Insights on the interaction of Zn²⁺ cation with triazoles: Structures, bonding, electronic excitation and...

At present, we investigate the structures, the stability, the bonding and the spectroscopy of the Zn²⁺-triazole complexes (Zn²⁺-Tz), which are subunits of triazolate based porous materials and Zn-enzymes. This theoretical work is performed using ab initio methods and density functional theory (DFT) where dispersion correction is included. Through these benchmarks, we establish the ability and reliability of M05-2X+D3 and PBE0+D3 functionals for the correct description of Zn²⁺-Tz bond since these DFTs lead to close agreement with post Hartree-Fock methods. Therefore, M05-2X+D3 and PBE0+D3 functionals are recommended for the characterization of larger organometallic complexes formed by Zn and N-rich linkers. For Zn²⁺-Tz, we found two stable σ-type complexes: (i) a planar structure where Zn²⁺ links to unprotonated nitrogen and (ii) an out-of-plane cluster where carbon interacts with Zn²⁺. The most stable isomers consist on a coordinated covalent bond between the lone pair of unprotonated nitrogen and the vacant 4s orbital of Zn²⁺. The roles of covalent interactions within these complexes are discussed after vibrational, NBO, NPA charges and orbital analyses. The bonding is dominated by charge transfer from Zn²⁺ to Tz and intramolecular charge transfer, which plays a vital role for the catalytic activity of these complexes. These findings are important to understand, at the microscopic level, the structure and the bonding within triazolate based macromolecular porous materials and Zn-enzymes.