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 CoIII(ammine) complex cations in acidic aqueous solutions of VO3 -, 10 crystalline solids with seven different oxovanadate anions were isolated: [H2V10O28]4- (in 1 and 5), [V10O28]6- (in 2, 6 and 7), [H3V10O28Na(H2O)2] n n- (in 3), [H2V10O28Na(H2O)8]3- (in 4), [V4O12]4- (in 8), [V2O6Na(H2O)] n n- (in 9), and [V3O9]n 3- (in 10), as well as two molybdovanadate anions [H2Mo8V5O40Na2(H2O)8]2- (in 11) and [HMo2V7O27]6- (12). 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] n n- 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 1, 2, 5, 10, and molybdovanadate 11 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, 1, 2, 5, and 6, 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.