Coordination Chemistry of Mixed-Donor Pyridine-Containing Macrocyclic Ligands: From Optical to Redox Chemosensors for Heavy Metal Ions

2,8-Dithia-5-aza-2,6-pyridinophane (<b>L1</b>) has been used as a receptor unit in the construction of the conjugated redox chemosensor 5-ferrocenylmethyl-2,8-dithia-5-aza-2,6-pyridinophane (<b>L3</b>). In order to further explore the coordination chemistry of <b>L1</b>, and comparatively, that of its structural analogue 2,11-dithia-5,8-diaza-2,6-pyridinophane (<b>L2</b>), featuring two secondary nitrogen atoms in the macrocyclic unit, the crystal structures of the new synthesised complexes [Pb(<b>L1</b>)(ClO₄)₂]·½CH₃CN, [Cu(<b>L2</b>)](ClO₄)₂·CH₃CN and [Cd(<b>L2</b>)(NO₃)]NO₃ were determined by X-ray diffraction analysis. The electrochemical response of <b>L3</b> towards the metal ions Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Pb²⁺ was investigated by cyclic voltammetry (CV) in CH₂Cl₂/CH₃CN 0.25:1 (<i>v</i>/<i>v</i>) mixture. Upon addition to <b>L3</b> of increasing amounts of the aforementioned metal cations, the wave corresponding to the Fc⁺/Fc redox couple of the un-complexed <b>L3</b> was gradually replaced by a new reversible wave at more positive potentials and corresponding to the Fc⁺/Fc redox couple of the complexed ligand. The maximum anodic shift of the ferrocene oxidation wave is observed in the presence of Pb²⁺ (230 mV), to which corresponds a reaction coupling efficiency (RCE) value as large as 7.9 × 10³. The response selectivity of <b>L3</b> is discussed in reference to the optical selectivity observed for conjugated chemosensors featuring <b>L1</b> as receptor unit and different fluorogenic fragments as signalling units.