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 salt s, bridged by 2,3,5,6-tetramethylbenzene-1,4-diyl (1(+)PF(6)(-)), biphenyle ne-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 (Bu4+B F6-)-Bu-n is added, indicating that ion pairing increases E-op. The E-op da ta fit a simple ion pairing equilibrium, giving ion pairing equilibrium con stants at 293 K of 3100, 3100, and 6100 M-1, respectively. Electron-transfe r 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 (Bu4+BF6-)- Bu-n in methylene chloride. Prediction of k(ET) from the optical spectrum o f 2(+)PF(6)(-) containing excess (Bu4+BF6-)-Bu-n was made both assuming the optical ET is endoenthalpic by an amount calculated from the increase in E -op, and that Delta G degrees = 0 (that is, that the ion pairing effect may be lumped into the electron transfer coordinate along with the vertical an d solvent reorganization effects). The predicted rate constant for the latt er is only a factor of 2.5 times larger the former, so both agree rather we ll with the ESR-derived rate constant.