Synthesis and redox properties of crowded triarylphosphines carrying a nitroxide radical and related compounds

Article abstract of DOI:10.1246/bcsj.80.1791

Crowded triarylphosphines carrying a nitroxide radical were synthesized to investigate intramolecular interaction between the crowded triarylphosphine redox centers and a neutral radical. A crowded triarylphosphine carrying a bromoaryl group was developed as a key synthetic intermediate. Lithiation of the (bromoaryl)phosphine, followed by reaction with 2-methyl-2-nitrosopropane, afforded a crowded triarylphosphine carrying a nitroxide after aerobic oxidation. Similar procedure using isoamyl nitrite (isoamyl = 3-methylbutyl) gave nitroxide and azobenzene derivatives with two crowded triarylphosphine moieties. The azobenzene derivative was alternatively prepared by photolysis of the corresponding (azidoaryl)phosphine. EPR spectra of the nitroxide radicals showed hyperfine couplings with ³¹P nuclei, which were larger than previously reported phosphinoaryl nitroxide radicals. Cyclic voltammograms of the newly synthesized triarylphosphines showed reversible redox waves corresponding to the phosphorus redox centers. The two triarylphosphine moieties connected by a nitroxide linkage showed a reversible two-step oxidation reflecting considerable interaction between the phosphorus redox sites across the nitroxide linkage. Oxidation of the crowded triarylphosphines carrying a nitroxide and photolysis of the chemically oxidized (azidoaryl)phosphme was studied by EPR spectroscopy.

Full text of DOI:10.1246/bcsj.80.1791

Ó 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 9, 1791–1798 (2007) 1791
Synthesis and Redox Properties of Crowded Triarylphosphines
Carrying a Nitroxide Radical and Related Compounds
y
ꢀ
Shigeru Sasaki, Kiyotoshi Kato, and Masaaki Yoshifuji
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578
Received February 16, 2007; E-mail: sasaki@mail.tains.tohoku.ac.jp
Crowded triarylphosphines carrying a nitroxide radical were synthesized to investigate intramolecular interaction
between the crowded triarylphosphine redox centers and a neutral radical. A crowded triarylphosphine carrying a bro-
moaryl group was developed as a key synthetic intermediate. Lithiation of the (bromoaryl)phosphine, followed by re-
action with 2-methyl-2-nitrosopropane, afforded a crowded triarylphosphine carrying a nitroxide after aerobic oxidation.
Similar procedure using isoamyl nitrite (isoamyl = 3-methylbutyl) gave nitroxide and azobenzene derivatives with two
crowded triarylphosphine moieties. The azobenzene derivative was alternatively prepared by photolysis of the corre-
sponding (azidoaryl)phosphine. EPR spectra of the nitroxide radicals showed hyperfine couplings with 31P nuclei, which
were larger than previously reported phosphinoaryl nitroxide radicals. Cyclic voltammograms of the newly synthesized
triarylphosphines showed reversible redox waves corresponding to the phosphorus redox centers. The two triarylphos-
phine moieties connected by a nitroxide linkage showed a reversible two-step oxidation reflecting considerable interac-
tion between the phosphorus redox sites across the nitroxide linkage. Oxidation of the crowded triarylphosphines carry-
ing a nitroxide and photolysis of the chemically oxidized (azidoaryl)phosphine was studied by EPR spectroscopy.
Crowded triarylphosphines have attracted broad interest due
to their unique structure, properties, and reactivities. Introduc-
tion of bulky aryl groups leads to structural changes around
the phosphorus represented by large C–P–C bond angles and
unique dynamic process. Because structural changes raise the
HOMO and the bulky aryl groups kinetically protect the phos-
phorus cation radical center, some crowded triarylphosphines
can be reversibly oxidized at low potential to the stable cation
radicals.1 Trimesitylphosphine (1)2 has long been known as a
typical crowded triarylphosphine and its structure,3 dynamic
behavior,4 and redox properties1 have been studied multidi-
mensionally. Recently, several triarylphosphines more crowd-
ed than 1 have been reported. Tri(9-anthryl)phosphine has a
crowded structure5 and unique photophysical properties,6 aris-
ing from the anthryl groups. We have reported the synthesis
and the redox properties of crowded triarylphosphines, such as
tris(2,4,6-triisopropylphenyl)phosphine (2),7 and have demon-
strated the relation between structure and properties (Chart 1).
Comparison of triphenylphosphine (average C–P–C bond
angle: 103.0ꢁ,8 Eox ¼ 1:01 V vs. Ag/Agþ), 1 (109.7ꢁ,3 E1=2
¼
0:39 V), and 2 (111.5ꢁ, 0.14 V)7 clearly shows influence of the
bulky aryl groups on the structure and properties. Steric effect
of ortho-alkyl groups on the structure has been discussed for
some crowded triarylphosphines.9 In order to examine if the
crowded triarylphosphines work as components of the func-
tional molecules, we have synthesized crowded triarylphos-
phines carrying various functional sites. In addition to devel-
opment of the practical materials, studies, such as how the
newly developed redox sites behave in the model systems,
are expected to clarify properties of the particular structure.
y Present address: Department of Chemistry, The University of
Alabama, Tuscaloosa, AL 353487-0336, U.S.A.
Chart 1. Crowded triarylphosphines.
Published on the web September 13, 2007; doi:10.1246/bcsj.80.1791

1792 Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007)
Crowded Triarylphosphines Carrying Nitroxide
Scheme 1. Synthesis of triarylphosphines carrying a nitroxide and related compounds. Reagents and conditions: a, i) isoamyl nitrite/
H2SO4/AcOH/0 ꢁC, ii) KI/H2O/0 to 20 ꢁC (92%). b, i) n-BuLi/THF/ꢂ78 ꢁC, ii) CuCl/ꢂ78 to 20 ꢁC, iii) chlorobis(2,4,6-triiso-
propylphenyl)phosphine/ꢂ78 ꢁC to reflux (50%). c, i) t-BuLi/THF/ꢂ78 ꢁC, ii) 2-methyl-2-nitrosopropane/ꢂ78 to 20 ꢁC, iii) air
(9%). d, i) t-BuLi/THF/ꢂ78 ꢁC, ii) isoamyl nitrite/ꢂ78 to 20 ꢁC, iii) air (7: 33%, 8: 41%). e, i) n-BuLi/THF/ꢂ78 ꢁC, ii) TsN3/
ꢂ78 to 20 ꢁC (49%). f hꢂ/THF/0 ꢁC (quant.).
Aminophosphinobenzene 3 shows a two-step nearly reversible
redox waves only at low temperature.10 On the other hand, in-
troduction of the crowded triarylphosphine moieties similar to
2 improves stability of the phosphorus redox sites. Diphos-
phinobiphenyl 411 and (ferrocenylaryl)phosphines 512 work as
multi-step redox systems at room temperature. In this article,
we report synthesis and redox properties of the crowded tri-
arylphosphines 6 and 7 carrying a nitroxide radical. There have
been several research works on the triarylphosphines carrying
nitroxide radicals. Spin delocalization on the phosphorus
atoms13 has been reported, and the phosphine moieties have
been employed as coordination sites to transition metals to
construct magnetic materials.14 However, the phosphine moie-
ties of 6 and 7 are severely shielded by six isopropyl groups
and work as redox and/or radical sites rather than coordination
sites. We discuss the delocalization of an unpaired electron on
the phosphorus atoms and communication between the two
phosphorus redox centers across the nitroxide linkage. Azo-
benzene 8 with the two triarylphosphine moieties was obtained
in the course of the synthesis of 7. Azide 9 was synthesized as
a precursor to the triplet nitrene with a cation radical of the
crowded triarylphosphine as well as an alternative synthetic in-
termediate for 8. EPR studies on the generation of the triaryl-
phosphine cation radicals with a nitroxide by oxidation of
6 and 7 and those with a triplet nitrene by oxidation of 9,
followed by photolysis, are also presented.
employed as a key synthetic intermediate, and the nitroxide
moiety was introduced in the final step (Scheme 1). The diazo-
tization of 4-bromo-2,6-diisopropylaniline,15 followed by reac-
tion with potassium iodide, afforded 5-bromo-2-iodo-1,3-
diisopropylbenzene. The reaction of an arylcopper(I) reagent,
prepared from the bromoiodobenzene, with chlorobis(2,4,6-
triisopropylphenyl)phosphine16 gave 10. (Bromoaryl)phos-
phine 10 was lithiated with t-butyllithium and allowed to react
with 2-methyl-2-nitrosopropane, followed by aerobic oxida-
tion during work-up, to afford nitroxide 6 as a purple solid.
The lithiation of 10, followed by reaction with isoamyl
nitrite,17 gave nitroxide 7 as a green solid after work-up, and
azobenzene 8 was obtained as a by-product. Azobenzene 8
possibly forms by reduction of the initially produced nitroso-
benzene derivative. Formation of azobenzenes by reduction
of nitrosobenzenes has been reported.18 Photolysis of azide
9, prepared by the lithiation of 10, followed by the reaction
with p-toluenesulfonyl azide, also afforded 8 quantitatively.
Azobenzene 8 was obtained as a single isomer (probably
1
(E)) and no isomerization was detected by H, 31P NMR and
UV–vis spectroscopy upon photolysis with Xe-lamp (ꢀ >
290 nm) in THF-d8 at 0 ꢁC. Azide 9 is less stable as compared
with typical aryl azides, and a colorless solution of 9 became
brown during evaporation. Nitroxides 6, 7, azobenzene 8, and
azide 9 were purified by column chromatography over Al2O3.
The UV–vis spectra of 6 and 7 reflect characteristics of the
crowded triarylphosphine and nitroxide moieties (Fig. 1). Tri-
Results and Discussion
ꢀ
arylphosphines 6 and 7 exhibit ꢁꢁ absorption typical of such
Synthesis and Structure. As it is unlikely that the nitrox-
ide radicals survive under the reaction conditions for construc-
tion of the crowded triarylphosphines,7 such as refluxing a
mixture of the arylcopper(I) reagents and the phosphorus
chlorides in tetrahydrofuran, (bromoaryl)phosphine 10 was
crowded triarylphosphines as 2 (ꢀmax ("): 6: 330 (17100); 7:
338 (32200); 2: 327 (13500) nm). t-Butyl aryl nitroxides gen-
ꢀ
erally exhibit strong absorption corresponding to ꢁꢁ transi-
ꢀ
tion of the aromatic ring (ca. 290 nm) and weak nꢁ absorp-
tion at longer wavelength (ca. 500 nm).19 Diaryl nitroxides

S. Sasaki et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007) 1793
Fig. 1. UV–vis spectra of the crowded triarylphosphines in
dichloromethane.
exhibit visible absorption at longer wavelength (ca. 520 nm).20
4-Aminophenyl nitroxides have been reported to exhibit broad
band absorption up to 800 nm with ꢀmax at ca. 500 nm.21 Both
t-butyl derivative 6 and diaryl derivative 7 showed absorption
in the longer wavelength region (ꢀmax ("): 6: 572 (932); 7: 615
(3450); 446 (8150) nm) as compared to typical nitroxides, sug-
gesting a significant interaction between the electron-donating
crowded triarylphosphine and the electron-accepting nitroxide.
On the other hand, azobenzene 8 showed red-shifted absorp-
tion corresponding to the azobenzene moiety (486 (9770) nm)
Fig. 2. EPR spectra of (a) 6 and (b) 7 in dichloromethane
at 293 K, and simulated spectra of (c) 6 (g ¼ 2:0059,
a(14N) = 1.297 (1N), a(31P) = 0.440 (1P), a(1H) =
0.189 mT (2H(ortho))) and (d) 7 (g ¼ 2:0056, a(14N) =
0.990 (1N), a(31P) = 0.600 (2P), a(1H) = 0.188 mT (4H-
(ortho))).
ꢀ
as well as ꢁꢁ transition similar to 2 (322 (16800) nm). The
absorption of azobenzene moiety of 8 was similar to the corre-
sponding absorption of analogous (E)-4,40-bis(dimethylamino)-
azobenzene (ꢀmax (", MeOH) 422 (19800), 457 (30600) nm)22
rather than the parent (E)-azobenzene (ꢀmax (", CH2Cl2) 318
(21000), 443 (490) nm).23
Chart 2. Nitroxide radicals.
Table 1. Redox Potentials of Crowded Triarylphosphinesa)
red
Compound 1E1=2/Vb) 2E1=2/Vb)
E
/Vb) ꢀE/Vc)
The EPR spectra of nitroxides 6 and 7 were interpreted
1
1=2
by assigning hyperfine couplings by 14N, 31P, and H nuclei
2
4
0.14
0.19
0.09
0.10
0.23
0.22
0.27
(6: g ¼ 2:0059, a(14N) = 1.297 (1N), a(31P) = 0.440 (1P),
a(1H) = 0.189 mT (2H(ortho)); 7: g ¼ 2:0056, a(14N) = 0.990
(1N), a(31P) = 0.600 (2P), a(1H) = 0.188 mT (4H(ortho)))
(Fig. 2). Nitroxides 6 and 7 exhibited hyperfine couplings by
14N and 1H nuclei similar to the previously reported nitroxides
carrying a phosphino group, t-butyl 4-diphenylphosphinophen-
yl nitroxide (11) (a(14N) = 1.160 (1N), a(31P) = 0.226 (1P),
a(1H) = 0.222 (2H(ortho)), 0.089 (2H(meta) mT)13 and simi-
lar nitroxides as t-butyl phenyl nitroxide (12) (a(14N) = 1.195
(1N), a(1H) = 0.213 (3H(ortho, para)), 0.086 (2H(meta))
mT)24 and diphenyl nitroxide (13) (a(14N) = 0.950 (1N),
a(1H) = 0.191 (6H(ortho, para)), 0.082 (4H(meta)) mT)25
(Chart 2). On the other hand, the hyperfine couplings with
31P nuclei, which reflect unpaired electron density in the phos-
phorus 3s orbitals, were considerably larger than those of 11.
Taking higher p-character or higher hybridization ratio of
the phosphorus lone pair of the crowded triarylphosphines
into consideration, the phosphorus atoms of 6 and 7 were
estimated to have a higher spin density than that of 11. More
effective ꢁ-conjugation arising form higher p-character of
the phosphorus lone pair or intramolecular charge transfer due
to low oxidation potential of the crowded triarylphosphines
might be responsible for enhanced spin delocalization on
the phosphorus.
0.35
0.16
6
7
8
0.44
0.35
ꢂ1:11
0.34
0.12
9
10
a) Measured by cyclic voltammetry. Conditions: ca. 10ꢂ4
mol dmꢂ3 in dichloromethane with 0.1 mol dmꢂ3 n-Bu4NClO4
as a support electrolyte; working electrode: glassy carbon;
counter electrode: Pt wire; reference electrode: Ag/0.01
mol dmꢂ3 AgNO3 in acetonitrile with 0.1 mol dmꢂ3 n-Bu4-
NClO4 (E1=2(ferrocene/ferricinium) = 0.18 V); scan rate: 30
1
mV sꢂ1; temp.: 293 K. b) Reversible. c) ꢀE ¼ 2E1=2 ꢂ E1=2
.
Redox Properties and Oxidation to Cation Radical. Cy-
clic voltammograms of 6, 7, 8, 9, and 10 showed that reversi-
ble oxidation of the crowded triarylphosphine moieties to the
corresponding cation radicals occurred (Table 1). Phosphine
6 displayed one reversible oxidation at E1=2 ¼ 0:09 V vs.
Ag/Agþ. On the other hand, 7 displayed two-step reversible
oxidation (E1=2 ¼ 0:10, 0.44 V) as well as reduction of the
nitroxide moiety (E1=2 ¼ ꢂ1:11 V) (Fig. 3, Scheme 2). The
small difference between the oxidation and reduction poten-
tials (ꢀE ¼ 1:21 V) suggests that intramolecular charge

1794 Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007)
Crowded Triarylphosphines Carrying Nitroxide
Fig. 4. Cyclic voltammograms of azobenzene 8 in di-
chloromethane with 0.1 mol dmꢂ3 n-Bu4NClO4. Condi-
tions: see Table 1.
Fig. 3. Cyclic voltammograms of nitroxide 7 in dichloro-
methane with 0.1 mol dmꢂ3 n-Bu4NClO4. Conditions:
see Table 1.
Scheme 2. Oxidation and reduction of 7.
transfer occurs between the phosphine and the nitroxide.
Azobenzene 8 displayed two-step reversible oxidation with
a slight difference of the potentials (E1=2 ¼ 0:23, 0.35 V)
(Fig. 4). The first oxidation potentials of the triarylphosphines
are slightly affected by the substituents, and the substitution of
the nitroxide at para to the phosphorus appears to lower the
potential as compared with 2 (E1=2 ¼ 0:14 V), which has an
isopropyl group, whereas bromo, azido, and azo groups raise
the potentials. Nitroxide 7 showed considerable difference
between the first and the second oxidation potentials (ꢀE ¼
0:34 V). The difference was larger than those of 8 (ꢀE ¼
0:12 V) and 46 (ꢀE ¼ 0:16 V), suggesting significant electron-
ic interaction between the two phosphorus atoms through the
nitroxide linkage. The phosphorus cation radical center of
Fig. 5. EPR spectra of 6 and 7 obtained after oxidation with
AgClO4 in dichloromethane: 6 (a) at 293 K, (b) expansion
around g ¼ 2 at 293 K, (c) at 77 K. 7 (d) at 293 K, (e) ex-
pansion around g ¼ 2, and (f) at 77 K.
(Fig. 5). Addition of AgClO4 to a purple solution of 6 or a
green solution of 7 in dichloromethane gave purple solutions.
The EPR spectra of both solutions exhibited new doublet sig-
nals corresponding to the cation radical of the triarylphosphine
(6: g ¼ 2:007, a(31P) = 22.6 mT; 7: g ¼ 2:007, a(31P) = 23.3
mT) and reduced and less resolved signals corresponding to
the nitroxide moieties. In the course of oxidation of 6, the sig-
nals for nitroxides decreased and a weak doublet correspond-
ing to the cation radicals of triarylphosphines was observed.
On the other hand, the spectra of 7 were dominated by
7
þꢃ induces the cationic character of the nitrogen atom through
ꢁ-conjugation or intramolecular electron transfer, which medi-
ates repulsion of the positive charges on the phosphorus atoms
in the second oxidation. Irreversible oxidation peaks were
observed for phosphines 6, 7, 9, and 10 (Ep ¼ 1:09 (6), 1.29
(7), 1.25 (9), 1.41 (10) V).
In order to generate cation radicals of the crowded triaryl-
phosphines with a nitroxide radical, chemical oxidation of 6
and 7 was carried out and monitored by EPR spectroscopy

S. Sasaki et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007) 1795
Scheme 3. Oxidation of 6 and 7.
the signals for the phosphorus cation radicals rather than
nitroxides. Although the central signals displayed considerable
change of subsidiary couplings, probably arising from decom-
position on the phosphorus, unresolved signals did not allow
detailed analysis, and there were no decisive signals indicative
of the exchange interaction. The frozen solutions at 77 K ex-
hibited superposition of the anisotropic EPR spectra of the cat-
ion radical of the triarylphosphine (6: g== ¼ 2:005, a== (31P) =
38.8 mT, g ¼ 2:011, a (31P) = 17.0 mT; 7: g== ¼ 2:006,
Fig. 6. EPR spectra of 9 in tetrahydrofuran obtained
(a) after photolysis (ꢀ > 290 nm) at 77 K, (b) after oxida-
tion with tris(4-bromophenyl)aminium perchlorate at 293
K, (c) after oxidation with tris(4-bromophenyl)aminium
perchlorate at 293 K and photolysis (ꢀ > 290 nm) at 77
K, and (d) expansion around g ¼ 2.
?
?
a== (31P) = 40.3 mT,
g
?
= 2.013, a (31P) = 11.1 mT) on
?
those of nitroxides (6: gx ¼ 2:024, gy ¼ 2:008, gz ¼ 1:989;
7: gx ¼ 2:019, gy ¼ 2:009, gz ¼ 1:993), and did not show other
meaningful signals, such as a ꢀms ¼ 2 transition. Absence of
new signals arising from dipole–dipole interaction as well as a
ꢀms ¼ 2 transition does not support formation of the triplet
diradical or quartet triradical carrying the phosphorus cation
radical and the nitroxide with detectable dipole–dipole interac-
tion. Most probably, the cation radical of the triarylphosphine
carrying nitroxides, such as 6þꢃ and 72ðþꢃÞ, is very reactive and
decomposition of the nitroxide moiety does not allow direct
observation in such conditions, although 6 and 7 exhibited re-
versible cyclic voltammograms and the cation radicals of the
triarylphosphine are not so reactive to nitroxides at least inter-
molecularly. Even if the oxidation affords cationic species 6þꢃ
and 72ðþꢃÞ as expected, the phosphorus and the nitroxide radi-
cal centers could be independent or interact antiferromagneti-
cally to give open-shell or closed-shell singlet moieties, such
as quinoiminium N-oxides 14 and 15 (Scheme 3).
Photolysis of a cation radical of 9 is expected to give a trip-
let nitrene carrying a triarylphosphine cation radical moiety. In
contrast to the cation radicals of 6 and 7, which could lead to
EPR silent singlet state, interaction of a triplet nitrene with
the phosphorus cation radical is expected to give a quartet
or doublet state. Photolysis of a frozen solution of 9 in
THF with a Xe lamp (ꢀ > 290 nm) at 77 K led to the forma-
tion of a green solid, and signals corresponding to a triplet
nitrene26 16 (D ¼ 0:991, E ¼ 0 cmꢂ1) were observed (Fig. 6,
Scheme 4). The green frozen solution became a red solution
upon warming to 293 K, and azobenzene 8 was obtained.
The oxidation of a solution of 9 with tris(4-bromophenyl)ami-
nium perchlorate afforded a purple solution, and formation
of the cation radical 9þꢃ was confirmed by EPR (g ¼ 2:007,
a(31P) = 22.8 mT). The sample was photolyzed with a Xe
Scheme 4. Photolysis of 9.
lamp (ꢀ > 290 nm) at 77 K to give a green frozen solution.
The EPR spectrum after the photolysis was the superposition
of an axial symmetric powder pattern of the triarylphosphine
cation radical (g== ¼ 2:003, a== (31P) = 39.2 mT, g ¼ 2:010,
?
a (31P) = 11.3 mT) and a typical signal of the triplet nitrene
?
(D ¼ 0:991, E ¼ 0 cmꢂ1). Additional signals around g ¼ 2
were not decisive and neither a ꢀms ¼ 2 transition nor a signal
typical of a radical-substituted arylnitrene27 was observed. The
green frozen solution became a red solution upon warming to
293 K. Although 9 does not undergo a Staudinger reaction, the
azide and the crowded triarylphosphine moieties are compati-
ble with each other, and the azide group survived at least to
some extent after the oxidation: The stability of the cation radi-
cal of the crowded triarylphosphines with an azido group 9þꢃ
is suspect. Azide 9 itself is less stable as compared to typical
aryl azides as mentioned above. Lack of the growing signals
typical of a radical-substituted arylnitrene,26 during the pho-
tolysis, excludes the formation of a triplet nitrene 17, which
is ferromagnetically coupled with the phosphorus cation
radical center through the arylene linkage, although limited

1796 Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007)
Crowded Triarylphosphines Carrying Nitroxide
mol dmꢂ3 was dissolved in dichloromethane with 0.1 M n-
Bu4NClO4 as a supporting electrolyte and the solution was de-
gassed by bubbling with nitrogen gas. X-band EPR spectra were
measured on a Bruker ESP300E spectrometer equipped with a
JEOL ES-UCD2X liquid nitrogen Dewar. Samples were dissolved
in dichloromethane, which was distilled over calcium hydride, de-
gassed by the freeze-and-thaw cycles, and transferred to a sample
by bulb-to-bulb distillation. Oxidation was carried out in an H-
shaped sealed tube. An Ushio UXL500D-O Xe lamp in a Model
UI501C lamp house and a cut filter (UV-290) were used for the
photolysis.
ꢁ-conjugation as compared with the nitrogen radical systems
and a large hyperfine coupling in the phosphorus system could
lead to negligible dipole–dipole interaction or magnetically in-
dependent system. It is not so easy to pick up signals corre-
sponding to a doublet molecule, such as 18, resulting from
antiferromagnetic interaction of the nitrene with the phospho-
rus cation radical center among other signals in the anisotropic
spectra. However, lack of apparent growing and decreasing
signals around g ¼ 2 excludes the formation of such a doublet
molecule.
Synthesis. 5-Bromo-2-iodo-1,3-diisopropylbenzene: Acetic
acid (75 mL) and sulfuric acid (25 mL) were added to 4-bromo-
2,6-diisopropylaniline (7.26 g, 28.3 mmol), and the resultant hot
mixture was stirred for 30 min. Isoamyl nitrite (6.8 mL, 56.4
mmol) was added at 0 ꢁC, and the mixture was stirred for 20
min. A solution of potassium iodide (6.23 g, 37.5 mmol) in water
(100 mL) was added at 0 ꢁC, and the mixture was gradually
warmed to 20 ꢁC and stirred for 12 h. Aqueous solution of sodium
hydrogensulfite was added to reduce iodine, and the mixture was
extracted with hexane, washed with saturated sodium hydrogen-
carbonate solution and saturated sodium chloride solution, and
dried over anhydrous magnesium sulfate. The drying agent was
removed by filtration, and the filtrate was concentrated under re-
duced pressure. The product was purified by column chromatogra-
phy (SiO2/hexane) to give 5-bromo-2-iodo-1,3-diisopropylben-
zene (9.54 g, 26.0 mmol, 92%). 5-Bromo-2-iodo-1,3-diisopropyl-
benzene: colorless oil; 1H NMR (400 MHz, CDCl3, 296 K) ꢃ 7.18
Conclusion
We synthesized crowded triarylphosphines with a nitroxide
radical, an azido group, or an azobenzene moiety from a com-
mon crowded (bromoaryl)phosphine. The (bromoaryl)phos-
phine is expected to be a useful intermediate for the synthesis
of the crowded triarylphosphines carrying various functional
sites. The crowded triarylphosphines with a nitroxide moiety,
which have a phosphorus lone pair of enhanced p-character,
exhibited considerable delocalization of an unpaired electron
on the phosphorus atom as compared to the conventional tri-
arylphosphines with a nitroxide. The nitroxide linkage signifi-
cantly contributes to long-range communication between the
two phosphorus redox centers and further extension of these
units is expected to be unique electronic system. We could
not fully clarify the intramolecular interaction between the cat-
ion radical of the triarylphosphine and a nitrogen radical cen-
ter, probably due to instability of the phosphorus cation radical
carrying a nitroxide as well as an azide. Synthetic studies
towards further stabilization of the phosphorus radical centers
is in progress to clarify interaction between the phosphorus and
other radical centers.
(s, 2H, arom.), 3.37 (sept, JHH ¼ 6:80 Hz, 2H, CH), 1.21 (d, JHH
¼
6:80 Hz, 12H, CH(CH3)2); 13C NMR (101 MHz, CDCl3, 296 K) ꢃ
153.17 (s, 1,3-arom.), 126.86 (s, 4,6-arom.), 123.29 (s, 5-arom.),
106.99 (s, 2-arom.), 39.23 (s, CH), 23.16 (s, CH(CH3)2); IR (neat)
2964, 2927, 2885, 2870, 1558, 1462, 1423, 1404, 1387, 1363,
1340, 1317, 1298, 1238, 1169, 1153, 1130, 1107, 1068, 999,
933, 924, 887, 862, 800, 731, 701, and 665 cmꢂ1; LRMS (EI,
70 eV) m=z (rel intensity) 368 (Mþ þ 2; 98), 366 (Mþ; 100), 353
(Mþ ꢂ CH3 þ 2; 63), 351 (Mþ ꢂ CH3; 65), 226 (Mþ ꢂ CH3 ꢂ
I þ 2; 11), 224 (Mþ ꢂ CH3 ꢂ I; 11), 211 (Mþ ꢂ 2CH3 ꢂ I þ 2;
7), 209 (Mþ ꢂ 2CH3 ꢂ I; 4), 145 (Mþ ꢂ CH3 ꢂ BrI; 14), 115
(Mþ ꢂ 3CH3 ꢂ IBr; 13); HRMS (EI, 70 eV) Calcd for C12H16-
79BrI: 365.9479. Found: 365.9471; Calcd for C12H1681BrI:
367.9459. Found: 367.9462; Anal. Found: C, 39.85; H, 4.42%.
Calcd for C12H16BrI: C, 39.27; H, 4.39%.
Experimental
General. 1H, 13C, and 31P NMR spectra were measured on a
Bruker AV400 spectrometer. 1H and 13C NMR chemical shifts are
expressed as ꢃ downfield from external tetramethylsilane and cali-
brated to the residual proton of the deuterated solvents (ꢃ 7.25 for
chloroform-d, ꢃ 7.20 for benzene-d6, ꢃ 5.32 for dichloromethane-
d2) or the carbon of the deuterated solvent (ꢃ 77.0 for chloroform-
d, ꢃ 128.5 for benzene-d6). 31P NMR chemical shifts are expressed
as ꢃ downfield from external 85% H3PO4. Mass spectra were mea-
sured on a Hitachi M-2500S with electron impact (EI) ionization
at 70 eV or a JEOL HX-110 with fast atom bombardment (FAB)
ionization using the m-nitrobenzyl alcohol matrix. FT-ICR mass
spectra were measured on a Bruker APEX III with electrospray
ionization (ESI). Melting points were measured on a Yanagimoto
MP-J3 apparatus without correction. Infrared and UV–vis spectra
were measured on a Horiba FT-300 and a Hitachi U-3210 spec-
trometer, respectively. Microanalyses were performed at Research
and Analytical Center for Giant Molecules, Graduate School of
Science, Tohoku University. Merck silica gel 60 and Sumitomo
basic alumina (KCG-30) were used for the column chromatogra-
phy. All reactions were carried out under argon unless otherwise
specified. Tetrahydrofuran was distilled from sodium diphenyl-
ketyl under argon just prior to use. Cyclic voltammetry was per-
formed on a BAS CV-50W controller with a glassy carbon, Pt
wire, and Ag/0.01 mol dmꢂ3 AgNO3/0.1 mol dmꢂ3 n-Bu4NClO4/
CH3CN as a working, counter, and reference electrode, respec-
tively (ferrocene/ferricinium = 0.18 V). A substrate ca. 10ꢂ4
(4-Bromo-2,6-diisopropylphenyl)bis(2,4,6-triisopropylphen-
yl)phosphine (10). To a solution of 5-bromo-2-iodo-1,3-diiso-
propylbenzene (2.49 g, 6.78 mmol) in tetrahydrofuran (25 mL)
was added butyllithium (1.56 mol Lꢂ1 in hexane, 4.4 mL, 6.86
mmol) at ꢂ78 ꢁC, and the mixture was stirred for 30 min. Cop-
per(I) chloride (716 mg, 7.23 mmol) was added at ꢂ78 ꢁC, and the
mixture was stirred at ꢂ78 ꢁC for 30 min and at 20 ꢁC for 2 h. A
solution of chlorobis(2,4,6-triisopropylphenyl)phosphine (3.05 g,
6.45 mmol) in tetrahydrofuran (7 mL) was added to the mixture
at ꢂ78 ꢁC. The mixture was stirred for 20 min, gradually warmed,
and refluxed for 10 h. The solvent was removed under reduced
pressure, and hexane was added to the residue. Insoluble materials
were removed by filtration, and the filtrate was concentrated under
reduced pressure. The product was purified by column chromatog-
raphy (Al2O3/hexane) followed by recrystallization (ethanol/hex-
ane) to give 10 (2.30 g, 3.39 mmol, 50%). 10: pale yellow prisms;
mp 143.0–144.0 ꢁC; 1H NMR (400 MHz, CDCl3, 296 K) ꢃ 7.15
(d, JPH ¼ 2:82 Hz, 2H, m-arom.(Ar)), 6.90 (d, JPH ¼ 1:81 Hz, 4H,
m-arom.(Tip)), 3.46–3.36 (m, 6H, o-CH), 2.82 (sept, JHH
¼

S. Sasaki et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007) 1797
6:90 Hz, 2H, p-CH), 1.20 (d, JHH ¼ 6:90 Hz, 12H, p-CH(CH3)2),
1.15 (d, JHH ¼ 6:59 Hz, 6H, o-CH(CH3)2), 1.14 (d, JHH ¼ 6:33
Hz, 6H, o-CH(CH3)2), 1.12 (d, JHH ¼ 6:59 Hz, 6H, o-CH(CH3)2),
0.73 (d, JHH ¼ 6:65 Hz, 6H, o-CH(CH3)2), 0.67 (d, JHH ¼ 7:31
Hz, 6H, o-CH(CH3)2), 0.65 (d, JHH ¼ 7:57 Hz, 6H, o-CH(CH3)2);
13C NMR (101 MHz, CDCl3, 296 K) ꢃ 155.43 (d, JPC ¼ 18:8 Hz,
o-arom.(Ar)), 153.05 (d, JPC ¼ 18:5 Hz, o-arom.(Tip)), 152.94
(d, JPC ¼ 18:3 Hz, o-arom.(Tip)), 149.58 (s, p-arom.(Tip)), 135.03
(d, JPC ¼ 28:8 Hz, ipso-arom.(Ar)), 131.04 (d, JPC ¼ 22:7 Hz,
ipso-arom.(Tip)), 127.11 (d, JPC ¼ 4:0 Hz, m-arom.(Ar)), 123.43
(s, p-arom.(Ar)), 122.12 (d, JPC ¼ 5:0 Hz, m-arom.(Tip)), 122.06
UV–vis (CH2Cl2, c ¼ 1:120 ꢄ 10ꢂ5 mol Lꢂ1) ꢀmax (")/nm 615
(3450), 446 (8150), 338 (32200); FT-ICR-MS (ESI) Calcd for
½C84H124NOP2 þ Hꢅþ: 1225.9231. Found: 1225.9227; Anal.
Found: C, 81.98; H, 10.40; N, 1.15%. Calcd for C84H124NOP2:
C, 82.30; H, 10.20; N, 1.14%.
(4-Azido-2,6-diisopropylphenyl)bis(2,4,6-triisopropylphen-
yl)phosphine (9). To a solution of (bromoaryl)phosphine 10 (197
mg, 0.291 mmol) in tetrahydrofuran (3 mL) was added butyllithi-
um (1.58 mol Lꢂ1 in hexane, 0.3 mL, 0.474 mmol) at ꢂ78 ꢁC, and
the mixture was stirred for 30 min. A solution of p-toluenesulfonyl
azide (116 mg, 0.544 mmol) in tetrahydrofuran (2 mL) was added,
and the mixture was stirred at ꢂ78 ꢁC for 5 min and at 20 ꢁC
for 12 h. The mixture was concentrated under reduced pressure
and purified by column chromatography (Al2O3/hexane, hexane/
ethyl acetate = 10/1) and GPC (Jaigel 1H + 2H, CHCl3). A frac-
tion containing 9, which was obtained as a colorless solution, was
concentrated under reduced pressure to afford 9 (92.0 mg, 0.143
(d, JPC ¼ 6:6 Hz, m-arom.(Tip)), 34.07 (s, p-CH), 32.08 (d, JPC
17:8 Hz, o-CH), 32.03 (d, JPC ¼ 18:0 Hz, o-CH), 32.00 (d, JPC
¼
¼
17:1 Hz, o-CH), 24.51 (s, o-CH(CH3)2(Tip)), 24.38 (s, o-CH-
(CH3)2(Ar)), 23.91 (s, p-CH(CH3)2), 23.24 (s, o-CH(CH3)2(Tip)),
22.98 (s, o-CH(CH3)2(Tip)), 22.67 (s, o-CH(CH3)2(Ar)); 31P NMR
(162 MHz, CDCl3, 296 K) ꢃ ꢂ50:9 (s); IR (KBr) 3045, 2966,
2933, 2904, 2868, 1601, 1560, 1462, 1419, 1383, 1362, 1308,
1236, 1155, 1103, 1057, 933, 879, 866, 802, 754, 650, 557, and
525 cmꢂ1; UV–vis (CH2Cl2, c ¼ 8:389 ꢄ 10ꢂ6 mol Lꢂ1) ꢀmax
(")/nm 329 (13000); LRMS (EI, 70 eV) m=z (rel intensity) 678
(Mþ þ 2; 100), 676 (Mþ; 93), 635 (Mþ ꢂ i-Pr þ 2; 8), 633
(Mþ ꢂ i-Pr; 7); FT-ICR-MS (ESI) Calcd for ½C42H62PBr þ Hꢅþ:
677.3845. Found: 677.3850; Anal. Found: C, 74.23; H, 9.20; Br,
11.61%. Calcd for C42H62PBr: C, 74.42; H, 9.22; Br, 11.79%.
t-Butyl 3,5-Diisopropyl-4-bis(2,4,6-triisopropylphenyl)phos-
phinophenyl Nitroxide (6). To a solution of (bromoaryl)phos-
phine 10 (200 mg, 0.296 mmol) in tetrahydrofuran (3 mL) was
added t-butyllithium (1.40 mol Lꢂ1 in pentane, 0.42 mL, 0.588
mmol) at ꢂ78 ꢁC, and the mixture was stirred for 30 min. A solu-
tion of 2-methyl-2-nitrosopropane dimer (28.1 mg, 0.162 mmol) in
tetrahydrofuran (0.5 mL) was added at ꢂ78 ꢁC, and the mixture
was stirred at ꢂ78 ꢁC for 5 min and at 20 ꢁC for 12 h. The mixture
was concentrated under reduced pressure and purified by column
chromatography (Al2O3/hexane, hexane/ethyl acetate = 20/1)
to afford 6 (18.5 mg, 0.027 mmol, 9%). 6: purple solid; mp
49.5–52.0 ꢁC; EPR (CH2Cl2) g ¼ 2:0059, a(14N) = 1.297 (1N),
a(31P) = 0.440 (1P), a(1H) = 0.189 (2H) mT; IR (KBr) 2960,
2931, 2908, 2868, 1701, 1599, 1552, 1462, 1417, 1383, 1362,
mmol, 49%) as brown solid. 9: brown solid; mp 108.5–110.0 ꢁC
1
(decomp.); H NMR (400 MHz, CDCl3, 296 K) ꢃ 6.90 (d, JPH
¼
3:12 Hz, 4H, m-arom.(Tip)), 6.71 (d, JPH ¼ 2:74 Hz, 2H, m-
arom.(Ar)), 3.52–3.40 (m, 6H, o-CH), 2.82 (sept, JHH ¼ 6:82
Hz, 2H, p-CH), 1.20 (d, JHH ¼ 6:82 Hz, 12H, p-CH(CH3)2),
1.15 (d, JHH ¼ 7:04 Hz, 6H, o-CH(CH3)2), 1.14 (d, JHH ¼ 6:64
Hz, 6H, o-CH(CH3)2), 1.13 (d, JHH ¼ 7:04 Hz, 6H, o-CH(CH3)2),
0.72 (d, JHH ¼ 6:67 Hz, 6H, o-CH(CH3)2), 0.67 (d, JHH ¼ 6:69
Hz, 6H, o-CH(CH3)2), 0.65 (d, JHH ¼ 6:70 Hz, 6H, o-CH(CH3)2);
13C NMR (101 MHz, CDCl3, 296 K) ꢃ 155.32 (d, JPC ¼ 19:0 Hz,
o-arom.), 153.06 (d, JPC ¼ 18:4 Hz, o-arom.), 152.88 (d, JPC
¼
18:2 Hz, o-arom.), 149.47 (s, p-arom.(Tip)), 139.88 (s, p-
arom.(Ar)), 132.60 (d, JPC ¼ 26:7 Hz, ipso-arom.(Ar)), 131.32
(d, JPC ¼ 23:1 Hz, ipso-arom.(Tip)), 122.09 (d, JPC ¼ 4:6 Hz,
m-arom.(Tip)), 122.01 (d, JPC ¼ 4:6 Hz, m-arom.(Tip)), 114.65
(d, JPC ¼ 4:4 Hz, m-arom.(Ar)), 34.06 (s, p-CH), 32.08 (d, JPC
17:4 Hz, o-CH), 32.03 (d, JPC ¼ 18:0 Hz, o-CH), 32.01 (d, JPC
¼
¼
18:2 Hz, o-CH), 24.51 (s, o-CH(CH3)2 ꢄ 2), 24.37 (s, o-CH-
(CH3)2), 23.90 (s, p-CH(CH3)2), 23.26 (s, o-CH(CH3)2), 22.98
(s, o-CH(CH3)2), 22.72 (s, o-CH(CH3)2); 31P NMR (162 MHz,
CDCl3, 296 K) ꢃ ꢂ51:1 (s); IR (KBr) 3045, 2960, 2933, 2904,
2866, 2104, 1601, 1589, 1554, 1462, 1427, 1419, 1383, 1362,
1311, 1192, 1165, 1103, 1068, 935, 879, 758, 650, and 525 cmꢂ1
;
1325, 1281, 1163, 1101, 937, 877, 856, 723, and 650 cmꢂ1
;
UV–vis (CH2Cl2, c ¼ 8:473 ꢄ 10ꢂ6 mol Lꢂ1) ꢀmax (")/nm 572
(932), 330 (17100); FT-ICR-MS (ESI) Calcd for ½C46H71NOP þ
Hꢅþ: 685.5346. Found: 685.5341; Anal. Found: C, 79.19; H,
10.51; N, 1.90%. Calcd for C46H71NOP: C, 80.65; H, 10.45; N,
2.05%.
UV–vis (CH2Cl2, c ¼ 1:433 ꢄ 10ꢂ5 mol Lꢂ1) ꢀmax (")/nm 334
(14500); FT-ICR-MS (ESI) Calcd for ½C42H62N3P þ Hꢅþ:
640.4754. Found: 640.4756; Anal. Found: C, 78.97; H, 9.83; N,
5.81%. Calcd for C42H62N3P: C, 78.83; H, 9.77; N, 6.57%.
(E)-4,40-Bis[bis(2,4,6-triisopropylphenyl)phosphino]-3,30,5,50-
tetraisopropylazobenzene (8). A solution of azide 9 in tetra-
hydrofuran-d8 (0.5 mL) was irradiated with Xe lamp (ꢀ > 290
nm) at 0 ꢁC. Quantitative conversion from 9 to 8 was confirmed
by 31P NMR after 5.5 h, and the solution was concentrated
under reduced pressure and purified by column chromatography
(Al2O3/hexane) to afford 8 quantitatively. 8: red solid; mp
215.0–218.0 ꢁC (decomp.); 1H NMR (400 MHz, CDCl3, 296 K)
ꢃ 7.68 (d, JPH ¼ 3:14 Hz, 4H, m-arom.(Ar)), 6.92 (d, JPH ¼ 3:26
Hz, 8H, m-arom.(Tip)), 3.53–3.45 (m, 12H, o-CH), 2.83 (sept,
JHH ¼ 6:90 Hz, 4H, p-CH), 1.24 (d, JHH ¼ 6:68 Hz, 12H, o-
CH(CH3)2), 1.21 (d, JHH ¼ 6:90 Hz, 24H, p-CH(CH3)2), 1.16 (d,
JHH ¼ 6:60 Hz, 24H, o-CH(CH3)2), 0.79 (d, JHH ¼ 6:62 Hz, 12H,
o-CH(CH3)2), 0.73 (d, JHH ¼ 6:65 Hz, 12H, o-CH(CH3)2), 0.66
(d, JHH ¼ 6:62 Hz, 12H, o-CH(CH3)2); 13C NMR (101 MHz,
CDCl3, 296 K) ꢃ 154.15 (d, JPC ¼ 18:6 Hz, o-arom.), 153.17 (d,
JPC ¼ 17:9 Hz, o-arom.), 153.10 (d, JPC ¼ 18:4 Hz, o-arom.),
152.96 (s, p-arom.(Ar)), 149.61 (s, p-arom.(Tip)), 140.15 (d,
Bis[3,5-diisopropyl-4-bis(2,4,6-triisopropylphenyl)phos-
phinophenyl] Nitroxide (7). To a solution of (bromoaryl)phos-
phine 10 (200 mg, 0.296 mmol) in tetrahydrofuran (3 mL) was
added t-butyllithium (1.40 mol Lꢂ1 in pentane, 0.42 mL, 0.588
mmol) at ꢂ78 ꢁC, and the mixture was stirred for 30 min. To
the resultant pale yellow solution was added isoamyl nitrite
(0.02 mL, 0.149 mmol) at ꢂ78 ꢁC, and the mixture was stirred
at ꢂ78 ꢁC for 5 min and at 20 ꢁC for 12 h. The mixture was con-
centrated under reduced pressure and purified by column chro-
matography (Al2O3/hexane, hexane/ethyl acetate = 20/1) to
afford 7 (60.0 mg, 0.0489 mmol, 33%). A fraction eluted with
hexane was further purified by GPC (Jaigel 1H + 2H, chloro-
form) to give azobenzene 8 (74.0 mg, 0.0605 mmol, 41%). 7:
green solid; mp 131.5–134.0 ꢁC; EPR (CH2Cl2) g ¼ 2:0056,
a(14N) = 0.990 (1N), a(31P) = 0.600 (2P), a(1H) = 0.188 (4H)
mT; IR (KBr) 2958, 2927, 2906, 2868, 1601, 1552, 1462, 1419,
1383, 1362, 1309, 1101, 937, 879, 735, 650, and 523 cmꢂ1
;

1798 Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007)
Crowded Triarylphosphines Carrying Nitroxide
JPC ¼ 29:2 Hz, ipso-arom.(Ar)), 131.30 (d, JPC ¼ 21:8 Hz, ipso-
J. Organomet. Chem. 2002, 652, 3.
S. Sasaki, K. Sutoh, F. Murakami, M. Yoshifuji, J. Am.
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a) J. J. Daly, J. Chem. Soc. 1964, 3799. b) B. J. Dunne,
A. G. Orpen, Acta Crystallogr., Sect. C 1991, 47, 345. c) J.
Bruckmann, C. Krueger, F. Luts, Z. Naturforsch., B: Anorg.
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arom.(Tip)), 122.14 (d, JPC ¼ 4:2 Hz, m-arom.), 122.10 (d, JPC
¼
7
4:3 Hz, m-arom.), 118.59 (d, JPC ¼ 3:8 Hz, m-arom.), 34.09 (s, p-
CH), 32.22 (d, JPC ¼ 16:1 Hz, o-CH), 32.17 (d, JPC ¼ 18:0 Hz,
o-CH), 24.59 (s, o-CH(CH3)2), 24.56 (s, o-CH(CH3)2), 24.54
(s, o-CH(CH3)2), 23.91 (s, p-CH(CH3)2), 23.34 (s, o-CH(CH3)2),
23.01 (s, o-CH(CH3)2), 22.73 (s, o-CH(CH3)2); 31P NMR (162
MHz, CDCl3, 296 K) ꢃ ꢂ49:5 (s); IR (KBr) 2960, 2931, 2906,
2868, 1601, 1552, 1462, 1419, 1383, 1362, 1311, 1234, 1165,
8
1120, 1101, 1066, 1057, 935, 901, 877, 756, 652, and 525 cmꢂ1
;
UV–vis (CH2Cl2, c ¼ 5:237 ꢄ 10ꢂ6 mol Lꢂ1) ꢀmax (")/nm 486
(9770), 322 (16800); FT-ICR-MS (ESI) calcd for ½C84H124N2P2 þ
Hꢅþ: 1223.9313. Found: 1223.9349; Anal. Found: C, 81.50; H,
10.17; N, 2.19%. Calcd for C84H124N2P2: C, 82.44; H, 10.21;
N, 2.29%.
9
2651.
R. T. Boere, Y. Zhang, J. Organomet. Chem. 2005, 690,
´
10 a) S. Sasaki, F. Murakami, M. Yoshifuji, Tetrahedron Lett.
1997, 38, 7095. b) S. Sasaki, F. Murakami, M. Murakami, R.
Chowdhury, K. Sutoh, M. Yoshifuji, Phosphorus, Sulfur Silicon
2002, 177, 1477. c) S. Sasaki, F. Murakami, M. Yoshifuji,
Organometallics 2006, 25, 140.
11 S. Sasaki, F. Murakami, M. Murakami, M. Watanabe,
K. Kato, K. Sutoh, M. Yoshifuji, J. Organomet. Chem. 2005,
690, 2664.
12 K. Sutoh, S. Sasaki, M. Yoshifuji, Inorg. Chem. 2006,
45, 992.
13 K. Torssel, Tetrahedron 1977, 33, 2287.
14 a) D. B. Leznoff, C. Rancurel, J.-P. Sutter, S. J. Rettig, M.
Pink, O. Kahn, Organometallics 1999, 18, 5097. b) D. B. Leznoff,
C. Rancurel, J.-P. Sutter, S. J. Rettig, M. Pink, C. Paulsen, O.
Kahn, J. Chem. Soc., Dalton Trans. 1999, 3593.
15 Z.-L. Lu, A. Mayr, K.-K. Cheung, Inorg. Chim. Acta 1999,
284, 205.
We thank Grants-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan (Nos. 17310063, 15550025, and 13304049) for
financial support, Research and Analytical Center for Giant
Molecules, Graduate School of Science, Tohoku University,
for taking mass spectra and elemental analysis. This work was
partially carried out in the Advanced Instrumental Laboratory
for Graduate Research of Department of Chemistry, Graduate
School of Science, Tohoku University.
Supporting Information
1H, 13C, and 31P NMR spectra and cyclic voltammograms.
This material is available free of charge on the web at http://
www.csj.jp/journals/bcsj/.
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