734
R. F. Lane and A. T. Hubbard
Electrochemistry of Chemisorbed Molecules. 5. Role of Nonaqueous Solvents in Ligand-Bridged Electrochemical lnterconversion of Platinum Complexes' Ross F. Lane"' and Arthur 1. Hubbard+ Deparfment of Chemistry, University of Hawaii, Honolulu, Hawaii 96822 (Received August 9, 1976)
Downloaded by CENTRAL MICHIGAN UNIV on September 10, 2015 | http://pubs.acs.org Publication Date: April 1, 1977 | doi: 10.1021/j100523a012
Publication costs assisted by the National Science Foundation and the Petroleum Research Fund
Halide-bridged electrode reactions of complexes of Pt(1V) and Pt(I1) proceed with difficulty or not at all in typical nonaqueous solvents. Thin layer voltammetric data are presented which demonstrate that dimethyl sulfoxide, acetonitrile,pyridine,sulfolane, and p-dioxane chemisorb irreversibly on Pt electrodes and markedly influence the rates of these reactions when examined in aqueous solutions. The dependence of the kinetics of the electron transfer process on surfactantcoverage is shown to be quantitatively attributable to steric hindrance of the approach of the reactant to the electrode surface, and to electrostatic alteration of the (42) potential and ionic concentrations at the reaction plane. Predominance of steric hindrance over electrostatic effects at high fractional converages of the organic surfactants accounts for the observed lack of reactivity in the nonaqueous electrolytes. Pretreatment of Pt surfaces with aqueous iodide provides electrodes which prevent adsorption of the organic solvents and allows the reactions to occur, suggesting their use for electrochemical investigations in strongly adsorbing aprotic media for which the influence of the compact region of the electrochemical double layer is pronounced.
Introduction In a preliminary study' an attempt was made to exploit the relative inertness toward electrochemical reduction at platinum electrodes of aprotic solvents such as dimethyl sulfoxide, acetonitrile, propylene carbonate, dioxane, and dimethylf~rmamide~ for studies of the electrode reactions of platinum c ~ m p l e x e s . ~For - ~ reasons ~ that were not at first apparent the usual halide-assisted electrochemical interconversion of complexes of Pt(I1) and Pt(1V) in aqueous electrolytes, eq 1,proceeded with difficulty or not
+ x- t
-e-,
-
PtIIL,
-e-,
Y
fast
+e-, slow
slow
I
V
I1
VI
I11
IV
VI1
VI11
[PtII'L,XY]
+ e - , fast
trans-PtIVL,XY
(X- = Cl-, Br-, I-; Y = X-, H,O; L = X-, Y, NR,, CN-, NO;, SCN-)
a t all in typical nonaqueous electrolytes. This difference in behavior is the subject of the present study.
Results and Discussion Aqueous Electrolytes a t Electrodes Treated with Nonaqueous Solvents. Experiments were carried out to determine the extent to which the electroinactivity of Pt(I1) and Pt(1V) complexes in nonaqueous media was due to blocking of the compact layer by adsorbed solvent. Figure 1shows current-potential curves for Pt electrodes in 1 M HC104 after rinsing with dilute aqueous solutions of typical nonaqueous solvent materials: acetonitrile, I; dimethyl sulfoxide (Me2SO),11; sulfolane, 111; p-dioxane, IV; pyridine, V; dimethylformamide, VI; propylene carbonate, VII; and acetic acid, VIII. The current-potential curve for the acetonitrile-treated Pt surface, Figure l A , shows the typical features observed with surfactant-coated surface^:^ suppression of surface oxidation in the potential range from 0.4to 0.8 V vs. NaCE, followed by a broad peak stemming from oxidation of the chemisorbed material, and
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Present address, Department of Chemistry, University of California, Santa Barbara, Calif. 93106. The Journal of Physical Chemistry, Vol. 8 1 , No. 8 , 1977
suppression of the small peaks occurring in the solvent reduction region from 0.0 to -0.3 V. Equally pronounced adsorption was observed for MezSO, sulfolane, and pyridine, but not for the others. Dimethylformamide decomposes in aqueous solutions to yield dimethylamine and carbon monoxide, eq 2, with the result that electrodes 0
/I
HCN(CH,),
+
NH(CH,),
+ CO
(2)
treated with its aqueous solutions acquire a layer of chemisorbed CO; accordingly, a peak is observed at 0.4V vs. NaCE93l3for oxidation of adsorbed CO. Chemisorption was not observed for dimethylamine. Fractional coverage of the surface by MezSO, acetonitrile, and pyridine has been determined by means of a procedure described in ref 4, 5, 6, and 9, in which the surfactant-treated surface is exposed to dilute aqueous KI and the density of unoccupied sites calculated from the I-coverage data subsequently obtained by oxidation of adsorbed iodine to dissolved IO
