Influence of the Nature of the Azo Ligand on the Structural, Electronic, and Spectroscopic Properties of Ruthenium RuCl2L2 Complexes: A Comparative DFT and TD-DFT Study
This study examines the influence of the number of nitrogen atoms in azo ligands on the geometric, thermodynamic, electronic, and spectroscopic properties of the trans isomers (γ and δ) of ruthenium (II) complexes of the formula RuCl2L2 (L = azben, azpy, or papm). The calculations were performed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) at the B3LYP/Lanl2DZ level to determine the optimized structures, thermodynamic parameters, reactivity descriptors, electronic interactions, and electronic absorption properties. The results show that substituting pyridine with pyrimidine has little effect on the geometry of the complexes. However, the substitution of pyridine with phenyl, in the case of the γ-RuCl2(azben)2 isomer, profoundly alters the complex’s octahedral structure. Also, it significantly alters their stability and reactivity. At 298.15 K, the formation of the RuCl2(papm)2 and RuCl2(azpy)2 isomers is spontaneous and exothermic, whereas the formation of the RuCl2(azben)2 isomers is thermodynamically unfavorable under the same conditions. The RuCl2(azpy)2 complexes appear to be the most thermodynamically stable. Analysis of frontier orbitals and global reactivity descriptors indicates that replacing azopyridine with azobenzene increases the complexes’ electronic reactivity. In particular, the γ-RuCl2(azben)2 isomer exhibits the greatest affinity for interactions with DNA bases, making it a promising candidate for anticancer applications. Furthermore, TD-DFT calculations reveal that all complexes absorb in the visible spectrum via MLCT (Metal-to-Ligand Charge Transfer) transitions. These optical properties suggest that these compounds could serve as promising photosensitizers for dynamic cancer phototherapy.
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