Physical nature of interactions in ZnII complexes with 2,2′-bipyridyl : quantum theory of atoms in molecules (QTAIM), interacting quantum atoms (IQA), noncovalent interactions (NCI), and extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV) comparative studies

Abstract

In the present account factors determining the stability of ZnL, ZnL2, ZnL3 complexes (L = bpy, 2,2-bipyridyl) were characterized based on various techniques: the Quantum Theory of Atoms in Molecules (QTAIM), energy decomposition schemes based on Interacting Quantum Atoms (IQA) and Extended Transition State coupled with Natural Orbitals for Chemical Valence (ETSNOCV). Finally, the Non-covalent Interactions (NCI) index was also applied. All methods consistently indicated that the strength of the coordination bonds, Zn–O, Zn–N, decreases from ZnL to ZnL3. Importantly, it has been identified that the strength of secondary intramolecular heteropolar hydrogen bonding interactions, CH•••O, CH•••N, increases when going from ZnL to ZnL3. A similar trend appeared to be valid for the π -bonding as well as electrostatic stabilization. In addition to the above leading bonding contributions, all techniques suggested the existence of very subtle, but non-negligible additional stabilization from the CH•••HC electronic exchange channel; these interactions are the weakest among all considered here. From IQA it was found that the local diatomic interaction energy, H,H int E , amounts at HF to –2.5, –2.7 and –2.9 kcal mol–1 for ZnL, ZnL2 and ZnL3, respectively (–2.1 kcal mol–1 for ZnL at MP2). NOCV-based deformation density channels showed that formation of CH--HC contacts in Zn-complexes causes significant polarization of (C–H) bonds, which accordingly leads to charge accumulation in the CH•••HC bay region. Charge depletion from (C–H) bonds were also reflected in the calculated spin-spin 1J(C–H) coupling constants, which decrease from 177.06 Hz (ZnL) to 173.87 Hz (ZnL3). This last result supports our findings of an increase in the local electronic CH•••HC stabilization from ZnL to ZnL3 found from QTAIM, IQA, and ETS-NOCV. Finally, this work unites for the first time the results from four methods that are widely used for description of chemical bonding.