Hexatriynediyl Chain Spanning Two Cp*(dppe)M Termini (M = Fe,
Ru): Evidence for the Dependence of Electronic and Magnetic
Couplings on the Relative Orientation of the Termini
Alexandre Burgun,
†,‡
Fre ́ de ́ ric Gendron,
†
Christopher J. Sumby,
‡
Thierry Roisnel,
†
Olivier Cador,
†
Karine Costuas,
†
Jean-Franç ois Halet,*
,†
Michael I. Bruce,*
,‡
and Claude Lapinte*
,†
†
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite ́ de Rennes 1, F-35042 Rennes, France
‡
School of Chemistry and Physics, University of Adelaide, Adelaide, South Australia 5005, Australia
* S Supporting Information
ABSTRACT: The binuclear complexes Cp*(dppe)FeC
CCCCCM(dppe)Cp* (6, M = Fe; 8, M = Ru) were
obtained in good yield by treatment of the iron chloro
complex Cp*(dppe)Fe-Cl (5) in the presence of KF with the
bis(silylated) hexatriyne Me
3
SiCCCCCCSiMe
3
and
the ruthenium complex Cp*(dppe)RuCCCCC
CSiMe
3
(7), respectively. The oxidized species 6(PF
6
)
n
(n =
1, 2) and 8(PF
6
) were obtained in ca. 80% yield by treatment
of the parent neutral compounds with 1 or 2 equiv of [Cp
2
Fe](PF
6
) in THF or dichloromethane at -78 °C. The CV of these
compounds show three reversible waves with a separation larger than 0.5 V. The salts 6(PF
6
)
n
(n = 1, 2), and 8(PF
6
) were
characterized by XRD. Quantum chemistry calculations performed at the DFT level on the oxidized species show a strong
contribution of the -C
6
- spacer to the delocalization of the spin density. IR spectra analyzed with the support of TD-DFT
calculations are consistent with the delocalization of the odd electron on the fast IR time scale for the two mixed-valence
complexes 6(PF
6
) and 8(PF
6
). Combined ESR measurements on rigid glass and on single crystal samples clearly establish that
the electronic properties of MV species and particularly their magnetic anisotropies depend on the conformation of the
molecules. In the case of the doubly oxidized species 6(PF
6
)
2
, which carries two unpaired electrons, it is shown that the singlet vs
triplet ground states can be inverted by the rotation of one metal end with respect to the other around the all-carbon chain axis.
Very strong NIR bands are found for the symmetric 6(PF
6
) and nonsymmetric 8(PF
6
) MV (mixed-valence) derivatives allowing
the determination of very large electronic couplings (H
ab
= 3070 and 4025 cm
-1
, respectively).
■
INTRODUCTION
A wide variety of organometallic molecular wires in which two
redox-active metal termini are connected through a bridging
ligand has been developed. The wire-like performance of these
linear species has been investigated in detail by means of
various physical evaluation methods
1-4
including rare measure-
ments at the molecular level.
5
Among these molecular wires, polyynediyl complexes,
[Cp′(dppe)M(CC)
m
M(dppe)Cp′]
n+
(X
-
)
n
[Cp′ = Cp
(cyclopentadienyl), Cp* (pentamethylcyclopentadienyl); M =
Fe, Ru; X = PF
6
, BF
4
], exhibit the best performance with
respect to interaction between the two metal centers through
the bridge (see examples in Chart 1).
6-11
These compounds are ideally suited for conveying electronic
coupling between the two redox centers. These assemblies
usually proved to be stable (and isolable) in different oxidation
states for short carbon bridges (m = 1, 2).
12-15
However,
previous studies have shown that the oxidized species from
complexes with longer polyyne linkers have poor chemical
stability.
15,16
Indeed, characterized mixed-valence (MV) com-
plexes containing a carbon chain longer than C
4
are extremely
rare. Up to now, there were no examples of MV with a C
6
-
bridge,
17
and the complex [Cp*(dppe)Fe(CC)
4
Fe-
(dppe)Cp*]
n+
(PF
6
-
)
n
is the unique example of a kinetically
stable mixed-valence complex with a C
8
-bridge.
8,17
Recently, guided by stimulating results on the reactivity of
mononuclear iron and ruthenium complexes containing
butadiynyl ligands,
18,19
we have investigated the reactivity of
the symmetric mixed-valence complex [Cp(dppe)Ru(C
C)
3
Ru(dppe)Cp](PF
6
)(3(PF
6
)). Despite a large compro-
portionation constant determined from voltammetry experi-
ments, thus establishing the thermodynamic stability of 3
+
, the
mixed-valent radical cation could not be isolated. In situ ESR
characterization of 3(PF
6
) confirmed full delocalization, and
thus its classification as a Robin-Day
20
Class-III complex.
However, the compound is kinetically unstable since above -10
°C an oxidative dimerization takes place providing the
tetraruthenium complex 4(PF
6
)
2
in essentially quantitative
yield (Chart 2).
21
The reaction is regiospecific with the
asymmetric isomer resulting from the initial (C
α
+C
γ
) coupling
as the unique product of the reaction.
Received: March 26, 2014
Published: May 7, 2014
Article
pubs.acs.org/Organometallics
© 2014 American Chemical Society 2613 dx.doi.org/10.1021/om500328y | Organometallics 2014, 33, 2613-2627