Aromaticity rules. A comment article in Nature Chemistry

Since the formulation of the Hückel’s rule, a variety of rules have been proposed to determine if a molecule is aromatic. These allow chemists to better understand molecules and their behavior, as well as identify the formation or elimination of (anti)aromatic species in a reaction, which helps understand and predict possible outcomes. In this comment article, Prof. Miquel Solà first briefly discusses the most widespread rules associated with different types of aromaticity, then draws attention to their limitations, and finally propose future directions for the development of this fascinating topic.

The paper can be read in Nature Chemistry journal through the following link:

https://www.nature.com/articles/s41557-022-00961-w

3-D aromaticity is not as usual as it may seem

The aromaticity of 3-D closo boranes, zintl ions, and charged fullerenes is widely accepted. On the other hand, several fully p-conjugated macrocycles with puckered or cage-type structures were recently found to exhibit aromatic character according to both experiments and computations. We examine their electronic structures and put them in relation to 3D-aromatic molecules (e.g., closo-boranes) and to 2D-aromatic polycyclic aromatic hydrocarbons. Using qualitative theory combined with quantum chemical calculations, we find that the macrocycles explored hitherto should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic. We establish the conditions that have to be fulfilled to classify a compound as 3D-aromatic. Indeed, we show that it is not easy to design new 3D-aromatic compounds. This work was carried out by Dr. Ouissam El Bakouri, Dr. Kjell Jorner, Dr. Rabia Ayub and Prof. Henrik Ottosson from the Ångström Laboratory of the Uppsala University, Prof. Patrick Bultinck from the Department of Chemistry of Chent University and Dr. Dariusz W. Szczepanik and Prof. Miquel Solà from the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona

El Bakouri, D. W. Szczepanik, K. Jorner, R. Ayub, P. Bultinck, M. Solà* and H Ottosson*. Three-Dimensional Fully p-Conjugated Macrocycles: When 3D-Aromatic and when 2D-Aromatic-in-3D? J. Am. Chem. Soc., 2022, DOI: 10.1021/jacs.1c13478.

The article has been highlighted and discussed in the following entry of Chemistry World:

Many molecules mislabelled as 3D aromatic

 

A new non-covalent interaction: the nido cage-···nido cage- interaction

Carboranes are boron–carbon clusters with important applications in fields of materials, catalysis, pharmaceuticals, etc. However, the non-covalent interactions that could determine the solid-state structures and properties of such boron clusters have been rarely investigated. Herein, inspired by the coordinate bond in metallacarborane or ferrocene, the boron cluster-based non-covalent interaction (denoted as cage···cage interaction) between two nido-carborane clusters was successfully realized by using a pyridinium-based molecular barrier. The X-ray diffraction studies uncover that the cage×××cage interaction has a contacting distance of 5.4-7.0 Å from centroid to centroid in the systems reported here. Theoretical calculations validate the formation of the non-covalent interaction and disclose its repulsive bonding nature that is overcome thanks to the positively charged pyridinium-based framework. Interestingly, such bulk crystalline materials containing the cage···cage interaction show relevant properties such as full-color absorption in the visible light range and important  photothermal effect, which are absent for the control compound without carboranes. This study may offer fundamental insights into the boron cluster-based non-covalent interactions and open a new research avenue to rationally design boron cluster-based materials. Finally, we have computationally shown that this π···π interaction is also possible in classical organic systems.

Synthesis of the clusters were done by the group of Prof. Hong Yan in Nanjing University and calculations were performed in the IQTCUB institute by Prof. Jordi Poater and in the DiMoCat group by Prof. Miquel Solà.

The paper has been highlighted by the Spanish Biophysical Society (https://sbe.es/paperhighlights-nov2021-2/).

The paper can be read in JACS Au journal through the following link:

https://pubs.acs.org/doi/10.1021/jacsau.1c00348

The adaptive aromaticity of metallapentalenes in the Front Cover of Chem. Eur. J.!

The Chemistry: A European Journal features in its FRONT COVER of issue number 57 of volume 26 the recently published article “Proving the origin of adaptive aromaticity in 16-valence-electron metallapentalenes”. The work has been carried out by Prof. Jun Zhu at the University of Xiamen and Dandan Chen, Dr. Dariusz W. Szczepanik, and Prof. Miquel Solà members of the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona. The cover is the result of the artistic inspiration of Dandan Chen. The paper has been highlighted in Chem. Eur. J. 26 (2020) 12902

The aromaticity of boron clusters in the Front Cover of JACS!

The Journal of the American Chemical Society (JACS) features in its FRONT COVER the recently published article “Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes”. The work has been carried out by Prof. Francesc Teixidor, Prof. Clara Viñas, and Dr. Ines Bennour of the Institute of Materials Science of Barcelona (ICMAB-CSIC), Prof. Jordi Poater at the University of Barcelona (previous DiMoCat member) and Sílvia Escayola and Prof. Miquel Solà members of the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona. The cover is the result of the artistic inspiration of Sílvia Escayola.

We paper is also highlighted by the editor in the Spotlights on Recent JACS Publications.

You can go to the JACS website to read the abstract and the full article “Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes”.

Jordi Poater, Clara Viñas, Ines Bennour, Sílvia Escayola, Miquel Solà*, Francesc Teixidor*
J. Am. Chem. Soc. 2020, 1429396–9407.
DOI: https://doi.org/10.1021/jacs.0c02228

 

Aromaticity in Boron Clusters Survives Radical Structural Changes

Whereas the aromaticity of closo boranes is widely accepted, less is known about the aromaticity of nido boranes. This work carried out by Prof. Francesc Teixidor, Prof. Clara Viñas, and Dr. Ines Bennour of the Institute of Materials Science of Barcelona (ICMAB-CSIC), Prof. Jordi Poater at the University of Barcelona (previous DiMoCat member) and Sílvia Escayola and Prof. Miquel Solà members of the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona, experimentally shows that deboronation of m-C2B9H12 is a difficult task, whereas deboranation of o-C2B9H12 is quite easy. Moreover, it is widely known that o-C2B10H12 isomerizes to m-C2B10H12 upon heating at 400 ºC. These two facts indicate that m-C2B10H12 is more stable than o-C2B10H12. To find a reason for the different stability of these two isomers, authors have analyzed the thermodynamic stability and aromaticity of these closo carboranes and their nido counterparts. Results show that the higher thermodynamic stability of m-C2B10H12 is not related to aromaticity differences but to the location of the C atoms in the carborane structure. It is also demonstrated that the aromaticity observed in closo boranes and carboranes is also present in their nido counterparts and, consequently, authors conclude that aromaticity in boron clusters survives radical structural changes. Further, sandwich metallocenes (e.g. ferrocene) and sandwich metallacarboranes (e.g. [Co(C2B9H11)2]) have traditionally been considered similar. In this work, it is shown that they are not. Metallacarboranes display global aromaticity, whereas metallocenes present local aromaticity in the ligands. Remarkable and unique is the double probe given by 1H- and 11B-NMR tracing the reciprocally antipodal endocyclic open face Hec and B1. These magnetic studies have permitted to correlate both nuclei and relate them to a diatropic current in the plane at the middle of the nido-[C2B9H12]. This observation is the first and unique data that proves experimentally the existence of diatropic currents, thence aromaticity, in nido clusters and is comparable to the existence of diatropic currents in planar aromatic compounds.

 

 

Poater, J.; Viñas, C.; Bennour, I.; Escayola, S.; Solà*, M.; Teixidor*, F. Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes. J. Am. Chem. Soc., 2020, DOI: 10.1021/jacs.0c02228

Financial support: This work has been supported by the Ministerio de Economía y Competitividad (MINECO) of Spain (Projects CTQ2017-85341-P, CTQ2016-77558-R, and MDM-2017-0767) and the Generalitat de Catalunya (projects 2017SGR39 and 2017SGR348). Excellent service by the Supercomputer center of the Consorci de Serveis Universitaris de Catalunya (CSUC) is gratefully acknowledged.

Discovered a new non-covalent interaction between boron nido clusters and aromatic rings

Exploration and comprehension of chemical bonding is one of the central tasks in chemistry. Here, a new non‐covalent interaction, a nido‐cage···π bond, is discovered in a collaborative work between Deshuang Du and Prof. Yan from Nanjing University and Prof. Jordi Poater at the University of Barcelona (previous DiMoCat member) and Prof. Miquel Solà from the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona. The new interaction found is between the boron cluster C2B9H12 and several aromatic π systems. The X‐ray diffraction studies indicate that the nido‐cage···π bonding presents parallel‐displaced or T‐shaped geometries. The theoretical calculations confirms that this nido‐cage···π  bond shares a similar nature to the conventional anion···π or π···π bonds found in classical aromatic ring systems. Besides, such a nido‐cage···π interaction induces variable photophysical properties such as aggregation‐induced emission and aggregation‐caused quenching in one molecule. This work offers an overall understanding towards the boron cluster‐based non‐covalent bond and opens a door to investigate its properties.

Tu, D.; Yan, H.; Poater, J.; Solà, M. “nido‐Cage···π Bond: A Non‐covalent Interaction between Boron Clusters and Aromatic Rings and Its Applications” Angew. Chem. Int. Ed. 2020,  DOI: 10.1002/anie.201915290

Two new PhD students in the DiMoCat group

We have recently incorporated two new PhD students in our group with two cotutelle agreements.

First, Daniel Eduardo Trujillo that will perform a theoretical study of the electronic structure and reaction mechanisms of reactions of multiple-bound boron compounds that can activate C-C and C-H bonds. Daniel will be supervised by J. Oscar C. Jimenez-Halla and Gerardo Gonzalez in the University of Guanajauto (Mexico) and Miquel Solà in the University of Girona.

A photo of Daniel E. Trujillo

Second, Dandan Chen that will be working in the study of aromatic species with special emphasis in metalloaromatic species in excited states. Dandan will be supervised by Jun Zhu in Xiamen University (China) and Miquel Solà in the University of Girona.


A photo of Dandan Chen

We wish Daniel and Dandan a great success with their PhD research!

Two recent DiMoCat works highlighted by ChemistryWorld

ChemistryWorld has recently highlighted two works of our group:

The jellium model assumes a uniform distribution of positive charge corresponding to the cluster atomic nuclei and their innermost electrons in which the interacting valence electrons move. The energy levels of valence electrons for such a model are 1S21P61D102S21F142P61G182D103S2…, where S, P, D, F, and G letters denote the angular momentum and numbers 1, 2, 3 indicate the radial nodes. The abundance found in experimental mass spectra of alkali, alkaline earth metals, and gold clusters of 2, 8, 18, 20, 34, 40… atoms are justified taken into account that these numbers correspond to closed-shell electronic structures in the jellium model. We have proven that if the last energy level of valence electrons for the jellium model is half-filled with same-spin electrons, the system has also an aromatic character that provides extra stability. This situation is reached for the magic numbers of valence electrons of 1 (S =1/2), 5 (S = 3/2), 13 (S = 5/2), 19 (S = 1/2), 27 (S = 7/2), 37 (S = 3/2), 49 (S = 9/2)… This new set of magic numbers may become a powerful tool for researchers who work in the quest for stable single high-spin molecules for their use as single-molecule based magnets. The paper has been published in Chem. Commun. and can be found in the following link Chem. Commun., 55 (2019) 5559-5562.

In this recent Chem. Commun. paper, we have proved that C18 (a cycle of 18 carbon atoms connected with alternating single and triple bonds) it is an electron acceptor of similar characteristics as C60. C18  when coupled with a range of donor molecules can readily accept electrons. Electron acceptors are important components in molecular electronic devices and solar cells, and, therefore, C18 is added to the list of organic electron acceptors that can be potentially useful in photovoltaics.