Amino acid-containing reduced Schiff bases as the building blocks for metallasupramolecular structures

Metal coordination compounds derived from several closely related yet different multidentate reduced Schiff base ligands (obtained by reducing the CN bond in the Schiff bases formed by the condensation of aldehyde and various natural/unnatural amino acids) are discussed in terms of their mode of binding and coordination to supramolecular network structures. These multidentate ligands have flexible backbone with hydrogen bond donors and acceptors, and readily form metal complexes and coordination polymers with metal ions such as Cu(II), Zn(II), and Ni(II). Various solid-state metallasupramolecular network structures are delineated ranging from hydrogen-bonded linear polymers and helical coordination polymers, 2D sheets to 3D network architectures constructed via N–H⋯O, CO⋯H–Osolvent, O–H⋯O, N–H⋯OC hydrogen bonds and CO⋯π, C–H⋯π, and π⋯π stacking interactions. This review gives an account of the observed structural diversity in relation to the role of different donors and acceptors, aqua ligand and solvents, nature of the ligands and metal ions, coordination geometry around the metal ions and counter ions besides the experimental conditions such as temperature, pH, etc. in directing the formation of supramolecular structures in the solid state. Some other related and interesting examples from the literature are also mentioned.