1,2-azaborine cations

Article abstract of DOI:10.1002/anie.201004084

Positively brilliant: The first examples of 1,2-azaborine cations have been prepared and their structure and optoelectronic properties characterized (see picture). 1,2-Azaborine cations exhibit solid-state fluorescence that is distinct from their neutral all-carbon analogues, and the 1,2-azaborine moiety is a critical component for the optoelectronic properties. There is a potential for the utility of these complexes in materials applications.

Full text of DOI:10.1002/anie.201004084

Communications
DOI: 10.1002/anie.201004084
Fluorescent BN-Heterocycles
1,2-Azaborine Cations**
Adam J. V. Marwitz, Jesse T. Jenkins, Lev N. Zakharov, and Shih-Yuan Liu*
1,2-Dihydro-1,2-azaborine is a six-membered aromatic het-
erocycle that is isoelectronic with benzene through the
boron atom (e.g., OTf) could render it susceptible to
nucleophilic attack by weaker neutral nucleophiles. In the
course of our studies, we discovered that silver reagents
facilitate the ligand exchange at the boron position in 1,2-
azaborines.[13] We were thus pleased to discover that treat-
ment of 1,2-azaborine 1 with AgOTf produced the substituted
1,2-azaborine 2 in 59% yield as an extremely moisture-
sensitive liquid (Scheme 2). The 1,2-azaborine 2 was charac-
=
replacement of a C C unit in benzene with an isoelectronic
[1,2]
B N unit.
Since the pioneering work by Dewar et al.,[3,4]
À
significant advances have been made in the synthesis and
reactivity studies of this family of heterocycles.[5–7] Our
continued exploration of the 1,2-azaborine motif[8–15] has led
us to consider the synthesis of cationic 1,2-azaborines, for
which no examples have been reported. In particular, we
envisioned that substitution of 1,2-azaborine on the boron
atom with pyridine derivatives would furnish cationic biaryl-
type structures[16] having the potential for use in materials
applications (Scheme 1). Herein we report the synthesis,
1
terized by H, 11B, and 13C NMR spectroscopy as well as IR
spectroscopy.
Scheme 2. Synthesis of 2. Tf=trifluoromethanesulfonyl.
Heterocycle 2 readily reacts with para-substituted pyr-
idines to form the desired cationic 1,2-azaborines 3. As can be
seen from Scheme 3, the substitution reaction is independent
of the electronic nature of the nucleophile. Excellent yields
have been obtained with both electron-rich and electron-poor
pyridines.
Scheme 1. 1,2-Azaborine cations.
structural characterization, and optoelectronic properties of
pyridine-substituted 1,2-azaborine cations, including a cat-
ionic heterocyclic analogue of para-terphenyl.
We have previously established nucleophilic substitution
À
of the B Cl bond in 1,2-azaborines by anionic nucleophiles
Scheme 3. Synthesis of 1,2-azaborine cations 3.
(with ClÀ serving as the leaving group).[8] Less reactive neutral
nucleophiles did not displace the chloride from the boron
atom. We hypothesized that a better leaving group on the
The 1,2-azaborine cations 3 are highly crystalline solids
that fluoresce under UV light. We thus explored the solid-
state fluorescence of the pyridine-substituted 1,2-azaborine
cations 3. The solid-state fluorescence and quantum yields of
aromatic hydrocarbons, including para-terphenyl, have been
reported using an integrating sphere.[17] We have recorded the
fluorescence spectra of crystalline samples of 3a–e (Figure 1),
all of which were freshly recrystallized prior to making the
fluorescence measurements. The solid-state fluorescence of
3a (R = Me) shows a peak at lem = 436 nm (FPL = 0.03) and is
visibly less fluorescent than samples of 3b and 3c under a UV
lamp (l = 365 nm; see Figure 1, right). The solid-state emis-
sion spectrum of 3b (R = Ph) showed a relatively narrow
band at lem = 448 nm (FPL = 0.86). The high quantum yield
observed for 3b is quite similar to the values obtained for
[*] A. J. V. Marwitz, J. T. Jenkins, Dr. L. N. Zakharov, Prof. Dr. S.-Y. Liu
Department of Chemistry
University of Oregon
Eugene, OR 97403-1253 (USA)
E-mail: lsy@uoregon.edu
[**] Support for this research has been provided by the University of
Oregon and the National Institutes of Health (National Institute of
General Medical Sciences, Grant R01-GM094541). Funding for the
University of Oregon Chemistry Research and Instrumentation
Services has been furnished in part by the National Science
Foundation (CHE-0234965). Correspondence concerning X-ray
crystallography should be directed to Dr. L. N. Zakharov (lev@-
uoregon.edu).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004084.
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Figure 1. Normalized solid-state fluorescence spectra and images
(under UV irradiation) of 1,2-azaborine cations 3.
Figure 2. Normalized absorption (dashed line) and emission (solid
line) spectra of 3b in CH2Cl2.
para-terphenyl, though the emission maximum of the cationic
3b is bathochromically shifted relative to the all-carbon para-
terphenyl by approximately 75 nm.[17] We found that solid
samples of 3c (R = H) were blue-green fluorescent (see
Table 1 summarizes the photophysical properties of
pyridine-substituted 1,2-azaborine cations 3a–e. To assess
whether the extended conjugation provided by the 1,2-
azaborine ring is critical for the observed optoelectronic
properties of the 1,2-azaborine cations 3, we prepared the
corresponding protonated pyridinium species 4a–e
(Scheme 4). We determined that under the same conditions
Figure 1) with a relatively broad emission maximum at lem
=
469 nm (FPL = 0.30). Compound 3d (R = CF3) exhibits
yellow-green emission at lem = 527 nm (FPL = 0.11), which
contrasts the blue emission observed for geometrically similar
3a. The data illustrated in Figure 1 suggest that the para
substituent on pyridine has a substantial effect upon the
emissive properties of 1,2-azaborine cations. DMAP-substi-
tuted 3e (R = NMe2;DMAP = 4-dimethylaminopyridine)
does not fluoresce in the solid state (Figure 1, right), which
is consistent with the reported fluorescence quenching by the
presence of a nitrogen lone pair of electrons.[18]
We also determined the absorption properties of 1,2-
azaborine cations 3 in solution. The absorption maximum of
3a (R = Me) in CH2Cl2 was found at l = 287 nm with an
extinction coefficient of e = 12713mÀ1 cmÀ1. The absorption
spectrum of 3b showed a broad, featureless peak at l =
292 nm (e = 21869mÀ1 cmÀ1), which is close to that observed
for 3a, but is slightly bathochromically shifted from the
absorption maximum of para-terphenyl (observed at l =
280 nm in CH2Cl2).[19] The absorption spectrum of 3c (R =
Table 1: Photophysical data for 3a–3e.
Compound
Absorbance
[nm][a]
e
Emission
[nm]
Fpl
[mÀ1 cmÀ1 [a]
]
3a
3b
287
292
8126
21869
436[b]
448[b]
360[a]
382[c]
469
0.03[b]
0.86[b]
0.06[a]
0.17[c]
0.30[b]
0.11[b]
N/A
3c
3d
3e
286
287
283
8624
12713
21303
527
N/A
[a] In CH2Cl2 (10À5 m). [b] Solid-state emission. [c] In MeCN (10À5 m).
N/A=not applicable.
H) also showed
a broad peak at l = 286 nm (e =
8624mÀ1 cmÀ1). The observed absorption peaks of 3d at l =
285 nm (e = 8126mÀ1 cmÀ1) and 3e at l = 283 nm (e =
21303mÀ1 cmÀ1) are relatively unchanged from the other
derivatives of 3.
Interestingly, of the prepared cationic derivatives, only
terphenyl analogue 3b was found to be fluorescent in
solution. The fluorescence spectrum of 3b in CH2Cl2
showed an emission peak at lem = 360 nm (FPL = 0.06), and
the absorption maximum was found at l = 292 nm in CH2Cl2
(Figure 2). The large Stokes shift of 68 nm for 3b is indicative
of considerable reorganization between the ground and the
excited state. We also observed a bathochromic shift in the
Scheme 4.
for 3, pyridinium triflate salts 4a–e do not exhibit solid-state
fluorescence emission in the visible region. This is consistent
with a critical role of the 1,2-azaborine moiety in the observed
emission properties of 1,2-azaborine cations 3.
emission peak when MeCN was used as the solvent (lem
=
382 nm, FPL = 0.17) instead of CH2Cl2. Furthermore, the
fluorescence of 3b in MeCN was quenched upon the addition
of NaI,[20] highlighting the potential of these materials in
sensing applications.
We have obtained the X-ray crystal structures of 1,2-
azaborine cations 3 (except for 3c), thus unambiguously
establishing their structural identity. As a representative
example, the solid-state structure of 3b is illustrated in
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Communications
In summary, we prepared the first examples of 1,2-
azaborine cations through a nucleophilic substitution reaction
between pyridine nucleophiles and the highly Lewis acidic
1,2-azaborine 2. 1,2-Azaborine cations 3a–3d exhibit solid-
state fluorescence that is distinct from the neutral all-carbon
analogues. Furthermore, 4-phenylpyridine-substituted 1,2-
azaborine cation 3b, an analogue of terphenyl, displays
solution-phase fluorescence in addition to solid-state emis-
sion. Control experiments establish the 1,2-azaborine ring as
an essential component for the observed optoelectronic
properties. This study highlights the unique properties of
1,2-azaborine cations and underscores the potential utility of
these complexes in materials applications.
Figure 3. ORTEP illustration of 3b, with thermal ellipsoids drawn at
the 35% probability level (hydrogen atoms have been omitted for
clarity). Bond distances (in ꢀ): B-N(1) 1.413(2), B-N(2) 1.531(2),
B-C(3) 1.496(2), C(3)-C(4) 1.369(2), C(4)-C(5) 1.408(3), C(5)-C(6)
1.358(2), C(6)-N(1) 1.3736(19); Torsion angles: 1,2-azaborine-pyri-
dine=50.58; pyridine-phenyl=18.08.
Experimental Section
3b: In a glove box, a solution of 4-phenylpyridine (0.073 g, 0.47 mmol
in 1.0 mL CH2Cl2) was added to a stirred solution of 2 (0.100 g,
0.392 mmol in 1.0 mL CH2Cl2). The mixture was stirred for 1 h at
room temperature. At the conclusion of the reaction, the solution was
cooled to À208C and left at that temperature for 24 h. The desired
product precipitated out of the solution as a crystalline solid. The
supernatant was decanted and the crystallized product was washed
with n-pentane (3 ꢁ 5 mL). Residual solvents were removed under
reduced pressure to provide 3b as clear, colorless crystals (0.155 g,
Figure 3 and reveals that the pyridine nitrogen atom is bound
to the boron atom with the triflate group serving as a
noncoordinating anion.[21] As expected, the dative exocyclic
À
B N bond (B-N(2) = 1.531(2) ꢀ) in 3b is significantly longer
1
3
97%). H NMR (600 MHz, CD2Cl2): d = 8.82 (d, JHH = 6.9 Hz, 2H),
8.44 (d, 3JHH = 6.9 Hz, 2H), 8.02 (dd, 3JHH = 9.8, 6.6 Hz, 1H), 7.97 (dd,
3JHH = 8.1 Hz, 4JHH = 1.7 Hz, 2H), 7.68 (m, 3H), 7.55 (d, 3JHH = 6.6,
1H), 6.85 (app t, 3JHH = 7.5 Hz, 2H), 3.83 (q, 3JHH = 7.3 Hz, 2H),
1.38 ppm (t, 3JHH = 7.3 Hz, 3H). 13C NMR (75 MHz, CD2Cl2): d =
158.1, 149.5, 145.8, 139.6, 134.3, 133.2, 130.5, 128.7, 125.7, 124 (br),
115.7, 47.7, 18.2 ppm. 11B NMR (192.5 MHz, CD2Cl2): d = 31.0 ppm.
FTIR (thin film) 3220, 3138, 3078, 2915, 1638, 1612, 1513, 1488, 1474,
1442, 1412, 1377, 1349, 1292, 1233, 1218, 1174, 1151, 1029, 833, 765,
736, 693 cmÀ1. HRMS (EI) calcd for C7H9BNO3SF3 [M+] 255.03484,
found 255.03528.
À
than the covalent exocyclic B NPh2 bond in a 1,2-azaborine
recently reported in our group (B-N(2) = 1.486(2) ꢀ).[9] The
À
exocyclic B N bond in cationic 3b is slightly shorter than the
À
B N bond in the charge-neutral borabenzene-4-phenylpyr-
idine adduct (B-N = 1.551(3) ꢀ) reported by Fu and co-
workers.[22] The 1,2-azaborine ring in 3b is completely planar
and is twisted by approximately 508 relative to the pyridine
ring. In contrast, the phenyl ring of 3b is only slightly twisted
relative to the pyridine ring (188).
We were also interested in examining the structural
À
features of the 1,2-azaborine ring in 3b.The intra-ring B N
Received: July 5, 2010
Published online: September 6, 2010
À
bond is short (B-N(1) = 1.413(2) ꢀ), as is the intra-ring B C
bond (B-C(3) = 1.496(2) ꢀ), which is consistent with bond
distances observed for electron-deficient 1,2-azaborines.[13]
Selected bond parameters for 1,2-azaborine cations 3 are
given in Table 2. The para substituent in the pyridine ring has
little influence on the observed bond lengths, which are
virtually identical for all derivatives. The torsion angles
between the pyridine and 1,2-azaborine ring are similar for
derivatives 3a, 3b, and 3d, although in 3e (R = NMe2) the 1,2-
azaborine ring is nearly perpendicular to the pyridine ring. It
noteworthy that the 1,2-azaborine cations 3 represent a new
family of borenium cations for which only a few members
have been structurally characterized by single-crystal diffrac-
tion.[23]
Keywords: boron · cations · fluorescence · heterocycles
.
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Table 2: Selected bond distances [in ꢀ] and angles [8] for 1,2-azaborine
cations 3.
Compound
B-N(1) [ꢀ]
B-C(3) [ꢀ]
B-N(2) [ꢀ]
Ring torsion[a]
3a
3b
3d
3e
1.418(5)
1.413(2)
1.416(4)
1.423(5)
1.481(5)
1.496(2)
1.489(4)
1.489(6)
1.528(5)
1.531(2)
1.526(4)
1.527(5)
57.18
50.58
58.08
77.88
[a] Torsion angle between the 1,2-azaborine and pyridine ring.
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7446 www.angewandte.org
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Angewandte
Chemie
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crystal size 0.36 ꢁ 0.31 ꢁ 0.24 mm3, monoclinic, space group
P21/n, a = 9.3812(9), b = 14.3671(13), c = 14.2217(13) ꢀ, b =
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,
m =
0.223 mmÀ1, F(000) = 848, MoKa-radiation l = 0.71073 ꢀ, T=
173(2) K, 2Vmax = 54.008, 17178 reflections measured [Rint
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[19] See the Supporting Information for details.
[20] Fluorescence quenching by iodide has been reported in para-
terphenyl, see: A. R. Watkins, J. Phys. Chem. 1974, 78, 2555 –
2558.
=
0.0255], 4101 reflections observed, 325 refined parameters,
R1 = 0.0376, wR2 = 0.1050 for reflections with I > 2s(I), R1 =
0.0451, wR2 = 0.1131, and GOF = 1.024 for all data, max/min
residual electron density + 0.439/À0.201 eꢀÀ3. CCDC 782514
(3b) 782515 (3a), 782517 (3d), and 782516 (3e) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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