Aqueous colloidal cobalt nanoparticles

Later, Chung et al. successfully developed an intramolecular Pauson-Khand reaction in water without any cosolvent by using aqueous colloidal cobalt nanoparticles as catalysts. The catalyst was prepared by reducing an aqueous solution of cobalt acetate containing sodium dode-cyl sulfate (SDS) surfactant. The cobalt nanoparticle could be reused eight times without any loss of catalytic activity (Eq. 4.57).107... 

S. Son, S. Lee, Y. Chung, S. Kim, and T Hyeon, The first intramolecular Pauson—Khand reaction in water using aqueous colloidal cobalt nanoparticles as catalysts. Organ. Lett. 4,277—279 (2002). 

Several non-carbonyl cobalt sources used recently show high efficiency in the catalysis of the PKR. Chung has reported different reusable catalysts like cobalt supported on mesoporous silica or on charcoal that work under high CO pressures [ 127]. Most recently they have described milder conditions with the use of colloidal cobalt nanoparticles, which react at lower CO pressures and can be used in aqueous media [128]. 

As described at the end of the last section, we came upon the study of metal oxide nanoparticles in aqueous solution accidentally. Our only earlier experience with this class of nanoparticles, although in organic solvents such as THF, was the investigation of the controlled 02-mediated oxidation of tetraalkylammonium bromide stabilized cobalt colloids prepared size-selectively by the electrochemical method (Fig. 8.5) [53]. 

Two systems were used by Chen et al. [428] for synthesis of cobalt metal nanoparticles. One of them was the well-known system AOT/isooctane/aqueous solution. Two reverse microemulsions (w = 11) were prepared with 0.3M C0CI2 or 0.6M NaBH4 dissolved in water as the aqueous phase. The microemulsions, when mixed, yielded a black colloid. Removal of the isooctane led to the formation of a paste containing Co particles in AOT. The surfactant was removed by washing with water. The product in this form, or as a powdery material, yielded 3-4 nm sized Co particles. Heating at 550°C/2 h produced crystalline Co. 

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