Poly(biradicals): Oligomers of 1,3-diphosphacyclobutane-2,4-diyl units

Article abstract of DOI:10.1002/anie.200801461

(Chemical Equation Presented) Biradical chromophore bridge: Di- and trimethylenephenyl moieties facilitate a through-space interaction between sterically demanding biradical subunits (see picture), enabling the design of new electronic materials. The solid-state structure of a bis(biradical) derivative illustrates the proximal positioning of two biradical subunits, allowing for direct interaction between them.

Full text of DOI:10.1002/anie.200801461

Communications
DOI: 10.1002/anie.200801461
Biradicals
Poly(Biradicals): Oligomers of 1,3-Diphosphacyclobutane-2,4-diyl
Units**
Shigekazu Ito,* Joji Miura, Noboru Morita, Masaaki Yoshifuji,* and Anthony J. Arduengo, III
Studies on stable organic biradicals have been of great
interest from the perspective of fundamental molecular
science and exploration of innovative materials.[1] The 1,3-
diphosphacyclobutane-2,4-diyl I is a remarkable biradical and
nation will lead to stable polyradical systems without strong
or shared interaction between individual biradical compo-
nents.
Therefore, on the basis of our synthetic method for 1,3-
diphosphacyclobutane-2,4-diyls,[5a] we demonstrate multi-bi-
radical derivatives in which the P-heterocyclic biradical
moieties are catenated with di- or trimethylenephenyl
groups. According to our previous report[5a] (Scheme 1),
ꢀ
anion
1 was prepared from phosphaalkyne Mes*C P
occupies a special place as a product of low-coordinated
organophosphorus chemistry.[2] This biradical has a range of
potential applications owing to its considerable stability, as
described by Niecke et al.[3,4] and our group.[5] Some of our
air-stable 1,3-diphosphacyclobutane-2,4-diyls of type I are
quite electron-rich and are easily oxidized to the correspond-
ing P-heterocyclic cation radicals (II).[6] The 1,3-diphospha-
cyclobutane-2,4-diyl moiety may thereby serve as a module
for redox-functionalized molecular systems. In addition to the
biradical derivatives, we succeeded in preparing a neutral
monoradical III by use of a synthetic intermediate for I (1).[7]
Covalent assembly of redox-active biradical units derived
from I is an attractive strategy to develop new materials for
molecular electronics and spintronics applications.[8] As an
example of double catenation of stable biradical units,
Bertrand and co-workers succeeded in binding two 1,3-
dibora-2,4-diphosphoniocyclobutane-1,3-diyl moieties (IV)
with the m- or p-phenylene unit to build p-conjugative
communication between the P2B2 biradical centers.[9] We are
interested in distinct “multi-biradical” systems in which each
biradical unit experiences a static interaction through a
nonconjugative covalent spacer. Such nonconjugative cate-
Scheme 1. Preparation of bis(biradical) derivatives 2 and 3 from P-
heterocyclic anion 1.
(Mes* = 2,4,6-tBu3C6H2) and one half of an equivalent of
tert-butyllithium. Subsequently, 1 was allowed to react with
a,a’-dibromo-m-xylene to afford the corresponding bis(bi-
radical) derivative 2 in moderate yield. Similarly, 3 was
obtained from 1 and a,a’-dichloro-p-xylene. The bis(biradi-
cal) derivatives 2 and 3 are stable blue solids and showed no
decomposition in air at room temperature over six months.
However, 3 is only sparingly soluble in common solvents, and
no useful spectroscopic data were obtained in solution. It
should be noted that, in contrast to the successful preparation
of 2 and 3, the reaction of 1 with a,a’-dibromo-o-xylene failed
to give the desired bis(biradical) compound, probably owing
to steric congestion.
[*] Dr. S. Ito, J. Miura, Prof. Dr. N. Morita
Department of Chemistry, Graduate School of Science
Tohoku University, Aoba, Sendai 980-8578 (Japan)
Fax: (+81) 22 795 6562
E-mail: shigeito@m.tains.tohoku.ac.jp
The successful catenation of two 1,3-diphosphacyclobu-
tane-2,4-diyl moieties prompted us to combine three P-
heterocyclic units (Scheme 2). Since a one-pot synthesis as in
Scheme 1 was unsuccessful, we employed a stepwise proce-
dure. Initially, anion 1 was allowed to react with 0.5 equiv-
alents 1,3,5-tris(bromomethyl)benzene to obtain bis(biradi-
cal) 4, and subsequently 4 was treated with another equivalent
of 1 to give the desired tris(biradical) 5 as an air-stable deep
blue solid. As observed for the bis(biradical)s 2 and 3,
tris(biradical) 5 possesses extremely high tolerance to air.
Table 1 summarizes selected spectroscopic data for 2 and
5 together with those of their elementary model 6. Each
Prof. Dr. M. Yoshifuji, Prof. Dr. A. J. Arduengo, III
Department of Chemistry, The University of Alabama
Tuscaloosa AL 35487-0336 (USA)
[**] This work was supported in part by Grants-in-Aid for Scientific
Research (Nos. 13304049 and 140440120) from the Ministry of
Education, Culture, Sports, Science and Technology (Japan), the
Program Research, Center for Interdisciplinary Research, Tohoku
University, and the Support for Young Researchers, Graduate School
of Science, Tohoku University The authors thank Dr. Chizuko
Kabuto, Tohoku University, for obtaining the X-ray data.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801461.
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Table 2: Redox potentials (V vs. Ag/AgCl) for 2, 5, and 6.
ox1
1=2
ox2
1=2
ox3
1=2
E
E
E
EoPx
ErPed
2
5
6
+0.35
+0.31
+0.38
+0.48
+0.45
+1.76
+1.78
+1.69
À0.71
À0.82
À0.76
+0.52
biradical unit, shows a reversible oxidation potential around
+ 0.4V vs. Ag/AgCl (Figure 1c). Bis(biradical) 2 appears to
show two oxidation potentials around + 0.4V vs. Ag/AgCl
(Figure 1a), and tris(biradical) 5 possesses three oxidation
potentials around + 0.4V vs. Ag/AgCl (Figure 1b, see also
the Supporting Information). Thus, the number of reversible
oxidation potentials corresponds to the number of biradical
units. Similar oxidation waves of bis(tetrathiafulvalenyl)
derivatives such as 7 are reported to be the result of the
interaction between the tetrathiafulvalene units through a
Coulombic force.[10] The separation between the first and
second oxidation potentials in 5 is almost the same as the
corresponding separation in 2, whereas the separation
between the second and third oxidation of 5 is considerably
smaller. The larger separations of the reversible oxidation
potentials observed in 2 and 5 may indicate structural
similarities (especially P2C2 conformations). Furthermore,
the lowest oxidation potentials of 2 and 5 are lower than that
of 6, possibly owing to interaction between the biradical units
analogous to that reported for 7.[10] The irreversible oxidation
potentials of 2 and 5 around 1.7 V are higher than that for 6,
probably indicating a Coulombic effect on multiple oxida-
tions. However, the reductions of 2 and 5 relative to 6 show no
obvious cumulative effect.
Scheme 2. Synthesis of tris(biradical) 5 by a stepwise procedure from
1.
biradical unit in 2 and 5 is identical, and one AB pattern and
one resonance for C(sp2) carbon atoms were observed in the
31P and 13C NMR spectra, respectively. No significant differ-
ences were observed in the NMR spectra of 2, 5, and 6. On the
other hand, the UV/Vis data (Table 1) indicate nonconjuga-
tive covalent interactions between the biradical units in 2 and
5.
Table 1: Spectroscopic data for 2, 5, and 6.
1
dP
2JPP
[Hz]
dC (sp2) JPC
[ppm]
lmax
e
[ppm]
[Hz]
[nm] [103 mÀ1 cmÀ1
]
2
5
57.7, 1.0
335.2 109.1
15.5
15.5
616 2.1
626 4.4
63.5, À0.7 327.2 110.0
6[5c,d] 58.1, 0.6
334.8 108.6
16.8, 15.8 610 1.6
Figure 2 displays an ORTEP drawing of the molecular
structure of 2.[11] Disordered substituent groups and the
presence of numerous solvent molecules in the crystals limit
the quality of structural data that can be obtained from single
crystal X-ray diffraction measurements. Nonetheless, it is
possible to discuss gross conformational features of the
molecule. Each four-membered biradical unit displays a
structure similar to the known 1,3-diphosphacyclobutane-
2,4-diyls[5a,c,d] with a distortion in the sterically encumbered
Mes* groups, as we have reported.[12] Interestingly, in spite of
the steric congestion, two bulky biradical units are positioned
on the same side of the xylyl moiety in a syn-type conforma-
tion; the shortest P···P separation
Electrochemical investigation by cyclic voltammetry (CV)
for the multi-biradicals revealed another intriguing property
arising from intramolecular interaction between the biradical
units (Figure 1, Table 2). Model compound 6, containing one
is 6.5 Š.
Model calculations were con-
ducted on 8 in which the Mes* and
tert-butyl groups of
2
were
replaced with methyl groups. In
contrast to the X-ray analysis, and
as predicted from the steric
effects, the anti conformation is
calculated as slightly more stable
than the syn form, but only by
0.13 kcalmolÀ1
(B3LYP/6-
Figure 1. Cyclic voltammograms for a) 2, b) 5, and c) 6. 1 mm in CH2Cl2; supporting electrolyte: 0.1m
tetrabutylammonium perchlorate (TBAP); working electrode: glassy carbon; counter electrode:
platinum wire; reference electrode: Ag/AgCl (E1/2 =+0.60 V vs. ferrocene/ferricinium) at 208C; scan
31G(d),[13] see the Supporting
Information). Also, semiempirical
calculations for the anti conforma-
rate: 50 mVsÀ1
.
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CV investigations of the multi-biradicals 2 and 5 indicate
nonconjugative covalent interaction between the concaten-
ated biradical units. The X-ray structure of 2 also supports the
possible interaction between the bulky biradical units in terms
of through-space pathways. Therefore, the results described
herein suggest approaches to novel open-shell materials for
molecular electronic applications by utilizing the interaction
between the biradical subunits, the 1,3-diphosphacyclobu-
tane-2,4-diyl moieties.[17]
Experimental Section
2: tert-Butyllithium (0.18 mmol, 1.4m solution in pentane) was added
to
a
solution of 2-(2,4,6-tri-tert-butylphenyl)-1-phosphaethyne
ꢀ
(Mes*C P, Mes* = 2,4,6-tBu3C6H2; 100 mg, 0.35 mmol) in THF
(5 mL) at À788C and stirred for 10 min. The reaction mixture was
allowed to warm to room temperature and stirred for 1 h. The
solution containing 1 was mixed with a solution of a,a’-dibromo-m-
xylene (0.18 mmol) in THF (1 mL) and subsequently stirred for 1 h.
The volatile materials were removed in vacuo, and the residue was
extracted with hexane. The hexane extract was concentrated in vacuo,
and the residual solid was washed with ethanol to afford 2 as a deep
blue solid (51 mg, 43%). Compound 2 was recrystallized from CH2Cl2
at 08C. M.p. 164–1658C (decomp). See the Supporting Information
for 1H NMR (400 MHz, CDCl3), 31P{1H} NMR (162 MHz, CDCl3),
13C{1H} NMR (101 MHz, CDCl3), UV/Vis (CH2Cl2), and ESI-MS.
Figure 2. ORTEP drawings for the molecular structure of 2; thermal
ꢀ
3: In a similar manner to the preparation of 2, Mes*C P
ellipsoids are set at the 40% probability level, and hydrogen atoms are
omitted for clarity. The solvent molecules (CH2Cl2) in the crystal are
omitted. The central C6H4 aryl ring is disordered, and the minor
structure is omitted. Two p-tert-butyl groups in the Mes* groups are
disordered, and the atoms with the predominant occupancy factors
(0.58, 0.69) are displayed.
(0.52 mmol), tert-butyllithium (0.27 mmol), and a,a’-dichloro-p-
xylene (0.27 mmol) were employed to afford 3 as a deep blue
insoluble solid (80 mg, 44% yield). M.p. 148–1508C (decomp); ESI-
MS calcd for C92H142P4: m/z 1371.0057; found: m/z 1371.0061.
ꢀ
4: In a similar manner to the preparation of 2, Mes*C P
(0.35 mmol), tert-butyllithium (0.18 mmol), and 1,3,5-tris(bromome-
thyl)benzene (0.18 mmol) were employed to afford 4 as a deep blue
solid (87 mg, 68% yield). See the Supporting Information for
1H NMR (400 MHz, CDCl3) and 31P{1H} NMR (162 MHz, CDCl3).
5: A solution of 1 (0.55 mmol) in THF (2 mL) was prepared in a
similar manner to 2 and 4 and was allowed to mix with a solution of 4
(0.55 mmol). The mixture was stirred for 1.5 h at room temperature,
and the volatile materials were removed in vacuo. The residual solid
was extracted with chloroform, and the chloroform extract was
concentrated in vacuo. The resulting solid was washed with ethanol to
afford 5 as a deep blue solid (67 mg, 72%). M.p. 140–1418C
tion on another model compound 9, in which the Mes* groups
of 2 were replaced with 2,6-di-tert-butylphenyl groups, turned
out to be slightly more stable than the syn conformation by
3.93 kcalmolÀ1 (AM1),[14] suggesting that steric congestion
raises the energetic difference between these two model
conformations (see the Supporting Information). However, it
could be concluded that both the anti and syn forms of 8 and 9
possess similar stability, and the syn geometry found in the X-
ray structure of 2 need not be energetically disfavored.
Multiple explanations are possible for the syn conformation
of the structure of 2. There might be CH/p interactions[15]
between the alkyl groups and aromatic rings in the biradical
units, or crystal packing forces could provide sufficient
stabilization to overcome the slight energetic disadvantage
suggested by the preliminary model calculation. Furthermore,
this observed syn conformation of 2 may cause sharing of the
p electrons upon oxidation because of the proximal position-
ing of the biradical subunits, which might lower the first
reversible oxidation potentials while reducing the highest
occupied molecular orbital (HOMO) levels.[10,16] Further-
more, the two different separations of the reversible oxidation
potentials of 5 indicate the presence of both syn and anti
conformations.
1
(decomp). See the Supporting Information for H NMR (400 MHz,
CDCl3), 31P{1H} NMR (162 MHz, CDCl3), 13C{1H} NMR (101 MHz,
CDCl3), UV/Vis (CH2Cl2), and ESI-MS.
Received: March 27, 2008
Published online: July 14, 2008
Keywords: biradicals · conformational analysis ·
.
electrochemistry · phosphorus heterocycles · steric hindrance
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[16] The HOMO or the energetically quite close HOMOÀ1 for 8 is
composed of a pair of 1,3-diphosphacyclobutane-2,4-diyl moi-
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[17] We recently found that multi-biradicals in which the tBuP groups
of 2, 3, and 5 are replaced with sBuP exhibit similar physico-
chemical properties to the those described herein. Details will be
reported elsewhere.
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