Ion pairing effects on the kinetic of the intramolecular electron transfer reaction [FeII(CN)5pzCoIII(NH3)5] → [FeIII(CN)5pzCoII(NH3)5]

Article abstract of DOI:10.1016/j.cplett.2006.03.009

The kinetics of the title reaction was studied in different salt solutions (NaNO₃, LiNO₃, Ca(NO₃)₂ and Na₂SO₄). The results are interpreted as a consequence of ion pairing effects. It has been shown that the anion of the salts is the ion associating with the binuclear complex. This association of the anion produces a less favorable reaction free energy and consequently a diminution of the reaction rate.

Full text of DOI:10.1016/j.cplett.2006.03.009

Chemical Physics Letters 422 (2006) 382–385
www.elsevier.com/locate/cplett
Ion pairing effects on the kinetic of the intramolecular electron transfer
reaction [FeII(CN)5pzCoIII(NH3)5] ! [FeIII(CN)5pzCoII(NH3)5]
*
´
´
´
F. Muriel, R. Jimenez, P. Perez-Tejeda, F. Sanchez
The Department of Physical Chemistry, Universidad de Sevilla, C/Professor Garcı´a Gonza´lez s/n, 41012 Sevilla, Spain
Received 11 January 2006; in final form 1 March 2006
Available online 10 March 2006
Abstract
The kinetics of the title reaction was studied in different salt solutions (NaNO3, LiNO3, Ca(NO3)2 and Na2SO4). The results are inter-
preted as a consequence of ion pairing effects. It has been shown that the anion of the salts is the ion associating with the binuclear com-
plex. This association of the anion produces a less favorable reaction free energy and consequently a diminution of the reaction rate.
ꢀ 2006 Elsevier B.V. All rights reserved.
1. Introduction
correspond to optical electron transfer reactions in highly
charged binuclear complexes [9]. Perhaps this circumstance
arises from the fact that most of the electron transfer pro-
cesses in which the reactants are inorganic species are
bimolecular reactions. For these reactions, separation of
the salt effects on the precursor complex formation process
from ion pairing effects on the true electron transfer pro-
cess (once the precursor complex is formed) is not a simple
matter.
Ion pairing influences kinetics, and in some cases the
mechanism, of the electron-transfer reactions. Theoretical
models for the effects of ion pairing on the kinetics of these
kinds of reactions have been proposed by Marcus [1] and
Piotrowiak [2]. In these models, the influence of the coun-
terion dynamics on the reorganization and reaction free
energies in several scenarios is considered. Piotrowiak
et al. [3], have checked the model in reference [2], through
the study of salt effects on long-lived photoinduced charge-
separated states of p-ANBP and p-ANTP. On the other
hand, Nelsen et al. have studied ion pairing effects on the
optical (intervalence) electron transfer processes in radical
cations [4,5].
The effect of ion pairing on the kinetic of thermal elec-
tron transfer has been considered by Fukuzumi et al.
[6,7]. They showed that ion pair formation not only influ-
ences the kinetics, but, depending on the Lewis acidity of
the ions participating in the ion association, the formation
of an ion pair can produce a change in the type of reaction
(from a cycloaddition to a hydride transfer) [8].
For this reason we considered of interest the study of
salt effects on the kinetics of the intramolecular electron
transfer reaction [10]:
½FeIIðCNÞ pzCoIIIðNH3Þ ꢀ ! ½FeIIIðCNÞ pzCoIIðNH3Þ ꢀ
5
5
5
5
ð1Þ
where pz = pyrazine.
The FeII/CoIII binuclear complex can be formed in situ
from [FeII(CN)5 H2O]3ꢁ and [CoIII (NH3)5pz]3+ through a
rapid reaction, which does not cause interference with the
electron transfer reaction within the binuclear complex.
This latter was selected because the reactant and product
states, although polar, are neutral. Consequently, long-
range ion–ion interactions must have less influence than
in cases where the precursor complex is charged. In this
way, we hoped that short-range, ion pairing, effects could
be more clearly seen.
In the field of Inorganic Chemistry ion pairing effects
have been less studied. In fact, most of studies in this field
*
Corresponding author. Fax: +34 954557174.
´
E-mail address: gcjrv@us.es (F. Sanchez).
0009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.03.009

F. Muriel et al. / Chemical Physics Letters 422 (2006) 382–385
383
2. Experimental
E1/2, which, for a reversible redox couple, represents the
standard formal redox potential of the couple, that is,
2.1. Materials
RT
F
cox
E1=2 ¼ E0 þ
ln
ð4Þ
cred
The complexes [FeII(CN)5H2O]3ꢁ, [CoIII(NH3)5pz]3+
and [RuIII(NH3)5pz]3+ were prepared and purified follow-
ing the procedures described in references [11–14], respec-
tively. Salts were commercial products (Merck) and were
used as received. Water was obtained from a Millipore
Milli Q water system, its conductivity being less than
10ꢁ6 S mꢁ1. It was deoxygenated before use.
where E0 is the standard potential of the couple and cox and
cred the activity coefficients of the oxidized and reduced
forms of the couple, respectively. The estimated uncer-
tainty in the measured potentials is about 3 mV.
3. Results and discussion
Table 1 shows the electron transfer rate constant, ket,
corresponding to the process given by Eq. (1). Notice that
this is a true unimolecular rate constant and thus, it is free
of the influence of the formation/dissociation processes of
the precursor complex.
2.2. Kinetic measurements
In order to follow the reaction in Eq. (1), the binu-
clear complex was prepared in situ using solutions of
[FeII(CN)5H2O]3ꢁ and [CoIII(NH3)5pz]3+
,
In fact, the variations of ket are, in all the cases, the
expected ones according to two states model [18] that is
according to two reaction paths corresponding to the reac-
tion in free binuclear complex and to the binuclear complex
associated to one of the ions of the background electrolyte.
These forms of the binuclear complex, free and ion paired,
are in equilibrium (see Scheme 1):
In this scheme BC is the binuclear complex and A the
ion associated with it. According to this, the measured rate
constant, ket, would be given by
3ꢁ
3þ
½FeIIðCNÞ H2Oꢀ þ ½CoIIIðNH3Þ pzꢀ
5
5
! ½FeIIðCNÞ pzCoIIIðNH3Þ ꢀ
ð2Þ
5
5
As this reaction is rapid in relation to the following elec-
tron transfer reaction (Eq. (1)), their kinetics are well sep-
arated, in such a way that they can be followed without
mutual interference.
Kinetic runs were carried out in a stopped flow spectro-
photometer from Hi-tech, monitoring the absorbance
changes at 620 nm, which is the wavelength corresponding
to the maximum absorbance of the binuclear complex.
The concentrations of the precursor reactants,
[FeII(CN)5H2O]3ꢁ and [CoIII(NH3)5pz]3+, after mixing,
were 5 · 10ꢁ5 mol dmꢁ3 and 5 · 10ꢁ4 mol dmꢁ3, respec-
tively. In preliminary experiments we checked that these con-
centrations ensured a complete formation of the binuclear
complex. All the experiments were carried out at
298.2 0.1 K. The uncertainty in the rate constant is less
than 5%.
kfet þ Kkaet½Aꢀ
ket
¼
ð5Þ
1 þ K½Aꢀ
Figs. 1 and 2 show that, in fact, the experimental data can
be fitted by Eq. (5) with the values of the parameters given
in the legends of these figures.
It is interesting to note that, although an association of
the binuclear complex with the two ions of a given salt (the
anion and the cation) could be possible given the charges of
the iron and cobalt moieties, Eq. (5) implies that only one
of the ions of the background electrolyte is, in fact, associ-
2.3. Electrochemistry
Table 1
Electron transfer rate constants (103 · ket/sꢁ1) for the intramolecular
process [FeII(CN)5 pzCoIII(NH3)5] ! [FeIII(CN)5pzCoII (NH3)5] in several
electrolyte solutions at 298.2 K
The standard formal redox potentials of the iron and
ruthenium centers in the binuclear complex, [FeII(CN)5
pzRuII(NH3)5]ꢁ [15], were obtained by differential pulse
voltammetry (DPV) as described elsewhere [16]. This binu-
clear complex was also prepared in situ using the same pro-
cedure as in the case of the iron–cobalt binuclear complex.
The measurements were carried out at 298.2 0.1 K
using a concentration of the binuclear complex of
[Salt] (mol dmꢁ3
)
LiNO3
NaNO3
Ca(NO3)2
Na2SO4
0.25
0.5
1.0
1.5
2.0
3.0
4.0
5.0
36.8
31.0
23.0
–
14.3
9.5
39.0
32.0
24.0
–
14.0
11.0
8.7
22.0
15.0
9.3
6.6
4.7
–
9.6
5.4
3.0
1.9
1.3
–
2 · 10ꢁ4 mol dmꢁ3
.
7.3
4.6
–
–
–
–
Using the DPV technique, the relationship between the
peak and half-wave potentials for a reversible redox system
is given by [17]
6.7
DE
2
Epeak ¼ E1=2
þ
ð3Þ
where DE is the voltage amplitude of the pulse (5 mV in our
experiments). As DE is small, Epeak can be identified with
Scheme 1.

384
F. Muriel et al. / Chemical Physics Letters 422 (2006) 382–385
Table 2
Standard formal redox potentials vs. ENH for the iron E1/2(Fe) and
ruthenium
E1/2(Ru)
centers
in
the
binuclear
complex
[FeII(CN)5pzRuII(NH3)5]ꢁ in several aqueous solutions of NaNO3 at
298.2 K
36
24
12
[Salt] (mol dmꢁ3
)
E1/2(Fe) (V)
E1/2(Ru) (V)
0.25
0.50
1.0
2.0
3.0
4.0
5.0
6.0
0.737
0.732
0.725
0.719
0.716
0.714
0.713
0.712
0.508
0.496
0.481
0.463
0.453
0.447
0.442
0.439
0.0
1.5
3.0
[NO3 ] / mol dm
4.5
6.0
would imply a stronger interaction with the anion and thus
a more marked decrease in cox.
-
-3
Fig. 1. Plot of ket against NOꢁ3 concentration for LiNO3 (triangles),
As to the variation of the redox potentials of the iron
center, a decrease is observed when the salt concentration
increases. This fact is opposed to the expected variation
of the redox potentials if the iron center was ion-paired
to a cation. In this case, following arguments similar to
those employed for the case of ruthenium center an
increase of the standard formal redox potential of the iron
center would be expected. Notice that the decrease of the
redox potential of the iron center is not due to a long-range
ion-atmosphere effect, because this effect would also pro-
duce an increase in the redox potential of the iron center.
However, the anion associated to the ruthenium center
would destabilize the iron center, and more strongly in
the case of reduced form of this couple. Thus, according
to Eq. (4), the diminution of the redox potential of this cen-
ter is an expected result.
NaNO3 (circles), and Ca(NO3)2 (diamonds). The points are experimental
data and the line represents the best fit to Eq. (5) with kfet ¼ 0:05 sꢁ1
,
K = 1.3 molꢁ1 dm3 and keat ꢂ 0.
12
9
6
3
Now the negative salt effect observed can be rational-
ized: it is a consequence of the less favorable free energy
of the electron transfer process originated by the associa-
tion of the cobalt center with the anions of the electrolyte.
This association decreases both, the redox potentials of the
donor and the acceptor, but more strongly in the case of
the latter because of the close vicinity of the associated
ion, an anion, to it (see also data in Table 2). Of course,
an increase of the reorganization energy, k (caused by the
reorganization of the ion atmosphere), cannot be discarded
as contributing to the negative salt effect [20]. However,
this effect, if present, according to the neutral charge of
the binuclear complex and to the results obtained for the
redox potential of the iron center would be a minor contri-
bution to the negative salt effect. Moreover, a contribution
of kionic, due to the displacement of the paired counterions
ðNOꢁ3 or SO24ꢁÞ, in the sense defined by Piotrowiak [2], is
not expected in the present case, because, after electron
transfer, the acceptor still bears a positive charge.
0.5
1.0
1.5
2.0
[Na2SO4] / mol dm-3
Fig. 2. Plot of ket against Na2SO4 concentration. The points are
experimental data and the line represents the best fit to Eq. (5) with
kfet ¼ 0:04 sꢁ1, keat ꢂ 0, K = 14.6 molꢁ1 dm3.
ated with the binuclear complex. In fact, a double associa-
tion would produce a quadratic dependence of the salt con-
centration [6,19]. The fact that all the nitrate salts can be
put on the same curve, independently of the cation of the
salt, suggest that is the anion that is associated with the
binuclear complex.
This idea is also supported by the results given in Table
2, which corresponds to the standard formal redox poten-
tials of the two centers in the binuclear complex [FeII(CN)5
pzRuII(NH3)5]ꢁ. The redox potentials of the ruthenium
center decrease when the concentration of the salt
increases. This is a consequence (see Eq. (4)) of the fact that
in the presence of an anion both cox and cred will decrease
as a consequence of the stabilization of the ruthenium cen-
ter. However, cox will decrease more than cred because of
the greater charge of the oxidized form of the couple that
Acknowledgments
This work was financed by D.G.I.C.Y.T. (Projects
´
BQU2002-01063 and BQU2001–3205) and the Consejerıa
´
´
de Educacion y Ciencia de la Junta de Andalucıa.

F. Muriel et al. / Chemical Physics Letters 422 (2006) 382–385
385
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