Abstract
Two new redox systems, sym. 1,2,4,1¢,2¢,4¢-hexamethylferrocene/cation sym. 1,2,4,1¢,2¢,4¢-hexamethyl ferricinium and sym. octamethylferrocene/cation sym. Octamethylferricinium (MenFc/MenFc+, n = 6, 8) were studied by the cyclic voltammetry method. The observed difference between potentials of anodic and cathodic peaks of 0.063 to 0.075 V, and its independence on the potential scan rate, the straight-line dependence of the current value of anodic (and cathodic) peak on square root of the potential scan rate, as well as shapes of the recorded cyclic voltammograms indicate that both redox systems in acetonitrile meet the most important requirement of IUPAC regarding internal reference redox systems (IRRS) - electrochemical reversibility of electron transfer reaction. The same method under identical conditions was used to study the effect of the number of methyl groups on the redox potential of MenFc/MenFc+ systems, n = 0, 6, 8, 10. It was shown that the successive displacement of half-wave potential in the series of Fc/Fc+ - Me6Fc/Me6Fc+ - Me8Fc/Me8Fc+ -Me10Fc/Me10Fc+ towards negative potentials is attributed to the electron-donor property of methyl groups. The location of the redox potentials values of new systems [n=6 (111 mV), n=8 (23 mV)] between redox potentials of systems of n = 0 (431 mV) and n = 10 (-77 mV) means that the redox potential of the systems of MenFc/MenFc+(n = 6, 8) has an optimal position on the electrode potential scale, i.e. meets another of the IUPAC criteria for IRRS.
Highlights
Periodic reports on its instability [4,5], have initiated numerous researches to find, from the point of view of the IUPAC criteria [1,6], more promising systems consisting of methyl homologues of ferrocene and ferricinium cation [7,8]
As in the sequence Me6Fc/Me6Fc+ → Me8Fc/Me8Fc+→ Me10Fc/Me10Fc+, in the series of Fc/Fc+ → Me2Fc/Me2Fc+ → ... → Me10Fc/Me10Fc+, the electron effect of methyl groups has an additive character: in case of decreasing ionization potential of d-electron of 1,1 -dimethylferocene in comparison with ionization potential of ferrocene it is equal to 0.2 eV, this value is 5 times higher for decamethylferrocene and it is 1 eV, as noted above. It was shown by cyclic voltammetry method that the electron transfer reaction of redox systems Me6Fc/Me6Fc+ and Me8Fc/Me8Fc+ in an organic solvent is a reversible one
The range of their redox potential is in the optimal region of electrode potential scale
Summary
This feature of MenFc/MenFc+ (n = 6, 8, 10) redox systems is due to electron-donor properties and spatial dimensions of methyl groups. The purpose of this work is to define the electrochemical reversibility of the electron transfer reaction proceeding in the redox systems Me6Fc/Me6Fc+ and Me8Fc/Me8Fc+ by the method of cyclic voltammetry.
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