Structure and conformational analysis of co-ordination complexes. Part III. A diastereoisomeric pair of αβ-chloro(tetraethylenepentamine)-cobalt(III) cations

Abstract

The crystal structures of the αβR- and αβS-chlorotetraencobalt(III) diperchlorates (tetraen = 1,4,7,10,13-pentaazatridecane) have been determined from photographic data. Both unit cells are orthorhombic with Z= 4 and space group P2₁2₁2₁; αβS : a= 17·37(1), b= 12·44(1), and c= 8·43(1); R 0·135 (1601 reflections) and for αβR a= 17·26(2), b= 12·03(1), and c= 8·72(3), R 0·129 (1529 reflections). The absolute configurations of both isomers were determined by the anomalous scattering technique. In both cations the ligand chain is bound by nitrogen co-ordination successively to the cobalt octahedral sites. The cations only differ by an inversion at the secondary nitrogen atom. Quantitative conformational analysis, using energy minimization, has been applied to the isomers and their molecular geometry was well reproduced in the absence of packing distortions. The calculations confirm the experimental result that the αβS isomer is more stable than αβR by ca. 2 kcal mol^–1.