Chiral Quadridentate Ligands Derived from Amino Acids and Some Zinc Complexes Thereof

Abstract

A series of three new quadridentate ligands was synthesized by reaction of the haloacetylated amino acid esters L-BrAc−Phe−OMe (4a), L-BrAc−Lys(Z)−OMe (4b), and ClAc−Gly−OEt (4c), respectively, with bis(picolyl)amine (bpa, 5). The obtained products L-bpaAc−Phe−OMe (3a), L-bpaAc−Lys(Z)−OMe (3b), and bpaAc−Gly−OEt (3c) were treated with Zn(OTf)2 (OTf = CF3SO3−) to yield the trigonal-bipyramidal complexes [(L-bpaAc−Phe−OMe)ZnOTf](OTf) (6a), [(L-bpaAc−Lys(Z)−OMe)ZnOTf](OTf) (6b), and [(bpaAc−Gly−OEt)ZnOTf](OTf) (6c). Crystal structures of 6c and the hydrolysis product [(L-bpaAc−Phe−OMe)(H2O)ZnOTf](OTf)2 (7a) are reported. The results suggest the formation of a chiral pocket at the metal center provided by the benzyl substituent in the L-phenylalanine derivative 7a. This observation is further supported by 1H-NMR and circular dichroism spectroscopic data. These methods indicate a significant ordering effect within the ligands upon metal coordination as well as interactions between the first coordination spheres and their chiral environments provided by the L-phenylalanine and L-lysine moieties of 3a and 3b, respectively. Our results are discussed with respect to the development of chiral building blocks for transition metal catalysts and biomimetic assemblies.