Interacting quantum fragments-rooted preorganized-interacting fragments attributed relative molecular stability of the Be-II complexes of nitrilotriacetic acid and nitrilotri-3-propionic acid

Abstract

A method designed to investigate, on a fundamental level, the origin of relative stability of molecular systems using BeII complexes with nitrilotriacetic acid (NTA) and nitrilotri-3-propionic acid (NTPA) is described. It makes use of the primary and molecular fragment energy terms as defined in the IQA/F (Interacting Quantum Atoms/Fragments) framework. An extensive classical-type investigation, focused on single descriptors (bond length, density at critical point, the size of metal ion or coordination ring, interaction energy between BeII and a donor atom, etc.) showed that it is not possible to explain the experimental trend. The proposed methodology is fundamentally different in that it accounts for the total energy contributions coming from all atoms of selected molecular fragments, and monitors changes in defined energy terms (e.g., fragment deformation, inter- and intra-fragment interaction) on complex formation. By decomposing combined energy terms we identified the origin of relative stability of BeII(NTA) and BeII(NTPA) complexes. We found that the sum of coordination bonds’ strength, as measured by interaction energies between BeII ion and donor atoms, favours BeII(NTA) but the binding energy of BeII ion to the entire ligand correlates well with experimental trend. Surprisingly, the origin of BeII(NTPA) being more stable is due to less severe repulsive interactions with the backbone of NTPA (C and H-atoms). This general purpose protocol can be employed not only to investigate the origin of relative stability of any molecular system (e.g., metal complexes) but, in principle, can be used as a predictive tool for, e.g., explaining reaction mechanism.