Molecular structure, bonding analysis and redox properties of transition metal-Hapca [bis(3-aminopyrazine-2-carboxylic acid)] complexes: A theoretical study

Density functional theory (DFT) calculations were carried out on M(Hapca)(2)(H2O)(2) (M = Mn, Fe, Co, Ni and Hapca = bis(3-aminopyrazine-2-carboxylic acid) by means of the BP86 and B3LYP functional using the TZP basis set. The optimized structures were obtained by imposing C-2 or C-i symmetries. The C-i structural arrangement consists of a slightly distorted octahedron centered by a transition-metal with bidentate Hapca ligands situated in equatorial positions and water molecules in axial ones. However, the C-2 structure consists of a distorted arrangement with n con-planar Hapca ligands. The influence induced by including double polarized functions in the (TZ2P) basis set is small on the geometrical parameters. A bonding analysis of these species showed the weakness of M-O(H2O) bonds compared to M-O(Hapca) ones. The obtained MO diagrams showed substantial HOMO-LUMO gaps for the 18-MVE closed-shell configuration. Reduction of M(Hapca)(2)(H2O)(2) led to the loss of the two water molecules, inducing four electrons downwards for [Co](-) and [Ni] species. The calculated ionization potentials (IPs) and electronic affinities (EAs) showed the oxidation and reduction ease of the manganese species contrarily to the cobalt ones. A diffuse function in the basis set (QZ3P-ndiffuse, n = 1 or 2) reduces remarkably the adiabatic electron affinities (AEAs). (C) 2015 Elsevier Ltd. All rights reserved.