Assembly Mechanism of Zr-Containing and Other TM-Containing Polyoxometalates

The mechanism by which Zr-substituted and other transition metal-substituted polyoxometalates (POMs) form covalently linked dimers has been analyzed by means of static density functional theory (DFT) calculations with a continuous solvent model as well as Car-Parrinello molecular dynamics (CPMD) simulations with explicit solvent molecules. The study includes different stages of the process: the formation of the active species by alkalination of the solution and formation of intercluster linkages. CPMD simulations show that the Zr-triaqua precursor, [W₅O₁₈Zr(H₂O)₃]^2-, under basic conditions, reacts with hydroxide anions to form Zr-aqua-hydroxo active species, [W₅O₁₈Zr(OH)(H₂O)]^3-. We computed the DFT potential energy profile for dimerization of [W₅O₁₈TM(OH)]^n- [TM = Zr^IV(H₂O), Zr^IV, Ti^IV, and W^VI] anions. The resulting overall energy barrier is low for Zr^IV, moderate for Ti^IV, and high for W^VI. The computed thermodynamic balance favors the dibridged (μOH)₂ linkages for Zr^IV, the monobridged (μOH) linkage for Ti^IV, and the monomeric forms for W^VI, in agreement with experimentally observed trends. The lowest energy barrier and largest coordination number of Zr-substituted POMs are both a consequence of the flexible coordination environment and larger radius of Zr.