Collective Behavior of Zr-Substituted Polyoxometalates in Solution: Insights from Molecular Dynamics Simulations

The collective behavior of zirconium-substituted polyoxometalates (POMs) in aqueous solution and at the interface with organic solvents has been analyzed by means of molecular dynamics (MD) simulations with explicit solvent molecules. MD simulations using tetrabutylammonium as counterion (TBA+) and Zr-hydroxo-aqua [W5O18Zr(OH)(H2O)]3- anions (W5Zr) indicate that these anions do not form permanent noncovalent contacts, but the interaction is directional occurring preferentially through Zr···Zr moiety contacts. In biphasic chloroform/water systems, the hydrophobic TBA+ countercations accumulate at the interface, creating a positively charged layer that attracts W5Zr anions. At the layer of TBAs, the W5Zr anions sit with hydrophilic Zr-aqua-hydroxo moiety pointing toward the bulk aqueous solution, favoring the frequency of W5Zr···W5Zr and Zr···Zr contacts further increased due to the local concentration. Detailed analysis of W5Zr···W5Zr contacts in bulk solution revealed that interactions are driven by intercluster hydrogen bonding between the two Zr-aqua-hydroxo moieties. Finally, potential of mean force (PMF) simulations are performed to evaluate the free-energy change when two W5Zr anions approach each other in solution, showing that the free-energy penalty is low (2-3 kcal·mol-1) and can be easily overcome at ambient temperature.