Synthesis, comprehensive characterization, crystallographic and density functional theory (DFT) studies of tridentate amic acid compounds

• A series of tridentate amic acid ligands L1 – L5 were successfully synthesized and fully characterized. • The first crystal structures of copper(II) complexes with amic acids are reported. • DFT calculations complemented the structural studies and confirm findings. • Ligand sterics and electronics influence metal coordination and cavity geometry. Amic acids constitute a class of organic acids which have both carboxylic as well as amide functional groups within their structure and their synthesis has gained significant interest due to their applications in medicine and hydrometallurgical industries. In this study, a series of novel tridentate amic acid-based ligands were synthesized via a bimolecular nucleophilic substitution (SN 2 ) reaction between 2-chloro-N,N-dioctylacetamide and glycine derivatives to afford (2-(dioctylamino)-2-oxoethyl)glycine ( L1 ), N-(2-(dioctylamino)-2-oxoethyl)-N-methylglycine ( L2), N-(2-(dioctylamino)-2-oxoethyl)-N-ethylglycine (L3) , N-(2-(dioctylamino)-2-oxoethyl)-N-isobutylglycine ( L4 ) and (2-(dihexylamino)-2-oxoethyl) methylglycine (L5) . Structural modifications were introduced by varying the alkyl substituents on the central amine nitrogen to investigate their influence on coordination behaviour. Ligands L1 – L5 were characterized using various spectroscopic techniques such as FT-IR ATR, NMR ( 1 H and 13 C), together with mass spectrometry to confirm their successful synthesis. The diagnostic peak for the methylene protons in the region of (3.8-4.0) ppm in the 1 H-NMR spectra corroborates the formation of a new bond between the amino acid moiety and the amide carbonyl carbon across all compounds. To further validate the coordination properties of these ligands, copper(II) complexes of two representative ligands ( L2 and L5 ) were obtained and structurally characterized using single-crystal X-ray diffraction. The crystal structures of copper(II) complexes of L2 and L5 which adopt the general molecular formula of Cu(L2) 2 and Cu(L5) 2 also affirm the successful synthesis of the tridentate amic acids. These are the first reported crystal structures of copper complexes involving tridentate amic acid extractants. The complexes adopt a distorted octahedral geometry with the Cu²⁺ ion coordinated by two ligands in a tridentate fashion—through carboxylic, amide, and amine donor atoms—resulting in four five-membered chelate rings per complex Notably, the observed bite angles were smaller than 90°, indicating a constrained coordination environment and supporting a distorted geometry. To complement the experimental findings, Density Functional Theory (DFT) calculations at B3LYP (Becke-Lee-Yang-Parr) method with the LANL2DZ and 6-31G (d, p) basis set were performed to model the electronic and structural properties of the ligands and their Cu(II) complexes. The calculations not only corroborated the crystallographic bond lengths and angles but also highlighted the role of alkyl substituent sterics and electronics in modulating complex stability. The results showed that electron-donating alkyl chains affect the bond metrics at the amine nitrogen, which in turn influences the metal-binding cavity and stability of the resulting complex. Overall, this work provides the first comprehensive correlation between synthetic, spectroscopic, crystallographic, and theoretical data for this class of ligands. It establishes a foundation for tailoring amic acid ligands with improved selectivity and efficiency for base metal ion separation.