Voltammetric studies on decaphenylferrocene, substituted decaphenylferrocenes, and their oxidized forms in dichloromethane and ionic liquids

Article abstract of DOI:10.1021/om049117c

Cyclic voltammetric studies have been undertaken on highly insoluble decaphenylferrocene (DPFc), on substituted forms of the compound [methyl (Me₂DPFc, Me₆DPFc, and Me₁₀DPFc), bromide (Br₂DPFc), and ethynyl (Ethin₂DPFc)], and on their more soluble one-electron-oxidized DPFc⁺ forms, Studies with DPFc ⁺ and its derivatives dissolved in dichloromethane (DCM) enable the reversible potentials (E^0′) of one-electron reduction processes, DPFc⁺(DCM) + e^- ? DPFc(DCM), to be established. E^0′ values obtained from these measurements exhibit the expected substituent dependence. Voltammetric studies with arrays of insoluble DPFc microparticles adhered to an electrode surface in contact with DCM (0.1 M Bu₄NPF₆) or ionic liquids are connected to the oxidized solid or dissolved forms of DPFc^⊥ via a range of pathways that are determined by the kinetics and thermodynamics of dissolution and reprecipitation processes. Comparative data obtained with more soluble ferrocene, ferricinium, and decamethylferrocene microparticles support the hypothesis that dissolution processes play an important role in determining the nature of the voltammetric response of adhered microparticles.

Full text of DOI:10.1021/om049117c

2188
Organometallics 2005, 24, 2188-2196
Voltammetric Studies on Decaphenylferrocene,
Substituted Decaphenylferrocenes, and Their Oxidized
Forms in Dichloromethane and Ionic Liquids
Jie Zhang and Alan M. Bond*
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
Herbert Schumann* and Klaudia Su¨hring
Institut fu¨r Chemie, Technische Universita¨t Berlin, Strasse des 17 Juni 135,
D-10623 Berlin, Germany
Received November 16, 2004
Cyclic voltammetric studies have been undertaken on highly insoluble decaphenylferrocene
(DPFc), on substituted forms of the compound [methyl (Me2DPFc, Me6DPFc, and Me10DPFc),
bromide (Br2DPFc), and ethynyl (Ethin2DPFc)], and on their more soluble one-electron-
oxidized DPFc+ forms. Studies with DPFc+ and its derivatives dissolved in dichloromethane
(DCM) enable the reversible potentials (E0′) of one-electron reduction processes, DPFc+(DCM)
+ e- / DPFc(DCM), to be established. E0′ values obtained from these measurements exhibit
the expected substituent dependence. Voltammetric studies with arrays of insoluble DPFc
microparticles adhered to an electrode surface in contact with DCM (0.1 M Bu4NPF6) or
ionic liquids are connected to the oxidized solid or dissolved forms of DPFc+ via a range of
pathways that are determined by the kinetics and thermodynamics of dissolution and
reprecipitation processes. Comparative data obtained with more soluble ferrocene, ferri-
cinium, and decamethylferrocene microparticles support the hypothesis that dissolution
processes play an important role in determining the nature of the voltammetric response of
adhered microparticles.
Introduction
No electrochemical data are available on substituted
forms of DPFc. The challenge to be met in order to
obtain such data and hence establish the substituent
effect is to find media that eliminate or take into account
the presence of dissolution or precipitation processes8,9
Electrochemical studies on ferrocene (Fc) compounds
are very widespread.1 Typically, a well-defined revers-
ible Fc h Fc+ + e- process is detected when both Fc
and Fc+ are soluble in the media of interest. In the case
of decaphenylferrocene (DPFc), the preparation and
characterization of the symmetrical form of the highly
insoluble ferrocene derivative proved to be highly
challenging.2-4 Furthermore, lack of solubility in many
media has meant electrochemical studies in organic
solvents have been restricted2,3 to reduction of the more
soluble DPFc+ to DPFc in dichloromethane (DCM) or
acetonitrile,2,5 although the complex oxidation of DPFc
and DPFc+ adhered to an electrode surface in contact
with aqueous (or water/aceotonitrile (70:30) mixed
solvent) electrolyte solutions has been described.6,7
that almost certainly will accompany a DPFc+ + e-
/
DPFc type electron transfer process. Ionic liquids10 that
have been used as an alternative to organic solvents
with electrolyte in electrochemical studies on Fc and
other compounds11-13 may provide an opportunity to
achieve this goal with respect to the DPFc system.
(7) Bond, A. M.; Lamprecht, A.; Tedesco, V.; Marken, F. Inorg. Chim.
Acta 1999, 291, 21.
(8) (a) Schro¨der, U.; Scholz, F. Inorg. Chem. 2000, 39, 1006. (b)
Scholz, F.; Meyer, B. In Electroanalytical Chemistry; Bard A. J., Ed.;
Marcel Dekker: New York, 1998; Vol. 20, p 1, and references therein.
(c) Scholz, F.; Meyer, B. Chem. Soc. Rev. 1994, 23, 341, and references
therein.
(9) (a) Zhang, J.; Bond, A. M.; Richardt, P. J. S.; Wedd, A. G. Inorg.
Chem. 2004, 43, 8263. (b) Zhang, J.; Bond, A. M. J. Electroanal. Chem.
2005, 574, 299. (c) Bond, A. M. Broadening electrochemical horizons:
principles and illustration of voltammetric and related techniques;
Oxford University Press: Oxford, 2002, and references therein. (d)
Bond, A. M.; Feldberg, S. W.; Miao, W.; Oldham, K. B.; Raston, C. L.
J. Electroanal. Chem. 2001, 501, 22.
(10) (a) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002,
102, 3667. (b) Endres, F. ChemPhysChem 2002, 3, 144. (c) Sheldon, R.
Chem. Commun. 2001, 2399. (d) Welton, T. Chem. Rev. 1999, 99, 2071.
(11) (a) Zhang, J.; Bond, A. M. J. Phys. Chem. B 2004, 108, 7363.
(b) Zhang, J.; Bond, A. M. Anal. Chem. 2003, 75, 6938. (c) Zhang J.;
Bond, A. M.; Belcher, W. J.; Wallace, K. J.; Steed, J. W. J. Phys. Chem.
B 2003, 107, 5777. (d) Zhang J.; Bond, A. M. Anal. Chem. 2003, 75,
2694. (e) Hultgren, V. M.; Mariotti, A. W. A.; Bond, A. M.; Wedd, A. G.
Anal. Chem. 2002, 74, 3151.
* Corresponding authors. E-mail: alan.bond@sci.monash.edu.au;
schumann@chem.tu-berlin.de.
(1) (a) Bunz, U. H. F. J. Organomet. Chem. 2003, 683, 269. (b)
Kadkin O, Nather C, Friedrichsen W. J. Organomet. Chem. 2002, 649,
161. (c) Bildstein, B. J. Organomet. Chem. 2001, 617, 28.
(2) (a) Schumann, H.; Lentz, A.; Weimann, R.; Pickardt, J. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1731. (b) Schumann, H.; Su¨hring, K.
Z. Naturforsch. 2005, in press.
(3) Field, L. D.; Lindall, C. M.; Maschmeyer, T.; Masters, A. F. Aust.
J. Chem. 1994, 47, 1127.
(4) Field, L. D.; Hambley, T. W.; Humphrey, P. A.; Masters, A. F.;
Turner, P. Inorg. Chem. 2002, 41, 4618.
(5) Bond, A. M.; Colton, R.; Fiedler, D. A.; Field, L. D.; He, T. A.;
Humphrey, P. A.; Lindall, C. M.; Marken, F.; Masters, A. F.; Schu-
mann, H.; Suhring, K.; Tedesco, V. Organometallics 1997, 16, 2787.
(6) Snook, G. A.; Bond, A. M.; Fletcher, S. J. Electroanal. Chem.
2003, 554-555, 157.
10.1021/om049117c CCC: $30.25 © 2005 American Chemical Society
Publication on Web 03/24/2005

Voltammetric Studies on Decaphenylferrocene
Organometallics, Vol. 24, No. 9, 2005 2189
Table 1. Physical Properties of Ionic Liquids
saturated
water contain/
mass%
viscosity/
cP
melting
point/°C
conductivity/
ms cm-1
potential
window/V
solubility in
water/mass%
ionic liquid
density
a
BMIM‚PF6
emim‚tfsaf
P14‚tfsag
312b
35
1.37b
1.520
1.41i
10
-3
-18
6.56c
9.6
4.5d
4.5
5.5
2.297e
1.8
2.116e
1.4
85h
2.2h
a Data are from ref 10a. b Measured at 30 °C. c Measured at 60 °C. d Data are from ref 10b. e Data are from ref 22. f Data are from ref
14 and were measured at 20 °C except that the value of density was obtained at 22 °C. g Data are from ref 15. h Measured at 20 °C.
i Measured at 25 °C.
rophosphate (BMIM‚PF6, >97%, Sigma-Aldrich) were used as
received from the manufacturer. The other ionic liquids,
1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)
amide (emim‚tfsa) and N-butylmethylpyrrolidinium bis(trif-
luoromethanesulfonyl) amide (P14‚tfsa), were synthesized
according to literature procedures.14,15 The structures of the
ionic liquids and their physical properties are summarized in
Chart 2 and Table 1, respectively. Decaphenylferrocene (DPFc),
[DPFc+][BF4-], and its derivatives (see Chart 1) were synthe-
sized, purified, and characterized as described in the litera-
ture.2 To study the voltammetry of DPFc+ derivatives, sus-
pensions of these compounds were oxidized with a slight molar
excess of NOPF6 (Strem Chemicals) in dichloromethane (DCM,
HPLC grade, BDH) containing 0.1 M Bu4NPF6 (GFS, recrys-
tallized twice before use) as the electrolyte.
Chart 1. Structures of DPFc and Its Derivatives
Instrumentation and Procedures. Voltammetric mea-
surements were undertaken with a BAS 100B (Bioanalytical
Systems, Indiana) electrochemical workstation. A conventional
three-electrode cell was employed, with 1 mm diameter glassy
carbon (GC) working and Pt wire counter electrodes. In
experiments involving adhered [Fc][PF6] microparticles, a 50
µm diameter Pt microdisk electrode was used as the working
electrode. An Ag wire dipped in a tube containing BMIM‚PF6,
P14‚tfsa, or emim‚tfsa, and which was separated from the test
solution by a glass frit, was used as a quasi-reference electrode
in studies employing bulk ionic liquids. The potential of this
quasi-reference electrode was then calibrated against that of
the IUPAC recommended [Co(Cp)2]+/0 process using a 1 mM
[Co(Cp)2][PF6] solution (Co(Cp)2 ) cobaltocene) as an internal
reference.16 For voltammetric measurements in thin layer ionic
liquids in contact with aqueous electrolyte solution, Ag/AgCl
(3 M NaCl) was used as the reference electrode.
Chart 2. Structures of Ionic Liquids Used in the
Present Studies
Similar to the case with other neural organometallic
compounds,11a-d the dissolution kinetic of DMFc in BMIM‚PF6
is extremely slow. To prepare a 2 mM BMIM‚PF6 solutions of
DMFc, the solvent containing solid DMFc was left in an
ultrasonic cleaner for ca. 3 h at 40 °C.
To prepare a microparticle modified electrode, a few mil-
ligrams of solid compounds was placed on weighing paper. The
GC electrode was then pressed onto the paper substrate and
rubbed over the solid so that some of the microcrystalline solid
adhered to the electrode surface. Typically, on a 1 mm
diameter electrode, the equivalent of a solid layer containing
microgram quantities of sample was achieved by this method.
After preparation as described above, the chemically modi-
fied electrode was placed directly in contact with ionic liquid
solution. Alternatively, it was coated with a layer of ionic
liquid11b (ca. 1 µL for a thick layer and much less than 1 µL
for a thin layer) prior to being placed in contact with the
conventional 1-2 mL volume of aqueous electrolyte solutions.
The basic principles relevant to the latter technique that
involves formation of electrode/adhered material/ionic liquid/
aqueous electrolyte interfaces have been described in detail
elsewhere11b and resemble those used in studies of electron
transfer reactions across immiscible organic solvent/aqueous
electrolyte interfaces as pioneered by Anson and co-workers.17
The schematic diagram shown in Figure 1 provides a repre-
sentation of the working electrode configuration and interfaces
present in three different configurations. Prior to the attach-
ment of solid, the GC working electrode was polished with a
In this study, we report the voltammetry of DPFc and
its substituents adhered to electrode in contact with
DCM (0.1 M Bu4NPF6) or ionic liquids and on DPFc+
derivatives dissolved in DCM (0.1 M Bu4NPF6). The
combination of approaches allows the role of dissolution/
reprecipitation processes to be elucidated and also to
establish conditions that minimize their influence.
Experimental Section
Chemicals. Cobalticinium hexafluorophosphate ([Co(Cp)2]-
[PF6], 98%, Strem Chemicals), ferricinium hexafluorophos-
phate ([Fc][PF6], Sigma-Aldrich), ferrocene (Fc, >98%, Merck),
decamethylferrocene (DMFc, 97%, Sigma-Aldrich), KPF6 (98%,
Sigma-Aldrich), and 1-butyl-3-methylimidazolium hexafluo-
(12) (a) Lagunas, M. C.; Pitner, W. R.; van den Berg, J.-A.; Seddon,
K. R. In Ionic Liquids as Green Solvents: Progress and Prospects;
Rogers, R. D., Seddon, K. R., Eds.; American Chemical Society:
Washington, DC, 2003. (b) Boxall, D. L.; Osteryoung, R. A. J.
Electrochem. Soc. 2002, 149, E185.
(13) For review, see for example: Buzzeo, M. C.; Evans, R. G.;
Compton, R. G. ChemPhysChem 2004, 5, 1107.

2190 Organometallics, Vol. 24, No. 9, 2005
Zhang et al.
Figure 2. Cyclic voltammogram of 0.5 mM Br2DPFc+
dissolved in DCM (0.1 M Bu4NPF6) at a 1.5 mm diameter
GC electrode (v ) 0.2 V s-1).
Figure 1. Schematic illustration of the different forms of
chemically modified GC electrode/solution interfaces used
in this study. Scheme 1: microparticle array in contact with
dichloromethane (electrolyte) media. Scheme 2: micropar-
ticle in contact with ionic liquid. Scheme 3: microparticle/
ionic liquid/aqueous electrolyte configuration.
0.3 µm Al2O3 (Buehler) slurry, washed successively with water
and acetone, and finally dried with tissue paper.
All experiments were conducted at a temperature of 20 ( 1
°C, and solutions were degassed with high-purity nitrogen for
at least 5 min prior to undertaking voltammetric measure-
ments.
Results and Discussion
Voltammetry of Dissolved Oxidized DPFc+ and
Its Derivatives in Dichloromethane. Direct mea-
surements of the reversible DPFc(dissolved) / DPFc+-
(dissolved) + e- oxidation process are not possible
because of the insolubility of DPFc and its derivatives
in most media. However, the oxidized DMFc+ forms of
the compounds are soluble in nonpolar organic solvents
such as dichloromethane (DCM). Consequently, initial
voltammetric data in this paper are reported on the
reduction DPFc+(dissolved) using solutions prepared
quantitatively by the reaction
Figure 3. Cyclic voltammograms of 0.25 mM Me10DPFc+
dissolved in DCM (0.1 M Bu4NPF6) at a 1.5 mm diameter
GC electrode (v ) 1 (a) and 0.1 (b) V s-1).
reaction scheme
DPFc(solid) + NO+ f
DPFc+(dissolved) + NOv (1)
in DCM (0.1 M Bu4NPF6).
In principle, the electrochemical reduction of DMFc+
and derivatives in DCM would be expected to follow the
However, if the rate of the DPFc(dissolved) f DPFc-
(solid) process is slow on the voltammetric time scale,
then the simpler
(14) Bonhoˆte, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram,
K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168.
(15) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M.
J. Phys. Chem. B 1999, 103, 4164.
DPFc+(dissolved) + e- / DPFc(dissolved) (3)
(16) Stojanovic, R. S.; Bond, A. M. Anal. Chem. 1993, 65, 56.
(17) Shi, C.; Anson, F. C. Anal. Chem. 1998, 70, 3114.

Voltammetric Studies on Decaphenylferrocene
Organometallics, Vol. 24, No. 9, 2005 2191
Table 2. E0′ Values Obtained by Cyclic Voltammetry for the One-Electron Reduction of DPFc+ and
Derivatives Dissolved in DCM (0.1 M Bu4NPF6) at a Glassy Carbon Electrode
DPFc+
Me10DPFc+
Me6DPFc+
Me2DPFc+
Ethin2DPFc+
Br2DPFc+
E0′/V vs [Co(Cp)2]+/0
E0′/V vs Fc0/+
1.310
-0.040
1.160
-0.190
1.200
-0.150
1.270
-0.080
1.340
-0.010
1.400
0.050
process would be detected. Under these circumstances,
the reversible potential (E0′) for the reaction in eq 3 can
be readily obtained.
Figure 2 shows a cyclic voltammogram for the
Br2DPFc(DCM)+/0 process obtained via reduction of
Br2DPFc(DCM)+ dissolved in DCM (0.1 M Bu4NPF6).
This voltammogram exhibits all the characteristics
expected for a solution phase electrochemically revers-
ible process. Thus, the peak current for the reduction
component of the process Irped depends linearly on the
square root of the scan rate (v ) 0.05 to 1 V s-1) and
the magnitude of the ratio of peak current for the
reduction and oxidation (Iopx), Ipred/Iopx, is unity. A mid-
point potential, Em, of 1.400 ( 0.005 V vs [Co(Cp)2]+/0
was obtained from the average of the reduction Epred
and oxidation Epox peak potentials [(Epred + Eopx)/2]. As
expected for a reversible process, Em is independent of
scan rate (0.05 to 1 V s-1) and hence provides an
excellent approximation of E0′. The peak-to-peak sepa-
ration, ∆Ep, value of 90 mV at a scan rate of 0.2 V s-1
is larger than the theoretically expected value of 56 mV
at 20 °C.18 However, the known reversible process for
oxidation of ferrocene exhibits a similar ∆Ep value under
the same conditions. Consequently, ∆Ep broadening is
attributed to the presence of an uncompensated resis-
tance in the high-resistance DCM media. Thus, reduc-
tion of Br2DPFc+ under voltammetric conditions is
consistent with the simple reaction Br2DPFc+(dissolved)
+ e- / Br2DPFc(dissolved) for which E0′ ) 1.400 (
Figure 4. Linear relationship between E0′ and the Ham-
mett substituent parameter for decaphenylferrocene and
designated derivatives.
blocked surfaces or pinholes gives rise to radial diffusion
and hence a steady-state (potential-independent) com-
ponent.19
E0′ values found for the DPFc+(dissolved) + e-
/
0.005 V vs [Co(Cp)2]+/0 or 0.050 ( 0.005 V vs Fc0/+
.
DPFc(dissolved) process are summarized in Table 2.
Using the E0′ value for the unsubstituted DPFc of 1.310
V vs [Co(Cp)2]+/0 as a reference point reveals that
progressive substituation of a Me group onto the phenyl
ring makes the cations easier to reduce (MexDPFc
harder to oxidize). Conversely, substitution by Br makes
reduction of the cation more difficult (oxidation of
Br2DPFc easier). These substituent effects are as ex-
pected on the basis of the electron-withdrawing effect
of Br and electron-donating influence of the Me group.
The substituent effect found in the case of Ethin2DPFc
is relatively small. Hammett (σp) substituent param-
eters are calculated from literature data20 (for Fe(C5Hn-
(C6H4R)5-n)2: R ) H, σp ) 0.0; R ) Br, σp ) 0.23; R )
Me, σp ) -0.34; R ) ethyne, σp ) 0.23). The expected
linear relationship between E0′ and ∑σp (∑σp is calcu-
lated as (5 - n)σp in the present case) is obtained, as
shown in Figure 4.
Voltammetric Oxidation of Adhered Micropar-
ticles of DPFc and Derivatives in Contact with
DCM. Since DPFc and its derivatives are insoluble in
DCM, it is possible to study the voltammetry of micro-
particles of these compounds adhered to a GC electrode
surface in contact with DCM (0.1 M Bu4NPF6). Volta-
mmograms obtained with microparticles of adhered
DPFc, Me2DPFc, Me6DPFc, or Ethin2DPFc all exhibit
Other derivatives of DPFc+ exhibit reversible reduc-
tive voltammetry analogous to that for [Br2DPFc]+, with
the notable exception of Me10DPFc. In the case of a 0.25
mM solution of [Me10DPFc]+, only at scan rates of
greater than 1 V s-1 (Figure 3a) is a simple reversible
Me10DPFc+(dissolved) + e- / Me10DPFc(dissolved)
process detected. The E0′ value of 1.160 ( 0.005 V vs
[Co(Cp)2]+/0 obtained under these conditions means that
[Me10DPFc]+ is easier to reduce than [Br2DPFc]+ by 240
mV so that a significant substituent effect is evident.
At slower scan rates and for a concentration of 0.25 mM,
the voltammetry is more complex. Thus, at a scan rate
of 0.1 V s-1, an almost potential-independent current
is observed at potentials more positive than Eopx (see
Figure 3b) in the reverse or oxidative potential sweep
direction. Furthermore, the shape of the oxidation
component is no longer that expected for a (planar)
diffusion-controlled process. This close to potential-
independent component also is detected upon repetitive
cycling of the potential at a scan rates of 0.2 V s-1 but
is less evident when the concentration of Me10DPFc+ is
decreased to 0.1 mM. It would seem that under slow
scan rate conditions, reduction of Me10DPFc+(dissolved)
occurs as in eq 3 to give insoluble Me10DPFc, which
partially blocks the electrode surface, thereby giving rise
to an array of pinholes. Voltammetry at partially
(19) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem.
1983, 147, 39.
(20) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(18) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.;
Wiley: New York, 2001.

2192 Organometallics, Vol. 24, No. 9, 2005
Zhang et al.
the following characteristics. In the initial cycle of the
potential, a large oxidation peak current is detected at
potentials that are considerably more positive than E0′.
In contrast, the significantly smaller reduction compo-
nent has a peak potential that occurs at the value
expected for dissolved DPFc+. The reduction component
also exhibits many other characteristics expected for a
solution phase, diffusion-controlled, one-electron-trans-
fer process. In the second and subsequent cycles of the
potential, the large oxidation peak current is replaced
by a much smaller limiting current. In contrast, the
reduction process remains at the same potential. How-
ever, the peak current for reduction and the oxidative
limiting current both progressively decrease in magni-
tude with cycling of the potential.
Figure 5a illustrates the particular case for voltam-
metric oxidation of adhered microparticles of Me2DPFc.
A close relationship with cyclic voltammograms ob-
tained for reduction of Me10DPFc+(DCM) (Figure 3) is
detected when both experiments are examined under
conditions of repetitive cycling. This analogous behavior
suggests that oxidation of DPFc(surf) to DPFc+(surf) is
followed by dissolution to give DPFc+(DCM). In turn,
reduction of DPFc+(DCM) to DPFc(DCM) is followed by
its reprecipitation, giving rise to an electrode surface
containing solid DPFc, in a film format that allows the
process DPFc+(dissolved) + e- / DPFc(dissolved) to
occur in second and subsequent potentials at pinholes
of a bare GC electrode. A reaction scheme consistent
with the voltammetric oxidation of an array of micro-
particles of DPFc is summarized in eq 4.
In the case of oxidation of microparticles of Br2DPFc,
cyclic voltammograms (Figure 5b) exhibit many of the
characteristics of a solution phase, diffusion-controlled,
one-electron-transfer process even in the first cycle of
the potential. However, a rapid peak current decay
accompanies cycling of the potential. In this case,
oxidation of Br2DPFc(solid) rapidly leads to formation
of Br2DPFc+(DCM), which is then reduced to Br2DPFc-
(DCM), which remains soluble (at least in the region of
the electrode surface rather than reprecipitating on the
electrode surface) on the voltammetric time scale.
Equation 5 describes the oxidation of microparticles of
Br2DPFc(DCM).
Figure 5. Cyclic voltammograms obtained at a scan rate
of 0.2 V s-1 when microparticles of (a) Me2DPFc, (b) Br2-
DPFc, and (c) Me10DPFc adhered to a 1.5 mm diameter
GC electrode are in contact with DCM (0.1 M Bu4NPF6).
Me10DPFc+/0(DCM) reaction, as are many of the other
voltammetric characteristics associated with this solu-
tion phase process. Consequently, process 1 is assigned
to the reaction scheme in eq 6. The second process is
then logically assigned to a fully solid-state process (eq
7), the implication being that the very high local
concentration of the dissolved Me10DPFc+ prevents
complete dissolution of [Me10DPFc][PF6], at least on the
voltammetric time scale.9b
For oxidation of Me10DPFc, two well-resolved pro-
cesses (labeled 1 and 2 in Figure 5c) are exhibited in
all cycles of the potential with Em values of 1.150 and
1.295 V vs [Co(Cp)2]+/0. The Em value for process 1 is
very close to the E0′ value of 1.160 V established for the
Em values for the first oxidation process are sum-
marized in Table 3 and in all cases are close to the E0′
values established for the DPFc+/0(DCM) process.
Voltammetric Studies with Ionic Liquids. The
equivalence of data obtained from adhered organome-

Voltammetric Studies on Decaphenylferrocene
Organometallics, Vol. 24, No. 9, 2005 2193
Scan rate (0.02 to 1 V s-1)-independent ∆Ep and peak
widths at half-height, W1/2, values detected in cyclic
voltammograms (Figures 6 and 7) are significantly
larger than expected for either fully dissolved or fully
surface-confined processes. The peak current depen-
dence on the scan rate is intermediate between linear
and square root and hence does not fully comply with
theoretical predictions for either of the fully dissolved
or surface-confined cases. These voltammetric features
resemble those encountered for the DCM situation with
adhered particles of Me10DPFc, which implies that two
unresolved processes are present (eqs 8 and 9). Cyclic
voltammograms for the reduction of TCNQ (TCNQ )
7,7,8,8-tetracyanodimethylquinone) also exhibit two
well-resolved analogous processes, in addition to a
dissolved TCNQ0/-(ionic liquid) process.9b In the present
case, the dissolved DPFc(ionic liquid)0/+ process is not
detected, presumably because of the low solubility of
both DPFc and DPFc+ in BMIM‚PF6.
tallic microparticles and dissolved compound has been
reported in the studies involving ionic liquids when the
product is thermodynamically soluble and dissolves
rapidly.11 However, in this case, DPFc and derivatives
are very insoluble in the ionic liquids and the oxidized
forms are only sparingly soluble. The latter feature was
ascertained by oxidation of DPFc and derivatives by a
slight molar excess of NOPF6. Consequently, only the
voltammetry of adhered solid could be obtained in ionic
liquid studies.
Voltammetry of DPFc and [DPFc+][BF4-] Micro-
particles Adhered to an Electrode in Contact with
BMIM‚PF6. Voltammetric oxidation of DPFc and re-
duction of [DPFc+][BF4-] microparticles adhered to a
GC electrode surface have been studied using the
electrode/solid/BMIM‚PF6 configuration shown in Figure
1. Voltammograms obtained in the first cycle of potential
exhibit only small currents. In contrast, voltammograms
obtained for second and subsequent cycles are well-
defined and essentially constant for at least 20 cycles
of the potential. Figures 6a and 7a show the second cycle
of the potential when microparticles of DPFc or [DPFc+]-
[BF4-], respectively (adhered to a GC electrode), are in
contact with bulk BMIM‚PF6. Similar voltammetric
profiles are obtained when the solids are adhered to Pt
or Au electrodes. Furthermore, voltammograms (Figures
6b and 7b) obtained with a modified GC electrode coated
with a thin or thick layer of ionic liquid interfaced to
an aqueous (0.1 M KPF6) electrolyte (Figure 1) are
essentially indistinguishable from those obtained when
the electrode is in contact with bulk BMIM‚PF6.
Voltammetry of DPFc and Derivatives in Con-
tact with Ionic Liquids. The shapes of cyclic volta-
mmograms obtained with adhered microparticles of
DPFc and derivatives in contact with BMIM‚PF6, emim‚
tfsa, and P14‚tfsa are compound and ionic liquid de-
Table 3. Comparison of Em (vs [Co(Cp)2]+/0) Values Obtained from Voltammograms When Microparticles of
DPFc and Its Derivatives Are Adhered onto a GC Electrode Surface That Is Then Placed in Contact with
DCM (0.1 M Bu4NPF6) and E0′ (vs [Co(Cp)2]+/0) Values for [DPFc]+/0(DCM) Processes
DPFc
Me10DPFc
Me6DPFc
Me2DPFc
Ethin2DPFc
Br2DPFc
Em/V
E0′/V
1.320
1.310
1.150
1.160
1.210
1.200
1.290
1.270
1.350
1.340
1.420
1.400
Table 4. Peak and Midpoint Potential Values (vs [Co(Cp)2]+/0) Obtained in Cyclic Voltammetric Studies
When Microparticles of DPFc and Its Derivatives Are Adhered to a 1 mm Diameter GC Electrode in
Contact with an Ionic Liquid (v ) 0.1 V s-1
)
ionic liquid
emim‚tfsa
BMIM‚PF6
P14‚tfsa
Ep/mV
Ep/mV
Ep/mV
compound
ox.
red.
Em/mV
∆Ep/mV
ox.
red.
Em/mV
∆Ep/mV
ox.
red.
Em/mV
∆Ep/mV
DPFc
[DPFc][BF4]
Me10DPFc
1527
1527
1218
1544a
1490
1410
1410
1211
1330a
1292
1469
1469
1215
1437
1391
117
117
107
214
198
1423
1426
1425
1410
1410
1389
1417
1418
1407
13
16
36
1500
1510
1288
1360
1360
1170
1430
1435
1229
140
150
118
Me6DPFc
Me2DPFc
1263
1433a
1450
1244
1395a
1293
1254
1414
1372
19
38
157
1413
1550a
1420
1622a
1567
1730
1363
1534a
1320
1388
1542
1370
50
16
100
1415
1366
1391
49
Ethin2DPFc
Br2DPFc
1408
1509
1319
1429
1364
1469
89
80
1440
1553
1360
1425
1400
1489
80
128
1494
1537
1531
1634
73
193
a Two resolved processes detected.

2194 Organometallics, Vol. 24, No. 9, 2005
Zhang et al.
-
Figure 7. Cyclic voltammograms of (a) [DPFc+][PF6
]
Figure 6. Cyclic voltammograms of (a) DPFc micropar-
ticles adhered to a 1 mm diameter GC electrode (v ) 0.1 V
s-1) in contact with bulk BMIM‚PF6 (scheme 2, Figure 1)
and (b) as for (a) but with the microparticles being coated
with a thin layer of BMIM‚PF6 prior to being placed in
contact with aqueous (0.1 M KPF6) electrolyte solution
(scheme 3, Figure 1).
microparticles adhered to a 1 mm diameter GC electrode
(v ) 0.1 V s-1) in contact with bulk BMIM‚PF6 (scheme 2,
Figure 1) and (b) as for (a) but with the microparticles being
coated with a thin layer of BMIM‚PF6 prior to being placed
in contact with aqueous (0.1 M KPF6) electrolyte solution
(scheme 3, Figure 1).
Em values, where converted to the [Co(Cp)2]+/0 scale,
are not highly ionic liquid dependent (Table 4), but are
generally more positive (less than 200 mV) than E0′
values obtained in dichloromethane (Table 2) or when
microparticle-modified electrodes are placed in contact
with DCM (Table 3). It also may be noted that cyclic
voltammograms of adhered DPFc (or DPFc+BF4-) cov-
ered by a thin layer of either emim‚tfsa or P14‚tfsa in
contact with 0.1 M KPF6 aqueous solution (saturated
with appropriate ionic liquid) are indistinguishable from
those obtained when the microparticle array is directly
in contact with the bulk ionic liquid. This result implies
that the thin layer method (Figure 1, scheme 3) can be
generally applied to obtain the voltammetric charac-
teristics of an adhered solid in contact with ionic liquid,
as supported by data available in a previous study.11b
Voltammetry of Adhered Fc, [Fc+][PF6-], and
DMFc Microparticles. The voltammetry of Fc in
BMIM‚PF6 has been reported.11d In contrast to the
situation prevailing with DPFc or [DPFc][BF4] (Figures
6 and 7), voltammetric characteristics when Fc- or [Fc+]-
[PF6-]-modified electrodes are placed in contact with
BMIM‚PF6 may be indistinguishable from those ob-
tained for a reversible diffusion-controlled system with
an E0′ value of 1334 ( 5 mV vs [Co(Cp)2]+/0 (Figures 9a
and 9b) because of the rapid dissolution of Fc+ and
absence of reprecipitation of Fc11d (Fc and [Fc][PF6] are
moderately soluble in BMIM‚PF6). Importantly, the
peak currents in experiments shown in Figure 9 at a
pendent. For example, when DPFc derivatives are
adhered to a GC electrode surface in contact with P14‚
tfsa (Figure 8, second cycle of potential), clear evidence
for partially resolved processes is obtained in the cases
of oxidation of Me6DPFc and Me2DPFc, unresolved
overlapping processes are evident in the case of
Me10DPFc, and a single essentially reversible process
is found in the cases of Ethin2DPFc and Br2DPFc.
Peak potential data obtained at a scan rate of 0.1 V
s-1 for oxidation of DPFc and derivatives in contact with
the three ionic liquids of interest in these studies are
summarized in Table 4. All data are essentially inde-
pendent of scan rate, over the range 0.02 to 1 V s-1
,
and in all cases the peak height dependence on the scan
rate is intermediate between linear and square root.
Examples involving observation of two well-resolved
peaks (e.g., data as in Figures 8b and 8c) are highlighted
in Table 4. These cases, as well as those where broad
peaks are detected, are attributed to the coexistence of
two reaction schemes of the kind summarized in eqs 8
and 9. Also noteworthy is the fact that ∆Ep is signifi-
cantly less than the theoretically predicted value of 56
mV for a solution phase one-electron reversible process
at 20 °C18 for some compound-ionic liquid combinations.
Ideal thin film, fully surface-confined, reversible volta-
mmetry would exhibit a ∆Ep value of zero, so that “thin
layer” type behavior probably contributes to some vol-
tammetric responses.

Voltammetric Studies on Decaphenylferrocene
Organometallics, Vol. 24, No. 9, 2005 2195
Figure 8. Cyclic voltammograms obtained when microparticles of DPFc derivatives (v ) 0.1 V s-1) are adhered to a GC
electrode in contact with P14‚tfsa (scheme 2, Figure 1) for (a) Me10DPFc, (b) Me6DPFc, (c) Me2DPFc, (d) Ethin2DPFc, and
(e) Br2DPFc.
50 µm diameter Pt microelectrode are of comparable
magnitude to those found for DPFc and DPFc+ (Figures
6 and 7) at a much larger 1 mm diameter electrode,
implying that the high solubility of Fc relative to DPFc
is a significant factor.
Studies were also undertaken with DMFc, which has
an intermediate level of solubility in BMIM‚PF6. When
DMFc is dissolved in BMIM‚PF6, a reversible, DMFc-
(ionic liquid) h DMFc+(ionic liquid) + e- process is
detected with an E0′ value of 0.856 V vs [Co(Cp)2]0/+ or
-0.478 V vs Fc0/+ (Figure 10a). This potential may be
slightly less negative than found in DCM (-0.544 V vs
Fc0/+) or acetonitrile (-0.504 V vs Fc0/+).21 However,
voltammograms for oxidation of DMFc (dissolution in
BMIM‚PF6 is kinetically slow) microparticles adhered
to a 1 mm GC electrode surface in contact with BMIM‚
PF6 exhibit many of the characteristics reported for
oxidation of microparticles of DPFc. Thus, the initial
oxidation process (labeled process a in Figure 10b)
occurs at a more positive potential than E0′. Addition-
ally, the peak current decreases on cycling of the
potential, as expected if dissolution of DMFc+(solid) is
rapid. In contrast with the voltammogram obtained
from microparticles of DPFc, a new oxidation process
(process b in Figure 10b) is detected in second and
subsequent cycles of potential at a potential around 0.87
vs [Co(Cp)2]0/+ and is assigned to the oxidation of
dissolved DMFc (DMFc+ is more soluble than DPFc+
in BMIM‚PF6). The key processes in the voltammetric
(21) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay,
P. A.; Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713.
(22) Fadeev, A. G.; Meagher, M. M. Chem. Commun. 2001, 295.

2196 Organometallics, Vol. 24, No. 9, 2005
Zhang et al.
Figure 10. Cyclic voltammogram at a 1 mm diameter GC
electrode (v ) 0.1 V s-1) for (a) DMFc (2 mM) dissolved in
BMIM‚PF6 and (b) DMFc adhered to the electrode surface
and placed in contact with BMIM‚PF6.
Figure 9. Voltammograms obtained on repetitive cycling
of the potential at a scan rate of 0.1 V s-1 when an array
of solid Fc (a) or [Fc][PF6] (b) microparticles are adhered
to a 50 µm diameter Pt disk electrode that is placed in
contact with BMIM‚PF6.
species is less favorable, but still a major pathway, Em
values obtained upon repetitive cycling of the potential
may still equal the E0′ value for the solution phase
process. In contrast, for situations where both oxidized
and reduced forms of the ferrocene are sparingly soluble,
Em reflects contributions from multiple reaction schemes
that may involve dissolution and reprecipitation steps
and conversion from thick film to thin film behavior. In
these cases, the kinetics of dissolution and precipitation
play an important role in determining the number of
processes detected and the difference between Em and
E0′ values.
Data obtained in this study confirm that the use of a
microchemical technique in which the solid particle
array on an electrode surface is covered by a thin layer
of water immiscible ionic liquid in contact with aqueous
electrolyte solution provide an efficient method of
establishing the electrochemical behavior of adhered
solids or dissolved species in ionic liquids, but via a more
cost-effective approach than use of bulk ionic liquid,
because only microliter volumes of expensive ionic
liquids are required.
oxidation of microparticles of DMFc therefore can be
represented by eq 10.
Conclusions
Cyclic voltammetric experiments with DPFc+ and its
derivatives enable E0′ values for the solution phase
DPFc+ + e- / DPFc process to be detected in DCM
because the rate of precipitation of DPFc is slow on the
voltammetric time scale. The expected substituent effect
is observed. On the other hand, studies with solid DPFc,
derivatives of DPFc, DMFc, and Fc, and their one-
electron-oxidized forms adhered to an electrode surface
in contact with DCM (0.1 M Bu4NPF6) or an ionic liquid
reveal that the nature of the cyclic voltammetric re-
sponse detected under these conditions is critically
dependent on the solubility and the kinetics of dissolu-
tion and reprecipitation of the adhered solid in both the
oxidized and reduced forms. In particular, dissolution
processes may give rise to the detection of multiple
reaction pathways. For example, if the dissolution of
oxidized solid is both thermodynamically and kinetically
favorable, Em ) E0′. If the dissolution of the oxidized
Acknowledgment. The authors gratefully acknowl-
edge the Australian Research Council and the Monash
Research Fund for financial support of this project.
Professor Douglas MacFarlane (School of Chemistry,
Monash University) and his research group are thanked
for generous provision of ionic liquids.
OM049117C