Modern Trends in Catalyst and Process Design for Alkyne Hydrogenations

Incorporation of Pd Single-Atom Sites in Perovskite with an Excellent Selectivity toward Photocatalytic Semihydrogenation of Alkynes

Semihydrogenation is a crucial industrial process. Noble metals such as Pd have been extensively studied in semihydrogenation reactions, owing to their unique catalytic activity toward hydrogen activation. However, the overhydrogenation of alkenes to alkanes often happens due to the rather strong adsorption of alkenes on Pd active phases. Herein, we demonstrate that the incorporation of Pd active phases as single-atom sites in perovskite lattices such as SrTiO3 can greatly alternate the electronic structure and coordination environment of Pd active phases to facilitate the desorption of alkenes rather than further hydrogenation. Furthermore, the incorporated Pd sites can be well stabilized without sintering by a strong host-guest interaction with SrTiO3 during the activation of H species in hydrogenation reactions. As a result, the Pd incorporated SrTiO3 (Pd-SrTiO3) exhibits an excellent time-independent selectivity (>99.9 %) and robust durability for the photocatalytic semihydrogenation of phenylacetylene to styrene. This strategy based on incorporation of active phases in perovskite lattices will have broad implications in the development of high-performance photocatalysts for selective hydrogenation reactions.

Encapsulation of Pd Single-Atom Sites in Zeolite for Highly Efficient Semihydrogenation of Alkynes

Palladium (Pd)-based single-atom catalysts (SACs) have shown outstanding selectivity for semihydrogenation of alkynes, but most Pd single sites coordinated with highly electronegative atoms (such as N, O, and S) of supports will result in a decrease in the electron density of Pd sites, thereby weakening the adsorption of reactants and reducing catalytic performance. Constructing a rich outer-shell electron environment of Pd single-atom sites by changing the coordination structure offers a novel opportunity to enhance the catalytic efficiency with excellent alkene selectivity. Therefore, in this work, we first propose the in situ preparation of isolated Pd sites encapsulated within Al/Si-rich ZSM-5 structure using the one-pot seed-assisted growth method. Pd1@ZSM-5 features Pd-O-Al/Si bonds, which can boost the domination of d-electron near the Fermi level, thereby promoting the adsorption of substrates on Pd sites and reducing the energy barrier for the semihydrogenation of alkynes. In semihydrogenation of phenylacetylene, Pd1@ZSM-5 catalyst performs the highest turnover frequency (TOF) value of 33582 molC═C/molPd/h with 96% selectivity of styrene among the reported heterogeneous catalysts and nearly 17-fold higher than that of the commercial Lindlar catalyst (1992 molC═C/molPd/h). This remarkable catalytic performance can be retained even after 6 cycles of usage. Particularly, the zeolitic confinement structure of Pd1@ZSM-5 enables precise shape-selective catalysis for alkyne reactants with a size less than 4.3 Å.

Structure evolution and specific effects for the catalysis of atomically ordered intermetallic compounds

Atomically ordered intermetallic compounds (IMCs) have been extensively studied for exploring catalysts with high activity, selectivity, and longevity. Compared to random alloys, IMCs present a more pronounced geometric and electronic effect with desirable catalytic performance. Their well-defined structure makes IMCs ideal model catalysts for studying the catalytic mechanism. This review focuses especially on elemental composition, electron transfer, and structure/phase evolution under high temperature treatment conditions, providing direct evidence for the migration and rearrangement of metal atoms through electron microscopy. We then present the outstanding applications of IMCs in growing single-walled nanotubes, hydrogenation/dehydrogenation reactions, and electrocatalysis from the perspective of electronic, geometric, strain, and bifunctional effects of ordered IMCs. Finally, the current obstacles associated with the use of in situ techniques are proposed, as well as future research possibilities.

Metal-Ligand Cooperation with Thiols as Transient Cooperative Ligands: Acceleration and Inhibition Effects in (De)Hydrogenation Reactions

ConspectusOver the past two decades, we have developed a series of pincer-type transition metal complexes capable of activating strong covalent bonds through a mode of reactivity known as metal-ligand cooperation (MLC). In such systems, an incoming substrate molecule simultaneously interacts with both the metal center and ligand backbone, with one part of the molecule reacting at the metal center and another part at the ligand. The majority of these complexes feature pincer ligands with a pyridine core, and undergo MLC through reversible dearomatization/aromatization of this pyridine moiety. This MLC platform has enabled us to perform a variety of catalytic dehydrogenation, hydrogenation, and related reactions, with high efficiency and selectivity under relatively mild conditions.In a typical catalytic complex that operates through MLC, the cooperative ligand remains coordinated to the metal center throughout the entire catalytic process, and this complex is the only catalytic species involved in the reaction. As part of our ongoing efforts to develop new catalytic systems featuring MLC, we have recently introduced the concept of transient cooperative ligand (TCL), i.e., a ligand that is capable of MLC when coordinated to a metal center, but the coordination of which is reversible rather than permanent. We have thus far employed thiol(ate)s as TCLs, in conjunction with an acridanide-based ruthenium(II)-pincer catalyst, and this has resulted in remarkable acceleration and inhibition effects in various hydrogenation and dehydrogenation reactions. A cooperative thiol(ate) ligand can be installed in situ by the simple addition of an appropriate thiol in an amount equivalent to the catalyst, and this has been repeatedly shown to enable efficient bond activation by MLC without the need for other additives, such as base. The use of an ancillary thiol ligand that is not fixed to the pincer backbone allows the catalytic system to benefit from a high degree of tunability, easily implemented by varying the added thiol. Importantly, thiols are coordinatively labile enough under typical catalytic conditions to leave a meaningful portion of the catalyst in its original unsaturated form, thereby allowing it to carry out its own characteristic catalytic activity. This generates two coexisting catalyst populations─one that contains a thiol(ate) ligand and another that does not─and this may lead to different catalytic outcomes, namely, enhancement of the original catalytic activity, inhibition of this activity, or the occurrence of diverging reactivities within the same catalytic reaction mixture. These thiol effects have enabled us to achieve a series of unique transformations, such as thiol-accelerated base-free aqueous methanol reforming, controlled stereodivergent semihydrogenation of alkynes using thiol as a reversible catalyst inhibitor, and hydrogenative perdeuteration of C═C bonds without using D2, enabled by a combination of thiol-induced acceleration and inhibition. We have also successfully realized the unprecedented formation of thioesters through dehydrogenative coupling of alcohols and thiols, as well as the hydrogenation of organosulfur compounds, wherein the cooperative thiol serves as a reactant or product. In this Account, we present an overview of the TCL concept and its various applications using thiols.

Structural Design of Single-Atom Catalysts for Enhancing Petrochemical Catalytic Reaction Process

Petroleum, as the "lifeblood" of industrial development, is the important energy source and raw material. The selective transformation of petroleum into high-end chemicals is of great significance, but still exists enormous challenges. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites, promise a broad application in petrochemical processes. Herein, the research systematically summarizes the recent research progress of SACs in petrochemical catalytic reaction, proposes the role of structural design of SACs in enhancing catalytic performance, elucidates the catalytic reaction mechanisms of SACs in the conversion of petrochemical processes, and reveals the high activity origins of SACs at the atomic scale. Finally, the key challenges are summarized and an outlook on the design, identification of active sites, and the appropriate application of artificial intelligence technology is provided for achieving scale-up application of SACs in petrochemical process.

Efficient Alkyne Semihydrogenation Catalysis Enabled by Synergistic Chemical and Thermal Modifications of a PdIn MOF

Recently, there has been a growing interest in using MOF templating to synthesize heterogeneous catalysts based on metal nanoparticles on carbonaceous supports. Unlike the common approach of direct pyrolysis of PdIn-MOFs at high temperatures, this work proposes a reductive chemical treatment under mild conditions before pyrolysis (resulting in PdIn-QT). The resulting material (PdIn-QT) underwent comprehensive characterization via state-of-the-art aberration-corrected electron microscopy, N2 physisorption, X-ray absorption spectroscopy, Raman, X-ray photoelectron spectroscopy, and X-ray diffraction. These analyses have proven the existence of PdIn bimetallic nanoparticles supported on N-doped carbon. In situ DRIFT spectroscopy reveals the advantageous role of indium (In) in regulating Pd activity in alkyne semihydrogenation. Notably, incorporating a soft nucleation step before pyrolysis enhances surface area, porosity, and nitrogen content compared to direct MOF pyrolysis. The optimized material exhibits outstanding catalytic performance with 96% phenylacetylene conversion and 96% selectivity to phenylethylene in the fifth cycle under mild conditions (5 mmol phenylacetylene, 7 mg cat, 5 mL EtOH, R.T., 1 H2 bar).

Encapsulation of Palladium Carbide Subnanometric Species in Zeolite Boosts Highly Selective Semihydrogenation of Alkynes

The selective hydrogenation of alkynes to alkenes is a crucial step in the synthesis of fine chemicals. However, the widely utilized palladium (Pd)-based catalysts often suffer from poor selectivity. In this work, we demonstrate a carbonization-reduction method to create palladium carbide subnanometric species within pure silicate MFI zeolite. The carbon species can modify the electronic and steric characteristics of Pd species by forming the predominant Pd-C4 structure and, meanwhile, facilitate the desorption of alkenes by forming the Si-O-C structure with zeolite framework, as validated by the state-of-the-art characterizations and theoretical calculations. The developed catalyst shows superior performance in the selective hydrogenation of alkynes over mild conditions (298 K, 2 bar H2 ), with 99 % selectivity to styrene at a complete conversion of phenylacetylene. In contrast, the zeolite-encapsulated carbon-free Pd catalyst and the commercial Lindlar catalyst show only 15 % and 14 % selectivity to styrene, respectively, under identical reaction conditions. The zeolite-confined Pd-carbide subnanoclusters promise their superior properties in semihydrogenation of alkynes.

Photoinduced NiH Catalysis with Trialkylamines for the Stereodivergent Transfer Semi-Hydrogenation of Alkynes

We report a selectivity-switchable nickel hydride-catalyzed methodology that enables the stereocontrolled semi-reduction of internal alkynes to E- or Z-alkenes under very mild conditions. The proposed transfer semi-hydrogenation process involves the use of a dual nickel/photoredox catalytic system and triethylamine, not only as a sacrificial reductant, but also as a source of hydrogen atoms. Mechanistic studies revealed a pathway involving photo-induced generation of nickel hydride, syn-hydronickelation of alkyne, and alkenylnickel isomerization as key steps. Remarkably, mechanistic experiments indicate that the control of the stereoselectivity is not ensuing from a post-reduction alkene photoisomerization under our conditions. Instead, we demonstrate that the stereoselectivity of the reaction is dependent on the rate of a final protonolysis step which can be tuned by adjusting the pKa of an alcohol additive.

In Situ DRIFTS Analysis during Hydrogenation of 1-Pentyne and Olefin Purification with Ag Nanoparticles

The catalytic performance of nanoparticles (NPs) of Ag anchored on different supports was evaluated during the selective hydrogenation of 1-pentyne and the purification of a mixture of 1-pentene/1-pentyne (70/30 vol %). The catalysts were identified: Ag/Al (Ag supported on ɣ-Al2 O3 ), Ag/Al-Mg (Ag supported on ɣ-Al2 O3 modified with Mg), Ag/Ca (Ag supported on CaCO3 ) and Ag/RX3 (Ag supported on activated carbon-type: RX3). In addition, in situ DRIFTS analysis of 1-pentyne adsorption on each support, catalyst, and 1-pentyne hydrogenation were investigated. The results showed that the synthesized catalysts were active and very selective (≥85 %) for obtaining the desired product (1-pentene). Different adsorbed species (-C≡C- and -C=C-) were observed on the supports and catalysts surface using in situ DRIFT analysis, which can be correlated to the activity and high selectivity reached. The role of the supports and electronic properties over Ag improve the H2 dissociative chemisorption during the hydrogenation reactions; promoting the selectivity and the high catalytic performance. Ag/Al and Ag/Al-Mg were the most active catalysts. This was due to the synergism between the active Ag/Ag+ species and the supports (electronic effects). The results show that Ag/Al and Ag/Al-Mg catalysts have favorable properties and are promising for the alkyne hydrogenation and olefin purification reactions.

Modulation of metal species as control point for Ni-catalyzed stereodivergent semihydrogenation of alkynes with water

A base-assisted metal species modulation mechanism enables Ni-catalyzed stereodivergent transfer semihydrogenation of alkynes with water, delivering both olefinic isomers smoothly using cheap and nontoxic catalysts and additives. Different from most precedents, in which E-alkenes derive from the isomerization of Z-alkene products, the isomers were formed in orthogonal catalytic pathways. Mechanistic studies suggest base as a key early element in modulation of the reaction pathways: by adding different bases, nickel species with disparate valence states could be accessed to initiate two catalytic cycles toward different stereoisomers. The practicability of the method is showcased with nearly 70 examples, including internal and terminal triple bonds, enynes and diynes, affording semi-hydrogenated products in high yields and selectivity.

Single-Atom Catalysts for Hydrogen Activation

Selective hydrogenation is one of the most important reactions in fine chemical industry, and the activation of H₂ is the key step for hydrogenation. Catalysts play a critical role in selective hydrogenation, and some single-atom catalysts (SACs) are highly capable of activating H₂ in selective hydrogenation by virtue of the maximized atom utilization and the highly uniform active sites. Therefore, more research efforts are needed for the rational design of SACs with superior H₂ -activating capabilities. Herein, the research progress on H₂ activation in typical hydrogenation systems (such as alkyne hydrogenation, hydroformylation, hydrodechlorination, hydrodeoxygenation, nitroaromatics hydrogenation, and polycyclic aromatics hydrogenation) is reviewed, the mechanisms of SACs for H₂ activation are summarized, and the structural regulation strategies for SACs are proposed to promote H₂ activation and provide schemes for the design of high-selectivity hydrogenation catalysts from the atomic scale. At the end of this review, an outlook on the opportunities and challenges for SACs to be developed for selective hydrogenation is presented.

Advances in Selective Electrocatalytic Hydrogenation of Alkynes to Alkenes

Selective electrochemical hydrogenation of alkynes to alkenes under ambient conditions is a promising alternative to the traditional energy-intensive and high-cost thermocatalytic hydrogenation. However, the systematic summary on the electrocatalysts and electrolyzers remains lacked. Herein, we demonstrate a comprehensive review about recent achievements in the electrocatalysts including noble metal and non-noble-metal materials. Several effective strategies of catalyst design were developed to improve alkyne conversion, and alkene selectivity, for example, accelerating the formation of active hydrogen species, enhancing alkyne adsorption and suppressing the side reactions. Furthermore, the advantages and disadvantages of various electrolyzers are systematically discussed. Accordingly, major challenges and future trends in this field are proposed.

Controllable Deposition of Bi onto Pd for Selective Hydrogenation of Acetylene

The rational regulation of catalyst active sites at atomic scale is a key approach to unveil the relationship between structure and catalytic performance. Herein, we reported a strategy for the controllable deposition of Bi on Pd nanocubes (Pd NCs) in the priority order from corners to edges and then to facets (Pd NCs@Bi). The spherical aberration-corrected scanning transmission electron microscopy (ac-STEM) results indicated that Bi₂O₃ with an amorphous structure covers the specific sites of Pd NCs. When only the corners and edges of the Pd NCs were covered, the supported Pd NCs@Bi catalyst exhibited an optimal trade-off between high conversion and selectivity in the hydrogenation of acetylene to ethylene under ethylene-rich conditions (99.7% C₂H₂ conversion and 94.3% C₂H₄ selectivity at 170 °C) with remarkable long-term stability. According to the H₂-TPR and C₂H₄-TPD measurements, the moderate hydrogen dissociation and the weak ethylene adsorption are responsible for this excellent catalytic performance. Following these results, the selectively Bi-deposited Pd nanoparticle catalysts showed incredible acetylene hydrogenation performance, which provides a feasible perspective to design and develop highly selective hydrogenation catalysts for industrial applications.

Direct generation of polypyrrole-coated palladium nanoparticles inside a metal-organic framework for a semihydrogenation catalyst

Herein, the direct synthesis of polypyrrole (PPy)-coated palladium nanoparticles (PdNPs) inside a metal-organic framework (MIL-101) was successfully demonstrated. Owing to the PPy coating of PdNPs, the resulting composites exhibited higher semihydrogenation capability (selectivity: up to 96%) than the analog composite without PPy coating.

Chromium-catalyzed stereodivergent E- and Z-selective alkyne hydrogenation controlled by cyclic (alkyl)(amino)carbene ligands

The hydrogenation of alkynes allows the synthesis of olefins, which are important feedstock for the materials, pharmaceutical, and petrochemical industry. Thus, methods that enable this transformation via low-cost metal catalysis are desirable. However, achieving stereochemical control in this reaction is a long-standing challenge. Here, we report on the chromium-catalyzed E- and Z-selective olefin synthesis via hydrogenation of alkynes, controlled by two carbene ligands. A cyclic (alkyl)(amino)carbene ligand that contains a phosphino anchor enables the hydrogenation of alkynes in a trans-addition manner, selectively forming E-olefins. With an imino anchor-incorporated carbene ligand, the stereoselectivity can be switched, giving mainly Z-isomers. This ligand-enabled geometrical stereoinversion strategy by one metal catalysis overrides common methods in control of the E- and Z-selectivity with two different metal catalysis, allowing for highly efficient and on-demand access to both E- and Z-olefins in a stereo-complementary fashion. Mechanistic studies indicate that the different steric effect between these two carbene ligands may mainly dominate the selective forming E- or Z-olefins in control of the stereochemistry.

(NHC-olefin)-nickel(0) nanoparticles as catalysts for the (Z)-selective semi-hydrogenation of alkynes and ynamides

Nickel(0) nanoparticles coordinated to NHC ligands bearing N-coordinated cinnamyl moieties were readily prepared by reduction of a [NiCpBr(NHC-cinnamyl)] complex with methyl magnesium bromide. The combination of a strong σ-donor NHC ligand with a π-coordinating appended cinnamyl moiety likely prevents nickel(0) nanoparticle aggregation to larger inactive species, and allows the effective and (Z)-selective semi-hydrogenation of alkynes and ynamides.

Parts-Per-Million of Soluble Pd0 Catalyze the Semi-Hydrogenation Reaction of Alkynes to Alkenes

The synthesis of cis-alkenes is industrially carried out by selective semi-hydrogenation of alkynes with complex Pd catalysts, which include the Lindlar catalyst (PdPb on CaCO₃) and c-Pd/TiS (colloidal ligand-protected Pd nanoparticles), among others. Here, we show that Pd⁰ atoms are generated from primary Pd salts (PdCl₂, PdSO₄, Pd(OH)₂, PdO) with H₂ in alcohol solutions, independently of the alkyne, to catalyze the semi-hydrogenation reaction with extraordinarily high efficiency (up to 735 s^-1), yield (up to 99%), and selectivity (up to 99%). The easy-to-prepare Pd⁰ species hold other potential catalytic applications.

E-selective Semi-hydrogenation of Alkynes under Mild Conditions by a Diruthenium Hydride Complex

The synthesis, characterization and catalytic activity of a new class of diruthenium hydrido carbonyl complexes bound to the tBu PNNP expanded pincer ligand is described. Reacting tBu PNNP with two equiv of RuHCl(PPh3 )3 (CO) at 140 °C produces an insoluble air-stable complex, which was structurally characterized as [Ru2 (tBu PNNP)H(μ-H)Cl(μ-Cl)(CO)2 ] (1) using solid-state NMR, IR and X-ray absorption spectroscopies and follow-up reactivity. A reaction with KOtBu results in deprotonation of a methylene linker to produce [Ru2 (tBu PNNP* )H(μ-H)(μ-OtBu)(CO)2 ] (3) featuring a partially dearomatized naphthyridine core. This enables metal-ligand cooperative activation of H2 analogous to the mononuclear analogue, [Ru(tBu PNP*)H(CO)]. In contrast to the mononuclear system, the bimetallic analogue 3 catalyzes the E-selective semi-hydrogenation of alkynes at ambient temperature and atmospheric H2 pressure with good functional group tolerance. Monitoring the semi-hydrogenation of diphenylacetylene by 1 H NMR spectroscopy shows the intermediacy of Z-stilbene, which is subsequently isomerized to the E-isomer. Initial findings into the mode of action of this system are provided, including the spectroscopic characterization of a polyhydride intermediate and the isolation of a deactivated species with a partially hydrogenated naphthyridine backbone.

Unveiling the Local Structure and Electronic Properties of PdBi Surface Alloy for Selective Hydrogenation of Propyne

Building a reliable relationship between the electronic structure of alloyed metallic catalysts and catalytic performance is important but remains challenging due to the interference from many entangled factors. Herein, a PdBi surface alloy structural model, by tuning the deposition rate of Bi atoms relative to the atomic interdiffusion rate at the interface, realizes a continuous modulation of the electronic structure of Pd. Using advanced X-ray characterization techniques, we provide a precise depiction of the electronic structure of the PdBi surface alloy. As a result, the PdBi catalysts show enhanced propene selectivity compared with the pure Pd catalyst in the selective hydrogenation of propyne. The prevented formation of saturated β-hydrides in the subsurface layers and weakened propene adsorption on the surface contribute to the high selectivity. Our work provides in-depth understanding of the electronic properties of surface alloy structure and underlies the study of the electronic structure-performance relationship in bimetallic catalysts.

Emerging single-atom iron catalysts for advanced catalytic systems

Due to the elusive structure-function relationship, traditional nanocatalysts always yield limited catalytic activity and selectivity, making them practically difficult to replace natural enzymes in wide industrial and biomedical applications. Accordingly, single-atom catalysts (SACs), defined as catalysts containing atomically dispersed active sites on a support material, strikingly show the highest atomic utilization and drastically boosted catalytic performances to functionally mimic or even outperform natural enzymes. The molecular characteristics of SACs (e.g., unique metal-support interactions and precisely located metal sites), especially single-atom iron catalysts (Fe-SACs) that have a similar catalytic structure to the catalytically active center of metalloprotease, enable the accurate identification of active centers in catalytic reactions, which afford ample opportunity for unraveling the structure-function relationship of Fe-SACs. In this review, we present an overview of the recent advances of support materials for anchoring an atomic dispersion of Fe. Subsequently, we highlight the structural designability of support materials as two sides of the same coin. Moreover, the applications described herein illustrate the utility of Fe-SACs in a broad scope of industrially and biologically important reactions. Finally, we present an outlook of the major challenges and opportunities remaining for the successful combination of single Fe atoms and catalysts.

Tuning crystal-phase of bimetallic single-nanoparticle for catalytic hydrogenation

Bimetallic nanoparticles afford geometric variation and electron redistribution via strong metal-metal interactions that substantially promote the activity and selectivity in catalysis. Quantitatively describing the atomic configuration of the catalytically active sites, however, is experimentally challenged by the averaging ensemble effect that is caused by the interplay between particle size and crystal-phase at elevated temperatures and under reactive gases. Here, we report that the intrinsic activity of the body-centered cubic PdCu nanoparticle, for acetylene hydrogenation, is one order of magnitude greater than that of the face-centered cubic one. This finding is based on precisely identifying the atomic structures of the active sites over the same-sized but crystal-phase-varied single-particles. The densely-populated Pd-Cu bond on the chemically ordered nanoparticle possesses isolated Pd site with a lower coordination number and a high-lying valence d-band center, and thus greatly expedites the dissociation of H2 over Pd atom and efficiently accommodates the activated H atoms on the particle top/subsurfaces.

Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations

Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly determined by the uniformity of active sites. However, the non-identical number of metal atoms, geometric shape, and morphology of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examined. In particular, their unique behavior in boosting the catalytic performance in various chemical transformations is discussed, including selective hydrogenation, selective oxidation, Suzuki coupling, and other catalytic reactions. In addition, the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.

Controlled Selectivity through Reversible Inhibition of the Catalyst: Stereodivergent Semihydrogenation of Alkynes

Catalytic semihydrogenation of internal alkynes using H2 is an attractive atom-economical route to various alkenes, and its stereocontrol has received widespread attention, both in homogeneous and heterogeneous catalyses. Herein, a novel strategy is introduced, whereby a poisoning catalytic thiol is employed as a reversible inhibitor of a ruthenium catalyst, resulting in a controllable H2-based semihydrogenation of internal alkynes. Both (E)- and (Z)-alkenes were obtained efficiently and highly selectively, under very mild conditions, using a single homogeneous acridine-based ruthenium pincer catalyst. Mechanistic studies indicate that the (Z)-alkene is the reaction intermediate leading to the (E)-alkene and that the addition of a catalytic amount of bidentate thiol impedes the Z/E isomerization step by forming stable ruthenium thiol(ate) complexes, while still allowing the main hydrogenation reaction to proceed. Thus, the absence or presence of catalytic thiol controls the stereoselectivity of this alkyne semihydrogenation, affording either the (E)-isomer as the final product or halting the reaction at the (Z)-intermediate. The developed system, which is also applied to the controllable isomerization of a terminal alkene, demonstrates how metal catalysis with switchable selectivity can be achieved by reversible inhibition of the catalyst with a simple auxiliary additive.

Low-Valent Molybdenum PNP Pincer Complexes as Catalysts for the Semihydrogenation of Alkynes

Low-valent molybdenum PNP pincer complexes were studied as catalysts for the semihydrogenation of alkynes. For that purpose, tBu-substituted PNP complexes PNP tBuMo(CO)2 (6a) and PNP tBuMo(CO)3 (6c) and the NNP complex NNP iPrMo(CO)2(PPh3) ((rac)-7) were synthesized and characterized. By utilizing the cyclohexyl-substituted complex PNPCyMo(CO)2(CH3CN) (5a), several diphenylacetylene derivatives are transformed to the corresponding (Z)-alkenes with good to very good diastereoselectivities (up to 91:9). Mechanistic experiments indicate an outer-sphere mechanism including metal-ligand cooperativity.

Iridium-Catalyzed Selective trans-Semihydrogenation of 1,3-Enynes with Ethanol: Access to (E,E)-1,4-Diarylbutadienes

A trans-semihydrogenation of 1,3-enynes with ethanol as the hydrogen source was developed using a new (PCN)Ir complex as the precatalyst and tBuNH₂ as the cocatalyst. This catalyst system provides an efficient and atom-economical access to unsymmetrical (E,E)-1,4-diarylbutadienes with high yields and stereoselectivities. Monitoring the process revealed that a sequence of cis-semihydrogenation of the triple bond of 1,3-enynes (to form (E,Z)-butadienes) and (E,Z)-to-(E,E) isomerization occurs to form (E,E)-butadienes.

Acetylene hydrogenation catalyzed by bare and Ni doped CeO2(110): the role of frustrated Lewis pairs

Ceria (CeO₂) has recently been found to catalyze the selective hydrogenation of alkynes, which has stimulated much discussion on the catalytic mechanism on various facets of reducible oxides. In this work, H₂ dissociation and acetylene hydrogenation on bare and Ni doped CeO₂(110) surfaces are investigated using density functional theory (DFT). Similar to that on the CeO₂(111) surface, our results suggest that catalysis is facilitated by frustrated Lewis pairs (FLPs) formed by oxygen vacancies (O_vs) on the oxide surfaces. On bare CeO₂(110) with a single O_v (CeO₂(110)-O_v), two surface Ce cations with one non-adjacent O anion are shown to form (Ce³⁺-Ce⁴⁺)/O quasi-FLPs, while for the Ni doped CeO₂(110) surface with one (Ni-CeO₂(110)-O_v) or two (Ni-CeO₂(110)-2O_v) O_vs, one Ce and a non-adjacent O counterions are found to form a mono-Ce/O FLP. DFT calculations indicate that Ce/O FLPs facilitate the H₂ dissociation via a heterolytic mechanism, while the resulting surface O-H and Ce-H species catalyze the subsequent acetylene hydrogenation. With CeO₂(110)-O_v and Ni-CeO₂(110)-2O_v, our DFT calculations suggest that the first hydrogenation step is the rate-determining step with a barrier of 0.43 and 0.40 eV, respectively. For Ni-CeO₂(110)-O_v, the reaction is shown to be controlled by the H₂ dissociation with a barrier of 0.41 eV. These barriers are significantly lower than that (about 0.7 eV) on CeO₂(111), explaining the experimentally observed higher catalytic efficiency of the (110) facet of ceria. The change of the rate-determining step is attributed to the different electronic properties of Ce in the Ce/O FLPs - the Ce f states closer to the Fermi level not only facilitate the heterolytic dissociation of H₂ but also lead to a higher barrier of acetylene hydrogenation.

A Magnetically Separable Pd Single-Atom Catalyst for Efficient Selective Hydrogenation of Phenylacetylene

Selective hydrogenation of alkynes to alkenes plays a crucial role in the synthesis of fine chemicals. However, how to achieve high selectivity and effective separation of the catalyst and substrate while obtaining high activity is the key for this reaction. In this work, a Pd single-atom catalyst is anchored to the shell of magnetic core-shell particles that consist of a Ni-nanoparticles core and a graphene sheets shell (Ni@G) for semi-hydrogenation of phenylacetylene, delivering 93% selectivity to styrene at full conversion with a robust turnover frequency of 7074 h^-1 under mild reaction conditions (303 K, 2 bar H₂ ). Moreover, the catalyst can be recovered promptly from the liquid phase due to its magnetic separability, which makes it present good stability for enduring five cycles. Experimental and theoretical investigations reveal that H₂ and substrates are activated by atomically dispersed Pd atoms and Ni@G hybrid support, respectively. The hydrogenation reaction occurs on the surface of Ni@G via hydrogen spillover from the metal to the support. Such a strategy opens an avenue for designing highly active, selective, and magnetically recyclable catalysts for selective hydrogenation in liquid reaction systems.

1,2-Dialkynyldiboranes(4): B-B versus CC bond reactivity

The reactivity of three 1,2-dialkynyl-1,2-diaminodiborane(4) derivatives, B₂(NMe₂)₂(CCR)₂ (R = H, Me, SiMe₃), towards small molecules known to react with both B-B and CC bonds was studied. With arylazides nitrene insertion into the B-B bond with concomitant loss of N₂ was kinetically favoured in all cases. While reactions with sterically unhindered hydroboranes proceeded unselectively, sterically encumbered dimesitylborane cleanly added to both alkynyl moieties, resulting in the first examples of 1,2-divinyldiboranes(4). In the presence of catalytic amounts of Pd/C room-temperature hydrogenation at 1 bar led to oxidative B-B bond cleavage and yielded the fully hydrogenated alkyl(amino)hydroborane products. These could be prevented from dimerising and isolated by complexation with an NHC ligand. Finally, stepwise halogenation of the B-B bond and the alkynyl groups afforded first the corresponding alkynyl(amino)haloboranes and then the amino(halo)(1,2-dihalovinyl)boranes.

Electrocatalytic Semihydrogenation of Alkynes with [Ni(bpy)3]2

Electrifying the production of base and fine chemicals calls for the development of electrocatalytic methodologies for these transformations. We show here that the semihydrogenation of alkynes, an important transformation in organic synthesis, is electrocatalyzed at room temperature by a simple complex of earth-abundant nickel, [Ni(bpy)₃]²⁺. The approach operates under mild conditions and is selective toward the semihydrogenated olefins with good to very good Z isomer stereoselectivity. (Spectro)electrochemistry supports that the electrocatalytic cycle is initiated in an atypical manner with a nickelacyclopropene complex, which upon further protonation is converted into a putative cationic Ni(II)-vinyl intermediate that produces the olefin after electron-proton uptake. This work establishes a proof of concept for homogeneous electrocatalysis applied to alkyne semihydrogenation, with opportunities to improve the yields and stereoselectivity.

E-Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H2 Directly Employed or In Situ-Generated

Selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)] (dippe = 1,2-bis(di-iso-propylphosphino)ethane) as a precatalyst is described. The required hydrogen gas is either directly employed or in situ-generated upon alcoholysis of KBH₄ with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl, and terminal alkynes was readily hydrogenated to yield E-alkenes in good to excellent isolated yields. The reaction proceeds at 60 °C for directly employed hydrogen or at 60-90 °C with in situ-generated hydrogen and catalyst loadings of 0.5-2 mol %. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups, including halides, phenols, nitriles, unprotected amines, and heterocycles. The reaction can be upscaled to the gram scale. Mechanistic investigations, including deuterium-labeling studies and density functional theory (DFT) calculations, were undertaken to provide a reasonable reaction mechanism, showing that initially formed Z-isomer undergoes fast isomerization to afford the thermodynamically more stable E-isomer.

Recent Progress in Pd-Based Nanocatalysts for Selective Hydrogenation

Selective hydrogenation plays an important role in the chemical industry and has a wide range of applications, including the production of fine chemicals and petrochemicals, pharmaceutical synthesis, healthcare product development, and the synthesis of agrochemicals. Pd-based catalysts have been widely applied for selective hydrogenation due to their unique electronic structure and ability to adsorb and activate hydrogen and unsaturated substrates. However, the exclusive and comprehensive summarization of the size, composition, and surface and interface effect of metal Pd on the performance for selective hydrogenation is still lacking. In this perspective, the research progress on selective hydrogenation using Pd-based catalysts is summarized. The strategies for improving the catalytic hydrogenation performance over Pd-based catalysts are investigated. Specifically, the effects of the size, composition, and surface and interfacial structure of Pd-based catalysts, which could influence the dissociation mode of hydrogen, the adsorption, and the reaction mode of the catalytic substrate, on the performance have been systemically reviewed. Then, the progress on Pd-based catalysts for selective hydrogenation of unsaturated alkynes, aldehydes, ketones, and nitroaromatic hydrocarbons is revealed based on the fundamental principles of selective hydrogenation. Finally, perspectives on the further development of strategies for chemical selective hydrogenation are provided. It is hoped that this perspective would provide an instructive guideline for constructing efficient heterogeneous Pd-based catalysts for various selective hydrogenation reactions.

Cu/O Frustrated Lewis Pairs on Cu Doped CeO2(111) for Acetylene Hydrogenation: A First-Principles Study

In this work, the H2 dissociation and acetylene hydrogenation on Cu doped CeO2(111) were studied using density functional theory calculations. The results indicated that Cu doping promotes the formation of oxygen vacancy (Ov) which creates Cu/O and Ce/O frustrated Lewis pairs (FLPs). With the help of Cu/O FLP, H2 dissociation can firstly proceed via a heterolytic mechanism to produce Cu-H and O-H by overcoming a barrier of 0.40 eV. The H on Cu can facilely migrate to a nearby oxygen to form another O-H species with a barrier of 0.43 eV. The rate-determining barrier is lower than that for homolytic dissociation of H2 which produces two O-H species. C2H2 hydrogenation can proceed with a rate-determining barrier of 1.00 eV at the presence of Cu-H and O-H species., While C2H2 can be catalyzed by two O-H groups with a rate-determining barrier of 1.06 eV, which is significantly lower than that (2.86 eV) of C2H2 hydrogenated by O-H groups on the bare CeO2(111), showing the high activity of Cu doped CeO2(111) for acetylene hydrogenation. In addition, the rate-determining barrier of C2H4 further hydrogenated by two O-H groups is 1.53 eV, much higher than its desorption energy (0.72 eV), suggesting the high selectivity of Cu doped CeO2(111) for C2H2 partial hydrogenation. This provides new insights to develop effective hydrogenation catalysts based on metal oxide.

The modulation mechanism of geometric and electronic structures of bimetallic catalysts: Pd13−mAgm (m=0–13) clusters for acetylene semi-hydrogenation

The modulation mechanism of the second metal in bimetallic catalysts is examined by taking acetylene semi-hydrogenation over Pd13−mAgm clusters, in which a metastable Pd6Ag7 structure exhibits excellent activity/selectivity to ethylene.

Room temperature Z-selective hydrogenation of alkynes by hemilabile and non-innocent (NNN)Co(ii) catalysts

Room temperature chemo- and stereoselective hydrogenation of alkynes is described using a well-defined and phosphine-free hemilabile cobalt catalyst.