A new redox-responsive 14-membered tetraazamacrocycle with ferrocenylmethyl arms as receptor for sensing transition-metal ions

A new redox-responsive receptor, 3,11-bis(ferrocenylmethyl)-7-methyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene, L¹, has been synthesized. The protonation constants of this compound and the stability constants of its complexes with Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ were determined at 25.0 °C, in methanol–water (1∶1, v/v), and at ionic strength 0.10 mol dm⁻³ in KNO₃. The values of the protonation constants of L¹ are similar to those of the parent macrocycle, 7-methyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene L², except for K₂ which is 1.82 log units lower than that of L². Structural reasons are proposed to explain this behaviour. The stability constants of the metal complexes of L¹ are lower than those of L² as expected on the basis of its lower overall basicity, but the Cd²⁺ complex is exceptional exhibiting a higher stability for L¹. The complexation of L¹ with different metals shifts anodically the ferrocene–ferrocenium half-wave potential in relation to that of the free compound, the largest shift being observed for Cu²⁺, followed by Ni²⁺, and then Zn²⁺ and Cd²⁺. No shift was observed for Pb²⁺. It was verified that L¹ is a copper-selective sensor in the presence of Ni²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ and although it is not the first compound for which this property is claimed it is probably the easiest to synthesize. The electronic spectra of [CuL¹]²⁺ reveal that the four nitrogen atoms of the macrocycle form a square-planar arrangement with a pronounced tetrahedral distortion. The single crystal structures of [CuL¹Cl][CuL¹I]Cl₂·1.25H₂O 1 and [ZnL¹I]Cl·2H₂O 2 have shown that these complex cations have distorted square pyramidal co-ordination spheres, the basal being planes formed by the four nitrogen atoms of the macrocyclic framework in 1a⁺ and 1b⁺ while in 2⁺ it is defined by three nitrogen atoms of L¹ and one iodine atom. The apical positions are occupied by a chlorine atom in 1a⁺ and by an iodine atom in 1b⁺, and by the nitrogen donor atom of the macrocycle trans to the pyridine ring in complex 2⁺. To achieve the geometric arrangement described for 2⁺ the macrocycle folds considerably through the line defined by the two nitrogen atoms contiguous to the pyridine group. It was also found that the intramolecular distances between the ferrocene groups and transition metal receptor centres studied play an important role in the redox behaviour of these complexes.