Pd‐Catalyzed Z‐Selective Semihydrogenation of Alkynes: Determining the Type of Active Species

Abstract

A protocol was developed to distinguish between well‐defined molecular and nanoparticle‐based catalysts for the Pd‐catalyzed semihydrogenation reaction of alkynes to Z‐alkenes. The protocol applies quantitative partial poisoning and dynamic light scattering methods, which allow the institution of additional validation experiments. For the quantitative partial poisoning method, tetramethylthiourea (TMTU) was developed as an alternative for the standard poison ligand CS2, and was found to be superior in its applicability. The protocol and the TMTU poison ligand were validated using the well‐described [PdII(phenanthroline)]‐catalyzed copolymerization of styrene and CO, confirming that this system is clearly operating as a well‐defined molecular catalyst. The protocol was subsequently applied to three catalyst systems used for the semihydrogenation of alkynes. The first was proposed to be a molecular [Pd0(IMes)] catalyst that uses molecular hydrogen, but the data gathered for this system, following the new protocol, clearly showed that nanoparticles (NPs) are catalytically active. The second catalyst system studied was an N‐heterocyclic carbene (NHC) Pd system for transfer semihydrogenation using formic acid as the hydrogen source, which was proposed to operate through an in situ generated molecular [Pd0(IMes)] catalyst in earlier studies. The investigations showed that only a small fraction of the Pd added becomes active in the catalytic reaction and that NPs are formed. However, despite these findings, a clear distinction between catalytic activity of NPs versus a molecular catalyst could not be made. The third investigated system is based on a [PdII(IMes)(η3‐allyl)Cl] precatalyst with additive ligands. The combined data gathered for this system are multi‐interpretable, but suggest that a partially deactivated molecular catalyst dominates in this reaction.

Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes

“…For instance, Medlin and co-workers have demonstrated that the catalytic properties of Pd and Pt catalysts can be well controlled by coating n-alkane thiols on their surfaces. [33][34][35][36] Although the importance of thiols or other sulfur-containing species in promoting palladium catalysis has been well understood in some Pd-complex systems, [37][38][39] determination of the detailed surface and interface structure of thiolated-protected Pd nanocatalysts, and thus understanding the molecular mechanism behind the catalytic enhancements, remains challenging.…”

Pd‐Nanocomposites Formed by Calcination of [Pd(2‐pymo)2]n Framework as Catalysts of Phenylacetylene Semihydrogenation in Water

“…It should be noted that a distinction between catalytic activity of Pd NPs and a molecular catalyst in phenylacetylene semihydrogenation is often problematic. [41] Interaction of the [Pd-(pymo) 2 ] n with BH 4 À resulted in formation of hydrido species, active in molecular hydrogenation. However, during formation of styrene, the MOF structure collapses forming Pd NPs which next catalyze the reaction.…”

Hydrogenation of carboxylic acids with a homogeneous cobalt catalyst

“…The To further investigate the reaction mechanism, we performed partial poisoning studies using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a radical inhibitor and TMTU (tetramethylthiourea) as a strongly binding poison (28). With both reagents, the reaction was completely inhibited by one equivalent per catalyst, and for TMTU, only half an equivalent was sufficient ( fig.…”