Experimental and Theoretical Studies of Transition Metal Coordination Crystals With Tetramethylethylenediamine and Aromatic Carboxylic Acids

Two new mononuclear heteroleptic complexes (a) Cu (II) complex using 3,5-dihydroxybenzoic acid and tetramethylethylenediamine and (b) Ni (II) complex with benzoic acid and tetramethylethylenediamine were synthesized and characterized spectroscopically. Moreover, the compounds were characterized by single-crystal X-ray diffraction (SC-XRD), which showed that the benzoate group chelated the metal center in the Cu (II) complex, whereas the non-chelating response of the benzoate group was observed in the Ni (II) complex. A disordered octahedron geometry was found in both complexes according to SC-XRD results. Furthermore, the supramolecular assembly was investigated by Hirshfeld surface analysis. DFT analysis was employed to investigate the electronic characteristics, assess charge transfer processes and examine the NLO properties of the synthesized Cu (II) and Ni (II) complexes. A band gap of 5.083 and 5.116 eV was observed for complexes of copper and nickel, respectively. The global reactivity results indicated that all complexes exhibited positive electron affinity values, implying their participation in charge transfer reactions. Notably, complex Cu (II) was identified as more reactive, while complex Ni (II) was found to be more stable. Both complexes showed absorption peaks in the UV region of the spectrum. Remarkably, the obtained compound Cu (II) demonstrated exceptional NLO characteristics, with a maximum linear polarizability <α> of 4.312 × 10⁻²³ esu and a second hyperpolarizability (γ_total) of 2.482 × 10⁻³⁵ a.u. when in an acetonitrile solution. These synthesized chromophores displayed remarkable NLO properties, primarily attributed to their reduced charge transport resistance. The findings from this NLO study could pave the way for researchers to explore novel NLO materials that hold promise for cutting-edge applications in the modern era.