π-conjugated phosphasilenes stabilized by fused-ring bulky groups

Article abstract of DOI:10.1021/ja9051153

(Chemical Equation Presented) πConjugated phosphasilenes with a variety of aryl substituents on the silicon atom have been synthesized by the use of a 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) group. X-ray structural analysis shows the highly co

Full text of DOI:10.1021/ja9051153

Published on Web 08/28/2009
π-Conjugated Phosphasilenes Stabilized by Fused-Ring Bulky Groups
Baolin Li,† Tsukasa Matsuo,*,† Daisuke Hashizume,‡ Hiroyuki Fueno,§ Kazuyoshi Tanaka,§ and
Kohei Tamao*,†
Functional Elemento-Organic Chemistry Unit and AdVanced Technology Support DiVision, RIKEN AdVanced
Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, and Department of Molecular Engineering,
Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan
Received June 22, 2009; E-mail: matsuo@riken.jp; tamao@riken.jp
Scheme 1
Organic π-conjugated architectures featuring multiple bonds of
the heavier main group elements1 have attracted much attention
because of their potentially useful properties and unique functions.2
For example, a number of oligomers and polymers encapsulating
PdE (E ) C, P)3 and SidSi4 bonds have been investigated, which
would open the way to new elemento-organic hybrid materials.
Recently, we5 and the Scheschkewitz group6 independently reported
the synthesis of disilene analogues of oligo(p-phenylenevinylene)s
(Si-OPVs). In our case, the fused-ring bulky Eind groups (Eind )
1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) effectively protected
the reactive SidSi fragments, thus producing the highly coplanar
arrangement of the Si-OPV framework. Notably, our 1,4-bis(di-
silenyl)benzene derivative shows a unique fluorescence at room
temperature based on the efficient π-delocalization over the
skeleton.
Although various phosphasilenes have been reported by taking
stability; 1a-c have melting points of >250 °C under argon
advantage of the steric protection with appropriate bulky substit-
uents, their electronic properties have rarely been explored;7,8 weak
atmosphere in a sealed tube.
The crystal structures of 1a-c and 2 were revealed by X-ray
absorptions probably due to the n-π* transition have been reported
only in rare cases.8c,g We now report the synthesis and characteriza-
tion of a series of π-conjugated phosphasilenes 1a-c and 2
stabilized by the Eind groups. In our case, strong π-π* absorptions
have been observed due to the highly coplanar framework
incorporating the SidP unit and the aromatic ring on the silicon
atom.
analysis, as represented by 1c and 2 shown in Figure 1. The aryl
and the arylene groups are entirely coplanar with the SidP bond(s),
having a P-Si-C-C torsion angle of 3.04(13)° (1a), 1.1(4)° (1b),
2.0(2)° (1c), and 0.8(2)° (2), respectively, which is well suited for
the 2pπ-3pπ conjugation. This coplanarity is apparently attained
by the two rigid Eind groups, oriented trans and perpendicular with
respect to the SidP bond, as observed in the disilene case.5 For 1b
and 1c, the arrangement between the SidP bond and the polycyclic
aromatic substituents about the intervening SisC bond is described
as an s-cis conformational geometry in the crystals.9 The SidP
bond lengths (ca. 2.09-2.10 Å) are in the range of those for typical
phosphasilenes.7,8
Phosphasilenes 1a-c and 2 were synthesized starting from the
bulky phosphine 3 (Scheme 1). The coupling reaction between
lithium phosphide 4 and dibromosilanes 5a-c and subsequent
removal of HBr from phosphasilanes 6a-c with 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), afforded phosphasilenes
1a-c, as yellow to yellow-orange crystals in 25-65% yields. We
have also isolated the orange crystals of 1,4-bis(phosphasilenyl)-
benzene 2 using 1,4-bis(dibromosilyl)benzene 7 in 29% yield, in
which two SidP bonds are connected by a phenylene linker. The
formation of the SidP bond was clearly confirmed by the 29Si and
31P NMR spectra. In the 29Si NMR spectra of 1a-c, one signal
was observed at 161.7-162.1 ppm as a doublet with a 1JSiP coupling
constant of 171-174 Hz. The characteristic 31P NMR signal was
observed at 89.2-89.7 ppm. Solid-state NMR experiments have
been utilized for characterization of 2, due to its poor solubility in
organic solvents. The NMR data indicated a highly symmetrical
structure for 2 in the solid state, which is in agreement with the
crystal structure (vide infra). The resulting phosphasilenes were
found to be stable in air for months in the solid state and for a
week as a dilute solution, demonstrating the efficient protection
ability of the Eind groups. Each compound has high thermal
Figure 1. Molecular structures of 1c (a) and 2 (b) (50% probability level).
Hydrogen atoms are omitted for clarity.
Some photophysical and electrochemical properties are sum-
marized as follows. (1) The UV-vis spectrum of 1a shows an
absorption peak at 385 nm (ꢀ ) 1.2 × 104), which is bathochro-
mically shifted about 50 nm and 2-orders stronger relative to the
longest wavelength absorption of the previously reported phospha-
silenes.8c,g The introduction of the 2-naphthyl (1b) and the 2-anthryl
(1c) groups to the silicon atom causes further red-shifts to 405 nm
(ꢀ ) 1.1 × 104) and 430 nm (sh, ꢀ ) 1.3 × 104), respectively.
† RIKEN, Functional Elemento-Organic Chemistry Unit.
‡ RIKEN, Advanced Technology Support Division.
§ Kyoto University.
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10.1021/ja9051153 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Moreover, the absorption maximum of 2 is observed at 449 nm (ꢀ
) 2.2 × 104), which is 64 nm red-shifted from 1a, indicating the
extension of π-conjugation over the 1,4-bis(phosphasilenyl)benzene
skeleton. (2) The extended 2 shows a weak, but distinct emission
in the solid state. The emission maximum of 2 is observed at 592
nm. The large Stokes shift may be attributed to the structural change
from the coplanar structure in the ground state to the twisted
structure in the excited state.10 (3) The irreversible oxidation waves
at +0.94 V (1a) and +0.97 V (2) as well as the quasi-reversible
and the irreversible reduction waves at -2.55 V (1a) and -2.12 V
(2) were observed by cyclic voltammetry, respectively, as the first
experimental electrochemical studies of the phosphasilenes.
To delineate the nature of the bonding in the phosphasilenes,
DFT computations at the B3LYP/6-31G** level were carried out
for 1a and 2 using the Gaussian 03 program package.11 The
optimized structures reproduced well the X-ray crystal structures.
Three pertinent MOs of 1a are shown in Figure 2. While the HOMO
of 1a is primarily represented by the 3pπ(Si-P) orbital, the LUMO
Acknowledgment. We thank the Ministry of Education, Culture,
Sports, Science, and Technology of Japan for the Grant-in-Aid for
Specially Promoted Research (No. 19002008). We also thank Dr.
Yayoi Hongo and Mr. Takashi Nakamura (RIKEN) for their kind
help with mass spectrometry and solid-state NMR spectroscopy.
Numerical calculations were partly performed at the Supercomputer
Laboratory, Institute for Chemical Research, Kyoto University.
Supporting Information Available: Experimental details, crystal-
lographic data of 1a-c and 2 (CIF), details for calculations of 1a and
2, and full listing for ref 11. This material is available free of charge
via Internet at http://pubs.acs.org.
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the
substantial
contribution
of
the
3pπ*(Si-P)-2pπ*(phenyl) conjugation. The lower-lying HOMO-1
involves the 3n-2pπ conjugation due to the lone pair on the
phosphorus atom and the π-orbital on the Eind benzene ring; the
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Figure 2. Selected molecular orbitals of 1a (top view).
The wavelengths were calculated to be 364 (1a) and 456 (2) nm
based on TD-DFT calculations, comparable to the observed values.
The absorption at 364 nm of 1a is assignable to the mixed version
of the HOMOfLUMO (π-π*) and HOMO-1fLUMO ((n + π
of Eind benzene)-π*) transitions with almost equal weighting. In
contrast, the absorption at 456 nm of 2 is mainly assigned to the
HOMOfLUMO (π-π*) transition.
In conclusion, we have established for the first time that the SidP
bond can conjugate with the carbon π-electron system by introduc-
tion of a variety of aryl substituents on the silicon atom and the
use of the Eind groups as protecting groups. Further work to clarify
the promising coordination abilities of these phosphasilenes is
currently underway.
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