Mimicking the magnetic properties of rare earth elements using superatoms

Boosted recovery of rare earth elements from mining wastes and discarded NdFeB magnets by tributyl phosphate-grafted ZIF-8

The escalating demand for rare earth elements (REEs) highlights the necessity for their sustainable recovery from waste streams and secondary resources. However, this process requires materials with exceptional selectivity and capacity for REEs due to their low concentration and high presence of interfering ions. Herein, we synthesized zeolitic imidazolate framework-8 (ZIF-8) and subsequently modified it with tributyl phosphate (TBP) to enhance its affinity and selectivity for the recovery of REEs. The distribution coefficients (Kd) of ZIF-8-TBP for REEs (neodymium, Nd, and dysprosium, Dy) were orders of magnitude higher than the Kd of main interfering ions (e.g., Mg, Ni Al, and Fe). Particularly, the maximum sorption capacities (qm) for Nd and Dy were 475 and 529 mg g-1, respectively. In addition, the separation factor between Dy (a representative of heavy REE) and Nd (a representative of light REE) reached 24, greatly exceeding the figures reported previously. Importantly, the outstanding ability of ZIF-8-TBP for selective separation and recovery of REEs was demonstrated via its application to real samples including mining wastewater, and leaching solutions from REE filter cakes and discarded NdFeB magnets. Multiscale simulations reveal that ZIF-8-TBP possessed a stronger binding strength and greater sorption energy for REE ions. These results indicate that ZIF-8-TBP could effectively harvest REEs from wastes and offers an efficient alternative for industrial applications in REE recovery.

d-p Hybridization Induced Open-Shell Planar Four-Membered Transition Metal Carbide Clusters with Double Möbius Aromaticity

Aromaticity is one of the most classical concepts in the field of modern chemistry and has been employed to explain and design substances with special stability. Although the knowledge about Hückel's and Baird's rules has been well established, the understanding of Möbius aromaticity remains extremely limited. In this letter, by employing density functional theory (DFT) calculations, we demonstrated that the four-membered VIB transition metal (TM) carbide clusters possess a highly stable open-shell planar tetrameric structure and exhibit double Möbius aromaticity, which was evidenced by analyzing multiple aromaticity criteria, including the electronic, magnetic, and energetic indicators. Each cluster was characterized by four delocalized π electrons and four delocalized σ electrons, forming a novel class exhibiting double Möbius aromaticity. Intriguingly, the unexpected stability of these open-shell clusters was suggested to arise from the hybridization of d-p atomic orbitals, as revealed by analysis of the composition of delocalized orbitals. Our findings highlight the significance of hybridization between the d orbitals of transition metals and the p orbitals of main group elements in the creation of dual Möbius aromatic species, which offers new avenues for the design of single-molecule magnetic inorganic materials.

Dual External Field Strategy in Regulating the Superhalogen Characteristics of the Non-Noble Metal Constituted Tantalum Oxide Clusters

The identification of the non-noble metal constituted TaO cluster as a potential analogue to the noble metal Au is significant for the development of tailored materials. It leverages the superatom concept to engineer properties with precision. However, the impact of incrementally integrating TaO units on the electronic configurations and properties within larger TaO-based clusters remains to be elucidated. By employing the density functional theory calculations, the global minima and low-lying isomers of the TanOn (n = 2-5) clusters were determined, and their structural evolution was disclosed. In the cluster series, Ta5O5 was found to possess the highest electron affinity (EA) with a value of 2.14 eV, based on which a dual external field (DEF) strategy was applied to regulate the electronic property of the cluster. Initially, the electron-withdrawing CO ligand was affixed to Ta5O5, followed by the application of an oriented external electric field (OEEF). The CO ligation was found to be able to enhance the Ta5O5 cluster's electron capture capability by adjusting its electron energy levels, with the EA of Ta5O5(CO)4 peaking at 2.58 eV. Subsequently, the introduction of OEEF further elevated the EA of the CO-ligated cluster. Notably, OEEF, when applied along the +x axis, was observed to sharply increase the EA to 3.26 eV, meeting the criteria for superhalogens. The enhancement of EA in response to OEEF intensity can be quantified as a functional relationship. This finding highlights the advantage of OEEF over conventional methods, demonstrating its capacity for precise and continuous modulation of cluster EAs. Consequently, this research has adeptly transformed tantalum oxide clusters into superhalogen structures, underscoring the effectiveness of the DEF strategy in augmenting cluster EAs and its promise as a viable tool for the creation of superhalogens.

Beyond Shell-Filling: Strong Enhancement of Electron Affinity of Metal Clusters through a Noninvasive Oriented External Electric Field

Traditional electron counting rules, like the Jellium model, have long been successfully utilized in designing superhalogens by modifying clusters to have one electron less than a filled electronic shell. However, this shell-filling approach, which involves altering the intrinsic properties of the clusters, can be complex and challenging to control, especially in experiments. In this letter, we theoretically establish that the oriented external electric field (OEEF) can substantially enhance the electron affinity (EA) of diverse aluminum-based metal clusters with varying valence electron configurations, leading to the creation of superhalogen species without altering their shell arrangements. This OEEF approach offers a noninvasive alternative to traditional superatom design strategies, as it does not disrupt the clusters' geometrical structures and superatomic states. These findings contribute a vital piece to the puzzle of constructing superalkalis and superhalogens, extending beyond conventional shell-filling strategies and potentially expanding the range of applications for functional clusters.

Magnetic Metal Clusters and Superatoms

Metal clusters with tunable magnetism and chemical activity are ideal models to study magnetic order changes from microstructures to macroscopic substances, to understand the spin effect in diverse catalytic reactions, and to create information carriers of qubits in quantum computation. Precise preparation, reaction, and characterization of magnetic clusters provide a platform to understand spin-exchange interactions and geometrical/electronic structure-property relationships; thus, they are beneficial for the rational design and development of new cluster-genetic materials and spintronics microdevices. Advances in this field have discovered some high-spin magnetic clusters and superatoms, expanding the understanding of magnetism, aromaticity, cluster stability, and electron delocalization. Herein we present a perspective of the experimental and theoretical progress regarding magnetic clusters and superatoms, with the expectation of stimulating more research interest in this field.

Dual External Field-Engineered Hyperhalogen

Hyperhalogens, a superatom featuring the highest known electron affinity (EA), have promising applications in the synthesis of superoxidizers. Contributions regarding the identified numbers and corresponding design strategies of hyperhalogens, however, are scarce. Herein, a novel and noninvasive dual external field (DEF) strategy, including the ligand field and oriented external electric field (OEEF), is proposed to construct hyperhalogens. The DEF strategy was shown to possess the power to increase Au₈'s EA, forming the hyperhalogen. Strikingly, the ligation process can increase the cluster's stability, while OEEF can realize the precise and continuous regulation of the cluster's EA. Moreover, besides the model Au₈ system, an experimentally synthesized Ag₁₇ nanocluster was also investigated, further demonstrating the reliability of the proposed strategy. Considering the crucial role of ligands in the liquid synthesis of clusters and the convenient source of OEEF, such a DEF strategy may greatly increase the synthesis and applications of hyperhalogens in the condensed phase.

High-Stability Light-Element Magnetic Superatoms Determined by Hund's Rule

Achieving stable high-magnetism light-element structures at nanoscale is vital to the field of magnetism, which has traditionally been ruled by transition-metal elements with localized d or f electrons. By first-principles calculations, we show that superatoms made of pure earth-abundant light elements (i.e., boron and nitrogen) exhibit desired magnetic properties that rival those of rare-earth elements, and the magnetism is dictated entirely by Hund's maximum spin rule. Importantly, the chemical and structural stabilities of the superatoms are not jeopardized by its high spins and are in fact better than those of transition-metal-element-embedded clusters. Our work thus establishes the basic principles for designing novel light-element, high-stability, and high-moment magnetic superatoms.

Probing the Nature of the Transition-Metal-Boron Bonds and Novel Aromaticity in Small Metal-Doped Boron Clusters Using Photoelectron Spectroscopy

Photoelectron spectroscopy combined with quantum chemistry has been a powerful approach to elucidate the structures and bonding of size-selected boron clusters (B_n^-), revealing a prevalent planar world that laid the foundation for borophenes. Investigations of metal-doped boron clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The current review focuses on recent advances in transition-metal-doped boron clusters, including the discoveries of metal-boron multiple bonds and metal-doped novel aromatic boron clusters. The study of the RhB^- and RhB₂O^- clusters led to the discovery of the first quadruple bond between boron and a transition-metal atom, whereas a metal-boron triple bond was found in ReB₂O^- and IrB₂O^-. The ReB₄^- cluster was shown to be the first metallaborocycle with Möbius aromaticity, and the planar ReB₆^- cluster was found to exhibit aromaticity analogous to metallabenzenes. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

On the Precise and Continuous Regulation of the Superatomic and Spectroscopic Behaviors of the Quasi-Cubic W4C4 Cluster by the Oriented External Electric Field

Designing and realizing novel superatoms with controllable and tunable electronic properties is vital for their potential applications in cluster-assembly nanomaterials. Here, we investigated the effect of the oriented external electric field (OEEF) on the geometric and electronic structures as well as the spectroscopic properties of the quasi-cubic W₄C₄ cluster by utilizing the density functional theory (DFT) calculations. Compared with traditional models, the OEEF was observed to hold the special capability in continuously and precisely modulating the electronic properties of W₄C₄, that is, remarkably increasing its electron affinity (EA) (1.58 eV) to 5.61 eV under the 0.040 au OEEF (larger than any halogen atoms in the periodic table), which possesses the superhalogen behavior. Furthermore, the downward movement of the lowest unoccupied molecular orbital level of the cluster accompanied by the enhancement of the OEEF intensity was demonstrated to be the origin of the EA increment. Additionally, the photoelectron spectra (PES) of W₄C₄^- were also simulated under different OEEF intensities, where the PES peaks move to a higher energy area following the enhancement of the OEEF strength, exhibiting the blue-shift behavior. These findings observed here open a new avenue in conveniently and precisely adjusting the electronic properties of clusters, which will be beneficial for the rational design of superatoms or superatom-assembled nanomaterials under the external field.

Deciphering the Superatomic Behavior of Group V Metal Monoxides

The valence orbitals of Group V metal monoxides exhibit atomic-like properties which mimic that of coinage metal element atoms. The electronic structures of MO^-1/0 (M = V, Nb, and Ta) have been determined by negative ion photoelectron velocity map imaging. Electron affinities and vibrational frequencies for the ground state and excited states of MO (M = V, Nb, and Ta) molecules have been identified as well as photoelectron angular distributions. On the basis of the equivalent-electron principle, MO^- (M = V, Nb, and Ta) molecules bear valence electron configurations similar to those of coinage metal elemental atoms, despite having more complicated electronic states for molecules, and concomitant mimicry of magnetic superatom. Generally, other than low-spin states of coinage metal atoms, Group V metal monoxides demonstrate a high-spin state except for TaO, possessing the potential applications to inexpensive superatoms in industrial catalysis.

Expanded Inverse-Sandwich Complexes of Lanthanum Borides: La2B10- and La2B11. J Phys Chem A 2021;125:2622-2630. [PMID: 33739102 DOI: 10.1021/acs.jpca.1c01149]

Inverse-sandwich structures have been observed recently for dilanthanide boride clusters, in which two Ln atoms sandwich a monocyclic B_x ring for x = 7-9. An interesting question is if larger B_x rings are possible to form such inverse-sandwich clusters. Here we address this question by investigating La₂B₁₀^- and La₂B₁₁^- using photoelectron spectroscopy and ab initio quantum chemical calculations. Photoelectron spectra of La₂B₁₀^- and La₂B₁₁^- show complicated, but well-resolved, spectral features that are used to compare with theoretical calculations. We have found that global minimum structures of the two clusters are based on the octa-boron ring. The global minimum of La₂B₁₀^- consists of two chiral enantiomers with C₁ symmetry, which can be viewed as adding a B₂ unit off-plane to the B₈ ring, whereas that of La₂B₁₁^- can be viewed as adding a B₃ unit in-plane to the B₈ ring in a second coordination shell. Chemical bonding analyses reveal localized B-B bonds on the edge of the clusters and delocalized bonds in the expanded boron frameworks. The interactions between the La atoms and the boron frameworks include the unique (d-p)δ bonding, which was found to be the key for inverse-sandwich complexes with monocyclic boron rings. The current study confirms that the largest monocyclic boron ring to form the inverse-sandwich structures is B₉ and provide insights into the structural evolutions of larger lanthanide boride clusters.

On the Role of Alkali-Metal-Like Superatom Al12 P in Reduction and Conversion of Carbon Dioxide

Developing efficient catalysts for the conversion of CO₂ into fuels and value-added chemicals is of great significance to relieve the growing energy crisis and global warming. With the assistance of DFT calculations, it was found that, different from Al₁₂ X (X=Be, Al, and C), the alkali-metal-like superatom Al₁₂ P prefers to combine with CO₂ via a bidentate double oxygen coordination, yielding a stable Al₁₂ P(η² -O₂ C) complex containing an activated radical anion of CO₂ (i.e., CO₂ ^.- ). Thereby, this compound could not only participate in the subsequent cycloaddition reaction with propylene oxide but also initiate the radical reaction with hydrogen gas to form high-value chemicals, revealing that Al₁₂ P can play an important role in catalyzing these conversion reactions. Considering that Al₁₂ P has been produced in laboratory and is capable of absorbing visible light to drive the activation and transformation of CO₂ , it is anticipated that this work could guide the discovery of additional superatom catalysts for CO₂ transformation and open up a new research field of superatom catalysis.

Revealing the effect of the oriented external electronic field on the superatom-polymeric Zr3O3 cluster: Superhalogen modulation and spectroscopic characteristics

Seeking novel strategies for designing superatoms is of significance for the potential applications in cluster-assembled nanomaterials. Herein, by employing the density functional theory (DFT) calculations, the effect of the oriented external electronic field (OEEF) on the electronic and photoelectron spectroscopic properties of the superatom-polymeric Zr₃O₃ cluster was explored. We present the evidence that the increment of the OEEF along all directions results in the remarkable enhancement of the electron affinity (EA) of Zr₃O₃, which turns it into superhalogen with an EA value of 4.02 eV under 0.020 au OEEF along +y direction. Strikingly, this EA value is larger than that of any halogen atoms in the periodic table. The downward shift of the electronic spectrum induced by the OEEF was confirmed to be the origin of the observed EA enhancement. Furthermore, the investigation of the OEEF's effect on the molecular orbitals (MOs) and photoelectron spectra (PES) of the cluster reveals that the OEEF could alter the electron distribution as well as promoting the blue shift of the PES without changing the spacings between different energetic levels. The OEEF highlighted here provides a new strategy in designing superatoms together with tuning their electronic and spectroscopic properties conveniently and precisely. We wish this finding could stimulate more efforts in designing novel superatoms or superatom-assembled materials from both theory and experiments.

Tuning the Electronic Properties and Performance of Low-Temperature CO Oxidation of the Gold Cluster by Oriented External Electronic Field

Conventional electronic rules, including Jellium and Wade-Mingos rules and so on, have long been successfully dedicated to design superatoms. These rules, however, rely on altering the intrinsic properties, for example, the compositions or the number of valence electrons, of clusters, which is relatively complicated and inconvenient to manipulate, especially in experiments. Herein, by employing density functional theory calculations, the oriented external electric field (OEEF) was demonstrated to possess the capability of precisely and continuously regulating the electronic properties of clusters at will, representing a novel and noninvasive methodology in constructing stable superatoms because it hardly changes the geometries of clusters. More interestingly, the active sites formed by the charge redistribution upon the introduction of an OEEF could significantly promote the catalytic performance of the low-temperature CO oxidation over clusters. Considering the convenient source of the OEEF, the findings highlighted here may boost the potential applications of superatom-assembly nanomaterials in catalysis and materials science.

Polymeric tungsten carbide nanoclusters: structural evolution, ligand modulation, and assembled nanomaterials

Seeking novel superatoms with tunable electronic and magnetic properties has attracted much interest due to their potential application in cluster assembly nanomaterials. By employing density functional theory (DFT) calculations, the recently observed superatomic WC cluster was adopted as the basic unit to construct larger polymeric clusters, namely (WC)_n (n = 2-7), and their structural evolution was explored to understand the growth pattern of these superatomic clusters into nanoscale materials. An unusual odd-even pattern in structural evolution was disclosed, in which the (WC)₂ unit is considered as the basic building block. Moreover, W₄C₄ is found to possess a cubic structure, based on which the CO and PH₃ ligands were attached to examine their ligation effects on W₄C₄. Theoretical results show that the electronic properties of W₄C₄ can be dramatically altered during the ligation process. Intriguingly, the continuous attachment of CO and PH₃ ligands strongly increases and decreases the electron affinities (EA) and ionization potentials (IP) of the ligated W₄C₄ clusters, respectively, leading to the formation of superhalogen and superalkali species with high magnetic moments. The observed ligand induced strategy highlighted here could serve as an effective way to tune the electronic and magnetic properties of clusters resulting in the formation of novel superatoms. Finally, studies on the geometrical and electronic structures of the W₄C₄ cluster solid unveil its special 3-D cubic honeycomb geometry and metallic properties with predominant contribution from the 5d of W, which may have potential applications in electro-catalysis.

Unveiling the electronic structures and ligation effect of the superatom-polymeric zirconium oxide clusters: a computational study

Discovering the non-noble ZrO cluster as an analog of the noble metal catalyst Pd is of significance toward designing functional materials with fine-tuned properties using the superatom concept. The effect of gradually assembling the ZrO superatomic unit on the electronic structures and chemical bonding of larger ZrO-polymeric clusters, however, is unclear. Herein, by using density functional theory (DFT) calculations, the lowest-energy structures and low-lying isomers of the (ZrO)n-/0 (n = 2-5) clusters were optimized, in which every O atom in these clusters tends to connect its adjacent two Zr atoms forming metal oxygen bridge bonds. Insights into the electronic characteristics of these clusters were obtained by analyzing their molecular orbitals (MOs) and density of states (DOS). More importantly, our studies on the CO (electron acceptor) and PH3 (electron donor) ligated Zr3O3 clusters unveil that the ligation process can substantially alter the electronic properties of the clusters by tuning the HOMO and LUMO states, which may have potential applications in photovoltaics. Strikingly, the successive attachment of PH3 on Zr3O3 dramatically lowers the adiabatic ionization potential (AIP) of the ligated clusters, resulting in the formation of stable superalkali clusters with large HOMO-LUMO gaps. Furthermore, the potential of constructing the superalkali Zr3O3(PH3)5 based 1-D cluster assembled material was also examined.

Probing the Geometric and Electronic Structures of the Monogadolinium Oxide GdO n-1/0 ( n = 1-4) Clusters

The existence of abundant 4f electrons significantly increases the complexity and difficulty in precisely determining the geometric and electronic structures of the lanthanide oxide clusters. Herein, by combining the photoelectron imaging spectroscopy and density functional theory (DFT) calculations, the electronic structure of GdO was investigated. An electron affinity (EA) of 1.16 ± 0.09 eV is obtained, and the measured anisotropy parameter (β) provides direct experimental evidence about the orbital symmetry of the detached electron in GdO^-. DFT calculations have been employed to acquire the optimized geometries of the GdO _n^-1/0 ( n = 2-4) clusters, and multiple activated oxygen species, which are radical, peroxide, superoxide, triradical, and ozonide radical, are found in these oxide clusters. Simulated photoelectron spectra (PES) of the GdO _n^-1/0 ( n = 2-4) clusters are examined, which may stimulate further experimental investigations on the gadolinium oxide clusters. In addition, the valence molecular orbitals (MOs) of these clusters are also discussed to reveal the interaction between the lanthanide metal (Gd) and O atoms.

Theoretical investigations on the d-p hybridized aromaticity, photoelectron spectroscopy and neutral salts of the LaX2- (X=Al, Ga, In) clusters

We present an extensive density functional theory (DFT) calculations on the geometrical and electronic structures of the triatomic LaX₂^- (X=Al, Ga, In) clusters. Various trail structures and spin states have been attempted to determine the lowest-energy geometries of these La-doped metal clusters. The ground states of all three clusters are calculated to possess the trigonal structures with the singlet multiplicities. The calculations on molecular orbitals (MOs) and nucleus-independent chemical shift (NICS) values have been performed to examine the aromatic characteristics of the LaX₂^- (X=Al, Ga, In) clusters. The present calculations disclose that all these metal clusters are doubly aromatic, namely d-p hybridized σ and π aromaticity resulting from the effective overlap between the 5d atomic orbital of the La atom and the p orbitals of the IIIA group elements. Theoretical vertical detachment energies (VDEs) were also calculated to simulate the photoelectron spectra (PES) of the clusters. In addition, by adding the alkali cations (Li⁺ and Na⁺) into the LaX₂^- (X=Al, Ga, In) clusters, the geometries and electronic structures of the corresponding neutral salts have also been investigated to gain more insights in the potential of using these aromatic anions as building blocks.

On the Potential of Using the Al7 Superatom as an Excess Electron Acceptor To Construct Materials with Excellent Nonlinear Optical Properties

With the aid of density functional theory (DFT) calculations, we found that, when alkali metal approaches the Al7 superatom, its outermost s-value electron can be trapped by Al7 to give the superatom compound MAl7 (M = Li, Na, K) with an excess electron. Different analyses including natural bond orbital (NBO), electron localization function (ELF), and energy decomposition analysis (EDA) show that the resulting M-Al bond is strong and has a polar covalent character. The optimizations of self-assemblies (MAl7)n (n = 2, 3) have been performed to explore the stability of MAl7 in the solid state. The results reveal that only NaAl7 can keep its structural integrity as a building block upon self-assembling, while serious aggregations between Al7 clusters occur in the dimers and trimers of LiAl7 and KAl7, despite the fact that the Li-Al7 and K-Al7 bond energies are comparable to that of Na-Al7. Born-Oppenheimer molecular dynamics (BOMD) simulations for (NaAl7)n (n = 2, 3) indicate that these species are stable toward fragmentation at 300 K. The β0 values of (NaAl7)n (n = 1, 2, and 3) predicted at the CAM-B3LYP/6-311+G(3df) level of theory are in the range of 1.6 × 10(4)a.u. to 7.5 × 10(4) a.u.. This theoretical study implies that NaAl7 is a promising candidate for nolinear optical (NLO) materials. We provide theoretical evidence for the possibility of using the Al7 superatom as an excess electron acceptor to construct materials with excellent NLO properties. Further experimental research is invited.

Direct experimental observation of weakly-bound character of the attached electron in europium anion

Direct experimental determination of precise electron affinities (EAs) of lanthanides is a longstanding challenge to experimentalists. Considerable debate exists in previous experiment and theory, hindering the complete understanding about the properties of the atomic anions. Herein, we report the first precise photoelectron imaging spectroscopy of europium (Eu), with the aim of eliminating prior contradictions. The measured EA (0.116 ± 0.013 eV) of Eu is in excellent agreement with recently reported theoretical predictions, providing direct spectroscopic evidence that the additional electron is weakly attached. Additionally, a new experimental strategy is proposed that can significantly increase the yield of the lanthanide anions, opening up the best opportunity to complete the periodic table of the atomic anions. The present findings not only serve to resolve previous discrepancy but also will help in improving the depth and accuracy of our understanding about the fundamental properties of the atomic anions.