Synthesis and properties of 4,5,6-triphospha[3]radialene

Article abstract of DOI:10.1002/anie.201200374

[3]Radialenes: 4,5,6-Tris(2,4,6-tri-tert-butylphenyl)phospha[3]radialene has been synthesized (see picture). The compound can be easily handled in air under ambient conditions, despite the [3]radialene moiety containing Pi-C bonds, and exhibits red-shifte

Full text of DOI:10.1002/anie.201200374

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DOI: 10.1002/anie.201200374
Radialene Chemistry
Synthesis and Properties of 4,5,6-Triphospha[3]radialene**
Hideaki Miyake, Takahiro Sasamori,* and Norihiro Tokitoh*
Radialenes are a unique class of unsaturated hydrocarbons
and have attracted much attention from the viewpoint of their
cross- conjugated p-electron electronic structures.[1] For
example, [3]radialene (1, Scheme 1),[2] the smallest radialene,
low-coordinated phosphorus compounds prompted us to
research the chemistry of the phosphorus analogues of
radialenes, which are an attractive research target as a result
of the synergy between the unique nature of radialenes and
=
P C bonds. Especially, we became interested in the chemistry
of 4,5,6-tris(2,4,6-tri-tert-butylphenyl)triphospha[3]radialene,
as its skeletal isomers 2,4,6-tri-tert-butyl-1,3,5-triphosphaben-
zene (3)[5] and 1,3,6-tris(2,4,6-tri-tert-butylphenyl)triphospha-
fulvene (4)[6] have already been synthesized as stable com-
pounds. However, studies of heteroatom-containing [3]radi-
alene derivatives have been quite limited because of their
instability,[4b,7] whereas some [4] or [6]radialene derivatives
that contain nitrogen,[8] oxygen,[9] phosphorus,[10] or sulfur
atoms[9b] in the framework have been isolated, and their
characteristic properties that are derived from their carbon–
heteroatom p-bond demonstrated. Herein, we report the
synthesis of a stable triphospha[3]radialene derivative 5, in
which the three exo-carbon atoms of the [3]radialene skeleton
are replaced by phosphorus atoms.
Scheme 1. Structures of related and isomeric compounds of 4,5,6-
triphospha[3]radialene.
exhibits rather strong UV absorption at longer wavelengths
than dimethylenecyclopropane (2),[1b] which reflects the
extension of the p-conjugated system. Compound 1 is an
important research target because the difference in properties
between 1 and its structural isomer, benzene, is a fundamental
issue of p-electron conjugation and p-electron cross-conju-
gation. A number of [3]radialene derivatives that contain
extended p-conjugated systems have been synthesized and
their unique optical and electrochemical properties were
reported.[3]
The reaction of tetrachlorocyclopropene[11] with the bulky
(2,4,6-tri-tert-butylphenyl)phosphine[12] in the presence of 2,6-
diisopropylaniline as a base gave a diastereomeric mixture of
4-phosphatriafulvenes[13] 6 as a stable orange solid in 66%
yield (Scheme 2). Oxidation of 6 with iodine in the presence
On the other hand, there has been much interest in the
=
chemistry of compounds that contain P C bonds, because p-
bonds that contain a heavier main-group element, such as
phosphorus, are known to have unique electronic character-
istics.[4] Although compounds that contain a double bond to
a heavier main-group element are known to be highly reactive
and difficult to isolate as a stable compound, during the past
=
three decades a number of phosphaalkenes that contain a P
C bond have been synthesized as stable compounds by taking
advantage of kinetic stabilization.[4] As a result, it was found
Scheme 2. Synthesis of 5 and 7. Mes*=2,4,6-(tBu)3C6H2;
Dip=2,6-(iPr)2C6H3.
=
that P C bonds have unique characteristics that are different
=
from those of C C bonds, for example, they have lower
=
=
LUMO levels relative to compounds with N N or C C
double bonds. These unique characteristics of radialenes and
of triethylamine afforded 5 as a deep-purple solid in 92%
yield. Compound 5 is a stable phosphorus-containing [3]radi-
=
alene derivative that contains cross-conjugated P C bond
[*] Dr. H. Miyake, Prof. Dr. T. Sasamori, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
moieties. It is stable enough to be handled in air without any
decomposition. On the other hand, it was found that treat-
ment of 6 with 0.1 equivalent of 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU) gave a diastereomeric mixture of diphosphini-
denecyclopropanes 7 as a yellow solid in 90% yield by a [1,3]-
proton shift reaction. Compound 7 is thought to be a good
E-mail: sasamori@boc.kuicr.kyoto-u.ac.jp
tokitoh@boc.kuicr.kyoto-u.ac.jp
Homepage: http://boc.kuicr.kyoto-u.ac.jp/www/index-e.html
[**] This work was partially supported by Grants-in-Aid for Scientific
Research (B) (No. 22350017), Young Scientist (A) (No. 23685010),
and the Global COE Program B09 from the Ministry of Education,
Culture, Sports, Science and Technology (Japan). JSPS Fellows were
supported by grant number 23 3938 from the Japan Society for the
Promotion of Science.
= ꢀ =
example of a P C C P conjugated p-electron system relative
to the cross-conjugated system of 5.
Compounds 5, 6, and 7 were fully characterized by NMR
spectroscopy, MS, and elemental analysis, and their structures
were finally determined by X-ray crystallographic analysis
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200374.
=
(Figure 1). The lengths of the three P C bonds of the central
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Chemie
Figure 2. UV/Vis absorption spectra of 5 (solid line) and 7 (dashed
line).
Figure 1. Molecular structures of (a) 5 and (b) 7 with thermal
ellipsoids set at 50% probability). Selected bond lengths [ꢀ] and
ꢀ
ꢀ
ꢀ
angles [8]: (a) P1 C1 1.6817(14), P2 C2 1.6870(15), P3 C3
ꢀ
ꢀ
ꢀ
1.6870(14), C1 C2 1.436(2), C1 C3 1.438(2), C2 C3 1.432(2); C2-C1-
ꢀ
of a phosphorus-containing p-conjugated system. Moreover,
the absorption of 5 was detected at considerably longer
wavelengths relative to that of 7 (lmax = 377 nm, e = 1.35 ꢁ
104), which suggests effective cross-conjugation among the
C3 59.76(10), C3-C2-C1 60.19(10), C2-C3-C1 60.05(10); (b) P1 C1
1.8635(17), P2 C2 1.6679(17), P3 C3 1.6737(18), C1 C2 1.507(2),
C1 C3 1.507(2), C2 C3 1.406(2); C3-C1-C2 55.67(11), C3-C2-C1
62.07(12), C2-C3-C1 62.26(12).
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
=
three P C bonds of 5. A similar red-shift was detected in the
absorption wavelength of dimethylenecyclopropane (2,
232 nm) versus [3]radialene (289 nm).[1b] To elucidate the
nature of the characteristic absorption of 5, theoretical
calculations for model compound tris(2,6-di-tert-butyl-
phenyl)-substituted triphospha[3]radialene derivative 5’
were carried out at the HF/6-31 + G(3d)[P]:6-31G(d)[C,H]//
B3PW91/6-31 + G(3d)[P]:6-31G(d)[C,H] level. The calcu-
lated frontier molecular orbitals of 5’ are shown in Figure 3.
framework of 5 are almost identical to each other in the range
of 1.68–1.69 ꢀ, which is much shorter than the typical length
ꢀ
of a P C bond (1.85–1.90 ꢀ) and slightly longer than that of
=
=
a nonconjugated P C bond (for example, (E)-Mes*P CHPh:
1.660(6) ꢀ, (Z)-Mes*P CHPh: 1.674(2) ꢀ).[14] The lengths of
=
ꢀ
C C bonds in the cyclopropane skeleton (1.43-1.44 ꢀ) of 5
are considerably shorter than that of cyclopropane
(1.510(2) ꢀ).[15] These structural features are similar to
=
ꢀ
those of [3]radialene (1, C C: 1.343(20), C C:
1.453(20) ꢀ).[2b] The bond lengths of the 1,4-diphosphabuta-
ꢀ
ꢀ
diene moiety of
7
(P2 C2: 1.6679(17) ꢀ; P3 C3:
ꢀ
1.6737(18) ꢀ; C2 C3: 1.406(2) ꢀ) are slightly shorter than
=
ꢀ
those of the exocyclic P C bonds and the endocyclic C C
bonds of 5, respectively.
On the basis of the analogy with all-carbon systems, the
slight difference in the bond lengths between 5 and 7 is
a result of cross-p-conjugation in the triphospha[3]radialene
skeleton, that is, the calculated bond lengths of the butadiene
Figure 3. Frontier molecular orbitals of 5’. Ar=2,6-(tBu)2C6H3.
=
ꢀ
moiety of 2 (C C: 1.312, C C: 1.421 ꢀ) were also reported to
The calculated HOMO, HOMOꢀ1, and LUMO of 5’ are
composed of the p-conjugated system of the triphospha[3]-
radialene framework. The time-dependent DFT (TD-DFT)
calculations for 5’ at the B3PW91/6-311 + G(3d)[P]:6-31 +
=
ꢀ
be slightly shorter than those of 1 (C C: 1.316, C C:
1.442 ꢀ).[16] Data from 31P NMR, 13C NMR, and 1H NMR
spectra suggest that 5 has a C3 symmetric structure in solution.
In the 31P NMR spectrum of 5, a characteristic signal was
detected at d = 187.5 in C6D6. The value is within the range for
those of phosphaalkenes, but is slightly shifted to higher field
G(d)[C,H]//B3PW91/6-31 + G(3d)[P]:6-31G(d)[C,H]
level
found two excitation states at l = 467.8 (f = 0.2325) and l =
467.7 nm (f = 0.2315) were dominant as a result of the
HOMO!LUMO and HOMOꢀ1!LUMO transitions,
respectively. The transitions seem to correspond to the
absorption of 5 at lmax = 526 nm. The lmax value of 5 is at
a longer wavelength than that of other long p-conjugated
systems, such as tetra(phosphaalkenyl)benzene derivative 8
=
from that of simple phosphaalkene, such as Mes*P CMe2
(d = 223.0 in CDCl3)[17] or 1,4-diphosphabutadiene Mes*P
=
ꢀ
=
CH CH PMes* (d = 243–285 for three different geometries
in CDCl3).[18] In the 31P NMR spectrum of 7, the signals from
the low-coordinate phosphorus atoms were detected in
a similar region (d 192–226 in C6D6). The JPC coupling
constant of the P C bonds in 5 is 54 Hz, the value of which is
1
(442 nm, e = 1.92 ꢁ 104)[20] as well as the P C-bond-containing
=
p-conjugated polymers 9 (323–338 nm)[21] and 10 (445 nm,[22]
Scheme 3), even though the p-system of 5 is limited to the
C3P3 triphospha[3]radialene core.
=
=
within the typical range for phosphaalkenes (Mes*P CPh2:
45.4 Hz,[14] (E)-Mes*P CHPh: 53.1 Hz,[19] (Z)-Mes*P
=
=
CHPh: 62.9 Hz[15]).
The cyclic voltammograms of 5 and 7 in THF at room
temperature have reversible reduction couples at E1/2 = ꢀ1.55
and ꢀ2.10 V (versus ferrocene/ferrocenium+ (FcH/FcH+)),
respectively, which shows the higher electron-accepting
ability of these two compounds relative to a simple phos-
The UV/Vis spectrum of 5 in n-hexane has a strong
absorption at lmax = 526 nm (e = 1.92 ꢁ 104, Figure 2), which is
at a much longer wavelength than that of [3]radialene (1)
(lmax = 289 nm, e = 9390 ꢁ 1170)[2c] and reflects the character
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Communications
dominantly composed of the HOMO of 7’ (ꢀ8.50 eV). The
HOMO of 11 (ꢀ9.18 eV) can be overlapped with HOMOꢀ3
of 7’ (ꢀ10.98 eV) in the same phase, which makes the HOMO
of 5’’ (ꢀ8.67 eV) and is located at higher energy. However, the
energy level is still similar to that of HOMO of 7’. On the
other hand, the LUMOs of 7’ (0.04 eV) and 11 (2.19 eV) can
make an effective association in the same phase, which makes
the LUMO level of 5’’ much lower (ꢀ1.38 eV). Thus, the
LUMO level of 5’’ is much lower than that of 7’, although their
HOMO levels are similar to each other. These results support
the higher electron-accepting ability of 5 (E1/2 = ꢀ1.55 V vs
FcH/FcH+) relative to 7 (E1/2 = ꢀ2.10 V vs FcH/FcH+) on the
basis of the cyclic voltammetry.
The molecular orbitals of 1,3,5-triphosphabenzene (3’),
the structural isomer of 5’’, and their carbon analogues,
[3]radialene (1) and benzene, were also calculated at the same
level (Figure 5). The calculated LUMO level of 5’’ (ꢀ1.38 eV)
is much lower than that of 3’ (0.79 eV). In fact, the color of 3
was reported to be pale yellow,[5] which is in sharp contrast to
the deep-purple color of 5 and shows that the HOMO–
LUMO gap in 5 is smaller than that in 3. In the case of the all-
carbon systems, the LUMO energy of 1 is also lower than that
of benzene. Furthermore, the calculated LUMO levels of the
phosphorus-containing compounds 5’’ and 3’ are much lower
than those of the corresponding carbon analogues 1 and
benzene. On the other hand, all of the calculated HOMO
levels were at similar levels, as shown in Figure 5. Thus, both
the [3]radialene framework and the phosphorus substitution
should significantly lower the LUMO levels without changing
the HOMO level so much. The unique electronic structure of
triphospha[3]radialene can be explained by the synergistic
effect of both of the factors.[25]
=
Scheme 3. p-Conjugated compounds that contain P C bonds.
Ar=4-(tBu)C6H4.
=
phaalkene, such as (E)-Mes*P CHPh (E1/2 = ꢀ2.54 V versus
FcH/FcH+).[23,24] The much higher reduction potential of 5
relative to 7 shows that the electron-accepting ability is
effectively raised by the cross-conjugated system in 5. On the
other hand, the cyclic voltammogtrams of 5 and 7 both have
an irreversible oxidation wave at almost the same potential.
Compound 5 can be divided into 1,4-diphosphabutadiene
moiety (Fragment A) and phosphaethene moiety (Frag-
ment B, Figure 4). To elucidate the electronic structure of 5,
the molecular orbitals of triphospha[3]radialene 5’’, diphos-
phinidenecyclopropane 7’, and phosphinidencyclopropane 11,
the p-conjugated model of 5, Fragments A, and B, were
computed at the HF/6-31G(d,p)//B3PW91/6-31G(d,p) level.
The structural properties in the central frameworks of the
optimized structures of 5’’ and 7’ are in good agreement with
those of 5 and 7, respectively. The molecular orbitals of 5’’ can
be explained by the interaction between the two conjugated
p-electron systems of 7’ and 11. The interaction between the
HOMOs of 7’ and 11 are mismatched in their phase with no
effective overlapping. Thus, the HOMOꢀ1 of 5’’ (ꢀ8.67 eV) is
In summary, 4,5,6-triphospha[3]radialene derivative 5 was
successfully synthesized by a simple procedure and can be
easily handled in the air under ambient conditions, despite the
=
[3]radialene moiety containing P C bonds. Furthermore, we
have revealed that 5 exhibits red-shifted absorption and high
electron-accepting ability, which reflects the unique phospho-
rus-containing cross-conjugated skeleton. These results indi-
cate the potential utility of heteroatom-containing radialenes
for molecular electronics.
Experimental Section
Synthesis of 6: Tetrachlorocyclopropene (0.12 mL, 0.98 mmol) was
added to a solution of Mes*PH2 (0.84 g, 3.0 mmol) and 2,6-diisopro-
Figure 4. Frontier molecular orbitals of 5’ and 7’and the corresponding
Figure 5. HOMO and LUMO levels of some phosphorus-containing p-
energies.
conjugated compounds (C3P3H3) and their carbon analogues (C6H6).
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Chemie
pylaniline (0.95 mL, 5.0 mmol) in dichloromethane (5 mL) at room
temperature in an argon atmosphere. After the reaction mixture was
stirred for 5 h, the solvents were removed under reduced pressure. n-
Hexane was added to the residue, and then the mixture was filtered
through Celite. After removal of the volatile materials, the residue
was filtered through a short-path silica gel pad (n-hexane). After
removal of the solvent, reprecipitation of the residue from dichloro-
methane/EtOH afforded 6 as a diastereomeric mixture (0.57 g,
0.66 mmol, 66%). 6: orange crystals, m.p. 168–1708C (decomp).
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1H NMR (300 MHz, C6D6): d = 1.28–1.77 (m, 81H), 5.47 (d, JPH
=
246 Hz), 5.75 (d, JPH = 244 Hz), 6.78 (d, 1JPH = 342 Hz), 7.40–7.67 ppm
(m, 6H); 13C{1H} NMR (75 MHz, C6D6): d = 31.5 (CH3, s), 31.6 (CH3,
s), 32.1 (CH3, s), 33.8–34.1 (CH3, m), 34.9–35.3 (C, m), 37.9–38.7 (C,
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Angew. Chem. Int. Ed. 2000, 39, 2781.
m), 119.4–119.8 (CH, m), 121.9–122.8 (CH, m), 124.9 (C, d, JPC
23 Hz), 125.1 (C, d, JPC = 21 Hz), 126.0 (C, d, JPC = 26 Hz), 126.7 (C, d,
PC = 25 Hz), 142.1 (C, d, JPC = 59 Hz), 142.3 (C, d, JPC = 59 Hz), 148.1
(C, s), 151.2 (C, s), 152.0 (C, s), 155.0–157.5 (C, m), 160.9 (C, dd, JPC
53, 21 Hz), 164.5 ppm (C, dd, JPC = 87, 6 Hz); 31P NMR (120 MHz,
C6D6) major isomer: d = ꢀ74.6 (d, JPH = 246 Hz), ꢀ72.9 (ddd, JPH
246 Hz, JPP = 5, 7 Hz), 12.5 ppm (d, JPP = 5 Hz); minor isomer: d =
=
[7] a) R. West, D. C. Zecher, S. K. Koster, D, Eggerding, J. Org.
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=
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=
ꢀ77.0 (dd, JPH = 242 Hz, JPP = 16 Hz), ꢀ76.0 (ddd, JPH = 244 Hz, JPP
=
16, 7 Hz), 14.8 ppm (d, JPP = 7 Hz); HRMS (FAB): m/z: calcd for
C57H89P3: 866.6177. Found 866.6116 [M+]. Elemental analysis calcd
(%) for C57H89P3: C 78.94, H 10.34; found: C 78.34, H 10.16.
Synthesis of 5: Iodine (50 mg, 0.20 mmol) was added to a solution
of 6 (87 mg, 0.10 mmol) and triethylamine (70 mL, 0.50 mmol) in
dichloromethane (2 mL) at room temperature. After the reaction
mixture was stirred for 30 min, the volatile materials were removed
under reduced pressure. The residue was extracted with n-hexane and
aqueous ammonium chloride, and the organic phase was washed with
aqueous sodium sulfite. After removal of the solvent, reprecipitation
of the residue from dichloromethane/EtOH afforded 5 (80 mg,
92 mmol, 92%). 5: purple crystals, m.p. 2258C (decomp). 1H NMR
(300 MHz, C6D6): d = 1.38 (s, 27H), 1.54 (s, 54H), 7.60 ppm (s, 6H);
13C{1H} NMR (75 MHz, C6D6): d = 31.6 (CH3, s), 33.4 (CH3, s), 35.0
(C, s), 38.2 (C, s), 122.5 (CH, s), 138.4 (C, dd, JPC = 40, 22 Hz), 150.4
(C, s), 153.9 (C, s), 160.9 ppm (C, ddd, JPC = 54, 32, 11 Hz); 31P NMR
(120 MHz, C6D6): d = 187.5 ppm (s); HRMS (FAB): m/z: calcd for
C57H87P3: 864.6021. Found 864.5930 [M+]. Elemental analysis calcd
(%) for C57H87P3: C 79.13, H 10.14; found: C 78.86, H 10.23.
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Received: January 14, 2012
Revised: February 6, 2012
Published online: February 22, 2012
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Keywords: cross-conjugation · electronic structure ·
heterocycles · phosphorus · radialenes
.
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