Ion pairing effects on bis(hydrazine) intervalence radical cations

Three bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl) radical cation salts, bridged by 2,3,5,6-tetramethylbenzene-1,4-diyl (1^+PF_6^-), biphenylene-4,4‘-diyl (2^+PF_6^-), and 9,9-dimethyl-fluorene-2,7-diyl (3^+NO_3^-) groups, have been studied in methylene chloride. The transition energy at band maximum (E_(op)) increases as concentration increases and when ^nBu_4+BF_6^- is added, indicating that ion pairing increases E_(op). The E_(op) data fit a simple ion pairing equilibrium, giving ion pairing equilibrium constants at 293 K of 3100, 3100, and 6100 M^(-1), respectively. Electron-transfer rate constants measured by ESR are reported for 0.19 mM 2^+PF_6^- and for 1 mM 2^+PF_6^- and 3^+NO_3^- in the presence of 20 mM ^nBu_4+BF_6^- in methylene chloride. Prediction of k_(ET) from the optical spectrum of 2^+PF_6^- containing excess ^nBu_4^+BF_6^- was made both assuming the optical ET is endoenthalpic by an amount calculated from the increase in E_(op), and that ΔG° = 0 (that is, that the ion pairing effect may be lumped into the electron transfer coordinate along with the vertical and solvent reorganization effects). The predicted rate constant for the latter is only a factor of 2.5 times larger the former, so both agree rather well with the ESR-derived rate constant