Effect of conformational changes on a one-electron reduction process: Evidence of a one-electron P-P bond formation in a bis(phosphinine)

Article abstract of DOI:10.1002/chem.200400073

EPR spectra show that one-electron reduction of bis(3-phenyl-6, 6(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the sam

Full text of DOI:10.1002/chem.200400073

FULL PAPER
Effect of Conformational Changes on a One-Electron Reduction Process:
ꢀ
Evidence of a One-Electron P P Bond Formation in a Bis(phosphinine)
Sylvie Choua,[a] Cosmina Dutan,[a] Laurent Cataldo,[a] Thÿo Berclaz,[a]
Michel Geoffroy,*[a] Nicolas Mÿzailles,[b] Audrey Moores,[b] Louis Ricard,[b] and
Pascal Le Floch*[b]
ꢀ
Abstract: EPR spectra show that one-
electron reduction of bis(3-phenyl-6,6-
(trimethylsilyl)phosphinine-2-yl)dime-
thylsilane (1) on an alkali mirror leads
to a radical anion that is localized on a
single phosphinine ring, whereas the
radical anion formed from the same re-
action in the presence of cryptand or
from an electron transfer with sodium
naphthalenide is delocalized on the
two phosphinine rings. Density func-
tional theory (DFT) calculations show
that in the last species the unpaired
electron is mainly confined in a loose
ray spectroscopy, the P P distance in
neutral 1 is large (5.8 ä). As shown by
crystal structure analysis, addition of a
second electron leads to the formation
phosphinine ring. It is suggested that
the structure of this pair hinders inter-
ꢀ
nal rotation around the C Si bonds
and prevents 1 from adopting a confor-
ꢀ
ꢀ
of a classical P P single bond (P P
2.389 ä). Spectral modifications in-
duced by the presence of cryptand or
by a change in the reaction tempera-
ture are consistent with the formation
of a tight ion pair that stabilizes the
radical structure localized on a single
mation that shortens the intramolecu-
ꢀ
lar P P distance. The ability of the
phosphinine radical anion to reversibly
ꢀ
form weak P P bonds with neutral
phosphinines in the absence of steric
hindrance is confirmed by EPR spectra
obtained for 2,6-bis(trimethylsilyl)-3-
phenylphosphinine (2). Moreover, as
shown by NMR spectroscopy, in this
system, which contains only one phos-
phinine ring, further reduction leads to
an intermolecular reaction with the for-
Keywords: aromaticity
functional calculations
¥
density
¥
electron
ꢀ
P P bond (3.479 ä), which results
transfer
¥
EPR spectroscopy
¥
from the overlap of two phosphorus p
orbitals. In contrast, as attested by X-
fluxionality ¥ P ligands
ꢀ
mation of a classical P P bond.
Introduction
lecular systems.[2] Unfortunately, electron-transfer reactions
are often complex. For example, reduction of aromatic mol-
ecules by an alkali metal mirror gives rise to several types of
ion pairs and aggregates.[3] Therefore, it is important to de-
termine the factors that govern the selective formation of a
reduction compound. The conformation of the electron-ac-
ceptor system is expected to play a crucial role because in
the first step it can affect the distance between the reductant
and the reduction site, while in the second step it can stabi-
lize the reduction species by allowing the additional electron
to be delocalized better.
We have recently shown that the one-electron reduction
of some macrocyclic species can lead to an appreciable in-
teraction between two aromatic subunits. As a result, a large
delocalization of the unpaired electron over the whole mole-
cule occurs and a marked change in the macrocyclic struc-
ture is observed.[4] In particular, incorporation of two phos-
phinine rings into a macrocycle forces the radical anion to
adopt a conformation that allows the formation of a rare
phosphorus phosphorus one-electron bond.[5,6] Recently,
Hoeffelmeyer and GabbaÔ have shown that the sterically
congested structure of 1,8-bis(diphenylboryl)naphthalene
Topological modifications that accompany electron-transfer
processes are currently attracting extensive interest.[1] These
modifications involve various fields of chemistry, such as bi-
ochemical mechanisms, and molecular recognition, and are
also investigated in the context of molecular electronics and
sensors because they are expected to yield switchable mo-
[a] Dr S. Choua, C. Dutan, Dr L. Cataldo, Dr T. Berclaz,
Prof. M. Geoffroy
Department of Physical Chemistry, University of Geneva
30 Quai Ernest Ansermet, 1211 Geneva 4 (Switzerland)
Fax: (+41)022-379-65-52
E-mail: michel.geoffroy@chiphy.unige.ch
[b] Dr N. Mÿzailles, A. Moores, Dr L. Ricard, Prof. P. Le Floch
Laboratoire ™Hÿtÿroÿlÿments et Coordination∫
UMR CNRS 7653, Dÿpartement de Chimie, Ecole Polytechnique
91128 Palaiseau Cedex (France)
Fax: (+33)169-33-39-90
E-mail: lefloch@poly.polytechnique.fr
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
4080
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200400073
Chem. Eur. J. 2004, 10, 4080 4090

4080 4090
leads to the formation of a boron boron one-electron bond
upon reduction.[7]
light of DFT calculations. Interpretations of the EPR spec-
tra obtained for the one-electron reduction species 1Cꢀ rely,
in part, on the analysis of the spectra recorded after reduc-
tion of monophosphinine 2 (which models half of 1), which
was also investigated in the present study. Comparisons
have also been made with the results previously reported
for macrocycle 3.[5]
In the present study, our purpose is to investigate the
effect of geometric constraints on the outcome of a reduc-
tion. A system that is able to adopt several conformations,
and thus potentially lead to different relative orientations of
the reduction centers, was chosen. Molecule 1, which contains
two phosphinine rings linked by an sp3 silicon atom, possess-
es the desired features for such an investigation. Indeed, the
two reduction centers of the molecule, namely the two phos-
phinine rings, may accommodate different relative orienta-
tions between two limiting geometrical structures 1a and 1b.
In this approach, structural information is obtained from
EPR spectra for the paramagnetic species and from X-ray
diffraction data and NMR spectra for the diamagnetic com-
pounds. Most of these structures are also discussed in the
Results
Neutral species: synthesis,NMR spectra,and crystal struc-
ture
Monophosphinine 2: Monophosphinine 2 was synthesized
according to the diazaphosphinine method developed sever-
al years ago in our laboratory,[8] and relies on the high reac-
tivity of 1,3,2-diazaphosphinine 4 with alkynes. In the first
step, compound 4 is allowed to react with one equivalent of
trimethylsilylphenylacetylene in a [4+2]/retro [4+2] se-
quence to give 1,2-monoazaphosphinine 5 (Scheme 1). Al-
though compound 5 was not isolated, its formation was es-
tablished by 31P NMR spectroscopy. Compound 5 was then
allowed to react with trimethylsilylacetylene in another
[4+2]/retro [4+2] sequence to yield the desired asymmetric
phosphinine 2 in good yield. Compound 2 was isolated as a
white solid and its formulation was proven by usual NMR
techniques as well as elemental analysis.
Abstract in French: Les spectres RPE montrent que la rÿduc-
tion monoÿlectronique du bis(3-phenyl-6,6(trimethylsilyl)-
phosphinine-2-yl)dimethylsilane (1) sur miroir de potassium
conduit ‡ un radical anion localisÿ sur l’un des deux cycles
phosphinine alors que dans le radical anion obtenu par rÿac-
tion identique en prÿsence de cryptant ou par transfert ÿlec-
tronique du naphthalõnure de sodium, l’ÿlectron non appariÿ
est dÿlocalisÿ sur les deux cycles phosphinine. Des calculs
DFT confirment que cet ÿlectron se situe principalement dans
ꢀ
une liaison P P assez longue (3.479 ä) rÿsultant du recou-
vrement des orbitales p des deux atomes de phosphore. A
ꢀ
titre de comparaison la distance P P dans le composÿ neutre
a ÿtÿ mesurÿe ‡ 5.8 ä par spectroscopie RX. La rÿduction
Bis(phophinine) 1: Bis(phosphinine) 1 was synthesized by
using a previously reported method.[9] The room-tempera-
ture 31P NMR spectrum exhibits only one signal; this sug-
gests that the phosphinine subunits undergo fast rotation
par un second ÿlectron conduit ‡ la formation d’une liaison
ꢀ
ꢀ
P P classique, comme l’atteste la structure RX (P P=
2.389 ä). Les modifications observÿes en RPE en prÿsence
de cryptant ou en changeant la tempÿrature rÿactionnelle cor-
roborent l’hypothõse de la formation d’une paire d’ions qui
stabilise la structure radicalaire localisÿe sur un cycle. La prÿ-
sence de cette paire d’ions qui rend difficile la rotation autour
du lien inter-phosphinine, pourrait expliquer l’impossibilitÿ
pour le composÿ 1 d’adopter une conformation oú les atomes
de phosphore sont proches. La possibilitÿ pour un anion de
phosphinine de former de faÁon rÿversible une liaison faible
ꢀ
ꢀ
around the C5 Si2 and C6 Si2 bonds. Lowering the temper-
ature to ꢀ958C did not induce broadening of the peak
either in the 31P or 1H NMR spectrum, and this indicates
that the rotational barrier is very low. Therefore, we were
interested to see which of the two conformers would be fa-
vored in the solid state. Single crystals were grown by slow
diffusion of methanol into a solution of 1 in toluene. The re-
sults of the X-ray crystal analysis are shown in Figure 1. Sig-
nificant geometrical parameters and crystal data are gath-
ered in Table 1 and 2. In the solid state, the most stable con-
formation is closer to 1a, since the two phosphinine planes
are perpendicular to each other (91.598). The C5-Si2-C6
angle shows that the system is not strained (111.85(7)8). In
this conformer the interphosphorus distance is particularly
ꢀ
P P avec une phosphinine neutre en l’absence de contrainte
stÿrique a ÿtÿ mise en ÿvidence par l’ÿtude RPE du composÿ
2,6-bis-trimethylsilyl-3-phenyl-phosphinine (2). Par ailleurs, il
a ÿtÿ montrÿ par RMN que la rÿduction de monophosphini-
nes de ce type conduisait, via une rÿaction intermolÿculaire, ‡
ꢀ
la formation de liaisons P P classiques.
Chem. Eur. J. 2004, 10, 4080 4090
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4081

M. Geoffroy, P. Le Floch et al.
FULL PAPER
Scheme 1. Synthesis of compound 2.
Table 2. Crystal data and structural refinement details for the structure
of compound 1 and [1]2ꢀ¥2Na+.
Compound
1
[1]2ꢀ¥2Na+
formula
Mr
C30H38P2Si3
544.81
C38H57Na2O2P2Si3
738.03
crystal size [mm]
crystal system
space group
a [ä]
b [ä]
c [ä]
0.30î0.30î0.20
monoclinic
P21/c
0.22î0.16î0.16
triclinic
P1
≈
17.334(5)
20.775(5)
17.415(5)
90.000(5)
100.400(5)
90.000(5)
6168(3)
8
1.173
2320
0.275
150.0(10)
30.02
12.974(5)
12.976(5)
14.390(5)
112.060(5)
103.570(5)
93.600(5)
2151.7(14)
2
1.139
790
0.234
150.0(10)
27.48
Figure 1. Structure of neutral compound 1 (ORTEP view). The number-
ing is arbitrary and different from that used in the assignment of the
NMR data.
a [8]
b [8]
g [8]
V [ä3]
Z
Table 1. Selected bond lengths [ä] and angles [8] for 1.
1calcd [gcmꢀ3
F(000)
]
ꢀ
ꢀ
P1 C1
1.730(2)
1.728(2)
1.893(2)
1.393(2)
1.393(2)
1.404(2)
1.387(2)
105.10(7)
121.1(1)
124.4(2)
121.9(1)
123.6(1)
121.9(1)
P1 C5
ꢀ
P2 C6
1.743(2)
1.742(2)
1.896(2)
1.384(2)
1.409(2)
1.400(2)
1.395(2)
104.84(8)
111.85(7)
124.9(2)
122.7(2)
114.4(1)
m [cmꢀ1
T [K]
qmax
]
ꢀ
P2 C10
ꢀ
ꢀ
Si2 C5
Si2 C6
ꢀ
ꢀ
C1 C2
C2 C3
ꢀ
ꢀ
C3 C4
C4 C5
GOF1.062
reflections measured
1.010
29546
ꢀ
ꢀ
C6 C7
C7 C8
14353
ꢀ
ꢀ
C8 C9
C9 C10
independent reflections 17988
9822
C10-P2-C6
C2-C1-P1
C2-C3-C4
C4-C5-P1
C4-C5-Si2
C7-C6-P2
C1-P1-C5
C5-Si2-C6
C3-C2-C1
C3-C4-C5
P1-C5-Si2
Rint
0.0256
0.0253
reflections used
parameters refined
12563
647
0.0438
0.1282
7606
480
0.0460
0.1263
[a]
R1
[b]
wR2
difference peak/hole
0.466(0.060)/
ꢀ0.410(0.060)
0.637(0.054)/
ꢀ0.394(0.054)
[eäꢀ3
]
[a] R1 =ꢀkFo jꢀjFc k/ꢀjFo j. [b] wR2 =[(ꢀw(jFo jꢀjFc j)2/ꢀwjFo j )]1/2
.
2
large (5.8 ä). Distances and angles within the phosphinine
rings are in the normal range and do not deserve any further
comment.
[5]
ꢀ
a P P bond (Scheme 2). X-ray structure analysis of com-
pound 7 shows that two sodium counterions complete the
structure by coordinating with the para-carbon atom of one
phosphinine unit and the phosphorus atom of the other
Identification and structure of the dianionic species
Formation of a dimeric dianion
by reduction of monophosphi-
nine 2: Reduction of 2 at room
temperature with one equiva-
lent of sodium naphthalenide
afforded a very moisture-sensi-
tive
green
solution.
The
31P NMR spectrum of the crude
mixture indicated the formation
of two diamagnetic species in a
1:4 ratio (d=ꢀ77.12 and
ꢀ82.8 ppm, respectively). For-
mation of diamagnetic species
is consistent with previous re-
sults; these show that reduction
of symmetric phosphinines 6 af-
fords dimer 7 by the creation of
Scheme 2. Synthesis and conformation of the dimeric dianions formed by reduction of monophosphinines.
4082
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4080 4090

Effect of Conformational Changes on a One-Electron Reduction Process
4080 4090
phosphinine unit. As the 31P NMR signal for 7 is in the same
region (d=ꢀ68.5 ppm) as the signal observed after the re-
duction of 2, we postulated that the same type of structure
was present in the solution of 2 following the reduction.
However, the unsymmetrical nature of 2 (only one Ph group
meta to P) affords cis and trans isomers [22]2ꢀ¥2Na+
(Scheme 2).
Formation and crystal structure of the bis(phosphinine) dia-
nion 12ꢀ: Consistent with the above mentioned results, the
reaction of bis(phosphinine) 1 in THFat room temperature
with two equivalents of sodium naphthalenide afforded a
burgundy-red solution whose NMR spectra indicated the
formation of dianionic species 12ꢀ. Single crystals of this
compound suitable for an X-ray diffraction study were
grown at ꢀ188C, and the structure is shown in Figure 2,
while significant geometrical parameters are given in
Table 3. In contrast to the conformer of 1 (Figure 1), inter-
2ꢀ
Figure 2. Structure of the dianionic compound [1]2ꢀ¥2Na+ (ORTEP
view). The numbering is arbitrary and different from that used in the as-
signment of the NMR data. The diethyl ether and THFligands coordi-
nated to the sodium cations have been omitted for clarity.
ꢀ
ꢀ
nal rotations around the C5 Si2 and C6 Si2 bonds in 1 al-
lowed the two phosphorus atoms to form a phosphorus
phosphorus bond (2.389 ä). In this structure, the phosphi-
nine rings are almost planar (angle between the C1-P1-C5
and C1-C2-C4-C5 planes is 4.58, while the corresponding
angle for the second phosphinine is 3.08). The C5-Si2-C6
angle is substantially smaller (98.968) than that found in the
neutral structure, and accounts for the cyclic strain within
this molecule. The angle between the two phosphinine
planes (48.168) is also much smaller than in the neutral com-
pound 1. In [1]2ꢀ¥2Na+, the sodium cations stabilize the
structure by coordinating to the phosphorus atom of one
phosphinine unit as well as to the other phosphinine ring in
Table 3. Selected bond lengths [ä] and angles [8] for [1]2ꢀ¥2Na+.
ꢀ
ꢀ
P1 C1
ꢀ
P1 P2
1.809(2)
2.386(1)
1.800(2)
2.818(1)
1.879(2)
2.702(2)
102.7(1)
85.45(6)
127.29(7)
98.96(8)
P1 C5
1.837(2)
3.151(1)
1.843(2)
1.867(2)
2.628(2)
2.831(2).
117.58(7)
129.87(6)
74.01(3)
ꢀ
P1 Na1
ꢀ
ꢀ
P2 C10
P2 C6
ꢀ
ꢀ
P2 Na1
Si2 C6
ꢀ
ꢀ
Si2 C5
Na(1) C(3)
ꢀ
ꢀ
Na(1) C(2)
Na(1) C(4)
C1-P1-C5
C5-P1-P2
C6-P2-Na1
C6-Si2-C5
C1-P1-P2
C10-P2-Na1
P1-P2-Na
a
h6-fashion (P2 Na1 2.818(1) ä, C1,C2,C3,C4,C5 Na1
ꢀ
ꢀ
ꢀ
range between 2.628 and 3.020 ä, P1 Na1 3.151(1) ä). One
molecule of solvent (one molecule of diethyl ether for Na1,
half a molecule of diethyl ether and half a molecule of THF
for Na2) completes the coordination sphere of each sodium
cation.
heated and spectra were recorded at various temperatures.
The spectra were particularly temperature sensitive between
160 and 200 K. The above-mentioned doublet (42 G) was
clearly detected above 200 K and, as shown in Figure 3b,
the spectra at 245 and 300 K exhibit a well-resolved struc-
ture (6 G) with an additional spin 1/2 nucleus. The sample
EPR spectra of the monoanionic species
Spectra obtained after reduction of monophosphinine 2: A
solution of 2 in THF(10 ꢀ2 m)
and in the presence of cryptand
was reduced at room tempera-
ture with sodium naphthalenide
and was then rapidly cooled to
200 K. At this temperature, the
EPR spectrum contained a dou-
blet (splitting: 42 G) of poorly
resolved doublets.[10] Therefore,
the solution was cooled to
120 K. The resultant frozen sol-
ution spectrum is shown in Fig-
ure 3a and is characterized by
two external lines marked A
and separated by 145 G, and by
a central triplet marked B (cou-
pling constant 31 G). The
Figure 3. EPR spectra obtained after reduction of a solution of 2 at room temperature with Na naphthalenide
sample was then carefully in the presence of cryptand: a) frozen solution spectrum (120 K); and b) liquid solution spectrum (245 K).
Chem. Eur. J. 2004, 10, 4080 4090 www.chemeurj.org ¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4083

M. Geoffroy, P. Le Floch et al.
FULL PAPER
Table 4. Experimental EPR parameters obtained for a solution of 1 re-
duced on a potassium mirror in the absence of cryptand.
was then cooled once again and showed that the tempera-
ture dependence of the spectra is reversible.[11]
g tensor 31P coupling
1H coupling
[Gauss]
phosphorus
spin densities
The same liquid-phase spectrum was also obtained upon
reduction of 2 with sodium naphthalenide in the absence of
[Gauss]
iso =39.5[a] and 45.8[b] jAiso j=5.6[a] and 2.3[b] 1s =0.009
[12]
A
cryptand in a THFsolution.
2.0033
2.0106
2.0058
t1 =ꢀ29.5
t2 =ꢀ64.4
t3 =94.1
t1 =ꢀ8.7
t2 =0.3
t3 =8.4
1p =0.37
EPR spectra obtained after reduction of bis(phosphinine) 1:
Reduction on a potassium mirror at 200 K in the absence of
cryptand: The spectrum recorded at 200 K (Figure 4a) con-
sists of a rather broad doublet (large splitting, Aiso =39.5 G)
and is similar to the spectrum recorded at the same temper-
ature after reduction of 2. However, at 95 K the spectrum
(Figure 4b) drastically differs from the frozen solution spec-
trum obtained for 2. An optimization program was used to
find the parameters that would give rise to a satisfactory
simulation of this spectrum (Figure 4b’),[13] and these param-
eters are reported in Table 4. The large ™parallel∫ element
(Tk =Aiso +t3 =140 G) of the axial 31P hyperfine tensor
leads to the external signals marked A1 and A2, while the
[a] In liquid solution. [b] In frozen solution.
(2.0066). The temperature dependence of the spectrum was
found to be reversible.
Reduction of 1 at 200 K on a potassium (or sodium) mirror
in the presence of cryptand or by reaction with sodium naph-
thalenide: The spectrum shown in Figure 5a was recorded at
200 K after a solution of 1 in THFwas allowed to react on a
potassium mirror in the presence of cryptand. Any increase
in temperature between the time when the reduction was
carried out and the EPR spectra were recorded was careful-
ly avoided. In contrast to the doublet of doublets obtained
small hyperfine ™perpendicular∫ 31P couplings (T?1,2 =Aiso
+
t1,2 =16.3 and ꢀ18.6 G), together with the g anisotropy and
the small coupling with an additional spin 1/2 nucleus,
afford a complex central part
whose intensity is similar to the
signals A1 and A2.[14] Compari-
son of these hyperfine tensors
with the 31P atomic constants
yields the spin densities report-
ed in Table 4.[15]
Upon increasing the temper-
ature to 290 K, a well-resolved
doublet of doublets whose pa-
rameters are also given in
Table 4 was obtained. This
spectrum is very similar to that
shown for 2 in Figure 3b. The g
value measured in liquid solu-
tion (2.0081) was very similar
to the average value obtained
Figure 5. EPR spectra obtained after reduction of a solution of 1 at 200 K on a potassium mirror in the pres-
ence of cryptand: a) liquid solution spectrum (200 K); b) frozen solution spectrum (100 K); and b’) spectrum
from the simulation of the
frozen solution
spectrum simulated by using the dipolar couplings obtained from DFT calculations.
Figure 4. EPR spectra obtained after reduction of a solution of 1 at 200 K on a potassium mirror in the absence of cryptand: a) spectrum recorded at
200 K; b) frozen solution spectrum (95 K); and b’) simulated spectrum.
4084
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4080 4090

Effect of Conformational Changes on a One-Electron Reduction Process
4080 4090
when the reaction was carried out with alkali metal in ab-
sence of cryptand, this spectrum consisted of a triplet with a
small coupling constant (Aiso =4 G).
DFT structures of neutral and anionic mono and bis(phos-
phinines)
When the temperature was decreased to 100 K, an asym-
metric triplet (Figure 5b) with a large splitting (~38 G) was
obtained. As explained in the DFT section (vide infra), this
type of spectrum can be simulated (Figure 5b’) by using the
dipolar hyperfine tensors calculated for the bis(phosphinine)
radical anion 1’Cꢀ (1’ corresponds to 1 without the phenyl
rings). Upon warming the solution once again to 200 K, the
same spectrum as that shown in Figure 5a was obtained.
However, after one minute at 220 K additional signals
appear and the spectrum rapidly becomes complex. These
modifications were found to be irreversible and indicate
that the compound had undergone partial decomposition.
Very similar spectra were obtained at 100 K when sodium
naphthalenide was used, at 200 K, as a reducing agent in-
stead of a ™potassium mirror and cryptand∫.[10]
Monophosphinine 2 and its radical anion: The optimized
structures of 2 and 2Cꢀ show that addition of an electron to 2
causes an increase in the P1 C5 (D=0.06 ä) and C1 P1
ꢀ
ꢀ
(D=0.09 ä) bond lengths, and a small shortening of the
ꢀ
ꢀ
C1 C2 (D=ꢀ0.02 ä) and C4 C5 (D=ꢀ0.01 ä) bonds in
accord with the p* character of the SOMO shown in
Figure 7
Reduction of 1 on a potassium mirror at 298 K and fast cool-
ing to 200 K: The spectrum shown in Figure 6a was obtained
after a THFsolution of 1 was allowed to react on a potassi-
um mirror at 298 K in the absence of cryptand and was sub-
sequently rapidly cooled to 200 K. The spectrum exhibits
the two patterns observed in Figure 3b and Figure 5a, re-
spectively. In particular, it contains a doublet of doublets,
and in the central part, a triplet with a splitting of 4 G.
Upon decreasing the temperature to 100 K, a spectrum (Fig-
ure 6b) that was practically identical to that observed for
the frozen solution of 1 after it had been reduced at 200 K
on a potassium mirror in the presence of cryptand or with
naphthalenide (Figure 5b) was obtained. As a result of the
low intensity of the ™parallel∫ transitions, the external lines
marked A1 and A2 in Figure 4a are not detected in Fig-
ure 6b.[16] Subsequent warming of the solution afforded the
original spectrum observed at 200 K (Figure 6a), but above
220 K new signals appeared that indicated the formation of
secondary radical species. As observed above, these transfor-
mations were found to be irreversible.
Figure 7. Representation of the SOMO in 2Cꢀ.
In the radical anion 2Cꢀ, the phosphinine is almost planar
(C3-C2-C1-P1 1.88, C5-C4-C3-C2 ꢀ2.98) and makes an
angle of 1028 with the plane of the phenyl ring. The pres-
ence of the phenyl group does not appreciably modify the
spin densities previously reported for the methylphosphinine
radical anion.[5] The isotropic and anisotropic coupling con-
stants for 31P and for the proton located para to the phos-
phorus atom are given in Table 5 along with some gross or-
bital spin populations. As expected, the 31P isotropic cou-
pling constants calculated with the TZVP basis set are
larger than those calculated with the 6-31G* basis set and
Figure 6. EPR spectra obtained after reduction of a solution of 1 at room temperature on a potassium mirror in the absence of cryptand: a) liquid solu-
tion spectrum (200 K); and b) frozen solution spectrum (100 K).
Chem. Eur. J. 2004, 10, 4080 4090
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4085

M. Geoffroy, P. Le Floch et al.
FULL PAPER
Table 5. DFT calculated coupling constants and spin densities for 2Cꢀ.
sodium atom was located in
close proximity to one of the
two phosphinine rings in 1’. The
hyperfine tensors and the
SOMO were obtained from a
single-point calculation per-
formed for a conformation of 1’
Basis set
Hyperfine couplings [Gauss]
31P 1H (para)
anisotropic anisotropic
Spin densities[a]
phosphorus pp carbon (para) pp
isotropic
isotropic
6-31+G*
TZVP
26
33
ꢀ53, ꢀ49, 102
ꢀ56, ꢀ59, 115
ꢀ6.6
ꢀ6.0
ꢀ3.5, 0.0, 3.5
ꢀ3.8, 0.3, 3.5
0.48
0.53
0.27
0.24
[a] From gross orbital spin populations.
are in reasonable accord with the splitting observed in the
EPR spectrum recorded above 245 K (Figure 3b).
On the hypothesis that the g tensor is isotropic, the hyper-
fine tensors calculated with the TZVP basis set give rise to
the spectrum shown in Figure 8. The spectrum is composed
Figure 9. Ion pair 1’ꢀNa+: a) geometry used to calculate the hyperfine
tensors; and b) representation of the SOMO.
that was very similar to that obtained for the crystal struc-
ture of 1 (see Experimental Section).
When d=2.4 ä (d=distance between the ring and the
sodium atom), a Mulliken analysis clearly indicates that a
charge separation occurs. Positively charged regions are cen-
tered on the sodium atoms, and to a smaller extent on the
silicon atoms, while the negatively charged region is mainly
located on the phosphinine ring closest to the sodium atom.
As shown by the SOMO illustrated in Figure 9b, the un-
paired electron is principally localized on that particular
phosphinine ring and is reminiscent of the SOMO reported
for 2Cꢀ (Figure 7). The isotropic and anisotropic hyperfine
Figure 8. Simulation of the EPR powder spectrum using the DFT calcu-
1
constants are given in Table 6. The 31P and H (para) cou-
lated hyperfine tensors for 2Cꢀ and an isotropic g tensor (g=2.0023).
Table 6. EPR parameters for the ion pair 1’CꢀNa+.
of two external lines (marked
M’) separated by ~128 G and a
central band (marked M’’)
whose intensity is similar to that
of the external lines. An axial
distortion of g (with gk aligned
Basis set
Hyperfine couplings [Gauss]
31P 1H (para)
anisotropic
Spin densities[a]
phosphorus pp carbon (para) pp
isotropic
isotropic
anisotropic
6-31+G*
TZVP
10
18
ꢀ39, ꢀ35, 74
ꢀ42, ꢀ38, 81
ꢀ8
ꢀ6, 0, 6
ꢀ6, 0, 6
0.41
0.33
0.34
0.36
ꢀ8.5
31
ꢀ
along P Tk) would cause a [a] From gross orbital spin populations.
broadening of M’’ and a de-
crease in the intensity ratio M’’/
M’. Although the positions and
shapes of the M’ lines are similar to A in the experimentally
obtained spectrum (Figure 3a), the central M’’ pattern is dif-
ferent from B in Figure 3a.
plings are similar to those calculated for 2Cꢀ as well as to the
experimental values measured upon reduction of 1 on a
sodium or potassium mirror in the absence of cryptand. The
accord between experimental and calculated 31P couplings is
slightly better with the TZVP basis set. The single discrep-
ancy lies in the calculated isotropic 23Na coupling which is
rather large (10 G) but not actually detected. Nevertheless,
taking into account the somewhat arbitrary location of the
sodium atom in these DFT calculations, it can be concluded
that the spectra shown in Figure 4 do not conflict with the
formation of a [1]ꢀNa+ ion pair. The ability of the phosphi-
nine ring to form a tight ion pair has already been reported
for disilylphosphinines that bear two methyl groups meta to
the phosphorus atom.[5] In that particular example the ap-
proach of the sodium cation was not hindered either by a
Phosphinine 1 and its radical anion: Compound 1 is far too
large to expect quantitative results from the available non-
empirical methods. Therefore, DFT calculations were car-
ried out on the model system 1’, in which the phosphinine
ring does not contain a phenyl group. The purpose of these
calculations is to assess to what extent the relative orienta-
tion of the two phosphinine rings can lead to hyperfine in-
teractions consistent with the experimental results.
Ion pair 1’ꢀNa+: As shown in Figure 9a, we first attempted
to estimate the hyperfine couplings for an ion pair when a
4086
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4080 4090

Effect of Conformational Changes on a One-Electron Reduction Process
4080 4090
phenyl group or the second phosphinine ring, and a coupling
with 23Na was observed.
effects are likely to occur, consistency between calculated
and experimental data will mainly be based on the contribu-
tion of the anisotropic couplings. As shown in Figure 6b, the
spectrum simulated by adjusting the g tensor when the cal-
culated dipolar tensors and experimental Aiso values (liquid
solution) were used is very similar to the experimental
frozen solution spectrum.[18] Moreover, the average value of
g is very similar to the value measured in liquid solution.
Isolated radical anion 1Cꢀ: The geometry of the radical anion
1’Cꢀ was optimized by using the geometry obtained from the
crystal structure of dianion 12ꢀ as an initial conformation.
The resultant optimized conformation is shown in Fig-
ure 10a, while the corresponding parameters, together with
those optimized for 1’2ꢀ, are reported in Table 7.
Discussion
Before discussing the reduction behavior of system 1 in
ꢀ
which the two phosphinine rings are linked by a single O
ꢀ
Si O chain, it is worthwhile recalling the results[5] previously
obtained for compound 3, which contains two phosphinines
ꢀ ꢀ
linked by two O Si O chains, and interpreting the EPR
spectra described above for system 2, which contains a
single phosphinine ring. As previously reported in reference
[5], reduction of 3 at room temperature readily afforded a
rather persistent radical anion whose EPR spectrum was
characterized by an isotropic coupling of ꢁ4 G between
two equivalent 31P nuclei. At 120 K the frozen solution spec-
trum could be simulated by using two aligned 31P coupling
tensors (Tx =ꢀ20.4 G, Ty =ꢀ31.4 G, Tz =40.9 G) and a g
tensor (gx =2.0097, gy =2.0088, gz =2.0039). The DFT calcu-
lations for the radical anion 3Cꢀ were fully consistent with
the experimental tensors. In the reduced species, the SOMO
is delocalized over the two phosphinine rings and a one-
Figure 10. a) Schematic representation of the conformation of 1’Cꢀ; and b)
SOMO for 1’Cꢀ.
Table 7. Local minima for [1’]ꢀ and [1’]2ꢀ
.
Radical ion
P1¥¥¥P2 [ä] C5 Si2 C6 [8] C P C [8]
x
[a] [8]
ꢀ
108.74
100.99
98.96
ꢀ
ꢀ ꢀ
102.0
99.4
1’Cꢀ
3.479
2.582
62.6
44.75
48.2
1’2ꢀ
1
2ꢀ (experimental) 2.386
102.7
ꢀ ꢀ
[a] x=angle formed by the normals to the two C P C planes.
ꢀ
Although the model dianion does not contain phenyl sub-
stituents, the optimized parameters for 1’2ꢀ are consistent
with those obtained from the crystal structure of 12ꢀ. How-
ever, it should be noted that the optimized geometries of
the mono and dianion species contain some notable differ-
ences. The addition of a second electron to the system short-
ens the P¥¥¥P distance by 0.9 ä and decreases the interphos-
electron P P bond is formed by the axial overlap of the two
phosphorus pp orbitals.
The DFT calculations for 2Cꢀ (Table 5) determined the 31P
couplings to be appreciably larger than those measured for
3Cꢀ, but were consistent with the doublet of doublets ob-
tained for compound 2 in the liquid phase (Figure 3b).
However, for the frozen solutions, whereas the two external
signals (marked A on Figure 3a) corresponded to the exter-
nal lines (M’) predicted by DFT calculations (Figure 8), the
intense central part (triplet marked B) was clearly different.
In fact, the central triplet B, which exhibits a hyperfine split-
ting of 30 G with two spin 1/2 nuclei, corresponds well with
the frozen solution spectrum expected for a radical such as
3Cꢀ. This indicates that in the aggregation process that
occurs upon a decrease in temperature, an appreciable
amount of the radical anions 2Cꢀ react with neutral mono-
ꢀ ꢀ
phinine C5 Si C6 angle by 88. Moreover, the angle between
ꢀ ꢀ
the normals to the two C P C planes decreases by 208. The
SOMO for 1’Cꢀ is shown in Figure 10b and clearly shows the
bonding overlap between the two phosphorus 3pp orbitals,
which contain 58% of the spin. The remainder is delocalized
on the ortho and para carbons of the two phosphinine rings.
A natural bond orbital study (NBO[17]) carried out (B3LYP
functional/TZVP basis set) for this optimized geometry de-
ꢀ
termined that the occupancy of the P P bond is equal to
0.93 electron.
ꢀ
phosphinines to form weak one-electron P P bonds to give
The dipolar and isotropic couplings calculated for 1’Cꢀ are
given in Table 8. Since Fermi contact is generally more diffi-
cult to predict than dipolar couplings when spin polarization
[22]Cꢀ. As shown by EPR experiments at various tempera-
tures, the formation of this intermolecular bond is reversi-
ble. As is consistent with NMR spectra, further reduction of
this radical monoanion leads to
the formation of a classical di-
Table 8. Calculated anisotropic (t) and isotropic (Aiso) hyperfine couplings [Gauss] for 1’Cꢀ and the adjusted g
tensors.
phosphine bond to give [22]2ꢀ.
We can now interpret the re-
sults obtained for the system
that contains a single interphos-
phinine link. In the liquid phase,
reduction of 1 by reaction at
200 K on a potassium or sodium
mirror leads to a doublet of
P1 and P2 couplings
eigenvectors
Ha (para) and Hb (para) couplings
Adjusted g tensor
eigenvalues
eigenvalues
eigenvectors
l
m
n
l
m
n
ti =ꢀ20.6
tj =ꢀ22.9
tk =43.5
0.643 (ꢂ)0.765 ꢀ0.043 t’i =ꢀ2.24
0.589 (ꢂ)0.804 ꢀ0.073 gx =2.0070
ꢀ0.554 (ꢂ)0.426 ꢀ0.716 t’j =ꢀ0.06
0.529 (ꢂ)0.484 ꢀ0.697 t’k =2.3
ꢀ0.596 (ꢂ)0.494
0.546 (ꢂ)0.330
0.634 gy =2.0030
0.770 gz =2.0007
A
iso =5
A
iso =ꢀ4
Chem. Eur. J. 2004, 10, 4080 4090
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4087

M. Geoffroy, P. Le Floch et al.
FULL PAPER
doublets that is similar to the one recorded upon reduction
of 2 (Figure 3b), and in which the radical anion is localized
on a single phosphinine ring. Additional signals are not ob-
served between 190 and 290 K. Furthermore, the frozen sol-
ution spectrum (Figure 4a) also corresponds to this species.
In contrast, when compound 1 is reduced on an alkali
mirror in the presence of cryptand, the EPR spectrum corre-
tron from the alkali metal to the phosphinine, a counter-
cation (K+ or Na+) is immediately formed in the vicinity of
the reduced phosphinine ring. Then, if cryptand is absent, a
tight ion pair is formed; this stabilizes the radical anion lo-
calized on a single phosphinine ring, hinders internal rota-
ꢀ
tion, and prevents the formation of a P P bond. This pro-
posed mechanism is consistent with the spectra obtained
when a solution of 1 at room temperature is allowed to
react only briefly, and the spectra are rapidly recorded at
200 K. Under these conditions, at the time when the elec-
tron transfer occurs, several conformations of 1 probably co-
exist in solution. Those with a short P¥¥¥P distance are likely
ꢀ
sponds to a species which contains a one-electron P P bond
[triplet with small hyperfine couplings in the liquid phase
(Figure 5a) and a broad asymmetric spectrum with a split-
ting of ~30 G at 100 K (Figure 5b)]. Above 210 K the spec-
trum progressively becomes more complex and several lines
appear. Reduction of 1 with sodium (or potassium) naphtha-
lenide at 200 K gives rise to the same spectra as those re-
ported when reduction occurs on a potassium mirror in the
presence of cryptand at temperatures below 210 K. Only sig-
nals similar to those observed for 3Cꢀ are observed. A simple
way to rationalize these results is to assume that at low tem-
peratures (<200 K) compound 1 adopts a conformation in
which the two phosphorus atoms are far apart (in accord
with the crystal structure). Therefore, when the electron
transfer occurs under conditions which do not lead to the
formation of a tight ion pair between an alkali cation and a
phosphinine anion (e.g. electron transfer between naphtha-
lenide and the phosphinine moiety, or reduction with an
alkali mirror in the presence of cryptand), an electron
passes from the reducing agent to one of the two phosphi-
nine rings and the system immediately relaxes and under-
ꢀ
to give rise to a P P bond (triplet with a small splitting),
while those with a large P¥¥¥P distance give rise to the local-
ized radical anion (doublet of doublet). Figure 6a clearly ex-
hibits these two contributions in the spectrum recorded in
the liquid phase. When the solution is frozen only the sig-
nals due to the delocalized species are detected, those due
to the localized anion are expected to be considerably small-
er since only the external weak ™parallel∫ transitions could
be observed. Although there are not many alternatives to
the mechanism shown in Scheme 3 which would explain the
spectra observed when alkali mirrors are used as reductants,
in instances when naphthalenide anions are used as reduc-
tants, we cannot exclude that the naphthalene ring favors
ꢀ
the formation of the one-electron P P bond by participating
in the electron-transfer transition state.
It is worth noting that in contrast to the localized radical
anion (e.g. Na+1Cꢀ), which is stable at room temperature, or
to the delocalized anion 3Cꢀ, the delocalized species 1Cꢀ rap-
idly undergoes decomposition above 210 K to give secon-
dary radical species. This probably explains why it is almost
impossible to obtain clear and unambiguous spectra of the
electrochemical reduction of 1. A triplet is indeed formed
immediately after electrolysis is begun at 200 K, but it is
quickly accompanied by several species that have larger
couplings.
ꢀ
goes further modifications. In particular, the C Si bond un-
dergoes internal rotation, the unpaired electron on the two
phosphinine rings is delocalized, and subsequently a one-
ꢀ
electron P P bond is formed (Scheme 3).
The angle between the two calculated 31P-t eigenvectors
k
(~608) indicates a modest overlap between the two p orbi-
tals. Therefore, the phosphorus phosphorus interaction is
rather weak. Indeed, this occurs because of the cyclic con-
straint imposed by the interphosphinine link shown in
[1]2ꢀ¥2Na+. This radical anion is, in fact, an intermediate
ꢀ
which subsequently goes on to form a ™classical∫ P P bond
upon being reduced by a second electron. However, in con-
trast to the reaction that occurs with naphthalenide, when
the reduction is performed by directly transferring an elec-
Conclusion
The EPR spectra obtained for the reduction of a system
that bears two phosphinine
rings clearly show that when
two equivalent reduction cen-
ters are present the reduction
process is affected by the rela-
tive orientation of the two elec-
tron-acceptor sites. Therefore,
some of the factors that govern
the electron-transfer mecha-
nism include: the presence of
cumbersome substituents that
hinder internal rotations and
prevent the molecule from
adopting certain conformations;
temperature, which determines
the equilibrium between vari-
Scheme 3. Proposed pathways for the formation of the bis(phosphinine) monoanion.
ous rotamers; the presence of
4088
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4080 4090

Effect of Conformational Changes on a One-Electron Reduction Process
4080 4090
were obtained by slow recrystallization from a mixture of diethyl ether
and THF(1:1) at ꢀ188C in a tube sealed under vacuum. CCDC-228879
and CCDC-228880 contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge via www.ccdc.ca-
m.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44)1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk).
metal ions which can form ion pairs and stabilize a particu-
lar structure; and the presence of cage molecules or chelat-
ing agents which preclude the formation of ion pairs. All
these factors are likely to be involved in many biological
systems and to participate in the regulation of electron-
transfer processes.
EPR measurements: EPR spectra were recorded on a Bruker 200D-SRC
or a Bruker ESP-300 spectrometer (X-band, 100 kHz field modulation)
equipped with a Bruker ER-4111 VT and an ER-081S variable tempera-
ture controller, respectively. Freshly distilled THF was used as a solvent,
and the solutions were carefully degassed on a vacuum line. Potassium
and sodium mirrors were formed by sublimation of the metal under high
vacuum (10ꢀ6 Torr). Reduction reactions on mirrors were carried out
under vacuum in sealed tubes. Chemical reductions with sodium naphtha-
lenide were performed under an atmosphere of nitrogen.
Experimental Section
All syntheses were carried out under an inert atmosphere using Schlenk
techniques.
Compounds: Bis(3-phenyl-6 6(trimethylsilyl)phosphinine-2-yl)dimethylsi-
lane (1) was synthesized according to a previously reported method.[9]
DFT calculations: Spin-unrestricted calculations were carried out with
the Gaussian 98 and Gaussian 03[19] packages using the B3LYP function-
al.[20] Geometries for the neutral systems (1’, 2) were optimized using the
6-31G* basis set functions. The geometry of 1’Cꢀ was optimized using the
6-31+G* basis set, while the hyperfine couplings and molecular orbitals
were calculated using the 6-31+G* and TZVP[21] functions. The 6-31+G*
and TZVP basis sets were also used for optimization and calculation of
the properties of the radical anion 2Cꢀ.
Calculations for 1’CꢀNa+ were carried out on 1’Na, and it was assumed
that except for the phosphinine ring close to the sodium atom, which was
assumed to adopt the geometry previously optimized[10] (B3LYP//6-
31+G+) for the phosphinine radical anion (C5PH5)ꢀ, 1’ would adopt the
conformation optimized for molecule 1. The sodium atom was placed in
the symmetry plane of the C5PH5 ring, which was kept fixed in a slight
™boat∫ deformation. Calculations were performed for several Na¥¥¥P dis-
tances (d) and for various distances (L) between sodium and the plane
formed by the ortho and meta carbon atoms. The minimum energy was
found to occur when d=2.25 ä and L=2.4 ä. The hyperfine tensors and
SOMO were calculated for this geometry using either the 6-31+G+ or
the TZVP basis set for phosphorus, while for all other atoms the 6-
31+G+ basis set was used.
2,6-Bis(trimethylsilyl-3-phenylphosphinine) (2): A solution of diazaphos-
phinine[8] (0.38 g, 1.8 mmol) and phenyl trimethylsilyl acetylene (280 mL,
1.8 mmol) in toluene (12 mL) was stirred at 658C while the reaction was
followed by 31P NMR spectroscopy. Formation of azaphosphinine (d=
303.9 ppm, s) was complete after 18 h. Trimethylsilyl acetylene (1.3 mL,
9.2 mmol, 5 equiv) was then added and the solution was heated for a fur-
ther 5 h at 908C. The solution was cooled to room temperature, celite
(1 g) was added, and toluene was removed in vacuo. The resultant phos-
phinine was purified by chromatography using hexane as eluent. The first
fraction contained 2,6-bis(trimethylsilylphosphinine), while the desired
product was in the second fraction. Upon removal of the solvent, the
product was isolated as a white powder. Yield: 285 mg (50%); 1H NMR
(CDCl3, 300 MHz): d=0.08 and 0.41 (2s, 18H; Si(CH3)3), 7.25 7.44 (m,
6H; C6H5 and C4H), 8.07 ppm (t, J(H,H)=J(H,P)=8.8 Hz, 1H; C5H);
13C NMR (CDCl3, 75.5 MHz): d=0.2 (d, J(C,P)=5.8 Hz; Si(CH3)3), 2.2
(d, J(C,P)=10.3 Hz; Si(CH3)3), 127.5, 127.9, and 129.1 (3s; C6H5), 129.4
(d, J(C,P)=24.3 Hz; C4H), 138.1 (d, J(C,P)=10.1 Hz; C5H), 146.0 (d,
J(C,P)=9.0 Hz; ipso-C of C6H5), 153.7 (d, J(C,P)=12.5 Hz; C3 in C6H5),
168.0 and 168.3 ppm (2d, J(C,P)=91.7 and 83.8 Hz; C2-TMS); 31P NMR
(CH2Cl2, 121.5 MHz): d=261.6 ppm (s); elemental analysis calcd (%) for
C17H25PSi2 (316.53): C 64.51, H 7.96; found: C 64.70, H 8.21.
The Molekel program[22] was used to represent the molecular orbitals.
Chemical reductions
Reduction of 1: To a solution of bis-phosphinine (25 mg, 0.046 mmol) in
THF(1 mL) at room temperature was added a solution of sodium naph-
thalenide (0.9 mL, 0.046 mmol, 50mm in THF). The color of the reaction
mixture immediately changed to burgundy red. The solvent was removed
in vacuo and the product was isolated as a very moisture sensitive brown
powder. 1H NMR ([D8]THF, 300 MHz): d=ꢀ0.85 (s, 6H; Si(CH3)2), 0.12
(s, 18H; Si(CH3)3), 3.94 (d, J(H,H)=7.7 Hz, 2H; C4H), 6.13 (dt,
J(H,H)=7.5 Hz, J(H,P)=6.4 Hz, 2H; C5H), 7.09 7.16 ppm (m, 10H;
C6H5); 13C NMR ([D8]THF, 75.5 MHz): d=ꢀ0.9 (s, Si(CH3)2), 1.7 (s,
Si(CH3)3), 90.8 (t, J(C,P)=9.5 Hz; C2 in Si(CH3)2), 96.2 (t, J(C,P)=
28.1 Hz; C6 in Si(CH3)3), 98.6 (t, J(C,P)=7.9 Hz; C4H), 126.4 130.6 (m,
C6H5), 147.25 (s, C5H), 152.0 (s, ipso-C of C6H5), 162.6 ppm (s, C3Ph);
31P NMR (THF, 121.5 MHz): d=22.0 ppm (s).
Acknowledgments
This work was supported by the CNRS and the Ecole Polytechnique
(Palaiseau). The authors would like to thank IDRIS (Paris XI Orsay Uni-
versity, France) for the allowance of computer time. Support of this work
by the Swiss National Science Foundation and the Swiss Center for Sci-
entific Computing is gratefully acknowledged.
[1] a) D. Astruc, in Electron Transfer and Radical Processes in Transi-
tion-Metal Chemistry, Wiley-VCH, 1995, New York; b) Z. V. Todres,
Organic Ion Radicals, Marcel Dekker, 2003, New York.
[2] a) A. E. Kaifer, M. Gomez-Kaifer, in Supramolecular Electrochem-
istry, Wiley-VCH, 1999, Weinheim; b) J.-P Sauvage, Molecular Ma-
chines and Motors, Springer, 2001, Berlin.
[3] a) M. Szwarc, Acc. Chem. Res. 1969, 2, 87 96; b) M. Szwarc, Acc.
Chem. Res. 1972, 5, 169 176; c) H. Bock, M. Ansari, N. Nagel, Z.
Havlas, J. Organometallic Chem. 1995, 499, 63 71; d) C. N‰ther,
H. Bock, Z. Havlas, T. Hauck, Organometallics 1998, 17, 4707
4715; e) T. Nishinaga, D. Yamasaki, H. Stahr, A. Wakamiya, K. Ko-
matsu, J. Am. Chem. Soc. 2003, 125, 7324 7335; f) P. B. Hitchkock,
M. F. Lappert, A. V. Protchenko, J. Am. Chem. Soc. 2001, 123, 189
190.
Reduction of 2: To a solution of phosphinine (34.2 mg, 0.11 mmol) in
THF(1 mL) at room temperature was added a solution of sodium naph-
thalenide (1.1 mL, 0.11 mmol, 0.1m in THF). The color of the reaction
mixture immediately changed to dark green. The solvent was removed in
vacuo and the product was isolated as a very moisture sensitive green
powder. NMR spectroscopy revealed that the solution was a mixture of
cis and trans isomers (1:4). 1H NMR ([D8]THF, 300 MHz): d=ꢀ0.21 (s,
4.5H; Si(CH3)3 minor), ꢀ0.15 (s, 18H; Si(CH3)3 major), 0.11 (s, 18H;
Si(CH3)3 major), 0.15 (s, 4.5H; Si(CH3)3 minor), 5.21 (d, J(H,P)=6.5 Hz,
0.5H; C4H minor), 5.25 (d, J(H,P)=6.8 Hz, 2H; C4H major), 7.00 7.30
(m, 12.5H; C6H5), 7.56 (d, J(H,P)=6.4 Hz, 0.5H; C3H minor), 7.65 ppm
(d, J(H,P)=6.3 Hz, 2H; C3H major); 13C NMR ([D8]THF, 75.5 MHz):
d=0.4, 2.0, 3.6, 6.7 (4s, Si(CH3)3), 102.7 (s, C4H), 128.1 (s, ipso-C of
C6H5), 128.8 (s, C3 of C6H5), 133.1 (s, C5H), 146.2 (s, C2 of C6H5), 152.2 (s,
C1 of C6H5), 160.7 ppm (s, C3 of C6H5); 31P NMR (THF, 121.5 MHz): d=
ꢀ82.8 (brs, major product), ꢀ77.12 ppm (brs, minor product).
[4] C. Dutan, S. Choua, T. Berclaz, M. Geoffroy, N. Mÿzailles, A.
Moores, L. Ricard, P. Le Floch, J. Am. Chem. Soc. 2003, 125, 4487
4494.
Crystallization and crystal structures: Single crystals of compound 1 suit-
able for crystallography were obtained by diffusing methanol into a tolu-
ene solution of the compound. Single crystals of compound [1]2ꢀ¥2Na+
[5] L. Cataldo, S. Choua, T. Berclaz, M. Geoffroy, N. Mÿzailles, L.
Ricard, F. Mathey, P. Le Floch, J. Am. Chem. Soc. 2001, 123, 6654
6661.
Chem. Eur. J. 2004, 10, 4080 4090
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4089

M. Geoffroy, P. Le Floch et al.
FULL PAPER
[6] For other odd-electron bonds studies see: Y. Canac, D. Bourissou,
A. Baceiredo, H. Gornitzka, W. W. Scholer, G. Bertrand, Science
1998, 279, 2080 2082; H. Gr¸tzmacher, F. Breher, Angew. Chem.
2002, 114, 4178 4184; Angew. Chem. Int. Ed. 2002, 41, 4006 4011.
[7] J. D. Hoefelmeyer, F. P. GabbaÔ, J. Am. Chem. Soc. 2000 122, 9054
9055.
[8] N. Avarvari, P. Le Floch, F. Mathey, J. Am. Chem. Soc. 1996, 118,
11978 11979.
[9] N. Avarvari, P. Le Floch, L. Ricard, F. Mathey, Organometallics
1997, 16, 4089 4098.
[17] E. D. Glendening, A. E. Reed, J. E. Carpenter, F. Weinhold. Natural
Bond Orbital Analysis, NBO 3.1; Theoretical Chemistry Institute,
University of Wisconsin, Madison, WI,
[18] Although the calculated isotropic coupling with the para protons is
around 4 G, the corresponding splitting is not detected in the spec-
trum. This can arise either because of a slight difference between
the real and calculated geometries (absence of phenyl in the model
system) or because of molecular motion, which alters the line shape.
When couplings are calculated with a triple zeta basis set the 31P iso-
tropic couplings increase, while the 1H isotropic couplings decrease.
[19] Gaussian 03, Revision B.03, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomer-
y, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyen-
gar, J. Tomasi,V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Fores-
man, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Na-
nayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Pittsburgh
PA, 2003.
[10] See Supporting Information.
[11] Additional experiments showed that the intensity ratio (A/B) meas-
ured on the frozen solution spectra was sensitive to additional fac-
tors such as the concentration ratio of sodium naphthalenide/phos-
phinine 2.
[12] At 100 K, only the B lines in Figure 3a were observed after reduc-
tion was carried out with sodium naphthalenide in the absence of
cryptand. Although a frozen solution spectrum could not be clearly
recorded upon reduction of 2 with a sodium mirror, a broad doublet
(40 G) was observed at 200 K. The signals observed upon reduction
with naphthalenide or alkali metal were also detected after in situ
electrolysis of a THFsolution of 2 in the EPR cavity. However, with
this method additional lines were observed that can tentatively be
attributed to protonation of the radical anion by traces of water in
the electrolytic cell.
[13] E. Souliÿ, C. Faure, T. Berclaz, M. Geoffroy, in Automatic Differen-
tiation of Algorithms, Springer, New-York 2002, pp. 99 106.
[14] The drastic diminution of the central part of the frozen solution
spectrum as a result of an overlap of the ™perpendicular∫ transitions
of an axial 31P tensor is reminiscent of previous observation made
for phosphinyl radicals.
[20] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648 5652;b) C. Lee, W.
Yang, R. G. Parr, Phys. Rev. 1988, 54, 785.
[21] N. Godbout, D. R. Salahub, J. Andzelm, E. Wimmer, Can. J. Chem.
1992, 70, 560.
[15] J. Morton and K. Preston, J. Magn. Reson. 1978, 30, 577 582.
ꢀ
[16] We cannot exclude that an intermolecular one-electron P P bond is
[22] Flukiger, P. Development of Molecular Graphics Package MOLE-
KEL, Ph. D. Thesis, University of Geneva, Switzerland, 1992.
not formed in the frozen solution. Such a reaction between a local-
ized radical anion and a neutral phosphinine would decrease the
amount of localized radical anions responsible for signals of type A1
and A2.
Received: January 21, 2004
Published online: July 5, 2004
4090
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4080 4090