The crystal and molecular structures of the nickel(II) complexes of malonamide-derived unsubstituted 14- and 13-membered tetraazamacrocycles and NMR study of the complexes in aqueous solution

Abstract

The crystal structures of the nickel(II) complexes of the 14- and 13-membered malonamide-derived macrocycles [NiL¹]·6H₂O and [NiL²]·5H₂O (H₂L¹ = 1,4,8,11-tetraazacyclotetradecane-5,7-dione, H₂L² = 1,4,7,10-tetraazacyclotridecane-11,13-dione) have been determined. Two deprotonated amide and two amine donors form the approximately square planar environment of the metal in both complexes. The averaged Ni–N_amide and Ni–N_amine bonds are longer in the complex of the 14-membered macrocycle as compared to the 13-membered one (1.889 vs. 1.839 Å and 1.941 vs. 1.889 Å for [NiL¹] and [NiL²] respectively). Water molecules do not co-ordinate but form an extended network of hydrogen bonds in the crystal lattices. The ligand in [NiL¹]·6H₂O has the N-meso and in [NiL²]·5H₂O the N-rac configuration of the secondary amino groups. The analysis of ¹H NMR spectra reveals that the solid state conformation of both the 14- and 13-membered co-ordinated ligands is retained in aqueous solution though a substantial amount of the N-meso isomer is also detected for the latter.