Conformational analyses of the isomers of the systems [Co(men)n(en)3-n]3+(men =N-methylethane-1,2-diamine, en = ethane-1,2-diamine; n= 1–3)

Abstract

A molecular mechanics study of the configurational/conformational isomers of the system Λ-[Co(men)(en)₂]³⁺(men =N-methylethane-1,2-diamine, en = ethane-1,2-diamine)(16 isomers), and the geometric/configurational/conformational isomers of the systems Λ-[Co(men)₂(en)]³⁺(72 isomers) and Λ-[Co(men)₃]³⁺(88 isomers) has been undertaken using the MM2 force field. Calculated steric energies (adjusted for statistical factors) are used to predict isomer ratios in the three systems, which are compared with experimentally determined themodynamic stabilities. Comparisons are also made between calculated geometries and X-ray molecular structures where these are available. For the Λ-[Co(men)₂(en)]³⁺ system the calculations not only correctly predict the order of stability of the three possible geometric isomers, but also the most stable conformation for each of these cases. For the Λ-[Co(men)₃]³⁺ system the two most stable forms observed experimentally are among the three forms of the molecule (of the 88 possible) calculated to be the most stable. Comparison of the geometric parameters for the two crystal structures and corresponding energy-minimized structures shows that the agreements are very good. The significance of statistical factors in thermodynamic comparisons of this type is discussed.