Redox chemosensors:: coordination chemistry towards CuII, ZnII, CdII, HgII, and PbII of 1-aza-4,10-dithia-7-oxacyclododecane ([12]aneNS2O) and its N-ferrocenylmethyl derivative

The coordination chemistry of the mixed donor 12-membered macrocyclic ligand 1-aza-4,10-dithia-7-oxacyclododecane ([12]aneNS(2)O) with Cu-II, Zn-II, Cd-II, Hg-II, and Pb-II has been investigated both in water solution and in the solid state. The protonation constant for [12] aneNS(2)O and stability constants with the aforementioned metal ions have been determined potentiometrically and compared with those reported for other mixed N/S/O-donor tetradentate 12-membered macrocycles. The measured values are consistent with trends observed previously for aza macrocycles as secondary N-donors are replaced by O- and S-donors. In particular our results show that Hg-II in water has the highest affinity for [12]aneNS(2)O followed by Cu-II, Cd-II, Pb-II, and Zn-II. For each considered metal ion, 1 : 1 complexes of [12]aneNS(2)O have been isolated in the solid state; those of Cu-II, Hg-II, and Cd-II have also been characterised by X-ray crystallography. In the cases of copper(II) and cadmium(II) complexes the ligand adopts a folded [2424] conformation, whereas a more planar [3333] conformation is observed in the case of the mercury(II) complex. The macrocycle [12]aneNS(2)O and its structural analogue [12]aneNS(3) have then been used as receptor units in the design and synthesis of the new ferrocene-containing redox-active ionophores N-ferrocenylmethyl 1-aza-4,10-dithia-7-oxacyclododecane (L-1) and N-ferrocenylmethyl 1-aza-4,10,7-trithiacyclododecane (L-2). Electrochemical studies carried out in MeCN in the presence of increasing amounts of Cu-II, Zn-II, Cd-II, Hg-II, and Pb-II showed that the wave corresponding to the Fc/Fc(+) couple of the uncomplexed ionophores L-1 and L-2 is gradually replaced by a new reversible wave at more positive potentials and corresponding to the Fc/Fc(+) couple of the complexed ionophores. The maximum shift of the ferrocene oxidation wave was found for L-1 in the presence of Zn-II (230 mV) and Pb-II (220 mV), whereas for L-2 a selective sensing response for Cu-II over the other guest metal cations was observed with an oxidation peak shift of 230 mV.