Do nonmetallic superalkali cations exist?

Interaction of N2, O2 and H2 Molecules with Superalkalis

Superalkalis (SAs) are exotic clusters having lower ionization energy than alkali atoms, which makes them strong reducing agents. In the quest for the reduction of diatomic molecules (X2) such as N2, O2, and H2 using Møller-Plesset perturbation theory (MP2), we have studied their interaction with typical superalkalis such as FLi2, OLi3, and NLi4 and calculated various parameters of the resulting SA-X2 complexes. We noticed that the SA-O2 complex and its isomers possess strong ionic interaction, which leads to the reduction of O2 to O2 - anion. On the contrary, there are both ionic and covalent interactions in SA-N2 complexes such that the lowest energy isomers are covalently bonded with no charge transfer from SA. Further, the interaction between SA and H2 leads to weakly bound complexes, which results in the adsorption of H2 molecules. The nature of interaction is found to be closely related to the electron affinity of diatomic molecules. These findings might be useful in the study of the activation, reduction, and adsorption of small molecules, which can be further explored for their possible applications.

Superalkalis with Hydrogen as Central Electronegative Atom and their Possible Applications: Ab Initio and DFT Study

Superalkalis are unusual species having ionization energies lower than that of the alkali metals. These species with various applications are of great importance in chemistry due to their low ionization energies and strong reducing property. A typical superalkali contains a central electronegative core decorated with excess metal ligands. In the quest for novel superalkalis, we have designed the superalkalis HLi2, HLiNa and HNa2 using hydrogen as central electronegative atom for the first time employing high level ab initio (CCSD(T), MP2) and density functional theory (ωB97X-D) methods. The superalkalis exhibit very low ionization energies, even lower than that of cesium. Stability of these species is verified from binding energy and dissociation energy values. The superalkalis are capable of reducing SO2, NO, CO2, CO and N2 molecules by forming stable ionic complexes and therefore can be used as catalysts for the reduction or activation of systems possessing very low electron affinities. The superalkalis form stable supersalts with tailored properties when interact with a superhalogen. They also show remarkably high non-linear optical responses, hence could have industrial applications. It is hoped that this work will enrich the superalkali family and spur further theoretical and experimental research in this direction.

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 advances in in silico design and characterization of superalkali-based materials and their potential applications: A review

In the advancement of novel materials, chemistry plays a vital role in developing the realm where we survive. Superalkalis are a group of clusters/molecules having lower ionization potentials (IPs) than that of the cesium atom (3.89 eV) and thus, show excellent reducing properties. However, the chemical industry and material science both heavily rely on such reducing substances; an in silico approach-based design and characterization of superalkalis have been the focus of ongoing studies in this area along with their potential applications. However, although superalkalis have been substantially sophisticated materials over the past couple of decades, there is still room for enumeration of the recent progress going on in various interesting species using computational experiments. In this review, the recent developments in designing/modeling and characterization (theoretically) of a variety of superalkali-based materials have been summarized along with their potential applications. Theoretically acquired properties of some novel superalkali cations (Li3 +) and C6Li6 species, etc. for capturing and storing CO2/N2 molecules have been unveiled in this report. Additionally, this report unravels the first-order polarizability-based nonlinear optical (NLO) response features of numerous computationally designed novel superalkali-based materials, for instance, fullerene-like mixed-superalkali-doped B12N12 and B12P12 nanoclusters with good UV transparency and mixed-valent superalkali-based CaN3Ca (a high-sensitivity alkali-earth-based aromatic multi-state NLO molecular switch, and lead-founded halide perovskites designed by incorporating superalkalis, supersalts, and so on) which can indeed be used as a new kind of electronic nanodevice used in designing hi-tech NLO materials. Understanding the mere interactions of alkalides in the gas and liquid phases and the potential to influence how such systems can be extended and applied in the future are also highlighted in this survey. In addition to offering an overview of this research area, it is expected that this review will also provide new insights into the possibility of expanding both the experimental synthesis and the practical use of superalkalis and their related species. Superalkalis present the intriguing possibility of acting as cutting-edge construction blocks of nanomaterials with highly modifiable features that may be utilized for a wide-ranging prospective application.

Superalkali Coated Rydberg Molecules

A series of complexes of Na, K, NH₄, and H₃O with [bpy.bpy.bpy]cryptand, [2.2.2]cryptand, and spherical cryptand were investigated via DFT and ab initio methods. We found that by coating Rydberg molecules with the “organic skin” one could further decrease their ionization potential energy, reaching the values of ∼1.5 eV and a new low record of 1.3 eV. The neutral cryptand complexes in this sense possess a weakly bounded electron and may be considered as very strong reducing agents. Moreover, the presence of an organic cage increases the thermodynamic stability of Rydberg molecules making them stable toward the proton detachment.

Designing Special Nonmetallic Superalkalis Based on a Cage-like Adamanzane Complexant

In this study, to examine the possibility of using cage-like complexants to design nonmetallic superalkalis, a series of X@3⁶adz (X = H, B, C, N, O, F, and Si) complexes have been constructed and investigated by embedding nonmetallic atoms into the 3⁶adamanzane (3⁶adz) complexant. Although X atoms possess very high ionization energies, these resulting X@3⁶adz complexes possess low adiabatic ionization energies (AIEs) of 0.78–5.28 eV. In particular, the adiabatic ionization energies (AIEs) of X@3⁶adz (X = H, B, C, N, and Si) are even lower than the ionization energy (3.89 eV) of Cs atoms, and thus, can be classified as novel nonmetallic superalkalis. Moreover, due to the existence of diffuse excess electrons in B@3⁶adz, this complex not only possesses pretty low AIE of 2.16 eV but also exhibits a remarkably large first hyperpolarizability (β₀) of 1.35 × 10⁶ au, indicating that it can also be considered as a new kind of nonlinear optical molecule. As a result, this study provides an effective approach to achieve new metal-free species with an excellent reducing capability by utilizing the cage-like organic complexants as building blocks.

Novel and Polynuclear K- and Na-Based Superalkali Hydroxides as Superbases Better Than Li-Related Species and Their Enhanced Properties: An Ab Initio Exploration

Hydroxides of superalkalis (particularly, K- and Na-related species) are shown for the first time to function as superbases. A new small series of hydroxides (XM n+1OH) is designed based on superalkali species (XM n+1) where M (K and Na) is alkali metal atoms, n is the maximal formal valence of the central atom X (F, O, and N), and n ≥ 1. To probe whether such fascinating polynuclear superalkali hydroxides (SAHs), especially the K- and Na-associated moieties are as basic as the representative alkali metal hydroxides (KOH, NaOH, and LiOH) as well as similar Li-based SAHs, a comprehensive computational exploration (in the gas phase) has been reported using the framework of an ab initio method. The ab initio calculations reveal that both the K- and Na-related SAHs consisting of larger gas-phase proton affinity (PA) and gas-phase basicity (GB) values demonstrate stronger basic character compared to the LiOH and Li-based SAHs. However, the available SAHs act as strong bases as well as superbases; among the proposed K- and Na-based SAHs, remarkably, the OK3OH moiety having the highest PA (1168.4 kJ/mol) and GB (1146.9 kJ/mol) values shows the evidence of the strongest basicity (i.e., superbase/hyperbase), which exceed enough (ΔPA: 142.1 kJ/mol and ΔGB: 146.9 kJ/mol) the IUPAC-defined superbasicity threshold values (PA: 1026.3 kJ/mol and GB: 1000 kJ/mol) of 1,8-bis(dimethylamino)naphthalene (DMAN). Furthermore, theoretical signatures have been predicted via the electronic structure calculation approach in probing the dissociation energy, ionization potential, electron affinity, HOMO-LUMO gap, and chemical hardness as well as the NCI plot and QTAIM tools are used for the bonding feature analysis and such parameters are well linked with the basicity analyzing parameters. In this study, the ab initio-based computational experiments provide some new insights into the basicity features and understanding of the structural and electronic features of a small series of designed K- and Na-related SAHs. Design and synthesis of such theoretically examined SAHs may pave alternative routes for the experimentally rewarding applications.

DFT and QTAIM studies on the reduction of carbon monoxide by superalkalis

Carbon monoxide (CO) is a toxic gas molecule with no positive electron affinity, which makes it difficult to reduce it into CO¯. In this work, we perform density functional theory (DFT) and quantum theory of atoms in molecule (QTAIM) based studies on the interaction of CO molecule with superalkali (SA) clusters. Our findings suggest that this interaction results in SA(CO) complexes, which are stabilized by purely ionic as well as partially covalent bonds although their binding energy decreases with the increase in the size of SA clusters. In these ionic complexes, the electron is transferred from the SA cluster to the CO molecule. This suggests the single-electron reduction of the CO molecule by interacting with superalkalis. This work may offer some novel insights into the detection and reduction of stable CO molecule and related systems.

N4Mg6M (M = Li, Na, K) superalkalis for CO2 activation

Superatoms have exciting properties, including diverse functionalization, redox activity, and magnetic ordering, so the resulting cluster-assembled solids hold the promise of high tunability, atomic precision, and robust architectures. By utilizing adamantane-like clusters as building blocks, a new class of superatoms N₄Mg₆M (M = Li, Na, K) is proposed here. The studied superalkalis feature low adiabatic ionization energies, an antibonding character in the interactions between magnesium and nitrogen atoms, and highly delocalized highest occupied molecular orbital (HOMO). Consequently, the N₄Mg₆M superalkalis might easily lose their HOMO electrons when interacting with superhalogen electrophiles to form stable superatom [superalkali]⁺[superhalogen]^- compounds. Moreover, the studied superalkalis interact strongly with carbon dioxide, and the resulting N₄Mg₆M/CO₂ systems represent two strongly interacting ionic fragments (i.e., N₄Mg₆M⁺ and CO₂ ^-). In turn, the electron affinity of the N₂ molecule (of -1.8 eV) is substantially lower than that observed for carbon dioxide (EA = -0.6 eV) and consequently, the N₂ was found to form the weakly bound [N₄Mg₆M][N₂] complex rather than the desired ionic [N₄Mg₆M]⁺[N₂]^- product. Thus, the N₄Mg₆M superalkalis have high selectivity over N₂ when it comes to CO₂ reduction and also are themselves stable. We believe that the results described within this paper will be useful for understanding CO₂ activation, which is the first step for producing fuels from CO₂. Moreover, we demonstrate that designing novel superatomic systems and exploring their physicochemical features might be used to create desirable functional materials.

Record Low Ionization Potentials of Alkali Metal Complexes with Crown Ethers and Cryptands

Electronic properties of series of alkali metals complexes with crown ethers and cryptands were studied via DFT hybrid functionals. For [M([2.2.2]crypt)] (M=Li, Na, K) extremely low (1.70-1.52 eV) adiabatic ionization potentials were found. Such low values of ionization energies are significantly lower than those of alkali metal atoms. Thus, the investigated complexes can be defined as superalkalis. As a result, our investigation opens up new directions in the designing of chemical species with record low ionization potentials and extends the explanation of the ability of the cryptates and alkali crown ether complexes to stabilize multiple charged Zintl ions.

Recent Progress on the Design, Characterization, and Application of Superalkalis

Superalkalis are clusters or molecules featuring lower ionization energies (IEs) than that of cesium atoms, and thus exhibit excellent reducing properties. Such special species have great potential to be used in the synthesis of unusual charge-transfer salts and cluster-assembled nanomaterials with tailored properties, in the reduction of carbon dioxide, or as hydrogen storage materials and noble-gas-trapping agents, etc. In this regard, ongoing efforts have been devoted to designing and characterizing superalkalis of new types. The recent progress on the study of superalkalis in terms of theoretical design, characterization, and potential application is summarized in this minireview. We hope this review will not only provide a broad overview of this research field, but also highlight the prospect of further extending the experimental synthesis and practical application of superalkalis.

Design of the NnH3n+1+ series of “non-metallic” superalkali cations

A new series of non-metallic superalkali cations, N_nH_3n+1⁺ by using ammonium (NH₄⁺) cations, possessing vertical electron affinity (EA_v), 4.39 eV for n = 1 to 2.39 eV for n = 5 has been proposed. This series can be continued for obtaining new superalkali cations, for instance N₉H₂₈⁺ with an EA_v of 1.84 eV. The EA_v of N_nH_3n+1⁺ cations is governed by the electron localization on the central N-atom. The EA_v of N_nH_3n+1⁺ cations decays exponentially with an increase in n.

The effect of hydration on the electronic structure and stability of the superalkali cation Li3+

Insights into the interaction between the superalkali cation Li₃⁺ and water molecules and the stability of the resulting hydrates.

On the making of aromatic organometallic superalkali complexes

First principle calculation on Au₃(Py)₃ and Au₃(IMD)₃ systems shows that they are superalkali and exhibit good nonlinear optical property.

Rational design of super-alkalis and their role in CO2 activation

Super-alkalis are clusters of atoms. With ionization potentials smaller than those of the alkali atoms, they are playing an increasing role in chemistry as highlighted by recent applications in solar cells as well as in Li-ion batteries. For the past 40 years superalkalis were designed using inorganic elements with the sp orbital character. Here, we show that a large class of superalkalis composed of only simple metal atoms, transition metal complexes as well as organic molecules can be designed by making use of electron counting rules beyond the octet rule. Examples include Al₃⁺, Mn(B₃N₃H₆)₂⁺, B₉C₃H₁₂⁺, and C₅NH₆⁺ which obey the jellium shell closure rule, the 18-electron rule, the Wade-Mingos rule, and Hückel's aromatic rule, respectively. We further show that the ability of superalkalis to transfer an electron easily can be used to activate a CO₂ molecule by transforming it from a linear to a bent structure. These results, based on density functional theory with generalized gradient approximation for exchange-correlation potential, open the door to a new class of catalysts for CO₂ activation.

Reduction of carbon dioxide with a superalkali

The ability of the superalkali Li₃F₂ to reduce CO₂ and N₂ is investigated using the CBS-QB3 composite method and intriguing results are presented.

Complex metal borohydrides: multifunctional materials for energy storage and conversion

With the limited supply of fossil fuels and their adverse effect on the climate and the environment, it has become a global priority to seek alternate sources of energy that are clean, abundant, and sustainable. While sources such as solar, wind, and hydrogen can meet the world's energy demand, considerable challenges remain to find materials that can store and/or convert energy efficiently. This topical review focuses on one such class of materials, namely, multi-functional complex metal borohydrides that not only have the ability to store sufficient amount of hydrogen to meet the needs of the transportation industry, but also can be used for a new generation of metal ion batteries and solar cells. We discuss the material challenges in all these areas and review the progress that has been made to address them, the issues that still need to be resolved and the outlook for the future.

Organo-Zintl Clusters [P7R4]: A New Class of Superalkalis

Zintl ions composed of Group 13, 14, and 15 elements are multiply charged cluster anions that form the building blocks of the Zintl phase. Superalkalis, on the other hand, are cationic clusters that mimic the chemistry of the alkali atoms. It is, therefore, counterintuitive to expect that Zintl anions can be used as a core to construct superalkalis. In this paper, using density functional theory, we show that this is indeed possible. The results are compared with calculations at the MP2 level of theory. A systematic study of a P7(3-) Zintl core decorated with organic ligands [R = Me, CH2Me, CH(Me)2 and C(Me)3] shows that the ionization energies of some of the P7R4 species are smaller than those of the alkali atoms and hence can be classified as superalkalis. This opens the door to the design and synthesis of a new class of superalkali moieties apart from the traditional ones composed of only inorganic elements.

Organic heterocyclic molecules become superalkalis

A new strategy has been developed to create superalkali molecules from aromatic heterocycles. C₃N₂(CH₃)₅ has the ionization energy close to 3.0 eV.