C–H Bond Activation Mechanism by a Pd(II)–(μ-O)–Au(0) Structure Unique to Heterogeneous Catalysts

Abstract

We focused on identifying a catalytic active site structure at the atomic level and elucidating the mechanism at the elementary reaction level of liquid-phase organic reactions with a heterogeneous catalyst. In this study, we experimentally and computationally investigated efficient C–H bond activation for the selective aerobic α,β-dehydrogenation of saturated ketones by using a Pd–Au bimetallic nanoparticle catalyst supported on CeO 2 (Pd/Au/CeO 2 ) as a case study. Detailed characterization of the catalyst with various observation methods revealed that bimetallic nanoparticles formed on the CeO 2 support with an average size of about 2.5 nm and comprised a Au nanoparticle core and PdO nanospecies dispersed on the core. The formation mechanism of the nanoparticles was clarified through using several CeO 2 -supported controlled catalysts. Activity tests and detailed characterizations demonstrated that the dehydrogenation activity increased with the coordination numbers of Pd–O species in the presence of Au(0) species. Such experimental evidence suggests that a Pd(II)–(μ-O)–Au(0) structure is the true active site for this reaction. Based on density functional theory calculations using a suitable Pd 1 O 2 Au 12 cluster model with the Pd(II)–(μ-O)–Au(0) structure, we propose a C–H bond activation mechanism via concerted catalysis in which the Pd atom acts as a Lewis acid and the adjacent μ-oxo species acts as a Brønsted base simultaneously. The calculated results reproduced the experimental results for the selective formation of 2-cyclohexen-1-one from cyclohexanone without forming phenol, the regioselectivity of the reaction, the turnover-limiting step, and the activation energy.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

“…502 The dehydrogenation of cyclohexanone is greatly enhanced by modifying the surface of Au nanoparticles with Pd(II) species, because the Pd(II) sites can activate the C−H bonds and the metallic Au surface can facilitate the β-H elimination step (see Figure 50). 498,503,504 One difference between core−shell bimetallic structures and metal-oxide interfacial sites is the chemical states of the oxidized species which can serve as the Lewis acid sites, leading to the generation of bifunctional catalysts. This synergy is reflected with the bimetallic Ru-ReOx/SiO 2 catalyst developed for production of secondary alcohols from 1,2-alkanediol by C−O hydrogenolysis.…”

“…The dehydrogenation of cyclohexanone is greatly enhanced by modifying the surface of Au nanoparticles with Pd­(II) species, because the Pd­(II) sites can activate the C–H bonds and the metallic Au surface can facilitate the β-H elimination step (see Figure ). ,, Interestingly, the synergy of Au and Pd cannot be achieved with AuPd bimetallic nanoparticles, because the rate-limiting step, activation of C–H bonds, is not favorable on bimetallic AuPd surface.…”

“…(c) Identification of the structure of the active sites in bimetallic Pd–Au/CeO 2 catalyst. Reproduced with permission from ref . Copyright 2022 American Chemical Society under CC-BY license ().…”

Decarbonylation of 1,2-Diketones to Diaryl Ketones via Oxidative Addition Enabled by an Electron-Deficient Au–Pd Nanoparticle Catalyst

“…Au–Pd/CeO 2 was prepared via an inert atmosphere reduction of a CeO 2 -supported Au and Pd hydroxide precursor [Au­(OH) x –Pd­(OH) x /CeO 2 ] using NaBH 4 in water (see the Supporting Information for details). , The Pd K-edge X-ray absorption near-edge structure (XANES) spectrum of Au–Pd/CeO 2 was similar to that of Pd foil, indicating the zero-valent nature of the Pd species (Figure a). Similarly, the presence of zero-valent Au species was confirmed by comparing the Au L III -edge XANES spectrum of Au–Pd/CeO 2 with that of Au foil (Figure b).…”

Supported Anionic Gold Nanoparticle Catalysts Modified Using Highly Negatively Charged Multivacant Polyoxometalates

“…This remarkable difference in regioselectivity of 2 b is possible to be explained by the priority on the reaction pathway (Figure S14); it is likely that amine oxidation followed by deprotonation is a dominant pathway using non-anionic Au/C leading to no specific selectivity, whereas, α-CÀ H activation followed by β-hydride elimination is preferred using anionic Au-SiW9/C, resulting in a high selectivity to 2 b via preferential formation of stable intermediates like enolate species at the methyl-substituted position. [49]…”