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[19.3.1.1^9,13]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.