Are luminescent Ru2+ chelated complexes selective coordinative sensors for the detection of heavy cations?

Abstract

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 Pb²⁺. 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 Hg²⁺, 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 Ru²⁺ 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 Pd²⁺ in aqueous media. Indeed, it does not possess the optical and photophysical characteristics necessary to discriminate Pd²⁺ ions over a variety of other cations.