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

Abstract

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.

Keywords: Carboxylic acid; Cerium; Manganese; Zirconium.

Figure 1

Modification of metal components in multi‐metal cluster for decarboxylative functionalization of carboxylic acids.

Figure 2

Molecular structure of 4a determined by X‐ray diffraction study (a, left) and DFT study (b, right). ^t Bu groups on the carboxylate ligands, and hydrogen atoms except for relating to the hydrogen bonding interaction are omitted for clarity. Green for cerium, blue for zirconium, red for oxygen, and gray for carbon atoms. Seven hydrogen bonding interactions are described as dot lines. a) Atoms in the CeZr₅O₄(OH)₄ core and carboxylate oxygen relating to the hydrogen bonding interaction with carboxylic acids are shown as thermal ellipsoids with 50% probability. Stick style is applied for other carboxylate moieties. Selected bond lengths (Å) of 4a: Ce − O1 2.170(3), Ce − O2 2488(3), Ce − O3 2160 (3), Ce − O4 2.350(3), Zr1 − O10 2.206(3), Zr1 − O11 2.108(3), Zr1 − O12 2.212(3), Zr1 − O13 2.034(3).

Figure 3

Time profiles for the formation of 3b.

Scheme 1

Plausible mechanism for decarboxylative hydrazination catalyzed by 4a and manganese salt.