A butadiyne-linked diruthenium molecular wire self-assembled on a gold electrode surface

Article abstract of DOI:10.1039/c0cc03048a

A 1,3-butadiyne-linked diruthenium complex 4 is successfully brought onto the gold surface in a lying flat mode to form self-assembled monolayers (SAMs) showing reversible multiple redox behaviors on the electrode surface. The diruthenium species with dif

Full text of DOI:10.1039/c0cc03048a

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A butadiyne-linked diruthenium molecular wire self-assembled on a gold
electrode surfacew
Li-Yi Zhang,a Hua-Xin Zhang,b Shen Ye,b Hui-Min Wen,a Zhong-Ning Chen,*a
Masatoshi Osawa,b Kohei Uosakic and Yoichi Sasaki*bc
Received 5th August 2010, Accepted 20th October 2010
DOI: 10.1039/c0cc03048a
A 1,3-butadiyne-linked diruthenium complex 4 is successfully
brought onto the gold surface in a lying flat mode to form
self-assembled monolayers (SAMs) showing reversible multiple
redox behaviors on the electrode surface. The diruthenium
III,III
species with different oxidation states, particularly the Ru2
state which is unstable and impossible to isolate from the
solution, can be detected by in situ IR spectroscopy.
Linear organometallic complexes with unsaturated carbon
chains spanning redox-active metal centers are attractive
candidates for molecular wires that facilitate intramolecular
electron transfer along the molecular backbone.1–3 It is known
that self-assembly of functional molecules onto the surface is a
key step for their applications in molecular electronics.4–7
Inspired by the findings that some thiol-functionalized metal
complexes display specific spectroscopic and redox properties
Scheme 1 Synthetic routes for [1]+–[4]2+. (i) Me3SiCRCCRCSiMe3,
KF; (ii) piperidine; (iii) 5-(1,2-dithiolan-3-yl)pentanoic acid and
upon the formation of self-assembled monolayers (SAMs)
onto gold surfaces,8–11 we are interested in bringing such
organometallic systems onto the solid surface, and examining
the intramolecular electron transfer as well as spectroscopic
properties. Herein we reported a butadiyne-linked diruthenium
system immobilized onto a gold surface in a lying flat mode
(Scheme 1) using disulfide-functionalized terpyridine as an
ancillary ligand.
As shown in Scheme 1, [2]2+ was prepared by reaction
of Me3SiCRCCRCSiMe3 with two equivalents of [1]+ via
fluoride-catalyzed desilylation in a microwave synthesizer at
N,N0-dicyclohexylcarbodiimide; (iv) self-assembled onto a gold surface.
[2]2+ exhibits three redox waves at E1/2 = 0.24, 0.76, and
1.34 V in a 0.1 M n-Bu4NPF6–CH2Cl2 solution (see ESIw).
These peaks can be ascribed to successive 1-eꢀ redox processes
III,IV
of Ru2II,II/Ru2II,III, Ru2II,III/Ru2III,III, and Ru2III,III/Ru2
,
respectively.12 The DE1/2 between the subsequent redox waves
was approximately 0.52 V. The comproportionation constant
III,II
Kc for the formation of Ru2
species was 6.18 ꢁ 108,
suggesting a quite large Ruꢂ ꢂ ꢂRu electronic interaction.12
The IR spectra of [2](ClO4)2 and [2a](ClO4)3 isolated
gave a C–C stretching mode for the CRC bond at 1967
and 1855 cm–1, respectively, in the KBr matrix (see ESIw). The
UV-Vis spectrum of [2]2+ in CH2Cl2 displayed an intense
absorption in the UV region together with a broad band at
642 nm, arising likely from dp(Ru) - p*(tpy) MLCT
(metal-to-ligand charge transfer)12 (see ESIw) transition. Once
[2]2+ was oxidized to [2a]3+, the MLCT band was blue-shifted
to 602 nm with a significantly reduced intensity, and two new
broad bands appeared at 820 and 1010 nm. The former
(820 nm) was probably ascribable to a ligand - RuIII LMCT
(ligand-to-metal charge transfer) transition whereas the latter
(1010 nm) results most likely from RuII - RuIII intervalence
charge transfer (IVCT) transition.12 Solvent independence and
a high extinction coefficient (e = 25100 Mꢀ1 cmꢀ1) of the IVCT
II,II
100 1C for 20 min in ca. 30% yield. By oxidation of Ru2
complex [2]2+ with [Cp2Fe](ClO4) or electrolysis at 0.5 V
(vs. Ag/AgCl), we were able to isolate a 1-eꢀ oxidized complex
II,III
[2a]3+ with a mixed-valence state of Ru2
as a perchlorate
salt in 80% yield. Isolation of further oxidized Ru2III,III species
was unattainable. Removal of amino-protecting groups in
[2]2+ using piperidine gave [3]2+ which directly reacted
with 5-(1,2-dithiolan-3-yl)pentanoic acid in the presence of
N,N0-dicyclohexyl-carbodiimide, giving the target compound
of [4]2+ in 50% yield.
a State Key Laboratory of Structural Chemistry,
Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou 350002, China.
E-mail: czn@fjirsm.ac.cn
b Catalysis Research Center, Hokkaido University, Sapporo 001-0021,
Japan. E-mail: ysasaki@sci.hokudai.ac.jp
band together with a large Kc of [2a]3+ reveal an electronically
c Division of Chemistry, Graduate School of Science, Hokkaido
University, Sapporo 060-0810, Japan
II,III
delocalized Ru2
system along the molecular backbone.
These results indicate that [2]2+ and [2a]3+ with substituted
phenyl terpyridyl ligands preserve the mixed-valence properties
similar to their analogs.12a
w Electronic supplementary information (ESI) available: Experimental
details and figures giving additional spectral and electrochemical
properties. See DOI: 10.1039/c0cc03048a
c
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Chem. Commun., 2011, 47, 923–925 923

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The target compound [4]2+ gave two redox peaks at 0.21
and 0.75 V in a 0.1 M n-Bu4NPF6–CH2Cl2 solution, which can
II,III
be attributed to the stepwise 1-eꢀ processes of Ru2II,II/Ru2
and Ru2II,III/Ru2III,III, respectively (see ESIw), with DE1/2 of
ca. 0.54 V comparable to that of [2]2+ (0.52 V). Another
quasi-reversible wave that appeared at 1.14 V was ascribable
to that of Ru2III,III/Ru2III,IV. The negative shift of the redox
potentials of [4]2+ compared with those of [2]2+ indicates the
higher electron donation from the alkyl disulfide groups to Ru
central ions.
As [4]2+ was self-assembled onto a gold electrode surface
(denoted as 4Au), the peak currents of both redox waves are
correlated linearly with the scan rates (Fig. 1, inset), indicating
that [4]2+ is immobilized onto the gold surface. The redox
II,III
III,III
and Ru2II,III/Ru2
waves due to Ru2II,II/Ru2
processes
are observed at 0.25 and 0.88 V, respectively (Fig. 1). The peak
separations between the anodic and cathodic peaks are around
50 mV, obviously smaller than those of [4]2+ in solution
(70–80 mV), indicating an improved redox reversibility of
[4]2+ after immobilization on the gold surface. The DE1/2
(0.63 V) of 4Au is distinctly larger that in solution (0.54 V),
implying that the SAMs of [4]2+ on surface are more favorable
II,III
for the mixed-valence state of Ru2
.
The surface coverage of 4Au estimated from the electric
charge of the anodic peak around 0.25 V (Fig. 1) is 1.5 ꢁ
10ꢀ11 mol cmꢀ2. This value is approximately 43% of the
theoretical value (ca. 3.5 ꢁ 10ꢀ11 mol cmꢀ2) assuming that
the unit area is 13 A ꢁ 33 A, based on the crystal structure of
analogs.12 Since [4]2+ contains four triphenyl phosphine and
two phenyl terpyridyl ligands, the head groups of the molecule
are very rigid and bulky, it is likely that [4]2+ arrays on
the gold surface with its two extending tilt arms of alkyl
disulfide groups.
Fig. 2 (a) Time-resolved IR spectra of 4Au recorded sequentially
during a potential sweep from +0.50 V - +1.10 V - +0.50 V - 0
V
- +0.50 V at a scan rate of 5 a 0.1 M
mV sꢀ1 in
n-Bu4NPF6–CH2Cl2 solution. The time resolution is 5 s corresponding
to a sampling interval of 25 mV. Each spectrum is co-added from
50 interferograms. The background spectrum was recorded at 0.50 V.
(b) Potential dependence of the normalized IR peak intensity
(Ai/Amax) of three n(CRC) bands and electric charges passed during
the potential cycles observed in (a). Ai is the IR peak intensity of
CRC in each spectrum in (a), and Amax is the maximum value.
To further elucidate the structure and electron distribution
of 4Au on the gold electrode surface, in situ surface-enhanced
IR absorption spectroscopy (SEIRAS) in the attenuated total
reflection mode has been employed during its redox process
(see ESIw for detailed experimental conditions for the in situ
IR measurements).9e,f
The upward and downward peaks correspond to the species
formed and disappeared, respectively, in comparison with the
reference potential.
Fig. 2a shows a series of in situ IR differential spectra
(2200–1650 cmꢀ1) simultaneously recorded with a potential
sweeping (5 mV sꢀ1) as +0.50 V - +1.10 V - +0.50 V -
0 V- +0.50 V, with respect to an IR spectrum obtained at 0.50 V
as a reference where 4Au exists as Ru2II,III (see the arrow in Fig. 1).
As shown in Fig. 2a, two IR bands appear at 1863 and
1747 cmꢀ1 in the potential cycle between 0.5 and 1.10 V, and
two IR bands at 1969 and 1863 cmꢀ1 appear in the subsequent
potential cycle between 0.5 and 0 V. Fig. 2b shows the IR band
intensities (symbols) as well as the electric charge (solid lines)
passed as a function of electrode potential. During the cycle in
the higher potential region, a downward band at 1863 cmꢀ1
and an upward band at 1747 cmꢀ1 appear from ca. 0.78 V and
change quickly with potential and become almost constant
when the potential is higher than 0.96 V. In the subsequent
negative-going sweeping, these two IR bands change in a
reversed way with a small hysteresis to the positive-going
sweeping. The potential dependence of the IR band intensities
is in agreement with that of charges recorded at each potential
during the IR measurement, indicating that the IR bands
III,III
correspond exactly to the redox process of Ru2III,II/Ru2
Fig.
1
Cyclic voltammograms (CVs) of 4Au in a 0.1 M
in the SAM (Fig. 1 and 2b). The pair of IR bands and charges
in the potential cycle between 0.5 and 0 V show similar
potential behaviors (Fig. 2b) and correlate well with the redox
n-Bu4NPF6–CH2Cl2 solution. The scan rates are 50, 100, 200, 300,
400, and 500 mV sꢀ1, respectively. Inset: a plot of peak current of the
anodic wave (E1/2 = 0.25 V) with scan rates.
c
924 Chem. Commun., 2011, 47, 923–925
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II,III
II,III
peaks of Ru2II,II/Ru2
in the SAM (Fig. 1). No downward
of the mixed-valence state Ru2
is even higher on the
or upward peaks are observed when the potential is swept
back to the reference potential (0.50 V), indicating that these
redox species are stable and no decomposition takes place in
the SAM during the potential cycles.
surface (Kc is 4.47 ꢁ 1010 against 1.34 ꢁ 109 in solution).
The information on the electronic structures of the SAMs
in three oxidation states, particularly the more stabilised
II,III
III,III
mixed-valence Ru2
state and the Ru2
state which is
Based on the comparison with the IR spectra for each
isolated diruthenium complex in the KBr matrix and CV
results (Fig. 1), IR bands at 1863 and 1969 cmꢀ1 can be
inaccessible in solution, has been provided by the in situ IR
absorption spectroscopy.
We thank financial supports from the NSFC (20625101,
20773128, 20821061 and 20931006), the 973 project
(2007CB815304) from MSTC, and the NSF of Fujian
Province (2008I0027). H.-X. Zhang is grateful to JSPS for a
postdoctoral fellowship.
II,III
II,II
assigned to n(CRC) of the Ru2
and Ru2
species,
respectively. Although the IR band at 1747 cmꢀ1 observed
on the surface could not be found in the KBr matrix, this band
III,III
can be definitely assigned to that of n(CRC) in the Ru2
species which is unable to isolate from solution. Therefore, all
three oxidation states (Ru2II,II, Ru2II,III, Ru2III,III) of 4Au have
been distinctly identified from the in situ IR measurements. As
Notes and references
II,III
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soon as the electron transfer takes place from that of Ru2
at 0.50 V, a new peak appeared at 1747 cmꢀ1 (1eꢀ oxidation,
Ru2III,III) or 1969 cmꢀ1 (1eꢀ reduction, Ru2II,II) with the
disappearance of itself at 1863 cmꢀ1. As the RuII central ions
are oxidized to RuIII, electrons are expected to flow from
CRC to RuIII, the bond order of CRC is thus reduced
and the IR absorption band is shifted to lower frequency
(red-shift).12 Conversely, the reduction process of Ru ion will
induce a blue-shift. These results suggest that the oxidation of
the Ru ions can significantly affect the electronic state and thus
can give useful information for conjugation states of CRC
with the Ru ions. Firstly, it is interesting to note that the IR
II,III
band for the CRC bond at 0.5 V (corresponding to Ru2
)
gave a single narrow IR band, indicating that the electrons on
Ru ions should be definitely delocalized between RuII
and RuIII (i.e. RuII.5RuII.5 is a more reasonable notation for
the real mixed valence state of RuIIRuIII). Secondly, the peak
shifts for RuII,III/RuIII,III, RuII,III/RuII,II are 116 and 106 cmꢀ1
,
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respectively (Fig. 2). In the previous in situ IR studies on SAM of
the triruthenium cluster [Ru3(m3-O)(m-CH3COO)6(CO)(L1)(L2)]
(L1 = [(NC5H4)CH2NHC(O)(CH2)10S–]2, L2 = 4-methyl-
pyridine),9a–c Ye et al. reported that the first 1eꢀ oxidation
of the Ru central ions in the SAM induces a large blue-shift
for CO ligand (120 cmꢀ1) in comparison with its second 1eꢀ
oxidation (blue-shift, 54 cmꢀ1) and first 1eꢀ reduction
processes (red-shift, 50 cmꢀ1). The large blue-shift in the first
1eꢀ oxidation step has been successfully explained by the
electron localization on the Ru central ions where CO ligand
is directly binded. The similar IR peak shifts (116/106 cmꢀ1
)
and band width (ca. 20 cmꢀ1) for the oxidation and the
II,III
reduction processes of Ru2
in 4Au SAM indicate similar
influence on the CRC bonds by the two electron transfer
state
III,III
II,III
steps from Ru2II,III. This confirms that the Ru2
should be a delocalized system from which Ru2
II,II
and
are formed through 1eꢀ charge transfer steps with
Ru2
11 F. A. Murphy, S. Suarez, E. Figgemeier, E. R. Schofield and
´
S. M. Draper, Chem.–Eur. J., 2009, 15, 5740.
similar band shifts (Fig. 2).
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Butadiyne-linked diruthenium complex is successfully
immobilized onto a gold surface to form SAMs, in which
the Ru-centered redox chemistry is improved and stable mixed-
valence state is observed in a better quality. The stability
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 923–925 925