Synthesis of 2,6-di(1,8-naphthyridin-2-yl)pyridines functionalized at the 4-position: Building blocks for suitable metal complex-based dyes

Article abstract of DOI:10.1080/00397911.2019.1597123

This study reports the synthesis of a methoxy-substituted 2,6-di(1,8-naphthyridin-2-yl)pyridine using Friedl?nder methodology. The functionalization at the 4-carbon of the methoxy-substituted derivative was confirmed by X-ray structural analysis. Finally, the methyl ether protecting group was cleaved to obtain 2,6-di(1,8-naphthyridin-2-yl)pyridine-4-ol. Using the compounds, coordination behavior to ruthenium(II) center was also examined.

Full text of DOI:10.1080/00397911.2019.1597123

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Synthesis of 2,6-di(1,8-naphthyridin-2-yl)pyridines
functionalized at the 4-position: Building blocks for
suitable metal complex-based dyes
Shunsuke Nakamura, Tsugiko Takase & Dai Oyama
To cite this article: Shunsuke Nakamura, Tsugiko Takase & Dai Oyama (2019): Synthesis of 2,6-
di(1,8-naphthyridin-2-yl)pyridines functionalized at the 4-position: Building blocks for suitable metal
complex-based dyes, Synthetic Communications, DOI: 10.1080/00397911.2019.1597123
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2019.1597123
Synthesis of 2,6-di(1,8-naphthyridin-2-yl)pyridines
functionalized at the 4-position: Building blocks for
suitable metal complex-based dyes
Shunsuke Nakamuraa, Tsugiko Takaseb, and Dai Oyamac
b
aGraduate School of Science and Engineering, Fukushima University, Fukushima, Japan; Institute of
c
Environmental Radioactivity, Fukushima University, Fukushima, Japan; Department of Industrial
Systems Engineering, Cluster of Science and Engineering, Fukushima University, Fukushima, Japan
ABSTRACT
ARTICLE HISTORY
Received 21 November 2018
This study reports the synthesis of a methoxy-substituted 2,6-di(1,8-
naphthyridin-2-yl)pyridine using Friedlander methodology. The func-
€
tionalization at the 4-carbon of the methoxy-substituted derivative
was confirmed by X-ray structural analysis. Finally, the methyl ether
protecting group was cleaved to obtain 2,6-di(1,8-naphthyridin-2-
yl)pyridine-4-ol. Using the compounds, coordination behavior to
ruthenium(II) center was also examined.
KEYWORDS
Crystal structure;
€
Friedlander condensation;
naphthyridine; polypyridine;
ruthenium complex
GRAPHICAL ABSTRACT
OR
OR
2+
N
N
N
N
Ru2+/py
N
N
N
N
Ru(py)
2
N
N
R = Me, H
OH
2
4-substituted-2,6-di(1,8-naphthyridin-2-yl)pyridines
"Ru-complex"
Introduction
In recent years, environmental pollution and exhaustion of fossil fuels have become a
serious problem, which have rendered the practical utilization of various renewable
energy resources as a grand challenge in modern chemistry. Among these renewable
energy resources, sunlight is considered the most useful resource. For example, dye-sen-
sitized solar cells (DSSC), which are based on semiconductors such as TiO2, have been
studied as the next generation of solar cells.[1] The performance of DSSC is greatly
affected by the molecular structure of the dye used.[2] This is exemplified by a variety of
well-known ruthenium(II) complexes containing both thiocyanate ions (SCNꢀ) and
2,20:60,200-terpyridine derivatives (1 in Fig. 1), which are typical polypyridine com-
pounds.[3] Among these complexes, the SCNꢀ ion, which functions as an electron
acceptor site, is an ambidentate ligand, and therefore, it can bind to the Ru center with
CONTACT Dai Oyama
daio@sss.fukushima-u.ac.jp
Department of Industrial Systems Engineering, Cluster of
Science and Engineering, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan.
Supplemental data for this article is available online at on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
S. NAKAMURA ET AL.
R
N
N
N
N
N
N
N
N
1
2a (R = H), 2b (R = tBu),
2c (R = OMe), 2d (R = OH)
Figure 1. The structures of polypyridyl compounds containing the terpyridine skeleton.
CHO
NH2
R1
R2
R1
R2
MeOH/KOH
EtOH, reflux
+
N
O
N
N
3
4
2
Scheme 1. General synthetic procedure for synthesizing 1,8-naphthyridine derivatives (2) by the
€
Friedlander reaction.
either a sulfur (jS) or a nitrogen atom (jN). Resultantly, the precise control of the
coordination mode in such complexes is one of the key factors affecting their final per-
formance because the resulting two linkage isomers often show different properties.[4]
We have reported that the coordination mode of the SCN ligand can be completely
controlled using 2,6-di(1,8-naphthyridin-2-yl)pyridine (dnp; 2a in Fig. 1), which is
obtained by the condensation of two more pyridines to the terpyridine skeleton.[5] In
addition, dnp and its derivatives have been shown to play an important role in electron
transfer reactions because they are redox-active compounds.[6] The introduction of dnp
in a dye molecule could be expected to improve the photoelectric conversion efficiency
(g) of DSSCs; however, the lack of a suitable anchor group in dnp for the adsorption
on TiO2 hinders its application as a dye. Generally, such anchor groups are phospho-
nates,[7] carboxylates,[8] or catechols,[9] whose oxygen atoms strongly adsorb on the sur-
face of transition metal oxides. Although dnp derivatives containing a t-butyl
substituent at the 4-position (2b in Fig. 1) have been reported,[10] no dnp skeleton mol-
ecules having the abovementioned anchor groups have been synthesized yet.
From the viewpoint of the importance of multifunctional dnp compounds, especially
for their utilization as dye molecules, we herein describe our efforts toward the synthesis
and characterization of dnp compounds containing oxygenated groups at the 4-position,
as shown in Fig. 1 (2c and 2d). For the synthesis of 1,8-naphthyridine derivatives (2)
€
such as dnp (Scheme 1), Friedlander condensation is the most commonly used
method,[11] which involves the reaction between 2-aminonicotinaldehyde (3) and acyl
derivatives (4). We applied this methodology to the proposed study using 4-methoxy-
2,6-diacetylpyridine as the corresponding acyl derivative, which would provide a dnp
derivative with a protected hydroxy group at the 4-position. As a preliminary study, we
also examined the coordination chemistry between 2c and a ruthenium(II) center.

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SYNTHETIC COMMUNICATIONSV
3
Results and discussion
First, we synthesized 2-aminonicotinaldehyde 3 through the known reaction of nicotina-
mide with ammonium amidosulfate.[11d,12] Aldehyde 3 must be freshly prepared and
used immediately after isolation to avoid self-condensation side reactions. Meanwhile,
the desired acyl precursor (4) must contain OR substituents, where R ¼ alkyl or aryl, as
protecting groups at the para position of the starting pyridine derivative. Since chelida-
mic acid has frequently been used as a starting material for the preparation of polypyri-
dines,[13] and considering that it bears a functionality in the appropriate position, we
selected chelidamic acid as the precursor for the synthesis of the desired 2,6-diacetylpyr-
idine 4.
The most convenient method for the preparation of 2,6-diacetylpyridine and its ana-
logs is by conducting Claisen condensation of dialkyl pyridine-2,6-dicarboxylates and
ethyl acetate in the presence of a strong base.[14] Therefore, we have tried to synthesize
the desired 2,6-diacetylpyridine 4 according to the synthetic route.[14a,15] For the incorp-
oration of the para-OMe substituent, chelidamic acid was first made to react with thi-
onyl chloride and then with methanol to give a para-hydroxy methyl ester intermediate.
After deprotonation and methylation with potassium carbonate and methyl iodide to
avoid tautomerism between pyridine and pyridone, para-OMe methyl ester was
obtained. The methyl ester was then converted to the para-OMe substituted diacetylpyr-
idine 4 using standard methods.[16]
Next, the novel compound, 4-methoxy-2,6-di(1,8-naphthyridin-2-yl)pyridine
€
(dnpOMe) 2c, was obtained by Friedlander condensation under a nitrogen atmosphere
using aldehyde 3 and diacetylpyridine 4 (the first step in Scheme 2). The isolation yield
(21%) of 2c after purification was lower than that obtained for its non-substituted coun-
terpart (62% yield).[11f] This is most likely due to the difficulty in recrystallization rather
than the difference in the reactivity of the respective acyl derivatives because of the low
solubility of 2c in most organic solvents. The structure of dnpOMe 2c in solution was
1
1
estimated from the corresponding H and 13C NMR spectroscopic data. Thus, the H
NMR spectrum of 2c showed a singlet corresponding to three protons at d ¼ 4.13 ppm
as well as signals specific to dnp in the aromatic region.[11f] The proton-decoupled 13C
NMR spectrum of 2c showed 11 distinct resonances, which are consistent with the pro-
posed structure. Additionally, the methoxy carbon resonated at d ¼ 55.93 ppm.
Meanwhile, the percentage of C, H, and N in the bulk sample of 2c was determined by
microanalysis, and it was in agreement with the calculated values.
CHO
OMe
N
OH
N
2
N
NH2
EtOH/KOH
(21%)
pyridine/HCl
(85%)
3
N
N
N
N
N
N
N
N
OMe
2c (dnpOMe)
2d (dnpOH)
Me
Me
N
O
O
4
Scheme 2. Synthesis of 4-substituted-2,6-di(1,8-naphthyridin-2-yl)pyridines 2c and 2d.

4
S. NAKAMURA ET AL.
C6
O1
C3
C4
C2
N5
N3
N2
N4
C5
C1
C22
C19
C18
C14
C15
C11
C10
C7
N1
C21
C13
C8
C16
C20
C17
C12
C9
Figure 2. X-ray crystallographic structure of 2c (CCDC: 1869682) with atom labels and displacement
ellipsoids for non-hydrogen atoms drawn at the 50% probability level. The asymmetric unit of the
crystal contains two chemically identical molecules, of which only one is shown. Hydrogen atoms are
shown as small spheres of arbitrary radius.
Furthermore, the structure of 2c was confirmed by single-crystal X-ray crystallog-
raphy (Fig. 2 and Supplementary Material). Compound 2c contains two molecules in
the asymmetric unit, but both structures are chemically identical. All hydrogen atoms
attached to the carbon atoms are clearly visible in the difference map, and the assign-
ment of nitrogen atoms in aromatic rings is unambiguous. The central pyridine N atom
exists in a trans conformation with respect to the naphthyridine N atoms. It was found
that the OMe group in compound 2c was bonded to the 4-carbon atom, and the bond
lengths and angles were within normal ranges.[17] Moreover, 2c contains some
C–H … N hydrogen bonds and intermolecular p … p interactions (Supplementary
Material). Due to these effects, the two aromatic groups tend to be coplanar.[18]
Subsequently, conversion of 2c to 2,6-di(1,8-naphthyridin-2-yl)pyridine-4-ol (dnpOH)
2d was conducted by deprotection using pyridine and hydrochloric acid at a reflux tem-
perature of 220 ꢁC (the second step in Scheme 2).[13] This deprotection reaction pro-
ceeded smoothly with high yield (85%). For both 1H and 13C NMR spectra of
compound 2d, the signals derived from the OMe group (d ¼ 4.13 and 55.93 ppm,
respectively) completely disappeared, which indicated the progress of the deprotection
reaction. Additionally, the dnpOH 2d signal pattern indicates that 2d retains the dnp
framework. Unfortunately, in the case of compound 2d, insufficient material was avail-
able for microanalysis because its weight increased accompanied by deliquescence when
exposed to air. Nevertheless, the composition suggested by the corresponding HRMS
data in solution supported the proposed structure. It is worth mentioning that we also
attempted the deprotection of 2c using hydrobromic acid.[19] Although spectroscopic
monitoring suggested a satisfactory outcome of this reaction, the isolation of 2d proved

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SYNTHETIC COMMUNICATIONSV
5
py
2+
MeO
N
N
2c/py
N
[Ru(dmso)4Cl2]
N
N
Ru
py
(21%)
OH2
Scheme 3. Formation of a Ru-complex containing 2c.
to be extremely difficult. Therefore, we confirmed that the deprotection using pyridine
and hydrochloric acid was superior in this system.
An attempt to prepare a family of ruthenium complexes containing 2 was successful.
[Ru(dmso)4Cl2] (dmso ¼ dimethyl sulfoxide)[20] was adopted as the precursor for the
preparation of ruthenium(II) complexes having tridentate polypyridines such as dnp.[21]
Actually, the aqua complex ([Ru(2c)(py)2(OH2)]2þ) was directly synthesized without the
formation of the corresponding chlorido complex (Scheme 3). Two naphthyridine units
promote hydration both by electronic repulsion of the chloride leaving group and stabil-
ization of the entering water through hydrogen bonds.[21] The complex was character-
1
1
ized by ESI-MS, H NMR, and microanalysis. In H NMR spectrum, a signal observed
at 9.2 ppm disappeared upon the addition of D2O, identifying it as the protons on the
coordinated water (Supplementary Material). Furthermore, a preliminary X-ray ana-
lysis was performed to obtain structural information of the complex. Although the qual-
ity of the crystals of the complex was not good enough for a detailed analysis of the
bond parameters, the ligand environment around the ruthenium atom was identical to
the related complex with dnp (Supplementary Material).[21] Currently, the effort is being
made to synthesize/characterize a series of ruthenium complexes with 2c or 2d and to
explore their application to the photosensitizer. These results will be reported in subse-
quent papers.
Conclusions
We have synthesized two kinds of functionalized dnp compounds, 2c and 2d, through
€
Friedlander condensation reaction, providing access to the desired dnp skeleton contain-
ing a hydroxy group at the 4-position. Also, we have demonstrated the one-pot forma-
tion of the ruthenium complex containing 2c as a ligand. Considering the fact that the
dye performance improves with an increase in the symmetry of the constituent mol-
ecule,[22] it can be expected that compound 2d could be suitable as a dye molecule
from a structural viewpoint. Therefore, we hope that this compound will create new
research pathways for effective dye molecules in DSSC applications.
Experimental
All chemicals were used without further purification. All solvents used for synthesis
were anhydrous and used without further purification. Nicotinamide, ammonium ami-
dosulfate, and chelidamic acid were purchased from Nacalai Tesque and Tokyo
Chemical Industry, respectively. The compounds 2-aminonicotinaldehyde 3, 4-metohxy-

6
S. NAKAMURA ET AL.
2,6-diacetylpyridine 4, and [Ru(dmso)4Cl2] were prepared according to previously
reported procedures.[11d,12a,12b,14a,15,20]
Microanalysis data were obtained on a Perkin Elmer 2400II series CHN analyzer.
1
Melting points were determined on a Barnstead International 1201 D instrument. H
13
1
and C f Hg NMR spectra were recorded on a JEOL JMN-AL300 spectrometer (25 ꢁC)
1
operating at H and 13C frequencies of 300 and 75 MHz, respectively. Both spectra were
referenced to external tetramethylsilane via residual protons of the solvent (1H) or
the solvent itself (13C). Electrospray ionization mass spectrometry (ESI-MS) data were
obtained on a Bruker Daltonics microTOF. High-resolution mass spectrometry (HRMS)
data were obtained on a JEOL JMS-T100GC spectrometer.
Synthesis of 4-methoxy-2,6-di(1,8-naphthyridin-2-yl)pyridine (2c)
A freshly prepared 2-aminonicotinaldehyde 3 (0.89 g, 7.3 mmol) and 4-methoxy-2,6-
diacetylpyridine 4 (0.64 g, 3.3 mmol) were added to ethanol (30 mL). After addition
of a freshly prepared ethanolic KOH solution (0.1 g/15 mL), the mixture was refluxed
for 5 h under nitrogen. The solution was cooled to room temperature and then was
allowed to stand at 4 ꢁC overnight. The resultant yellow precipitate was collected by
filtration, washed with diethyl ether, and then dried in vacuo. The crude product
was recrystallized from methanol. Yield: 249 mg (21%); Colorless powder; mp
1
265–268 ꢁC. H NMR (300 MHz, DMSO-d6): d ¼ 4.13 (s, 3H, –OCH3), 7.69 (dd, 2H,
60–H, J¼ 8.1, 3.9 Hz), 8.31 (s, 2H, 3 and 5–H), 8.57 (dd, 2H, 50–H, J ¼ 8.1, 1.8 Hz),
8.71 (d, 2H, 40–H, J ¼ 8.4 Hz), 9.00 (d, 2H, 30–H, J ¼ 8.7 Hz), 9.16 (dd, 2H, 70–H,
1
J ¼ 3.9, 1.8 Hz). 13C f Hg NMR (75 MHz, DMSO-d6): d ¼ 55.93 (–OCH3), 108.27,
119.79, 122.78, 123.02, 137.52, 139.01, 154.22, 155.10, 156.46, 157.68, 167.60. Anal.
Calcd. for C22H15N5OꢂH2O: C, 68.92; H, 4.47; N, 18.27. Found: C, 68.77; H, 4.68;
and N, 18.14.
Synthesis of 2,6-di(1,8-naphthyridin-2-yl)pyridin-4-ol (2d)
4-Methoxy-2,6-di(1,8-naphthyridin-2-yl)pyridine 2c (190 mg, 0.52 mmol) was dissolved
in pyridine (14 mL). A concentrated HCl solution (12 M, 15 mL) was slowly added to
the solution at 0 ꢁC. The reaction temperature was slowly increased to 220 ꢁC, after
which time the mixture was refluxed for 3.5 h. Addition of water (50 mL) to the solution
resulted in the formation of a white precipitate of 2d. The product was collected by fil-
tration, washed with cold water, and dried in vacuo. Yield: 155 mg (85%); Colorless
1
solid; mp 336–340 ꢁC (dec.). H NMR (300 MHz, DMSO-d6): d ¼ 7.68 (dd, 2H, 60–H,
J ¼ 8.1, 3.9 Hz), 8.20 (s, 2H, 3 and 5–H), 8.56 (dd, 2H, 50–H, J ¼ 7.8, 2.1 Hz), 8.69 (d,
2H, 40–H, J ¼ 9.0 Hz), 8.98 (d, 2H, 30–H, J ¼ 8.4 Hz), 9.15 (dd, 2H, 70–H, J ¼ 4.2, 2.1 Hz).
1
13C f Hg NMR (75 MHz, DMSO-d6): d ¼ 110.33, 120.16, 122.78, 123.14, 138.76, 139.04,
153.48, 154.10, 156.00, 158.30, 166.38. HRMS (EI): m/z [M]þ calcd for C21H13N5O
351.1120; found: 351.1110.

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SYNTHETIC COMMUNICATIONSV
7
Synthesis of [Ru(dnpOMe-j3N)(py)2(OH2)](PF6)2
[Ru(dmso)4Cl2] (117 mg, 0.24 mmol) and 2c (86 mg, 0.24 mmol) were dissolved in ethyl-
ene glycol (6 mL) under N2, and the mixture was refluxed for 10 min. On cooling, Et3N
(0.2 mL), EtOH (10 mL), H2O (6 mL), and pyridine (0.6 mL, 7.5 mmol) were added to
the solution. The solution was refluxed for a further 26 h. The mixture was allowed to
cool, and the aqueous solution was extracted with three 80 mL portions of dichlorome-
thane. Blue powders precipitated when a saturated aqueous solution of KPF6 was added
to the mixture and allowed to cool overnight. The precipitate was collected by filtration,
washed with cold water and diethyl ether, and then dried in vacuo. The crude product
was purified by column chromatography on Al2O3 (eluent: acetone). Yield: 40 mg
1
(21%); Blue powder. H NMR (300 MHz, acetone-d6): d ¼ 4.33 (s, 3H, –OCH3), 7.00 (t,
4H, J ¼ 7.2 Hz), 7.57 (t, 2H, J ¼ 7.8 Hz), 8.07 (dd, 2H, J ¼ 8.4, 4.5 Hz), 8.16 (dd, 4H,
J ¼ 6.6, 1.5 Hz), 8.83–8.86 (m, 4H), 8.97 (d, 2H, J ¼ 8.4 Hz), 9.07 (d, 2H, J ¼ 8.7 Hz),
9.23 (br, 1H, OH2), 9.48 (dd, 2H, J ¼ 4.2, 2.4 Hz). MS (ESI): m/z 321.5 ([M]2þ), 312.5
([M–OH2]2þ). Anal. Calcd. for C32H27N7O2F12P2Ru: C, 41.21; H, 2.92; N, 10.51. Found:
C, 40.99; H, 2.67; and N, 10.27.
1
Supporting information H and 13C NMR spectra and X-ray crystallographic data.
This material can be found via the “Supplementary Content” section of this
article’s webpage.
Acknowledgments
We would like to thank Dr. Kyeonga Inomata at Tohoku University for HRMS measurements of
2d and Rigaku Corporation for the X-ray measurement of the Ru-complex.
Funding
This work was supported by Japan Society for the Promotion of Science (JSPS) under
[Grant JP17K05799].
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