Syntheses and Properties of the Polymethylenebis(diphenylcyclopropenium) Dications

A series of polymethylenebis(diphenylcyclopropenium) dications with the polymethylene chain of one to six carbons (4n=1–6) were synthesized and characterized by spectral and analytical means. Among the dications 4n=3–6 which were isolated as perchlorates, 4n=3 was shown by the pKR+ measurements to be destabilized by 1.3 pK units as compared with 4n=4–6. The values of reduction potential exhibited the same tendency. The chromium(II) ion reduction of 4n=6, a representative dication, afforded a polymeric material containing a hexylbenzene unit. The dication 4n=2 was isolable only as a complex salt with SbCl6− and Cl−: the conventional hydride abstraction from 1,2-dicyclopropenylethane using trityl perchlorate resulted in rearrangements affording 4,9,10-triphenylphenanthrene and the unknown monocation C32H25+ containing the tropylium ring. A possible mechanism for these rearrangements is presented. On the other hand, the dication 4n=1 was observable only in a highly acidic solution due to the ready deprotonation yielding a new fully conjugated monocation 17. This cation, characterized by remarkable stability (pKR+ 8.9 in 50% aq acetonitrile), is formally represented as a triafulvene with a cyclopropenium ring substituted at the 4-position, but is shown to have a partially polarized electronic structure by 13C NMR spectroscopy.