Revisiting ion-pair interactions in phase transfer catalysis: from ionic compounds to real catalyst systems.

Ion-pairing is a fundamental phenomenon that significantly influences phase-transfer catalysis. In this study, we conduct a comprehensive investigation of ion-pair interactions, aiming to establish a comprehensive understanding of their nature and implications. The study begins with the examination of polar ionic compounds to define the concept of an ion-pair in the context of phase-transfer catalysis. Subsequently, a diverse range of ion-pair catalyst models were explored to gain insight into the factors governing their interactions. Finally, the focus shifts towards the characterisation of real phase-transfer catalysts, bridging the gap between theoretical models and practical applications. Through a combination of computational approaches and theoretical analysis, this work provides valuable insight into the nature of ion-pair interactions within phase-transfer catalysis fields.

Simultaneous Hydrogen Bonds with Different Binding Modes: The Acceptor "Rules" but the Donor "Chooses".

Invited for the cover of this issue is the group of Cristina Trujillo at Trinity College Dublin and the University of Manchester. The image depicts a market run by hydrogen bond acceptors in which hydrogen-bond-donor "customers" are choosing their preferred binding mode "vegetable". Read the full text of the article at 10.1002/chem.202203577.

Simultaneous Hydrogen Bonds with Different Binding Modes: The Acceptor "Rules" but the Donor "Chooses".

This computational work studies the different hydrogen bond (HB) binding modes that can be established between neighbouring HB donors and acceptors in structures with relevance in catalysis and biology. To analyse the electronic effect on the σ-hole, unsubstituted HB donors and ones with two different substituents, an electron withdrawing (EWG), and an electron donating (EDG) group, were studied. Upon complexation, three different binding modes were observed: bifurcated, parallel, and zigzag. It was found that, as a general trend, HBs within a parallel pattern are the strongest followed by those within bifurcated and zigzag binding modes, leading to a "competition" between the last two. Similar patterns and trends have been found in experimental structures found in a search within the CSD. In conclusion, even though the HB acceptors "rule" the pattern and strength of the HB interactions within the dimers, when there is an option for different binding modes within a particular dimer, the HB donors "choose" the type of binding established.

Efficiency and Suitability when Exploring the Conformational Space of Phase-Transfer Catalysts.

In this study, a complete exploration of the conformational space of different phase-transfer catalysts by means of computational method benchmarking is presented. For this particular research work, only the most significant and relevant conformational analysis approaches have been chosen to characterize the main Cinchona alkaloid-based phase-transfer catalysts. This particular guiding study aims to rigorously compare the performance of different conformational methods, determining the strengths of each method and providing recommendations regarding suitable and efficient choices of methods for analysis.

Theoretical Perspectives in Organocatalysis.

It is clear that the field of organocatalysis is continuously expanding during the last decades. With increasing computational capacity and new techniques, computational methods have provided a more economic approach to explore different chemical systems. This review offers a broad yet concise overview of current state-of-the-art studies that have employed novel strategies for catalyst design. The evolution of the all different theoretical approaches most commonly used within organocatalysis is discussed, from the traditional approach, manual-driven, to the most recent one, machine-driven.

Reactivity of Coinage Metal Hydrides for the Production of H2 Molecules.

Invited for this month's cover picture is the group of Dr. Cristina Trujillo at the Trinity Biomedical Sciences Institute in Dublin (Ireland) and Prof. Ibon Alkorta at the CSIC in Madrid (Spain). The cover picture shows the possibility of reversibly storing hydrogen using metallic compounds as reactants to produce H2 gas and metallic dimers. Computational studies have been carried out to investigate these processes, as shown in the concept art by the Schrödinger equation. It was found that the formation and release of H2 is energetically favorable. These results are a promising starting point for further research of using coinage metals for storing hydrogen in light compounds. Read the full text of their Full Paper at 10.1002/open.202100108.

Reactivity of Coinage Metal Hydrides for the Production of H2 Molecules.

The formation of molecular hydrogen as well as the possibility of using coinage metal hydrides as a prospective complex to produce hydrogen was presented in this work. Therefore, the reactions involving the interaction between two coinage metal hydrides, MH (M=Cu, Ag and Au, homo and heterodimers), were studied. The free energy profiles corresponding to aforementioned complexation were analysed by means of ab initio methods of quantum chemistry. The characteristics of these intermediates, final complexes and the electron density properties of the established interactions were discussed.

Evaluation of Electron Density Shifts in Noncovalent Interactions.

In the present paper, we report the quantitative evaluation of the electron density shift (EDS) maps within different complexes. Values associated with the total EDS maps exhibited good correlation with different quantities such as interaction energies, Eint, intermolecular distances, bond critical points, and LMOEDA energy decomposition terms. Besides, EDS maps at different cutoffs were also evaluated and related with the interaction energies values. Finally, EDS maps and their corresponding values are found to correlate with Eint within systems with cooperative effects. To our knowledge, this is the first time that the EDS has been quanitatively evaluated.

Improving phase-transfer catalysis by enhancing non-covalent interactions.

A wide variety of asymmetric transformations catalysed by chiral catalysts have been developed for the synthesis of valuable organic compounds in the past several decades. Within the asymmetric catalysis field, phase-transfer catalysis has been recognized as a powerful method for establishing useful procedures for organic synthesis. In the present study intermolecular interactions between a well-known alkaloid quinine-derived phase transfer catalyst and four different anions were characterised, analysing the competition between the pure ion-pair interaction and the intermolecular hydrogen bond established upon complexation. Finally, a theoretical study of the free-energy profile corresponding to the enantioselective conjugate cyanation of an α,ß-unsaturated ketone in the presence of two different catalysts was performed.

Anion Recognition by Neutral and Cationic Iodotriazole Halogen Bonding Scaffolds.

A computational study of the iodide discrimination by different neutral and cationic iodotriazole halogen bonding hosts was carried out by means of Density Functional Theory. The importance of the size of the scaffold was highlighted and its impact observed in the binding energies and intermolecular X···I distances. Larger scaffolds were found to reduce the electronic repulsion and increase the overlap between the halide electron lone pair and the corresponding I-C antibonding orbital, increasing the halogen bonding interactions. Additionally, the planarity plays an important role within the interaction, and can be tuned using hydroxyl to perform intramolecular hydrogen bonds (IMHB) between the scaffold and the halogen atoms. Structures with IMHB exhibit stronger halogen bond interactions, as evidenced by the shorter intramolecular distances, larger electron density values at the bond critical point and more negative binding energies.

Cations brought together by hydrogen bonds: the protonated pyridine-boronic acid dimer explained.

According to the Cambridge Structural Database, protonated pyridine-boronic acid dimers exist in the solid phase, apparently defying repulsive coulombic forces. In order to understand why these cation-cation systems are stable, we carried out M06-2X/6-311++G(3df,2pd) electronic structure calculations and used a set of computational tools (energy partitioning, topology of the electron density and electric field maps). The behavior of the charged dimers was compared with the corresponding neutral systems, and the effect of counterions (Br- and BF4-) and the solvent (PCM model) on the binding energies has been considered. In the gas-phase, the charged dimers present positive binding energies but are local minima, with a barrier (16-19 kJ mol-1) preventing dissociation. Once the environment is included via solvent effects or counterions, the binding energies become negative; remarkably, the strength of the interaction is very similar in both neutral and charged systems when a polar solvent is considered. Essentially, all methods used evidence that the intermolecular region where the HBs take place is very similar for both neutral and charged dimers. The energy partitioning explains that repulsion and electrostatic terms are compensated by the desolvation and exchange terms in polar solvents, thus giving stability to the charged dimer.