Conformational analysis of coordination compounds. II. Substituted five-membered diamine chelate rings

The conformations of chelate rings involving ethylenediamine, propylenediamine, and N-methylethylenediamine have been studied by detailed a priori conformational energy calculations. It has been found that for ethylenediamine there are a number of energetically equivalent symmetrical and unsymmetrical “minimum-energy” conformations. This is in contrast with the presently accepted view that the ring is limited to two mirror-image symmetrical conformations. The substituted ethylenediamine chelate rings with the substituent axial possess a different range of conformations which does not include a symmetrical conformation. The energy differences between the equatorial and axial conformations have been computed using Hill ’s, Bartell’s, and Mason and Kreevoy’s nonbonded interaction energy expressions in order to compare the results obtained when typical low-, medium-, and high-energy expressions are employed. The results regarding the geometrical parameters are in agreement with data determined by X-ray structural studies