Synthesis, structure, theoretical calculation of electronic absorption spectrum, and spectroscopic property analysis of Cu(II) complex with pyrazine-2-carboxylic acid based on DFT theory

Abstract

An octahedral Cu(II) complex with pc− and water, [Cu(pc)2(H2O)2] (Hpc = pyrazine-2-carboxylic acid), was synthesized and its geometric structure, electronic structure, electronic absorption spectrum, and spectroscopic properties were thoroughly investigated by X-ray crystallography, density functional theory (DFT), and time-dependent DFT (TDDFT) theoretical calculations. The geometric structure and electronic structure were studied in detail on the basis of DFT results, partial density of states (PDOS), and overlap population density of states (OPDOS) analyses. According to theory, the electronic spectrum is in the UV–Vis region, and the spectroscopic properties are ascribed to σ → (d x2−y2, d xy) ligand-to-metal charge transfer transition for strong absorption bands with charge transfer from pc− σ orbitals to Cu(II) d x2−y2 and d xy orbitals, and π/σ → π* intra-ligand charge transfer transition for weak absorption bands with charge transfer from pc− σ/π orbitals to pc− π* antibonding orbitals. The coordinative water is not associated the optical transitions. Moreover, the Cu(II) central d → d charge transfer bands may be hard to observe in the visible-light region owing to the transition-forbidden selection rule. Both the Cu(II)–O/N (pc−) and Cu(II)–O (H2O) interactions are mainly ionic, but not covalent.