J . Am. Chem. SOC.1985, 107, 2632-2635
2632
Kinetics of Reduction of Eight Viologens by Dithionite Ion K. Tsukahara and R. G. Wilkins* Contribution from the Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003. Received October 18, 1984
Abstract: The rate constants are reported for reduction by dithionite of methyl viologen, diquat, and six other diquaternary salts of 4,4'-bipyridine, 2,2'-bipyridine, and 1,lO-phenanthroline. The active reductant is the SO,-radical, and rate constants vary from >5 X lo* to 8.5 X lo' M-ls-I w'ith increasing negative reduction potential of the viologen. It is concluded that self-exchange rate constants for the viologens (Xz'/' couple) are lo8 M-I s-I, and this is supported by the results of a cross-reaction involving two viologens, the second-order rate constant being measured by pulse radiolytic techniques.
-
W e have been interested in the reduction by dithionite of inorganic1 and bioinorganic2 compounds. In many of these systems the effective reducing agent is the SO2- radical which exists in small concentration in dithionite solutions. Recently, we have compared the rate constants for reduction by SO; with the superoxide ion, o ~ . ~ The viologens are diquaternary salts of 4,4'-bipyridine, 2,2'bipyridine, and 1,lO-phenanthroline. They undergo interesting one-electron reduction, to give strongly colored radical cation^.^^^ However, no kinetic studies of the reduction of viologens by dithionite, one of the most used reducing agents, have apparently been reported. W e describe the kinetics of reduction of the eight viologens 1-3 which cover a range of structural types and reduction la, n
=
2 (EPQ~+)
Table I. Electrochemical, Spectral, and Kinetic Characteristics of
One-Electron Reduced Viologens
parent viologen l b (PPQ")
EO,
V
-0.13b,c
la (EPQ,')
-0.25b
2a (DQ2')
-0.34'-"
3b (BV2')
-0.36s3h
3a (MV2+) 2b (MDQ2')
-0.45cf-h -0.49'f
2c (PDQ2')
-0.55cf-"*q
spectral characteristics of radical ipsak,x 10-3, nm M-I cm-l 570 490 400 520 490 390 760 370 603
lb, n = 3 (PPQ2+)
w R
2d (BDQ2+) -0.64bf-h
R
+N\
/
+
>a, R
=
It; n =
z
(DQ~+)
Zb, K = C H 3 ; n = 2 ( M D Q ~ + ) 2 c , R = H ; n = 3 (PDQ2+)
Zd, R
=
H; n = 4 (BDQ2+)
386 600 760 370 493 375 520 500 460 390 383
3.3d@
k," M-I s-' > 5 x 108
1.2e
6.3d3e 3.5drc 1.3e 5.Idse 2.7,e 3.1' 32,' 3EdJ 14,' 15dJ,k 17'3m 43dJ 13,' 14,d,'14" 2.9e 32' 2.0,' 3.2dJ 15dJ 3.0dJ 2.5d,e 0.6e 7.Od
