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

Delocalized radical cations having two nitrogen-centered charge bearin g units bridged by pi systems may be considered Class III intervalence compounds. The transition energy of the longest wavelength band may t herefore be equated with the E-op of Hush theory, which is twice the e lectronic interaction matrix element V. The optically estimated value of V (V-op = E-op/2) drops significantly (6.0 kcal/mol) when the methy l groups of tetramethyl-p-phenylenediamine radical cation (1(+), V-op = 23.3 kcal/mol) are replaced by phenyl groups in the tetraphenyl comp ound (3(+)) and detectably (0.6 kcal/mol) when they are replaced by bi cyclic alkyl groups in is(9-azabicyclo[3.3.1]non-9-yl)-p-phenylenediam ine (4(+)). The V-op values observed for n bonds connecting the nitrog ens follow the relationship V = V-o exp(-beta(n)(n-1)/2) rather well f or dinitrogen (n = 3), p-phenylene (n = 5), and biphenylene (n = 9) br idges with beta(n) similar to 0.3. AM1-NCG calculations are fairly suc cessful at predicting changes in E-op for these compounds and 1,5-dime thyl-1,5-dihydrophenazine radical cation (8(+)) but fail totally for m ethylviologen radical cation (10(+)). AM1 calculations predict [Me2N(C H=CH)(y)NMe2](+) to localize at y = 6 and [Me2N(C drop C)(y)NMe2](+) a t y = 5, and in both cases calculated V drops below calculated lambda/ 2, as expected. V values for nitrogen-centered and transition metal-ce ntered intervalence compounds are compared. Significantly larger V val ues for the nitrogen-centered examples cause charge delocalization to occur for larger pi systems than for transition metal-centered example s.