A series of phenylene-bridged bis(pyridinium) compounds was prepared by reaction of the appropriate alkyl or aryl amine with the corresponding bis(pyrylium) species.  For most p-phenylene-bridged derivatives, cyclic voltammetry reveals a fully reversible reduction, and peak separations are suggestive of a two-electron process.  This has been confirmed by the covalent attachment of ferrocenyl moieties as internal electron counters.  For the m-phenylene-bridged derivatives, initial reduction is split into two closely spaced one-electron processes. Both p- and m-phenylene-bridged isomers also display a second, generally quasireversible multielectron reduction peak, the position of which is sensitive to the nature of the N substituent.Quantum calculations were employed to examine the molecular and electronic structures of the parent bis(pyridinium) dications, as well as the corresponding one-electron and two-electron reduced species (cation radicals and neutrals, respectively).  The lowest energy state of the neutral species for all meta derivatives was a triplet, while for para derivatives, it was a singlet.  Further, the neutral para systems show a distinct and pronounced bond length alternation pattern, indicative of a quinoidal structure.Finally, a detailed spectroelectrochemical investigation of the p-phenylene-bridged compounds indicates the presence of radical cation only at low conversion, suggesting that the inherent stability of the quinoidal form drives a rapid and efficient disproportionation.

Multielectron transfer processes in strongly coupled bis(pyridinium) systems

247th National Meeting (2014)

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