Metalloxocubes: A new class of neutral Co13O8 clusters with cubic aromaticity

September 16, 2020

Building materials with a clear composition and stable structure is one of the primary challenges in chemistry and cluster science. Ongoing extensive efforts have been paid to exploring new stable clusters thus to construct new materials and to understand structural evolution bridging atoms and macroscopic matter. This is interesting yet challenging in chemistry. In the past decades, researchers have achieved significant advances, such as the discovery of fullerene C60, tetrahedral Au20, and all-boron fullerene B40, etc. In particular, a few "magic " metal clusters have unique stability, such as Al13?, surviving the reaction with oxygen. Such metal clusters often possess closed-shell electronic structure and geometric structure, which embodies the near free electron gas (NFEG) theory of metal and epitomizes the jellium model of clusters. In other cases, cluster stability could be related to aromaticity or superatom characteristics, pertaining to a large HOMO-LUMO gap, electron detachment energy and spin excitation energy. Besides, it was found that the doping of ligands, such as hydrogen or halogen, could bring forth changes to the stability of metal clusters, enabling to passivate or activate the metal clusters. This principle has been greatly embodied in understanding the stability of ligand-protected metal clusters.

In a new overview published in the Beijing-based National Science Review, scientists in Institute of Chemistry Chinese Academy of Sciences, and Peking University Shenzhen Graduate School, etc. present an interesting finding of a very stable neutral Co13O8 cluster which possess unique cubic structure akin to perovskite and exhibit cubic aromaticity. This new class of neutral oxygen passivated metal cluster is expected to be used as genetic material, and the researchers named it "metalloxocubes".

Using the home-made deep ultraviolet laser ionization mass spectrometry (DUV-LIMS) with high ionization efficiency of neutral cobalt clusters, they observed the reactions of Con (n=2-30) with oxygen. Interestingly, a particularly stable species, Co13O8, shows dominant mass abundance in the presence of a large flow rate of oxygen gas (as shown in the Figure below).

In order to study its geometric structure, authors used three independent methods to search and identify the global minimum structure of the Co13O8 clusters, and obtained a consistent result unveiling its unique stability related to the cubic structure and a large HOMO-LUMO gap (2.14 eV). Ab-initio molecular dynamics (AIMD) simulations indicate that Co13O8 has outstanding thermal stability. The thermodynamics and reaction kinetics demonstrated the structural evolution from icosahedral Co13 to cubic Co13O8 clusters.

Further, the authors found a large value of negative nuclear independent chemical shift (NICS), with NICS(0) = -54.0 ppm at the Co4O4 plane center of the cubic Co13O8 and NICS(1) = -22.0 ppm with 1.0 Å above the plane surface, indicating remarkable aromaticity of this cluster. The integrate net current of 5.14 nA/T with an external magnetic field perpendicular to the Co4O4 plane. The electronic structure and orbital analyses revealed that Co13O8 has cubic aromaticity in accordance with the '6n+2' rule of electron counting.

It is interesting and challenging in chemistry to understand the structure evolution from atoms to macroscopic matter and construct new materials with well-defined components and regular polyhedron structures. Considering the unique stability and cubic aromaticity of Co13O8, this class of neutral oxygen- passivation metal clusters is named as "metalloxocubes", expecting to become a suitable candidate for gene materials.
Financial support for this work was provided by the National Project Development of Advanced Scientific Instruments Based on Deep Ultraviolet Laser Source (No. Y31M0112C1), by the National Natural Science Foundation of China (Nos. 21722308, 21802146, 91536105, 91645203, and 21590792), by National Key R&D Program of China (2016YFB0700600), and by CAS Key Research Program of Frontier Sciences (QYZDBSSW-SLH024).

See the article:

Lijun Geng, Mouyi Weng, Cong-Qiao Xu, Hanyu Zhang, Chaonan Cui, Haiming Wu, Xin Chen, Mingyu Hu, Hai Lin, Zhen-Dong Sun, Xi Wang, Han-Shi Hu, Jun Li, Jiaxin Zheng, Zhixun Luo*, Feng Pan* & Jiannian Yao*

Co13O8--Metalloxocubes: A new class of perovskite-like neutral clusters

with cubic aromaticity

National Science Review, 2020,

Science China Press

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