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.