Synthesis of terpyridine-containing Pd(II) complexes and evaluation of their catalytic activity

Article abstract of DOI:10.1016/j.molstruc.2019.126925

Herein, we prepare two terpyridine-containing Pd(II) complexes, [PdClL₁]?solvent (A₁) and [PdClL₂]?2H₂O (A₂) (L₁ = 4’-(4-carboxyl-phenyl)-2,2’:6′,2″-terpyridine, L₂ = 2,6-bis(2-pyrazinyl)-4-(4-carboxyl-phenyl)pyridine), from 4’-(4-cyanophenyl)-2,2’:6′,2″-terpyridine(L_1a)/2,6-bis(2-pyrazinyl)-4-(4-cyanophenyl)pyridine (L_2a) and Pd(II) acetate and characterise them by several instrumental techniques. A₁ and A₂ are shown to be good catalysts for the coupling of 2-iodobiphenyl with iodobenzenes to afford triphenylenes, which is known to involve dual C–H bond activation and double C–C bond formation. The obtained data suggest that the mechanism of A₁-and A₂-mediated coupling may be similar to the reference Pd catalysts, A1 and A2 are also suitable catalysts for this cyclization process. Study on this kind of complexes is of importance to the development of novel Pd-based catalysts and triphenylene synthesis techniques.

Full text of DOI:10.1016/j.molstruc.2019.126925

Journal of Molecular Structure 1198 (2019) 126925
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis of terpyridine-containing Pd(II) complexes and evaluation of
their catalytic activity
Sanmao Yin, Xiaomin Wang, Jun Jiang, Hongping Xiao, Xinhua Li*
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang, 325035, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein, we prepare two terpyridine-containing Pd(II) complexes, [PdClL1]∙solvent (A1) and [PdClL2]
∙2H2O (A2) (L1 ¼4’-(4-carboxyl-phenyl)-2,2’:60,200-terpyridine, L2 ¼ 2,6-bis(2-pyrazinyl)-4-(4-carboxyl-
Received 3 July 2019
Received in revised form
9 August 2019
Accepted 12 August 2019
Available online 14 August 2019
phenyl)pyridine),
from
4’-(4-cyanophenyl)-2,2’:60,200-terpyridine(L1a)/2,6-bis(2-pyrazinyl)-4-(4-
cyanophenyl)pyridine (L2a) and Pd(II) acetate and characterise them by several instrumental tech-
niques. A1 and A2 are shown to be good catalysts for the coupling of 2-iodobiphenyl with iodobenzenes
to afford triphenylenes, which is known to involve dual CeH bond activation and double CeC bond
formation. The obtained data suggest that the mechanism of A1-and A2-mediated coupling may be
similar to the reference Pd catalysts, A1 and A2 are also suitable catalysts for this cyclization process.
Study on this kind of complexes is of importance to the development of novel Pd-based catalysts and
triphenylene synthesis techniques.
Keywords:
Terpyridine-containing Pd(II) coordination
Crystal structure
Catalyst
© 2019 Elsevier B.V. All rights reserved.
Cross-coupling reaction
Triphenylene synthesis
1. Introduction
available stepwise assembly processes, seldom Pd2þ-terpyridine
complexes have been reported to date [29e33]. Herein, we describe
Multitopic tectons based on [2,2’:60,2’’]-terpyridine (tpy), a
ubiquitous tridentate chelating ligand, have recently attracted
increased attention in view of their ability to coordinate diverse
the synthesis, characterization, and catalytic activity of two func-
tional complexes of Pd2þ containing 4’-(4-carboxyl-phenyl)-
2,2’:60,200-terpyridine
(L1)
and
2,6-bis(2-pyrazinyl)-4-(4-
metals, facilitating both labile (Zn2þ, Cd2þ) and non-labile (Ru2þ
,
carboxylphenyl) pyridine (L2) (Scheme 1). The square planar
Pd2þ-terpyridine complexes are very good cross-coupling catalysts.
Complexes A1 and A2 were separately prepared using 4’-(4-
cyanophenyl)-2,2’:60,200-terpyridine (L1a) and 2,6-bis(2-pyrazinyl)-
4-(4-cyanophenyl)pyridine (L2a) as precursor ligands and Pd(II)
acetate (Pd(OAc)2) as a Pd source, and the structures of these
complexes were probed by several instrumental techniques.
Moreover, we investigated the ability of A1 and A2 to catalyse the
formation of triphenylene via CeC coupling of 2-iodobiphenyl with
iodobenzenes.
Os2þ) tpy-M2þ-tpy connectivity and thus permitting control over
bond strength and, ultimately, over molecular architecture [1].
Consequently, pseudo-octahedral tpy-M2þ-tpy moieties were
widely employed as monomers of diverse metallodendrimers [2],
racks [3], and grids [4], and were also popular components of
macrocycles, cages, and diverse polyhedra [5]. Moreover, the
unique structure and properties of terpyridine-based functional
complexes have been used in the fields of optics [6e10], magnetic
material synthesis [11,12], supramolecular chemistry [13,14],
biochemistry [15], catalysis [16,17], and anticancer drug design
[18,19].
2. Results and discussion
Pentacoordinated polypyridyl Cu(II) coordination complexes
can efficiently cleave DNA [20e23], while square planar Pt-
terpyridine complexes are very good G-quadruplex binders
[24e28]. However, despite the abundance of functional complexes
comprising terpyridine-based monomers and the large variety of
2.1. Structural characterization
Fig. S1 shows the experimental and simulated powder XRD
patterns of A1. The experimental and simulated patterns were in
good agreement, which indicated that the title compound featured
one homogeneous phase. TGA of L1a and A1 (Fig. S2) showed that
the latter was more thermally stable than the former, i.e., A1 was
confirmed to be a coordination complex of Pd. The observation of
* Corresponding author.
E-mail address: lixinhua01@126.com (X. Li).
https://doi.org/10.1016/j.molstruc.2019.126925
0022-2860/© 2019 Elsevier B.V. All rights reserved.

2
S. Yin et al. / Journal of Molecular Structure 1198 (2019) 126925
complexes has seldom to been reported yet.
Since triphenylenes can be synthesised from readily available 2-
iodobiphenyls and iodobenzenes under the conditions of Pd
catalysis [41], we attempted to employ A1 and A2 as catalysts of the
cross-coupling reaction between 2-iodobiphenyl and iodobenzenes
under mild conditions without the use of expensive and toxic dppf
ligand and a protective N2 atmosphere. To our delight, the reaction
of 2-iodobiphenyl with iodobenzene catalysed by A1([PdClL1]∙sol-
vent) and A2 ([PdClL2]∙2H2O) in dimethylformamide (DMF) under
mild conditions afforded triphenylene in isolated yields of 34 and
22%, respectively (Table 1, entry 1). The obtained product was
characterised by NMR, and its physical and spectral data were
compared to those reported in literature (Fig. S7) [41]. The above
coupling reaction was used to determine the optimal reaction
conditions. First, we investigated the effect of catalyst loading
(Table 1), showing that the highest yield was obtained at 10 at% A1
or A2, with A1 featuring a higher catalytic activity than A2. Hence,
subsequent experiments were performed with 10 at% A1.
Scheme 1. Structures of complexes A1, A2 and precursor ligands L1a and L2a
.
CeO (1399e1608 cmꢀ1) and C]O (1696.6 cmꢀ1) bands in the FT-IR
spectrum of A1 (Fig. S3) demonstrated that A1 comprised Pd ions
and 4’-(4-carboxyl-phenyl)-2,2’:60,200-terpyridine (L1), i.e., the eCN
group was converted into a eCOOꢀ group [34]. In support of this
assumption, the C]O band was blue-shifted because of the
conjugation between the carboxyl group and the benzene ring. The
absence of characteristic eCOOH bands at 1710 cmꢀ1 indicated that
carboxylate groups in A1 were completely deprotonated, as was
further confirmed by single-crystal XRD.
Single-crystal XRD analysis revealed that A1 crystallised in the
monoclinic C2/c space group and featured a structural unit con-
taining one crystallographic Pd2þ ion, one L1 ligand, and one Clꢀ
anion (Fig. 1). Each Pd2þ ion exhibited a distorted square-planar
geometric configuration and was coordinated by Clꢀ (Cl1) and
three nitrogen atoms (N1, N2, and N3) of the terpyridine moiety of
L1. Thus, the Pd2þ ion was coordinatively unsaturated, which was of
key importance for the catalytic activity of A1. L1 acted as a tri-
dentate ligand, and its carboxylate group was non-coordinated but
deprotonated to achieve charge balance. PdeN bond lengths lied in
the range of 1.929(6)e2.016(7) Å. The torsion angle between the
phenyl and the terpyridine planes was 37.53(8) degree, and the
torsion angle between the carboxylate and the phenyl was
174.61(7) degree. The structure of A2 was found to be similar to that
of A1 (Figs. S4eS6). The PdeCl bond lengths in A1 and A2 were
2.284(2) Å and 2.025 (3) Å, respectively.
Second, we probed the effect of base type/loading, revealing that
although strong bases were more effective than weak bases,
decreased yields were observed for very strong bases (Table 2,
entry 2). As has been described in literature [41], the highest yield
was obtained in the presence of Na2CO3 and CsOAc (entry 1).
Finally, we optimised reaction time, temperature, and solvent, and
the optimum reaction conditions were identified as iodobenzene (2
equiv), 2-iodobiphenyl (1 equiv), Na2CO3 (2 equiv), CsOAc (3 equiv),
A1 or A2 (10 at%) in DMF at 120 ꢁC.
Having developed an efficient procedure for the synthesis of
triphenylene, we investigated the corresponding substrate scope.
As A1 featured a higher catalytic activity than A2, only the former
catalyst was employed in the following reactions.
2.2. Dual CeH bond activation and double CeC bond formation
Table 1
There has been ongoing interest in the development of triphe-
nylenes [35], as triphenylenes were widely employed to prepare
functional organic materials such as discotic liquid crystals and
organic light-emitting diodes [36e39]. Among the variety of tri-
phenylene synthesis strategies, those based on Pd catalysis have
gained great attention, offering ease of implementation and high
efficiency [40]. However, CeC coupling catalysed by Pd-terpyridine
Effect of catalyst loading on triphenylene yield.
Entry
Catalyst (mmol%)
Product (A3)
yield A1 (%)
yield A2 (%)
1
2
3
5
10
15
3aa
3aa
3aa
34.2
50.8
45.4
22(0.5a)
38(0.8)
26(0.6)
a
Margins of error for multiple experiments.
Fig. 1. Coordination environment of Pd(II) in A1 (left) and A2 (right), with displacement ellipsoids drawn at the 30% probability level.

S. Yin et al. / Journal of Molecular Structure 1198 (2019) 126925
3
Table 2
6e8), and reactions of iodobenzenes containing m- or p-methyl
groups furnished the same product, 3gg (entries 7 and 8). The re-
actions of iodobenzenes bearing electron-withdrawing groups such
as CF3 and Cl were low-yielding (entries 5 and 7), which was mainly
ascribed to the large extent of concomitantly occurring homocou-
pling to form biphenyl byproducts. In fact, the yield of tripheny-
lenes could be improved in all cases when iodobenzenes were used
in excess. While the detailed mechanism of the catalysis reaction
remains to be investigated and other reaction pathways cannot be
ruled out, based on our previous works and reference [41], a similar
tentative mechanism was proposed as shown in Supporting
Information.
Effect of base type and loading on triphenylene yield.
Entry
Base
Yield(%)
1
2
3
4
Na2CO3(2equiv) þ CsOAc(3equiv)
NaOH(4equiv) þ CsOAc(3equiv)
Na2CO3(2equiv) þ Cs2CO3 (1.5equiv)
Cs2CO3 (3.5equiv)
51(0.6)
38(0.9)
trace
trace
Table 3 lists the results obtained when several iodobenzenes
were coupled with 2-iodobiphenyl to assess the catalytic activity of
A1. Notably, 2-iodobiphenyl smoothly reacted with most iodo-
benzene derivatives to afford triphenylenes in moderate to good
yields, and iodobenzenes bearing electron-donating groups were
found to be more reactive than those with electron-withdrawing
groups. Both chloro and fluoro groups were tolerated (entries
3. Material and methods
3.1. Chemicals
All chemicals (AR grade) were obtained from commercial
sources and used without further purification. Petroleum ether
refers to the fraction with a boiling point of 60e90 ꢁC. Reaction
progress was monitored by thin layer chromatography (silica gel,
Polygram SILG/UV 254 plates). Column chromatography was per-
formed using Silicycle silica gel (200e300 mesh).
Table 3
Results of CeC coupling reactions catalysed by A1.
3.2. Preparation of A1 and A2
Entry
1
Substrate(2a)
Product
Yield(%)
71(0.6)
Pd(OAc)2 and (0.205 g, 1 mmol) was dissolved in HOAc (3 mL) to
afford solution A, while L1a/L2a (0.336 g, 1 mmol/0.334 g, 1 mmol)
was suspended in pure water (15 mL) at room temperature to afford
solution B. Subsequently, solution A was dropwise added to solu-
tion B, and the resulting mixture was treated with 30 wt% HCl
(2 mL) and stirred for 30 min to afford a light red suspension. The
suspension was transferred to a polytetrafluoroethylene (PTFE)-
lined 50-mL stainless steel reactor and heated to 150 ꢁC at a rate of
10 ꢁC hꢀ1. The above temperature was maintained for 72 h, and tꢀh1e
2
3
73(0.2)
56 (0.3)
reactor was then cooled to room temperature at a rate of 5 ꢁC h
.
The product was washed with 10 wt% sodium hydroxide solution to
furnish light red acicular crystals (0.170 g/0.166 g, yield ¼ 83%/81%
based on Pd(OAc)2) that were dried at 100 ꢁC overnight.
4
31(0.8)
3.3. Characterization
Melting points (uncorrected) were determined on an Omega
MPS10 digital melting point apparatus. 1H and 13C NMR spectra
were recorded on a Bruker DRX-500 (500 MHz) spectrometer in
CDCl3 using tetramethylsilane (TMS) as an internal reference. All
chemical shifts were reported relative to that of TMS (0.0 ppm). All
compounds (known and unknown) were identified by 1H and 13C
NMR. The physical and spectral data of known compounds were
compared with those reported in literature. Single-crystal X-ray
diffraction (XRD) patterns were recorded on a Bruker Smart-1000
CCD diffractometer, while powder XRD measurements were per-
formed on a Bruker D8 Advance diffractometer at 40 kV and 40 mA
5
6
49(0.7)
53 (0.6)
7
39(1.0)
using Cu Ka
(
l
¼ 15.406 nm) radiation, a scan step of 0.01ꢁ, and a
scan speed of 0.1 s stepꢀ1. Thermogravimetric analysis (TGA) was
performed under N2 at a heating rate of 5 ꢁC minꢀ1 using a TA-Q600
system (TA Instruments, USA). The thermal stability of the as-
synthesised complex precursor was investigated in the range be-
tween ambient temperature and 800 ꢁC. Fourier transform infrared
(FT-IR) spectra were recorded on a Thermo Scientific Nicolet iS10
spectrometer in the range of 4000e400 cmꢀ1 using the KBr pellet
technique. Elemental analyses (C and H) were carried out on a
Carlo-Erba 1112 Elemental Analyzer.
8
3gg
45 (0.9)
Unless otherwise noted, the following conditions were used: 1: 0.2 mmol, 2:
0.4 mmol, catalyst: 10 at%, Na2CO3: 2.0 equiv, CsOAc: 3.0 equiv, DMF: 3 mL, 120 ꢁC,
24 h.

4
S. Yin et al. / Journal of Molecular Structure 1198 (2019) 126925
(2003) 10257e10265.
3.4. General experimental procedure for the catalytic coupling of 2-
iodobiphenyl with iodobenzenes
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tubular reactor was charged with Na2CO3 (0.0424 g,
0.4 mmol), CsOAc (0.1152 g, 0.6 mmol), 2-iodobiphenyl (0.0560 g,
0.2 mmol), iodobenzene (0.0816 g, 0.4 mmol), A1 (10 at%), and sol-
vent (3 mL), sealed, and heated to 120 ꢁC in an oil bath for 24 h.
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brine (20 mL) and ethyl acetate (40 mL). The organic layer was
retained, and the aqueous phase was extracted with ethyl acetate
(2 ꢂ 40 mL). All organic phases were combined, dried over anhy-
drous MgSO4, and filtered. The desired product was separated via
vacuum distillation and was further purified by silica gel thin layer
chromatography using petroleum ether as an expansion agent. The
product-containing silica gel was scraped off and pulverised. The
powder was leached with ethyl acetate for 4 h, and the leachate was
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In conclusion, we prepared and characterised Pd(II)-terpyridine
coordination complexes A1 and A2, showing that they effectively
catalyse the cross-coupling of iodobenzenes and 2-iodobiphenyl to
afford asymmetrically functionalised triphenylenes. The square-
planar and unsaturated Pd2þ ion of Pd2þ-terpyridine complexes
play key role for their catalysis properties. The above reaction is
thought to involve two Pd-catalysed CeH bond activations and two
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and step economy. Contrary to previously reported Pd-based cat-
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toxic dppf as a ligand and a protective N2 atmosphere. Therefore,
this study reports another catalyst for this cyclization process and it
is valuable to the development of novel routes to polycyclic aro-
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Notes
The authors declare no competing financial interest.
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