Cubic cryptates. A molecular mechanics study

Abstract Molecular mechanics calculations on cubic [222] cryptand conformations derived from the coordinates of Mn 2+ /222 are reported. The cubic cryptates are unusual in that the metal-nitrogen distance is equal to the metal-oxygen distance. The calculations show that the cubic conformation is achieved with no increase in strain energy compared with the more usual elongated cryptand conformations. The relatively short metal-nitrogen distance in cubic cryptates is attributed to the strong metal-nitrogen bonds in d -block transition-metal ion inclusion complexes. Results of calculations using starting coordinates from the Ca 2+ /222 and Sm 3+ /222 structures are also presented and discussed.