Triarylimidazole Redox Catalysts: Electrochemical Analysis and Empirical Correlations

A series of triarylimidazoles was synthesized and characterized
electrochemically. The synthetic route is general, providing a pathway
to 30 redox mediators that exhibit a > 700 mV
range
of accessible potentials. Most of the triarylimidazoles display three
oxidation peaks where the first redox couple is quasi-reversible.
The electronic character of the substituents affects the oxidation
potential. This is exemplified by a linear correlation between the
first oxidation potential and the sum of the Hammett σ+ substituent constants, as well as with a series of calculated ionization
potentials. We close by putting forward a rule of thumb stating that
for a given mediator, the upper limit of accessible potentials can
be extended by at least 500 mV beyond the largest recorded value.
A rationale, the conditions under which the rule is likely to apply,
and an example are provided.