Synthesis and properties of oligo(biradicals) composed of 1,3-diphosphacyclobutane-2,4-diyl units and benzyl-type linkers

Article abstract of DOI:10.1002/hc.20625

We have succeeded in catenating two sterically encumbered 1,3-di-t-butyl-2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2, 4-diyl units with a spacer 1,2-(CH₂)₂C₆H ₄ to obtain bis(biradicals) as consi

Full text of DOI:10.1002/hc.20625

Heteroatom Chemistry
Volume 21, Number 6, 2010
Synthesis and Properties of Oligo(biradicals)
Composed of 1,3-Diphosphacyclobutane-
2,4-diyl Units and Benzyl-type Linkers
Shigekazu Ito,1 Joji Miura,2 Noboru Morita,2 Masaaki Yoshifuji,3
and Anthony J. Arduengo, III3
1Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of
Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan
2Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai
980-8578, Japan
3Department of Chemistry, the University of Alabama, Tuscaloosa, Alabama 35487-0336, USA
Received 12 May 2010; revised 10 June 2010
tion for electronic and magnetic materials [1,2].
ABSTRACT: We have succeeded in catenating two
However, the cyclic biradical moiety is inherently
unstable, and it is necessary to carry out studies on
such biradicals by using cryogenic and inert con-
sterically encumbered 1,3-di-t-butyl-2,4-bis(2,4,6-
tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl
units with
a spacer 1,2-(CH2)2C6H4 to obtain
ditions. On the other hand, in 1995 Niecke and
co-workers reported room-temperature stable 1,3-
diphosphacyclobutane-2,4-diyl B by utilizing reac-
tivity of kinetically stabilized phosphaalkenes [3].
The successful isolation of B indicated that phos-
phorus atom can facilitate the synthesis of sta-
ble organic biradical derivatives. In fact, in 2002,
Bertrand and co-workers also succeeded in the syn-
thesis of an isolable cyclic biradical composed of
phosphorus and boron involving an apparently dif-
ferent stabilization mode at phosphorus (C) [4].
So far, several isolable cyclic biradicals have been
developed by utilizing heavier main group ele-
ments such as germanium, tin, and aluminum [5–
7]. A 1,3-diphosphacyclobutane-2,4-diyl species pos-
sesses properties of a singlet biradical and exhibits a
number of intriguing characteristics because of the
low-energetic difference between HOMO and LUMO
and the unique open-shell electronic structure (alter-
natively named as biradicaloid) [3–7].
bis(biradicals) as considerably stable compounds.
We have discussed physicochemical properties of
the dimer, together with DFT calculations of model
compounds. Spectroscopic data, redox properties,
and X-ray structures of the oligo(biradicals) deriva-
tives including other spacers like 1,3-(CH2)2C6H4,
1,4-(CH2)2C6H4, and 1,3,5-(CH2)3C6H3, reveal that
the P-heterocyclic biradical moieties interact through
nonconjugative pathways. These properties of
oligo(biradicals) will facilitate to design novel molec-
ꢀ
C
ular systems for electronics.
2010 Wiley Periodicals,
Inc. Heteroatom Chem 21:404–411, 2010; View this article
online at wileyonlinelibrary.com. DOI 10.1002/hc.20625
INTRODUCTION
Cyclobutane-1,3-diyl A (Fig. 1) is a prototypical
cyclic biradical system and has been studied from
the perspectives of molecular science and applica-
In the course of our research on chemistry of
low-coordinated phosphorus compounds originat-
ing from a sterically stabilized phosphaalkyne 1,
we found an efficient synthetic method for 1,3-
diphosphacyclobutane-2,4-diyl 3 from the cyclic
Correspondence to: Shigekazu Ito; e-mail: ito.s.ao@m.titech
.ac.jp.
c
ꢀ
2010 Wiley Periodicals, Inc.
404

Synthesis and Properties of Oligo(biradicals) Composed of 1,3-Diphosphacyclobutane-2,4-diyl Units and Benzyl-type Linkers 405
FIGURE 1 Structure of cyclic biradicals A–C.
FIGURE 3 Structure of bis(biradicals) 4 and 5.
phosphaallyl anion
2 [8,9] (Fig. 2). The P-
heterocyclic biradicals (3) thus obtained showed
sufficiently high stability to enable handling even
in air at room temperature. Effective protection of
the biradical moiety by electronic perturbation of
phosphorus atoms and steric protection of the Mes*
(2,4,6-tri-t-butylphenyl) groups on carbon atoms is
believed responsible for this enhanced stability. This
high stability of 3 is advantageous for exploration of
chemical properties of the P-heterocyclic biradicals
such as redox properties under ambient conditions.
We found that the P-heterocyclic biradical shows a
considerable electron-donating ability, resulting in a
stable radical cation species [8,9]. We have success-
fully observed the corresponding radical cation via
chemical oxidation [10]. These findings indicate that
the P-heterocyclic biradicals are suitable candidates
for exploration of novel electro-functional materials.
Taking previous attempts to utilize biradi-
cal moieties for electronically functionalized com-
pounds into account, binding several biradical units
together in the same molecules would facilitate the
exploration of novel and unique properties. In an
attempt to construct high-spin organic molecules,
Dougherty and co-workers reported generation of
ferromagnetic molecular species by binding two
cyclic biradicals with a 1,3-dimethylenecyclobutane
spacer (4) [11]. Bertrand and co-workers succeeded
in binding their cyclic biradical units composed of
phosphorus and boron with p- and m-phenylene
groups (5), indicating that the amplitude of the com-
munication is influenced by the structure of the cate-
nating unit (P2B2 four-membered ring) [12], specif-
ically where biradical units are, through π-electron
systems in the linking groups (Fig. 3). On the other
hand, the catenation of biradical units with noncon-
jugative spacers is an alternative approach to explore
novel characteristics of biradical species in terms to
electronic functionality. In fact, dual catenation of
tetrathiafulvalene (TTF) moieties with an alkyl chain
induced through-space interaction between the TTF
chromophores [13].
In this work, we examined catenation of two and
three P-heterocyclic biradical units with noncon-
jugative 1,2-, 1,3-, and 1,4-dimethylene- and 1,3,5-
trimethylenebenzene spacers because such cate-
nation would derive characteristics from the P-
heterocyclic biradicals [14,15]. Studies of a simple
series of oligo(biradicals) provide key information
about through-space interaction between the birad-
ical units. Additionally, substituents at the terminal
phosphorus of the P2C2 four-membered oligomers
affect the conformation of chains. As indicated in
the examples of dual TTF catenation [13], such cate-
nation of 1,3-diphosphacyclobutane-2,4-diyl units
without π-conjugative bridges between the biradical
chromophores is expected to be suitable to propa-
gate characteristics from the electron-donating P-
heterocyclic biradicals. Furthermore, the benzyl-
type substituents are expected to raise stability of
the biradical structure [9], which is advantageous to
explore novel functional materials.
RESULTS AND DISCUSSION
Catenation of Two P-Heterocyclic Biradical
Units
Following to our previous results [8], cyclic
phosphaallyl anion 2a was prepared from
1
and t-butyllithium, and allowed to react with
bis(halomethyl)benzenes to afford the correspond-
ing bis(biradicals) 6a–8a [14] (Fig. 4). Several re-
newed attempts to obtain bis(biradicals) 6a (whose
FIGURE
2
Preparation of biradicals
3
from phos-
phaalkyne 1.
Heteroatom Chemistry DOI 10.1002/hc

406 Ito et al.
FIGURE 5 Structure of mono(biradicals) 9.
eties 6b–8b were obtained from 2b [15]. In the
case of the stepwise synthesis of 6b from a com-
bination of 2b and 1,2-bis(bromomethyl)benzene
through 9b, an inseparable mixture of 6b and
1,3-di-s-butyl-2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-
diphosphacyclobutane-2,4-diyl was obtained [15]
(Fig. 5). Unlike 8a, compound 8b was soluble enough
in common organic solvents for spectroscopic anal-
yses in solution. Due to reduction of steric encum-
brance around phosphorus atoms, however, 6b–8b
show somewhat inferior stability to 6a–8a and de-
composed within a few days in air [15].
Catenation of Three P-Heterocyclic Biradical
Units
As reported earlier, the reaction of
2 with
1,3,5-tris(bromomethyl)benzene afforded the corre-
FIGURE 4 Structure of o-, m-, p-bis(biradicals) 6–8.
sponding tris(biradicals) 10 [14,15] (Fig. 6). In the
synthesis was unsuccessful in the previous studies)
[14] were made and we found conditions appropri-
ate to prepare 6a (namely, utilization of a chelat-
ing diamine to increase nucleophilicity of the phos-
phaallyl moiety). Furthermore, slow addition of a
solution of 1,2-bis(bromomethyl)benzene into a re-
action mixture containing 2 was required to avoid
the puzzling formation of a byproduct containing
one biradical unit (9a) because 9a only reluctantly
reacts with 2a to give 6a. Probably due to exces-
sive steric hindrance, 6a showed relatively low sta-
bility and decomposed within a few days in air
in the crystalline state. Nevertheless, it is essential
to obtain pure 6a for understanding the through-
space interaction in the bis(biradicals) series as de-
scribed below. By contrast to the ortho-substitution
in 6a, 7a (bearing a 1,3-dimethylenebenzene spacer)
showed high stability in air over 1 year in solid
state. The molecular structure for 7a was unam-
biguously confirmed (vide infra) by its spectra and
a solid-state structural determination [14]. The p-
linked bis(biradicals) 8a shows low solubility in any
common solvents and could not be employed for an-
alyzing physicochemical properties in solution. Sim-
ilarly to 6a–8a, bis(biradicals) bearing s-BuP moi-
FIGURE 6 Structure of tris(biradicals) 10 and bis(biradicals)
11a.
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Synthesis and Properties of Oligo(biradicals) Composed of 1,3-Diphosphacyclobutane-2,4-diyl Units and Benzyl-type Linkers 407
TABLE 1 Spectroscopic Data for 6–8, 10, and 12
NMR
UV-Vis
max/nm
R
δ
P
2JPP/Hz
δ
C
λ
log ε
t-Bu
t-Bu
t-Bu
t-Bu
s-Bu
6a
54.9, −0.8
57.7, 1.0
63.5, −0.7
58.1, 0.6
27.2, 0.31
27.0, 0.28
27.7, 1.3
25.8, 2.1
25.9, 2.1
36.1, −2.1
26.6, 1.5
337.8
335.2
327.2
334.8
374.4
373.5
377.9
380.2
380.2
365.6
379.1
107.3
109.1
110.0
108.6
nd
614
616
626
610
nd
3.1
3.3
3.6
3.2
nd
7aa
10aa
12ab
6bc
s-Bu
s-Bu
7bc
8bc
109.2, 109.9
nd
610
608
3.4
3.3
s-Bu
s-Bu
10bc
12bc
110.6, 111.5
109.0, 109.6
620
606
3.5
3.1
nd = not determined.
aRef [14].
bRef [9].
cRef [15].
synthesis of 10a, it was impossible to introduce three
biradical units in a single step due to excess steric
bulk. Thus, stepwise procedures were employed to
accomplish threefold introduction of 2a [14]. Two
equivalents of phosphaallyl anion 2a were allowed
to react with 1,3,5-tris(bromomethyl)benzene to af-
ford the corresponding bis(biradicals) 11a and sub-
sequently 11a was treated with 2a to afford the cor-
responding tris(biradicals) 10a in a separate step.
However, 10b could be successfully synthesized
from 2b and 1,3,5-tris(bromomethyl)benzene in a
one-pot manner [15].
supra). The chemical shifts of the RP (R = t- or s-
Bu) phosphorus in tris(biradicals) 10 show lower-
field shifts compared with the other compounds,
suggesting that the RP phosphorus atoms may ex-
hibit slightly less pyramidalized geometries (possi-
bly due to steric bulk of the three biradical units).
The extent of pyramidalization also correlates with
the difference of 31P chemical shifts between the t-
butyl (a) and the s-butyl (b) derivatives [15]. The 2 JPP
constants also seem to be affected by the pyramidal-
ization determining interaction between the two 31
P
nuclei. The sp2 carbon atoms, on which an unpaired
electron is formally located, are observed around
δC = 110 ppm. UV-Vis spectra showed that the ab-
sorption maxima are dependent on number of bi-
radical units. Compound 6a showed slightly smaller
red shift in comparison with 7a, indicating that the
nonconjugative, shorter distance between the birad-
ical units hardly affects the photo-absorption prop-
erties. Thus, in spite of the absence of obvious and
direct conjugating structures, the P-heterocyclic bi-
radical units appear to experience an intramolecular
interaction.
NMR and UV-Vis Spectroscopic Data of
Oligo(biradicals)
Table 1 summarizes the available NMR and UV-
Vis data of 6–9 together with 12a and 12b [14,15]
(Fig. 7). In the 31P NMR spectra, the expected signals
of a typical AB pattern were observed. Compound
6a showed almost the same 31P NMR data as 3a,
indicating that the geometry around the phospho-
rus atoms remains undistorted, although its stabil-
ity is considerably reduced as compared to 3a (vide
Electrochemical Properties
Redox properties of oligo(biradicals) 6–8 and 10
were examined by voltammetric analysis to confirm
that, in spite of the absence of conjugative structures
through the spacer, the biradical units interact each
other showing the expected stepwise releases of elec-
trons [14,15]. As shown in a cyclic voltammogram
of 6a (Fig. 8), two reversible oxidation potentials
were characterized due to presence of intramolec-
ular interaction between the redox-active biradical
FIGURE 7 Structure of mono(biradicals) 12.
Heteroatom Chemistry DOI 10.1002/hc

408 Ito et al.
tives by 0.1 V, consistent with a reduced electron-
donating property of s-butyl group relative to t-
butyl. The differences of oxidation potentials enable
estimation of stabilization energy of the radical
cation and the dication species by using the relation-
ship: −RTlnK = −ꢀEF, where F is the Faraday con-
stant. Thus the stabilization energies for 6a·+, 7a·+,
7b·+, 8b·+, and 10a·+ were calculated as 3.5, 3.0, 1.8,
2.5, and 3.2 kcal·mol−1, respectively, and similarly
1.6 kcal·mol−1 for 10a2+. On the other hand, the fact
ox3
that E1/2 was not observed for 10b likely reflects a
low stability of 10b·+ and 10b2+ [15].
Molecular Structures of Oligo(biradicals)
We obtained crystals of 7a suitable for X-ray anal-
ysis by recrystallization from dichloromethane [14].
Although the quality of the crystals was rather poor
due to the inclusion of solvent molecules and the
disorders that are often problematic in these highly
t-butylated systems, it is obvious that two bulky bi-
radical units are positioned on the same side of the
central C6H4 aromatic ring in a syn-fashion. In the
crystalline state, the edge of the C6H4 aromatic plane
(1,2-position) of the catenating bridge faces the other
C6H4 edge of a second bis(biradical) molecule with
FIGURE 8 Cyclic voltammogram of 6a. Conditions: 1 mM
in CH2Cl2; supporting electrolyte: 0.1 M tetrabutylammo-
nium perchlorate (TBAP); working electrode: glassy car-
bon; counter electrode: platinum wire; reference electrode:
Ag/AgCl (E1/2 = +0.49 V vs. ferrocene/ferricinium) at 20◦C;
scan rate: 50 mVs−1
.
units. Table 2 summarizes reversible oxidation po-
tentials for oligo(biradicals) 6–8 and 10. Separations
of the oxidation potentials (ꢀEox1−ox2) for 6a are sim-
ilar to those of 7a and 8b, indicating that the extent
of intramolecular interaction between the biradical
units is independent of π-conjugation structure of
the benzyl-type spacer. In other words, the magni-
tude of the intramolecular interaction is almost con-
stant in bis(biradicals) 6–8. As for tris(biradicals),
the ꢀEox1−ox2 value of 10 reflects the same structural
characteristics as 7 (utilizing the same spacer com-
ponent), whereas the ꢀEox2−ox3 split indicates smaller
subsequent interaction [14]. It should be noted that
oxidation potentials of the s-butyl derivatives 7b, 8b,
and 10b are higher than those for the t-butyl deriva-
˚
a nonbonded C···C distance of 3.87 A (see Fig. 9).
Figure 10 displays two optimized structures for
13 as a model compound for 6a [B3LYP/6–31G(d)]
[16], where t-Bu and Mes* groups are replaced by
methyl (Fig. 11). Intramolecular P···P distances of
˚
˚
3.92 A (syn) and 4.95 A (anti) are shorter than the
corresponding data of model compounds for 7 and
8 [14,15]. In spite of the theoretically predicted short
intramolecular P···P distance of 13, the splitting of
oxidation potentials (ꢀEox1−ox2) of 6a was almost the
same as 7a and 8b (see Table 2). Therefore, the
magnitude of the putative through-space interac-
tion between the 1,3-diphiosphacyclobutane-2,4-diyl
units may be almost constant in bis(biradicals) 6–8.
TABLE 2 Redox Properties of Oligo(biradicals) (V vs. Ag/AgCl)a
ox1
1/2
ox2
1/2
ox3
R
E
E
E
1/2
ꢀEox1−ox2
ꢀEox2−ox3
t-Bu
t-Bu
t-Bu
s-Bu
s-Bu
s-Bu
6a
7ab
+ 0.35
+ 0.35
+ 0.31
+ 0.45
+ 0.47
+ 0.43
+ 0.50
+ 0.48
+ 0.45
+ 0.53
+ 0.56
+ 0.59
0.15
0.13
0.14
0.08
0.11
0.16
10ab
7bc
+ 0.52
0.07
nd
8bc
10bc
nd
nd = not determined.
aFerrocene/ferricinium = +0.60 V.
bRef [14].
cRef [15].
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Properties of Oligo(biradicals) Composed of 1,3-Diphosphacyclobutane-2,4-diyl Units and Benzyl-type Linkers 409
FIGURE 9 (a) A pair of 7a in solid state. Hydrogen atoms and solvent molecules are omitted for clarity. (b) Crystal packing in
7a.
AM1 calculations for 14 as a “near-experimental”
molecule bearing 2,6-t-Bu2C6H3 and t-butyl groups
characterized energetic preference of the anticon-
formation by 5.98 kcal·mol−1. [Electronic supple-
mentary material is available on request from the
authors: DFT calculations for 13 and 15, and AM1
calculations for 14 and 16.]
In an attempt to obtain structural aspects of the
triply catenated derivatives 10, DFT calculations for
15 as a model compound were undertaken (Fig. 12).
Two types of structures (alternate-type and cone-
type) were characterized revealing only small ener-
getic differences. In taking the presence of six Mes*
groups in 10 into account, the alternate-type struc-
ture might predominate in the experimentally char-
acterized tris(biradicals). Similar to the discussions
for 7a [14], AM1 calculation for 16 with 2,6-di-t-
butylphenyl groups showed that the alternate-type
is energetically preferable to the cone-type by 2.05
kcal·mol−1.
CONCLUSION
We have succeeded in catenating two and three
sterically encumbered 1,3-diphospacyclobutane-2,4-
diyl units with benzyl-type spacers to obtain
oligo(biradicals) 6–8 and 10 as remarkably stable
compounds. Spectroscopic data, redox properties,
and molecular structures of the oligo(biradicals)
derivatives revealed that the P-heterocyclic biradical
FIGURE 10 Optimized anti (a) and syn (b) structures of 13
at the B3LYP/6–31G* level. Relative energies are 0.00 and
1.88 kcal·mol−1, respectively.
FIGURE 11 Structure of model compounds 13–16.
Heteroatom Chemistry DOI 10.1002/hc

410 Ito et al.
ethanol to afford 6a as a deep blue solid (56 mg,
47%). Mp 140–142◦C (decomp); 1H NMR (300 MHz,
3
CDCl3) δ 0.72 (d, JPH = 14.0 Hz, 18H, t-BuP), 1.28
(s, 36H, p-t-Bu), 1.33 (s, 36H, o-t-Bu), 1.63 (s, 36H,
4
o-t-Bu), 3.65 (pt, (2 JPH + JPH)/2 = 4.2 Hz, 6H, CH2),
3
3
6.60 (t, JHH = 6.0 Hz, 2H, C6H4), 6.75 (d, JHH
=
6.0 Hz, 2H, C6H4), 7.14 (s, 4H, m-Mes*), 7.36 (s,
4H, m-Mes*); 13C{ H} NMR (75.5 MHz, CD2Cl2) δ
1
29.3 (pt, (2 JPC + JPC)/2 = 3.8 Hz, PCMe3), 30.8 (dd,
4
1 JPC = 37.0, JPC = 3.0 Hz, CH2), 31.1 (s, p-CMe3),
1
5
33.4 (d, JPC = 10.6 Hz, o-CMe3), 34.4 (s, p-CMe3),
34.6 (brs, o-CMe3), 37.5 (s, o-CMe3), 38.4 (s, o-CMe3),
FIGURE 12 Drawings of alternate-type (a) and cone-type
(b) structures of 15 at the B3LYP/6–31G(d) level. Relative
energies are 0.00 and 0.38 kcal·mol−1, respectively.
1
3
47.8 (dd, JPC = 52.9 Hz, JPC = 11.3 Hz, PCMe3),
107.3 (pt, (1 JPC + JPC)/2 = 7.6 Hz, CP2), 121.0 (d,
1
4 JPC = 2.7 Hz, m-Mes*), 122.5 (s, m-Mes*), 126.5
(d, JPC = 2.3 Hz, C6H4), 128.8 (d, JPC = 1.5 Hz,
C6H4), 132.9 (d, 3 JPC = 6.0 Hz, ipso-Mes*), 136.0 (d,
JPC = 4.5 Hz, C6H4), 145.3 (s, p-Mes*), 148.6 (brs,
moieties interact through nonconjugative pathways.
The properties of oligo(biradicals) hold promise for
the design control of spin–spin interactions in novel
molecular architectures.
o-Mes*), 150.5 (t, JPC = 4.5 Hz, o-Mes*); 31P{ H}
3
1
2
NMR (121 MHz, CDCl3) δ 54.9 (d, JPP = 337.8 Hz,
2
t-BuP), −0.75 (d, JPP = 337.8 Hz, CH2P); UV-Vis
(CH2Cl2) λmax (nm) (log ε): 614 (3.1), 386 (4.0), 339
(4.2); ESI-MS calcd. for C92H142P2 [M+] 1371.0057,
found 1371.0078.
EXPERIMENTAL
All manipulations were carried out under an argon
atmosphere by the standard Schlenk technique. All
solvents employed were dried by appropriate meth-
1
ods. H, 13C, and 31P NMR spectra were recorded
Spectroscopic data of 9a
with a Bruker Avance 400 or AMX 300 spectrometer
with Me4Si (1H, 13C) and H3PO4 (31P) as internal and
external standards. Mass spectra were recorded with
a Bruker APEX3 or a JEOL JMS T100LC spectrome-
ter. UV-Vis spectra were recorded with a Hitachi U-
3210 spectrometer. Electrochemical analyses were
performed with a BAS CV-50W voltammetric ana-
lyzer. Phosphaalkyne 1 was prepared according to
the method reported previously from 2,2-dibromo-
1-(2,4,6-tri-t-butylphenyl)-1-phosphaethene (Mes*P
= CBr2) [8,9]. Preparation of 7a, 8a, 10a, 6b, 7b,
8b, and 10b was described in our previous reports
[14,15].
1H NMR (300 MHz, CDCl3) δ 0.81 (d, 3 JPH = 14.1 Hz,
9H, t-BuP), 1.31 (s, 18H, p-t-Bu), 1.45 (s, 18H, o-t-
4
Bu), 1.71 (s, 18H, o-t-Bu), 3.74 (pt, (2 JPH + JPH)/2
= 3.0 Hz, 2H, CH2), 4.33 (s, 2H, CH2Br), 6.91–7.05
(m, 4H, C6H4), 7.22 (s, 2H, m-Mes*), 7.41 (s, 2H, m-
1
Mes*); 31P{ H} NMR (121 MHz, CDCl3) δ 56.4 (d,
2
2 JPP = 323.2 Hz, t-BuP), −1.6 (d, JPP = 323.2 Hz,
CH2P); ESI-MS calcd. for C50H75BrP2 [M+] 816.4522,
found 816.4516.
ACKNOWLEDGMENTS
This work was supported in part by a Grant-in-
Aid for Scientific Research (No. 13304049) from
the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan; Program Research,
Center for Interdisciplinary Research, and Support
for Young Researchers, Graduate School of Sci-
ence, Tohoku University; the Casio Science Promo-
tion Foundation; the National Science Foundation
(CHE-0413521, CHE-0342921); the U.S. Department
Energy Grant (DE-FG02-86ER13465); and the De-
partment of Energy, Office of Energy Efficiency and
Renewable Energy, under the Hydrogen Storage
Grand Challenge (DE-PS36-03GO93013). AJA grate-
fully acknowledge the support of the Saxon en-
dowment of the University of Alabama. We thank
Preparation of 6a
t-Butyllithium (0.18 mmol, 1.4 M solution in pen-
tane) was added to a solution of 1 (100 mg, 0.35
mmol) in THF (5 mL) and TMEDA (0.18 mmol) at
−78◦C and stirred for 10 min. The reaction mix-
ture was allowed to warm to room temperature
and stirred for 1 h. To the solution containing 2a
was added a solution of α,αꢁ-dibromo-o-xylene (0.088
mmol) in THF (1 mL) over a period of 1 h, and sub-
sequently 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
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Properties of Oligo(biradicals) Composed of 1,3-Diphosphacyclobutane-2,4-diyl Units and Benzyl-type Linkers 411
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