New pyridyl-functionalized organoiron alkynyl complexes. Easy access to polymetallic architectures featuring an electroactive site by simple co-ordination reactions

Article abstract of DOI:10.1016/S0022-328X(98)00961-9

The functionalized complexes 3a-d [(dppe)Cp*Fe(CC-L)] (L=4-Py, 3-Py, 2-Py) were isolated with good yields from the alkynyl complex 1 [(dppe)Cp*Fe(CCH)] (dppe=1,2-bis(diphenylphosphino)ethane) from the corresponding bromopyridines using a Sonogashira-coupling reaction. These redox active compounds are stable under two redox states and react as usual pyridyl ligands towards methyl iodide or (THF)W(CO)₅. They allow easy access to the corresponding pyridinium salt 4a or to the dimetallic iron-tungsten complex 5a.

Full text of DOI:10.1016/S0022-328X(98)00961-9

Journal of Organometallic Chemistry 572 (1999) 189–192
New pyridyl-functionalized organoiron alkynyl complexes. Easy access
to polymetallic architectures featuring an electroactive site by simple
co-ordination reactions
Sylvie Le Stang, Dirk Lenz, Fre´de´ric Paul, Claude Lapinte *
UMR CNRS 6509, Department of Chemistry, Campus de Beaulieu, Uni6ersity of Rennes 1, 35042 Rennes Cedex, France
Received 6 May 1998; received in revised form 15 July 1998
Abstract
The functionalized complexes 3a–d [(dppe)Cp*Fe(CꢀCꢁL)] (L=4-Py, 3-Py, 2-Py) were isolated with good yields from the
alkynyl complex 1 [(dppe)Cp*Fe(CꢀCH)] (dppe=1,2-bis(diphenylphosphino)ethane) from the corresponding bromopyridines
using a Sonogashira-coupling reaction. These redox active compounds are stable under two redox states and react as usual pyridyl
ligands towards methyl iodide or (THF)W(CO)5. They allow easy access to the corresponding pyridinium salt 4a or to the
dimetallic iron–tungsten complex 5a. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Organoiron(II); Organoiron(III); Iron acetylide; Sonogashira-coupling; Substituted pyridine; Pyridinium salt; Dinuclear
complex
1. Introduction
the availability of similar organoiron building blocks
presenting a remote pyridyl co-ordination group ap-
peared very appealing [13]. Recently, the authors re-
ported on the use of the Sonogashira-coupling to
covalently graft the ‘(dppe)Cp*FeꢁCꢀC’ moiety on var-
ious haloaromatic units [12]. In this communication, we
now report on the easy access to new pyridyl-function-
alized organoiron alkynyl complexes and the ability of
the new coupling products to behave as redox ligands
toward a metal center.
Whereas the coupling of organic -ynes with nitrogen
heterocycles using the Sonogashira- or derived-coupling
reactions has been known for a long time [14–21], the
coupling of transition metal |-co-ordinated -ynes is
much more rare, and quite new for transition metal
alkynyls [22,23]. As depicted in Scheme 1, the Sono-
Disposition in space of organic or organometallic
molecular units possessing a pyridyl moiety by use of
different metal centers acting as templates is a conve-
nient access to supramolecular assemblies, well-defined
in size and shape [1–3]. For instance, the complexation
of such a unit to Pd(II) or Pt(II) complexes proved to
be sufficiently strong and specific regarding geometric
isomers to allow successful prediction of the structures
obtained [4,5]. We are strongly involved in the synthesis
of organometallic architectures possessing several elec-
troactive ‘(dppe)Cp*Fe’ units and in the study of their
potential as new nanoscopic devices for molecular elec-
tronics [6–11]. Regarding its very promising electronic
and magnetic properties, they are currently developing
synthetic means to allow the incorporation of this
fragment into various molecular assemblies [12]. Thus,
gashira-coupling
reaction
between
the
(dppe)Cp*FeCꢀCH complex (1) [24] and several bro-
mopyridyl substrates allows the straightforward isola-
tion of the corresponding organoiron-functionalized
pyridines 3a–c with fair yields [25].
* Corresponding author. Tel.: +33-9928-6361; fax: +33-9928-
1646; e-mail: claude.lapinte@univ-rennesl.fr.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)00961-9

190
S. Le Stang et al. / Journal of Organometallic Chemistry 572 (1999) 189–192
Scheme 1.
electronic interaction between the nitrogen atom of the
These complexes were fully characterized by means
heterocycle and the iron nucleus.
of usual spectrometric and elemental analyses. The
presence of the triple bond was clearly evidenced by the
resonances of its quaternary carbon atoms and by a
characteristic IR frequency. The h carbon resonance,
being coupled with the two phosphorus atoms of the
dppe, appears as a triplet with a constant of ca. 38 Hz
(Table 1). The multiplicity of the triple bond stretching
mode may arise from Fermi coupling with other vibra-
tional modes as already stated for this family of
molecules [26], since the purity of the isolated molecules
was assessed by the spectroscopic data, only one signal
being observed for the dppe phosphorus atoms, and by
the correct elemental analysis. As for their non-hetero-
cyclic analogs previously reported, these complexes pos-
sess a second stable redox state (3a–c+) in an
accessible potential range as evidenced by reversible
cyclic voltammograms. The mono-oxidized state can be
successfully isolated and characterized after chemical
oxidation using ferrocinium hexafluorophosphate
(Table 1) [27]. In accordance with the data gathered for
similar compounds, the paramagnetic oxidized com-
plexes present a low spin iron(III) center with a single
unpaired electron [24].
These complexes appear to behave as typical pyridi-
nes. Thus, 3a reacts with methyl iodide to afford the
corresponding pyridinium salt 4a with good yields [28].
The 13C resonance of the acetylenic Ch carbon at low
field and FTIR spectroscopy (Table 1) suggest an inter-
mediate electronic structure between a pure alkynyl
form with the positive charge located on nitrogen and
an allenylidene form derived from a dihydropyridine
ring with the positive charge located on iron. However,
Mo¨ssbauer spectra at 80 K (IS=0.245 mm s−1, QS=
2.020 mm s−1) is typical for an iron(II) site with a
|-iron–carbon bond. A significantly weaker QS is ex-
pected for compounds of this series with a iron–carbon
double bond, therefore, the alkynyl structure is as-
sumed to be preserved in 4a [29]. The large shift stated
for the oxidation potential of the iron-centered process
in 4a relative to 3a is noticeable and indicates a good
In order to assess the potential of these molecules as
ligands, we attempted co-ordination of one of these
complexes to a metal center. Tungsten hexacarbonyl
seemed to be well-suited for this purpose [13,18,21,22],
thus, the W(CO)6 complex was reacted with 3a under
UV-irradiation. Photochemical displacement of one
carbonyl from tungsten center occurred and the desired
complex 5a was isolated in good yields [30]. The pres-
ence of the tungsten pentacarbonyl center was evi-
denced by apparition of carbonyl stretching modes
(2069, 1924 and 1883 cm−1) in FTIR and by carbonyl
resonances at 217 and 216 ppm (relative intensities 1:4)
in 13C-NMR, as well as by a change in the redox
potential of 5a relative to complex 3a. The shift to-
wards more positive values indicates that the iron-cen-
tered redox process is influenced by the electropositive
tungsten center and suggests the occurrence of an elec-
tronic interaction between the metals. Additionally, as
indicated by the observation of a single reversible redox
system, we established that the pyridine ligand re-
mained co-ordinated to the tungsten center in the oxi-
dized state of complex 5a. Moreover, the complex 5a+
prepared following the procedure described for oxida-
tion of 3 displays the same cyclic voltammogram as
that of 5a and a wCꢀC bond stretching at lower energy
(Table 1). The CO stretching frequencies of 5a+ are
very close to those of 5a (2070, 1927 and 1892 cm−1).
See Scheme 2 for corresponding reactions.
In conclusion, the new organoiron-functionalized
pyridyl compounds are good ligands, even in their
oxidized states. Thus, the premise that 3/3+ com-
pounds might be disposed in space using templating
metal centers seems to be valid. Several other metal
centers giving rise to stable complexes with pyridine are
currently being tested in our group in order to establish
the potential of such an approach and to investigate a
possible co-operative electronic or magnetic behavior
between remote electrons [31] Additionally, regarding
the NLO properties of very similar complexes

S. Le Stang et al. / Journal of Organometallic Chemistry 572 (1999) 189–192
191
Table 1
Yields and selected spectroscopic data for complexesa
Compound Yield(%)
FTIR: wCꢀC in CH2Cl2 (cm−1
)
Cyclic voltammetryb
31P{1H}-NMRc: dppe 13C{1H}-NMRd: Ch and Ci
3a
3a+
3b
3b+
3c
73
87
79
87
80
87
90
73
68
2059, 2024
1990
−0.032
−0.032
−0.105
−0.105
−0.083
−0.083
0.221
100.9
–
101.1
–
101.7
–
98.3
100.3
–
155.0 (38 Hz)
–
147.5 (39 Hz)
–
145.7 (38 Hz)
–
204.4 (38 Hz)
172.3 (37 Hz)
–
120
–
117
–
2057, 2031
1991d
2053, 2028
2006d
1991
2016
1976
122.5
–
3c+
4ae
141.5f
121.7
–
5a
5a+
0.055
0.055
a NMR data from 31P, 1H and 13C spectra recorded at 81, 200 and 50 MHz respectively.
b E° in V vs. SCE (ferrocene–ferrocenium couple was used as an internal calibrant for the potential measurements, E°=0.460 V vs. SCE), Pt
electrode, 25°C, 0.1 M NBu4PF6.
c In C6D6.
d Very weak absorption.
e NMR in acetone-d6 for this complex.
f Proposed assignment.
Scheme 2.
[2] P.J. Stang, B. Olenyuk, Acc. Chem. Res. 30 (1997) 502.
[3] W.T.S. Huck, A. Roher, A.T. Anilkumar, R.H. Fokkens,
[21,32,33], compounds as 3a–d, 5a and analogs possess-
ing a stable oxidized state appear as interesting models
of redox-switchable NLO devices [25,31].
N.M.M. Nibbering, F.C.J.M. van Vegel, D.N. Reinhoudt, New
J. Chem. 2 (1998) 165.
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Am. Chem. Soc. 120 (1998) 611.
Acknowledgements
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S. Le Stang et al. / Journal of Organometallic Chemistry 572 (1999) 189–192
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