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

Abstract

The protonation as well as the metal-binding constants with Ni2+, Cu2+, Zn2+, Cd2+ and Pb2+ for the macrocyclic hexadentate ligands L1{3,6,14,17,23,24-hexaazatricyclo[17.3.1.1.8,12]tetracosa-1(23),8,10,12(24),19,21-hexaene} and L2{3,7,15,19,25,26-hexaazatricyclo[19.3.1.19,13]hexacosa-1(25),9,11,13(26),21,23-hexaene} have been determined at 25 °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 L1 showed a medium dependency, being much higher in KNO3(aq) than in KCl (aq). The crystal structure of [H4L1][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 L1 and L2. Electronic spectral data indicate octahedral six-co-ordination for the complexes of Cu2+ and Ni2+ with L1 and L2. Copper(II) also forms a monoprotonated mononuclear complex and a dinuclear complex with L2. The metal-ion affinities of L1 are compared to those of the fully saturated 1,4,7,10,13,16-hexaazacyclooctadecane L3 using thermodynamic and electronic spectral data. The larger stability of Cu2+ with L1 relative to L3 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 L2 is lower than that of L1 due to the less exothermic heats of complexation.