Synthesis of Colloidal DMF-Protected Cobalt Nanoparticles for Alkene Hydrosilylation Catalyst: Effect of Cobalt Precursors and Recycling Process

We report the synthesis of cobalt nanoparticles (Co NPs) via a DMF-protecting method and their use as catalysts in hydrosilylation reactions. Various types of cobalt nanoparticles were synthesized from different precursors, namely, cobalt(II) acetate, cobalt(III) acetylacetonate, and cobalt(II) bromide, to give Co NPs-OAc, Co NPs-acac, and Co NPs-Br, respectively. A range of methods, e.g., annular dark-field scanning transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption fine structure, were used to investigate their chemical properties. The results indicated that these cobalt nanoparticles involved a structure of Co₃O₄ regardless of the type of metal precursor. Alternatively, the particle size distribution and their cohesion with each particle strongly depended on the metal precursor, resulting in different catalytic activity for alkene hydrosilylation. The Co NPs-OAc exhibited the highest activity for the transformation with at least 0.005 mol % catalyst loading in the Co NPs, where the turnover number was 13 800. In addition, we succeeded in catalyst recycling by using a convenient liquid-liquid extraction method. The recycled catalysts retained their catalytic activity for alkene hydrosilylation, comparable to that of the pristine catalysts.