[n]Cyclo-para-biphenylmethine Polyradicaloids: [n]Annulene Analogs and Unusual Valence Tautomerization

Annulenes and their charged analogs play an important role in validating Hückel's aromaticity rule. Whereas annulenes with [4n + 2] π electrons tend to delocalize π electron density, annulenes with [4n] π electrons prefer to form local double bonds (bond alternation) resulting in two interconvertible valence isomers. However, it is very challenging to experimentally detect such a valence tautomerization process as a result of the very fast conversion. In this work, we synthesized a series of [n]cyclo-para-biphenylmethine macrocycles ([n]CPBM-An, n = 3–8) as analogs of [n]annulenes and observed the unusual valence tautomerization process by variable-temperature NMR measurements for [4]CPBM-An, which showed a moderate interconversion barrier (11.22 kcal/mol at coalescence temperature) via a C4 tetraradicaloid transition state. These macrocycles showed [n]annulene-like geometries but with unique open-shell polyradical character. Their optical, electrochemical, and magnetic properties were systematically investigated and compared with those of the recently studied [n]cyclo-para-phenylenes.