Oxidizing Metal Oxides with Polynuclear Superhalogen: An ab Initio Study

Design and Investigation of Superatoms for Redox Applications: First-Principles Studies

A superatom is a cluster of atoms that acts like a single atom. Two main groups of superatoms are superalkalis and superhalogens, which mimic the chemistry of alkali and halogen atoms, respectively. The ionization energies of superalkalis are smaller than those of alkalis (<3.89 eV for cesium atom), and the electron affinities of superhalogens are larger than that of halogens (>3.61 eV for chlorine atom). Exploring new superalkali/superhalogen aims to provide reliable data and predictions of the use of such compounds as redox agents in the reduction/oxidation of counterpart systems, as well as the role they can play more generally in materials science. The low ionization energies of superalkalis make them candidates for catalysts for CO2 conversion into renewable fuels and value-added chemicals. The large electron affinity of superhalogens makes them strong oxidizing agents for bonding and removing toxic molecules from the environment. By using the superatoms as building blocks of cluster-assembled materials, we can achieve the functional features of atom-based materials (like conductivity or catalytic potential) while having more flexibility to achieve higher performance. This feature paper covers the issues of designing such compounds and demonstrates how modifications of the superatoms (superhalogens and superalkalis) allow for the tuning of the electronic structure and might be used to create unique functional materials. The designed superatoms can form stable perovskites for solar cells, electrolytes for Li-ion batteries of electric vehicles, superatomic solids, and semiconducting materials. The designed superatoms and their redox potential evaluation could help experimentalists create new materials for use in fields such as energy storage and climate change.

Recent progress on the design and applications of superhalogens

The research on superhalogens has successfully completed four decades. After their prediction in 1981 and experimental verification in 1999, such species have attracted attention due to their unusual structures and intriguing applications. Superhalogens are species whose electron affinity exceeds that of halogen or whose anions possess a larger vertical detachment energy than that of halides. Initially, these species were designed using s and p block atoms having a central electropositive atom as the core with excess electronegative atoms as ligands such as F, Cl, O etc. The last decade has witnessed enormous progress in the field of superhalogens. The transition metal atoms have played the role of the central core and a variety of new ligands have been explored. Further, new classes of superhalogens such as polynuclear superhalogens, magnetic superhalogens, aromatic superhalogens, etc. have been reported. The first application of superhalogens as strong oxidizers appeared much before their conceptualization. In the last decade, however, their applications have spanned a variety of fields such as energy storage, superacids, organic superconductors, ionic liquids, liquid crystals, etc. This makes research in the field of superhalogens truly interdisciplinary. This article is intended to highlight the progress on the design and applications of superhalogens in the last decade.

Tripodal Podand Ligand with a Superhalogen Nature as an Effective Molecular Trap

Tris(2-methoxyethyl) fluoroborate anion (TMEFA), anovel tripodal ligand based on the BF4− superhalogen anion, is proposed and was investigated theoretically using ab initio MP2 (second-order Møller-Plesset perturbational method) and OVGF (outer valence Green function) methods. The studied molecule comprises three 2-methoxyethoxy groups (-O-CH2-CH2-O-CH3) connected to a central boron atom, which results in the C3-symmetry of the compound. The resulting anion was stable against fragmentation processes and its vertical electron detachment energy was found to be 5.72 eV. Due to its equilibrium structure resembling that of classical tripodal podands, the [F-B(O-CH2-CH2-O-CH3)3]− anion is capable of binding metal cations using its three arms, and thus may form strongly bound ionic complexes such as [F-B(O-CH2-CH2-O-CH3)3]−/Li+ and [F-B(O-CH2-CH2-O-CH3)3]−/Mg2+. The binding energies predicted for such compounds far exceed those of the similar neutral classical podand ligands, which likely makes the [F-B(O-CH2-CH2-O-CH3)3]− system a more effective molecular trap or steric shielding agent with respect to selected metal cations.

Greatly Enhanced Electron Affinities of Au2n Cl Clusters (n = 1-4): Effects of Chlorine Doping

Au2n Cl- (n = 1-4) clusters are investigated by both laser ablation mass spectrometry and theoretical calculations. It is interesting to find that the electron affinities of neutral Au2n Cl (n = 1-4) clusters are much larger than those of corresponding pure Au2n clusters. Among them, the electron affinity of Au2Cl is 4.02 eV, which can be defined as a very unique superhalogen that is quite different from classical ones of M n X m (M = metal, X = halogen, and n < m). Natural bond orbital and highest occupied molecular orbital analyses indicate that the extra electron is predominantly delocalized over the positively charged metal moiety in these anionic Au2n Cl- clusters, which is the main reason for the large electron affinities of the corresponding neutral species.

Magnesium-Based Clusters as Building Blocks of Electrolytes in Lithium-Ion Batteries

Superhalogens, owing to their large electron affinity (EA, exceeding those of any halogen atom), play an essential role in physical chemistry as well as new material design. They have applications in hydrogen storage and lithium-ion batteries. Owing to the unique geometries and electronic features of magnesium-based clusters, their potential to form a new class of lithium salts has been investigated here theoretically. The idea is assessed by conducting ab initio computations on Li⁺ /Mg_n F_2n+1-2m O_m ^- compounds (n=2, 3; m=0-3) and analyzing their performance as potential Li-ion battery electrolytes. The Mg₃ F₇ ^- cluster, with large electron binding energy (EA of 7.93 eV), has been proven to serve as a building block for lithium salts. It is shown that, apart from high electronic stability, the new superhalogen-based electrolytes exhibit a set of desirable properties, including a large band gap, high electrolyte stability window, easy mobility of the Li⁺ , and favorable insensitivity to water.

Magnesium-Based Oxyfluoride Superatoms: Design, Structure, and Electronic Properties

The ability of mixed ligands to form stable dinuclear and trinuclear magnesium-based superatoms has been investigated theoretically. The Mg₂F_{5-2 m}O _m and Mg₃F_{7-2 m}O _m systems (where m = 1-3) were found able to form stable and strongly bound anionic clusters, and those assumptions were validated by (i) the analysis of the geometrical stability; (ii) the estimated Gibbs free energies for the most probable disproportion paths these clusters might be vulnerable to (which allows examining their thermodynamic stabilities); (iii) the localization of the electron density; and (iv) the adiabatic electron affinity (AEA), vertical electron detachment energy (VDE), and adiabatic electron detachment energy (ADE) values calculated for the studied systems. It is demonstrated that the stability of the anionic daughters of these clusters increases with the number of electronegative ligands, and Mg _nF_{2 n+1-2 m}O _m^- ( n = 2, 3; m = 1-3) clusters are stable against electron emission. The largest electron binding energy was found for the Mg₃F₅O^- anion (VDE = 6.826 eV). The strong VDE dependence on (i) the geometrical structure, (ii) the number of central atoms, (iii) ligand type, and (iv) bonding/antibonding character of the highest molecular orbital (HOMO) was also remarked upon and discussed.