Cobalt Complex-Directed Self-Assembly of a Polyoxometalate-Based Species: Influence of Synthetic Methods on the Structure and Properties of Hybrid Assemblies

A study of the influence of a counterion, mono- and dicarboxylic acid, the addition of the MoO₄ ^2- anion, and synthetic routes on the formation of polyoxo(molybdo)vanadates was presented. Using four Co^III(ammine) complex cations in acidic aqueous solutions of VO₃ ^-, 10 crystalline solids with seven different oxovanadate anions were isolated: [H₂V₁₀O₂₈]^4- (in <b>1</b> and <b>5</b>), [V₁₀O₂₈]^6- (in <b>2</b>, <b>6</b> and <b>7</b>), [H₃V₁₀O₂₈Na(H₂O)₂] <i>_n</i> ^<i>n</i>- (in <b>3</b>), [H₂V₁₀O₂₈Na(H₂O)₈]^3- (in <b>4</b>), [V₄O₁₂]^4- (in <b>8</b>), [V₂O₆Na(H₂O)] <i>_n</i> ^<i>n</i>- (in <b>9</b>), and [V₃O₉]_n ^3- (in <b>10</b>), as well as two molybdovanadate anions [H₂Mo₈V₅O₄₀Na₂(H₂O)₈]^2- (in <b>11</b>) and [HMo₂V₇O₂₇]^6- (<b>12</b>). The oxovanadate species found in solids mostly correspond with the predominant species present in solution at a specific pH. Mechanochemically accelerated vapor-assisted aging and transformation from the amorphous precipitate to the crystalline products was demonstrated to be the efficient method for isolating the intermediates. Thus, in the reaction involving [Co(C₂O₄)(NH₃)₄]⁺, ammonium vanadate, sodium molybdate, and any carboxylic acid, the protonated decavanadate anions [H₂V₁₀O₂₈]^4- and [H₂V₁₀O₂₈Na(H₂O)₈]^2- transform to [H₃V₁₀NaO₂₈(H₂O)₂] <i>_n</i> ^<i>n</i>- and [H₂Mo₈V₅O₄₀]^5-. All of the products were characterized in the solid state via single-crystal X-ray diffraction, infrared spectroscopy, and thermogravimetric and elemental analyses. The oxovanadates <b>1</b>, <b>2</b>, <b>5</b>, <b>10</b>, and molybdovanadate <b>11</b> were also examined as catalysts for the oxidation of benzyl alcohol. The results of catalytic reactions showed that polyoxometalates as catalysts exhibit good selectivity but limited activity. In addition, for four decavanadates, <b>1</b>, <b>2</b>, <b>5</b>, and <b>6</b>, the electrostatic potential was mapped on the calculated electron isodensity surfaces. The reaction profiles for their synthesis were investigated in detail using quantum chemical calculation.