It is widely known that most metal complexes will preferentially complex to form five-membered chelate rings (5m-CR) when compared to six-membered chelate rings (6m-CR) except when binding to small metal cations. A characteristic example of this phenomenon can be found in the complexes of nitrilotri-3-propionic acid (NTPA, it forms 6m-CR) and nitrilotriacetic acid (NTA, it forms 5m-CR) where the only known metal that complexes preferentially to form the 6m-CR is Be. This has largely been attributed to lone pair donor atom repulsion, steric hindrance and to a lesser extent inductive effects.
In this work the complexes of BeII and ZnII complexes of NTA and NTPA will be explored. Competition reactions will be used to validate the solvent optimized structures and then in depth analysis will be performed to understand the relative complex stability including: (i) geometric analysis to measure bond strength and steric repulsion based on interatomic distances, (ii) The Quantum Theory of Atoms in Molecules (QTAIM) to show the presence of atomic interaction lines (AILs) and topological properties to understand the strength and nature of coordination bonds and weak intramolecular interactions, (iii) the Non-Covalent Interactions (NCI) scheme to determine the presence of additional interactions not visualized in QTAIM, (iv) 1D cross-sections along the ?2 eigenvector of the Hessian matrix to understand the local behaviour of the topology of electron density within an interaction, and (v) the theory of Interacting Quantum Atoms (IQA) to measure the strength of and understand the nature of interactions of interest. From this we will conclude from our data, we cannot explain the relative metal complex stability based on the local properties of interactions. In addition, we will suggest that rather than destabilize the molecule, the CH HC contributes to stabilizing the molecule.
Using a simplified two step model, similar to that proposed in the ETS-NOCV scheme, in which (i) the preorganization of the ligands and the preorganization of the metal fragment (in the case of ZnII complexes), and (ii) binding between the preorganized fragments with the associated affinity energy will be extensively evaluated using QTAIM, NCI, IQA and the Interacting Quantum Fragments (IQF). This analysis will show that the preferential complex stability is not due to strain as a result of CH HC interactions, but rather due to the formation of unfavourable interactions between the lone-pair donor atoms such as oxygen and nitrogen and the resultant weaker interactions between the metal centre and water molecules (specifically when the ligands bind to a polyatomic metal fragment).