Metal complexes of a tetraazacyclophane: solution and molecular modelling studies

Abstract

An alternative synthetic approach to yield the compound 2,3,5,6,8,9,11,14-octahydrobenzo[l][1,4,7,10]tetraazacyclotetradecine (bz[14]N₄) is presented. The protonation constants of bz[14]N₄ and the stability constants of its complexes with Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺ were determined in H₂O at 25 °C with ionic strength 0.10 mol dm⁻³ in KNO₃ and they were compared with structurally related macrocycles cyclam (1,4,8,11-tetraazacyclotetradecane) and cyclen (1,4,7,10-tetraazacyclododecane). These studies indicate that only 1 ∶ 1 (M ∶ L) species are formed in solution, and the ligand exhibits a high affinity for larger ions such as Cd²⁺ and Pb²⁺. The X-ray study of [bz[14]N₄H₃]³⁺ shows that an inclusion compound with a chloride counter-anion is formed through NH⋯Cl hydrogen bonds. Spectroscopic data in solution (electronic and NMR spectra) showed that the macrocycle adopts a planar arrangement upon metal complexation. Molecular mechanics calculations reveal that in spite of the presence of the benzene ring in the macrocyclic framework this ligand can encapsulate metal ions with different stereo-electronic sizes in square planar arrangements. Our results indicate that the presence of the benzene ring in the backbone of the bz[14]N₄ confers a coordination behaviour intermediate between that of cyclam and cyclen.