Isolation and crystal structures of two singlet bis(triarylamine) dications with nonquinoidal geometries

We report the first structural data for bis(diarylamine) "bipolarons": we have isolated and crystallographically characterized salts of the dications obtained by two-electron oxidation of E-4,4′-bis-[di(p-anisyl)amino]stilbene and E,E-2,5-bis{4-[di(p-anisyl) amino]styryl}-3,4-di(n-butoxy)thiophene, [1]²⁺ and [2]²⁺ respectively. ESR, NMR, and magnetometry suggest both species have singlet ground states. X-ray structures, together with ¹H NMR coupling constants for [2]²⁺, indicate geometries in which the bond lengths are shifted toward a quinoidal pattern relative to that in the neutral species, but not to a fully quinoidal extent. In particular, the bond-length alternations across the vinylene bridging groups approach zero. DFT calculations with closed-shell singlet configurations reproduce the observed structures well. Our results indicate that singlet species for which one might expect quinoidal geometries (with differences of ca. 0.1 Å between formally single and double bonds) on the basis of a limiting valence-bond representation of the structure can, in fact, show structures with significantly different patterns of bond lengths.