Mixed-Metal Ce-Zr-Mn Clusters as Photo-Catalysts for Decarboxylative Functionalization of Carboxylic Acids

Decarboxylative hydrazination of carboxylic acids was achieved using a 1:5:2 ratio of three metal salts, Ce(O^tBu)₄, Zr(O^tBu)₄, and Mn(OAc)₃, as a catalyst under visible light irradiation. The catalytic activity, compared with our previously developed Ce₆ cluster photo-catalysts, was enhanced by the formation of single cerium-incorporated hexanuclear mixed-metal clusters containing a [CeZr₅O₄(OH)₄]¹²⁺ core. The manganese salts further accelerated the overall reaction rate (10 times faster reaction rate with the manganese salt than that of the manganese-free conditions). Using the isolated cluster, CeZr₅O₄(OH)₄(OCOCH₂ ^tBu)₁₂(HOCOCH₂ ^tBu)₄ (4a), with Mn(OAc)₃, phenol and thiophenol-containing carboxylic acids were transformed to their decarboxylative hydrazinated products in moderate to high yields, while a mixture of Ce₆O₄(OH)₄(OCOCH₂ ^tBu)₁₂(HOCOCH₂ ^tBu)₄ (4c) and Mn(OAc)₃ or Ce(O^tBu)₄, Zr(O^tBu)₄, and Mn(OAc)₃ yielded lower amounts of the products. These findings highlight the importance of incorporating cerium(IV) into the zirconium-based core to tolerate these easily oxidizable functional groups. Upon exposure of 4a to blue LED light under an argon atmosphere, the CeZr₅ cluster produced 2,2,5,5-tetramethylhexane, a radical coupling product derived from the carboxylate ligand on 4a, in half an equivalent per cluster, consistent with the photo-reduction of cerium(IV) and inertness of the oxo- and hydroxo-bridged Zr₅ motif as a metallo-ligand around the cerium(IV) site. Moreover, decarboxylative oxygenation of carboxylic acids under air followed by treatment with NaBH₄ resulted in the production of one-carbon shortened alcohols in excellent yields when using Ce(O^tBu)₄ and Zr(O^tBu)₄ or Hf(O^tBu)₄ in a 1:5 ratio: the reaction rates were 8-10 times higher than that of the previously developed cerium-catalyzed reaction under identical conditions.