Synthesis and catalytic activity of cationic dinuclear palladium (II) complexes supported by thioether ligands containing two di-(2-picolyl) amine arms

Article abstract of DOI:10.1016/j.poly.2020.114489

The design, synthesis and characterization of a series of dithioether ligands featuring two di-(2-picolyl)amine arms (2a–2d) and their corresponding cationic dinuclear palladium (II) complexes (3a–d) are reported. Crystal structures of ligand 2b and complexes 3b and 3d were determined by X-ray diffraction studies. The molecular structures of 3b and 3d display each of the two di-(2-picolyl)amine fragments coordinated to one palladium(II) atom in a (κ³-N,N,N) tridentate fashion and with the cationic metal centers displaying square-planar geometries. Weak interactions between the metal centers and the thioether fragments are observed. All bimetallic complexes 3a–d were tested as catalytic precursors in the Suzuki couplings of different o- or p-substituted iodo- or bromoaryls with boronic acids. The overall catalytic results indicate that complex 3b is the best precursor of the series demonstrating even more efficient performance compared to commercial palladium sources such as Pd(OAc)₂ and the Nájera Catalysts.

Full text of DOI:10.1016/j.poly.2020.114489

Polyhedron 182 (2020) 114489
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and catalytic activity of cationic dinuclear palladium (II)
complexes supported by thioether ligands containing two di-(2-picolyl)
amine arms
a
b
c
César Ignacio Sandoval-Chávez , René Velázquez-Jiménez , Diego Martínez-Otero ,
b
b
b,
⇑
Verónica Salazar-Pereda , Noemi Andrade-López , Simplicio González-Montiel
a
CONACYT Research Fellow – Universidad Autónoma del Estado de Hidalgo, Ciudad del Conocimiento, C.P. 42184, Mineral de la Reforma, Hidalgo, Mexico
Área Académica de Química, Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Hidalgo, km. 14.5 Carretera Pachuca-Tulancingo, Ciudad del Conocimiento,
b
C.P. 42184, Mineral de la Reforma, Hidalgo, Mexico
c
Centro Conjunto de Investigación en Química Sustentable, UAEM-UNAM, Carretera Toluca-Atlacomulco km. 14.5, C.P. 50200 Toluca, Estado de México, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The design, synthesis and characterization of a series of dithioether ligands featuring two di-(2-picolyl)
amine arms (2a–2d) and their corresponding cationic dinuclear palladium (II) complexes (3a–d) are
reported. Crystal structures of ligand 2b and complexes 3b and 3d were determined by X-ray diffraction
Received 18 October 2019
Accepted 2 March 2020
Available online 5 March 2020
studies. The molecular structures of 3b and 3d display each of the two di-(2-picolyl)amine fragments
3
coordinated to one palladium(II) atom in a (
j
-N,N,N) tridentate fashion and with the cationic metal cen-
Keywords:
Dinuclear Pd(II) complex
Di-(2-picolyl)amine
Suzuki–Miyaura cross-coupling reactions
Molecular structure
Dithiolato ligand
ters displaying square-planar geometries. Weak interactions between the metal centers and the thioether
fragments are observed. All bimetallic complexes 3a–d were tested as catalytic precursors in the Suzuki
couplings of different o- or p-substituted iodo- or bromoaryls with boronic acids. The overall catalytic
results indicate that complex 3b is the best precursor of the series demonstrating even more efficient per-
2
formance compared to commercial palladium sources such as Pd(OAc) and the Nájera Catalysts.
Ó 2020 Elsevier Ltd. All rights reserved.
1
. Introduction
ted mainly to cooperative interactions between proximal metal
centers where features such as intermetallic distances, three-
dimensional structures, and the enhanced probability of interac-
tion with the substrates play a key role [13].
Among the wide presence of tridentate-type ligands in the liter-
ature, ligands attached one or more di-(2-picolyl)amine (DPA) frag-
ments has been successfully utilized as a feasible tool to develop
cationic or neutral multinuclear complexes with a great variety
of metal centers mainly due to the ability of di-(2-picolyl)amine
group to act as a N,N,N-donor ligand towards the metal center
[14]. DPA forms metal complexes via the formation two 5-mem-
bered chelate rings which have been employed in several applica-
tions [15].
Herein, we report the design, synthesis, characterization of a
series dicationic bimetallic Pd(II) complexes bearing a dithioether
ligand functionalized with two di-(2-picolyl)amine fragments.
The new ligands (2a–d) and their respective bimetallic complexes
(3a–d) were fully characterized in solution and in several cases
their molecular structures were determined by X-ray diffraction
studies. Discussion related to the structural features of the new
complexes and the comparison of their catalytic performance in
the Suzuki-Miyaura reaction will be addressed.
In recent years, great interest has raised in the design and syn-
thesis of ligands with two or more arms capable to coordinate sev-
eral metal centers and consequently generate multimetallic
compounds [1–6]. The successful preparation of homo- or hetero-
multimetallic complexes generally dependens on the specificity
functional from ligand, flexibility, or the degree of delocalization
associated with the metals and the bridged fragments [7]. Bimetal-
lic complexes are involved in different application areas, such as
materials science, energy storage [8], optoelectronics [9], photody-
namic therapeutical behaviors [10], and in the fluorescence detec-
tion of heavy metal ions [11]. One area of great interest for
bimetallic complexes is homogeneous catalysis. The increasing
interest on developing cleaner chemical processes has risen partic-
ular attention in multifunctional complexes owing to its superior-
ity in terms of enhanced reactivity and selectivity over their
monometallic counterparts [12]. The improvement in the catalytic
and selectivity achieved by these multinuclear systems is attribu-
⇑
Corresponding author.
E-mail address: gmontiel@uaeh.edu.mx (S. González-Montiel).
https://doi.org/10.1016/j.poly.2020.114489
277-5387/Ó 2020 Elsevier Ltd. All rights reserved.
0

2
C.I. Sandoval-Chávez et al. / Polyhedron 182 (2020) 114489
2
. Experimental details
2.3. General procedure for the preparation binuclear palladium(II)
complexes 3a–d
2.1. Materials and instrumentation
Compounds 3a–d were synthesized according to the following
general method: A mixture of palladium dichloride in 50 mL of
hot acetonitrile was refluxed until the salt was dissolved. The reac-
tion mixture was allowed to cool to room temperature and ligands
2a–d were added. The mixture was refluxed for 24 h resulting in a
yellow-orange suspension that was cooled to room temperature.
The yellow-orange precipitate was filtered and washed with
one portion of 20 mL of cold acetonitrile to yield bispalladium(II)
complexes 3a–d as yellow solids.
All chemicals were purchased from commercial sources and
used without further purification. Melting points were measured
in a Mel-Temp II instrument and are uncorrected. Elemental anal-
yses were determined on a Perkin–Elmer Series II CHNS/O Analyzer
1
13
1
H, and C{ H} spectra were recorded at 20 °C in CDCl
3
and
DMSO d on a Varian Inova 400 and a Bruker Ascend 400 NMR
6
spectrometers, and spectra were referenced with the residual
protio-solvent or TMS. The dibromide-dithioether derivatives
(
1a–d) were synthesized as previously described [16–17].
Di-(2-picolyl)amine (DPA), PdCl , Pd(OAc) and C26 Pd
Nájera Catalysts) were obtained from Sigma-Aldrich and used as
received.
Compound 3a was prepared from PdCl
and 2a (0.705 mmol, 480 mg) according to the procedure described
above. Yellow-orange powder (90%). mp 182–186 °C dec. C41
Cl Pd (1037.59). Calc. C, 47.46; H, 4.08; Found: C, 45.98; H,
.45%. H NMR (DMSO d ): d = 8.31 (4H, d), 7.96 (4H, ddd), 7.61
2H, d), 7.52 (4H, d), 7.37 (4H, t), 7.09 (2H, ddd), 7.04 (2H, ddd),
.89 (2H, ddd), 5.70 (2H, d), 4.80 (4H, d), 4.17 (4H, s), 3.00 (4H,
2
(1.41 mmol, 250 mg)
2
2
H16Cl
6
N
2
O
2
2
(
42
H -
4
N
6
2 2
S
1
4
(
6
6
2
.2. General procedure for the preparation of dithioether ligands
functionalized with two di-(2-picolyl)amine fragments (2a–d)
1
3
t), 1.79 (2H, quint) ppm. C NMR (DMSO d
6
): d = 165.4, 150.6,
1
2
41.7, 138.7, 135.1, 1311.2, 129.4, 126.6, 125.5, 124.0, 68.2, 33.4,
8.6 ppm.
Compound 3b was prepared from PdCl (1.41 mmol, 250 mg)
2
A mixture of the corresponding dibromide-dithioether deriva-
tive 1a–d (2.0 mmol, 1.0 equiv), di-(2-picolyl)amine (4.2 mmol,
.1 equiv), and anhydrous K CO (10.0 mmol) in acetone (50 mL)
2 3
2
and 2a (0.705 mmol, 491 mg) according to the procedure described
above. Yellow-orange powder (92%). mp 174–176 °C dec. C42
Cl Pd (1051.62). Calc. C, 47.97; H, 4.22; Found: C, 47.68; H,
.36%. H NMR (DMSO d ): d = 8.43 (4H, d), 8.07 (4H, ddd), 7.67
2H, d), 7.61 (4H, d), 7.47 (4H, dd), 7.24 (2H, d), 7.14 (2H, dd),
.85 (2H, dd), 5.70 (2H, d), 4.78 (4H, d), 4.78 (4H, s), 3.09 (4H,
was refluxed for 48 h. After cooling, the resulting suspension was
filtered, and the solvent was removed under reduced pressure to
give a deep brown oil. The deep brown oil was then dissolved in
44
H -
4
N
6
2 2
S
1
4
(
6
6
dichloromethane (3 ꢀ 10 mL), dried over Na
2 4
SO then filtered
through a bed of Celite, and evaporated to dryness to give deep a
brown solid. The deep brown solids were purified by column chro-
matography on silica gel using chloroform/methanol as eluents to
afford compounds 2a–d as light-yellow power or light-brown resi-
nous liquid.
1
3
m), 1.73 (2H, m) ppm. C NMR (DMSO d
6
): d = 164.9, 150.2,
1
6
41.2, 138.8, 134.6, 130.9, 130.7, 128.6, 125.8, 125.0, 123.5, 67.8,
4.2, 33.3, 27.9 ppm.
2
Compound 3c was prepared from PdCl (1.41 mmol, 250 mg)
3
Compound 2a. CC over silica gel (eluent: CHCl -MeOH (19:1).
and 2a (0.705 mmol, 501 mg) according to the procedure described
above. Yellow-orange powder (90%). mp 170–174 °C dec. C43
Cl Pd (1065.64). Calc. C, 48.47; H, 4.35; Found: C, 48.12; H,
.75%. H NMR (DMSO d ): d = 8.36 (4H, d), 7.99 (4H, ddd), 7.60
2H, dd), 7.55 (4H, d), 7.41 (4H, ddd), 7.14 (2H, dd), 7.07 (2H,
ddd), 6.88 (2H, dd), 5.70 (2H, d), 4.80 (4H, d), 4.18 (4H, s), 2.98
Light-brown resinous liquid Yield: 87% (0.939 g). C41
42 6 2
H N S
1
46
H -
(
(
(
1
1
3
682.95) Calcd. C, 72.11; H, 6.20 Found C, 71.23; H, 6.25. H NMR
CDCl ): d = 8.50–8.48 (4H, m), 7.62–7.52 (10H, m), 7.26–7.23
2H, m), 7.17–7.07 (8H, m), 3.83 (4H, s), 3.82 (8H, s), 2.96 (4H, t),
4
N
6
2 2
S
3
1
4
(
6
.88 (2H, quint) ppm. 13C NMR (CDCl
3
): d = 159.5, 148.8, 138.7,
36.4, 136.3, 129.8, 128.3, 127.5, 125.6, 123.2, 121.9, 60.1, 56.3,
2.3, 28.0 ppm.
1
3
(
4H, t), 1.61 (4H, quint), 1.42 (2H, m) ppm. C NMR (DMSO d
d = 164.8, 150.1, 141.2, 138.8, 134.5, 130.8, 130.7, 128.3, 125.7,
25.0, 123.5, 67.7, 64.1, 33.6, 28.1, 28.0 ppm.
Compound 3d was prepared from PdCl (1.41 mmol, 250 mg)
and 2a (0.705 mmol, 511 mg) according to the procedure described
above. Yellow powder (90%). mp 225–228 °C dec. C44 Pd
(1079.67). Calc. C, 48.95; H, 4.48; Found: C, 48.35; H, 4.86%. H
NMR (DMSO d ): d = 8.39 (4H, d), 8.03 (4H, ddd), 7.64 (2H, dd), 7.58
4H, d), 7.45 (4H, ddd), 7.19 (2H, dd), 7.11 (2H, ddd), 6.92 (2H, ddd),
6
):
Compound 2b. CC over over silica gel (eluent: CHCl
19:1). Light-brown solid (70%). mp 113–115 °C dec. C421
696.98) Cald. C, 72.38; H, 6.36 Found C, 72.48; H, 6.40. H NMR
CDCl ): d = 8.51–8.49 (4H, m), 7.64–7.57 (10H, m), 7.28–7.25
2H, m), 7.19–7.10 (8H, m), 3.83 (12H, s), 2.86 (4H, t), 1.72 (4H,
3
-MeOH
1
(
(
(
(
44 6 2
H N S
2
3
H48Cl
4
N
6
2
-
1
1
3
S
2
quint) ppm. C NMR (CDCl
3
): d = 159.4, 148.8, 138.5, 136.4,
1
2
36.3, 129.7, 128.2, 127.5, 125.5, 123.1, 121.9, 60.0, 56.8, 32.0,
8.1 ppm.
6
(
5
1
1
3
.70 (2H, d), 4.75 (4H, d), 4.22 (4H, s), 3.03 (4H, t), 1.60 (4H, m),
.42 (4H, m) ppm. C NMR (DMSO d ): d = 164.9, 150.2, 141.2,
6
39.0, 134.6, 130.8, 130.7, 128.4, 125.7, 125.0, 123.5, 67.8, 64.2,
3.8, 28.5, 28.3 ppm.
Compound 2c. CC over over silica gel (eluent: CHCl
3
-MeOH
(711.00)
Cald. C, 72.64; H, 6.52 Found C, 72.72; H, 6.59. H NMR (CDCl ):
d = 8.50–8.48 (4H, m), 7.63–7.55 (10H, m), 7.29–7.26 (2H, m),
1
3
(
46 6 2
19:1). Light-brown resinous liquid (64%) C43H N S
1
3
7
1
1
3
.19–7.08 (8H, m), 3.84(12H, s), 2.85 (4H, t), 1.60 (4H, quint),
1
3
.52 (2H,m) ppm. C NMR (CDCl
3
): d = 159.6, 148.8, 138.5,
2.4. Crystallographic analyses
36.9, 136.2, 129.5, 128.2, 127.4, 125.4, 123.1, 121.9, 60.1, 56.7,
3.3, 28.5, 20.1 ppm.
Crystals of 2b suitable for X-ray diffraction analysis were grown
by slow evaporation of a chloroform solution, while crystals of 3b,
Compound 2d. CC over over silica gel (eluent: CHCl
19:1). Light-brown solid (44%). mp 90–92 °C dec. C441
725.03) Cald. C, 72.89; H, 6.67 Found C, 72.99; H, 6.76. H NMR
CDCl ): d = 8.50–8.48 (4H, m), 7.63–7.55 (10H, m), 7.29–7.26
2H, m), 7.19–7.08 (8H, m), 3.84(12H, s), 2.85 (4H, t), 1.60 (4H,
3
-MeOH
0
(
(
(
(
H
48
N
6
S
2
3b and 3d were grown by slow evaporation of an acetonitrile solu-
tion at room temperature. X-ray diffraction data for all compounds
were collected at room temperature on an Oxford Diffraction
GEMINI CCD diffractometer with graphite-monochromated Mo
3
13
quint), 1.40 (2H, m) ppm. C NMR (CDCl
3
): d = 159.6, 148.8,
38.4, 137.0, 136.2, 129.5, 128.1, 127.4, 125.3, 123.1, 121.9, 60.1,
6.7, 33.4, 28.8, 28.4 ppm.
K
a
radiation (k = 0.71073 Å) for 2b and Cu K
a
(k = 1.54184 Å)
0
1
5
for 3b, 3b and 3d. Data were processed using the CRYSALIS soft-
ware package [18]. Using Olex2 [19], the structures were solved

C.I. Sandoval-Chávez et al. / Polyhedron 182 (2020) 114489
3
with the ShelXT [20] structure solution program using Intrinsic
Phasing and refined with the XL [21] refinement package using
Least Squares minimization.
The asymmetric unit of crystal of ligand 2b contained half a
molecule which is related at a crystallographic inversion center.
In 3b, the two chlorine anions are disordered over different posi-
tions with relative occupancies 0.2542 for Cl3, 0.2452 for Cl4,
in situ from the reaction of PdCl
2
with acetonitrile) in a 1:2 ratio,
afforded the dinuclear Pd(II) complexes 3a–d. Ligands 2a–d are
soluble in acetonitrile, acetone, dichlorometane, and chloroform,
while compounds 3a–d are soluble in DMSO and DMF.
3.2. NMR spectroscopy
0
0
.5878 for Cl5, 0.2435 for Cl6, 0.2365 for Cl7, 0.2422 for Cl8 and
.1907 for Cl9, while 0.83 from molecule water is disordered over
All compounds 2a–d and 3a–d were elucidated by 1H and 13
C
NMR in solution by 1D and gCOSY, gHSQC, gHMBC ROESY by 2D
NMR studies in CDCl or DMSO d . The chemical shift of all com-
three different positions with relative occupancies 0.33:0.33:0.16
for O2:O3:O4 atoms, respectively. The asymmetric units of crystals
of complex 3b contained half a molecule which are related at a
crystallographic inversion center, the chlorine anion Cl2 is disor-
dered over three positions with relative occupancies 0.60 (3):0.22
3
6
pounds is given in the experimental section and spectra are show-
ing in Supporting information.
0
1
13
(
0
3):0.18(3) for the isotropically refined for Cl2:Cl2a:Cl2b, while
.60 from acetonitrile molecule is observed, respectively. The
3.2.1. H and C NMR spectroscopy of ligands 2a–d
1
The H NMR spectra of all ligands 2a–d were obtained from
structure of 3d was solved by the HKLF5 file. The final refinement
generated a fractional occupancy 0.7821(9) and 0.2179(9), respec-
tively. In 3d, the chlorine anion Cl3 is disordered over two posi-
tions with relative occupancies 0.67(2):0.33(4) for the
isotropically refined for Cl3:Cl3a, respectively. The asymmetric
unit of 3d, is contained three water molecules. Crystallographic
details and structure refinements are presented in Table 1.
3
CDCl . The resonances for the pyridinyl protons and ortho-aromatic
moieties are resolved into four sets located between d ꢂ 8.50–
7.00 ppm. The multiple signal at d ꢂ 8.49 ppm is assigned to the
ortho-proton of the pyridine ring. The other three multiplets
observed at d 7.65–7.53, 7.30–7.26 and 7.20–7.0 ppm, are attribu-
ted to the rest of the pyridine hydrogens and the ortho-aromatic
ring protons. Resonances at d ꢂ 3.85–3.81 ppm (two broad singlets
in 2a and one broad singlet in 2b–d) are assigned to methylene
groups of the dipyridyl fragments (N-CH
lene group bridging the dipyridyl with the ortho-aromatic ring
Ph-CH -N). The alkyl chain protons are displayed as one triplet
(Ar-S-CH and one quintet (Ar-S-CH -CH in 2a and 2b
(d ꢂ 2.96 and 1.88 ppm for 2a and d ꢂ 2.86 and 1.72 ppm for
b) while one triplet (Ar-S-CH ) and two multiplets (Ar-S-CH
CH ; Ar-S-CH -CH -CH ) signals were observed in ligands 2c and
2
-Py) and to the methy-
3
. Results and discussion.
(
2
3.1. Synthesis and characterization of ligands 2a–d and dinuclear
2
)
2
2
)
palladium(II) complexes 3a–d
2
2
2
-
The synthesis of ligands 2a–d and their palladium (II) com-
plexes 3a–d is illustrated in Scheme 1. Ligands 2a–d were obtained
from the reaction between the corresponding dibromide-
dithioether derivatives (1a–d), di-(2-picolyl)amine (1:2 ratio,
2
2
2
2
2d (d ꢂ 2.85, 1.60 and 1.52 ppm for 2c and d ꢂ 2.85, 1.60 and
1.40 ppm for 2d), respectively.
1
3
3
The C NMR spectra of ligands 2a–d in CDCl displayed eleven
respectively) and K
reaction of compounds 2a–d with PdCl
2
CO
3
(in excess) using acetone as solvent. The
signals between d ꢂ 160.0 and 121.0 ppm. Five of those signals
2
ꢁ2CH
3
CN (generated
were attributed to the dipyridyl fragments (d ꢂ 159.5, 1448.8,
Table 1
0
Details of crystal data and structure refinement parameters for 2b, 3b, 3b and 3d.
Compound
2b
3b
3b’
3d
Empirical formula
Mr
T [K]
C
42
H
44
N
6
S
2
C
42
H
44
N
6
S
2
Cl
4
Pd
2
2
ꢃ0.83 H O
C
42
H
44N6S
2
Cl
4
Pd
2
ꢁ1.2 CH
3
CN
C
44
H
48
N
6
O
3
S
2
Cl
4
2
Pd ꢁ3 H
2
O
696.95
293(2)
0.71073
1066.56
293(2)
1.54184
1100.95
293(2)
1.54184
1133.65
293(2)
1.54184
wavelength [Å]
Crystal size [mm ]
3
0.43 ꢀ 0.18 ꢀ 0.12
0.52 ꢀ 0.15 ꢀ 0.09
0.31 ꢀ 0.23 ꢀ 0.18
0.21 ꢀ 0.17 ꢀ 0.13
Crystal system
Space group
a [Å]
b [Å]
c [Å]
Monoclinic
Triclinic
Triclinic
Triclinic
P2
1
/c
Pꢀı
Pꢀı
Pꢀı
14.3542(13)
6.5130(5)
20.5590(19)
90
97.950(9)
90
1903.6(3)
2
1.216
9.1365(3)
17.3553(8)
17.5972(7)
111.734(4)
92.626(3)
95.821(3)
2568.1(2)
2
8.17637(18)
9.5477(2)
16.0887(3)
81.4307(19)
80.8255(17)
75.744(2)
1193.80(5)
1
12.4223(4)
13.7306(5)
15.2065(6)
90.452(3)
102.139(3)
107.436(3)
2412.20(16)
2
a
[°]
b [°]
[°]
c
3
V [Å ]
Z
q
l
3
calc [g/cm ]
1.379
8.598
1.531
9.262
1.561
9.223
ꢄ1
[mm
]
0.178
F(0 0 0)
740.0
1077.0
556.0
1152.0
2
H
range for data collection [°]
6.522 – 52.736
ꢄ17 ꢅ h ꢅ 17
ꢄ8 ꢅ k ꢅ 8
ꢄ25 ꢅ l ꢅ 25
Analytical
60,377
3884, 0.0823
3884/0/226
1.018
6.114 – 155.074
ꢄ10 ꢅ h ꢅ 11
ꢄ21 ꢅ k ꢅ 21
ꢄ22 ꢅ l ꢅ 22
Analytical
37,436
10785, 0.0602
10785/87/595
1.060
9.622 – 136.498
ꢄ9 ꢅ h ꢅ 9
ꢄ10 ꢅ k ꢅ 11
ꢄ19 ꢅ l ꢅ 19
Analytical
67,830
4364, 0.0730,
4364/67/301
1.099
6.768 – 140.256
ꢄ15 ꢅ h ꢅ 15
ꢄ16 ꢅ k ꢅ 16
ꢄ18 ꢅ l ꢅ 18
Analytical
19,387
19,387
19387/9/573
0.973
Index ranges
Abs. correction
Reflections collected
Unique reflections, Rint
Data/restraints/parameters
Goodness-of-fit (GOF) on F
2
R
R
1
, wR
, wR
2
[I ꢆ 2
r
(I)]
0.0423, 0.0953
0.0806, 0.1119
0.15/ꢄ0.16
0.0695, 0.1961
0.0890, 0.2185
1.35/ꢄ0.63
0.0573, 0.1607
0.0623, 0.1684
1.70/ꢄ0.57
0.0643, 0.1730
0.0965, 0.1959
1.61/ꢄ0.53
1
2
[all data]
ꢄ3
Largest diff. peak/hole [e Å
]

4
C.I. Sandoval-Chávez et al. / Polyhedron 182 (2020) 114489
2 3 2
Scheme 1. Synthetic route of ligands 2a–d and bimetallic palladium (II) complexes 3a–d. (i) di-(2-picolyl)amine, K CO , acetone, reflux, 48 h; (ii) PdCl , acetonitrile, reflux,
2
4 h.
1
36.2, 123.1 and 121.9 ppm) and the remaining six signals belong
to the ortho-aromatic rings (d ꢂ 138.5, 136.4–137.0, 129.5, 128.1,
27.4 and 125.3 ppm). At lower frequency, the aliphatic chain pro-
tons (Ar-S-CH and Ar-S-CH -CH ) in compounds 2a and 2b are
located between d ꢂ 32.3 and 28.0 ppm while, the dipyridyl
methylene fragments (N-CH -Py) and the methylene group bridg-
ing the dipyridyl and ortho-aromatic moieties (Ph-CH -N) are
are attributed to the dipyridyl fragments (d ꢂ 164.9, 150.2, 141.2,
125.0 and 123.5 ppm) and six signals for ortho-aromatic ring
(d ꢂ 138.6, 134.6, 130.9, 130.7, 128.6 and 125.9 ppm), respectively.
At lower frequency four signals are observed for compounds 3a
and 3b with d ꢂ 33.1 and 28.0 ppm were assigned to aliphatic
1
2
2
2
2
chain (Ar-S-CH
2
2 2
and Ar-S-CH -CH ) while that the signals
2
d ꢂ 67.8 and 64.4 ppm were associated at methylene groups of
the dipyridyl fragments (N-CH -Py) and to methylene group that
is bridged the dipyridyl fragments with ortho-aromatic ring (Ph-
CH -N), while the compounds 3c and 3d were displaying a plus
one signal to show five signals attributed to the addition to another
methylene group into the aliphatic chain (Ar-S-CH -CH -CH ). The
H and CNMR data thus suggest that the two halves [(CH -S-
S-CH -N-(CH -C N) PdCl ] are magnetically identical in
solution.
observed at d ꢂ 60.1 and 56.9 ppm. As expected, and attributed
2
to the addition to another methylene group into the aliphatic chain
(
Ar-S-CH
located at d ꢂ 28.0 ppm. The symmetrical H and C NMR patterns
indicates that the two halves [(CH –S–C S–CH –N–(CH
] in the ligands 2a–d are magnetically identical in solution.
2
-CH
2
-CH
2
), compounds 2c and 2d display an extra signal
2
1
13
2
)
n
H
6 4
2
2
–
2
2
2
1
13
C
H
5 4
N)
2
2 n
)
C H
6 4
2
2
5
H
4
2
2
1
13
3.2.2. H and C NMR spectroscopy of bimetallic Pd(II) complexes 3a–
d
The 1H NMR spectra of complexes 3a–d were recorded in
3.3. Molecular structures of compounds 2b, 3b, 3b and 3d
0
DMSO d
6
. All spectra shown eight resonances to high frequencies
0
(d ꢂ 8.43–6.91 ppm) that were attributed to two ABCD patterns;
The molecular structures of compounds 2b, 3b, 3b and 3d are
0
one corresponding to ortho-substituted aromatic rings and another
to dipyridinyl fragments. The signal at d ꢂ 8.43–8.30 ppm was
attributed to the ortho-proton of the pyridine ring and the other
three proton sets at d ꢂ 8.00, 7.61, 7.47 ppm completed the ABCD
pattern assigned to the dipyridinyl fragments. Another ABCD pat-
tern is also formed by four proton sets with d ꢂ 7.67, 7.20, 7.13
and 6.95 ppm corresponding to the ortho-aromatic rings. A pair
of doublets typical of an AX or AB spin system centered in the
depicted in Figs. 1–4. Compounds 2b and 3b are centrosymmetric.
Selected bond lengths and angles are given in Supporting
information.
The structure of ligand 2b is shown in Fig. 1 and confirms the
presence of two di-(2-picolyl)amine fragments which are adopting
an anti-arrangement. In each di-(2-picolyl)amine fragment, the
two pyridinic rings and the thiophenolic ring are positioned on
the same side and the distances between their centroids have val-
ues of 5.978 Å/6.199 Å/6.584 Å (see Supporting Information).
The structures of complexes 3b, 3b and 3d are depicted in
Figs. 2–4 and they confirm that each di-(2-picolyl)amine fragment
in 2b and 2d is coordinated to one Pd ion in a tridentate fashion
j -N,N,N). The palladium(II) cen-
ters in 3b, 3b and 3d complete their square planar geometry with
a Cl atom located trans to the N(amine) atom. In general, the bond
distances Pd–Namine are slightly longer than those to the Pd–Npy
range of d 5.70–4.71 ppm with a 2
to the methylene groups of the di-(2-picolyl)amine fragment (N-
CH -Py). The typical methylene AX or AB patterns observed in
JH–H = 16.0 Hz was attributed
0
2
+
2
complexes 3a–d are indicative of diasterotopic protons (consistent
with the coordinating form of the ligand toward the Pd(II) atoms)
which is in agreement with similar palladium(II) complexes bear-
ing the di(2-picolyl)amine group [14h–j]. The resonances of the ali-
phatic chain protons (n-propyl for 3a, n-butyl for 3b, n-pentyl for
3
through its three nitrogen atoms (
0
.
3
1
c and n-hexyl for 3d) are located at low frequencies (d
.4 ppm) and depicted as triplet and quintuplet signals according
neighbors. The methylene group that is link-
H
ꢂ 3.2–
The three Pd–N and Pd-Cl distances are unexceptional and very
similar to those reported for compounds bearing di-(2-picolyl)
amine-Pd fragments in their structure [14h–j].
to the number of CH
2
ing the thiophenol moieties and the dipyridinyl fragments is
observed as a singlet with d ꢂ 2.25 ppm. The above described is
consistent with a highly symmetry in solution in all the bimetallic
Pd(II) complexes.
Both complexes 3b and 3d adopt a syn-arrangement as the two
di-(2-pyridylmethyl)amine-Pd groups and the two sulphur atoms
are positioned on the same side. The Pd1ꢁ ꢁ ꢁPd2 distances are
7.411 Å in 3b and 8.398 Å in 3d, and the S1ꢁ ꢁ ꢁS2 distances are
6.076 Å in 3b and 7.846 Å in 3d, respectively, which is consistent
with the increase of two methylene groups in the aliphatic chain
All 13C NMR spectra of compounds 3a–d in DMSO d
displayed
eleven signals between d ꢂ 165.0 and 123.5 ppm which five signals
6

C.I. Sandoval-Chávez et al. / Polyhedron 182 (2020) 114489
5
Fig. 1. A view of the molecular structure of 2b. Ellipsoids shown at 50% of probability.
Fig. 2. Molecular structure of the bimetallic Pd(II) complex 3b. Solvent molecules, and chloride counteranions have been omitted. Ellipsoids shown at 50% of probability.
0
0
that is bridging the two ortho-aromatic rings. Complex 3b displays
N4–Cl2–N5–N6, respectively), while 3b the two di-(2-pyridyl-
an anti-arrangement with the two di-(2-pyridylmethyl)amine-Pd
methyl)amine-Pd groups are antiparallel because they run in
opposite directions.
fragments and the two sulphur atoms positioned on the opposite
i
i
0
side, and with Pd1ꢁ ꢁ ꢁPd1 and S1ꢁ ꢁ ꢁS1 distances of 12.788 Å and
The molecular structures of complexes 3b, 3b and 3d display
6
.860 Å, respectively. The main structural difference between the
several intramolecular noncovalent interactions including C–
Hꢁ ꢁ ꢁCl–Pd which are promoted by a hydrogen atom of a pyridinyl
ring and chloride atom bonding to palladium atom. Additionally,
0
two polymorphous 3b and 3b is the location acquired by the di-
(
2-pyridylmethyl)amine-Pd fragments. In the case of 3b, the two
di-(2-pyridylmethyl)amine-Pd groups are close to coplanarity with
an angle of 16.75° (formed by the next planes N1–Cl1–N2–N3 and
Sꢁ ꢁ ꢁPd and
p–p stacking interactions between another pyridinyl
ring and the thiophenolic ring are also observed [C–Hꢁ ꢁ ꢁCl–Pd; d

6
C.I. Sandoval-Chávez et al. / Polyhedron 182 (2020) 114489
0
Fig. 3. A view of the molecular structure of the bimetallic complex 3b . Solvent molecules, and chloride counteranions have been omitted. Ellipsoids shown at 50% of
probability.
Fig. 4. A view of the molecular structure of the bimetallic complex 3d. Solvent molecules, and chloride counteranions have been omitted. Ellipsoids shown at 50% of
probability.
(
H31ꢁ ꢁ ꢁCl1) = 2.864 Å, ](C31–H31ꢁ ꢁ ꢁCl1) = 126.86°, ](C31–
H31ꢁ ꢁ ꢁCl1–Pd1) 154.74°, d(H12ꢁ ꢁ ꢁCl2) 2.894 Å, ](C12–
H12ꢁ ꢁ ꢁCl2) = 126.86° and ](C12–H12ꢁ ꢁ ꢁCl2–Pd2) = 156.71° in for
b; d(H34ꢁ ꢁ ꢁCl1) = 2.728 Å, ](C34–H34ꢁ ꢁ ꢁCl1) = 153.50°, ](C34–
H34ꢁ ꢁ ꢁCl1–Pd1)
=
95.59°, d(H2ꢁ ꢁ ꢁCl2)
=
2.977 Å, ](C2–
=
=
H2ꢁ ꢁ ꢁCl2) = 134.4° and ](C2–H2ꢁ ꢁ ꢁCl2–Pd2) = 140.02° in 3d],
[Sꢁ ꢁ ꢁPd; d(S1ꢁ ꢁ ꢁPd1) = 3.236 Å and d(S2ꢁ ꢁ ꢁPd2) = 3.153 Å in 3b; d
0
3
(S1ꢁ ꢁ ꢁPd1) = 3.167 Å in 3b ; d(S1ꢁ ꢁ ꢁPd1) = 3.269 Å and d

C.I. Sandoval-Chávez et al. / Polyhedron 182 (2020) 114489
7
(
S2ꢁ ꢁ ꢁPd2) = 3.213 Å in 3d] and [
p
ꢄ
p
; d(Cgar1ꢁ ꢁ ꢁCgpy1) = 3.876 Å,
cross-coupling reaction leading to desired product in moderate
yield similar at entry 5. Different bases were tested (entries 7–9)
]
]
]
]
]
(Cgar1ꢁ ꢁ ꢁCgpy1
)
=
35.78°, d(Cgar2ꢁ ꢁ ꢁCgpy2
)
=
3.715 and
Å
(Cgar2ꢁ ꢁ ꢁCgpy2) = 29.82° for 3b; d(Cgar1ꢁ ꢁ ꢁCgpy1) = 3.808 Å,
and according to this, K
2 3
CO was selected as the best base for the
0
(Cgar1ꢁ ꢁ ꢁCgpy1) = 33.75° for 3b ; d(Cgar1ꢁ ꢁ ꢁCgpy1) = 4.099 Å,
reaction. The different yields between Na and K carbonates bases
(Cgar1ꢁ ꢁ ꢁCgpy1
)
=
40.10°, d(Cgar2ꢁ ꢁ ꢁCgpy2
)
=
3.707
Å
and
can be due to the different bases character of the MeOH/H
2
O solu-
(Cgar2ꢁ ꢁ ꢁCgpy2) = 27.55° for 3d, respectively] (see Supporting
tions or by affinity of counter cations for halide ions [27].
0
information). The Sꢁ ꢁ ꢁPd distances found for 3b, 3b and 3d are sig-
nificantly shorter than the corresponding sum of the van der Waals
radii [RrvdW (Pd, S) = 3.43 Å] [22] and are similar to those reported
for complexes that display this type of bond [23], the centroide-to-
centroide distances were found to be similar to those proposed for
the plane separated by face-to-face and offset face-to-face interac-
tions [24], the values found for C–Hꢁ ꢁ ꢁCl–Pd interactions are con-
sistent with compounds that exhibit similar type of interactions
Upon optimization of the reaction conditions, the catalytic
activity of 3b, 3c and 3d for Suzuki coupling reactions of iodoben-
zene and phenylboronic acid was investigated (entries 10–12)
(Table 2) and those complexes can generate yields from 55% to
91%, which 3b displayed the highest efficiency. Additionally, two
experiments at different reaction times were carried out (entries
13 and 14) and the reaction is less efficient at 8 h while at 12 h
the yield was very similar than when the reaction was performed
at 18 h. Based in the above results, the next studies in the
Suzuki-Miyaura coupling were performed only employing the 3b
complex. The versatility of the bispalladium(II) complexes 3a–d
as precursors catalytic in Suzuki-Miyaura cross-coupling reaction
was compared with source commercials such as Pd(OAc) and
2
Najera catalysts, and it is noteworthy to mention that these palla-
dium salts given lower yield (entries 15 and 16).
The scope of the catalytic activity of complex 3b towards differ-
ent substrates was investigated, and the results are summarized in
Table 3, all the substrates were readily converted to the corre-
sponding coupling product. Electron-donor groups in aryl iodides
afforded moderate yields (Table 3, entries 1 and 2) and for elec-
tron-withdrawing the corresponding byphenyl was obtained in
[
25].
3
3
.4. Catalysis
.4.1. Suzuki–Miyaura cross coupling with complexes 3a–3d
Complexes 3a–d were tested as catalytic precursors in the
Suzuki-Miyaura cross-coupling reaction. To begin the catalytic
exploration, the Suzuki coupling between iodobenzene and
phenylboronic acid was chosen as a model reaction employing as
catalyst the 3a complex and to examine the influence of solvent,
dimethylformamide, methanol, and methanol/water were studied.
The initial reaction conditions involved 1 mol% loading of 3a,
2 3 3
K CO , PPh , and DMF as solvent at 110 °C for 18 h. According to
8
1% (entry 3). When iodo-toluene was used as substrate with boro-
Table 1 (entry 1), the desired cross-coupling product was obtained
with 10% yield. When the reaction was realized at 75 °C for 18 h,
the yield increases to 20% (entry 2). Switching the solvent to
methanol increases the yield to 35% (Entry 3). With the premise
that cross-coupling reactions are usually accelerated by the use
of aqueous solutions of sodium or potassium carbonate, phosphate,
or hydroxide anions [26]. The next reactions tests were performed
using mixtures of methanol-water in different ratios observing
that the best yield methanol-water ratio was employed (entries 4
and 5) (Table 2). The catalytic activity of complex 3a was evaluated
nic acids containing both electro-donating and electro-withdraw-
ing groups, the cross-coupling product was obtained in similar
yields (70–75%).
To explore the aryl bromides scope several reactions were car-
ried out with phenyl boronic acid. As depicted in Table 3, the reac-
tion from aryl bromides substituted with both electron-donating
and electron-withdrawing groups provided good results (entries
6
–10). As expected, the best yields were obtained when using aryl
bromides with electron-withdrawing groups. When the boronic
acid with an electron deficient group and bromobenzene were cou-
pling partners, the product was obtained in 83% (Entry 11). The
without the presence of PPh
amount of catalyst was 0.5 mol% Pd (entry 6) to conduct
3
and this decreased. When the
Table 2
a
Catalytic Suzuki coupling reactions of iodobenzene with phenyl boronic acid by dicationic palladium(II) complexes (3a–d).
Entry
Complex
%Mol [Pd]
Temperature (°C)
Time (h)
Solvent
Base
Yieldb
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3b
3b
1
1
1
1
110
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
18
18
18
18
18
18
18
18
18
18
18
18
8
DMF
DMF
MeOH
K
K
K
K
K
K
Na
K
NEt
K
K
K
K
K
K
K
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
3
3
3
3
3
10%
20%
35%
45%
57%
55%
40%
25%
30%
91%
81%
76%
55%
89%
22%
27%c
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
MeOH/H
2
O (1:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
2
O (2:1)
1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
3
CO
2 3
3
PO
4
3
0
1
2
3
4
5
6
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
3
12
12
12
Pd(OAc)
Nájera Catalysts
2
a
All reactions were performed with 1 mmol of aryl iodide. 1.2 mmol of phenyl boronic acid, 5 mL of solvent, 1.2 mmol of base and PPh
Isolated yield after silica-gel chromatography.
3
mol 5%.
b

8
C.I. Sandoval-Chávez et al. / Polyhedron 182 (2020) 114489
Table 3
a
Substrate scope for the catalytic Suzuki-Miyaura using complex 3b.
Entry
X
R
1
R
2
R
3
Yieldb
TONc
TOFd
1
2
3
4
5
6
7
8
9
I
I
I
I
H
OMe
COMe
CH
CH
H
OMe
NO
CHO
H
COMe
H
H
H
H
OMe
NO
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
NH
OH
91%
68%
82%
75%
70%
78%
71%
70%
79%
79%
83%
22%
27%
164
135
162
150
140
150
142
155
137
166
155
43
14
11
13
12
12
13
12
13
11
14
13
4
3
I
3
2
Br
Br
Br
Br
Br
Br
Br
Br
2
10
11
12
13
NO
H
H
2
2
H
H
53
4.4
a
b
c
All reactions were carried out using 1 mmol of aryl halide. 1.2 mmol of phenyl boronic acid, 5 mL of solvent, 1.2 mmol of base and PPh
Isolated yields.
TON = ratio of moles of product formed to moles of catalyst used.
TOF = TON/t (h).
3
2
5% mol, in Methanol/H O (2:1).
d
overall results highlight the good catalytic performance of complex
b in the Suzuki-Miyaura reaction employing a wide range of start-
Declaration of Competing Interest
3
ing materials.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
The Suzuki-cross coupling reaction of phenylboronic acid was
also carried out with 2-bromo aniline and 2-bromophenol (Entries
1
2 and 13). However, the ortho substituted aryl bromides were less
reactive generating the cross-coupling products in just 22 and 27%
yields, respectively. Furthermore, exploration of the C-Cl bond acti-
vation using aryl chlorides as coupling partners, resulted in decom-
position of complex 3b to palladium black.
Acknowledgments
This work was supported by the CONACyT – México (Grant CB-
2
011-01-167873 to S.G.M., and Grant Cátedra-CONACyT-2016-222
to C.I.S.C) and by PRODEP-SEP (Grant 511-6/18-8661 to R.V.J.)
4
. Conclusion.
Appendix A. Supplementary data
In conclusion, new symmetric ligands containing two di-(2-
picolyl)amine fragments (2a–d) and their corresponding cationic
dinuclear palladium (II) complexes were synthesized and charac-
terized in solution by NMR spectroscopy and in several cases by
single-crystal X-ray diffraction. The molecular structures of 3b
and 3d demonstrate that the two di-(2-picolyl)amine fragments
are coordinated to a Pd(II) center in a tridentate fashion through
CCDC 1959732, 1959733, 1959734 and 1959735 contains the
supplementary crystallographic data for 2b, 3b, 3b and 3d, respec-
0
tively. These data can be obtained free of charge via http://www.
ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2020.114489.
3
0
00
its three nitrogen atoms (
j
-N,N ,N ) and weak Sꢁ ꢁ ꢁPd interactions
via thiophenolic ring were observed. The catalytic activity of all
cationic bimetallic palladium (II) complexes was explored in
Suzuki-Miyaura coupling at a low catalyst loading (0.5 mol %)
and mild reaction conditions. Complex 3b exhibits the best cat-
alytic activity in the Suzuki-Miyaura coupling reaction of different
aryl iodides or aryl bromides with phenyl boronic acids.
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