Enantiopure and racemic alanine as bridging ligands in Ca and Mn chain polymers.

Abstract

Under accelerated and controlled evaporation, chain polymers crystallize from aqueous solutions of Ca(II) and Mn(II) halides with enantiopure L-alanine or racemic DL-alanine. In all ten solids thus obtained zwitterionic amino acid ligands bridge neighbouring cations. The exclusively O-donor-based coordination sphere around the metal cations is completed by aqua ligands; the halides remain uncoordinated and act as counter-anions for the cationic strands. Despite the differences in ionic radii and electronic structure between the main group and the transition metal cation, their derivatives with L-alanine share a common structure type. In contrast, the solids derived from DL-alanine differ and adopt structures depending on the metal cation and the halide. Homochiral chains of either chirality or heterochiral chains with different arrangements of crystallographic inversion centres along the polymer strands are encountered. On average, the six-coordinated Ca(II) cations, devoid of any ligand field effect, show more pronounced deviation from idealized octahedral geometry than the d-block cation Mn(II).