Unlike their SCS analogues, SNS pincer complexes are poorly studied for their use in coupling reactions. Accordingly, a series of water soluble cationic Pd(II) SNS pincer complexes have been successfully synthesised and investigated in detail for their catalytic activity in Suzuki–Miyaura coupling reactions. By using only 0.5 mol % loading of the complexes, the coupling of inactivated aryl bromides and activated aryl chlorides with various boronic acids in water was achieved in excellent yields and the catalysts were found to be reusable for three cycles without a significant loss of activity. The investigation of the mechanism of the reaction revealed that a Pd(II) to Pd(IV) route is the more likely pathway which was further supported by computational studies.

Detailed insight into molecular diffusion in zeolite frameworks is crucial for the analysis of the factors governing their catalytic performance in methanol-to-hydrocarbons (MTH) reactions. In this work, we present a molecular dynamics study of the diffusion of methanol in all-silica and acidic zeolite MFI and Beta frameworks over the range of temperatures 373–473 K. Owing to the difference in pore dimensions, methanol diffusion is more hindered in H-MFI, with diffusion coefficients that do not exceed 10×10−10 m2s−1. In comparison, H-Beta shows diffusivities that are one to two orders of magnitude larger. Consequently, the activation energy of translational diffusion can reach 16 kJ·mol−1 in H-MFI, depending on the molecular loading, against a value for H-Beta that remains between 6 and 8 kJ·mol−1. The analysis of the radial distribution functions and the residence time at the Brønsted acid sites shows a greater probability for methylation of the framework in the MFI structure compared to zeolite Beta, with the latter displaying a higher prevalence for methanol clustering. These results contribute to the understanding of the differences in catalytic performance of zeolites with varying micropore dimensions in MTH reactions.

We present a Hubbard-corrected density functional theory (DFT+U) study of the adsorption and reduction reactions of oxygen on the pure and 25% Ca-doped LaMnO3 (LCM25) {100} and {110} surfaces.

The mechanistic pathways for the tandem sequential [4 + 2] / [3 + 2] and [3 + 2]/[4 + 2] cycloaddition reaction of functionalized‐acetylenes with cyclooctatetraene (COTE) and nitrile imines for the formation of the biologically‐active tricyclic cyclobutane‐condensed pyrazoline systems, and the subsequent cycloreversion/thermolysis of these adducts, have been studied using density functional theory (DFT) at the M06‐2X/6‐31G(d) and 6‐311G(d,p) levels with the aim of providing mechanistic rationale for the regioselectivities and stereoselectivities. Along the [4 + 2] / [3 + 2] addition sequence, it has been found that the initial 6π‐electrocyclic ring‐closure of the COTE is the rate‐determining step irrespective of the type of substituent on the parent acetylene or the nitrile imine. The mechanistic channels along the [4 + 2] / [3 + 2] addition sequence show that the addition of the dipole across the unsubstituted olefinic bond in the cyclohexadiene moiety in the endo fashion is the most favored, which is in agreement with experimentally observed selectivity. The results show that the thermolysis proceeds with relatively high activation barriers toward formation of the monocyclic pyrazolines among other products. The decomposition of the tandem adducts has been found to be controlled solely by kinetic factors. An exploration of a [3 + 2] / [4 + 2] addition sequence as a mechanistic possibility reveals that in contrast to the [4 + 2] / [3 + 2] addition sequence, the Diels‐Alder addition step is the rate‐determining. The [3 + 2] / [4 + 2] addition order is found as an approach with better selectivity as it leads to formation of only tandem adducts where the nitrile imines are attached to the substituted olefinic bond in the cyclohexadiene subunit. The results show that the monocyclic pyrazolines obtained from the thermolysis of the [4 + 2] / [3 + 2] tandem adducts could easily be obtained from a direct 1,3‐dipolar addition of the alkynes with the dipoles which has been found to proceed rapidly with low activation barriers. The results are rationalized with perturbation molecular orbital theory.

The oxo complexes of group VII B are of great interest for their potential toward epoxidation and dihydroxylation. In this work, the mechanisms of oxidation of ethylene by rhenium-oxo complexes of the type LReO3 (L = O(-), Cl, CH3, OCH3, Cp, NPH3) have been explored at the B3LYP/LACVP* level of theory. The activation barriers and reaction energies for the stepwise and concerted addition pathways involving multiple spin states have been computed. In the reaction of LReO3 (L = O(-), Cl, CH3, OCH3, Cp, NPH3) with ethylene, it was found that the concerted [3 + 2] addition pathway on the singlet potential energy surfaces leading to the formation of a dioxylate intermediate is favored over the [2 + 2] addition pathway leading to the formation of a metallaoxetane intermediate and its re-arrangement to form the dioxylate. The activation barrier for the formation of the dioxylate on the singlet PES for the ligands studied is found to follow the order O(-) > CH3 > NPH3 > CH3O(-) > Cl(-) > Cp and the reaction energies follow the order CH3 > O(-) > NPH3 > CH3O(-) > Cl(-) > Cp. On the doublet PES, the [2 + 2] addition leading to the formation the metallaoxetane intermediate is favored over dioxylate formation for the ligands L = CH3, CH3O(-), Cl(-). The activation barriers for the formation of the metallaoxetane intermediate are found to increase for the ligands in the order CH3 < Cl(-) < CH3O(-) while the reaction energies follow the order Cl(-) < CH3O(-) < CH3. The subsequent re-arrangement of the metallaoxetane intermediate to the dioxylate is only feasible in the case of ReO3(OCH3). Of all the complexes studied, the best dioxylating catalyst is ReO3Cp (singlet surface); the best epoxidation catalyst is ReO3Cl (singlet surface); and the best metallaoxetane formation catalyst is ReO3(NPH3) (triplet surface).

No abstract

The ABO perovskite lanthanum ferrite (LaFeO) is a technologically important electrode material for nickel-metal hydride batteries, energy storage and catalysis. However, the electrochemical hydrogen adsorption mechanism on LaFeO surfaces remains under debate. In the present study, we have employed spin-polarized density functional theory calculations, with the Hubbard U correction (DFT+U), to unravel the adsorption mechanism of H on the LaFeO(010) surface. We show from our calculated adsorption energies that the preferred site for H adsorption is the Fe-O bridge site, with an adsorption energy of -1.18 eV (including the zero point energy), which resulted in the formation of FeOH and FeH surface species. H adsorption at the surface oxygen resulted in the formation of a water molecule, which leaves the surface to create an oxygen vacancy. The H molecule is found to interact weakly with the Fe and La sites, where it is only physisorbed. The electronic structures of the surface-adsorption systems are discussed via projected density of state and Löwdin population analyses. The implications of the calculated adsorption strengths and structures are discussed in terms of the improved design of nickel-metal hydride (Ni-MH) battery prototypes based on LaFeO.

We have used spin polarized density functional theory calculations to perform extensive mechanistic studies of CO2 dissociation into CO and O on the clean Fe (100), (110) and (111) surfaces and on the same surfaces coated by a monolayer of nickel. CO2 chemisorbs on all three bare facets and binds more strongly to the stepped (111) surface than on the open flat (100) and close-packed (110) surfaces, with adsorption energies of -88.7 kJmol -1 , -70.8 kJmol -1 and -116.8 kJmol -1 on the (100), (110) and (111) (110) and (111) facets respectively. We have also investigated the thermodynamics and activation energies for CO2 dissociation into CO and O on the bare and Ni-deposited surfaces.Generally, we found that the dissociative adsorption states are thermodynamically preferred over molecular adsorption, with the dissociation most favoured thermodynamically on the closepacked (110) facet. The trends in activation energy barriers were observed to follow that of the trends in surface work functions; consequently, the increased surface work functions observed on the Ni-deposited surfaces resulted in increased dissociation barriers and vice versa. These results suggest that measures to lower the surface work function will kinetically promote the dissociation of CO2 to CO and O, although the instability of the activated CO2 on the Ni-covered 2 surfaces will probably result in CO2 desorption from the nickel-doped iron surfaces, as is also seen on the Fe(110) surface. Graphical abstract Introduction