Classifying and Understanding the Reactivities of Mo-Based Alkyne Metathesis Catalysts from ⁹⁵Mo NMR Chemical Shift Descriptors

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X3Mo≡CR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the context of alkyne metathesis that 95Mo chemical shift tensors encode detailed information on the electronic structure of these catalysts. Analysis by first-principles calculations of 95Mo chemical shift tensors extracted from solid-state 95Mo NMR spectra shows a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2 + 2]-cycloaddition step, thus linking 95Mo chemical shifts to reactivity. In particular, the 95Mo chemical shifts are driven by ligand electronegativity (σ-donation) and electron delocalization through Mo–O π interactions, thus explaining the reactivity patterns of the silanolate canopy catalysts. These results further motivate exploration of transition metal NMR signatures and their relationships to electronic structure and reactivity

Strain in Silica-Supported Ga(III) Sites : Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Altres ajuts: Acord transformatiu CRUE-CSICWell-defined Ga(III) sites on SiO are highly active, selective, and stable catalysts in the propane dehydrogenation (PDH) reaction. In this contribution, we evaluate the catalytic activity toward PDH of tricoordinated and tetracoordinated Ga(III) sites on SiO by means of first-principles calculations using realistic amorphous periodic SiO models. We evaluated the three reaction steps in PDH, namely, the C-H activation of propane to form propyl, the β-hydride (β-H) transfer to form propene and a gallium hydride, and the H-H coupling to release H, regenerating the initial Ga-O bond and closing the catalytic cycle. Our work shows how Brønsted-Evans-Polanyi relationships are followed to a certain extent for these three reaction steps on Ga(III) sites on SiO and highlights the role of the strain of the reactive Ga-O pairs on such sites of realistic amorphous SiO models. It also shows how transition-state scaling holds very well for the β-H transfer step. While highly strained sites are very reactive sites for the initial C-H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need for the right balance in strain to be an effective site for PDH. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites toward the PDH reaction and paves the way toward the design and prediction of better single-site catalysts on SiO for the PDH reaction. We performed computational calculations of Ga(III) single sites on realistic amorphous models of SiO to evaluate their catalytic activity toward the propane dehydrogenation reaction. Our results show that a balance in strain is key, in which neither too stiff nor too loose Ga−O bonding is needed to obtain the highest catalytic activity

Alkyne gem‐Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character

Parahydrogen (p‐H2) induced polarization (PHIP) NMR spectroscopy showed that [CpXRu] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem‐hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich CpX rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT‐assisted analysis of the chemical shift tensors determined by solid‐state 13C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis‐active Grubbs‐type catalysts

Phase Coexistence and Structural Dynamics of Redox Metal Catalysts Revealed by Operando TEM

Metal catalysts play an important role in industrial redox reactions. Although extensively studied, the state of these catalysts under operating conditions is largely unknown, and assignments of active sites remain speculative. Herein, an operando transmission electron microscopy study is presented, which interrelates the structural dynamics of redox metal catalysts to their activity. Using hydrogen oxidation on copper as an elementary redox reaction, it is revealed how the interaction between metal and the surrounding gas phase induces complex structural transformations and drives the system from a thermodynamic equilibrium toward a state controlled by the chemical dynamics. Direct imaging combined with the simultaneous detection of catalytic activity provides unparalleled structure–activity insights that identify distinct mechanisms for water formation and reveal the means by which the system self-adjusts to changes of the gas-phase chemical potential. Density functional theory calculations show that surface phase transitions are driven by chemical dynamics even when the system is far from a thermodynamic phase boundary. In a bottom-up approach, the dynamic behavior observed here for an elementary reaction is finally extended to more relevant redox reactions and other metal catalysts, which underlines the importance of chemical dynamics for the formation and constant re-generation of transient active sites during catalysis

Elucidation of Metal Local Environments in Single‐Atom Catalysts Based on Carbon Nitrides

The ability to tailor the properties of metal centers in single-atom heterogeneous catalysts depends on the availability of advanced approaches for characterization of their structure. Except for specific host materials with well-defined metal adsorption sites, determining the local atomic environment remains a crucial challenge, often relying heavily on simulations. This article reports an advanced analysis of platinum atoms stabilized on poly(triazine imide), a nanocrystalline form of carbon nitride. The approach discriminates the distribution of surface coordination sites in the host, the evolution of metal coordination at different stages during the synthesis of the material, and the potential locations of metal atoms within the lattice. Consistent with density functional theory predictions, simultaneous high-resolution imaging in high-angle annular dark field and bright field modes experimentally confirms the preferred localization of platinum in-plane in the corners of the triangular cavities. X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and dynamic nuclear polarization enhanced 15N nuclear magnetic resonance (DNP-NMR) spectroscopies coupled with density functional theory (DFT) simulations reveal that the predominant metal species comprise Pt(II) bound to three nitrogen atoms and one chlorine atom inside the coordination sites. The findings, which narrow the gap between experimental and theoretical elucidation, contribute to the improved structural understanding and provide a benchmark for exploring the speciation of single-atom catalysts based on carbon nitrides

¹⁸³W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

Triarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for alkyne metathesis but lead to disappointing results and poor stability in the tungsten series. 1H,183W heteronuclear multiple bond correlation spectroscopy, exploiting a favorable 5J‐coupling between the 183W center and the peripheral protons on the alkylidyne cap, revealed that these ligands upregulate the Lewis acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2] cycloaddition step is over‐stabilized and the catalytic turnover brought to a halt. Guided by the 183W NMR shifts as a proxy for the Lewis acidity of the central atom and by an accompanying chemical shift tensor analysis of the alkylidyne unit, the ligand design was revisited and a more strongly π‐donating all‐alkoxide ligand prepared. The new expanded chelate complex has a tempered Lewis acidity and outperforms the classical Schrock catalyst, carrying monodentate tert‐butoxy ligands, in terms of rate and functional‐group compatibility

Strain in Silica-Supported Ga (III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Well-defined Ga(III) sites on SiO2 are highly active, selective, and stable catalysts in the propane dehydrogenation reaction. In this contribution, we evaluate the catalytic activity towards propane dehydrogenation of tri-coordinated and tetra-coordinated Ga(III) sites on SiO2 by means of first principles calculations using realistic amorphous periodic SiO2models. We evaluated the three reaction steps in propane dehydrogenation, namely the C-H activation of propane to form propyl, the beta-hydride elimination transfer to form propene, and a Ga-hydride, and the H-H coupling to release H2, regenerating the initial Ga-O bond and closing the catalytic cycle. Our work shows how Brønsted-Evans-Polanyi relationships are followed for these three reaction steps on Ga(III) sites on SiO2 and highlights the role of the strain of the reactive Ga-O pairs on such sites of realistic amorphous SiO2 models. While highly strained sites are very reactive sites for the initial C-H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need of a right balance in strain to be an effective site for propane dehydrogenation. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites towards the propane dehydrogenation reaction and paves the road towards the design and prediction of better single-site catalysts on SiO2 for the propane dehydrogenation reaction.</p