Solution chemistry of macrocycles. Part 4. Thermodynamics of protonation and complexation of several N2S2 macrocycles

Abstract

The protonation of six N₂S₂ macrocycles L¹—L⁶ and the open-chain ligand 2,13-dithia-6,9-diazatetradecane (L⁷) as well as their complexation with Cu²⁺ have been studied by pH titrations, spectrophotometric titrations, and calorimetry at 25 °C and I= 0.5 mol dm^–3(KNO₃). The first protonation constant, pK_H,1, of these ligands is in the range of slightly acidified secondary aliphatic ammonium ions, whereas the second pK_H,2 is essentially determined by electrostatic factors. So the difference pK_H,1– pK_H,2 is larger for the cis than for the trans derivatives and decreases with ring size for both cis and trans compounds. The ligands give mononuclear species [ML]²⁺ with stabilities of 10^10.6–10¹⁶ dm³ mol^–1 for Cu²⁺ and 10^7.8–10^9.1 dm³ mol^–1 for Ni²⁺. In addition for the 12-membered ligands (L¹ and L²) and the open-chain ligand L⁷ partially hydrolysed species [CuL(OH)]⁺ are also observed. The Cu²⁺ complex of 1,4-dithia-8,11-diazacycIotetradecane is more stable than that of the open-chain ligand L⁷. This extra stability (macrocyclic effect) is due to both favourable enthalpic and entropic terms.