Angewandte
Chemie
for P1; 319(1)8 for P2). This difference might indicate some
for 6, which is similar to the corresponding value of 7
(À0.68 V).
We were interested in the reactivity of 6 on thermolysis.
The biradical 6 was heated to reflux in toluene to produce 4,
which was isolated in a moderate yield. This result, that is, the
influence of the dimethylenephosphorane structure on the P1
side,[5c] although the P1 C1/C2bond lengths are indeed
À
longer than the isolated dimethylenephosphoranes
[1.65–1.67 ].[13] The two Mes* rings make the mean angle
to the P2C2 plane of 60(1)8 (C1) and 59(1)8 (C2), respectively.
The lack of reactivity of 6 is due to the steric congestion
around the radical centers formed by the bulky Mes* groups.
The NMR data was also used to characterize the singlet
biradical structure of 6. In the 13C NMR spectrum, the
(radical) sp2 carbon was observed at dC = 111.3, which is
similar to that of 2 (dC = 98.8)[4] and its derivatives.[5a,b] In the
31P NMR spectrum of 6, two kinds of phosphorus atoms were
observed at dP = 55.9 (tBuP) and À11.3 (MeP) with a large
À
breaking of the P C(alkyl) bond together with the breakup of
the four-membered ring, are in sharp contrast to the reactivity
of 2.[4] Any other products, such as the eliminated tert-butyl or
methyl group that would prove the reaction mechanism, have
not been identified. We did not observe any isomerization of 6
upon irradiation (medium pressure 100 W Hg lamp at 08C for
15 h) but only recovered 6.
In conclusion, we demonstrated an alternative method to
prepare 1,3-diphosphacyclobutane-2,4-diyl from phosphaal-
kyne 4, and the novel compound 6 showed quite high stability.
The structural elucidation of 6 revealed the effective steric
protection given by the Mes* groups around the radical
centers. The small HOMO–LUMO gap found in 6, indicated
by the absorption spectrum, suggests a triplet state, as
demonstrated by its carbon congener (cyclobutane-1,3-diyl).
1
J(P,P) coupling constant (362.8 Hz). In H NMR spectrum, a
larger 3J(P,H) value (13.6 Hz) was observed for the tBuP
2
group rather than the J(P,H) for the MeP part (5.8 Hz). In
UV/Vis spectrum of 6 the absorption was observed at 612nm,
which is a large red shift compared with the absorptions of 2
(478 nm)[4] and 3 (446 nm).[6] This difference indicates that the
energetic separation between the HOMO and LUMO in 6 is
smaller than those of 2 and 3. The ground state of 1,3-
diphosphacyclobutane-2,4-diyl is theoretically assigned as a
singlet due to the interaction between the symmetrical
nonbonding molecular orbital (NBMO) and the lonepair of
the phosphorus atoms.[4,5c] As discussed on cyclobutane-1,3-
diyl, spin-multiplicity is controlled by substituents on the 2,4-
positions,[14] and therefore the spin-multiplicity of 1,3-diphos-
phacyclobutane-2,4-diyl could operate to configure the triplet
state. The red-shift absorption of 6 might indicate the small
energetic gap between the singlet and triplet states.
The present synthetic method should be
a promising
approach in the development of novel stable biradical species.
Experimental Section
Preparation of 6:
A solution of tert-butyllithium in pentane
(0.56 mmol, 1.4m) was added to a solution of 4 (300 mg, 1.0 mmol)
in THF (8 mL) at À788C and stirred for 15 min. The solution was then
warmed to room temperature, and stirred for 1 h, during which the
solution turned blue. Iodomethane (0.64 mmol) was added to the
reaction mixture and the solvent was removed in vacuo. The residue
was extracted with hexane and the solvent was removed by
evaporation to afford almost pure 6 (yield: 640 mg, 98%). Compound
6 was crystallized in dichloromethane at 08C: deep-blue/violet prisms,
mp 158–1608C; 31P{1H} NMR (162MHz, CDCl 3): d = 55.9 (d,
2J(P,P) = 362.8 Hz, tBuP), À11.3 ppm (d, 2J(P,P) = 362.8 Hz, MeP);
1H NMR (400 MHz, CDCl3): d = 7.45 (s, 2H, m-Mes*), 7.26 (s, 2H, m-
Mes*), 1.86, (d, 2J(P,H) = 5.8 Hz, 3H, MeP), 1.68 (s, 18H, o-tBu), 1.66
We performed cyclic voltammetry (CV) on 6. A reversible
ox
oxidation peak (1E ) is observed at + 0.35 V (versus Ag/
1=2
AgCl, Figure 2) and an irreversible oxidation peak (2Epox)
appears at + 1.56 V. The first oxidation probably occurs at the
radical centers, which corresponds to the HOMO, and the
other irreversible oxidation peak might indicate the oxidation
at the phosphorus atoms. Thus a stable cationic species is
generated by the oxidation. In the case of 7, an irreversible
oxidation peak was observed at + 1.36 V. As for the
reduction, the irreversible peak Erped is observed at À0.70 V
3
(s, 18H, o-tBu), 1.36 (s, 18H, p-tBu), 0.69 ppm (d, J(P,H) = 13.6 Hz,
9H, tBuP); 13C{1H} NMR (101 MHz, CDCl3): d = 150.2(pt, ( 2J(P,C) +
2J(P,C))/2 = 9.3 Hz, ipso-Mes*), 148.0 (d, 3J(P,C) = 9.3 Hz, o-Mes*),
144.7 (s, p-Mes*), 133.8 (d, 3J(P,C) = 2.8 Hz, o-Mes*), 123.0 (s, m-
Mes*), 120.0 (s, m-Mes*), 111.3 (dd, 1J(P,C) = 10.7 Hz, 1J(P,C) =
1
3
3.3 Hz, CP2), 46.7 (dd, J(P,C) = 52.9 Hz, J(P,C) = 15.8 Hz, PCMe3),
38.9 (s, o-CMe3), 37.7 (s, o-CMe3), 35.4 (s, p-CMe3), 33.8 (s, o-CMe3),
33.7 (s, o-CMe3), 31.9 (s, p-CMe3), 29.4 (dd, 2J(P,C) = 5.6 Hz, 4J(P,C) =
3.7 Hz, PCMe3), 13.6 ppm (dd, 1J(P,C) = 58.5 Hz, 3J(P,C) = 24.1 Hz,
PMe); UV-Vis (hexanes) lmax (e) = 326 (16700), 382 (17500), 612
(1500) nm.
Thermolysis of 6: A solution of 6 (0.092mmol) in toluene (8 mL)
was heated to reflux for 3 h and the solution changed from blue/
purple to yellow. The mixture was cooled to room temperature and
analyzed by 31P NMR spectroscopy, which indicated that 4 was the
major product, although the spectrum also had unidentified trace
resonance signals. The solvent was removed in vacuo and the residual
product was purified by silica gel column chromatography (hexane) to
afford 4 (14 mg, 0.049 mmol).
X-Ray data for 6: C43H70P2: Mr = 648.97, deep-blue/violet prism
crystallized from dichloromethane at 08C. Crystal dimensions 0.30
3
¯
Figure 2. Cyclic volammogram of 6: 1 mm in dichloromethane; sup-
porting electrolyte: 0.1m tetrabutylammonium perchlorate (TBAP);
working electrode: glassy carbon; counter electrode: platinum wire;
reference electrode: Ag/AgCl (E1/2 (ferrocene/ferricinium)=+0.60 V)
at 208C; scan rate: 50 mVsÀ1; E=potential, I=current.
0.25 0.10 mm , triclinic, space group P1 (no. 2), a = 14.553(2), b =
14.480(3), c = 8.878(3) , a = 96.64(1), b = 104.94(2), g = 106.812(7)8,
V= 2044.2(8) 3, Z = 2, 1calcd = 1.054 gcmÀ3
, F(000) = 716.00, m =
0.133 mmÀ1, T= 120 K. A Rigaku RAXIS-IV imaging plate detector
with graphite-monochromated MoKa radiation (l = 0.71070 ) was
Angew. Chem. Int. Ed. 2003, 42, 3802 –3804
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3803