Protonation equilibria of image complexes possessing electron donating sites on the diimine ligand

The protonation equilibria of [Ru(LL)(CN)4]2− complexes, where LL = 4,4′-dicarboxy-2,2′-bipyridine (dcb) and 2,3-bis(2-pyridyl)-pyrazine (bppz), have been investigated by spectrophotometric titration and 13C NMR spectroscopy in the acidic region. Two protonation steps with pKa values of 2.0 and 0.5 and the absorption spectra of protonated species are distinguished for [Ru(bppz)(CN)4]2− in water. The 13C NMR spectra unambiguously prove the first protonation step occurring on the free pyridyl nitrogen of bppz. The second protonation step occurring on the cyanide ligand under pH = 2 results in a blue shift of the MLCT absorption maximum. The weakly emitting excited state of [Ru(bppz)(CN)4]2− is presumably more acidic than the ground state and a complete quenching of luminescence is observed with a diffusion controlled quenching rate of 4 ± 0.5 × 1010 s−1. Four separate protonation steps with pKa values of 3.2, 2.2, 1.1, −0.2 and the absorption spectra of the different protonated complex of [Ru(dcb)(CN)4]4− have also been determined. The protonation of carboxyl groups gives rise to an upfield shift of 13C NMR lines (6.1 and 4.8 ppm) of the C4 and COO− carbon atoms. The chemical shift of cyanide ligands are also affected by the protonation of carboxyl group and 1.5 ppm upfield shift was measured between pH = 4 and 2. The emission intensity and apparent lifetimes of [Ru(dcb)(CN)4]4− decrease to ca. 10% of the original values at pH = 2, while the position of emission maxima shifts from 666 to 715 nm. The change of emission intensity versus pH suggests that the monoprotonated form of [Ru(dcbH)(CN)4]3− is a weaker emitter than [Ru(dcbH2)(CN)4]2−.