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, [WOZr(HO)], under basic conditions, reacts with hydroxide anions to form Zr-aqua-hydroxo active species, [WOZr(OH)(HO)]. We computed the DFT potential energy profile for dimerization of [WOTM(OH)] [TM = Zr(HO), Zr, Ti, and W] anions. The resulting overall energy barrier is low for Zr, moderate for Ti, and high for W. The computed thermodynamic balance favors the dibridged (μOH) linkages for Zr, the monobridged (μOH) linkage for Ti, and the monomeric forms for W, 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.