Simon K. Beaumont

Sonogashira coupling on an extended gold surface in vacuo: reaction of phenylacetylene with iodobenzene on Au(111).

Temperature-programmed reaction measurements supported by scanning tunneling microscopy have shown that phenylacetylene and iodobenzene react on smooth Au(111) under vacuum conditions to yield biphenyl and diphenyldiacetylene, the result of homocoupling of the reactant molecules. They also produce diphenylacetylene, the result of Sonogashira cross-coupling, prototypical of a class of reactions that are of paramount importance in synthetic organic chemistry and whose mechanism remains controversial. Roughened Au(111) is completely inert toward all three reactions, indicating that the availability of crystallographically well-defined adsorption sites is crucially important. High-resolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy show that the reactants are initially present as intact, essentially flat-lying molecules and that the temperature threshold for Sonogashira coupling coincides with that for C-I bond scission in the iodobenzene reactant. The fractional-order kinetics and low temperature associated with desorption of the Sonogashira product suggest that the reaction occurs at the boundaries of islands of adsorbed reactants and that its appearance in the gas phase is rate-limited by the surface reaction. These findings demonstrate unambiguously and for the first time that this heterogeneous cross-coupling chemistry is an intrinsic property of extended, metallic pure gold surfaces: no other species, including solvent molecules, basic or charged (ionic) species are necessary to mediate the process.

Size-Controlled Model Co Nanoparticle Catalysts for CO2 Hydrogenation: Synthesis, Characterization, and Catalytic Reactions

Model cobalt catalysts for CO(2) hydrogenation were prepared using colloidal chemistry. The turnover frequency at 6 bar and at 200-300 °C increased with cobalt nanoparticle size from 3 to 10 nm. It was demonstrated that near monodisperse nanoparticles in the size range of 3-10 nm could be generated without using trioctylphosphine oxide, a capping ligand that we demonstrate results in phosphorus being present on the metal surface and poisoning catalyst activity in our application.

Identity of the Active Site in Gold Nanoparticle-Catalyzed Sonogashira Coupling of Phenylacetylene and Iodobenzene

XPS, TEM, and reaction studies were used to examine the catalytic behavior of gold species deposited on lanthana toward the cross-coupling of phenylacetylene and iodobenzene. Atomically dispersed Au(I) and Au(III) were catalytically inert, whereas metallic Au(0) nanoparticles were both active and very selective. Thus it is metallic gold and not ionic gold that provides the catalytically active sites. Au(0) nanoparticles supported on silica, gamma-alumina, and BaO were active but relatively unselective; however, as with lanthana, ceria-supported Au(0) nanoparticles showed high selectivity. This strong promoting effect of the lanthanide oxide supports on Sonogashira selectivity cannot be accounted for in terms of acid/base, redox, or SMSI effects; it may be tentatively ascribed to metal --> support hydrogen spillover.

Sonogashira coupling catalyzed by gold nanoparticles: Does homogeneous or heterogeneous catalysis dominate?

A variety of measurements indicates that Au nanoparticle‐ catalyzed Sonogashira coupling of iodobenzene and phenylacetylene is predominantly a heterogeneous process. Large gold particles are much more selective than small ones, which is consistent with this view. Substantial leaching of Au into the solution phase occurs during the reaction, but the resulting supernatant liquid exhibits immeasurably low catalytic activity; TONs for the nanoparticles are orders of magnitude higher than those for the leached Au, once more pointing to the primacy of heterogeneous chemistry. These properties are independent of the support material, implying that they are intrinsic to metallic Au nanoparticles. Reaction data and quantitative analysis of the solid and solution phases by XPS and ICP‐MS, respectively, showed that catalytic activity ceased when all the metallic Au had dissolved. Conversely, when starting with a soluble Au complex, a long inactive induction phase is followed by the sharp onset of reaction and steadily increasing catalytic activity, consistent with the eventual nucleation and growth of gold particles. Again, the implication is that, for the nanoparticle‐catalyzed reaction, heterogeneous catalysis is by far the most important process.

Aspects of heterogeneous enantioselective catalysis by metals

Some aspects of metal-catalyzed heterogeneous enantioselective reactions are reviewed with specific reference to four different systems where the phenomena that control enantioselection appear to be very different. In the case of glucose electro-oxidation, it is clear that any intrinsic chirality present at the metal surface plays a vital role. With α-keto hydrogenation, achiral surfaces modified by the adsorption of chiral agents become effective enantioselective catalysts and the formation of extended arrays of chiral species appears not to be of importance: instead a 1:1 docking interaction controlled by hydrogen bonding between the adsorbed chiral modifier and the prochiral reactant determines the outcome. Hydrogen bonding also plays a central role in β-ketoester hydrogenation, but here fundamental studies indicate that the formation of ordered arrays involving the reactant and chiral ligand is of importance. Asymmetric C═C hydrogenation, though relatively little studied, has the potential for major impact in synthetic organic chemistry both on the laboratory scale and in the manufacture of fine chemicals and pharmaceuticals. The structural attributes that determine whether a given chiral ligand is effective have been identified; the ability to form strong covalent bonds with the metal surface while also resisting hydrogenation and displacement by the strongly adsorbing reactant under reaction conditions is an essential necessary condition. Beyond this, ligand rigidity in the vicinity of the chirality center coupled with resistance to SAM formation is a critically important factor whose absence results in racemic chemistry.

Spatially orthogonal chemical functionalization of a hierarchical pore network for catalytic cascade reactions

The chemical functionality within porous architectures dictates their performance as heterogeneous catalysts; however, synthetic routes to control the spatial distribution of individual functions within porous solids are limited. Here we report the fabrication of spatially orthogonal bifunctional porous catalysts, through the stepwise template removal and chemical functionalization of an interconnected silica framework. Selective removal of polystyrene nanosphere templates from a lyotropic liquid crystal-templated silica sol-gel matrix, followed by extraction of the liquid crystal template, affords a hierarchical macroporous-mesoporous architecture. Decoupling of the individual template extractions allows independent functionalization of macropore and mesopore networks on the basis of chemical and/or size specificity. Spatial compartmentalization of, and directed molecular transport between, chemical functionalities affords control over the reaction sequence in catalytic cascades; herein illustrated by the Pd/Pt-catalysed oxidation of cinnamyl alcohol to cinnamic acid. We anticipate that our methodology will prompt further design of multifunctional materials comprising spatially compartmentalized functions.

Combining in situ NEXAFS spectroscopy and CO2 methanation kinetics to study Pt and Co nanoparticle catalysts reveals key insights into the role of platinum in promoted cobalt catalysis

The mechanistic role of platinum and precious metals in promoting cobalt hydrogenation catalysts of the type used in reactions such as Fischer-Tropsch synthesis is highly debated. Here we use well-defined monometallic Pt and Co nanoparticles (NPs) and CO2 methanation as a probe reaction to show that Pt NPs deposited near Co NPs can enhance the CO2 methanation rate by up to a factor of 6 per Co surface atom. In situ NEXAFS spectroscopy of these same Pt NP plus Co NP systems in hydrogen shows that the presence of nearby Pt NPs is able to significantly enhance reduction of the Co at temperatures relevant to Fischer-Tropsch synthesis and CO2 methanation. The mechanistic role of Pt in these reactions is discussed in light of these findings.

A nanoscale demonstration of hydrogen atom spillover and surface diffusion across silica using the kinetics of CO2 methanation catalyzed on spatially separate Pt and Co nanoparticles.

Hydrogen spillover is of great importance to understanding many phenomena in heterogeneous catalysis and has long been controversial. Here we exploit well-defined nanoparticles to demonstrate its occurrence through evaluation of CO2 methanation kinetics. Combining platinum and cobalt nanoparticles causes a substantial increase in reaction rate, but increasing the spatial separation between discrete cobalt and platinum entities results in a dramatic ∼ 50% drop in apparent activation energy, symptomatic of H atom surface diffusion limiting the reaction rate.

Determination of Molecular Surface Structure, Composition, and Dynamics under Reaction Conditions at High Pressures and at the Solid–Liquid Interface†

In the last two decades, surface-science experiments and techniques have been developed to focus on obtaining molecular information under reaction conditions at high pressures (near or above 1 bar) and liquid interfaces. This Minireview describes the results of these studies obtained by surface-sensitive laser spectroscopies, scanning tunneling microscopy, and X-ray spectroscopies usually practiced at a synchrotron light source. The use of model surfaces, single crystals, and monodisperse nanoparticles with variable size (1-10 nm) and shape facilitates meaningful interpretation of the experimental data. These methods allow evaluation of the molecular structures of intermediates, oxidation states of metals, and mobility of adsorbants. New techniques that are likely to make major contributions to the investigation of surfaces under reaction conditions are also discussed.

Heterogeneously Catalyzed Asymmetric Hydrogenation of C═C Bonds Directed by Surface-Tethered Chiral Modifiers

Asymmetric hydrogenation of C=C bonds is of the highest importance in organic synthesis, and such reactions are currently carried out with organometallic homogeneous catalysts. Achieving heterogeneous metal-catalyzed hydrogenation, a highly desirable goal, necessitates forcing the crucial enantiodifferentiating step to take place at the metal surface. By synthesis and application of six chiral sulfide ligands that anchor robustly to Pd nanoparticles and resist displacement, we have for the first time accomplished heterogeneous enantioselective catalytic hydrogenation of isophorone. High resolution XPS data established that ligand adsorption from solution occurred exclusively on the Pd nanoparticles and not on the carbon support. All ligands contained a pyrrolidine nitrogen to enable their interaction with the isophorone substrate while the sulfide functionality provided the required interaction with the Pd surface. Enantioselective turnover numbers of up to approximately 100 product molecules per ligand molecule were found with a very large variation in asymmetric induction between ligands: observed enantiomeric excesses increased with increasing size of the alkyl group in the sulfide. This likely reflects varying degrees of ligand dispersion on the surface: bulky substituent groups hinder close approach of ligand molecules to each other, inhibiting close-packed island formation, favoring dispersion as separate molecules, and leading to effective asymmetric induction. Conversely, small substituents favor island formation leading to very low asymmetric induction. Enantioselective reaction most likely involves initial formation of an enamine or iminium species, confirmed by use of an analogous tertiary amine, which leads to racemic product. Ligand rigidity and resistance to self-assembled monolayer formation are important attributes that should be designed into improved chiral modifiers.