Abstract
Ground state geometries, spectral (IR and UV-Vis) properties, analysis of frontier molecular orbitals (FMOs), natural bond orbital (NBO) analysis and molecular electrostatic potential (MEP) surfaces of three transition metal complexes [Cu(AOYP)2(OH2)2] (A), [Ni(AOYP)2(OH2)2] (B) and [Zn-(AOYP)2(OH2)2] (C), have been studied theoretically by the Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) methods. AOYP is the oxadiazole ligand 2-(5-amino-[1,3,4]-oxadiazol-2-yl)phenol. The geometries of these complexes were initially optimized using two basis sets: LAN2DZ and a generic basis set, the latter of which was selected for subsequent analysis. The stability of the complexes arising from intramolecular interactions and electron delocalization was estimated by natural bond orbital (NBO) analysis. The NBO results showed significant charge transfer from lone pair orbitals on the AOYP donor atoms O19, O21, N15 and N36 to central metal ions in the complexes, as well as to the benzene and oxadiazole rings. The electronic spectrum of (A) showed bands at 752 and 550 nm mainly attributable to ligand-to-metal charge transfer (LMCT) transitions, and a band at 446 nm assigned to a d-d transition. The electronic spectrum of (B) consisted of bands at 540, 463 and 395 nm mainly due to d-d transitions. Calculated electronic bands for (C) occurred at 243, 238 and 235 nm, arising from intraligand charge transfer (ILCT) transitions within AOYP. A good agreement in terms of band positions was found between experimental and calculated absorption spectra of the complexes.
Highlights
Oxadiazoles constitute a very important class of ligands in coordination chemistry due to their wide applications in the synthesis of a large variety of transition metal complexes with diverse biological activities such as anti-inflammatory, antifungal, antibacterial, antiviral, anti-HIV and anticancer activities [1]-[4]
Careful selection of basis sets is very important for accurate prediction of the properties of transition metal complexes
The complexes were optimized in the gas phase and solution phase at two levels of theory (B3LYP/LANL2DZ and B3LYP/ GEN) for comparison, after which B3LYP/GEN was chosen for all remaining calculations
Summary
Oxadiazoles constitute a very important class of ligands in coordination chemistry due to their wide applications in the synthesis of a large variety of transition metal complexes with diverse biological activities such as anti-inflammatory, antifungal, antibacterial, antiviral, anti-HIV and anticancer activities [1]-[4]. In 2013, Wanale and co-workers synthesized and characterized complexes of 2-(5amino-[1,3,4]-oxadiazol-2-yl)phenol with Cu(II), VO(IV), Ni(II), Zn(II) and Cd(II), but did not determine their geometrical parameters (bond lengths, bond angles and dihedral angles). These parameters can be conveniently determined theoretically, by quantum chemical calculations. To the best of our knowledge, a theoretical study of the geometries and properties of these complexes has not been reported in the literature This inadequacy encouraged us to pursue theoretical studies on the work of Wanale and co-workers in order to determine stable geometries and shapes of [Cu(AOYP)2(OH2)2] (A), [Ni(AOYP)2(OH2)2] (B) and [Zn(AOYP)2(OH2)2] (C) as well as various microscopic properties. A rigorous control of molecular geometry and shape is crucial to the drug design process because different geometric, steric and conformational properties of a biologically active molecule can give rise to different potencies, types of activity and unwanted effects [9]
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