Chromium tricarbonyl complex of phosphaalkene: Crystal structure and electrochemistry of the Cr(CO)3 complex of PhC(H)PMes*, EPR and DFT studies of its radical anion

Article abstract of DOI:10.1016/j.jorganchem.2005.08.045

We report the synthesis, crystal structure and electrochemical behaviour of a complex in which the Ph group of the phosphaalkene PhC(H)PMes* (Mes*: 2,4,6-tri-tert-butylphenyl) is coordinated to a chromium tricarbonyl group. The EPR spectra resulting from electrochemical and chemical reductions are described and the experimental g and hyperfine tensors ( ³¹P)T, as determined from the EPR data, are compared with those predicted by DFT calculations for the radical anion (Cr(CO)₃, PhC(H)PMes)^-. The structural changes caused by the addition of an electron to the neutral complex are described, together with an estimation of the contribution of Cr(CO)₃ to the stabilization of the radical anion.

Full text of DOI:10.1016/j.jorganchem.2005.08.045

Journal of Organometallic Chemistry 691 (2006) 72–78
www.elsevier.com/locate/jorganchem
Chromium tricarbonyl complex of phosphaalkene: Crystal structure
and electrochemistry of the Cr(CO)3 complex of PhC(H)@PMes*,
EPR and DFT studies of its radical anion
a
b,
a
a,
*
*
´
Cyril Gouverd , Marcin Brynda , Theo Berclaz , Michel Geoffroy
a
Department of Physical Chemistry, University of Geneva 30, Quai Ernest Ansermet, 1211 Geneva, Switzerland
b
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
Received 25 July 2005; received in revised form 30 August 2005; accepted 30 August 2005
Available online 11 October 2005
Abstract
We report the synthesis, crystal structure and electrochemical behaviour of a complex in which the Ph group of the phosphaalkene
PhC(H)@PMes* (Mes*: 2,4,6-tri-tert-butylphenyl) is coordinated to a chromium tricarbonyl group. The EPR spectra resulting from elec-
trochemical and chemical reductions are described and the experimental g and hyperfine tensors (31P)T, as determined from the EPR
data, are compared with those predicted by DFT calculations for the radical anion (Cr(CO)3, PhC(H)@PMes)Åꢀ. The structural changes
caused by the addition of an electron to the neutral complex are described, together with an estimation of the contribution of Cr(CO)3 to
the stabilization of the radical anion.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Phosphaalkene; Radical anion; Cr(CO)3 complexes; EPR; DFT; Electrochemistry
1. Introduction
metal in the vicinity of the phosphaalkene moiety. To this
purpose, the properties of tricarbonylchromium group
seem, a priori, well suited. Indeed, Cr(CO)3, referred to as
a ‘‘hermaphrodite’’ group, has been shown to produce sta-
bilization of both benzylic anions and cations [7–9]. In the
present study, we report the synthesis and crystal structure
of a phosphaalkene complex, in which the phenyl ring
bound to the phosphaalkenic carbon is coordinated to
Cr(CO)3. The electron-acceptor properties of this system
(1) are studied by cyclic voltammetry and EPR spectros-
copy. Experimental hyperfine couplings and DFT calcula-
tions are used to assess the structural modifications that
accompany the one-electron reduction of 1.
Phosphaalkenes (e.g. 10) are well known for their ability
to accept an extra electron in their p* orbital [1–4]; some sys-
tems containing a P@C bond have already been investigated
for their potential applications in molecular electronics [5].
However, the reduction potentials of phosphaalkenes are
rather low (Eo = ꢀ1980 mV/SCE for 10) and the stability
of their radical anion remains modest. Therefore, it is desir-
able to modify their electronic structure to facilitate the
reduction process and to increase the stability of the result-
ing radical anion. One approach is to directly coordinate the
phosphorus atoms of two phosphaalkene moieties to a tran-
sition metal in a chelating structure. This method, which
was successfully used some years ago, requires the rather
delicate synthesis of pincer type ligands [6]. Another easier
approach is to take advantage of the presence of a transition
2. Experimental
2.1. Analytical instrumentation
1H, 13C and 31P NMR spectra were recorded on a Bru-
ker 400 or 500-MHz, and Varian-XL 200 spectrometers.
High-resolution mass spectra (HR-MS) were measured
*
Corresponding authors. Tel.: +41 22 379 65 52; fax: +41 22 379 61 03.
E-mail addresses: mabrynda@ucdavis.edu (M. Brynda), michel.
geoffroy@chiphy.unige.ch (M. Geoffroy).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.08.045

C. Gouverd et al. / Journal of Organometallic Chemistry 691 (2006) 72–78
73
on a VG analytical 7070E instrument (data system 11250,
resolution 7000).
using C6H5–13C(H)O for the synthesis of tricarbonyl
[1,2,3,4,5,6-g-benzaldehyde]chromium.
31P{1H} NMR (200 MHz, C6D6): d = 266.04 (d,
tBu
1
5
JPH = 24.0 Hz). H NMR (500 MHz, C6D6): 1.37 (s, 9H,
tBu
5
tBu
Cpara–CMe3), 1.62 (s, 18H, Cortho–CMe3), 4.29 (t, 2H,
J = 6.2 Hz, Harom), 4.51 (t, 2H, J = 6.2 Hz, Harom), 5.11
tBu
H
3
1
P
C 7
9
3
2
8
1
H
P
C
(d, 2H, J = 6.2 Hz, Harom), 7.35 (d, 1H, JPH = 24.2 Hz
8
9
tBu
7
10
P@C–H), 7.65 (s, 2H, Harom). 13C NMR (spectra recorded
with the 13C enriched compound, 125 MHz, C6D6): d =
31.44 (s, Cpara–CMe3), 34.10 (s, Cortho–CMe3), 35.08 (s,
13
tBu
10
13
11
12
Cr
11
12
CO
CO
Cpara–CMe3), 38.44 (s, Cortho–CMe3), 89.99 (s, Carom),
CO
92.32 (s, Carom), 92.47 (s, Carom), 93.49 (s, Carom), 122.24
(s, Cmeta), 137.66 (d, JPC = 52.5 Hz, Cipso–P), 150.62 (s,
1
1'
1
Cpara), 154.95 (s, Cortho), 171.28 (d, JPC = 35.8 Hz,
12
2.2. Syntheses
C@P), 233.20 (s, C@O). HR-MS: calculated for C28-
1H3531P16O352Cr+ 502.17289, measured 502.17390.
All manipulations involving organometallics were car-
ried out under an atmosphere of purified nitrogen and with
an inert-gas-vacuum double manifold and standard
Schlenk technique. Cr(CO)6 was supplied by FLUKA
and used as received. Tetrahydrofuran, diethyl ether, and
dibutyl ether were distilled from sodium-benzophenone
ketyl immediately before use. Hexane was distilled from
metallic sodium. n-BuLi (FLUKA) was titrated before
use according to the method of Chong [10]. Mes*PH2
[11,12] and tricarbonyl [1,2,3,4,5,6-g-benzaldehyde]chro-
mium [13,14] were synthesized following methods descri-
bed in the literature. The synthesis of the 13C-enriched
compound was performed using 13C-labelled benzaldehyde
supplied by Cambridge Isotope Labs. Column chromato-
graphy was carried out by using the flash method described
by Still [15]. All other reagents were commercial.
2.3. Crystal structure determination
Red needles of 1 suitable for X-ray analysis were ob-
tained by slow evaporation of a solution in diethyl ether.
Cell dimensions and intensities were measured at 200 K
on a STOE IPDS diffractometer with graphite-monochro-
˚
mated Mo Ka radiation (k = 0.71073 A). Data were cor-
rected for Lorentz polarization effects and for absorption.
A summary of crystal data, intensity measurement and
structure refinement is reported in Table 1. The structure
Table 1
Summary of crystal data, intensity measurement and structure refinement
for compound 1
Formula
C28H35CrO3P
Early studies on reaction of low-coordinated organo-
phosphorus systems with chromium(0) carbonyls have
shown that diaryldiphosphenes complex tricarbonylchro-
mium via their benzene ring [16,17], whereas 10 forms a
phosphorus-coordinated complex with Cr(CO)5 [18]. In
the present work, we could synthesize 1 by reacting
Mes*PH2 with tricarbonyl [1,2,3,4,5,6-g-benzaldehyde]-
chromium. 3.4 mL of n-BuLi (1.6 M in hexane, 5.4 mmol)
was added dropwise at ꢀ78 ꢁC to a carefully degassed solu-
tion of 1.5 g of Mes*PH2 (5.4 mmol) in 12 mL of THF.
After 5 min, the solution was allowed to warm to room tem-
perature and stirred for another 15 min. After addition of
0.815 g of t-BuMe2SiCl (5.4 mmol) the reaction mixture
was stirred for 30 min. Thereafter, 3.4 mL of n-BuLi
(1.6 M in hexane) was added at ꢀ78 ꢁC while stirring vigor-
ously. After warming to room temperature, 1.83 g (7.5 mmol)
of tricarbonyl [1,2,3,4,5,6-g-benzaldehyde]chromium was
added to the solution which was stirred overnight. The
solvents were removed under vacuum, and the resulting
brown residue was purified by column chromatography on
SiO2 using hexane as an eluant. This yielded 1.2 g of an
orange powder (yield = 44%). Red crystals were obtained
after recrystallisation from Et2O. M.p. 155–157 ꢁC.
Molecular weight
Crystal Size (mm)
Crystal system
Space group
502.6
0.057 · 0.185 · 0.35
Monoclinic
P21/n
5.9621(3)
26.5621(10)
16.8347(10)
96.729(7)
2647.7(3)
4
1.261
0.518
0.8971, 0.9705
0.637
25704
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
Dcalc (g cmꢀ3
)
l(Mo Ka) (mmꢀ1
Tmin, Tmax
)
ꢀ1
˚
((sinh)/k)max (A
)
N (measured)
N (independent)
N (observed)
5535
3124
|Fo| > 4r(Fo)
Full-matrix
298
Criterion for observed
Refinement (on F)
N (parameters)
Weighting scheme pa
Maximum D/r
0.0001
0.0013
ꢀ3
˚
Dqmin., Dqmax. (e A
)
0.424, ꢀ0.327
Sb (all data)
1.49(2)
Rc, xRd
0.033, 0.032
a
x = 1/[r2(Fo) + p(Fo)2].
P
b
c
S = [ {((Fo ꢀ Fc)/r(Fo))2}/(Nref ꢀ Nvar)]1/2
.
P
P
The 13C-enriched compound Cr(CO)3, Ph13C(H)@PMes*
was obtained by following the same procedure and by
R = ||Fo| ꢀ |Fc||/ |Fo|.
P
P
d
xR = [ (x|Fo| ꢀ |Fc|)2/ x|Fo|2]1/2
.

74
C. Gouverd et al. / Journal of Organometallic Chemistry 691 (2006) 72–78
was solved by direct methods (SIR-97) [19]; all other calcu-
lations were performed with the XTAL [20] system.
Crystallographic data (excluding structure factors) for
the structure in this paper have been deposited with the
Cambridge Crystallographic Data Centre as Supplemen-
tary Publication No. CCDC-277168. Copies of the data
can be obtained, free of charge, on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, UK [fax: +44(0)-
1223-336033 or deposit@ccdc.cam.ac.uk)].
The crystal structure of the non-complexed phosphaalk-
ene 10 was first reported by Appel et al. [21]. The structures
of the E and Z isomers of 10 were discussed by Yoshifuji
et al. [22].
all reactants and products at their geometry optimized with
basis set 2. Molecular orbital visualisations were performed
using the MOLEKEL program [29].
3. Results
3.1. Crystal structure
An ORTEP representation of 1 is given in Fig. 1. The
phosphaalkene part of the complex adopts an E conforma-
tion and is hardly affected by the presence of Cr(CO)3: The
P@C bond length = 1.666(3) A (1.674 A in 10), the
C1,P,C7,C8 atoms lie in the same plane (torsion angle
C1–P–C7–C8 = ꢀ177.3(2)ꢁ (177.8ꢁ in 10) oriented perpen-
dicular to the Mes* phenyl ring (torsion angle C7–P–C1–
C6 = ꢀ87.7(2)ꢁ). The complexed phenyl ring remains
planar. The presence of the Cr(CO)3 group causes an
appreciable rotation of this phenyl ring around the C7–
C8 bond (P–C7–C8–C9 torsion angle = 25.4(3)ꢁ in 1 and
16.0ꢁ in 10) as well as a small increase in the C7–C8 bond
˚
˚
2.4. Electrochemistry
Cyclic voltammetry measurements were performed on a
BAS station (model CV-50W). A standard three-electrode
cell was used, with positive feedback IR compensation.
All measurements were carried out under dry argon. The
0
˚ ˚
length (1.477(3) A in 1, 1.456 A in 1 ). The Cr–Cphenyl
˚
solvent was THF, dried in a glove box over activated 4 A
˚
˚
molecular sieves, the supporting electrolyte was n-Bu4PF6
(0.1 M), twice recrystallized and melt-dried under vacuum.
The electrodes were 1 mm diameter platinum wire as work-
ing and counter-electrode, and a SCE reference electrode.
The standard potential of ferrocene in THF is equal to
575 mV in our experimental conditions.
distances lie in the range 2.197 A (Cr–C13) to 2.237 A
(Cr–C8).
3.2. Electrochemistry
At room temperature, cyclic voltammetry of a solution
of 1 in THF shows a quasi-reversible one-electron reduc-
tion at the platinum electrode (Fig. 2). The anodic and
cathodic peak potential values are Ep(a) = ꢀ1456.5 mV
2.5. EPR spectroscopy
Spectra were recorded on a Bruker 200D spectrometer
(X-band, 100 kHz field modulation) with a Bruker ER-
4111 VT variable temperature controller. Samples were
usually prepared by electrolysis in situ of a thoroughly ar-
gon-degassed solution of the compound. A two-platinum
electrodes cell was used, adjusting the voltage until a signal
appeared.
and Ep(c) = ꢀ1643.5 mV vs. SCE, respectively (E1/2
ꢀ1554.5 mV vs. SCE, D = 187 mV). Ip(a) = Ip(b).
=
3.3. EPR
An example of EPR spectrum obtained at 290 K after
reaction at the same temperature of a solution of 1 in
THF on a potassium mirror is shown in Fig. 3(a). This
spectrum is characterized by a 31P isotropic coupling of
217 MHz and by a g-value equal to 2.0049. In the frozen
solution, at 110 K, the spectrum spreads over more than
2.6. Computations
All the computations, including the geometry opti-
mizations, were carried out with the GAUSSIAN 03 set of
programs [23], using the B3LYP functional (BeckeÕs
three-parameter nonlocal exchange potential [24] coupled
to the nonlocal correlation functional of Lee, Yang and
Parr [25]) combined with several basis sets. The following
designations for the basis sets are used throughout the text
for negatively charged species: (1) 6-31+G* for all atoms
(basis set 1), (2) LANL2DZ [26] for Cr and, for the other
atoms, either 6-31+G* (basis set 2), IGLO3 [27] (basis set
3) or TZVP [28] (basis set 4). The same designations are
used for neutral species, except that the 6-31+G* basis
set is replaced by the 6-31G* set. Minima on potential en-
ergy surfaces were characterized with harmonic frequency
calculations (no imaginary frequencies). The stabilization
energy (homodestic reaction a, vide infra) was performed
by calculating, with the 6-31+G* basis set, the energy of
Fig. 1. ORTEP view of 1 with atom numbering. Ellipsoids are represented
with 50% probability level.

C. Gouverd et al. / Journal of Organometallic Chemistry 691 (2006) 72–78
75
3.4. DFT calculations
Due to the large size of 1, the DFT calculations were
carried out on model systems 1a, in which the cumbersome
Mes* group was replaced by a mesityl group. The various
basis sets (vide supra) lead to very similar optimized geom-
etries for this complex. A representation of the optimized
structure (B3LYP/ basis set 2) for the neutral complex 1a
is given in Fig. 4. The corresponding geometrical parame-
ters, together with those obtained from the optimization
of the non-complexed phosphaalkene PhC(H)@PMes
(10a) [31], are given in Table 3.
Taking into account the fact that bulky t-butyl groups
have been replaced by methyl groups, these parameters
agree reasonably well with the crystal structures of 1 and
10, respectively. As shown by the dihedral angle P–C7–
C8–C9, the complexed phenyl ring and the adjacent
C–C@P moiety form an angle that is appreciably larger
in the determined crystal structure than in the optimized
structure. Nevertheless, both the crystal structure and the
DFT calculations indicate that complexation to Cr(CO)3
causes a drastic change of this dihedral angle.
Fig. 2. Cyclic voltammogram of 1.
200 G (Fig. 3(b)) showing a large anisotropy of the 31P
hyperfine interaction. This spectrum is well simulated by
using the axial g and hyperfine (31P)T tensors given in
Table 1. The isotropic 31P coupling constant (221 MHz)
and the average g-value (2.0045) obtained from the spec-
trum of the frozen solution agree well with the values obtai-
ned at 290 K. The same spectra were obtained by reducing
a THF solution of 1 with sodium naphthalenide or by
direct electrolysis of this solution, in situ, in the EPR
cavity.
A second set of calculations was performed with the
reduced species. The optimized geometrical parameters
calculated for (1a)Åꢀ and (10a)Åꢀ are also included in Table
3. Reduction of 1a and 10a causes an elongation of the
P@C7 bond and a shortening (d) of the C7–C8 bond which
˚
is more pronounced for the complex (d = 0.055 A) than for
˚
the isolated phosphaalkene (d = 0.037 A). The most impor-
The isotropic and anisotropic coupling constants s(31P),
deduced from the T(31P) coupling tensor, are reported in
Table 2. The experimental spin densities can be obtained
by comparing these values with the Fermi contact and
dipolar interactions characterizing an unpaired electron
located in a 3s and 3p orbital of atomic phosphorus,
respectively [30]. The resulting qs(P) and qp(P) are given
in Table 2. No modification of the spectrum could be
detected as compared to 1, when the 13C enriched complex
Cr(CO)3, C6H513C(H)@PMes* was used instead for the
in situ reduction.
tant structural changes due to the electron addition affect
the Cr(CO)3 phenyl moiety of 1a: this phenyl ring adopts
an envelope conformation (C10–C12–C13–C8 = 9.9ꢁ)
which removes the coplanarity of the C12, C13, C8, C7
atoms and contributes to an elongation of the Crꢁ ꢁ ꢁC8 dis-
˚
tance (from 2.301 to 2.638 A), moreover the chromium
atom moves away from C9 and C13 (Crꢁ ꢁ ꢁC9 increases
˚
from 2.252 to 2.382 A, Crꢁ ꢁ ꢁC13 increases from 2.255 to
˚
2.380 A) while the Cr–C10, Cr–C11, Cr–C12 distances
˚
remain close to 2.25 A. These geometrical modifications
clearly indicate that the anion formation causes a decrease
Fig. 3. Liquid solution (a) and frozen solution (b) EPR spectra obtained with a reduced solution of 1 in THF.

76
C. Gouverd et al. / Journal of Organometallic Chemistry 691 (2006) 72–78
Table 2
Experimental EPR parametersa and spin densities obtained for the reduction product of 1
g Values
31P hyperfine coupling (MHz)
Phosphorus spin densities
T(31P)
Aiso
s(31P)
qs(P)
qp(P)
Liquid solution
Frozen solution
g = 2.0049
218 (152)
221
gi = 1.9986 (2.003)
g^ = 2.0075 (2.006)
gaverage = 2.0045
Ti = 603 (455)
T^ = 29.5 (0.5)
si = 382 (303)
0.016
0.52
s
^ = ꢀ191.5 (ꢀ151.5)
a
In parentheses experimental values for the non-complexed radical anion (10)Åꢀ (see [1]).
Fig. 4. Optimized structures for 1a and 1aÅꢀ
.
Table 3
Optimized parameters for the neutral and anionic complexes 1a and (1a)Åꢀ and for the neutral and anionic non-complexed phosphaalkenes 10a and (10a)Åꢀ
˚
Bond lengths (A)
Bond angle (ꢁ)
Torsion angle (ꢁ)
P@C7
C7–C8
P–C1
C7–P–C1
P–C7–C8–C9
C7–P–C1–C2
1a (Crystal 1)a
10a (Crystal 10)b
(1a)Åꢀ
1.694 (1.666)
1.695 (1.674)
1.763
1.460 (1.477)
1.462 (1.456)
1.404
1.848 (1.852)
1.856 (1.848)
1.862
102.2 (100.5)
101.6 (100.9)
100.7
11.2 (25.4)
ꢀ0.1 (ꢀ16)
2.7
70.4 (89.9)
92.2 (83.9)
117.6
ꢀ131.4
(10a)Åꢀ
1.765
1.425
1.853
102.3
6.5
a
This work.
b
From [21].
in the coordination number of chromium. As shown in
Fig. 5 the relative orientation of the Cr(CO)3 tripod and
The g values and the hyperfine coupling constants have
been calculated for the optimized structures of the com-
plexed species (1a)Åꢀ. For comparison, the coupling con-
stants of the non-complexed phosphaalkene radical anion
Cipso–C(phosphaalkene)-bond changes with the reduction
state: the system adopts a syn eclipsed conformation in
the reduced species but is close to the anti-eclipsed confor-
mation in the neutral species.
Åꢀ
1a0 are also included in Table 4.
4. Discussion
As shown by cyclic voltammetry, Cr(CO)3 complexation
of the phenyl ring bound to a phosphaalkene carbon facil-
itates the reduction of such a system. While the reduction
of 10 occurs at ꢀ1980 mV/SCE, the reduction wave of 1
occurs at ꢀ1550 mV/SCE.
The EPR parameters determined after electrochemical
or chemical reduction of the Cr(CO)3 complexed phos-
phaalkene 1 are appreciably different from those previously
reported for the non-complexed species 1a. As expected,
Fig. 5. Orientation of the Cr(CO)3 tripod with respect to the phenyl ring
bound to the phosphaalkene carbon.

C. Gouverd et al. / Journal of Organometallic Chemistry 691 (2006) 72–78
77
Table 4
EPR parameters calculated by DFT for (1a)Åꢀ
Basis set used for
(
31P)Aiso (MHz)
Anisotropic couplings (MHz)
qp
Diagonalized g-tensor
Optimization
Set 1
Magnetic properties
6-31+G*
IGLO3
TZVP
TZVPb (for (10a)Åꢀ
Set 2
102.3
112.8
130.8
92.1
141.5
142.1
142.1
ꢀ170.2
ꢀ182.7
ꢀ186.5
ꢀ119.5
ꢀ169.5
ꢀ189.0
ꢀ189.0
ꢀ158.3
ꢀ169.3
ꢀ174.8
ꢀ111.1
ꢀ157.6
ꢀ176.3
ꢀ176.3
328.5
352.0
361.3
230.6
327.0
365.3
365.3
49.2
45.0
48.0
1.9907, 2.0069, 2.0081a
Set 2
Set 3
Set 4
a
48.6
49.9
49.9
Set 3
Set 4
Calculation with the 6-311++G** basis set: 1.9905, 2.0070, 2.0081.
b
Calculations (optimization and hyperfine couplings) for the non-complexed model (10a)Åꢀ
.
which mainly contributes to the structure of (1a)Åꢀ is a
phosphinyl radical in which the phosphorus atom is bound
to both a chromium-complexed benzylic anion and a Mes
group. The SOMO calculated for (1a)Åꢀ is represented in
Fig. 6.
The homodestic equation (a) [34] was used to assess the
effect of chromium tricarbonyl complexation on the stabil-
ization of phosphaalkene radical anion:
H
Mes
H
Mes
H
C
Mes
Mes
H
C
P
C
P
P
C
P
+
+
ðaÞ
Cr(CO)3
Cr(CO)3
.-
.-
(1'a)
(1a)
(1a)
(1'a)
The molecular orbital stabilization energy, calculated
by using the energy values calculated for the optimized
structures of the reactants and products, is equal to
19 kcal molꢀ1, therefore indicating that the complexation
with chromium is a key stabilizing factor for the reduced
phosphaalkene. This value is, however, less than the
stabilization (33 kcal molꢀ1) previously reported for the
benzylic anion. This stabilization is achieved through geo-
metrical modifications that accompany the reduction of
the complex 1a: the chromium atom moves away from
the exocyclic C8–C7 bond which gets some double bond
character (the corresponding Wiberg bond order increases
from 1.3 to 1.5 [31]); after rotation, the chromium tricar-
bonyl tripod adopts a syn eclipsed conformation with
respect to the phosphaalkene group. These geometric dis-
torsions allow a part of the negative charge to be localized
on the Cr(CO)3 moiety (charge on Cr(CO)3 from NBO
analysis: ꢀ0.4) [31].
Fig. 6. Representation of the SOMO for 1aÅꢀ
.
the presence of Cr(CO)3 causes an increase of the g anisot-
ropy (Dg = gi ꢀ g^ increases from 0.0038 to 0.0089). The
main difference between the two species lies in the enhance-
ment of the phosphorus pp spin density which leads to a
marked enhancement in both the 31P isotropic coupling
(Aiso increases from 152 to 217 MHz) and anisotropic
constants (si increases from 303 to 386 MHz). The DFT
calculations on the model compounds confirm the presence
of the radical anion: the g tensor calculated for 1aÅꢀ very
well agrees with the experimental values, while the calcu-
lated 31P dipolar coupling tensor is very close to the tensor
reported in Table 2 for 1Åꢀ. The accord between experimen-
tal and calculated (31P)Aiso values is rather modest; but it is
well known that the standard DFT methods perform rather
poorly in predicting the Fermi contact interaction with 31
P
[32,33] The DFT results confirm the effect of the presence
of Cr(CO)3 on the spin distribution, in particular, the
stronger localisation of the unpaired electron on the phos-
phorus atom (qtotal,phosphorus increases from 0.37 to 0.64).
The rather small spin density on the phosphaalkene carbon
explains why no spectral modification was observed with
the corresponding 13C enriched compound. These proper-
ties, together with the NBO charge analysis (charge on
the Cr(CO)3-phenyl moiety = ꢀ0.577, on the phosphaalk-
ene carbon C7 = ꢀ0.45 [31] suggest that a mesomeric form
5. Concluding remarks
Cr(CO)3 complexation of the phenyl ring bound to a
phosphaalkenic carbon facilitates appreciably the reduc-
tion of the corresponding phosphaalkene. In the resulting
radical anion the presence of Cr(CO)3 increases the spin
density on the phosphorus atom as expected for a phos-
phinyl radical whose phosphorus is bound to a Cr(CO)3
benzylic anion. Geometrical modifications related to the

78
C. Gouverd et al. / Journal of Organometallic Chemistry 691 (2006) 72–78
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Acknowledgements
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Support from the Swiss National Science Foundation is
gratefully acknowledged. We thank Prof. P. Kundig for
¨
help and fruitful discussion.
Appendix A. Supplementary data
[22] M. Yoshifuji, K. Toyota, I. Matsuda, T. Niitsu, N. Inamoto,
Tetrahedron 44 (1988) 1363.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2005.08.045.
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