Synthesis, characterization, and crystal structure determination of palladacycles of para -substituted 2-thiobenzylazobenzenes

Article abstract of DOI:10.1080/00958972.2014.922683

Bichelated neutral palladacycles (1-3), [Pd(L)Cl], were synthesized from reaction of the new potential tridentate (C,N,S) ligands, 2-thiobenzylazobenzene (L1), 4′-methyl-2-thiobenzylazobenzene (L2), and 4′-chloro-2- thiobenzylazobenzene (L3) with sodium t

Full text of DOI:10.1080/00958972.2014.922683

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Synthesis, characterization, and
crystal structure determination of
palladacycles of para-substituted 2-
thiobenzylazobenzenes
Tirtha Bhattacharjeea, Mukul Kalitaa, Debamitra Chakravartyb,
Pranjit Barmana & Vedavati G. Puranikb
a Department of Chemistry, National Institute of Technology,
Silchar, India
b Center for Materials Characterizations, National Chemical
Laboratory, Pune, India
Accepted author version posted online: 12 May 2014.Published
online: 09 Jun 2014.
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To cite this article: Tirtha Bhattacharjee, Mukul Kalita, Debamitra Chakravarty, Pranjit Barman
& Vedavati G. Puranik (2014) Synthesis, characterization, and crystal structure determination of
palladacycles of para-substituted 2-thiobenzylazobenzenes, Journal of Coordination Chemistry,
67:10, 1702-1714, DOI: 10.1080/00958972.2014.922683
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 10, 1702–1714, http://dx.doi.org/10.1080/00958972.2014.922683
Synthesis, characterization, and crystal structure
determination of palladacycles of para-substituted
2-thiobenzylazobenzenes
TIRTHA BHATTACHARJEE*†, MUKUL KALITA†, DEBAMITRA CHAKRAVARTY‡,
PRANJIT BARMAN† and VEDAVATI G. PURANIK‡
†Department of Chemistry, National Institute of Technology, Silchar, India
‡Center for Materials Characterizations, National Chemical Laboratory, Pune, India
(Received 10 October 2013; accepted 21 March 2014)
Bichelated neutral palladacycles (1–3), [Pd(L)Cl], were synthesized from reaction of the new poten-
tial tridentate (C,N,S) ligands, 2-thiobenzylazobenzene (L1), 4′-methyl-2-thiobenzylazobenzene (L2),
and 4′-chloro-2-thiobenzylazobenzene (L3) with sodium tetrachloropalladate(II), Na2[PdCl4], in etha-
nol. The compounds were characterized by elemental analysis, FT-IR, 1H NMR, UV–visible, and
thermogravimetric analysis. The crystal structures of L2 and 1–3 were determined by single-crystal
X-ray diffraction. In 1–3, the geometry around palladium remains almost square planar, coordinated
to carbon, nitrogen, and sulfur of the ligand forming a bichelated cyclopalladate complex. The
C–H…Cl type intermolecular hydrogen bonds, weak π…π, C–H…π, and van der Waals interactions
are believed to be the stabilizing forces for the crystal packing of these palladacycles.
Keywords: Palladacycles; 2-Thiobenzylazobenzene; CNS-donor ligand; Thermogravimetric analysis;
Crystal structure
1. Introduction
The chemistry of Pd(II) complexes [1–4] especially palladacycles (palladium complexes
containing at least one metal-carbon bond intra-molecularly stabilized by at least one
neutral donor) [5–8] have attracted researchers owing to their facile synthesis [9], thermal
*Corresponding author. Email: Tirtha_07@yahoo.com
© 2014 Taylor & Francis

Palladacycles
1703
stability, possibility to modulate steric and electronic properties and potential applications in
organic synthesis [10], homogeneous catalysis [11], photochemistry [12], optical resolution
[13], luminescence properties [14], antibacterial studies, supramolecular architecture [15],
etc. Again, organic ligands containing hard (N donors) and soft (S or phosphine) donor sites
[16–19] have attracted interest due to their coordination [20–23] in Pd(II) complexes and
potential hemilability [18, 19].
Palladacycles have been reported to bind with bidentate [24] or tridentate donors [25]
and more frequently with five-membered nitrogen-containing rings. Palladacycles are
known for nearly all classes of organic ligands using different preparative methodologies
[26] and mechanisms [27]. Their syntheses are facile and it is possible to modulate their
electronic and steric properties simply by changing the size of the cyclic ring (3–10 mem-
ber), the nature of the metallated carbon (aliphatic, aromatic, vinylic, etc.), the type of neu-
tral donor (N, S, O containing group), and its substituent (alkyl, aryl, etc.). These factors
determine its dimeric, monomeric, neutral, or cationic nature. The flexibility of the palla-
dium binding pattern with different ligands and their consequences on structure confers a
plethora of potential applications in synthetic organic and catalytic chemistry.
Synthesis of only a few tridentate (C,N,S) thioazobenzenes [28, 29] and their palladacy-
cles [25, 30–34] has been reported. Recently palladacycles of thioazobenzenes have been
mentioned in C–H bond activation and aromatic metal-oxylation (Ar–Pd → Ar–O–Pd trans-
formation) [25, 30] reactions. For supramolecular chemistry, such palladacycles have
opened up the possibility of stacking interactions in crystal packing and polymorphism
[31]. The thioazobenzenes generally attain trans-π-diastereoisomeric configuration [28, 29]
due to repulsive force between nucleophilic thioether and arylazo moiety which have an
impact in the structures of palladacycles [32, 33].
In order to ascertain the nature of these catalytically active palladacycles, we have
attempted crystallization of palladacycles to isolate them as solids and subject them to
unambiguous structural elucidation. In this article, the one-pot synthesis and characteriza-
1
tion [elemental, FT-IR, H NMR, UV–visible, and thermogravimetric analysis (TGA)] of
p-substituted 2-thiobenzylazobenzenes [potential tridentate (C,N,S) ligands] and their
palladacycles are reported. Crystal structures of 4′-methyl-2-thiobenzylazobenzene and
palladacycles have been investigated by single-crystal X-ray diffraction.
2. Experimental
2.1. Materials and methods
Sodium tetrachloropalladate(II) Na2[PdCl4] was purchased from Merck. 2-Benzylsulfanyl
aniline was prepared according to a literature method [35]. Nitrosobenzene, 4-methylnitr-
osobenzene, and 4-chloronitrosobenzene were synthesized according to the literature pro-
cedure [36]. Elemental analyses were recorded on a Perkin-Elmer Model 240C elemental
analyzer. Electronic spectra were measured on a Cary 100 Bio UV–visible spectropho-
tometer. Infrared spectra of the ligand and complexes were recorded on an IR-affinity-I
1
FTIR Spectrometer SHIMADZU as KBr pellets. H NMR spectra were recorded on an
Ultrasonic Bruker 300 MHz FT NMR spectrometer using TMS as internal standard and
CDCl3 as solvent. All other reagents and solvents used were of commercial grade and
employed as received or purified by standard methods prior to use. Melting points were
recorded on a Veego melting point apparatus and were uncorrected. Thermogravimetric

1704
T. Bhattacharjee et al.
analyses were performed on a TGA 4000 Thermal Analyzer (Perkin Elmer) with a
heating rate of 10 °C min−1 under nitrogen.
2.2. Synthesis
2.2.1. Synthesis of 2-thiobenzylazobenzene (L1). In a typical procedure, a solution of 2-
benzylsulfanyl aniline (1.29 g, 6 mM) in glacial acetic acid (5 mL) was added to a solution
of nitrosobenzene (0.64 g, 6 mM) in glacial acetic acid (50 mL) and stirred for 45 min. Dur-
ing stirring, temperature was maintained between 50 and 70 °C. Then the solution was kept
overnight at room temperature. Orange crystals of 2-thiobenzylazobenzene were filtered off,
washed with dilute acetic acid to remove any impurities, and dried (the purity was checked
by TLC). Yield 76%; m.p. 131 °C. IR (KBr pellet, cm−1): 1578(m), 769(s). 1H NMR
(CDCl3, 300 MHz) δ: 7.9 (1H, d, J = 6.3 Hz), 7.7 (1H, d, J = 7.5 Hz), 7.2–7.5 (12H, m), 4.2
(2H, s). Anal. Calcd for C19H16N2S: C, 75.00; H, 5.26; N, 9.21. Found: C, 74.92; H, 5.28;
N, 9.18.
2.2.2. Synthesis of 4′-methyl-2-thiobenzylazobenzene (L2). The preparation of L2 fol-
lows the same condensation procedure as that of L1 except that 4-methylnitrosobenzene
(0.73 g, 6 mM) was used. The orange needle-like crystals of 4′-methyl-2-thiobenzylazoben-
zene suitable for X-ray diffraction that formed were filtered off, washed with dilute acetic
acid to remove impurities, and dried (The purity was checked by TLC). Yield 85%; m.p.
1
111 °C. IR (KBr pellet, cm−1): 1580(m), 768(s). H NMR (CDCl3, 300 MHz) δ: 8.0 (1H, d,
J = 7.8 Hz), 7.7 (1H, d, J = 7.8 Hz), 7.0–7.4 (11H, m), 4.2 (2H, s), 2.3 (3H, s). Anal. Calcd
for C20H18N2S: C, 75.47; H, 5.66; N, 8.80. Found: C, 75.37; H, 5.65; N, 8.72.
2.2.3. Synthesis of 4′-chloro-2-thiobenzylazobenzene (L3). The preparation of L3 follows
the same condensation procedure as that of L1 except that 4-chloronitrosobenzene (0.85 g,
6 mM) was used. The reaction scheme for the synthesis of L1–L3 is given in scheme 1.
1
Yield 81%; m.p. 134 °C. IR (KBr pellet, cm−1): 1582(m), 765(s). H NMR (CDCl3, 300
MHz) δ: 7.8 (1H, d, J = 8.6 Hz), 7.6 (1H, d, J = 8.4), 7.1–7.4 (11H, m), 4.2 (2H, s). Anal.
Calcd for C19H15N2SCl: C, 67.35; H, 4.43; N, 8.27. Found: C, 67.28; H, 4.40; N, 8.19.
2.2.4. Synthesis of chlorido-{1-[2-(benzylsulfanyl)-phenyldiazenyl]phenyl-k3C,N,S}-
palladium(II) (1). In a typical procedure, a solution of sodium tetrachloropalladate(II)
Na2[PdCl4] (147 mg, 0.50 mM) in ethanol (20 mL) was added dropwise to a warm ethanolic
solution (25 mL) of 2-thiobenzylazobenzene (136 mg, 0.45 mM). Immediately the color of
the solution changed to red. The resulting solution was stirred for one hour over a water
bath (90 °C) and then allowed to cool. The crystalline solid formed was filtered, washed
several times with ethanol to remove impurity, and dried. The crystals of chlorido-{1-[2-
(benzylsulfanyl)-phenyldiazenyl]phenyl-k3C,N,S}-palladium(II) (1), suitable for single-
crystal X-ray diffraction, were obtained by recrystallization from methylene chloride-hexane
solvent system (10 : 1) followed by slow evaporation of the solvent (2 days). Yield 90%. IR
1
(KBr pellet, cm−1): 1569(m), 763(s). H NMR (CDCl3, 300 MHz) δ: 8.0 (1H, d, J = 7.8
Hz), 7.8 (2H, d, J = 8.1 Hz), 7.1–7.5 (10H, m), 4.3 (2H, s). UV–visible [Methylene chloride,

Palladacycles
1705
λmax (nm) (εmax, M−1 cm−1)]: 341 (28,202), 415 (4977), 490 (2123). Anal. Calcd for
C19H15ClN2PdS: C, 51.24; H, 3.37; N, 6.29. Found: C, 51.20; H, 3.34; N, 6.25.
2.2.5. Synthesis of chlorido-{4-methyl-1-[2-(benzylsulfanyl)-phenyldiazenyl] phenyl-
k3C,N,S}-palladium(II) (2). The preparation of 2 follows the same procedure as that of 1
except that 4′-methyl-2-thiobenzylazobenzene (143 mg, 0.45 mM) was used as ligand. Yield
1
94%. IR (KBr pellet, cm−1): 1573(m), 765(s). H NMR (CDCl3, 300 MHz) δ: 8.0 (1H, d,
J = 7.8 Hz), 7.7 (1H, d, J = 7.8 Hz), 7.6 (1H, s), 7.4 (2H, d, J = 7.2 Hz), 7.1–7.2 (6H, m),
7.0 (1H, d, J = 8.1 Hz), 4.3 (2H, s), 2.3 (3H, s). UV–visible [Methylene chloride, λmax (nm)
(εmax, M−1 cm−1)]: 354 (28,977), 376 (23,390), 418 (18,310), 495 (9850). Anal. Calcd for
C20H17ClN2PdS: C, 52.29; H, 3.70; N, 6.10. Found: C, 52.25; H, 3.72; N, 6.12.
2.2.6. Synthesis of chlorido-{4-chloro-1-[2-(benzylsulfanyl)-phenyldiazenyl] phenyl-
k3C,N,S}-palladium(II) (3). The preparation of 3 follows the same procedure as that of 1
except that 4′-chloro-2-thiobenzylazobenzene (152 mg, 0.45 mM) was used as ligand and
the time required for completion of reaction was 2 h. Yield 84%. IR (KBr pellet, cm−1):
1
1563 (m), 765(s). H NMR (CDCl3, 300 MHz) δ: 7.9 (1H, d, J = 8 Hz), 7.7 (1H, d, J = 8.4
Hz), 7.6 (1H, s), 7.1–7.4 (9H, m), 4.3 (2H, s). UV–visible [Methylene chloride, λmax (nm)
(εmax, M−1 cm−1)]: 325 (24,423), 379 (7403), 414 (4413), 485 (3067). Anal. Calcd for
C19H14Cl2N2PdS: C, 47.55; H, 2.92; N, 5.84. Found: C, 47.58; H, 2.88; N, 5.70.
2.3. Crystal structure determination and refinement
Single-crystal X-ray diffraction data for L2, and 1–3 were collected at 296 K with Mo Kα
radiation (λ = 0.71073 Å) using a Bruker Smart Apex II CCD diffractometer equipped with
a graphite monochromator. SMART [37] software was used for data collection and also for
indexing the reflections and determining the unit cell parameters. Collected data were inte-
grated using SAINT [37] software. Structures were solved by direct methods and refined by
full-matrix least-square calculations using SHELXTL [38] software. Absorption corrections
were done by multiscan method (SADABS) [37]. All the non-H atoms were refined in the
anisotropic approximation against F2 of all reflections. The hydrogens were placed at their
calculated positions and refined in the isotropic approximations. SHELXTL [38], ORTEP
III [39], and Mercury [40] were used for plotting the asymmetric unit and unit cell contents.
Crystal data and structure refinements are given in table 1.
3. Results and discussion
To investigate the utility of p-substituted 2-thiobenzylazobenzenes as potential tridentate (C,
N,S) ligands, they were employed for the synthesis of palladacycles with special emphasis
on synthesis, mode of coordination of palladium, and the structure motifs.
3.1. Synthesis
The three potential tridentate (C,N,S) ligands, L1–L3, were synthesized by condensation
between equimolar quantities of 2-thiobenzylaniline and para-substituted nitrosobenzenes

1706
T. Bhattacharjee et al.
Table 1. Crystal data and structure refinement details for L2 and 1–3.
Compound
L2
1
2
3
CCDC entry no.
Empirical formula
Formula weight
T (K)
781,797
C20H18N2S
318.42
941,204
C19H15Cl N2PdS
445.24
941,205
C20H17ClN2PdS
459.27
948,613
C19H14Cl2N2PdS
479.68
296(2)
296(2)
296(2)
296(2)
λ (Å)
0.71073
Monoclinic
P21/c
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
P21/n
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
16.0637(15)
5.0183(4)
21.4121(18)
90
9.6859(2)
19.4386(3)
9.7220(1)
90
11.0579(3)
10.9588(2)
15.4009(3)
90
11.1340(1)
10.8880(1)
15.3779(2)
90
α (°)
β (°)
98.324(6)
90
1707.9(3)
4
1.238
0.190
108.1170(9)
90
1739.71(5)
4
1.700
1.342
100.5020(6)
90
1835.04(7)
4
1.662
1.275
100.6580(7)
90
1832.06(3)
4
1.739
1.422
γ (°)
V (Å3)
Z
DCalcd (g/cm3)
μ (mm−1
F (0 0 0)
)
672
888
920
952
Crystal size (mm3)
0.23 × 0.15 × 0.13
0.33 × 0.11 × 0.03
2.10−25.50
0.41 × 0.25 × 0.16
2.10−25.00
0.27 × 0.14 × 0.09
2.09–25.00
θ Range for data collection 1.92−25.99
(°)
Index ranges
−19 ≤ h ≤ 19
−6 ≤ k ≤ 4
−23 ≤ l ≤ 26
15,295
−11 ≤ h ≤ 11
−23 ≤ k ≤ 23
−11 ≤ l ≤ 11
12,106
−13 ≤ h ≤ 12
−13 ≤ k ≤ 13
−16 ≤ l ≤ 18
12,013
−13 ≤ h ≤ 13
−12 ≤ k ≤ 12
−18 ≤ l ≤ 15
13,389
Reflections collected
Independent reflections
3349 (0.046)
3107 (0.0756)
3229 (0.0698)
3231 (0.0522)
(Rint
)
Completeness (%)
100
95.9
99.9
100.0
Absorption correction
none
multiscan
(SADABS)
multiscan
(SADABS)
multiscan
(SADABS)
Max and min transmission
–
0.9583 and 0.6695 0.8181 and 0.6223 0.8875 and 0.7017
Refinement method
Full-matrix least-
squares on F2
3349/0/209
1.035
Full-matrix least-
squares on F2
3107/0/217
Full-matrix least-
squares on F2
3229/0/227
Full-matrix least-
squares on F2
3231/0/226
Data/restraints/parameters
Goodness-of-fit on F2
1.150
1.078
1.041
Final R indices [I > 2σ(I)]
R1 = 0.0514,
wR2 = 0.1054
R1 = 0.1093,
wR2 = 0.1266
0.18 and −0.17
R1 = 0.0595,
wR2 = 0.1621
R1 = 0.0751,
wR2 = 0.2253
1.32 and −1.71
R1 = 0.0529,
wR2 = 0.1517
R1 = 0.0585,
wR2 = 0.1604
1.062 and −0.842
R1 = 0.0276,
wR2 = 0.0720
R1 = 0.0328,
wR2 = 0.0770
0.270 and −0.575
R indices (all data)
Largest difference in peak
and hole (e Å−3
)
in acetic acid as shown in scheme 1. One-pot condensation, simple work-up, and good
yields make this method a convenient one.
The C,N,S-donor chelating nature of these para-substituted 2-thiobenzylazobenzenes
was studied by employing them in the synthesis of three new palladacycles, chlorido-
{1-[2-(benzylsulfanyl)-phenyldiazenyl] phenyl-k3C,N,S}-palladium(II) (1), chlorido-{4-
methyl-1-[2-(benzylsulfanyl)-phenyldiazenyl]
phenyl-k3C,N,S}-palladium(II)
(2),
and
chlorido-{4-chloro-1-[2-(benzylsulfanyl)-phenyldiazenyl]phenyl-k3C,N,S}-palladium(II) (3).
Na2[PdCl4] smoothly reacts with L1–L3 in ethanol affording the dark purple palladacycles
[Pd(L)Cl] in decent yields (scheme 2).
For the chloro-substituted ligand, formation of the cyclopalladate complex required more
time (2 h) for completion than the other ligands, and direct evidence of the influence of
donor/acceptor groups (in the aryl ring) on the formation of the cyclopalladate complex.

Palladacycles
1707
Methyl groups will favor the electrophilic ortho-metallation during cyclization while deacti-
vating chloride in the aryl ring will render it more difficult.
3.2. Characterization
1
L1–L3 and 1–3 were characterized by elemental analysis, FT-IR, H NMR, UV–visible, and
TGA. The structure of L2 and cyclopalladate complexes 1–3 were studied by single-crystal
X-ray diffraction. Elemental analyses indicate that palladium(II) reacts with the tridentate
(C,N,S) ligands in a 1 : 1 M ratio to afford neutral palladacycles [Pd(L)Cl]. These com-
plexes are crystalline solids, stable in air, and soluble in some organic solvents (CHCl3,
CH3CN, and DCM).
Cyclopalladate complexes 1–3 afford light brown solutions in methylene chloride and
display characteristic spectra in the UV–visible region. The electronic spectral data of 1–3
are given in the Experimental section. Strong absorptions observed in the visible region are
due to metal to ligand charge transfer [d(Pd) → π(azo)] transitions [41]. Bands observed in
the high-energy region with high molar extinction coefficients arise from intraligand charge
transfer transitions [42].
In FT-IR spectra, the ν(N=N) band of 1 shifted to lower frequency (1569 cm−1) compared
to ligand L1 (1578 cm−1) indicating coordination through nitrogen of the azo moiety [43–45].
Similar ν(N=N) bands are observed for 2 and 3. The ν(C–S) bands for 1–3 appear at 763–
765 cm−1.
The structures of para-substituted 2-thiobenzylazobenzenes L1–L3 and cyclopalladate
1
complexes 1–3 were established by H NMR (300 MHz) spectra recorded in CDCl3. For
L1, aromatic (Ar–H) protons appear at δ 7.2–7.9 ppm and benzylic protons (PhCH2) appear
as a singlet at 4.2 ppm. The methyl protons (CH3) for L2 are a singlet at 2.3 ppm. Aromatic
and benzylic protons for L1–L3 have a more or less similar spectral pattern. In 1–3, the
ortho-metallation (Pd–Caromatic) is evident as the signal of one aromatic proton (Ar–H) is
absent in the spectra. In 2, due to Pd–Caromatic formation (palladacycles), the aromatic pro-
ton (Ar–H) between methyl and Pd–C moiety is a singlet at δ 7.6 ppm. For 1–3, due to S-
coordination of palladium (Pd–S), benzyl protons (PhCH2) appear slightly downfield at δ
4.3 ppm (~0.2 ppm).
3.3. X-ray crystallography
L2 and palladacycles 1–3 were investigated by single-crystal X-ray diffraction. Crystals of
all the complexes suitable for X-ray diffraction were obtained by slow evaporation of meth-
ylene chloride-hexane (10 : 1). Perspective views of the molecular structure of L2 and 1–3
are shown in figures 1–4. The crystal data and structure refinement details of 1–3 are given
in table 1. L2 crystallizes in the monoclinic space group P21/c with four molecules in the
unit cell. The molecule consists of a benzyl sulfanyl group bonded to the ortho position of
the azo group of the 4-methylazobenzene. Due to repulsion between the nucleophilic ary-
lazo moiety and sulfanyl S1, the molecule attains stable trans-π-diastereoisomeric configura-
tion (figure 1) and the benzyl sulfanyl unit is moved away from the 4-methylazobenzene
unit, creating the possibility of coordination of the ligand through its N, S-donor sites. The
sulfanyl sulfur used sp3 hybrid orbitals and not pure p-orbitals for bond formation, which is
evident from the bond angle C13–S1–C14 [102.82(12)°]. The C14–S1 bond length [1.811
(2) Å] is consistent with the C–S covalent bond and the S1–C13 bond length [1.762(2) Å]

1708
T. Bhattacharjee et al.
Figure 1. Molecular structure of 4′-methyl-2-thiobenzylazobenzene (L2) shown with 50% probability ellipsoids.
is in the expected range for a sp2C–sp3S bond, so the d-resonance between the nucleophilic
sulfur and the aromatic π-cloud is insignificant. The N1=N2 bond length of 1.253(2) Å is in
the expected range for an azo N=N bond [28, 29]. The molecule is non-planar due to steric
repulsion. The tolyl and benzyl rings are oriented at dihedral angles of 6.71(12)° and 89.49
(6)°, respectively, with respect to the thiophenyl ring. There are no significant intra and
intermolecular hydrogen bonding interactions, and van der Waals interactions stabilize the
crystal packing. The structure of L1 and L3 [28, 29] are similar to that of L2 with minor
deviations of N=N bond lengths and sulfanyl (S) and azo (N) distances depending on the
substituents attached to the arylazo moiety and there is no prominent heterocyclization
(ortho azo-sulfur interaction) as found in 4′-methylazobenzene-2-sulfenyl thiocyanate
(derivative of L2) [46], which in turn favors ortho-metallation (Pd–Caromatic) with Pd(II)
salts.
In 1, the palladium is coordinated to C1 of the phenyl ring, N2 of the azo moiety and S
of the thioether, forming two five-membered chelate rings with bite angles of 79.7(3)°
(N2−Pd−C1) and 84.93(15)° (N2−Pd−S) (table 2). The C,N,S-donor set, palladium(II) cen-
ter, and chloride constitute an almost square-planar geometry. From the S−C13−C14 bond
angle, it is evident that in all these palladacycles, the plane bearing the Pd(II) chelate is
capped by the benzyl unit which is unique among other palladacycles reported (figure 2).
The Pd−S and Pd−Cl bond distances are in the expected range [31]. The observed
N1−N2 bond distance of the coordinated azo (diazene) is longer than that of the free dia-
zene group [29]. This may be attributed to back bonding from the filled metal orbitals to

Palladacycles
1709
Table 2. Selected bond lengths (Å) and angles (°) for 1–3.
Bond lengths
Bond angles
Complex 1
N1−N2
Pd−N2
Pd−S
Pd−Cl
Pd−C1
1.268(7)
1.963(5)
2.390(18)
2.303(2)
1.978(6)
N2−Pd−C1
N2−Pd−Cl
C1−Pd−Cl
N2−Pd−S
C1−Pd−S
Cl−Pd−S
79.7(3)
175.46(14)
97.2(2)
84.93(15)
164.6(2)
98.16(8)
Complex 2
N1−N2
Pd−N2
Pd−S
Pd−Cl
Pd−C2
1.266(6)
1.975(5)
2.3768(12)
2.3002(15)
2.000(5)
N2−Pd−C2
N2−Pd−Cl
C2−Pd−Cl
N2−Pd−S
C2−Pd−S
Cl−Pd−S
79.35(18)
178.01(11)
98.87(15)
85.23(12)
163.83(15)
96.62(5)
Complex 3
N1−N2
Pd−N2
Pd−S
Pd−Cl1
Pd−C2
1.271(3)
1.975(2)
2.3691(7)
2.2975(8)
1.991(2)
N2−Pd−C2
N2−Pd−Cl1
C2−Pd−Cl1
N2−Pd−S
C2−Pd−S
Cl1−Pd−S
79.55(10)
177.53(6)
98.70(8)
85.25(6)
163.98(8)
96.63(3)
Figure 2. The molecular structure of 1 shown with 50% probability ellipsoids. Hydrogens are omitted for clarity.
the vacant π* orbital localized on the azo moiety. In 2 and 3, the geometry around
palladium(II) remains almost square planar. Selected geometric parameters are listed in
table 2.

1710
T. Bhattacharjee et al.
Figure 3. The molecular structure of 2 shown with 50% probability ellipsoids. Hydrogens are omitted for clarity.
Figure 4. The molecular structure of 3 shown with 50% probability ellipsoids. Hydrogens are omitted for clarity.
In 1, the C13-H13B…Cl intermolecular hydrogen bond between the benzylic proton
H13B and Cl forms a dimer as shown in figure 5. Apart from this, weak π…π (3.327 Å)
and C8–H8…π (3.781 Å) interactions also have an impact in the crystal architecture. In 2,
due to favorable orientation of benzyl, the intermolecular hydrogen bond is prominent
between chloride, benzylic proton H13B, and between Cl and aromatic proton H11. These
two C–H…Cl type hydrogen bonds form a virtual seven-membered ring structure of graph

Palladacycles
1711
Figure 5. Perspective view of the dimeric structure formed by intermolecular hydrogen bond shown as dashed lines.
(A) For 1, symmetry code (i) –1/2 + x, 3/2 – y, –1/2 + z; (B) For 2, symmetry code (i) 3/2 – x, –1/2 + y, 1/2 – z.
set motif R21ð7Þ in the dimer as shown in figure 5. The short interaction between H5 and the
centroid (C14, C15, C16, C17, C18, C19) (2.979 Å) is significant while considering the
packing of the crystals.
Likewise, in 3, only one intermolecular hydrogen bond is prominent between chloride
and hydrogen of the benzylic carbon. The short C–H…π (2.927 Å) interaction between aro-
matic proton H5 and the centroid (C14, C15, C16, C17, C18, C19) is also significant for
orientation of the benzyl moiety in the formation of palladacycles. Hydrogen bond parame-
ters for palladacycles 1–3 are given in table 3. In palladacycles 1–3 no polymorphism is
found unlike chlorido-{4-chloro-1-[2-(methylsulfanyl) phenyldiazenyl] phenyl-k3C,N,S}pal-
ladium(II) reported by Bagchi et al. [32, 33]. Again, due to the capped benzyl moiety above
the square plane, the non-bonded S…S interaction (9.6 Å) is insignificant in dimer
formation.
3.4. Thermogravimetric analysis
In order to examine the thermal stability of 1–3, thermal analyses were carried out in nitro-
gen. Complexes of approximately 3.5 mg were heated from 35 to 910 °C at 10 °C min−1
under nitrogen (flow 90 mL min−1). Complex 1 is stable up to 200 °C (figure 6), decompos-
ing between 203 and 743 °C in a three-step process. The first step takes place between 203
and 249 °C with 18.39% weight loss (Calcd for C6H5, 17.29%). The second step is
observed from 249 to 590 °C with 47.28% weight loss (Calcd for C12H8N2S, 47.61%), and
Table 3. Hydrogen bond geometry for 1–3.
D−H…A
d(D−H) (Å)
d(H…A) (Å)
d(D…A) (Å)
<(D−H…A) (°)
Complex 1
C13−H13B…Cli
0.97
2.71
3.601(8)
153
Complex 2
C11−H11…Cli
C13−H13B…Cli
0.93
0.97
2.80
2.76
3.721(6)
3.703(6)
169
163
Complex 3
C13–H13A…Cl1i
0.97
2.76
3.687(2)
161
Note: Symmetry codes for 1: –1/2 + x, 3/2 – y, –1/2 + z; for 2: 3/2 – x, –1/2 + y, 1/2 – z; for 3: 1/2 – x, 1/2 + y, 3/2 – z.

1712
T. Bhattacharjee et al.
Figure 6. TG curve for 1 under nitrogen.
Scheme 1. Synthesis of p-substituted 2-thiobenzylazobenzenes (L1–L3).
Scheme 2. Synthesis of palladacycles (1–3).

Palladacycles
1713
the third step is between 590 and 743 °C with 0.36% weight loss. The total weight loss is
66.03%. These data show that the percentage of the residue is 33.97% (PdCl, 31.87%). All
the complexes exhibit more or less similar weight loss patterns in the thermograms.
4. Conclusion
One-pot efficient methodology for synthesis of three tridentate (C,N,S) ligands, para-substi-
tuted 2-thiobenzylazobenzenes L1–L3 and their neutral palladacycles 1–3 are described. L2
1
and 1–3 are unambiguously characterized by spectroscopic methods (elemental, FT-IR, H
NMR, and UV–visible) and TGA (for complexes 1–3). XRD study reveals that L2 due to
repulsion between sulfanyl sulfur and azo moiety attains stable trans-π-diastereoisomeric
configuration which favors the possibility of coordination with palladium ion through its N,
S-donor sites. In palladacycles 1–3, the palladium metal coordinates to sp2 C of the phenyl
ring, N of diazene, and S of thioether forming two stable five-membered chelate rings. The
geometry around palladium remains almost square planar, capped by the benzyl moiety.
Due to favorable orientation of the benzyl, C–H…Cl intermolecular hydrogen bonds are
formed. The intermolecular hydrogen bonds along with some short interactions (C–H…π
and π…π) and van der Waals interactions are believed to be the stabilizing force for crystal
packing. Thermal stability of the neutral palladacycles is established from the TG curve
(first decomposition above 200 °C).
Supplementary material
CCDC 781797, 941204, 941205 and 948613 contain the supplementary crystallographic
data for L2, 1, 2, and 3. These data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Center, 12
Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336 033; or E-mail: deposit@ccdc.
cam.ac.uk. Supplementary data associated with this article can be found in the online ver-
sion of the article [http://dx.doi.org/10.1080/00958972.2014.922683].
Acknowledgements
Authors thank the Council of Scientific and Industrial Research (CSIR), New Delhi for the
award of Senior Research Fellowship (T.B.) and Director of NIT, Silchar for financial assis-
tance (M.K.). Authors also thank SAIF, IIT Bombay; IASST, Guwahati and Tezpur Univer-
sity, Tezpur for spectral and TGA analysis.
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