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

Detailed structural analyses, including solid-state (X-Ray), solution-state (NMR) and theoretical (AM1) studies of the two most stable protonated forms - di 3H(2)(2+) and tetraprotonated 3H(4)(4+) - of a C-2 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 C-2 symmetry stabilized by bifurcated intramolecular hydrogen bonds is obtained for 3H(2)(2+), whereas for 3H(4)(4+) a dynamic system in solution and a loss of the C-2 symmetry in the solid state are found. The effects of temperature and the counter-ion are also discussed.