Synthesis and Characterization of Co(II) Substituted Keggin-Type Polyoxometalates as Novel Catalysts for the Hydroformylation of 1-Hexene in a Thermomorphic Solvent System

Polyoxometalates (POMs) are a unique class of metal oxides, being of huge interest for the catalysis society. Co^II-substituted phosphomolybdate POMs (namely H₇[PCoMo₁₁O₄₀], H₁₁[PCo₂Mo₁₀O₄₀] and H₁₅[PCo₃Mo₉O₄₀]) have been successfully synthesized for the first time. The structure of the new Co^II substituted POMs was solved by single-crystal X-ray diffraction as well as vibrational (FT-IR and Raman) and nuclear magnetic/electron paramagnetic resonance (NMR and EPR) spectroscopy and found to be of the Keggin-type. The Co-POMs were then applied as molecular catalysts for the hydroformylation of 1-hexene in a thermomorphic solvent mixture of water and 1-butanol. This is the very first example of a 1^st row transition metal POM acting as a hydroformylation catalyst achieving 46 % heptanal yield. The best results were obtained using H₁₁[PCo₂Mo₁₀O₄₀] as a catalyst at 140 °C, 150 bar CO/H₂ 1 : 2, and a reaction time of 4 h, paving the way for new POM-based catalysts in hydroformylation reactions.