Synthesis and characterization of boron complexes of imidazo[1,5-a] pyridylalkyl alcohols

Article abstract of DOI:10.1246/cl.130274

Imidazo[1,5-a]pyridin-1-ylalkyl alcohols were reacted with diphenylborinic acid to give boron complexes in which nitrogen and oxygen atoms were coordinated to a boron atom. The use of imidazo[1,5-a]pyridin-1-ylalkyl alcohol with two 2-pyridyl groups also

Full text of DOI:10.1246/cl.130274

doi:10.1246/cl.130274
828
Published on the web May 29, 2013
Synthesis and Characterization of Boron Complexes of Imidazo[1,5-a]pyridylalkyl Alcohols
Toshiaki Murai,*1,2 Eri Nagaya,1 Keitaro Miyahara,1 Fumitoshi Shibahara,1 and Toshifumi Maruyama1
1Department of Chemistry, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193
2JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
(Received March 29, 2013; CL-130274; E-mail: mtoshi@gifu-u.ac.jp)
Imidazo[1,5-a]pyridin-1-ylalkyl alcohols were reacted with
diphenylborinic acid to give boron complexes in which nitrogen
and oxygen atoms were coordinated to a boron atom. The use
of imidazo[1,5-a]pyridin-1-ylalkyl alcohol with two 2-pyridyl
groups also gave a boron complex, but the nitrogen atom in a
2-pyridyl group, rather than that in an imidazopyridyl group,
was coordinated to the boron atom. The molecular structures
of these complexes were revealed by X-ray analyses. The
fluorescence spectra of the complexes were generally observed
at wavelengths shorter than those for the starting materials and
well-known boron complexes of 8-hydroxyquinolines.
Ph
Ph
Ph
Ph
10% HClaq
B OH
B OCH2CH2NH2
Et2O, rt, 15 min
3
2
OH
Ph
B O
1
N
Ph
N
R2
Ar
3
R1
R2
R1
N
N
Ar
1
4
Et2O/CH2Cl2, rt, 2 h
Scheme 1.
The design and development of new chelating ligands are of
great interest since they can be used to finely tune the properties
of the obtained metal complexes. Such ligands include mono-
anionic chelating N,O-ligands, which possess hard nitrogen and
oxygen donors and which can coordinate not only to late
transition metals but also to main group metals. In addition,
unlike phosphine-based ligands, they are expected to be stable
under oxidative conditions. As an example of N,O-ligands,
8-hydroxyquinolate ligands form complexes with a wide variety
of metals such as B,1 Al,2 Ga,3 Rh,4 Ir,5 and Pt.6 The wide
applicability of the resulting metals as catalysts and drug
candidates, and in OLED devices and photovoltaic cells has
been extensively studied. Meanwhile, increasing attention has
been paid to nitrogen-containing 10³-electron heterocycles, i.e.,
imidazo[1,5-a]pyridines,7 because of their unique photophysi-
cal8 and biological properties.9 They have also been used as
neutral nitrogen-containing metal ligands.10 During our studies
related to these heterocycles11 starting from our thiocarbonyl
chemistry,12 we recently established an efficient method for
the synthesis of imidazo[1,5-a]pyridin-1-ylalkyl alcohols 1
(Scheme 1),13 which may be available as monoanionic chelating
N,O-ligands. As an initial study of their abilities as a metal
ligand, we report here the synthesis and characterization of
boron complexes of 1.
Table 1. Reaction of imidazo[1,5-a]pyridylalkyl alcohols 1
with diphenyborinic acid (3) generated in situa
Ligand 1
Product
Yield/%
Entry
Ar
R1
R2
4
1
2
3
4
5
6
7
1a 4-MeC6H4
1b 4-MeC6H4
1c 4-MeC6H4 Ph
1d 4-MeC6H4
1e 4-MeOC6H4 Ph
1f 4-ClC6H4
H
H
t-Bu
Ph
Ph
9-fluorenyl
Ph
4a
4b
4c
4d
4e
4f
®
®
63
52
76
73
63
Ph
Ph
1g 2-naphthyl C6H4Cl-4 C6H4Cl-4 4g
aAlcohols 1 were reacted with borinic acid 3 generated in situ
in CH2Cl2 at room temperature for 2 h.
O
B
C1
N1
C2
Initially, borinic acid 3 generated in situ from 2-aminoethyl
diphenylborinate (2) was reacted with ligands 1a and 1b in
which a hydrogen atom is attached to a carbon atom having
a hydroxy group (Scheme 1, Entries 1 and 2 in Table 1). The
reaction took place, and the disappearance of the starting
material 1 was confirmed by TLC analyses. However, during
purification of the desired products 4a and 4b, decomplex-
ation partially occurred to give mixtures of 1 and 4. Attempts
to obtain 4 in good purity failed. The ligand with two phenyl
groups 1c was then reacted with 3 (Entry 3). The desired
compound 4c was isolated in good yield with purity higher
than 95% by recrystallization using CH2Cl2/hexane. The
structure of 4c was unequivocally determined by X-ray analysis
(Figure 1).14
Figure 1. ORTEP drawing of boron complex 4c. Selected
bond lengths (¡), bond angle (°), and dihedral angle (°): B-O =
1.490, B-N1 = 1.616, O-B-N1 = 97.7, N1-C2-C1-O = ¹5.3.
The nitrogen and oxygen atoms of the starting 1c coordinate
to a boron atom, and the boron atom adopts a fairly deviated
tetrahedral structure. The bond length between boron and
oxygen atoms are slightly shorter than those in the diphenylboryl
Chem. Lett. 2013, 42, 828-830
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

829
Table 2. Photophysical data for 4 in CHCl3
Ph
Ph
Ph
Ph
B OH
UV-visa Fluorescenceb
B
Compound 4
OH
O
3
N
N
-
/nm
-
em
/nm
max
N
N
Ar
R1
R2
log ¾
ΦF
N
N
N
N
CH2Cl2
rt, 2 h
4c 4-MeC6H4 Ph
4d 4-MeC6H4
4e 4-MeOC6H4 Ph
Ph
9-fluorenyl
Ph
314 4.15 441 0.11
316 4.19 449 0.07
300 4.10 445 0.10
315 4.09 434 0.12
1h
5
Scheme 2.
4f 4-ClC6H4
Ph
Ph
4g 2-naphthyl C6H4Cl-4 C6H4Cl-4 320 4.16 439 0.33
N2
b
aMeasured in CHCl3 (1.0 © 10¹5 M). Excited at 350 nm.
N1
C1
B
Table 3. Photophysical data for 1 in CHCl3
C2
UV-visa Fluorescenceb
O
Compound 1
-
/nm
-
em
/nm
max
Ar
R1
R2
log ¾
ΦF
1c 4-MeC6H4 Ph
1d 4-MeC6H4
1e 4-MeOC6H4 Ph
Ph
9-fluorenyl
Ph
316 4.05 453 0.05
311 4.07 460 0.07
315 3.81 439 0.14
330 4.12 445 0.06
Figure 2. ORTEP drawing of boron complex 5. Selected bond
lengths (¡), bond angle (°), and dihedral angle (°): B-
O = 1.486, B-N2 = 1.632, O-B-N2 = 97.22, N1-C1-C2-
O = 159.30.
1f 4-ClC6H4
Ph
Ph
1g 2-naphthyl C6H4Cl-4 C6H4Cl-4 332 4.19 455 0.11
aMeasured in CHCl3 (1.0 © 10¹5 M). Excited at 350 nm.
b
complex of 8-hydroxyquinolinone (B-O bond length: 1.55 ¡),
whereas that between boron and nitrogen atoms is nearly the
same (B-N bond length: 1.61 ¡).15 For the synthesis of boron
complexes 4, ligands 1d-1g were also used (Entries 4-7) so that
it would become more stable and so that the photophysical
properties can be fine-tuned. Notably, the introduction of a
2-naphthyl group at the C1-position of imidazo[1,5-a]pyridine
and chlorine atoms to the phenyl group attached to the carbon
atom with a hydroxy group enabled us to isolate the complex 4g
by chromatographic purification.
Ph
Ph
B
O
C6H4Cl-4
C6H4Cl-4
N
N
4g
Figure 3. Visualized fluorescence of 4g: left in the solid state;
right in CHCl3.
Eventually, imidazo[1,5-a]pyridine with two 2-pyridyl
groups 1h was reacted with borinic acid 3 (Scheme 2). In this
case, existence of two pyridyl and imidazoyl groups may give
a mixture of coordinated complexes. Nevertheless, the reaction
gave a single product. Purification by column chromatography
on silica gel followed by recrystallization gave highly pure
product 5, the formation of which was confirmed by X-ray
analysis of its molecular structure (Figure 2). This implies that
the ability of a pyridyl group to coordinate with a boron atom is
greater than that of an imidazopyridyl group.16 The C3-2-pyridyl
group is located nearly on the same plane as an imidazopyridyl
group, whereas the 2-pyridyl group that forms a complex with a
boron atom is oriented almost perpendicular to an imidazopyr-
idyl group.
at shorter wavelengths than those of boron complexes of
8-hydroxyquinolines.1 The relative quantum yield of 4g was
about three times higher than that of the starting ligand 1g.
In summary, we have demonstrated the first example of
boron complexes of imidazo[1,5-a]pyridin-1-ylalkyl alcohols.
The efficiency of the synthetic reaction and the stability of
obtained boron complexes depend on the substituents. The
introduction of two aryl groups to the carbon atom at the C1-
positions allowed for isolation of the complexes as colorless
solids, and their structures were clearly characterized by X-ray
analyses. Further applications of our monoanionic N,O-chelating
ligands and boron complexes will be reported in due course.17
The photophysical properties of the obtained boron com-
plexes are shown in Table 2 (See also Figures S1 and S2 in
the Supporting Information).17 In the UV-visible spectra, the
longest wavelengths of maximum absorption were observed at
312 « 12 nm, which is in nearly the same region as those for
the starting ligands 1 (Table 3). Therefore, all the compounds
are colorless solids, unlike the diphenylboryl complex of
8-hydroxyquinolinone, which is yellow. All of the complexes
4 showed blue fluorescence from 429 to 449 nm both in solution
and as a solid state, as shown in Figure 3. The fluorescence
wavelengths of complexes 4, except for 4e, were blue-shifted by
ca. 13 nm compared with the starting ligands 1 and were also
This work was supported in part by a Grant-in-Aid for
Scientific Research on Innovative Areas (Integrated Organic
Synthesis) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
This paper is dedicated to Professor Teruaki
Mukaiyama in celebration of the 40th anniversary of the
Mukaiyama aldol reaction.
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Chem. Lett. 2013, 42, 828-830
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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Chem. Lett. 2013, 42, 828-830
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/