Luca Bellarosa

Highly Efficient Redox Isomerisation of Allylic Alcohols Catalysed by Pyrazole‐Based Ruthenium(IV) Complexes in Water: Mechanisms of Bifunctional Catalysis in Water

The catalytic activity of ruthenium(IV) ([Ru(η(3):η(3)-C(10)H(16))Cl(2)L]; C(10)H(16) = 2,7-dimethylocta-2,6-diene-1,8-diyl, L = pyrazole, 3-methylpyrazole, 3,5-dimethylpyrazole, 3-methyl-5-phenylpyrazole, 2-(1H-pyrazol-3-yl)phenol or indazole) and ruthenium(II) complexes ([Ru(η(6)-arene)Cl(2)(3,5-dimethylpyrazole)]; arene = C(6)H(6), p-cymene or C(6)Me(6)) in the redox isomerisation of allylic alcohols into carbonyl compounds in water is reported. The former show much higher catalytic activity than ruthenium(II) complexes. In particular, a variety of allylic alcohols have been quantitatively isomerised by using [Ru(η(3):η(3)-C(10)H(16))Cl(2)(pyrazole)] as a catalyst; the reactions proceeded faster in water than in THF, and in the absence of base. The isomerisations of monosubstituted alcohols take place rapidly (10-60 min, turn-over frequency = 750-3000 h(-1)) and, in some cases, at 35 °C in 60 min. The nature of the aqueous species formed in water by this complex has been analysed by ESI-MS. To analyse how an aqueous medium can influence the mechanism of the bifunctional catalytic process, DFT calculations (B3LYP) including one or two explicit water molecules and using the polarisable continuum model have been carried out and provide a valuable insight into the role of water on the activity of the bifunctional catalyst. Several mechanisms have been considered and imply the formation of aqua complexes and their deprotonated species generated from [Ru(η(3):η(3)-C(10)H(16))Cl(2)(pyrazole)]. Different competitive pathways based on outer-sphere mechanisms, which imply hydrogen-transfer processes, have been analysed. The overall isomerisation implies two hydrogen-transfer steps from the substrate to the catalyst and subsequent transfer back to the substrate. In addition to the conventional Noyori outer-sphere mechanism, which involves the pyrazolide ligand, a new mechanism with a hydroxopyrazole complex as the active species can be at work in water. The possibility of formation of an enol, which isomerises easily to the keto form in water, also contributes to the efficiency in water.

On the Mechanism Behind the Instability of Isoreticular Metal-Organic Frameworks (IRMOFs) in Humid Environments

Increasing the resistance to humid environments is mandatory for the implementation of isoreticular metal-organic frameworks (IRMOFs) in industry. To date, the causes behind the sensitivity of [Zn(4)(μ(4)-O)(μ-bdc)(3)](8)(IRMOF-1; bdc=1,4-benzenedicarboxylate) to water remain still open. A multiscale scheme that combines Monte Carlo simulations, density functional theory and first-principles Born-Oppenheimer molecular dynamics on IRMOF-1 was employed to unravel the underlying atomistic mechanism responsible for lattice disruption. At very low water contents, H(2)O molecules are isolated in the lattice but provoke a dynamic opening of the terephthalic acid, and the lattice collapse occurs at about 6% water weight at room temperature. The ability of Zn to form fivefold coordination spheres and the increasing basicity of water when forming clusters are responsible for the displacement of the organic linker. The present results pave the way for synthetic challenges with new target linkers that might provide more robust IRMOF structures.

Solvent‐Dependent Cation Exchange in Metal–Organic Frameworks

We investigated which factors govern the critical steps of cation exchange in metal-organic frameworks by studying the effect of various solvents on the insertion of Ni(2+) into MOF-5 and Co(2+) into MFU-4l. After plotting the extent of cation insertion versus different solvent parameters, trends emerge that offer insight into the exchange processes for both systems. This approach establishes a method for understanding critical aspects of cation exchange in different MOFs and other materials.

State-of-the-art and challenges in theoretical simulations of heterogeneous catalysis at the microscopic level

Theoretical simulations of systems that represent heterogeneous catalysts constitute one of the main tools in the research for new catalytic materials. Theory plays a role in the three stages of the development ladder: characterisation, understanding and prediction. Due to the complexity of the computational methods, there is a strong need to integrate different models and cover the relevant scales in heterogeneous catalysis. This requirement constitutes an important drawback as scientists need training in several aspects of the problem including chemical, physical and engineering views of the modelling while keeping the experimental and industrial interests and needs in perspective. Here we present some of the latest developments in the field of theoretical simulations at the microscopic level while illustrating suitable examples that show how theory can shed light on several aspects of characterisation, activity, selectivity and long-term stability.

How ligands improve the hydrothermal stability and affect the adsorption in the IRMOF family

Metal-Organic Frameworks are considered to be the next generation of sorbents both because of their synthetic versatility and high selectivity potential. In the first generation (IRMOF), the main drawback for commercial implementation is the lack of hydrothermal stability. Even if several studies have been conducted to elucidate the reasons behind their structural weakness in humid environments, how apparently small changes in the stoichiometry of the building units affect the stability of the lattice is still poorly understood. Using density functional theory and ab initio molecular dynamics we investigated the reason behind the different behaviour of several substituted IRMOF-1 structures. We show that hydrophilic variations in the organic linkers work as new basins of attraction for the incoming water molecules, thus depleting the water content at the metal center. To confirm this, we performed Monte Carlo simulations to provide insights into the adsorption energies and check the effectiveness of the adsorption sites in the substituted structures for a variety of polar and non-polar molecules. The results show that linker modification affects molecular adsorption and can improve the overall stability of the lattice redirecting water to the new sites in the case of hydrophilic units. Three key parameters have been singled out to rationalize this behaviour, and used to predict the favoured adsorption sites in the case of gas mixtures.

Diversity at the Water–Metal Interface: Metal, Water Thickness, and Confinement Effects

The structure and properties of water films in contact with metal surfaces are crucial to understand the chemical and electrochemical processes involved in energy-related technologies. The nature of thin water films on Pd, Pt, and Ru has been investigated by first-principles molecular dynamics to assess how the chemistry at the water–metal surface is responsible for the diversity in the behavior of the water layers closer to the metal. The characteristics of liquid water: the radial distribution functions, coordination, and fragment speciation appear only for unconfined water layers of a minimum of 1.4 nm thick. In addition, the water layer is denser in the region closest to the metal for Pd and Pt, where seven- and five-membered ring motifs appear. These patterns are identical to those identified by scanning tunneling microscopy for isolated water bilayers. On Ru densification at the interface is not observed, water dissociates, and protons and hydroxyl groups are locked at the surface. Therefore, the acid–base properties in the area close to the metal are not perturbed, in agreement with experiments, and the bulk water resembles an electric double layer. Confinement affects water making it closer to ice for both structural and dynamic properties, thus being responsible for the higher viscosity experimentally found at the nanoscale. All these contributions modify the solvation of reactants and products at the water–metal interface and will affect the catalytic and electrocatalytic properties of the surface.

How Theoretical Simulations Can Address the Structure and Activity of Nanoparticles

Theoretical simulations in the field of heterogeneous catalysis started about two decades ago when the main goal was to understand the activation of small molecules on infinite surfaces. The improvements in the accuracy and the large availability of computers with increasing power have raised the quality of the calculations, the reliability of the results and prompted the interest in their predictions. Such changes have also allowed the study of nanoparticles by the combined investigation of different facets or by taking into account the complete structures. As for the reactivity, theoretical simulations allow the comparison of different synthetic conditions within the same approximation. Consequently, large systematic studies with the same theoretical models can provide databases for properties, structures, prove and disprove hypothetical reaction paths, identify intermediates, and complete the understanding of reaction mechanisms. In some cases, simulations support and give explanations to experiments but new emerging aspects such as the prediction of new properties or the analysis of complex systems are possible. Several challenges are ahead the simulations of reactions on nanoparticles: (i) how to drive the synthesis to achieve the desired architectures and (ii) how to stabilize the active phase under reaction conditions.

Structure, Activity, and Deactivation Mechanisms in Double Metal Cyanide Catalysts for the Production of Polyols

Polyether polyols are used widely in the plastic and coating industries in the form of polyurethanes. The polymerization of epoxides can be catalyzed by double metal cyanides (DMCs), Zn3[Co(CN)6]2. These catalysts were first reported in the 1960s by General Tire Inc. and provide products with excellent technical features, which are better than those that result from traditional alkaline polymerization as side reactions are alleviated. However, DMC‐catalyzed polymerization is not free of drawbacks as high‐molecular‐weight side products (1–3 wt %) can form in the propylene process. These tails are detrimental to the subsequent use of these polymers, in particular to foam stability. Despite the wide industrial interest in DMCs, there are only a few experimental studies and a complete lack of theoretical research of their structure, activity, and performance. The present work is thus the first attempt to describe the nature of the active site, the main polymerization mechanism, and two potential origins for the high‐weight tails from a theoretical standpoint by analyzing three crucial steps in the polymerization process.