Metallacarboranes as Photoredox Catalysts in Water

Metallacarboranes with the shape of the Greek letter θ, such as [Co(C₂ B₉ H₁₁ )₂ ]^- , were tested, for the first time, as efficient photoredox catalysts in the oxidation of aromatic and aliphatic alcohols in water. Their efficiency is linked to their high solubility in water, their high oxidizing power (Co^4+/3+ ), and their absence of fluorescence on excitation, among others. In most of the studied examples, using a catalyst load of 0.4 mol % gave high yields of 90-95 % with selectivity greater than 99 %. By reducing the catalyst load to 0.01 mol %, quantitative conversion of reactants to products was achieved, in some cases with greater than 99 % yield, high catalyst efficiency reaching a turnover number of 10 000, and a higher yield with a 45 times lower concentration of catalyst. The metallacarboranes can be recovered easily by precipitation on addition of [NMe₄ ]Cl. A pathway for the photoredox-catalyzed oxidation of alcohols is proposed.