Tuning of the optical properties and Lewis acidity of dibenzopnictogenaborins by modification on bridging main group elements

Article abstract of DOI:10.1021/ic061055q

Dibenzophosphaborin 1 and dibenzoazaborine 7 have been synthesized from the corresponding dibromides and MesB(OMe)₂. Dibenzophosphaborin P-sulfide 2, the P-selenide 3, and the phosphonium salt 4 were obtained by the reaction of 1 with S₈, elemental selenium, and benzyl bromide, respectively. Crystallographic analysis of 1-3 and 7 showed that the dibenzophosphaborin framework does not have a planar structure, which is caused by the pyramidalization around the phosphorus atoms, unlike in 7. Compound 1 showed a blue-shifted and weak UV-vis absorption relative to 7, indicating a weak electronic interaction between the phosphorus lone pair electrons and the π-orbitals on the dibenzopnictogenaborin framework. The longest absorption bands of 2 and 3 are attributable to the intramolecular electron transfer from the lone pair electrons on the chalcogen atoms to the π* orbitals on the dibenzophosphaborin framework, and the existence of these electronic transitions is supported by TD-DFT calculations. The UV-vis and ¹¹B NMR spectra of 4 showed a temperature-dependent change due to the coordination of a counter bromide anion to the boron center, indicating a strong Lewis acidity of the boron center. We also investigated the complex formation ability of dibenzopnictogenaborin to fluoride or chloride anions. The complex formation constants increased in accordance with the decrease in LUMO energy levels calculated using the DFT method. Compound 4 exhibited an anomalously high complex formation ability among these compounds, due to the strong electron-withdrawing effect of the phosphonium cation moiety.

Full text of DOI:10.1021/ic061055q

Inorg. Chem. 2006, 45, 9137−9144
Tuning of the Optical Properties and Lewis Acidity of
Dibenzopnictogenaborins by Modification on Bridging Main Group
Elements
Tomohiro Agou, Junji Kobayashi, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
Received June 13, 2006
Dibenzophosphaborin 1 and dibenzoazaborine 7 have been synthesized from the corresponding dibromides and
MesB(OMe)2. Dibenzophosphaborin P-sulfide 2, the P-selenide 3, and the phosphonium salt 4 were obtained by
the reaction of 1 with S8, elemental selenium, and benzyl bromide, respectively. Crystallographic analysis of 1
and 7 showed that the dibenzophosphaborin framework does not have a planar structure, which is caused by the
pyramidalization around the phosphorus atoms, unlike in 7. Compound 1 showed a blue-shifted and weak UV vis
absorption relative to 7, indicating a weak electronic interaction between the phosphorus lone pair electrons and
the -orbitals on the dibenzopnictogenaborin framework. The longest absorption bands of 2 and 3 are attributable
to the intramolecular electron transfer from the lone pair electrons on the chalcogen atoms to the * orbitals on
the dibenzophosphaborin framework, and the existence of these electronic transitions is supported by TD-DFT
calculations. The UV
vis and 11B NMR spectra of 4 showed a temperature-dependent change due to the coordination
−3
−
π
π
−
of a counter bromide anion to the boron center, indicating a strong Lewis acidity of the boron center. We also
investigated the complex formation ability of dibenzopnictogenaborin to fluoride or chloride anions. The complex
formation constants increased in accordance with the decrease in LUMO energy levels calculated using the DFT
method. Compound 4 exhibited an anomalously high complex formation ability among these compounds, due to
the strong electron-withdrawing effect of the phosphonium cation moiety.
Introduction
molecules with small HOMO-LUMO gaps is exemplified
by the application of silacyclopentadienes (siloles) to fab-
ricate electron carrier materials for use in organic light-
emitting devices (OLEDs), as well as OLEDs constructed
from phosphole-based materials.
Main group element substituents confer interesting elec-
tronic and optical properties to π-conjugated molecules
because the donor or acceptor orbitals of these elements can
interact electronically with π-orbitals (hyperconjugation) to
decrease the energy gaps between the HOMOs and LUMOs.
A number of hetero π-conjugated molecules containing
nitrogen or sulfur atom donor centers have been known for
several decades and these have been studied extensively, but
the use of other main group elements, such as phosphorus,
silicon, and boron, as key components to produce an
electronic modulator has only been investigated recently.1
These main group elements have low-lying acceptor orbitals
(e.g.: Si, σ*(Si-R); B, vacant 2p; P, σ*(P-R or P-P)),
and these lower the LUMO levels of π-systems, in contrast
to donor atoms, which raise the HOMO levels. The advantage
of using such a new strategy to construct π-conjugated
Recently, boron-containing π-conjugated molecules have
been investigated as novel organic materials for optical or
(1) (a) Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R. A. Synth.
Met. 1998, 96, 177-189. (b) Yamaguchi, S.; Tamao, K. In The
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* To whom correspondence should be addressed. E-mail: takayuki@
chem.s.u-tokyo.ac.jp.
10.1021/ic061055q CCC: $33.50
Published on Web 09/29/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 22, 2006 9137

Agou et al.
electronic applications, because boron has two superior char-
acteristics over other main group elements, including silicon
and phosphorus: (1) Boron atoms have vacant 2p orbitals
that can act as acceptor orbitals, and the electronic interaction
between boron atoms and π-orbitals, constructed from carbon
2p orbitals, is generally strong. (2) Complex formation with
Lewis bases transforms the coordination mode of boron
atoms from trigonal planar to tetrahedral, and the electronic
interaction between the vacant 2p orbitals and the π orbitals
no longer occurs. Thus, the electronic state of a boron-con-
taining π-systems can be controlled using an external stim-
ulus (e.g., a Lewis base).2 Such properties are the main
justification to investigate the properties of novel π-conju-
gated molecules bearing boron atoms for applications such
as fluorescent materials,3 electroluminescence devices,4 non-
linear optics,5 organic conductors,6 and Lewis base sensors.7
In addition to the properties described above, phosphorus-
containing π-conjugated molecules have another interesting
feature: they exhibit electronic modulation by substitution
of the phosphorus atoms. Phosphorus atoms usually function
as both a weak donor and an acceptor, because they have
lone pair electrons and low-lying σ*(P-C or P-P) orbitals,
respectively. Such electron-withdrawing properties can be
dramatically enhanced by the oxidation of a phosphorus
atom, e.g., as in chalcogenation, alkylation, or complexation
with a metal (P-substitution). The modulation of the elec-
tronic state of a π-conjugated molecule by P-substitution has
been shown to be effective through intensive research into
the electronic and optical properties of diarylphospholes.8
Dibenzoheteraborins are boron-containing π-conjugated
molecules featuring a dihydroanthracene framework. The
rigid, planar structure of dibenzoheteraborins is expected to
allow for increased electronic interactions between a bridging
main group element and the π-orbitals on the benzene rings
leading to an enhancement of fluorescence intensity. The
optical properties of nitrogen- or oxygen-containing diben-
zoheteraborins and dibenzoborin have been reported. The
nitrogen-containing molecule (dibenzoazaborine) absorbs
violet light strongly and emits a moderately intense
fluorescence.3c,9 We have also reported on the synthesis,
structure, and photophysical properties of dibenzophos-
phaborins, a new member of the dibenzoheteraborin family
containing phosphorus atoms as electron donors.10 Unlike
azaborine, the fluorescence of dibenzophosphaborin is very
weak and shows a large Stokes shift (100-200 nm),
indicating a substantial structural relaxation around the
phosphorus atoms on photoexcitation. However, the optical
properties of other heteraborins have not been investigated
systematically. In addition, the effect of bridging main group
elements on the electronic structure and the Lewis acidity
of heteraborins has not been studied, despite the promise of
constructing a broad library of boron-containing π-conjugated
molecules with a common molecular structure. The electronic
state of dibenzophosphaborin can be regulated by the
substitution on the phosphorus atom.
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ties, and the Lewis acidity of dibenzoheteraborins bearing
nitrogen or phosphorus atoms as the bridging main group
elements. Azaborine has a rigid, nearly planar molecular
structure, resulting in a decreased HOMO-LUMO gap and
an intense fluorescence.3c In contrast, phosphaborin and its
P-derivatives show a weak fluorescence due to pyramidal-
ization around the phosphorus atoms and a lack of molecular
rigidity, but a modification on the phosphorus atom sites can
change the Lewis acidity of the boron centers dramatically.
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ic060709s.
9138 Inorganic Chemistry, Vol. 45, No. 22, 2006

Dibenzopnictogenaborins
7.6 Hz, 2H), 7.63-7.72 (m, 6H), 8.34 (dd, J ) 15.0, 7.4 Hz, 2H);
13C{1H} NMR (100 MHz, CDCl3) δ 21.30 (s), 22.61 (s), 22.88
(s), 126.79 (s), 127.14 (d, J ) 14.0 Hz), 128.36 (d, J ) 12.4 Hz),
130.88 (d, J ) 2.5 Hz), 131.13 (s), 131.23 (s), 133.48 (d, J ) 14.0
Hz), 134.06 (s), 134.35 (d, J ) 14.1 Hz), 134.78 (s), 137.46 (d, J
) 9.1 Hz), 138.27 (s), 139.44 (d, J ) 10.7 Hz), 140.27 (s); 31P
NMR(162 MHz, CDCl3) δ 17.5 (1JPSe ) 732 Hz); 11B NMR(128
MHz, CDCl3) δ 67.6 (h1/2 ) 1500 Hz); 77Se{1H} NMR (76 MHz,
CDCl3) δ -268 (d, 1JPSe ) 732 Hz); UV-vis (cyclohexane) [λmax
(log ꢀ)] 369 (3.10), 391 (3.07). Anal. Calcd for C27H24BPSe: C,
69.11; H, 5.16. Found: C, 68.90; H, 5.13
(c) Phosphonium Salt 4. To a solution of 1 (1.0 g, 2.6 mmol)
in toluene (20 mL) was added benzyl bromide (BnBr) (1.2 mL, 10
mmol), and the mixture was refluxed for 12 h. After cooling of the
mixture to room temperature, resulting precipitates were collected
by filtration and washed with pentane to give 4 as a colorless solid
(1.2 g, 84%).
4: colorless solid; mp 214-215 °C; 1H NMR (400 MHz, CDCl3)
δ 1.27 (s, 3H), 1.88 (s, 3H), 2.34 (s, 3H), 5.55 (d, J ) 14.4 Hz,
2H), 6.70 (dd, J ) 7.2, 2.0 Hz, 2H), 6.82-6.87 (m, 4H), 6.97 (td,
J ) 7.3, 2.4 Hz, 1H), 7.67-7.75 (m, 7H), 7.95 (brs, 2H), 8.35 (dd,
J ) 13.0, 7.2 Hz, 2H), 8.58 (dd, J ) 13.8, 7.0 Hz, 2H); 13C{1H}
NMR (100 MHz, CDCl3) δ 21.17 (s), 22.28 (s), 22.75 (s), 32.24
(d, J ) 44.8 Hz), 120.32 (s), 121.13 (s), 124.60 (s), 125.45 (s),
126.75 (d, J ) 7.6 Hz), 127.29 (s), 127.84 (d, J ) 3.8 Hz), 128.43
(d, J ) 3.8 Hz), 130.44 (d, J ) 12.4 Hz), 130.56 (d, J ) 5.7 Hz),
134.07 (d, J ) 10.5 Hz), 134.30 (d, J ) 1.9 Hz), 134.74 (d, J )
2.9 Hz), 134.94 (d, J ) 12.4 Hz), 135.42 (d, J ) 13.4 Hz), 137.86
(d, J ) 63.9 Hz), 138.09 (s), 140.85 (d, J ) 10.5 Hz); 31P NMR
(162 MHz, CDCl3) δ 6.5; 11B NMR (128 MHz, CDCl3) δ 62.3
(h1/2 ) 2300 Hz); HRMS (FAB+) m/z calcd for C34H31BP 481.2256,
found 481.2279 ([M - Br-]+); UV-vis (CH2Cl2) [λmax/nm (log
ꢀ)] 381 (2.82), 312 (3.90), 307 (3.88). Anal. Calcd for C34H31BBrP‚
H2O: C, 70.49; H, 5.74. Found: C, 70.71; H, 5.41.
X-ray Data Collection and Structure Refinement. The intensi-
ties of reflections were collected at 120 K on a Rigaku MSC
Mercury CCD diffractometer with a graphite-monochromated Mo
KR radiation (λ ) 0.710 70 Å) using CrystalClear (Rigaku Corp.).
The structure was solved by direct methods (SHELXS) and ex-
panded using Fourier techniques. The structure was refined by full-
matrix least-squares methods on F2 (SHELXL-97).11 All non-hydro-
gen atoms were refined anisotropically. Hydrogen atoms were as-
signed idealized positions and were included in structure factor cal-
culations. Crystallographic data are listed in Table 1. Selected bond
lengths and angles are given in Table 2. CCDC-605853 (7), CCDC-
605854 (2), and CCDC-605855 (3) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ,
U.K. Fax: (+44) 1223-336-033. E-mail: deposit@ccdc.cam.ac.uk).
UV-Vis Titration Experiments. A CH2Cl2 solution of dibenzo-
pnictogenaborins (1.0 × 10-4 M, 2.0 mL) was placed in a quartz
cell with a septum cap. To this solution was added a solution of
n-Bu4NF in CH2Cl2 (0.10 M) by portions, and the absorption change
was monitored at λmax of each compounds. The collected data were
fitted using a reported method.7f
Experimental Section
General Comments. General chemicals were used as received.
Cyclohexane and CH2Cl2 for spectrochemical or fluorometric grade
(Dojindo) were used for optical measurement. All manipulations
were carried out using Schlenk technique under an argon atmo-
sphere. Solvents were purified by MBRAUN MB-SPS system
(Et2O, THF, and toluene) or distillation from CaH2 (CH2Cl2). Gel
permeation liquid chromatography (GPLC) was performed using
LC-918 or LC-908 with JAIGEL 1H+2H columns (Japan Analyti-
cal Industry) using chloroform or toluene as solvents, respectively.
NMR spectra were recorded by a JEOL AL-400 spectrometer (1H,
400 MHz; 13C, 100 MHz; 11B, 128 MHz) or JEOL ECX-400
spectrometer (1H, 400 MHz, 13C, 100 MHz). Chemical shifts are
1
reported in δ. H NMR spectra are referenced to residual protons
in deuterated solvent, 13C NMR spectra are referenced to carbon-
13 in the deuterated solvent, and 11B NMR spectra are referenced
to an external standard of BF3‚Et2O. High-resolution mass spectra
were recorded by a JEOL JMS-700P using PEG600 as an internal
standard. UV-vis spectra were recorded on a JASCO V-530
spectrophotometer. Fluorescence spectra were recorded on a Hitachi
F4500 fluorescence spectrophotometer. All melting points were
measured with a Yanaco MP-S3 and uncorrected. Elemental
analyses were performed by the Microanalytical Laboratory of
Department of Chemistry, Faculty of Science, The University of
Tokyo. The syntheses of dibenzophosphaborin 19 and diben-
zoazaborine 73c have been reported previously.
Preparation of Compounds. (a) Dibenzophosphaborin Sulfide
2. In a 5 mm o.d. NMR tube, to a solution of 1 (30.6 mg, 78 µmol)
in toluene (0.5 mL) was added S8 (10.0 mg, 0.30 mmol as S). After
a few freeze-pump-thaw cycles, the tube was sealed in vacuo
and heated at 75 °C for 6 h. After confirmation of disappearance
of 1 by 31P NMR, the sealed tube was opened. The residual S8 was
filtered off, and the solvent was removed under reduced pressure.
The residue was subjected to GPLC to give 2 as a pale yellow
solid (21.2 mg, 64%).
1
2: pale yellow solid; mp 198-199 °C; H NMR (400 MHz,
CDCl3) δ 1.93 (s, 3H), 2.01 (s, 3H), 2.40 (s, 3H), 6.91 (s, 1H),
6.95 (s, 1H), 7.27-7.36 (m, 3H), 7.49 (t, J ) 7.4 Hz, 2H), 7.62
(dd, J ) 13.4, 8.2 Hz, 2H), 7.68-7.73 (m, 4H), 8.34 (dd, J )
14.6, 7.4 Hz, 2H); 13C{1H} NMR (100 MHz, CDCl3) δ 21.23 (s),
22.53 (s), 22.83 (s), 126.86 (s), 127.19 (d, J ) 11.4 Hz), 128.42
(d, J ) 12.2 Hz), 130.66 (d, J ) 10.5 Hz), 130.86 (d, J ) 2.9 Hz),
131.34 (d, J ) 2.9 Hz), 132.57 (d, J ) 13.4 Hz), 135.44 (s), 136.24
(s), 137.52 (d, J ) 11.4 Hz), 138.59 (brd, J ) 9.5 Hz), 139.52 (d,
J ) 10.5 Hz), 140.38 (s), 141.22 (s); 31P NMR (162 MHz, CDCl3)
δ 8.6; 11B NMR (128 MHz, CDCl3) δ 65.8 (h1/2 ) 1700 Hz); HRMS
(FAB+) m/z calcd for C27H24BPS 422.1429, found 422.1433 (M+);
UV-vis (cyclohexane) [λmax (log ꢀ)] 353 (3.31), 360 (3.15). Anal.
Calcd for C27H24BPS‚0.33H2O: C, 75.71; H, 5.80. Found: C, 75.80;
H, 5.83.
(b) Dibenzophosphaborin Selenide 3. In a 5 mm o.d. NMR
tube, to a solution of 1 (28.3 mg, 73 µmol) in toluene (0.5 mL)
was added elemental selenium (23.1 mg, 0.29 mmol). After a few
freeze-pump-thaw cycles, the tube was sealed in vacuo and heated
at 75 °C for 6 h. After confirmation of disappearance of 1 by 31P
NMR, the sealed tube was opened. The residual selenium was
filtered off, and the solvent was removed under reduced pressure.
The residue was subjected to GPLC to give 3 as a yellow solid
(20.8 mg, 61%).
Results and Discussion
Dibenzoheteraborins 1 and 7 bearing a phosphorus or
nitrogen atom were synthesized in moderate yields by the
3: yellow solid; mp 207-209 °C; 1H NMR (400 MHz, CDCl3)
1
δ H NMR (400 MHz, CDCl3) δ 1.92 (s, 3H), 2.01 (s, 3H), 2.39
(11) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(s, 3H), 6.91 (s, 1H), 6.95 (s, 1H), 7.29-7.35 (m, 3H), 7.58 (t, J )
Inorganic Chemistry, Vol. 45, No. 22, 2006 9139

Agou et al.
Table 1. Crystal Data and Structure Refinement for 2, 3, and 7
param
2
3
7
empirical formula
fw
C
27.5H26BO0.5PS
C27H24PBSe
469.20
C24H26BN
339.27
0.20 × 0.20 × 0.20
monoclinic
P21/c
16.548(2)
12.8158(14)
19.342(3)
431.34
cryst size/mm3
cryst system
space group
a/Å
0.40 × 0.40 × 0.40
0.70 × 0.50 × 0.20
triclinic
triclinic
P1h
P1h
9.304(2)
14.747(4)
17.336(5)
86.360(8)
81.222(8)
75.856(7)
2278.5(10)
4
9.254(3)
14.834(7)
17.483(8)
86.106(15)
80.210(14)
76.791(13)
2301.2(17)
4
b/Å
c/Å
R/deg
â/deg
108.9558(10)
γ/deg
V/Å3
3879.6(9)
8
1.162
Z
Dc/g cm-3
F(000)
1.324
960
1.354
960
1456
2θ/deg
6.10-54.96
17 486/9887 (Rint ) 0.0199)
570/0
1.058
0.0394
6.04-54.96
15 924/8838 (Rint ) 0.0241)
547/0
1.066
0.0333
6.10-54.96
reflcns collcd/unique
params/restraints
goodness of fit on F2
R1 (I > 2σ(I))
wR2 (all data)
29 694/8712 (Rint ) 0.0567)
481/0
0.911
0.0569
0.1508
0.1215
0.0882
Table 2. Selected Bond Lengths (Å) and Angles (deg) of 1-3 and 7
param
1a,b
2c (planar)
2c (butterfly)
3d (planar)
3d (butterfly)
7e
B-C1
1.554(5)
1.553(5)
1.570(5)
1.819(3)
1.811(3)
1.836(3)
1.557(2)
1.565(2)
1.574(2)
1.8094(16)
1.8058(16)
1.8165(16)
1.9535(6)
1.563(2)
1.560(2)
1.570(2)
1.8062(16)
1.8111(16)
1.8147(15)
1.9507(6)
1.563(3)
1.565(3)
1.582(3)
1.819(2)
1.809(2)
1.818(2)
2.1087(9)
1.568(3)
1.564(3)
1.570(3)
1.813(2)
1.816(2)
1.820(2)
2.1004(9)
1.522(3)
1.524(3)
1.584(3)
1.392(2)
1.395(2)
1.465(2)
B-C4
B-CMes
Pn-C2
Pn-C3
Pn-CR
P-Ch
C1-B-CMes
C4-B-CMes
C1-B-C4
C2-Pn-CR
C3-Pn-CR
C2-Pn-C3
C2-P-Ch
C3-P-Ch
CPh-P-Ch
120.6(3)
119.3(3)
119.9(3)
103.37(14)
100.33(14)
103.22(16)
121.06(13)
117.93(13)
120.93(13)
105.18(7)
103.79(7)
106.74(7)
113.87(5)
114.11(6)
112.25(5)
120.18(14)
119.88(14)
119.79(13)
103.49(7)
103.39(7)
105.29(7)
115.02(5)
115.48(5)
112.80(5)
120.68(18)
118.40(19)
120.88(19)
105.85(10)
103.47(10)
106.59(10)
113.26(8)
114.39(8)
112.43(8)
120.6(2)
119.65(19)
119.8(2)
102.74(10)
103.64(10)
105.42(10)
115.31(8)
115.29(8)
113.02(8)
123.20(17)
122.16(16)
114.58(17)
118.57(15)
118.77(16)
122.65(15)
a Pn ) P and R ) Ph. b Reference 4. c Pn ) P, R ) Ph, and Ch ) S. d Pn ) P, R ) Ph, and Ch ) Se. e Pn ) N and R ) Me. Structural data for one
of the two independent structures are shown.
Scheme 1. Syntheses of Dibenzopnictogenaborinsa
2).10,11 The diheteroanthracene ring of nitrogen-containing
7 deviated slightly from the planar structure due to the
twisting of a benzene ring. In contrast, dibenzophosphaborin
1 had a butterfly-type structure owing to the pyramidalization
at the phosphorus atom.10 Such a nonplanar structure suggests
that the lone pair orbital is not aligned perpendicular to the
dibenzoborin framework, resulting in an inferior electronic
interaction compared to that seen in 7.
a (i) n-BuLi (for 5) or t-BuLi (for 6), 0 °C; (ii) MesB(OMe)2, reflux.
reaction of the dilithio derivatives prepared from the corre-
sponding dibromides 5 and 6, respectively, with MesB-
(OMe)2 by refluxing in Et2O (Scheme 1). These molecules
were stable against air and moisture both in the solid state
and in solution because of the steric protection of the mesityl
groups. In addition, the ring structures of these mole-
cules fix the position of the mesityl groups orthogonal to
the pnictogenaborin framework, so that the steric protection
of the mesityl groups is effective compared to other tri-
arylboranes bearing one mesityl group that are oxidized in
air.
The P-derivatives of 1 were synthesized using usual
organophosphorus chemistry methods (Scheme 2). The
sulfide 2 and selenide 3 were obtained by the reaction of 1
with the elemental chalcogen in toluene. These reactions
proceeded quantitatively, judging from the 31P NMR spectra,
but the moderate solubility of 2 and 3 may decrease the
isolated yields. The reaction with BnBr gave the phospho-
nium salt 4. These results indicate that the phosphorus atom
of 1 has the same reactivity as that of ordinary triarylphos-
phines. The sulfide 2 and phosphonium salt 4 were pale
yellow and colorless solids, respectively, but the selenide 3
was obtained as a yellow solid. We also tried to synthesize
the dibenzophosphaborin P-oxide from the oxidation of 1
using various oxidants, but the decomposition of the frame-
Single crystals of 1 and 7 suitable for X-ray crystal-
lographic analysis were obtained by slow evaporation of their
C6H6 and CHCl3/EtOH solutions, respectively (Figures 1 and
9140 Inorganic Chemistry, Vol. 45, No. 22, 2006

Dibenzopnictogenaborins
Scheme 2. Syntheses of Dibenzophosphaborin P-Derivatives
the crystals: one molecule had a near planar structure, but
the other had a boatlike distorted ring with a central
phosphaborin moiety. One MeOH molecule/two molecules
of 2 was included in the unit cell, and a hydrogen bond was
formed between MeOH and the boatlike structural isomer
of 2. Although no solvent molecules were found in the single
crystals of 3, there was no significant difference in the crystal
structures between these two compounds.
The crystal data for 2, 3, and 7 are summarized in Table
1, and selected bond lengths and angles of 2, 3, and 7, as
well as 1, are listed in Table 2.
The UV-vis spectra of the dibenzopnictogenaborins and
the dibenzophosphaborin P-derivatives were recorded in
CH2Cl2 at 298 K (Table 3). Dibenzoazaborine 7 showed an
Figure 1. ORTEP drawings of dibenzophosphaborin 1 (50% probability):10
(a) top view; (b) side view (mesityl and phenyl groups not shown). Hydrogen
atoms and a solvent molecule are omitted for clarity.
Figure 2. ORTEP drawings of dibenzoazaborine 7 (50% probability): (a)
top view; (b) side view (mesityl and methyl groups not shown). One of
two independent structures is shown here. Hydrogen atoms are omitted for
clarity.
work occurred due to the instability of the triarylborane
moiety in the oxidative conditions.
Single crystals of the chalcogenides 2 and 3 were obtained
by recrystallization from CHCl3/MeOH and CHCl3/EtOH,
respectively.11 The crystal structures of 2 and 3 were
investigated using X-ray crystallographic analysis, and the
molecular structures of 2 and 3 are shown in Figures 3 and
4, respectively. Two independent structures were found in
Figure 3. ORTEP drawings of dibenzophosphaborin sulfide 2 (50%
probability): (a) isomer with a planar structure; (b) isomer with a butterfly
structure. The hydrogen bond between MeOH and the sulfur atom is shown
as a dotted line (MeOH-S: 1.92(5) Å).
Inorganic Chemistry, Vol. 45, No. 22, 2006 9141

Agou et al.
Figure 5. Dibenzoborin 8 and dibenzooxaborin 9.
Figure 6. VT UV-vis spectra of phosphonium salt 4 in CH2Cl2 (1.0 ×
10-4 M).
of 8 or 9, respectively. Theoretical calculations on the
phosphaborins also suggested that the P-Ph bonds of the
phosphaborins do not function as acceptor orbitals (see
Supporting Information for the orbital plots of the phos-
phaborins). The dibenzophosphaborin chalcogenides 2 and
3 showed a weak absorption in near UV region, with intense
absorption bands in the UV region. The absorption maximum
of the selenide 3 was red-shifted compared to that of the
sulfide 2. These absorptions were assigned to the intramo-
lecular charge transfer from the lone pair orbitals on the
chalcogen atoms to the LUMOs that are constructed mainly
from the boron 2p orbitals. This idea is supported by TD-
DFT calculations (vide infra).
Figure 4. ORTEP drawings of dibenzophosphaborin selenide 3 (50%
probability): (a) isomer with a planar structure; (b) isomer with a butterfly
structure.
Table 3. UV-Vis Spectral Data for Dibenzopnictogenaborins in
CH2Cl2 at 298 K
The UV-vis spectra of the phosphonium salt 4 (1.0 ×
10-4 M in CH2Cl2) showed a temperature dependency (Figure
6). The intensity of the absorption band around 300 nm
decreased on cooling. This absorption was attributed to the
π-π* transition of the dibenzoborin moiety, and no π-con-
jugation via the vacant 2p orbital on the boron atom occurred
due to the coordination of the bromide anion at lower
temperatures. The temperature dependence of the coordina-
tion of the bromide anion was also shown in the VT 11B
NMR spectra of 4 in CD2Cl2 (7.0 × 10-2 M) (Figure 7).
Above a temperature of 273 K, a single signal at δB 56,
corresponding to triarylborane, was observed. When the
sample was cooled to temperature below 253 K, the signal
at δB 56 decreased in intensity, and a new signal was
observed at around δB 0. This signal was attributed to the
bromoborate that is formed by the coordination of the
bromide anion to the boron atom.
compd
λ/nm (ꢀ/M-1 cm-1
)
1
7
2
3
4
368 (5000)
404 (11 600), 385 (9900)
353 (2000), 360 (1400)
369 (1300), 391 (1200)
312 (8000), 307 (7600), 381 (660)
intense and sharp absorption profile in the violet to near UV
region, but the UV-vis absorption of the phosphaborin 1
showed a weak and broad absorption, indicating a weaker
donor-acceptor interaction between the phosphorus and
boron atoms through π-conjugation, which was expected
from the crystal structure described above. The phosphonium
salt 4 showed an absorption maximum around 300 nm that
is close to that of dibenzoborin 8 (λmax 313 nm) and
dibenzoxaborine 9 (λmax 321 nm) (Figure 5).9 Although the
σ*(P-Ph) orbital of 4 potentially works as an electron
acceptor orbital, like the σ*(Si-R) orbital of siloles or the
σ*(P-R) orbital of phospholes (vide supra), these spectral
data suggest that such electronic interactions are very weak
in the case of 4, and the phosphorus atom of 4 effectively
functions as a bridging unit, like the carbon or oxygen atoms
Dibenzoazaborine 7 exhibited a moderately strong fluo-
rescence in cyclohexane at 298 K (λem 421 nm, Φ 0.48),
with a small Stokes shift (∆λ < 20 nm), indicating a retention
of the rigid molecular structure in the excited state.12
9142 Inorganic Chemistry, Vol. 45, No. 22, 2006

Dibenzopnictogenaborins
Table 4. Calculated Frontier Orbital Energy and Excited Statesa
compd
HOMO/eV
LUMO/eV
excited statesb
1
7
2
-5.74
-5.40
-5.81c
-5.42c
-9.04
-1.78
-1.42
-2.34
-2.34
-5.32
π* r π
π* r π
π* r n(Ch)
π* r n(Ch)
π* r π(B-Ph)
π* r π
3
4d
a All the calculations were performed at the B3LYP/6-31G(d) level of
theory. The mesityl groups on boron atoms were replaced by phenyl groups
to reduce computation time. b Excited states were estimated by TD-DFT
method implemented with the Gaussian 03 program package. c HOMO was
the lone pair orbital of a chalcogen atom. d Only the cation part was
calculated.
Table 5. Complex Formation Constants K(X-) (X ) F, Cl) of
Dibenzopnictogenaborins
compd
K(F-)/M-1
K(Cl-)/M-1
a
1
7
2
3
4
2.1(3) × 104
-
-
Figure 7. VT 11B NMR of phosphonium salt 4 in CD2Cl2 (7.0 × 10-2
a
14(3)
1.4(2) × 105
3.3(7) × 106
11(1)
M).
14(3)
b
-
1.7(1) × 104
However, the fluorescence of dibenzophosphaborin 1 and
its P-derivatives was very weak (Φ < 0.01), and the
broadened spectral profiles, as well as the large Stokes shift
(∆λ > 100 nm), revealed the existence of a substantial
structural change after photoexcitation.
a Complex formation was not observed. b The value of K(F-) was too
large to be determined.
in the near UV region are thought to be intramolecular
charge-transfer bands from the lone pair orbitals on the
chalcogen atoms to the vacant 2p orbitals on the boron atoms.
TD-DFT calculations also reveal that the lowest excited states
of 2 and 3 correspond to the π* r n(Ch) transitions. Among
the compounds investigated, compound 4 has anomalously
low-lying frontier orbitals, reflecting the strong electron-
withdrawing property of the phosphonium moiety (Table 4).
The longest UV absorption bands of 4 are attributable to
the intramolecular charge transfer from the mesityl group
on the boron atom to the π* orbital on the dibenzophos-
phaborin framework, and the second longest absorption band
originates from the π* r π transition of the dibenzophos-
phaborin moiety.
The complex formation constants between the dibenzo-
heteraborins and halide anions were determined from
UV-vis titration experiments, and the results are summarized
in Table 5. The complex formation constants of dibenzo-
azaborine 7, phosphaborin 1, and its sulfide 2 with fluoride
anion increased in this order, reflecting the order of the
calculated LUMO levels.15 The complex formation constant
of the phosphonium salt 4 was too large to be determined
under the same conditions. Next, we tested the complex
formation with chloride anion. It is difficult to capture halide
anions with triarylboranes, other than fluoride anion,16 and
the dibenzoheteraborins having relatively high LUMO levels
did not form complexes with chloride anion. However, the
Theoretical calculations at the B3LYP/6-31G(d) level of
theory were performed to investigate the electronic structure
of the dibenzoheteraborins, and the calculated energy levels
of the frontier molecular orbitals and excited states estimated
using the TD-DFT method are summarized in Table 4.13,14
Orbital plots are shown in the Supporting Information. All
the LUMOs of these molecules were mainly constructed from
the vacant 2p orbitals on the boron atoms. Both the HOMO
and LUMO of dibenzoazaborine 7 were relatively high in
energy because of the strong electron-donating properties of
the nitrogen atom. When the main group element was
changed to phosphorus, the energy level of the frontier
orbitals decreased by 0.3 eV due to the weaker electron
donation. The effect of modification of the phosphorus atom
of dibenzophosphaborin was studied for the sulfide 2,
selenide 3, and phosphonium salt 4. The HOMOs of 2 and
3 were attributed to the lone pair orbitals on the chalcogen
atoms, and the UV-vis absorption bands of these molecules
(12) Fluorescence quantum yields were determined using anthracene in
EtOH (Φ 0.27) as a standard.
(13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(14) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(15) Optical sensing of fluoride anion has been achieved taking advantage
of various intermolecular interactions: (a) Cooper, C. R. Chem.
Commun. 1998, 1365-1366. (b) Black, C. B.; Andrioletti, B.; Try,
A. C.; Ruiperez, C.; Sessler, J. L. J. Am. Chem. Soc. 1999, 121,
10438-10439. (c) Dusemund, C.; Sandanayake, K.; Shinkai, S. J.
Chem. Soc., Chem. Commun. 1995, 333-334. (d) Yamamoto, H.; Ori,
A.; Ueda, K.; Dusemund, C.; Shinkai, S. Chem. Commun. 1996, 407-
408. (e) Liu, B.; Tian, H. J. Mater. Chem. 2005, 15, 2681-2686. (f)
Cho, E. J.; Ryu, B. J.; Lee, Y. J.; Nam, K. C. Org. Lett. 2005, 7,
2607-2609.
Inorganic Chemistry, Vol. 45, No. 22, 2006 9143

Agou et al.
dibenzophosphaborin chalcogenides 2 and 3 formed com-
plexes with chloride anion, and the phosphonium salt 4
showed a high complex formation constant with chloride
anion, reflecting the anomalously low LUMO energy
level. Although we also tried titration experiments using
n-Bu4NBr under the same experimental conditions, complex
formation was not observed. However, the VT 11B NMR
data indicate that bromide anion can coordinate to the boron
atom at low temperatures (vide supra). As described above,
the Lewis acidity of dibenzophosphaborin was remarkably
enhanced by substitution on the phosphorus atom site.
and its P-derivatives emitted a weak fluorescence due to the
lack of rigidity, but titration with halide anions revealed that
the Lewis acidity of the P-derivatives is tunable due to the
change in substituent on the phosphorus atom site. Among
these compounds, the phosphonium salt has the strongest
Lewis acidity, and even bromide anion can coordinate to
the boron center at low temperatures.
Acknowledgment. This work was supported by Grants-
in-Aid for The 21st Century COE Program for Frontiers in
Fundamental Chemistry (T.K.) and for Scientific Research
(T.K.) from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan and Research Fellowships
of the Japan Society for the Promotion of Science for Young
Scientists (T.A.). We thank Prof. Dr. Hiroshi Nishihara, Dr.
Masaki Murata, and Dr. Shoko Kume (The University of
Tokyo) for their valuable advice for optical measurements.
We also thank Tosoh Finechem Corp. for the generous gifts
of alkyllithium.
Conclusions
In summary, we have synthesized dibenzopnictogenaborins
bearing a nitrogen or phosphorus atom. These molecules
showed different optical properties and Lewis acidity cor-
responding to the bridging main group element. The aza-
borine exhibited relatively strong UV-vis absorption bands
and fluorescence, with a small Stokes shift, reflecting the
rigid, planar structure. On the other hand, the phosphaborin
Supporting Information Available: X-ray experimental details,
atomic coordinates, interatomic distances and angles, anisotropic
thermal parameters, and hydrogen parameters for compounds 2, 3,
and 7 in CIF format and orbital plots of 1-4 and 7. This material
is available free of charge via the Internet at http://pubs.acs.org.
(16) Chloroborates of highly Lewis acidic triarylborane B(C6F5)3 are stable
enough for structural determination: (a) Bosch, B. E.; Erker, G.;
Fro¨hlich, R.; Meyer, O. Organometallics 1997, 16, 5449-5456. (b)
Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.; Thornton-Pett,
M.; Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223-237.
IC061055Q
9144 Inorganic Chemistry, Vol. 45, No. 22, 2006