Amino-acid complexation to a cobalt(III) complex of optically-active (2R,5R,8R,11R)-2,5,8,11 -tetraethyl-1,4,7,10-tetra-azacyclododecane (L). Preparation, characterization, and structure determinations of [CoIII{(R)-or(S)-alaO}L]Br[ClO4]·H2O

Abstract

The complex [Co^IIIBr(H₂O)L] Br₂[L = optically-active (2R,5R,8R,11R)-2,5,8,11-tetraethyl-1,4,7,10-tetra-azacyclododecane] reacts with amino acids under mild reaction conditions, at room temperature and at pH 8.0, to give the corresponding amino acidato complexes. The crystal structures of two such complexes, [Co{(R)alaO}L]Br[ClO₄]·H₂O (2) and [Co{(S)-alaO}L]Br[ClO₄]·H₂O (3), were determined by X-ray analyses. Crystal data for (2): tetragonal, space group P4₃, a=14.320(6), c= 13.377(2)Å, and Z= 4; for (3): tetragonal, space group P4₃, a= 14.319(3), c= 13.325(7)Å, and Z= 4. Least-squares refinement converged at R= 0.044 and 0.042 for 2 618 and 2 413 unique reflections respectively. Both complexes show a cis-octahedral geometry, with the macrocycle co-ordinated in a folded manner, and the chiralities of the four nitrogen atoms of the macrocycle are SSSR as found in the starting complex, In both alaninato complexes, each amino acid co-ordinates in the β form with ε and δ conformations, respectively. The release of alanine from complex (3) is also discussed.