Spectroscopic and electrochemical study of the adsorption of [Co(en) 2Cl2]Cl on γ-alumina: Influence of the alumina ligand on Co(III)/(II) redox potential

Article abstract of DOI:10.1021/jp058224x

UV-visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize cis- and trans-[Co^(III)(en) ₂Cl₂]Cl (en = ethylenediamine) complexes and the γ-alumina-supported cis-Co^(III) complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by UV-visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step has been determined, and experimental results reveal that cis and trans complexes present similar electrochemical characteristics. It is also shown that the deposition of trans-[Co^(III)(en)₂Cl₂]⁺ on γ-alumina leads to an inner-sphere complex (ISC) in a cis configuration in which Cl^- ligands are substituted by OH or O^- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial decrease of its apparent redox potential since it is -0.63 V/SCE (saturated calomel electrode) for the γ-alumina-supported cis-Co^(III) complex, whereas values of -0.17 and -0.35 V/SCE were determined in dimethyl sulfoxide (DMSO) for the trans and cis precursor complexes, respectively.

Full text of DOI:10.1021/jp058224x

900
J. Phys. Chem. B 2006, 110, 900-906
Spectroscopic and Electrochemical Study of the Adsorption of [Co(en)2Cl2]Cl on
γ-Alumina: Influence of the Alumina Ligand on Co(III)/(II) Redox Potential
Vincent Vivier,*,†,‡ Franc¸ois Aguey,† Jeanine Fournier,† Jean-Franc¸ois Lambert,†
Fethi Bedioui,§ and Michel Che†,|
Laboratoire de Re´actiVite´ de Surface, UMR 7609-CNRS, UniVersite´ Pierre & Marie Curie, 4 place Jussieu,
Casier 178, 75252 Paris Cedex 05, France, Laboratoire Interfaces et Syste`mes Electrochimiques, UPR
15-CNRS, UniVersite´ Pierre & Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France, and Laboratoire
d′Electrochimie et Chimie Analytique, UMR 7575-CNRS, ENSCP, 11 rue Pierre & Marie Curie,
75231 Paris Cedex 05, France.
ReceiVed: August 11, 2005; In Final Form: October 11, 2005
UV-visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize
cis- and trans-[Co(III)(en)2Cl2]Cl (en ) ethylenediamine) complexes and the γ-alumina-supported cis-Co(III)
complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage
mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by
UV-visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step
has been determined, and experimental results reveal that cis and trans complexes present similar
electrochemical characteristics. It is also shown that the deposition of trans-[Co(III)(en)2Cl2]+ on γ-alumina
leads to an inner-sphere complex (ISC) in a cis configuration in which Cl- ligands are substituted by OH or
O- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial
decrease of its apparent redox potential since it is -0.63 V/SCE (saturated calomel electrode) for the γ-alumina-
supported cis-Co(III) complex, whereas values of -0.17 and -0.35 V/SCE were determined in dimethyl sulfoxide
(DMSO) for the trans and cis precursor complexes, respectively.
1. Introduction
scopic properties upon adsorption have been reported and used
as evidence for an ISC mechanism.9 ISC formation should have
other consequences, one of the most important of which is a
change in the redox properties of the complex. Indeed, it is well-
known in the field of “classical” coordination chemistry (i.e.,
in homogeneous media) that redox potentials of couples
involving transition metal complexes depend on the nature of
the ligands in the coordination sphere.11-14 However, very few
data are available to assess whether ISC formation does modify
the redox potential of grafted TMCs. This may appear surprising
since electron-transfer reactions (ETRs) between TMCs in
solution have been reported since Taube’s pioneering work15
in many studies at the solid-gas interface.16,17
Some attempts have been made to study the electrochemical
behavior of oxide-supported species using classical electro-
chemical techniques on semiconductor electrodes18-20 or with
modified electrodes.21-23 For the latter, the solid under study
(containing the electrochemically active species supported on
materials such as zeolites or clays) may be mixed with highly
pure graphite to obtain chemically modified working elec-
trodes.22 The electrochemical response observed with these
electrodes is clearly due to the supported species, since it is
drastically different in the absence of the solid under study.
There is, however, still a lot of debate about the exact nature of
the electronic processes involved.22,23 Moreover, it will be
interesting to compare the electrochemical behavior of the initial
complex in solution and that of the resulting catalyst at different
steps of its synthesis.
Many attempts have been made to rationalize the preparation
of supported metal catalysts, where the first step is usually the
deposition of a precursor of the catalytically active phase, a
transition metal complex (TMC), onto an oxide surface. The
properties of the final catalyst depend on this deposition step
and on the nature of the established TMC/oxide interaction.1-5
To understand the phenomena that occur during deposition, the
concept of interfacial coordination chemistry has been pro-
posed,6-8 allowing the classification of the adsorption mecha-
nisms of metal complexes on oxide supports.9 One may
distinguish between the following complexes: (a) TMCs, which
are electrostatically adsorbed in the diffuse layer and compensate
the electric charge of the oxide surface (Figure 1a);3 (b) outer-
sphere complexes (OSCs), where the surface groups of the
support enter the solvation sphere of the TMC and interact with
the complex by hydrogen bonds or van der Waals forces (Figure
1b);10 (c) inner-sphere complexes (ISCs), where the surface
groups substitute some of the original ligands of the TMC in a
ligand-exchange reaction, also called grafting (Figure 1c).1-5
In the latter case, the oxide can be considered to be a
supermolecular ligand. Since the coordination sphere of the
TMC is modified by an inner-sphere interaction, the TMC
properties should be significantly altered with respect to those
of “free” TMC in solution. Indeed, modifications of spectro-
* Corresponding Author. E-mail: vivivier@ccr.jussieu.fr.
† Laboratoire de Re´activite´ de Surface, Universite´ Pierre & Marie Curie.
‡ Laboratoire Interfaces et Syste`mes Electrochimiques, Universite´ Pierre
& Marie Curie.
Two different approaches can be proposed. The standard
redox potential of the grafted complex can be evaluated by the
use of successive redox reactions with known potentials. The
§ Laboratoire d′Electrochimie et Chimie Analytique.
| Institut Universitaire de France.
10.1021/jp058224x CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/16/2005

Electron Transfer with Co(III) Complexes
J. Phys. Chem. B, Vol. 110, No. 2, 2006 901
TABLE 1: Elemental Analysis of the Solid Co(III) Reference
Complexes and the γ-Alumina-Supported Complex
%Co
%N
%C
%Cl
%H
theoretical values
20.64
19.18
21.80
0.91
19.63
18.45
17.20
0.70
16.82
15.86
15.67
1.06
37.24
35.34
38.94
0.20
5.65
5.57
5.06
nda
cis-[Co(en)2Cl2]Cl
trans-[Co(en)2Cl2]Cl
trans-[Co(en)2Cl2]Cl
deposited on Al2O3
a Not determined.
defined system by adsorption of the cobalt complex on
γ-alumina and the determination of the adsorption mechanism,
and (iii) the characterization of its electrochemical behavior, in
particular its redox potential, as a function of its adsorption state.
2. Experimental Section
2.1. Synthesis. 2.1.a. Trans and Cis Precursor Complexes.
The trans-[Co(en)2Cl2]Cl complex (en ) ethylenediamine) was
synthesized along a previously published procedure.29 A 25 mL
portion of a 1.48 mol‚L-1 ethylenediamine (en, Acros, 99%)
solution was added to 50 mL of an aqueous CoCl2 (Acros,
99.99%) solution (0.6 mol‚L-1). The mixture was heated at 100
°C for 1 h under air bubbling. Co(II) was fully converted into
Co(III) due to the presence of oxygen from the air atmosphere.
Then, 25 mL of hydrochloric acid (HCL, Prolabo, 37%) was
added and the resulting solution was heated at 100 °C for 1 h.
During the heat treatment, a green crust of small crystals
progressively appeared. After slow cooling of the system to
room temperature, the obtained green crystals were thoroughly
rinsed successively with absolute ethanol (Prolabo, 99.9%) and
diethyl ether (SDS, 99.7%). Elemental analysis (Table 1)
allowed us to check whether the final product had the composi-
tion [Co(en)2Cl2]Cl, and spectroscopic studies reported in the
results section evidenced that the trans isomer has indeed been
formed.
The cis-[Co(en)2Cl2]Cl complex was synthesized from the
trans-[Co(en)2Cl2]Cl compound. A 50 mg portion of the trans-
[Co(en)2Cl2]Cl complex was dissolved in 20 mL of distilled
water and gently evaporated while the solution was heated at
100 °C. This stage of the synthesis was repeated twice. The
residual product was then mixed with very cold water, filtered,
and abundantly rinsed successively with absolute ethanol and
diethyl ether. The final solid product was pink. Elemental
analysis (Table 1) confirmed that it had the same composition
as the trans isomer, and spectroscopic studies reported in the
results section evidenced that it was the cis isomer.
Figure 1. Representation of the various interactions between a TMC
and an oxide with the formation (a) of TMCs electrostatically adsorbed,
(b) of an OSC, and (c) of an ISC: (Mn+) transition metal (III); (L)
ligand; (A-) counter anion.
The Co(en)3Cl3 complex was synthesized from 100 mL of a
solution of 0.1 mol‚L-1 CoCl2 containing ethylenediamine (1
mol‚L-1) and HCl (1 mol‚L-1) under air bubbling (3 h). The
mixture was heated at 100 °C until a crust was obtained at the
liquid surface. After adding concentrated HCl (15 mL) and
absolute ethanol (30 mL), the mixture was cooled to room
temperature. The ochre crystals were then thoroughly rinsed
with absolute ethanol and diethyl ether, successively.
2.1.b. γ-Alumina-Supported Complex. The γ-alumina-
supported Co complex was synthesized via a selective adsorption
procedure at room temperature from a trans-[Co(III)(en)2Cl2]Cl
solution. This solution was prepared by adding 2.5 g of the
complex salt to 50 mL of distilled water and adjusting the pH
to 10 with a concentrated sodium hydroxide solution. A 5 g
portion of γ-alumina (Institut Franc¸ais du Pe´trole, ref: Ec 1285,
nonporous, 200 m2‚g-1, 150 µm < φ < 250 µm) was then
suspended in this solution. The mixture was stirred for 4h, then
centrifuged, and washed with distilled water until the supernatant
occurrence of electron transfer can be checked by spectroscopic
techniques. For instance, starting with a grafted Co(III) low spin
octahedral complex (with two UV-visible bands), its reduction
into a Co(II) high spin octahedral complex (with three UV-
visible bands) can be readily followed by UV-visible spec-
troscopy.24 In this way, it is possible to approach, by trial and
error, the value of the standard potential. The second possibility
reported here is to take advantage of electrochemical techniques
such as cyclic voltammetry.
In the present study, we have selected cobalt complexes
because they have been at the basis of Taube’s pioneering work
on ETRs, and they have also been reported to form ISCs with
oxide surfaces.25,26 Furthermore, they are practically relevant
to many catalytic systems.27,28 Here, we report on (i) the
spectroscopic characterization and the electrochemical behavior
of the bis-ethylenediaminedichloro cobalt complexes cis- and
trans-[Co(en)2Cl2]Cl in solution, (ii) the preparation of a well-

902 J. Phys. Chem. B, Vol. 110, No. 2, 2006
Vivier et al.
had no visible coloration. The pale-pink solid obtained was dried
at room temperature. Elemental analysis of the γ-alumina-
supported cobalt complex is reported in Table 1.
With the same protocol, the cis-[Co(III)(en)2Cl2]Cl precursor
was grafted on γ-alumina. A pale-pink solid was also obtained
using this isomer as the precursor. Elemental analysis of this
solid is similar to that cited in Table 1.
2.2. Spectrometry Measurements. UV-visible spectra were
recorded at room temperature on a Cary 5E spectrometer
(Varian) equipped with two sources. The first one was a
deuterium lamp allowing study the 150-310 nm range, whereas
the second one was a tungsten lamp used to study the 310-
2500 nm range.
Raman spectra were recorded at room temperature on a
Hololab 5000R spectrometer (Kaiser Optical System) with a
785 nm excitation line.
2.3. Electrochemical Measurements. All electrochemical
experiments were performed at room temperature under an inert
atmosphere (Ar) with a conventional three-electrode cell. The
working electrode was either a 2 mm diameter Au-disk or Pt-
disk electrode (Tacussel). For the study of the alumina-supported
complex, the working electrode was a composite-pressed
graphite electrode prepared by pressing 25 mg of graphite
powder (Carbone Lorraine, 99.999%) and 25 mg of the solid
(alumina-supported complex) on a platinum grid.22 The reference
electrode was a saturated calomel electrode (SCE), and the
counter electrode was made of a platinum wire. Both were
placed in separate compartments. Cyclic voltammetry measure-
ments were performed with an Autolab PT30 potentiostat (Eco
Chemie) equipped with a FRA2 module for electrochemical
impedance measurements.
Solutions were freshly prepared with analysis grade solvents
[N,N-dimethyl formamide (DMF, Carlo Erba, 99.8%); dimethyl
sulfoxide (DMSO, SDS, 99.5%); and acetonitrile (CH3CN,
BDH, 99.7%)] and deaerated by bubbling with argon gas for
15 min before each experiment. Potassium tetrafluoroborate
(KBF4, Fluka, 97%) was used as an electrolytic salt for all
experiments performed in DMF and DMSO.
Figure 2. UV-visible spectra of powder materials: (a) cis-[Co(en)2Cl2]-
Cl; (b) trans-[Co(en)2Cl2]Cl. UV-visible spectra of trans-[Co(en)2Cl2]-
Cl (8 mmol‚L-1) in various solvents: (c) distilled water; (d) CH3CN;
(e) DMSO; (f) DMF.
Figure 2 shows the UV-visible spectra of trans-[Co(en)2Cl2]-
Cl in different solvents (curves c-f). It appears that, in DMF
or DMSO (curves e and f), the main bands (465, 610, and 680
nm) are close to those of the solid compound (Figure 2b). On
the other hand, the use of acetonitrile (curve d) induces
noticeable band displacements (particularly for the band located
at 590 nm) and the disappearance of the transition at 465 nm.
In distilled water (curve c), only a single broad band at 480 nm
is observed. In these last two solvents, the dissolution of the
complex is accompanied by chemical reactions that modify its
coordination sphere. Indeed, cis- and trans-[Co(en)2Cl2]Cl can
be hydrolyzed.36 As a result, only DMF and DMSO will be
used hereafter for electrochemical studies.
3. Results and Discussion
3.1. Spectrochemical Characterization. 3.1.a. Trans and
Cis Precursor Complexes. Curves a and b in Figure 2 show
the UV-visible spectra of the cis- and trans-[Co(en)2Cl2]Cl
complexes in the solid state, respectively. For the latter (curve
b), four main transitions are evidenced. The bands at 630 and
460 nm correspond to the allowed d-d transitions of a low-
spin d6 complex in D4h symmetry (1A1g f 1Eg and 1A1g f 1A2g,
respectively).30,31 The two bands at 660 and 690 nm are due to
spin-forbidden (singlet f triplet) transitions.32 The spectrum
of the cis-[Co(en)2Cl2]Cl complex (curve a) is characterized by
two large bands at 390 and 550 nm, corresponding to the two
spin-allowed d-d transitions of a d6 complex in pseudo-Oh
3.1.b. γ-Alumina-Supported Complex. The UV-visible
spectra shown in Figure 4 indicate that the adsorbed complex
always has the cis configuration, although the precursor in
solution was either the cis or the trans isomer.31 This was also
confirmed by Raman spectroscopy (Figure 3c), which evidenced
very similar vibration modes to those of the cis isomer (Table
2). Measurements at different pH values showed that the amount
of adsorbed Co is not straightforwardly correlated to the surface
charge of the alumina support and, particularly, remains
significant (0.2 wt %) even under pH conditions for which the
surface charge is positive (data not shown). That rules out a
purely electrostatic adsorption of the complex (Figure 1a).
Elemental analysis (see the Experimental Section) of the
adsorbed complexes (OSC, Figure 1b; ISC, Figure 1c) reveals
the existence of only one ligand per cobalt atom instead of two
ligands being present in the precursor complexes. Moreover, it
also indicates that little chlorine is coadsorbed, indicating that
the formal charge of the adsorbed complex is neutral or at most
weakly positive. However, from these data, it is actually not
possible to obtain a satisfactory model to describe the formation
of these adsorbed complexes.
1
1
1
symmetry (1A1g f T2g and A1g f T1g).32
The Raman spectra of the solid cis-[Co(en)2Cl2]Cl complex
(Figure 3a) and the solid trans-[Co(en)2Cl2]Cl complex (Figure
3b) exhibit well-defined vibration modes that allow us to
distinguish between the two isomers. The characteristic Raman
frequencies are reported in Table 2. The two main bands at 282
and 517 cm-1 are assigned in both cases to δ(N-Co-N) and
νs(Co-N), respectively.34,35 The chlorine-cobalt stretching
vibrations are located in the 320-390 cm-1 range. A comparison
of the spectra permits us to easily distinguish these isomers:
particularly the bands at 386, 439, and 458 cm-1 for the cis
isomer (Figure 3a) and the band at 424 cm-1 for the trans isomer
(Figure 3b).

Electron Transfer with Co(III) Complexes
J. Phys. Chem. B, Vol. 110, No. 2, 2006 903
TABLE 2: Values of Characteristic Frequencies of Raman Spectra for the Solid Reference Complexes Cis- and
Trans-[Co(en)2Cl2]Cl
Wavenumber (cm-1
ν(Co-Cl)a
)
vibration mode
δ(N-Co-N)a
νas(Co-N)a
νs(Co-N)a
cis-[Co(en)2Cl2]Cl
trans-[Co(en)2Cl2]Cl
supported complex
282
319-350-386
334
327-389
439-458
422
441-464
517
517
521
251-261-282
278
a From refs 34 and 35.
3.2. Electrochemical Characterization. 3.2.a. Trans and
Cis Precursor Complexes. We have first performed the
complete electrochemical characterization of the cis and trans
isomers in solution. To the best of our knowledge, although
these two complexes have been known for many decades, no
work deals with their electrochemical characterization.
It is noticeable that, in both cases, the obtained spectra are
characteristic of a cis configuration. Moreover, an intervalence
band located at 670 nm appears after the coulometric reduction
at -0.60 V/SCE of the starting trans-[Co(III)(en)2Cl2]+ solu-
The cyclic voltammetry of trans-[Co(en)2Cl2]Cl (2 mmol‚L-1
)
in a DMSO + 0.1 mol‚L-1 KBF4 solution is reported on Figure
5a. During the reduction sweep, two cathodic peaks C1 and C2
are observed. The first one appears to correspond to a quasi-
reversible system and is attributed to the redox couple trans-
[Co(III)(en)2Cl2]+/trans-[Co(II)(en)2Cl2] at ca. -0.17 V/SCE
(Table 3). A coulometric study at -0.50 V/SCE shows that this
first step involves the exchange of 0.5 electron per complex
unit. When applying -0.80 V/SCE, the two steps involve the
exchange of 1 electron per complex unit. This suggests that the
trans-[Co(II)(en)2Cl2] complex formed during the electrochemical
reduction could react with the trans-[Co(III)(en)2Cl2]+ complex
to form a mixed-valence dimer that is reduced at ca. -0.60
V/SCE. This was confirmed by UV-visible spectroscopy
performed after each coulometry study (Figure 6).
Figure 4. UV-visible spectra of the trans-[Co(en)2Cl2]Cl complex
in aqueous solution (solid line) and the Co complex adsorbed on
γ-alumina, having a cis configuration (dotted line).
Figure 5. Cyclic voltammograms of (a) trans-[Co(en)2Cl2]Cl (C ) 2
mmol‚L-1, V ) 100 mV‚s-1); (b) cis-[Co(en)2Cl2]Cl (C ) 5 mmol‚L-1
,
Figure 3. Raman spectra of powder materials (excitation line λ )
785 nm): (a) cis-[Co(en)2Cl2]Cl; (b) trans-[Co(en)2Cl2]Cl; (c) the Co
complex adsorbed on γ-alumina.
V ) 50 mV‚s-1); (c) [Co(en)3]Cl3 (C ) 1 mmol‚L-1, V ) 100 mV‚s-1
)
in DMSO + 0.1 mol‚L-1 KBF4 with a 2 mm diameter Au-disk
electrode.

904 J. Phys. Chem. B, Vol. 110, No. 2, 2006
Vivier et al.
TABLE 3: Values of the Apparent Redox Potentials Determined for Complexes in Solution and for the ISC Grafted Complex
Potential (V/SCE)
dimer from
dimer from
redox couple
cis-[Co(en)2Cl2]0/+
cis-[Co(en)2Cl2]0/+
trans-[Co(en)2Cl2]0/+
trans-[Co(en)2Cl2]0/+
ISC grafted complex
DMSO
DMF
-0.35
-0.32
-0.65
-0.62
-0.17
-0.14
-0.60
-0.57
-0.63
nda
a Not determined.
tion.37,38 This is typical of an electronic exchange between two
metallic ions of a dimer. Thus, the overall reduction mechanism
of the complex in solution can be described as follows:
First electrochemical step
contrast, the electrochemical behavior of the dimer remains
unchanged. Similar observations were made when using a
platinum electrode, whereas, with a carbon electrode, the A1/
C1 system appears to always be irreversible.
The electrochemical behavior of cis-[Co(en)2Cl2]Cl in DMSO
is shown in Figure 5b and may be contrasted to that of trans-
[Co(en)2Cl2]Cl shown in Figure 5a. In the case of the cis isomer,
both monomer and dimer reductions occur at more negative
potentials than those of the trans isomer. The first one is
cathodically shifted by about 180 mV (EaDpMp SO(cis-[Co-
(en)2Cl2]+/0) ) -0.35 V/SCE), and the second one is shifted
by about 50 mV (EaDpMp SO(dimer) ) -0.65 V/SCE, Table 3). It
should be mentioned that similar voltammetric results are
obtained when using DMF as a solvent. The entire voltammo-
gram is then shifted toward more positive potentials of ca. 30
mV, and the apparent standard potentials were determined as
trans-[Co(III)(en)2Cl2]+ + e- a trans-[Co(II)(en)2Cl2]0
at -0.17 V/SCE
Conformation change
trans-[Co(III)(en)2Cl2]0 + e- a cis-[Co(II)(en)2Cl2]0
Dimer formation (chemical reaction)
cis-[Co(II)(en)2Cl2]0 + trans-[Co(III)(en)2Cl2]+ f
[(en)2Co(II)Cl2Co(III)(en)2]3+ + 2Cl-
EaDpMp F(trans-[Co(en)2Cl2]+/0) ) -0.14 V/SCE,
During this step, the configuration of the Co(III) center should
change from trans to cis.
Second electrochemical step
app
and E (dimer) ) -0.57 V/SCE
DMF
Finally, Figure 5c shows the cyclic voltammetry of the Co-
(en)3 complex in DMSO. It clearly shows a single reversible
couple of peaks at -0.60 V/SCE related to the well-known
Co(III)/Co(II) redox process.
[(en)2Co(II)Cl2Co(III)(en)2]3+ + e- a
[(en)2Co(II)Cl2Co(II)(en)2]2+ at -0.60 V/SCE
The apparent standard potential Eapp of each step was
determined at low scan rates as the mean value between the
cathodic and anodic peak potentials. It was found that E
3.2.b. γ-Alumina-Supported Complex. Figure 7 shows the
cyclic voltammograms obtained with the modified electrode
containing the raw γ-alumina support (curve a) and the
γ-alumina-supported cobalt complex (curve b) in DMSO in the
presence of KBF4 (0.1 mol‚L-1). Curve a shows that γ-alumina
exhibits no electroactivity. Curve b, obtained with the γ-alumina-
supported cobalt complex, exhibits only one well-defined couple
of reversible peaks at -0.59 V (reduction) and -0.52 V
(oxidation). It is important to note that the intensities of these
peaks remain stable for 4 h over repeated cycling.
app
DMSO
-
(trans-[Co(en)2Cl2]+/0) ) -0.17 V/SCE and EaDpMp SO(dimer) )
-0.60 V/SCE. However, the electrochemical behavior of the
trans-[Co(en)2Cl2]+/0 redox process is greatly influenced by the
nature of the electrode materials. On a freshly polished gold
electrode and within the first tens of minutes of immersion of
the electrode in the electrolytic solution, the anodic peak A1
located at -0.12 V/SCE disappears and the cathodic peak C1
shifts toward more negative values to reach -0.30 V/SCE. In
For reference, we have verified above that the cyclic
voltammograms of the precursor, the trans-[Co(III)(en)2Cl2]+
Figure 7. Cyclic voltammograms of the composite-pressed graphite
powder electrode in a DMSO + 0.1 mol‚L-1 KBF4 solution: (a)
containing raw alumina; (b) containing cobalt complexes adsorbed on
γ-alumina.
Figure 6. UV-visible spectra of the electrolytic solution containing
the trans-[Co(en)2Cl2]+ complex: (a) before the reduction step; (b) after
a reduction step at -0.5 or -0.8 V/SCE.

Electron Transfer with Co(III) Complexes
J. Phys. Chem. B, Vol. 110, No. 2, 2006 905
it is transfer into a fresh solution; the couple of peaks observed
in Figure 8 (inset) can then be attributed to the Co(III)/Co(II) redox
process of the ISC. Therefore, the Co(III)/Co(II) redox process
of the ISC can be evaluated to have Eapp ) -0.63 V/SCE.
Comparing this value with those of the precursor complexes
in DMSO (for the trans isomer, -0.17 V/SCE; for the cis
isomer, -0.35 V/SCE), it appears that ISC formation has caused
a net decrease of the redox potential, which is very close to
that of the dimer in solution (Eapp ) -0.60 V/SCE). In other
words, the barrier of energy to reduce the compound is much
more important when the complex is grafted on alumina than
when it is free in solution.
4. Conclusion
The deposition of trans-[Co(III)(en)2Cl2]+ on γ-alumina leads
to an inner-sphere complex (ISC) in a cis configuration in which
Cl- ligands are substituted by the OH surface groups of alumina.
Neither Cl- substitution nor cis/trans isomerization lead to large
enough redox potential shifts of the Co(III)/Co(II) process to
account for the effect observed for the alumina supported
complex. Therefore, it seems reasonable to interpret the latter
effect in terms of the grafting of the precursor complex onto
the alumina support. These changes in the coordination sphere
of the complex induce a substantial decrease of its redox
potential (Table 3), i.e., the oxidized form Co(III) is definitely
more stabilized than the reduced form Co(II) upon ISC formation.
This might be correlated to the electron-donating properties of
the Al-OH ligand; this ligand is expected to be both a σ- and
π-donor, especially if it is deprotonated (AlO-),6,7 and thus, its
interaction should be most favorable with the more highly
charged cobalt cation.
The lowering in the redox potential of the ISC (Eapp ) -0.63
V/SCE) should also translate into a significant difference in its
reactivity toward redox reactions, compared to that of its free
counterpart in solution (Eapp ) -0.17 and -0.35 V/SCE for
the trans and the cis isomer, respectively). These findings are
likely to be generalized to other redox couples involving
adsorbed transition metal complexes and have obvious implica-
tions for the design of supported redox catalysts for liquid-phase
reactions.
From a more fundamental point of view, they constitute a
first step toward the understanding of electron-transfer reactions
at the oxide-solution interface, with potential applications, for
instance, in the field of toxicochemistry.39 The elemental analysis
of the adsorbed complex and the redox potential of the ISC in
solution seem to show that the ISC presents a complex
formulation. However, at this stage of the study, a definitive
model of the grafted complex cannot be established and this
effort requires further investigations. Finally, the results obtained
could help develop a sounder basis for the notion of metal
“reducibility” that is often invoked in the preparation of
supported metal catalysts.
Figure 8. Evolution of the cyclic voltammograms upon adding en to
a composite-pressed graphite powder electrode containing cobalt
complexes adsorbed on γ-alumina in a DMSO + 0.1 mol‚L-1 KBF4
solution. Inset: stable cyclic voltammogram of the composite electrode
in a fresh DMSO + 0.1 mol‚L-1 KBF4 solution after it has been rinsed
and transfered from an en containing solution.
complex, the cis isomer, and the corresponding trans-diaqua
complex (the redox couple is trans-[Co(en)2(H2O)2]3+/2+; data
are not shown for this complex) exhibit in DMSO solution
nonreversible cathodic peaks related to the Co(III)/Co(II) reduction
at -0.17, -0.29, and -0.11 V/SCE, respectively. These
observations allow us to suggest that the reversible redox process
for the γ-alumina-supported complex may be related to the
Co(III)/Co(II) redox process of OSCs and/or ISCs that are
significantly different from all the above complexes.
By adding en ligands to the electrolytic solution, the intensity
of the cyclic voltammogram continuously decreased after the
first cycle (Figure 8). This loss of electroactivity was ac-
companied by a pink coloration of the electrolytic solution. This
can be explained by the displacement of weakly adsorbed OSCs
from the alumina surface to form [Co(III)(en)3]3+ that diffuses
in solution far away from the electrode. Elemental analysis and
a UV-visible spectrum of the electrolytic solution confirm the
formation of the [Co(III)(en)3]3+ complex (data not shown),
probably upon substitution of the two OH ligands of the OSC
by one en ligand.
Eight hours after the addition of en into the solution, the
intensity of the cyclic voltammogram was stabilized. The
composite electrode was then thoroughly rinsed and transferred
to a fresh DMSO-KBF4 (0.1 mol‚L-1) solution (without en),
and the cyclic voltammogram was recorded. It exhibited one
well-defined and stable couple of reversible peaks at Eapp
)
-0.66 V/SCE (reduction) and -0.58 V/SCE (oxidation) (inset
in Figure 8). There was no further evolution in the shape or
intensity of the peaks even after repeating scans for 4 h. The
fact that no loss of electroactivity occurred after several hours
of immersion and/or potential scanning of the electrode in the
fresh electrolytic solution means that there was no loss of the
redox-active complexes via any chemical or physical process.
Indeed, no traces of [Co(III)(en)3]3+ were detectable in solution,
at this stage of the experiments. This provides evidence that
the observed faradic response does not contain a diffusional
component.
Acknowledgment. The authors gratefully acknowledge Prof.
H. Kno¨zinger and Dr. S. Kuba (Institute of Physical Chemistry,
University of Munich, Germany) and J.-M. Kraft (Laboratoire
de Re´activite´ de Surface, Paris, France) for their help in the
interpretation of Raman spectra.
Experiments performed with a pressed-powder composite
electrode containing γ-alumina and preformed [Co(III)(en)3]3+
complex allow us to rule out the possibility that the electro-
chemical response may arise from complexes freed from the
solid but somehow remaining in the composite electrode. It is
likely that no more OSCs are left on the alumina surface after
Supporting Information Available: Supporting results on
the electrochemical impedance spectroscopy characterization of
the trans-[Co(III)(en)2Cl2]+ complex providing more details on
the Co(III)/Co(II) redox process. This material is available free
of charge via the Internet at http://pubs.acs.org.

906 J. Phys. Chem. B, Vol. 110, No. 2, 2006
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