Pulse radiolysis studies of the protonation of reduced\ud Ruthenium(II) complexes containing 2,2´-Bipyrimidine

The pKa values associated with the protonation of the one-electron reduced forms of a series of bis(heteroleptic)\ud [(pp)2RuII(bpm)]2+ and tris(heteroleptic) [(pp)RuII(bpz)(bpm)]2+ complexes were measured using pulse radiolysis\ud techniques (pp = bidentate polypyridyl ligand; bpm = 2,2´-bipyrimidine; bpz = 2,2´-bipyrazine). A linear correlation was observed between pKa and E°red(1) (the redox potential for the first ligand-based reduction) for series of complexes for which the same protonatable ligand was the site of the reduction. For the tris(heteroleptic) complexes studied in which two protonatable ligands were present (bpm and bpz), the plots indicated the protonation was associated with the bpz (rather than the bpm) ligand, consistent with the lower π* energy of bpz which would lead\ud to its preferential reduction