Intervalence transfer of ferrocene moieties adsorbed on electrode surfaces by a conjugated linkage

Article abstract of DOI:10.1016/j.cplett.2009.02.057

Effective intervalence transfer occurred between the metal centers of ferrocene moieties that were adsorbed onto a ruthenium thin film surface by ruthenium-carbene π bonds, a direct verification of Hush's four-decade-old prediction. Electrochemical measur

Full text of DOI:10.1016/j.cplett.2009.02.057

Chemical Physics Letters 471 (2009) 283–285
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Intervalence transfer of ferrocene moieties adsorbed on electrode surfaces
by a conjugated linkage
Wei Chen, Lauren E. Brown, Joseph P. Konopelski, Shaowei Chen *
Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, CA 95064, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Effective intervalence transfer occurred between the metal centers of ferrocene moieties that were
Received 22 January 2009
In final form 20 February 2009
Available online 26 February 2009
adsorbed onto a ruthenium thin film surface by ruthenium–carbene
p bonds, a direct verification of
Hush’s four-decade-old prediction. Electrochemical measurements showed two pairs of voltammetric
peaks where the separation of the formal potentials suggested a Class II behavior. Additionally, the poten-
tial spacing increased with increasing ferrocene surface coverage, most probably as a consequence of the
enhanced contribution from through-space electronic interactions between the metal centers. In con-
trast, the incorporation of a sp3 carbon spacer into the ferrocene–ruthenium linkage led to the diminish-
ment of interfacial electronic communication.
Ó 2009 Elsevier B.V. All rights reserved.
Intervalence charge transfer (IVCT) is an important process in
both chemical and biological systems and has been studied exten-
sively in the past few decades [1–5]. This phenomenon is typically
observed with organometallic complexes in which at least two re-
dox centers are linked with a conjugated spacer to allow extensive
intramolecular charge delocalization, leading to the emergence of
new optical and electronic characteristics. Quantum mechanically,
the extent of such intramolecular electronic communication is
mainly determined by two factors: (i) through-space interactions
that entail direct overlap of the orbitals of the metal centers and
electrostatic interactions between the metal centers; and (ii)
through-bond interactions that arise from metal–ligand–metal
The results strongly suggest that with a conjugated chemical
linker, intervalence transfer indeed occurs between the ferrocene
functional groups, whereas with a saturated alkyl spacer, the ferro-
cene moieties behave independently. Such an electrochemical sys-
tem provides a more direct verification of Hush’s hypothesis and
we believe that this is the first of its kind.
The experimental procedure is outlined in Scheme 1. First, a
thin film of ruthenium was electrodeposited onto a gold disk elec-
trode surface. Here a polycrystalline gold disk electrode (sealed in a
glass tubing) was firstly polished with alumina slurries (0.05
and then cleansed by sonication in 0.1 M HNO , H SO , and Nano-
pure water (18 M cm) for 10 min successively. The gold electrode
was then electrochemically polished in 0.1 M H SO by rapid po-
lm)
3
2
4
X
overlap and may involve
r
or
p
metal–ligand bonds. Both of these
2
4
factors may be readily manipulated by the chemical structure of
the bridging ligands. Forty years ago, Hush [6] predicted that effec-
tive electronic communication might also occur between metal
centers at the electrode/electrolyte interface when they were
bound onto the electrode surface by conjugated chemical linkers.
Yet, there has been little experimental verification of this hypoth-
esis until recently when we demonstrated that when ferrocene
moieties were bound onto ruthenium nanoparticle surface by
tential sweeps within the potential range of +0.90 V and ꢀ0.5 V
+
(vs Fc /Fc). The clean gold electrode was then immersed into a
3 2 4
RuCl solution (0.2 mM with 0.1 M H SO ), and a Ru thin film
+
was formed by electrodeposition at ꢀ0.75 V (vs Fc /Fc) for 5 min
(longer deposition time did not result in drastic difference in the
eventual electrochemical measurements). Based on the charge
accumulated during the electrodeposition process, the average
thickness of the Ru thin film was evaluated to be 10 to 20 monolay-
ers [9]. The resulting electrode was denoted as Au/Ru. The freshly
prepared Ru film was then immediately immersed into a dichloro-
methane solution containing 10 mM octyl diazoacetate (ODA, the
synthesis of which has been detailed previously [10]) for 12 h
where the strong affinity of the diazo moiety to ruthenium surfaces
ruthenium–carbene
p bonds, intervalence transfer was observed
in both electrochemical and near-infrared spectroscopic measure-
ments, which was further confirmed by density functional theory
calculations [7]. This suggests that effective intraparticle charge
delocalization indeed occurred and the particles behaved as a Class
II compound [8]. In this Letter, we expand the study to two-dimen-
sional metal films by carrying out electrochemical studies of ferro-
cene moieties that are bound onto a ruthenium thin film surface.
led to the formation of Ru@C
p bonds (and the concurrent release
of nitrogen) and hence the formation of a carbene self-assembled
monolayer on the ruthenium surface (denoted as Au/Ru@C8)
[
7,10,11]. The electrode was then incubated into a solution of
either vinylferrocene or allylferrocene in dichloromethane typi-
cally for 2 days where olefin metathesis reactions [7,10,11] were
*
Corresponding author.
E-mail address: schen@chemistry.ucsc.edu (S. Chen).
0
009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.02.057

2
84
W. Chen et al. / Chemical Physics Letters 471 (2009) 283–285
metric peaks remained quite well-defined at the Au/Ru@C8 elec-
Ru electro-
deposition
Ru thin film
trode suggests that the ODA monolayer was not sufficiently
compact and/or the gold electrode was not fully coated by the
ruthenium film. Previously, we found that carbene-passivated
ruthenium nanoparticles were stable enough to survive repetitive
drying and dispersion, indicative of the formation of a compact car-
bene protecting layer [7,10]. Thus, it is likely that the electrodepos-
ited ruthenium did not cover the entire gold electrode surface and
the surface defects gave rise to the apparent voltammetric features
of the ferricyanide/ferrocyanide couple. Such ‘porous’ surface
structures were anticipated to allow ready access of vinyl-termi-
nated derivatives such as vinylferrocene and allyferrocene to the
ruthenium film surface for olefin metathesis reactions. Their vol-
tammetric characteristics were then examined and compared.
Fig. 2 shows the cyclic voltammograms (CVs) of the Au/Ru@CH–
Au
N
2
Fc and Au/Ru@CHꢀCH
2
ꢀFc electrodes in dichloromethane with
Scheme 1. Schematic of electrode surface functionalization.
0
.1 M tetrabutylammonium perchlorate (TBAP). It can be seen that
their voltammetric profiles are drastically different. Specifically,
two pairs of voltammetric peaks can be clearly seen with the
exploited for the incorporation of the respective ferrocene moi-
ety into the surface monolayer, with the electrode referred to as
Au/Ru@CH–Fc and Au/Ru@CH–CH –Fc, respectively. The resulting
2
0
Au/Ru@CHꢀFc electrode with the formal potentials (E° ) of
+
0
ꢀ
0.105 V and +0.155 V (vs Fc /Fc) and a potential spacing (DE° )
of 260 mV. Additionally, the peak currents increase linearly with
electrode was then rinsed with a copious amount of dichlorometh-
ane and dried with a gentle stream of nitrogen before being subject
to electrochemical characterizations.
The formation of an ODA monolayer on the Ru surface was first
examined. Similar to the well-known alkanethiol self-assembled
monolayers that effectively suppress interfacial faradaic reactions,
the ODA monolayer is anticipated to exhibit significant hindrance
to charge transfer of solution redox couples to the electrode
potential sweep rates, in agreement with surface-confined systems
(
not shown). These may be ascribed to the redox reaction of the
+
ferrocene moieties, Fc + e M Fc. The appearance of two instead of
one pair of voltammetric peaks strongly suggests that interfacial
intervalence transfer occurred between the surface-adsorbed fer-
0
rocene metal centers, and the potential spacing (
D
E° ) of 260 mV
was highly comparable to those observed with biferrocene deriva-
tives [12–14], suggestive of Class II behaviors as defined by Robin
and Day [8]. Note that this potential spacing is somewhat larger
than that found when the ferrocene moieties are bound onto
ruthenium nanoparticle surface through a similar Ru@C conju-
gated linkage (ꢁ200 mV). This may be accounted for, at least in
part, by two contributing factors. First, intervalence charge transfer
is better facilitated by a ruthenium thin film than by a nanometer-
sized ruthenium particle, probably because the former behaves as
a better sink for charge delocalization [7]. Second, the close prox-
imity of the surface-confined ferrocene moieties on ruthenium thin
film surfaces is anticipated to lead to enhanced through-space
electronic interactions between the metal centers. It should be
noted that direct orbital overlap decays exponentially with
surface. We used potassium ferricyanide K
probe molecule. Fig. 1 shows the cyclic voltammograms of the
Au/Ru and Au/Ru@C8 electrodes in 10 mM K [Fe(CN) ] at a poten-
tial sweep rate of 0.1 V/s. It can be seen that at the Au/Ru electrode
black curve), a pair of very well-defined voltammetric peaks can
3 6
[Fe(CN) ] as the
3
6
(
0
+
be seen with a formal potential (E° ) of ꢀ0.144 V (vs Fc /Fc) and
peak splitting ( ) of 72 mV. These are ascribed to the electron-
transfer reaction of the ferricyanide/ferrocyanide couple,
DE
p
3
ꢀ
4ꢀ
FeðCNÞ6 þ e $ FeðCNÞ . At the Au/Ru@C8 electrode (red curve),
6
0
similar voltammetric features were observed with E° = ꢀ0.133 V
and
p
DE = 87 mV, yet the peak currents were drastically sup-
pressed by about 50%, suggesting the successful formation of an
ODA monolayer on the Ru surface. Note that surface derivation
by ODA was exclusive to ruthenium and none occurred on the gold
surface [11]. The fact that the ferricyanide/ferrocyanide voltam-
0
0
.35
.30
0
0
0
.4
.2
.0
Au/Ru
Au/Ru=C8
Au/Ru=CH-CH2-Fc
Au/Ru=CH-Fc
1
5
0
5
0
5
0.25
.20
0.15
0
-
-
0.2
0.4
1
0
0
0
.10
.05
.00
2
Au/Ru=CH-CH -Fc
-
1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9
-
-
0.05
0.10
Au/Ru=CH-Fc
-
Au/Ru
Au/Ru=C8
-0.15
-0.20
-
10
-
1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
+
E (V vs Fc /Fc)
-15
-
0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
Fig. 2. Cyclic voltammograms (CV) and square wave voltammograms (SWV, inset)
of Au/Ru@CH–Fc and Au/Ru@CH–CH –Fc electrodes in dichloromethane containing
.1 M tetrabutylammonium perchlorate (TBAP). The SWV profiles for the Au/Ru and
+
E (V vs Fc/Fc)
2
0
Fig. 1. Cyclic voltammograms (CV) of the Au/Ru and Au/Ru@C8 electrodes in an
Au/Ru@C8 electrodes were also included in the figure inset. In CV, potential sweep
rate 0.1 V/s. In SWV, increment of potential 4 mV, amplitude 25 mV and frequency
15 Hz.
aqueous solution containing 10 mM potassium ferricyanide and 0.1 M KCl. Potential
2
sweep rate 0.1 V/s. Gold electrode surface area 0.72 mm .

W. Chen et al. / Chemical Physics Letters 471 (2009) 283–285
285
[
[
14,17] and allylferrocene-functionalized ruthenium nanoparticles
7].
The variation of the voltammetric features can also be mani-
0
0
0
0
0
0
.5
.4
.3
.2
.1
.0
fested in square wave voltammetric (SWV) measurements (inset).
Again, two pairs of voltammetric peaks are very well-defined with
the Au/Ru@CHꢀFc electrode, whereas only a single pair of peaks
are observed with the Au/Ru@CHꢀCH ꢀFc electrode. Furthermore,
2
the Au/Ru and Au/Ru@C8 only exhibited featureless voltammetric
profiles within the same potential range.
In summary, olefin metathesis reactions were exploited to
incorporate redox-active ferrocene moieties onto carbene-func-
tionalized ruthenium thin film surfaces. When the ferrocene
groups were directly bound onto the electrode surface by a conju-
gated linkage, effective intervalence transfer occurred, as mani-
fested by the appearance of two pairs of voltammetric peaks and
the potential spacing suggested Class II characteristics. In contrast,
0
5 10 15 20 25 30
-
10
2
ΓFc (10 mol/cm )
0
3
Fig. 3. Variation of the separation of the formal potentials (DE° ) of the Au/Ru@CH–
with the insertion of a sp carbon spacer, the ferrocene moieties
Fc electrode with ferrocene surface coverage. Symbols are experimental data
obtained from voltammetric measurements as exemplified in Fig. 2 and line is
linear regression.
were found to behave independently and only one pair of voltam-
metric peaks were observed. In essence, these studies provide a
first direct verification of Hush’s hypothesis [6] made four decades
ago that effective electronic communication might take place
when redox-active moieties were anchored onto an electrode sur-
face by a conjugated linker.
distance and thus is prominent only at short range, whereas elec-
trostatic interactions decrease inversely with distance and repre-
sent a long-range contributing factor. In fact, the potential
spacing has been found to enlarge with increasing surface coverage
of ferrocene moieties (estimated from the voltammetric peak
areas), as manifested in Fig. 3. For instance, one can see that when
the ferrocene surface coverage increases from 4.6 ꢂ 10 mol/cm
E° increases from 260 mV to 430 mV
note that the surface coverages were most likely to be overesti-
mated as they were normalized to the geometrical area of the elec-
trode; and the scattering of the data points probably arose from the
variation of the structure and quality of the ruthenium films which
were deposited onto a polycrystalline gold surface).
Extrapolation to zero surface coverage by linear regression
E° of ca. 250 ± 50 mV, which most probably reflects
the through-bond interactions between the metal centers. Interest-
ingly, in previous studies [15,16] with two-dimensional self-
assembled monolayers of
face, a potential splitting of ca. 100 mV was sometimes observed,
which might serve as a first-order approximation of the through-
space contributions, in reasonable agreement with the present
observations. The through-space effect was much less prominent
when the ferrocene moieties were bound onto nanoparticle sur-
faces possibly because of the three-dimensional curvatures of the
nanocrystalline cores [7].
In contrast, at the Au/Ru@CHꢀCH
pair of voltammetric peaks can be seen (formal potential
0.090 V), very similar to that of ferrocene monomers. This sug-
Acknowledgment
ꢀ10
2
This work was supported in part by the National Science Foun-
dation (CHE-0832605).
ꢀ
9
2
0
to 1.3 ꢂ 10 mol/cm ,
D
(
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(
gests that the intervalence transfer is effectively impeded because
of the sp carbon spacer, in accord with previous studies of biferro-
3
[
cene derivatives linked by an ethylene spacer (ꢀCH
2
ꢀCH ꢀ)
2