Synthesis, optical properties, and reactivities of a dibenzophosphaborin and its derivatives

Article abstract of DOI:10.1080/10426500701735296

Dibenzo-1,4-phosphaborins were synthesized and characterized by X-ray crystallographic analysis. The phosphaborins exhibited UV/vis absorption derived from intramolecular charge transfer from the phosphorus atom to the boron atom. The phosphaborins showed different fluorescent properties depending on the substituents on the phosphorus atom. The phosphaborin was easily derivatized to P-sulfide, P-selenide and phosphonium salt. The phosphaborin and its P-derivatives formed complexes with fluoride ion. The complex formation constants were shown to increase in accordance with the decrease in their LUMO energy levels. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500701735296

This article was downloaded by: [Northeastern University]
On: 27 December 2014, At: 11:06
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Phosphorus, Sulfur, and Silicon
and the Related Elements
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gpss20
Synthesis, Optical Properties,
and Reactivities of A
Dibenzophosphaborin and Its
Derivatives
Junji Kobayashi a , Tomohiro Agou a & Takayuki
Kawashima a
a Department of Chemistry, Graduate School of
Science , The University of Tokyo, Hongo , Bunkyo-
ku, Tokyo, Japan
Published online: 20 Jun 2008.
To cite this article: Junji Kobayashi , Tomohiro Agou & Takayuki Kawashima (2008)
Synthesis, Optical Properties, and Reactivities of A Dibenzophosphaborin and Its
Derivatives, Phosphorus, Sulfur, and Silicon and the Related Elements, 183:2-3,
389-392, DOI: 10.1080/10426500701735296
To link to this article: http://dx.doi.org/10.1080/10426500701735296
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,

and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Phosphorus, Sulfur, and Silicon, 183:389–392, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701735296
Synthesis, Optical Properties, and Reactivities of A
Dibenzophosphaborin and Its Derivatives
Junji Kobayashi, Tomohiro Agou, and
Takayuki Kawashima
Department of Chemistry, Graduate School of Science, The University of
Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan
Dibenzo-1,4-phosphaborins were synthesized and characterized by X-ray
crystallographic analysis. The phosphaborins exhibited UV/vis absorption derived
from intramolecular charge transfer from the phosphorus atom to the boron
atom. The phosphaborins showed different fluorescent properties depending
on the substituents on the phosphorus atom. The phosphaborin was easily
derivatized to P-sulfide, P-selenide and phosphonium salt. The phosphaborin
and its P-derivatives formed complexes with fluoride ion. The complex formation
constants were shown to increase in accordance with the decrease in their LUMO
energy levels.
Keywords Fluorescence; fluoride sensing; heteraborins; hetero-π-conjugated molecules
INTRODUCTION
π−Conjugated molecules with novel electronic and optical properties
are desirable for constructing organic functional materials including
organic semiconductors and light emitting devices. Recently, incorpo-
ration of main group elements into π-conjugated systems has been re-
ported as an efficient methodology to decrease HOMO-LUMO energy
gaps because of π-conjugation between π-orbitals and donor or accep-
tor orbitals on main group elements. Among these classes of molecules,
boron-containing hetero π-conjugated molecules have attracted much
attention because they have a vacant 2p orbital on the boron atom,
which can decrease the LUMO level and respond to external stimuli (i.e.
Lewis bases). Herein we report the syntheses and the properties of a
dibenzophosphaborin and its derivatives as the novel boron-containing
hetero π-conjugated molecules.
Address correspondence to Takayuki Kawashima, Department of Chemistry, Gradu-
ate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan. E-mail: takayuki@chem.s.u-tokyo.ac.jp
389

390
J. Kobayashi et al.
RESULTS AND DISCUSSION
B-Mesityl-P-phenylphosphaborin
1
was synthesized by the
reaction of dilithio derivative prepared from the bis(2-
bromophenyl)phenyphosphane with MesB(OMe)2 in Et2O under
reflux conditions (Scheme 1). Phosphaborin 1 was obtained as crys-
talline solids, which was stable against air and moisture in the solid
state.1
SCHEME 1
The P-derivatives of 1 were synthesized using usual organophos-
phorus chemistry methods (Scheme 2).2 The sulfide 2 and selenide 3
were obtained by the reaction of 1 with the corresponding elemental
chalcogen in toluene. These reactions proceeded quantitatively, judg-
ing from the 31P NMR spectra, but the moderate solubility of 2 and
3 may decrease the isolated yields. The reaction of 1 with BnBr gave
the phosphonium salt 4. These results indicate that the phosphorus
atom of 1 has the same reactivity as that of ordinary triarylphosphines.
The sulfide 2 and phosphonium salt 4 were pale yellow and color-
less solids, respectively, but the selenide 3 was obtained as a yellow
solid.
SCHEME 2

Synthesis, Optical Properties, and Reactivities of A Dibenzophosphaborin
391
FIGURE 1 ORTEP drawing of dibenzophosphaborin 1.
Single crystals of 1 suitable for X-ray crystallographic analysis were
obtained by recrystallization from benzene/Et2O (Figure 1). Unlike the
azaborine ring, the phosphaborin ring of 1 deviates from planarity due
to pyramidalization at the phosphorus atom. The sums of the bond
angles around the phosphorus atom (307◦) and the boron atom (360◦)
are similar to those of ordinary triarylphosphanes and triarylboranes,
respectively.
The UV-vis spectra of the dibenzopnictogenaborins and the diben-
zophosphaborin P-derivatives were recorded in CH2Cl2 at 298 K (Ta-
ble I). Dibenzoazaborine 5 showed an intense and sharp absorption pro-
file in the violet to near UV region, but the UV-vis absorption of the phos-
phaborin 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 max-
imum around 300 nm that is close to those of dibenzoborin 6 (λmax
313 nm) and dibenzoxaborine 7 (λmax 321 nm). Although the σ∗(P Ph)
TABLE I UV-vis Spectral Data for Dibenzopnictogenaborins in
CH2Cl2 at 298 K
Compound
λ/nm (ε/M−1 cm−1
)
1
5
2
3
4
368 (5000)
404 (11600), 385 (9900)
353 (2000), 360 (1400)
369 (1300), 391 (1200)
312 (8000), 307 (7600), 381 (660)

392
J. Kobayashi et al.
TABLE II Complex Formation Constants
K(F−) of Dibenzopnictogenaborins
Compound
K(F−)/M−1
1
5
2
3
4
2.1(3) ×104
14(3)
1.4(2) ×105
3.3(7) ×106
a
−
a
The vale of K(F−) was too large to be determined.
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, 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 of 6 or 7, respectively.
Theoretical calculations on the phosphaborins also suggested that the
P Ph bonds of the phosphaborins do not function as acceptor orbitals.
The dibenzophosphaborin chalcogenides 2 and 3 showed a weak absorp-
tion 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 intramolec-
ular charge transfer from the lone pair orbitals on the chalcogen atoms
to the LUMOs that are constructed mainly from the boron 2p orbitals.
Dibenzoazaborine 5 exhibited moderately strong fluorescence in cy-
clohexane at 298 K (λem 421 nm,ꢀ 0.48), with a small Stokes shift (ꢁλ
20 nm), indicating a retention of the rigid molecular structure in the
excited state. However, the fluorescence of dibenzophosphaborin 1 and
its P-derivatives was very weak (ꢀ < 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.
The complex formation constants between the dibenzoheteraborins
and fluoride ion were determined from UV-vis titration experiments,
and the results are summarized in Table II. The complex formation
constants of dibenzoazaborine 5, phosphaborin 1 and its sulfide 2 with
fluoride ion increased in this order, reflecting the order of the calculated
LUMO levels. The complex formation constant of the phosphonium salt
4 was too large to be determined under the same conditions.
REFERENCES
[1] T. Agou, J. Kobayashi, and T. Kawashima, Org. Lett., 7, 4373 (2005).
[2] T. Agou, J. Kobayashi, and T. Kawashima, Inorg. Chem., 45, 9137 (2006).