Square planar Pd(II) complexes derived from 1-ethyl-3-phenylthiourea, 3-mercapto-4-methyl-1,2,4-triazole and 2-mercapto-5-methyl-1,3,4-thiadiazole: Syntheses, spectral, structural characterization and photoluminescence properties

Article abstract of DOI:10.1016/j.ica.2016.01.003

The reaction of PdCl₂ with 1-ethyl-3-phenyl-thiourea (Heptu), 3-mercapto-4-methyl-4H-1,2,4-triazole (Hmmtrz) and 2-mercapto-5-methyl-1,3,4-thiadiazole (Hmthd) respectively, yielded three new complexes [Pd(eptu)₂] (1), [Pd(Hmmtrz)

Full text of DOI:10.1016/j.ica.2016.01.003

Inorganica Chimica Acta 443 (2016) 160–169
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Square planar Pd(II) complexes derived from 1-ethyl-3-phenylthiourea,
3
4
-mercapto-4-methyl-1,2,4-triazole and 2-mercapto-5-methyl-1,3,
-thiadiazole: Syntheses, spectral, structural characterization and
photoluminescence properties
a
b
a
a
a
a,
⇑
P. Bharati , A. Bharti , P. Nath , S. Kumari , N.K. Singh , M.K. Bharty
a
Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
Department of Chemistry, Kirori Mal College, University of Delhi, Delhi 110007, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of PdCl2 with 1-ethyl-3-phenyl-thiourea (Heptu), 3-mercapto-4-methyl-4H-1,2,4-triazole
Hmmtrz) and 2-mercapto-5-methyl-1,3,4-thiadiazole (Hmthd) respectively, yielded three new com-
plexes [Pd(eptu) ] (1), [Pd(Hmmtrz) ]Cl (2) and [Pd(Hmthd) ]Cl (3). These complexes have been
ꢀ2CHCl
Received 28 October 2015
Received in revised form 31 December 2015
Accepted 2 January 2016
(
2
4
2
4
2
3
isolated in pure form and characterized by elemental analyses, IR, NMR and single crystal X-ray diffrac-
tion technique. In all compounds the metal ion adopts a square-planar geometry. Two nitrogen and two
sulfur atoms from the monoanionic eptu ligand in complex 1 are bonded to Pd(II) centre in trans fashion.
Complexes 2 and 3 are salt-like and the metal centre is bonded through four neutral ligands. The electri-
cal neutrality in these complexes is maintained by the presence of two chlorides as counter ions.
Migration of hydrogen, within the ligand framework in complexes 2 and 3, from sulfur to nitrogen is
observed that resulted in the thione form of the ligand and its coordination to the metal centre in both
the complexes. Complex 2 is stabilized by intermolecular CAHꢀ ꢀ ꢀN and intramolecular NAHꢀ ꢀ ꢀCl
hydrogen bonding leading to an extended structure. Complex 3 is stabilized by intermolecular CAHꢀ ꢀ ꢀS
hydrogen bonding. Complexes 1–3 are fluorescent materials which upon excitation at 31000, 38600
Available online 7 January 2016
Keywords:
Thiadiazole and triazole complexes
Pd(II) complexes
Square planar complexes
Photoluminescence properties
ꢁ1
ꢁ1
and 32300 cm exhibit an emission at 25200, 27000 and 26700 cm , respectively.
Ó 2016 Elsevier B.V. All rights reserved.
1
. Introduction
bonded trinuclear centres (VI and VII) (Scheme 1) [7–10]. Unsym-
metrically substituted thiourea RNH(C@S)NHR , because of the
0
The coordination behavior of thiourea and its derivatives have
presence of one NH group on each side of (C@S) moiety, may exhi-
bit two tautomeric forms viz. RN = C(SH)NHR and RNHC(SH) = NR
[11,1]. Thus, they may act as hard and soft donor providing a
multitude of bonding possibilities.
0
0
been reported during last few decades [1–4]. Substituted thioureas
exhibit efficient capacity for coordination with transition metals
producing interesting colored compounds. Thioureas act as analyt-
ical reagents, for the determination of metals in the presence of
Mercapto-triazoles are important ligands with nitrogen-sulfur
donor centres which produce supramolecular network with novel
topologies [12–16]. Two types of mercapto-triazoles are found in
the literature, 1,2,3 and 1,2,4-triazoles exhibit different bonding
behaviour. They act as bidentate-bridging or monodentate ligands
depending on the position and the nature of the substituent at the
triazole ring [17]. The N1 and N2 positions of the triazole ring if not
substituted may coordinate to the metal ion forming N1, N2 bridg-
ing polynuclear complexes [18–20]. The mercapto group (thiol
form) can also function as a donor site exhibiting uninegative mon-
odentate or neutral bridging modes between two metal centres or
as a bridge between three metal centres in the azolato or thiolato
form. Studies on thiadiazole and its derivatives, which belong to an
important group of nitrogen sulfur donor ligands, are the major
0
other metal ions [5,6]. N,N -Substituted thioureas may show differ-
ent bonding possibilities (Scheme 1). They may act as monoanionic
monodentate ligand binding through thiolato sulfur atom (I), as
neutral monodentate bonded through thione sulfur atom (II), dian-
ionic N,S bidentate ligand binding through sulfur and nitrogen
atoms (III) or as dianionic N,S bidentate ligand in dinuclear com-
plexes involving sulfur bridging (IV). They can also act as neutral
bridging ligand through the sulfur atom (V). Other bonding possi-
bilities of substituted thioureas may be mononegative tridentate
form that utilize thiolato form to form bridges between MAM
⇑
Corresponding author. Tel.: +91 5426702447; fax: +91 0542 2368127.
E-mail address: mkbharty@bhu.ac.in (M.K. Bharty).
http://dx.doi.org/10.1016/j.ica.2016.01.003
020-1693/Ó 2016 Elsevier B.V. All rights reserved.
0

P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
161
M
R
N
S
M
S
S
R
L
N
C
C
R
S
M
C
N
HN
N
HN
N
H
M
N
M
R
L
S
N
N
(I)
R
(
II)
(III)
(
IV)
M
M
M
M
R
M
S
S
R
NH
M
C
M
S
C
NH
M
HN
N
HN
N
R = -CH CH3
2
R
(
V)
(VI)
(VII)
Scheme 1. Bonding possibilities of 1-ethyl-3-phenylthiourea (Heptu) in its neutral, monoanionic and dianionic forms involving thiol–thione tautomers.
area of research because of their diverse biological activities, such
as anti-tubercular, anti-inflammatory, analgesic, antipyretic,
anticonvulsant, antibacterial and antifungal [21–26]. Triazole and
thiadiazole ligands can exist in tautomeric equilibrium in solution,
which enables two possible configurations of the ANHCS moiety as
thione and thiol forms (Scheme 2). On complexation with ring
nitrogen, only the thione form of the ligands exists in the solid-
state. This tautomerism may cause the proton transfer or migration
on the triazole/thiadiazole ring, and consequently the ligand binds
in various modes with different metal ions (Scheme 3) [27,28]. Pal-
ladium complexes show high efficiency as homogenous catalysts
in a variety of coupling reactions in organic synthesis [29–33]. Pd
purchased from Sigma Aldrich. The ligand, 1-ethyl-3-phenyl-
thiourea (Heptu) was prepared by the literature method [37].
Other chemicals were of reagent grade and used as purchased
without further purification. All the synthetic manipulations were
carried out in open atmosphere at room temperature. The solvents
were distilled before use following the standard procedure.
2.2. Physical measurements
Carbon, hydrogen, nitrogen and sulfur contents were measured
on a CHN Model CE-440 Analyzer and on an Elementar Vario EL III
Carlo Erba 1108 instrument. Infrared spectra (IR) were recorded in
(
II) complex of a triazole-based ligand has been reported as a
ꢁ1
the 4000–400 cm region as KBr pellets on a Varian Excalibur
catalyst for the Heck reaction [34]. Pd(II) complexes with 1,3-bis
1
13
3
100 FT-IR spectrophotometer. H and C NMR spectra were
0
(
2 -imidazolinyl)benzene have high catalytic activity in the Frie-
recorded in DMSO-d and CDCl on a JEOL AL 300 FT NMR spec-
6
3
del–Crafts alkylation reaction [35]. Pd(II) complex of thioamide
ligand, 2,6-bis(butylaminothio-carbonyl)pyridine is considered to
be efficient catalysts for Negishi coupling reaction [36]. In view
of the above interesting coordination behaviour of the nitrogen-
sulfur ligands and catalytic nature of palladium(II) complexes, we
have synthesized and fully characterized three new palladium(II)
trometer using TMS as an internal reference. The emission and
excitation spectra were recorded at ambient temperature with a
Varian Cary Eclipse spectrophotometer in CHCl
3
solution.
2
.3. Synthesis of [Pd(eptu) ] (1)
2
complexes, [Pd(eptu)
Hmthd) ]Cl (3) from 1-ethyl-3-phenylthiourea (Heptu),
ꢀ2CHCl
-mercapto-4-methyl-4H-1,2,4-triazole (Hmmtrz) and 2-mer-
capto-5-methyl-1,3,4-thiadiazole (Hmthd), respectively.
2
]
(1), [Pd(Hmmtrz)
4
]Cl
2
(2) and [Pd
PdCl
2
(0.177 g, 1 mmol) was added to a solution of Heptu
(
3
4
2
3
(
0.360 g, 2 mmol) in 10 mL methanol–chloroform mixture and stir-
red for 12 h at room temperature. A clear red solution was
obtained which was filtered off and kept for crystallization.
Block-shaped crystals suitable for X-ray analysis were obtained.
2
. Experimental
Yield: 70%; m.p. 199 °C. Anal. Calc. for C18H22N PdS2 (464.94): C,
4
4
6.45; H, 4.73; N, 24.08, S, 13.76; Found: C, 46.49; H, 4.69; N,
ꢁ1
2.1. Chemicals and starting materials
24.06; S, 13.74%. IR (
m
cm , KBr):
(PdAS) 541; H NMR (CDCl
ppm): 11.06 (s, 2H, NH), 7.64–6.73 (m, 10H, aromatic protons),
m
(NH) 3121;
m
(C@N) 1556;
1
m(C@S) 901;
m
(NAN) 1099;
m
3
; d
The ligands, 3-mercapto-4-methyl-4H-1,2,4-triazole (Hmmtrz)
1
3
and
2-mercapto-5-methyl-1,3,4-thiadiazole
(Hmthd)
were
2 3 3
4.49 (q, 4H, CH ), 1.30 (t, 6H, CH ). C NMR (CDCl ; d ppm):
1
1
79.55 (C@S), 141.70–123.15 (aromatic carbons), 63.85 (CH
2
),
ꢁ1
ꢁ1
4.21 (CH ). UV–Vis. [CHCl , kmax, nm; emax, M cm ]: 245
3 3
N
N
N
NH
5
4
4
(
1.14 ꢂ 10 ), 278 (7.0 ꢂ 10 ), 323 (3.8 ꢂ 10 ).
2.4. Synthesis of [Pd(Hmmtrz) ]Cl (2)
2
A mixture of chloroform suspension (10 mL) of PdCl (0.177 g,
R
SH
R
S
X
X
4
2
Thiol form
Thione form
X= N-CH ; R=H (Hmmtrz)
3
X = S; R= CH3 (Hmthd)
1 mmol) and methanol solution of Hmmtrz (0.461 g, 4 mmol)
was stirred for 6 h at room temperature. An orange-red solution
was obtained, which was filtered off and kept for crystallization.
Scheme 2. Thiol and thione forms of Hmmtrz and Hmthd ligands.

1
62
P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
M
M
M
M
M
N
NH
N
NH
M
N
N
N
N
R
S
M
R
S
M
X
S
S
X
R
M
R
X
X
M
X= N-CH ; R=H (Hmmtrz)
3
X = S; R= CH3 (Hmthd)
Scheme 3. Possible bonding modes of Hmmtrz and Hmthd ligands.
Upon slow evaporation of the solvents, orange-red crystals suitable
for X-ray analysis were obtained after 20 days. Yield: 50%; m.p.
and 3 are salt-like, contain two chlorides as counter ions and the
ligands coordinate in the neutral form. Similar complexes have
been reported in the literature [41,42]. Schemes 4–6 depict
formation of the complexes which also show that the thiourea
ligand acts as monoanionic N,S bidentate chelating ligand
while 3-mercapto-4-methyl-4H-1,2,4-triazole and 2-mercapto-5-
methyl-1,3,4-thiadiazole coordinate in the thione form after the
migration of proton from S to N during complex formation.
Complexes 1, 2 and 3 are soluble in chloroform and melt at 199,
145 and 183 °C, respectively.
1
3
2
m
(
45 °C. Anal. Calc. for C12
2 4
H20Cl N12PdS (637.94): C, 22.57; H,
.13; N, 26.33; S, 20.06; Found: C, 22.60; H, 3.10; N, 26.31; S,
ꢁ
1
0.02%. IR (
(C@N) 1556;
CDCl ; d ppm): 13.44 (s, 4H, NH), 7.91 (s, 4H, CH, triazole ring
proton), 3.58 (s, 12H, CH
m
cm , KBr):
m
(NAH) 3121,
m(CAH) 2944–3019;
1
m
(NAN) 1079 s;
m
(CAS) 782; m(PdAS) 541; H NMR
3
1
3
3
). C NMR (CDCl
3
; d ppm): 166.75
3
carbon). UV–Vis.
(
[
NCS), 140.93 (aromatic carbons), 31.53 (CH
ꢁ1
ꢁ1
4
CHCl
.5. Synthesis of [Pd(Hmthd)
To a methanol solution (15 mL) of Hmthd (0.468 g, 4 mmol) was
3
, kmax, nm;
e
max, M cm ]: 259 (9.0 ꢂ 10 ).
2
4
]Cl (3)
2
ꢀ2CHCl
3
3.1. Magnetic moments and electronic spectra
The complexes [Pd(eptu)
(Hmthd) ]Cl (3) are diamagnetic indicating the presence
ꢀ2CHCl
of low spin Pd(II) centres. They show absorptions in the region of
2 4 2
] (1), [Pd(Hmmtrz) ]Cl (2) and [Pd
added methanol–chloroform suspension (10 mL) of PdCl
a
2
4
2
3
(
0.177 g, 1 mmol) and stirred for 10 h at room temperature which
ꢁ1
yielded an orange-red solution. This was filtered off and kept for
crystallization. Upon slow evaporation of the solvents at room
temperature, rod-shaped crystals were obtained after 30 days.
31000–33300 cm due to intraligand/charge transfer transitions
8
characteristics of low spin d Pd(II) complexes with square planar
geometry [43]. The complex 1 shows three bands at 40800, 36000
ꢁ1
Yield: 60%; m.p. 183 °C. Anal. Calc. for C14
H18Cl
8
N
8
PdS
8
(944.84):
and 31000 cm while complexes 2 and 3 show only one band at
ꢁ1
C, 17.78; H, 1.90; N, 11.85; S, 27.09; Found: C, 17.82; H, 1.49; N,
38600 and 32300 cm , respectively. The three bands observed
ꢁ
1
⁄
⁄
1
(
(
1
(
[
1.92; S, 27.19%. IR (
C@N) 1554; (CAS) 766;
DMSO-d ; d ppm): 13.96 (s, 4H, NH); 7.59 (s, 2H, CHCl
2H, CH
CAN), 77.46 (carbon of CHCl
m
cm , KBr):
m
(NAH) 3052,
(PdAS) 432; H NMR
), 3.37 (s,
; d ppm): 188.90 (CAS), 158.47
), 15.89 (CH carbon). UV–Vis.
m
(CAH) 2973;
m
in complex 1 are assigned to p ? p (C@S) and n ? p transitions
1
m
m
(NAN) 1066s;
m
while the bands in complexes 2 and 3 are attributed to the
⁄
p
?
p
transition of C@S group [44].
6
3
1
3
3 6
). C NMR (DMSO-d
3.2. IR spectra
3
ꢁ1
3
ꢁ1
4
CHCl
.6. X-ray crystallography
Structural measurements of complexes 1, 2 and 3 were per-
3
, kmax, nm;
e
max, M cm ]: 310 (8.7 ꢂ 10 ).
The IR spectrum of the ligand 1-ethyl-3-phenyl-thiourea
ꢁ
1
(
Heptu) shows bands at 3215 and 3117 cm , due to
m
(NH), and
2
ꢁ1
ꢁ1
a band at 956 cm for
due to
of an NH group adjacent to the ethyl group which does not take
part in bonding. However, the disappearance of the (NH) band
m(C@S). Presence of a band at 3121 cm
m
(NH) in the spectrum of complex 1 indicates the presence
formed on a computer-controlled Oxford Gemini diffractometer
equipped with a CrysAlis CCD software using a graphite monochro-
mated Mo Ka (k = 0.71073 Å) radiation source at 293 K. Multi-scan
m
ꢁ1
around 3215 cm suggests loss of the thioamide proton adjacent
to the phenyl group and participation of the deprotonated
absorption correction was applied to the X-ray data collection for
all the compounds. The structures were solved by direct methods
(
SHELXS-08) and refined against all data by full matrix least-square
H
N
H
N
2
on F using anisotropic displacement parameters for all non-hydro-
gen atoms. All hydrogen atoms were included in the refinement at
geometrically ideal position and refined with a riding model [38].
The MERCURY package and ORTEP-3 for Windows program were used
for generating molecular graphics [39,40].
H2
C
EtOH
RT
CH3
+
PhNCS
C
H
H2N
CH3
2
S
PdCl2
MeOH-CHCl3
3
. Results and discussion
H3C
HN
CH2
The ligand, 1-ethyl-3-phenyl-thiourea (Heptu) reacts with pal-
S
N
ladium(II) chloride to form orange-red complex, [Pd(eptu)
in methanol-chloroform solution. The ligand 3-mercapto-4-
methyl-4H-1,2,4-triazole reacts with PdCl to yield orange-red
precipitate of [Pd(Hmmtrz) ]Cl (2) which was dissolved upon
addition of chloroform. Similarly, 2-mercapto-5-methyl-1,3,
-thiadiazole affords mononuclear complex, [Pd(Hmthd)
Cl (3) with a distorted square planar geometry at Pd(II)
ꢀ2CHCl
centre utilizing four neutral monodentate ligands. Complexes 2
2
] (1)
Pd
NH
H2C
N
2
S
4
2
CH3
4
a
4
]
1
2
3
2
Scheme 4. Synthesis of [Pd(eptu) ] (1) complex.

P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
163
N
N
HN
CH3
N
N
N
NH
PdCl2
N
S
HN
SH
N
MeOH-CHCl3
N
S
Pd
S
N
CH3
H C
3
Cl2
S
Hmmtrz
CH3
NH
N
N
H C
3
2
Scheme 5. Synthesis of [Pd(Hmmtrz)
4
]Cl
2
(2) complex.
H C
3
N
NH
S
N
N
S
N
NH
S
PdCl2
HN
N
H C
SH
3
MeOH-CHCl3
H3C
S
Pd
S
S
Hmthd
CH3
Cl .2CHCl3
2
S
S
S
HN
CH3
N
3
Scheme 6. Synthesis of [Pd(Hmthd)
4
]Cl
2
ꢀ2CHCl
3
(3) complex.
thioamide nitrogen in bonding with the metal ion. The band due to
13.44 ppm due to NAH proton indicates that the ligand is bonded
in the thione form in complex 2. The C NMR spectrum of [Pd
ꢁ1
13
m
(C@S) suffers a negative shift of 55 cm suggesting bonding of
the ligand via the thioamide sulfur to the metal ion [37]. The ligand
adopts thione form in the complex which is supported by the
(Hmmtrz)
4
]Cl
2
(2) shows signals at
d
166.75, 140.93 and
carbons,
respectively. The H NMR spectrum of the ligand 2-mercapto-5-
methyl-1,3,4-thiadiazole in DMSO-d shows signals at 12.05, 3.32
31.53 ppm due to C@S, C@N (triazole ring) and CH
3
ꢁ
1
1
presence of
m
(NAH) at 3121 cm . The thione form of the ligand
(C@S) and (PdAS) at 852 and
respectively. The ligand 2-mercapto-5-methyl-1,3,
is supported by the occurrence of
m
m
6
ꢁ1
5
41 cm
-thiadiazole shows bands at 2868 and 1554 cm due to
(C@N), respectively. The IR spectrum of complex 3 shows
absence of the (SH) band and appearance of (NH) at
052 cm , indicating bonding through the thione sulfur after tau-
,
and 2.49 ppm due to the thiadiazole NH, SH and methyl protons,
respectively, indicating existence of the ligand in the thiol/thione
tautomeric forms. The H NMR spectrum of [Pd(Hmthd)
ꢁ1
4
m
(SH)
1
and
m
4
]Cl
2
m
m
ꢀ2CHCl
3
(3) exhibits signals at d 7.59 and 3.37 ppm for the CHCl
3
ꢁ
1
3
and CH
3
protons respectively, and a peak at d 13.96 ppm for NH
tomerization during the complex formation. The coordinated
ligand adopts the thione form as supported by the occurrence of
indicates the presence of thione form of the ligand in the complex.
Appearance of a signal for NH in both the complexes suggests
bonding of the neutral soft sulfur atom with the soft Pd(II) in com-
ꢁ1
stretching vibrations of C@S and PdAS at 766 and 432 cm
,
1
3
respectively.
plexes 2 and 3. The C NMR spectrum of [Pd(Hmthd)
4
]Cl
2
ꢀ2CHCl
3
(3) shows signals at d 188.90 and 158.47 ppm due to C@S and CAN
1
13
carbons, respectively.
3
.3. H and C NMR spectra
1
The H NMR spectrum of free Heptu ligand exhibits signals at d
3.4. Photoluminescence studies
1
1.06 and 4.49 ppm for the NH protons attached to the ethyl group
and phenyl ring, respectively. The phenyl ring protons appear as a
multiplet in the region d 7.11–7.64 ppm. The ACH and ACH pro-
tons of the ethyl group appear at d 2.49 and 1.32 ppm, respectively.
In the present work, we have examined the photoluminescence
properties of complexes 1, 2 and 3 at room temperature and have
tried to correlate them with the emission patterns of their respec-
2
3
1
The H NMR spectrum of the complex [Pd(eptu)
2
] (1) shows the
tive free ligands. Complex [Pd(eptu)
2
] (1) shows photolumines-
ꢁ1
ꢁ1
absence of a signal for the ANH proton attached to the phenyl ring,
while the ANH proton attached to ethyl group appears at d
cence at 25200 cm
upon excitation at 31000 cm . The free
ligand, 3-mercapto-4-methyl-4H-1,2,4-triazole (Hmmtrz) displays
ꢁ
1
ꢁ1
1
1.06 ppm, indicating that the metal is bonded to the ligand via
photoluminescence at 18650 cm upon excitation at 29700 cm
⁄
the nitrogen atom attached to the phenyl ring after loss of a proton.
which has been ascribed as originating from intraligand
transition [45]. Upon excitation of complex 2 at 38600 cm , the
corresponding emission was observed at 27000 cm
and 1b) with a blue shift of 8350 cm as compared to the free
p–p
1
3
ꢁ1
The C NMR spectrum of [Pd(eptu)
2
] (1) shows signals at 179.55,
ꢁ1
6
3.85 and 14.21 ppm due to C@S, CH
2
and CH
3
carbons, respec-
]Cl (2) in CDCl
(Figs. 1a
1
ꢁ1
tively. The H NMR spectrum of [Pd(Hmmtrz)
4
2
3
exhibits signals at d 3.58 and 7.91 ppm due to methyl and methine
protons, respectively. In addition, the presence of a signal at d
ligand (Hmmtrz). Complexes [Cd(H
(H EDTA)] [27] exhibit blue shifts of 1300 and 600 cm
2
2 2 2 2
trzS) Cl ] and [Cd(H trzS)
ꢁ
1
,

1
64
P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
Fig. 1a. UV–Vis spectra of [Pd(eptu)
Cl (3).
ꢀ2CHCl
2
] (1), [Pd(Hmmtrz)
4
]Cl
2
(2) and [Pd(Hmthd)
4
]
Fig. 1c. Normalized excitation spectra of complexes 1, 2 and 3 at excitation
wavelengths of 397, 370 and 375 nm, respectively.
2
3
3.5. Crystal structure description
The solid state structures of complexes 1–3 were determined by
single crystal X-ray diffraction data. The details of data collection,
structure solution and refinement are listed in Table 1. ORTEP dia-
gram of complexes 1–3 with atom numbering schemes are shown
in Figs. 2, 4 and 7, respectively. Selected bond lengths and angles
are included in Tables 2–4. Hydrogen bonding parameters of
complexes 2 and 3 are given in Tables 5 and 6, respectively.
3
.5.1. Crystal structure description of [Pd(eptu)
Fig. 2 shows ORTEP diagram of [Pd(eptu)
2
](1)
] (1) with atom
2
numbering scheme. Palladium(II) centre in 1 forms two four mem-
bered PdNCS chelate rings using two monoanionic thiourea ligand
utilizing thioamide sulfur and nitrogen atoms. In general, the
ligand is expected to lose a proton from the thioamide nitrogen
adjacent to the phenyl ring due to the resonance effect, and thus
acts as
a uninegative bidentate ligand. The carbon–sulfur
(
1.748 Å) and carbon–nitrogen (1.276 Å) distances within the che-
late rings are comparable to the bond lengths in Heptu but longer
than C@S and C@N double bonds, which suggest delocalization of
the negative charge within the chelate ring and suggests coordina-
tion with sulfur and nitrogen atoms of the thiourea ligand [47]. The
chelate and phenyl rings are tilted towards each other, making a
dihedral angle of 34.30°. The PdAS bond distances are found to
be 2.337(3) and 2.345(3) Å for the two chelate rings, which are
comparable to that reported for palladium thiourea complexes
Fig. 1b. Emission spectra of [Pd(eptu)
2 4 2
] (1), [Pd(Hmmtrz) ]Cl (2) and [Pd
(
Hmthd) ]Cl (3) at the excitation wavelengths of 323, 259 and 310 nm
4
2
ꢀ2CHCl
3
for complexes 1, 2 and 3, respectively.
respectively as compared to the respective free ligand, which has
been ascribed to the tautomerization of the triazole ring from thiol
(
in the free ligand) to the thione form in the above complexes
[
Pd(tu)
Cl (H O)(CH
for the two chelate rings are 2.001(9) and 2.053(10) Å which are
close to those reported for the palladium complex [PdL
ClO O (2.029 Å) [44]. The bond angles for N(3)APd(1)A
ꢀ2C
S(1) (109.8(3)°), N(1)APd(1)AS(1) (70.1(2)°), N(3)APd(1)AS(2)
69.8(3)°) and N(1)APd(1)AS(2) (110.3(3)°) suggest a distorted
4
]I
2
(2.338 and 2.336 Å) [48] and trans-[Pd(PPh
3 2 2
) (Dmtu) ]
which may increase the HOMO–LUMO energy gap. Similar phe-
nomenon is also occurring in complex 2, because single crystal
X-ray diffraction data of complex 2 supports thiol-thione tau-
tomerization during complex formation. The free ligand, 2-mer-
capto-5-methyl-1,3,4-thiadiazole (Hmthd) is found to be an
inferior fluorescent material in itself but it works as ‘‘turn-on” sen-
sor upon complexation with Zn(II) [46]. Free Hmthd displays an
2
2
3
OH)0.5 (2.323 and 2.336 Å) [49]. The PdAN distances
2
]
(
4
)
2
3 6
H
(
ꢁ1
ꢁ1
square planar geometry around Pd(II) centre [50]. Molecular pack-
ing diagram of complex 1 shows that four complex molecules are
present in one unit cell (Fig. 3).
emission at 22900 cm upon excitation at 32600 cm (Suppl.
Figs. 1a and 1b). Photoluminescence studies indicate that complex
[
Pd(Hmthd)
4
]Cl
2
ꢀ2CHCl
3
(3) is a fluorescent material with a maxi-
ꢁ1
ꢁ1
mum emission at 26700 cm with a blue shift of 3800 cm as
compared to the free ligand (Hmthd). The excitation spectra of
complexes 1, 2 and 3 show maxima at 31800, 36000 and
3.5.2. Crystal structure description of [Pd(Hmmtrz) ]Cl (2)
4
2
Fig. 4 shows ORTEP diagram of [Pd(Hmmtrz) ]Cl (2) with the
4
2
ꢁ1
3
2600 cm which are close to those observed in the absorption
atom numbering scheme. The ligand acts as monoanionic
spectra (Fig. 1c).
monodentate in complexes [Ag(mmtrz) (PPh ) ]ꢀHmmtrz and
2
3 2

P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
165
Table 1
Crystallographic data for complexes 1, 2 and 3.
Parameters
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
T (K)
C
18
H
22
N
4
PdS
2
C
12
H
20Cl
2
N
12PdS
4
C
14 8 8 8
H18Cl N PdS
464.94
637.94
triclinic
P 1ꢀ )
293(2)
944.84
monoclinic
P 21/n
monoclinic
P 1 21/n 1
293(2)
293(2)
k, Mo K
a (Å)
b (Å)
a
(Å)
0.71073
10.3338(14)
7.4314(11)
24.891(2)
90.00
89.95(10)
90.00
1911.5(4)
4
0.71073
0.71073
8.750(5)
11.648(5)
17.649(5)
90.00(5)
104.05(5)
90.00(5)
1745.0(13)
2
8.2109(19)
8.619(3)
9.915(2)
103.26(2)
110.43(2)
107.64(2)
580.8(3)
1
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z
3
q
l
calcd (g/cm )
2.082
2.250
1.824
1.417
1.798
1.647
(mm 1
ꢁ
)
F(000)
Crystal size (mm )
1172
320
936
3
0.30 ꢂ 0.28 ꢂ 0.24
0.35 ꢂ 0.25 ꢂ 0.14
0.30 ꢂ 0.27 ꢂ 0.23
h range for data collections (°)
Index ranges
No. of reflections collected
No. of independent reflections (Rint
No. of data/restrains/parameters
3.19–29.27
3.64–28.89
2.97–29.15
ꢁ8 6 h 6 13, ꢁ10 6 k 6 8, ꢁ31 6 l 6 34 ꢁ10 6 h 6 10, ꢁ9 6 k 6 11, ꢁ13 6 l 6 7 ꢁ11 6 h 6 11, ꢁ9 6 k 6 15, ꢁ21 6 l 6 24
6197
3606
3606/0/226
1.134
0.0800, 0.2097
0.1305, 0.2378
4124
2544
7501
4688
)
2544/0/152
0.947
0.0424, 0.0791
0.0698, 0.0894
0.823, ꢁ0.785
4688/0/173
1.113
0.0608, 0.1462
0.0872, 0.1653
0.897, ꢁ0.811
2
Goodness-of-fit (GOF) on F
a
a
b
b
R
R
1
, wR
, wR
2
[(I > 2
r
(I)]
(all data)
1
2
Largest difference in peak/hole (e Åꢁ3) 1.049, ꢁ0.983
a
R
R
1
=
= [
R
R
||F
w (|F
o
| ꢁ |F
c o
||R|F |.
b
2
2
2
2|2]1/2.
2
o
| ꢁ |F
|) /Rw|F
c o
Fig. 2. ORTEP diagram of [Pd(eptu)
2
] (1) with atomic numbering scheme at 30% probability level.
Table 3
4 2
Interatomic distances (Å) and angles (°) for [Pd(Hmmtrz) ]Cl (2).
Table 2
Interatomic distances (Å) and angles (°) for complex [Pd(eptu)
2
] (1).
Bond length(Å)
Bond angles (°)
Bond length (Å)
Bond angles (°)
Pd(1)AS(1A)
Pd(1)AS(1A)#1
Pd(1)AS(1B)
Pd(1)AS(1B)#1
S(1A)AC(1A)
S(1B)AC(1B)
N(1B)AN(2B)
2.330(13)
2.330(13)
2.341(11)
2.341(11)
1.710(4)
1.696(4)
1.356(5)
S(1A)APd(1)AS(1A)
S(1A)APd(1)AS(1B)
S(1A)APd(1)AS(1B)
S(1A)APd(1)AS(1B) #1
S(1A)APd(1)AS(1B) #1
C(1A)AS(1A)APd(1)
C(1B)AS(1B)APd(1)
180.0
93.43(4)
86.57(4)
86.57(4)
93.43(4)
107.51(14)
104.31(12)
Pd(1)AN(3)
Pd(1)AN(1)
Pd(1)AS(1)
Pd(1)AS(2)
S(1)AC(7)
2.001(9)
2.053(10)
2.337(3)
2.345(3)
1.727(10)
1.749(11)
N(3)APd(1)AN(1)
N(3)APd(1)AS(1)
N(1)APd(1)AS(1)
N(3)APd(1)AS(2)
N(1)APd(1)AS(2)
S(1)APd(1)AS(2)
178.9(4)
109.8(3)
70.1(2)
69.8(3)
110.3(3)
177.81(14)
S(2)AC(16)

1
66
P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
Table 4
Interatomic distances (Å) and angles (°) for [Pd(Hmthd)
4
]Cl)
2
ꢀ2CHCl
3
(3).
Bond length (Å)
Bond angles (°)
Pd(1)AS(2)
Pd(1)AS(3)
S(2)AC(3)
S(4)AC(6)
S(4)AC(5)
S(1)AC(3)
S(1)AC(2)
2.326(16)
2.331(2)
1.699(6)
1.721(6)
1.747(7)
1.728(6)
1.743(6)
S(2)APd(1)AS(2)
S(2)APd(1)AS(3)
S(2)APd(1)AS(3)
S(2)APd(1)AS(3)
S(2)APd(1)AS(3)
S(3)APd(1)AS(3)
C(3)AS(2)APd(1)
180.0(1)
88.38(6)
91.62(6)
91.62(6)
88.38(6)
180.0
107.3(2)
Symmetry transformations used to generate equivalent atoms: #1 ꢁx + 1, ꢁy + 1,
z + 1.
ꢁ
[
Hg(mmtrz)
well as neutral bidentate in complex [Hg
connects two Hg(II) centres via its bridging behaviour [51]. In the
crystallographic asymmetric unit of the complex [Pd(Hmmtrz)
Cl , the metal centre is coordinated with four neutral ligands using
thione sulfur and two chlorides as counter ions. In complex 2, Pd
II) ion resides in a distorted square planar geometry with bond
angles for S(1A)APd(1)AS(1B) (93.43(4)°) and S(1A)APd(1)AS(1B)
1 (86.57(4)°). The Pd(II) centre in 2 is bonded to sulfur atoms of
2
]
n
while it behaves as a monoanionic monodentate as
2
(mmtrz) (Hmmtrz) ] and
4
2
4
]
2
(
#
4
Fig. 4. ORTEP diagram of [Pd(Hmmtrz) 2 with atomic
]2+ cation of complex
numbering scheme at 30% probability level. Chlorine atoms are omitted for clarity.
four triazole molecules at the Pd(1)AS(1A) and Pd(1)AS(1B) dis-
tances of 2.330(13) and 2.341(11) Å respectively, that are compa-
rable to similar Pd(II) complexes reported in the literature [52].
The exocyclic S(1A)AC(1A) and S(1B)AC(1B) {[1.710(4) and 1.696
shorter than other two Pd(1)AS(1B) which indicates that the for-
mer pair of sulfur atoms are strongly bonded with Pd(II) than the
latter. In the solid state, complex 2 exhibits weak intermolecular
CAHꢀ ꢀ ꢀN interactions between hydrogen of one triazole and nitro-
gen of the other triazole ring (Fig. 5) forming an extended network.
The complex 2 is further stabilized by intramolecular NAHꢀ ꢀ ꢀCl
hydrogen bonding (Fig. 6).
(
4) Å]} distances are also in agreement with that of typical carbon
sulfur double-bonds (C@S)] [53], indicating that the exocyclic CAS
bond is of double bond character and ligand is present in the thione
form. This is further supported by the longer N(1A)AC(1A) and N
(
1B)AC(1B) bond lengths [1.319(5) and 1.315(5) Å], than the
N@C double bonds [1.27 Å] [54]. Two Pd(1)AS(1A) distances are
Table 5
Weak intermolecular interactions [Å and °] in [Pd(Hmmtrz)
4
]Cl
2
(2).
d(Hꢀ ꢀ ꢀA)
DAHꢀ ꢀ ꢀA
d(DAH)
d(Dꢀ ꢀ ꢀA)
<(DHA)
Symmetry equivalent operators
N(1A)AH(1A)ꢀ ꢀ ꢀCl(1)#1
N(1B)AH(1B)ꢀ ꢀ ꢀCl(1)
0.88(4)
0.75(4)
2.15(5)
2.35(4)
3.028(4)
3.088(4)
175.0(5)
170.0(4)
ꢁx + 1, ꢁy + 1, ꢁz + 1
x + 1, y + 1, z + 1
Table 6
Weak intermolecular interactions [Å and °] in [Pd(Hmthd)
4
]Cl)
2
ꢀ2CHCl
3
(3).
DAHꢀ ꢀ ꢀA
d(DAH)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
<(DHA)
Symmetry equivalent operators
C(4)AH(4A)ꢀ ꢀ ꢀS(1)
0.96(4)
2.95(5)
3.72(4)
138.0(5)
0.5 + x, 0.5 ꢁ y, ꢁ1 + z
Fig. 3. Molecular packing diagram of complex 1.

P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
167
Fig. 5. CAHꢀ ꢀ ꢀN interaction in complex 2 leading to 1D structure.
Fig. 6. Packing diagram of complex 2 showing intramolecular NAHꢀ ꢀ ꢀCl hydrogen bonding.
3
.5.3. Crystal structure description of [Pd(Hmthd)
4
]Cl
2
ꢀ2CHCl
3
(3)
(3) with
Fig. 7 shows ORTEP diagram of [Pd(Hmthd) ]Cl
4
2
ꢀ2CHCl
3
the atom numbering scheme. In this complex, Pd(II) ion adopts a
distorted square planar geometry with S(2)APd(1)AS(3) and
S(2)APd(1)AS(3) bond angles of 91.62(6) and 88.38(6)°, respec-
tively. The Pd(II) centre is bonded with four sulfur atoms of thiadi-
azole ring at the Pd(1)AS(2) and Pd(1)AS(3) distances of 2.326(16)
and 2.331(2) Å, respectively. It is reported that the ligand behaves
as monoanionic monodentate in its Cu(II), Hg(II), Cd(II) and Zn(II)
complexes [46]. Interaction of ligand in the Cu(II) complex is ionic
in nature, whereas it is covalently bonded through sulfur in its Cd
(
II) complex [46]. The ligand behaves as neutral monodentate in
the complex [Pd(Hmthd) ]Cl (3) and is bonded through
ꢀ2CHCl
the thione sulfur to the Pd(II) centre. The exocyclic S(2)AC(3)
4
2
3
[
1.699(6) Å] bond length is shorter than endocyclic S(1)AC(3)
[
1.728(6) Å] bond length, and is close to the carbon sulfur dou-
ble-bonds (S@C), which indicates that the exocyclic sulfur has dou-
ble bond character and the ligand is present in the thione form.
This is further supported by the N(2)AC(3) bond length [1.320(8)
Å], which is longer than the N@C double bond [1.270 Å] [54]. The
crystal structure of the complex revealed the presence of inter-
molecular NAHꢀ ꢀ ꢀCl and CAHꢀ ꢀ ꢀCl hydrogen bonding interactions
between CH hydrogen of thiadiazole NH and methyl CH and chlo-
rine atom present in the outer sphere of the complex (Fig. 8). The
crystal structure is further supported by weak intermolecular
CAHꢀ ꢀ ꢀS hydrogen bonding occurring between the CH hydrogen
of methyl and sulfur atom of thiadiazole ring (Fig. 9).
]2+ cation of complex
numbering scheme at 30% probability level. Chlorine and chloroform are not
shown for clarity.
Fig. 7. ORTEP diagram of [Pd(Hmthd)
2
3
with atomic

1
68
P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
Fig. 8. Showing intermolecular NAHꢀ ꢀ ꢀCl, CAHꢀ ꢀ ꢀCl and CAHꢀ ꢀ ꢀS hydrogen bonding leading to a linear chain structure in complex 3. Chlorine atoms are presented in ball and
stick model.
Fig. 9. Showing intermolecular CAHꢀ ꢀ ꢀS hydrogen bonding leading to a cross-linked linear chain structure in complex 3.
To know the tautomeric form present in both the complexes,
bond lengths within the triazole/thiadiazole ring were compared
with the reported complexes. It is found that the carbon–sulfur
and carbon–nitrogen bond lengths in the two polymorphic struc-
tures correspond to the thione form. In fact, the average carbon
sulfur bond length of 1.696–1.710 Å for complex 2 and 1.698–
planar geometry around Pd(II) centre in these complexes. The
thiourea ligand, Heptu is bonded to the metal ion through nitrogen
and sulfur atoms and acts as a uninegative bidentate in complex 1.
The triazole (Hmmtrz) and thiadiazole (Hmthd) ligands act as neu-
tral monodentate, coordinated to the metal ion through the thione
sulfur in complexes 2 and 3. The complexes are stabilized via inter-
molecular as well as intramolecular hydrogen bonding. Complexes
1, 2 and 3 are fluorescent materials which upon excitation at
1
.747 Å for complex 3 are slightly longer than the carbon sulfur
double bond whereas carbon–nitrogen distances of 1.319 and
.284 Å for complex 2 and 1.324 and 1.289 for complex 3 are
ꢁ1
1
31000, 38600 and 32300 cm
exhibit an emission at 25200,
ꢁ1
shorter than the single bond, indicating electron delocalization in
the ring. The above results suggest that the neutral thione form
of the ligand is bonded to the metal and there is extensive electron
delocalization within the triazole/thiadiazole ring [55].
27000 and 26700 cm , respectively as a consequence of intrali-
⁄
gand
p–p
transitions.
Acknowledgements
4
. Conclusions
P. Bharati thanks ‘CSIR’ – ‘India’ for the award of a SRF. Dr. N.K.
Singh is thankful to ‘UGC’ – ‘India’ for the award of UGC-Emeritus
Fellowship (2014-2015). Dr. M.K. Bharty is thankful to the ‘Science
and Engineering Research Board’ – ‘India’ for the award of a Project
(No. SERB/F/372/2015-16).
This paper reports on the syntheses, spectral and crystal struc-
ture investigations of three new Pd(II) complexes, [Pd(eptu)
Pd(Hmmtrz) ]Cl (2) and [Pd(Hmthd) ]Cl (3). The single
ꢀ2CHCl
crystal X-ray structures show four coordinate distorted square
2
] (1),
[
4
2
4
2
3

P. Bharati et al. / Inorganica Chimica Acta 443 (2016) 160–169
169
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