Structure and conformational analysis of co-ordination complexes. Part II. X-Ray crystal structure of cis-[carbonatobis(trimethylenediamine)cobalt(III)] perchlorate and conformational analysis of trimethylenediamine complexes

Abstract

The crystal structure of cis-[carbonatobis(trimethylenediamine)cobalt(III)] perchlorate, cis-[Co(tn)₂CO₃]ClO₄(tn = trimethylenediamine), has been determined by X-ray structure analysis. The crystals are monoclinic space group P2₁/c, with a= 9·51, b= 13·22, c= 12·03 Å and β= 111·17°. The structure was solved by Patterson and Fourier methods and refined by least squares to R 0·092 for 1347 observed reflections. The cobalt-tn rings are not related by molecular symmetry and have different and slightly distorted chair conformations. These distortions are well reproduced by a conformational strain energy minimization calculation using a comprehensive force-field. This conformational isomer of the cis-complex is not the one of least strain energy. Two other cis-isomers in which the ligands are related by a two-fold axis are possible; one has a lower and the other a higher strain energy, but neither of the forms could be detected in several preparations of the complex. The strain energy minimization of the tris-chair, tris-lel-skew-boat, and tris-ob-skew-boat forms of Co(tn)₃⁺³ reveals an energy difference of 5·06 kcal mol^–1 between the tris-ob-boat and tris-chair conformers, the tris-chair being the energetically more favourable. However the tris-lel-skew-boat was found to be 0·77 kcal mol^–1 lower in energy than the trischair isomer.