Direct arylations for study of the air-stable p-heterocyclic biradical: From wide electronic tuning to characterization of the localized radicalic electrons

Article abstract of DOI:10.1021/ja409625x

We have developed methods for installing aryl substituents directly on the phosphino groups of the 1,3-diphosphacyclobutane-2,4-diyl system. The aryl substituents tuned the electronic and structural characteristics of the biradical unit both in solution and in the solid state. 1-tert-butyl-2,4-bis(2, 4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobuten-4-yl anion, prepared from phosphaalkyne (MesCi - P; Mes* = 2,4,6-tBu₃C ₆H₂) and t-butyllithium, was allowed to react with an electron-deficient N-heterocyclic reagent. The corresponding N-heteroaryl-substituted P-heterocyclic biradicals were produced via S _NAr reactions. Biradicals bearing perfluorinated pyridyl substituents exhibited photoabsorption properties comparable to those of previously reported derivatives because the highest occupied and lowest unoccupied molecular orbit levels were reduced by a similar amount. In contrast, the triazine substituent reduced the band gap of the biradical unit, and the large red shift in the visible absorption and high oxidation potential were further tuned via subsequent S_NAr and Negishi coupling reactions. The amino-substituted triazine structure provided a strongly electron-donating biradical chromophore, which produced unique p-type semiconducting behavior even though there was no obvious π-overlap in the crystalline state. The single-electron transfer reaction involving Mes Ci - P, phenyllithium, and iodine afforded 1,3-diphenyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2, 4-diyl via the intermediate P-heterocyclic monoradical. The tetraaryl- substituted symmetric biradical product was used to determine the electron density distribution from the X-ray diffraction data. The data show highly localized radicalic electrons around the skeletal carbon atoms, moderately polarized skeletal P-C bonds in the four-membered ring, and no covalent transannular interaction.

Full text of DOI:10.1021/ja409625x

Article
pubs.acs.org/JACS
Direct Arylations for Study of the Air-Stable P‑Heterocyclic Biradical:
From Wide Electronic Tuning to Characterization of the Localized
Radicalic Electrons
Shigekazu Ito,*,† Yasuhiro Ueta,† Trang Thi Thu Ngo,† Makoto Kobayashi,† Daisuke Hashizume,‡
Jun-ichi Nishida,§ Yoshiro Yamashita,§ and Koichi Mikami†
†Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama,
Meguro, Tokyo 152-8552, Japan
‡Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198,
Japan
§Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
S
* Supporting Information
ABSTRACT: We have developed methods for installing aryl
substituents directly on the phosphino groups of the 1,3-
diphosphacyclobutane-2,4-diyl system. The aryl substituents
tuned the electronic and structural characteristics of the
biradical unit both in solution and in the solid state. 1-tert-
butyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobut-
en-4-yl anion, prepared from phosphaalkyne (Mes*CP;
Mes* = 2,4,6-tBu3C6H2) and t-butyllithium, was allowed to
react with an electron-deficient N-heterocyclic reagent. The
corresponding N-heteroaryl-substituted P-heterocyclic biradicals were produced via SNAr reactions. Biradicals bearing
perfluorinated pyridyl substituents exhibited photoabsorption properties comparable to those of previously reported derivatives
because the highest occupied and lowest unoccupied molecular orbit levels were reduced by a similar amount. In contrast, the
triazine substituent reduced the band gap of the biradical unit, and the large red shift in the visible absorption and high oxidation
potential were further tuned via subsequent SNAr and Negishi coupling reactions. The amino-substituted triazine structure
provided a strongly electron-donating biradical chromophore, which produced unique p-type semiconducting behavior even
though there was no obvious π-overlap in the crystalline state. The single-electron transfer reaction involving Mes*CP,
phenyllithium, and iodine afforded 1,3-diphenyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl via the
intermediate P-heterocyclic monoradical. The tetraaryl-substituted symmetric biradical product was used to determine the
electron density distribution from the X-ray diffraction data. The data show highly localized radicalic electrons around the skeletal
carbon atoms, moderately polarized skeletal P−C bonds in the four-membered ring, and no covalent transannular interaction.
INTRODUCTION
■
Highly stabilized reaction intermediates such as radicals1 have
been important in furthering molecular science and in
developing various functional materials for electronics and
biology. Biradicals, which bear two radical centers, are usually
Figure 1. Isolable 1,3-diphosphacyclobutane-2,4-diyls.
too unstable to isolate under ordinary conditions. For example,
an observable cyclobutane-1,3-diyl under cryogenic conditions
is promptly converted to the corresponding bicyclo[1.1.0]-
Although A exhibits considerable thermal stability, it is highly
air- and moisture-sensitive, probably because of insufficient
butane upon heating.2,3 However, Niecke and co-workers
isolated the first room temperature stable 1,3-diphosphacyclo-
butane-2,4-diyl A (Mes* = 2,4,6-tri-tert-butylphenyl), which is a
congener of cyclobutanediyl, by using sterically encumbered
2,2-dichloro-1-phosphaethene (Figure 1, left).4,5 The isolation
of A confirmed that the electronic effects of the heavier main
group elements6−8 can be exploited to produce stable
biradicals.
steric protection around the radicalic carbon centers.
Subsequent studies of P-heterocyclic biradicals suggested that
bulky Mes* groups adjacent to the radical centers stabilize the
biradical skeleton effectively (Figure 1, right). We used
Received: September 20, 2013
Published: October 16, 2013
© 2013 American Chemical Society
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phosphaalkyne chemistry to synthesize air-tolerant crystalline
1,3-diphosphacylobutane-2,4-diyl B, and the high stability of B
was valuable for investigating functional materials based on the
biradical moiety.9−11 Very recently, we reported the scope of
the through-space electrostatic interaction between the
electron-donating biradical chromophores, which would be
useful for developing functional materials based on mixed-
valent structures.12
Scheme 1. SNAr Reaction of 2 with Perfluoropyridine
Understanding the conjugative interaction between the P-
heterocyclic moiety and π-electron systems is important for
developing the chemistry of stable biradicals. We expect that
the conjugative interaction between the P-aryl moiety and the
cyclic biradical unit will be unusual and useful for wider
applications.13 However, tuning the biradical unit with direct
P−aryl bonding has not been investigated extensively. Niecke
and co-workers used the Mes*P moiety as the reactive
functional group, which provided the monoradical upon
irradiation5d and the dianion upon alkaline reduction.5e Direct
P−aryl bonding to the biradical system has not been
thoroughly investigated because of the lack of synthetic
methods.
Furthermore, although the number of isolable congeners of
cyclobutanediyl has increased, the stable biradical structure is
still debated on the basis of the contribution of the resonance
structures4,8,14 and the effect of the possible transannular
interaction.15 Even though a number of stable cyclic biradical
derivatives have been reported, their structures have not been
investigated well. Therefore, the structural characteristics of the
P-heterocyclic biradical skeleton have yet to be determined.
Very recently, Stalke and co-workers succeeded in experimental
charge density determination of an isomer of hexasilabenzene,
which is topologically related to 1,3-diphosphacyclobutane-2,4-
diyl.16
elimination of fluoride. In fact, 2 can also behave as a leaving
group,11b,18 and the attempted synthesis of similar aryl-
substituted 1,3-diphosphacyclobutane-2,4-diyls with hexafluor-
obenzene, pentafluorobenzene, pentafluoroiodobenzene, 1-
fluoro-2,4-dinitrobenzene, 2-chloropyridine, 2-chloro-5-
(trifluoromethyl)pyridine, 2-chloro-5-(trifluoromethyl)pyridine
N-oxide, and 3,5-dichloro-2,4,6-trifluoropyridine as electro-
philes failed. Reactions of 2 with pentafluorobenzonitrile and
octafluorotoluene gave small amounts of the corresponding
products together with nonseparable byproducts (see the
Supporting Information).
We expected the fluorine atoms in 3 to be available for
further substitution reactions.17c However, the presence of the
electron-rich P-heterocyclic system prevented the expected
SNAr reactions of 3 with 2-pyridiyllithium, [4-(trifluoromethyl)-
phenyl]lithium, lithium 2-phenylacetylide, sodium tert-butoxide,
diethylamine, and butyllithium. The SNAr reaction of two
fluorine atoms in 3 took place when 3 was treated with more
than 2 equiv of phenyllithium, and the corresponding product 4
was obtained in 71% isolated yield (Scheme 2). Crystalline
compound 4 could be stored in air for 6 months.
Scheme 2. SNAr Reaction of 3 with Phenyllithium
In this study, we demonstrate that electron-deficient N-
heteroaryl groups can be installed directly on the P-heterocyclic
biradical skeleton via a nucleophilic aromatic substitution
(SNAr) reaction. The N-heteroaryl groups altered the
physicochemical properties and crystalline structure of the
biradicals. Furthermore, we discuss the p-type semiconducting
properties induced by the strongly electron-donating amino-
substituted s-triazine group with the properties of B. We
obtained X-ray diffraction data from single crystals of the
symmetrical tetraaryl-substituted 1,3-diphosphacyclobutane-
2,4-diyl, synthesized by a single-electron transfer (SET)
reaction, which showed the radicalic electrons localized around
the skeletal carbons as well as the polarized P−C bonds and no
transannular bonding.
Table 1 compares the UV−vis data and the oxidation
potentials of 3 and 4 with the highest occupied and lowest
Table 1. Physical Properties of the Pyridyl-Substituted
Biradicals
a
b
c
b
product
λmax /nm
ΔEHOMO−LUMO /eV
Epa /V
EHOMO /eV
3
608
4.76
0.61
0.84
−5.82
RESULTS AND DISCUSSION
■
d
4
596
4.80
0.48
−5.68
c
SNAr Reaction for Pyridyl-Substituted 1,3-Diphospha-
cyclobutane-2,4-diyls. Phosphaalkyne 1 was treated with 0.5
equiv of tert-butyllithium in THF to generate the corresponding
cyclic anion 2, and the room temperature stable THF solution
of 2 was allowed to react with perfluoropyridine (Scheme 1).17
After the reaction mixture was stirred for 36 h, the SNAr
reaction was terminated, and the perfluoropyridyl-substituted
1,3-diphosphacyclobutane-2,4-diyl 3 was obtained as a deep-
blue crystalline compound in 43% yield. Crystalline compound
3 showed high stability in air over at least 6 months, probably
because of the steric and electronic effects of the fluorinated N-
heteroaryl structure. We expected that the perfluorination of
pyridine would be a suitable SNAr reaction for constructing the
direct phosphorus−perfluoroaryl linkage through the facile
a
b
Visible region in dichloromethane. M06-2X/6-31G(d) level. Volts
vs Ag/AgCl. Conditions: 1 mM in dichloromethane, 0.1 M TBAP,
d
working electrode GC, counter electrode Pt, scan rate 50 mV s−1. Epc
= 0.39 V.
unoccupied molecular orbitals (HOMO and LUMO, respec-
tively) of the DFT-optimized structures [M06-2X/6-31G(d)
level].19,20 Compounds 3 and 4 showed a HOMO−LUMO
transition absorption comparable to that of biradical B.10h The
perfluoropyridyl groups reduce the LUMO level [ELUMO
=
−1.06 eV (3), −0.881 eV (4)], and the HOMO level was also
affected by the σ-electron-withdrawing effect of fluorine. The
presence of four electronegative fluorine atoms meant that 3
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displayed a higher, irreversible oxidation potential and an extra
peak, probably from the decomposition product of the first
oxidation. In contrast, 4 showed a lower oxidation potential
because it contained fewer fluorine atoms, and the HOMO
level seemed to be increased. Despite the electronegative
substituents, 3 and 4 displayed irreversible reduction potentials
around −0.80 V, probably because the P−aryl bond is unstable
upon reduction.5e,11b,18 In the 19F NMR spectrum, 4 showed a
large JPF constant (25.7 Hz), indicating the conjugation
extended between the lone pair and the teraryl structure.
SNAr Reaction for Triazine-Substituted 1,3-Diphos-
phacyclobutane-2,4-diyls. The SNAr reaction of 2 with
perfluorinated pyridine directly introduced the P-heterocycle
and perfluoropyridyl groups, although the reaction was slow,
and most postfunctionalization was not possible. Furthermore,
the physicochemical properties of 3 and 4 indicated that the
fluorinated pyridyl groups are restrictive. Therefore, we chose
2,4,6-trichloro-1,3,5-triazine (cyanuric chloride) as a reagent for
direct arylation of 2. Cyanuric chloride is used for the synthesis
of a number of functionalized s-triazine derivatives,21 and we
expected that the highly electron-deficient N-heteroaromatic
structure would allow us to install the P-heterocyclic biradical
moiety via nucleophilic aromatic substitution. Furthermore,
postfunctionalization of the triazine skeleton by subsequent
SNAr reaction of the chlorine atoms should proceed
smoothly.22
with the amine. The 31P NMR chemical shift of the tert-butyl-
substituted phosphorus nuclei in 5 depended strongly on the
substituents at the 3- and 5-positions of the 2,4,6-triazinyl
group, and electron-withdrawing substituents induced lower 31P
2
chemical shifts. Furthermore, the JPP spin−spin coupling
constants were reduced by electron-deficient substituents (see
the Supporting Information). However, the phosphorus atoms
on the triazine ring in 5 showed similar chemical shifts,
probably because of the unique electronic properties of the
singlet biradical bearing the skeletal tBuP structure.10i
Table 2 summarizes the UV−vis absorption properties,
oxidation potentials, HOMO−LUMO energy differences, and
Table 2. Physical Properties of the Triazine-Substituted
Biradicals
a
a
b
c
b
ox
λmax /nm λedge /eV ΔEHOMO−LUMO /eV E1/2 /V EHOMO /eV
5a
5b
5c
5d
5f
692
659
630
627
643
674
1.18
1.33
1.43
1.44
1.33
4.38
4.45
4.52
4.54
4.51
4.37
0.50
0.35
0.31
0.18
0.34
0.40
−5.76
−5.52
−5.43
−5.35
−5.51
−5.59
d
e
d
e
5g
1.28
b
a
c
In dichloromethane. M06-2X/6-31G(d) level. Volts vs Ag/AgCl.
Conditions: 1 mM in dichloromethane, 0.1 M TBAP, working
electrode GC, counter electrode Pt, scan rate 50 mV s−1. Ferrocene/
d
ferricinium +0.50 V. Instead of tert-butoxy groups, methoxy groups
Anion 2 was allowed to react with cyanuric chloride at 0 °C
to room temperature in THF, and the desired SNAr reaction
took place to afford the corresponding triazine-substituted
biradical 5a in a moderate yield as a deep green solid (Scheme
3, step 1). Although crystalline 5a could be handled in air, it
e
were employed for DFT calculation. Instead of (trimethylsilyl)ethynyl
groups, ethynyl groups were employed for DFT calculation.
HOMO energies at the M06-2X/6-31G(d) level19 of 5. The
absorption maxima around 600−700 nm, assigned as a HOMO
to LUMO transition, corresponded to the ΔE value between
the HOMO and LUMO. Compared with B, the visible
absorption of 5 showed a substantial red shift caused by the
triazine, and the chlorine atom and ethynyl group reduced the
HOMO−LUMO gap of the P-heterocyclic singlet biradical.
The LUMO of 5a in Figure 2a indicates a considerable
Scheme 3. SNAr Reaction of 2 with 2,4,6-Trichloro-1,3,5-
triazine
Figure 2. LUMOs of (a) 5a and (b) 5g.
contribution from the s-triazine moiety, and thus, the electron-
deficient N-heteroaryl group reduces the band gap. However, in
5g the ethynyl groups mainly contributed to the LUMO, and
coefficients of the P-heterocyclic biradical skeleton are reduced
in the LUMO (Figure 2b). Thus, the HOMO−LUMO
transition of 5g possesses charge-transfer excitation properties.
In contrast, introducing π-electron-donating substituents, such
as alkoxy and amino groups, effectively increased the HOMO−
LUMO gap. The amino groups were particularly effective in
maintaining the electron-donating properties of the P-
heterocyclic biradical moiety; thus, the oxidation potential of
5d was low.
gradually decomposed under ambient conditions and requires
storage under an inert atmosphere at −20 °C. The subsequent
dual SNAr reaction was also successful for the reactions of 5a
with phenyllithium, sodium tert-butoxide, diethylamine, and
butylamine, and the corresponding P-heterocyclic biradicals
5b−e were obtained in moderate to high yields (Scheme 3, step
2). The Negishi cross-coupling reaction23 was used for the less
thermally stable P-heterocyclic biradical 5a, and the dimethyl-
and bis(trimethylsilyl)ethynyl-substituted derivatives 5f and 5g
were obtained in moderate yields (Scheme 3, step 2).
Crystalline 5b−f can be handled in air at room temperature,
although they require storage under inert conditions. In
contrast, Sonogashira coupling conditions were not suitable
for preparing 5g because of an undesirable Hoffmann reaction
Figures 3 and 4 show the structures of 5b and 5d,
respectively. The metric parameters are summarized in Table
S1 (Supporting Information). The structure of 5b indicates that
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Figure 5. Transfer plots of an FET based on 5d by using a bare silica
dielectric surface.
Figure 3. ORTEP drawing of the molecular structure of 5b (40%
probability levels). Hydrogen atoms and the solvent molecule
(dichloromethane) are omitted for clarity. The 4-C(tBu) group is
disordered, and the atoms of higher occupancy factor (0.58) are
shown.
relatively low threshold voltage of −6 V. However, the mobility
of 2.3 × 10−8 cm2 V−1 s−1 and the on/off ratio of 7 were too
small for the material to be of practical use (Figure 5; Figure S4,
Supporting Information). The crystal structure of 5d indicates
no obvious intermolecular π−π interaction (Figure S2,
Supporting Information), which would explain the poor
mobility and on/off ratio. Nevertheless, the facile electron
release may allow semiconducting behavior. The ionization
potential (IP) of crystalline 5d (Figure 6) is as low as that of an
evaporated thin film of air-stable dianthra[2,3-b:2′,3′-f ]thieno-
[3,2-b]thiophene (DATT), which is composed of eight fused π-
aromatic rings (5.14 eV).25
Figure 4. ORTEP drawing of the molecular structure of 5d (50%
probability levels). Hydrogen atoms are omitted for clarity. The
phosphorus atoms are disordered, and the atoms of higher occupancy
factor (0.96) are shown.
the phenyl rings with the adjacent triazine ring and the teraryl
skeleton adopted a π-stacked dimeric structure (see Figure S1,
Supporting Information). The nitrogen atoms in the
diethylamino groups of 5d are planar because of the
conjugation between the lone pairs and the triazine π-system.
These structural features around the triazine ring are consistent
with the substituent effects. However, the sums of the bond
angles around the P−aryl phosphino groups [317.54° (5b),
319.11° (5d)] indicate that the phosphorus atoms are
considerably less hybridized and are very pyramidal. The lone
pairs of the phosphorus atoms would still allow conjugation
with the directly connected π-conjugate structure, because the
PCPC ring skeleton would require enough electrons to stabilize
the biradical system.24 The crystal structure of 5e contains H-
bonds which form a dimeric structure (Figure S3, Supporting
Information).
Semiconductor Properties of 1,3-Diphosphacyclobu-
tane-2,4-diyl. The low oxidation potential of 5d suggests
facile electron release from the biradical chromophore would
generate corresponding holes. In this study we focused on the
field-effect transistor (FET) properties of 5d. An FET device
was fabricated by drop-casting a toluene solution of 5d on a Si/
SiO2 wafer with interdigitated Au bottom electrodes. The silica
dielectric surface was not treated. The measurements were
conducted in situ, and Figure 5 shows the transfer plot. The 5d
FET device exhibited p-type semiconducting behavior with a
Figure 6. Photoelectron yield spectroscopy in air of crystalline 5d.
The semiconducting character of 5d is similar to that of B
(Vth = −0.1 V, μ = 1.67 × 10−7 cm2 V−1 s−1, on/off ratio 70;
Figures S5 and S6, Supporting Information). The slightly
superior properties of B compared with those of 5d may arise
from the nonsolvate crystal structure, which contains an offset
intermolecular π−π interaction26 (Figure S15, Supporting
Information).
Preparation of a Tetraaryl-Substituted 1,3-Diphos-
phacyclobutane-2,4-diyl via SET Processes. In addition to
nucleophilic substitution of the P-heterocyclic anion, the
corresponding air-tolerant crystalline 1,3-diphosphacyclobut-
en-4-yl radicals27 can be accessed through a one-electron
oxidation. We speculated that coupling the P-heterocyclic
monoradical with other radical species would be an effective
method for constructing P-heterocyclic biradicals.
Phosphaalkyne 1 was allowed to react with 1 equiv of
phenyllithium in THF to generate the corresponding cyclo-
butenyl anion 2′. The formation of 2′ [δP (THF/C6D6) 275.7,
2
50.0 ppm; JPP = 89.5 Hz] indicates that the intermediate
phosphavinyl anion promptly reacted with 1. The mixture of 2′
and phenyllithium was subsequently treated with iodine at
room temperature to generate monoradical 6 and the phenyl
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radical. After removal of volatile materials and washing with
acetonitrile, novel biradical 7 was obtained as a deep-blue solid
in a moderate yield (Scheme 4). Crystalline biradical 7 showed
extremely high stability in air, and the structure was not altered
after exposure to air for 1 year.
However, the direct P−aryl linkage would not necessarily be a
disadvantage for suitable molecular and crystal structures of P-
heterocyclic biradicals with semiconducting properties, and P-
heterocyclic biradicals may still be promising candidates for
organic field-effect transistors, offering some structural
improvements and better device fabrication.
Electron Density Distribution of the Symmetrical 1,3-
Diphosphacyclobutane-2,4-diyl. Single-crystal X-ray dif-
fraction data of 7 were obtained to analyze the electron
density distribution in detail. The diffraction data were
measured up to [(sin θ)/λ]max = 1.22 Å−1 using Mo Kα
radiation at 90 K (see the Supporting Information). A 3D plot
of the static model density of 7 (Figure 8; Figure S10,
Scheme 4. SET-Based Preparation of 7 from 1 through
Intermediates 2′ and 6
Figure 7 shows an ORTEP drawing for 7 determined by
conventional crystallographic analysis. The molecular structure
Figure 8. Three-dimensional view plot of 7 displaying electron-
increased (blue) and electron-decreased (red) areas by construction of
the molecule.
Supporting Information) indicates that all the covalent bonds
except for those in the four-membered ring were conventional
σ- and π-bonds. Figure 9 shows static model density maps of 7
on the PCPC plane (top) and the cross section of the PCPC
plane through the skeletal carbons (bottom). The bent skeletal
P−C bonds are clearly visible in the map on the PCPC plane.
Figure 7. ORTEP drawing for the X-ray structure of 7 (50%
probability levels). Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): P1−C1, 1.7463(2); P1i−C1,
1.7502(2); C1−C2, 1.4883(3); P1−C3, 1.8055(3); C1−P1−C1i,
93.12(2); C1−P1−C3, 121.88(1); C3−P1−C1i, 117.78(1); P1−C1−
P1i, 86.88(1); P1−C1−C2, 139.55(2); P1i−C1−C2, 129.54(2)
(superscript “i” indicates symmetry code 1 − x, 1 − y, 1 − z).
possesses a symmetric point, and the two [Mes*CPPh]
moieties are equivalent. The skeletal P−C lengths of 7 are
equivalent, and the rhomboidal four-membered ring showed
high planarity (τ = 0.0°). The phosphino groups exhibited an
sp3-like structure, and the skeletal carbon atoms were sp2
hybridized (sum of bond angles: P, 332.5°; C1, 356.1°). The
dihedral angle between the Mes* aromatic ring and the
biradical skeleton of τ = 69.1° indicates a small contribution
from the conjugation effect. The Cipso−Cortho and Cortho−tBu
lengths in the Mes* moiety indicate a distortion corresponding
to a boat conformation,28 probably arising from the steric
hindrance. The X-ray structure of 7 is almost identical to the
DFT-calculated structure [M06-2X/6-31G(d)].19
A drop-cast device composed of 7 displayed no FET
response. The electron-donating properties of 7 are slightly less
than those of 5d and B because the phosphorus atoms are
directly bound to electron-withdrawing phenyl groups. In the
crystal structure, no interaction between the aryl substituents
was observed, whereas the centroid−centroid distance between
the four-membered rings of 8.701 Å is shorter than that of B.
Figure 9. Static model maps of 7 on the PCPC plane (top) and the
cross section of the PCPC plane through the skeletal carbons
(bottom). Solid and broken lines indicate positive and negative
densities, respectively. The contours are drawn at a 0.05 e Å−3 interval.
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The map also shows high endocyclic electron density around
the skeletal carbons. The p-like shape of the radicalic electron
orbitals was characterized by the plane perpendicular to the
PCPC plane. The p-like structure of the vertically unsym-
metrical distribution at the rims of skeletal carbons was
unusually distorted. The distortion may be caused by the small
influence of the sp3-hybridization. The pyramidalized phos-
phorus atoms indicate a structure similar to that of usual
trisubstituted phosphines and the delocalization of the lone
pairs in both the s- and sp3-shaped orbitals.
stable 1,3-diphosphacyclobuten-4-yl anion 2 with electron-
deficient N-heteroaromatic reagents. We subsequently used
SNAr and the Negishi cross-coupling reactions to obtain 4 and
5b−g. The direct link between the P-heterocyclic biradical
skeleton and N-heteroaromatic structure enabled effective
tuning of the MOs to vary the physicochemical properties of
the stable P-heterocyclic biradical moiety via the considerable
conjugative interaction. The six-membered conjugative N-
heterocyclic structures were highly electronegative, and the
amino-substituted triazine produced a strongly electron-
donating biradical unit that exhibited unique p-type semi-
conductor behavior. The SET reaction afforded tetraaryl-
substituted 1,3-diphosphacyclobutane-2,4-diyl 7 as a highly air-
stable crystalline compound, indicating that the radical-induced
method is a practical approach for accessing various P-
heterocyclic biradicals. Single-crystal X-ray diffraction of 7
revealed unusual electron density distributions, including very
localized radicalic electrons. Precise structural analyses would
explain the unique physicochemical properties and also allow
further studies of singlet open-shell molecular entities. This
study has demonstrated the utility of the P−aryl moiety for
tuning the electronic and structural properties of the P-
heterocyclic biradical system. We are currently developing
functional and practical organic materials based on this work.
Figure 10 shows the experimentally determined bond paths,
bond critical points (BCPs), and a ring critical point (RCP) for
ASSOCIATED CONTENT
* Supporting Information
■
Figure 10. Gradient trajectories (fine lines), bond paths (broken
lines), bond critical points (BCPs; circles), and ring critical point
(RCP; square) in the four-membered heterocycle plane of 7. Bond
ellipticity at the BCPs: P(1)−C(1), 0.37; P(1)−C(1)i, 0.57; C(1)−
S
Experimental details for preparation of 3−5 and 7,
physicochemical data of 3−5 and 7, X-ray crystallographic
data of 5b, 5d, 5e, 7, and nonsolvate B, DFT and AIM
calculation data of 5, 7, and B, copies of NMR charts, and CIF
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
CMes*, 0.03.
the four-membered cycle of 7. The RCP indicates there were
no bonding interactions between the transannular C···C and
P···P areas. Slightly bent bond paths were observed in the four-
membered ring, which are common in small rings. The bond
ellipticity of the skeletal P−C bonds was created by a large
accumulation of electron density at the skeletal carbon atoms
by the adjacent electropositive phosphorus atoms and the
covalent bonds. Hence, the P−C linkages in the four-
membered ring29 were distinct from those of acyclic
phosphorus ylides.30 The electron density map and the BCP
data for 7 indicate the contribution of the two canonical forms
which describe 1,3-diphosphacyclobutane-2,4-diyl on the basis
of the difference between carbon and phosphorus. Additionally,
Figure S11 (Supporting Information) shows a Laplacian
distribution plot on the PCPC plane. The low bond ellipticity
of the bond between the skeletal carbon and Mes* group may
indicate a trace conjugative interaction between the four-
membered ring and the aryl ring. Therefore, the π-overlap
between the PCPC ring and the Mes* aryl plane is small, and
the π-delocalization between the PCPC ring and Mes* aryl
skeleton is negligible. The extreme stability probably depends
on the steric hindrance around the radical centers. The
experimentally determined bond characteristics of 7, including
BCPs and RCPs, were also determined by the AIM calculations
based on the DFT data (Figure S12, Supporting Information).
These electron density distribution characteristics were also
confirmed by studies of B (see the Supporting Information).
AUTHOR INFORMATION
Corresponding Author
ito.s.ao@m.titech.ac.jp
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by Grants-in-Aid for Scientific
Research (Nos. 22350058 and 23655173 for S.I. and No.
25109543 for D.H.) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan, and Nissan Chemicals
Co. Ltd. for S.I. We thank Prof. Dr. Hiroharu Suzuki and Dr.
Masataka Oishi of the Tokyo Institute of Technology for
support of X-ray crystallographic analyses and Dr. Yoshiyuki
Nakajima of Riken Keiki Co., Ltd. for measurements of
photoelectron yield spectroscopy in air. UV−vis spectroscopic
measurements were supported by Prof. Yuji Wada and Dr.
Masato Maitani of the Tokyo Institute of Technology.
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We synthesized N-heteroaryl-substituted 1,3-diphosphacyclo-
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