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).

Crystal structures of amino acids: from bond lengths in glycine to metal complexes and high-pressure polymorphs

“…Alternatively, metal ions may link the amino acids into chain polymers. [186] 18. Conclusions Amino acids are relatively simple, small organic molecules, but, as should be evident from this review, their world of crystal structures is extremely diverse.…”

Incidental Polymorphism, Non-Isomorphic and Isomorphic Substitution in Calcium-Valine Coordination Polymers

“…On the other hand, a series of molecular, isomorphous products in 1:2 stoichiometry was obtained with racemic proline [1]. Enantiopure alanine even allowed for isomorphous substitution of the heavy manganese and calcium halides to form a series of chiral coordination polymers in 1:3 stoichiometry, while racemic alanine yields different structures for the two metals [2]. Calcium, a cation without crystal field effects exerted from partially filled d-orbitals, exhibited its versatility in the coordination chemistry with proline: two polymorphs were found for reactions of CaCl 2 with both enantiopure and racemic proline.…”