Nickel(II) complexes of amino acids in comparison with the cobalt(III) analogues: interesting structural aspects relative to intermolecular hydrogen bonds and chiral recognition

Abstract

The crystal and packing structures of three racemic compounds with the formula, [Ni(tren)( dl-aminoacidato)]Cl· nH 2O have been investigated, aiming to compare the homochiral–heterochiral assembly process of the nickel(II) chiral complex cations with their well-studied cobalt(III) analogues. Due to the inherent hydrogen bonding donors–acceptors carried by the complex cations, infinite 1-dimensional (1-D) chains are generated by complementary inter-cationic H- bonds in both the Ni(II) and Co(III) analogues, but with different H- bond patterns. In all the Co(III) analogues studied by this group, the cations are linked together via the same pattern and strength of H- bonds which are designated as C6[R 2 1(6)]. They are homochiral, irrespective of the nature of the amino acidato ligands. In the Ni(II) analogues, the cations are linked by H- bonds described as C 2 1(6)[R 2 2(12)]. However, when the side chain of the amino acid is bulkier, the homochiral assembly of the cations becomes ambiguous due to the observed disorder of the asymmetric carbon. Additionally, it is interesting to note that the deprotonated carboxylic group of the amino acid coordinates with Ni(II) and Co(III) in different modes, as a delocalized and localized anion, respectively.