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

Abstract

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 C2 or Ci symmetries. The Ci 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 C2 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).