Design of a Novel Series of Hetero-Binuclear Superhalogen Anions MM′X₄⁻ (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br)

Abstract

A series of hetero-binuclear superatom motifs involving chloride/bromide ligands, that is, MM′X₄⁻ (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br) anions, have been characterized by using many-body perturbation theory calculations. Large vertical electron detachment energies (VDEs, 5.470–6.799 eV) confirm the superhalogen identity of these anions. A larger VDE value can be obtained by introducing small M or large M′ central atoms and small halogen ligand atoms. Thus, one isomer of LiCaCl₄⁻ possesses the largest VDE value. Besides, when the extra electron is shared by all ligand atoms or three bridging ligand atoms, the isomers have relatively larger VDE values.

Introduction

Superhalogens are unusual molecules possessing higher electron affinity (EA) than those of any halogen atom (Gutsev and Boldyrev, 1981). They were first proposed by Gutsev and Boldyrev in 1981 and verified by a variety of theoretical chemical methods (Gutsev and Boldyrev, 1981). Meanwhile, a new class of highly stable anions (superhalogen anions) were also reported. Now, superhalogen anions have been proved to possess very large vertical electron detachment energies (VDEs) (Anusiewicz et al., 2003; Koirala et al., 2010; Yang et al., 2017; Li et al., 2019; Li et al., 2020; Zhao et al., 2020), even approaching 14 eV in certain systems (Freza and Skurski, 2010). Moreover, it is found that the superhalogen anions have much high stability, such as BF₄⁻, AlCl₄⁻, and AsF₆⁻, and other superhalogen anions have been confirmed to be stable in crystalline solids or gaseous molecules.

In 1981, Gutsev and Boldyrev proposed the representative formula MX_k+1 for a class of superhalogens, in which M represents the central main group atom, K is the highest valence of M atom, and X is the halogen atom (Gutsev and Boldyrev, 1981). In 1999, the superhalogen anions MX₂⁻ (M = Li, Na; X = Cl, Br, and I) were reported by Wang et al., and their VDE values were experimentally measured for the first time and theoretically calculated applying the outer valence Green function (OVGF) method, which are consistent with each other well (Wang et al., 1999). Shortly afterward, the EA value of the BO₂ superhalogen (Zhai et al., 2007) and the VDE value of the MX₃⁻ (M = Be, Mg, Ca; X = Cl, Br) superhalogen anion (Elliott et al., 2005) were determined by the same experimental means. During subsequent studies on superhalogens and their corresponding anions, the central atom M of MX_k+1 formula was no longer limited to the main group metal atoms (Anusiewicz et al., 2003; Elliott et al., 2005), and the transition metal atoms (Gutsev et al., 1999; Gutsev et al., 2001; Yang et al., 2003), coinage metal atoms (Feng et al., 2011; Lu et al., 2019), and nonmetal atoms (Arnold et al., 2002) could act as central atoms to construct superhalogens. In addition, the researchers found that increasing the number of central atoms benefits the dispersion of extra negative charges without increasing the repulsion between ligands. Therefore, some binuclear/multinuclear superhalogen anions have been proposed by experimental synthesis or theoretical prediction (Anusiewicz and Skurski, 2007; Anusiewicz, 2008; Czapla, 2017; Yang et al., 2017; Yang et al., 2018).

Besides, it is realized that halogen atoms are not the necessary units for the construction of superhalogens. In recent years, the ligands of superhalogens have been extended from halogen atoms to oxygen atoms (Gutsev et al., 1999; Zhai et al., 2007), acid functional groups (Anusiewicz, 2009b), various monovalent groups (Smuczynska and Skurski, 2009), nine-electron ligands (Sikorska et al., 2011), hydroxyl groups (Świerszcz and Anusiewicz, 2011), and electrophilic substituents (Anusiewicz, 2009a). In addition, M@Nk-type superhalogens with inclusion complexes of metal (Zhai et al., 2004) and carborane cage superhalogens (Pathak et al., 2011) have also been proposed. Recently, a new class of cluster was designed in which the central atom was modified by superhalogen ligands replacing the halogen ligands. These clusters have higher EAs than their superhalogen ligands; thus, they are termed “hyperhalogen" (Willis et al., 2010). Subsequently, many hyperhalogens with various geometries of superhalogen ligands have been proposed (Paduani et al., 2011; Gutsev et al., 2012; Sun et al., 2015; Sun et al., 2016; Yang et al., 2021; Dong et al., 2022).

Superhalogens play an important role in chemistry given the strong oxidation capability. For example, they can be used as capable oxidants to oxidize substances that have relatively high ionization potentials (e.g., O₂ (Bartlett and Lohmann, 1962), noble gas atom (Bartlett, 1962), and (H₂O)n clusters (Marchaj et al., 2013)). They can also be used to synthesize and prepare noble gas compounds (Saha et al., 2018; Chang et al., 2019), supersalts (Giri et al., 2014a), ion battery electrolytes (Giri et al., 2014b), ionic liquids (Srivastava et al., 2021), liquid crystalline molecules (Srivastava, 2021), solar cells (Kim et al., 2022), and so on. Therefore, exploring various new species classified as superhalogens and studying their structures, stability, and properties has become a significant and attractive research topic in recent years.

To the best of our knowledge, most hitherto proposed superhalogens are mono- or homo-nuclear. The hetero-nuclear superhalogens involving different central atoms, however, have received very little attention. The investigation on the influence of different ligands on hetero-nuclear superhalogen properties, however, has not been reported yet. In this research, we aim to design a new class of superhalogen anions with two different central atoms using chloride or bromine atoms as ligands. Consequently, the MM′X₄⁻ (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br) anions have been proposed and systematically investigated. The considerable VDE values of these anions confirm their superhalogen identity. The geometric features and relative stability of these anions were analyzed. Meanwhile, the correlations between their VDEs and structural features, ligand and central atoms, and extra electron distribution are also revealed. The present investigation predicts a new member of superhalogens and conduces to the development of new strong oxidizing agents.

Computational Details

Initially, the structures of the MM′X₄⁻ (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br) anions were built by considering all the possible connection between M, M′, and X atoms. Then, all the constructed structures of anions were optimized using the Møller–Plesset perturbation method (MP2) (Møller and Plesset, 1934) together with the 6–311+G (3df) basis set (Yang et al., 2017; Yang et al., 2018). Meanwhile, frequency analysis was performed at the same computational level to ensure that the obtained structures are stable on potential energy surfaces without imaginary frequency. Natural bond orbital (NBO) (Reed et al., 1985) and single-point energy calculations were carried out at the same level.

The vertical electron detachment energies (VDEs) of the MM′X₄⁻ anions were calculated applying the outer valence Green function (OVGF) approximation (Cederbaum, 1975) with the 6–311+G (3df) basis set. The smallest pole strength (PS) in our study is 0.90, justifying the validity of the OVGF method (Zakrzewski et al., 1996).

The above-mentioned calculations were performed using the GAUSSIAN 16 program package (Frisch et al., 2016). The plots of molecular structures and orbitals were generated with the GaussView program (Dennington et al., 2016).

Results and Discussion

Geometrical Structures and Relative Stability

The optimized geometries of MM'X₄⁻anions are depicted schematically in Figure 1. The relative energies, the lowest vibrational frequencies, bond lengths, and angles are listed in Tables 1, 2. As shown in the figure, each MM'X₄⁻ anion has two types of structures, that is, central atoms M and M′ are connected by two or three bridging X atoms. Notably, these two structures are also presented in the superhalogen anions with F ligands (Yang et al., 2017). Unlike MM'F₄⁻ anions (Yang et al., 2017), the structures involving one bridging ligand atom are not stable, which turn to the above two types of structures after optimization. In terms of the number of bridging X atoms, the isomers of MM'X₄⁻ are termed MM'X₄⁻-2 and MM'X₄⁻-3, respectively. For the sake of convenience, the terminal X atoms that bind with M and M′ atoms are named X_t and X_t’, respectively, and the bridging X atom that connects M and M′ atoms is named X_b.

FIGURE 1

It can be seen in Figure 1 that the MM'Cl₄⁻-2 and MM'Cl₄⁻-3 isomers possess planar and three-dimensional structures with C_2v and C_3v symmetries, respectively. From the data in Table 1, it is found that for MM'Cl₄⁻-2 and MM'Cl₄⁻-3 isomers, when M′ atom varies from Be to Ca, the Cl_t-M and M-Cl_b bond lengths change very little, while the ∠Cl_bM'Cl_t angle showed a tendency of increasing; for example, the orders of ∠Cl_bM'Cl_t angle in NaM'Cl₄⁻-2 and NaM'Cl₄⁻-3 are 122.4° < 127.4° < 131.8° and 115.0° < 121.0° < 125.8° with varying M′ atoms, respectively. Thus, the MM'Cl₄⁻-2 and MM'Cl₄⁻-3 structures tend to elongate along the M-M′ axis with the increasing radius of M′ atoms. Besides, the Cl_t-M bond is shorter than the M-Cl_b bond in MM'Cl₄⁻-2 isomers. For instance, the Cl_t-Na bonds are about 2.80 Å, while the Na-Cl_b bonds are 2.50 Å in NaM'Cl₄⁻-2 isomers. On the other hand, when the M atom goes from Li to Na, the Cl_b-M′ and M′-Cl_t’ bond lengths also show minor difference in MM'Cl₄⁻ anions, but the ∠Cl_bM'Cl_t’ angles show a decrease, e.g., the ∠Cl_bM'Cl_t’ angles of LiBeCl₄⁻-2 and LiBeCl₄⁻-3 are 1.4 and 1.6° larger than that of the corresponding NaBeCl₄⁻-2 and NaBeCl₄⁻-3, respectively. In addition, the total NBO charges of Cl_tM subunits are in the range of -0.080–0.088|e|, which are close to zero (see Table 1); consequently, the total NBO charges of M'Cl₃ subunits approximate -1. In this sense, the MM'Cl₄⁻ structures can be regarded as a combination of an MCl molecule and a superhalogen anion M'Cl₃⁻.

MM'Br₄⁻ series show similar structural characteristics with MM'Cl₄⁻ anions. From the data in Tables 1, 2, it is noticed that ∠X_bM'X_t’ angles of MM'Cl₄⁻ anions are always larger than that of the corresponding MM'Br₄⁻ anions; for example, the ∠Cl_bM'Cl_t’ angles of NaMgCl₄⁻-3 and NaMgCl₄⁻-2 are 121.0° and 127.4°, respectively, which are larger than the ∠Br_bM'Br_t’ angles of NaMgBr₄⁻-3 (120.1°) and NaMgBr₄⁻-2 (126.5°), respectively.

It is reported that the isomers could exhibit higher stability with more bridging ligands in the previous studies on the dual-nuclear superhalogen anions with F ligands, such as homonuclear Mg₂F₅⁻(Anusiewicz and Skurski, 2007) and heteronuclear ones NaM'F₄⁻(M' = Mg, Ca) (Yang et al., 2017). MM'X₄⁻ anions also follow this rule except the MBeX₄⁻ series. However, MBeX₄⁻ anions show a reverse trend; that is, structure 2 is more stable than structure 3. In other words, Be atoms are more likely to bond with three ligands than four ligands. This may be due to the smaller atomic radius of the central Be atom, the three bridging ligands are more crowded in structure 3 of MBeX₄⁻ anions than in structure 3 of MMgX₄⁻ and MCaX₄⁻. This can be confirmed by the shorter Cl_b-Cl_b and Br_b-Br_b distance in structures 3 of MBeX₄⁻ than MMgX₄⁻ and MCaX₄⁻; for example, the Cl_b-Cl_b and Br_b-Br_b distances in LiBeCl₄⁻-3 and LiBeBr₄⁻-3 are 0.226 Å and 0.229 Å shorter than that in LiMgCl₄⁻-3 and LiMgBr₄⁻-3, respectively. Thereby, MBeX₄⁻-3 is less stable than the MBeX₄⁻-2.

Vertical Electron Detachment Energies (VDEs)

FIGURE 2

FIGURE 3

Conclusion

Our systematic investigation of the MM′X₄⁻ (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br) species has theoretically proposed a series of hetero-binuclear superhalogen anions. The results show that these heteronuclear superhalogen anions could gain larger VDE values by involving a smaller alkali metal atom M, a larger alkaline earth metal atom M′, and a higher electronegative ligand atom X. Thereby, of all the anions studied, an isomer of LiCaCl₄⁻ anions possesses the largest VDE value (6.799 eV). Moreover, the extra electron distribution is a very influential factor in the VDE values of structural isomers. For the NaBeX₄⁻ anions, the isomers have larger VDE values when the extra electron is distributed on the terminal X_t ligand atom instead of the X_t’ ligand atom. For the other anions, the isomers possess larger VDE values when the extra electron is shared by all ligand atoms or three bridging ligand atoms.

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