10536 J. Am. Chem. Soc., Vol. 123, No. 43, 2001
Lebeau et al.
Scheme 2a
°
a In V vs NHE, pH 7, 25 C, I ) 0.1 M.
system. Sodium phosphate monobasic monohydrate, NaH2PO4‚H2O,
sodium phosphate dibasic heptahydrate, Na2HPO4‚7H2O, sodium
phosphate tribasic dodecahydrate, Na3PO4‚12H2O, and phosphoric acid,
H3PO4, were obtained from Fisher Scientific and used without further
purification in the preparation of buffer solutions. All other materials
were reagent grade and used without additional purification. The salts
cis-[RuII(bpy)2(py)(H2O)](ClO4)2, cis-[RuIII(bpy)2(py)(OH)](ClO4)2, cis-
[RuIV(bpy)2(py)(O)](ClO4)2, [OsII(bpy)3](PF6)2, [OsIII(bpy)3](PF6)3, and
[OsII(bpy)2(4-CO2H-4′-CH3bpy)](PF6)2 {4-CO2H-4′-CH3-bpy ) 4′-
methyl-2,2′-bipyridine-4-carboxylic acid} were all prepared according
to previously reported methods.16-19
Instrumentation. Routine UV-visible spectra were recorded on a
Hewlett-Packard 8452A diode array spectrophotometer in standard
quartz cells. Kinetic measurements were carried out on a Hi-Tech
Scientific SF-61MX stopped-flow multimixing spectrophotometer. The
temperature of the reactant solutions was controlled to within (0.02
°C by using a Neslab RTE-110 water bath circulator. The pH of
solutions used for kinetics measurements was determined by using a
Radiometer model 62 pH meter and a Ross model 81-02 combination
electrode after calibration with standard buffer solutions.
Figure 1. pH dependences for the couples cis-[RuIV(bpy)2(py)(O)]2+
/
cis-[RuIII(bpy)2(py)(OH)]2+ (RuIVdO2+/RuIIIsOH2+) and cis-[RuIII-
(bpy)2(py)(OH)]2+/cis-[RuII(bpy)2(py)(OH2)]2+ (RuIIIsOH2+/RuIIs
OH22+) (vs NHE, 25 °C, µ ) 0.1 M). The potentials for the
pH-independent [Ru(NH3)5(py)]3+/2+ and [Os(bpy)3]2+/3+ couples are
also shown.
Kinetic Measurements. Rate data in water were collected by
following visible spectral changes at a series of pH values. Wavelengths
were chosen where large spectral changes were observed or where
component absorbances could be isolated. These included isosbestic
pointss345 and 445 nm (RuIVdO2+ and RuIIIsOH2+), 400 nm (RuIIs
OH22+ and RuIIIsOH2+), and 450 nm (RuIIsOH22+ and OsII)sand 472
cis-[RuIV(bpy)2(py)(O)]2+ is 0.74 V, but initial oxidation to cis-
12,13
[RuIV(bpy)2(py)(OH)]3+ requires >1.6 V.
In this article, we explore the kinetic nuances and mechanistic
implications of outer-sphere electron-transfer reactions in which
there are changes in proton content. The study was based on
the reactions between the couples in Scheme 2 and the couples
2+
and 630 nm where RuIIsOH2 or OsII, respectively, dominates
absorbance changes. In the complete study, greater than 500 separate
kinetic traces were collected on approximately 60 separate solutions
at a series of pH values from pH 0.6 to 8.3.
[Os(bpy)3]3+/2+ and [Ru(NH3)5(py)]3+/2+ 14,15 The OsIII/II couple
.
is especially appropriate because its potential is near those for
the Ru couples, but independent of pH. As illustrated by the
pH dependences of the couples in Figure 1, its reactions can be
investigated in either direction by varying the pH.
In the stopped-flow experiments, data were acquired at single
wavelengths because measurements in the diode array configuration
were complicated by photochemical processes. They included pho-
toreduction of RuIVdO2+ and RuIIIsOH2+ and pyridine photolabilization
from RuIIsOH2
.
2+
The following abbreviations will be used throughout:
Triple mixing was employed at basic pH values (pH > 6) because
of the instability of [OsIII(bpy)3]3+ for extended periods under these
conditions. The instability arises from self-reduction by ligand oxidation
as the pH is increased, as has been reported for many pyridyl and
RuIVdO2+ ) cis-[RuIV(bpy)2(py)(O)]2+
RuIIIsOH2+ ) cis-[RuIII(bpy)2(py)(OH)]2+
RuIIIsOH23+ ) cis-[RuIII(bpy)2(py)(H2O)]3+
RuIIsOH+ ) cis-[RuII(bpy)2(py)(OH)]+
RuIIsOH22+ ) cis-[RuII(bpy)2(py)(H2O)]2+
Experimental Section
polypyridyl complexes, including [RuIII(bpy)3]3+ and [FeIII(bpy)3]3+ 20-22
.
Creutz and Sutin20 describe the kinetics of decomposition of [RuIII-
(bpy)3]3+ in basic media as dominated by rate-determining nucleophilic
attack by hydroxide on the bound bypyridine ligand. The pH jump
experiments minimize complications from decomposition of [OsIII-
(bpy)3]3+
.
(16) Moyer, B. A.; Meyer, T. J. Inorg. Chem. 1981, 20, 436-444.
(17) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17,
3334-3341.
Materials. High-purity deionized water was obtained by passing
distilled water through a Nanopure (Barnstead) water purification
(18) Kober, E. M.; Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg.
Chem. 1988, 27, 4587-4598.
(12) Binstead, R. A.; McGuire, M. E.; Dovletoglou, A.; Seok, W. K.,
Roecker, L. E.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114, 173-186.
(13) Trammell, S. A.; Wimbish, J. C.; Odobel, F.; Gallagher, L. A.;
Narula, P. M.; Meyer, T. J. J. Am. Chem. Soc. 1998, 120, 13248-13249.
(14) Braddock, J. L.; Cramer, J. L.; Meyer, T. J. J. Am. Chem. Soc. 1975,
97, 1972.
(19) Dupray, L. M.; Meyer, T. J. Inorg. Chem., 1996, 35, 6299-6307.
(20) Creutz, C.; Sutin, N. Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 2858-
2862.
(21) Roecker, L. R.; Kutner, W.; Gilbert, J. A.; Simmons, M.; Murray,
R. W.; Meyer, T. J. Inorg. Chem. 1985, 24, 3284.
(22) Nord, G.; Pedersen, B.; Bjergbakke, E. J. Am. Chem. Soc. 1983,
105, 1913-1919.
(15) Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1977, 96, 5615.