and 3.2 eV for the SET to 3 and to 4, respectively, as well as Z =
M s . Although DG foll, and accordingly k values, are not
definitive, important here is the difference in k between the SET
disk electrode (RDE) voltammetry was carried out on a BAS
RDE-1 equipped with an ALS electrochemical analyzer Model
620A.
1
1
−1 −1
0
10
to 3 and that to 4 (1.3 eV). A simulation shows that this difference
0
is almost unchanged if 50% tolerance is taken for DG foll
.
Materials
Tributylphosphine (1) was purchased (Tokyo Chemical Industry
Ltd.). Viologens 3 and 4 were synthesized through the Men-
ꢀ
schutkin reaction of 2,2 -bipyridine and 1,10-phenanthroline,
respectively, with an alkyl dihalide in acetonitrile according to
21
1
the literature procedure. The products were characterized by H
NMR and elemental analysis. The spectral data of 3a are given
1
below; H NMR (200 MHz, CD
7
6
3
CN) d (ppm) 3.45 (4H, t, J =
.2 Hz), 4.98 (4H, t, J = 7.2 Hz), 7.36 (10H, m), 8.42 (4H, d, J =
.6 Hz), 8.85 (4H, d, J = 7.0 Hz). Anal. calcd. for C26
26 2 2 8
H N B F :
C 57.82, H 4.85, N 5.19, found: C 58.02, H 4.92, N 5.19%.
Electrochemical measurements
A solution of viologens 2–4 (5.0 × 10− M) and tetraethylam-
monium tetrafluoroborate (0.10 M) as the supporting electrolyte
in acetonitrile was subjected to RDE measurement at room
temperature with a rotating disk platinum electrode (1000 rpm)
3
Scheme 4
Viologen 3 assumes a twisted geometry and is reduced through
SET to the radical cation whose two pyridinium rings are coplanar
Scheme 4). That is, the SET to 3 is accompanied by a significant
6,7
+
(
as the working electrode and Ag/Ag as the reference electrode.
structural change. The acyclic viologen 2 also undergoes a
significant structural change from the twisted geometry to the
coplanar one upon the SET. On the other hand, 4 assumes a
Spectroscopy
7
A solution of 1 (0.15 M) and 3 or 4 (2.0 × 10− M) in acetonitrile
4
geometry in which two pyridinium rings are coplanar due to the
fused ring system. Therefore, only a slight structural change occurs
upon the SET, if any. The difference in k is thus attributable to the
difference in the structural change of the viologen accompanying
the SET.
containing a large excess of methanol (MeCN : MeOH = 1 :
1
(v/v)) was kept in a quartz cell of a spectrophotometer under an
argon atmosphere. The resulting absorption was recorded after an
appropriate time.
The rate constants kSET for the SET from sodium dithionite
10
Kinetics
Na
2
S
2
O
4
to 2–4 have been reported. When logkSET is plotted
0
against DG , also in Fig. 1, the plots fall on the single line predicted
by the Rehm–Weller theory (dotted line). These points can also
be traced by the Marcus equation (eqn (2)) to afford the best
fit as shown in the solid line in Fig. 1 when Z = 10
and k = 0.6 eV. This kinetic behavior is expected because the
SET from Na takes place in a completely reversible and
outer-sphere manner. Undoubtedly, the structural change in 2–
discussed above occurs also in the SET from Na . It is
A kinetics study for the reaction of 3 was carried out on a spec-
trophotometer according to the procedure described previously.
9
For the reactions of 4, the stock solutions of 1 and 4 were prepared
in acetonitrile containing a large excess of methanol (MeCN :
MeOH = 1 : 1 (v/v)) such that the concentrations of 1 and 4
1
1
−1
−1
M
s
S
2
O
4
−2
−4
2
became (0.2–7.5) × 10 and 2.0 × 10 M, respectively, after
mixing. The solutions were placed into separate reservoir cells
4
2
S
2
O
4
◦
of a stopped-flow spectrophotometer maintained at 45 C, and
therefore concluded that the unexpected kinetic behavior observed
in the SET from 1 to 2–4 does not result from the intrinsic nature of
these viologens, but from an event occurring within the encounter
complex between 1 and 2–4. The encounter complex is tight
enough, so that the structural change in the viologen results in
a significant change not only in the orientation of the redox pair
but also in the structure of the solvent cage. Importantly, k for the
SET from 1 to viologens 4 is still larger than k for the SET from
these solutions were mixed. After the mixing, the increase in the
absorbance at an appropriate wavelength was monitored. The
values of the rate constants obtained were reproducible in at least
three experiments carried out under the same conditions.
Acknowledgements
One of the authors (S. Y.) gratefully acknowledges the financial
support of a Tezukayama Research Grant in 2003.
Na
2
S
2
O , which means that even a slight change in 4 upon the SET
4
20
has some effects on the rate within the tight encounter complex.
References
Experimental
1
(a) S. Yasui, Rev. Heteroat. Chem., 1995, 12, 145; (b) S. Yasui, K. Shioji,
M. Tsujimoto and A. Ohno, J. Chem. Soc., Perkin Trans. 2, 1999, 855;
Instruments
(
c) S. Yasui, M. Tsujimoto, K. Itoh and A. Ohno, J. Org. Chem., 2000,
5, 4715; (d) S. Yasui, K. Itoh, M. Tsujimoto and A. Ohno, Bull. Chem.
6
UV–Vis spectra were recorded on a Hitachi U-3210 spectropho-
tometer. The kinetics of the rapid reaction was followed with a
Union Giken RA-401 stopped-flow spectrophotometer. Rotating
Soc. Jpn., 2002, 75, 1311; (e) S. Yasui, S. Tojo and T. Majima, J. Org.
Chem., 2005, 70, 1276.
2 R. L. Powell and C. D. Hall, J. Am. Chem. Soc., 1969, 91, 5403.
2
930 | Org. Biomol. Chem., 2006, 4, 2928–2931
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