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Impact of Skeletal Isomerization of Ultrasmall Gold Clusters on
Electrochemical Properties: Voltammetric Profiles of Non-spoked
Octanuclear Clusters.
Yutaro Kamei,1 Neil Robertson,3 Yukatsu Shichibu,1,2 Katsuaki
Konishi,1,2*
1 Graduate School of Environmental Science, Hokkaido University,
North 10 West 5, Sapporo 060-0810 (Japan). 2 Faculty of
Environmental Earth Science, Hokkaido University, North 10 West 5,
Sapporo 060-0810 (Japan). 3 School of Chemistry, University of
Edinburgh, West Mains Road, Edinburgh EH9 3JJ (UK)
*E-mail: [email protected]
KEYWORDS. cyclic voltammetry, geometrical structure,
nanocluster, noble metal, electronic structure
ABSTRACT. Electronic properties of ultrasmall gold clusters with
defined nuclearity and geometrical structures have been a recent
subject of interest not only with respect to the concept of
molecularity but also because of their potential applicability as
nanomaterials and catalysts. In this work, the electrochemical
properties of dppp-protected octagold clusters ([Au8L4]n+ (L =
dppp, n = 2 (1) and 4 (2), dppp = Ph2P(CH2)3PPh2) with
charge-dependent geometrical structures were investigated. Unlike
conventional sphere-like centered clusters held by multiple spokes,
the non-spoked Au8 clusters displayed irreversible electrochemical
profiles for the two-electron redox interconversion between 1 and
2, exhibiting a wide energy gap between the redox couples. This
electrochemical irreversibility could be attributed to the
significant alteration of electronic structures associated with the
redox-coupled isomerization of the non-spoked cluster structures.
In addition, we show that the coordinative interaction of Cl−
anions with the Au8 clusters notably affects both reduction and
oxidation courses, providing an example of coordination-coupled
electron transfer events.
INTRODUCTION
Gold clusters with sizes of less than ~2 nm have attracted
continuing interest because of their distinct properties and
reactivities that are not found in conventional colloidal
nanoparticles.1-5 Especially, there is keen interest in expanding
their redox chemistry not only from the fundamental aspects
associated with the molecularity of cluster compounds but also in
relation to the development of quantum capacitors 6-9 and redox
catalysts.10-14 Recent progress in atomic-level
characterization15-17 has offered opportunities for the concise
nuclearity- and structure-based investigation of the
electrochemical properties of several subnanometer clusters with
nuclearity of less than 40.17-24 Examples include
[Au9(PPh3)8]q,17-18 [Au25(SR)18]q,20-22 and [Au38(SR)24]q 23
clusters, which are known to possess conventional polyhedral cores
containing multiple gold-gold spokes radiating from inner
center(s). Voltammetric studies have indicated that they behave
like conventional molecules, exhibiting electrochemically
reversible patterns with a small potential gap between the redox
couples.
During the course of our recent study on diphosphine-coordinated
gold clusters,25-26 we found some examples with exceptional
non-spherical geometries displaying unique optical features.27-32
Among them, the octanuclear cluster species coordinated by four
dppp (Ph2P(CH2)3PPh2, L) ligands (Au8L4) is quite interesting,
since it offers two isomeric non-spoked structures depending on the
oxidation state of the cluster unit (Figure 1).28, 30
Crystallographic studies revealed that the reduced form [Au8L4]2+
(1) has a gold tritetrahedral unit, whereas the oxidized form
[Au8L4]4+ (2) adopts a [core+exo]-type structure composed of a
bitetrahedral Au6 core and two extra gold atoms. The coordination
sites of the gold atoms in 1 are fully occupied by eight phosphorus
atoms, whereas [Au8L4]4+ has two coordination unsaturated sites and
hence has been isolated as a divalent cation ([Au8L4X2]2+ (2-X2, X
= Cl, C≡CR)) by accommodating two anionic ligands (X). Because
these redox isomers exhibit distinctly different optical properties
(color and photoluminescence), it would be interesting to
investigate their electrochemical properties in relation to the
development of electrochromsim materials. Herein, we report unusual
irreversible voltammetric profiles of the Au8L4 clusters to
demonstrate that the electronic structures of the non-spoked
clusters are substantially altered by redox-induced isomerization
of the cluster units. Moreover, we provide an example of
coordination-coupled electrochemical processes by showing the
notable effects of chloride ions on the voltammetric profiles.
Figure 1. Crystallographically determined structures of two
redox isomers of the dppp-coordinated Au8 cluster
[Au8(dppp)4]n+.
EXPERIMENTAL SECTION
[Au8(dppp)4](NO3)2 (1·(NO3)2) and [Au8(dppp)4Cl2](PF6)2
(2-Cl2·(PF6)2) were prepared and identified, as reported
previously.28 Electrochemical measurements were performed on an ALS
600A electrochemical analyzer under nitrogen atmosphere in dry
dimethylformamide (DMF) containing 0.1 M tetrabutylammonium
tetrafluoroborate (TBABF4). A three-electrode system comprised a
1.6-mm diameter Pt-disk (working electrode), an Ag/Ag+ (reference
electrode) and a Pt coil (counter electrode) were employed.
Anhydrous DMF was obtained from Kanto Chemicals. TBABF4 (Aldrich,
99%) was recrystallized from methanol/diethyl ether three times,
and the crystals were crushed with a spatula and dried under
reduced pressure at 60 °C in an oil bath for 3 h prior to the
measurement. Tetrabutylammonium chloride (TBACl) (Aldrich, 97%),
tetrabutylammonium trifluoromethanesulfonate (TBACF3SO3) (Tokyo
Kasei, >97%), and tetrabutylammonium hexafluorophosphate
(TBAPF6) (Tokyo Kasei, >98%) were recrystallized from
methanol/diethyl ether. UV–vis spectra in the
spectroelectrochemical experiments were obtained on an ALS SEC2000
using an electrochemical cell of 0.5-mm path length incorporating
the three-electrode system.
RESULTS AND DISCUSSION
Cyclic voltammetry (CV) of [Au8L4]2+ (1). Electrochemical
properties of the reduced form 1·(NO3)2 was investigated by CV
measurement in DMF at room temperature in the presence of 0.1 M
tetrabutylammonium tetrafluoroborate (TBABF4) as the electrolyte.
When the solution was scanned to positive at a scan rate of 0.1 V/s
from −0.65 V (vs. Fc+/Fc), a single oxidation peak was observed at
−0.23 V (Figure 2a (i)). On the other hand, the consecutive back
scan gave two weak peaks at −1.25 and −1.53 V (ii). CV patterns
were almost reproduced without loss of current intensity even after
15 redox cycles (Figure S1a). The cluster species generated in the
CV cycle was identified by spectroelectrochemical (SEC) studies.
Prior to electrolysis, 1·(NO3)2 exhibited an absorption band at 520
nm together with a shoulder at 590 nm (Figure 2b (i)). Upon
electrolysis at −0.25 V for 8 min, the band at 520 nm shifted to
512 nm with the disappearance of the shoulder, giving a single-band
spectrum (ii), which is similar to those reported for the [Au8]4+
species having a [core+exo] structure (bitetrahedral Au6 core + two
exo Au atoms) accommodating two anionic ligands ([Au8L4X2]2+ (2-X2,
X = Cl, C≡CR)).28, 30 It is generally known that the absorption
spectral patterns of molecular gold clusters reflect the
geometrical structures of the metal moieties.27 Therefore, the
oxidation product should have a similar Au6+2Au structure, which
however accommodates no anionic ligands to exist as 2 because
the
Figure 2. Cyclic voltammogram of (a) [Au8L4](NO3)2 (1·(NO3)2) in
DMF (0.8 mM) containing 0.1 M TBABF4 (v = 0.1 V/s) under nitrogen
atmosphere at room temperature. The arrow indicates the initial
sweep direction. (b) Absorption spectra of 1 in DMF containing
TBABF4 before (i) and after (ii) electrolysis at −0.25 V for 8 min,
and (iii) the sample (ii) after electrolysis at −1.6 V for 10
min.
coexisting anions (NO3− and BF4−) have low (or no) coordination
abilities. Subsequent reduction of this solution by electrolysis at
−1.6 V regenerated the original spectrum of 1 (iii). No further
reduction of [Au8L4]2+occurs because no peaks were detected in the
measurement range when the initial scan was performed toward the
negative direction (Figure S2). Therefore, the CV profile in Figure
2a represents the interconversion between 1 and 2. Both the
oxidation and reduction processes appeared to proceed cleanly
without any detectable intermediate species because isosbestic
points were observed in the absorption spectra tracing experiments
(Figure S3).
Isomerization-coupled Electrochemical Irreversibility. As
mentioned above, the electrochemical two-electron redox cycles
between 1 and 2 occurred in a chemically reversible manner.
However, the oxidation and reduction courses were clearly
different. The anodic and cathodic waves were separated from each
other with a large potential gap (ΔE > ~1.0 V). The energy gap
value was hardly affected by the scan rate (Figure S4), indicating
that it does not arise from diffusion-controlled events.
Furthermore, the patterns of the redox waves were inequivalent. The
two-electron oxidation of 1 to 2 gave one peak indicative of a
single step (Figure 2a (i)), whereas the reverse reduction from 2
to 1 occurred in a one-by-one manner with significantly smaller
current intensities (ii). Thus, the redox course between 1 and 2 is
chemically reversible but is electrochemically irreversible.
The electrochemical irreversibility thus observed is possibly
due to a significant change in the electronic structure upon
redox-coupled isomerization of the non-spoked skeleton between
[Au8L4]2+ (1) and [Au8L4]4+ (2) (Figure 1). It should be noted that
such a large potential gap has not been reported for conventional
gold cluster compounds with spoked geometries, which behave like
common molecules exhibiting redox wave pairs with a small energy
gap. For example, [Au9(PPh3)8]3+, which has a centered toroidal
geometry, is reported to give a clear set of reversible CV waves
associated with two-electron transfer processes (ΔE = ~0.13 V at
0.1 V/s).18 A similar reversible redox profile has been reported
for the interconversion between Au25(SR)18 and [Au25(SR)18]− having
a centered icosahedral Au13 core.19-21 X-ray structures of the
reduced and oxidized forms of these clusters have shown that
structural modifications upon reduction/oxidation are not so
significant, which is presumably because the cluster structure is
held by multiple spokes radiating from the central atom.18, 33
Accordingly, the redox events may only have marginal influence on
their inherent electronic structures (e.g., HOMO and LUMO). In
contrast to these spoked clusters, the redox-induced
interconversion between 1 and 2 accompanies large geometrical
changes in the Au8 skeleton (Figure 1). This may cause a
substantial alteration in the electronic structure of the Au8 unit,
whereby a large energy gap between the redox couples results. Thus,
the observed electrochemical irreversibility can be regarded as a
specific feature of non-spoked clusters. Such energy gaps between
the redox couples have also been reported for several metal
complexes, for which the involvement of redox-induced changes of
the coordination environments (ligand reorganization) has been
suggested.34-36 On the other hand, in the present case, the
isomerization of the Au8 cluster unit, involving the rearrangement
of the gold atoms, is primarily responsible for the irreversible
nature.
CV of [Au8L4]4+ with two Cl ligands (2-Cl2). The electrochemical
properties of the oxidized counterpart ([Au8]4+) were also
investigated under similar conditions using 2-Cl2·(PF6)2, bearing
two built-in Cl ligands, as the starting cluster. As expected from
the abovementioned isomerization-coupled redox processes between 1
and 2, the reduction/oxidation waves were clearly separated to
exhibit an electrochemically irreversible feature (Figure 3), and
were reproducible after several cycles (Figure S1b). However, the
voltammetric pattern was
Figure 3. Cyclic voltammogram of [Au8L4Cl2](PF6)2 (2-Cl2·(PF6)2)
in DMF (~0.8 mM) containing 0.1 M TBABF4 (v = 0.1 V/s) under argon
at room temperature. The arrow indicates the initial sweep
direction.
Table 1. Cyclic voltammetry data of [Au8L4](NO3)2 (1·NO3) and
[Au8L4Cl2](PF6)2 (2-Cl2·PF6).a
entry
starting cluster
additive
Epa / V b
Epc / V b
1
[Au8L4]2+ (1)
none
−0.23
−1.25, −1.53
2
[Au8L4Cl2]2+ (2-Cl2)
none
−0.24, –0.47
−1.36
3
[Au8L4]2+ (1)
TBACl (4 eq.)
−0.58
−1.43
4
TBAOTf (4 eq.)
−0.24
−1.26, −1.52
5
TBAPF6 (4 eq.)
−0.25
−1.28, −1.55
6
[Au8L4Cl2]2+ (2-Cl2)
TBACl (4 eq.)
−0.51
−1.38
a In DMF containing 0.1 M TBABF4 at room temperature at 0.1 V/s.
b Versus Fc/Fc+. From three or more independent experiments. Errors
were estimated to be less than ±0.03 V.
considerably different from that obtained for the redox cycle
starting from pure 1 (Figure 2a). As summarized in Table 1, the
negative scan of 2-Cl2·(PF6)2 gave an intense single reduction peak
at −1.36 V (entry 2), which is in contrast to and lies between the
twin peaks (entry 1) observed in the reduction of the in situ
generated [Au8L4]4+ species (2) from 1 (Figure 2a (i)). The single
peak in the reduction of 2-Cl2 (Figure 3 (i)) indicated that the
two-electron transfer occurred in an all-at-once manner. As noted
in the previous section, 2 and 2-Cl2 should have similar
[core+exo]-type structures. Therefore, it is likely that the
coordination of Cl− anions to 2 drastically altered the reduction
process.
A difference in the voltammetric pattern was also observed in
the [Au8]2+ → [Au8]4+ oxidation process. The [Au8L4]2+ species (1),
which was generated in situ by the electrochemical reduction of
2-Cl2 (Figure 3 (i)), showed two weak waves at −0.47 and −0.24 V
(iii, iv), which were in contrast to the single peak observed for
the oxidation of pure 1 (Figure 2a (i)). However, it should be
noted that one of the two oxidation waves in Figure 3 (−0.24 V)
almost coincided in potential with that observed in Figure 2a
(−0.23 V) (Table 1, entries 1 and 2). The reductive formation of 1
from 2-Cl2 accompanies the liberation of Cl anions (Figure 3 (i)),
which exist as free anions during the subsequent oxidative process.
Therefore, it is likely that the peak at −0.47 V resulted from the
interaction of Cl− anions with 1 (Figure 3 (ii and iii)), whereas
the peak at −0.23 V may be assigned to the oxidation of “Cl-free” 1
(Figure 1a (i), Figure 3).
Effects of Anion Coordination on the Electrochemical Courses
between 1 and 2. As mentioned earlier, the marked difference in the
voltammetric profiles of the reduced form 1 and the Cl-bound
oxidized form 2-Cl2 could originate from the interaction of the Au8
clusters with Cl− anions. The perturbation effects of Cl− anions
were more explicitly observed in the electrochemical behavior of
1·(NO3)2. As noted in the previous section, the voltammogram of
1·(NO3)2 under chloride-free conditions showed an intense oxidation
peak at −0.23 V and two
Figure 4. Cyclic voltammograms of 1·(NO3)2 in DMF (0.8 mM)
containing 0.1 M TBABF4 (v = 0.1 V/s) under nitrogen atmosphere at
room temperature in the absence (a) and presence (b) of TBACl (4
molar equiv).
weak reduction peaks (Figure 2a) (Table 1, entry 1). When the CV
measurement was conducted in the presence of tetrabutylammonium
chloride (TBACl, 4 molar equiv.), the oxidation peak negatively
shifted to −0.58 V, which is closer to the lower-potential
oxidation peak found for 2-Cl2 (Figure 3 (iii)), and the two
reduction peaks merged into a single peak at −1.43 V with a larger
current intensity (Table 1, entry 3) (Figure 4). Such changes were
not observed when TBA salts of PF6− and CF3SO3− with weak
coordinating abilities were used as the additive instead of TBACl
(Table 1, entries 4 and 5). Accordingly, the CV measurements of
1·(NO3)2 in DMF containing 0.1 M TBACF3SO3 or TBAPF6 electrolyte
exhibited voltammetric patterns similar to that with TBABF4 (Figure
S5). Therefore, it is possible that the coordinative interaction
with Cl− anions critically affects the electrochemical properties
of the Au8 cluster. It should also be noted that the effects of Cl−
anions were observed both for the isomeric reduced ([Au8]2+) and
oxidized ([Au8]4+) forms. For the oxidation of 1, the binding of
Cl− anions with the tritetrahedral core of 1 could be involved
(Figure 3 (ii)) but apparently 1 has no coordination sites.
Therefore, weak binding or ligand exchange of Cl− anions may reduce
the HOMO energy level to facilitate electron abstraction from the
cluster (oxidation) at a lower potential. On the other hand, the
effect of Cl− anions on the reduction process is reasonable because
2 has vacant coordination sites to readily accommodate Cl− anions.
The change in the voltammetric pattern and increase in the current
intensities implied that the binding of Cl anions to 2 not only
alters the cluster electronic structure but also promotes the
electron transfer process. In accordance with these observations,
the reduction courses of 2-Cl2 bearing built-in Cl ligands were not
significantly affected by Cl− anions (Table 1, entry 2 vs. 6),
whereas the effects of Cl− anions on the oxidation course was
ambiguous because the profiles at > ~ −0.2 V were obscured by
the overlap with the Cl− oxidation wave in the presence of excess
TBACl (Figure S5).
CONCLUSION
Unlike colloidal nanoparticles, gold clusters in the
subnanometer range have individual geometrical structures which
exhibit molecular-like behaviors depending critically on their
nuclearity and geometrical features. Among them, non-spoked
clusters are an exceptional but interesting family because of their
unique optical/electronic structures that are distinctly different
from conventional spherical spoked clusters. In this paper, we have
provided an example of the unusual properties of non-spoked
clusters in their electrochemical irreversibility, demonstrating
that the redox-coupled skeletal isomerization of the Au8 unit
causes a substantial impact on the electronic structures. We have
also shown that the electronic structures are influenced by anion
coordination, providing an implication that the interaction of
coexisting species with the metal moieties must be taken into
consideration in the study of electronic properties of small
cluster compounds. The unique electrochemical properties of
non-spoked clusters presented in this paper are interesting in
relation to the development of molecular memories and switching
devices, considering that the two states can be easily
discriminated by simple spectroscopic techniques (color and
photoluminescence). In this respect, the present results expand the
scope of the utility of ultrasmall metal clusters for functional
materials. The electrochemical properties of a series of related
ultrasmall clusters are worthy of further investigation.
Supporting Information. CV profiles under several different
conditions. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]
ACKNOWLEDGMENT
This work was supported by JSPS Institutional Program for Young
Researcher Overseas Visits for Y.K. Supports from MEXT/JSPS
Grant-in-Aids (20111009 and 24350063 for K.K. and 24750001 for
Y.S.) and the Asahi Glass Foundation (K.K.) are also
acknowledged.
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Table for Contents Image
Impact of Skeletal Isomerization of Ultrasmall Gold Clusters on
Electrochemical Properties: Voltammetric Profiles of Non-spoked
Octanuclear Clusters
1
18
– 2e–
-1.5 -0.5 0.0-1.0E / V vs Fc+/Fc
50 µA / cm2
[Au8L4]2+ (1) + n Cl– [Au8L4]2+ ···(Cl–)n
[Au8L4]2+ ···(Cl–)n
(ii)
(i) [Au8L4]2+ (1) + 2Cl–[Au8L4Cl2]2+ (2-Cl2)2e–
(iii) [Au8L4Cln](4-n)+ (2-Cln)
(i) 2-Cl2 → 1
(iii)
(iv) [Au8L4]4+ (2)[Au8L4]2+ (1)– 2e–
(iv)
-1.5 -0.5 0.0-1.0
E / V vs Fc+/Fc
50 µA / cm2
(a)
(b)
-1.5
-0.50.0-1.0
E / V vs Fc
+
/Fc
50 µA / cm
2
(a)
(b)
[Au8L4]2+ (1) [Au8L4]4+ (2)
Wavelength / nm500 600
(i)
(ii)
(iii)
(b)
Abs
520
512
590
(i) 1 → 2
(ii) 2 → 1
50 µA / cm2
-1.5 -0.5 0.0-1.0E / V vs Fc+/Fc
(ii)
(i) [Au8L4]4+ (2)[Au8L4]2+ (1)– 2e–
e–[Au8L4]4+ (2) [Au8L4]3+ e–
[Au8L4]2+ (1)
(a)
