Functionalized amino acid complexation to a cobalt(III) complex of (2R,5R,8R,11R)-2,5,8,11-tetraethyl-1,4,7,10-tetra-azacyclododecane. Crystal structures of complexes containing (R)- or (S)-serine and (R)-α-methylserine, and release of amino acid from the complexes

Abstract

The complex [CoBr(H₂O)L] Br₂(1)[L =(2R,5R,8R,11R)-2,5,8,11-tetraethyl-1,4,7,10-tetra-azacyclododecane] reacts with a racemic neutral amino acid having a hydroxymethyl group to give a pair of diastereomeric complexes. One of these crystallized preferentially as its appropriate salt. The less-soluble complex co-ordinated its (R)-amino acid. Structure determinations of (R)- or (S)-serinato (serO) and (R)-α-methylserinato (mseO) complexes, (3)–(5), showed that each hydroxyl group in the amino acid forms a hydrogen bond. The OH group of (R)-amino acids takes part in an intramolecular hydrogen bond to one of the secondary nitrogen atoms in L, and that of (S)-serO in (4) interacts with a cocrystallized water molecule and with an anion. Such different hydrogen-bonding formation results in different solubility between the diastereomers, and allows them to be isolated. Only a powdery solid was obtained for the complex co-ordinating (S)-mseO. Slightly racemized (R)- and (S)-ser and optically pure mse were obtained upon release from the corresponding complexes. Further, complex (1) was regenerated in good yield. The resolution expressed as the different solubilities between two salts in this series is ascribed to the effect of a three-point attachment.