trans-Methylpyridine cyclen versus cross-bridged trans-methylpyridine cyclen. Synthesis, acid–base and metal complexation studies (metal = Co2+, Cu2+, and Zn2+)

Abstract

The synthesis of the cross-bridged cyclen CRpy₂ {4,10-bis((pyridin-2-yl)methyl)-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane}, a constrained analogue of the previously described trans-methylpyridine cyclen Cpy₂ is reported. The additional ethylene bridge confers to CRpy₂proton-sponge type behaviour which was explored by NMR and potentiometric studies. Transition metal complexes have been synthesized (by complexation of both ligands with Co²⁺, Cu²⁺ and Zn²⁺) and characterized in solution and in the solid state. The single crystal X-ray structures of [CoCpy₂]²⁺, [CuCpy₂]²⁺ and [ZnCpy₂]²⁺ complexes were determined. Stability constants of the complexes, including those of the cross-bridged derivative, were determined using potentiometric titration data and the kinetic inertness of the [CuCRpy₂]²⁺ complex in an acidic medium (half-life values) was evaluated by spectrophotometry. The pre-organized structure of the cross-bridged ligand imposes an additional strain for the complexation leading to complexes with smaller thermodynamic stability in comparison with the related non-bridged ligand. The electrochemical study involving cyclic voltammetry underlines the importance of the ethylene cross-bridge on the redox properties of the transition metal complexes.