Fluorescent chemosensors for heavy metal ions based on bis(terpyridyl) ruthenium(II) complexes containing aza-oxa and polyaza macrocycles

Reactions of 4′-[4-(bromomethyl)phenyl]-2,2′:6′,2′′-terpyridine with 4,10-diaza-15-crown-5 and 1-aza-12-crown-4 in dichloromethane yielded the ligands L1 and L3, respectively. Reaction of an excess of 4′-[4-(bromomethyl)phenyl]-2,2′:6′,2′′-terpyridine with 4,10-diaza-15-crown-5 yielded L2, while treatment of the same terpyridine ligand with 1,4,7,10,13-pentaazacyclopentadecane afforded L4. Reactions of L1, L3, and L4 with Ru(mtpy)Cl3 (mtpy = 4′-methyl-2,2′:6′,2′′-terpyridine) in methanol yielded the metallo receptors [Ru(L1)(mtpy)][PF6]2, [Ru(L3)(mtpy)][PF6]2, and [Ru(L4)(mtpy)][PF6]2 after precipitation with ammonium hexafluorophosphate and column chromatography. On treating L3 with RuCl3, the homoleptic ruthenium complex [Ru(L3)2][PF6]2 was obtained. The synthesized metallo receptors contain oxa-aza crown or polyazacycloalkane moieties as recognition sites and [Ru(tpy)2]2+ cores as the signal-generating centre. The electronic spectra of the complexes are as expected for an Ru(tpy)22+ chromophore, with the main Ru[d(π)] tpy(π*) MLCT transition at ca. 484 nm and intense ligand-centred transitions in the UV region. One of the most interesting aspects of these ruthenium complexes is their multicomponent nature, as they contain both coordination sites and fluorescent Ru(tpy)22+ cores. The cations [Ru(L1)(mtpy)]2+, [Ru(L3)(mtpy)]2+, and [Ru(L3)2]2+ display an emission maximum at ca. 650 nm, the intensity of which is pH dependent, showing an enhancement upon protonation. The metallo receptor [Ru(L1)(mtpy)]2+ selectively senses Hg2+ in preference to Cu2+, Cd2+, and Pb2+. The emission intensity vs. pH curve for [Ru(L2)(mtpy)]2+ in the presence of Cu2+ and Hg2+ ions is close to that of the free receptor, but the presence of Cd2+ or Pb2+ enhances the emission intensity in the range pH 4−6. For the [Ru(L3)2]2+ complex, Cd2+, Pb2+, and Hg2+ induce an enhancement of the fluorescence of the Ru(tpy)22+ core in the range pH 3.5−7.5. These results are compared with those obtained for the metallo receptor [Ru(L4)(mtpy)]2+ containing a polyazacycloalkane moiety as the binding domain.