Protonation Studies of Reduced Ruthenium(II) Complexes with Polypyridyl Ligands

The pKa values associated with protonation of the one-electron reduced forms of series of [L‘2RuIIL]2+ complexes
{L‘ = bidentate polypyridyl ligand; L = bidentate polypyridyl ligand with additional uncoordinated N atoms in
the aromatic ring system:  e.g., dpp = 2,3-bis(2-pyridyl)pyrazine, bpz = 2,2‘-bipyrazine} were assessed using
pulse radiolysis techniques by the measurement of spectral variations as a function of pH. A linear correlation
was observed between pKa and E°(RuL‘2L2+/+) for complexes in which the protonatable ligand was at the same
time the site of reduction. In complexes where one or more of the nonprotonatable ligands (L‘) had very low π*
energy levels {e.g. (CF3)4bpy}, reduction occurs on a nonprotonatable ligand and a dramatic decrease in the pKa
values was observed for the reduced species. In complexes where the energies of the protonatable and
nonprotonatable ligands were comparable, the protonation behavior was consistent with some orbital mixing/delocalization of the electronic charge.