Redox chemosensors: coordination chemistry towards Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II) of 1-aza-4,10-dithia-7-oxacyclododecane ([12]aneNS2O) and its ferrocenylmethyl derivative

The coordination chemistry of the mixed donor 12-membered macrocyclic ligand 1-aza-4,10-dithia-7-oxacyclododecane ([12]aneNS2O) with CuII, ZnII, CdII, HgII, and PbIIhas been investigated both in water solution and in the solid state. The protonation constant for [12]aneNS2O 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 HgIIin water has the highest affinity for [12]aneNS2O followed by CuII, CdII, PbII, and ZnII. For each considered metal ion, 1 : 1 complexes of [12]aneNS2O have been isolated in the solid state; those of CuII, HgII, and CdIIhave 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]aneNS2O and its structural analogue [12]aneNS3have then been used as receptor units in the design and synthesis of the new ferrocene-containing redox-active ionophores N-ferrocenylmethyl I-aza-4,10-dithia-7-oxacyclododecane (L1) and N-ferrocenylmethyl 1-aza-4,10,7-trithiacyclododecane (L2). Electrochemical studies carried out in MeCN in the presence of increasing amounts of CuII, ZnII, CdII, HgII, and PbIIshowed that the wave corresponding to the Fc/Fc+couple of the uncomplexed ionophores L1and L2is 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 L1in the presence of ZnII(230 mV) and PbII(220 mV), whereas for L2a selective sensing response for CuIIover the other guest metal cations was observed with an oxidation peak shift of 230 mV.