Synthesis of pyridine N-oxide-BF2CF3 complexes and their fluorescence properties

Article abstract of DOI:10.1002/asia.201301688

Pyridine N-oxide-BF₂CF₃ and-BF₂C ₂F₅ complexes and their derivatives were synthesized. Most of the complexes show fluorescence both in solution and in the solid state. By expanding the π-conjugated skeleton, the color of the fluorescence could be changed dramatically. A fluorophore with a high solvent dependency could also be produced. Since such compounds can be synthesized on a gram scale in high yield, and are stable to oxygen, water, and heat, the complexes hold great potential as organic functional materials. Too complex for simple answers: Complexes of pyridine N-oxide with BF₂CF₃ and BF ₂C₂F₅, and their derivatives were synthesized. Most of the complexes fluoresce, both in solution and in the solid state. By expanding the π-conjugated skeleton, the color of the fluorescence could be changed dramatically. Owing to their properties such complexes hold potential as organic functional materials.

Full text of DOI:10.1002/asia.201301688

COMMUNICATION
DOI: 10.1002/asia.201301688
Synthesis of Pyridine N-Oxide–BF2CF3 Complexes and Their Fluorescence
Properties
Tomoaki Nishida,[a] Aiko Fukazawa,[c] Eriko Yamaguchi,[c] Hiroya Oshima,[c]
Shigehiro Yamaguchi,[c] Motomu Kanai,*[a, b] and Yoichiro Kuninobu*[a, b]
Abstract: Pyridine N-oxide–BF2CF3 and –BF2C2F5 com-
plexes and their derivatives were synthesized. Most of the
complexes show fluorescence both in solution and in the
solid state. By expanding the p-conjugated skeleton, the
color of the fluorescence could be changed dramatically. A
fluorophore with a high solvent dependency could also be
produced. Since such compounds can be synthesized on
a gram scale in high yield, and are stable to oxygen, water,
and heat, the complexes hold great potential as organic
functional materials.
amples, however, contain thermally labile intermolecular
pyridine N-oxide–borane complexes. Recently, Murakami
and co-workers reported intramolecular complexes of pyri-
dine N-oxide with boranes that have an improved stabili-
ty.[5f] Here we report the successful synthesis of pyridine N-
oxide–BF2CF3 complexes and their derivatives as highly
electron-deficient compounds. These intermolecular com-
plexes are stable to oxygen, water, and heat. In addition,
most of the complexes fluoresce not only in solution but
also in the solid state.[6–8]
First, we investigated the complex formation of 4-phenyl-
pyridine N-oxide (2a) with various borane compounds, in-
cluding BF3·OEt2, BF2C6F5, and BF2(4-CF3C6F4) (see the
Supporting Information). In all cases, we observed quantita-
tive conversion of 2a to the corresponding borane complex.
These products, however, were unstable under standard
aqueous workup conditions or silica gel column chromatog-
raphy. Their stability was highly dependent on the substitu-
ents on the boron center. Pyridine N-oxide complexes with
BF2C6F5 and BF2(4-CF3C6F4) were more stable than those
with BF3 (Figure S1, Supporting Information).
Pyridine, quinoline, and their related skeletons are impor-
tant partial structures, not only of natural products[1] and
drugs,[2] but also of functional materials.[3] Their higher elec-
tron-accepting ability compared with other heteroaromatic
rings is particularly noteworthy, and makes these rings im-
portant components of n-type semiconducting materials for
organic electronics.[4] We are interested in producing more
electron-deficient pyridine and quinoline derivatives. Con-
ventional approaches to obtain such compounds are deriva-
tion of the pyridines and quinolines to pyridinium salts and
pyridine and quinoline N-oxides. Another promising method
for producing more electron-deficient pyridine and quino-
line rings is the activation of pyridine and quinoline N-
oxides using a Lewis acid. Several examples of pyridine N-
oxide–borane complexes have been reported, some of which
show an enhanced electron-accepting property.[5] These ex-
Based on the finding that borane reagents with higher
Lewis acidity afford more stable products, we next tried the
synthesis of a pyridine N-oxide complex with BF2CF3, which
is a stronger Lewis acid than BF3. Thus, BF2CF3·OEt2 (1a)[9]
was prepared in situ by treating KACHTNURTGNEUNG[BF3CF3] with BF3·OEt2
in CH2Cl2 at 258C. Subsequent treatment of the mixture
with 2a for one hour afforded the 4-phenylpyridine N-
oxide–BF2CF3 complex 3a in 89% yield (Scheme 1). Com-
plex 3a is stable to air, water, and silica gel purification, and
could be stored for at least 3 months.[10,11] When BF2Me,
BF2(1-hexyn-1-yl), or BF2Ph was used in place of 1a, a 4-
phenylpyridine N-oxide–BF3 complex was obtained, thus in-
dicating that Lewis acidic boranes stronger than BF3 are es-
[a] T. Nishida, Prof. Dr. M. Kanai, Prof. Dr. Y. Kuninobu
Graduate School of Pharmaceutical Sciences
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: kuninobu@mol.f.u-tokyo.ac.jp
kanai@mol.f.u-tokyo.ac.jp
[b] Prof. Dr. M. Kanai, Prof. Dr. Y. Kuninobu
ERATO (Japan) Science and Technology Agency (JST)
Kanai Life Science Catalysis Project
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
[c] Dr. A. Fukazawa, E. Yamaguchi, H. Oshima, Prof. Dr. S. Yamaguchi
Department of Chemistry
Graduate School of Science, Nagoya University
and Institute for Transformative Bio-Molecules (WPI-ITbM)
Nagoya University
Furo, Chikusa, Nagoya 464-8602 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301688.
Scheme 1. Preparation of a BF2CF3 complex with 4-phenylpyridine N-
oxide (2a).
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Motomu Kanai, Yoichiro Kuninobu et al.
boron atom is 1.514(3) ꢁ,
which is significantly shorter
than a typical B–O coordina-
tion bond (e.g., B–O in
BF3·OMe2, 1.719 ꢁ).[12] The
boron center adopts a slightly
distorted tetrahedral geometry
with
a tetrahedral character
(%THC) of 87%.[13] This find-
ing indicates a strong Lewis
acid–base interaction between
BF2CF3 and acridine N-oxide
moieties.
The series of pyridine N-
oxide–borane complexes thus
prepared has several useful
characteristics. First, complexa-
tion of pyridine N-oxides with
more Lewis acidic boranes en-
hances the thermal stability
(Table S1, Supporting Informa-
tion). In addition, increasing
the Lewis basicity of the pyri-
dine N-oxides improves the sta-
bility. For example, the 4-phe-
nylpyridine N-oxide complex
with the more Lewis acidic
BF2C2F5 has
a
significantly
higher decomposition point of
2758C than that of the BF2CF3
congener (1628C). The decom-
position point of 3c (1768C) is
also higher than that of 3b
Scheme 2. Synthesis of various pyridine N-oxide–BF2CF3 complexes 3.
sential to produce the desired pyridine N-oxide–borane
complexes based on this synthetic protocol.
This synthetic method is applicable to the preparation of
the BF2CF3 complexes with various pyridine N-oxide deriva-
tives, as summarized in Scheme 2. Pyridine N-oxides bearing
electron-donating or electron-withdrawing groups, as well as
sterically encumbered derivatives, smoothly produced the
corresponding BF2CF3 complexes 3b–n in moderate to ex-
cellent yields. Ring-fused analogues, such as quinoline, iso-
quinoline, acridine, and benzo[h]quinoline N-oxides, also
produced the corresponding BF2CF3 complexes 3o–t in
yields of 60–94%. The BF2C2F5 complex 4 was also ob-
tained, with a yield of 87%.
Figure 1. ORTEP drawing of acridine N-oxide–BF2CF3 complex 3r (50%
probability for thermal ellipsoids. Hydrogen atoms are omitted for clari-
ty).
Notably, the synthesis can be performed on a gram scale.
Treatment of 750 mg of 4-phenylpyridine N-oxide (2a) with
BF2CF3 produced 1.17 g of 3a in 92% yield, which is compa-
rable to the yield shown in Scheme 1 (2a: 145 mg).
Among the BF2CF3 complexes thus produced, the struc-
ture of the acridine derivative 3r was determined by single-
crystal X-ray structure analysis (Figure 1). The oxygen atom
in the acridine N-oxide coordinates to the boron center of
BF2CF3. The distance between the oxygen atom and the
(1398C). Natural bond orbital (NBO) calculations (MP2/6-
31+G(d)//B3LYP/6-31G+(d)) suggested that the Wiberg
À
bond index (WBI) of the B O bond increases as the Lewis
acidity of the fluoroboron moiety increases (WBI for pyri-
dine N-oxide complexes with BF3, BF2CF3, and BF2C2F5
were 0.4711, 0.5226, and 0.5304, respectively), consistent
with the observed trend of the thermal stability of these
À
complexes. The NBO analyses also indicated that O B
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Motomu Kanai, Yoichiro Kuninobu et al.
bonds in the pyridine N-oxide complexes with highly Lewis
acidic BF2CF3 and BF2C2F5 have significant covalent bond
properties, whereas the corresponding BF3 complex induces
a coordination interaction (see the Supporting Information).
Second, complexation with the highly Lewis acidic fluo-
roalkylborane enhances the electron-accepting character of
the pyridine N-oxide-containing p-electron systems. The
cyclic voltammetry of 4-phenylpyridine N-oxide–BF2CF3
complex 3a showed an irreversible reduction peak with
a peak potential of À1.87 V (vs. Fc/Fc+) in THF (Figure S2,
Supporting Information).[14] The LUMO energy level esti-
mated by the onset potential of the reduction peak was
À3.30 eV.
showed the highest FF of 0.53 among the pyridine N-oxide–
BF2CF3 complexes. These features might be attributable to
the rigid molecular skeleton that prohibits a structural
change both in ground and singlet excited states.
Because 3a has a high electron-accepting ability, we an-
ticipated that the introduction of an electron-donating group
to this skeleton should render the molecule with interesting
photophysical properties.[18] Indeed, 4-(4-diphenylamino)-
phenylpyridine N-oxide–BF2CF3 complex 3k showed a signif-
icant extent of solvatochromism in fluorescence (Figure 2).
Third, introduction of the BF2CF3 moiety gives rise to
fluorescence emission. During the synthesis of 3a, we found
that 3a showed a purple fluorescence in acetonitrile solu-
tion,[8b,c,15] whereas 4-phenylpyridine and its N-oxide 2a
showed quite weak fluorescence (in CH3CN, 4-phenylpyri-
dine, and 4-phenylpyridine N-oxide, FF <0.005). Although
pyridine N-oxide–BF2CF3 complexes 3b–h showed virtually
no fluorescence, the other aryl-, alkenyl-, and alkynyl-substi-
tuted complexes 3i–n as well as the ring-fused derivatives
3o–t also showed intense emissions, presumably due to the
effective extension of p-conjugation. The photophysical
properties of several BF2CF3 complexes, 3a, 3m, and 3r, are
summarized in Table 1. Both 3a and 3m showed only subtle
Figure 2. Fluorescent spectra of 4-(4-diphenylamino)phenylpyridine N-
oxide–BF2CF3 complex 3k in various solvents.
Varying the solvent from cyclohexane to CH2Cl2 induced
a red shift of more than 80 nm (see the Supporting Informa-
tion for details). According to the density functional theory
(DFT) calculation at the B3LYP/6-31+G* level of theory,
the HOMO of 3k is localized on the 4-Ph2NC6H4 moiety,
while the LUMO mainly localizes on the pyridine moiety
(Figure 3). The time-dependent DFT (TD-DFT) calculation
at the same level of theory indicated that the emission of 3k
occurs from an excited state generated by the intramolecu-
lar charge-transfer (ICT) transition from the HOMO to the
LUMO.
Table 1. Photophysical data of various pyridine N-oxide-BF2CF3 com-
plexes 3a, 3m, and 3r
Absorption
Emission
lem FF
Stokes
shift
Cmpd Solvent labs
e/104
[nm][a]
[MÀ1 cmÀ1
]
[nm]
[cmÀ1
]
3a
3m
3r
THF
290
2.14
2.15
2.91
2.93
0.25
0.22
355[b]
360[b]
439[c]
449[c]
455[d]
456[d]
0.0094[e] 6300
MeCN 289
THF 330
MeCN 329
THF 438
MeCN 438
0.35[e]
0.10[f]
6800
7500
0.0075[f] 8100
0.53[f]
0.53[f]
900
900
[a] The longest wavelength absorption band. [b] Excited at 270 nm.
[c] Excited at 290 nm. [d] Excited at 310 nm. [e] Determined using 2-ami-
nopyridine in aq. H2SO4 (FF =0.66) as a standard.[16] [f] Determined
using quinine in aq. H2SO4 (FF =0.55) as a standard.[17]
positive solvatochromism in their fluorescence spectra,
which indicates that these molecules contain comparable
dipole moments between the ground state and the excited
state. Their fluorescence quantum yields (FF) are dependent
on the solvent polarity. Compound 3a had significantly
higher FF of 0.35 in the more polar solvent acetonitrile than
in THF (FF =0.0094), whereas FF of 3m decreased with in-
creasing solvent polarity (FF =0.10 and 0.0075 in THF and
acetonitrile, respectively), although the reason remains un-
clear. On the other hand, acridine N-oxide–BF2CF3 complex
3r displayed completely different features compared to
those of 3a and 3m (Figures S6 and S7, Supporting Informa-
tion). Both absorption and the fluorescence properties of 3r
are almost independent on the solvent. Compound 3r
Figure 3. Kohn–Sham molecular orbitals of 3k calculated at the B3LYP/
6-31+G* level.
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The BF2CF3 complexes also showed intense fluorescence,
Experimental Section
even in the solid state. Powder samples of 3a, 3m, 3r, and
3k showed purple, blue, green, and yellow fluorescence with
emission maxima at 354, 394, 497, and 545 nm, respectively
(Figure 4). The quantum yields were 0.51, 0.25, 0.28, and
Preparation of 4-Phenylpyridine N-oxide–BF2CF3 Complex
BF3·OEt2 (595 mL, 4.82 mmol, 1.1 equiv) was added to a mixture of potas-
sium trifluoro(trifluoromethyl)borate (848 mg, 4.82 mmol, 1.1 equiv) in
CH2Cl2 (9.0 mL), and the mixture was stirred at 258C for 20 min. Then,
4-phenylpyridine N-oxide (750 mg, 4.38 mmol) was added to the reaction
mixture and the mixture was stirred at 258C for 3 h. After dilution of the
reaction mixture with CH2Cl2/acetone (1:1), the insoluble solid was fil-
tered off and washed with CH2Cl2/acetone (1:1). Subsequently, the sol-
vent was removed under reduced pressure, and the crude product was
purified by column chromatography on silica gel (CH2Cl2, then CH2Cl2/
acetone, 20:1) to give difluoro((4-phenylpyridin-1-ium-1-yl)oxy)(trifluor-
omethyl)borate (3a, 1.17 g, 92% yield).
Acknowledgements
This work was supported in part by ERATO from JST (M.K. and Y.K.).
Keywords: boranes · charge transfer · fluorescence · Lewis
acid–base interaction · pyridine N-oxide
Figure 4. Fluorescence spectra of 3a, 3m, 3r, and 3k in the solid state.
Inset: Photographs taken under irradiation with a hand-held UV lamp.
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Conclusions
In summary, using a simple synthetic method, a series of N-
oxide–BF2CF3 and –BF2C2F5 complexes of pyridines, quino-
line, acridine, and their derivatives can be synthesized as
a new category of highly electron-deficient pyridines and re-
lated compounds. Most of the complexes showed fluores-
cence not only in solution but also in the solid state. By ex-
panding the p-conjugated systems, the color of the fluores-
cence could be changed dramatically. Cyclic voltammetry
measurements indicated that the reduction potential was
low enough to be a good electron acceptor. In fact, a fluoro-
phore exhibiting a large solvatochromism could be produced
by introducing an electron-donating group into the pyridine
skeleton. Because the pyridine N-oxide–BF2CF3 complexes
and their derivatives can be synthesized on a gram scale in
high yield, and are stable to oxygen, water, and heat, the
complexes hold great potential as organic functional materi-
als. Our findings provide useful insights for the field of fluo-
rescent materials. It is expected that such compounds could
be used as organic electroluminescence materials.
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Received: December 19, 2013
Published online: February 12, 2014
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