Exploring Solvation Effects in Ligand-Exchange Reactions via Static and Dynamic Methods.

We investigate ligand-exchange reactions of a biomimetic Co(II)-based heterocubane complex in aqueous solution by means of various approaches for consideration of solvent effects. Static calculations based on geometry optimizations carried out in vacuum, with solvent continuum models, or with several explicit solvent molecules have been carried out as well as density functional theory (DFT)-based molecular dynamics simulations. In addition, reaction pathways and barriers have been elucidated via nudged elastic band calculations and metadynamics. The results show that static approaches with approximate consideration of the solvent environment lead to reaction energies, which may change drastically depending on the method employed. A more sophisticated approach is DFT-molecular dynamics at ambient conditions with full solvation, i.e. enough solvent molecules to retain bulk water properties far from the solute, which, however, comes with a much higher computational cost. The investigated example of the exchange of an acetate ligand by a hydroxide demonstrates that entropic contributions can be vital and consideration of electronic energies alone may be a rather rough approximation.