COORDINATION INTERACTION BETWEEN ANTHRANILIC LIGAND AND D-ELEMENT SALTS DURING CRYSTAL FORMATION: A STRUCTURAL AND SPECTROSCOPIC APPROACH

Аннотация

This study focuses on the crystallization of coordination complexes formed between anthranilic acid ligand and d-block metal salts (Fe(III), Cu(II), Ni(II), Co(II)). The coordination occurs via the ligand’s carboxylic and amino groups, resulting in the formation of stable chelate complexes. The crystalline structure of the compounds was characterized by X-ray diffraction analysis, while bonding characteristics were examined through IR and UV-Vis spectroscopy. The results suggest that these crystalline complexes hold potential for use in pharmaceuticals, catalysis, and biosensor technologies. Keywords: Anthranilic acid, coordination complex, d-block metal ions, chelation, crystallization, bidentate ligand, thermal stability, ligand field, transition metals. Introduction: Anthranilic acid (2-aminobenzoic acid) serves as a structurally versatile ligand due to its functional groups – the carboxylic acid (-COOH) and amino (-NH2). These groups enable bidentate coordination to metal centers, especially d-block elements which are known for their variable oxidation states and coordination geometries. Investigating the crystallization behavior of such complexes provides insights into structure–property relationships critical for applied chemistry. Literature review: The coordination chemistry of aromatic amino acids, particularly anthranilic acid (2-aminobenzoic acid), has been widely investigated due to the ligand’s dual donor sites — the amino group (-NH₂) and the carboxylic acid group (-COOH). Early studies by Cotton and Wilkinson (1999) established the foundational principles of bidentate ligand behavior in d- block metal coordination complexes, emphasizing the importance of geometric preferences and ligand field stabilization. Nakamoto (2009) further elaborated on the spectroscopic signatures of coordinated carboxylate and amine functionalities in metal-organic complexes, providing a reference framework for interpreting IR and UV-Vis spectral data. In particular, shifts in the ν(C=O) band and changes in ligand field transition bands have been extensively used as indicators of coordination. The use of anthranilic acid in forming stable chelate rings has been documented in studies by Housecroft & Sharpe (2012), where its coordination to transition metals such as Cu(II), Fe(III), and Ni(II) was shown to result in thermally stable crystalline complexes. These studies confirmed that the crystallization behavior is highly dependent on the metal’s ionic radius, electronic configuration, and the nature of the solvent environment. More recent research has shifted toward application- focused analysis. Complexes of anthranilic acid with transition metals have been reported as potential precursors in catalysis (Lever, 1984), biosensor fabrication, and even drug delivery systems, owing to their structural predictability and functional group compatibility.Despite this foundational work, detailed investigations of the crystal structure variations among different metal 199 RESPUBLIKA ILMIY-AMALIY KONFERENSIYASI 7-8-MAY 2025-YIL complexes of anthranilic acid remain limited. The present study aims to bridge this gap by systematically analyzing the crystallization outcomes, structural motifs, and spectroscopic characteristics of anthranilic acid complexes with Fe(III), Cu(II), Ni(II), and Co(II) under controlled experimental conditions. Materials and Methods: The experiments were conducted using anthranilic acid and four transition metal salts: FeCl₃·6H₂O, CuCl₂·2H₂O, NiSO₄·6H₂O, and CoCl₂·6H₂O. Stoichiometric ratios of 1:2 metal to ligand were reacted in ethanol–water solution (1:1 v/v) under ambient temperature with continuous stirring. Crystallization was achieved through slow evaporation over a period of 7–14 days. Characterization was performed using the following methods: X-ray diffraction (XRD): to confirm the formation of crystalline structures and determine unit cell parameters. UV-Visible spectroscopy: to identify metal–ligand charge transfer bands and ligand field transitions, typically observed between 510–580 nm. Infrared spectroscopy (IR): to detect shifts in vibrational frequencies, especially the C=O stretch moving from 1700 cm⁻¹ in free ligand to 1620 cm⁻¹ upon coordination, and NH₂ vibrations near 3300 cm⁻¹. Results and Discussion: The resulting complexes formed distinct crystalline phases. XRD analysis revealed that Fe and Co complexes crystallized in a monoclinic system, whereas Cu and Ni complexes preferred a triclinic arrangement. These differences suggest a metal-dependent influence on lattice packing and symmetry. UV-Vis spectra showed distinct absorption bands for each metal complex, indicating successful coordination and electronic interaction between the metal center and the ligand. IR spectra confirmed the involvement of both amino and carboxyl groups in coordination, as evidenced by the characteristic shifts. The thermal behavior of the crystals, inferred from decomposition onset points above 250 °C, s