Effect of Lewis Acids on the Catalyst Activity for Alkene Metathesis, Z-/E- Selectivity and Stability of Tungsten Oxo Alkylidenes

Altres ajuts: Acord transformatiu CRUE-CSICLewis acids increase the catalytic activity of classical heterogeneous catalysts and molecular d tungsten oxo alkylidenes in a variety of olefin metathesis processes. The formation of labile adducts between the metal complex and the Lewis acid has been observed experimentally and suggested to be involved in the catalyst activity increase. In this contribution, DFT (M06) calculations have been performed to determine the role of Lewis acids on catalyst activity, Z-/E- selectivity and stability by comparing three W(E)(CHR)(2,5-dimethylpyrrolide)(O-2,6-dimesithylphenoxide) (E = oxo, imido or oxo-Lewis acid adduct) alkylidenes. Results show that the formation of the alkylidene-Lewis acid adducts influences the reactivity of tungsten oxo alkylidenes due to both steric and electronic effects. The addition of the Lewis acid on the E group increases its bulkiness and this decreases catalyst Z-selectivity. Moreover, the interaction between the oxo ligand and the Lewis acid decreases the donating ability of the former toward the metal. This is important when the oxo group has either a ligand in trans or in the same plane that is competing for the same metal d orbitals. Therefore, the weakening of oxo donating ability facilitates the cycloaddition and cycloreversion steps and it stabilizes the productive trigonal bipyramid metallacyclobutane isomer. The two factors increase the catalytic activity of the complex. The electron donating tuneability by the coordination of the Lewis acid also applies to catalyst deactivation and particularly the key β-hydride elimination step. In this process, the transition states show a ligand in pseudo trans to the oxo. Therefore, the presence of the Lewis acid decreases the Gibbs energy barrier significantly. Overall, the optimization of the E group donating ability in each step of the reaction makes tungsten oxo alkylidenes more reactive and this applies both for the catalytic activity and catalyst deactivation

Metal coordination determines the catalytic activity of IrO2 nanoparticles for the oxygen evolution reaction

Acord transformatiu CRUE-CSICH2 production through water electrolysis is a promising strategy for storing sunlight energy. For the oxygen evolution reaction, iridium oxide containing materials are state-of-the-art due to their stability in acidic conditions. Moreover, precious metal content can be reduced by using small nanoparticles that show high catalytic activities. We performed DFT calculations on a 1.2 nm large IrO2 Wulff-like stoichiometric nanoparticle model (IrO2) with the aim of determining the factors controlling the catalytic activity of IrO2 nanoparticles. Results show that at reaction conditions tetra- and tricoordinated iridium centers are not fully oxidized, the major species being IrO(OH) and IrO(OH)2, respectively. Although the computed overpotential show that all centers present relatively similar reactivities, low coordinated iridium centers tend to be more active than the pentacoordinates sites of the well-defined facets. These low coordination sites are likely more abundant on amorphous nanoparticles, which could be one of the factors explaining the higher catalytic activity observed for non-crystalline materials

Importance of the oxyl character on the IrO2 surface dependent catalytic activity for the oxygen evolution reaction

Acord transformatiu CRUE-CSICThe oxygen evolution reaction catalyst optimization is hindered because in the desirable acidic conditions the sole active catalysts are RuO and IrO. Thus, the understanding of the factors controlling the reactivity of these materials is mandatory. In this contribution, DFT (PBE-D2) periodic calculations are performed to analyze the catalytic activities of the main ((1 1 0), (0 1 1), (1 0 0) and (0 0 1)) IrO surfaces. Results show that the reaction only occurs if the Ir=O species on the surfaces exhibit an oxyl character. The water nucleophilic attack mechanism is the most favorable pathway on the (1 1 0), (1 0 0) and (0 0 1) surfaces. In contrast, for the (0 1 1) facet the oxo-coupling is preferred. The required overpotentials for the four IrO surfaces depend on the feasibility to oxidize the Ir-OH to Ir-O species and this is tuned by the coordination of the unsaturated iridium sites: the (1 0 0) and (0 0 1) surfaces appear to be more active than the (1 1 0) and (0 1 1)

Controlling the formation of two concomitant polymorphs in Hg(II) coordination polymers

Acord transformatiu CRUE-CSICAltres ajuts: J.P. acknowledges financial support from the CB615921 project, the CB616406 project from "Fundació La Caixa"Controlling the formation of the desired product in the appropriate crystalline form is the fundamental breakthrough of crystal engineering. On that basis, the preferential formation between polymorphic forms, which are referred to as different assemblies achieved by changing the disposition or arrangement of the forming units within the crystalline structure, is one of the most challenging topics still to be understood. Herein, we have observed the formation of two concomitant polymorphs with general formula {[Hg(Pip)2(4,4'-bipy)]·DMF}n (P1A, P1B; Pip = piperonylic acid; 4,4'-bipy = 4,4'-bipyridine). Besides, [Hg(Pip)2(4,4'-bipy)]n (2) has been achieved during the attempts to isolate these polymorphs. The selective synthesis of P1A and P1B has been successfully achieved by changing the synthetic conditions. The formation of each polymorphic form has been ensured by unit cell measurements and decomposition temperature. The elucidation of their crystal structure revealed P1A and P1B as polymorphs, which originates from the Hg(II) cores and intermolecular associations, especially pinpointed by Hg···π and π···π interactions. Density functional theory (DFT) calculations suggest that P1B, which shows Hg(II) geometries that are further from ideality, is more stable than P1A by 13 kJ·mol-1 per [Hg(Pip)2(4,4'-bipy)]·DMF formula unit, and this larger stability of P1B arises mainly from metal···π and π···π interactions between chains. As a result, these structural modifications lead to significant variations of their solid-state photoluminescence

Controlling the Formation of Two Concomitant Polymorphs in Hg(II)Coordination Polymers

Controlling the formation of the desired product in the appropriate crystalline form is the fundamental breakthrough of crystal engineering. On that basis, the preferential formation between polymorphic forms, which are referred to as different assemblies achieved by changing the disposition or arrangement of the forming units within the crystalline structure, is one of the most challenging topics still to be understood. Herein, we have observed the formation of two concomitant polymorphs with general formula {[Hg(Pip)2(4,4′-bipy)]·DMF}n (P1A, P1B; Pip = piperonylic acid; 4,4′-bipy = 4,4′-bipyridine). Besides, [Hg(Pip)2(4,4′-bipy)]n (2) has been achieved during the attempts to isolate these polymorphs. The selective synthesis of P1A and P1B has been successfully achieved by changing the synthetic conditions. The formation of each polymorphic form has been ensured by unit cell measurements and decomposition temperature. The elucidation of their crystal structure revealed P1A and P1B as polymorphs, which originates from the Hg(II) cores and intermolecular associations, especially pinpointed by Hg···π and π···π interactions. Density functional theory (DFT) calculations suggest that P1B, which shows Hg(II) geometries that are further from ideality, is more stable than P1A by 13 kJ·mol–1 per [Hg(Pip)2(4,4′-bipy)]·DMF formula unit, and this larger stability of P1B arises mainly from metal···π and π···π interactions between chains. As a result, these structural modifications lead to significant variations of their solid-state photoluminescence

A Hg(I) corrugated sheet assembled by auxiliary dioxole groups and Hg··· π interactions

The formation of a new double-stranded staircase Hg(I) supramolecular assembly is reported. It is arranged into 2D corrugated sheets supported by Hg(I)⋯Odioxole and Hg⋯π interactions, resulting from the comproportionation reaction between Hg(II) and Hg(0) species in DMF as a solvent

Engineering photocatalytic porous organic materials for directing redox versus energy transfer processes

Two organic materials containing phenanthroline and triazine fragments, but connected in different ways, are presented. The imine-based material Phen–Tz–covalent organic framework (COF) preferentially shows photocatalytic activity through an energy transfer pathway as observed for olefin photoisomerization. However, an analogous covalent triazine framework (Phen–CTF) behaves as a powerful photoredox catalyst able to activate C-X (X=Br, Cl) bonds. The analysis of this phenomenon by means of theoretical calculations enables the rationalization of the different photocatalytic behavior observed. Phen–CTF behaves as a donor–acceptor material resulting in efficient charge separation upon excitation, while the imine groups present in Phen–Tz–COF hamper charge separation contributing to the rapid recombination between electrons and holes. This justifies a better activation via electron transfer in Phen–CTF and via energy transfer in Phen–Tz–COFPID2021-122299NB-I0, PID2019-110637RB-I00, ID2020-112715GB-I00, PID2022-141016OB-I00, TED2021-130470B-I0, TED2021-129999B-C32, S2018/NMT-4367, Y2020/NMT64

Organocatalytic vs. Ru-based electrochemical hydrogenation of nitrobenzene in competition with the hydrogen evolution reaction

The electrochemical reduction of organic contaminants allows their removal from water. In this contribution, the electrocatalytic hydrogenation of nitrobenzene is studied using both oxidized carbon fibres and ruthenium nanoparticles supported on unmodified carbon fibres as catalysts. The two systems produce azoxynitrobenzene as the main product, while aniline is only observed in minor quantities. Although PhNO2 hydrogenation is the favoured reaction, the hydrogen evolution reaction (HER) competes in both systems under catalytic conditions. H2 formation occurs in larger amounts when using the Ru nanoparticle based catalyst. While similar reaction outputs were observed for both catalytic systems, DFT calculations revealed some significant differences related to distinct interactions between the catalytic material and the organic substrates or products, which could pave the way for the design of new catalytic materials

Enhanced photocatalytic activity of gold nanoparticles driven by supramolecular host-guest chemistry

Functionalization of gold nanoparticles with supramolecular hosts allows their plasmon-based photocatalytic activity to be enhanced. This is mainly ascribed to the formation of labile host-guest complexes with the reagent molecules on the metal surface, thus promoting nanoparticle-substrate approximation without interfering with the light-induced catalytic process