Protonation and metal-ion complexation in aqueous solution by pyridine-containing hexaaza macrocycles

The protonation as well as the metal-binding constants with Ni2+, Cu2+, Zn2+, Cd2+ and Pb2+ for the macrocyclic hexadentate ligands L(1) {3,6,14,17,23,24-hexaazatricyclo[17.3.1.1.(8,12)]tetracosa- 1(23),8,10,12(24),19,21-hexaene} and L(2) {3,7,15,19,25,26-hexaazatricyclo[19.3.1.1(9,13)]hexacosa- 1(25),9,11,13(26),21,23-hexaene} have been determined at 25 degrees C in 0.1 mol dm(-3) KNO3 or KCl. The heats of protonation and of complexation with Cu2+, Cd2+ and Pb2+ were determined calorimetrically. The value for the fourth protonation step of L(1) showed a medium dependency, being much higher in KNO3 (aq) than in KCl (aq). The crystal structure of [H(4)L(1)][NO3](4) was determined and consists of discrete centrosymmetric three-ring tetrapositive cations and two crystallographically independent nitrate ions. Hydrogen bonds between the four endo-nitrogens of the eighteen-atom ring and the nitrate ions are formed. Mononuclear 1:1 metal-ion complexes were identified with L(1) and L(2). Electronic spectral data indicate octahedral six-co-ordination for the complexes of Cu2+ and Ni2+ with L(1) and L(2). Copper(n) also forms a monoprotonated mononuclear complex and a dinuclear complex with L(2). The metal-ion affinities of L(1) are compared to those of the fully saturated 1,4,7,10,13,1 6-hexaazacyclooctadecane L(3) using thermodynamic and electronic spectral data. The larger stability of Cu2+ with L(1) relative to L(3) is entirely due to a more favourable entropy change, indicating that ligand pre-organization is the main reason for the increased stability. The metal-ion affinity of L(2) is lower than that of L(1) due to the less exothermic heats of complexation.