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, L-1, has been synt hesized. The protonation constants of this compound and the stability const ants of its complexes with Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ were determined at 25.0 degrees C, in methanol-water (1:1, v/v), and at ionic strength 0.1 0 mol dm(-3) in KNO3. The values of the protonation constants of L-1 are si milar to those of the parent macrocycle, 7-methyl-3,7,11,17-tetraazabicyclo [11.3.1]heptadeca-1(17),13,15-triene L-2, except for K-2 which is 1.82 log units lower than that of L-2. Structural reasons are proposed to explain th is behaviour. The stability constants of the metal complexes of L-1 are low er than those of L-2 as expected on the basis of its lower overall basicity , but the Cd2+ complex is exceptional exhibiting a higher stability for L-1 . The complexation of L-1 with different metals shifts anodically the ferro cene-ferrocenium half-wave potential in relation to that of the free compou nd, the largest shift being observed for Cu2+, followed by Ni2+, and then Z n2+ and Cd2+. No shift was observed for Pb2+. It was verified that L-1 is a copper-selective sensor in the presence of Ni2+, Zn2+, Cd2+ and Pb2+ and a lthough 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 s tructures of [(CuLCl)-Cl-1][(CuLI)-I-1]Cl-2. 1.25H(2)O 1 and [(ZnLI)-I-1]Cl . 2H(2)O 2 have shown that these complex cations have distorted square pyr amidal co-ordination spheres, the basal being planes formed by the four nit rogen atoms of the macrocyclic framework in 1a(+) and 1b(+) while in 2(+) i t is defined by three nitrogen atoms of L-1 and one iodine atom. The apical positions are occupied by a chlorine atom in 1a(+) and by an iodine atom i n 1b(+), and by the nitrogen donor atom of the macrocycle trans to the pyri dine ring in complex 2(+). To achieve the geometric arrangement described f or 2(+) the macrocycle folds considerably through the line defined by the t wo nitrogen atoms contiguous to the pyridine group. It was also found that the intramolecular distances between the ferrocene groups and transition me tal receptor centres studied play an important role in the redox behaviour of these complexes.