ESR/DFT study of bis-iminophosphorane cation radicals

Article abstract of DOI:10.1002/mrc.2079

Bis-iminophosphoranes containing various types of linkers between two R₃P=N moieties were electrochemically oxidized at controlled potential in situ in the electron spin resonance (ESR) cavity. For linkers constituted of phenylenes, conjugated phenylenes or merely a dicyanoethylenic bond, this oxidation led to well-resolved ESR spectra which were characterized by their g values and by their ¹H, ¹⁴N and ³¹P isotropic hyperfine constants. These coupling constants agree with those calculated by DFT for the corresponding cation radicals. Experimental and theoretical results clearly indicate that in these species the unpaired electron is mostly delocalized on the bridge and on the nitrogen atoms while the spin density on the phosphorus atoms is particularly small. Cyclic voltammetry and ESR spectra show that the nature of the bridge between the two iminophosphoranes considerably influences the oxidation potential of the compound as well as the stability of the radical cation. Information about the conformation of the precursor containing two Ph₃P=N moieties separated by a -C(CN)=C(CN)-group was obtained from its crystal structure. Copyright

Full text of DOI:10.1002/mrc.2079

MAGNETIC RESONANCE IN CHEMISTRY
Magn. Reson. Chem. 2007; 45: 1011–1017
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/mrc.2079
ESR/DFT study of bis-iminophosphorane cation
radicals
1
2
1
2
´
Adil Matni, Leila Boubekeur, Philippe Grosshans, Nicolas Mezailles,
Gerald Bernardinelli, Pascal Le Floch and Michel Geoffroy1∗
3
2∗
´
1
Department of Physical Chemistry, University of Geneva, 30 quai Ernest Ansermet, 1211 Geneva 4, Switzerland
Laboratoire ‘‘He´ te´ roe´ le´ ments et Coordination’’, Ecole Polytechnique, CNRS, 91128 Palaiseau Ce´ dex, France
Laboratory of Crystallography, 24 Quai Ernest Ansermet, University of Geneva, 1211 Geneva 4, Switzerland
2
3
Received 11 June 2007; Revised 3 August 2007; Accepted 10 August 2007
Bis-iminophosphoranes containing various types of linkers between two R3P N moieties were
electrochemically oxidized at controlled potential in situ in the electron spin resonance (ESR) cavity.
For linkers constituted of phenylenes, conjugated phenylenes or merely a dicyanoethylenic bond, this
1
oxidation led to well-resolved ESR spectra which were characterized by their g values and by their H,
14N and 31P isotropic hyperfine constants. These coupling constants agree with those calculated by DFT
for the corresponding cation radicals. Experimental and theoretical results clearly indicate that in these
species the unpaired electron is mostly delocalized on the bridge and on the nitrogen atoms while the spin
density on the phosphorus atoms is particularly small. Cyclic voltammetry and ESR spectra show that the
nature of the bridge between the two iminophosphoranes considerably influences the oxidation potential
of the compound as well as the stability of the radical cation. Information about the conformation of the
precursor containing two Ph3P N moieties separated by a –C(CN) C(CN)–group was obtained from its
crystal structure. Copyright  2007 John Wiley & Sons, Ltd.
Supplementary electronic material for this paper is available in Wiley InterScience at http://www.interscience.wiley.com/
jpages/0749-1581/suppmat/
KEYWORDS: ESR; DFT; 1H; 14N; 31P; hyperfine structure; spin delocalization; bis-iminophosphoranes; cation radical
investigations are carried out to produce new cation radicals,
involving nitrogen or phosphorus atoms, and likely to
be incorporated in more complex molecular systems.8–10
While early studies had shown that one-electron oxidation
of simple iminophosphoranes is an irreversible process,11
it was recently shown by cyclic voltammetry and optical
studies that a system constituted by two R3P N groups
separated by one or two phenylene rings undergoes a
reversible one-electron oxidation between 0.04 and 0.4 V.8,9
We have recently reported electron spin resonance (ESR)
spectra and density functional theory (DFT) calculations
which are consistent with the formation of a cation radical
widely delocalized on a N–Phen–N bridge.12 In this context,
an important question is, of course, to know if this type
of cation radical can also be formed by using other linkers
between the two iminophosphorane groups. The purpose of
the present study is to assess the influence of this bridge
in the formation, the stability and the spin delocalization
of bis-iminophosphorane cation radicals. We have therefore
studied the ESR response to one-electron oxidation of the
bis-iminophosphoranes shown in Scheme 1. In a first class
of compounds, the bridge is constituted by two adjacent
phenylene rings (A1, A2) or by two phenylene rings
separated either by an ethylenic bond (B) or by an sp3
hybridized carbon (C). In a second class of compounds
(D and E), the two iminophosphoranes are merely linked
INTRODUCTION
Organic ion radicals play a crucial role in various domains
of modern chemistry.1 As reaction intermediates, their
structure and reactivity often govern the mechanism of
chemical transformations. This property is important not
only to improve the yield of some organic syntheses2
but also to understand the biological activity of many
compounds (e.g. carcinogenic properties of benz[a]pyrene,3
role of flavins for repairing damage in DNA4). More recently,
electrical and magnetic properties of ion radicals have
led to new developments in materials science. This is
the case, for example, of cation radicals of TTF systems,
well-known for their performances as superconductors.5
With the development of organic light emitting diodes
(OLED),6 considerable efforts have been made to devise
new molecules and molecular edifices able to generate
ion radicals in the charge transporting layers of these
devices. While most of the cation radicals currently used
as hole transporters are derived from triarylamines,7 many
ŁCorrespondence to: Michel Geoffroy, Department of physical
Chemistry, 30 quai Ernest Ansermet, 1211 Geneva 4, Switzerland.
E-mail: michel.geoffroy@chiphy.unige.ch
Pascal Le Floch, Laboratoire ‘‘He´te´roe´le´ments et Coordination’’,
Ecole Polytechnique, 91128 Palaiseau Ce´dex, France.
E-mail: lefloch@poly.polytechnique.fr
Copyright  2007 John Wiley & Sons, Ltd.

1012
A. Matni et al.
Ph
Ph
Ph
Ph
Me
6
2
5′ 3′
4′ 2′
2
4
5
5′ 3′
P
6′
P
1′
1
6′
1
1′
Ph
N
Ph
N
N
Ph
Ph
N
Ph
Ph
6
P
P
3
5
3
4′
2′
Me
Ph
Ph
A1
A2
Ph
Ph
H
3′
2′
5′
Ph
Ph
P
1′
6′
2
4
5′ 3′
N
C
H2
C
Ph
1′
2
4
6
6′
P
1
7′
N
Ph
N
1
6
4′
Ph
Ph
Ph
C
N
P
7
3
5
4′
2′
Ph
Ph
H
3
5
P
Ph
B
C
1
C
1
N
1′
C
1′
1′
1
1
C
1′
N
PPh3
Ph3P
C
N
N
PMe3
Me3P
CN
CN
CN
CN
D
E
Scheme 1. Investigated bis-iminophosphoranes.
Table 1. Electrochemical parameters obtained for solutions of
bis-iminophosphoranes
by a carbon–carbon double bond; ESR is expected to show
if an ethylenic bond is sufficient to stabilize the structure of
a bis-iminophosphorane cation radical. As far as we know,
compounds A2, B, C and E were not known prior to this
work and this is the first report of the crystal structure of a
bis-iminophosphorane with a C C bridge.
In these investigations, most of the experimental informa-
tion is provided by the hyperfine interaction; the measured
coupling constants are interpreted in terms of spin delo-
calization by using DFT calculations. Correlations with the
oxidation potentials are also presented.
Oxidation potentials E1/2 (V/SCE)
Compound
EI1/2
EII
1/2
A1a,b
A2b
Bb
0.366
0.719
0.387
0.940
0.397
0.296
0.543
–
–
1.17
0.842
0.454
Cb
Db
Ec
a Values already reported in literature: E1/2 D 0.36, E1/2
D
RESULTS
0.54 V9.
b Solution in CH2Cl2.
c Solution in dimethyl formamide (DMF).
Cyclic voltammetry
As shown in Table 1, bis-iminophosphoranes A1, B, D
and E undergo a first one-electron oxidation at a rather
low potential (0.3 < E1/2 < 0.4 V SCE). The oxidations of
A2 and C occur at higher potential (E D 0.719 V for A2
and E D 0.94 V for C). For A1, A2, B and D, the peak
potential separation (E much greater than 58 mV), and the
peak current ratio (ipc/ipa different from 1 and dependent
upon the scan rate), indicate that, at the surface of the
electrode, the concentrations of the neutral species and of the
corresponding radical cation are not maintained at the values
required by the Nernst equation (quasi-reversibility). For E,
the reverse peak is hardly detected; for C, the electrochemical
process is irreversible. A second oxidation wave (quasi
reversible) is observed for A1, C, D and E.
quite stable; their spectra are still observed for a rather long
time after the voltage has been switched off (at 220 K: 60 min
for A1, 10 min for A2; at 300 K: 20 min for B and D, but only
30 s for E). Chemical oxidation of these solutions with one
equivalent of [Cp2Fe] PF6 gives rise to the same ESR spectra
as those obtained by electrolysis. The intensities in these
spectra remain constant as long as the oxidized solution is
kept under nitrogen atmosphere.
For each bis-iminophosphorane, the formation of the
radical is accompanied by a change in the colour of the
solution: For A1, B and D, the yellow solution turns to red;
for A2, the yellow solution becomes green; for E, the brown
solution turns to red. No ESR signal could be obtained by
electrochemical or chemical oxidation of C.
ESR spectra
Examples of ESR spectra obtained with B and D are
shown in Fig. 1. Simulations of the experimental spectra lead
to the g values and isotropic hyperfine coupling constants
reported in Table 2.
The small variations in the g values correlate with the
differences in the radical structures: g slightly increases when
Oxidations of solutions of A1, A2, B, D and E, in situ in the
ESR cavity, at a voltage slightly higher than the potential
of the first oxidation wave detected by cyclic voltammetry
lead to ESR spectra exhibiting a rich hyperfine structure and
centred around g D 2.006. Most of these radical species are
Copyright  2007 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2007; 45: 1011–1017
DOI: 10.1002/mrc

ESR of bis-iminophosphorane cation radicals
1013
Figure 1. ESR spectrum obtained by oxidation with [Cp2Fe] PF6 of a solution of B (a) and D (b) in CH2Cl2.
Table 2. ESR parameters measured after oxidation of bis-iminophosphoranes and DFT-predicted coupling constants
14N–Aiso (Gauss)a
DFT
31P–Aiso (Gauss)a
DFT
1H–Aiso (Gauss)a
Compound
g
Expb
Expb
Expb
DFT
A1
2.0060
2.5 (2ð)
2.8 (2ð)
5.7 (2ð)
ꢀ5.9ꢀ2ðꢁ
0.8 (4ð)
0.7 (2ð)
ꢀ1.5ꢀ2, 20, 3, 30ꢁ
ꢀ1.0 ꢀ5, 50ꢁ
ꢀ0.7 ꢀ4, 40ꢁ
ꢀ1.6 ꢀ3, 30ꢁ
ꢀ1.5 ꢀ2, 20ꢁ
ꢀ0.6 ꢀ5, 50ꢁ
0.11 (av. Me)
ꢀ2.6 ꢀ7, 70ꢁ
ꢀ1.2 ꢀ5, 50ꢁ
ꢀ1.0 ꢀ3, 30ꢁ
ꢀ0.9 ꢀ2, 20ꢁ
ꢀ0.8 ꢀ4, 40ꢁ
A2
B
2.0062
2.0059
2.4 (2ð)
2.2 (2ð)
2.9 (2ð)
5.8 (2ð)
6.2 (2ð)
ꢀ6.3 ꢀ2ðꢁ
1.0 (4ð)
0.5 (2ð)
2.1 (2ð)
ꢀ4.1ꢀ2ðꢁ
1.0 (2ð)
0.9 (2ð)
0.8 (2ð)
0.2 (2ð)
0.1 (2ð)
D
E
D1.C
D2.C
4.7, 4.2
1.2, 0.8
E2.C
4.5, 4.0
1.3, 0.7
D1.C
D2.C
2.0064
2.0065
4.6 (2ð)
PN:4.3 (2ð)
CN:1.0 (2ð)
E1.C
10.1(2ð)
10.0(2ð)
ꢀ7.8 ꢀ2ðꢁ
ꢀ8.9, ꢀ6.9
0.56 (2ð)
E1.C
E2.C
5.2 (2ð)
0.6 (2ð)
PN:4.4 (2ð)
ꢀ9.2ꢀ2ðꢁ
ꢀ9.0, ꢀ8.2
CN: 0.9 (2ð)
a In parentheses: number of equivalent nuclei, and, for the DFT results, numbering of the carbon atoms bound to the protons (see
Scheme 1).
b Only the absolute value of the coupling is measured.
passing from cation radicals delocalized on several aromatic
rings (A1, B) to systems containing only an ethylenic bond
between the two phosphorane moieties. (D, E).
this structure, given in Fig. 2, illustrates the type of rotamer
that crystallized (hereafter D2).
DFT calculations
Crystal structure of D
The structures of the bis-iminophosphoranes A1, A2, B and of
the corresponding monocation radicals have been optimized
by DFT; some of the resulting geometrical parameters are
reported in Table 3. (Structure optimizations together with
calculation of properties have also been performed for other
rotamers. The results are given as supplementary electronic
material).
Although the non-symmetric structure D2 is observed
in the solid state (see crystal structure), the symmetric
conformation D1 (see Scheme 2) is likely to exist in solution.
Analogous conformations E1 (symmetrical) and E2 (non-
symmetrical) were also taken into account for the methyl
Owing to rotation around the C1–N1 and C10 –N10
bonds, bis-iminophosphoranes can adopt several conforma-
tions. Recently, Lucht et al. have shown by X-ray diffrac-
tion that the structure of the compound with a single
phenylene as a linker – the NN0, p-phenylene bis (triph-
enyl)iminophosphorane – possess an inversion centre.9 We
have assumed the same type of rotamers for A1, A2 and B
(see Scheme 1). For compound D, we could obtain suitable
crystals for X-ray diffraction; the resulting structure clearly
indicates that the two P N–C C dihedral angles are differ-
°
°
ent (ꢀ174.5ꢀ2ꢁ and 38.9(3) ). The ORTEP representation of
Copyright  2007 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2007; 45: 1011–1017
DOI: 10.1002/mrc

1014
A. Matni et al.
for the neutral and cationic species. The hyperfine constants
calculated for these isomers are very similar to those given in
Table 2; the resulting values are reported as supplementary
electronic material.
Although no ESR spectrum could be observed by
oxidation of C, the coupling constants of C.C have been
calculated by DFT. They are not drastically different from
the other constants calculated for the other cation radicals:
31P: ꢀ6.7 and ꢀ6.2 G, 14N: 3.1 and 3.0 G.
DISCUSSION
Consistent with the formation of a cation radical, the first
oxidation wave of all bis-iminophosphoranes mentioned
in Table 1, except compound C, gives rise to a clear ESR
response. The resulting hyperfine coupling constants being
in good accordance with those calculated by DFT, they can
readily be discussed in terms of electronic structures.
As shown in Tables 3 and 4, removing an electron
from bis-iminophosphoranes leads to an increase of the
N–P bond lengths and to a decrease of the PN–C bond
lengths. Additional changes are noteworthy: diminution of
the interphenylene bond distances C6–C60 in A1 and A2
and elongation of the central double bond (C7–C70 in B,
C1–C10 in D1,2 and E1,2). Moreover, as indicated by the
various dihedral angles reported in Tables 3 and 4, the bis-
iminophosphoranes become more planar when oxidized.
Figure 2. ORTEP representation of the crystal structure of D2.
containing compound E. The structures of the two rotamers
D1 and D2 as well as those of E1 and E2 were optimized by
DFT. Some of their geometrical parameters are reported in
Table 4.
The 31P, 14N and 1H isotropic coupling constants
predicted by DFT are reported in Table 2. The small negative
values obtained for 31P suggest that these couplings are due
to spin polarization through the ꢂ N–P bond. The singly
occupied molecular orbitals (SOMOs) of B.C and D2.C are
shown in Fig. 3.
As expected, in contrast with the symmetrical rotamers
D1.C and E1.C, the two nitrogen nuclei and the two
phosphorus nuclei of D2.C and E2.C are not magnetically
equivalent. Nevertheless, the two 14N and the two 31P
coupling constants calculated for each rotamer D2.C and
E2.C are only very slightly different. Moreover, the average
values are close to the couplings calculated for D1.C and E1.C
respectively. Although the 31P couplings are slightly less than
the experimental values (31P–Aiso[experimental] D 10 G), the
agreement between calculated and experimental coupling
constants for D and E is quite reasonable.
Ph
Ph
Ph
P
N
Ph
Ph
Ph
Ph
Ph
N
P
N
Ph
Ph
P
N
P
N
C
C
C
C
Ph
Ph
C
C
C
C
N
N
N
D1
D2
Additional rotamers exist for A1, A2, B (rotation around
a C–N bond) and for D (rotation around the central C
bond). The corresponding structures have been optimized
C
Scheme 2. Symmetric (D1) and non-symmetric (D2) conforma-
tions for D.
Table 3. Optimized geometries for neutral and cationic bis-iminophosphoranes with a phenylenes-containing bridgea
A1 A2
B
Neutral
Cation
Neutral
Cation
Neutral
Cation
N—P
N–C1
C1–C2
C2–C4
C4–C6
C6–C60
C6–C7
C7–C70
PNC1C3
C4C6C60C50b
C6C7C70C60
1.599
1.388
1.424
1.397
1.419
1.484
1.636
1.354
1.440
1.386
1.433
1.458
1.598
1.390
1.423
1.405
1.426
1.499
1.633
1.356
1.437
1.394
1.445
1.469
1.601
1.385
1.428
1.393
1.423
1.636
1.354
1.444
1.381
1.435
1.461
1.368
ꢀ8.63
0.99
1.435
1.390
ꢀ4.66
ꢀ0.22
179.8
9.02
ꢀ31.7
2.68
ꢀ12.49
ꢀ8.69
ꢀ78.8
1.42
ꢀ46.0
ꢀ179.7
a
˚
Numbering of the atoms given in Scheme 1, bond lengths in A, angles in degree.
b C4C6C7C70 for B.
Copyright  2007 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2007; 45: 1011–1017
DOI: 10.1002/mrc

ESR of bis-iminophosphorane cation radicals
1015
Table 4. Optimized geometries for neutral and cationic bis-iminophosphoranes with an ethylenic bond as a bridgea
D1
E1
D2
E2
Neutral
1.609
Cation
1.666
Neutral
1.592
Cation
1.651
Neutral
Crystal Structureb
Cation
Neutral
Cation
N–P
1.621
1.596
1.393
1.368
1.406
ꢀ169.4
1.2
1.593(2)
1.575(2)
1.383(3)
1.374(3)
1.380(3)
ꢀ174.5(2)
ꢀ1.1(4)
38.9(3)
1.677
1.644
1.344
1.326
1.456
179.6
8.0
1.602
1.587
1.400
1.365
1.385
180.0
0.0
1.661
1.633
1.343
1.315
1.442
180.0
0.0
N–C1
1.376
1.333
1.378
1.328
C1–C10
1.409
ꢀ171.9
ꢀ2.95
1.458
175.3
ꢀ4.3
1.385
179.9
0.0
ꢀ179.9
1.439
180.0
0.0
PNC1C10
NC1C10N
P0N0C10C1
ꢀ162.5
175.9
180.0
19.1
19.4
0.0
0.0
a Numbering of the atoms given in Scheme 1.
b Parameters obtained from the crystal structure of the neutral compound.
˚
lost its double-bond character elongates to 0.06 A. Similar
properties are observed for the other bis-iminophosphoranes
of Table 2.(Calculated charge distribution and spin densities
are reported as supplementary electronic material). As
exemplified by E1.C, simple mesomeric structures, written
by neglecting partial charges, correctly describe the spin
delocalization (Scheme 3):
It is worthwhile mentioning that the coupling constants
for A1.C and A2.C are quite similar while departure from
planarity is much more pronounced for the latter cation than
for the former one. This difference in conformation, due to
the presence of the Me groups in A2 (see the values of the
C4C6C60C50 angles for A1, A1.C, A2 and A2.C in Table 3) is
however clearly reflected by the electrochemical behaviour of
the two bis-iminophosphoranes: the first oxidation potential
of A2 is equal to 0.719 V, 0.35 V above the oxidation potential
of A1. Moreover, as shown by the ESR spectra, A1.C is more
stable than A2.C. The nature of the bridge determines the
formation and the stabilization of the cation radical; when
delocalization between the two iminophosphorane groups is
not possible the oxidation potential considerably increases
and the lifetime of the cation radical becomes very short.
An extreme case corresponds to C where a sp3 carbon
hinders conjugation between the two phenylene rings of
the bridge. For this system, oxidation is irreversible and
occurs 0.57 V above that of A1. Moreover, no ESR spectrum
could be detected for the oxidation product of this bis-
iminophosphorane.
Figure 3. SOMOs for B.C (top) and D2.C (bottom).
Such changes clearly reflect the properties of the SOMOs
(see, e.g. Fig. 3).
For all cation radicals, the nitrogen 2pꢃ forms an
antibonding combination with the 2pꢃ atomic orbital of the
adjacent carbon. Clearly, the ꢃ character of the SOMO leads
to a wide delocalization of the unpaired electron : the spin
density on each iminophosphorane nitrogen progressively
decreases from 27 to 17% and 13% when passing from D.C (or
E.C), to A.C and to B.C; for A.C and B.C the spin delocalization
on each phenylene ring lies in the range 25–30% and that on
the central double bond is between 18% (for B.C) and 38% (for
D.C or E.C). It is well-known that the P–N bond of a neutral
monoiminophosphorane is strongly PC —Nꢀ polarized, this
property is still observed for bis-iminophosphoranes. For
example, in E1 the Mulliken charge analysis leads to a
charge of C0.59 on each Me3P, ꢀ0.58 on each nitrogen
atom, C0.50 on the C C bond and ꢀ0.26 on each CN
group; the fact that in E1.C this charge distribution changes
into C0.90, ꢀ0.54, 0.58 and ꢀ0.15 shows that removing an
electron drastically modifies only the charge of the R3P
group. Spin density on the resulting phosphonium groups
is practically negligible, and the P–N bond which has
N
N
R3P
PR3
N
P R3
N
R3P
C
C
C
N
C
C
C
C
C
N
N
N
N
P R3
N
C
R3P
R3P
N
N
PR3
C
C
C
C
C
C
C
N
N
N
N
Scheme 3. Mesomeric structures for E1.C
.
Copyright  2007 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2007; 45: 1011–1017
DOI: 10.1002/mrc

1016
A. Matni et al.
415 variables and 3977 contributing reflections. CCDC-
647 368 contains the supplementary crystallographic data
for D2. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (C 44) 1223-336-033; or
deposit@ccdc.cam.ac.uk).
CONCLUSION
In contrast to monoiminophosphoranes, bis-iminophos-
phoranes can undergo a one-electron oxidation to form per-
sistent cation radicals. This property is, however, dependent
upon the nature of the bridge between the two R3P N moi-
eties. Planar ꢃ systems facilitate this oxidation and stabilize
the resultant radical. A simple ethylenic bond is sufficient
to lead to a persistent small cation radical while conjugated
phenylenes can be used to facilitate spin delocalization on
long distances. In these paramagnetic species, the phospho-
rus atoms bear the positive charge and the spin densities
on these atoms are almost negligible. The unpaired electron
is delocalized on the two pꢃ nitrogen orbitals and on the ꢃ
system of the linker. These results confirm that incorporation
of phosphorus atoms in conjugated systems can lead to new
organic materials with interesting and unique properties.13
Syntheses
All solvents were dried on aluminium oxide. Et3N was
freshly distilled on KOH. All reactions were performed
under nitrogen atmosphere. Bis-iminophosphoranes were
synthesized by using one of the two following procedures:
procedure a described in Ref. 16, or procedure b described
in Ref. 17. 4,40-Diamino-3,30-dimethyl-1,10-binaphthyl was
purchased from Riedel de Hae¨n, 2,20-Dimethyl-4,40-diamino-
biphenyl from Chrikev Compagny Inc., 4,40-methylenebis-
benzenamine and diaminomaleonitrile from Fluka, 4,40-
diaminostilbene dihydrochloride from Aldrich and all were
used without further purification. Mass spectroscopy mea-
surements were carried out on the Applied Biosystems API
150EX Turboionspray. NMR spectra were recorded on a
EXPERIMENTAL
Cyclic voltammetry and ESR
Cyclic voltammograms were recorded at room temperature
with a BAS station, model CV-50W, using a SCE reference
electrode and 1 mm diameter platinum wire as working
electrode and counter electrode. The solvents (CH2Cl2
or dimethyl formamide, DMF) were carefully dried and
degassed, tetra-n-butyl ammonium hexafluorophosphate
(0.1 M) was used as supporting electrolyte. The following
scan rates (V sꢀ1) were used: 1.0 for A2, 1.2 for C, 1.5 for A1
and D, 2.2 for B, 4.0 for E.
1
Bruker DRX400 spectrometer operating at 400 MHz for H,
100.61 MHz for 13C and 161.9 MHz for 31P or on a Bruker
AC 300 Advance operating at 200.13 MHz for 1H, 50.32 MHz
for 13C and 81.01 MHz for 31P. Solvent peaks are used as
internal references relative to Me4Si for 1H and 13C chemical
shifts (ppm); 31P chemical shifts are relative to a 85% H3PO4
external reference.
The synthesis of A1 has already been published.9,16
ESR spectra were recorded on Bruker 200 and Bruker
300 X-band spectrometers. Radical cations were generated
in situ in the ESR cavity using a three-electrode quartz
cell and a EGG PAR 362 potentiostat. This home-made
electrolytic cell used Ag/AgCl as a pseudo electrode, and
platinum wires as working electrode and counter electrode.
All the solutions were degassed by several freeze-pump-
thaw cycles. Measurements were performed under nitrogen
atmosphere. CH2Cl2 solutions, used for chemical oxidation
with [Cp2Fe] PF6, were prepared in a glove box for B, D
and E oxidation, and ESR measurements were performed at
room temperature; for A1 and A2, oxidation was performed
at 220 K and the spectra were recorded at 220 and 250 K
respectively.
Synthesis of A2: 2,20-Dimethyl-4,40-di iminophosphorane
biphenyl (A2) was synthesized using procedure a with 2,20-
dimethyl-4,40-diamino-biphenyl as a precursor (Yield 68%. m
1
°
p 95–100 C). NMR H (CDCl3, ppm): 1.8 (s, 6H); 6.5 (d, 2H,
J D 10.65 Hz); 6.73 (d, 2H, J D 10.65 Hz); 7.44–7.55 (m, 9H);
7.78–7.85 (m, 6H). NMR 13C (CDCl3, ppm): 20.2; 199.9; 128.7;
130.3; 131.7; 131.9; 132.4; 132.8; 132.9; 136.7; 149.1. NMR 31P
(CDCl3, ppm): 1.8. MS m/z: 733.5.
Synthesis of B: The synthesis of 4,40-di iminophosphorane
stilbene (B) was carried out following procedure a with 4,40-
diaminostilbene dihydrochloride as a precursor (Yield: 72%.
1
°
mp 230–235 C). NMR H (CDCl3, ppm) : 6.74 (m, 6H); 7.11
(m, 4H); 7.50 (m, 20H); 7.75 (m, 12H). NMR 13C (CDCl3,
ppm) : 123.5; 123.7; 125.1; 126.8; 128.8; 128.9; 131.9; 132.8;
132.9. NMR 31P (CDCl3, ppm) : 2.2. MS m/z: 731.8.
Synthesis of C: 4,40-Methylenebis-benzene-di(triphenyl)
iminophosphorane (C) was obtained by following proce-
dure a and by using 4,40 methylenebis-benzenamine as a
X-ray diffraction
Crystallographic data for D2: C40H30N4P2; Mr D 628.7,
Monoclinic, P21/n, a D 9.3810ꢀ4ꢁ, b D 15.0736ꢀ7ꢁ, c D
3
precursor (Yield 55%. mp 200–210 C). NMR 1H (CDCl3,
˚
˚
°
°
22.6029ꢀ12ꢁ A, ˇ D 91.834ꢀ6ꢁ , V D 3194.5ꢀ3ꢁ A ; Z D 4,
ꢄ D 0.172 mmꢀ1, dx D 1.307 g cmꢀ3, Mo K˛ radiation
ppm): 3.65 (2H, s), 6.46 (4H, dd, J D 6.06 and 1.14 Hz), 6.63
(4H, dd, J D 6.06 and 1.14 Hz), 7.45–7.60 (18H, m), 7.75–7.80
(12H, m). NMR 13C (CDCl3, ppm): 39.8; 125.6; 128.8; 128.9;
130.8; 131.8; 131.9; 132.0; 132.9. NMR 31P (CDCl3, ppm): 1.7.
MS: 719.5.
˚
(ꢅ D 0.71073 A); 35 622 reflections measured at 200 K on a
STOE IPDS diffractometer, 6780 unique reflections of which
3628 were observables (jFoj > 4ꢂ (Fo)). Data were corrected
for Lorentz and polarization effects and for absorption (Tmin
,
Tmax D 0.9623, 0.9837). The structure was solved by direct
methods (SIR97).14 All calculations were performed with
the XTAL system.15 Full-matrix least-squares refinement
based on F using weights of 1/ꢀꢂ2ꢀFoꢁ C 0.00015ꢀFo2ꢁꢁ
gave final values R D ωR D 0.033, and S D 1.17ꢀ1ꢁ for
Synthesis of D: Although the synthesis of cis-di(triphenyl-
phosphorane)maleonitrile (D) was reported in literature,18
we obtained this compound by following procedure b and
reacting cis-diamino maleonitrile with triphenylphosphine in
presence of diethyldiazodicarboxylate (DEAD) (Yield 88%.
Copyright  2007 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2007; 45: 1011–1017
DOI: 10.1002/mrc

ESR of bis-iminophosphorane cation radicals
1017
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Hall HK, Hpokins T, Mash EA, Wang J, Padias A, Thayumana-
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°
mp: 250–260 C). NMR H (CDCl3, ppm): 7.44–7.73 (m, 15H).
NMR 13C (CDCl3, ppm): 16.7, 17.4, 114.7, 114.9, 121.0, 121.2.
NMR 31P (CDCl3, ppm): 9.7 ppm. MS m/z: 733.
Synthesis of E: Cis-di(trimethylphosphorane)maleonitrile
(E) was obtained from procedure b reacting cis-diamino-
maleonitrile with trimethylphosphine in presence of DEAD.
(Yield 53%. mp: decomposition at 230–240 C). NMR 1H
°
(CDCl3, ppm): 1.7 (d, 18H, J D 12.64 Hz). NMR 13C (CDCl3,
ppm): 16.7, 17.4, 114.7, 114.9, 121.0, 121.2. NMR 31P (120 MHz,
CDCl3): 20.2. MS: 256.
13. (a) Smith RC, Chen X, Protasiewicz JD. Inorg. Chem. 2003; 42:
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DFT calculations
Optimizationsofthe structuresofthebis-iminophosphoranes
and of the corresponding radical cations were performed
with the TURBOMOLE package19,20 using the B-P86 exchange-
correlation functional21 and the SVP basis set.22 A frequency
analysis was carried out for each optimized structure (all
frequencies real). Molecular properties were calculated with
the Gaussian 03 set of programs23 by running single point
calculations at the optimized geometries. These calculations
were carried out using the B3LYP functional24 together with
the standard 6–31GŁ basis set.
Supplementary material
Supplementary electronic material for this paper is available
in Wiley InterScience at: http://www.interscience.wiley.
com/jpages/0749-1581/suppmat/
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20. Electronic Structure Calculations on Workstation Computers :
the Program System TURBOMOLE. V5.8. Lehrstuhl fu¨r Theoretische
Chemie, Universita¨t Karlsruhe D-76128 (Germany) 2005.
21. Perdew JP. Phys. Rev. B 1986; 33: 8822.
Acknowledgements
Support of this work by the Swiss National Science Foundation
(grant: 200020-113544/1) is gratefully acknowledged. The authors in
Palaiseau thank the Ecole Polytechnique and the CNRS for financial
support of this work, and the IDRIS (project 71616).
22. Scha¨fer A, Horn H, Ahlrichs R. J. Chem. Phys. 1992; 97: 2571.
23. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb A,
Cheeseman JR, Montgomery Jr JA, Vreven T, Kudin KN,
Burant JC, Millam JM, Iyengar SS, Tomasi J, Barone V, Men-
nucci B, Cossi M, Scalmani G, Rega N, Petersson GA, Nakat-
suji H, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J,
Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Klene M,
Li X, Knox JE, Hratchian HP, Cross JB, Adamo C, Jaramillo J,
Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R,
Pomelli C, Ochterski JW, Ayala PY, Morokuma K, Voth GA, Sal-
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Komaromi I, Martin RL, Fox DJ, Keith T, Al-Laham MA,
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Copyright  2007 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2007; 45: 1011–1017
DOI: 10.1002/mrc