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

Abstract

Abstract Decarboxylative hydrazination of carboxylic acids was achieved using a 1:5:2 ratio of three metal salts, Ce(O t Bu) 4 , Zr(O t Bu) 4 , and Mn(OAc) 3 , as a catalyst under visible light irradiation. The catalytic activity, compared with our previously developed Ce 6 cluster photo‐catalysts, was enhanced by the formation of single cerium‐incorporated hexanuclear mixed‐metal clusters containing a [CeZr 5 O 4 (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, CeZr 5 O 4 (OH) 4 (OCOCH 2 t Bu) 12 (HOCOCH 2 t Bu) 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 Ce 6 O 4 (OH) 4 (OCOCH 2 t Bu) 12 (HOCOCH 2 t Bu) 4 ( 4c ) and Mn(OAc) 3 or Ce(O t Bu) 4 , Zr(O t Bu) 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 CeZr 5 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 Zr 5 motif as a metallo‐ligand around the cerium(IV) site. Moreover, decarboxylative oxygenation of carboxylic acids under air followed by treatment with NaBH 4 resulted in the production of one‐carbon shortened alcohols in excellent yields when using Ce(O t Bu) 4 and Zr(O t Bu) 4 or Hf(O t Bu) 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.