Protonation equilibria of [formula omitted] complexes possessing electron donating sites on the diimine ligand

Abstract

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 p K 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 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 × 10 10 s −1. Four separate protonation steps with p K 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 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(dcbH 2)(CN) 4] 2−.