Molecular mechanics studies of the conformations of metal complexes of 1,4,7,10,13,16-hexaazacyclooctadecane: Calculations of macrocyclic cavity size

Molecular mechanics calculations are reported on metal complexes of 18-azacrown-6. Four possible conformations are considered which encapsulate the metal in different geometric environments, namely, chiral octahedral, meso octahedral, trigonal prismatic, and hexagonal bipyramidal. Crystal structures are available for all but the last geometry, and molecular mechanics calculations show an excellent fit to the experimental data. The strain energy of the macrocycle has been calculated for a range of M-N distances, and thus the hole size of the macrocycle has been obtained for various conformations. Two different methods were used to calculate the strain energy of the metal environment. We either used the conventional molecular mechanics term for angle bending,k _b — ₀)², and chose the ideal angles to fit the particular metal geometry, or we set the angle force constants to zero and introduced 1,3 van der Waals interactions between the ligand donor nitrogen atoms. Results from the two methods are compared.