Anal. Chem. 2000, 72, 3245-3252
Probing the Limits: Ultraslow Diffusion and
Heterogeneous Electron Transfers in Redox
Polyether Hybrid Cobalt Bipyridine Molten Salts
Joseph C. Crooker and Royce W. Murray*
Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
homogeneous1,2 electron-transfer rates and on developing the
required electrochemical methodology.5
This paper describes microelectrode voltammetry mea-
surements of self-diffusion coefficients and of heteroge-
neous Co(II/ III) electron-transfer rate constants (ko) in
undiluted molten salts of three cobalt tris(bipyridine)
perchlorate complexes in which the bipyridine ligands are
“tailed” with poly(propylene oxide) and poly(ethylene
oxide) oligomers. The self-diffusion coefficients are mea-
sured with potential step chronoamperometry and range
from 1 0 -1 2 to 1 0 -1 7 cm2 / s, while the quasi-reversible
reaction rate constants are measured using cyclic volta-
mmetry and small potential steps and range from 1 0 -7 to
1 0 -1 2 cm/ s. The ko measurements are unusual in that
when rate constants become smaller, the reaction remains
quasi-reversible, because of concurrently decreasing self-
diffusion rates. The measurements are, furthermore,
accomplished in the face of uncompensated resistances
that range from mega- to gigaohms, which is made
possible by the combination of microelectrode properties
and small diffusivities. The melt in which self-diffusion
and ko values are smallest is at a temperature below its
nominal glassing transition and in the regime of molecule-
scale diffusion profiles.
This paper describes measurements of diffusion coefficients
and of heterogeneous Co(II/ III) electron-transfer rate constants
(ko) in cobalt tris(bipyridine) perchlorate melts in which the
ligands are “tailed” with very short poly(propylene oxide) and poly-
(ethylene oxide) oligomerssfrom one to seven units in lengths
as shown below:
The abbreviations to be used for the three ligands and
associated metal complexes are shown. In these materials, the
intrinsic viscosity increases, and self-diffusion coefficients and ionic
conductivity decrease, with decreasing polyether chain length. We
have pointed2a out that this chain length dependence is unusual,
being opposite to that seen in conventional polymers.6 The
explanation is that the volume fraction of the polyether component
(and associated free volume content of the melt) decreases with
decreasing chain length.
This laboratory has designed and synthesized a number of
metal bipyridine salts in which either the ligand1 or the counterion2
is are combined (“tailed”) with short polyether chains. Labeled
“redox polyether hybrids”, these highly viscous, semisolid, molten
salts are, in electrochemical studies, used mainly in their undiluted
states. The central objectives of the electrochemical investigations,
among others, are ascertaining the effects of rigidified environ-
ments on diffusion dynamics1-3 and on heterogeneous4 and
What is exceptional about the ko measurements presented here
is that they are of rate constants approaching 10-12 cm/ s, yet these
reactions are quasi-reversible, and are obtained in the face of
uncompensated resistances that are very large, ranging from
(1) (a) Vela´zquez, C. S.; Hutchison, J. E.; Murray, R. W. J. Am. Chem. Soc. 1993,
115, 7896. (b) Williams, M. E.; Masui, H.; Long, J.; Malik, J.: Murray, R.
W. J. Am. Chem. Soc. 1997, 119, 1997-2005. (c) Emmenegger, F.; Williams,
M. E.; Murray, R. W. Inorg. Chem. 1 9 9 7 , 36, 3146-3151. (d) Williams, M.
E.; Lyons, L. J.; Long, J. W.; Murray, R. W. J. Phys. Chem. 1 9 9 7 , 101, 7584-
7591. (e) Masui, H.; Murray, R. W. Inorg. Chem. 1 9 9 7 , 36, 5118-5126. (f)
Williams, M. E.; Masui, H.; Murray, R. W. Submitted for publication.
(2) (a) Dickinson, V. E.; Masui, H.: Williams, M. E.; Murray, R. W. J. Phys.
Chem. 1 9 9 9 , 103, 11028-11035. (b) Dickinson, V. E.; Williams, M. E.;
Hendrickson, S. M.; Masui, H.; Murray, R. W. J. Am. Chem. Soc. 1 9 9 9 ,
121, 613-616.
(5) (a) Porat, A.; Crooker, J. C.; Zhang, Y.; LeMest, Y.; Murray, R. W. Anal.
Chem. 1 9 9 7 , 69, 5073. (b) Longmire, M. L.; Watanabe, M.; Zhang, H.;
Wooster, T. T.; Murray, R. W. Anal. Chem. 1 9 9 0 , 62, 747. (c) Wooster, T.
T.; Longmire, M. L.; Watanabe, M.; Murray, R. W. J. Phys. Chem. 1 9 9 1 ,
95, 5315. (d) Wooster, T. T.; Longmire, M. L.; Zhang, H.; Watanabe, M.;
Murray, R. W. Anal. Chem. 1 9 9 2 , 64, 1132.
(3) Watanabe, M.; Longmire, M. L.; Murray, R. W. J. Phys. Chem. 1 9 9 0 , 94,
2614.
(6) (a) Gary, F. M. Solid Polymer Electrolytes, Fundamentals and Technological
Applications; VCH Publishers: New York, 1991. (b) MacCallum, J. R.;
Vincent, C. A. Polymer Electrolyte Reviews; Elsevier Applied Science: Oxford,
U.K., 1989: Vols. 1 and 2.
(4) (a) Williams, M. E.; Crooker, J. C.; Pyati, R.; Lyons, J. L.; Murray, R. W. J.
Am. Chem. Soc. 1 9 9 7 , 119, 10249. (b) Pyati, R.; Murray, R. W. J. Am. Chem.
Soc. 1 9 9 6 , 118, 1743.
10.1021/ac000231o CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/16/2000
Analytical Chemistry, Vol. 72, No. 14, July 15, 2000 3245