Unexpected isolation and structural characterization of a redox-active Fe(II) piano-stool complex with an η1-dppe ligand

Article abstract of DOI:10.1016/j.jorganchem.2006.10.047

A new cationic complex [(η²-bipy)(η⁵-C₅Me₅)Fe(η¹-dppe)][BPh₄] featuring a pendant free diphenylphosphine arm has been isolated in good yield after reaction of 2,2′-bipyridine with the known chloro precursor complex (η²-dppe)(η⁵-C₅Me₅)FeCl in presence of NaBPh₄. This complex which can be envisioned as a new redox-active building-block has been structurally characterized.

Full text of DOI:10.1016/j.jorganchem.2006.10.047

Journal of Organometallic Chemistry 692 (2007) 917–920
www.elsevier.com/locate/jorganchem
Note
Unexpected isolation and structural characterization of a
redox-active Fe(II) piano-stool complex with an g1-dppe ligand
a,1
a,
a,
*
Safaa Ibn Ghazala , Frederic Paul *, Loic Toupet b,2, Claude Lapinte
´ ´
a
Sciences Chimiques de Rennes, Universite´ de Rennes I, CNRS (UMR 6226), Campus de Beaulieu, Baˆt. 10C, 35042 Rennes, France
´
b
`
´ ´
Groupe Matiere Condensee et Materiaux, Universite de Rennes I, CNRS (UMR 6626), Campus de Beaulieu, 35042 Rennes, France
Received 26 September 2006; received in revised form 21 October 2006; accepted 22 October 2006
Available online 27 October 2006
Abstract
A new cationic complex [(g2-bipy)(g5-C5Me5)Fe(g1-dppe)][BPh4] featuring a pendant free diphenylphosphine arm has been isolated
in good yield after reaction of 2,20-bipyridine with the known chloro precursor complex (g2-dppe)(g5-C5Me5)FeCl in presence of
NaBPh4. This complex which can be envisioned as a new redox-active building-block has been structurally characterized.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Piano-stool; Iron(II) complex; 2,20-Bipyridine; g1-Diphosphine; Redox-active
1. Introduction
behaved reaction proved unselective in this instance and
led to mixtures of several unidentified compounds. In order
to rationalize the unexpected course of this reaction, we
have subsequently performed several control experiments.
We thereby could evidence that the reaction between the
starting (g2-dppe)(g5-C5Me5)FeCl complex (2) and neat
2,20-bipyridine (3) in presence of NaBPh4 led quantitatively
to the formation of the new compound 4[BPh4] in which
the dppe ligand adopts a rather rare g1-coordination mode.
We report in the following the characterization of this new
complex.
The study of redox families obtained from dinuclear
complexes where two redox-active ‘‘(g2-dppe)(g5-
C5Me5)FeC„C–’’ endgroups are connected together via
an unsaturated organic bridge brings decisive information
about the capability of the central spacer to convey elec-
trons [1–3]. In the hope of isolating a new family of such
dinuclear compounds featuring a 2,20-bipyridyl unit in
the bridge like A (Scheme 1) we have reacted the corre-
sponding diethynyl-2,20-bipyridine (1) with 2 equiv. of the
(g2-dppe)(g5-C5Me5)FeCl complex (2) in presence of
KPF6 or NaBPh4 and a strong base under various condi-
tions [2,3]. However, instead of obtaining the expected
alkynyl complex, only mixtures of unidentified compounds
were obtained in each case, some of which being paramag-
netic [4]. Thus, much to our surprise, this usually well
2. Experimental
All manipulations were carried out under inert atmo-
spheres using Schlenk techniques. Transmittance-FTIR
spectra were recorded using a Bruker IFS28 spectrometer
(400–4000 cmꢀ1). UV-Vis spectra were recorded on an
UVIKON XL spectrometer. NMR spectra were recorded
on AVANCE 200 MHz and 500 MHz spectrometers and
were calibrated against the deuterated solvent used as inter-
nal reference. Elemental analyses were performed at the
‘‘Centre Regional de Mesures Physiques de l’Ouest’’
(CRMPO, University of Rennes).
*
Corresponding authors. Tel.: +33 02 23 23 59 62; fax: +33 02 23 23 56
37.
E-mail addresses: frederic.paul@univ-rennes1.fr (F. Paul), claude.la-
pinte@univ-rennes1.fr (C. Lapinte).
1
Tel.: +33 02 23 23 59 62; fax: +33 02 23 23 56 37.
Tel.: +33 02 23 23 64 97; fax: +33 02 23 23 52 92.
2
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.10.047

918
S. Ibn Ghazala et al. / Journal of Organometallic Chemistry 692 (2007) 917–920
Ph2P
C Fe
N
N
PPh2
Fe
C
C
C
C
C
Ph2P
Ph
N
N
PPh2
Ph
Fe
N
P
BPh4
A
1
N
PPh2
HC
CH
4[BPh4]
Scheme 1.
2.1. Synthesis of [(g2-bipy)(g5-C5Me5)
Fe(g1-dppe)][BPh4] (4[BPh4])
154 frames via 2.0ꢁ omega rotation and 80 s per frame) pro-
vided 38115 reflections for 4[BPh4], from which 13394 were
unique [6]. Subsequent data reduction with Denzo and Scale-
pack [5] gave 10428 independent reflections [I > 2r(I)]. The
structure was solved with SIR-97 which revealed the non-
hydrogen atoms [7]. After anisotropic refinement, the
remaining atoms were found in Fourier difference maps.
The complete structure was next refined with SHELXL97 [8]
by the full-matrix least-square technique (use of F square
magnitude; x, y, z, bij for Fe, P, C, N and/or O atoms, x, y,
z in riding mode for H atoms with 716 variables for 10428
observations and w = 1/[r2(Fo)2 + (aP)2 + bP] where P ¼
½F 2o þ F 2cꢁ=3 with a = 0.0587 and b = 2.3717). Atomic scat-
tering factors were taken from the literature [9]. ORTEP
views of 4+ were realized with PLATON98 [10].
In a Schlenk tube, 0.125 g of 2,20-bipyridine (3;
0.80 mmol) and 0.274 g of NaBPh4 (0.80 mmol) were added
to 0.505 g of (g2-dppe)(g5-C5Me5)FeCl (2; 0.80 mmol) in
30 mL of methanol. The reaction mixture was stirred for
ca. 20 h during which time a deep red powder precipitated
in the reaction medium. The later was recovered by filtration
and washed with methanol several times, before being dried
in vacuo, providing the new complex 4[BPh4] as a deep red
solid (0.600 g, 70%). This compound could be crystallized
by slow diffusion of n-pentane in a dichloromethane solution
of the crude material previously isolated (total yield: 60%).
MS (ESI+, m-NBA) m/z 745.2565 ([M]+, 100%), m/z calc
for [C46H47N2P256Fe]+ = 745.22564. Anal. Calc. for
C70H67BFeN2P2: C, 78.95; H, 6.34; N, 2.63. Found: C,
3. Results and discussion
79.00; H, 6.35; N, 2.70%. FT-IR (KBr/Nujol, m in cmꢀ1
)
1578 cmꢀ1, (m, bipy). 31P{1H} NMR (81 MHz, CDCl3, dp
This new complex 4[BPh4] was formed overnight by reac-
tion of 1 equiv 2,20-bipyridine (3) with the known Fe(II)
chloride precursor 2 in presence of NaBPh4 (Scheme 2)
[11]. Dark red crystals of this compound, which constitutes
the unique product of the reaction according to mass spec-
trometry (MS-ESI), were obtained by slow diffusion of n-
pentane into a saturated CH2Cl2 solution of the above com-
plex. The quite quantitative formation of 4[BPh4] reveals the
high affinity of the cationic Fe(II) centre for diimine-like
ligands. Actually, the complex formation takes most likely
place after reaction of 2,20-bipyridine (3) with the 16e inter-
mediate [(g2-dppe)(g5-C5Me5)Fe][BPh4] (5[BPh4]) tran-
siently generated after abstraction of the chloride ligand
from 2 by the sodium salt in methanol. Likewise, the coordi-
nation of the bipyridyl site of 1 on 5+ is possibly at the origin
1
in ppm) 60.0 ppm (s, dppe). H NMR (200 MHz, CDCl3,
dH in ppm) 8.22 (m, 2H, Hbipy); 7.60–7.00 (m, 46H, aromat-
ics); 1.60–1.10 (m, 19H, C5Me5 + 2CH2). UV–Vis. (CH2Cl2)
k
max (e/103 dm3 Mꢀ1 cmꢀ1) 228 (sh, 58); 298 (27.4); 356 (sh,
3.9); 486 (2.5); 518 (2.6). CV (CH2Cl2, 0.1 M n-Bu4N+, PF
20 ꢁC, 0.1 V sꢀ1) E0 (DEp, ia/ic) 0.34 V (0.06, 1).
,
ꢀ
6
2.2. Crystallography
X-ray data were collected on a NONIUS Kappa CCD
with graphite monochromatized MoKa radiation. The cell
parameters were obtained with Denzo and Scalepack [5] with
10 frames (psi rotation: 1ꢁ per frame) on a 0.22 ·
0.12 · 0.08 mm crystal. The data collection (2hmax = 54ꢁ,
Ph
Ph
P
Fe
N
BPh4
NaBPh4, 20 h
MeOH
N
N
Fe Cl
PPh2
+
N
Ph2P
PPh2
3
2
4[BPh4]
Scheme 2.

S. Ibn Ghazala et al. / Journal of Organometallic Chemistry 692 (2007) 917–920
919
Table 1
Crystal data, data collection, and refinement parameters for
4[BPh4] Æ CH2Cl2
Empirical formula
Formula weight
T (K)
FeP2C46H47N2,BC24H20 Æ CH2Cl2
1150.79
120(1)
Crystal system
Space group
Triclinic
ꢀ
P1
˚
a (A)
11.8235(2)
13.9849(2)
19.3985(3)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
94.919(1)
106.535(1)
105.172(1)
2922.88(8)
3
˚
V (A )
Z
2
Absorption coefficient (mmꢀ1
)
0.450
H Range (ꢁ)
2.47–25.52
Index ranges
0 6 h 6 15; ꢀ18 6 k 6 17; ꢀ25 6 l 6 23
R = 0.047; Rw = 0.117
R = 0.067; Rw = 0.130
1.022
Final R indices
R indices (all data)
Goodness of fit/F2(Sw)
of the unselective course taken by the previous reactions
between 4,40-diethynyl-2,20-bipyridine (1) and 2 under simi-
lar conditions [12].
Fig. 1. ORTEP representations of the cation in 4[BPh4] Æ CH2Cl2 at 50%
probability level. Selected bond distances (A): Fe1–(Cp )centroid, 1.740;
As shown by the solid state structure of 4[BPh4]
(Table 1), the 2,20-bipyridyl ligand is essentially planar, with
an inter-cycle torsion angle of ca. 5ꢁ (Fig. 1). This ligand has
replaced the chloride ion in the coordination sphere as well
as one of the diphenylphosphine arms of the dppe ligand.
However, in solution at 25 ꢁC, the dppe ligand appears flux-
ional, as evidenced by the observation of a single peak near
60 ppm and by a much broader peak near ꢀ11 ppm in 31P
NMR, sometimes quite difficult to detect. Also, fluxionality
is suggested by broad CH2 signals, which partly merge with
the methyl signals of the cyclopentadienyl ligand on the 1H
NMR spectrum. Note that this fluxional motion is appar-
ently frozen when an excess 2,20-bipyridine (3) is introduced
in the solution, since a clean AX system (3JPP = 24 Hz) can
then be observed by 31P NMR [13]. The ESI-MS spectrum
of the crude isolated product 4+, while confirming the
mononuclear structure proposed, did not allow for observa-
tion of other species. The other experimental characteriza-
tions (see experimental part) are also consistent with the
diamagnetic organoiron(II) formulation of 4[BPh4] [3].
Such g1-ligated dppe piano-stool complexes are however
quite unusual for electron-rich Fe(II) centres. To our
knowledge, related electron-rich Fe(II) complexes featuring
a g1-dppe ligand with a pendant free diphenylphosphine
arm are not ubiquitous species and were reported only in
seldom cases [14–18]. Notably, cyclic voltammetry indicates
that 4[BPh4] undergoes a chemically reversible oxidation at
0.34 V vs. ECS i.e. slightly lower than the oxidation poten-
tial of ferrocene [19].
*
˚
Fe1–P1, 2.2571(6); Fe1–P2, 6.104; Fe1–N1, 1.9602(18); Fe1–N2,
1.9642(17) (Cp* = pentamethylcyclopentadienyl ligand). Selected bond
angles (deg): N1–Fe1–N2, 80.84(7); P1–Fe1–N1, 87.87(5); P1–Fe1–N2,
92.59(5).
good yields. Although initially unwanted, this new redox-
active complex presents an available coordinating site
(i.e. the free diphenylphosphino unit) and exhibits a revers-
ible oxidation. Likewise to A, it might therefore also consti-
tute an appealing building block for the realization of
polynuclear assemblies using simple complexation reac-
tions [17,20], provided its fluxional behaviour in solution
can be properly understood. Efforts in this direction shall
be pursued. Coming back to the failure to isolate the com-
plex A (Scheme 1) from the diethynyl-2,20-bipyridyl ligand
1, it seems that prior protection of the diimine site consti-
tutes an important requirement when (poly)ethynyl 2,20-
bipyridine ligands are to be reacted with the iron(II) chlo-
ride precursor 2. Work along these lines is currently under-
way in our group in order to selectively synthesize the
target compound A.
Acknowledgement
We thank ATCO(Caen) for financial support.
Appendix A. Supplementary material
In conclusion, we have shown here that diimine ligands
like 2,20-bipyridine are able to displace one phosphine
group of the dppe ligand and the halogen ligand in the iron
chloride complex 2 to give the cationic complex 4[BPh4] in
CCDC 619568 contains the supplementary crystallo-
graphic data for this paper. The data can be obtained free
of charge via htpp://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data

920
S. Ibn Ghazala et al. / Journal of Organometallic Chemistry 692 (2007) 917–920
[10] A.L. Spek, PLATON. A Multipurpose Crystallographic Tool, Utrecht
University, 1998.
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2006.10.047.
[11] C. Roger, P. Hamon, L. Toupet, H. Rabaaˆ, J.-Y. Saillard, J.-R.
Hamon, C. Lapinte, Organometallics 10 (1991) 1045–1054.
[12] Control experiments revealed that neither (g2-dppe)(g5- C5Me5)Fe–
C„C–Ph nor (g2-dppe)(g5- C5Me5)Fe–C„C–3-Py did appreciably
react with excess 2,20-bipyridine under similar conditions, suggesting
that if formed, the Fe(II)-acetylide complex A (Scheme 1) should have
been quite stable under the reaction conditions used.
[13] Until yet, we were unable to rationalize the role played by the
bipyridine ligand in this equilibrium by running 1H and 31P variable
temperature experiments. Upon suggestion from a referee, we have
nevertheless verified that the bipyridine ligand does not simply act as
a base in the medium. Thus, the spectrum of 4[BPh4] in chloroform is
quite unchanged upon addition of triethylamine, a stronger base than
2,20-bipyridine.
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