A new pyridine-based 12-membered macrocycle functionalised with different fluorescent subunits; coordination chemistry towards CuII, ZnII, CdII, HgII, and PbII

The coordination chemistry of the new pyridine-based, N₂S₂-donating 12-membered macrocycle 2,8-dithia-5-aza-2,6-pyridinophane (L¹) towards Cu^II, Zn^II, Cd^II, Hg^II, and Pb^II has been investigated both in aqueous solution and in the solid state. The protonation constants for L¹ and stability constants with the aforementioned metal ions have been determined potentiometrically and compared with those of ligand L², which contains a N-aminopropyl side arm. The measured values show that Hg^II in water has the highest affinity for both ligands followed by Cu^II, Cd^II, Pb^II, and Zn^II. For each metal ion considered, 1:1 complexes with L¹ have also been isolated in the solid state, those of Cu^II and Zn^II having also been characterised by X-ray crystallography. In both complexes L¹ adopts a folded conformation and the coordination environments around the two metal centres are very similar: four positions of a distorted octahedral coordination sphere are occupied by the donor atoms of the macrocyclic ligand, and the two mutually cis-positions unoccupied by L¹ accommodate monodentate NO₃⁻ ligands. The macrocycle L¹ has then been functionalised with different fluorogenic subunits. In particular, the N-dansylamidopropyl (L³), N-(9-anthracenyl)methyl (L⁴), and N-(8-hydroxy-2-quinolinyl)methyl (L⁵) pendant arm derivatives of L¹ have been synthesised and their optical response to the above mentioned metal ions investigated in MeCN/H₂O (4:1 v/v) solutions.