Investigation of the Deactivation and Reactivation Mechanism of a Heterogeneous Palladium(II) Catalyst in the Cycloisomerization of Acetylenic Acids by In Situ XAS 

Abstract

A well-studied heterogeneous palladium(II) catalyst used for the cycloisomerization of acetylenic acids is known to be susceptible to deactivation through reduction. To gain a deeper understanding of this deactivation process and to enable the design of a reactivation strategy, in situ X-ray absorption spectroscopy (XAS) was used. With this technique, changes in the palladium oxidation state and coordination environment could be studied in close detail, which provided experimental evidence that the deactivation was primarily caused by triethylamine-promoted reduction of palladium(II) to metallic palladium nanoparticles. Furthermore, it was observed that the choice of the acetylenic acid substrate influenced the distribution between palladium(II) and palladium(0) species in the heterogeneous catalyst after the reaction. From the mechanistic insight gained through XAS, an improved catalytic protocol was developed that did not suffer from deactivation and allowed for more efficient recycling of the catalyst.

Aligning Metal Coordination Sites in Metal–Organic Framework-Enabled Metallaphotoredox Catalysis

“…The first coordination sphere of Ni was further examined by EXAFS. The Fourier-transformed EXAFS of NH 2 -UiO-66, Ni@(NH 2 ) 2 -UiO-67, and Ni@NH 2 -MIL-125 displays dominant peaks at 1.59, 1.62, and 1.56 Å, respectively, typical of contributions from Ni–N­(O) scatterings (Figure e, please note that Ni–O and Ni–N could not be distinguished unambiguously by EXAFS) . The absence of the characteristic Ni–Ni peak at 2.12 Å (Ni foil) confirmed the isolated Ni sites in NH 2 -MOFs.…”

“…The Fourier-transformed EXAFS of NH 2 -UiO-66, Ni@(NH 2 ) 2 -UiO-67, and Ni@NH 2 -MIL-125 displays dominant peaks at 1.59, 1.62, and 1.56 Å, respectively, typical of contributions from Ni−N(O) scatterings (Figure 4e, please note that Ni−O and Ni−N could not be distinguished unambiguously by EXAFS). 42 The absence of the characteristic Ni−Ni peak at 2.12 Å (Ni foil) confirmed the isolated Ni sites in NH 2 -MOFs. The fitting of EXAFS gives a coordination number (CN) of 4.2 and an average Ni−N/O distance of 2.06 Å for Ni@NH 2 -UiO-66 (Figure 4f,g and Table 1), supportive of a (MOF) N/O and methanol (introduced during synthesis) ligated Ni (also predicted by DFT calculation, Figure 1a).…”

Cellulose-Supported Heterogeneous Gold-Catalyzed Cycloisomerization Reactions of Alkynoic Acids and Allenynamides

“… 6 Catalytic intramolecular cycloisomerization of alkynoic acids provides an easy atom-economical way for the synthesis of functionalized enol lactones. Different transition-metal complexes (Pd, 7 Au, 8 Ag, 9 Pt 10 ) have been shown to be able to catalyze the intramolecular cyclization reaction with high degrees of regio- and chemoselectivity.…”