Resumen de Carborane compounds as efficient light-driven in oxidation catalysis
This thesis studies the behavior of different metallacarboranes such as Na[Co(C2B9H11)2] Na[1] and their chlorinated derivatives, Na[3,3’-Co(8-Cl-1,2-C2B9H10)2] Na[Cl2-1] i Na[3,3’-Co(8,9,12-Cl3-1,2-C2B9H8)2] Na[Cl6-1], as photoredox catalysts in the oxidation of different alcohols and alkenes in aqueous media, which take place through single-electron transfer (SET) processes. In the major part of the catalytic experiments, high yield and selectivity values (>99%) have been obtained, even reducing the catalyst load. These results have been possible thanks to the high solubility of the catalysts in water, their high oxidizing power of the CoIV/III redox couple, their lack of fluorescence on excitation, and the surfactant behavior of the catalyst. Additionally, the metallacarboranes were easily recovered by precipitation on addition of [NMe4]Cl. Based on the results obtained, a mechanism for the photoredox catalytic procedure was proposed.
These catalysts have been supported on silica-coated amino functionalized magnetic nanoparticles, through non-covalent interactions, (MSNPs-NH2@H[1]), showing high water stability. The heterogeneous catalytic system was highly efficient for the photooxidation of alcohols, using catalyst loads of 0.1 and 0.01 mol% and cetyl trimethyl ammonium chloride (CTAC) as surfactant to prevent the aggregation of MNPs, providing stability without hindering the easy magnetic separation. The recyclability of the catalyst was succesful through 12 runs, maintaining the activity of the catalyst, as well as its easy magnetic separation. These results have demonstrated the robustness, efficiency and reusability of the supported cobaltabis(dicarbollide) compound, resulting in a green and sustainable heterogeneous catalytic system.
We have also synthesized the cooperative photoredox catalyst, [RuII(trpy)(bpy)(H2O)][3,3’-Co(1,2-C2B9H11)2]2, where cation (oxidation catalyst) and anion (photoredox catalyst) are linked by non-covalent interactions that persist in aqueous medium. An important electronic coupling between [RuII(trpy)(bpy)(H2O)]2+ and [3,3'-Co (1,2-C2B9H11)2]-, was detected by electrochemical studies in water. The cooperative photoredox system was efficient (TON = 20000) in the oxidation of alcohols in water, through proton-coupled electron transfer (PCET) processes, at room temperature under UV irradiation, using 0.005 mol% of catalyst, demonstrating better catalytic yield than the compounds separately. In addition, a mechanism for the photoredox catalytic process was proposed.
The metallacarborane Na[1] has been studied as photoredox catalyst for the oxidation of alkenes in water, obtaining high conversion vàlues, in short reaction times and moderate selectivities with respect to the epoxide, due to the formation of diols as by-products. Epoxide selectivity increased significantly when the catalyst concentration decreased, obtaining also high TON values. A comparison with the photosensitizer of Ru(II), [Ru(bpy)3]2+, showed the high efficiency of Na[1] compared to [Ru(bpy)3]2+, for the photooxidation of alkenes in water. Preliminary photooxidation experiments using methyl oleate as substrate, led to selective epoxidation of the substrate. These data represent a promising starting point for the development of practical methods for the processing of unsaturated fatty acids, such as the valorisation of animal fat waste by using this sustainable photoredox catalyst.
We have also investigated the photocatalytic behaviour of [3,3’-Fe(1,2-C2B9H11)2]- [2]- in the oxidation of alcohols and alkenes in water, using low loads of catalyst, resulting an efficient and selective photoredox catalysts in both processes. This behaviour is possible thanks to the properties of Na[2], as their high solubility in water, and its surfactant behaviour, its molecular compactness, its lack of photoluminescence, and the high oxidizing power for the couple Fe4+/3+ high enough to oxidize substrates, and at the same time to be able to obtain high selectivity values for the corresponding aldehydes, ketones or epoxides. Our results demonstrate that Na[2] is an efficient photocatalyst based in a green, cheap and abundant metal that operates in water in relatively short reaction times, and really small amounts of photocatalyst.
