COMPARISON OF V VALUES FOR SOME NITROGEN- AND METAL-CENTERED PI -BRIDGED MIXED-VALENCE COMPOUNDS

Delocalized radical cations having two nitrogen-centered charge bearing units bridged by π systems may be considered Class III intervalence compounds. The transition energy of the longest wavelength band may therefore be equated with the Eop of Hush theory, which is twice the electronic interaction matrix element V. The optically estimated value of V (Vop = Eop/2) drops significantly (6.0 kcal/mol) when the methyl groups of tetramethyl-p-phenylenediamine radical cation (1+, Vop = 23.3 kcal/mol) are replaced by phenyl groups in the tetraphenyl compound (3+) and detectably (0.6 kcal/mol) when they are replaced by bicyclic alkyl groups in bis(9-azabicyclo[3.3.1]non-9-yl)-p-phenylenediamine (4+). The Vop values observed for n bonds connecting the nitrogens follow the relationship V = V0 exp(−βn(n−1)/2) rather well for dinitrogen (n = 3), p-phenylene (n = 5), and biphenylene (n = 9) bridges with βn ∼ 0.3. AM1-NCG calculations are fairly successful at predicting changes in Eop for these compounds and 1,5-dimeth...