Mono- and tetranuclear cyclopalladated complexes with N′-(9-anthracenylidene)benzothiohydrazide: Syntheses, structures and catalytic applications

Article abstract of DOI:10.1016/j.jorganchem.2016.09.032

Reaction of PdCl₂, LiCl, N′-(9-anthracenylidene)benzothiohydrazide (H₂L, 2 Hs represent the thioamide NH proton and the 9-anthracenylidene peri proton) and NaOAc·3H₂O in 1:2:1:1?mole ratio in methanol produced [Pd₄(L)₄] (1) in 75% yield. Treatment of 1 with PPh₃(1:4.5?mole ratio) in acetone provided [Pd(L)(PPh₃)] (2) in 72% yield. The molecular formulas of the diamagnetic and non-electrolytic 1 and 2 were established by elemental analyses. Molecular structures of 1 and 2 were determined by single crystal X-ray diffraction studies. In the tetranuclear 1, the palladium(II) centers are in distorted square-planar CNS₂coordination environments created by four (L)^2?, each of which acts as 5,6-membered fused chelate rings forming thioamidate-S, azomethine-N and 9-anthracenylidene peri-C donor to one metal center and uses the thioamidate-S atom to bridge a second metal center. In the mononuclear 2, (L)^2?and PPh₃assemble a distorted square-planar CNSP coordination environment around the palladium(II) center. Spectroscopic (IR, NMR and UV-Vis) measurements were also used to characterize 1 and 2. The catalytic properties of both complexes in the oxidative phenylacetylene homocoupling reaction were examined.

Full text of DOI:10.1016/j.jorganchem.2016.09.032

Journal of Organometallic Chemistry 824 (2016) 42e47
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Mono- and tetranuclear cyclopalladated complexes with
N0-(9-anthracenylidene)benzothiohydrazide: Syntheses, structures
and catalytic applications
G. Narendra Babu, Samudranil Pal*
School of Chemistry, University of Hyderabad, Hyderabad, 500 046, India
a r t i c l e i n f o
a b s t r a c t
Reaction of PdCl2, LiCl, N0-(9-anthracenylidene)benzothiohydrazide (H2L, 2 Hs represent the thioamide
NH proton and the 9-anthracenylidene peri proton) and NaOAc$3H2O in 1:2:1:1 mole ratio in methanol
produced [Pd4(L)4] (1) in 75% yield. Treatment of 1 with PPh3 (1:4.5 mole ratio) in acetone provided
[Pd(L)(PPh3)] (2) in 72% yield. The molecular formulas of the diamagnetic and non-electrolytic 1 and 2
were established by elemental analyses. Molecular structures of 1 and 2 were determined by single
crystal X-ray diffraction studies. In the tetranuclear 1, the palladium(II) centers are in distorted square-
planar CNS2 coordination environments created by four (L)2ꢀ, each of which acts as 5,6-membered fused
chelate rings forming thioamidate-S, azomethine-N and 9-anthracenylidene peri-C donor to one metal
center and uses the thioamidate-S atom to bridge a second metal center. In the mononuclear 2, (L)2ꢀ and
PPh3 assemble a distorted square-planar CNSP coordination environment around the palladium(II)
center. Spectroscopic (IR, NMR and UV-Vis) measurements were also used to characterize 1 and 2. The
catalytic properties of both complexes in the oxidative phenylacetylene homocoupling reaction were
examined.
Article history:
Received 11 August 2016
Received in revised form
28 September 2016
Accepted 30 September 2016
Available online 1 October 2016
Keywords:
Cyclopalladates
N0-(9-anthracenylidene)benzothiohydrazide
Crystal structures
Spectroscopic properties
C(sp)ꢀC(sp) coupling
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
present work, to examine whether N0-(9-anthracenylidene)ben-
zothiohydrazide (H2L) behaves in the same way or not we have
Since the discovery of the first cyclometallated complex [1],
such species remain of considerable contemporary interest due to
their potential applications in a variety of research areas such as
organic synthesis, materials science and biological and medicinal
chemistry [2ꢀ9]. As a consequence, there are continuous efforts
towards the synthesis of new cyclometallated complexes. Over the
past several years, we have been working on cyclometallates of
platinum metal ions with aroylhydrazones and thiosemicarbazones
of various mono- and polycyclic aromatic aldehydes [10ꢀ16]. It has
been found that five-membered ring forming azomethine-N and
amidate-O or thioamidate-S chelation by aroylhydrazonates or
thiosemicarbazonates facilitates cyclometallation via not only
C(sp2)ꢀH [10ꢀ15] but also C(sp3)ꢀH [16] activation. The aroylhy-
drazonates or the thiosemicarbazonates act as pincer-like CNO- or
CNS-donor in the final cyclometallated complexes formed. In the
explored its chemistry with palladium(II) and isolated a mono-
nuclear cyclopalladate and a teranuclear cyclopalladate (Scheme 1).
Herein we report the syntheses, crystal structures and spectro-
scopic properties of these two complexes and their catalytic ap-
plications in oxidative homocoupling of phenylacetylene.
2. Experimental
2.1. Materials
The Schiff base N0-(9-anthracenylidene)benzothiohydrazide
(H2L) was synthesized from equimolar amounts of 9-anthraldehyde
and thiobenzhydrazide by following a slightly modified (using
methanol instead of ethanol as solvent) reported procedure [17]. All
other chemicals used in this work were of reagent grade available
commercially and were used as received without any further pu-
rification. Standard methods [18] were employed for purification of
the solvents used.
* Corresponding author.
E-mail address: spal@uohyd.ac.in (S. Pal).
http://dx.doi.org/10.1016/j.jorganchem.2016.09.032
0022-328X/© 2016 Elsevier B.V. All rights reserved.

G.N. Babu, S. Pal / Journal of Organometallic Chemistry 824 (2016) 42e47
43
1 was collected and evaporated to dryness. Yield: 170 mg (75%).
Anal. Calcd for C88H56N8S4Pd4: C, 59.40; H, 3.17; N, 6.30. Found: C,
59.27; H, 3.23; N, 6.38. Selected IR data:
n
(cmꢀ1) ¼ 1594 & 1581
(N¼CeC¼N), 945 (CeS). UV-Vis in CH2Cl2: lmax (nm) (104 x ε
(Mꢀ1 cmꢀ1)) ¼ 562 (0.93), 525 (1.10), 490 (1.04), 392 (1.33), 376
(1.19). 1H NMR in CDCl3:
8.47 (7) (d, 2H), 7.96 (9) (d, 1H), 7.77 (s, 1H), 7.75 (2) (d, 1H), 7.63 (s,
d
(ppm) (J (Hz)) ¼ 9.37 (s, 1H), 8.87 (s, 1H),
1H), 7.43 (4) (d, 2H), 7.21e7.13 (m, 3H), 6.57 (8) (t, 1H).
2.4. Synthesis of [Pd(L)(PPh3)] (2)
To a suspension of [Pd4(L)4] (1) (135 mg, 0.076 mmol) in acetone
(20 ml) solid PPh3 (90 mg, 0.34 mmol) was added. The resulting
mixture was stirred at room temperature (298 K) for one day. The
orange red solid obtained was filtered off, dissolved in minimum
amount of dichloromethane and transferred to a silica gel column
(packed with a silica gel slurry in n-hexane). Elution with n-hex-
aneꢀethylacetate (9:1) gave an orange red band containing 2. This
band was collected and evaporated to dryness. Yield: 155 mg (72%).
Anal. Calcd for C40H29N2SPPd: C, 67.94; H, 4.13; N, 3.96. Found: C,
67.85; H, 4.18; N, 4.07. Selected IR data:
n
(cmꢀ1) ¼ 1596 & 1581
(N¼CeC¼N), 941 (CeS), 742, 691 and 536 (PPh3). UV-Vis in CH2Cl2:
lmax (nm) (104 x ε (Mꢀ1 cmꢀ1) ¼ 550sh (0.73), 511 (1.27), 485sh
(1.14), 390 (0.65), 370 (0.56). 1H NMR in CDCl3:
d (ppm) (J
(Hz)) ¼ 9.99 (10) (d, 1H), 8.68 (9) (d, 1H), 8.66 (s, 1H), 8.17 (8) (d,
2H), 8.11 (8) (d,1H), 7.71e7.65 (m, 8H), 7.60e7.56 (m, 2H), 7.47e7.42
(m, 3H), 7.39e7.35 (m, 9H), 6.72 (8) (t, 1H). 31P{1H} NMR in CDCl3:
d
(ppm) ¼ 36.26.
2.5. X-ray crystallography
Single crystals of [Pd4(L)4] (1) were grown by slow evaporation
of its solution in chloroform-acetonitrile (1:1), while slow evapo-
ration of an acetonitrile solution of [Pd(L)(PPh3)] (2) provided its
single crystals. Both 1 and 2 crystallized as solvates ꢀ 1$CHCl3 and
2$CH3CN, respectively. Unit cell determination and intensity data
collection for both crystals were performed at 298 K on an Oxford
Diffraction Xcalibur Gemini single crystal X-ray diffractometer us-
Scheme 1. (i) PdCl2, LiCl and NaOAc$3H2O (1:2:1 mole ratio) in methanol at 298 K. (ii)
PPh3 (4.5 molar equivalents) in acetone at 298 K.
2.2. Physical measurements
ing graphite monochromated Mo K
a
radiation (
l
¼ 0.71073 Å). The
CrysAlisPro software [19] was used for data collection, reduction
and absorption correction. The structures of both 1$CHCl3 and
2$CH3CN were solved by direct methods and refined on F2 by full-
matrix least-squares procedures. In the case of 1, the phenyl rings of
the thiobenzoyl moieties of two (L)2ꢀ were refined with geometric
restraints. A few additional significant residual electron density
peaks (38 eꢀ per unit cell in a total potential solvent-accessible void
volume of 1940.2 Å3) which could not be refined as disordered
solvent molecules were dealt with SQUEEZE procedure [20] as
implemented in the Platon package [21]. In both structures, all non-
hydrogen atoms were refined anisotropically. The hydrogen atoms
were placed in geometrically idealized positions and refined by
using a riding model. SHELX-97 programs [22] used for structure
solution and refinement were accessed through the WinGX pack-
age [23]. Thermal ellipsoid plots were prepared using the Mercury
[24] package. Selected crystallographic data for both structures are
summarized in Table 1.
Elemental (CHN) analysis data were obtained using a Thermo
Finnigan Flash EA1112 series elemental analyzer. A Shimadzu LCMS
2010 liquid chromatograph mass spectrometer was used to verify
the purity of H2L. Magnetic susceptibility measurements at room
temperature were performed with a Sherwood scientific balance. A
Thermo Scientific Nicolet 380 FT-IR spectrophotometer was
employed to collect the infrared spectra. A Digisun DI-909 con-
ductivity meter was used for solution electrical conductivity mea-
surements. Electronic spectra were recorded on a Shimadzu
UV3600 UV-Vis-NIR spectrophotometer. The 1H (400 MHz, SiMe4
as an internal standard) and 31P{1H} (160 MHz, 85% H3PO4 as an
external standard) NMR spectra were recorded with the help of a
Bruker NMR spectrometer. A Shimadzu GCMS-QP2010 gas chro-
matograph mass spectrometer was used for GC-MS analysis.
2.3. Synthesis of [Pd4(L)4] (1)
A mixture of PdCl2 (90 mg, 0.5 mmol), LiCl (43 mg, 1 mmol), H2L
(171 mg, 0.5 mmol) and NaOAc$3H2O (69 mg, 0.5 mmol) in 40 ml
methanol was stirred at room temperature (298 K) for 2 days. The
brick red solid separated was collected by filtration, dissolved in
minimum amount of dichloromethane and added to a silica gel
column (packed with a n-hexane slurry of silica gel). Elution with n-
hexaneꢀethylacetate (3:7) mixture initially provided a light yellow
band which was discarded. The following brick red band containing
2.6. Procedure for the homocoupling of phenyl acetylene
To a mixture of phenylacetylene (1 mmol), K3PO4 (1.5 mmol)
and cocatalyst CuI (0.5 mol%) in acetonitrile (1 ml) a dime-
thylformamide solution (0.1 ml) of catalyst (1 or 2) (0.1 mol%) was
added. The reaction mixture was stirred at 60 ꢁC for the required
time and then cooled to room temperature. It was transferred to a
separating funnel containing water (20 ml) and extracted with

44
G.N. Babu, S. Pal / Journal of Organometallic Chemistry 824 (2016) 42e47
LC-MS and spectroscopic (IR and 1H NMR) measurements. Reaction
of Li2PdCl4 (prepared in situ), H2L and NaOAc$3H2O in 1:1:1 mole
ratio in methanol afforded a tetranuclear palladium(II) complex of
molecular formula [Pd4(L)4] (1) in 75% yield. Treatment of 1 with ca.
4.5 molar equivalent of PPh3 in acetone produced the mononuclear
palladium(II) complex having the molecular formula [Pd(L)(PPh3)]
(2) in 72% yield (Scheme 1). Elemental analysis data of 1 and 2 are in
good agreement with their corresponding molecular formulas.
Both complexes are diamagnetic. Thus the palladium centers in
them are bivalent and they are in square-planar coordination
environment. The tetranuclear complex (1) is brick-red, while the
mononuclear complex (2) is orange-red in color. They are highly
soluble in dichloromethane, chloroform, dimethylsulfoxide and
dimethylformamide; sparingly soluble in acetonitrile, methanol
and toluene and insoluble in n-hexane. The electrically non-
conducting behavior of both 1 and 2 in solution is consistent with
them being neutral molecular species.
Table 1
Selected crystal data and structure refinement summary.
Complex
1$CHCl3
2$CH3CN
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C89H57Cl3N8S4Pd4
1898.62
Tetragonal
I41/a
44.5189(10)
44.5189(10)
15.3784(5)
90
C42H32N3PSPd
748.14
Monoclinic
P21/n
12.4885(18)
12.9202(19)
21.517(3)
90
105.596(9)
90
3344.0(8), 4
1.486
0.701
12756
5880
b (Å)
c (Å)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
90
90
V (Å3), Z
30478.9(14), 16
1.655
1.198
29976
13418
rcalcd (g cmꢀ3
m
)
(mmꢀ1
)
Refl. collected
Refl. unique
Refl. [I ꢃ 2
s
(I)]
8547
973
4324
434
Parameters
R1, wR2 [I ꢃ 2
s
(I)]
0.0688, 0.1369
0.1143, 0.1531
1.062
0.769/ꢀ0.509
0.0417, 0.0841
0.0656, 0.0944
1.026
0.502/ꢀ0.439
R1, wR2 [all data]
3.2. X-ray molecular structures
GOF on F2
Max./Min. Dr (e Åꢀ3
)
The molecular structures of 1 and 2 are illustrated in Figs. 1 and
2, respectively. The bond lengths and angles related to the metal
centers are listed in Tables 2 and 3. In both complexes, the metal
centers are in distorted square-planar coordination environments.
In the tetranuclear 1, each (L)2ꢀ binds one palladium atom through
the thioamidate-S, the azomethine-N and the 9-anthracenylidene
peri-C atoms to form a 5,6-membered fused chelate rings motif
and uses the thioamidate-S again to coordinate a second palladium
atom (Scheme 1 and Fig. 1). As a result, all four palladium centers
are in planar CNS2 (rms deviations from the mean plane are in the
ethylacetate (20 ml). The ethylacetate extract was washed with
water (2 ꢂ 10 ml), dried over anhydrous Na2SO4 and finally sub-
jected to GC-MS analysis for confirmation of the product and
determination of its yield.
3. Results and discussion
3.1. Synthesis and characterization
range 0.03e0.06 Å) environments. The {Pd4(m-S)4} core has a
cradle-like structure (Fig. 1, inset). The metal/metal distances in
the four PdeSePd fragments are in the range 3.8859(9)ꢀ4.0912(8)
Å, while those in the two pairs of unbridged palladium centers are
The Schiff base N0-(9-anthracenylidene)benzothiohydrazide
(H2L) was synthesized in ca. 90% yield by the condensation reaction
of equimolar amounts of benzothiohydrazide and 9-anthraldehyde
in methanol in presence of a few drops of acetic acid [17]. The
identity and purity of H2L were confirmed by elemental analysis,
comparatively much shorter (Pd(1)/Pd(2), 3.3636(9)
Å and
Pd(3)/Pd(4), 3.2592(9) Å). It may be noted that a tetrapalladiu-
m(II) complex with a CNS-donor thiosemicarbazonate ligand [25]
Fig. 1. ORTEP representation of [Pd4(L)4] (1) and the cradle-like {Pd4(m-S)4} core (inset). All non-hydrogen atoms are shown at their 40% probability thermal ellipsoids. All non-
carbon atoms and only the metallated carbons are labeled for clarity.

G.N. Babu, S. Pal / Journal of Organometallic Chemistry 824 (2016) 42e47
45
environment in 1. This slight non-planarity is perhaps due to the
steric effect of the PPh3. Overall, the metal centered bond lengths in
1 and 2 are comparable with those observed for palladium(II)
complexes having similar coordinating atoms [25ꢀ28].
3.3. Spectroscopic properties
Infrared spectra of all the compounds were recorded in the
range 4000ꢀ400 cmꢀ1 using KBr pellets. Each compound displayed
a large number of bands of various intensities. Except for the
following few selected bands no attempt was made to assign the
other bands. The spectrum of the free Schiff base H2L showed the
thioamide NeH and C¼S stretches as a broad strong band at
3183 cmꢀ1 and as a sharp strong band at 954 cmꢀ1, respectively
[17,26,29]. The disappearance of the NeH stretching band and
appearance of the C¼S stretching band at lower frequency (ca.
943 cmꢀ1) with less intensity in the spectra of 1 and 2 indicate the
thioamidate-S coordination to the metal center in both complexes.
A medium intensity band observed for H2L at 1594 cmꢀ1 is attrib-
uted to the C¼N stretching. On the other hand, both 1 and 2 dis-
played two closely spaced medium to strong bands at ca. 1595 and
1581 cmꢀ1. These bands are most likely associated with the one end
coordinated conjugated eN¼CeC¼Nꢀ moiety of the ligand (L)2ꢀ
(Scheme 1). The presence of the coordinated PPh3 in the mono-
nuclear 2 is indicated by the characteristic three strong bands at
742, 691 and 536 cmꢀ1 [15,16,27].
Fig. 2. ORTEP representation of [Pd(L)(PPh3)] (2) with the atom labeling scheme. The
Electronic absorption spectra of 1 and 2 were recorded in
dichloromethane. Both complexes exhibited a strong band in the
visible region (ca. 518 nm) flanked on both sides by two shoulders
and two more strong bands in the ultraviolet region (ca. 391 and
373 nm) (Fig. 3). It may be noted that the spectrum of the free Schiff
base (H2L) displayed a shoulder at 435 nm and a group of four
rather closely spaced strong bands in the range 395ꢀ335 nm fol-
lowed by a very strong absorption at 257 nm [17]. This spectral
profile of H2L is very similar to that of anthracene containing Schiff
bases and free anthracene [11,30,31]. Except for the red shift, the
visible region absorption profiles of 1 and 2 are somewhat similar
to the spectral profile observed for H2L [17]. Thus it is very likely
that the structured visible region absorption as well as the couple of
higher energy absorptions displayed by the complexes 1 and 2 are
largely due to ligand centered transitions only [11,13ꢀ15]. Further,
thermal ellipsoids of all non-hydrogen atoms are drawn at the 40% probability level.
Table 2
Selected bond lengths (Å) and angles (ꢁ) for [Pd4(L)4]$CHCl3 (1$CHCl3).
Pd(1)eC(1)
Pd(1)eN(1)
Pd(1)eS(1)
Pd(1)eS(3)
Pd(2)eC(23)
Pd(2)eN(3)
Pd(2)eS(2)
Pd(2)eS(4)
C(1)ePd(1)ꢀN(1)
C(1)ePd(1)ꢀS(1)
C(1)ePd(1)ꢀS(3)
N(1)ePd(1)ꢀS(1)
N(1)ePd(1)ꢀS(3)
S(3)ePd(1)ꢀS(1)
C(23)ePd(2)ꢀN(3)
C(23)ePd(2)ꢀS(2)
C(23)ePd(2)ꢀS(4)
N(3)ePd(2)ꢀS(2)
N(3)ePd(2)ꢀS(4)
S(2)ePd(2)ꢀS(4)
2.026(7)
1.995(6)
2.3376(19)
2.312(2)
2.028(8)
2.001(6)
2.329(2)
2.338(2)
91.7(3)
175.4(2)
93.1(2)
84.16(17)
173.33(19)
91.18(7)
91.7(3)
Pd(3)eC(45)
Pd(3)eN(5)
Pd(3)eS(3)
Pd(3)eS(2)
Pd(4)eC(67)
Pd(4)eN(7)
Pd(4)eS(4)
Pd(4)eS(1)
C(45)ePd(3)ꢀN(5)
C(45)ePd(3)ꢀS(3)
C(45)ePd(3)ꢀS(2)
N(5)ePd(3)ꢀS(3)
N(5)ePd(3)ꢀS(2)
S(3)ePd(3)ꢀS(2)
C(67)ePd(4)ꢀN(7)
C(67)ePd(4)ꢀS(4)
C(67)ePd(4)ꢀS(1)
N(7)ePd(4)ꢀS(4)
N(7)ePd(4)ꢀS(1)
S(4)ePd(4)ꢀS(1)
2.044(7)
1.994(6)
2.319(2)
2.345(2)
2.035(8)
1.987(7)
2.331(2)
2.357(2)
91.6(3)
174.2(2)
94.6(2)
83.73(19)
173.37(19)
90.25(7)
91.4(3)
173.5(2)
94.2(2)
82.95(19)
174.1(2)
91.14(7)
173.0(3)
96.0(3)
82.9(2)
172.6(2)
89.70(7)
also has the molecular structure very similar to that of 1. In the
mononuclear 2, (L)2ꢀ acts as CNS-donor and the P-atom of PPh3
occupies the fourth coordination site (Fig. 2). The CNSP coordina-
tion environment around the metal center in 2 is not so planar (rms
deviation from the mean plane is 0.13 Å) as that of the CNS2
Table 3
Selected bond lengths (Å) and angles (ꢁ) for [Pd(L)(PPh3)]$CH3CN (2$CH3CN).
Pd(1)eC(1)
Pd(1)eN(1)
C(1)ePd(1)ꢀN(1)
C(1)ePd(1)ꢀS(1)
C(1)ePd(1)ꢀP(1)
2.021(3)
2.005(3)
90.43(12)
169.77(11)
96.37(10)
Pd(1)eS(1)
Pd(1)eP(1)
N(1)ePd(1)ꢀS(1)
N(1)ePd(1)ꢀP(1)
S(1)ePd(1)ꢀP(1)
2.3126(9)
2.2659(10)
82.61(8)
170.91(9)
91.39(3)
Fig. 3. Electronic spectra of [Pd4(L)4] (1) (_______) and [Pd(L)(PPh3)] (2) (ꢀ ꢀ ꢀ) in
dichloromethane.

46
G.N. Babu, S. Pal / Journal of Organometallic Chemistry 824 (2016) 42e47
Table 4
metallation of the aromatic ring is also known to cause red shift of
Optimization of reaction conditions.a
absorption bands related to
p
ꢀp* transitions [11,13ꢀ15,28,32].
The 1H NMR spectra of 1 and 2 were recorded in CDCl3. The
spectrum of 1 indicated that its four {Pd(L)} units are equivalent to
each other. None of the two complexes displayed the singlet reso-
nance of the thioamide NeH proton observed for the free H2L at
Entry Base
Solvent
Temp. (ꢁC) Time (h) Yield (%)
d
13.45 ppm [17]. Thus in each of 1 and 2 the thioamide fragment is
deprotonated. The azomethine proton of H2L resonates as a singlet
at 9.96 ppm. A singlet observed at 9.37 ppm for 1 is attributed to
1
2
3
4
5
6
7
8
No Base
CH3CN
CH3CN
CH3CN
CH3CN
60
60
60
60
60
60
60
60
60
60
60
60
60
60
24
16
16
18
24
24
10
12
12
24
04
04
10
12
04
04
12
04
04
24
16
04
04
04
23
47
43
55
37
31
82
72
66
74
94
93
81
66
88
80
68
79
85
62
78
94
48
29
K2CO3 (2 mmol)
Na2CO3 (2 mmol)
NaOAc (2 mmol)
NaHCO3 (2 mmol) CH3CN
NaOH (2 mmol)
DBU (2 mmol)
DABCO (2 mmol)
Et3N (2 mmol)
PPh3 (2 mmol)
K3PO4 (2 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.0 mmol)
K3PO4 (0.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
K3PO4 (1.5 mmol)
d
d
the proton of the azomethine group that is coordinated to the metal
via the N-atom. In contrast, the azomethine proton in 2 appeared as
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
(CH3)2NCHO 60
C6H5CH3
H2O
CH3OH
(CH2)4O
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
a doublet (J ¼ 10 Hz) at
d 9.99 ppm due to coupling with the PPh3 P-
atom at the coordination site trans to the azomethine-N. The
9
chemical shift ranges and the intensities of the signals corre-
sponding to the aromatic protons of both 1 (
10
11
12
13
14
15
16
17
18
19
20
21
22
23b
24c
d 8.87e6.57 ppm) and 2
(d
8.68e6.72 ppm) are unexceptional and as expected. The 31P{1H}
NMR spectrum of 2 exhibited a singlet at
sponding to the PPh3 ligand present in it.
d 36.26 ppm, corre-
60
60
60
60
25
40
80
60
60
3.4. Catalytic activities
Conjugated 1,3-diynes derived via cross coupling of sp-hybrid-
ized carbons have been found to be very useful and effective syn-
thons for
a variety of biologically active natural products,
pharmaceuticals and also functional materials [33ꢀ38]. The first
oxidative homocoupling reaction of the terminal alkyne phenyl-
acetylene using CuCl in ammoniacal ethanol medium in presence of
air was reported way back in 1869 by Glaser [39,40]. Since then the
original procedure has been modified every now and then by
varying the catalyst composition, base, oxidant and solvent to
broaden its applications. Among various catalysts employed so far,
quite a few were copper/palladium based [33ꢀ38]. Majority of the
palladium cocatalysts used were either palladium(II) salts or
palladium(0/II) coordination complexes. To the best of our knowl-
edge there are only two reports on the use of cyclopalladated
complexes as catalysts [41,42]. Hence, we examined the catalytic
properties of the present cyclometallated complexes 1 and 2 in the
homocoupling of phenylacetylene. Optimization of the reaction
conditions was performed by varying the base and its amount,
solvent (1 ml) and temperature using 1 mmol of the substrate
phenylacetylene and 2 (0.1 mol% in 0.1 ml dimethylformamide)
with CuI (0.5 mol%) as the catalyst system in presence of air
(Table 4). In each attempt, the reaction was considered complete
when there was no change of the substrate concentration. The best
condition was found to be the use of K3PO4 (1.5 mmol) as the base
and acetonitrile as the solvent at a temperature of 60 ꢁC (entry 12).
Under this condition in 4 h the reaction was complete and 93% yield
was obtained. In the control experiments, 48% yield was obtained
without 2 (entry 23) and 29% yield was obtained in the absence of
CuI (entry 24). In the previously reported first instance of cyclo-
palladate catalyzed homocoupling of terminal alkynes, a dinuclear
a
Phenylacetylene (1 mmol), 2 (0.1 mol%) in (CH3)2NCHO (0.1 ml), CuI (0.5 mol%),
base, solvent (1 ml).
b
Without 2.
Without CuI.
c
Table 5
Variation of catalyst and its loading.a
Entry
Catalyst
Mol%
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
1
2
1
2
1
2
1
2
0.1
0.1
4 h
4 h
5 h
5 h
6 h
6 h
10 h
10 h
96
93
91
91
86
88
78
73
0.05
0.05
0.01
0.01
0.001
0.001
a
Reaction conditions: Phenylacetylene (1 mmol), catalyst in (CH3)2NCHO
(0.1 ml), CuI (0.5 mol%), K3PO4 (1.5 mmol), CH3CN (1 ml), 60 ꢁC.
the decrease of the catalyst loading from 1 mol% to 0.1 mol%. It was
also found that the mononuclear complex was more active and
efficient than the dinuclear complex. In the present study, change
of the catalyst from the mononuclear 2 to the teranuclear 1 keeping
the same loading (0.1 mol%) did not affect the reaction time and the
yield in any significant way (Table 5, entries 1 and 2). Further, the
extents of the increase in the reaction time and the decrease in the
yield with the decrease of the catalyst loading were very similar for
both complexes (entries 3e8). The reaction time increased 2.5-fold
and the yield decreased by about 20% for both 1 and 2 with the
decrease of the catalyst loading from 0.1 to 0.001 mol%. Although
both 1 and 2 displayed very similar catalytic activities, but the
difference in the nuclearities of 1 and 2 suggests that 2 is a better
performing catalyst than 1.
ortho-metallated complex [Pd2(
m
-Cl)2(L0)2] (0.05 mol%) (HL' ¼ 4,4'-
dichlorobenzophenone oxime) and CuI (5 mol%) were used as the
catalyst mixture [41]. The reactions performed in N-methyl-
pyrrolidinone solvent in presence of pyrrolidine or Bu4NOAc as
base provided 90e99% yields of the coupled products in 2e3 h, but
at a much higher temperature of 110 ꢁC. While in the second such
example, both the dinuclear [Pd2(
m
-Cl)2(L00)2] and the mononuclear
[Pd(L00)(PPh3)Cl] (HL00
¼
4-methyl-N-(ferrocenylidene)aniline)
cyclopalladates (1 mol%) along with CuI (2.5 mol%) were examined
as the catalyst systems in presence of KOAc as the base for the
terminal alkyne homocoupling reactions in dimethylformamide
solvent at 40 ꢁC [42]. The reaction showed a broad substrate scope
with yields in the range of 42e96%, but the reaction time varied
from 2 to 44 h. The reaction time also increases significantly with
4. Conclusions
The capability of N'-(9-anthracenylidene)benzothiohydrazide
(H2L) to produce cyclometallated species has been demonstrated.

G.N. Babu, S. Pal / Journal of Organometallic Chemistry 824 (2016) 42e47
Rev. 257 (2013) 2784e2797.
47
The cyclopalladated complexes isolated were characterized as the
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tetranuclear [Pd4(L)4] (1) and the mononuclear [Pd(L)(PPh3)] (2). X-
ray crystal structures of 1 and 2 revealed the pincer-like 5,6-
membered fused chelate rings forming thioamidate-S, azome-
thine-N and 9-anthracenylidene peri-C coordination mode of (L)2ꢀ
and its additional ability to bridge a second metal atom via the
thioamidate-S in 1, leading to a cradle-like {Pd4(m-S)4} core. The
spectroscopic characteristics of 1 and 2 are consistent with the
corresponding molecular structures. Both complexes exhibited
comparable and decent catalytic activities in the oxidative homo-
coupling reaction of phenylacetylene.
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G. N. Babu thanks the Council of Scientific and Industrial
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provided by the Department of Science and Technology, New Delhi
(FIST and PURSE programs) and the University Grants Commission,
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Appendix A. Supplementary material
CCDC 1497686 and 1497687 contain the supplementary crys-
tallographic data for 1$CHCl3 and 2$CH3CN, respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallographic
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