Solution and Solid-State Studies of Doubly Trimethylene-Bridged Tetraalkyl <i>p</i>-Phenylenediamine Diradical Dication Conformations

X-ray crystallographic structures are reported for 1(Me)2+(SbCl6−)2·2CH3CN, 2(Et)2+(SbF6−)2·2CH3CN·2CH2Cl2, and 1(iPr)2+(SbF6−)2, which also contained unresolved solvent and is in a completely different conformation than the methyl- and ethyl-substituted compounds. A quite different structure of 1(Me)2+(SbF6−)2 than that previously published was obtained upon crystallizing it from a mixture rich in monocation. It does not contain close intramolecular PD+,PD+ contacts but has close intermolecular ones. Low temperature NMR spectra of 1(Me)2+ and 1(Et)2+ in 2:1 CD3OD/CD3CN showed that both contain three conformations of all-gauche NCCC unit material with close intramolecular PD+,PD+ contacts. In addition to the both PD+ ring syn and anti material that had been seen in the crystal structure of 1(Me)2+(SbF6−)2·2CH3CN published previously, an unsymmetrical conformation having one PD+ ring syn and the other anti (abbreviated uns) was seen, and the relative amounts of these conformations were significantly different for 1(Me)2+ and 1(Et)2+. Calculations that correctly obtain the relative amounts of both the methyl- and ethyl-substituted material as well as changes in the optical spectra between 1(Me)2+ and 1(Et)2+, which contains much less of the uns conformation, are reported.