Stereoselectivity in lanthanide complexes of malic acid

Abstract

A variety of spectroscopic and potentiometric studies have been carried out detailing the nature of the complex formation between lanthanide ions and malic acid. Formation constants calculated below pH 7 (where the complexes are mononuclear) demonstrate that both Gd 3+ and Dy 3+ complexes of DL-malic are more stable than corresponding complexes of either D- or l-malic acid. Studies of intermolecular energy transfer, on the other hand, indicate that above pH 7 (where the complexes are found to self-associate) complexes of resolved ligand associate to a greater extent than complexes prepared from racemic ligand. Supporting evidence is obtained from measurements of differential absorption. It appears that the weaker D- or L-MAL complexes are more prone to hydrolysis than the DL-MAL complexes, and evidence is presented from measurements of luminescent titrations that dimeric complex units are apparently bridged by either one or two -OH bridges. No evidence was found at 1:5 metal-to-ligand ratios for the existence of 1:3 lanthanide.malic acid complexes and no experimental effects could be detected which pointed toward participation of the malic acid hydroxyl group in the complex bonding.