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-[CoIIIBr(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 CoIII 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.