Enthalpic and entropic contributions in the reactions of hydrogen, copper(II) and nickel(II) ions with the macrocycle 1,5,9,13-tetra-azacyclohexadecane

Abstract

Enthalpies of protonation of 1,5,9,13-tetra-azacyclohexadecane (L¹) have been determined by a continuous-titration calorimetric technique and the corresponding entropy values obtained by combination with the free-energy data. The stepwise enthalpies are all very similar, and are interpreted in terms of the ability of the amino-groups to behave independently of each other towards protons. The stability constants of the complexes [CuL¹]²⁺ and [Cu(HL¹)]³⁺ have been determined by potentiometry at 25 °C in 0.5 mol dm^–3 K[NO₃]. The enthalpies of formation of the copper(II) and nickel(II) complexes of L¹ have been determined by batch microcalorimetry, and are discussed in terms of the unfavourable free-ligand conformation and the presence of hindered chelate rings in the complexes. Finally the stability of the [CuL¹]²⁺ complex is compared with that of the corresponding complex with the acyclic ligand 4,8-diazaundecane-1,11-diamine; the greater stability of the former complex is attributed to a very favourable entropy term and to a slightly favourable enthalpy term.