Synthesis, characterization, and catalytic performance of cobalt and nickel-substituted polyoxometalates organic-inorganic nanohybrids

Two organic-inorganic hybrids, PDA/[SiW₉M₃(PDA)₃O₃₇], were synthesized through the self-assembly of the [SiW₉O₃₄]^10- anion and M = Co(II) (1), Ni(II) (2), in the presence of para-phenylenediamine (PDA) as the organic component, and characterized. The structure of the hybrids was investigated by means of FT-IR spectroscopy, X-ray diffraction analysis (XRD), field emission scanning electron (FE-SEM), energy-dispersive X-ray spectroscopy (EDX) and N₂ adsorption-desorption studies. To further investigate structural connectivity, the composites 1a and 2a were obtained by calcining 1 and 2 at 400 °C and studied. A comparison of nitrogen adsorption/desorption studies of 2a and 2 showed that the BET surface area and pore texture were improved by calcination. Catalytic activity studies showed that the as-synthesized organic-inorganic hybrids efficiently catalyzed the synthesis of various 1 H-5-substituted tetrazole derivatives in good to excellent yields (40-98%) under mild conditions. The catalysts were reused for up to five consecutive cycles with only a slight decrease in catalytic activity under the tested conditions. These results demonstrate that the prepared catalytic system can serve as a useful heterogeneous catalyst for tetrazole synthesis.