Structural Stability and Evolution of Medium-Sized Tantalum-Doped

Dec 11, 2017 - Structural Stability and Evolution of Medium-Sized Tantalum-Doped Boron Clusters: A Half-Sandwich-Structured TaB12–Cluster. Bo Le Che...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Structural Stability and Evolution of Medium-Sized Tantalum-Doped Boron Clusters: A Half-Sandwich-Structured TaB12− Cluster Bo Le Chen,† Wei Guo Sun,† Xiao Yu Kuang,*,† Cheng Lu,*,‡,§ Xin Xin Xia,† Hong Xiao Shi,† and George Maroulis*,⊥ †

Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China Department of Physics, Nanyang Normal University, Nanyang 473061, China § Department of Physics and High Pressure Science and Engineering Center, University of Nevada, Las Vegas, Nevada 89154, United States ⊥ Department of Chemistry, University of Patras, GR-26500 Patras, Greece ‡

S Supporting Information *

ABSTRACT: Transition-metal (TM)-doped boron clusters have received considerable attention in recent years, in part, because of their remarkable size-dependent structural and electronic properties. However, the structures of medium-sized boron clusters doped with TM atoms are still not well-known because of the much increased complexity of the potential surface as well as the rapid increase in the number of low-energy isomers, which are the challenges in cluster structural searches. Here, by means of an unbiased structure search, we systematically investigated the structural evolution of medium-sized tantalum-doped boron clusters, TaBn0/− (n = 10−20). The results revealed that TaBn0/− (n = 10−15) clusters adopt half-sandwich molecular geometries, with the notable exception of TaB10−, while for n = 16−18 and 19−20, the lowest-energy clusters are characterized by drum-type geometries and tubular molecules with two B atoms on the top, respectively. Good agreement between the calculated and experimental photoelectron spectra strongly support the validity of our global minimum structures. Molecular orbital and adaptive natural density partitioning analyses indicate that the enhanced stability of half-sandwich TaB12− is due to the strong interaction of the Ta atom (5d orbitals) with surrounding B atoms (2p orbitals) and σ B−B bonds in the B12 moiety. shapes.12,13 In 2007, Oger reported a planar-to-cylindrical structure transition for cationic boron clusters occurring at n = 16.14 While in anionic species Bn− clusters display planar or quasi-planar structures up to 25 B atoms,15−21 even higher anionic boron clusters with 27, 29, 30, 35, and 36 atoms13,22−24 continue to display a (quasi-)planar pattern with a borospherene characteristic, that is, a central hexagonal hole. Subsequently, the new borospherene B40 with D2d symmetry has been reported.25,26 A possible way to extend our knowledge of the structural and electronic properties of boron clusters consists of doping, that is, attaching selected heteroatoms. Among the small-sized boron clusters, wheel-like B8− and B9− are distinctively characterized by the presence of a central B atom.8 This has motivated systematic explorations where the central B atom is substituted by other adjacent or isoelectronic atoms. However, studies on CB62−, CB7−, CB8, and CB8− reported that the C atom tends to occupy an edge and not the central position because of its more electronegative character.27 In addition, a joint experimental and theoretical investigation found that replacing the central B atom by a valence isoelectronic Al atom

1. INTRODUCTION Boron and its compounds have attracted considerable interest in recent years because of the fact that a large number of unusual chemical structures and novel properties exist.1−6 It is well-known that boron shows a strong ability to bond with most elements via a favored delocalized multicenter bond. This bonding capacity is predominantly due to the electron deficiency of the B atom; the electron configuration of boron is 2s22p1, while it has four valence electron orbitals, leaving an orbital empty. The unique nature of this electronic defect leads to intriguing architectures for boron clusters, such as planar, tubular, and fullerene-like (borospherenes). In recent years, important theoretical investigations of the structural and electronic properties of boron clusters have been reported. Neutral and charged Bn clusters, with n ranging from 7 to 15, have been experimentally and theoretically studied,7−9 and all of them have been found to have aromatic planar structures with the notable exception of the global minimum state of neutral B14,10 which is a complicated three-dimensional (3D) structure redefined by a later report. Nearly contemporary work reveals that B16 and B17 are planar or quasi-planar.11 Remarkably, experimental and computational simulation works clearly indicate that the B20, B26, and B27 clusters are double-ring tubular forms but B28 and B29 adopt quasi-planar © XXXX American Chemical Society

Received: October 9, 2017

A

DOI: 10.1021/acs.inorgchem.7b02585 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry just results in umbrella-like AlB7− and AlB8− structures.28 Keigo and colleagues theoretically demonstrated that Co, Fe, and Ni encapsulated by a B8 or B9 ring have perfectly planar hypercoordinate structures.29 Subsequently, they performed density functional theory (DFT) calculations on MBn [M represents the first row of transition metals (TMs); n = 7− 10].30 On the basis of the above-mentioned studies and considering the character of the TM participating delocalized bonds, Romanescu et al.31 advanced the hypothesis that TMs are more suitable candidates to occupy the center of wheel-like Bn clusters. They proposed a guiding design principle taking into account relevant electronic and geometric factors. Following this principle, a series of TM-centered boron rings have recently been reported: CoB8−, FeB9−, RuB9−, RhB9, IrB9, TaB10−, NbB10−, and VB92− are perfectly wheel-like molecules with unprecedented symmetry and aromaticity.31−37 Hence, a thought-provoking question arises: how are these extraordinary structures formed?37 As a canonical example, Wang and coworkers38 investigated the formation mechanisms of a decagonal TaB10− based on the photoelectron spectroscopy (PES) techniques and DFT calculations and found that the B8 ring is too small to enclose a Ta atom, which is in agreement with the previous report.30 Enlarging the ring by adding a B atom still results in a pyramidal structure.35 With a Ta atom in the center of the B10 ring, the perfect wheel-like molecule with D10h symmetry is formed.37 Then, theoretical studies of ditantalum doped into Bn (n = 2−5) clusters revealed that the strong B−B bonds have a decisive impact on the lowestenergy structures of Ta2Bn0/− (n = 2−5),39 which are all of bipyramidal shape. Subsequently, it is reported that Ta2B60/− is also of bipyramidal form, with the composition of two Ta atoms and a planar B6 ring, which can be viewed as structural motifs.40 A recent theoretical investigation in combination with PES experimental observations on TaB20− revealed that the global minimum represents a tubular structure with two B atoms on top of the drum moiety, not the perfect D10d drumlike one.41 Although many studies about TM-doped boron clusters have been reported to date, remarkably little is known of the 5d TM tantalum doped into boron clusters, particularly on the structural evolution of tantalum doped into middle-sized Bn (n = 10−20) clusters. What kinds of structural evolution patterns do the tantalum-doped middle-sized boron clusters follow? It is of crucial importance for cluster science to determine the transformation mechanisms of planar wheellike37-to-tubular molecular shape.41 In this work, we provide a systematic study on neutral and anionic tantalum-doped boron clusters in order to elucidate the structural evolution patterns and bonding characteristics of TaBn0/− (n = 10−20).

structure searches and obtained about 1000 trial isomers for each size and charge state. Selected low-lying isomers are then refined by optimization at the PBE051 level in conjunction with the LANL2DZ52 basis set for the Ta atom and the all-electron 6-311+G(d)53 basis set for B atoms, as implemented in the Gaussian 09 package.54 Different spin multiplicities (singlet, doublet, triplet, quartet, quintet, and sextet) are tested for the system considered during the optimization process. The calculation of harmonic vibrational frequencies ensures that the cluster geometries are true local minima on the potential energy surface (no imaginary frequencies obtained). The theoretical PES spectra of the global minimum anionic clusters are simulated using the time-dependent DFT method,55 via calculations of the excitation energies of the neutral clusters at the corresponding anionic structure. Relying on the natural bond orbital (NBO) and adaptive natural density partitioning (AdNDP)56 methods, we carried out chemical bonding analysis in order to achieve a better understanding of the bonding mechanism. The choice of the LANL2DZ basis set for TMs is based on other similar systems, which provided accurate results.57−60 In addition, we used 6-311+G(d) for B atoms and the Stuttgart basis set61 for the Ta atom to calculate the single-point energy, and the results show that the energy orders obtained are found to be almost the same by using two different basis sets. Therefore, we believe that the results achieved at the PBE0 functional couple with the LANL2DZ basis set for the Ta atom and the 6-311+G(d) basis set for B atoms are reliable, and the discussion below is on the basis of this level.

3. RESULTS AND DISCUSSION 3.1. Geometric Configurations. To facilitate understanding via visualization, two different views of the lowestenergy structures of TaBn0/− (n = 10−20) clusters are displayed in Figures 1 and 2. The low-lying isomers of TaBn0/− (n = 10− 20) along with their corresponding symmetries are depicted in Figures S1 and S2. Meanwhile, Table 1 summarizes the electronic states, average binding energies, highest occupied molecular orbital−lowest unoccupied molecular orbital

2. COMPUTATIONAL DETAILS The unbiased structure search of neutral and anionic tantalum-doped boron clusters are based on a global minimum search of the potential energy surfaces using the CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization)42−44 code. It has been severely tested in accurate determinations of molecular geometries of various systems, as shown in some detail elsewhere.45−50 Several major techniques have been implemented in the CALYPSO method to achieve a high efficiency of structure prediction, such as symmetry constraints, bond characterization matrices, and atom-centered symmetrical functions.43 In each generation of structure searches, 60% of the structures are generated by a particle swarm optimization algorithm and the others are generated randomly. In the course of the structure prediction of TaBn0/− (n = 10−20) clusters, we followed 50 generations of initial

Figure 1. Top and front views of the lowest-energy structures of TaBn (n = 10−20) clusters, along with the point group symmetry. B

DOI: 10.1021/acs.inorgchem.7b02585 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

nearly identical and their growth patterns are quite similar. The ground state of TaB11 (singlet, Cs) and TaB11− (doublet, Cs) possess an intermediate structure of wheel and drum, which can be seen as a B8 circular ring with three edge B atoms slightly out of the plane, half-surrounding a Ta atom in the center. The B12 moiety of TaB120/− is analogous to the bare B12 cluster. Just as in the case of MB12− (M = Co, Rh, Ir),62,63 our global minimum computational approach revealed that TaB12− also adopts a half-sandwich geometry with C3v symmetry, while the neutral TaB12 cluster degenerates to Cs symmetry. The lowestenergy structures of TaB13 and TaB13− clusters are Cs (singlet) and C1 (doublet), respectively. Both of them are quasi-planar with a decorating Ta atom above it, showing a half-sandwich structure. As for TaB14−, the ground state is a Cs half-sandwich structure with the Ta atom in the center. TaB14 (doublet) possesses the same global minimum geometry shape and point symmetry as TaB14−. In analogy to the case of n = 14, TaB15 (singlet) and TaB15− (quartet) also have identical lowest-energy structure configurations and symmetries (Cs). With the number of B atoms increasing from 16 to 18, it is obvious that the central Ta atom is eventually sandwiched by the two monocyclic rings of B atoms, adopting a slightly distorted drum-type structure characterized by the lowest spin multiplicity, with the exception of TaB18 (quartet). The global minima of TaB19 and TaB19− can be visualized as two B atoms capped on the Ta-centered drum. Both of them are of Cs symmetry and possess lowest spin multiplicity, singlet and doublet, respectively. The lowest-energy structures of TaB20 (doublet, Cs) and TaB20− (doublet, Cs) can be built upon TaB19 and TaB19− by adding a B atom on the drum. The optimized lowest-energy structure of TaB20− in this work is largely in agreement with the tubular molecular rotor reported by the previous theoretical and experimental studies.41 From the results presented above, it is evident that the Ta atom is invariably located on the center, surrounded by B atoms. The global minimum structures visually display a growth pattern going from half-sandwich to drum-type and then to tubular molecular rotor. 3.2. PES of TaBn− Clusters. At this stage, it is imperative to confirm the validity of the determined ground-state structures. Also, we know that PES is an important technology to extract electronic binding energies from the lowest-energy structures, which can provide more information about the underlying electronic structures. Herein, the simulated spectra of groundstate structures combined with the experimental PES spectra are displayed in Figure 3. The vertical detachment energy (VDE) was defined by the location of each peak in the spectra, which represents the electronic detachment transition from the anionic ground state to the counterpart neutral ground or excited state at the optimized anionic geometry. In addition, the first VDE values of each anionic cluster are given in Table S1 compared with the available experimental values. It can be seen from the simulated spectra of TaB10− that three distinct peaks are located in the region of 4.0−6.0 eV, with the first peak located at 4.00 eV, which is in excellent agreement with the corresponding experimental result of 4.04 eV.37 The experimental spectra of TaB10− are almost simulated by our calculations, with just a slight shift to the higher binding energy for the third peak. Additionally, for the theoretical PES of TaBn− clusters with n = 11−19, we observed that the first peak is approximately in the binding energy range of 3.0−4.0 eV. In addition, there are several prominent peaks from 4.0 to 6.0 eV except for TaB18−, which has just one prominent peak in

Figure 2. Top and front views of the lowest-energy structures of TaBn− (n = 10−20) clusters, along with the point group symmetry.

Table 1. Electronic States, Average Bonding Energies Eb (eV), HOMO−LUMO Energy Gaps Egap (eV), and the Charges on the Ta Atom Q(Ta) (e) of the Lowest-Energy TaBn0/− (n = 10−20) Clusters TaBn−

TaBn n 10 11 12 13 14 15 16 17 18 19 20

state 2

A1 1 A′ 2 A′ 1 A′ 2 A′ 1 A′ 2 A1 1 A′ 4 Au 1 A′ 2 A′

Eb

Egap

Q(Ta)

5.03 5.01 5.18 5.09 5.22 5.23 5.28 5.31 5.39 5.41 5.45

2.20 2.23 1.79 2.64 1.84 2.88 1.53 2.12 1.36 3.01 1.51

0.60 0.41 0.73 0.47 0.20 0.26 −0.47 −0.30 −0.18 −0.23 −0.70

state 1

Ag 2 A″ 1 A1 2 A 1 A′ 4 A″ 1 A1 2 A′ 1 A 2 A′ 1 A′

Eb

Egap

Q(Ta)

5.25 5.26 5.42 5.31 5.40 5.33 5.41 5.48 5.57 5.52 5.58

2.59 1.92 2.65 1.93 2.25 0.57 1.03 1.65 1.72 1.82 2.82

0.41 0.22 0.60 0.30 0.11 −0.09 −0.49 −0.38 −0.19 −0.29 −0.78

(HOMO−LUMO) energy gaps, and charges on the Ta atom of the determined lowest-energy structures. Clear structure diagrams show that both neutral and anionic species are 3D structures, except for TaB10−. For n = 10, TaB10− obeys the design rules of TM-centered boron molecular wheels, which contains the geometry size and electron number 2 parts.35 Therefore, the lowest-energy structure of TaB10− is a perfect wheel-like molecule with D10h symmetry, of almost the same shape as that reported by Galeev et al.37 However, the D2h wheel-like molecule of TaB10 is just a metastable isomer, being higher in energy than its lowest-energy C2v bowl-like structure by 1.67 eV (Figure S1). In the B atom size range of n from 11 to 20, the configurations of neutral and anionic clusters are C

DOI: 10.1021/acs.inorgchem.7b02585 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Simulated PES (outer, blue) of lowest-energy TaBn− (n = 10−20) clusters with the experimental PES (inset, red) from Galeev et al.37 and Li et al.41 for comparison.

Figure 4. Size dependences of the average binding energies Eb (a), second-order difference Δ2E (b), and HOMO−LUMO gaps Egap (c) for the global minimum TaBn0/− (n = 10−20) clusters.

−

the binding energy region of 4.0−6.0 eV. For TaB20 , the first peak coincides well with the experimental measurement value,41 and our simulation virtually reproduces the essential features of the experimental spectra. In spite of the slight deviation between the theoretical and experimental spectra, which may be attributed to the coexistence of potential isomers in the experimental measurement, the first VDE values of the TaB10− and TaB20− spectra coincide well with the experimental data. On the basis of the reliable results obtained, we believe that the predictive PES will facilitate further theoretical and experimental studies. 3.3. Relative Stabilities and Charge Transfer. Two significant parameters associated with thermodynamic and relative stabilities, average binding energy, and second energy difference have been computed to analyze the relative stability of the determined global minimum geometries of the TaBn0/− clusters via the following formulas, where E represents the energy of the corresponding atom or cluster. The results are plotted in parts a and b of Figure 4, respectively. E b(TaBn) = [E(Ta) + nE(B) − E(TaBn)]/(n + 1)

of odd−even alteration. The obvious peak found in n = 12 indicates that TaB12 and TaB12− are more thermodynamically stable than their neighboring clusters. For n > 14, the two curves increase gradually with the B atom number n, with the exception of the slight decreases in TaB15− and TaB19−, suggesting a stability enhancement effect with the increase of the number of B atoms. From Figure 4b, we infer that both neutral and anionic clusters exhibit odd−even oscillation behavior. Distinct peaks occur at n = 12, 14, 16, and 18, implying that the TaB120/−, TaB140/−, TaB160/−, and TaB180/− clusters are more stable than their neighbors. It is also evidenced that TaB12− and TaB14− are salient on the binding energy curve of Figure 4a. The energy difference between HOMO and LUMO can be considered as an indicator of chemical stability. A large HOMO−LUMO Egap value signifies a high-energy cost for an electron excited from the HOMO to the LUMO and also suggests that the corresponding structure is chemically inert. From Figure 4c, the obvious oscillation behavior of neutral species indicates that the neutral tantalum-doped boron clusters with an odd number of B atoms are more stable than those with an even one. This may relate to the fact that the neutral clusters with an odd number of B atoms possess paired even-number valence electrons, resulting in a closed-shell electronic configuration. The same oscillatory behavior can be seen in anionic species with a number of B atoms in the 10−14 range, with a sharp decline appearing at n = 15. From n > 15, the curve shows a gradual rising trend. Several nonnegligible peaks are found in TaB10−, TaB12−, TaB14−, and TaB20−, implying that these clusters are relatively more stable than other anionic ones. Therefore, combining average binding energy and second-order

(1)

E b(TaBn−) = [E(Ta) + (n − 1)E(B) + E(B−) − E(TaB−n )]/(n + 1)

(2)

Δ2 E(TaBn 0/ −) = E(TaBn − 10/ −) + E(TaBn + 10/ −) − 2E(TaBn 0/ −)

(3)

As evidenced from Figure 4a, the average binding energy values of anionic clusters are overall more sizable than the corresponding neutral ones. A larger value of Eb indicates a high relative stability of the counterpart. From the definition of Eb, we can say that TaBn− clusters are more stable than the corresponding neutral clusters. In the size range of n = 10−14 B atoms, both neutral and anionic clusters show the same trends D

DOI: 10.1021/acs.inorgchem.7b02585 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry difference analyses, we locate magic clusters at TaB12−, TaB14−, and TaB20−. To investigate charge transfer between the Ta atom and the host of the boron cluster, we employed natural population analysis (NPA) for the lowest-energy TaBn0/− (n = 10−20). The results of the charge on the Ta atom are shown in Figure 5

Figure 6. MOs of the TaB12− cluster.

as shown in Figure 1. Then, HOMO−2 (e) and HOMO−3 (e) are characterized with π + δ orbitals and primarily originate from B 2p and Ta 5d. The percentages of Ta 5d orbital contributions for HOMO−2 (e) and HOMO−3 (e) are 19.04% and 12.28%, respectively. Finally, HOMO−4 and HOMO−4′ with σ-orbital features are predominantly responsible for the B−B σ bonds of the peripheral B atoms, and HOMO−6 (e) with π-orbital features is primarily composed of p orbitals of the B atoms. In addition, the contributions of Ta 5d to the two nondegenerate orbitals, HOMO−1 and HOMO−5, are 23.07% and 7.7%, respectively. Through the above analyses, we conclude that the 5d orbitals of the Ta atom have certain contributions to the occupied MOs and strengthen the interactions between the B12 moiety and the central Ta atom. We employed the AdNDP method to achieve quantitative insight into the nature of the bonding of TaB12−. AdNDP is a generalized NBO search method for analysis of the localized and delocalized multicenter bonds (coded as nc-2e, that is, an n-center two-electron bond). As depicted in Figure 7, the chemical bonding pattern can be divided into four categories. First, the peripheral B9 ring is characterized by nine 2c-2e

Figure 5. Natural charge populations of the Ta atom for the lowestenergy structures of the TaBn0/− (n = 10−20) clusters.

as a function of the boron size and summarized in Table 1. In the size range of n = 10−12 for anion species, Q(Ta) values of TaB10−, TaB11−, and TaB12− are 0.41, 0.22, and 0.60 e, respectivily, while for n > 12, the Q(Ta) value begins to decrease, suggesting that the Ta atom presents a trend of obtaining electrons with the size n increase. In the case of neutral tantalum-doped boron clusters, the charge transition situations are similar to those of the anion species. It is worth noting that there is an obvious peak at n = 12 for both anionic and neutral clusters. Coincidentally, TaB120/− are both more stable than their neighboring clusters by relative stability analysis. Another notable piece of information is that the charge on the Ta atom from positive to negative occurs at n = 15 and 16 for neutral and anion species, and our structural evolution results indicate that the geometric transition point of halfsandwich to drum-type is also at n = 16. Thus, we conclude that charge transfer between the Ta atom and the host of the boron cluster perhaps relates to the follwing factors: cluster size, geometric configuration, and relative stability. In addition, we also investigated the natural electron configuration (NEC) for the lowest-energy TaBn0/− (n = 10−20) structures and list the results in Tables S2 and S3. 3.4. Chemical Bonding Analysis. On the basis of the comprehensive relative stability analysis in the previous section, the anionic TaB12− cluster with a largest HOMO−LUMO energy gap of 2.65 eV presents an exceptional stability. Thus, we probed the bonding characteristics between the Ta atom and the B12 shell of the TaB12− cluster for an in-depth understanding provided by molecular orbital (MO) and AdNDP analyses. Figure 6 displays some MOs that located near the highest occupied orbital, and we divide them into two categories: doubly degenerate and nondegenerate. Doubly degenerate orbitals contain HOMO (e), HOMO−2 (e), HOMO−3 (e), HOMO−4 (e), and HOMO−6 (e). First, HOMO and HOMO′ of TaB12− are two σ and δ orbitals formed with the components of Ta 5d and B 2p orbitals and have contributions from Ta 5d of 23.07%. Because of the degenerated feature of HOMO, one electron detached from it, together with a slight structure distortion caused by the Jahn− Teller effect, may lead to a lower point symmetry of TaB12 (Cs),

Figure 7. AdNDP analysis of TaB12−. ON stands for the occupation number. E

DOI: 10.1021/acs.inorgchem.7b02585 Inorg. Chem. XXXX, XXX, XXX−XXX

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localized σ B−B bonds, and the occupation number (ON; the ideal value is 2.00 |e|) of each bond is 1.90−1.91 |e|. Then, there are four delocalized 3c-2e σ bonds in the B12 moiety. One of them with ON = 1.95 |e| consists of three inner B atoms with a regular triangle shape. The remaining three delocalized 3c-2e σ bonds (ON = 1.80 |e|) are responsible for the connection between the inner B atoms and the outer B9 ring and lead to the enhanced stability of the B12 moiety. The second row of Figure 7 shows the third class, i.e., the 4c-2e bond. The “+” symbol represents an overlap between the delocalized bonds (σ and π bonds). Strong interactions between the Ta and B atoms, as reflected in the four delocalized σ bonds, render the structure more kinetically stable. The last category includes four totally delocalized 13c-2e bonds with ideal ON = 2.00 |e|. Overall, there are strong interactions between the central Ta atom and the B12 moiety via delocalized bonds to stabilize the halfsandwich TaB12− cluster.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 11574220, 11304167, and 2 16 7 11 14 ) , t h e 9 73 P r o g r am o f Ch i n a ( G ra n t 2014CB660804), the Special Program for Applied Research on Super Computation of the NSFC−Guangdong Joint Fund (the second phase; Grant U1501501), and the Program for Science & Technology Innovation Talents in Universities of Henan Province (Grant 15HASTIT020). Parts of the calculations were performed using the Cherry Creek Supercomputer of UNLV’s National Supercomputing Institute.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02585. Low-lying isomers of TaBn0/− (n = 10−20) clusters, NEC of the lowest-energy TaBn0/− (n = 10−20) clusters, calculated VDE values of the lowest-energy TaBn− (n = 10−20) clusters, and Cartesian coordinates of the lowestenergy geometry of the half-sandwich TaB12− (C3v, 1A1) cluster (PDF)

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4. CONCLUSIONS In conclusion, we performed an extensive theoretical study on the structural evolution behavior of tantalum-doped Bn clusters in the size range of n = 10−20 by using unbiased structure searches coupled with DFT calculations. It is clearly shown that the structure evolution pattern of tantalum-doped boron clusters, from half-sandwich to drum-type at n = 16 and from drum-type to tubular molecular rotor at n = 19, is followed by both neutral and anionic clusters. Interestingly, for n = 11−20, neutral and anionic clusters present identical structural evolution patterns and essentially similar configurations. The calculated results of the relative stability reveal that TaB12− possesses a high HOMO−LUMO energy gap of 2.65 eV and shows a goodish relative stability within the size range of n = 10−20, which is due to the fact that the Ta atom interacts strongly with the seashell B12 moiety. We hope that the results presented here will inspire new experiments and theoretical studies to explore the structural and electronic properties of TM doping of boron clusters.

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AUTHOR INFORMATION

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*E-mail: [email protected] (X.Y.K.). *E-mail: [email protected] (C.L.). *E-mail: [email protected] (G.M.). ORCID

Xiao Yu Kuang: 0000-0001-7489-9715 Cheng Lu: 0000-0003-1746-7697 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.7b02585 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02585 Inorg. Chem. XXXX, XXX, XXX−XXX