A successful homogeneous photoredox catalyst has been fruitfully heterogenized on magnetic nanoparticles (MNPs) coated with a silica layer, keeping intact its homogeneous catalytic properties but gaining others due to the easy magnetic separation and recyclability. The amine-terminated magnetic silica nanoparticles linked noncovalently to H[3,3'-Co(1,2-C2B9H11)2]- (H[1]), termed MSNPs-NH2@H[1], are highly stable and do not produce any leakage of the photoredox catalyst H[1] in water. The magnetite MNPs were coated with SiO2 to provide colloidal stability and silanol groups to be tethered to amine-containing units. These were the MSNPs-NH2 on which was anchored, in water, the cobaltabis(dicarbollide) complex H[1] to obtain MSNPs-NH2@H[1]. Both MSNPs-NH2 and MSNPs-NH2@H[1] were evaluated to study the morphology, characterization, and colloidal stability of the MNPs produced. The heterogeneous MSNP-NH2@H[1] system was studied for the photooxidation of alcohols, such as 1-phenylethanol, 1-hexanol, 1,6-hexanediol, or cyclohexanol among others, using catalyst loads of 0.1 and 0.01 mol %. Surfactants were introduced to prevent the aggregation of MNPs, and cetyl trimethyl ammonium chloride was chosen as a surfactant. This provided adequate stability, without hampering quick magnetic separation. The results proved that the catalysis could be speeded up if aggregation was prevented. The recyclability of the catalytic system was demonstrated by performing 12 runs of the MSNPs-NH2@H[1] system, each one without loss of selectivity and yield. The cobaltabis(dicarbollide) catalyst supported on silica-coated magnetite nanoparticles has proven to be a robust, efficient, and easily reusable system for the photooxidation of alcohols in water, resulting in a green and sustainable heterogeneous catalytic system.
Keywords: heterogeneous catalysis; metallacarboranes; nanoparticles; photooxidation; surface functionalization.