Computed versus experimental energy barriers in solution: Influence of the type of the density functional approximation.

Mechanistic investigations at the density functional theory level of organic and organometallic reactions in solution are now broadly accessible and routinely implemented to complement experimental investigations. The selection of an appropriate functional among the plethora of developed ones is the first challenge on the way to reliable energy barrier calculations. To provide guidelines for the choice of an initial and reliable computational level, the performances of commonly used non-empirical (PBE, PBE0, PBE0-DH) and empirical density functionals (BLYP, B3LYP, B2PLYP) were evaluated relative to experimental activation enthalpies. Most reactivity databases to assess density functional performances are primarily based on high level calculations, here a set of experimental activation enthalpies of organic and organometallic reactions performed in solution were selected from the literature. As a general trend, the non-empirical functionals outperform the empirical ones. The most accurate energy barriers are obtained with hybrid PBE0 and double-hybrid PBE0-DH density functionals, both providing similar performance. Regardless of the functional under consideration, the addition of the GD3-BJ empirical dispersion correction does not enhance the accuracy of computed energy barriers.

Metadynamics simulations reveal alloying-dealloying processes for bimetallic PdGa nanoparticles under CO2 hydrogenation.

Supported bimetallic nanoparticles (NPs) often display improved catalytic performances (activity and/or selectivity). Yet, structure-activity relationships are difficult to derive due to the multitude of possible compositions, interfaces and alloys. This is notably true for bimetallic NPs used in the selective hydrogenation of CO2 to methanol, where the NPs respond dynamically to the chemical potential of the reactants and products. Herein, we use a combined computational and experimental approach that leverages ab initio Molecular Dynamics (AIMD) and Metadynamics (MTD) in conjunction with in situ X-ray absorption spectroscopy, chemisorption and CO-IR, to explore the dynamic structures and interactions with adsorbates under various CO2 hydrogenation conditions in highly active and selective silica-supported PdGa NPs. We find that PdGa alloying generates isolated Pd sites at the NP surface, changing the dominant binding modes of relevant adsorbates compared to pure Pd NPs: CO molecules mainly occupy atop sites and hydrides switch from mainly internal to atop and bridge sites. Under more oxidizing conditions, akin to CO2 hydrogenation, Ga is partially oxidized, forming a GaOX layer on the NP surface, with a partially dealloyed PdGa core and some remaining isolated Pd surface sites. Overall, these bimetallic NPs show high structural dynamics and a variable extent of alloying depending on the adsorbates relevant to CO2 hydrogenation. This work highlights that AIMD/MTD is a powerful approach to elucidate structural dynamics at a single particle level in complex catalytic systems.

Electrochemically Driven Nickel-Catalyzed Halogenation of Unsaturated Halide and Triflate Derivatives.

A robust electrochemically driven nickel-catalyzed halogen exchange of unsaturated halides and triflates (Br to Cl, I to Cl, I to Br, and OTf to Cl) is reported. A combination of NiCl2 ⋅ glyme as the precatalyst, 2,2'-bipyridine as a ligand, NMP as the solvent, and electrochemistry allowed the generation of a nickel species that promotes reductive elimination of the desired product. This paired electrochemical halogenation is compatible with a range of unsaturated halides and triflates, including heterocycles, dihaloarenes, and alkenes with good functional-group tolerance. Joint experimental and theoretical mechanistic investigations highlighted three catalytic events: i) oxidative addition of the aryl halide to a Ni(0) species to deliver a Ni(II) intermediate; ii) halide metathesis at Ni(II); iii) electrochemical oxidation of Ni(II) to Ni(III) to enable the formation of the desired aryl halide upon reductive elimination. This methodology allows the replacement of heavy halogens (I or Br) or polar atoms (O) with the corresponding lighter and more lipophilic Cl group to block undesired reactivity or modify the properties of drug and agrochemical candidates.

Shedding Light on the Hidden Roles of Lithium in the Nickel-Catalyzed Cross-Coupling of Aryl Ethers.

The Ni-catalyzed cross-coupling of aryl ethers is a powerful synthetic tool to transform widely available phenol derivatives into functionalized aromatic molecules. Recent theoretical and experimental mechanistic studies have identified the involvement of heterobimetallic nickelates as key intermediates that facilitate the challenging transformation under mild conditions and often without the need for external ligands or additives. In this work, based on calculations performed at the density functional theory (DFT) level and by comparison with spectroscopic and kinetic data, we investigate the mechanism of the Ni(COD)2-catalyzed cross-coupling of 2-methoxynaphthalene with PhLi and assess the speciation of lithium nickelate intermediates. The crucial role of solvent on the reaction is explained, and the multiple roles played by lithium are unveiled. Experimental studies have identified key lithium nickelate species which support and help to evolve the calculated reaction mechanism and ultimately complete the catalytic cycle. Based on this new mechanistic knowledge, a well-known experimental challenge of these transformations, the so-called "naphthalene problem" which restricts the use of electrophilic coupling partners to π-extended systems, can be addressed to enable the cross-coupling of unbiased aryl ethers under mild conditions.

Ti-Doping in Silica-Supported PtZn Propane Dehydrogenation Catalysts: From Improved Stability to the Nature of the Pt-Ti Interaction.

Propane dehydrogenation is an important industrial reaction to access propene, the world's second most used polymer precursor. Catalysts for this transformation are required to be long living at high temperature and robust toward harsh oxidative regeneration conditions. In this work, combining surface organometallic chemistry and thermolytic molecular precursor approach, we prepared well-defined silica-supported Pt and alloyed PtZn materials to investigate the effect of Ti-doping on catalytic performances. Chemisorption experiments and density functional calculations reveal a significant change in the electronic structure of the nanoparticles (NPs) due to the Ti-doping. Evaluation of the resulting materials PtZn/SiO2 and PtZnTi/SiO2 during long deactivation phases reveal a stabilizing effect of Ti in PtZnTi/SiO2 with a kd of 0.015 h-1 compared to PtZn/SiO2 with a kd of 0.022 h-1 over 108 h on stream. Such a stabilizing effect is also present during a second deactivation phase after applying a regeneration protocol to the materials under O2 and H2 at high temperatures. A combined scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory study reveals that this effect is related to a sintering prevention of the alloyed PtZn NPs in PtZnTi/SiO2 due to a strong interaction of the NPs with Ti sites. However, in contrast to classical strong metal-support interaction, we show that the coverage of the Pt NPs with TiOx species is not needed to explain the changes in adsorption and reactivity properties. Indeed, the interaction of the Pt NPs with TiIII sites is enough to decrease CO adsorption and to induce a red-shift of the CO band because of electron transfer from the TiIII sites to Pt0.

Regime Switch in the Dual-Catalyzed Coupling of Alkyl Silicates with Aryl Bromides.

Metallaphotoredox catalyzed cross-coupling of an arylbromide (Ar-Br) with an alkyl bis(catecholato)silicate (R-Si⊖ ) has been analyzed in depth using a continuum of analytical techniques (EPR, fluorine NMR, electrochemistry, photophysics) and modeling (micro-kinetics and DFT calculations). These studies converged on the impact of four control parameters consisting in the initial concentrations of the iridium photocatalyst ([Ir]0 ), nickel precatalyst ([Ni]0 ) and silicate ([R-Si⊖ ]0 ) as well as light intensity I0 for an efficient reaction between Ar-Br and R-Si⊖ . More precisely, two regimes were found to be possibly at play. The first one relies on an equimolar consumption of Ar-Br with R-Si⊖ smoothly leading to Ar-R, with no side-product from R-Si⊖ and a second one in which R-Si⊖ is simultaneously coupled to Ar-Br and degraded to R-H. This integrative approach could serve as a case study for the investigation of other metallaphotoredox catalysis manifolds of synthetic significance.

Lithium Promotes Acetylide Formation on MgO During Methane Coupling Under Non-Oxidative Conditions.

A prototypical material for the oxidative coupling of methane (OCM) is Li/MgO, for which Li is known to be essential as a dopant to obtain high C2 selectivities. Herein, Li/MgO is demonstrated to be an effective catalyst for non-oxidative coupling of methane (NOCM). Moreover, the presence of Li is shown to favor the formation of magnesium acetylide (MgC2 ), while pure MgO promotes coke formation as evidenced by solid-state 13 C NMR, thus indicating that Li promotes C-C bond formation. Metadynamic simulations of the carbon mobility in MgC2 and Li2 C2 at the density functional theory (DFT) level show that carbon easily diffuses as a C2 unit at 1000 °C. These insights suggest that the enhanced C2 selectivity for Li-doped MgO is related to the formation of Li and Mg acetylides.

Role and dynamics of transition metal carbides in methane coupling.

Transition metal carbides have numerous applications and are known to excel in terms of hardness, thermal stability and conductivity. In particular, the Pt-like behavior of Mo and W carbides has led to the popularization of metal carbides in catalysis, ranging from electrochemically-driven reactions to thermal methane coupling. Herein, we show the active participation of carbidic carbon in the formation of C2 products during methane coupling at high temperature that is associated with the dynamics of Mo and W carbides. A detailed mechanistic study reveals that the catalyst performance of these metal carbides can be traced back to its carbon diffusivity and exchange capability upon interaction with methane (gas phase carbon). A stable C2 selectivity over time on stream for Mo carbide (Mo2C) can be rationalized by fast carbon diffusion dynamics, while W carbide (WC) shows loss of selectivity due to slow diffusion leading to surface carbon depletion. This finding showcases that the bulk carbidic carbon of the catalyst plays a crucial role and that the metal carbide is not only responsible for methyl radical formation. Overall, this study evidences the presence of a carbon equivalent to the Mars-Van Krevelen type mechanism for non-oxidative coupling of methane.

Assigning 1H chemical shifts in paramagnetic mono- and bimetallic surface sites using DFT: a case study on the Union Carbide polymerization catalyst.

The Union Carbide (UC) ethylene polymerization catalyst, based on silica-supported chromocene, is one of the first industrial catalysts prepared by surface organometallic chemistry, though the structure of the surface sites remains elusive. Recently, our group reported that monomeric and dimeric Cr(ii) sites, as well as Cr(iii) hydride sites, are present and that their proportion varies as a function of the Cr loading. While 1H chemical shifts extracted from solid-state 1H NMR spectra should be diagnostic of the structure of such surface sites, unpaired electrons centered on Cr atoms induce large paramagnetic 1H shifts that complicate their NMR analysis. Here, we implement a cost-efficient DFT methodology to calculate 1H chemical shifts for antiferromagnetically coupled metal dimeric sites using a Boltzmann-averaged Fermi contact term over the population of the different spin states. This method allowed us to assign the 1H chemical shifts observed for the industrial-like UC catalyst. The presence of monomeric and dimeric Cr(ii) sites, as well as a dimeric Cr(iii)-hydride sites, was confirmed and their structure was clarified.

Union carbide polymerization catalysts: from uncovering active site structures to designing molecularly-defined analogs.

The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of their active sites. Based on a combination of IR, EPR spectroscopies, labeling experiments, and DFT modeling, we identified monomeric surface-supported Cr(iii) hydrides, ([triple bond, length as m-dash]SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involve a C-H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(iii) alkyl species that insert ethylene through a Cossee-Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analog, CpCr(CH(SiMe3)2)2 grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure to the UC catalyst, further supporting the proposed active site structure.

Copper-Catalyzed Homocoupling of Boronic Acids: A Focus on B-to-Cu and Cu-to-Cu Transmetalations.

Controlling and understanding the Cu-catalyzed homocoupling reaction is crucial to prompt the development of efficient Cu-catalyzed cross-coupling reactions. The presence of a coordinating base (hydroxide and methoxide) enables the B-to-Cu(II) transmetalation from aryl boronic acid to CuIICl2 in methanol, through the formation of mixed Cu-(µ-OH)-B intermediates. A second B-to-Cu transmetalation to form bis-aryl Cu(II) complexes is disfavored. Instead, organocopper(II) dimers undergo a coupled transmetalation-electron transfer (TET) allowing the formation of bis-organocopper(III) complexes readily promoting reductive elimination. Based on this mechanism some guidelines are suggested to control the undesired formation of homocoupling product in Cu-catalyzed cross-coupling reactions.

A Robust and Efficient Propane Dehydrogenation Catalyst from Unexpectedly Segregated Pt2Mn Nanoparticles.

The increasing demand for short chain olefins like propene for plastics production and the availability of shale gas make the development of highly performing propane dehydrogenation (PDH) catalysts, robust toward industrially applied harsh regeneration conditions, a highly important field of research. A combination of surface organometallic chemistry and thermolytic molecular precursor approach was used to prepare a nanometric, bimetallic Pt-Mn material (3 wt % Pt, 1.3 wt % Mn) supported on silica via consecutive grafting of a Mn and Pt precursor on surface OH groups present on the support surface, followed by a treatment under a H2 flow at high temperature. The material exhibits a 70% fraction of the overall Mn as MnII single sites on the support surface; the remaining Mn is incorporated in segregated Pt2Mn nanoparticles. The material shows great performance in PDH reaction with a low deactivation rate. In particular, it shows outstanding robustness during repeated regeneration cycles, with conversion and selectivity stabilizing at ca. 37 and 98%, respectively. Notably, a material with a lower Pt loading of only 0.05 wt % shows an outstanding catalytic performance─initial productivity of 4523 gC3H6/gPt h and an extremely low kd of 0.003 h-1 under a partial pressure of H2, which are among the highest reported productivities. A combined in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, electron paramagnetic resonance, and metadynamics at the density functional theory level study could show that the strong interaction between the MnII-decorated support and the unexpectedly segregated Pt2Mn particles is most likely responsible for the outstanding performance of the investigated materials.

Salt-Enhanced Oxidative Addition of Iodobenzene to Pd: An Interplay Between Cation, Anion, and Pd-Pd Cooperative Effects.

Halide salts facilitate the oxidative addition of organic halides to Pd(0). This phenomenon originates from a combination of anionic, cationic, and Pd-Pd cooperative effects. Exhaustive computational exploration at the density functional theory level of the complexes obtained from [Pd0(PPh3)2] and a salt (NMe4Cl or LiCl) showed that chlorides promote phosphine release, leading to a mixture of mononuclear and dinuclear Pd(0) complexes. Anionic Pd(0) dinuclear complexes exhibit a cooperativity between Pd(0) centers, which favors the oxidative addition of iodobenzene. The higher activity of Pd(0) dimers toward oxidative addition rationalizes the previously reported kinetic laws. In the presence of Li+, the oxidative addition to mononuclear [Pd0L(Li2Cl2)] is estimated barrierless. LiCl coordination polarizes Pd(0), enlarging both the electrophilicity and the nucleophilicity of the complex, which promotes both coordination of the substrate and the subsequent insertion into the C-I bond. These conclusions are paving the way to the rational use of the salt effects in catalysis for the activation of more challenging bonds.

Heterogeneous alkane dehydrogenation catalysts investigated via a surface organometallic chemistry approach.

The selective conversion of light alkanes (C2-C6 saturated hydrocarbons) to the corresponding alkene is an appealing strategy for the petrochemical industry in view of the availability of these feedstocks, in particular with the emergence of Shale gas. Here, we present a review of model dehydrogenation catalysts of light alkanes prepared via surface organometallic chemistry (SOMC). A specific focus of this review is the use of molecular strategies for the deconvolution of complex heterogeneous materials that are proficient in enabling dehydrogenation reactions. The challenges associated with the proposed reactions are highlighted, as well as overriding themes that can be ascertained from the systematic study of these challenging reactions using model SOMC catalysts.

Metal-Free Visible-Light Synthesis of Arylsulfonyl Fluorides: Scope and Mechanism.

The first metal-free procedure for the synthesis of arylsulfonyl fluorides is reported. Under organo-photoredox conditions, aryl diazonium salts react with a readily available SO2 source (DABSO) to afford the desired product through simple nucleophilic fluorination. The reaction tolerates the presence of both electron-rich and -poor aryls and demonstrated a broad functional group tolerance. To shed the light on the reaction mechanism, several experimental techniques were combined, including fluorescence, NMR, and EPR spectroscopy as well as DFT calculations.

Copper-catalyzed transformation of alkyl nitriles to N-arylacetamide using diaryliodonium salts.

This work reports a simple and efficient method for the copper-catalyzed redox-neutral transformation of alkyl nitriles using eco-friendly diaryliodonium salts and leading to N-arylacetamides. The method features high efficiency, broad substrate scope and good functional group tolerance.

Copper Reactivity Can Be Tuned to Catalyze the Stereoselective Synthesis of 2-Deoxyglycosides from Glycals.

We demonstrate that tuning the reactivity of Cu by the choice of oxidation state and counterion leads to the activation of both "armed" and "disarmed" type glycals toward direct glycosylation leading to the α-stereoselective synthesis of deoxyglycosides in good to excellent yields. Mechanistic studies show that CuI is essential for effective catalysis and stereocontrol and that the reaction proceeds through dual activation of both the enol ether as well as the OH nucleophile.

Direct Amination of Alcohols Catalyzed by Aluminum Triflate: An Experimental and Computational Study.

Among the best-performing homogeneous catalysts for the direct amination of activated secondary alcohols with electron-poor amine derivatives, metal triflates, such as aluminum triflate, Al(OTf)3 , stand out. Herein we report the extension of this reaction to electron-rich amines and activated primary alcohols. We provide detailed insight into the structure and reactivity of the catalyst under working conditions in both nitromethane and toluene solvent, through experiment (cyclic voltammetry, conductimetry, NMR spectroscopy), and density functional theory (DFT) simulations. Competition between aniline and benzyl alcohol for Al in the two solvents explains the different reactivities. The catalyst structures predicted from the DFT calculations were validated by the experiments. Whereas a SN 1-type mechanism was found to be active in nitromethane, we propose a SN 2 mechanism in toluene to rationalize the much higher selectivity observed when using this solvent. Also, unlike what is commonly assumed in homogeneous catalysis, we show that different active species may be active instead of only one.

Evidence for a Cooperative Mechanism Involving Two Palladium(0) Centers in the Oxidative Addition of Iodoarenes.

Oxidative addition of iodoarenes (ArI) to Pd0 ligated by 1-methyl-1H-imidazole (mim) in polar solvents leads to cationic arylpalladium(II) complexes [ArPd(mim)3 ]+ . Kinetic analyses evidence that this reaction is second order with respect to the concentration of Pd0 , and a mechanism involving the cooperative intervention of two Pd0 centers has been postulated to explain this finding. This unusual behavior is also observed with other nitrogen-containing ligands and it is general for iodobenzenes substituted with electron-donating or weakly electron-withdrawing groups. In contrast, bromoarenes and electron-poor iodoarenes display first-order kinetics with respect to Pd0 . Theoretical calculations performed at the density functional theory (DFT) level suggest the existence of mim-ligated ArI-Pd0 complexes, in which the iodoarene is bound to the metal in a halogen-bond-like fashion. Coordination weakens the C-I bond and facilitates the oxidative insertion of another Pd0 center across this C-I bond. This conceptually novel mechanism, involving the cooperative participation of two distinct metal centers, allows a full explanation of the experimentally observed kinetics.