Studies on metal complexes of chiral cyclen. Part 14. Configurational isomerism in a complex of cobalt(III)

Abstract

The chiral 12-membered cyclic tetramine (2R,5R,8R,11R)-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane (L) reacts with CoBr₂ to give mainly cis-SSSR-[Co^IIIBr(H₂O)L]Br₂1. Another isomeric complex 2 has recently been isolated from the reaction mixture obtained under mild alkaline conditions. In order to clarify the geometry, the structures of cis-(RSRS)-[Co{(S)-alaO}L][ClO₄]₂4(alaO = alaninate) and cis-(RSRS)-[Co{(S)-thrO}L][ClO₄]₂·3H₂O 6(thrO = threoninate) derived from 2 have been determined by X-ray analysis. All complexes prepared are cis-octahedral, each Co^III is surrounded by four N from L and by N and O of the amino acid, and the configurations of the four asymmetric nitrogen atoms are RSRS. This means that the cobalt ion in 2 co-ordinates to the crowded face of the N₄ plane of L, and the ligations of the metal to L in 1 and 2 occur from opposite directions. The absorption band maxima of the respective visible spectra of all the RSRS complexes shift approximately 400 cm^–1 to higher energy than those of the corresponding SSSR ones. A slow exchange reaction between the fifth and sixth ligands is observed for 1, and release of the amino acid from a SSSR-amino acidato complex takes place in Na₂CO₃ solution. No such reactions occur in 2 and in RSRS-amino acidato complexes. These results are probably related to the ligand-field strength.