Group 10 metal complexes with a tetradentate thiosemicarbazonate ligand: Synthesis, crystal structures and computational insights into the catalysis for C–C coupling via Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.molstruc.2019.126997

A series of mononuclear complexes has been synthesized by reactions of group 10 metal ions with bis(4-phenyl-3-thiosemicarbazone) (H₂bPht), affording compounds of general formula [M^II(bPht)] (M = Ni, Pd and Pt). Their characterization involved FTIR, UV–Vis, ¹H NMR, CV, DPV and elemental analysis. Furthermore, the crystal structures of all complexes have been determined, showing that the thiosemicarbazonate ligand is coordinated as a tetradentate N,N,S,S-donor forming three five-membered chelate rings. The catalytic activity of [M^II(bPht)] in Heck's C–C coupling reaction using styrene and iodobenzene to obtain stilbenes has been evaluated. It was verified that the Ni^II and Pt^II complexes present low catalytic activity, while the Pd^II complex showed a conversion of 99% within 24 h. Trans-stilbene was identified as the major product of the coupling reaction, up to 90%. DFT studies were also performed in order to better understand the catalytic behavior of these complexes giving support for a new route for Mizoroki-Heck reaction.

Full text of DOI:10.1016/j.molstruc.2019.126997

Journal of Molecular Structure 1199 (2020) 126997
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Group 10 metal complexes with a tetradentate thiosemicarbazonate
ligand: Synthesis, crystal structures and computational insights into
the catalysis for CeC coupling via Mizoroki-Heck reaction
a
b
b
a
Jackelinne C. Lima , Rebecca D. Nascimento , Luana M. Vilarinho , Alice P. Borges ,
c
c
b
d
Leonardo H.F. Silva , Jhonathan R. Souza , Luis R. Dinelli , Victor M. Deflon ,
c
,
e
b
a, *
Antonio E. da Hora Machado , Andr eꢀ L. Bogado , Pedro I.S. Maia
a
Departamento de Química, Universidade Federal do Tri a^ ngulo Mineiro, Av. Dr. Randolfo Borges 1400, 38025-440, Uberaba, MG, Brazil
Instituto de Ci ^e ncias Exatas e Naturais do Pontal, Universidade Federal de Uberl a^ ndia, Rua vinte, 1600, 38304-402, Ituiutaba, MG, Brazil
Instituto de Química, Universidade Federal de Uberl a^ ndia, Av. Jo a~ o Naves de Avila 2121, 38400-902, Uberl a^ ndia, MG, Brazil
Instituto de Química de S a~ o Carlos, Universidade de S a~ o Paulo, Av. Trabalhador S a~ ocarlense, 400, 13566-590, S a~ o Carlos, SP, Brazil
Universidade Federal de Catal a~ o, Av. Lamartine Pinto de Avelar, 1120, CEP 75704-020, Catal a~ o, GO, Brazil
b
c
ꢀ
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2019
Received in revised form
A series of mononuclear complexes has been synthesized by reactions of group 10 metal ions with bis(4-
II
phenyl-3-thiosemicarbazone) (H
2
bPht), affording compounds of general formula [M (bPht)] (M ¼ Ni, Pd
1
and Pt). Their characterization involved FTIR, UVeVis, H NMR, CV, DPV and elemental analysis.
Furthermore, the crystal structures of all complexes have been determined, showing that the thio-
semicarbazonate ligand is coordinated as a tetradentate N,N,S,S-donor forming three five-membered
chelate rings. The catalytic activity of [M (bPht)] in Heck's CeC coupling reaction using styrene and
iodobenzene to obtain stilbenes has been evaluated. It was verified that the Ni and Pt complexes
24 August 2019
Accepted 27 August 2019
Available online 28 August 2019
II
II
II
Keywords:
II
present low catalytic activity, while the Pd complex showed a conversion of 99% within 24 h. Trans-
Homogenous catalysis
Thiosemicarbazones
Crystal structures
Heck catalysts
DFT
stilbene was identified as the major product of the coupling reaction, up to 90%. DFT studies were also
performed in order to better understand the catalytic behavior of these complexes giving support for a
new route for Mizoroki-Heck reaction.
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
Most of the TSC complexes tested for such reactions are based
on the mixed-ligand approach containing phosphines as the
Transition metal complexes derived from thiosemicarbazones
TSCs) have been widely explored in relation to their bioinorganic
ancillary ligand (see Fig. 1, II-V, VII) [12,14e20], whose design has
been probably related to the classic catalysts for CeC cross coupling
(
medicinal chemistry [1e6]. More recently, there has been an
increasing interest in their catalytic applications [7e10]. Although
palladium complexes have been employed successfully as catalysts
in various CeC cross coupling reactions such as Negishi, Suzuki,
Mizoroki-Heck and Sonogashira [11,12], the first work regarding the
catalysis using a thiosemicarbazonate derived complex (Fig. 1, I) for
Heck reaction was published only in 2004 [13]. Since then, several
3 4
reactions, such as [Pd(PPh ) ]. However, their sensitivity to oxygen
requires generally inert conditions for use in catalysis [13].
Although in a lower number, homoleptic palladium complexes
with TSCs have also shown to be active for cross-coupling reactions
(Fig. 1, I and VI) [21,22]. Interestingly, there is only one report in the
literature dealing with a bis(TSC) derivative (a dinuclear palladiu-
m(II) complex, Fig. 1, VII) studied as a catalyst for Mizoroki-Heck
reaction [23].
II
mononuclear Pd complexes with bidentate and tridentate TSCs
(
Fig. 1) have been studied as pre-catalysts for a variety of cross
Despite of the success of palladium complexes in catalysis, the
high costs of palladium catalysts remain as a significant disadvan-
tage, leading to the investigation of less expensive systems that do
coupling reactions [14e22].
not decrease the efficiency. In this sense, nickel has emerged as a
II
cheaper alternative to palladium and some Ni
thio-
*
Authors to whom Correspondence should be addressed.
E-mail address: pedro.maia@uftm.edu.br (P.I.S. Maia).
semicarbazonate complexes have already been investigated
https://doi.org/10.1016/j.molstruc.2019.126997
022-2860/© 2019 Elsevier B.V. All rights reserved.
0

2
J.C. Lima et al. / Journal of Molecular Structure 1199 (2020) 126997
Fig. 1. Some of the PdII thiosemicarbazone catalysts for CeC coupling.
[
24e27]. On the other hand, to the best of our knowledge, although
PerkinElmer FT-IR spectrophotometer in the region between 220
and 4000 cm . The samples were analyzed in the solid state using
the Attenuated Total Reflectance (ATR) accessory with diamond
II
ꢀ1
many Pt complexes with TSCs have been developed for medicinal
purposes, none of them have been applied as catalyst for CeC
coupling until now, undoubtedly due to the expected formation
of inert complexes.
crystal. The electronic spectra were measured with a Shimadzu UV-
1
1800 spectrophotometer from DMF solutions. The H NMR spectra
Considering the context exposed above, herein we describe the
were acquired on a Bruker Avance III HD, operating at 400 MHz. The
characteristic peaks of the solvents or TMS were used as internal
standards. The high resolution ESI-TOF mass spectra were obtained
in a Bruker Compass micrOTOF-Q Mass Spectrometer and detected
on a positive mode.
II
II
II
synthesis and characterization of analogue Ni , Pd and Pt com-
plexes derived from a potentially tetradentate bis(thiosemicarba-
zone) ligand and their evaluation as catalyst precursors for
Mizoroki-Heck coupling using aryl iodide and styrene for forma-
tion of stilbenes. Based on these preliminary results, it was possible
to purpose a potential catalytic pathway which was investigated by
means of computational studies. To our knowledge, this paper
represents the first study concerning the application of mono-
nuclear homoleptic complexes with a bis(thiosemicarbazone)
ligand in the Heck reaction.
2.2. Synthetic procedures
2.2.1. Synthesis of the nickel and palladium complexes
These compounds were synthesized from equimolar reactions
of H
bPht (102 mg) was added to a solution containing 0.2 mmol of
NiCl $6H O (64 mg) or [PdCl (MeCN) ] (52 mg) dissolved in EtOH
5 mL). The resulting mixtures were refluxed for 3 h and then kept
2
bPht and the desired metal precursor. Briefly, 0.2 mmol of
H
2
2
. Experimental
2
2
2
2
(
ꢁ
2.1. Materials and instruments
under stirring for 24 h. After that, the solutions were left at ꢀ15 C
for 24 h. The precipitates formed were filtered off, washed with n-
hexane and dried under vacuum.
NiCl
2
$6H
2
O
(Kinetics),
4-phenylthiosemicarbazide,
1,5-
II
cyclooctadiene (COD) and benzil (Sigma Aldrich) and solvents
were obtained from commercial sources and used without further
Benzil bis(4-phenyl-3-thiosemicarbazonate)nickel(II) [Ni (bPht)]
ꢁ
(1): Color: brown. Yield 87% (40 mg). M.P.: >300 C. Anal. Calcd. for
purification. The precursors [PtCl
2
(COD)] and trans-[PdCl
2
(MeCN)
2
]
28 22 6 i 2
C H N N S (613.06 g/mol): C: 59.49; H: 3.92; N: 14.87; S: 11.34%.
Found: C: 58.68; H: 4.09; N: 14.71; S: 11.27%. IR (ATR/cm ) 3298
ꢀ1
as well as the H bPht ligand were synthesized according to pro-
2
1
cedures described in the literature [28e30]. The melting points
were determined using a PF1500 FARMA-GEHAKA instrument. The
conductivities of the complexes were measured in 10 mol L
dichloromethane or dimethylsulfoxide solutions using a CG1800-
GEHAKA conductometer. The elemental analyses (CHNS) were
determined using a PerkinElmer CHNS/O 2400 Series II equipment.
The FTIR spectra were measured on a Frontier Single Range-MIR
n
(NeH), 1596
DMSO‑d /ppm): 10.21 (s, 2H NHPh), 7.46 (d, 4H, Ph), 7.35e7.24
(m, 10H, Ph), 7.17 (t, 4H, Ph), 6.98 (t, 2H, Ph). Molar conductivity
n(C]N), 1493 n(C]C), 720 n(CeS). H NMR (400 MHz,
6
, d
ꢀ3
ꢀ1
ꢀ3
(1 ꢂ 10 mol/L in CH
2 2
Cl ): 0.00 mS/cm. UVeVis bands from DMF
solution max/nm (log ε): 321 (3.95), 418 (3.51), 491 (3.68).
Benzil bis(4-phenyl-3-thiosemicarbazonate)palladium(II) [Pd
(bPht)] (2): Color: dark green. Yield 92% (112 mg). M.P.:> 300 C.
l
II
ꢁ

J.C. Lima et al. / Journal of Molecular Structure 1199 (2020) 126997
3
Elemental analysis calculated for C28
H
22
N
6
PdS
2
(613.06 g/mol): C:
calculated in idealized positions and treated with the “riding
model” option of the SHELXL2014 program [31]. Crystallographic
data and experimental details of the structural analysis are sum-
marized in Table 1.
5
4.86; H: 3.62; N: 13.71; S: 10.46%. Found: C: 53.95; H: 3.63; N:
ꢀ
1
1
3.34; S: 10.18%. IR (ATR/cm ): 3315
n
(NeH), 1593
(C]N). H NMR (400 MHz, DMSO‑d
ppm): 10.28 (s, 2H, NHPh), 7.44 (d, 4H, Ph), 7.33e7.27 (m, 10H, Ph),
n(C]N), 1446
1
n
(C]C), 720
n
(CeS), 1647
n
6
, d/
ꢀ
3
7
.15 (t, 4H, Ph), 6.97 (t, 2H, Ph). Molar conductivity (1 ꢂ 10 mol/L
in CH Cl ): 0.12 S/cm. UVeVis bands from DMF solution
log ε): 310 (4.30), 415 (4.00), 481 (3.96).
2.4. Electrochemical studies
2
2
m
lmax/nm
(
The cyclic and differential pulse voltammograms were obtained
from a potentiostat/galvanostat Autolab model mAutolab III coupled
2
.2.2. Synthesis of the platinum complex
.2 mmol of H bPht (102 mg) was added to a solution containing
.2 mmol of [PtCl (COD)] (75 mg) dissolved in MeCN (5 mL). After
N, the resulting mixture was kept under
to a graphical interface GPES version 4.9. The measurements were
carried out in a glass electrochemical cell with a capacity of 5 mL
with a 3 electrode system consisting on a reference electrode (Ag/
AgCl), a platinum working and counter electrodes. Tetrabuty-
lammonium hexafluorophosphate (TBAH) at the concentration of
0
2
0
2
addition of 0.5 mL of Et
stirring at room temperature for 24 h. The precipitate formed was
filtered off, washed with n-hexane and dried under vacuum.
3
ꢀ1
2 2
0.1 mol L in CH Cl was used as supporting electrolyte.
Benzil
bis(4-phenyl-3-thiosemicarbazonate)platinum(II)
ꢁ
[
Pt(bPht)] (3): Color: green. Yield 57% (80 mg). M.P:> 300 C.
2.5. Gas chromatography
Elemental analysis calculated for C28
22 6 2
H N PtS (701.73 g/mol): C:
4
7.92; H: 3.16; N: 11.98; S: 9.14%. Found: C: 46.83; H: 3.36; N: 11.59;
The catalytic yields were determined by gas chromatography
using hexadecane (14.6 mL) as an internal standard. The operating
ꢀ1
S: 8.94%. IR (ATR/cm ): 3310
n
(NeH), 1588
n
(C]N), 1453
Cl ) 0.06
/ppm): 10.35 (s, 2H, NHPh), 7.41
d, 4H, Ph), 7.34e7.26 (m, 10H, Ph), 7.15 (t, 4H, Ph), 6.95 (t, 2H, Ph).
UVeVis bands from DMF solution max/nm (log ε): 317 (3.54), 400
n
(C ¼ C),
ꢀ
3
724
n
1
(CeS). Molar conductivity (1 ꢂ 10 mol/L in CH
2
2
m
S/
conditions were optimized in an isotherm for the separation of
reaction products, obtaining the following parameters: furnace
6
cm H NMR (400 MHz, DMSO‑d , d
ꢁ ꢁ
temperature 170 C, detector temperature 250 C, injector tem-
(
ꢁ
ꢀ1
l
perature 220 C, gas flow 2.50 mL min constant and analysis time
of 20 min. Characteristics of the NST-100 column used (Capillary
column for gas chromatography, polar phase (Polyethylene Glycol -
PEG), dimensions: length 30 m, internal diameter 0.25 mm, film
(
3.21), 477 (3.54).
2.3. Crystal structure determinations
thickness 0.25 mm). The chromatograms were also obtained in a
Good quality crystals of the complexes 1e3 were obtained from
recrystallization from dichloromethane:ethanol (2:1) at room
temperature. The data collections were performed with Mo-K
radiation (
Standard procedures were applied for data reduction and absorp-
tion correction. The structures were predicted by direct methods
using SHELXS-97 [31] and refined by using SHELXL2014 [31]. The
OeH bond distances for the water molecule in the crystal structure
of the complex 1 were fixed using the DFIX command of
SHELX2014. The positions of the other hydrogen atoms were
Shimadzu model, CG-17 A coupled to a graphical interface, Shi-
madzu CG-Glass software version 2.0.
a
l
¼ 0.71073 Å) on a BRÜKER APEX II duo diffractometer.
2.6. Theoretical calculations
The structures were optimized using the Density Functional
Theory (DFT) functional M06 [32] combined with the basis set:
DZP-DKH [33]. The structures, after optimization, did not present
imaginary vibrational frequencies, indicating that they were
structures of minimal energy. The crystallographic data were
Table 1
2 2
Refinement data for complexes [Ni(bPht)]$H O (1$H O), [Pd(bPht)] (2) and [Pt(bPht)] (3).
1
$H
2
O
2
C
3
C
Formula
Fw
T (K)
C
28
H
24
6
N NiOS
2
28
H
22
N
6
PdS
2
28 22 6 2
H N PtS
701.72
296(2)
583,36
296(2)
613.03
296(2)
Crystal system
Space group
a (Å)
Monoclinic
P2 /c
Monoclinic
C2/c
15.5945(3)
13.8463(3)
13.3872(3)
119.1070(10)
2525.59(9)
4
Monoclinic
C2/c
15.5929(5)
13.8590(4)
13.3162(4)
118.9930(10)
2517.03(13)
4
1
22.1848(8)
5.6617(2)
23.3488(8)
117.816(2)
2593.82(16)
4
b (Å)
c (Å)
ꢁ
b
( )
3
V (Å )
Z
r
calcd (g∙cmꢀ3)
1.494
0.944
1.612
0.931
1.852
5.771
ꢀ
1
m
(mm
)
Theta range for data collection
Index ranges
1.038 to 25.111
2.097 to 25.091
ꢀ16)h)18,
ꢀ16)k)11,
ꢀ15)l)10
7575
2214/0.0189
2214/0/168
Multi-scan
0.7452/0.6933
0.0220
2.095 to 26.422
ꢀ19)h)19,
ꢀ17)k)17,
ꢀ16)l)16
22549
2588/0.1213
2588/0/169
Multi-scan
0.7452/0.5770
0.0226
ꢀ26)h)26,
ꢀ
5)k)6,
ꢀ27)l)27
Reflections Collected
Reflections unique/Rint
Data/restraints/param.
Absorption correction
Max/min. transmission
16811
4602/0.0368
4602/2/349
Multi-scan
0.7452/0.6716
0.0390
R
1
[I > 2
s
(I)]
(I)]
wR [I > 2
GOF on F , S
2
s
0.1108
1.082
0.0505
1.041
0.0542
1.048
2
Extinction coefficient
Not refined
Not refined
0.00507(17)

4
J.C. Lima et al. / Journal of Molecular Structure 1199 (2020) 126997
Scheme 1. Synthesis of the complexes.
compared to the optimized structures. All calculations were per-
formed considering the isolated molecules, using the software
Gaussian 09 revision E.01 [34]. The molecular orbitals, Mulliken
charges and structures were represented with the aid of the soft-
ware GaussView 5.0.8 [35]. In addition, the calculations related to
the transition states and intermediates were executed using the
DGDZVP atomic basis set and model IEFPCM [36], considering N,N-
dimethylformamide as solvent at 393.15 K [37].
EtOH, while the NiII complex is soluble in all these solvents. The
ꢀ
3
ꢀ1
molar conductivity for 10 mol L solutions of the complexes 1e3
ꢀ
1
in dichloromethane presented values close to 0 mS cm , which
corresponds to the neutral form of the compounds. The elemental
analyses were consistent with the general formula [M(bPht)].
II
The infrared spectra of the analogous complexes [M (bPht)]
show only small differences. The spectrum of the free ligand shows
ꢀ
1
two bands at 3337 and 3260 cm for the
n(NH) stretch, while in the
spectra of complexes 1e3 (see SI) only one absorption was found, at
ꢀ
1
3
298, 3315 and 3310 cm , respectively. This fact is expected due to
3
. Results and discussion
deprotonation of the ligand upon coordination. The vibrations
ꢀ
1
attributed to the
undergo small shifts upon formation of the complexes 1, 2 and 3,
n ]N) stretches, found at 1599 cm for H bPht,
(
C
2
3.1. Synthesis and spectroscopic characterization
ꢀ
1
being observed respectively at 1596, 1593 and 1588 cm . The
n ]S) band, observed at 874 cm for H bPht, is also shifted to
(C 2
smaller wavenumbers after coordination, which is in agreement
with the formation of the CeS bond after coordination, corrobo-
rating with NNSS-coordination mode of the ligand [3e5].
The complexes derived from benzyl bis(4-phenyl-3-
ꢀ
1
thiosemicarbazone) (H
precursors NiCl $6H O, [PdCl
to Scheme 1. The Pd and Pt products present low solubility in
2
bPht) were obtained by reaction with the
2
2
2
(MeCN) ] or [PtCl (COD)] according
2
2
II
II
DMSO, CHCl
3
and CH
2
Cl
2
, and are insoluble in MeCN, MeOH or
1
The H NMR spectrum of the free ligand (Fig. S5) shows two
signals related to the NH groups, at 10.43 and 10.48 ppm, while the
spectra of the complexes 1e3 (Figs. S6e8) present only one signal
in the range 10.2e10.4 ppm, according to the deprotonation of the
ligand as indicated by the IR data. The signals related to the phenyl
rings, found in the range 7.21e7.86 ppm for the free ligand, suffer a
Table 2
ꢁ
Selected bond lengths (Ǻ) and angles ( ) for the complexes [Ni(bPht)]$H
2
O (1$H
2
O),
[Pd(bPht)] (2) and [Pt(bPht)] (3).
Bond lengths (Å)
(1$H
2
O)
(2)
(3)
0
shielding effect upon complexation which is dependent on the
M-N(1)/N(1 )
1.856(2)/1.839(2)
2.1324(9)/2.1513(9)
1.753(3)/1.769(3)
1.313(4)/1.312(4)
1.317(4)/1.301(4)
1.9757(17) 1.974(2)
0
13
M-S(1)/S(1 )
C(1)eS(1)/C(1 )-S(1 )
C(2)eN(1)/C(2 )-N(1 )
2.2957(6)
1.772(2)
1.310(3)
1.309(3)
2.2907(7)
metal center. Due to the low solubility, the characterization by
NMR was not possible.
C
0
0
1.771(3)
1.315(4)
1.309(4)
0
0
0
0
C(1)eN(2)/C(1 )-N(2 )
Bond Angles (º)
3.2. Crystal structures
0
N(1)-M-N(1 )
83.69(11)
80.72(10)
164.71(5)
84.36(5)
93.51(7)
80.46(13)
164.60(7)
84.44(7)
0
0
N(1)-M-S(1 )/N(1 )-M-S(1)
171.29(8)/171.49(8)
87.85(8)/87.73(8)
94.56(10)/93.98(10)
0
0
The crystal structures of the complexes 1, 2 and 3 were deter-
N(1)-M-S(1)/N(1 )-M-S(1 )
C(1)-S(1)-M/C(1 )-S(1 )-M
N(2)-N(1)-M/N(2 )-N(1 )-M 123.51(19)/124.01(19) 123.75(14) 123.81(18)
S(1)-M-S(1 )
0
0
93.48(10)
mined by single crystal X-ray diffraction. Selected bond lengths and
angles are shown in Table 2. Figs. 2 and 3 show the molecular
structures of the complexes [Ni(bPht)] and [Pd(bPht)] as
0
0
0
100.71(4)
110.71(3)
110.79(4)
Fig. 2. ORTEP representation of the complex [Ni(bPht)] (1) with thermal ellipsoids at 50% probability. Water molecule has been omitted for clarity.

J.C. Lima et al. / Journal of Molecular Structure 1199 (2020) 126997
5
Fig. 3. ORTEP representation of the complex [Pd(bPht)] (2) with thermal ellipsoids at 50% probability. Symmetry operation used to generate equivalent atoms ('): xþ3, -yþ1, -zþ1.
representative of these compounds. [Pd(bPht)] is isostructural with
the platinum derivative and crystallizes in the monoclinic system
and space group C2/c, while [Ni(bPht)] crystallizes in the mono-
suffers an increase to 1.769(32) Ǻ, 1.772(2) and 1.85(4) Ǻ upon
formation of the complexes 1, 2 and 3, respectively, which can be
explained by the deprotonation of the nitrogen atoms N2/N2’ and
consequent coordination in the thioenolate form of the ligand. This
is also in accord with the decrease carbon-hydrazinic nitrogen bond
distance from 1.372(2) Ǻ in the free ligand to approximately 1.31 Å
in the complexes.
1
clinic and space group P2 /c. An ORTEP representation of the
complex 3 can be observed in the SI (Fig. S9). For the complexes 2
and 3 only half molecule is observed in the asymmetric unit.
II
Another crystal structure for the Pt derivative containing acetone
as solvate in triclinic system and space P ꢁı has already been pub-
The crystal structure of the complex 1 contains a co-crystallized
water molecule. The water molecule forms an intermolecular
hydrogen bond network involving the oxygen atom O1s as H-donor
lished by our group [38].
The coordination geometry around the Ni , PdII and PtII metal
centers is best described as a distorted square-planar with high
distortions at the N(21)-M-S(11) angle (171.49(8), 164.71(5) and
II
0
and receptor to the sulfur atoms S2 and nitrogen atom N3’,
respectively (see Fig. 4). The second NH group also works as a
hydrogen donor to the sulfur atom S1b from a symmetry related
molecule. In the crystalline net of the complexes 2 and 3 no water
molecule was observed and also no H-bond.
166.5(9) º for M ¼ Ni, Pd and Pt, respectively). In all the complexes
the thiosemicarbazone ligand coordinates to the metal center as a
NNSS-donor with double deprotonation of the coordinating back-
bone, forming three five-membered chelate rings, with metal ions
lying in an approximate plane with the four donor atoms (de-
viations: 0.0179, 0.0167 and 0.0157 Å, for M ¼ Ni, Pd and Pt,
respectively). The NieS and NieN bond lengths are significantly
shorter than those for the Pd and Pt complexes which is consistent
3.3. Electrochemical studies
The redox potential of a complex is determinant for its catalytic
activity. By the redox potential, it is possible to verify the stability of
the complexes against oxidation or reduction, since complexes
with higher redox potential will be more stable with respect to the
formation of species with higher oxidation states. In addition, there
II
with the smaller ionic radius of Ni . This fact also reflects on the
considerable differences among the angles involving the metal
centers. The CeS bond length, of 1.6764(18) Ǻ for H
2
bPht [28],
0
0
0
ꢁ
Fig. 4. Hydrogen bonds found in the crystalline structure of [Ni(bPht)]ꢃH
2
O. [N(3 ) … O(1S ) ¼ 2.919(5) Å, N(23)eH(23) … O(1S ) ¼ 173.6 ], [N(3) … S(1b) ¼ 3.467(3) Å, N(3)eH(3)
ꢁ
0
0
…
S(1b) ¼ 172.5 ] and [O(1S)/S(1 ) ¼ 3.529(5) Å, O(1S)e H(1S)/S(1 ) ¼ 142(4) º]. Symmetry operations: (b) x, yþ1, z and (c) -xþ1, -yþ2, -zþ1.

6
J.C. Lima et al. / Journal of Molecular Structure 1199 (2020) 126997
Table 3
looking more deeply into the CV of these complexes it is possible to
observe also discrete cathodic potentials. In order to confirm this
remark, the differential pulse voltammetry analysis, a more sensi-
tive technique, was used, showing two cathodic and two anodic
processes with large intensity, as verified for the nickel derivative
(Fig. 5 and Fig. S12). Thus, it was verified that all the complexes
studied here present two reversible or quasi-reversible processes
ꢀ1
Electrochemical data of complexes 1, 2 and 3 (Ag/AgCl, 0.1 mol L HTBA solution in
CH Cl , 50 mV/s).
2
2
a
b
Complex
E
ap (V)
E
cp (V)
D
E
p
(mV)
E1/2 (V)
1
0.99
1.03
1.31
1.13
1.43
1.04
1.43
40
40
60
60
90
40
1.01
1.29
1.10
1.40
1.00
1.41
1
.27
1.07
.37
0.95
.39
2
3
1
II
III
III
IV
supposed to be related to the M /M and M /M redox couples.
Only one reference describing the electrochemical behavior of
palladium complexes with thiosemicarbazones was found in the
literature, but the complexes have a different behavior, presenting
1
a
D
E
p
¼ Eap e Ecp
.
b
E1/2 ¼ (Eap þ Ecp)/2.
II
III
one redox process at positive potentials, attributed to the Pd /Pd
pair, and a second one at negative potentials, which was ascribed as
II
I
are few reports in the literature involving electrochemical studies
with complexes of group 10 metals and thiosemicarbazones. In
order to understand the redox behavior of the compounds pre-
sented in this work, cyclic voltammetry experiments for the free
ligand and its complexes 1e3 were carried out in CH Cl solution
2 2
using tetrabutylammonium hexafluorophosphate (TBAH) as sup-
porting electrolyte.
No redox potential was verified for the free ligand within the
analyzed range (Fig. S10). Table 3 shows the data obtained for the
complexes. Fig. 5 presents the voltammograms of the complexes 1
and 2 as representative for the complexes. In the cyclic voltam-
Pd /Pd [42]. It was also verified that the metal centers influence the
redox potentials and that palladium complex (2) showed the
highest oxidation potential for the first process (E1/2 ¼ 1.10 V) and
higher than that of the nickel complex (1) for the second process
(E1/2 ¼ 1.40 V). Overall, the high oxidation potentials (close to 1 V)
found for the complexes here show that the electron withdrawing
groups of the ligand lead to a high stability of the oxidation state of
the metal centers.
3.4. Catalytic efficiency in Mizoroki-Heck reaction
II
III
mogram of the complex [Ni(bPht)] (1) two anodic (Ni /Ni and
II
II
II
III
IV
IV
III
III
II
Ni /Ni ) and two cathodic (Ni /Ni and Ni /Ni ) processes have
been clearly observed, characterizing two reversible processes (Ipa
pc ratio z 1 and < 59 mV) [39]. This electrochemical behavior
Our methodology employed Ni , Pd and Pt complexes as pre-
catalysts as described in Scheme 2, there is the possibility of for-
mation of three products, however, the diphenylethene was not
observed by gas chromatography. The reactions of iodobenzene
with styrene were studied by varying the amount of pre-catalyst in
order to verify a possible catalytic activity of the complexes.
Tests for coupling reactions of the Heck type, performed to
obtain products with the structure of stilbenes, indicated that
complexes derived from bis(thiosemicarbazones) exhibit catalytic
/
I
DE
p
is similar to that observed for other nickel complexes derived from
thiosemicarbazones [40], however, it can be modified according to
the substituents of the ligand [41]. In the CV of the complexes 2 and
3
(see Fig. S11), apparently, only one clear anodic process was
recorded at approximately 1.11 and 1.03 V, respectively. However,
ꢀ
1
2 2
Fig. 5. Differential pulse voltammograms (DPV) and cyclic voltammograms (CV) of the complexes 1 and 2 (Ag/AgCl, 0.1 mol L HTBA solution in CH Cl , 50 mV/s).

J.C. Lima et al. / Journal of Molecular Structure 1199 (2020) 126997
7
II
Scheme 2. Heck reaction between styrene and iodobenzene catalyzed by [M (bPht)] (M ¼ Ni, Pd or Pt).
activity. The achieved conversions are shown in Table 4.
commercial cis-stilbene and trans-stilbene were used as standards,
which had retention times of 10.35 and 21.45 min, respectively (see
Table S1). Therefore, it is possible to state that the products ob-
tained from the catalyzed reactions were cis-stilbene and trans-
stilbene.
It was possible to observe that the palladium pre-catalyst has a
good activity in DMF as solvent, at 3.5 mol %, within 12 h of reaction.
The analysis of the aliquot indicates 85% of conversion to products
when the Pd pre-catalyst was used, while in the presence of Ni and
Pt pre-catalysts no reaction was observed at the same time. When
the reaction was carried out in a period of 24 h, under the same
conditions, a conversion of 99% was verified for the Pd pre-catalyst.
The Ni and Pt catalysts were also active with 24 h of reaction, but
presented lower conversion rates when compared to the Pd system.
Since the Pd pre-catalyst presented interesting results, further
studies were performed. The first experiment was to attest the
influence of the concentration of pre-catalyst in the reaction.
Hence, the amount of the Pd pre-catalyst was doubled (7 mol %)
and reduced to (1.75 mol %). When in a higher concentration, it was
possible to verify the formation of 99% of the products after 12 h of
reaction. However, no catalytic activity was detected upon reduc-
tion of the amount of catalyst to 1.75%. Standard catalysts of
A new test was carried out to verify the influence of the time on
the CeC coupling reaction. The aliquots were withdrawn at 0,1, 3, 5,
7, 9 and 24 h. From the calculations performed it was possible to
II
determine the conversion rate, yield and selectivity of the Pd pre-
catalyst in the Heck reaction of styrene and iodobenzene. These
data are presented in Table 5. Surprisingly, after the catalytic re-
action, the solution which was left under slow evaporation afforded
green crystals. These crystals were not suitable for X-ray diffraction,
however, by mass spectrometry analysis (Fig. S15) it was possible to
þ
suggest that the Pd-complex seems to be regenerated since the ESI
þ
HRMS spectrum shows the molecular ion peak [M þ H] with the
expected isotopic pattern of the initial complex 2 (m/z (100% of
relative
isotopic
abundance)
found/calculated ¼ 613.0456/
palladium such as [Pd(PPh
rate in the range of 50e92% at 120 C [43], while a variation from
3
)
4
] and [Pd(OAC)
2
] have a conversion
613.0460) (see Fig. S16). Another peak found at m/z ¼ 1227.0760 is
ꢁ
þ
in accord with an adduct of [2 M þ H] (m/z (calcd) ¼ 1227.08467)
11% to 99% has been reported for non-standard palladium com-
(see Fig. S17).
plexes [44e47], which shows that the palladium catalyst used this
work is very promising.
3
3
.5. DFT studies
The catalytic activity of the palladium complex was also carried
out in situ reaction, using 3.5 mol % of Pd-precursor and the free
thiosemicarbazone ligand within 12 h of reaction, at reflux in DMF
solution, but no conversion to products was found. Nevertheless,
when the time was increased to 24 h, a conversion of 60% was
found. This fact indicates that the catalyst can be generated in situ,
although a long time of induction is required to produce the real
catalyst.
.5.1. Optimized structures and orbitals of the complexes
In order to better understand the catalytic activity of the com-
plexes, DFT calculations were performed. The optimized structures
can be observed in Figs. S18e20 (Supplementary Information),
presenting a high similarity among them. Selected theoretical bond
lengths and angles along with a comparison with experimental
values are shown together in Table S2. The calculated values for the
geometric parameters suggest good agreement with the structures
elucidated by X-ray crystallography, as shown by the calculated
deviations, demonstrating that the calculated structures are very
similar to the crystalline ones. The highest discrepancies were
observed for the Pt complex, in the immediate coordination
environment around the platinum center, with some over-
estimation of the angle between the metal and the adjacent ni-
trogen atoms (N(1)-Pt-N(1 ), 11.4%), and between the neighboring
nitrogen atoms and the metal (N(2)-N(1)-Pt, 7.10%). An inference
II
The products of the reaction catalyzed by Pd were analyzed by
H NMR (see Fig. S13). Hydrogen signals from ethene were
1
observed as a singlet peak at 7.10 ppm. A doublet at 7.50 ppm and
two triplet signals at 7.35 and 7.25 ppm correspond to the aromatic
hydrogen atoms. Signals at 1.26 and 0.86 ppm are due to the
presence of n-hexane used in product separation.
II
1
In addition to the H NMR analysis, the product formed in the
palladium reaction was also characterized by gas chromatography
0
(Fig. S14), being possible to observe two signals at the times of
10.35 and 21.45 min. For the identification of these products,
Table 5
Table 4
Heck reactions of iodobenzene and styrene in DMF (120 C) at 12 h and 24 h.
Heck reactions of iodobenzene and styrene using [Pd(bPht)] (3.5 mol%) as the
catalyst at different times using DMF (120 C) as solvent.
ꢁ
ꢁ
Complex
Mol%
Conversion (%) at 12 h
Conversion (%) at 24 h
Conversiona
Selectivity cis/trans (%)b
Time (h)
1
2
3
2
2
3.5
3.5
3.5
1.75
7
0
85
0
0
99
0
19
99
22
0
99
60
cis-stilbene (%)
trans-stilbene (%)
0
1
3
5
7
9
e
e
e
e
e
e
e
e
e
2
(in situ)
3.5
67.30
77.52
74.55
81.66
11.91
10.07
12.85
12.30
88.10
89.92
87.15
87.70
Reactions were carried out in DMF (3.0 mL), in the presence of iodobenzene (60
.54 mmol), styrene (90
19
Conversion of products were determinate by gas chromatography using n-hex-
adecane as internal standard.
m
L,
0
(
m
L, 0.79 mmol), NEt3 (96
m
L, 0.69 mmol) and pre-catalyst
2
4
ꢁ
m
mol) at 120 C. [M]/iodobenzene/styrene/NEt3 molar ratio ¼ 1/28/42/36. a
a
GC conversion determined by internal standard (IS ¼ n-hexadecane).
Selectivity ¼ yield of product/conversion ꢂ 100.
b

8
J.C. Lima et al. / Journal of Molecular Structure 1199 (2020) 126997
ꢁr
intermediate (Fig. 8). The variation of Gibbs free energy (DG ) in
ꢀ
1
this step was determined as e 3.18 kJ mol . The second step con-
sists of a nucleophilic attack from styrene and a consecutive aryl
migratory 1,2-insertion, providing an 18-electron
s
-intermediate.
ꢁ
6
ꢀ1
This last step has a remarkable
D
G
r
¼ ꢀ265 ꢂ 10 J mol . The third
step encompasses the formation of a pre-product by a reductive
elimination assisted by a -elimination by iodine attack to produce
b
II
ꢁ
ꢀ1
.
the initial Pd complex. This step showed a
Finally, the pre-product can react with NEt
D
G
r
¼ ꢀ1329 kJ mol
ꢀ
3
or with I to provide
the desired product and the respective conjugated acid. These steps
were labeled as 4.1 and 4.2 (Fig. 8) and showed
ꢁ
ꢀ1 ꢀ1
DG
r
¼ ꢀ139 kJ mol
and e67 kJ mol , respectively. All steps
showed a reversible work that may be performed to provide a
spontaneous Heck reaction of the iodobenzene and styrene cata-
lyzed by [Pd (bPht)]. However, attempts of reaction without addi-
II
Fig. 6. HOMO orbital for the complex [Pt(bPht)] (3).
tion of base have not succeeded, so the 4.1 step seems to be the
most plausible. It is important to highlight that the latter conclu-
sion is also supported by a comprehensive review of a large number
of studies that reported the formation of Pd intermediates during
CeC coupling reactions [49e55].
made involving a distinct set of atomic bases (result not shown)
suggests that the observed discrepancy may be related to the
inherent characteristics of the set of bases used which, however,
does not compromise the quality of the results.
IV
Additionally, we have also been investigating another proposal
The HOMO relative to the three complexes studied are repre-
sented below (Figs. 6 and 7). It can be observed that the electron
involving a classic hydride
intermediate, producing
b
a
-elimination from the 18-electron
[Pd(H)(I)(bPht)] intermediate (see
s-
II
distribution in the frontier state of the Pt complex differs signifi-
Table S4 in the SI), following by a reductive elimination assisted by
cantly from that observed for the others, most probably due to the
participation of electrons in the 4f sublevel of the metal center (see
II
3
base to produce the initial Pd -complex, the product and [HNEt I].
Nevertheless, the energy variation related to this pathway has not
shown to be accessible. For while, it was only investigated by DFT
calculations, but some experiments have been planned in order to
confirm them, including a kinetic study.
II
II
Fig. 6), which indicates that in catalytic terms the Ni and Pd
complexes should have similar trends (see Fig. 7), but differing from
II
II
II
the Pt one. Considering the Pd and Ni complexes, there is a clear
inversion in the polarity of the wave functions between them as
well as the existence of a dxz orbital on the metal center (which
II
presents a higher density for Pd ).
4. Conclusions
Besides, the analysis of Mulliken's partial atomic charges on the
metal ions and neighboring ligands (see Table S3 and Figs. S21e23)
The benzil bis(4-phenyl-3-thiosemicarbazone) ligand success-
II
II
II
II
II
shows positive charges on Ni (þ0.284) and Pd (þ1.154), being the
fully provided analogous complexes with Ni , Pd and Pt metal
centers. All compounds were suitably characterized. It was possible
to verify a catalytic activity in Heck CeC coupling reactions at 24 h
for all the complexes. The palladium complex showed promising
results affording an excellent conversion and selectivity. The major
product formed in the Heck reaction using the palladium complex
was characterized as trans-stilbene. Interestingly, the original
palladium complex could be partially recovered after use in the
catalytic reaction indicating that this catalyst is regenerated. Be-
sides, by means of DFT calculations it was possible to verify that the
catalytic behavior of the complexes can be emphasized by an
analysis of the frontier orbitals in each complex as well as from the
Mulliken's partial atomic charges on the metal centers. The acces-
II
II
charge on Pd four times. On the other hand, the value for Pt is
II
comparable to that of Ni , but with a negative signal (ꢀ0.273),
II
which could provide the different behavior for the Pt complex.
This fact is in accord with the first oxidation potentials of the
complexes 1 (0.99 V), 2 (1.07 V) and 3 (0.95 V). Besides, the fact that
II
the partial charge on Pd is about four times higher than that on
II
II
Ni , together with the higher efficiency of the Pd complex in the
catalytic process, indicates that the first step of the catalysis is
possibly an oxidative addition of iodobenzene via an ionic pathway
[
48].
3.5.2. Calculated reaction pathway
IV
The first step in the mechanism proposal involves an oxidative
sibility of Pd intermediates was computationally indicated. This
IV
addition in the Pd-complex from iodobenzene to provide a Pd
implies that tetradentate thiosemicarbazonate complexes
Fig. 7. HOMO orbital for the complexes [Ni(bPht)] (1, left) and [Pd(bPht)] (2, right).

J.C. Lima et al. / Journal of Molecular Structure 1199 (2020) 126997
9
II
ꢁr
values for each step.
Fig. 8. Proposal of the potential mechanism of the reaction between styrene and iodobenzene catalyzed by [Pd (bPht)] (2), including
DG
described here do not follow the typical reaction mechanism for
Heck catalysts. Further studies using other bases as well as the
investigation of experimental evidences to support the catalytic
reaction pathway proposed in the present work are now ongoing.
[8] A. Ratnam, M. Bala, R. Kumar, U.P. Singh, K. Ghosh, J. Organomet. Chem. 856
2018) 41e49.
(
[9] R. Ramachandran, G. Prakash, P. Viswanathamurthi, J.G. Malecki, Inorg. Chim.
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Acknowledgements
5
[
13] D. Kovala-Demertzi, P.N. Yadav, M.A. Demertzis, J.P. Jasiski, F.J. Andreadaki,
This work was supported by CNPq (Grants 305432/2017-6,
I.D. Kostas, Tetrahedron Lett. 45 (2004) 2923e2926.
4
5
24095/2018-1 and 307443/2015-9), CAPES, FAPESP (Grant 2009/
4011-8) and FAPEMIG (Grants APQ-03174-18, APQ-01988-14,
[14] P. Paul, S. Datta, S. Halder, R. Acharyya, F. Basuli, R.J. Butcher, S.-M. Peng, G.-
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62e73.
APQ-00583-13 and APQ-03017-16). This work is also a collabora-
tion research project of members of the Rede Mineira de Química
[15] R.N. Prabhu, R. Ramesh, Tetrahedron Lett. 58 (2017) 405e409.
[16] L. Lu, P. Chellan, G.S. Smith, X. Zhang, H. Yan, J. Mao, Tetrahedron 70 (2014)
(
RQ-MG) supported by FAPEMIG (Project: CEX-RED-00010-14) and
5980e5985.
[
17] P. Paul, P. Sengupta, S. Bhattacharya, J. Organomet. Chem. 724 (2013)
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