Synthesis and characterization of Palladium(II) complexes with thiosemicarbazones

Article abstract of DOI:10.14233/ajchem.2018.21117

Three new complexes viz., [PdCl₂(H-aatp)₂] (1), [Pd(dmbptz)2] (2) and [Pd(btz)] (3) have been synthesized by reacting [PdCl₂(cod)] with the ligands H-aatp, H-dmbptz and H₂-btz, respectively (where H-aatp = anisa

Full text of DOI:10.14233/ajchem.2018.21117

Asian Journal of Chemistry; Vol. 30, No. 6 (2018), 1205-1208
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21117
Synthesis and Characterization of Palladium(II) Complexes with Thiosemicarbazones
*
SAMIM SULTANA, JAYANTAJIT BARUAH and GEETIKA BORAH
Department of Chemistry, Dibrugarh University, Dibrugarh-786 004, India
*Corresponding author: Tel: +91 373 2370210; E-mail: geetikachem@yahoo.co.in
Received: 17 November 2017;
Accepted: 15 March 2018;
Published online: 30 April 2018;
AJC-18865
Three new complexes viz., [PdCl2(H-aatp)2] (1), [Pd(dmbptz)2] (2) and [Pd(btz)] (3) have been synthesized by reacting [PdCl2(cod)] with
the ligands H-aatp, H-dmbptz and H2-btz, respectively (where H-aatp = anisaldene-4-aminothiophenol; H-dmbptz = 2,5-dimethoxybenzyl-
4-phenylthiosemicarbazone and H2-btz = benzil-bisthiosemicarbazone). These have been characterized by various physico-chemical
techniques such as FTIR, UV-visible, 1H and 13C NMR, ESI(+)-mass spectroscopy, etc. The complexes were found to be diamagnetic,
four coordinated mononuclear neutral molecules with square planar or distorted square planar geometry.
Keywords: Anisaldene-4-aminothiophenol, 2,5-Dimethoxybenzyl-4-phenylthiosemicarbazone, Benzil-bisthiosemicarbazone.
antitumor activity comparable to cisplatin [16]. It has also been
reported that pyridoxal thiosemicarbazone ligand is also inter-
esting especially because of the presence of three different
existing forms such as (H2L), singly-deprotonated (HL)– or
doubly-deprotonated (L)2- form [17]. In this note, we wish to
report the synthesis and characterization of three new complexes
viz., [PdCl2(H-aatp)2], Pd(dmbptz)2] and [Pd(btz)] from the
metal precursor [PdCl2(cod)] in aerobic condition, which might
have potent biological activity.
INTRODUCTION
Derivatives of thiosemicarbazones are unique N,S-donors
because of their mixed hard-soft character, variable binding
modes, flexibility, selectivity and sensitivity towards the central
metal atom [1]. These ligands have gained significant attention
because of structural similarities with natural biological
substances due to the presence of imine group (-N=CH-) which
imparts biological activity including antifungal, antibacterial,
anti-inflammatory, antiviral, anticonvulsant, antidepressant,
anticancer along with good anti-HIV activity and as a result
received wide applications in pharmacology and nuclear
medicine [2-6]. These ligands can also act as analytical reagents
for determining, fixing and entrapping heavy metal ions [7,8].
The transition metal incorporation into thiosemicarbazones
leads to an enhancement in their pharmacological activities
[9] and the subsequent synergistic effects involving both metal
and the thiosemicarbazone leads to the improvement of their
biological activities and decreases the cytotoxicity of both the
metal ions and ligands [10]. They bind to the transition metal
center in a neutral or anionic form. It has been reported that
the substituent at the carbon atom of the imine group (-HC=N)
can affect the bonding to the metal halides, which determines
the formation of mono-, di- and poly-nuclear complexes.Various
transition metals such as organotin(IV), iron(II), copper(II),
zinc(II), nickel(II), palladium, platinum, ruthenium, etc. have
been incorporated into thiosemicarbazones and their applications
have been studied [11-15]. Palladium(II) derivativeswere found
to be more preferred for cancer treatment because of their struc-
tural analogy with Pt(II) complexes and some complexes show
EXPERIMENTAL
The chemicals used for synthesis and analysis were all of
AnalaR grade and procured from Sigma-Aldrich and Fluka.
The PdCl2 was purchased from E. Merck. They were used as
received without further drying or purification. The metal
precursor [PdCl2(cod)] was synthesized according to literature
method [18]. The aluminium-coated TLC (thin-layer chroma-
tography) plates and silica gel used for column chromato-
graphy were procured from E. Merck. All the reactions were
carried out under aerobic condition.
Physical measurements: The molar conductances of the
complexes were measured using digital conductivity bridge
(ELICO-CM-180) at room temperature. The melting point of
the complexes was determined using a Buchi B450 melting
point apparatus. The FTIR spectra (4000-240 cm–1) of the
complexes were obtained as KBr disc using Shimadzu Prestige-
21 FTIR spectrophotometer. Electronic spectra (800-200 nm)
of the complexes were taken by using Shimadzu-Graphicord
UV-1700 spectrometer with 1 cm3 quartz cell. The 1H and 13C
NMR spectra were recorded on a Bruker Avance II FT NMR

1206 Sultana et al.
Asian J. Chem.
spectrometer operating at 400 and 100.62 MHz, respectively
using TMS as internal standard. The solvents used were DMSO-
d6 and CD3CN. The ESI(+) mass spectra of the complexes were
recorded on a Water ZQ-4000 mass spectrometer in CH3CN
or CHCl3. Elemental analyses (C, H, N, S) were done by using
the Elementer Vario EL III Carlo Erba 1108.
Yield: 78 % m.p.: 160 °C; m.w.: 356 g mol-1. Anal. (%)
calcd. for C16H16N6S2: C, 53.93; H, 4.49; N, 23.60; S, 17.98.
Found: C, 54.46; H, 4.20; N, 23.14; S, 18.27 %. IR (KBr, cm–1):
3420, 3335 ν(N-H), 3142 ν(N-H), 1609 ν(C=N), 1186 ν(N-N),
839 ν(C=S). 1H NMR (300.14 MHz, DMSO-d6) δ: 10.0, 9.89
(d, N-H), 8.64,8.34 (s, N-H), 7.18-7.89 (m, Ph-H). 13C NMR
(100.62 MHz, DMSO-d6) δ: 179.9 (C=S), 142.4 (C=N), 126.9
(C4), 125.7 (C2′, C6′), 127.5 (C2,6), 129.6 (C3,5), 133.8 (C1).
Synthesis of [PdCl2(H-aatp)2] (1): To a mixture of
[PdCl2(cod)] (0.5 mmol, 0.143 g) and H-aatp (1 mmol, 0.243
g), 20 mL dichloromethane was added. The mixture was stirred
in air at room temperature for 3 h to get a reddish brown
coloured solution. It was filtered off and then washed with
chloroform. The product was dried over fused CaCl2 in a
dessicator and then recrystallized from DCM.
Synthesis
Anisaldene-4-aminothiophenol (H-aatp): To a mixture
of anisaldehyde (1.36 g, 10 mmol) and 4-aminothiophenol
(1.25 g, 10 mmol) 20 mL methanol was added followed by
the addition of few drops of conc. HCl. The mixture was stirred
at room temperature for 3 h, which results in the formation of
brown coloured precipitate. It was filtered, washed with
methanol and then dried over fused CaCl2 in a desiccator. It
was soluble in CH3CN, DCM, DMF, DMSO.
–1
Yield: 81 %. m.p.: 147 °C; m.w.: 663 g mol-1, Λm: 10 Ω
Yield: 77 %; m.p.: 122 °C; m.w. 243 g mol-1, Anal. (%)
calcd. for C14H13NOS: C, 70.14; H, 5.35; N, 5.66; S, 13.17.
Found: C, 69.68; H, 5.12; N, 5.98; S, 12.90. IR (KBr, cm–1):
2575 ν(S-H), 2837 ν(O-CH3), 1601 ν(C=N), 829 ν(C-S). 1H
NMR (300.14 MHz, DMSO-d6) δ: 8.55 (s, HC=N), 6.87-7.26
(m, 4H, Pha-H), 7.51-7.57 (m, 2H, Phb-H), 7.87-7.89 (m, 2H,
Phb-H), 3.84 (s, CH3), 3.42 (s, S-H). 13C NMR (100.62 MHz,
DMSO-d6) δ: 150.3 (C=N), 114.4 (Pha-C2, Pha-C6), 122.1 (C1a),
128.4 (C3a, C5a), 160.5 (C4a), 127.5 (C2b, C6b),129.7 (C3b, C5b),
134.3 (C1b), 191.3 (C4b), 55.2 (CH3).
cm2 mol–1. Anal. (%) calcd. for C28H26O2N2S2PdCl2: C, 50.65;
H, 3.92; N, 4.22; S, 9.65. Found: C, 50.71; H, 4.11; N, 4.50;
S, 9.77 %. IR (KBr, cm–1): 2837.29 ν(O-CH3), 2594.26 ν(S-
H), 1599.06 ν(C=N), 831.32 ν(C-S), 390.12 ν(Pd-S) and
362.68 ν(Pd-Cl). 1H NMR (300.14 MHz, DMSO-d6) δ: 9.81
(s, 2H, CH), 7.31-7.85 (m, 8H, Ph-H), 6.49-7.23 (m, 8H, Ph-
H), 3.67-3.89 (m, 6H, CH3), 2.49-2.53 (d, 2H, SH). 13C NMR
(100.62 MHz, DMSO-d6) δ: 151.2 (C=N), 115.3 (C2a,C6a),
122.8 (C1a), 129.4 (C3a,C5a), 161.2 (C4a), 124.7 (C2b,C6b), 127.6
(C3b,C5b), 131.3 (C1b), 187.5 (C4b), 55.4 (CH3).
2,5-Dimethoxybenzaldene-4-phenylthiosemicarbazone
(H-dmbptz): To a mixture of 2,5-dimethoxybenzaldehyde
(1.66 g, 10 mmol) and 4-phenylthiosemicarbazide (1.65 g, 10
mmol), 25 mL ethanol was added followed by the addition of
few drops of glacial acetic acid. The resulting mixture was
then refluxed at 65 °C for 2 h to obtain a white solution, which
was then allowed to cool to room temperature and a white
microcrystalline product was obtained. The solid product was
then filtered off, washed with diethyl ether and water and then
dried over fused CaCl2 in a desiccator. The crude product was
then recrystallized from ethanol for purification. The ligand
was soluble in ethanol, DMF and DMSO.
Synthesis of [Pd(dmbptz)2] (2):To a mixture of [PdCl2(cod)]
(0.5 mmol, 0.143 g) and H-dmbptz (1 mmol, 0.315 g), 20 mL
acetonitrile was added to make a solution. The mixture was
stirred in air at room temperature for 3 h to get a bright yellow
coloured precipitate, which was washed with acetonitrile and
then dried over fused CaCl2 in a desiccator.
–1
Yield: 82 %. m.p.: 166 °C; m.w.: 733 g mol-1 Λm.:12 Ω
cm2 mol–1. Anal. (%) calcd. for C32H32O4N6S2Pd: C, 52.43; H,
4.37; N, 11.47; S, 8.74. Found: C, 51.96; H, 4.13; N, 11.70; S,
8.92 %. IR (KBr, cm–1): 3129 ν(N-H), 2834 ν(O-CH3), 1587
ν(C=N), 1172 ν(N-N), 621 ν(C-S), 449 ν(Pd-N) and 380 ν(Pd-
1
S). H NMR (300.14 MHz, DMSO-d6) δ: 8.13 (s, 2H, CH),
Yield: 82 % m.p.: 220 °C; m.w.: 315 g mol-1. Anal. (%)
calcd. for C16H17N3O2S: C, 60.95; H, 5.40; N, 13.33; S, 10.16.
Found: C, 61.12; H, 5.29; N, 13.46; S, 10.37 %. IR (KBr, cm–1):
3319 ν(N-H), 3167 ν(N-H), 2854 ν(O-CH3), 1606 ν(C=N),
1172 ν(N-N), 830 ν(C=S). 1H NMR (300.14 MHz, DMSO-
d6) δ: 9.80 (s, 1H, N-H), 8.30 (s, HC=N), 9.45 (s, 1H, N-H),
6.96-7.22 (m, 3H, Ph-H), 7.35-7.84 (m, 5H, Ph-H), 3.55, 3.81
(s, CH3). 13C NMR (100.62 MHz, DMSO-d6) δ: 177.7 (C=S),
154.8 (C=N), 123.6, 125.7 (C6a), 129.3 (C1a, C3a, C4a), 154.1
(C5a), 139.8 (C2a), 125.3 (C3b,5b), 125.7 (C2b, C6b), 129.2 (C4b),
138.8 (C1b), 57.2, 56.5 (CH3).
Benzil-bisthiosemicarbazone (H2-btz): To a hot ethanolic
solution of thiosemicarbazide (1.82 g, 20 mmol) and benzil
(2.1 g, 10 mmol) few drops of conc. HCl were added. The
mixture was then refluxed at 65 °C for 3 h. The resulting mixture
was then degassed on a rotary evaporator over a water bath,
after which it was allowed to cool to get cream-coloured crystals.
It was then filtered, washed with cold ethanol and then dried
under vacuum. The product was characterized by comparing
the physical and spectroscopic data with the literature report
[19]. It was soluble in soluble in ethanol, DMF and DMSO.
8.73 (s, 2H, NH), 6.63-6.65 (m, 6H, Ph-H), 7.08-7.85 (m, 10H,
Ph-H), 3.56 (s, CH3), 3.81 (s, CH3). 13C NMR (100.62 MHz,
DMSO-d6) δ: 171.5 (C-S), 150.1 (C=N), 129.3, 130.2, 131.9
(C1a,3a,4a), 152.1 (C5a), 140.0 (C2a), 114.4 (C3b,5b), 116.7 (C2b,6b),
122.0 (C4b), 122.7 (C1b), 55.6 (CH3).
Synthesis of [Pd(btz)] (3): To a hot solution of the ligand
H2-btz (0.5 mmol, 0.178 g) in 10 mL ethanol [PdCl2(cod)]
(0.5 mmol, 0.143 g) solution in 10 mL dichloromethane was
added. The mixture was then refluxed at 45 °C in air for 3 h to
obtain a reddish brown coloured precipitate. It was filtered
off, washed, recrystallized in dichloromethane and then dried
over fused CaCl2 in a desiccator.
–1
Yield: 70 %. m.p.: 185 °C; m.w.: 460 g mol-1 Λm: 18 Ω
cm2 mol–1. Anal. (%) calcd. for C16H14N6S2Pd: C, 41.70; H,
3.48; N, 18.75; S, 13.90. Found: C, 42.26; H, 3.85; N, 18.24;
S, 13.52 %. IR (KBr, cm–1): 3130 ν(N-H), 1620 ν(C=N), 1153
1
ν(N-N), 627 ν(C-S), 467 ν(Pd-N), 393 ν(Pd-S). H NMR
(300.14 MHz, CD3CN) δ: 7.62, 7.60 (d, J = 2Hz, 2H, NH),
7.22-7.58 (m, 10H, Ph-H). 13C NMR (100.62 MHz, CD3CN)
δ: 181.2 (C-S), 155.1 (C=N), 129.6 (C4,4′), 130.0 (C2,2′,C6,6′),
130.2 (C3,3′, C5,5′), 131.9 (C1,1′).

Vol. 30, No. 6 (2018)
Synthesis and Characterization of Palladium(II) Complexes with Thiosemicarbazones 1207
at m/z 734 and 590.5 for the complex 2 could be attributed to
[M+1]+ and {M-[(CH3O)2-Ph-CH]+1}+ fragment, respectively.
The mass spectrum of complex 3 showed peaks at m/z 483
and 428 could be assigned to [M+23]+ and [M-S]+, respectively.
The electronic spectra of the complexes showed strong
bands in the range 300-400 nm attributed to intra-ligand and
LMCT transitions. The broad band at 611 nm in the UV-visible
RESULTS AND DISCUSSION
The reaction of [PdCl2(cod)] with H-aatp in DCM gives
reddish brown [PdCl2(H-aatp)2] (1), while that with H-dmbptz
in acetonitrile gives bright yellow [Pd(dmbptz)2] (2) and that
with H2-btz in hot ethanolic solution yields reddish brown
coloured [Pd(btz)] (3) complexes (Scheme-I). All the comp-
lexes were non-electrolyte, thermally stable and soluble in
ethanol, acetonitrile, DCM, DMF and DMSO. All the comp-
lexes were found to be four coordinated having square-planar
or distorted square planar geometry.
The FTIR spectrum of the complex 1 shows a weak absor-
ption at 390 cm–1 corresponding to stretching frequency of
ν(Pd-S). For the complex 2, the weak absorptions at 380 and
449 cm–1 corresponds to ν(Pd-S) and ν(Pd-N), respectively,
while that for the complex 3 at 393 and 467 cm–1 corresponds
to ν(Pd-S) and ν(Pd-N), respectively. These confirm the comp-
lexation of the ligands to the metal centres in all the complexes.
The ESI-mass spectra of all the Pd(II) complexes are in
well agreement with the proposed molecular formulations. The
complex 1 showed characteristic isotopic 35Cl and 37Cl peaks
at m/z 507, 509 and at 419, 421 due to the ions [M-Cl-(CH3O-
Ph-CH)]+ and [M-(H-aatp)]+ respectively, whereas the peaks
1
spectrum of complex 3 could be assigned to 1A1g→ A2g transition
*
[10,20]. A typical strong band attributed to intra-ligand n→π
transition was observed at 366, 319, 391 nm for the complexes
1, 2 and 3, respectively [10,20]. The UV-visible spectrum of
the complex 2 is depicted in Fig. 1.
From 13C and 1H NMR spectra of all the complexes, it has
been observed that the complexes 2 and 3 showed coordination
induced shift for C=S carbon signal (∆δ = 1.3-7.6 ppm),
indicates the coordination of the ligand through thioamide-S.
Thiophenol carbon (C-SH) of complex 1 displayed shift of
∆δ = 4.3 ppm, representing coordination of the ligand through
thiol-SH. The absence of -NH- proton signal in the complexes,
2 and 3 specifies deprotonation of the ligands on coordination
and confirms the coordination through the anionic thiolate S-
atom of the ligand to Pd(II) ion in these complexes. Complex
1 showed one doublet at 2.49-2.53 ppm (∆δ = 1.91 ppm)
O
HS
Cl
N
Pd
Cl
HS
N
O
(1)
3b
SH
2b
4b
1b
DCM, r.t
stir, 3 h
5b
N
6b
2a
2
3a
1a
4a
5a
6a
O
3b
4b
5b
2b
H-aatp
Cl
3a
1b
6b
O
Pd
2a
1a
2
4a
HN
Cl
N
O
N
S
[PdCl2(cod)]
5a
6a
H
H-dmbptz
4
3
4/
3/
2/
h
5
3
S
,
r
i
5/
t
2
s
1
1/
O
6
6/
HN N
NH2
N NH
H2N
O
S
S
NH
N
N
N
S
N
S
Pd
N
N
N
Pd
N
HN
S
H2-btz
H2N
NH2
O
(3)
O
(2)
Scheme-I: Synthetic route of the complexes [PdCl2(H-aatp)2] (1), [Pd(dmbptz)2] (2) and [Pd(btz)] (3) from [PdCl2(cod)]

1208 Sultana et al.
Asian J. Chem.
the ligands get deprotonated confirming complexation through
the thiolate S and azomethine N-atom.
256 nm
319 nm
ACKNOWLEDGEMENTS
The authors gratefully thank STIC, Kochi University, Kochi,
India for elemental analysis; SAIF, NEHU, Shillong, India for
ESI-mass; SAIF, CIL, Punjab University, Chandigarh, India
for 1H and 13C NMR facilities and SAP-DRS-I program (2016-
2021) for financial support.
387 nm
REFERENCES
200
500
800
Wavelength (nm)
1. R.N. Prabhu and R. Ramesh, Tetrahedron Lett., 57, 4893 (2016);
https://doi.org/10.1016/j.tetlet.2016.09.049.
Fig. 1. UV-visible spectrum of complex 2
2. T.S. Lobana, P. Kumari, G. Hundal, R.J. Butcher, A. Castineiras and T.
Akitsu, Inorg. Chim. Acta, 394, 605 (2013);
corresponding to the proton of thiol group (-SH), suggesting
coordination through neutral thiol (-SH) group of the ligand
H-aatp to the Pd2+ ion (Fig. 2). This complex also exhibited
coordination induced shift for all the proton signals. The 1H
NMR spectra of these complexes supported the proposed
molecular formulations of the complexes 1, 2 and 3.
https://doi.org/10.1016/j.ica.2012.09.021.
3. D. Kumar and V.K. Singh, J. Drug Disc. Therap., 2, 24 (2014).
4. P. Chellan, T. Stringer, A. Shokar, P.J. Dornbush, G. Vazquez-Anaya,
K.M. Land, K. Chibale and G.S. Smith, J. Inorg. Biochem., 105, 1562
(2011);
https://doi.org/10.1016/j.jinorgbio.2011.07.023.
5. M.X. Li, C.L. Chen, D. Zhang, J.Y. Niu and B.S. Ji, Eur. J. Med. Chem.,
45, 3169 (2010);
https://doi.org/10.1016/j.ejmech.2010.04.009.
6. S. Datta, M.G.B. Drew and S. Bhattacharya, Indian J. Chem., 50A, 1403
(2011)
7. J. Martínez, E.M. Cabaleiro-Lago, J.M. Ortigueira, M.T. Pereira, P.
Frieiro, F. Lucio and J.M. Vila, Inorg. Chim. Acta, 449, 20 (2016);
https://doi.org/10.1016/j.ica.2016.03.044.
8. A.D. Khalaji, E. Shahsavani, N. Feizi, M. Kuceraková, M. Dusek, R.
Mazandarani and A. Amiri, C.R. Chim., 20, 125 (2017);
https://doi.org/10.1016/j.crci.2016.05.009.
9. J.G. Da Silva, C.C.H. Perdigao, N.L. Speziali and H. Beraldo, J. Coord.
Chem., 66, 385 (2013);
https://doi.org/10.1080/00958972.2012.757762.
10. G. Borah, J. Baruah and D. Kardong, Indian J. Chem., 53A, 1520 (2014).
11. M.A. Salam, M.A. Hussein, I. Ramli and M.S. Islam, J. Organomet.
Chem., 813, 71 (2016);
https://doi.org/10.1016/j.jorganchem.2016.04.007.
12. A. Dobrova, S. Platzer, F. Bacher, M.N.M. Milunovic, A. Dobrov, G.
Spengler, É.A. Enyedy, G. Novitchi and V.B. Arion, Dalton Trans., 45,
13427 (2016);
11
10
9
8
7
6
5
4
3
2
1
0
ppm
Fig. 2. 1H NMR spectrum of complex 1
https://doi.org/10.1039/C6DT02784A.
13. N. Nanjundan, R. Narayanasamy, R.J. Butcher, J.P. Jasinski, K.
Velmurugan, R. Nandhakumar, M.D. Balakumaran, P.T. Kalaichelvan
and V.G. Gnanasoundari, Inorg. Chim. Acta, 455, 283 (2017);
https://doi.org/10.1016/j.ica.2016.10.035.
14. A. Molter and F. Mohr, Polyhedron, 120, 118 (2016);
https://doi.org/10.1016/j.poly.2016.07.016.
Conclusion
In summary, this work presents the simple method of synthe-
sizing N, S-donor ligands from readily available and less
expensive materials by a simple one-pot condensation reaction.
The ligands are found to be air and moisture stable, non-toxic
and can easily coordinate to the metal ions. On coordination
with metal ions the thiosemicarbazone derived ligands, viz.,
H-dmbptz and H2-btz can bond through the azomethine N and
thiolate/thione S atom and remain in both the neutral and ionic
form. The thiophenol derived ligand H-aatp was found to coor-
dinate to Pd(II) in the neutral form. Though two coordination
sites (N and S) were present in the ligand yet chelation was
not observed due to the rigid backbone structure and so it pre-
ferably bonded to the metal ions via the thiol (SH) sulphur
atom. These complexes [PdCl2(H-aatp)2] (1), [Pd(dmbptz)2]
(2) and[Pd(btz)] (3) were found to be four coordinated and
diamagnetic in nature. On coordination to the Pd(II) centre
15. R. Deb, P. Paul and S. Bhattacharya, Proc. Natl. Acad. Sci. India Sect.
A Phys. Sci., 86, 551 (2016);
https://doi.org/10.1007/s40010-016-0303-z.
16. E.Z. Jahromi,A. Divsalar,A.A. Saboury, S. Khaleghizadeh, H. Mansouri-
Torshizi and I. Kostova, J. Iran. Chem. Soc., 13, 967 (2016);
https://doi.org/10.1007/s13738-015-0804-8.
17. R. Manikandan, P. Anitha, P. Viswanathamurthi and J.G. Malecki,
Polyhedron, 119, 300 (2016);
https://doi.org/10.1016/j.poly.2016.09.005.
18. D. Drew and J.R. Doyle, Inorg. Synth., 13, 47 (1972).
19. S. Chandra, S. Raizada, M. Tyagi andA. Gautam, Bioinorg. Chem. Appl.,
Article ID 51483 (2007);
https://doi.org/10.1155/2007/51483.
20. D. Sutton, Electronic Spectra of Transition Metal Complexes: An
Introductory Text, McGraw-Hill Publishing Company Limited, p. 192
(1968).