FULL PAPER
DOI:10.1002/ejic.201402022
CLUSTER
ISSUE
Synthesis and Properties of a Mixed-Valence Compound
with Single-Step Tunneling and Multiple-Step Hopping
Behavior
Rim Makhoul,
[a]
Yuki Kumamoto,
[b]
Akira Miyazaki,*
[b]
Frédéric Justaud,
[a]
Frédéric Gendron,
[a]
Jean-François Halet,
[a]
Jean-René Hamon,*
[a]
and Claude Lapinte*
[a]
Keywords: Mixed-valent compounds / Hybrid materials / Iron / Molecular electronics / Conjugated systems
The organic precursor bis(trimethylsilylethynyl)TTFMe
2
(3,
TTF = tetrathiafulvalene) was prepared as a 1:1 mixture of
the cis and trans isomers. Pure samples of 3-cis and 3-trans
were obtained by crystallization and identified by XRD
analysis. The treatment of pure 3-trans and a 1:1 mixture of
3-cis/trans with (i) potassium carbonate, (ii) the iron complex
Cp*(dppe)FeCl [5, Cp* = η
5
-C
5
Me
5
, dppe = 1,2-bis(diphenyl-
phosphanyl)ethane] in the presence of KPF
6
, and (iii) tBuOK
provided Cp*(dppe)Fe–CC–TTFMe
2
–CC–Fe(dppe)Cp*
as the pure geometric isomer 6-trans (85 %) and as the 60:40
Introduction
Dimetallic complexes with a π-conjugated carbon bridge
have attracted great attention in recent years because of
their potential applications in molecular electronics as
building units in molecular scale electronic materials and
nanotechnological devices. Much of this interest in these
compounds, which are often called molecular organometal-
lic wires, has been prompted by the rich redox chemistry
of different metal termini
[1,2]
and the efficiency of diverse
bridges
[2,3]
to promote electronic communication in mixed-
valence species, that is, compounds with two or more redox
sites in different oxidation states. We have shown that spe-
cies containing redox-active Cp*(dppe)Fe fragments [Cp* =
η
5
-C
5
Me
5
, dppe = 1,2-bis(diphenylphosphanyl)ethane]
linked to π-conjugated polyynediyl ligands are ideally suited
for studies of electronic and magnetic coupling between the
redox centers.
[4–6]
These assemblies are usually stable (and
isolable) in different redox states, and the polyynediyl linker
[a] Institut des Sciences Chimiques de Rennes, UMR 6226,
CNRS – Université de Rennes 1,
Campus de Beaulieu, 35042 Rennes Cedex, France
E-mail: jean-rene.hamon@univ-rennes1.fr
claude.lapinte@univ-rennes1.fr
[b] Department of Environmental Applied Chemistry, University
of Toyama,
Toyama 930-8555, Japan
E-mail: miyazaki@eng.u-toyama.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402022.
Eur. J. Inorg. Chem. 2014, 3899–3911 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3899
mixture 6-cis/trans (63%), respectively. The oxidation of 6-
trans with [(C
5
H
5
)
2
Fe]PF
6
gave [6-trans][PF
6
]
n
(n = 1–3). Vis-
ible, IR, near-IR (NIR), and electron paramagnetic resonance
(EPR) spectroscopy together with DFT data show that [6-
trans][PF
6
] is a class II mixed-valence complex (H
ab
=
85 cm
–1
) in which the spin distribution depends on the con-
formation of the molecule. Intramolecular electron transfer
occurs through single-step tunneling and a multistep hoping
mechanism. The triplet state is thermally accessible for [6-
trans][PF
6
]
2
.
is a particularly efficient mediator of electron transfer from
one end to the other. The reason for this efficiency is the
good match in energy between the Fe dπ orbitals and the π
highest occupied molecular orbitals (HOMOs) of the car-
bon bridging ligand, which permits superexchange electron-
transfer processes.
[7]
However, previous studies have demon-
strated that the length expansion of polyynediyl linkers is
limited by the poor chemical stability of the oxidized spe-
cies.
[7,8]
Therefore, the introduction of aromatic rings such
as thiophene or benzene {[Cp*(dppe)Fe–CC–(1,4-C
6
H
4
)–
CC–Fe(dppe)Cp*][PF
6
]
n
(7[PF
6
]
n
)(n = 0–2) constitutes a
well-known example}
[9,10]
in the polyynediyl spacer is an
attractive alternative to circumvent this instability as well
as to tune the physical properties of the assemblies.
[9,11,12]
Consequently, the various new nonlinear geometries that
can be envisioned for different carbon-rich spacers lead to
structural variation, which can deeply modify the electronic
and magnetic properties of the molecules both in the bulk
and at the single-molecule level.
[5,7,13]
The incorporation of functional groups into the bridge
between the redox centers and control of the molecular top-
ology open routes to functional molecules with oriented
electron-transfer capabilities,
[14,15]
switching abilities,
[16]
multipath connections,
[12,17]
optical activities,
[18,19]
or mag-
netic properties.
[5,12,20]
Recently, it was shown that the 1',1'''-
biferrocenyl (bfc) unit in the family of complexes [Cp*-
(dppe)Fe–CC–bfc–CC–Fe(dppe)Cp*][PF
6
]
n
(8[PF
6
]
n
, n =
0–4) acts as a molecular relay and mediates electron transfer