Synthesis and properties of novel N,C,N terdentate skeleton based on 1,3-di(pyridin-2-yl)benzene moiety—new tricks for old dogs

Article abstract of DOI:10.1016/j.cclet.2019.08.003

Utilization of intermolecular Friedel–Crafts and intramolecular condensation reaction, novel 1,3-di(pyridine-2-yl)benzene(N,C,N terdentate) skeleton with electro-withdrawing group in 6' position of pyridyl and a cyclization between 6' position of pyridyl and 6 position of benzyl ring were firstly designed and synthesized. The structures of these novel N,C,N terdentate were confirmed by NMR, MS and X-ray single crystalanalyses. The photophysical properties of these compounds were briefly explored.

Full text of DOI:10.1016/j.cclet.2019.08.003

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CCLET 5154 No. of Pages 4
Chinese Chemical Letters xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Synthesis and properties of novel N,C,N terdentate skeleton based
on 1,3-di(pyridin-2-yl)benzene moiety—new tricks for old dogs
Shuai Tanb,1, Xiugang Wua,1, Yanqiong Zhengc, Yafei Wanga,b,
*
a
Jiangsu Engineering Laboratory of Light-Electricity-Heat Energy-Converting Materials and Applications, Jiangsu Key Laboratory of Environmentally Friendly
Polymeric Materials, Jiangsu Collaboration Innovation Center of Photovoltaic Science and Engineering, School of Materials Science & Engineering, Changzhou
University, Changzhou 213164, China
b
College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application of the Ministry of Education, Xiangtan University, Xiangtan 411105, China
Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, Shanghai 200072, China
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Available online xxx
Utilization of intermolecular Friedel–Crafts and intramolecular condensation reaction, novel 1,3-di
(pyridine-2-yl)benzene(N,C,N terdentate) skeleton with electro-withdrawing group in 6' position of
pyridyl and a cyclization between 6' position of pyridyl and 6 position of benzyl ring were firstly designed
and synthesized. The structures of these novel N,C,N terdentate were confirmed by NMR, MS and X-ray
single crystalanalyses. The photophysical properties of these compounds were briefly explored.
© 2019 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
1,3-Di(pyridin-2-yl)benzene skeleton
Platinum complex
Synthesis
Intramolecular cyclization
With rigid conjugated structure, unique metal-carbon bond, and
a molecular structurethat can be facilely modified,1,3-di(pyridin-2-
yl)benzene, the so-called “pincer” ligand, has wide applications in
the synthesis of inorganic and organometallic complexes [1–4]. It
possesses N,C,N terdentate potentially available for coordination,
where N is a neutral donor and C represents an anionic aryl carbon
atom. Over the past four decades, considerable amount of
organometallic derivatives, such as those of platinum, iridium,
palladium, ruthenium that are based on 1,3-di(pyridin-2-yl)
benzene pincer ligand have been reported as emissive materials,
dye-sensitizers of solar cells, catalysts and so on [5–17]. Notably,
modification of N,C,N pincer ligand has an obvious effect on the
metal center, and there are strategies for the functionalization of
1,3-di(pyridin-2-yl)benzene. As shown in Fig. 1, substituents
including electron-rich and electron-withdrawing units, such as
methyl, halogen, cyano, ester and so on, are commonly introduced
to the benzylic positions (1) (i.e., 4-, 5- and 6-position) [10,11,18–
26]. In contrast, researches on the integration of substituents into
the pyridyl positions (1) (i.e., 3'-, 4'- and 5'-position) are few
[18,23,27,28]. In addition, there are investigations on isoquinonyl
replacement of pyridyl in N,C,N pincer ligand (2) [23,29–31],
However, the N,C,N ligands with electro-withdrawing group in 6'
position of pyridyl and a cyclization between 6' position of pyridyl
and 6 position of benzylic ring are rare, most probably due to
difficulty of synthesis.
In this study, we prepared a series of N,C,N pincer ligands
bearing an electro-withdrawing group in the 6' position of pyridyl
(3) via intermolecular Friedel–Crafts (F–C), and recorded the first
cyclization example of N,C,N pincer ligand (4) through intramo-
lecular condensation reaction. These functionalized pincer ligands
provide access to a large group of interesting acceptor moiety/or
organometallic complexes for optoelectronic application. Further-
more, we chose platinum atom as metal center andsynthesized
two platinum complexes using these novel N,C,N terdentate
ligands.
Initially, we used commercially available 2-bromonicotinic acid
(5) as starting material to afford the cyclized pincer ligand
(Scheme 1). In view of the formation of byproducts in the reaction
between compound 5 and methanol, we prepared 2-bromonico-
tinate (6) via the following optimized procedures: i) The
intermediate of 2-bromonicotinoyl chloride was synthesized by
acyl chlorination between compound 5 and oxalyl chloride; ii)
Subsequent esterification reaction was carried out between the
acyl chloride intermediate and methanol at room temperature for
2 h, furnishing methyl 2-bromonicotinate (6) in 72% yield. Using Pd
(dppf)Cl2 as catalyst, 1,3-phenylene diboronic acid reacted with
compound 6 to afford compound 7 in 74% yield. The key precursor
(8) was generated through acidification of compound 7 in ethanol/
H2O under reflux condition. The acyl chloride intermediate was
leading to enhancement of conjugated
p system. All the
methodologies can realize fine-tuning of electronic properties.
* Corresponding author.
E-mail address: qiji830404@hotmail.com (Y. Wang).
The authors contributed equally to this work.
1
https://doi.org/10.1016/j.cclet.2019.08.003
1001-8417/© 2019 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Tan, et al., Synthesis and properties of novel N,C,N terdentate skeleton based on 1,3-di(pyridin-2-yl)
benzene moiety—new tricks for old dogs, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.08.003

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Fig. 1. Potential modification sites in 1,3-di(pyridine-2-yl)benzene.
Scheme 1. Synthesis of 5-11. Reaction conditions: (a) (i) (COCl)2, r.t., 2 h; (ii) MeOH,
r.t., 2 h; (b) 1,3-phenylenediboronic acid, Pd(dppf)Cl2, K2CO3, THF/H2O, 85 ꢀC, 24 h;
(c) NaOH aq., ethanol, 80 ꢀC, overnight; (d) (i) SOCl2, 70 ꢀC, 24 h; (ii) AlCl3,
chlorobenzene, 135 ꢀC, overnight.
prepared by reacting compound 8 with SOCl2, and was used for the
next step without further purification. The most critical step, the
intramolecular cyclization reaction, was conducted via F–C
reaction using chlorotoluene as solvent and AlCl3 as catalyst at
135 ꢀC overnight. Surprisingly, isomer products 9-11 were obtained
instead of the expected cyclization compound. In order to purify
these isomers, HPLC with water/acetonitrile (v/v = 30:70) eluent
was carried out. The ratio of the three isomers is roughly 22:10:1
after purification.
Scheme 2. Synthesis of compound 18. Reaction conditions: (a) KMnO4, BuOH, H2O;
(b) (COCl)2, methanol; (c) Pd(PPh3)2Cl2, 2-(tributylstannyl)pyridine, LiCl; (d) LiOH,
THF, methanol, H2O; (e) LDA, DIEPA, THF.
All new compounds were confirmed by 1H NMR, 13C{1H} NMR,
1
and TOF-MS (Supporting information). 1HÀ H COSY and NOESY
NMR spectra were also recorded to fully elucidate the structure of
compounds 9 and 10 and to unambiguously assign the resonance
peaks (Supporting information). Single crystal of compound 9 was
grown by slow evaporation of a methanol/CH2Cl2 solution of the
compound at room temperature (v/v = 1:1, Fig. 2 and Supporting
information). The crystal system is triclinic with P-1 space group,
which shows a distorted structure for compound 9.
To further gain insight into the cyclic N,C,N pincer ligand, 1,5-
dibromo-2,4-dimethylbenzene (12) was selected as a simplified
starting material. Indaceno[1,2-b:7,6-b']dipyridine-5,7-dione (18)
could be prepared by two synthetic routes (Scheme 2). First, the
oxidation of compound 12 catalyzed by KMnO4 afforded 4,6-
dibromoisophthalic acid (13), which was subsequentlysubjected to
esterification and Still coupling reaction using 2-(tributylstannyl)
Please cite this article in press as: S. Tan, et al., Synthesis and properties of novel N,C,N terdentate skeleton based on 1,3-di(pyridin-2-yl)
benzene moiety—new tricks for old dogs, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.08.003

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3
Fig. 3. Crystal structure of compound 18: (a) top view; (b) side view; (c) packing
diagram along the c-axis. H atoms are omitted.
pyridine, Pd(PPh3)2Cl2 and LiCl to yield compound 15. The key
precursor 17 was generated in good yield through the hydrolysis of
compound 15 using LiOH as base. Second, an optimized two-step
route for obtaining 17 was carried out. Still coupling reaction of 12
and 2-(tributylstannyl)pyridine catalyzed by Pd(PPh3)2Cl2 and LiCl
yielded compound 16 which was subjected to an oxidation reaction
to afford 17. Finally, the cyclizationproduct 18 was obtained from 17
using LDA and N,N-diisopropylethylamine in THF solution in 44%
yield.
All new compounds in Scheme 2 were confirmed by 1H NMR,
13C{1H} NMR, and TOF-MS (Supporting information). Single crystal
of product 18 suitable for X-ray crystallography were obtained
from a methanol/CDCl3 (v/v = 2:1) solution of 18. As shown in
Fig. 3, compound 18 exhibits a monoclinic crystal system with P21/
n space group which is different from that of compound 9.
Compound 18 displays a planar conjugated structure with four
molecules in a unit cell while there are only two molecules in a unit
cell for compound 9. Interestingly, compound 18 possesses a good
ordered packing structure with intermolecular distance of 3.49 Å.
In order to check the complexation property of these pincer
ligand, reacting N,C,N ligands 9, 10 and 18 with K2PtCl4 in acetic
acid and H2O at 120 ꢀC for three days, platinum complexes 20 and
21 were obtained and characterized by 1H NMR, 13C{1H} NMR, TOF-
MS and elemental analysis (Supporting information). In contrast,
the reaction of 18 with K2PtCl4 failed to produce complex 22
plausibly due tothe large NCN angle (143ꢀ, Scheme 3) of compound
18. For comparison, the NCN angle of 1,3-di(pyridin-2-yl)benzene)
is 121ꢀ.
Scheme 3. Synthesis of 20–22. (a) K2PtCl4, CH3COOH, H2O, 120 ꢀC, 3 days.
To explore the structure–property relationship of the N,C,N
ligands and their corresponding platinum complexes, their
photophysical properties were studied via density functional
theoretical (DFT) calculation using Gaussian 09 program
(
Supporting information), as well as by UV-vis absorption and
photoluminescence analyses. Analogous electron distribution of
highest occupied molecular orbital (HOMO) and lowest unoccu-
pied molecular orbital (LUMO) was observed for compounds 9 and
10 (Supporting information). The HOMOs were found to be mainly
localized on the entire 1,3-di(pyridin-2-yl)benzene moiety as well
as one of 4-acetylchlorobenzene units, whereas the LUMOs
completely localized on one of the pyridine and 4-acetylchlor-
obenzene units. After cyclization, both HOMO and LUMO
distributions in compound 18 are entirely localized on the s-
indaceno[1,2-b:7,6-b']dipyridine-5,7-dione skeleton. The electron
distribution of platinum complexes 20 and 21 are significantly
different from that of N,C,N ligand, with HOMOs predominantly
distribute over the 1,3-di(pyridin-2-yl)benzene moiety, platinum
atom and chloro ligand, while LUMOs distribute mainly over the
entire N,C,N pincer ligand.
Fig. 4. UV absorption (a, b) and PL spectra (c, d): (a) compounds 9, 10 and 18 in
CH2Cl2; (b) compounds 20 and 21 in CH2Cl2; (c) compounds 9 (lex = 310 nm), 10
(lex = 310 nm) and 18 (lex = 400 nm) in CH2Cl2; (d) lex = 450 nm, compounds 20 and
21 in CH2Cl2.
at 264 nm is attributed to
and 340 nm is ascribed to n-
p
-
p
* transition. The one between 280 nm
p
* transition, whereas the one at about
368 nm to intramolecular charge-transfer transition. The absorp-
tion spectra of the platinum complexes are significantly different
from that of the N,C,N ligand (Fig. 4b). Beside the signal of
p-
p*
transition at about 266 nm, there are overlapping bands of metal-
to-ligand and ligand-to-center charge transfer between 350 nm
and 480 nm. Additionally, complex 21 presents a little red-shifted
absorption bands in long-wavelength than that of complex 20. In
solution, both 9 and 10 display similar emission spectra with
maximum emission at 365 nm, whileclear red-shift emission
(518 nm) is observed over compound 18. Similar with the
absorption spectra, platinum complex 20 (510 nm and 534 nm,
As shown in Fig. 4, in solution compounds 9 and 10 show similar
absorption spectra with a maximum absorption peak at 262 nm
which is assignable to
presents a spectrum with three distinct absorption bands. The one
p-p
* transition of aryl skeleton. Cyclized 18
7.28
ms, see Supporting information) also shows a red-shifted
Please cite this article in press as: S. Tan, et al., Synthesis and properties of novel N,C,N terdentate skeleton based on 1,3-di(pyridin-2-yl)
benzene moiety—new tricks for old dogs, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.08.003

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emission spectrumin solution compared to complex 21 (540 nm,
8.44 s, Supporting information).
Appendix A. Supplementary data
m
The electrochemical properties of the new compounds (9, 10,
18, 20 and 21) were measured by using cyclic voltammetry (CV) in
CH3CN solution. All compounds only display the irreversible
Supplementarymaterialrelatedtothisarticlecanbefound, inthe
online version, at doi:https://doi.org/10.1016/j.cclet.2019.08.003.
oxidation potential (Eox) in the range of 0.5–1.3 V (vs. Fc/Fc+, Eox
References
FC/FC
+ = 0.48 V, Supporting information). According to the formula of
EHOMO = -(Eox+4.8) eV, the HOMO energy levels were calculated to
be -5.86 eV, -5.96 eV, -6.02 eV, -5.75 eV and -5.34 eV for com-
pounds 9, 10, 18, 20 and 21, respectively. Based on the optical band
gap and HOMO energy levels, the LUMO energy levels are -2.07 eV
for compound 9, -2.23 for compound 10, -4.73 eV for compound 18,
-3.15 eV for compound 20and -2.72 eV for compound 21.
In conclusion, novel 1,3-di(pyridin-2-yl)benzene skeleton with
electro-withdrawing group at 6' position of pyridyl moiety and the
cyclization derivative between 6' position of pyridyl and 6 position
of benzylic ring were synthesized by means of intermolecular F-C/
intramolecular condensation, respectively. Isomers 9–11 were
purified successfully via HPLC method. All the novel N,C,N
terdentate skeletons were confirmed by 1H NMR, 13C{1H} NMR,
MS or X-ray single crystal analysis. Further reaction of compounds
9 and 10 with potassium tetrachloroplatinate(II) resulted in the
formation of platinum complexes 20 and 21 as orange dyes. The
present study provides a protocol to afford novel acceptor moiety/
or organometallic complexes for optoelectronic application.
Further investigation on the complexation of the cyclization
ligand is in progress.
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Acknowledgments
Financial support was from the National Natural Science
Foundation of China (Nos. 51773021, 51911530197, U1663229,
51473140), Six talent peaks project in Jiangsu Province (No. XCL-
102), the Talent project of Jiangsu Specially-Appointed Professor,
Natural Science Fund for Colleges and Universities in Jiangsu
Province (No. 19KJA430002), the Open Fund of Key Laboratory of
Advanced Display and System Applications of Ministry of
Education, Shanghai University and Natural Science Foundation
of Hunan Province (No. 2017JJ2245).
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Please cite this article in press as: S. Tan, et al., Synthesis and properties of novel N,C,N terdentate skeleton based on 1,3-di(pyridin-2-yl)
benzene moiety—new tricks for old dogs, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.08.003