A new pyridine-based 12-membered macrocycle functionalised with different fluorescent subunits; coordination chemistry towards Cu-II, Zn-II, Cd-II, Hg-II, and Pb-II

The coordination chemistry of the new pyridine-based, N2S2-donating 12-membered macrocycle 2,8-dithia-5-aza-2,6-pyridinophane (L-1) 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 L-1 and stability constants with the aforementioned metal ions have been determined potentiometrically and compared with those of ligand L-2, 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 L-1 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 L-1 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 L-1 accommodate monodentate NO3- ligands. The macrocycle L-1 has then been functionalised with different fluorogenic subunits. In particular, the N-dansylamidopropyl (L-3), N-(9-anthracenyl) methyl (L-4), and N-(8-hydroxy-2-quinolinyl) methyl (L-5) pendant arm derivatives of L-1 have been synthesised and their optical response to the above mentioned metal ions investigated in MeCN/(HO)-O-2 (4:1 v/v) solutions.