increasing the stability of mg2 dobpdc metal organic framework in air through solvent removal

A simple way to increase MOFs stability in air? Solvent removal. Counterintuitive? No, it is not. The condensation of water is delayed by removing all the solvent (polar) molecules from the pores

Thermal Treatment Effect on CO and NO Adsorption on Fe(II) and Fe(III) Species in Fe₃O-Based MIL-Type Metal–Organic Frameworks: A Density Functional Theory Study

The properties of metal–organic frameworks (MOFs) based on triiron oxo-centered (Fe3O) metal nodes are often related to the efficiency of the removal of the solvent molecules and the counteranion chemisorbed on the Fe3O unit by postsynthetic thermal treatment. Temperature, time, and the reaction environment play a significant role in modifying key features of the materials, that is, the number of open metal sites and the reduction of Fe(III) centers to Fe(II). IR spectroscopy allows the inspection of these postsynthetic modifications by using carbon monoxide (CO) and nitric oxide (NO) as probe molecules. However, the reference data sets are based on spectra recorded for iron zeolites and oxides, whose structures are different from the Fe3O one. We used density functional theory to study how the adsorption enthalpy and the vibrational bands of CO and NO are modified upon dehydration and reduction of Fe3O metal nodes. We obtained a set of theoretical spectra that can model the modification observed in previously reported experimental spectra. Several CO and NO bands were previously assigned to heterogeneous Fe(II) and Fe(III) sites, suggesting a large defectivity of the materials. On the basis of the calculations, we propose an alternative assignment of these bands by considering only crystallographic iron sites. These findings affect the common description of Fe3O-based MOFs as highly defective materials. We expect these results to be of interest to the large community of scientists working on Fe(II)- and Fe(III)-based MOFs and related materials

Modeling Metal Influence on the Gate Opening in ZIF-8 Materials

Zeolitic imidazolate frameworks (ZIFs) undergo pressure-induced phase transitions that are peculiar to each ZIF. The phase transition is associated with the rotation of the imidazolate, and it is accompanied by an increase of their pore openings, affecting ZIFs performance in separation processes. This phenomenon is known as the gate opening or the swing effect. Here we report the metal dependence of the ambient pressure and high pressure (HP) phases of ZIF-8(M) with M = Mg, Fe, and Zn, determined by using periodic Kohn–Sham density functional models. We show that the substitution of Zn with Mg or Fe has a big influence on the gate opening energy, which significantly decreases, an opposite trend than what was previously reported upon functionalization of the linker. The lowest energy phase of ZIF-8(Fe) is different than for ZIF-8(Mg) and ZIF-8(Zn), and its structure is significantly closer to the HP phase. Multireference wave function methods have been used to study the electronic structure of ZIF-8(Fe), confirming the metal center to be high spin (S = 2) divalent iron in antiferromagnetic coupling. The high-spin nature of the iron species coupled with a band gap in the visible light range makes ZIF-8(Fe) an interesting material for catalysis and photocatalysis

Influence of First and Second Coordination Environment on Structural Fe(II) Sites in MIL-101 for C–H Bond Activation in Methane

Divalent iron sites in tri-iron oxo-centered metal nodes in metal–organic frameworks (MOFs) catalyze light alkane oxidation. The first two steps of the reaction sequence, which are also the most en..

Multireference Methods are Realistic and Useful Tools for Modeling Catalysis

Highly correlated systems, in particular those that include transition metals, are ubiquitous in catalysis. The significant static correlation found in such systems is often poorly accounted for using Kohn Sham density functional theory methods, as they are single determinantal in nature. Applications to catalysis of more rigorous and appropriate multiconfigurational methods have been reported in select instances, but their use remains rare. We discuss obstacles that hinder the routine application of multireference (MR) wave function theoretical calculations to catalytic systems and the current state of the art with respect to removing those obstacles

Mechanism of Benzene Hydroxylation on Tri-Iron Oxo-Centered Cluster-Based Metal–Organic Frameworks

High-valent Fe(IV)-oxo species derived upon reactions of N2O with Fe(II) centers─embedded in the framework of tri-iron oxo-centered-based metal–organic frameworks (MOFs)─ selectively affect the conversion of benzene-to-phenol via electrophilic addition to arene C–H bonds akin to oxygen transfer mechanisms in the P450 enzyme. The Fe(II) species identified by Mössbauer spectroscopy can be titrated in situ by the addition of NO to completely suppress benzene oxidation, verifying the relevance of Fe(II) centers. Observed inverse kinetic isotope effects in benzene hydroxylation preclude the involvement of H atom transfer steps from benzene to the Fe(IV)-oxo species and instead suggest that the electrophilic iron-oxo group adds to an sp2 carbon of benzene, resulting in a change in the hybridization from sp2-to-sp3. These mechanistic postulates are affirmed in Kohn–Sham density functional calculations, which predict lower barriers for additive mechanisms for arene oxidation than H atom abstraction steps. The calculations show that the reaction proceeds on the pentadectet spin surface and that a non-innocent ligand participates in the transfer of the H atom. Following precedent literature which demonstrates that these Fe(IV)-oxo species react with C–H bonds in alkanes via hydrogen atom abstraction to form alcohols, it appears that iron(IV)-oxo species in MOFs exhibit duality in their reactions with inert hydrocarbon substrates akin to enzymes─if the C–H bonds are in saturated aliphatic hydrocarbons, then activation occurs via hydrogen abstraction, while if the C–H bonds are aromatic, then activation occurs by addition rearrangement

X‐ray and Synchrotron FTIR Studies of Partially Decomposed Magnesium Borohydride

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