π-conjugated phosphasilenes stabilized by fused-ring bulky "rind" groups

Article abstract of DOI:10.1016/j.crci.2010.07.005

Recent progress in the chemistry of π-conjugated phosphasilenes stabilized by bulky protecting groups based on a fused-ring octa-R-substituted s-hydrindacene skeleton (Rind groups) is reviewed. The phosphasilenes with a variety of aryl substituents at the

Full text of DOI:10.1016/j.crci.2010.07.005

C. R. Chimie 13 (2010) 1104–1110
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Account/Revue
p-conjugated phosphasilenes stabilized by fused-ring bulky ‘‘Rind’’
groups
*
*
Tsukasa Matsuo , Baolin Li, Kohei Tamao
Functional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Recent progress in the chemistry of
p-conjugated phosphasilenes stabilized by bulky
Received 9 February 2010
Accepted after revision 5 July 2010
Available online 23 August 2010
protecting groups based on a fused-ring octa-R-substituted s-hydrindacene skeleton (Rind
groups) is reviewed. The phosphasilenes with a variety of aryl substituents at the silicon
atom are covered in detail. The X-ray crystallography analysis showed the highly coplanar
p
-framework reinforced by the perpendicularly-fixed Rind groups. Strong
absorptions have been observed, demonstrating the extension of -conjugation over
the skeleton. The DFT calculations indicated that the LUMO involves significant
contribution by the 3p *(Si–P)–2p *(carbon -electron system) conjugation. The
preparation of the gold(I) complexes of the -conjugated phosphasilenes is also included.
p–p*
Keywords:
p
Phosphorus
Silicon
p
p
p
Conjugation
Multiple bonds
Steric hindrance
Substituent effect
Gold
p
ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
PH group and its metallated compounds were also
reported [6e,f]. However, despite the unconventional polar
In 1981, a new concept of bulky protecting groups was
introduced for the kinetic stabilization of highly reactive
unsaturated bonds between the main group elements
heavier than the second row elements [1]. Since then,
many experimental efforts have been directed toward a
variety of unsaturated compounds of the heavy elements
[2], including the Si=P compounds, i.e., phosphasilenes [3].
In 1984, Bickelhaupt et al. reported the first stable
compound containing a Si=P double bond [4a]. Since then,
various phosphasilenes have been prepared by taking
advantage of the steric protection with appropriate bulky
substituents [4–7]. For example, the first X-ray structural
analysis of phosphasilene was reported by Niecke et al. in
1993 [5a]. A series of phosphasilenes having bulky silyl
groups at the phosphorus atom have been studied by
Driess et al. [6a–d]. The ‘‘half’’-parent phosphasilene with a
heavy double bonds, the electronic properties of the
phosphasilenes have been only slightly explored; weak
absorptions probably due to the n–p* transition have been
reported only in rare cases [4c, 6c]. Very recently, a ‘‘push-
pull’’ phosphasilene was synthesized by Sekiguchi and
´
Escudie groups [7], the EPR studies of its anion radical
species was reported.
Last year, we reported the first experimental evidence
for the
the carbon
[8]. Organic
p
-conjugation between the Si=P double bond and
-electron system linked to the silicon atom
-conjugated architectures comprised of
p
p
multiple bonds of the heavier main group elements have
attracted much attention because of their potentially
useful properties and unique functions [9]. For example, a
variety of oligomers and polymers encapsulating P=E
(E = C, P) [10] and Si=Si [11] bonds have been investigated,
which would open the way to new elemento-organic
hybrid materials. In 2007, Scheschkewitz’s group [12] and
we [13] independently reported the synthesis and
characterization of the disilene analogues of the oligo(p-
phenylenevinylene)s (Si-OPVs) (Scheme 1). In our case, the
* Corresponding authors.
E-mail addresses: matsuo@riken.jp (T. Matsuo), tamao@riken.jp
(K. Tamao).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.07.005

T. Matsuo et al. / C. R. Chimie 13 (2010) 1104–1110
1105
Scheme 1. Si-OPV dimers and Rind groups.
newly developed fused-ring bulky 1,1,3,3,5,5,7,7-
octaethyl-s-hydrindacen-4-yl groups, abbreviated as
‘‘Eind’’ groups (‘‘E’’ for ethyl), effectively protect the
reactive Si=Si fragments, thus providing the highly
coplanar arrangement of the Si-OPV dimer framework.
Notably, our 1,4-bis(disilenyl)benzene derivative showed
a unique fluorescence at room temperature based on the
Scheme 3. Synthesis of 5a–g.
efficient
p-delocalization over the skeleton.
After the initial findings of the effectiveness of the Eind
groups in the Si-OPVs, we concentrated on the develop-
mixture of the Eind-stabilized diphosphene 4 [16], phos-
phasilene 5a, and disilene 6 [13]. We have isolated 4 as
orange crystals, 5a as yellow crystals, and 6 as orange
crystals in 20, 33, and 30% yields, respectively, using silica
gel chromatography in a glove box filled with argon gas.
However, this synthetic approach seemed to be not suitable
for the large-scale synthesis of the phosphasilenes.
ment of
p-conjugated systems containing heavy unsatu-
rated bonds by taking advantage of the bulky ‘‘Rind’’
groups based on a fused-ring octa-R-substituted s-hydrin-
dacene skeleton [14], where R denotes the initial of the
substituents. For example, 1,1,7,7-tetraethyl-3,3,5,5-tetra-
methyl-s-hydrindacene-4-yl group, abbreviated as an
‘‘EMind’’ group (‘‘M’’ for methyl), is sometimes useful as
the less bulky group. In this report, we review our recent
The selective synthesis of phosphasilenes has been
achieved by the stepwise construction of the Si=P double
bonds, starting from the Eind-phosphine 7 as shown in
Scheme 3. The coupling reaction between the lithium
phosphide (8) and 1a and subsequent removal of HBr from
the resulting phosphasilane 9a with 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) afforded the Eind-stabilized
phosphasilene 5a in 65% yield. Similarly, we synthesized a
series of Eind-stabilized phosphasilenes 5b–g with various
aryl substituents at the silicon atom: 2-naphthyl (5b), 2-
anthryl (5c), 2-fluorenyl (5d) [17], 1-naphthyl (5e) [17], 2-
thienyl (5f) [17], and 2,2’-bithienyl (5 g) [17]. Each
compound was isolated as yellow to yellow-orange
crystals in 15–35% yields. We have also synthesized the
1,4-bis(phosphasilenyl)benzene 12, in which two Si=P
bonds are connected by a para-phenylene linker as shown
in Scheme 4. Thus, the reaction of 1,4-bis(dibromosilyl)-
benzene 10 with 2 equiv. of 8 and subsequent treatment
with DBU produced 12 as orange crystals in 29% yield.
The 29Si and 31P NMR spectral data of 5a–g are listed in
Table 1. In the 29Si NMR spectra of 5a–d, 5f and 5 g, one
results for a series of
p-conjugated phosphasilenes and
their gold(I) complexes.
2. Preparation of
p-conjugated phosphasilenes
At the beginning of this study, as shown in Scheme 2,
we examined the one-step coupling reaction between the
Eind-dichlorophosphine
3 and Eind-dilithiosilane 2a,
which was formed by the reaction of the dibromosilane
1a with lithium naphthalenide (LiNaph) in THF at low
temperature [7,15]. This reaction, however, led to a
signal is observed at
with
1JSiP coupling constant of 171–178 Hz. The
characteristic 31P NMR signal is found at
= 77.2–
d = 147.7–162.1 ppm as a doublet
a
d
89.7 ppm. Compound 5e exhibits two signals in both the
29Si and 31P NMR spectra, respectively, indicating the
existence of two rotational isomers around the Si–
Cipso(aryl) bond, the s-cis and s-trans, in solution as shown
Scheme 2. Synthesis of 4, 5a, and 6.
in Fig. 1. However, all attempts to separate the isomers

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T. Matsuo et al. / C. R. Chimie 13 (2010) 1104–1110
Fig. 1. Rotational isomers of 5e.
Scheme 4. Synthesis of 12.
Table 1
Selected NMR spectral data for phosphasilenes.
Compound
d
(Si, ppm)
1JSiP (Hz)
d (P, ppm)
5aa
5ba
5ca
5da
5ea
161.7
171
174
174
174
183
180
178
175
182
89.2
161.9
89.7
162.1
89.2
160.0
82.5
149.6 (major)
147.9 (minor)
147.7
84.0 (major)
84.4 (minor)
77.2
5fa
5ga
12b
148.6
81.5
160.9
109.1
97.9c
Fig. 2. Molecular structure of 5a. Hydrogen atoms are omitted for the
a
In benzene-d6 at 25 8C.
In the solid state.
clarity.
b
c
In THF-d8 at 65 8C.
have been unsuccessful. For compound 12, the solid-state
NMR experiments have been utilized due to its poor
solubility in organic solvents. Both the 29Si and 31P NMR
spectral peaks appear as single signals, indicating a highly
symmetrical structure for 12 in the solid-state, which is in
agreement with the crystal structure (vide infra).
The resulting phosphasilenes are 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 a high thermal
stability; 5a–g melted at > 200 8C without decomposition
under an argon atmosphere in a sealed tube. The dimer 12
has decomposition points of > 340 8C under an argon
atmosphere in a sealed tube.
Fig. 3. Molecular structure of 12. Hydrogen atoms are omitted for the
clarity.
3. X-ray structural aspects
which is well suited for the 2p –3p conjugation. This
p
p
coplanarity is apparently attained by the rigid Eind groups,
oriented trans and perpendicular with respect to the Si=P
bond, as observed in the disilene case [13]. The space-
filling model of 12 shows that the peripheral ethyl groups
effectively protect the Si=P moieties (Fig. 4). The ethyl side
chains on the rigid s-hydrindacene skeletons interlock
with one another above and below the Si=P moiety to
The molecular structures of 5a–c and 12 have been
determined by X-ray crystallography; their structural
parameters are summarized in Table 2, and two of them,
5a and 12, are reproduced in Figs. 2 and 3. The aryl and the
arylene groups are entirely coplanar with the Si=P double
bond(s), having the P–Si–C–C torsion angles of 3.04(13)
(5a), 1.1(4) (5b), 2.0(2) (5c), and 0.8(2)8 (12), respectively,
Table 2
Selected X-ray crystal structural data for phosphasilenes.
˚
Compound
Bond lengths (A)
Torsion angle (8)
Si=P
Si–C(Eind)
Si–C(aryl)
P–C
P–Si–C–C
5a
5b
5c
12
2.0917(5)
2.0958(16)
2.0915(8)
2.0980(9)
1.9006(12)
1.901(4)
1.899(2)
1.902(3)
1.8633(13)
1.855(4)
1.860(2)
1.857(3)
1.8706(12)
1.876(4)
1.872(2)
1.871(3)
3.04(13)
1.1(4)
2.0(2)
0.8(2)

T. Matsuo et al. / C. R. Chimie 13 (2010) 1104–1110
1107
Fig. 5. UV-vis absorption spectra of 5a–c and 12 in THF.
is about 50 nm bathochromically shifted and two-orders
stronger relative to the lmax and of the previously
Fig. 4. Space-filling model of 12.
e
reported phosphasilenes [4c,6c]. The introduction of the 2-
naphthyl (5b) and the 2-anthryl (5c) groups to the silicon
atom causes further lmax red-shifts to 405 nm
enforce the planar geometry. The resulting cavity sur-
rounded by the ethyl groups can fix the framework aryl
groups in a coplanar conformation with the P=Si plane,
(
e
= 1.06 ꢀ 104) and 430 nm (sh,
e
= 1.27 ꢀ 104), respective-
ly. Moreover, the lmax of 12 is observed at 449 nm
which is ideal for the extension of the
p
-conjugation.
(
e
= 2.20 ꢀ 104), which is 64 nm red-shifted from 5a,
˚
The Si=P bond lengths of 5a–c and 12 (ca. 2.09–2.10 A)
indicating the extension of the p-conjugation over the
are in the range of those for typical phosphasilenes [5–7].
1,4-bis(phosphasilenyl)benzene skeleton.
˚
The Si–Cipso(Eind) bond lengths (ca. 1.90 A) are somewhat
While 5a does not show any fluorescence, the p-
extended 12 shows a weak, but distinct emission in the
˚
longer than the Si–Cipso(aryl) bond lengths (ca. 1.86 A),
presumably due to the small steric repulsion between the
ethyl groups of the Eind groups. The P–C bond lengths (ca.
solid state, as shown in Fig. 6 [8]. The emission maximum
of 12 is observed at 592 nm. The large Stokes (5380 cmꢁ1
)
˚
1.87–1.88 A) are normal. Each silicon atom in 5a–c and 12
shift may be attributed to the structural change from the
coplanar structure in the ground state to the twisted
structure in the excited state.
adopts an essentially planar geometry; the sum of the bond
angles around the Si atom are ca. 3608.
For 5b and 5c, the arrangement between the Si=P bond
and the polycyclic aromatic substituents about the
intervening Si–Cipso(aryl) bond is described as an s-cis
conformational geometry [18] in the crystals.
Although there have been a number of reports
regarding the electrochemical properties of the dipho-
sphenes [19], those of phosphasilenes have rarely been
explored [7]. We performed the electrochemical studies of
5a and 12 by cyclic voltammetry [8]. The irreversible
oxidation waves at +0.94 V (5a) and +0.97 V (12) as well as
4. Photophysical and electrochemical properties
Phosphasilenes 5a–g and 12 exhibit an intense
absorption band in the electronic absorption spectrum.
The data are summarized in Table 3. The UV-visible
absorption spectra of 5a–c and 12 are shown in Fig. 5 [8].
The phenyl-substituted phosphasilene 5a shows an
absorption peak (
l
max) at 385 nm (
e
= 1.16 ꢀ 104), which
Table 3
Electronic absorption data for phosphasilenes in THF.
Compound
lmax (nm)
nmax (cmꢁ1
)
e )
(cmꢁ1 Mꢁ1
5a
5b
5c
5d
5e
5f
385
26,000
24,700
23,300
24,800
24,800
25,400
23,500
22,300
11,600
10,600
12,700
15,500
9,200
405
430 (sh)
403
404
394
9,500
5g
12
425
10,600
22,000
Fig. 6. Solid-state luminescence spectrum of 12 excited at 360 nm
(purple). UV-vis absorption spectrum of 12 in THF (blue).
449

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T. Matsuo et al. / C. R. Chimie 13 (2010) 1104–1110
Fig. 7. Cyclic voltammograms of 5a (black) and 12 (red).
Fig. 8. Selected molecular orbitals of 5a (top view).
the quasi-reversible and the irreversible reduction waves
at –2.55 V (5a) and –2.12 V (12) were observed for the
cyclic voltammograms, respectively (Fig. 7). These experi-
mental results exhibit good qualitative agreement with the
DFT calculations (vide infra).
5. Theoretical studies of
p-conjugated phosphasilenes
To delineate the nature of the chemical bonding in the
phosphasilenes, DFT computations at the B3LYP/6-31G**
level were carried out for 5a and 12. The optimized
structures well reproduced the X-ray crystal structures.
The three pertinent molecular orbitals (MOs) of 5a are
depicted in Fig. 8 [8]. While the HOMO of 5a is primarily
Fig. 9. Selected molecular orbitals of 12 (top view).
results. The significant 3n–2p conjugation are also found
p
in the HOMO–2 and HOMO–3 of 12.
The absorption wavelengths were calculated for the
ground-state geometries using the TD-B3LYP/6-31G**
method. The calculated wavelengths of 364 (5a) and 456
(12) nm are comparable to the observed values. The
absorption at 364 nm of 5a is assignable to the mixed
represented by the 3p (Si–P) orbital, the LUMO involves
p
the substantial contribution of the 3p *(Si–P)–2p *(phe-
p
p
nyl) conjugation. The lower-lying HOMO-1 involves the
3n–2p conjugation due to the lone pair at the phosphorus
version of the HOMO!LUMO
1!LUMO ((n + of Eind benzene)–
almost equal weighting. In contrast, the absorption at
456 nm of 12 is mainly assigned to the HOMO!LUMO (
*) transition.
(
p
–
p
*) and HOMO-
p
atom and the
p-orbital on the Eind benzene ring; the
p
p*) transitions with
maximum orbital interaction of these two orbitals is
attained by the perpendicularly-fixed Eind group.
p
–
Fig. 9 shows the selected MOs of the
p-extended 12.
p
Calculations for 12 have demonstrated that the LUMO level
is lowered [–1.552 (5a) and –2.108 (12) eV], thus keeping
the HOMO levels nearly constant [–5.141 (5a) and –5.137
(12) eV], in agreement with the orbital shapes for 5a and
the previously mentioned electrochemical experimental
6. Gold(I) complexes of
p-conjugated phosphasilenes
Although the P-zinciophosphasilene [6e] and P-plum-
byleniophosphasilene [6f] have been reported to be

T. Matsuo et al. / C. R. Chimie 13 (2010) 1104–1110
1109
red-shifted from 5a, with a higher molar extinction
coefficient (
degree of the
e
= 1.75 ꢀ 104). These data suggest that the
p
-conjugation has been increased by the
Au(I) ion complexation.
7. Conclusions
We have described our recent results concerning the
p-
conjugated stable phosphasilenes based on the Rind
groups in this report. The phosphasilenes with a variety
of aryl groups have been clearly characterized by NMR
spectroscopy and X-ray structural analyses. All of the
Scheme 5. Synthesis of 13 and 15.
phosphasilenes demonstrate the strong
resulting from the -conjugation between the Si=P bond
and the carbon -electron system. The fused-ring bulky
Rind groups successfully protect the reactive polar Si=P
p–p* absorptions
p
p
bonds, making the highly coplanar arrangement of the
framework. The unique electronic properties of the
p
p
-
-
conjugated phosphasilenes have been elucidated by UV-
visible absorption spectroscopy as well as DFT calculations.
The gold(I) complexes of the
silenes have also been isolated to demonstrate the
extension of -conjugation through the gold atom.
p-conjugated phospha-
p
Elucidation of the chemistry of the phosphasilene metal
complexes has just begun and further investigations are
needed in order to understand their properties and
reactivities.
Fig. 10. Molecular structure of 13. Hydrogen atoms are omitted for the
clarity.
Acknowledgment
obtained by the metallation of the corresponding hydro-
gen-substituted phosphasilene, the transition metal com-
plexes bearing neutral phosphasilene ligands had been
unexplored until we succeeded quite recently in preparing
some gold(I) complexes [20]. Thus, as a part of the studies
aimed at developing new conjugated hybrid materials
consisting of both the main group elements and transition
metals, we have focused on the synthesis of the gold(I)
We thank the Ministry of Education, Culture, Sports,
Science, and Technology of Japan for the Grant-in-Aid for
Specially Promoted Research (No. 19002008). The calcula-
tions and experiments discussed in this article have been
performed with many collaborators, whose names appear
in the references. The authors wish to express their thanks
to all of them for their kind collaboration, particularly
Professor Kazuyoshi Tanaka and Dr Hiroyuki Fueno (Kyoto
University), Dr Daisuke Hashizume, Dr Takeo Fukunaga
complexes of
p-conjugated phosphasilenes stabilized by
the bulky Rind groups.
´
´
(RIKEN), Dr Vincent Lemau de Talance (Universite de
Rennes 1), and Ms Qian Luo (Peking University). We also
thank Dr Yayoi Hongo and Mr Takashi Nakamura (RIKEN)
for their kind help with the mass spectrometry and solid-
state NMR spectroscopy. Numerical calculations were
partly performed at the Supercomputer Laboratory, Insti-
tute for Chemical Research, Kyoto University. Finally, the
authors express their sincere thanks to Pr Hayato Tsuji
(The University of Tokyo), Dr Yongming Li (The Chinese
Academy of Sciences), Dr Aiko Fukazawa, and Pr Shigehiro
Yamaguchi (Nagoya University) for their initial contribu-
tion to this study.
As shown in Scheme 5, the reaction of 5a with AuCl(tht)
(tht = tetrahydrothiophene) led to the quantitative forma-
tion of the phosphasilene gold(I) chloride complex 13 as a
yellow powder. We have also designed a new phospha-
silene 14 with the less bulky 1,1,7,7-tetraethyl-3,3,5,5-
tetramethyl-s-hydrindacene-4-yl (EMind) groups. The
corresponding AuCl complex 15 was prepared in 82% yield.
The molecular structure of 13 has been determined by
X-ray crystallography (Fig. 10). The Au atom has a linear
two-coordinate geometry with the P–Au–Cl angle of
178.83(5)8. The Au–P and Au–Cl distances are 2.2295(9)
˚
and 2.2739(10) A, respectively. The Si=P bond length of 13
˚
˚
is 2.0924(12) A, which is similar to that of 5a [2.0917(5) A].
The phenyl group is slightly twisted to the Si=P bond; the
P–Si–C–C torsion angle is 13.83(15)8.
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