Structural studies of two protonated forms of a C2 symmetrical optically active cyclam derivative

Abstract

Detailed structural analyses, including solid-state (X-Ray), solution-state (NMR) and theoretical (AM1) studies of the two most stable protonated forms — di 3H₂²⁺ and tetraprotonated 3H₄⁴⁺ — of a C₂ symmetrical, optically active cyclam derivative 3 have been carried out. Important conformational changes in the 14-membered and cyclohexane rings accompany the protonation. A rigid conformation with C₂ symmetry stabilized by bifurcated intramolecular hydrogen bonds is obtained for 3H₂²⁺, whereas for 3H₄⁴⁺ a dynamic system in solution and a loss of the C₂ symmetry in the solid state are found. The effects of temperature and the counter-ion are also discussed.