3D-aromatic compounds are scarce. The first known species with 3D-aromaticity were the closo boranes, such as [B₁₀H₁₀]^2- or [B₁₂H₁₂]^2- and derivatives. Not only closo boranes, but also carboranes, which result from single or double substitution of BH by isoelectronic CH⁺ units in closo boranes, are 3D-aromatic compounds. Apart from closo (car)boranes, fullerenes in certain spin and charge states and Zintl phases are other known 3D-aromatic compounds. On the other hand, doubly 3D-aromatic species are unknown.

In this work, we describe the experimental ¹¹B{¹H} NMR spectra of iodinate derivatives that result from the stepwise substitution of hydrogen atoms by iodine atoms in the neutral o-carborane (1,2-C₂B₁₀H₁₂). These NMR spectra allows us to conclude the existence of I···I non-covalent interactions in substituted boron-iodinated o-carboranes (I_n-1,2-C₂B₁₀H_12-n, n=1-10). The existence of such interactions in periodinated o-carboranes together with the well-known double 2D-aromaticity of C₆I₆⁺² and related species, lead us to wonder whether it would be possible to generate doubly 3D-aromatic species by oxidation of [B₁₂I₁₂]^2- to generate [B₁₂H₁₂] and [B₁₂H₁₂]²⁺. Magnetic properties of [B₁₂H₁₂]^0/2+ species indicate that these two oxidized forms of closo icosahedral dodecaiodo-dodecaborate cluster behave as doubly 3D-aromatic compounds. Analysis of the electronic properties of [B₁₂I₁₂]^0/2+ reveals an increase in delocalization within the iodine shell, yet it proves insufficient to fully support double 3D-aromaticity. Finally, an estimation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I₁₂ shell, in contrast to the I₆ ring in the hexaiodidebenzene dication, does not contribute to any stabilization of the system. Therefore, our conclusion is that the [B₁₂I₁₂]^0/2+ species cannot be considered as doubly 3D-aromatic.

This work has been carried out by Prof. Jordi Poater from the University of Barcelona, Prof. Francesc Teixidor and Prof. Clara Viñas from the Institute of Material Science (CSIC, Bellaterra), Prof. Henrik Ottosson from Uppsala University, and Sílvia Escayola, Dr. Albert Poater, and Prof. Miquel Solà from the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona.

J. Poater, S. Escayola, A. Poater, F. Teixidor, H. Ottosson*, C. Viñas* and M. Solà*.Single – not double – 3D-aromaticity in oxidized closo icosahedral dodecaiodo-dodecaborate cluster.

J. Am. Chem. Soc., ASAP, DOI: 10.1021/jacs.3c07335. IF: 15.0, Q1. Open access.

Aromatic polycyclic systems have been extensively utilized as structural subunits for the preparation of various functional molecules. Currently, aromatics-based polycyclic systems are predominantly generated from the extension of two-dimensional (2D) aromatic rings. In contrast, polycyclic compounds based on the extension of three-dimensional (3D) aromatics such as boron clusters are less studied. Here, we report three types of boron cluster-cored tricyclic molecular systems, which are constructed from a 2D aromatic ring, a 3D aromatic nido-carborane, and an alkyne. These new tricyclic compounds can be facilely accessed by Pd-catalyzed B−H activation and the subsequent cascade heteroannulation of carborane and pyridine with an alkyne in an isolated yield of up to 85% under mild conditions without any additives. Computational results indicate that the newly generated ring from the fusion of the 3D carborane, the 2D pyridyl ring, and an alkyne is nonaromatic. However, such fusion not only leads to a ¹H chemical shift considerably downfield shifted owing to the strong diatropic ring current of the embedded carborane but also devotes to new improved physicochemical properties including increased thermal stability, the emergence of a new absorption band, and a largely red-shifted emission band and enhanced emission efficiency. Besides, a number of bright, color-tunable solid emitters spanning over all visible light are obtained with absolute luminescence efficiency of up to 61%, in contrast to aggregation-caused emission quenching of, e.g., Rhodamine B containing a 2D-aromatics-fused structure. This work demonstrates that the new hybrid conjugated tricyclic systems might be promising structural scaffolds for the construction of functional molecules.

The paper has been highlighted by the Spanish Biophysical Society (https://sbe.es/paperhighlights-feb-2023-2/).

This work has been carried out by F. Sun, S. Tan, H.-J. Cao, C.-S. Lu, Dr. D. Tu and Prof. Hong Yan* from Nanjing University and Prof. Jordi Poater from the University of Barcelona and Prof. Miquel Solà from the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona.

Sun, S. Tan, H.-J. Cao, C.-S. Lu, D. Tu*, J. Poater*, M. Solà* and H. Yan*. Facile Construction of a New Hybrid Conjugate via Boron Cage Extension. Am. Chem. Soc., 145 (2023) 3577-3587, DOI: 10.1021/jacs.2c12526.

 

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