Are luminescent Ru 2+ chelated complexes selective coordinative sensors for the detection of heavy cations?

The ability of [Ru(bpy) ₂ (bpym)] ²⁺ (bpy = 2,2'-bipyridine; bpym = 2,2'-bipyrimidine) to probe specifically heavy cations has been investigated by means of density functional theory for transition metals, group 12 elements and Pb ²⁺ . On the basis of the calculated Gibbs free energies of complexation in water it is shown that all reactions are favorable with negative enthalpies except for Hg ²⁺ , with the transition metal cations forming stable bi-metallic complexes by coordination to the bpym ligand. Comparison between the optical and photophysical properties of the Ru ²⁺ probe and those of the coordination compounds does not demonstrate a high selectivity due to very similar characteristics of the absorption and emission spectra. Whereas by complexation the lowest metal-to-ligand-charge-transfer (MLCT) shoulder of [Ru(bpy) ₂ (bpym)] ²⁺ at 462 nm is more or less shifted to the red as a function of the cation, the second MLCT band at 415 nm, less sensitive to the complexation, gains in intensity and is slightly blue-shifted. The visible MLCT emission of [Ru(bpy) ₂ (bpym)] ²⁺ at 706 nm is altered by complexation leading to near IR (800-900 nm) emission in most of the coordination compounds. Complexation to some transition metal cations (Fe, Co, Rh and Pd) generates low-lying metal-centered (MC) excited states that quench luminescence. In contrast to the conclusion of experimental findings by Kumar et al. ( Chem. Commun. 2014, 50 , 8488-8490), [Ru(bpy) ₂ (bpym)] ²⁺ cannot be proposed as a fast and selective probe for monitoring Pd ²⁺ in aqueous media. Indeed, it does not possess the optical and photophysical characteristics necessary to discriminate Pd ²⁺ ions over a variety of other cations.