Crystal and molecular structures of related nickel(ii) complexes of open-chain and macrocyclic oxamide-based ligands and the peculiarities of water aggregates in their crystal lattices

Abstract

A comparison of the molecular structure of related nickel(II) complexes of the open-chain and 13-membered macrocyclic oxamide-derived ligands NiL¹·4H₂O and NiL²·3H₂O revealed that the formation of an additional 6-membered chelate ring in the complex results in rather small changes in the molecular structure of the ligands and the bite angles around the metal ion. Two deprotonated amide and two amine donors form an approximately square planar environment for the nickel(II) in both complexes and the only essential consequence of ligand cyclization is the contraction of the nickel–nitrogen distances by 0.012 and 0.021 Å for the Ni–N_amide and Ni–N_amine bonds, respectively. The packing modes of NiL¹ and NiL² in the crystalline state are essentially different—lattice water molecules form isolated monomolecular 2D sheets separating and gluing the metallocomplex layers in the former complex, while they are included in the formation of hybrid metallocomplex–water layers connected by van der Waals interactions in the latter. Analysis of the ¹H NMR spectra reveals that the solid state conformation of the macrocyclic complex is retained in aqueous solution.