Redox reactions of [VO(salen)]+/0 couple in acetonitrile: Volume analyses in relation to large chiral recognitions observed for electron self-exchange reactions of [VO(Schiff-Base)]+/0

Article abstract of DOI:10.1016/j.ica.2005.10.028

Kinetic measurements were carried out for the outer-sphere electron transfer reactions involving [VO(Schiff-Base)]^+/0 couples. Electron self-exchange rate constant for [VO(3-MeOsal-(RR)-chxn)]^+/0 couple was determined as k_ex

Full text of DOI:10.1016/j.ica.2005.10.028

Inorganica Chimica Acta 359 (2006) 346–354
www.elsevier.com/locate/ica
Redox reactions of [VO(salen)]+/0 couple in acetonitrile:
Volume analyses in relation to large chiral recognitions observed
for electron self-exchange reactions of [VO(Schiff-Base)]+/0
a
b
b
a
Yasuhide Sasajima , Motoharu Shimizu , Norie Kuroyanagi , Nobuyuki Kishikawa ,
a
a
c,
b,
*
Kyoko Noda , Sumitaka Itoh , Hideo D. Takagi *, Masahiko Inamo
a
Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8602, Japan
b
Department of Chemistry, Aichi University of Education, Kariya, Aichi, 448-8542 Japan
Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8602, Japan
c
Received 22 July 2005; received in revised form 5 October 2005; accepted 9 October 2005
Available online 1 December 2005
Abstract
Kinetic measurements were carried out for the outer-sphere electron transfer reactions involving [VO(Schiff-Base)]+/0 couples. Elec-
tron self-exchange rate constant for [VO(3-MeOsal-(RR)-chxn)]+/0 couple was determined as kex = (5.2 0.8) · 106 kg molꢀ1sꢀ1 at
25 ꢀC. It was found that added water in acetonitrile solvent retarded the electron transfer reactions between [VO(salen)]+ and [Co-
(o-phen)3]2+: the six-coordinate V(V) species, [VO(salen)(OH2)]+ was found to be ca. 3.5 times less reactive compared with the five-coor-
dinate species, [VO(salen)]+. This small difference between the reactivity of five- and six-coordinate species indicates that the reaction
through the direct O@V(V)–O@V(IV) interaction is outer-sphere in nature, contrary to the previously proposed inner-sphere mechanism.
Activation volumes corresponding to the electron self-exchange reactions for [VO(salen)]+/[VO(salen)]0 and [VO(salen)OH2]+/[VO-
(salen)]0 couples were estimated from the volume profiles of the reduction reactions of [VO(salen)]+ by [Co(o-phen)3]2+, with and without
added water in acetonitrile: DV* = ꢀ3.4 3.7 and ꢀ8.0 4.0 cm3 molꢀ1 for [VO(salen)]+/[VO(salen)]0 and [VO(salen)OH2]+/[VO-
(salen)]0 couples, respectively. It was suggested that previously reported chiral recognitions in the redox reactions involving
[VO(Schiff-Base)]+/0 redox couples take place within the encounter complex through the outer-sphere mechanism by maximizing the
direct coupling between the dp orbitals of V(IV) and V(V) species (Scheme 2).
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Redox reaction; Volume analyses; Oxovanadium(V)/(IV); Schiff-Base complexes; Chiral recognition
1. Introduction
bond formation between reducing and oxidizing reagents,
and the succeeding electron transfer process is very rapid.
In contrast to the inner-sphere electron transfer processes,
time allowed for the reacting complexes to recognize suit-
able molecular orientations before the electron transfer
process is limited for the outer-sphere reactions.
Most of the reported results concerning the molecular
recognition in the outer-sphere electron transfer reactions
involve either chiral Co(III) complexes [1–3] or chiral
Ru(III) complexes [4], since the racemization reactions of
these complexes are rather slow compared with the ET pro-
cesses. However, the enantiomeric excess (ee) observed for
Investigation of chiral recognitions in electron transfer
(ET) reactions is important both for the understanding of
biological processes and for the development of stereo-
selective catalytic systems. In the inner-sphere reaction sys-
tems, chiral recognition may take place at the step of the
*
Corresponding authors. Tel./fax: +81 566 26 2636 (M. Inamo), Tel./
fax: +81 52 789 5473 (H.D. Takagi).
E-mail addresses: h.d.takagi@nagoya-u.jp, htakagi@chem.nagoya-u.
ac.jp (H.D. Takagi), minamo@auecc.aichi-edu.ac.jp (M. Inamo).
0020-1693/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.10.028

Y. Sasajima et al. / Inorganica Chimica Acta 359 (2006) 346–354
347
these reactions are usually below 20%, that corresponds to
the difference in the rate constants of only 20% at its max-
imum for different molecular orientations. Therefore, it is
generally understood that the detection of such a small dif-
ference in the rate constants is very difficult to confirm.
Moreover, little structural information has been given to
date for the probable molecular orientations for such chiral
recognitions.
In 1995, Nakajima et al. [5] reported a large difference in
the rate constants for the electron transfer reactions
between the optically active, substituted oxovana-
dium(V)/(IV)-Schiff-Base complexes in acetonitrile: up to
50% difference between kRR–SS and kRR–RR was observed
for the following reaction.
method, when the reaction through the O@V(V)–O@V(IV)
interaction is truly inner-sphere in nature, since the inner-
sphere interaction should induce strong electronic coupling
that leads either to the rapid superexchange or the sequen-
tial transfer [6].
Chiral selection in the outer-sphere electron transfer
reaction originates from the recognition of molecular
shapes within a very short lifetime of the encounter com-
plex, although the lifetime of the encounter complex
depends largely on the charge-type of the two colliding spe-
cies [7]. However, chiral recognition through the molecular
re-orientation within the encounter complex is still possible
to occur for some of the outer-sphere reactions since the
average correlation time for molecular rotation is some-
what shorter (ꢂ10ꢀ10ꢂꢀ12 s) than the lifetime of the
encounter complex unless the reactants carries large like
charges [8].
½VOð3-MeOsal-ðRRÞor -ðSSÞ-chxnÞꢁ
þ ½VOðsal-ðRRÞ-chxnÞꢁþ
! ½VOð3-MeOsal-ðRRÞ or -ðSSÞ-chxnÞꢁþ
In this study, we carried out the reduction reactions of
various chiral Schiff-Base complexes of oxovanadium(V)
at ambient and elevated pressures to investigate the origin
of the relatively large ee (enatiomeric excess) reported for
the pseudo self-exchange process in the previous study [5].
þ ½VOðsal-ðRRÞ-chxnÞꢁ
ð1Þ
where structures of salchxn and 3-MeOsal-chxn complexes
are shown in Scheme 1. Although the reactions do not in-
volve any change in the coordination geometry around
V(IV) and V(V) complexes, they attributed this large ste-
reochemical selection to the inner-sphere O@V(V)–
O@V(IV) interaction. Such a speculation seems to have
emerged, as the interaction between the vacant dp-orbitals
with the filled pp-orbitals of the oxygen atom may be pos-
sible. On the other hand, a direct dp–dp interaction be-
tween the two vanadium ions is equally plausible along
the same reaction coordinate since the energy levels be-
tween dp orbitals on V(IV) and V(V) ions are closer to each
other.
2. Experimental
2.1. Materials
Acetonitrile was obtained from Wako Pure Chemicals
Inc., and purified by distillation from phosphorus pentox-
ide. As acetonitrile treated with phosphorus pentoxide con-
tains species that induce strong acid-base interaction with
ligands, the solvent was re-distilled twice from molecular
sieves before use. The content of the residual water in thus
purified acetonitrile was examined by a Mitsubishi Kasei
CA07 Karl-Fisher apparatus, by which the amount of
Therefore, it is interesting to examine the nature of the
electron transfer process in this reaction system: the self-
exchange rate constant estimated from the reaction of the
V(V)/(IV) couple with typical outer-sphere reagents should
be much smaller than that directly measured by the NMR
residual water was determined to be less than 1 mmol kgꢀ1
.
Tetra-n-butylammonium perchlorate (TBAP) was recrys-
tallized twice from the mixture of ethyl acetate/pentane
solution, and dried under reduced pressure. All other
chemicals used for the syntheses of metal complexes were
purchased from Wako, Aldrich, and Nakarai, and were
used without further purification.
The Schiff-Base ligands and corresponding vanadium(V)
complexes were synthesized according to the literature
methods [9,10], by using appropriate chiral diamines as
starting materials. Vanadium complexes thus obtained
were sufficiently pure for the electrochemical and kinetic
analyses, although it was shown by the NMR analyses that
purified V(V) complexes contained up to 5% of corre-
sponding V(IV) species while no significant deviations from
the calculated values were observed for the results of
microanalyses (Appendix 1). The microanalyses, UV–Vis
absorption spectra, and NMR spectra of these complexes
are summarized in Table 1. The structures and abbrevia-
tions of oxovanadium complexes used in this study are
summarized in Scheme 1.
Scheme 1. Oxovanadium complexes used in this study.

348
Y. Sasajima et al. / Inorganica Chimica Acta 359 (2006) 346–354
Table 1
Results of microanalyses, UV–Vis absorption spectra, NMR spectra, and the observed electron self-exchange rate constants for the vanadium(V)
complexes by using NMR
Vanadium(V) complex
Microanalysisa
UV–Vis
H (%) N (%) k (nm) e (Mꢀ1 cmꢀ1
1H NMR (CD3CN)
Self-exchange ratebconstant
C (%)
)
d (ppm) (TMS)
kex (kg molꢀ1 sꢀ1
(1.6 0.1) · 106
)
[VO(salen)(H2O)]ClO4
568.8
291.8
1.28 · 103
1.89 · 104
2.96 · 104
3.40 · 104
43.25
3.28
6.33
9.07 (s, PhCH@N)
7.92–7.09 (Ph)
4.48, 4.24 (s, CH2CH2–N)
(42.64) (3.85) (6.22) 249.4
224.8
[VO(3-MeOsal-(RR)-chxn)(H2O)]ClO4
[VO(3-Phsal-(RR)-chxn)(H2O)]ClO4
9.08, 8.81 (s, PhCH@N)
7.65–7.22 (Ph)
3.99 (s, OMe)
(5.2 0.8) · 106
46.65
4.51
4.91
255.6
2.77 · 104
3.93 · 104
(46.78) (4.64) (4.96) 221.4
3.62–1.49 (chxn)
599.8
318.6
1.04 · 103
1.28 · 104
4.30 · 104
8.82, 8.04 (s, PhCH@N)
7.61–6.73 (Ph)
3.67–0.92 (chxn)
57.70
4.56
5.76
(58.50) (4.60) (4.26) 241.4
a
Calculated values are in the bracket.
Determined from the NMR line-broadening method at 297.9 K.
b
[VO(salen)] was synthesized by mixing VOSO4 with
salen in methanol with the existence of triethylamine. The
green product was recrystallized from acetonitrile. Anal.
Calc. for VOC16H14N2O2: C, 57.67; N, 8.40; H, 4.23.
Found: C, 57.22; N, 8.12; H, 4.22%.
rocene couple in the same solvent. The reaction volumes
were determined from the pressure dependence of the redox
potentials at 25 ꢀC by using a BAS 100B/W Electrochemi-
cal Analyzer. The pressure vessel used for the measure-
ments is similar to that reported elsewhere [12]. We used
an all-glass electrochemical cell equipped with a 3 mmB
GC disk and a 1.0 mmB Pt wire as the working and counter
electrodes, respectively.
[Co(o-phen)3](ClO4)2
(o-phen = 1,10-phenanthroline)
and [Co(4,7-Me2phen)3](ClO4)2 Æ H2O (4,7-Me2phen =
4,7-dimethyl-1,10-phenanthroline) complexes were synthe-
sized by the literature method [11] Anal. Calc. for
CoC36H24N6Cl2O8: C, 54.15; N, 10.53; H, 3.03. Found:
C, 53.55; N, 10.47; H, 3.13%. Anal. Calc. for CoC42H38N6-
Cl2O9: C, 56.01; N, 4.25; H, 9.33. Found: C, 55.86; N, 4.18;
H, 9.46%. [Co(o-phen)3](ClO4)3 and [Co(4,7-Me2phen)3](P-
F6)3 Æ 2.5H2O complexes were also synthesized by the
reported method [11]. Anal. Calc. for CoC36H24N6Cl3O12:
C, 48.16; N, 9.36; H, 2.69. Found: C, 46.49; N, 9.69; H,
2.77%. Anal. Calc. for CoC42H41N6P3F18O2.5: C, 43.35;
N, 7.22; H, 3.55. Found: C, 43.76; N, 7.22; H, 3.64%.
A Hitachi U-3000 UV–Visible spectrophotometer and a
JASCO DIP-360 polarimeter were used for the measure-
ments of the absorption spectra and the angles of rotation
of sample solutions, respectively. A JEOL JMN-400 and a
Bruker AMX-400WB were used for the measurements of
1
the H NMR spectra.
3. Results and discussion
3.1. Self-exchange rate constants for [VO(salen)OH2]+/0
and [VO(3-MeOsal-(RR)-chxn)OH2]+/0 couples
2.2. Kinetic and electrochemical measurements
The crystal structure of [VO(salen)OH2]NO3 has been
˚
Kinetic measurements at ambient pressure were carried
out by a Unisoku RA401 stopped-flow apparatus, while
the measurements at elevated pressures were performed
by using a Hi-Tech HPSF-50 apparatus. Deaerated paraf-
fin oil was used as the pressurizing fluid. The reactions were
monitored by observation of the absorbance change at 370
nm (an absorption maximum in the case of [VO(salen)]
species).
Electrochemical measurements at ambient pressure were
carried out by using a BAS 100B/W Electrochemical Ana-
lyzer with a standard three-electrode configuration: a
3 mmB glassy carbon (GC) was used as the working elec-
trode, a platinum wire as the counter electrode, and a
Ag/AgNO3 electrode in 0.1 M solution of TBAP in aceto-
nitrile was used as the reference electrode. The redox
potentials of complexes are reported in Table 2 with refer-
ence to the redox potential observed for the ferricinium/fer-
reported: V–OH2 distance is 2.225 A [13]. On the other
hand, most of the corresponding V(IV) complexes are
five-coordinate with a square pyramidal geometry [14,15]
(see Appendix 2). It has also been known that the water
exchange rate constant for pentaaquaoxovanadium(IV) is
ca. 108 sꢀ1 at its axial position which is trans to the oxo
anion, while the water exchange at the equatorial position
is significantly slower (ca. 0.1–1 sꢀ1) [16]. The crystal struc-
ture of [VO(salen)OH2]NO3 indicates that the water mole-
cule trans to the oxo anion is the more tightly bound to the
metal ion in V(V) compared with the corresponding V(IV)
species: no coordinated ligand at the trans position to the
oxo anion in the crystal structure of V(IV) complex [15].
The electron self-exchange rate constants for [VO(salen)
OH2]+/0 and [VO(3-MeOsal-(RR)-chxn)OH2]+/0 couples
were estimated by the NMR line broadening method [17]
in dry acetonitrile at 297.9 0.3 K. All but the ring proton

Y. Sasajima et al. / Inorganica Chimica Acta 359 (2006) 346–354
349
the directly obtained kex = (5.2 0.8) · 106 kg molꢀ1 sꢀ1
for [VO(3-MeOsal-(RR)-chxn)]+/0 couple in this study.
The close coincidence between these independently
obtained self-exchange rate constants indicates that the
electron self-exchange and pseudo-exchange reactions pro-
ceed through the identical activation process for each
reactant.
Table 2
Redox potentials of complexes used in this studya
Redox couple
E1/2 (mV)b
[VO(salen)]+/0
85
49
21
[VO(3-MeOsal-(RR)-chxn)]+/0
[VO(3-Phsal-(RR)-chxn)]+/0
[Co(o-phen)3]3+/2+
ꢀ17
[Co(4,7-Me2phen)3]3+/2+
ꢀ186
Retardation of the electron transfer reaction by added
water was observed for the cross reaction of [VO(salen)]+
with [Co(o-phen)3]2+ in acetonitrile, as described in the
later section: the estimated formation constant of [VO(salen)-
OH2]+ from [VO(salen)]+, 8 3 kg molꢀ1, indicates that
dissociation of coordinated water takes place upon dissolu-
tion of [VO(salen)OH2]+ in dry acetonitrile (see Appen-
dixes 1 and 2). In the following sections, we will describe
V(V) complexes dissolved in dry acetonitrile without
including coordinated water, such as [VO(salen)]+: acetoni-
trile molecule, that probably replaces the coordinated
water molecule, was also omitted from the formula because
of the inferior basicity of this solvent molecule. The line-
a
I = 0.1 mol kgꢀ1 (TBAP), T = 298.2 K
vs. Eferricinium/ferrocene
b
.
signals of V(V) complexes broadened by adding small
amounts of V(IV), ([V(IV)]/[V(V)] < 10%), to the solution
of V(V). The –N@CH– proton signal was used for the esti-
mation of the electron self-exchange rate constant. As these
proton signals were observed at the identical positions for
solutions containing different amounts of V(IV), the slow
exchange limit was assumed [17] for the kinetic analyses.
Fig. 1 shows the dependence of pDm for the –N@CH–
proton of the V(V)-salen complex on [V(IV)], when extra
amounts of V(IV)-salen complex was added to the solution
(Dm stands for the difference between the observed line
width for the sample solution that contains extra amounts
of V(IV) and the natural line width, see Appendix 1): filled
circles represent the experiments for the sample solutions
with no appreciable amount of water in the solvent, while
open circles represent the results for the sample solutions
with more than 30 mmol dmꢀ3 of water (30–
80 mmol dmꢀ3). As shown in Fig. 1 (Tables S0), no signif-
icant effect by added water was observed for the rate of
electron self-exchange reaction (Appendix 1). Estimated
electron self-exchange rate constants for salen and 3-MeO-
sal complexes are summarized in Table 1.
1
width of the H signal for impure acetonitrile in the bulk
was very narrow (<1 Hz) and was unaffected by the exis-
tence of V(V), indicating either a very rapid exchange with
the coordinated acetonitrile or no coordination of acetoni-
trile to V(V) complex (see Appendixes 1 and 2).
3.2. Self-exchange rate constant for the [Co(o-phen)3]3+/2+
couple in acetonitrile
The electron self-exchange rate constant for [Co(o-
phen)3]3+/2+ couple was measured in acetonitrile, from
the reaction of optically active (+)-[Co(o-phen)3]3+ and
racemic [Co(o-phen)3]2+. As the racemization reaction of
Co(II) complex is rapid, the loss of the optical activity of
the reaction mixture is attributed to the slow electron
self-exchange reaction between [Co(o-phen)3]3+ and
[Co(o-phen)3]2+. A typical change of the optical rotation
with time is shown in Fig. 2. We used a home-made rapid
mixer to achieve equivolume mixing of (+)-[Co(o-phen)3]3+
and racemic [Co(o-phen)3]2+: the optical rotation measure-
ments required 10 cm3 of sample solutions for each mea-
surement. The temperature of the sample solutions were,
therefore, guaranteed to only 1 K, since the temperature
equilibration of samples with such large amount of solvent
takes relatively long time (up to several minutes). The
observed rate constant was kex = 2.16 0.12 kg molꢀ1 sꢀ1
at 298.2 1.0 K.
Nakajima et al. reported pseudo-electron self-exchange
reactions between chiral [VO(3-MeOsal-(RR or SS)-chxn)]+
and [VO(sal-(RR)-chxn)] in acetonitrile [5]: kRR–RR
=
(2.8 0.1) · 106 dm3 molꢀ1 sꢀ1 and kSS–RR = (5.6 0.7) ·
106 dm3 molꢀ1 sꢀ1. These values may be compared with
2þ
ðþÞ-½Coðo-phenÞ3ꢁ3þ þ rac-½Coðo-phenÞ3ꢁ
kobs
!ðþÞ-½Coðo-phenÞ3ꢁ2þ þ rac-½Coðo-phenÞ3ꢁ
3þ
ð2Þ
rapid
2þ
2þ
ðþÞ-½Coðo-phenÞ3ꢁ
rac-½Coðo-phenÞ ꢁ
ꢀ!
3
d½ðþÞ-CoðIIIÞꢁ
Fig. 1. Dependence of the first-order rate constant estimated from the
broadening of the line width of the imine proton signal on the concentration
of paramagnetic [VO(salen)] in acetonitrile at 298.2 K. (s): [VO(sa-
len)+] = (1.19–1.33) · 10ꢀ3 mol kgꢀ1. [H2O] = 3–7 · 10ꢀ2 mol kgꢀ1. (d):
ꢀ
¼ kobs½kðþÞ-CoðIIIÞꢁ kex ¼ 2kobs
dt
This value may be compared with that estimated for the
same reaction in water, 12 dm3 molꢀ1 sꢀ1 [6].
[VO(salen)+] = (1.25–1.35) · 10ꢀ3 mol kgꢀ1. [H2O] < 10ꢀ2 mol kgꢀ1
.

350
Y. Sasajima et al. / Inorganica Chimica Acta 359 (2006) 346–354
responding tables are included in the supporting materials
(Figs. S1–S4 and Tables S2–S5).
The self-exchange rate constants for V(V)/V(IV) couples
were estimated by using the Marcus/Ratner cross relation
[19]: the self-exchange rate constant for [Co(o-phen)3]3+/2+
,
2.16 0.12 kg molꢀ1 sꢀ1, determined in this study was
used for this purpose. Then, the self-exchange rate constant
for [Co(4,7-Me2phen)3]3+/2+ couple was estimated from the
cross reaction rate constants for the reactions with V(V)
complexes, by using the cross relation. The electron self-
exchange rate constants for the redox couples thus esti-
mated are summarized in Table 3, in which we assumed
two different distances for the closest approach between
the reactants: the shorter distance corresponds to the con-
tact of two reactants at the coordination site trans to the
oxo anion of V(V) complex (axial approach) while the
longer distance corresponds to that through the salen-type
ligand of V(V) complex (equatorial approach). It is seen
that the electron self-exchange rate constant for [VO(sa-
len)]+/0 couple obtained directly by the NMR method is
consistent with that estimated by using the cross relation,
especially when the axial approach was assumed.1 It should
be noted that Marcus/Ratner type cross relation holds for
both inner- and outer-sphere type reactions, as far as the
activation processes for each reactant are identical for the
cross and self-exchange reactions. From the results summa-
rized in Table 3, however, we can safely conclude that the
self-exchange reactions between V(V) and V(IV) complexes
proceed though the outer-sphere mechanism, since the esti-
mated self-exchange rate constants for the V(V)/(IV) redox
couples from the rate constants for cross reactions with
typical outer-sphere reagents are identical to those esti-
mated either by the NMR method or from the pseudo-
exchange reaction [5].
Fig. 2. Typical change of optical rotation for the reaction between
(+)-[Co(o-phen)3]3+ and racemic [Co(o-phen)3]2+ with time. [Coðo-phenÞ23þ] =
3þ
7.35 · 10ꢀ4 mol kgꢀ1
.
½ðþÞ-Coðo-phenÞ3 ꢁ ¼ 1:57 ꢃ 10ꢀ4 mol kgꢀ1
. I =
. T = 298 1 K.
1.1 mol kgꢀ1 (TBAP). [o-phen] = 1.0 · 10ꢀ3 mol kgꢀ1
Free o-phen was added to the solution to suppress decomposition of
[Co(o-phen)3]2+
.
3.3. Cross reactions of V(V) with [Co(o-phen)3]2+ and
[Co(4,7-Me2 phen)3]2+
Redox potentials of the complexes synthesized in this
study were examined in acetonitrile, and are summarized
in Table 2. Cross reactions of these V(V) complexes with
[Co(o-phen)3]2+ and [Co(4,7-Me2phen)3]2+ were examined
in acetonitrile, to confirm the activation mode of these spe-
cies for the redox reactions: it has been known that the
Marcus/Ratner cross relation holds only when the activa-
tion process of each reactant for the self-exchange reaction
is identical to that for cross reactions [18]. Fig. 3 (Table S1)
shows the typical dependence of the reduction rate con-
stant for [VO(3-MeOsal(RR)-chxn)]+ by [Co(o-phen)3]2+
on the concentration of excess [Co(II)] at various tempera-
tures. Plots for other cross reactions together with the cor-
3.4. Dependence of the rate constant for the reduction
reaction of [VO(salen)]+ by [Co(o-phen)3]2+ on the
concentration of water in acetonitrile
Dependence of the second-order rate constant for reac-
tion (3) on the water concentration was examined in
acetonitrile.
2þ
½VOðsalenÞꢁþ þ ½Coðo-phenÞ3ꢁ
3þ
! ½VOðsalenÞꢁ þ ½Coðo-phenÞ3ꢁ
ð3Þ
The reaction was retarded by added water, as shown in
Fig. 4 (Table S6). Such retardation of the electron transfer
rate by water may be explained by the following mecha-
nism in which a less reactive water adduct is formed by
the addition of water.
Fig. 3. Dependence of the rate constant for the reduction reaction of
[VO(3-MeOsal-(RR)-chxn)]+ by [Co(o-phen)3]2+ in acetonitrile at various
temperatures. [VO(3-MeOsal-(RR)-chxn)+] = 3.35 · 10ꢀ5 mol kgꢀ1. [o-
phen] = 1.05 · 10ꢀ3 mol kgꢀ1. I = 0.1 mol kgꢀ1 (TBAP). T = 288 (j),
293 (s), 298 (r), 303 (M), and 308 (.) K. Free o-phen was added to the
1
Since the solvent used for this experiment contains less than
4 mmol kgꢀ1 of water, it is probable that the axial position opposite to
the oxo anion in V(V) complex is available for the redox reactions (see
later section for the equilibrium constant for the aquation of this site,
solution to suppress decomposition of [Co(o-phen)3]2+
.
8
3).

Y. Sasajima et al. / Inorganica Chimica Acta 359 (2006) 346–354
351
Table 3
The electron self-exchange rate constants calculated for the V(V)/(IV) redox couples in two different types of interactionsa
Redox couple
Counter reagent
Axial approach (400 pm)
Equatorial approach (890 pm)
[VO(salen)]+/0
[Co(o-phen)3]3+/2+
1.60 · 106
7.54 · 106
2.07 · 106ꢀ1
1.05 · 10
9.68 · 105
4.59 · 106
1.27 · 106ꢀ2
9.86 · 10
[VO(3-MeOsal-(RR)-chxn)]+/0
[VO(3-Phsal-(RR)-chxn)]+/0
[Co(4,7-Me2phen)3]3+/2+
[Co(4,7-Me2phen)3]3+/2+
[Co(o-phen)3]3+/2+
[Co(o-phen)3]3+/2+
[VO(3-MeOsal-(RR)-chxn)]+/0
[VO(3-Phsal-(RR)-chxn)]+/0
6.61 · 10ꢀ2
6.32 · 10ꢀ2
I = 0.1 mol kgꢀ1 (TBAP), T = 298.2 K.
a
kg molꢀ1 sꢀ1
.
reaction is fast for the O@V–O@V conformation while it
is somewhat slower when a water molecule exists between
the reactants. However, such a O@V–O@V interaction
does not necessarily induce the inner-sphere type electron
transfer: the very small degree of acceleration by this con-
figuration strongly indicates that the reaction is outer-
sphere in nature, and that the electron transfer takes place
through the dp–dp interaction. As the water content of ace-
tonitrile solvent was small in the previous study [5], the rel-
atively large stereochemical selection reported for the
pseudo-exchange reactions between [VO(3-MeOsal-(RR
or SS)-chxn)] and [VO(sal-(RR)-chxn)]+ should have taken
place with the direct O@V(V)–O@V(IV) interaction.
3.5. Reaction volumes of [VO(salen)]+/0 and
[Co(o-phen)3]3+/2+ couples in acetonitrile
Fig. 4. Dependence of the rate constants for the reduction reaction of
[VO(salen)]+ by [Co(o-phen)3]2+ on the concentration of water in aceto-
2þ
nitrile. [Coðo-phenÞ3 ] = 4.4 · 10ꢀ4 mol kgꢀ1, [VO(salen)+] = 4.3 · 10ꢀ5 or
2.2 · 10ꢀ5 mol kgꢀ1, I = 0.1 mol kgꢀ1 (TBAP), [oꢀphen] = 1.0 · 10ꢀ3
It is necessary to know the reaction volumes of [VO(sa-
len)]+/0 and [Co(o-phen)3]3+/2+ couples for the purpose of
the volume analysis of cross reactions (5) and (6). The reac-
tion volumes were obtained by measuring the pressure
dependence of the redox potential of an acetonitrile solu-
tion containing [VO(salen)]+ or [Co(o-phen)3]3+ with
0.1 mol kgꢀ1 of tetra-n-butylammonium perchlorate (Table
S7). The observed reaction volume of a redox couple
includes the reaction volume of the reference electrode.
mol kgꢀ1, T = 298.2 K. Free o-phen was added to the solution to suppress
decomposition of [Co(o-phen)3]2+
.
K
½VOðsalenÞꢁþ þ H2O ꢀ ½VOðsalenÞOH2ꢁþ
ð4Þ
ð5Þ
ð6Þ
½VOðsalenÞꢁþ þ ½Coðo-phenÞ3ꢁ
2þ
k1
3þ
!½VOðsalenÞꢁ þ ½Coðo-phenÞ3ꢁ
½VOðsalenÞOH2ꢁþ þ ½Coðo-phenÞ ꢁ
2þ
3
V 0
n+/(n-1)+
M
∆
k2
3þ
!½VOðsalenÞOH2ꢁ þ ½Coðo-phenÞ3ꢁ
Mn+ + Ag
Ag+ + M(n-1)+
ð8Þ
The apparent first-order rate constant k is expressed by the
V 0
reference
following equation:
∆
k1 þ k2K½H2Oꢁ
k ¼
½Coðo-phenÞ23þꢁ
ð7Þ
We do not have to know the reaction volume for the Ag/
Ag+ reference couple ðDV 0referenceÞ, since it cancels when
the specific reaction volume is calculated for reaction (5).
The overall reaction volumes for reaction (8),
1 þ K½H2Oꢁ
The best-fit curve according to Eq. (7) is also shown in
Fig. 4, and the values of k1, k2, and K are (2.2 0.1) ·
104 kg molꢀ1 sꢀ1, (6.3 1.0) · 103 kg molꢀ1 sꢀ1, and 8
3 kg molꢀ1, respectively. The value of k1 is 3.5 times larger
than k2: [VO(salen)OH2]+ is less reactive compared with
[VO(salen)]+ only by a factor of only 3.5.
DV 0Mnþ=ðnꢀ1Þþ þ DV 0reference
,
were
ꢀ12.4 2.7
and
15.2 0.5 cm3 molꢀ1 for [VO(salen)]+/0 and [Co(o-
phen)3]3+/2+ couples, respectively. The reaction volumes
for the sample solutions containing less than 5 mmol kgꢀ1
of water and more than 100 mmol kgꢀ1 of water were iden-
tical to each other for both redox couples, within the exper-
imental uncertainty, indicating that the existence of water
molecule at the trans position of the oxo anion in V(V)
complex does not significantly alter the reaction volume:
the volume difference of these V(V)/(IV) couples is identical
The water molecule in [VO(salen)OH2]+ complex may
be positioned at the position trans to the oxo anion of
[VO(salen)]+, by considering that the electron transfer rate
generally decreases with increasing the distance between
reactants in the encounter complex [19]. Therefore, it
is highly probable that the rate of the electron transfer

352
Y. Sasajima et al. / Inorganica Chimica Acta 359 (2006) 346–354
DV ꢄ11 þ DV ꢄ22 þ DV 012
and the water molecule at the trans position of the oxo
anion is withheld during the electrode reaction (6). The spe-
cific reaction volume, DV 012, was estimated as ꢀ27.6
2.7 cm3 molꢀ1 for both reactions (5) and (6).
DV ꢄ12
¼
þ C
2
2
XDV 102 lnK12 ꢀ 2ðlnK12Þ ðDV ꢄ11 þ DV 2ꢄ2 ꢀ DV W11 ꢀ DV 2W2
Þ
C ¼
X2
ꢃ
ꢁ
ꢂ
ꢄ
k11k22
Z2
w11 þ w22
3.6. Reduction reaction of [VO(salen)]+ by
[Co(o-phen)3]2+ at elevated pressures
X ¼ 4 ln
þ
RT
ð9Þ
where DV ꢄ12 is the activation volume of the cross reaction,
DV ꢄ11, DV 2ꢄ2, k11, and k22 are the activation volumes and cor-
responding rate constants for each electron self-exchange
reaction, DV 012 is the reaction volume of the cross reaction,
and Z is the collision frequency which is usually taken to be
The dependence of the rate constant for cross reaction
(5) on pressure at 25 ꢀC in dry acetonitrile is shown in
Fig. 5. The activation volume was estimated from the slope
of the plot as ꢀ17.6 1.3 cm3 molꢀ1. In the acetonitrile
solution that contains ca. 100 mmol kgꢀ1 of water, the acti-
vation volume was estimated as ꢀ19.9 1.5 cm3 molꢀ1
(Fig. 5). By using the following volume cross relation
(Eq. (9)) [20,21], we obtained activation volumes for the
self-exchange reactions of [VO(salen)]+/[VO(salen)]0 and
[VO(salen)OH2]+/[VO(salen)]0 couples: DV eꢄx ¼ ꢀ3:4ꢅ
3:7 cm3 molꢀ1 and ꢀ8.0 4.0 cm3 molꢀ1, for the reactions
in dry acetonitrile and in acetonitrile containing
100 mmol kgꢀ1 of water, respectively. Under the latter con-
dition, ca. 50% of V(V) species are expected to exist as
[VO(salen)(OH2)]+. The relatively large mathematical
errors of the estimated activation volumes for the self-
exchange processes represent the accumulation of the max-
imum errors for measured activation and reaction volumes.
However, it has been shown that the comparison with the
theoretically calculated value can be made when the differ-
ence from the experimentally estimated activation volume
is within the 1 cm3 molꢀ1 range of the theoretical value,
as the error of 1 cm3 molꢀ1 is the allowed maximum that
originates from the ignorance of the inner-sphere contribu-
tion to the total activation volume [22,23].
1011
M
ꢀ1 sꢀ1, respectively. When the redox potential of the
cross reaction is close to zero, C is negligibly small.
For this calculation, a theoretically calculated value of
ꢀ4.2 cm3 molꢀ1 was used for the activation volume of
[Co(o-phen)3]3+/2+ couple [24,25]. The parameters used
for this calculation are summarized in Table 4. Theoretical
values of the activation volumes corresponding to the two
different configurations of activated complexes were esti-
mated by the following Stranks-Hush-Marcus equation
[26,28]:
DV 1ꢄ1 ¼ DV iꢄs þ ðDV ꢄos þ DV cꢄoul þ DV ꢄDHÞ þ bRT
ð10Þ
in which the b is the isothermal compressibility of the sol-
vent, and theꢄ inner-sphere contribution to the activation
volume, DV is, is usually very small and ignored
[22,23,26]. Other terms in the above equation is given by
the following expression:
ꢃꢁ
ꢂꢅ
ꢁ
ꢂꢆ
Ne2
1
1
1
o
1
1
DV ꢄos
¼
þ
ꢀ
ꢀ
16pe0 2a1 2a2
a
oP Dop Ds
T
ꢁ
ꢂꢄ
b
1
1
ꢀ
ꢀ
3a Dop Ds
Nz1z2e2
16pe0a
ꢃꢁ
ꢂ
ꢄ
o
1
b
DV ꢄcoul
DV ꢄDH
¼
¼
ꢀ
oP Ds
3Ds
T
ꢃꢁ
ꢂ
ꢄ
pffiffiffi
ꢇ
ꢈ
RTz1z2C l
o ln Ds
oP
pffiffiffi
3 þ 2Ba l ꢀ b
ꢇ
ꢈ
pffiffiffi
2
1 þ Ba l
T
ð11Þ
where a1 and a2 are the radii of oxidized and reduced spe-
cies (a = a1 + a2), respectively. Dop and Ds are the high-
and low-frequency dielectric constants of the solvent,
Table 4
Activation volumes for the self-exchange reaction of [VO(salen)]+/0
DV ꢄ12
DV 0
DV ꢄ22
DV ꢄ11
ðcm3 molꢀ1
Þ
ðcm3 molꢀ1
Þ
ðcm3 molꢀ1
Þ
ðcm3 molꢀ1
Þ
Fig. 5. Pressure dependence of the rate constants for the reduction reaction
ꢀ17.6 1.3a
ꢀ27.6 2.7
ꢀ4.2
ꢀ3.4 3.7
of [VO(salen)]+ by [Co(o-phen)3]2+ in acetonitrile. (s): [VO(salen)+] =
ꢀ19.9 1.5b
ꢀ8.0 4.0
2þ
4.04 · 10ꢀ5 mol kgꢀ1. [Coðo-phenÞ3 ] = 4.47 · 10ꢀ4 mol kgꢀ1. [H2O] < 10
mmol kgꢀ1
. ,
I = 0.1 mol kgꢀ1 (TBAP). [o-phen] = 1.0 · 10ꢀ3 mol kgꢀ1
Axial approach
Equatorial approach
ꢀ3.8 (890 pm)
ꢀ8.4 (400 pm)
T = 298 K. (h): [VO(salen)+] = 4.08 · 10ꢀ5 mol kgꢀ1. [Coðo-phenÞ23þ] =
8.20 · 10ꢀ4 mol kgꢀ1. [H2O] > 100 mmol kgꢀ1. I = 0.1 mol kgꢀ1 (TBAP).
[o-phen] = 9.5 · 10ꢀ4 mol kgꢀ1, T = 298 K. Free o-phen was added to the
a
[H2O] < 10 mmol kgꢀ1
[H2O] > 100 mmol kgꢀ1
.
b
solution to suppress decomposition of [Co(o-phen)3]2+
.
.

Y. Sasajima et al. / Inorganica Chimica Acta 359 (2006) 346–354
353
Scheme 2. Configurations of the encounter complexes: (a) closest approach for the RR-SS combination in the axial approach, (b) the closest approach for
the RR–RR combination in the axial approach, (c) and (d) possible close approach for the six coordinate V(V) and five coordinate V(IV) [the distance
between two vanadium centers is ca. 480–500 pm in this configuration where no specific interaction that induces the stereo selectivity]. The V(V)–V(IV)
distance in (a) is shorter by more than 100 pm than that for (b). When no water is added to the solvent, the self-exchange reaction proceeds through a-, b-,
and d-type interactions. The reported stereochemical selection can be explained by the largest rate constant for the ET process through (a): the largest
electronic coupling between V(V) and V(IV) is achieved in this configuration. On the other hand, the reaction takes place via (c) and (d) with the existence
of sufficient amounts of water in the solvent: the rate constant for the reaction involving this type of interaction is smaller because of the longer distance
between V(V) and V(IV).
respectively, and l is the ionic strength of the solution.
Each volume term in Eq. (10) was estimated for different
configurations of the encounter complex (Scheme 2) and
also listed in Table 4. As seen in Table 4, the activation vol-
ume calculated by assuming the axial approach of [VO(sa-
len)]+ and [VO(salen)] is consistent with the experimentally
obtained activation volume for the self-exchange reaction,
when [H2O]free < 10 mmol kgꢀ1. It is, therefore, suggested
that the electron self-exchange reaction of [VO(salen)]+/0
couple proceeds with the short O@V(V)–O@V(IV) interac-
tion distance as described in Scheme 2(a) when the water
content in the solvent is small. It seems that the oxo anion
on V(IV) complex approaches from the opposite side of the
oxo anion of V(V) complex, since V(IV) is stabilized in a
C4v form without a ligand at the trans position to the
oxo anion (Appendixes 1 and 2).
The relatively large enantiomeric excess reported for the
reaction of [VO(3-MeOsal-(RR or SS)-chxn)]0 with
[VO(sal-(RR)-chxn)]+ in acetonitrile with no added water
in the acetonitrile solvent, therefore, may be explained by
the preferred configuration described in Scheme 2: the dis-
tance between vanadium ions is some 100 pm shorter for
the RR–SS interaction (Scheme 2(a)) than that for the
RR–RR combination (Scheme 2(b)). In both cases, how-
ever, the electron transfer reactions proceed through the
outer-sphere mechanism.
Acknowledgment
This work was supported by a Grant-in-Aid for Scien-
tific Research (No. 16550053) from the Ministry of Educa-
tion, Science, Sports, and Culture of Japan.
On the other hand, the self-exchange reaction involving
[VO(salen)(OH2)]+ species should also take place with the
optimized geometry through which the electronic coupling
between two vanadium ions is maximized. In this case, the
maximum overlap between two dp orbitals on V(IV) and
V(V) complexes is achieved in the manner described in
Scheme 2(c) and (d): the distance between V(IV) and
V(V) centers are some 50–200 pm longer than the case in
Scheme 2(a). Since the rate of the electron transfer
decreases exponentially with increasing the distance
between metal centers, a somewhat smaller rate constant
for the k2 process (Eq. (6)) than the k1 process may be
explained by this small difference in the interaction dis-
tances between two metal ions. The preference of the
RR–SS combination for V(V) and V(IV) complexes to
the RR–RR combination may be explained by the differ-
ence of the steric repulsion between the salicylaldoimine
moieties on two vanadium complexes in these two combi-
nations (Scheme 2(a) and (b)).
Appendix 1
It was not possible to obtain pure V(V) complexes: sam-
ples with small but noticeable amounts of V(IV) were used
in this study. Although such a contamination by V(IV) did
not create a serious problem for the measurements of cross
reactions, the results of direct measurements of the self-
exchange rate were somewhat obscured: the natural line
widths for the V(V) solutions were significantly broadened
as shown by mref in Table S0. Addition of an extra amount
of V(IV) further broadened the signal, and the self-
exchange rate constant was determined by the slope of
the plot in Fig. 1: the rate constant was obtained by multi-
plying p after subtraction of mref from the observed line
widths, mobs, of the sample solution that contains extra
amounts of V(IV). A rather inferior V(V) complex in terms
of the purity (contaminated with ca 8% of V(IV)) was also
used (a plot shown by open circles in Fig. 1), showing no

354
Y. Sasajima et al. / Inorganica Chimica Acta 359 (2006) 346–354
significant deviation from the expected slope. Therefore, it
was concluded that no impurity other than V(IV) complex
was contained in the synthesized V(V) complexes provided
by the method described in Section 2.
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filled high-energy ligand p orbitals to the metal t2g and/or
eꢄg orbital. The ligand p level consists of t2u, t1u, and t1g
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The T1u normal mode (m4) vibration not only causes defor-
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˚
its a relatively long V(V)–OH2 distance (ca. 2.23 A) but
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Eight tables (Tables S0–S7) of all experimental data are
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