The importance of precursor and successor complex formation in a bimolecular proton-electron transfer reaction

The transfer of a proton and an electron from the hydroxylamine 1-hydroxyl-2,2,6,6-tetramethylpiperidine (TEMPOH) to [Co(III)(Hbim)(H(2)bim)(2)](2+) (H(2)bim = 2,2'-biimidazoline) has an overall driving force of DeltaG degrees = -3.0 +/- 0.4 kcal mol(-1) and an activation barrier of DeltaG(degrees) = 21.9 +/- 0.2 kcal mol(-1). Kinetic studies implicate a hydrogen-bonded "precursor complex" at high [TEMPOH], prior to proton-electron (hydrogen-atom) transfer. In the reverse direction, [Co(II)(H(2)bim)(3)](2+) + TEMPO, a similar "successor complex" was not observed, but upper and lower limits on its formation have been estimated. The energetics of formation of these encounter complexes are the dominant contributors to the overall energetics in this system: DeltaG degrees ' for the proton-electron transfer step is only -0.3 +/- 0.9 kcal mol(-1). Thus, formation of the precursor and successor complexes can be a significant component of the thermochemistry for intermolecular proton-electron transfer, particularly in the low-driving-force regime, and should be included in quantitative analyses.