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

Summary Annulenes and their charged analogs play an important role in validating Huckel's aromaticity rule. Whereas annulenes with [4 n  + 2] π electrons tend to delocalize π electron density, annulenes with [4 n ] 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 C 4 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.