Structural Changes in CuII Complexes of Potential Octadentate Ligands by Coordination with Carboxylate/Carboxylic Acid: DFT, TD-DFT, and Experimental Studies

The structural and spectroscopic studies of N,N,N?,N?,N?-pentakis-(benzimidazol-2-yl-methyl)diethylenetriamine (L1) and N,N,N?,N?-tetrakis-(benzimidazol-2-yl-methyl)-N?-(carboxylmethyl)diethylenetriamine (L2H) and [CuL1]2+, [CuL2H]2+, and [CuL2]+ were carried out by density functional theory (DFT) and time-dependant (TD)-DFT techniques. The results show that a geometrical change occurs when carboxylate/carboxylic acid coordinates with the metal ion. For example, the ligand L2H forms an octahedral geometry with CuII and in the structure, four nitrogens (N3, N13, N44, N47) are equatorially coordinated with the metal ion, and atoms O50 (COOH) and N41, which are weakly bonded at the axial positions, are in competition in the formation of an axial bond with CuII; however, for the ligand L2, only a square pyramidal (SP) geometry results with CuII because of the formation of a strong axial bond by O50 (COO) with CuII, which dictates non-bonding at its trans position. Molecular orbital analysis proves that both HOMO and HOMO 1 are localized over the carboxylate ion that favours a strong axial bond with the metal ion; thus, the SP geometry results in the X-ray structure of [CuL2]ClO4. Furthermore, for the complexes, since the electronic spectroscopic bands were unseparated in the spectra, the TD-DFT was used to identify the bands.