Protonation equilibria of [Ru(LL)(CN)4]2- 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 C-13 NMR spectroscopy in the acidic region. Two protonation steps with pK(a) 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 C-13 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 x 10(10) s(-1). Four separate protonation steps with pK(a) 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 C-13 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(dcbH(2))(CN)(4)](2-). (c) 2006 Elsevier B.V. All rights reserved.