the ligands in the 1H NMR and
disappearance of the
t(N–H) bands in the IR spectra. The splitting
(CSS) bands of the free ligands at 1022–1028 cmꢀ1 in the IR
Table 3
of the
t
Selected bond lengths (Å) and bond angles (°) for [Cd(HL2)2(py)2]ꢁpy2.
of the complexes indicates the involvement of the thioamide sulfur
in the complexation. This is supported by upfield shifts of the CSS
13C NMR signals on chelation. The shifts are significant for the com-
plexes of HL1 and HL2 (13–15 ppm), but relatively small for
[Cd(HL3)2] (ꢃ5 ppm). The assignment of the 13C signals are based
on the chemical shifts, intensity patterns and coordination induced
shifts, and are in agreement with those suggested by others [8,15].
[Ni(HL1)2]ꢁH2O is diamagnetic, thus the nickel(II) ion is chelated in
a square planar environment provided by two mono-anionic li-
gands. The water molecule is most likely hydrogen bonded to the
ligand of the complex. The presence of the indole NH signal in 1H
Bond lengths
Cd(1)–N(2)
Cd(1)–N(5)
Cd(1)–N(8)
Cd(1)–N(7)
Cd(1)–S(1)
2.321(5)
2.324(5)
2.522(5)
2.542(5)
2.5768(17)
Cd(1)–S(3)
S(1)–C(10)
S(3)–C(27)
N(3)–C(10)
N(6)–C(27)
2.5791(16)
1.727(6)
1.731(6)
1.305(8)
1.297(7)
Bond angles
N(2)–Cd(1)–N(5)
N(2)–Cd(1)–N(8)
N(5)–Cd(1)–N(8)
N(2)–Cd(1)–N(7)
N(5)–Cd(1)–N(7)
N(8)–Cd(1)–N(7)
N(2)–Cd(1)–S(1)
N(5)–Cd(1)–S(1)
173.73(17)
91.90(17)
94.30(17)
88.37(17)
85.50(16)
176.13(17)
76.39(12)
104.65(13)
N(8)–Cd(1)–S(1)
N(7)–Cd(1)–S(1)
N(2)–Cd(1)–S(3)
N(5)–Cd(1)–S(3)
N(8)–Cd(1)–S(3)
N(7)–Cd(1)–S(3)
S(1)–Cd(1)–S(3)
88.81(13)
87.50(12)
102.72(12)
76.34(12)
90.24(13)
93.46(12)
178.68(6)
NMR spectra of the obtained complexes and the related
bands in their IR spectra show that the indole nitrogens are not
bound to the metal atoms.
t(N–H)
There are a few reports in the literature on metal complexes of
7-imineindoles, in all of which the indole nitrogens are coordi-
nated to the metal ions [16,17]. This prompted us to apply more
basic reaction conditions to ease deprotonation of the indole NH
of H2L3 and take advantage of its nitrogen donor atom. Thus the
7-imineindole (H2L3) was treated with the selected metal(II) ions
in the presence of KOH in ethanol. Analyses of the obtained prod-
ucts showed the formation of metal complexes of the type
[M(L3)(H2O)] (M = CdII, ZnII, NiII). 1H NMR investigation of the cad-
mium(II) and zinc(II) complexes indicates deprotonation of the thi-
oamide NH and also indole NH on chelation, signifying the
coordination through the indole nitrogen. Similar to the complexes
of HL1 and HL2, the 13C NMR spectra of the complexes show signif-
icant upfield shifts (ꢃ20 ppm) of CSS signal from that in the free li-
gand. The nickel(II) complex, [Ni(L3)(H2O)], is paramagnetic with
magnetic susceptibility value of 1.50 B.M which is lower than the
expected value for a high spin Ni(II) complex. This might indicate
that the compound contains a mixture of high and low spin Ni(II)
in different coordination geometries. A similar observation made
by others was explained by the pronounced tendency of nickel
thiolate complexes to form higher aggregates by Ni–S–Ni bridging
[18]. It is worth mentioning that using triethylamine as the base in
the reaction of H2L3 with nickel(II) acetate, gave the complex
[Ni(L3)(Et3N)(H2O)] with magnetic susceptibility value of 1.13 B.M.
Fig. 4. The molecular structure of [Cd(HL3)2(py)2] with thermal ellipsoids drawn at
the 30% probability level.
3.2. Crystallographic analysis
3.2.1. Bis{benzyl N0-[(1H-indol-2-yl)methylene]dithiocarbazato-
j2N0,S}zinc(II)ꢁ[Zn(HL1)2]
Table 4
Selected bond lengths (Å) and bond angles (°) for [Cd(HL3)2(py)2].
The crystal structure of the zinc complex is depicted in Fig. 2
and selected bond lengths and angles are given in Table 2. The
crystal studied was a racemic twin with the twin parameter re-
fined to 0.506(13). The metal ion in the complex is chelated by
two monoanionic bidentate ligands via the azomethine nitrogen
atoms and the thiolate sulfur atoms, forming two five-membered
chelating rings. The dihedral angle between the two formed rings
is 81.81(7)°, thus the geometry is best considered distorted tetra-
hedral. While coordinating to the zinc(II) ion, deprotonation of
the ligand generates an iminothiolate ion with a negative charge
delocalized over the S–C–N chain (thioamide group). This is indi-
cated by the longer bond lengths of C10–S1 and C27–S3
[1.744(4) and 1.732(4) Å, respectively] compared to the corre-
sponding distance in the free ligand [1.6647(14) Å] [19]. Moreover,
the bond distances of N3–C10 [1.310(5) Å] and N6–C27
Bond lengths
Cd–N(4)
Cd–N(3)
Cd–S(1)
2.365(3)
2.409(3)
S(1)–C(10)
N(3)–C(10)
1.710(4)
1.318(4)
2.6732(10)
Bond angles
N(4)–Cd–N(4)#1
N(4)–Cd–N(3)
N(4)#1–Cd–N(3)
N(4)–Cd–N(3)#1
N(4)#1–Cd–N(3)#1
N(3)–Cd–N(3)#1
N(4)–Cd–S(1)#1
N(4)#1–Cd–S(1)#1
101.31(15)
93.22(10)
158.49(9)
158.49(9)
93.22(10)
77.85(14)
102.79(7)
89.83(8)
N(3)–Cd–S(1)#1
N(3)#1–Cd–S(1)#1
N(4)–Cd–S(1)
N(4)#1–Cd–S(1)
N(3)–Cd–S(1)
102.43(7)
61.10(7)
89.83(8)
102.79(7)
61.10(7)
102.43(7)
160.16(5)
N(3)#1–Cd–S(1)
S(1)#1–Cd–S(1)
Symmetry transformations used to generate equivalent atoms: #1 ꢀx + 1/2, ꢀy + 1/
2, z.

238
H. Khaledi et al. / Inorganica Chimica Acta 366 (2011) 233–240
Fig. 5. The molecular structure of [Ni(L3)(py)] with thermal ellipsoids drawn at the 50% probability level.
Table 5
six-coordinate cadmium(II) complex in a distorted octahedral
environment. The two mono-deprotonated Schiff base ligands act
as NS bidentate chelating agents in an almost planar arrangement
around the cadmium atom and form two five-membered rings
with the metal center. The octahedral geometry is completed by
the two trans-located pyridine ligands, the dihedral angle between
them being 77.4(2)°. In the crystal structure, the indole nitrogens
are hydrogen-bonded to the pyridine solvent molecules.
Selected bond lengths (Å) and bond angles (°) for [Ni(L3)(py)].
Bond lengths
Ni(1)–N(2)
Ni(1)–N(1)
Ni(1)–N(4)
1.884(7)
1.891(8)
1.904(6)
Ni(1)–S(1)
S(1)–C(10)
N(3)–C(10)
2.170(3)
1.718(9)
1.287(10)
Bond angles
N(2)–Ni(1)–N(1)
N(2)–Ni(1)–N(4)
N(1)–Ni(1)–N(4)
94.4(3)
171.0(3)
93.2(3)
N(2)–Ni(1)–S(1)
N(1)–Ni(1)–S(1)
N(4)–Ni(1)–S(1)
86.5(2)
177.4(3)
86.2(2)
3.2.3. Bis{benzyl N0-[(1H-indol-7-yl)methylene]dithiocarbazato-
j
2N,S}bis(pyridine-
The structure of the complex is represented in Fig. 4 and the se-
j
N)cadmium(II)ꢁ[Cd(HL3)2(py)2]
[1.297(5) Å] are shorter than the related one in the free ligand
[1.3397(19) Å]. The Zn–N distances of 2.034(3) and 2.044(3) Å
and Zn–S bond lengths of 2.2667(13) and 2.2698(14) Å are compa-
rable with the values reported in literature for the similar struc-
tures [1,6]. Intramolecular hydrogen bonding links the indole
nitrogens to the thioamide nitrogens.
lected bond lengths and angles are compiled in Table 4. The cad-
mium ion lies on a crystallographic 2-fold rotational axis of
symmetry and is six-coordinated by two ligands HL3 and two cis-
positioned pyridine ligands in a highly distorted octahedral geom-
etry. Similar to HL1 and HL2, the ligand HL3 coordinated to the me-
tal center as a monoanionic NS bidentate chelate, but unlike them,
the coordination occurred through the iminothiolate nitrogens and
sulfurs to form two four-membered chelate rings with bite angle of
61.09(6)°. The azomethine nitrogen is hydrogen bonded to the in-
dole N–H. The preference for four over five-membered chelate ring
is unusual among the complexes of S-alkyl/aryldithiocarbazones
3.2.2. Bis{benzyl N0-[(1H-indol-3-yl)methylene]dithiocarbazato-
j2N0,S}bis(pyridine-
jN)cadmium(II) pyridine
disolvateꢁ[Cd(HL2)2(py)2]ꢁpy2
A view of the complex structure is shown in Fig. 3 and selected
bond lengths and angles are listed in Table 3. The structure shows a
Fig. 6. Structures 1 and 2.

H. Khaledi et al. / Inorganica Chimica Acta 366 (2011) 233–240
239
Fig. 7. Coordination modes of the synthesized ligands.
and thiosemicarbazones, however there are a few reports on this
mode of coordination [20–25].
The Cd–S, Cd–Niminothiolate and Cd–Npy bond distances are
2.6732(9), 2.409(3) and 2.366(3) Å, respectively, which are similar
to those in [Cd(C7H4NS2)2 (py)2], the closest analogous structure
[26].
served between the azomethine hydrogen and the indole NH at
10.26 ppm. Therefore the unusual coordination mode observed in
the crystal structure of [Cd(HL3)2(py)2] is not the effect of steric
bulk of the co-ligand pyridines, but most likely is a consequence
of the intramolecular NH. . .N hydrogen bonding. This is in agree-
ment with the five-membered ring formation observed in
[Cd(HL2)2(py)2]ꢁpy2 in which two co-ligand pyridines exist, but
the analogous intramolecular hydrogen bonding is prevented.
3.2.4. {Benzyl N0-[(1H-indol-7-yl)methylene]dithiocarbazato-
j3N0,N’’,S}(pyridine-
j
N)nickel(II)ꢁ[Ni(L3)(py)]
A solution of [Ni(L3)(Et3N)(H2O)] in a mixture of DMSO and pyr-
idine gave red crystals of [Ni(L3)(py)]. The structure of the complex
is depicted in Fig. 5 and the selected bond lengths and angles are
listed in Table 5. The crystal studied was a racemic twin and the
twin parameter refined to 0.18(3). As the structure shows, L3 is
coordinated to the metal ion as a dianionic NNS tridentate ligand
through the indole nitrogen, azomethine nitrogen and iminothio-
late sulfur atoms, forming two five-membered and six-membered
rings with the metal atom. The pyridine nitrogen atom occupied
the fourth position, giving rise to a distorted square planar nicke-
l(II) complex with the metal center being 0.019(4) Å out of the
coordination plane, N1–N2–N4–S1. The pyridine ring makes a
dihedral angle of 74.99(15)° with the mean plane passing through
the coordination plane.
4. Conclusions
The synthesized S-benzyldithiocarbazones can ligate to the me-
tal ions in different coordination modes, as summarized in Fig. 7.
Like most dithiocarbazone esters, the ligands HL1 and HL2 with
the imine link at positions 2 and 3 of the indole system coordinated
to the metal ions through the azomethine nitrogen and thioamide
sulfur atoms and created five-membered chelate rings (3 and 4).
Similarly, the 7-imineindole (HL3) di-ligated the cadmium(II) as a
monoanionic bidentate ligand, however in an unusual manner
through the thioamide nitrogen and sulfur atoms, leading to the
formation of four-membered rings with the metal center (5). It
would appear that intramolecular NH. . .N hydrogen bonding is
the responsible for such observed coordination mode. In the pres-
ence of KOH, the ligand L3 can act as a dianionic NNS tridentate
chelate (6) and mono-ligate the metal ions to afford complexes
of the the type [M(L3)(H2O)].
3.3. NMR analysis of [Cd(HL3)2]
The formation of four-membered chelate ring observed in
[Cd(HL3)2(py)2], poses the question of the driving force behind
such an unusual coordination mode. Two factors which might be
responsible for this observation are the steric bulk of the two pyr-
idine ligands, and intramolecular hydrogen bonding between the
indole NH and the azomethine nitrogen atom. The first effect is
absent in [Cd(HL3)2], therefore the elucidation of its structure can
help us to find out the origin of the coordination mode. To do that,
a detailed 2D-NMR spectroscopy study, using NOESY, COSY and
HMQC techniques, was undertaken which allowed us to assign
most of the 1H and 13C NMR signals unambiguously. Owing to
the presence of the indole NH, five-membered chelate ring forma-
tion can only take place when the azomethine C@N bond is
directed away from the indole-NH (1, Fig. 6). On the other hand,
the conformation in which the C@N bond directed toward the in-
dole-NH, stabilized by NH. . .H hydrogen bonding, leads to the
four-membered ring (2, Fig. 6). The structure of [Cd(HL3)2] was
found to be 2 by 2D-NMR and NOE experiments, wherein the
azomethine hydrogen at 8.62 ppm has an NOE correlation with
the aromatic hydrogen at 7.16 ppm, and no NOE effect was ob-
Acknowledgments
Financial support by University of Malaya is highly appreciated
(UMRG grant RG024/09BIO).
Appendix A. Supplementary material
CCDC 787620, 787620, and 787623 contain the supplementary
crystallographic 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
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2010.11.008.
References
[1] M. Akbar Ali, H.J.H. Abu Bakar, A.H. Mirza, S.J. Smith, L.R. Gahan, P.V. Bernhardt,
Polyhedron 28 (2008) 71.
[2] S. Roy, T.N. Mandal, A.K. Barik, S. Pal, S. Gupta, A. Hazra, R.J. Butcher, A.D.
Hunter, M. Zeller, S.K. Kar, Polyhedron 26 (2007) 2603.

240
H. Khaledi et al. / Inorganica Chimica Acta 366 (2011) 233–240
[3] K.B. Chew, M.T.H. Tarafder, K.A. Crouse, A.M. Ali, B.M. Yamin, H.-K. Fun,
[15] A.M. Giuliani, E. Trotta, Polyhedron 7 (1988) 1211.
[16] P.K. Bowyer, D.S. Black, D.C. Craig, A.D. Rae, A.C. Willis, J. Chem. Soc., Dalton
Trans. (2001) 1948.
[17] J.H.H. Ho, D.S.C. Black, B.A. Messerle, J.K. Clegg, P. Turner, Organometallics 25
(2006) 5800.
Polyhedron 23 (2004) 1385.
[4] M. Akbar Ali, A.H. Mirza, R.J. Fereday, R.J. Butcher, J.M. Fuller, S.C. Drew, L.R.
Gahan, G.R. Hanson, B. Moubaraki, K.S. Murray, Inorg. Chim. Acta 358 (2005)
3937.
[5] M.H.S.A. Hamid, M. Akbar Ali, A.H. Mirza, P.V. Bernhardt, B. Moubaraki, K.S.
Murray, Inorg. Chim. Acta 362 (2009) 3648.
[18] A. Berkessel, G. Hermann, O.-T. Rauch, M. Büchner, A. Jacobi, G. Huttner, Chem.
Ber. 129 (1996) 1421.
[6] M.T.H. Tarafder, K.B. Chew, K.A. Crouse, A.M. Ali, B.M. Yamin, H.-K. Fun,
Polyhedron 21 (2002) 2683.
[7] M.H.E. Chan, K. A Crouse, M.I.M. Tahir, R. Rosli, N. Umar-Tsafe, A.R. Cowley,
Polyhedron 27 (2008) 141.
[8] H.L. Singh, A.K. Varshney, Bioinorg. Chem. Appl. (2006) 1.
[9] H. Khaledi, H. Mohd Ali, S.W. Ng, Acta Crystallogr., Sect. E 64 (2008) o2107.
[10] H. Khaledi, H. Mohd Ali, S.W. Ng, Acta Crystallogr., Sect. E 64 (2008) m1615.
[11] H. Khaledi, H. Mohd Ali, S.W. Ng, Acta Crystallogr., Sect. E 65 (2009) m139.
[12] Bruker APEX2 and SAINT, Bruker AXS Inc., Madison, WI, USA, 2007.
[13] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[14] L.J. Barbour, J. Supramol. Chem. 1 (2001) 189.
[19] H. Khaledi, H. Mohd Ali, S.W. Ng, Acta Crystallogr., Sect. E 64 (2008) o2445.
[20] A. Marchi, R. Rossi, L. Magon, A. Duatti, R. Pasqualini, V. Ferretti, V. Bertolasi, J.
Chem. Soc., Dalton Trans. (1990) 1411.
[21] F. Basuli, S.-M. Peng, S. Bhattacharya, Inorg. Chem. 36 (1997) 5645.
[22] F. Basuli, M. Ruf, C.G. Pierpont, S. Bhattacharya, Inorg. Chem. 37 (1998) 6113.
[23] F. Basuli, S.-M. Peng, S. Bhattacharya, Inorg. Chem. 39 (2000) 1120.
[24] I. Pal, F. Basuli, T.C.W. Mak, S. Bhattacharya, Angew. Chem., Int. Ed. 40 (2001)
2923.
[25] M.-X. Li, Y. Bai, B.-G. Zhang, C.-Y. Duan, J. Xu, O.-J. Meng, Inorg. Chem. 44
(2005) 5459.
[26] R. Baggio, M.T. Garland, M. Perec, J. Chem. Soc., Dalton Trans. (1993) 3367.