Comparison ofVValues for Some Nitrogen- and Metal-Centered π-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 E op of Hush theory, which is twice the electronic interaction matrix element V . The optically estimated value of V ( V op = E op /2) drops significantly (6.0 kcal/mol) when the methyl groups of tetramethyl- p -phenylenediamine radical cation ( 1 +, V op = 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 V op values observed for n bonds connecting the nitrogens follow the relationship V = V 0 exp(−β n ( n −1)/2) rather well for dinitrogen ( n = 3), p -phenylene ( n = 5), and biphenylene ( n = 9) bridges with β n ∼ 0.3. AM1-NCG calculations are fairly successful at predicting changes in E op for these compounds and 1,5-dimethyl-1,5-dihydrophenazine radical cation ( 8 + ) but fail totally for methylviologen radical cation ( 10 + ). AM1 calculations predict [Me 2 N(CH CH) y NMe 2 ] + to localize at y = 6 and [Me 2 N(C⋮C) y NMe 2 ] + at y = 5, and in both cases calculated V drops below calculated λ/2, as expected. V values for nitrogen-centered and transition metal-centered intervalence compounds are compared. Significantly larger V values for the nitrogen-centered examples cause charge delocalization to occur for larger π systems than for transition metal-centered examples.