Redox control of rotary motions in ferrocene-based elemental ball bearings

Article abstract of DOI:10.1021/ja209766e

Rotational motions of ferrocene-based carousels have been achieved by electron transfer centered on π-dimerizable 4,4′-bipyridinium substituents introduced on both cyclopentadienyl rings through covalent linkers of different size, geometry, and flexibilit

Full text of DOI:10.1021/ja209766e

Article
pubs.acs.org/JACS
Redox Control of Rotary Motions in Ferrocene-Based Elemental Ball
Bearings
Adriana Iordache,† Mircea Oltean,‡ Anne Milet,‡ Fabrice Thomas,† Benoît Baptiste,§ Eric Saint-Aman,†
and Christophe Bucher*,†
‡
§
†Laboratoire de Chimie Inorganique Red
́ ́ ́
ox, Laboratoire de Chimie Theorique, and Service de Crystallographie, Departement de
Chimie Moleculaire, Universite Joseph Fourier/CNRS, Grenoble, France
́
́
S
* Supporting Information
ABSTRACT: Rotational motions of ferrocene-based carousels have been achieved by electron transfer centered on π-
dimerizable 4,4′-bipyridinium substituents introduced on both cyclopentadienyl rings through covalent linkers of different size,
geometry, and flexibility. Detailed spectroscopic, electrochemical, and theoretical analyses demonstrate that rigid and fully
conjugated linkers allow the quantitative formation of intramolecular π-dimers resulting from optimized orbital overlaps within
the HOMO of the electrochemically generated bis-radical species. The tetra-cationic “charge-repelled” conformers, the self-
assembled π-dimers, and their electron triggered interconversions have been investigated by UV−vis, NMR, and ESR
spectroscopy, electrochemistry, X-ray diffraction analysis, and theoretical calculations. These studies support the conclusion that
the rotation of both cyclopentadienyl rings in ferrocene can be controlled electrochemically using noncovalent reversible
interactions arising from π-radical coupling processes.
serve as molecular switch, if one can reversibly flip a rotator
into different readable positions using an external impulse, or as
a machinery part, if the system is able to turn repeatedly and
periodically in the same direction.6−12
INTRODUCTION
■
The ability to reversibly trigger fast movements of large
amplitude in molecular objects has emerged in the past decade
as a major scientific objective which is mainly motivated by
exciting foreseen applications in nanosciences. From this
promising and highly competitive field has already emerged a
wide range of nanometer-scale analogues of macroscopic tools
known as molecular gears, tweezers, switches, rotors, shuttles,
brakes, and turnstiles, whose function is activated by light,
temperature, or pH or through a wide range of chemical
reactions.1 Radical cation π-interactions have, for instance, been
employed to generate mechanical motions in the form of
translation as well as circumrotation in the context of
mechanically interlocked molecules.2−4
Rotational motion is one of the most common and most
important mechanical processes involved in energy conversion
at different levels: from ATP synthases in mammalian cells,
relying on ion fluxes and rotary motions of large proteins, to a
range of macroscopic motors of fundamental importance for
mankind. The control of rotational motion about molecular
bonds is thus a primary but particularly challenging objective
which has been intensively pursued using artificial inorganic or
organic architectures.5 Depending on the targeted applications,
different types of rotary motions may be desirable. It might
Unsubstituted metallocenes, featuring two freely rotating
cyclopentadienyl ligands (Cp) linked to a metal center,13,14
have quite often been considered as archetypes of gear-wheels,
propellers, or molecular carousels.15−19 Wang or Crowley and
co-workers have, for instance, recently devised various ferrocene-
based rotors wherein intramolecular rotation is promoted
chemically, by successive protonation/deprotonation of carbox-
ylate or heteroaromatic fragments.16,17 Photochemically controlled
rotational motion of cyclopentadienyl ligands in ferrocene-based
carousels has, on the other hand, only seldom been achieved, and
most photo-operated systems have been developed by Aida and
co-workers using an azobenzene strap.20−24
In the present article we describe the first example of a
ferrocene-based redox-responsive molecular carousel whose
rotating motion can be triggered by simple electron transfer
centered on π-dimerizable bipyridinium groups introduced on
the cyclopentadienyl rings of a ferrocene skeleton.
Received: October 17, 2011
Published: December 12, 2011
© 2011 American Chemical Society
2653
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
Scheme 1. Reversible Conversion of a Generic Ferrocene-Based Pivot between an “Open” Charge-Repelled Form and a
a
“Closed” π-Dimerized State
a
X represents an organic linker.
The formation of π-bonded dimers from the spontaneous
and reversible association of π-radicals has been observed with a
wide range of polyaromatics, such as porphyrins, tetrafulva-
lenes, oligothiophenes, or viologenes.25−31 Such noncovalent
associations of π-radicals are characterized (i) by cofacial
arrangements of monomeric moieties at interplanar distances of
around 3.05 0.25 Å, matching the Cp−Cp distance found in
ferrocene,26,32,33 (ii) by diagnostic electronic absorption bands
in the near-IR region, (iii) by silent ESR signatures, and (iv) by
fast kinetics of dimerization. Observation of π-dimers usually
requires nonstandard experimental conditions (low temper-
atures or high concentrations of radicals), but their formation is
usually greatly favored in confined environments34 or using
structurally preorganized architectures35 such as the propyl-
linked bis-viologens allowing optimal overlap of two sets of
singly occupied π orbitals from two adjacent reduced
pyridinium rings.36−44
Our main motivation to carry out this work was to
demonstrate that the redox properties of π-dimerizable
bipyridiniums can be exploited to develop electron-motive
rotating modules which could prove useful as molecular
switches in electronics or as drivers for artificial molecular
motors. The concept developed in this article is illustrated in
Scheme 1. It involves the reversible conversion of a ferrocene
pivot between an “open” charge-repelled form and a “closed”
state wherein two reduced π-radicals adopt a cofacial
arrangement promoting an efficient orbital overlap in a
sandwich-like π-dimer.
has been achieved starting from ferrocene in an overall
7% yield, following a published procedure involving the
1,1′-di(chloromethyl)ferrocene 4 (Scheme 2) as a key
intermediate.48,51
The introduction of a longer alkyl chain could be achieved
through a classical homologation strategy starting from the
chloromethylated derivative 4. The latter could be converted in
five steps with an overall 25% yield into the 1,1′-di(2-
bromoethyl)ferrocene 10 via hydrolyzation of the 1,1′-
di(cyanomethyl)ferrocene 752,53 followed by reduction with
LiAlH4, mesylation of the resulting bis-alcohol 9,54 and
subsequent bromination in dry methylene chloride using
lithium bromide (Scheme 2). It needs to be mentioned that
the bis-brominated derivative 10 has previously been isolated
through a nucleophilic cyclopropane ring-opening in
spiro[2.4]hepta-4,6-diene with sodium [dicarbonyl(η5-
cyclopentadienyl)ferrate] whereas longer homologues could
be obtained by reduction of alkyl−carbonyl substituted
ferrocenes.55−57 In our hands, the direct formation of 10
from 9 could not be achieved using a classical brominating
agent such as PBr358,59 but only through the in situ formation of
a mesylated intermediate ultimately converted into the bis-
halogenated precursor 10. Reacting this intermediate with an
excess of 4,4′-bipyridine in acetonitrile produced the pyridine−
pyridinium derivative 11(PF6)2, readily isolated by simple
filtration and further quaternized using methyl iodide. The
targeted product 124+ was finally isolated as a PF6− salt through
anion metathesis by addition of aqueous KPF6 to the crude
iodide precursor dissolved in water.
RESULTS AND DISCUSSION
In a second approach, we aimed for derivatives wherein
viologens and cyclopentadienyl rings are held at short distances
through rigid connectors. As shown in Scheme 3, a benzene
linkage could be introduced using aniline substituted ferrocenes
as key intermediates. This ingenious strategy was mainly
developed by Allen in the seventies60−64 to cope with the poor
reactivity of aryl halides prohibiting the direct formation of aryl-
substituted pyridiniums.65
■
Synthesis. Viologen moieties can potentially be appended
to cyclopentadienyl (Cp) rings through a wide range of organic
linkers. Several examples of ferrocene derivatives connected,
through alkyl chains, to one 4,4′-bipyridinium moiety can be
found in the literature,45−50 but strategies allowing the
introduction of one viologen per Cp fragment have been
much less investigated.48,49,51
The monosubstituted 4-aminophenylferrocene 14 (Scheme 3)
could be obtained following previously reported proce-
dures66−69 involving the diazotization of p-nitroaniline to form
13, which was thereafter reduced in a Sn/HCl mixture. The
relative efficiency and expediency of this procedure led us to
hypothesize that a similar approach could be developed to
produce the 1,1′-bis(4-aminophenyl)ferrocene derivative 19,
bearing an aniline side group on each Cp. Increasing the ratio
The road toward meeting our objectives required selecting
linkers with suitable length, rigidity, and geometry to allow an
optimal orbital overlap in the “closed” sandwich-like species
(Scheme 1). In a first approach, we considered alkyl chains as
flexible linkers between the metallocene and the electron
deficient viologen (X in Scheme 1).
The synthesis of 6(PF6)4, featuring a simple methylenic
bridge between both bipyridiniums and the metallocene core,
2654
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
a
Scheme 2. Formation of the 1,1′-Di(2-Bromoethyl)ferrocene 10
a
Conditions: (a) n-BuLi/TMEDA/DMF; (b) NaBH4/MeOH; (c) PCl3/CH2Cl2; (d) 4,4′-bipyridine/CH3CN, KPF6 aq; (e) CH3I/CH3CN, KPF6 aq; (f)
KCN aq; (g) KOH/EtOH; (h) LiAlH4/THF; (i) MsCl/Et3N then LiBr/CH2Cl2; (j) 4,4′-bipyridine/CH3CN, KPF6 aq; (k) CH3I/CH3CN, KPF6 aq.
a
Scheme 3. Introduction of a Benzene Linkage Using Aniline Substituted Ferrocenes as Key Intermediates
a
Conditions: (a) 4-Nitroaniline/NaNO2/HCl; (b) SnCl2/HCl/EtOH; (c) 1-(2,4-dinitrophenyl)-4,4′-bipyridinium chloride, CH3CN/EtOH abs; (d)
CH3I/CH3CN/CH2Cl2; (e) n-BuLi/TMEDA/Et2O then B(OBu)3/Et2O; (f) 1-bromo-4-iodobenzene, Pd(dppf)Cl2, DME/NaOH 3 M; (g) Cu2O,
DMF/NH3 28% water = 1/1; (h) 1-(2,4-dinitrophenyl)-4,4′-bipyridinium chloride, CH3CN/EtOH abs; (i) CH3I/CH3CN/CH2Cl2.
2655
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
a
Scheme 4. Our Third Approach toward Rigidly Linked Viologen-Ferrocenes
a
Conditions: (a) LDA/THF/I2 then LDA/THF; (b) pyridine-4-boronic acid, Pd(PPh3)4/NaHCO3/DME/H2O; (c) trimethyl silylacetylene,
Pd(dppf)Cl2/THF/CuI/Et3N; (d) NaOH/MeOH; (e) t-BuLi/cyclohexane (nBu)3SnCl/THF then I2/CH2Cl2; (f) 26, Pd(dppf)Cl2/CuI/THF/
Et3N; (g) CH3I/CH3CN/CH2Cl2 KPF6 aq; (h) n-BuLi/TMEDA/Et2O (nBu)3SnCl/THF then I2/CH2Cl2; (i) 26, Pd(PPh3)4/THF/CuI/Et3N; (j)
CH3I/CH3CN/CH2Cl2, KPF6 aq Pd(dppf)Cl2 = dichloro-((bis(diphenylphosphino))ferrocenyl) palladium(II).
of diazotized p-nitroaniline to ferrocene, however, turned out to
systematically afford complex mixtures of polysubstituted
ferrocenes isolated in low yields through fastidious purifica-
tions. This failure thus led us to develop a different approach,
mainly inspired by the work of Braga and co-workers,70−72
relying on the use of ferrocene-1,1′-diboronic acid, easily
obtained from ferrocene and tributyl borate.73 This boronic
derivative (17) could be converted with a 5% yield into the
targeted 1,1′-bis(4-aminophenyl)ferrocene 19 through the
formation of the brominated intermediate 18. The formation
of aminoaryl derivatives from halogenated precursors is well
documented,74 but such a strategy has, to the best of our
knowledge, never been implemented with ferrocene derivatives.
The aniline-appended ferrocenes 14 and 19 were ultimately
converted in two steps into the targeted cationic species
16(PF6)2 and 21(PF6)4, respectively. The initial step of these
conversions requires the use of 1-(2,4-dinitrophenyl)-4,4′-
bipyridinium chloride reacting with each aniline subunit to
produce the targeted phenyl-substituted pyridine-pyridinium
derivatives 15(NO3) and 20(PF6)2, wherein the positively
charged nitrogen atom comes from the primary amine(s) in 14
or 19.75
64% yield through a Suzuki-like76 palladium-catalyzed cross-
coupling between the commercially available pyridine-4-
boronic acid and the 2-bromo-4-iodopyridine 23.77
A
Sonogashira coupling78 between 24 and trimethyl silylacetylene
subsequently afforded the intermediate 25, which was readily
deprotected to produce 26 in 73% yield. This viologen
precursor was then coupled to the iodoferrocene 2779 under
Sonogashira conditions to yield 28, which was quaternized with
methyl iodide in a mixture of CH3CN/CH2Cl2 and ultimately
submitted to an anion exchange procedure yielding the targeted
compound 29(PF6)2. A similar strategy developed from 1,1′-
diiodoferrocene (30)80−82 led to the rigidly linked bis-viologen
ferrocene 32(PF6)4.
NMR Studies. The targeted viologen-appended ferrocenes
have been characterized by NMR spectroscopy using 1D and
2D NMR experiments. Signals attributed to the ferrocene
fragment were observed between 4.0 and 4.2 ppm in the
spectrum of 12(PF6)4, between 4.2 and 4.6 ppm for 21(PF6)4,
and from 4.8 to 5.1 for the ethynyl bridged derivative 32(PF6)4.
The low-field shift experienced by these signals is unambigu-
ously attributed to the electron withdrawing character of the
bipyridinium fragments introduced on each Cp ligand. Such an
effect is further brought to light through comparison of the
latter values with the chemical shifts measured for Ha, Hb, and
Hi in the monosubstituted derivatives 16(PF6)2 or 29(PF6)2
Our third approach toward rigidly linked viologen-ferrocenes
is outlined in Scheme 4. The main objective was here to
introduce an organic connector allowing minimization of the
steric constraints between the metallocene and bipyridinium
moieties.
1
(Table 1). The number and the position of the H NMR
signals assigned to the hydrogen atoms of the pyridinium rings,
observed between 7 and 9.3 ppm, are also significantly
influenced by the nature and position of the organic linker,
introduced either directly on the nitrogen atom, as in 6(PF6)4,
The introduction of an ethynyl substituent on 4,4′-bipyridine
could be achieved in four steps involving the bromination of
one pyridine ring in the α-position to a nitrogen atom. This
novel brominated 4,4′-bipyridine (24) could be obtained in
2656
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
Table 1. 1H NMR Chemical Shifts of Protons from the Heterocyclic Fragments in 64+, DMV2+, 124+, 162+, 214+, 292+, 324+, 52+,
a
MBP+, 112+, 15+, 202+, 28, and 31
a
Recorded at 400 MHz in CD3CN except for 28 and 31, which have been recorded in CD2Cl2. Chemical shifts are reported in ppm using the solvent
residual peak as internal standard. Differences between analogous mono- and disubstituted ferrocenes are denoted Δδn (n = 1−5). DMV2+
dimethylviologen, MBP+ = 1-methyl-4,4′-bipyridinium. R1 and R2 are shown in Scheme 5.
=
12(PF6)4, and 21(PF6)4, or through one carbon atom of the
pyridine ring in 32(PF6)4.
Scheme 5. Representation of Two Rotamers of Ferrocene:
anti (α = 180°) and syn (α = 0°)
Prior to this work, several NMR investigations have
established that π−π interactions between aromatic substituents
introduced on both Cp rings of a ferrocene might favor syn-like
conformations (Scheme 5), with these intramolecular inter-
actions being most of the time revealed through low field shifts
in the aromatic region.16,83−87
1
The average resonance frequencies measured on the H
NMR spectra of the bis-viologen derivatives 6(PF6)4, 12(PF6)4,
21(PF6)4, or 32(PF6)4 result from the contribution of rotamers
and conformers in equilibrium at a given temperature.
Comparing these values with those corresponding to the
monosubstituted compounds, used as references, thus came out
as a straightforward way to assess the level of intramolecular
interactions between both bipyridinium moieties. A large
2657
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
Figure 1. Top (left) and side (right) Ortep94 views of 16(PF6)2. Hydrogen atoms have been omitted for clarity reasons. Ellipsoids are drawn at a
45% probability level.
population of molecules in syn-conformation (α ∼ 0° in
Scheme 5), imposed by intramolecular stacking between the R1
and R2 side groups, would indeed lead to significant differences
as opposed to anti-like conformations featuring non-interacting
side groups.
but it does not provide key information on their preferred
conformations. Further insights into these conformational
issues were obtained at the solid state by X-ray diffraction
analyses and by computational chemistry.
X-ray Diffraction Analyses. Monocrystals of 16(PF6)2
have been grown in acetonitrile by slow diffusion of diisopropyl
ether. The Ortep94 views depicted in Figure 1 show both
cyclopentadienes in an eclipsed conformation. The four
covalently linked cyclic subunits (Cp-Ph-Py-Py) do not adopt
a coplanar arrangement, as revealed by the large dihedral angles
of 10.5°, 44.6°, and 30.4° measured between the Cp and
benzene rings, between the benzene and the first pyridinium
ring, and between both terminal pyridinium heterocycles,
respectively.
Monocrystals of 11(I)2 have been grown by slow diffusion of
tetra-n-butylammonium iodide into a solution of 11(PF6)2 in
acetonitrile. Both Cp fragments adopt a slightly twisted
conformation with an interplanar angle of around 2.3°. The
angle measured between both bipyridinium axes (N8−N17) is
87.4°, and within each viologen, the pyridinium planes are
found almost coplanar with a dihedral angle of only 5.5°. A
careful examination of the crystal lattice furthermore reveals
rather short intermolecular distances between two adjacent
bipyridinium fragments (∼3.4 Å). This proximity, as well as the
coplanar conformation found within each viologen, indicates
that the cationic charge is efficiently delocalized over all the
bipyridinium units as compared to the case of the
monoquaternized derivative shown in Figure 2. It also suggests
that the conformation observed at the solid state is strongly
influenced by supramolecular interactions.
Monocrystals of 12(I)4 have been grown by slow diffusion of
tetra-n-butylammonium iodide into a solution of 12(PF6)4 in
acetonitrile. The Ortep views depicted in Figure 3 reveal that
the solid-state conformation of 12(I)4 is quite similar to that of
its dicationic precursor 11(I)2. The Cp ligands are found in an
eclipsed geometry (interplanar Cp−Cp angle ∼3.2°), and the
angle measured between both bipyridinium substituents (N17−
N8 and N108−N117) is 78.2° (Figure 3). As seen for 112+, the
conformation observed at the solid state is mainly imposed by a
wide range of intermolecular interactions involving the
cyclopentadiene and bipyridium units acting as a donor and
acceptor, respectively. The partial view of the crystal packing
reveals, for instance, a cofacial arrangement of both subunits
with intermolecular carbon−carbon distances as low as 3.25 Å
(Figure 3). The conformation of each molecule and their
organization in the crystal lattice is further imposed by a
compact network of hydrogen bonds involving the pyridinium
units, iodide anions, and solvent molecules (not shown in
Figure 4).
The chemical shifts of signals attributed to the viologen side
groups in the mono- and bis-substituted derivatives have been
carefully analyzed, and the data are collected in Table 1. We
find that the bipyridinium protons in the disubstituted alkyl-
linked derivatives 64+ and 124+ resonate at frequency values
matching those measured on the spectrum of 4,4′-dimethylbi-
pyridinium (DMV2+) used as a reference. A similar conclusion
can be drawn from comparison between 214+ and 162+ or
between 324+ and 292+. The picture which develops from this
close examination is that the biscationic side groups introduced
on each Cp's do not interact in acetonitrile, in agreement with
simple electrostatic laws suggesting that positively charged units
should repelled themselves and thus favor anti-like conforma-
tions. The mutual electrostatic repulsion of bipyridinium
dicationic units is a well studied phenomenon which has
been widely used, for instance by Stoddard’s group to control
mechanical motions in mechanically interlocked mole-
cules.88−91
As recently established by Bosnich and co-workers, the
conformation of polycationic 1,1′-diaryl ferrocenes is in fact
driven by the balance of π-stacking “attracting” and electrostatic
“repulsive” energies between positively charged aryl substitu-
ents introduced on the upper and lower rings of the
metallocene framework.16 To illustrate the utmost importance
of electrostatic repulsion, mainly imposed by the number of
positive charges borne by each substituents, the authors report
that in solvents of high dielectric constant, such as CH3CN, the
amount of torque generated by electrostatic repulsion in
dicationic systems is insufficient to drive the two substituents
away from populations where some π-overlap occurs. As an
extension of this statement, our findings now demonstrate that
the population of π-stacked states is negligible for tetra-cationic
1,1′-disubtituted ferrocenes, with the electrostatic repulsion
being in the latter case much larger than the van der Waals
attractive forces involved in π stacked assemblies. As expected
from these conclusions, a different behavior is observed with
1
the nonquaternized derivative 31, showing aromatic H NMR
signals significantly shielded as compared to the same
resonances in the monosubstituted derivative 28. Shielding is
strong for Hc (Δδ ∼ 0.27 ppm) and Hd (Δδ∼ 0.23) and weak
for Hf (Δδ ∼ 0.08), in agreement with a nonsymmetric
conformation wherein the closest pyridine to the Cp ring
experiences the greatest overlap. Such an arrangement is,
however, not surprising, knowing that the interactions between
dipolar or quadripolar moments of aromatic fragments are
known to favor offset-stacked geometries and disfavor perfectly
stacked ones.92,93
Overall, these results are in agreement with the absence of
intramolecular interactions between the bipyridium units, but
they can hardly be utilized as definitive proofs, revealing what is
the preferred conformation of the bis-viologen derivatives in
solution. Crystal packing effects and intermolecular interactions
1
This H NMR study thus reveals that the tetra-cationic
derivatives do not adopt syn-like arrangements in acetonitrile,
2658
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
Figure 2. Ortep94 views of 11(I)2. Top (left) and partial (right) views of the crystal packing. Hydrogen atoms have been omitted for clarity reasons.
Ellipsoids are drawn at a 50% probability level.
Figure 3. Ortep94 views of 12(I)4. Top (left) and partial (right) views of the crystal packing. Hydrogen atoms have been omitted for clarity reasons.
Ellipsoids are drawn at a 50% probability level.
Figure 4. (A) Voltammetric curves of a DMF (TBAP 0.1 M) solution of 12(PF6)4 (5 × 10−4 M) recorded at (solid line) a stationary platinum
working electrode (Ø = 2 mm, E vs Ag/Ag+ (10−2 M), ν = 0.1 V·s−1) and (dotted line) at a rotating platinum disk electrode (Ø = 2 mm, ν = 0.01
V·s−1, 550 rd/min). (B) Voltammetric curves of DMF (TBAP 0.1 M) solutions of 16(PF6)2 (solid line) and 21(PF6)4 (dotted line) (5 × 10−4 M)
recorded at (solid line) a stationary platinum working electrode (Ø = 2 mm, E vs Ag/Ag+ (10−2 M), ν = 0.1 V·s−1).
involved at the solid state are clearly too important to allow a
meaningful comparison with data collected in liquid phases.
Electrochemical Investigations. The electrochemical
activity of the mono- and disubstituted ferrocenes has been
studied in DMF (0.1 M TBAP) using cyclic voltammetry (CV)
and voltammetry at rotating disk electrodes (RDE). The CV
curves exhibit similar patterns showing both well-known
consecutive viologen-centered reversible reductions as well as,
in the positive potential domain, the reversible one-electron
oxidation of ferrocene (Figure 4). The number of electrons
exchanged in each process is easily estimated through RDE
measurements (dotted lines in Figure 4A), yielding three
diffusion waves whose relative intensities are directly propor-
tional to the number of viologens and ferrocenes in the
structure. The electrochemical data corresponding to each
derivative are collected in Table 2.
It should be pointed out that the half-wave potential of
ferrocene varies over a large domain from 0.08 V for 124+ up to
0.57 V measured with 324+. We believe that these changes
observed in the series result from a combination of “through
bond” and “through space” electron withdrawing effects of the
bipyridinium fragments on the ferrocene center. In 324+, the
ethynyl linkers introduced between the Cp rings and the
deficient bipyridinium units allow an efficient electron
2659
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
a
Table 2. Half Wave Potential (E1/2) Measured by Cyclic Voltammetry
d
e
E11/2 (V) V2+/V+•
ΔEp1 (mV) V2+/V+•
E12/2 (V) V+•/V0
E13/2 (V) Fc2+/Fc3+
ΔE0 (mV) (V2+/V+•
)
KDisp (×10−3)
KDim
DMV(PF6)2
6(PF6)4
−0.83(1)
−0.86(2)
−0.86(2)
−0.78(1)
−0.69(2)
−0.84(1)
−0.74(2)
64
70
64
62
39
64
46
−1.19(1)
−1.24(2)
−1.25(2)
−1.10(1)
b
0.34(1)
0.08(1)
0.11(1)
0.18(1)
48
42
155
196
12(PF6)4
16(PF6)2
21(PF6)4
29(PF6)2
32(PF6)4
−15
1818
613
1248
2756
c
−1.18(1)
−1.24(2)
0.32(1)
c
0.57(1)
13
a
Measured by CV, 5 × 10−4 M in DMF + TBAP (0.1 M), platinum working electrode Ø = 2 mm, E vs Ag/Ag+ (10−2 M), 298 K, CV: ν = 0.1 V·s−1;
RDE: ν = 0.01 V·s−1, 550 rd/min; ΔEp1 measured at ν = 0.02 V·s−1 using an automatic ohmic drop compensation procedure. ΔE0 = E10_1 − E10_2 was
estimated using a method established by Richardson et al.95 The number of electrons transferred for each electrochemical process is shown between
b
c
d
parentheses. Adsorption phenomena lead to ill-behaved voltammograms. Irreversible wave. KDisp calculated from log10(Kdisp) = − ΔE0/0.059.95,96
e
KDim estimated from Kdim = exp[(E11/2 − (E11/2)ref)/(RT/nF)] (see Supporting Information).
delocalization over the entire molecule; the shift of the
ferrocene oxidation potential toward more positive values
thus more likely results from this conjugation, which greatly
amplifies the bipyridiniums' withdrawing effects and consid-
erably lowers the electron density on the metallic center. This
effect is not so important with the less conjugated phenyl-
linked derivative 214+ and almost negligible with 124+, featuring
a saturated ethylene linker which limits the extent of the
electronic communication between the bipyridinium and
ferrocene centers. Following a similar reasoning, we believe
that the large positive oxidation potential measured with 64+
mainly results from “through space” electrostatic effects
imposed by the structure of the methylene linker leading to
relatively short distances between the dicationic bipyridiniums
and the ferrocene centers.
electrochemical data, in marked contrast with those reported
above for the alkyl-linked ferrocene-viologens, can be explained
considering a complex mechanism involving coupled electro-
chemical (E) and chemical (C) processes. The electrochemical
behavior of molecules with multiple redox centers has already
been thoroughly investigated.98,99 It has been demonstrated
that electron transfers to or from molecules containing
identical, non-interacting, electroactive centers should yield a
single current−potential CV curve similar to that observed with
a single one electron electroactive center (ΔEp = 58 mV at
25 °C) but with a magnitude determined by the total number
of redox centers. When each center is characterized by the same
standard potential Em0 and adheres to the Nernst equation, it is
possible to calculate the formal potentials corresponding to
each pair of n successive oxidation states of the multicenter
molecules. Considering fully non-interacting centers, the
theoretical shift between the first viologen-based formal
reduction potentials (E10_1 and E10_2 in Figure 5) should thus
We also find that the redox signature of N,N′-dimethyl-4,4′-
bipyridinium (DMV2+), studied as a reference featuring two
consecutive reversible one-electron reduction waves, exhibits
strong similarities with that of the alkyl-linked ferrocene
1
viologens 64+ and 124+. The E11/2 and ΔEp values measured
for these derivatives indeed suggest that the “communication”
between the bipyridinium and ferrocene groups is negligible
and that each redox-active subunit behaves as an independent
1
center. The ΔEp values found between 70 and 64 mV for 64+
and 124+, moreover, not only fall within the same range, they
also do correspond to what is expected for molecules featuring
two chemically equivalent and non-interacting redox-active
centers. In other words, these electrochemical data can be seen
as clear experimental evidence demonstrating that both
bipyridium moieties are not interacting in solution, neither in
their standard dicationic state nor after reduction, as cation
radical species.96,97
Figure 5. Successive one electron reductions of a generic 1,1′-
bis(bipyridinium)ferrocene L4+ (E10_1 and E10_2) and their potential
coupled chemical reactions: dimerization (KDim) and disproportiona-
tion (KDisp).
A fully different conclusion is inferred from the signature of
the “rigid” systems 214+ and 324+, wherein ferrocene and
bipyridinium moieties are connected through alkyne or phenyl
linkers: (i) The first reduction process proceeds in both cases at
significantly less negative potential values in the 1,1′-
disubstituted ferrocenes than in the monosubstituted ones.
equal ΔE10 = E10_2 − E10_1 = 35.6 mV, although both redox
processes are expected to appear as a single wave with peak
potentials satisfying ΔEp = 58 mV. Experimentally, the lowest
ΔEp value has been recorded with 214+ (ΔEp = 39 mV), in
1
which two chemically equivalent bipyridiniums, linked through a
ferrocene pivot, are successively reduced into 213+• and 212(+•)
(E10_1 and E10_2 in Figure 5). Such a low ΔEp value thus reveals
that the bipyridinium centers do not behave independently, as
postulated in the theoretical description discussed above, and
that they do interact as their reduced radical states to form the
The first viologen-centered reduction is observed at E11/2
=
−0.69 and −0.74 V for 21(PF6)4 and 32(PF6)4, respectively,
and at E11/2 = −0.78 and −0.84 V for the corresponding
monosubstituted analogues 16(PF6)2 and 29(PF6)2, respec-
1
tively (Figure 4B); (ii) The ΔEp values of 39 and 46 mV
π-dimer complexes denoted [L] in Figure 5.26,27
2+
Dim
measured on the first reduction wave of 21(PF6)4 and
32(PF6)4, respectively, are much smaller than those of 62 and
64 mV found under the same conditions with the
corresponding references 16(PF6)2 and 29(PF6)2. These
As mentioned in the introduction, these sandwich-like dimers
are, most of the time, only observed at very low temperatures
or at high concentrations of simple viologen radicals as well as
2660
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
in confined environments or with structurally preorganized
architectures.36−39,100,101
(1000 < ε < 2500 L·mol−1·cm−1).104 The connection between a
ferrocene donor and one or two organic electron-acceptors
through conjugated linkers leads to large hyperchrome and
bathochrome shifts in the visible range.105−107 It also results in
a significant photosensitivity going from moderate with the
phenyl-substituted derivatives 162+ and 214+ to relatively
important for the alkyne-linked systems 292+ and 324+. As a
consequence, these compounds were systematically handled
and studied in the absence of light.
The low ΔEp value recorded with 214+ and 324+ thus
unambiguously results from the intramolecular association
between both bipyridinium radicals (Kdim in Figure 5), which
shifts the second viologen-centered reduction (E10_2) at a less
negative potential value than the initial one (E10_1). As shown in
Figure 5, the exact mechanism involves two cathodic electron
transfer processes and two coupled chemical steps: the
intramolecular dimerization of two radicals producing a π-
dimer (Kdim) and a disproportionation equilibrium, with log
Kdisp = −ΔE0/0.059.95
In-situ spectroelectrochemical analyses were carried out to
obtain further insights into the nature of the electrogenerated
species. For these investigations, absorption spectra were
recorded periodically as the ferrocene-viologenes were
submitted to bulk electrolyses. Electrochemical reductions
of DMV2+ (up to 1 electron/molecule), 64+, and 124+ (up to
2 electrons/molecule), performed in anhydrous DMF electro-
lyte in the potentiostatic regime upon setting the potential of a
platinum working electrode at −1 V, produced similar
spectroscopic patterns with a large decrease in intensity of
the initial π−π* absorption band at the expense of new signals
growing at λmax = 400 nm (ε = 80000 L·mol−1·cm−1) and at
610 nm (ε = 30000 L·mol−1·cm−1) with shoulders from 530 to
800 nm (Figure 6). The blue color and UV−vis signature of the
In a first approximation, Kdisp, Kdim, as well as E10_2 and E10_1
have been calculated from the ΔEp and E1/2 values measured by
cyclic voltammetry.95 As shown in Table 2, the ΔE0 and KDisp
values estimated for the bis(bipyridinium) derivatives are in
agreement with our initial assumptions relying on simple
comparisons between the E1/2 and ΔEp values. The lowest
disproportionation constants are found with the alkyl linked
derivatives 64+ and 124+, behaving as an assembly of
independent redox centers, whereas both rigid species exhibit
much larger KDisp values (613 × 10−3 M−1 and 1818 × 10−3
M−1) due to more favorable intramolecular π-dimerizations
(Kdim, L2(+•) → [L] ), resulting in the displacement of the
2+
Dim
resulting solutions could thus be attributed to the non-
75
associated bipyridinium cation radical DMV+•
and bis-
disproportionation equilibrium toward the doubly reduced
species L2(+•)
.
radicals 62(+•) and 122(+•), respectively (L2(+•) in Figure 5).
The reversibility of the reduction process at the electrolysis
time scale was moreover checked by UV−vis absorption
spectroscopy and voltammetry at rotating disk electrodes upon
recovering the entire signature of the initial species after back
electron transfer.
It needs to be mentioned that the intramolecular
dimerization following the first electron transfers additionally
shifts the second reduction potential, leading to the neutral
bipyridinium species.88,102 The E12/2 value is, for instance, found
at a significantly lower value in 324+ than in the monomer 292+,
as a consequence of the stabilizing influence of the radical-
coupling involved in the π-dimerization process. A similar
potential shift is observed between 162+ and 214+ (Figure 4B),
although, in the later case, the ill-behaved shape of the second
reduction wave (V+•/V0) suggests the existence of adsorption
phenomena.
These electrochemical data thus support the notion that the
geometric and structural features of the rigid systems 214+ and
324+ allow an efficient redox-triggered rotation of the ferrocene
pivot driven by the formation of an intramolecular π-dimer.
Spectro-electrochemical Investigations. The UV−vis
absorption data of the bipyridinium-appended ferrocenes are
collected in Table 3. The alkyl-linked derivatives exhibit an
These assumptions were further confirmed upon analyzing
samples of the electrolyzed solutions by ESR spectroscopy
(Figure 7). The latter were collected after full electrochemical
reduction of each viologen fragment in 6(PF6)4, 12(PF6)4, or
DMV(PF6)2. The ESR spectra recorded at room temperature in
DMF electrolyte show one signal at g = 2.00 attributed to the
S = 1/2 organic radicals DMV+•, 62(+•), and 122(+•) (Figure 8). It
should, however, be noted that a hyperfine splitting108,109 is
observed only with DMV+•, whereas broad and unresolved
signals were obtained with the less symmetric species 62(+•)
and 122(+•)
.
These results point out the incapacity of both bipyridinium
radicals in 62(+•) and 122(+•) to self-assemble into π-dimer
complexes, at least in DMF at room temperature. In an attempt
to find experimental conditions which would favor the
dimerization, we studied the effect of temperature or the
influence of added water in the electrolyte.110 Solutions of
62(+•), 122(+•), and DMV+• in THF/DMF (50/50 v/v),
obtained by bulk electrolyses conducted at room temperature
under an argon atmosphere, were slowly cooled down to 193 K.
These drastic temperature changes turned out to have no
influence on the reference compound DMV+• and on 62(+•) as
opposed to 122(+•), whose absorption spectrum was seen to
gradually evolve upon cooling. As seen in Figure 8B, the
intensity of the main signals centered initially at 400 and 610
nm on the spectrum of 122(+•) goes down as the temperature is
lowered at the expense of new bands growing at lower
wavelengths, 360 and 520 nm, respectively, and of a new broad
signal in the near-infrared region (above 800 nm).
Table 3. Absorption Wavelengths and Extinction
Coefficients Associated with the Absorption Bands Observed
a
in the Spectra of the Ferrocene-Viologen Derivatives
λ (nm); ε (L·mol−1·cm−1)
DMV(PF6)2
6(PF6)4
264; 19500
265; 45300
266; 42000
275; 23300
266; 49000
267; 25000
265; 48200
472; 1000
477; 2300
357; 9500
355; 21000
368; 13300
368; 24000
12(PF6)4
16(PF6)2
21(PF6)4
29(PF6)2
32(PF6)4
546; 1800
565; 1700
567; 3200
541; 6200
a
All measurements have been performed in DMF electrolyte (0.1 M
TBAP) at a concentration of 10−4 M in viologen units.
intense viologen-centered π−π* transition103 in the UV range,
between 260 and 280 nm (42000 < ε < 45000 L·mol−1·cm−1),
and a less intense iron-based d−d transition around 460 nm
Similar results were obtained at room temperature using an
aqueous electrolyte. Although these investigations were greatly
hampered by the limited solubility of all the ferrocene-viologens
2661
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
Figure 6. UV−vis spectra recorded during the exhaustive one-electron reduction (per viologen) of (A) 6(PF6)4, (B) 12(PF6)4, and (C) DMV(PF6)2
in DMF (0.1 M TBAP) using a platinum plate working electrode (∼10 cm2) whose potential was fixed at Eap = −1.0 V (10−4 M in viologen units,
12 mL, l = 1 mm, t = ∼1 h).
Figure 7. X-band ESR spectra (A) DMV+•, (B) 62(+•), and (C) 122(+•), recorded at room temperature, 10−4 M in viologen subunits in DMF + TBAP
(0.1 M) (power = 4 mW, ν = 9.355 Hz, ModAmpl = 0.1 mT).
Figure 8. (A) UV−vis spectra recorded during the one-electron reduction (per viologen) of 12(PF6)4 (20 mL at ∼4 × 10−5 M in viologen)
conducted at 298 K in DMF/THF 50/50 (v/v) + TBAP (0.1 M) using a platinum working electrode (∼10 cm2, Eap = −0.9 V, t ∼ 1.4 h) and a
10 mm all quartz immersion probe. (B) UV−vis spectra of 122(+•) recorded in DMF/THF 50/50 (v/v) + TBAP (0.1 M) at 298 K (solid line) and
after cooling at 193 K (dotted line). (C) UV−vis spectra recorded during the one-electron reduction (per viologen) of 12(PF6)4 (6 × 10−5 M in
viologen) conducted at 298 K in H2O/DMF 90/10 (v/v) + KNO3 (0.1 M) using a carbon working electrode (Eap = −1 V, t ∼30 min) and a 10 mm
immersion probe.
in water, their exhaustive electrochemical reduction could be
successfully achieved in a water/DMF mixture (90/10: v/v)
using KNO3 as a supporting electrolyte and a carbone foam as
working electrode. Every attempt to increase the water ratio
systematically led to passivation phenomena resulting from the
physisorption of the electrogenerated cation radicals, which are
far less polar than the starting materials. Here again, only
experiments carried out with the ethylene bridged derivative
124+ yielded spectroscopic signatures suggesting the formation
of a novel species, i.e. showing absorption bands at different
wavelengths than those expected for a simple bipyridinium
radical. As shown in Figure 8C, the spectrum recorded after
reduction of 124+ carried out in a water/DMF electrolyte is
almost identical to that obtained at low temperature (Figure 8B),
with an overall blue shift of the signals corresponding to the
bis-radical species 122(+•) and a new broad absorption band
above 800 nm. The spectroscopic signatures observed at low
temperature or in aqueous media are thus in accordance with
2+
the formation of the dimerized species [12]
from the bis-
Dim
radical 122(+•), but the magnitude of the observed modifications
also reveals that the associated equilibrium (KDim, Figure 5) is
not fully displaced under these conditions and that the self-
2+
Dim
associated species [12]
and the nonassociated form 122(+•)
coexist in solution. It also needs to be mentioned that the
2662
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
Figure 9. UV−vis spectra recorded during the exhaustive one-electron reduction (per viologen) of (A) 16(PF6)2 (Eap = −0.90 V), (B) 21(PF6)4
2+
(Eap = −0.88 V), (C) 29(PF6)2 (Eap = −0.95 V), or (D) 32(PF6)4 (Eap = −0.95 V) or (E) superimposition of 16+• (solid line) and [21] (dashed
Dim
2+
line) and (F) superimposition of 29+• (solid line) and [32] (dashed line). All these experiments have been conducted at room temperature on
Dim
∼10 mL of a given receptor (10−4 M in viologen subunits) dissolved in DMF + TBAP (0.1 M), using a platinum plate working electrode (∼10 cm2,
duration of the experiments ∼1 h) and a 1 mm all quartz immersion probe.
intramolecular nature of this association process is supported
by the fact that no modifications of the UV−vis absorption
spectrum could be observed under the same experimental
conditions with the reference compound DMV+•, neither at
low temperature nor in an aqueous electrolyte.
visible range at 415 nm (ε = 26500 L·mol−1·cm−1) and between
570 and 720 nm (ε = 18000 L·mol−1·cm−1) (Figure 9A). These
changes are thus easily attributed to the simple and clean
transformation of 162+ into 16+•, proceeding via a well-defined
isosbestic point at 380 nm (Figure 9A).
Similar investigations were carried out with the phenyl- and
ethynyl-linked ferrocene-viologens 162+, 214+, 292+, and 324+,
expecting that rigid and less bulky linkers would allow an
efficient intramolecular dimerization of the bipyridinium
radicals, a process required to stabilize the syn conformation
of the targeted electrochemically driven pivot. The exhaustive
one electron reduction of the reference phenyl-linked mono
viologen-ferrocene 16(PF6)2 led to drastic changes in its
absorption spectra; the most remarkable ones are reminiscent
of those observed when producing the cation radical DMV+•
(Figure 6), being the growth of intense absorption bands in the
The same experiment carried out with the disubstituted ana-
logue 21(PF6)4 yielded a fully different spectrum (Figure 9B)
showing bands gradually developing at λmax = 410 nm (ε =
54000 L·mol−1·cm−1) and 570 nm (ε = 38000 L·mol−1·cm−1),
blue-shifted as compared to those observed through the
reduction of 16(PF6)2 (both final spectra are represented in
Figure 9E). Most importantly, the reduction of 214+ also
yielded a broad diagnostic band for the presence of π-dimer
centered at 980 nm (ε = 7400 L·mol−1·cm−1), whose energy is
directly related to the electronic-coupling between both
bipyridinium radicals.26,75,111 This spectrocopic imprint is
2663
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
Figure 10. X-band ESR spectra of (A) 16+• (solid line) and [212+]dim (dotted line); (B) 29+• (solid line) and [322+]dim (dotted line) recorded at
room temperature, 10−4 M in viologen subunits dissolved in DMF + TBAP (0.1 M) (power = 4 mW, frequency = 9.355 Hz, ModAmpl = 0.1 mT).
Figure 11. Side view of 64+ optimized at the CAM-BLYP/6-31G* level.
thus a clear evidence of the electron-triggered dimerization
leading to [21]D2+im, with the efficiency of this process being
demonstrated (i) by the absence of signals attributed to the
nonassociated bis-radical species 212(+•) in the spectrum of the
fully electrolyzed solution and (ii) by the observation of one
well-defined isosbestic point at 355 nm, suggesting a clean and
well-behaved transformation (Figure 9B). This behavior led us
to postulate a fast and quantitative electrochemically triggered
and 550 nm in [322+]dim are blue-shifted as compared to those
found with the nonassociated radical 29+• and that the broad
diagnostic band for the π-dimer appears conversely red-shifted
as compared to the signal of lower energy found in the
spectrum of 29+•. It needs to be mentioned that the simple and
well-defined transformations of 292+ into 29+• as well as that of
324+ into [322+]dim are further demonstrated by the observation
of isosbestic points at 305, 340, 360, and 310 nm in the stacked
spectra shown in Figure 9C and D, respectively.
transformation of the starting tetra-cationic species 214+ into
2+
Dim
the π-dimer [21] without experimental evidence suggesting
The presence of paramagnetic species in each electrolyzed
medium has been checked by ESR. The spectra were recorded
at room temperature from samples collected after electro-
chemical reduction (1 electron per viologen) of 162+, 214+,
292+, and 324+. As seen in Figure 10, the mono bipyridium-
ferrocenes 16+• and 29+• feature one intense signal at g = 2.00
whereas both 1,1′-disubstituted derivatives were found to be
ESR silent, in agreement with the electrochemical and
spectroelectrochemical data discussed above, suggesting the
the existence of the intermediate bis-radical 212(+•)
.
The electrochemical reduction of the bipyridinium subunits
in the ethynyl-bridged architectures leads to a different set of
signals (Figure 9C and D). The electrolysis of the reference
compound 292+ into 29+• gives rise to an intense and thin
signal at 400 nm (ε = 27000 L·mol−1·cm−1) growing along with
a larger one centered at 650 nm (ε = 11500 L·mol−1·cm−1) as
well as two bands of weak intensity at 840 nm (ε = 3800
L·mol−1·cm−1) and 950 nm (ε = 2600 L·mol−1·cm−1). The
observation of signals over 800 nm in the spectrum of 29+• is
quite unexpected, since only the π-dimer-diagnostic bands have
so far been observed in the near IR region.36,38−40,44 This
bathochromic shift, in fact, results from the lowering of the
π−π* HOMO−LUMO band gap due to the large electronic
delocalization occurring between the alkyne-linked donor and
acceptor subunits in 29+•.112
The spectroscopic signature recorded under the same
experimental conditions with the 1,1′-bis(ethynylviologene)-
ferrocene 324+ shows two main absorption bands at 375 nm
(ε = 45500 L·mol−1·cm−1) and 550 nm (ε = 19700 L·mol−1·cm−1)
(Figure 9D), blue-shifted as compared to the signals of the
reference radical 29+•, as well as a broad signal centered at
1050 nm. Here again, this spectroscopic response is attributed to
a fast and unequivocal formation of the π-dimer [322+]dim from
the tetra-cationic derivative 324+. The superimposition shown
in Figure 9F further reveals that the transitions at λmax = 375
formation of the diamagnetic π-dimers [212+]dim and [322+]dim
.
Computational Chemistry. The present study aims at
determining, from a computational point of view, whether the
redox properties of the viologens subunits in 64+, 124+, 214+,
and 324+ can be exploited to promote the rotation of a
ferrocene scaffold from an “open” charge-repelled form to a
“closed” conformation, wherein the π-orbitals of two
bipyridinium cation-radicals would overlap in a face-to-face
arrangement (Scheme 1).
Geometry optimization of the nonreduced species 64+, 124+,
214+, and 324+ conducted in the gas phase at the CAM-
B3LYP113 level with the 6-31* basis set (see the Experimental
Section and Supporting Information for more detailed
information on the calculation procedures) led invariantly to
open structures wherein both cyclopentadienes adopt a
staggered conformation with both dicationic pyridinium
substituents pointing toward opposite directions, as seen on
the optimized structure of 64+ depicted in Figure 11. 64+, 124+,
2664
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
214+, and 324+ were all found in open conformations, but their
exact structure can only be accurately described using the
dihedral angle α defined by the four carbon atoms involved in
the ferrocene substitution. For 64+ (C25−C14−C23−C25′ in
Figure 11) and for 324+, this angle was estimated at around
180° (anti conformation) while much lower values were found
for 124+ (165°) or 214+ (169°) (Table 4).
Table 4. α-Angle Values in Gas-Phase DMF (in deg)
a
Calculated for the Nonreduced Species (X4+), the
Reduced/Nondimerized Species ([X]2(+•)
open ), and the
)
b
2+
Reduced/Dimerized Species ([X]dim
α(X4+)
α([X]
)
α([X]dim
)
2(+•)
2+
open
Figure 12. Potential energy curve calculated for 214+ in DMF. The red
squares are the relaxed scan optimization calculated from 0° to 180°.
The blue squares have been obtained considering the C2 symmetry of
the complex. F(x) represents the functional form of the potential with
F(x) = A(1 + cos(Bx + C)) used for fitting.
21
32
12
6
169° (144°)
180° (142°)
165° (141°)
179° (177°)
144° (146°)
143°
8° (2°)
0°
147°
179°
0°
30°
a
b
CAM-B3LYP113/6-31G*. Values close to 180° or 0° correspond to
staggered conformations whereas values close to 144° correspond to
eclipsed conformations.
Detailed theoretical analyses were then carried out on the
singlet spin state of the doubly reduced species considered
2(+•)
either in open conformations, denoted [X]
(α values found
open
Calculations were then conducted upon considering DMF as
solvent in order to provide theoretical support to experimental
data collected in this medium. Under these conditions,
optimization of each nonreduced species afforded open
conformations with α-angle values found significantly lower
than those measured in the gas phase (Table 4). For instance,
angles measured on the minimized conformations of 214+
considered in the gas phase and in solution were estimated at
169 and 144°, respectively, with the difference being attributed
to the shielding effect of DMF, which decreases the charge
repulsion between both solvated dicationic bipyridinium
substituents. As opposed to the staggered conformation
found in the gas-phase, the angle calculated in DMF (α =
144°) corresponds to a situation wherein both iron-linked
cyclopentadienes adopt a fully eclipsed arrangement. It should
be recalled that the latter is known to be the most stable form
of ferrocene (Scheme 5, R1 = R2 = H), although the associated
stabilization energy has been found by electron diffraction
studies to be only ∼1 kcal/mol.114 This energy gap has been
confirmed in the present study with calculations conducted on
ferrocene at the CAM-B3LYP level yielding a stabilization for
the eclipsed conformation of 0.55 and 0.57 kcal/mol in the gas
phase and DMF solvent, respectively.
The relative energies corresponding to a number of selected
conformations adopted by 214+ in DMF were determined by a
relaxed scan calculation procedure involving setting the
dihedral angle between both bipyridinium substituents (α in
Scheme 5). The energy calculated for α = 0° (E0) was used as a
reference to plot the potential energy surface ΔE = E − E0 as a
function of the dihedral angle α. As seen in Figure 12, a
minimum is observed when α = 144° and the energy difference
found between this potential well and the less favorable
conformation (α = 0°) was found to reach 3.34 kcal/mol.
Fitting these values using the force field Fourier function
truncated at the leading term F(x) = A(1 + cos(Bx + C)
(dashed line in Figure 12) allowed us to estimate A =
−1.67481, B = 1.27674, and C = 3.14155. As seen with the
unsubstituted ferrocene, the minimum found on the potential
energy surface corresponds to an eclipsed conformation of both
cyclopentadienes.
for each “open” species are collected in Table 4), or as
π-dimerized conformers, denoted [X]dim (α = 0°).115,116
2+
2(+•)
α-Angles measured on the minimized conformations [X]
open
were found to range between 144° and 147°, except for the
2(+•)
methylene-linked derivative [6]
, exhibiting a much larger α
open
value of 179° (Table 4). Compound [6]o2p(e+n•), furthermore, exhibits
two cyclopentadiene rings (Cp) in a staggered conformation, as
opposed to [12]2(+•)
open , [21] , and [32] , featuring eclipsed
2(+•)
2(+•)
open open
Cp’s in their minimized states (147 ≥ α ≥ 143°). The antiparallel
orientation (α = 179°) of both bipyridinium substituents in
[6]o2p(+en•) may to some extent be seen as a consequence of a larger
charge repulsion effect resulting from the short length of the
methylene linker. In the minimized structures, the bisbipyridium
cations are furthermore found perpendicular to the Cp rings in
2(+•)
2(+•)
[12]
and [6]
whereas they appear almost parallel to the
open
open
2(+•)
2(+•)
Cp rings in [21]
and [32]
.
open
open
The large differences seen in Table 5 between the ΔE values
calculated for the reduced species in the gas phase and in
2+
dim
solution reveal that the closed dimeric conformations [X]
are strongly favored by solvent effects, with the largest
stabilization being observed for the singlet state π-dimer
[21]d2i(m+•). While looking for parameters that could explain these
theoretical results, we found that the large stabilization of the
dimeric forms in DMF can be attributed to polarity issues.
Dipolar moments computed in DMF solution, ranging from 16
2+
dim
2(+•)
to 37 D for [X] and only from 1 to 9 D for [X]
, indeed
open
reveal that the largest solvent effects (ΔEDMF − ΔEGP) are
observed with the most polar species, probably as the results of
strong dipole−dipole and ion−dipole interactions occurring
with polar DMF molecules (the dipolar moments calculated for
2+
[21]o2p(+en•) and [21]dim are represented in Figure 13). In the gas
2+
dim
2+
dim
2+
dim
phase, [32] , [21] , and [6] are found to be the most
stable forms by only −8.06, −2.26, and −2.15 kcal/mol,
respectively, while, in DMF, their stabilization energies range
from −17 to −28 kcal/mol (Table 5). The open conformation
2(+•)
[X]
is only found as the most stable form for the ethyl
open
linked ferrocene-viologen (X = 12) in the gas phase (ΔE =
+1.55 kcal/mol), but here again, inclusion of DMF allows
2+
stabilization of the dimer [12]dim . In DMF, the lowest ΔE
values have been obtained with the alkyl-linked derivatives 62+
and 122+, as the result of steric repulsion issues prohibiting both
2665
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
a
2(+•)
2+
Table 5. Energy Differences Calculated between the Open ([X]
, α = 144−179°, S = 0) and π-Dimerized ([X]dim , α = 0°,
open
S = 0) Forms of the Doubly Reduced Species 62(+•), 122(+•), 212(+•), and 322(+•) in the Gas Phase (ΔEGP) and in DMF Solution
b
(ΔEDMF
)
total dipolar moment in gas
phase (Debye)
total dipolar moment in
DMF (Debye)
2+
[X]
dim
c
c
2(+•)
2(+•)
2+
dim
X
ΔEGP = Edim − Eopen (kcal/mol) ΔEDMF = Edim open (kcal/mol) ΔEGP − ΔEDMF (kcal/mol)
α = 0°
[X]
[X]
α = 0°
[X]
open
open
21
−2.26
−8.04
+1.55
−2.15
−27.81
−27.74
−21.11
−18.47
25.55
19.70
22.66
16.32
28.4
8.2 (20.2)
α = 144°
5.6 (10.2)
α = 143°
3.0 (19.8)
α = 147°
0.2 (15.9)
α = 179°
36.9
9.3 (27.6)
6.4 (15.3)
2.7 (24)
32
12
6
15.8
22.8
16.1
21.7
26.7
19.3
1.1 (18.2)
a
b
c
CAM-B3LYP113/6-31G*. The closed conformation is favored when ΔE = Edim(0°) − Eopen(α) < 0. For each entry, the difference between the
dipolar moments calculated for the open ([X]o2p(+en•)) and closed ([X]dim ) conformations is shown between parentheses.
2+
2(+•)
2+
Figure 13. Dipolar moments calculated for [21]
(α = 144°) and [21]dim
.
open
reduced bipyridium substituents from adopting a cofacial
arrangement. These steric effects are clearly revealed on the
solvent leads to an important stabilization of the closed
conformation, with the largest ΔE values systematically found
for 214+ and 324+. Solvent effects seem overestimated, but the
trend in the series is reliable. The phenyl and alkyne linkers are
2+
2+
minimized structures of [6]
and [12] , for instance
dim
dim
through the deformation of the ferrocene scaffold with an
2+
angle between both Cp planes reaching 12.3° in [6] whereas
not only found best suited for the formation of a cofacial
dim
2+
dim
2+
dim
much lower values, below 6°, are measured for [32] and
[21] (Table 5). The optimized form of [6] (see Figure 26
arrangement in the π-dimer complex [X] ; they also allow a
2+
dim
2+
dim
more straightforward rotation from the charge repelled state to
the π-dimer state, with an overall reorganization which is less
energetically demanding than the same processes considered
with the flexible systems 64+ or 124+. Structural data found for
of the Supporting Information), moreover, reveals that the
steric hindrances between both methylene linkers (C25−H and
C25′−H) and between the adjacent ferrocene and pyridinium
rings prohibit the formation of a fully symmetric “face to face”
arrangement of both bipyridinum cation radicals and, hence,
prevent a good overlap of their HOMO orbitals.
214+ and 324+ and for [32] and [21] indeed reveal that
2+
dim
2+
dim
the conversions between the open and closed conformation
require a large rotation of the ferrocene pivot but involve only
little changes for the bipyridium substituents (The effects of
electron doping on the angles and bond distances measured on
Similar constraints due to the bulkiness of the alkyl chains are
2+
dim
observed on the minimized structure of [12] , and all our
theoretical investigations support the experimental findings
suggesting that alkyl linkers are not suited to promote the
intramolecular π-dimerizations of both bipyridium cation
radicals. The distance between both Cp rings appears indeed
too short to avoid the repulsion between the alkyl chains
located on the upper and lower Cp rings.
the viologen units are tabulated in Table ESI 2). The result of
2+
dim
our molecular orbital analyses carried out with [21] is shown
in Figure 14. The LUMO of the open species 214+ becomes the
2+
dim
HOMO of the doubly reduced species [21] with a bonding
character associated with the C8−C5 (C8′-C5′) bonds. The
same feature is observed for the C4−C3 (C6−C7) and C9−
C10 (C12−C13) bonds, which are also found shorter in the
reduced form, as opposed to C4−C5 (C5−C6), N2−C3 (N2−
C7), and N10−C11 (N11−C12), whose antibonding character
is revealed by their elongated bond lengths (see Table 2 of the
Supporting Information). The pictures shown in Figure 14
The theoretical predictions obtained for 214+ and 324+ using
the CAM-B3LYP113/6-31* functional are in good agreement
with what is observed experimentally. The stabilization energies
found for the flexible systems 64+ and 124+ are conversely quite
different from what might be expected form the experimental
observations. Similar overestimated values have also been
obtained for the closed conformations using other DFT
functionals (see ESI), namely B3LYP and M06-2X. These
discrepancies between experiment and theory are not well
understood; however, a common feature of the data obtained
with B3LYP, CAM-B3LYP, and M06-2X is that inclusion of
further reveal that the effective orbital overlaps in the HOMO
2+
of [21] are responsible for the efficient association between
dim
2+
dim
both radicals yielding the closed π-dimer [21] . In the latter
species, the significant electronic delocalization observed over a
large π system involving the pyridinium units, the phenyl linker,
2666
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
2+
Figure 14. Frontier molecular orbitals calculated for (top) 214+ and (bottom) [21] . The electron density isosurface is 0.02 au.
dim
equipment at a rotation rate of 550 rd·min−1 using a glassy carbon
RDE tip (Ø = 3 mm).
and the ferrocene leads to a stabilization of the closed
conformation overstepping the electrostatic repulsion.
Spectroelectrochemical measurements were carried out in a
glovebox with a standard potentiostat coupled to an MCS 500 UV-
NIR Zeiss spectrophotometer using 1 mm or 10 mm all quartz
immersion probes. Electrolyses were conducted at 25 °C using a
platinum plate working electrode and a large piece of carbon felt as a
counterelectrode isolated from the electrolytic solution through an
ionic bridge. A CH instrument Ag|AgNO3 (10−2 M + TBAP 10−1 M in
CH3CN) was used as a reference electrode.
CONCLUSION
■
We have described the first example of a ferrocene-based redox-
responsive molecular carousel whose rotating motion is
triggered by simple electron transfers centered on π-dimerizable
bipyridinium fragments introduced on both Cp's. Detailed
electrochemical, theoretical, and spectroscopic analyses reveal
that formation of intramolecular π-dimers requires the
introduction of rigid, conjugated organic linkers between the
rotating metallocene module and between the electron motive
π-dimerizable drivers. Evidence for intramolecular dimerization
came from electrochemistry data, supporting the existence of
chemical steps coupled to the electron transfer processes, and
from ESR and UV−vis spectroscopy with the observation of
diagnostic absorption bands, notably in the near-infrared
region. All our conclusions were supported by computational
investigations which provided key insights into the struc-
ture of the self-assembled intramolecular π-dimers and into
the conformation of the tetra-cationic “charge-repelled”
conformers.
ESR X-band spectra were recorded on a Bruker EMX, equipped
with the ER-4192 ST Bruker cavity.
Calculations were performed using the Gaussian09 suite of
programs.117 We used the hybrid exchange-correlation functional
CAM-B3LYP113 based on a B3LYP118,119 upgraded with the long-
range corrections proposed by Tawada et al.120 For all calculations, we
used the well-known basis set 6-31G* for the C,N,H atoms and the
SDD121 pseudopotential combined with SDD122 basis sets for the Fe
atom. Each structure was optimized in the gas phase and/or in N,N-
dimethylformamide solution (DMF, ε = 37.219) using the polarizable
continuum model (PCM).123−126
Syntheses. The ferrocene-1,1′-dicarboxaldehyde 2 was synthesized
according to a procedure described by Pel
́
inski.127 The synthesis of
6(PF6)4 was carried out according to a published procedure developed
by Olivier Reynes in our laboratory.51 The 1-ferrocenyl-4-nitrobenzene
13 and 1-ferrocenylaniline 14 were synthesized following procedures
described by D’Souza and Ito.68 The syntheses of 1,1′-ferrocenyl-bis-
boronic acid 17 and 1,1′-ferrocenyl-bis(4-bromophenyl) 18 were
performed according to a procedure described by Knapp and
Rehahn.73 2-Bromo-4-iodopyridine 23,77 1-iodoferrocene 27,128 and
1,1′-diiodoferrocene 3080,82,129 were synthetized following literature
procedures.
EXPERIMENTAL SECTION
■
General Methods. 1H NMR and 13C NMR spectra were recorded
1
on Bruker 300 or 400 MHz spectrometers. H chemical shifts (ppm)
were referenced to residual solvent peaks. UV−vis spectra were
recorded on an MCS 500 UV-NIR Zeiss spectrophotometer using
conventional quartz cells or all-quartz immersion probes (Hellma
Inc.). Elemental analyses (C, H, and N) were carried out on a Perkin-
Elmer 240 analyzer. ESI or DCI positive mode spectra were recorded
with an Esquire 3000 Plus, Bruker Daltonics.
All chemicals were used as received unless otherwise noted. 4,4-
Dipyridyl (98%) and 2-bromopyridine (99%) were purchased form
Acros and Aldrich, respectively. Anhydrous acetonitrile and DMF were
purchased from Rathburn Inc. and Acros, respectively.
Cyclic voltammetry (CV) and voltamperometry at rotating disk
electrodes (RDE) were recorded using a CH-600 potentiostat (CH-
instrument). All analytical experiments were conducted under an
argon atmosphere (glovebox or argon stream) in a standard one-
compartment, three-electrode electrochemical cell. Tetra-n-butylam-
monium perchlorate (TBAP) or potassium nitrate (KNO3) was used
as supporting electrolyte (0.1 M) in nonaqueous (DMF) or aqueous
media (DMF/H2O). An ohmic drop compensation was performed
when using cyclic voltammetry. CH-instrument vitreous carbon (Ø =
3 mm) or platinum (Ø = 2 mm) working electrodes were polished
with 1 μm diamond paste before each recording. A standard sweep
rate of 0.01 V·s−1 was used in RDE experiments. Voltamperometry at
rotating disk electrode (RDE) was carried out with radiometer
Synthesis of 10. The dialcohol 9 (0.1 g, 0.4 mmol) and dry
triethylamine (4 mmol) were dissolved in anhydrous methylene
chloride (10 mL). This solution was then purged for 15 mn with argon
gas and cooled down to −5 °C using an iced bath of sodium chloride
solution. A solution of methanesulfonyl chloride (MsCl) (0.32 mL,
4 mmol) in dry CH2Cl2 (2 mL) was then added dropwise for 20 min.
The resulting mixture was stirred at −5 °C for 2 h and then overnight
at room temperature and subsequently washed with water (3 × 5 mL)
and brine (5 mL). The organic phase was dried over anhydrous
Na2SO4, filtered, and evaporated under reduced pressure. After the
crude product was dissolved in CH2Cl2 (10 mL), LiBr (1.55 g, 17.8
mmol) was added under argon, and the solution was stirred in the dark
at room temperature for 48 h. The reaction mixture was then washed
with water, dried over anhydrous Na2SO4, and evaporated under
reduced pressure. The crude product was purified by column
chromatography on SiO2 (CH2Cl2/cyclohexane = 1/1 v/v) to afford
1
10 as a yellow powder. Yield: 47% (75 mg). H NMR (400 MHz,
CD3CN) δ: 4.11 (t, 3J = 1.8 Hz, 4H), 4.07 (t, 3J = 1.8 Hz, 4H), 3.49 (t,
3
3J = 7.4 Hz, 4H), 2.88 (t, J = 7.4 Hz, 4H). 13C NMR (400 MHz,
2667
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
CD3CN) δ: 85.57, 68.75, 68.00, 33.00, 32.67. MS (ESI+): m/z 399.8
([M]+, 100%). Anal. Calc % for C14H16Br2Fe: C, 42.04; H, 4.03.
Found %: C, 42,02; H, 4,49.
The combined organic layers were dried over anhydrous Na2SO4,
filtered, and evaporated to dryness. The crude product was purified by
column chromatography on SiO2 (CHCl3/MeOH = 98/2 v/v) and
dried to afford 19 as a goldish powder. Yield: 46% (54 mg). 1H NMR
(400 MHz, CD2Cl2) δ: 7.12 (d, 3J = 8.4 Hz, 4H), 6.55 (d, 3J = 8.4 Hz,
4H), 4.32 (t, J = 4 Hz, 4H), 4.09 (t, J = 4 Hz, 4H), 3.66 (br s, 4H,
NH2). 13C NMR (101 MHz, CD2Cl2) δ: 145.38, 128.28, 127.36,
115.25, 87.48, 70.09, 67.35. MS (ESI+): m/z 369 ([M + 1]+, 100%),
185 ([M + 2]2+, 60%). Anal. Calc % for C22H20FeN2: C, 71.75; H,
5.47; N, 7.61. Found %: C, 69.41; H, 5.38; N, 7.31.
Synthesis of 11(PF6)2. To a solution of 4,4′-bipyrridine (0.9 mg, 5.8
mmol) in dry acetonitrile (3 mL) was added dropwise the dibromide
derivate 10 (0.33 g, 0.08 mmol) in dry acetonitrile (2 mL). The
reaction mixture was protected from light and stirred at reflux under
argon atmosphere for 2 days. The pink precipitate was filtered and
washed with acetonitrile. The solid compound was dissolved in a
minimum of water, and the PF6 salt was precipitated by addition of an
aqueous saturated solution of KPF6. The pink−red solid was filtered
3
3
Synthesis of 20(PF6)2. The 1,1′-ferrocenyl-bis-4-aniline 19 (0.095 g,
0.26 mmol) and the N-(2,4-dinitrophenyl)-4,4′-bipyridinium chloride
(0.555 g, 1.55 mmol) were dissolved in a mixture of CH3CN/EtOH =
1/5 (v/v) (50 mL), and the resulting solution was stirred and heated
at 80 °C for 2 days. After cooling to room temperature, the solvents
were evaporated to dryness under reduce pressure and the crude
product was purified by column chromatography on SiO2 (THF/
H2O/KPF6 = 100/20/4 v/v) and dried to afford 20(PF6)2 as a dark
red powder. Yield: 41% (0.1 g). 1H NMR (400 MHz, CD3CN) δ: 8.97
(d, 3J = 6.9 Hz, 4H), 8.86 (d, 3J = 8 Hz, 4H), 8.43 (d, 3J = 6.9 Hz, 4H),
7.82 (d, 3J = 8 Hz, 4H), 7.66 (d, 3J = 8.7 Hz, 4H), 7.57 (d, 3J = 8.7 Hz,
1
and washed with water. Yield: 87% (56.6 mg). H NMR (400 MHz,
CD3CN) δ: 8.84 (d, 3J = 6.1 Hz, 4H), 8.53 (d, 3J = 6.9 Hz, 4H), 8.23
3
(d, 3J = 6.9 Hz, 4H), 7.76 (d, J = 6.1, 4H), 4.59 (t, 3J = 7.0 Hz, 4H),
4.09 (t, 3J = 1.8 Hz, 4H), 3.96 (t, 3J = 1.8 Hz, 4H), 3.01 (t, 3J = 7.0 Hz,
4H). 13C NMR (300 MHz, CD3CN) δ: 151.13, 144.71, 125.49,
121.58, 69.02, 62.19, 30.52. MS (ESI+): m/z 679 ([M − PF6−], 20%),
276 ([M − 2PF6−]2+, 100%). Anal. Calc % for C34H32F12FeN4P2: C,
48.48; H, 3.83; N, 6.65. Found %: C, 48.54; H, 3.79; N, 6.79.
Synthesis of 12(PF6)4. To a degassed solution of 11(PF6)2 (58 mg,
0.07 mmol) in acetonitrile (10 mL) was added dropwise CH3I (1 g,
0.5 mL, 7 mmol). This solution was then stirred under an argon
atmosphere in the dark at 40 °C for 2 days. After cooling, the brown
precipitate was filtered, washed thoroughly with acetonitrile, and
dissolved into a minimum of water. The addition of an aqueous
saturated KPF6 solution (∼1 mL) led to the precipitation of a light
pink solid which was filtered, washed with water, and dried. Yield: 50%
3
3
4H), 4.80 (t, J = 4 Hz, 4H), 4.42 (t, J = 4 Hz, 4H). 13C NMR (101
MHz, CD3CN) δ: 155.60, 152.29, 145.31, 143.68, 141.71, 128.54,
127.00, 125.05, 122.82, 118.31, 84.18, 73.24, 69.52. MS (ESI+): m/z
793 ([M − PF6−]+, 60%), 324 ([M − 2PF6−]2+, 100%). Anal. Calc %
for C42H32F12FeN4P2: C, 53.75; H, 3.44; N, 5.97. Found %: C, 52.71;
H, 3.44; N, 6.02.
1
3
(40 mg). H NMR (300 MHz, CD3CN) δ (ppm): 8.84 (d, J = 6.4
Hz, 4H), 8.67 (d, 3J = 6.4 Hz, 4H), 8.32 (m, 8H), 4.67 (t, 3J = 6.9 Hz,
4H), 4.40 (s, 6H, CH3), 4.11 (s, 4H), 3.99 (s, 4H), 3.05 (t, 3J = 6.9 Hz,
4H). MS (ESI+): m/z 1016.9 ([M − PF6−]+, 5%), 436 ([M −
2PF6−]2+, 20%). Anal. Calc % for C36H38F24FeN4P4: C, 37.20; H, 3.30;
N, 4.82. Found %: C, 36.27, H, 3.04, N, 4.64.
Synthesis of 21(PF6)4. To a degassed solution of 20(PF6)2 (0.05 g,
0.053 mmol) in dry acetonitrile (2 mL) was added dropwise 1 mL of
CH3I (2.3 g, 16 mmol). The resulting solution was stirred under an
argon atmosphere in the dark at 80 °C for 16 h. After cooling, the
precipitate formed was filtered and thoroughly washed with
acetonitrile. This solid was then dissolved into a mixture of
CH3CN/H2O/KPF6 = 100/10/2 (v/v) until complete dissolution.
This solution was concentrated under reduced pressure until a dark
pink solid was observed. This product was isolated by filtration,
Synthesis of 15(NO3). 1-Ferrocenyl-4-aniline 14 (0.1 g, 0.372
mmol) and N-(2,4-dinitrophenyl)-4,4′-bipyridinium chloride (0.1 g,
0.248 mmol) were dissolved in a dry mixture of CH3CN/EtOH = 1/3
(v/v) (20 mL). The resulting solution was then refluxed under stirring
for 16 h. After cooling, the solvents were removed under reduced
pressure and the crude product was purified by column chromatog-
raphy on SiO2 (THF/H2O/KNO3 = 100/20/4) to afford N-(4-
phenyl-N-(4,4′-pyridinium))ferrocene nitrate salt as dark red powder.
1
washed with water, and dried. Yield: 78% (52 mg). H NMR (400
MHz, CD3CN) δ: 9.20 (d, 3J = 6.8 Hz, 4H), 8.90 (d, 3J = 6.6 Hz, 4H),
3
3
3
8.59 (d, J = 6.8 Hz, 4H), 8.48 (d, J = 6.5 Hz, 4H), 7.77 (d, J = 8.7
3
Hz, 4H), 7.70 (d, J = 8.7 Hz, 4H), 4.83 (s, 4H), 4.43 (s, 6H, CH3),
4.37 (s, 4H). MS (ESI+): m/z 1113 ([M − PF6−]+, 40%), 484.0 ([M −
2PF6−]2+, 100%). Anal. Calc % for C44H38F24FeN4P4: C, 41.99; H,
3.04; N, 4.45. Found %: C, 40.72; H, 3.01; N, 4.61.
1
3
Yield: 90% (0.107 g). H NMR (400 MHz, CD3CN) δ: 9.03 (d, J =
7.0 Hz, 2H), 8.89 (dd, 3J = 4.5 Hz, 4J = 1.7 Hz, 2H), 8.48 (d, 3J = 7.0
Hz, 2H), 7.88 (dd, J = 4.5 Hz, J = 1.7 Hz, 2H), 7.84 (d, J = 8 Hz,
3
4
3
Synthesis of 24. A solution of 2-bromo-4-iodopyridine 23 (2.3 g,
8.1 mmol) and pyridine-4-boronic acid (1 g, 8.1 mmol), and NaHCO3
(2.72 g, 32.4 mmol) dissolved in a mixture of 1,2-dimethoxyethane
(DME) and water (150 mL of DME/H2O = 70:30 v/v), was degassed
through a freeze−pump−thaw procedure (3 cycles). Tetrakis-
(triphenylphosphine)palladium(0) (0.5 g, 0.43 mmol, 20% mol) was
then added, and the stirred mixture was heated at 120 °C under an
argon atmosphere for 16 h. After cooling to room temperature, the
solvents were removed under reduced pressure and the solid product
was dissolved in dichloromethane (100 mL) and then washed with 0.1
M aqueous NaOH (100 mL) and with water (3 × 100 mL). The
organic layer was dried over anhydrous Na2SO4, filtered, and
evaporated to dryness under reduced pressure to give a brown solid.
The crude product was purified by column chromatography on SiO2
(CH2Cl2/CH3OH/Et3N = 473/25/2) to afford 24 as a white powder.
Yield: 64% (0.64 g). 1H NMR (400 MHz, DMSO-d6) δ: 8.73 (dd, 3J =
3
3
3
2H), 7.65 (d, J = 8 Hz, 2H), 4.87 (t, J = 4 Hz, 2H), 4.49 (t, J = 4
Hz, 2H), 4.09 (s, 5H). 13C NMR (101 MHz, CD3CN) δ: 152.29,
145.48, 128.45, 126.99, 125.24, 122.90, 118.30, 71.24, 70.86, 68.13.
MS (ESI+): m/z 417 ([M − PF6−]+, 100%). Anal. Calc % for
C26H21F6FeN2P: C, 55.54; H, 3.76; N, 4.98. Found %: C, 51.97; H,
3.71; N, 4.77.
Synthesis of 16(PF6)2. To a degassed solution of 15(NO3) (0.05 g,
0.104 mmol) in a mixture of acetonitrile (10 mL) and CH2Cl2 (2 mL)
was added dropwise 0.65 mL of CH3I (1.48 g, 10.4 mmol). The
resulting solution was stirred under an argon atmosphere in the dark at
70 °C for 24 h. After cooling, the precipitate formed was filtered and
washed with acetonitrile. This solid was then dissolved into a mini-
mum of water, and an aqueous saturated solution of KPF6 (∼1 mL)
was added until observing a dark pink precipitate. This solid was iso-
lated by filtration, washed with water, and dried. Yield: 60% (45 mg).
1H NMR (400 MHz, CD3CN) δ: 9.17 (d, J = 7.1 Hz, 2H), 8.88
3
4
3
3
4.5 Hz, J = 1.6 Hz, 2H), 8.54 (d, J = 5.2 Hz, 1H), 8.12 (d, J = 1.2,
1H), 7.90 (dd, 3J = 5.2 Hz, 4J = 1.6 Hz, 1H), 7.86 (dd, 3J = 4.5 Hz, 4J =
1.6, 2H). 13C NMR (101 MHz, DMSO-d6) δ: 151.24, 150.58, 147.76,
142.83, 142.54, 125.40, 121.49, 121.14. MS (DCI): m/z 235 ([M]+,
100%). Anal. Calc % for C10H7BrN2: C, 51.09; H, 3.00; Br, 33.99; N,
11.92. Found %: C, 51.49; H, 3.15; N, 12.05.
3
3
3
(d, J = 6.7 Hz, 2H), 8.55 (d, J = 7.0 Hz, 2H), 8.45 (d, J = 6.6 Hz,
3
3
2H), 7.87 (d, J = 8.8 Hz, 2H), 7.68 (d, J = 8.8 Hz, 2H), 4.90−4.87
(m, 2H), 4.52−4.49 (m, 2H), 4.42 (s, 3H, CH3), 4.09 (s, 4H). MS
(ESI+): m/z 577 ([M − PF6−]+, 20%), 216 ([M − 2PF6−]2+, 100%).
Synthesis of 19. Into a sealed tube were added 1,1′-ferrocenyl-bis(4-
bromophenyl) 18 (160 mg, 0.32 mmol), Cu2O (5 mg, 0.032 mmol,
10% mol), and DMF/28% NH3 in water = 50/50 (v/v) (32 mL).
After sealing the tube, this suspension was stirred and heated at 80 °C
for 2 days. After cooling down to room temperature, water was added
(50 mL). The resulting mixture was extracted with Et2O (3 × 50 mL).
Synthesis of 25. 2-Bromo-4,4′-bipyridine 24 (2.14 g, 9.1 mmol) was
added into a two-neck round-bottom flask equipped with a condenser/
septum−stopcock and an addition funnel containing [PdCl2(1,1′-
bis(diphenylphosphino)ferrocene)] (0.2 g, 0.36 mmol, 4% mol) and
CuI (0.17 g, 0.91 mmol, 10% mol). This setup was purged 3 times
2668
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
with vacuum/argon cycles before a degassed mixture of triethyl amine
in dry THF was added (150 mL of Et3N/THF = 1/2 v/v). The Pd-
and Cu-based catatysts were then added and the stirred reaction
mixture, kept under an argon atmosphere, was heated at 45 °C for
1 day. After cooling to room temperature, the mixture was filtered and
the remaining solid was washed with CH3Cl (∼10 mL). The filtrate
was washed with water, dried over anhydrous Na2SO4 and then filtered
and evaporated under reduced pressure to give a brown oil. This crude
product was purified by column chromatography on SiO2 using ethyl
acetate/Et3N = 99/1 (v/v) to afford 25 as a light brown powder. Yield:
96% (2.2 g). 1H NMR (400 MHz, DMSO-d6) δ: 8.71 (d, 3J = 4.8 Hz,
2H), 8.69 (d, 3J = 5.2 Hz, 1H), 7.98 (s, 1H), 7.88 (d, 3J = 4.8 Hz, 2H),
and CuI (0.14 g, 0.74 mmol, 10% mol). This setup was purged 3 times
with vacuum/argon cycles and then a degassed solution of triethyl
amine in dry THF (60 mL of Et3N/THF = 1/2 v/v) was added over
the stirred mixture of 1,1′-diiodoferrocene and 2-ethynyl-4,4′-
bipyridine. The Cu- and Pd-based catalysts were then added, and
the reaction mixture was heated at 70 °C under stirring for 60 h. After
cooling to room temperature, unsoluble materials were filtered and
washed with CH3Cl (∼10 mL). The filtrate was washed with water
(3 × 10 mL) and dried over anhydrous Na2SO4. After filtration, the
solvents were removed under reduced pressure to afford a crude
product containing 2-ethynyl-4,4′-bipyridine, 1-(2-ethynyl-4,4′-bipyr-
idine)-1′-iodoferrocene, and 1,1′-bis(2-ethynyl-4,4′-bipyridine)-
ferrocene. This mixture was purified by column chromatography on
SiO2 (CH2Cl2/CH3OH = 95/5 v/v) to afford 31 as an orange powder.
Yield: 5% (21 mg). 1H NMR (400 MHz, CD2Cl2) δ: 8.68 (d, 3J = 4.8
Hz, 4H), 8.40 (d, 3J = 5.1 Hz, 2H), 7.55 (s, 2H), 7.43 (d, 3J = 4.4 Hz,
4H), 7.25 (d, 3J = 5.1 Hz, 2H), 4.69 (s, 4H), 4.44 (s, 4H). MS (ESI+):
m/z 543 ([M + 1]+, 100%), 272 ([M + 2]2+, 50%).
3
8.85 (d, J = 5.2 Hz, 1H), 0.27 (s, 9H, CH3). MS (ESI+): m/z 253
([M]+, 100%).
Synthesis of 26. A solution of 25 (2.2 g, 8.7 mmol) in methanol
(22 mL) and 1 N aqueous NaOH (9 mL) was stirred for 90 min at room
temperature. Acetic acid (9 mL) was then added, and the mixture was
brought to dryness under reduced pressure. The solid residue was
washed with diethyl ether (∼20 mL), and the filtrate was washed with
water (3 × 20 mL) and then with brine (1 × 20 mL), dried over
anhydrous Na2SO4, filtered, and finally evaporated under reduced
pressure. The crude product was purified by column chromatography
on SiO2 (CH2Cl2/CH3OH/Et3N = 96/3/1). Yield: 66% (1.04 g).
Synthesis of 32(PF6)4. CH3I was added dropwise (0.7 mL, 11.6
mmol) to a degassed solution of 1,1′-bis(2-ethynyl-4,4′-bipyridine)-
ferrocene 31 (0.018 g, 0.033 mmol) in a mixture of acetonitrile (3 mL)
and dichloromethane (4 mL). The resulting solution was stirred under
an argon atmosphere in the dark at 40 °C for 24 h. After cooling to
room temperature, the dark precipitate was filtered and washed with
acetonitrile. This solid was dissolved into a minimum of water, and the
addition of a saturated aqueous KPF6 solution (∼1 mL) resulted in the
precipitation of a purple solid which was filtered, washed with water,
3
1H NMR (400 MHz, DMSO-d6) δ: 8.74 (d, J = 6.4 Hz, 2H), 8.69
3
3
(d, J = 6.4 Hz, 1H), 8.01 (d, J = 1.7 Hz, 1H), 7.89−7.85 (m, 3H),
4.43 (s, 1H, ethynyl). 13C NMR (101 MHz, DMSO-d6) δ: 152.01,
151.58, 146.10, 146.04, 144.42, 143.71, 125.74, 122.37, 122.34,
83.99, 81.79, 55.91. MS (DCI): m/z 181 ([M + 1]+, 100%), 209
([M + C2H5]+, 30%).
1
and dried. Yield: 60% (23.4 mg). H NMR (400 MHz, D2O) δ: 9.08
3
3
3
(d, J = 6.8 Hz, 4H), 9.01 (d, J = 6.7 Hz), 8.64 (d, J = 1.8 Hz, 2H),
8.53 (d, 3J = 6.8, 4H), 8.36 (dd, 3J = 6.6 Hz, 4J = 2.2 Hz, 2H), 5.10 (t,
3J = 1.9, 4H), 4.88 (t, 3J = 1.9, 4H), 4.53 and 4.52 (s, 12H, CH3). MS
(ESI+): m/z 1037 ([M − PF6]+, 70%), 446 ([M − 2PF6]2+, 100%).
Synthesis of 28. 1-Iodoferrocene 27 (0.064 g, 0.355 mmol) and the
2-ethynyl-4,4′-bipyridine 26 (0.111 g, 0.355 mmol) were added into a
two-neck round-bottom flask equipped with a stopcock-fitted
condenser and an addition funnel containing dichloro((bis-
(diphenylphosphino))ferrocenyl)palladium(II) (0.03 g, 0.04 mmol,
10% mol) and CuI (0.01 g, 0.04 mmol, 10% mol). This setup was
purged 3 times with vacuum/argon cycles before a degassed mixture of
triethyl amine in dry THF (Et3N/THF = 1/2 v/v) (9 mL) was added
over the stirred mixture of 1-iodoferrocene and 2-ethynyl-4,4′-
bipyridine. The Cu- and Pd-based catalysts were added, and the
reaction mixture was stirred and heated at 80 °C for 24 h. After
cooling to room temperature, the solid phase was filtered and washed
with CH3Cl (∼10 mL). The filtrate was washed with water (3 ×
25 mL) and dried over anhydrous Na2SO4. After filtration, the solvents
were removed under reduced pressure, and the crude product was
purified by column chromatography on SiO2 (CHCl3/CH3OH = 98/2
v/v) to afford 28 as an orange powder. Yield: 26% (33 mg). 1H NMR
(300 MHz, CD2Cl2) δ: 8.76 (d, 3J = 6.2 Hz, 2H), 8.67 (d, 3J = 5.2 Hz,
ASSOCIATED CONTENT
■
S
* Supporting Information
Characterization data and procedure for estimation of Kdim
.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
christophe.bucher@ujf-grenoble.fr
ACKNOWLEDGMENTS
■
3
3
3
The authors would like to thank the CNRS, UJF, and “Reg
́
ion
ation
́
ropole” (Nanobio program) for funding.
1H), 7.73 (d, J = 3 Hz, 1H), 7.63 (d, J = 6.2 Hz, 2H), 7.48 (d, J =
3
3
Rho
̂
ne-Alpes”, as well as the “Communaute
Grenoble-Alpes Met
C.B. and A.I. would also like to express their gratitude to
Dr. Damien Jouvenot for sharing his skills in graphic arts.
́
d′agglomer
́
5.2 Hz, 1H), 4.61 (t, J = 3 Hz, 2H), 4.34 (t, J = 3 Hz, 2H), 4.28 (s,
5H). MS (ESI+): m/z 365 ([M + 1]+, 100%).
Synthesis of 29(PF6)2. CH3I (1.7 mL, 28 mmol) was added
dropwise to a degassed solution of 1-(2-ethynyl-4,4′-bipyridine)-
ferrocene 28 (0.029 g, 0.08 mmol) dissolved in a mixture of
acetonitrile (2 mL) and dichloromethane (0.5 mL). This solution was
stirred under an argon atmosphere in the dark at 40 °C for 24 h. After
cooling to room temperature, the dark precipitate was filtered and
washed thoroughly with acetonitrile. This intermediate was dissolved
into a minimum of water, and the addition of a saturated aqueous
KPF6 solution (∼1 mL) resulted in the precipitation of a purple solid.
This final product was filtered, washed with water, and dried. Yield:
REFERENCES
■
(1) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007,
46, 72.
(2) Spruell, J. M.; Coskun, A.; Friedman, D. C.; Forgan, R. S.; Sarjeant,
A. A.; Trabolsi, A.; Fahrenbach, A. C.; Barin, G.; Paxton, W. F.; Dey,
́
S. K.; Olson, M. A.; Benıtez, D.; Tkatchouk, E.; Colvin, M. T.; Carmielli,
1
R.; Caldwell, S. T.; Rosair, G. M.; Hewage, S. G.; Duclairoir, F.;
Seymour, J. L.; Slawin, A. M. Z.; Goddard III, W. A.; Wasielewski, M. R.;
Cooke, G.; Stoddart, J. F. Nat. Chem. 2010, 2, 870.
51% (27.5 mg). H NMR (400 MHz, DMSO-d6) δ: 9.30−9.27 (m,
3H), 8.91 (d, 3J = 4 Hz, 1H), 8.79 (d, 3J = 8 Hz, 2H), 8.61 (dd, 3J = 8
Hz, 4J = 4 Hz, 1H), 4.9 (t, 3J = 2 Hz, 2H), 4.69 (t, 3J = 2 Hz, 2H), 4.00
(m, 11H). MS (ESI+): m/z 539 ([M − PF6]+, 30%), 197 ([M −
2PF6]2+, 100%).
(3) Coskun, A.; Spruell, J. M.; Barin, G.; Fahrenbach, A. C.; Forgan,
R. S.; Colvin, M. T.; Carmieli, R.; Benítez, D.; Tkatchouk, E.;
Friedman, D. C.; Sarjeant, A. A.; Wasielewski, M. R.; Goddard, W. A.
III; Stoddart, J. F. J. Am. Chem. Soc. 2011, 133, 4538.
(4) Li, H.; Fahrenbach, A. C.; Coskun, A.; Zhu, Z.; Barin, G.; Zhao,
Y.-L.; Botros, Y. Y.; Sauvage, J.-P.; Stoddart, J. F. Angew. Chem., Int. Ed.
2011, 50, 6782.
Synthesis of 31. 1,1′-Diiodoferrocene 30 (0.325 g, 0.74 mmol) and
2-ethynyl-4,4′-bipyridine 26 (0.667 g, 3.7 mmol) were introduced
into a two-neck round-bottom flask equipped with a stopcock-
fitted condenser and an addition funnel containing tetrakis-
(triphenylphosphine)palladium(0) (0.171 g, 0.14 mmol, 5% mol)
2669
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
(5) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. Rev.
2005, 105, 1281.
(44) Imabayashi, S.-I.; Kitamura, N.; Tazuke, S.; Tokuda, K. J. Electroanal.
Chem. 1988, 243, 143.
(6) Hawthorne, M. F.; Ramachandran, B. M.; Kennedy, R. D.;
Knobler, C. B. Pure Appl. Chem. 2006, 78, 1299.
(7) Vacek, J.; Michl, J. Adv. Funct. Mater. 2007, 17, 730.
(8) Vicario, J.; Katsonis, N.; Ramon, B. S.; Bastiaansen, C. M.; Broer,
D. J.; Feringa, B. L. Nature 2006, 440, 163.
(9) Leigh, D. A.; Wong, J. K. Y.; Dehez, F.; Zerbetto, F. Nature 2003,
424, 174.
(10) Kelly, T. R.; De Silva, H.; Silva, R. A. Nature 1999, 401, 150.
(11) Koumoura, N.; Zijlstra, R. W. J.; Van Delden, R. A.; Harada, N.;
Feringa, B. L. Nature 1999, 401, 152.
(12) Van Delden, R. A.; Wiel, M. K. J. t.; Pollard, M. M.; Vicario, J.;
Koumura, N.; Feringa, B. L. Nature 2005, 437, 1337.
(13) Bohn, R. K.; Haaland, A. J. Organomet. Chem. 1966, 470.
(14) Gardner, A. B.; Howard, J.; Waddington, T. C.; Richardson, R. M.;
Tomkinson, J. Chem. Phys. 1981, 57, 453.
(45) Coutouli Argyropoulou, E.; Kelaidopoulou, A.; Sideris, C.;
Kokkinidis, G. J. Electroanal. Chem. 1999, 477, 130.
(46) Fontani, M.; Peters, F.; Scherer, W.; Wachter, W.; Wagner, M.;
Zanello, P. Eur. J. Inorg. Chem. 1998, 10, 1453.
(47) Kubo, T.; Shinada, T.; Kobayashi, Y.; Imafuku, H.; Toya, T.;
Akita, S.; Nishikitani, Y.; Watanabe, H. Solid State Ionics 2003, 165,
209.
(48) Meyerhans, A.; Pfau, W.; Memming, R.; Margaretha, P. Helv.
Chim. Acta 1982, 65, 2603.
(49) Nishikitani, Y.; Ikai, K.; Kobayashi, M.; Imafuku, H.; Minami,
M.; Kubo, T. PCT Int. Appl. WO 2000049454 A1 20000824, 2000.
(50) Fujihira, M.; Yamada, H. Thin Solid Films 1988, 160, 125.
(51) Reynes, O.; Bucher, C.; Moutet, J.-C.; Royal, G.; Saint-Aman, E.
Chem. Commun. 2004, 428.
(52) Gonsalves, K.; Zhan-ru, L.; Rausch, M. D. J. Am. Chem. Soc.
1984, 106, 3862.
(53) Ratajczak, A.; Czech, B.; Drobek, L. Syn. React. Inorg. Met. 1982,
12, 557.
(54) Christensen, T. B.; Ditte, R.; Daasbjerg, K.; Skrydstrup, T.
(15) Brydges, S.; Harrington, L. E.; McGlinchey, M. J. Coord. Chem.
Rev. 2002, 233/234, 75.
(16) Crowley, J. D.; Steele, I. M.; Bosnich, B. Chem.Eur. J. 2006,
12, 8935.
(17) Wang, X. B.; Dai, B.; Woo, H.-K.; Wang, L.-S. Angew. Chem., Int.
Chem. Commun. 1999, 20, 2051.
Ed. Engl. 2005, 44, 6022.
(55) Bhatt, J.; Fung, B. M.; Nicholas, K. M. Liq. Cryst. 1992, 12, 263.
(56) Bhattacharyya, S. J. Org. Chem. 1998, 63, 7101.
(57) Bhattacharyya, S. Synlett 1998, 8, 837.
(58) Hisatome, M.; Takano, S.-I.; Yamakawa, K. Tetrahedron Lett.
1985, 26, 2347.
(18) Tanaka, K.; Kinbara, K. Mol. Biosyst. 2008, 4, 512.
(19) Zhang, D.; Zhang, Q.; Su, J.; Tian, H. Chem. Commun. 2009,
1700.
(20) Muraoka, T.; Kinbara, K.; Aida, T. Nature 2006, 440, 512.
(21) Muraoka, T.; Kinbara, K.; Aida, T. J. Am. Chem. Soc. 2006, 128,
11600.
(22) Muraoka, T.; Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am. Chem.
Soc. 2003, 125, 5612.
(23) Muraoka, T.; Kinbara, K.; Aida, T. Chem. Commun. 2007, 1441.
(24) Kinbara, K.; Muraoka, T.; Aida, T. Org. Biomol. Chem. 2008, 6,
1871.
(59) Lapic, J.; Rapic, V. Croat. Chem. Acta 2000, 73, 755.
(60) Emmert, B.; Roh, N. Chem. Ber. 1925, 58, 503.
(61) Zincke, T. Liebigs Ann. Chem. 1904, 330, 361.
(62) Allen, J. G. Brit. Patent GB1399595 (A), 1975.
(63) Allen, J. G. Ger. Offen. DE2527638, 1976.
(64) Cheng, W.-C.; Kurth, M. J. Org. Prep. Proced. Int. 2002, 34, 585.
(65) Anderson, T. Liebigs Ann. Chem. 1855, 94, 358.
(66) Weinmayr, V. J. Am. Chem. Soc. 1955, 77, 3012.
(67) Carlescu, I.; Scutaru, A. M.; Apreutesei, D.; Alupei, V.; Scutaru,
D. Appl. Organomet. Chem. 2007, 21, 661.
(68) D’Souza, F.; Smith, P. M.; Gadde, S.; McCarty, A. L.; Kullman,
M. J.; Zandler, M. E.; Ito, M.; Araki, Y.; Ito, O. J. Phys. Chem. B 2004,
108, 11333.
(69) Beer, P. D.; Sikanyika, H. Polyhedron 1990, 9, 1091.
(70) Braga, D.; D’Addario, D.; Polito, M.; Grepioni, F. Organo-
metallics 2004, 23, 2810.
(25) Kochi, J. K.; Rathore, R.; Le Magueres, P. J. Org. Chem. 2000,
65, 6826.
(26) Lu, J.-M.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003,
125, 12161.
(27) Nishinaga, T.; Komatsu, K. Org. Biomol. Chem. 2005, 3, 561.
(28) Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 29, 417.
(29) Levillain, E.; Roncali, J. J. Am. Chem. Soc. 1999, 121, 8760.
(30) Hapiot, P.; Audebert, P.; Monnier, K.; Pernaut, J.-M.; Garcia, P.
Chem. Mater. 1994, 6, 1549.
(31) Fuhrhop, J. H.; Wasser, P.; Riesner, D.; Mauzerall, D. J. Am.
(71) Braga, D.; Polito, M.; Bracaccini, M.; D’addario, D.; Tagliavini,
E.; Proserpio, D. M.; Grepioni, F. Chem. Commun. 2002, 1080.
(72) Braga, D.; Polito, M.; Bracaccini, M.; D’Addario, D.; Tagliavini,
E.; Sturba, L.; Grepioni, F. Organometallics 2003, 22, 2142.
(73) Knapp, R.; Rehahn, M. J. Organomet. Chem. 1993, 452, 235.
(74) Bunnett, J. F.; Zahler, R. E. Chem. Rev. 1951, 49, 273.
(75) Monk, P. M. S. The viologens: physicochemical properties, synthesis
and applications of the salts of 4,4′ bipyridine; Wiley: 1998.
(76) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(77) Duan, X.-F.; Li, X.-H.; Li, F.-Y.; Huang, C.-H. Synthesis 2004,
16, 2614.
(78) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
16, 4467.
(79) Kagan, H. B.; Diter, P.; Gref, A.; Guillaneux, D.; Masson-
Szymczak, A.; Rebiere, F.; Riant, O.; Samuel, O.; Taudien, S. Pure
Appl. Chem. 1996, 68, 29.
Chem. Soc. 1972, 94, 7796.
(32) Koivisto, B. D.; Ichimura, A. S.; McDonald, R.; Lemaire, M. T.;
Thompson, L. K.; Hick, R. G. J. Am. Chem. Soc. 2006, 128, 690.
(33) Femoni, C.; Iapalucci, M. C.; Longoni, G.; Tiozzo, C.;
Wolowska, J.; Zacchini, S.; Zazzaroni, E. Chem.Eur. J. 2007, 13,
6544.
(34) Park, J. W.; Choi, N. H.; Kim, J. H. J. Phys. Chem. 1996, 100,
769.
(35) Trabolsi, A.; Khashab, N.; Fahrenbach, A. C.; Friedman, D. C.;
Colvin, M. T.; Cotí, K. K.; Benítez, D.; Tkatchouk, E.; Olsen, J.-C.;
Belowich, M. E.; Carmielli, R.; Khatib, H. A.; Goddard, W. A.;
Wasielewski, M. R.; Stoddart, J. F. Nat. Chem. 2010, 2, 42.
(36) Furue, M.; Nozakura, S.-I. Chem. Lett. 1980, 821.
(37) Deronzier, A.; Galland, B.; Viera, E. Nouv. J. Chim. 1982, 6, 97.
(38) Geuder, W.; Huenig, S.; Suchy, A. Tetrahedron 1986, 42, 1665.
(39) Kannappan, R.; Bucher, C.; Saint-Aman, E.; Moutet, J.-C.; Milet,
(80) Butler, I. R.; Hobson, L. J.; Macan, S. M. E.; Williams, D. J.
Polyhedron 1993, 12, 1901.
́
A.; Oltean, M.; Metay, E.; Pellet-Rostaing, S.; Lemaire, M.; Chaix, C.
New J. Chem. 2010, 34, 1373.
(81) Clarke, P.; Hounslow, A. M.; Keough, R. A.; Lincoln, S. F.;
Wainwright, K. P. Inorg. Chem. 1990, 29, 1793.
(82) Rausch, M. D.; Ciappenelli, D. J. J. Organomet. Chem. 1967, 10,
127.
(40) Neta, P.; Richoux, M.-C.; Harriman, A. J. Chem. Soc., Faraday
Trans. 2 1985, 81, 1427.
(41) Van Leeuwen, J. W.; Van Dijk, C.; Veeger, C. Eur. J. Biochem.
1983, 135, 601.
(83) Gelin, F.; Thummel, R. P. J. Org. Chem. 1992, 57, 3780.
(84) Inouye, M.; Hyodo, Y.; Nakazumi, H. J. Org. Chem. 1999, 64,
2704.
(42) Geuder, W.; Hunig, S.; Suchy, A. Angew. Chem., Int. Ed. Engl.
1983, 22, 489.
(43) Itoh, M.; Kosower, E. M. J. Am. Chem. Soc. 1968, 90, 1843.
̈
2670
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671

Journal of the American Chemical Society
Article
(85) Kasahara, A.; Izumi, T.; Yoshida, Y.; Shimizu, I. Bull. Chem. Soc.
Jpn. 1982, 55, 1901.
(86) Shimizu, I. Bull. Chem. Soc. Jpn. 1983, 56, 2023.
(87) Shimizu, I.; Kamei, Y.; Tezuka, T.; Izumi, T.; Kasahara, A. Bull.
Chem. Soc. Jpn. 1983, 56, 192.
Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
̈
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman,
J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.02;
Gaussian, Inc.: Wallingford, CT, 2009.
(118) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(119) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(120) Tawada, Y.; Tsuneda, T.; Yanagisawa, S.; Yanai, T.; Hirao, K.
J. Chem. Phys. 2004, 120, 8425.
(88) Li, H. L.; Fahrenbach, A. C.; Dey, S., K.; Basu, S.; Trabolsi, A.;
Zhu, Z.; Botros, Y. Y.; Stoddard, J. F. Angew. Chem., Int. Ed. 2010, 49,
8260.
(89) Hmadeh, M.; Fahrenbach, A. C.; Basu, S.; Trabolsi, A.; Benítez,
D.; Li, H.; Albrecht-Gary, A.-M.; Elhabiri, M.; Stoddard, J. F. Chem.
Eur. J. 2011, 17, 6076.
(121) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987,
86, 866.
(90) Trabolsi, A.; Fahrenbach, A. C.; Dey, S. K.; Share, A. I.;
Friedman, D. C.; Basu, S.; Gasa, T. B.; Khashab, N. M.; Saha, S.;
Aprahamian, I.; Khatib, H. A.; Flood, A. H.; Stoddard, J. F. Chem.
Commun. 2010, 46, 871.
(91) Li, H.; Zhao, Y.-L.; Fahrenbach, A. C.; Kim, S.-Y.; Paxton, W. F.;
Stoddart, J. F. Org. Biomol. Chem. 2011, 9, 2240.
(92) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem.
Soc., Perkin Trans. 2 2001, 651.
(122) Dunning, T. H., Jr., Hay, P. J. In Modern Theoretical Chemistry,
Third Edition; Schaefer, H. F., Ed.; Plenum: New York, 1976; Vol. 3,
pp 1−28.
(123) Cances
107, 3032.
̀
, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997,
(124) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Chem. Phys.
Lett. 1998, 286, 253.
(125) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151.
(126) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys.
2002, 117, 43.
(127) Bastin, S. Tetrahedron: Asymmetry 1999, 10, 1647.
(128) Guillaneux, D.; Kagan, H. B. J. Org. Chem. 1995, 60, 2502.
(129) Wright, M. E. Organometallics 1990, 9, 853.
(93) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525.
(94) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(95) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278.
(96) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49.
(97) Deronzier, A. Electrochim. Acta 1983, 28, 805.
(98) Ammar, F.; Saveant, J. M. J. Electroanal. Chem. 1973, 47, 115.
(99) Bard, A. J. Pure Appl. Chem. 1971, 25, 379.
(100) Atherton, S. J.; Tsukahara, K.; Wilkins, R. G. J. Am. Chem. Soc.
1986, 108, 3380.
(101) Pedatsur, N.; Richoux, M.-C.; Harriman, A. J. Chem. Soc.,
Faraday Trans. 2 1985, 81, 1427.
(102) Jeon, W. S.; Kim, H.-J.; Lee, C.; Kim, K. Chem. Commun. 2002,
1828.
(103) Di Matteo, A. Chem. Phys. Lett. 2007, 439, 190.
(104) Armstrong, A. T.; Smith, F.; Elder, E.; McGlynn, S. P. J. Chem.
Phys. 1967, 46, 4321.
(105) Sporer, C.; Ratera, I.; Ruiz-Molina, D.; Vidal-Gancedo, J.;
Ventosa, N.; Wurst, K.; Jaitner, P.; Rovira, C.; Veciana, J. Solid State
Sci. 2009, 11, 786.
(106) Sporer, C.; Ratera, I.; Wurst, K.; Vidal-Gancedo, J.; Ruiz-
Molina, D.; Rovira, C.; Veciana, J. Arkivoc 2005, ix, 104.
(107) Alain, V. Inorg. Chim. Acta 1996, 242, 43.
(108) Johnson, C. S. G. J. Chem. Phys. 1963, 39, 58.
(109) Rieger, A. L.; Rieger, P. H. J. Phys. Chem. 1984, 88, 5845.
(110) Water is known to favor the dimerization of viologen cation
radicals. This effect most probably results from the hydrophobicity of
the reduced forms. See ref 72 for more detailed informations.
(111) Lee, C.; Lee, Y. M.; Moon, M. S.; Park, S. H.; Park, J. W.; Kim,
K. G.; Jeon, S.-J. J. Electroanal. Chem. 1996, 416, 139.
(112) Gomila, R. M.; Garau, C.; Frontera, A.; Quinonero, D.;
̃
̀
Ballester, P.; Costa, A.; Deya, P. M. Tetrahedron Lett. 2004, 45, 9387.
(113) Yanai, T.; Tew, D.; Handy, N. Chem. Phys. Lett. 2004, 393, 51.
(114) Haaland, A. Acc. Chem. Res. 1979, 12, 415.
(115) Calculations conducted in the gas phase revealed that both
species are minima of the PES (Potential Energy Surface).
(116) For calculations including solvent effects, we used geometries
optimized in the gas phase to stress the influence of the solvent on the
stabilization energy of the closed form. It should be noted that
experiments conducted with compound 21 revealed that the changes
in geometry are small or negligible.
(117) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian,
H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada,
M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery,
J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;
Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
2671
dx.doi.org/10.1021/ja209766e | J. Am. Chem.Soc. 2012, 134, 2653−2671