Synthesis, spectral characterization, and anticancer activity of 6-methylpyridine-2-carbaldehdyethiosemicarbazone and its complexes; Crystal structure and DFT calculations of [Pd(mpyptsc)Cl]·DMSO

Article abstract of DOI:10.1080/00958972.2014.942224

Synthesis and characterization of new complexes of 6-methylpyridine-2-carbaldehydethiosemicarbazone (Hmpytsc) and 6-methylpyridine-2-carbaldehyde-N(4)-phenylthiosemicarbazone (Hmpyptsc) with [VO₂L], [Zn(HL)Cl₂], [Ru(PPh₃)<

Full text of DOI:10.1080/00958972.2014.942224

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Synthesis, spectral characterization,
and anticancer activity
of 6-methylpyridine-2-
carbaldehdyethiosemicarbazone and its
complexes; crystal structure and DFT
calculations of [Pd(mpyptsc)Cl]·DMSO
Shadia A. Elsayeda, Ian S. Butlerb, Denis F.R. Gilsonb, Bertrand J.
Jean-Claudec & Sahar I. Mostafad
a Faculty of Science, Department of Chemistry, Damietta
Click for updates
University, New Damietta, Egypt
b Department of Chemistry, McGill University, Montreal, Canada
c Department of Medicine, Royal Victoria Hospital, Montreal,
Canada
d Faculty of Science, Department of Chemistry, Mansoura
University, Mansoura, Egypt
Accepted author version posted online: 08 Jul 2014.Published
online: 08 Aug 2014.
To cite this article: Shadia A. Elsayed, Ian S. Butler, Denis F.R. Gilson, Bertrand J. Jean-Claude
& Sahar I. Mostafa (2014) Synthesis, spectral characterization, and anticancer activity of 6-
methylpyridine-2-carbaldehdyethiosemicarbazone and its complexes; crystal structure and DFT
calculations of [Pd(mpyptsc)Cl]·DMSO, Journal of Coordination Chemistry, 67:16, 2711-2727, DOI:
10.1080/00958972.2014.942224
To link to this article: http://dx.doi.org/10.1080/00958972.2014.942224
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 16, 2711–2727, http://dx.doi.org/10.1080/00958972.2014.942224
Synthesis, spectral characterization, and anticancer activity of
6-methylpyridine-2-carbaldehdyethiosemicarbazone and its
complexes; crystal structure and DFT calculations of
[Pd(mpyptsc)Cl]·DMSO
SHADIA A. ELSAYED†, IAN S. BUTLER*‡, DENIS F.R. GILSON‡, BERTRAND J.
JEAN-CLAUDE§ and SAHAR I. MOSTAFA*¶
†Faculty of Science, Department of Chemistry, Damietta University, New Damietta, Egypt
‡Department of Chemistry, McGill University, Montreal, Canada
§Department of Medicine, Royal Victoria Hospital, Montreal, Canada
¶Faculty of Science, Department of Chemistry, Mansoura University, Mansoura, Egypt
(Received 20 August 2013; accepted 13 June 2014)
New complexes of 6-methylpyridine-2-carbaldehydethiosemicarbazone (Hmpytsc) and 6-methylpyri-
dine-2-carbaldehyde-N(4)-phenylthiosemicarbazone (Hmpyptsc) and crystal structure of [Pd
(mpyptsc)Cl]·DMSO are reported. Hmpyptsc and its complexes were tested against human colon
cancer (HCT116) and prostate cancer (DU145) cell lines.
Synthesis and characterization of new complexes of 6-methylpyridine-2-carbaldehydethiosemicarbazone
(Hmpytsc) and 6-methylpyridine-2-carbaldehyde-N(4)-phenylthiosemicarbazone (Hmpyptsc) with
[VO2L], [Zn(HL)Cl2], [Ru(PPh3)2L2], and [MLCl] (M(II) = Pd, Pt; HL = Hmpytsc, Hmpyptsc) are
reported. Their structures are discussed on the basis of IR, Raman, UV–vis, NMR (1H, 13C, and 31P),
and mass spectroscopic data, as well as elemental analysis and molar conductivity. In the X-ray crystal
structure of the square-planar [Pd(mpyptsc)Cl]·DMSO, mpyptsc− is coordinated to Pd(II) in a tridentate
manner through pyridyl N, azomethine N, and thiol S, and the fourth coordination site is occupied by a
*Corresponding authors. Email: ian.butler@mcgill.ca (I.S. Butler); sihmostafa@gmail.com (S.I. Mostafa)
© 2014 Taylor & Francis

2712
S.A. Elsayed et al.
chloride. A theoretical study on [Pd(mpyptsc)Cl]·DMSO was undertaken through DFT conformational
analysis. The in vitro cytotoxic activity has been evaluated against human colon cancer (HCT116) and
prostate cancer (DU145) cell lines. Hmpyptsc and [Zn(Hmpyptsc)Cl2] were most active with mean IC50
values of 3.32 and 2.60 (HCT116), and 3.60 and 3.10 (DU145) μM, respectively.
Keywords: Hmpytsc; Hmpyptsc; Palladium; Zinc; Platinum; Anticancer
1. Introduction
The chemistry and biological activity of thiosemicarbazone derivatives and their complexes
are of considerable interest [1–6]. In our laboratory, the chemistry of thiosemicarbazones
has been an extremely active area of research because of the beneficial biological activities
(anticancer, antifungal, and antibacterial) of their transition metal complexes [7–9]. We have
reported the chemistry of 2-hydroxybenzophenone-N(4)-(Me, Ph and piperidyl) thiosemi-
carbazones, 2-formylpyridinethiosemicarbazone, and 6-methylpyridine-2-carbaldehyde-N
(4)-ethylthiosemicarbazone, and a number of their transition metal complexes [7–9]. The
anticancer activity of 6-methylpyridine-2-carbaldehyde-N(4)-ethylthiosemicarbazone and its
Zn(II) and Pd(II) complexes have been tested against colon cancer human cell lines
(HCT116) [9]. Over the past few years, our laboratory has been actively involved in the
synthesis of O,O-; O,N-; N,S-; N,N,S-; and O,N,S-donor transition metal complexes, which
have been evaluated as anticancer agents against Ehrlich ascites tumor cells [10–13] and
human cancer cell lines [9, 14, 15]. As a part of our continuing interest in the anticancer
activity of N,N,S-donor complexes, the present study includes the interaction of 6-methyl-
pyridine-2-carbaldehyde thiosemicarbazone (Hmpytsc) and 6-methylpyridine-2-carbalde-
hyde-N(4)-phenylthiosemicarbazone (Hmpyptsc) with V(V), Zn(II), Ru(II), Pd(II), and
Pt(II). Their structures are discussed on the basis of elemental analysis, molar conductivity,
IR, Raman, NMR, and mass spectroscopy. The X-ray crystal structure of [Pd(mpyptsc)
Cl]·DMSO is reported. The DFT conformational analysis of [Pd(mpyptsc)Cl]·DMSO was
undertaken. The results of the in vitro cytotoxic activity of Hmpytsc, Hmpyptsc and their
complexes [Zn(HL)Cl2], and [MLCl] (M(II) = Pd, Pt) have been evaluated against human
colon cancer (HCT116) and prostate cancer (DU145) cell lines.
2. Experimental
2.1. Materials and measurements
All reagents and solvents were purchased from Alfa-Aesar, and all manipulations were per-
formed under aerobic conditions using materials and solvents as received. The starting com-
plex, [Ru(PPh3)2Cl2], was prepared by the literature methods [16]. DMSO-d6 was used for
the NMR measurements, which were referenced against TMS.
The human colon cancer (HCT116) and prostate cancer (DU145) cell lines were obtained
from the American Type Culture Collection (ATCC catalog number). Cells were maintained
in Roswell Park Memorial Institute (RPMI-1640) medium (Wisent Inc., St-Bruno, Canada)
supplemented with 10% fetal bovine serum (FBS), 10 mM HEPES, 2 mM L-gutamine, and
100 μg/mL penicillin/streptomycin (GibcoBRL, Gaithersburg, MD). All assay cells were
plated 24 h before drug treatment.

Phenylthiosemicarbazone complexes
2713
Elemental analyses and X-ray crystallography were performed at the Department of
Chemistry, Montreal University. The crystal structure was measured at the X-ray Crystal
Structure Unit using a Bruker Platform diffractometer equipped with a Bruker MART 4 K
Charger-Coupled Device Area Detector using APEX II and a Nonius Fr591 rotating anode
(copper radiation) equipped with Montel 200 optics. The crystal-to-detector distance was 5
cm and the data collection was carried out in 512 × 512 pixel mode. The initial unit-cell
parameters were determined by least-squares fit of the angular setting of strong reflections,
collected by a 10.0-degree scan in 33 frames over three different parts of the reciprocal
space (99 frames total) and one complete sphere of data was collected. Infrared spectra were
recorded on a Nicolet 6700 Diamond ATR spectrometer from 4000 to 200 cm−1. Raman
spectra were measured on an In-via Renishaw spectrometer using 785-nm laser excitation.
NMR spectra were recorded on a VNMRS 500-MHz spectrometer in DMSO-d6 using TMS
as reference. Mass spectra, ESI-MS, and EI-MS were recorded using LCQ Duo and double-
focusing MS25RFA instruments, respectively. Electronic spectra were measured in DMF
using a Hewlett-Packard 8453 spectrophotometer. Molar conductivity measurements were
carried out at room temperature on a YSI Model 32 conductivity bridge. The GAUSSIAN
03 rev B-02 suite of programs was used for the DFT calculations using the DGDZVP/
B3LYP basis set [17].
2.2. Preparations
Both 6-methylpyridine-2-carbaldehydethiosemicarbazone (Hmpytsc) and 6-methylpyridine-
2-carboxaldehyde-N(4)-phenylthiosemicarbazone (Hmpyptsc) were prepared by modifica-
tion of the literature methods [18].
2.2.1. 6-Methylpyridine-2-carbaldehydethiosemicarbazone (Hmpytsc). 6-Methylpyri-
dine-2-carboxaldehyde (1.21 g, 10 mM) in ethanol (10 mL) was added to thiosemicarbazide
(0.91 g, 10 mM) in ethanol-water (1 : 1, V/V; 80 mL) followed by addition of drops of gla-
cial acetic acid. The reaction mixture was heated under reflux for 4 h. The white precipitate
was filtered off and washed with water and ethanol. It was re-crystallized from ethanol and
dried in vacuum. Yield: 89%, m.p. = 219 °C, Elemental Anal. Calcd C8H10N4S (%): C,
49.4; H, 5.2; N, 28.8; S, 16.5. Found (%): C, 49.3; H, 5.0; N, 28.6; S, 16.4. IR (Raman)
cm−1, 3392, νas(NH2); 3258, νs(NH2); 3152 ν(NH); 1612 (1808) ν(C=N); 990 (990)
1
ν(N–N); 842, ν(C=S); 623 (630), ρ(Py); H NMR (d6-DMSO/TMS, ppm), δ: 8.056 (d,1H,
H3); 7.66 (t, 1H, H4); 7.20 (d, 1H, H5); 8.01 (s, 1H, H7), 2.43 (s, 3H, CH3); 11.61 (s, 1H,
N(3)H); 8.12, 8.13 (s, 1H, N(4)H2); 13C NMR (d6-DMSO/TMS, ppm) δ: 158.3, C(2);
124.1, C(3); 137.4, C(4); 117.9, C(5); 153.5, C(6); 143.5, (CH=N); 178.9, (C=S); 24.5,
6-CH3.
2.2.2. 6-Methylpyridine-2-carboxaldehyde-N(4)-phenylthiosemicarbazone (Hmpyptsc).
The same procedure of preparation of Hmpytsc was applied using N(4)-phenylthiosemicar-
bazide (1.73 g, 10 mM) instead of thiosemicarbazide. Yield: 85%, m.p. = 184 °C. Elemental
Anal. Calcd C14H14N4S (%): C, 62.2; H, 5.2; N, 20.7; S, 11.8. Found (%): C, 62.0; H, 5.2;
N, 20.6; S, 11.7. IR (Raman) cm−1: 3319 (3099), ν(NH); 1588 (1606), ν(C=N); 994 (995),
1
ν(N–N); 784 (792), ν(CS); 643 (597), ρ(Py); H NMR (d6-DMSO/TMS, ppm), δH: 8.18 (d,
1H, H3); 7.72 (t, 1H, H4); 7.55 (d, 1H, H5); 8.13 (s, 1H, H7); 2.47 (s, 3H, CH3); 12.01(s,

2714
S.A. Elsayed et al.
1H, N(3)H); 10.30 (s, 1H, N(4)H); 7.20–7.40 (m, ph); 13C NMR (d6-DMSO/TMS, ppm) δ:
158.2, C(2); 125.8, C(3); 137.1, C(4); 118.5, C(5); 152.5, C(6); 143.6, (CH=N); 176.8
(C=S); 24.2 CH3; 124.1–138.4, Ph.
2.2.3. [VO2L] (L = mpytsc, mpyptsc). Solid [VO(acac)2] (0.078 g, 0.3 mM) was added to
Hmpytsc or Hmpyptsc (0.3 mM) in ethanol (10 mL). The reaction mixture was stirred and
heated under reflux for 1 h. Upon cooling, an orange precipitate was obtained, filtered off,
washed with ethanol and diethyl ether and dried in vacuum.
2.2.3.1. For [VO2(mpytsc)]. Yield: 75%. Elemental Anal. Calcd C8H9N4O2SV (%): C,
34.7; H, 3.3; N, 20.3; S, 11.6. Found (%): C, 34.7; H, 3.1; N, 20.2; S, 11.5. Conductivity data
(10−3 M in DMF): ΛM = 5.0 Ω−1. IR (Raman) cm−1: 3310, νas(NH2); 3277, νs(NH2); 1632
(1620), ν(C=N); 1020 (1023) ν(N–N); 785, ν(C=S); 634 (681), ρ(Py); 451 (451) ν(V–N);
1
340 (344), ν(V–S); 936, νs(VO2); 908, νas(VO2); H NMR (d6-DMSO/TMS, ppm), δ: 7.55
(d, 1H, H3); 8.10 (t, 1H, H4); 7.71 (d, 1H, H5); 8.48 (s, 1H, H7); 2.48 (s, 3 H, CH3); 7.87
(s, 2H, N(4)H2); 13C NMR (d6-DMSO/TMS, ppm) δ: 158.9, C(2); 123.8, C(3); 137.2, C(4);
117.8, C(5); 154.1, C(6); 144.2, (CH=N); 174.7, (C=S); 24.3, 6-CH3.
2.2.3.2. For [VO2(mpyptsc)]. Yield: 79%. Elemental Anal. Calcd C14H13N4O2SV (%): C,
47.7; H, 3.7; N, 15.9; S, 9.1. Found (%): C, 47.3; H, 3.4; N, 15.7; S, 9.0. Conductivity data
(10−3 M in DMF): ΛM = 6.0 Ω−1. IR (Raman) cm−1: 3340, ν(NH); 1599 (1613), ν(C=N);
1020 (1022), ν(N–N); 760 (758), ν(C=S); 649 (678), ρ(Py); 446 (449), ν(V–N); 329 (333),
1
ν(V–S); 940, νs(VO2); 912, νas(VO2); H NMR (d6-DMSO/TMS, ppm), δ: 7.80 (d, 1H,
H3); 8.15 (t, 1H, H4); 7.70 (d, 1H, H5); 8.82 (s, 1H, H7) 2.48 (s, 3H, CH3); 10.15 (s, 1H, N
(4)H); 7.07–7.63 (m, ph); 13C NMR (d6-DMSO/TMS, ppm) δ: 163.4, C(2); 124.2, C(3);
140.6, C(4); 121.2, C(5); 157.2, C(6); 153.1, (CH=N); 173.6, (C=S); 26.4, CH3, 123.7–
142.8, Ph.
2.2.4. [Zn(HL)Cl2] (HL = Hmpytsc or Hmpyptsc). Hmpytsc or Hmpyptsc (0.3 mM) in
ethanol (10 mL) was added to a methanolic solution of ZnCl2 (0.041 g, 0.3 mM; 10 mL).
The reaction mixture was heated under reflux for 2 h and the off-white product was filtered
off, washed with methanol, and dried in air.
2.2.4.1. For [Zn(Hmpytsc)Cl2]. Yield: 86%. Elemental Anal. Calcd C8H10Cl2N4SZn (%):
C, 29.2; Cl, 21.5; H, 3.0; N, 16.9; S, 9.7. Found (%): C, 29.1; Cl, 21.6; H, 2.8; N, 17.0; S,
9.4. Conductivity data (10-3 M in DMF): ΛM = 9.0 Ω−1. IR (Raman) cm−1: 3347, νas(NH2);
3254, νs(NH2); 3156, ν(NH); 1619 (1636), ν(C=N); 1009 (1012) ν(N–N); 799 (787),
ν(C=S); 621 (629), ρ(Py); 441 (445), ν(Zn–N); 317 (310), ν(Zn–S); 233 (252), ν(Zn–Cl)
1
cm−1; H NMR (d6-DMSO/TMS, ppm), δ: 8.07 (d, 1H, H3); 7.70(t, 1H, H4); 7.23 (d, 1H,
H5); 8.00 (s, 1H, H7), 2.48 (s, 3H, CH3); 11.63 (s, 1H, N(3)H); 8.13 (s, 1H, N(4)H2); 13C
NMR (d6-DMSO/TMS, ppm) δ: 158.2, C(2); 124.2, C(3); 137.7, C(4); 118.2, C(5); 153.2,
C(6); 143.1, (CH=N); 178.9, (C=S); 24.4, 6-CH3.
2.2.4.2. For [Zn(Hmpyptsc)Cl2]. Yield: 88%. Elemental Anal. Calcd C14H14Cl2N4SZn
(%): C, 41.3; Cl, 17.5; H, 3.4; N, 13.8; S, 7.9. Found (%): C, 41.2; Cl, 17.5; H, 3.2; N,
13.6; S, 7.8. Conductivity data (10−3 M in DMF): ΛM = 8.0 Ω−1. IR (Raman) cm−1: 3156,

Phenylthiosemicarbazone complexes
2715
ν(NH); 1619 (1636), ν(C=N); 1009 (1012), ν(N–N); 799 (787), ν(C=S); 621 (629), ρ(Py);
1
441 (445), ν(Zn–N); 317 (310), (Zn–S); 233 (252), ν(Zn–Cl); H NMR (d6-DMSO/TMS,
ppm), δ: 8.27 (d, 1H, H3); 7.81 (t, 1H, H4); 7.55 (d, 1H, H5); 8.13 (s, 1H, H7); 2.50 (s, 3H,
CH3); 12.10 (s, 1H, N(3)H); 10.27 (s, 1H, N(4)H); 7.21–7.37(m, ph): 13C NMR (d6-
DMSO/TMS, ppm) δ: 157.6, C(2); 125.7, C(3); 138.2, C(4); 118.5, C(5); 152.4, C(6);
142.8, (CH=N); 176.8, (C=S); 23.7, CH3; 123.3–139.3, Ph.
2.2.5. [Ru(PPh3)2L2] (L = mpytsc, mpyptsc). Hmpytsc or Hmpyptsc (0.2 mM) in ethanol
(10 mL) was added to solid [RuCl2(PPh3)3] (0.1 g, 0.1 mM) and Et3 N (0.02 mL, 0.2 mM)
was added. The reaction mixture was heated under reflux for 2 h. The yellow complex was
filtered off warm, washed with hot ethanol, and air-dried.
2.2.5.1. For [Ru(PPh3)2(mpytsc)2]. Yield: 54%. Elemental Anal. Calcd C52H48N8P2RuS2
(%): C, 61.7; H, 4.7; N, 11.1; S, 6.3. Found (%): C, 61.7; H, 4.5; N, 11.1; S, 6.1. Conduc-
tivity data (10−3 M in DMF): ΛM = 4.0 Ω−1. IR (Raman) cm−1: 3306, νas(NH2); 3281,
νs(NH2); 1635 (1587), ν(C=N); 1028 (1012), ν(N–N); 802 (809), ν(C=S); 622 (618), ρ(Py);
1
436 (438), ν(Ru–N), 325 (320), ν(Ru–S); H NMR (d6-DMSO/TMS, ppm), δ: 8.06 (d, 1H,
H3); 7.52 (t, 1H, H4); 7.20 (d, 1H, H5); 8.60 (s, 1H, H7), 2.47 (s, 3H, CH3); 6.67 (s, 2H, N
(4)H2); 13C NMR (d6-DMSO/TMS, ppm) δ: 159.3, C(2); 122.1, C(3); 137.7, C(4); 117.6, C
(5); 155.5, C(6); 146.7, (CH=N); 173.5, (C=S); 24.5, 6-CH3.
2.2.5.2. For [Ru(PPh3)2(mpyptsc)2]. Yield: 60%. Elemental Anal. Calcd C64H56N8P2RuS2
(%): C, 66.0; H, 4.8; N, 9.6; S, 5.5. Found (%): C, 65.9; H, 4.5; N, 9.2; S, 5.5. Conductiv-
ity data (10−3 M in DMF): ΛM = 3.0 Ω−1. IR (Raman) cm−1: 3270, ν(NH); 1592 (1599),
ν(C=N); 999 (998), ν(N–N); 740 (742), ν(C=S); 644 (642), ρ(Py); 494 (475), ν(Ru–N); 344
1
(343), ν(Ru–S); H NMR (d6-DMSO/TMS, ppm), δ: 7.97 (d, 1H, H3); 7.92 (t, 1H, H4);
7.54 (d, 1H, H5); 8.34 (s, 1H, H7); 2.64 (s, 3H, CH3); 10.24 (s, 1H, N(4)H); 7.20–7.42 (m,
ph); 13C NMR (d6-DMSO/TMS, ppm) δ: 163.9, C(2); 124.4, C(3); 140.4, C(4); 120.6, C
(5); 157.2, C(6); 152.6, (CH=N); 172.4, (C=S); 25.7, CH3; 123.9–140.7, Ph.
2.2.6. [MLCl] (M(II) = Pd, Pt; L = mpytsc, mpyptsc). An aqueous solution of K2PdCl4
or K2PtCl4 (0.3 mM, 5 mL) was added to a methanolic solution of Hmpytsc or Hmpyptsc
(0.3 mM) containing KOH (0.018 g, 0.3 mM; 15 mL). The reaction mixture was stirred at
room temperature for 24 h. The orange precipitate was filtered off, washed with water and
methanol, and air-dried.
2.2.6.1. For [Pd(mpytsc)Cl]. Yield: 77%, Elemental Anal. Calcd C8H9ClN4PdS (%): C,
28.7; Cl, 10.6; H, 2.7; N, 16.7; Pd, 31.8; S, 9.6. Found (%): C, 28.6; Cl, 10.4; H, 2.4; N,
16.5; Pd, 31.5; S, 9.3. Conductivity data (10−3 M in DMF): ΛM = 12.0 Ω−1. IR (Raman)
cm−1: 3407, νas(NH2); 3285, νs(NH2); 1635 (1604), ν(C=N); 1003 (1001), ν(N–N); 780
(778), ν(C=S); 621 (621), ρ(Py); 438 (435), ν(Pd–N); 343 (340), ν(Pd–S); 315 (320), ν(Pd–
1
Cl); H NMR (d6-DMSO/TMS, ppm), δ: 7.57 (d, 1H, H3); 7.89 (t, 1H, H4); 7.40 (d, 1H,
H5); 7.90 (s, 1H, H7), 2.47 (s, 3H, CH3); 7.80 (s, 2H, N(4)H2); 13C NMR (d6-DMSO/TMS,
ppm) δ: 163.5, C(2); 127.7, C(3); 140.6, C(4); 123.4, C(5); 157.7, C(6); 148.1, (CH=N);
172.1, (C=S); 24.7, 6-CH3.

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S.A. Elsayed et al.
2.2.6.2. For [Pd(mpyptsc)Cl]. Yield: 75%. Elemental Anal. Calcd C14H13ClN4PdS (%): C,
40.9; Cl, 8.6; H, 3.2; N, 13.6; Pd, 25.9; S, 7.80. Found (%): C, 40.9; Cl, 8.5; H, 3.1; N,
13.6; Pd, 25.9; S, 7.6. Conductivity data (10−3 M in DMF): ΛM = 10.0 Ω−1. IR (Raman)
cm−1: 3288, ν(NH); 1602 (1611), ν(C=N); 1006 (1007), ν(N–N); 752 (756), ν(N–N); 760
1
(683), ρ(Py); 439 (449), ν(Pd–N); 394 (391), ν(Pd–S); 271 (260), ν(Pd–Cl); H NMR (d6-
DMSO/TMS, ppm), δ: 7.64 (d, 1H, H3); 7.99(t, 1H, H4); 7.65 (d, 1H, H5); 7.28 (s, 1H, H7)
2.48 (s, 3H, CH3); 10.22 (s, 1H, N(4)H); 7.05–7.46 (m, ph); 13C NMR (d6-DMSO/TMS,
ppm) δ: 166.6, C(2); 126.7, C(3); 142.7, C(4); 123.5, C(5); 161.5, C(6); 155.3, (CH=N);
171.3, (C=S); 28.1, CH3; 126.2–142.9, Ph.
2.2.6.3. For [Pt(mpytsc)Cl. Yield: 62%. Elemental Anal. Calcd C8H9ClN4PtS (%): C,
22.6; Cl, 8.4; H, 2.1; N, 13.2; S, 7.6. Found (%): C, 22.4; Cl, 8.3; H, 2.0; N, 13.0; S, 7.5.
Conductivity data (10−3 M in DMF): ΛM = 8.0 Ω−1. IR (Raman) cm−1: 3300, νas(NH2);
3258, νs(NH2); 1599 (1615) ν(C=N); 1006 (1008), ν(N–N); 777 (779), ν(C=S); 686 (684),
1
ρ(Py); 450 (455), ν(Pt–N); 359 (362), ν(Pt–S); 330 (334), ν(Pt–Cl); H NMR (d6-DMSO/
TMS, ppm), δ: 7.59 (d, 1H, H3); 7.97 (t, 1H, H4); 7.45 (d, 1H, H5); 8.42 (s, 1H, H7), 2.49
(s, 3H, CH3); 7.80 (s, 2H, N(4)H2); 13C NMR (d6-DMSO/TMS, ppm) δ: 164.1, C(2);
128.9, C(3); 140.6, C(4); 123.5, C(5); 159.9, C(6); 148.5 (CH=N); 174.4 (C=S); 25.9,
6-CH3.
2.2.6.4. For [Pt(mpyptsc)Cl]. Yield: 57%. Elemental Anal. Calcd C14H13N4PtS (%): C,
33.6; Cl, 7.1; H, 2.6; N, 11.2; S, 6.4. Found (%): C, 33.4; Cl, 7.0; H, 2.4; N, 11.4; S, 6.3.
Conductivity data (10−3 M in DMF): ΛM = 7.0 Ω−1. IR (Raman) cm−1: 3288, ν(NH); 1599
(1603), ν(C=N); 1009 (1112), ν(N–N); 742 (749), ν(C=S), 655 (652), ρ(Py); 453 (455),
1
ν(Pt–N); 390 (391), ν(Pt–S); 295 (299), ν(Pt–Cl); H NMR (d6-DMSO/TMS, ppm), δ: 7.65
(d, 1H, H3); 8.01 (t, 1H, H4); 7.56 (d, 1H, H5); 8.77 (s, 1H, H7) 2.48 (s, 3H, CH3); 10.28
(s, 1H, N(4)H); 7.037–7.53 (m, ph). 13C NMR (d6-DMSO/TMS, ppm) δ: 166.6, C(2);
126.7, C(3); 142.7, C(4); 123.5, C(5); 161.5, C(6); 155.3, (CH=N); 171.3, (C=S); 28.1,
CH3; 126.21–142.98, Ph.
2.3. Biological assay
The human colon cancer (HCT116) and prostate cancer (DU145) cells were maintained in
Dulbecco’s modified eagle medium (DMEM), containing 10% FBS, penicillin, fungizone,
and streptomycin. Cells were maintained in an incubator with an atmosphere of 5% CO2 at
37 °C. In all assays, drug stocks of 25 and 50 mM were prepared by dissolving in DMSO
to ensure complete dissolution. The human colon cancer (HCT116) and prostate cancer
(DU145) cells were grown to 80% confluence and then incubated in DMEM onto 96-well
flat-bottomed micro-liter plates at a density of 5000 cells per well (100 μL/well) for 24 h at
37 °C. Subsequently, the cells were treated with Hmpytsc, Hmpyptsc, and their Zn(II), Pd
(II), and Pt(II) complexes at decreasing concentrations. Drug stocks were diluted in DMEM
supplemented with 10% FBS and antibiotics right before treatment. Cells in DMEM with-
out drug treatment were included in each plate in triplicate as negative controls. Cells were
fixed using 50 μL of cold trichloroacetic acid (50%) for 60 min at 4 °C, washed with water,
stained with 0.4% sulforhodamine B for 4 h at room temperature, rinsed with 1% acetic
acid, and allowed to dry overnight [19]. The resulting colored residue was dissolved in
200 μL Tris base (10 mM, pH 10.0) and optical density was recorded at 490 nm using a

Phenylthiosemicarbazone complexes
2717
microplate reader ELx808 (BioTek Instruments). The results were analyzed by Graph Pad
Prism (Graph Pad Software, Inc., San Diego, CA), and the sigmoidal dose response curve
was used to determine the 50% cell growth inhibitory concentration (IC50). Each point rep-
resents the average of two independent experiments performed in triplicate [19].
3. Results and discussion
The Experimental section describes the modified synthesis method of 6-methylpyridine-2-
carboxaldehydethiosemicarbazone (Hmpytsc) and 6-methylpyridine-2-carboxaldehyde-N(4)-
phenylthiosemicarbazone (Hmpyptsc) [18], and their V(V), Zn(II), Ru(II), Pd(II), and Pt(II)
complexes. Their elemental analyses are in excellent agreement with the assigned formulae.
The molar conductivities (ΛM) in DMF at room temperature suggest all complexes to be
non-electrolytes [20]. [VO2L] and [Zn(HL)Cl2] were prepared from the addition of [VO2(a-
cac)2] or ZnCl2 to ligands in ethanol or methanol, respectively. On the other hand, [Ru
(PPh3)2L2] and [MLCl] (M(II) = Pd, Pt) were obtained from the reaction of [Ru(PPh3)2Cl2]
or K2MCl4 and ligands in ethanol or aqueous methanol under basic conditions. All the
reported complexes are microcrystalline or powder-like, stable under normal laboratory con-
ditions, and soluble in DMF, DCM, and DMSO.
Orange needles, suitable for X-ray crystallography, were obtained by slow evaporation of
a solution of [Pd(mpyptsc)Cl] in DMSO-DCM (1 : 1) at room temperature. These crystals
were mounted on the diffractometer and the unit-cell dimensions and intensity data were
Table 1. Crystal data and structure refinement of [Pd(mpyptsc)Cl]·DMSO.
Empirical formula
Formula weight
Temperature
C16H19ClN4OPdS2
489.32
200 K
Wavelength
1.54178 Å
Crystal system
Space group
Monoclinic
P21/c
Unit-cell dimensions
a = 7.2407(2) Å, α = 90°
b = 11.1883(3) Å, β = 97.850(2)°
c = 23.5428(6) Å, γ = 90°
1889.36(9) Å3
Volume
Z
4
Density (Calcd)
Absorption coefficient
F (0 0 0)
1.720 g cm−3
11.396 mm−1
984
Crystal size
θ Range for data collection
Index ranges
0.16 × 0.08 × 0.04 mm
3.79–71.94
−8 ≤ h ≤ 8, −13 ≤ k ≤ 13, −28 ≤ l ≤ 28
23,587
3672 (Rint = 0.055)
Semi-empirical from equivalents
0.6339 and 0.3716
Full-matrix least-squares on F2
3672/0/233
Reflections collected
Independent reflections
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole
1.024
R1 = 0.0366, wR2 = 0.0931
R1 = 0.0514, wR2 = 0.1042
1.077 and −0.907 e Å−3

2718
S.A. Elsayed et al.
4
3
2
5
6
7
H3C
N
CH
H3C
N
CH
N
1
N
3
NH
2
N
8
4
NHR
NHR
S
HS
Thiol form
Thione form
Figure 1. Tautomeric forms of 6-methylpyridine-2-carbaldehydethiosemicarbazone (R = H, Hmpytsc) and 6-meth-
ylpyridine-2-carbaldehyde-N(4)-phenylthiosemicarbazone (R = Ph, Hmpyptsc).
measured at 200 K. The structure was solved by least-squares fit of the angular setting of
strong reflections based on F2. The relevant crystal data and experimental conditions along
with the final parameters are summarized in table 1.
3.1. Vibration spectra
We reported earlier the vibrational spectroscopic data of 2-formylpyridinethiosemicarbaz-
one, 6-methylpyridine-2-carbaldehyde-N(4)-ethylthiosemicarbazone, and a number of their
complexes [8, 9]. The characteristic IR and Raman bands observed for 6-methylpyridine-2-
carboxaldehydethiosemicarbazone (Hmpytsc) and 6-methylpyridine-2-carboxaldehyde-N(4)-
phenyl thiosemicarbazone (Hmpyptsc), and their new complexes, together with the pro-
posed vibrational assignments, are presented in the Experimental section and they are in
good agreement with the data reported earlier [18]. Hmpytsc and Hmpyptsc have the thio-
amide moiety [–NH–C(=S)–NH2], which exhibits thione–thiol tautomerism (figure 1) [8, 9,
18, 21]. The absence of ν(SH) band at ca. 2600 cm−1 indicates that Hmpytsc and Hmpyptsc
are predominantly in the thione form in the solid state [8, 9, 21, 22]. In solution and in the
presence of metal ions, however, it may convert to the deprotonated thiol form [8, 9, 21].
The spectra of the free Hmpytsc and Hmpyptsc exhibit strong IR bands at 1612 and
1588 cm−1 (1608 and 1606 cm−1 in Raman), respectively, which is characteristic for
ν(HC=N). The coordination of the azomethine N to the metal ion reduces the electron den-
sity in its link, and thus shifts its stretch to lower wavenumbers [8, 9, 23]. In the complexes,
this band is observed at 1635–1599 cm−1, supporting coordination through the azomethine
N [8, 9, 24]. The band near 3150 cm−1 in the free ligands (near 3100 cm−1 in Raman), aris-
ing from ν(NH), is not observed in the complexes [25]. The ν(C=S) stretches at 842 and
784 cm−1 (at 799 and 792 cm−1 in Raman) in free Hmpytsc and Hmpyptsc, respectively, are
less intense in the V(V), Pd(II), and Pt(II) complexes and shifted to lower frequency, sug-
gesting coordination of the metal through sulfur. The coordination of the thiolato sulfur was
further indicated by a decrease in the energy of the thioamide band as well as by the pres-
ence of a band near 350 cm−1 assigned to ν(M–S) [26]. In spectra of Hmpytsc, bands at
3392 and 3258 cm−1 assigned to νas(NH2) and νs(NH2) stretches, respectively, are shifted to

Phenylthiosemicarbazone complexes
2719
lower wavenumbers upon complex formation [8, 9, 12]. The free ligands show an intense
band near 992 cm−1 assigned to ν(N–N), which shifted to higher frequency in the com-
plexes [8, 9]. Furthermore, the pyridine ring deformation mode at 562 cm−1 in the free
ligands is shifted to near 590 cm−1 in the complexes, indicating participation of pyridyl N
in coordination [8, 9]. The significant changes in the ligand bands upon complexation are
the decrease in ν(C=N), increase in ν(N–N), and the absence of large systematic shifts of
νas(NH2) to lower frequencies (in case of mpytsc complexes). These data indicate coordina-
tion through azomethine nitrogen without interaction between the terminal amino nitrogen
and the metal ion. The IR and Raman data suggest mono-negative tridentate N,N,S– coordi-
nation of mpytsc− and mpyptsc− to the metal ions. In the case of [Zn(HL)Cl2]
(HL = Hmpytsc, Hmpyptsc), ν(NH) is shifted to lower wavenumbers since the thione S is
taking part in coordination. Also, the pyridine ring deformation mode is observed more or
less in the same position in the free ligands near 560 cm−1, i.e. Hmpytsc and Hmpyptsc are
neutral bidentate ligands through azomethine N and thione S [7–9]. The spectra of the Ru
(II) complexes show shifts of ν(HC=N) to 1635 and 1592 cm−1 in IR (1587 and 1599 cm−1
in Raman), while the ν(C=S) stretch shifts to lower energy (802 and 740 cm−1 in IR, and
890 and 747 cm−1 in Raman) for [Ru(mpytsc)(PPh3)2] and [Ru(mpyptsc)(PPh3)2], respec-
tively [8, 9]. Moreover, ν(N–N) shifts to higher energies while there is no shift observed in
the pyridyl ring deformation mode. These features support the mono-negative bidentate
coordination of the mpytsc− and mpyptsc− to Ru(II) through the azomethine N and thiolato
S [7–9, 27].
As expected, the presence of coordinated PPh3 in [RuL2(PPh3)2] is manifested by strong
IR bands near 1100 and 750 cm−1, attributed to the ν(P–C) and δ(C–CH) vibrations, respec-
tively [28]. The spectra of [VO2L] show two extra IR bands characteristic of the cis-VO2
core near 935 and 910 cm−1 (near 940 and 900 cm−1 in Raman) assigned to νas(O=V=O)
and νs(O=V=O) stretches, respectively [9]. Finally, extra-far IR and Raman bands are
observed near 450, 350, and 280 cm−1, attributed to ν(M–N), ν(M–S), and ν(M–Cl)
stretches, respectively [7–10, 28, 29].
3.2. NMR spectra
The 1HNMR spectral data of Hmpytsc, Hmpyptsc, and their metal complexes (in d6-
DMSO) are listed in the Experimental section (see figure 1 for numbering scheme); they
are in agreement with those reported earlier [18]. The spectra of free Hmpytsc and
Hmpyptsc exhibit singlets at δ 11.61 and 12.01 ppm, respectively, assigned to N(3)H. This
signal is missing in the complexes, indicating coordination of the deprotonated thiol S to
the central metal ions [8, 9]. As expected, in [Zn(HL)Cl2] complexes, this signal is observed
at δ 11.63 (Hmpytsc) and 12.100 (Hmpyptsc) ppm, confirming the coordination of thione S
to Zn(II) [7–9]. The singlet at δ 8.01 and 8.13 ppm in free Hmpytsc and Hmpyptsc, respec-
tively, arising from the azomethine CH(7)=N proton is shifted downfield in the complexes
due to the involvement of azomethine N in coordination [7]. In Hmpytsc, the two hydro-
gens in the terminal nitrogen, N(4)H2, are not equivalent and are assigned to a pair of sing-
lets at δ 8.12 and 8.13 ppm, indicating restricted rotation about the S=C-NH2 group; these
signals are shifted upfield upon complex formation [11]. In the spectrum of Hmpyptsc, the
N(4)H proton shows a singlet at δ 10.29 ppm, which is also shifted upfield in the com-
plexes. The 6-CH3 and pyridine protons appear near δ 2.45 and 7.202–8.190 ppm, respec-
tively. In the complexes, the pyridine ring protons are shifted downfield, except in case of

2720
S.A. Elsayed et al.
[Zn(HL)Cl2], due to the decrease in the electron density caused by electron withdrawal by
the metal ions from pyridyl N [14, 30]. 13C NMR spectral data of Hmptsc, Hmpyptsc, and
their complexes are reported in the Experimental section. It is clear that the resonances of
carbons adjacent to the coordination centers in the complexes, i.e. C(2), C(6), and C(7) shift
downfield relative to their positions in the free ligands [8, 9, 31]. In the free ligands, the
C=S resonance is observed near 177.0 ppm, which is shifted upfield 3–5 ppm in the com-
plexes, attributed to the complexation through sulfur, which apparently is related to an
increased electron density at this site, probably due to π-back bonding for thiolato S [32].
This feature is due to the increase in current brought about by coordination to pyridyl N,
azomethine N, and deprotonated thiol S [8, 9]. In spectra of [Zn(HL)Cl2] and
[RuL2(PPh3)2], the resonances arising from C(6) and C(2) are more or less in the same posi-
tions as in the free ligands, confirming coordination of Hmpytsc and Hmpyptsc to Zn(II) or
Ru(II) through thione S (Zn(II)) and thiolato S– (Ru(II)), and azomethine N without any
participation of the pyridyl N in coordination [8, 9].
31PNMR spectra of [Ru(PPh3)2L2] show sharp singlets at δ 52.34 (Hmpytsc) and
56.2 (Hmpyptsc) ppm. The observation of only one signal in the spectra of the com-
plexes suggests that the two triphenylphosphine groups are magnetically equivalent
[33].
3.3. Mass spectra
The mass spectrum of Hmpytsc (C8H10N4S) is in agreement with the assigned formula {m/
e 194.0 (Calcd 194.0)} with 100% abundances. Three more signals are observed at 134,
106, and 93 corresponding to C7H8N3 (Calcd 134.0), C7H8N (Calcd 106), and C6H7N
(Calcd 93) fragments, respectively. The spectrum of Hmpyptsc (C14H14N4S) shows a peak
at m/e 270.9 (Calcd 270.0) with 16% abundance. The peak at 178.8 (Calcd 178.0) corre-
sponds to C8H8N3S fragment.
The mass spectra of the complexes were measured and their molecular ion peaks are in
agreement with their calculated formulas. In the spectra of [Ru(L)2(PPh3)2] (L = mpytsc,
mpyptsc), the peaks at m/z 1011.8 (Calcd 1011.1) and 1164.0 (Calcd 1163.1) with 31 and
42% abundance are assigned to the molecular ions [Ru(mpytsc)2(PPh3)2]+ and [Ru
(mpyptsc)2(PPh3)2]+, respectively. The fragmentation patterns indicate the stepwise L− and
PPh3 ligand loss. Three more fragments for mpystc, [Ru(mpytsc)(PPh3)2]+ (818.8), [Ru
(mpystc)(PPh3)]+ (557.1), and [Ru(PPh3)]+ (363.5); while for mpyptsc two more fragments,
[Ru(mpyptsc)2(PPh3)]+ (902.2) and [Ru(PPh3)]+ (363.9), were obtained [34]. The mass
spectra of [VO2L] (L = mpytsc, mpyptsc) show signals at m/z 277.3 (Calcd 276.0) and
351.0 (Calcd 351.9) with 89 and 82% abundance, corresponding to [VO2(mpytsc)]+ and
[VO2(mpyptsc)]+, respectively. In the latter complex, three more peaks are observed at
286.0, 255.0, and 241.2 corresponding to [VO2(mpyptsc-C5H6)]+, [VO2(mpyptsc-C5H6S)]+,
and [VO2(mpyptsc-C6H6NS)]+, respectively. The spectrum of [Pt(mpytsc)Cl] displays a sig-
nal at 423.9 (Calcd 423.5) with 8% abundance, assigned to [Pt(mpyptsc)Cl]+. The fragmen-
tation pattern shows the loss of Cl at 387.2 (Calcd 388.0). The mass spectrum of [Zn
(Hmpyptsc)Cl2] shows a signal at 406.0 (Calcd 406.5) with 46% abundance, corresponding
to [Zn(Hmpyptsc)Cl2]+. The fragmentation pattern indicates the loss of Cl2 [35] and C4H5
fragments at 282.0 (Calcd 282.5).

Phenylthiosemicarbazone complexes
2721
3.4. Electronic spectra
The UV–vis spectra of Hmpytsc, Hmpyptsc, and their complexes were recorded in DMF.
The absorptions in the UV region are due to transitions within ligand orbitals, while those
in the visible region are assigned to transitions involving both metal and ligand orbitals
[36]. The spectra of Hmpytsc and Hmpyptsc show broadbands in the 290–360 nm region,
attributable to n–π* transitions of the pyridine and phenyl rings, as well as azomethine and
thioamide moieties [18, 37]. The electronic spectra of the diamagnetic Ru(II) complexes
1
1
show three bands near 500, 390, and 340 nm corresponding to A1g → 1T1g, A1g → 1T2g,
and ligand (p-dp) transitions, respectively [8, 10, 12]. These transitions are attributed to a
low-spin octahedral geometry around Ru(II) [10]. The electronic spectra of the diamagnetic
Pd(II) and Pt(II) complexes exhibit two absorptions near 380 and 465 nm, assigned to
1
1A1g → 1Bg and A1g → 1Eg transitions, respectively, confirming square-planar geometry
[11–15].
3.5. X-ray crystallography
Orange needles, suitable for X-ray crystallography of [Pd(mpyptsc)Cl]·DMSO, were
obtained from slow evaporation of a solution of [Pd(mpyptsc)Cl] in DMSO-DCM (1 : 1).
These crystals crystallize in a monoclinic lattice with space group symmetry P21/c. The crys-
tal data and structure refinement details are given in table 1.
The structure and atomic labeling are shown in figure 2. The complex is planar except
for the phenyl group, which is twisted out of the plane by 10.5°. The square-planar palla-
dium coordinates the pyridyl nitrogen N(1), the azomethine nitrogen N(7), and thiolato sul-
fur of mpyptsc− in a ZEZ configuration. The fourth coordination site is occupied by a
chloride. The coordinated ligand consists of heterocyclic, PdNCCN, and PdNNCS rings.
Figure 2. X-ray crystal structure of [Pd(mpyptsc)Cl]·DMSO.

2722
S.A. Elsayed et al.
Table 2. Selected bond lengths (Å) and angles (°) of [Pd(mpyptsc)Cl)]·DMSO.
Bond lengths
Bond angles
Pd(1)–N(7)
Pd(1)–N(1)
Pd(1)–S(8)
Pd(1)–Cl(1)
S(8)–C(8)
N(1)–C(2)
N(1)–C(6)
N(7)–C(7)
N(7)–N(8)
N(8)–C(8)
N(9)–C(8)
N(9)–C(9)
S(21)–O(21)
(21)–C(22)
S(21)–C(21)
1.970(3)
2.171(3)
2.2421(11)
2.3238(11)
1.774(4)
1.335(5)
1.375(5)
1.295(5)
1.372(4)
1.308(5)
1.352(5)
1.414(5)
1.509(4)
1.763(5)
1.771(5)
N(7)–Pd1–N(1)
N(7)–Pd1–S(8)
N(1)–Pd1–S(8)
N(7)–Pd1–Cl1
N(1)–Pd1–Cl1
S(8)–Pd1–Cl1
80.34(13)
83.59(10)
163.84(10)
172.83(10)
106.77(10)
89.34(4)
96.08(14)
118.5(4)
133.6(3)
107.9(3)
118.8(3)
116.6(3)
124.6(3)
111.7(3)
129.9(3)
120.5(4)
119.4(4)
124.0(3)
121.0(4)
115.0(3)
121.6(4)
106.1(3)
106.3(3)
97.2(3)
C(8)–S(8)–Pd1
C(2)–N(1)–C(6)
C(2)–N(1)–Pd1
C(6)–N(1)–Pd1
C(7)–N(7)–N(8)
C(7)–N(7)–Pd1
N(8)–N(7)–Pd1
C(8)–N(8)–N(7)
C(8)–N(9)–C(9)
N(1)–C(2)–C(3)
N(1)–C(2)–C(1)
N(8)–C(8)–S(8)
N(8)–C(8)–N(9)
N(9)–C(8)–S(8)
C(13)–C(14)–C(9)
O(21)–S(21)–C(22)
O(21)–S(21)–C(21)
C(22)–S(21)–C(21)
Selected bond angles (°) and bond lengths (Å) are listed in table 2. The bond lengths and
angles of the phenyl and pyridyl rings are normal. The bite angles N(1)–Pd–N(7), 80.34
(13)°; N(7)–Pd–S, 83.59(10)°; N(1)–Pd–Cl, 106.77(10)°; and S–Pd–Cl, 89.34(4)°, and the
bond lengths of Pd–N(1), Pd–N(7), Pd–S, and Pd–Cl are 2.171(3), 1.970(3), 2.2421(11),
and 2.3238(11) Å, longer than those reported for square-planar Pd(II) complexes with N,N,
S-donors [38] and similar to [Pd(fpytsc)X] (Hfpytsc = 2-formylpyridinethiosemicarbazone;
X = CN, PPh3) and [Pt(mpyetsc)Cl] (Hmpyetsc = 6-methylpyridine-2-carbaldehyde-N(4)-
ethylthiosemicarbazone) complexes, which we have reported earlier [8, 9]. The bond length
of Pd–N(1), 2.171(3) Å is longer than that of Pd–N(7), 1.970(3) Å; the weak coordination
of Pd(II) to the pyridyl nitrogen N(1) compared to azomethine N(7) is attributed to high
basicity of the latter [8, 9]. The bond distance, C(8)–S, 1.774(4) Å is intermediate between
C=S double bond (1.62 Å) and C–S single bond (1.82 Å), i.e. C(8)–S suggesting partial sin-
gle-bond character [8, 9, 32, 38]. In addition, the bond lengths of C(8)–N(8), 1.308(5) Å,
and C(7)–N(7), 1.295(5) Å, are close to a double bond (1.28 Å), while that of N(7)–N(8),
1.372(4) Å, is very close to a single bond [8, 9, 32]. The S–C bond distances are consistent
with increased single-bond character, while both thioamide C–N distances indicate
increased double-bond character. The negative charge of the deprotonated ligand is delocal-
ized over the thiosemicarbazonate. This is indicative of the coordinated thiosemicarbazone’s
Table 3. Bond lengths (Å) and angles (°) related to the hydrogen bonding for [Pd(mpyptsc)
Cl)]·DMSO.
D–H
…A
d(D–H)
d(H…A)
d(D…A)
∠DHA
N(9)–H(9)
O(21)#1
0.77(5)
2.08(5)
2.840(5)
169(4)

Phenylthiosemicarbazone complexes
2723
greater conjugation and more delocalized electron density [32]. The bond angles, N(7)–Pd–
Cl, 172.83(10)°, and N(1)–Pd–S, 163.84(10)°, deviate substantially from that expected for
regular square-planar geometry [8, 9].
The complex molecules are linked through intermolecular hydrogen bonds. The mono-
mers form hydrogen-bonded dimers linked by Ph–N(9)–H and DMSO–O(21). The observed
hydrogen bonding patterns are of the DH and DA [32]. Table 3 shows bond lengths and
angles related to the hydrogen bonding of the dimeric complex.
3.6. DFT computation study
The structure of Pd(mpyptsc)Cl as well as both thione and thiol forms of Hmpyptsc (figure
1) were calculated using the Gaussian 03 Rev B-02 programs [17]. The geometry optimiza-
tion was carried out using the DFT computational method with the DGDZVP/B3LYP basis
set. [Pd(mpyptsc)Cl] was found to be planar with all dihedral angles within 2° of 0° or
180° except for the phenyl ring which is twisted out of plane by 10.5° in the crystal struc-
ture data and 37.6° (in calculation) (figure 3). In the thiol and thione forms of Hmpyptsc,
the angle is 29° and 38°, respectively. This difference is presumably due to crystal packing
forces. The calculated bond lengths and angles are in agreement with the experimental val-
ues. Figure 3 shows the calculated structures of [Pd(mpyptsc)Cl], Hmpyptscthione and
Hmpyptscthiol. The distances between N(1) and N(9) are 6.059, 6.292, and 6.342 Å, respec-
tively. Thus, the basic structure of Hmpyptsc changes little upon coordination, and the
geometry around palladium is constrained by the rigidity of the ligand, accounting for the
Figure 3. The calculated geometries of (a) [Pd(mpyptsc)Cl], (b) Hmpyptsc in thione, and (c) thiol tautomeric
forms.

2724
S.A. Elsayed et al.
distorted geometry. Free Hmpyptsc is more stable in the thione form than the thiol one
(figures 1 and 3) by 9.54 kJ M−1 [35].
3.7. Biological applications
Cisplatin is one of the best-known small metal-containing drug molecules. It acts as an anti-
cancer agent for several human cancers, particularly testicular and ovarian cancers [11].
Generally, the side effects, especially nephrotoxicity, limit its widespread use in high doses
[39]. The need to develop new complexes with reduced nephrotoxicity and higher activity
has stimulated the synthesis of many new complexes. Over the past years, a renewed inter-
est in Zn(II), Pd(II), Ag(I), and Pt(II) complexes as potential anticancer agents has devel-
oped in our laboratory [9–15, 40].
The ultimate goal of this research is to develop new complexes with high efficacy against
cancer cells. Free Hmpytsc and Hmpyptsc (HL), and their complexes, [Zn(HL)Cl2], [Pd(L)
Cl], and [Pt(L)Cl], were evaluated in vitro for their inhibitory effects on cell growth prolif-
eration of human colon cancer (HCT116) and prostate (DU145) cell lines in comparison to
cisplatin as a reference (figure 4).
The cell growth proliferation was evaluated by DMEM assay. The final IC50 values for
cell growth proliferation after 24 h of incubation with free Hmpytsc and Hmpyptsc, and
their complexes, [Zn(Hmpytsc)Cl2], [Zn(Hmpyptsc)Cl2], [Pd(mpytsc)Cl], [Pd(mpyptsc)Cl],
Figure 4. Anticancer activity of Hmpyptsc and [Zn(Hmpyptsc)Cl2] against human colon cancer (HCT116) and
human prostate (DU145) cell line.

Phenylthiosemicarbazone complexes
2725
[Pt(mpytsc)Cl], and [Pt(mpyptsc)Cl], for human colon cancer (HCT116) cells were >100,
3.3 0.4, 84.8 0.3, 2.6 0.1, 46.0 0.2, 38 1, 87 1, and 84 1 μM, respectively. For
prostate colon cancer (DU145), cells were >100, 3.6 0.8, >100, 3.1 0.3, 87.5 0.8, 46
1, >100, and 93.5 0.6 μM, respectively. The IC50 values of cisplatin are 18.0 0.4
(HCT116) and 22.5 0.7 (DU145) μM. Thus, the order for cytotoxic activity of the drugs
is [Zn(Hmpyptsc)Cl2] > Hmpyptsc > [Pd(mpyptsc)Cl] > [Pd(mpytsc)Cl] > [Pt(mpyptsc)Cl] >
[Zn(Hmpytsc)Cl2] > [Pt(mpytsc)Cl]. We previously reported the in vitro anticancer activity
of 6-methylpyridine-2-carbaldehyde-N(4)-ethylthiosemicarbazone (HmpETSC) and its com-
plexes, [Zn(HmpETSC)Cl2] and [M(mpETSC)Cl] (M(II) = Pd, Pt), against human colon
cancer (HCT116) cell line [9]. Generally, nitrogen-containing ligands have important effects
on cytotoxicity [41]. [Zn(HmpETSC)Cl2] exhibits higher anticancer activity compared to
[Pd(mpETSC)Cl], which is more active than its Pt(II) analog. Generally, substituted groups
as well as the modes of chelation of HmpETSC in the complexes may play a role in their
anticancer activity towards human cancer cell lines [9, 41, 42]. An important aspect of the
anticancer activity of [Zn(Hmpyptsc)Cl2] is the high degree of conjugation of the (–CH=N–
N–C(=S)–NH(Ph)) moiety, i.e. phenyl when compared to its ethyl analog in [Zn(HmpETS)
Cl2] [28, 43]. Generally, the Zn(II) complexes show higher activity than their free ligands,
as a result of the polarity reduction of the complex molecule as a whole, when compared
with the free ligand, by partial sharing of their charges within the coordination compounds,
favoring their permeation through the cell membrane, hence resulting in a better cell uptake
of the active species [44].
Cepeda et al. reported that the binding of cisplatin to genomic DNA (gDNA) in the cell
nucleus is the main event responsible for its antitumor properties [45]. Thus, the damage
induced upon binding of cisplatin to gDNA may inhibit transcription and/or DNA replica-
tion mechanisms. Subsequently, these alterations in DNA processing would trigger cyto-
toxic processes that lead to cancer cell death. An important property of the complexes is the
ligand exchange process. The low activity of [Pd(mpyptsc)Cl] may be due to the poor solu-
bility, difficulty in hydrolysis to produce their cationic form, fast interaction with DNA
under physiological conditions [46], or the presence of intermolecular hydrogen bonds, as
indicated from the crystal structure of [Pd(mpyptsc)Cl] [15].
4. Conclusion
The new complexes, [VO2L], [Zn(HL)Cl2], [Ru(PPh3)2L2], and [MLCl] (M(II) = Pd, Pt;
HL = 6-methylpyridine-2-carbaldehydethiosemicarbazone (Hmpytsc), 6-methylpyridine-2-
carbaldehyde-N(4)-phenylthiosemicarbazone (Hmpyptsc)), were prepared and characterized.
In the complexes, [VO2L] and [MLCl] (M(II) = Pd, Pt), mpytsc− and mpyptsc− are mono-
anionic tridentate ligands bonding through pyridyl N, azomethine N, and thiolato S; for [Zn
(HL)Cl2], they function as neutral bidentate through azomethine N and thione S. In addi-
tion, they are mono-negative bidentate through azomethine N and thiolato S for [Ru
(PPh3)2L2]. The X-ray crystal structure and DFT conformational analysis of [Pd(mpyptsc)
Cl]·DMSO were discussed. [Pd(mpyptsc)Cl]·DMSO shows Pd(II) ion in a distorted square
plane, ligated by mono-anionic tridentate, pyridyl N, azomethine N, and thiolato S of
mpyptsc− moiety, and chloride. Free Hmpyptsc and [Zn(Hmpyptsc)Cl2] exhibit the highest
growth inhibitor activity against human colon cancer (CHT116) and prostate cancer
(DU145) with mean IC50 values of 3.32 and 2.60 (HCT116), and 3.60 and 3.10 (DU145)
μM, respectively.

2726
S.A. Elsayed et al.
CCDC numbers
CCDC number of the crystal obtained of [Pd(mpyptsc)Cl]·DMSO is 866624.
Acknowledgements
We are grateful to the Ministry of Higher Education in Egypt (E.S.) and NSERC (Canada)
Discovery grant (ISB) for the financial support of this work. We would also like to thank
Prof. M.A. Whitehead and Mr K. Conley for helpful discussions.
Supplemental data
Supplemental data for this article can be accessed here [http://dx.doi.org/10.1080/00958972.2014.942224]
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