Characterization of transient radicals in the reduction product of the -P=C=C=P- system: EPR and theoretical studies

Article abstract of DOI:10.1039/b205848k

The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar-P=C=C=P-Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent ³¹P nuclei. Additional couplings with two equivalent ¹³C are observed with Ar-P=¹³C=¹³C=P-Ar. The ³¹P anisotropic coupling constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initio SCI) were performed on the various isomers of the two radical anions: [H-P=C=C=P-H]?^-and [H-P=CH-CH=P-H]?^-. Although the optimized geometries of these two species are clearly di3erent, neither of them leads to ¹³C/³¹P hyperfine tensors in conflict with the experimental results. The absence of any ¹H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar-P=C=C=P-Ar]?^-more probable.

Full text of DOI:10.1039/b205848k

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Characterization of transient radicals in the reduction product
of the –P=C=C=P– system: EPR and theoretical studies
Helena Sidorenkova,a Mostafa Chentit,a Sylvie Choua,a Michel Geoffroy*a and Yves Ellinger*b
a
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211,
Geneva, Switzerland
b
´
`
Laboratoire d’Etude Theorique des Milieux Extremes, Universite de Nice-Sophia Antipolis
et Observatoire de la Cote d’Azur, 06108, Nice, France
´
ˆ
Received 17th June 2002, Accepted 20th August 2002
First published as an Advance Article on the web 18th September 2002
The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of
Ar–P=C=C=P–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent 31P nuclei. Additional
13
13
couplings with two equivalent 13C are observed with Ar–P= C= C=P–Ar. The 31P anisotropic coupling
constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron
is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initio
ꢀ
ꢁ
SCI) were performed on tꢀhe various isomers of the two radical anions: [H–P=C=C=P–H]
and [H–P=CH–CH=P–H] ꢁ. Although the optimized geometries of these two species are clearly different,
neither of them leads to 13C/31P hyperfine tensors in conflict with the experimental results. The absence of
1
any H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the
identification of [Ar–P=C=C=P–Ar] more probable.
ꢀ
ꢁ
hydrogen atoms on the C=C bond (the equivalence of the
two 31P couplings rules out a single addition). Such an addition
on an unsaturated carbon–carbon bond would be reminiscent
of a recent observation on 1,2-bis(3-pyridyl)acetylene.11 Infor-
mation about the reduction product of diphosphabutatriene is
obtained from liquid/frozen EPR spectra recorded after
Introduction
Low-coordinated phosphorus atoms at the extremity of delo-
calized systems have been shown to give interesting properties
to these molecules. It is mainly related to the presence of low-
lying empty orbitals suitable for electronic transfer. How these
properties are modified when the number of delocalized bonds
between the extremities increases has not yet been considered.
It has been found that moderately persistent radical anions are
formed when an electron is added to the antibonding level of a
P=C1 or a P=P2 double bond by means of chemical or electro-
chemical reduction. When the phosphorus atoms are separated
by a carbon (P=C=P) or part of 3-atom system (P=C=C) as in
phosphaallenes3 the addition of an electron leads to a drastic
structural change revealed by EPR studies. The spectrum
observed is not, as originally thought,4 that expected of an alle-
nic anion but rather that of an allylic neutral radical.5–7 The
reason is that the primary anion formed is so basic that it is
neutralized by a proton. This interpretation is based on a com-
plementary study using both EPR spectroscopy and quantum
chemical calculations. On the EPR side, it relies only upon the
analysis of the 31P and 13C hyperfine couplings since the addi-
tional proton has a very weak coupling and the line-width of
the EPR signal is large due to the presence of tri-tert-butyl phe-
nyl (Ar) groups necessary to protect the low-coordinated phos-
phorus.8 Theoretically the assignment to the allylic species is
supported by DFT and configuration interaction (CI) calucla-
tions on appropriate model compounds.
13
13
reduction of Ar–P=C=C=P–Ar and Ar–P= C= C=PAr.
These experimental results are interpreted in the light of
ab initio/DFT caꢀlꢁculations on representative ꢀmodel systems
ꢁ
[H–P=C=C=P–H] and [H–P=CH–CH=P–H]
.
Materials and methods
Compounds
Ar–P=C=C=P–Ar was synthesized by following the method
reported by Yoshifuji et al.:12 ArP=CCl213 was allowed to react
with n-BuLi in THF in presence of anhydrous CuCl2 . The
compound was purified by chromatography on a SiO2 column
using pentane as an eluant. (mp: 230 ꢂC, 31P{1H} NMR
(CDCl3), d ¼ 179.5 ppm (isomer E) and d ¼ 171.8 ppm (iso-
mer Z).
The 13C enrichment of the two central atoms of
13
ArP=C=C=P–Ar was performed by using ArP= CCl2 as a
reactant. This 13C-enriched compound was obtained by react-
ing ArPCl2 with 13C(H)Cl3 in the presence of n-BuLi as
described by Goede and Bickelhaupt.13
The present study is centered on diphosphabutatrienic com-
pounds,9,10 i.e., compounds in which the phosphorus atoms
are incorporated at the extremities of a 4-atom delocalized
fragment. The objective is to determine if the presence of a
third adjacent double bond will lead to the formation of an
observable negative ion by direct addition of an electron to
the neutral system or induce a reduction–protonation process
forming a phosphabutadienic anion after fixation of two
Measurements
Voltammograms were recorded on a BAS station using a pla-
tinum electrode and an SCE reference electrode. Freshly dis-
tilled THF was used as a solvent and Bu4NPF6 (0.2 M) as
an electrolyte.
EPR spectra were recorded on a Bruker 200-D EPR spectro-
meter (100 kHz field modulation) equipped with a variable
DOI: 10.1039/b205848k
Phys. Chem. Chem. Phys., 2002, 4, 4931–4936
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4931

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temperature attachment. Electrochemical reductions of solu-
tions of Ar–P=C=C=P–Ar in THF (Bu4NPF6 as an electrolyte)
were carried out in situ in the EPR cavity by using a quartz cell
and platinum electrodes. Chemical reductions were performed
in sealed tubes, under high vacuum, by reacting degassed THF
solutions of Ar–P=C=C=P–Ar at the surface of a potassium
mirror.
Calculations
The DFT calculations were performed with GAUSSIAN98;14
CI treatments were carried out with the MELDF package.15
Representations of the molecular structures and spin density
distributions were obtained with the Molekel program.16
As indicated by crystal structures, in ArP=C moieties, where
Ar is a bulky protective group (Ar ¼ tri-tert-butylphenyl), the
Ar plane is oriented almost perpendicular to the CP=C
plane.17–19 The spin delocalisation on the Ar group is therefore
small, and previous calculations on phosphorus compounds
containing allenic fragments have shown that replacement of
bulky aryl groups by hydrogens retains the essential character-
istics of the actual system. Density functional theory (DFT)
and ab initio post Hartree–Fock calculations were therefore
performed on the neutral molecule H–P=C=C=P–H and on
Fig. 1 EPR spectrum obtained at room temperature after electroche-
mical reduction of a solution of (a) Ar–P=C=C=P–Ar in THF; (b) Ar–
P= C= C=P–Ar in THF.
13
13
ꢀ
ꢁ
the model radicals [H–P=C=C=P–H]
and [H–P=CH–
which precludes a rapid reorientation of the radical and, there-
fore, a total averaging of the g and hyperfine anisotropy and a
1-2-1 distribution of the intensities cannot be observed. These
spectra are consistent with a large hyperfine coupling of 165
MHz with two equivalent 31P nuclei and a g value equal to
2.006. The spectrum obtained after chemical reduction is
clearly modified when the reduction is carried out with a solu-
tion of the 13C-enriched compound: the central band is split
into three lines as a result of a small hyperfine interaction (6
CH=P–H]ꢀꢁ. The actual systems are far beyond the possibility
of present methods if quantitative information is needed.
The optimizations of the various structures were performed
with the DFT method. We used hybrid B3LYP as functionals
(Becke’s three-parameter nonlocal exchange potential20
coupled to the nonlocal correlation functional of Lee, Yang
and Parr21). The basis set employed for the neutral closed-shell
phosphabutatriene H–P=C=C=P–H is standard 6-311G**; it
was extended with diffuse functions to 6-311++G** in the
1
2
MHz) with two equivalent spin nuclei (Fig. 1(b)); certainly
ꢀ
ꢁ
negative ions [H–P=C=C=P–H]
and [H–P=CH–CH=P–
due to the larger line-width of the phosphorus-MI ¼ ꢃ1 tran-
sitions, this additional splitting is not resolved on the external
lines. With the electrochemically reduced solution, the 13C
enrichment leads only to an increase of the line-widths. Freez-
H]ꢀꢁ to better describe the region, far from the nuclei, covered
by the unpaired electron. Vibrational frequencies have been
calculated (and scaled by 0.975 following Bauschlicher and
Langhoff22) for characterization of the energy minima. The
CI calculations were used for the determination of the isotro-
pic coupling constants. In order to avoid spin contamination
artefacts which, if marginal for the anisotropic parameters,
may severely affect the determination of the isotropic values,
these calculations employed a basis of spin-adapted configura-
tions that lead to a pure doublet wave function. The single-CI
treatment which was employed is known to give reliable
results for magnetic properties governed by spin-polarization
effects; it was used with the well-balanced aug-cc-pVDZ
atomic basis set.23
13
13
ing the solutions of Ar–P=C=C=P–Ar or Ar–P= C= C=P–
Ar after electrochemical or chemical reductions leads to similar
spectra, an example of which is given in Fig. 2. This spectrum
exhibits very large anisotropic couplings with two 31P nuclei,
and indicates that the corresponding two hyperfine tensors
are axial and parallel. The absence of any additional splitting
with the 13C-enriched compound indicates that the 13C coup-
lings are smaller than the line-widths. The g and hyperfine
parameters are given in Table 1.
Calculations
Results
Optimized structuresHP=C=C=PH. Two different minima,
hereafter referred to as cis and trans, are found for H–
P=C=C=P–H whose parameters are given in Table 2. Both
have a planar geometry; they belong to C2v and C2h symmetry
Experimental results
As shown by cyclic voltammetry, the reduction potential of
Ar–P=C=C=P–Ar is equal to ꢁ1.23 V vs. SCE reference
(DEp ¼ 90 mV, scan rate ¼ 10 mV sꢁ1) and corresponds to
a quasi-reversible process. Under electrolysis, the yellow solu-
tion of Ar–P=C=C=P–Ar in THF turns orange and leads to
the EPR spectrum shown in Fig. 1(a).
At room temperature, the maximum of intensity is reached
after 5 min of electrolysis; the signal disappears 5 min after
having cut the voltage. A similar spectrum is obtained after
reaction of a THF solution of Ar–P=C=C=P–Ar on a potas-
sium mirror; the line-widths are nevertheless smaller than by
electrolysis and the signals are stable for 4 h. Increasing the
temperature causes a narrowing of the signals and an appreci-
able augmentation of the intensity of the lateral signals. These
temperature effects are reversible. This broadening of the side-
bands is probably due to the presence of the bulky Ar groups
Fig. 2 Frozen solution EPR spectrum obtained after reduction of
Ar–P=C=C=P–Ar.
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13 13
Table 1 EPR parameters measured after reduction of a solution of ArP= C= C=PAr in THF
g
31P hyperfine couplinga /MHz
13C-hyperfine couplinga /MHz
Liquid
solution
Frozen
solution
Liquid
solution
Frozen
solution
Anisotropic
constantsd
Liquid
solution
Frozen
solution
13
13
b
b
c
c
ArP= C= C=PAr
2.006(3)
g
?
g
k
¼ 2.008
Aiso ¼ 165
T
T
¼ 26
t
t
¼ ꢁ139
¼ 279
6
?
k
?
k
¼ 2.003
¼ 444
a
b
Only the absolute value of the hyperfine coupling is determined from the EPR spectra. The perpendicular component was deduced from the
isotropic (liquid solution) and the ‘‘parallel’’ (frozen solution) values. c Not observed; taking the line-width of the lateral and central signals into
account, |T (13C)| and |T (13C)| are less than 17 and 10 MHz respectively. ti ¼ Ti ꢁ Aiso
.
d
?
k
Table 2 Selected optimized parameters of diamagnetic diphospha-
butatrienes (B3LYP/6-311G**)
Table 3 Selected optimized parameters of paramagnetic diphospha-
butatrienes (B3LYP/6-311++G**)
ꢀ
ꢀ
ꢁ
ꢁ
cis H–P=C=C=P–H
trans H–P=C=C=P–H
[cis H–P=C=C=P–H]
[trans H–P=C=C=P–H]
˚
˚
P=C/A
1.663
1.263
1.432
174.29
96.42
0.00
1.663
1.264
1.432
171.48
96.45
180.00
0.00
P=C/A
1.723
1.246
1.437
174.94
96.63
0.00
1.723
1.246
1.436
172.44
96.63
180.00
0.00
˚
˚
C=C/A
C=C/A
˚
˚
P–H/A
P–H/A
cPCC/ꢂ
cPCC/ꢂ
cHPC/ꢂ
cHPC/ꢂ
Di-HPPH/ꢂ
DE/kcal molꢁ1
Di-HPPH/ꢂ
DE/kcal molꢁ1
0.16
0.12
the same plane in the butatrienic structure (odd number of
bonds) whereas they are in perpendicular planes for the allenic
compounds (even number of bonds).
ꢀ
ꢀ
ꢁ
[HP=C=C=PH] ꢁ. In [H–P=C=C=P–H]
the electron is
added to the antibonding p* orbital of a butatrienic system.
This anion shows two planar isomers, cis and trans, as close
in energy as the parent compounds. The stability order is the
same with trans (C2h symmetry) more stable than cis (C2v).
The structural parameters are reported in Table 3.
With respect to the neutral parents, one can note the
increase in the P=C bond lengths coupled to the shortening
of the central C=C bond consecutive to the addition of the
electron in the lowest unoccupied molecular orbital (LUMO).
This is consistent with the very structure of the p* level which
is antibonding between the phosphorus and carbon atoms but
has a bonding character between the central carbons (see dis-
cussion below).
Fig. 3 The cis and trans conformers of diphosphabutatriene.
respectively (Fig. 3) and are very close in energy with a slight
preference for the trans isomer.
Comparison of the cumulene chain in P=C=C=P and
P=C=P4 shows very little structural difference: both systems
are quasi-linear. The central C=C bond is intermediate
between a double and a triple bond: the cPCC angles are lar-
[H–P=CH–CH=P–H]ꢀꢁ. In this radical anion the unpaired
electron lies in the antibonding p* of a butadienic system per-
pendicular to the molecular plane. Six different isomers can be
identified which are represented on Fig. 4. They belong to sym-
metry C2v (a and c), C2 (e and f) and Cs (b and d). The struc-
tural parameters of these ions are given in Table 4.
24
ger than the value of 170.73ꢂ of the P=C=P backbone. The
fundamental difference between these compounds is found in
the orientation of the end groups; due to the parity of the num-
ber of bonds, the substituents of the phosphorus atoms are in
Fig. 4 The six isomers of the diphosphabutadiene anion.
Phys. Chem. Chem. Phys., 2002, 4, 4931–4936
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Table 4 Selected optimized parameters of paramagnetic diphosphabutadienes (B3LYP/6-311++G**)
a
b
c
d
e
f
˚
P=C1/A
1.768
1.768
1.396
1.772
1.765
1.387
1.759
1.759
1.399
1.768
1.760
1.397
1.771
1.771
1.386
1.766
1.766
1.387
˚
P=C2/A
˚
C1–C2/A
cPC1C2/ꢂ
cPC2C1/ꢂ
DE/kcal molꢁ1
128.73
128.73
5.21
125.20
130.87
0.45
134.99
134.99
3.37
133.52
128.73
3.98
125.24
125.24
0.83
130.81
130.81
0.00
ꢀ
Compared to [H–P=C=C=P–H]ꢀꢁ, all the bonds of the
PCCP backbone are longer, which is consistent with the
addition of the hydrogen atoms breaking any possibility of
in-plane conjugation. The differences between PCC angles
reflect steric repulsion.
For [H–P=CH–CH=P–H]
, the calculated anisotropic
ꢁ
values show the same localisation of the unpaired electron in
a p* system, antisymmetric to the molecular plane as in the
non-hydrogenated anion. In the same way, the difference in
the t values is small (in the order of 10 MHz).
?
Vibration frequencies. The infrared frequencies related to the
P–C–C–P skeleton of the neutral compound and correspond-
ing negative ions are listed in Table 5. The sym-PC and
asym-PC stretching vibrations are shifted by 50 and 130
cmꢁ1 to lower frequencies (10%) which is a measure of the
weakness introduced in the P=C bond by addition of the extra
electron. At the same time the CC stretching frequency
increases by 50–60 cmꢁ1 in agreement with the shortening of
the CC bond in the negative ion. Concerning the protonated
species, the rupture of the in-plane conjugation appears mainly
in the CC frequency which is shifted by 500 cmꢁ1 to lower
Discussion
A preliminary description of the molecular orbital containing
the unpaired electron in the reduction product of Ar–
P=C=C=P–Ar can be obtained by decomposing the experi-
mental 31P hyperfine tensors into their Fermi contact and dipo-
lar interactions. The resulting values are reported in Table 1;
they lead, after comparison with the atomic coupling con-
stants25 31P-A*iso (13 360 MHz), and 31P-2Bo (733 MHz), to
the s and p characters of the phosphorus orbitals participating
in the SOMO. It is clear from the experimental spin densities
that 78% of the total spin is equally delocalized in two phos-
phorus p-orbitals. As indicated by the relative orientations of
the g and 31P coupling tensors, the two phosphorus p orbitals
are parallel and aligned along the g eigenvector associated to
the g value close to that of the free electron. These properties
are indicative of a p molecular orbital with a large contribution
of the two phosphorus atoms; this is in agreement with the
very small phosphorus s spin densities (rs ¼ 0.01) which are
probably due to inner shell polarization. The participation of
the central carbon atoms to this SOMO appears to be very
small: the dipolar 13C couplings are too weak to be resolved
on the frozen solution spectra and are therefore less than ca.
20 MHz; nevertheless, their presence is revealed by their small
isotropic coupling constant (6 MHz).
wavenumbers. It is an illustrꢀation of the fact that a valence
ꢁ
=
bond structure such as [H–P –C C–P –H] is no longer pos-
=
sible with the protonated ion which behaves like an ethylenic
compound.
Hyperfine interactions. The isotropic and anisotropic hyper-
fine constants, as obtained in the SCI/aug-cc-pVDZ
treatments, are presented in Tables 6 and 7 for [H–P=C=
ꢀ
ꢀ
ꢁ
ꢁ
C=P–H] and [H–P=CH–CH=P–H] respectively.
For [H–P=C=C=P–H]ꢀꢁ, the calculated anisotropic values t
show that the unpaired electron belongs to a p* system, anti-
symmetric to the molecular plane. The difference in the t
?
values is of the order of 10 MHz, which means that the atoms
linked to phosphorus have little influence. The isotropic coup-
lings are due to spin polarization of the inner shells by the p*
spin distribution that is essentially the same for both cis and
trans isomers.
This description of the SOMO derived from experiment is
entirely consistent with the DFT results obtained for [H–
P=C=C=P–H]ꢀꢁ. As shown in Fig. 5, the spatial distribution
of the spin density belongs to a p* orbital with a strong parti-
cipation of the phosphorus p-orbitals, and the calculated
coupling constants reported in Table 6 reasonably agree with
the experimental values.
Table
H–P=C=C=P–H,a of its radical anion [H–P=C=C=P–H]
5 Vibrational analysis of minimum energy structures of
ꢀ
ꢁ b
and the
ꢀ
ꢁ b
radical anion [H–P=CH–CH=P–H]
It is worthwhile remarking, however, that the 31P and 13C
coupling constants calculated for [H–P=CH–CH=P–H] are
ꢀ
ꢁ
Sym-PC
stretch n/cmꢁ1
Asym-PC
stretch n/cmꢁ1
CC stretch n/cmꢁ1
not in conflicꢀtꢁwith the experimental values. Although [H–
ꢁ
are obviously
ꢀ
(I/km molꢁ1
)
(I/km molꢁ1
)
(I/km molꢁ1
)
P=C=C=P–H]
and [H–P=CH–CH=P–H]
different, one being quasi linear, the other with a planar
Molecule H–P=C=C=P–Ha
cis
trans
488 (0)
494 (0)
1034 (8)
1034 (9)
1910 (3)
1907 (0)
Table 6 EPR parameters of paramagnetic diphosphabutatriene radi-
cal anions (MHz)
Anion [H–P=C=C=P–H].ꢁ b
cis
trans
436 (0)
446 (0)
900 (47)
902 (59)
1965 (3)
1963 (0)
ꢀ
ꢀ
ꢁ
ꢁ
[cis H–P=C=C=P–H]
[trans H–P=C=C=P–H]
Anion [HP=CH–CH=PH].ꢁ b
Aiso 31P
Aiso 13C
100
ꢁ3
100
ꢁ4
a
b
c
d
e
f
655 (2)
705 (0)
621 (1)
643 (1)
920 (0)
687 (0)
800 (25)
805 (48)
767 (12)
778 (15)
802 (48)
797 (28)
1362 (36)
1415 (2)
1368 (4)
1370 (13)
1412 (0)
1416 (0)
t31
P
ꢁ121
ꢁ110
231
ꢁ121
ꢁ110
231
t13
C
ꢁ12
ꢁ8
ꢁ12
ꢁ8
a
b
B3LYP/6-311G**. B3LYP/6-311++G**.
20
20
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Phys. Chem. Chem. Phys., 2002, 4, 4931–4936

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Table 7 EPR parameters of paramagnetic diphosphabutadiene radical anions (MHz)
ꢀ
ꢁ
[H–P=CH–CH=P–H]
a
ba
c
da
e
f
Aiso 31P
Aiso 13C
Aiso 1H
79
3
91
4
88
2
83
3
93
5
88
4
ꢁ11
ꢁ106
ꢁ117
223
ꢁ8
ꢁ10
ꢁ11
ꢁ11
ꢁ111
ꢁ112
223
ꢁ9
ꢁ10
ꢁ10
ꢁ109
ꢁ119
228
ꢁ10
ꢁ9
t31
P
ꢁ114
ꢁ113
227
ꢁ10
ꢁ9
ꢁ116
ꢁ106
222
ꢁ8
ꢁ118
ꢁ108
226
ꢁ10
ꢁ9
t13
C
ꢁ8
ꢁ8
ꢁ9
16
ꢁ2
19
16
ꢁ2
18
ꢁ2
19
19
t1H
ꢁ4
ꢁ3
ꢁ4
ꢁ5
ꢁ3
ꢁ4
ꢁ5
ꢁ4
ꢁ3
7
7
6
7
7
7
a
Average value for non symmetric compounds.
ꢀ
ꢁ
Fig. 5 Three-dimensional view of calculated spin density distribution in [H–P=C=C=P–H] radical anion (isosurface value: 0.08 au).
ꢀ
ꢁ
Fig. 6 Three-dimensional view of calculated spin density distribution in [H–P=CH–CH=P–H] radical anion (isosurface value: 0.08 au).
arrangement governed by the geometry of sp2 carbons, their
spin distributions (Figs. 5 and 6) present the same relative con-
tributions of the phosphorus and carbon p-orbitals and lead
therefore to similar couplings (Table 7). No difference can be
found in the phosphorus or carbon anisotropic tensors which
can help in discriminating between the hydrogenated and
non-hydrogenated anions. The isotropic couplings which are
here also due to spin polarization of the inner shells by the
p*ꢀ spin distribution are slightly lower in [H–P=CH–CH=P–
H] ꢁ, but the difference is too small to be decisive in regards
of the simplifications inherent to the model radicals.
The fact that this coupling is not observed, either after electro-
lysis of a solution of Ar–P=C=C=P–Ar or after reaction on a
potassium mirror, suggests that the secondary product Ar–
P=CH–CH=P–Ar has not been formed in the reduction pro-
5
cess. In contrast with Ar–P=C=CPh2 or Ar–P=C=P–Ar,7
the life time of the radical anion is sufficiently long to be
observed by EPR. This is consistent with the quasi-reversibility
of the reduction wave observed for Ar–P=C=C=P–Ar
whereas cyclic voltammetry had showed that reduction of
5
Ar–P=C=CPh2 or of fulvenephosphaallene26 was irreversible
as a consequence of the formation of the stable protonated
species.
The difference between the EPR signatures of the two radi-
cals has to be found in the additional proton couplings present
for [H–P=CH–CH=P–H]ꢀꢁ. The calculated values show that
the isotropic 1H coupling is larger than the 13C coupling; under
these conditions, the proton splittings would probably be
Acknowledgements
We thank the Swiss National Science Foundation for financial
support.
ꢀ
ꢁ
resolved on the EPR spectrum of [Ar–P=CH–CH=P–Ar]
.
Phys. Chem. Chem. Phys., 2002, 4, 4931–4936
4935

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