Mendeleev Commun., 2009, 19, 258–259
The exhaustive electrolysis of 4-NPHA occurred with the
consumption of 0.43 electron per substrate molecule, which is
somewhat lower than 0.66, which corresponds to the stoichio-
metry of reactions (1) and (2). During the electrolysis, the solu-
tion turned bright red, which is characteristic of the 4-NPHA
anion.11 After the electrolysis completed, the CV curves of the
catholyte contained peak corresponding to the reduction of 4-NA
and oxidation of 4-NPHA anion, and no reduction and oxidation
peaks of the 4-NPHA.
The acidification of the catholyte with a phosphate buffer
(pH 3), as shown by HPLC analysis,†† was accompanied by
4-NPHA regeneration in an amount of 49% of its initial amount
due to the protonation of the 4-NPHA anion. According to the
HPLC data, the yield of 4-NA was 20%. Thus, the products of
4-NPHA electrolysis correspond to the products of reaction (2).
The fact that the yields of the 4-NPHA anion and 4-NA are
noticeably lower than the theoretical values, which for this
process should be 66 and 33%, respectively, indicates that side
reactions occur during the electrolysis. Unlike the CV method,
whose time window is measured in seconds, the time necessary
for the exhaustive electroreduction (tens of minutes) is suffi-
cient for the contribution of side reactions to become appreciable.
The most probable reactions decreasing the product yield are
those of the 4-NPHA anion with the solution components,
including 4-NPHA itself.
0.08
0.00
–0.08
–0.16
–0.24
–1800 –1400 –1000 –600
–200
E/mV vs. SCE
Figure 3 Cyclic voltammogram of 25 mM 4-NPHA in DMF containing
0.1 M Bu4NClO4 at the carbositall electrode (d = 3 mm) and a potential
sweep of 0.1 V s–1 (T = 298 K) (line). Empty circles show corresponding
simulated curves for self-protonation of 4-NPHA with formation of 4-NA
and 4-NPHA anion.
The simulation of the CV curves (Figure 3) using this scheme
results in the satisfactory agreement between the experimental
and calculated data.¶ We used the value of the formation poten-
tial of first electron transfer, which was estimated from the
electron affinity of 4-NPHA calculated by the density functional
theory (DFT) using the B3LYP exchange-correlation functional
and the correlation dependence obtained earlier.13
The electroreduction of 2-NPHA is analogous to the above-
described behavior of 4-NPHA. The cathodic branch of the CV
curve for 2-NPHA (Figure 4) exhibits two peaks at –1.10 (P1')
and –1.30 V (P2'). As in the case of 4-NPHA, the first of these
peaks corresponds to the chemically irreversible reduction of
2-NPHA, and the second peak corresponds to the reversible
reduction of 2-NA. The anodic branch of the curve contains a
peak at –0.33 V (P3') corresponding to the oxidation of the
2-NPHA anion. The reduction curves of 2-NPHA reduction in
the presence of Et4NOH are also shown in Figure 4.
The electrolysis of 2-NPHA at the potentials of the limiting
current of the first reduction step is accompanied by the appear-
ance of a bright violet color characteristic of the 2-NPHA
anion.11 The yields of the 2-NPHA anion and 2-NA were 49
and 30%, respectively, based on the starting 2-NPHA.
Thus, we can conclude that the electroreduction of both
2- and 4-NPHA at the potentials of the first step proceeds via
the mechanism including the step of RA protonation with the
starting compound and affords NA and the NPHA anion.
For the additional identification of the reduction products,
we carried out electrolyses of 2- and 4-NPHA at the potentials
of the limiting current of the first electroreduction waves for
these compounds.
References
1
A. I. Rusakov, A. S. Mendkovich, V. P. Gul’tyai and V. Yu. Orlov,
Struktura i reaktsionnaya sposobnost’ organicheskikh anion-radikalov
(Structure and Reactivity of Organic Radical Anions), Mir, Moscow,
2005 (in Russian).
P3'
2
0.11
2
3
4
C. Amatore, G. Capobianco, G. Farnia, G. Sandona, J.-M. Saveant,
M. G. Severin and E. Vianello, J. Am. Chem. Soc., 1985, 107, 1815.
A. S. Mendkovich, O. Hammerich, T. Ya. Rubinskaya and V. P. Gultyai,
Acta Chem. Scand., 1991, 45, 644.
J. A. Morales-Morales, C. Frontana, M. Aguilar-Martinez, J. A. Bautista-
Martinez, F. J. Gonzalez and I. Gonzalez, J. Phys. Chem. A, 2007, 111,
8993.
3
1
0.04
–0.03
–0.10
–0.17
5
6
7
8
9
A. Baeza, J. L. Ortiz and I. Gonzalez, J. Electroanal. Chem., 1997,
429, 121.
D. S. Silvester, A. J. Wain, L. Aldous, C. Hardacre and R. G. Compton,
J. Electroanal. Chem., 2006, 596, 131.
C. L. Forryan and R. G. Compton, Phys. Chem. Chem. Phys., 2003, 19,
4226.
P2'
P1'
–1700 –1300
–900
–500
–100
E/mV vs. SCE
Figure 4 Cyclic voltammograms in DMF containing 0.1 M Bu4NClO4 at
the carbositall electrode (d = 3 mm) and a potential sweep of 0.1 V s–1
(T = 298 K): 15 mM 2-NPHA in the absence (1) and in the presence of
15 mM Et4NOH (2), 8 mM of 2-NA (3).
C. L. Forryan, N. S. Lawrence, N. V. Rees and R. G. Compton, J. Electro-
anal. Chem., 2004, 561, 53.
H. Lund, in Organic Electrochemistry, 4th edn., eds. H. Lund and
O. Hammerich, Marcel Dekker, New York, 2001, ch. 9.
10 F. G. Bordwell and W.-Z. Liu, J. Am. Chem. Soc., 1996, 118, 8777.
11 R. Kuhn and F. Weygand, Ber. Deutsch. Chem. Ges. B, 1936, 69, 1969.
13 M. A. Syroeshkin, M. N. Mikhailov, A. S. Mendkovich and A. I. Rusakov,
Izv. Akad. Nauk, Ser. Khim., 2009, 41 (in Russian).
¶
The CV curves were simulated using the DigiElch Professional program
(ElchSoft), v. 3 (Build 3.600).12 Mechanism used for the simulation
involved the following successive stages: electron transfer – protona-
tion – electron transfer – protonation – water molecule elimination resulted
in 4-NA formation. Since investigation of 4-NA reduction was beyond
the scope of this work, it was supposed for simplicity that degradation of
4-NA anion radical is a pseudo-first-order reaction. The limiting stage
was the first proton transfer (kp = 3×103 dm3 mol–1 s–1), and rate con-
stant for 4-NA anion radical degradation was 0.25 s–1. Other simulation
Received: 3rd April 2009; Com. 09/3315
†† HPLC was carried out on a Diasfer-110-S16 column (5 μm, 2.0×80 mm)
using a MeCN/0.1 M phosphate buffer mixture with pH 3 in a ratio of
25:75 as a mobile phase and an UV detector at wavelengths of 235 (2-NA)
and 350 nm (4-NA, 2- and 4-NPHA).
parameters: Er0e'd 4-NPHA = –1.26 V, Eo0'x 4-NPHA = 0.44 V, Eo0'x 4-NPHA anion
= –0.40 V, Er0e'd 4-NA = –1.43 V, T = 298.15 K, D4-NPHA = 6×10–6 cm2 s–1,
D4-NA = 1×10–5 cm2 s–1, ks = 1 cm s–1, a = 0.5.
=
– 259 –