Isostructural Molecular and Surface Mimics of the Active Sites of the Industrial WO₃/SiO₂ Metathesis Catalysts

In classical, ill-defined WO₃/SiO₂ alkene metathesis catalysts, isolated, silica-supported tungsten oxo alkylidene species are suspected to represent the catalytically active species. Here, we report the synthesis and the characterization of molecular and supported isostructural mimics of these active sites. The good activities at room temperature of these well-defined oxo alkylidene complexes indicate that the lower activity of WO₃/SiO₂, even after preactivation, is likely due to the difficulty to generate active alkylidene sites from W oxo species in the classical system

Alkyne Hydroamination Catalyzed by Silica-Supported Isolated Zn(II) Sites

Hydroamination is an atom-economical reaction to form C–N bonds, which are ubiquitous in organic compounds. Hydroamination has seen prolific advancements and has mostly focused on the development of homogeneous catalysts based on lanthanides or transition metals. Here, we have developed silica-supported, site-isolated Zn­(II) sites through a combined surface organometallic chemistry (SOMC) and thermolytic molecular precursor (TMP) approach and show that they catalyze the intramolecular hydroamination of alkynes. This material is applicable to a broad range of substrates. On the basis of kinetics and in situ IR spectroscopic studies, we propose that the mechanism involves coordination of the aminoalkyne onto Zn­(II) followed by the heterolytic activation of the N–H bond and subsequent cyclization and proton transfer

Highly Active Subnanometer Au Particles Supported on TiO₂ for Photocatalytic Hydrogen Evolution from a Well-Defined Organogold Precursor, [Au₅(mesityl)₅]

A highly efficient H₂ evolution photocatalyst based on TiO₂ supported subnanometer Au particles was developed on the basis of the reaction of a gold­(I) molecular precursor [Au₅Mes₅] (Mes = 2,4,6-trimethylphenyl), with titanium dioxide partially dehydroxylated at 120 °C. IR, UV–vis, elemental analysis, XANES, and STEM-EDX show that the deposition of [Au₅Mes₅] onto TiO₂ leads to the formation of both subnanometer Au particles and chemisorbed [Au₅Mes₅]. The remaining organic ligands are removed via a mild treatment under H₂, yielding subnanometer gold(0) particles. A range of Au loadings (0.3, 0.9, 2.4 wt %) with similar particle sizes (<1 nm) on TiO₂ are obtained and tested in methanol-assisted photocatalytic hydrogen production under UV light. These catalysts display significantly higher activity than a commercial reference Au-TiO₂ catalyst. The presence of chemisorbed [Au₅Mes₅] in the as-synthesized catalyst further improved activity, albeit at the expense of stability. This work demonstrates a simple synthetic route to obtain subnanometer Au particles on TiO₂ that display exceptional activity in photocatalysis

Strongly σ Donating Thiophenoxide in Silica-Supported Tungsten Oxo Catalysts for Improved 1‑Alkene Metathesis Efficiency

We report the synthesis of tungsten oxo alkylidene complexes bearing bulky thiophenoxide ligands [W­(O)­(CHtBu)­(SHMT)₂(PMe₂Ph)] and [W­(O)­(CHtBu)­(SHMT)₂] (SHMT = 2,6-dimesitylthiophenoxide) and their grafting on partially dehydroxylated silica, affording the supported complexes [(SiO)­W­(O)­(CHtBu)­(SHMT)­(PMe₂Ph)] and [(SiO)­W­(O)­(CHtBu)­(SHMT)]. While the molecular precursors are not significantly active in the metathesis of alkenes, the grafted analogue without bound phosphine ligands displays activity comparable to that of its aryloxide analogue [(SiO)­W­(O)­(CHtBu)­(OHMT)] (OHMT = 2,6-dimesitylphenoxide). It is worth noting that [(SiO)­W­(O)­(CHtBu)­(SHMT)] showed unprecedented activity in the metathesis of 1-alkenes, probably because of the lower stability of metallacyclobutane intermediates

Elucidating the Link between NMR Chemical Shifts and Electronic Structure in d⁰ Olefin Metathesis Catalysts

The nucleophilic carbon of d⁰ Schrock alkylidene metathesis catalysts, [M] = CHR, display surprisingly low downfield chemical shift (δ_iso) and large chemical shift anisotropy. State-of-the-art four-component relativistic calculations of the chemical shift tensors combined with a two-component analysis in terms of localized orbitals allow a molecular-level understanding of their orientations, the magnitude of their principal components (δ₁₁ > δ₂₂ > δ₃₃) and associated δ_iso. This analysis reveals the dominating influence of the paramagnetic contribution yielding a highly deshielded alkylidene carbon. The largest paramagnetic contribution, which originates from the coupling of alkylidene σ_MC and π*_MC orbitals under the action of the magnetic field, is analogous to that resulting from coupling σ_CC and π*_CC in ethylene; thus, δ₁₁ is in the MCH plane and is perpendicular to the MC internuclear direction. The higher value of carbon-13 δ_iso in alkylidene complexes relative to ethylene is thus due to the smaller energy gap between σ_MC and π*_MC vs this between σ_CC and π*_CC in ethylene. This effect also explains why the highest value of δ_iso is observed for Mo and the lowest for Ta, the values for W and Re being in between. In the presence of agostic interaction, the chemical shift tensor principal components orientation (δ₂₂ or δ₃₃ parallel or perpendicular to π_MX) is influenced by the MCH angle because it determines the orientation of the alkylidene CHR fragment relative to the MC internuclear axis. The orbital analysis shows how the paramagnetic terms, understood with a localized bond model, determine the chemical shift tensor and thereby δ_iso

Increased Back-Bonding Explains Step-Edge Reactivity and Particle Size Effect for CO Activation on Ru Nanoparticles

Carbon monoxide is a ubiquitous molecule, a key feedstock and intermediate in chemical processes. Its adsorption and activation, typically carried out on metallic nanoparticles (NPs), are strongly dependent on the particle size. In particular, small NPs, which in principle contain more corner and step-edge atoms, are surprisingly less reactive than larger ones. Hereby, first-principles calculations on explicit Ru NP models (1–2 nm) show that both small and large NPs can present step-edge sites (e.g., B₅ and B₆ sites). However, such sites display strong particle-size-dependent reactivity because of very subtle differences in local chemical bonding. State-of-the-art crystal orbital Hamilton population analysis allows a detailed molecular orbital picture of adsorbed CO on step-edges, which can be classified as <i>flat</i> (η¹ coordination) and <i>concave</i> (η² coordination) sites. Our analysis shows that the CO π-metal <i>d</i>_π hybrid band responsible for the electron back-donation is better represented by an oxygen lone pair on flat sites, whereas it is delocalized on both C and O atoms on concave sites, increasing the back-bonding on these sites compared to flat step-edges or low-index surface sites. The bonding analysis also rationalizes why CO cleavage is easier on step-edge sites of large NPs compared to small ones irrespective of the site geometry. The lower reactivity of small NPs is due to the smaller extent of the Ru–O interaction in the η² adsorption mode, which destabilizes the η² transition-state structure for CO direct cleavage. Our findings provide a molecular understanding of the reactivity of CO on NPs, which is consistent with the observed particle size effect

Role of Coordination Number, Geometry, and Local Disorder on ²⁷Al NMR Chemical Shifts and Quadrupolar Coupling Constants: Case Study with Aluminosilicates

²⁷Al solid-state NMR is a powerful tool for elucidating local geometries at Al sites in molecular and solid-state systems because they are typically associated with specific NMR signatures, namely, isotropic chemical shift (δ_iso) and quadrupolar coupling constant (<i>C</i>_Q). Assignment is however mostly empirical; hence, obtaining a detailed understanding of the origins of the NMR parameters would be a valuable step toward a structure–property/reactivity relationship. Here, we investigate the origin of the ²⁷Al NMR signatures in aluminosilicates using DFT calculations on cluster models complemented by natural chemical shift (NCS) analysis. In particular, NCS analysis shows that the chemical shift of Al is mostly associated with the coupling Al–O σ and σ* orbitals for σ₁₁ leading to deshielding as the coordination number of Al decreases, allowing the distinction between tri-, tetra-, penta-, and hexacoordinated sites. In contrast, <i>C</i>_Q can take a broad range of values (between 8.0 and 23.6 MHz) independently of the coordination number because it is greatly affected by slight variation of the bond distance of siloxane bonds coordinated to aluminum, which perturbs the electrostatic interaction with aluminum and thereby the <i>C</i>_Q

Oxo vs Imido Alkylidene d⁰‑Metal Species: How and Why Do They Differ in Structure, Activity, and Efficiency in Alkene Metathesis?

Density functional calculations have been carried out to analyze the origin of the differences in reactivity, selectivity, and stability toward deactivation in metathesis of d⁰ oxo alkylidene complexes vs their isoelectronic imido counterparts. DFT calculations show that the elementary steps and geometries of the extrema are similar for the oxo and imido complexes, but that the energy profiles are different, the greatest difference occurring for the deactivation pathway. For the alkene metathesis pathway, replacing the imido by an oxo ligand slightly lowers the energy barrier for alkene coordination but raises that for the [2+2]-cycloaddition and cycloreversion; it also destabilizes the trigonal bipyramidal (<b>TBP</b>) metallacyclobutane intermediate with respect to the separated reactants. The isomeric square-based pyramid (<b>SP</b>) metallacyclobutane is in general more stable, and its stability relative to the separated reactants is similar for oxo and imido systems. Consequently, the oxo complex is associated with a slightly larger energy difference between the lowest energy intermediate (<b>SP</b> or separated reactants) and the highest energy transition state (cycloreversion) than the imido complex, which accounts for a slightly lower activity. Changing the imido into an oxo ligand disfavors strongly the deactivation pathway by raising considerably the energy barrier of the β-H transfer at the <b>SP</b> metallacycle that begins the entry into the channel for deactivation and byproduct formation as well as that of the subsequent ethene insertion. This makes the oxo catalysts more selective and stable toward deactivation than the corresponding imido catalysts, when dimerization can be avoided

Strongly σ Donating Thiophenoxide in Silica-Supported Tungsten Oxo Catalysts for Improved 1‑Alkene Metathesis Efficiency

We report the synthesis of tungsten oxo alkylidene complexes bearing bulky thiophenoxide ligands [W­(O)­(CHtBu)­(SHMT)₂(PMe₂Ph)] and [W­(O)­(CHtBu)­(SHMT)₂] (SHMT = 2,6-dimesitylthiophenoxide) and their grafting on partially dehydroxylated silica, affording the supported complexes [(SiO)­W­(O)­(CHtBu)­(SHMT)­(PMe₂Ph)] and [(SiO)­W­(O)­(CHtBu)­(SHMT)]. While the molecular precursors are not significantly active in the metathesis of alkenes, the grafted analogue without bound phosphine ligands displays activity comparable to that of its aryloxide analogue [(SiO)­W­(O)­(CHtBu)­(OHMT)] (OHMT = 2,6-dimesitylphenoxide). It is worth noting that [(SiO)­W­(O)­(CHtBu)­(SHMT)] showed unprecedented activity in the metathesis of 1-alkenes, probably because of the lower stability of metallacyclobutane intermediates

<i>N</i>‑Trifluoromethyl NHC Ligands Provide Selective Ruthenium Metathesis Catalysts

We report the synthesis of ruthenium metathesis catalysts containing unsymmetrical <i>N</i>-trifluoromethyl NHC ligands. These complexes have been fully characterized, and a Ru–F interaction has been identified in the solid state by X-ray crystallographic analysis for three catalysts with Ru–F distances between 2.629(2) and 2.652(2) Å. The influence of the <i>N</i>-trifluoromethyl NHC ligands on the initiation rates and activation parameters was studied. The activity of these catalysts was evaluated in benchmark olefin metathesis reactions and compared to the standard second-generation Grubbs catalyst. Remarkably, <i>N</i>-trifluoromethyl catalysts display an unusually high selectivity for the formation of terminal olefins (up to 90%) in the ethenolysis of ethyl oleate. Much improved selectivity is demonstrated for alternating copolymerization of cyclooctene and norbornene as well. These results underline the importance of electronic effects exerted by the NHC ligand