Coordination chemistry of N-aminopropyl pendant arm derivatives of mixed N/S-, and N/S/O-donor macrocycles, and construction of selective fluorimetric chemosensors for heavy metal ions

Abstract

The coordination chemistry of the N-aminopropyl pendant arm derivatives (L^1c–4c) of the mixed donor macrocyclic ligands [12]aneNS₂O, [12]aneNS₃, [12]aneN₂SO, and [15]aneNS₂O₂ (L^1a–4a) towards Cu^II, Zn^II, Cd^II, Hg^II, and Pb^II in aqueous solution has been investigated. The protonation and stability constants with the aforementioned metal ions were determined potentiometrically and compared, where possible, with those of the unfunctionalised macrocycles. The measured values show that Hg^II and Cu^II in water have the highest affinity for all ligands considered, with the N-aminopropyl pendant arm weakly coordinating the metal centres. Crystals suitable for X-ray diffraction analysis were grown for the perchlorate salt (H₂L^1c)(ClO₄)₂·dmf, and for the 1 : 1 complexes [Cd(L^3a)(NO₃)₂] (1), [Cu(L^4a)(dmf)](ClO₄)₂ (2), [Zn(L^1c)(ClO₄)]ClO₄ (3), [Cd(L^1c)(NO₃)]NO₃ (4), and [Hg(L^2c)](ClO₄)₂ (5). Their structures show the macrocyclic ligands adopting a folded conformation, which for the 12-membered systems can be either [2424] or [3333] depending on the nature of the metal ion. L^1c–4c were also functionalised at the primary amino pendant group with different fluorogenic subunits. In particular the N-dansylamidopropyl (L^nd, n = 1–4), and the N-(9-anthracenylmethyl)aminopropyl (L^ne, n = 1, 2, 4, Scheme 1) pendant arm derivatives of L^1a–4a were synthesised and their optical responses to the above mentioned metal ions were investigated in MeCN/H₂O (4 : 1 v/v) solutions.