Open Access Article
Jingxin
Hu
,
Lin
Zhang
and
Zexing
Cao
*
State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, China 361005. E-mail: zxcao@xmu.edu.cn
First published on 12th March 2025
The low-energy isomers of boron sulfides (B2S3)n (n = 1–6) were constructed by using B2S3 as the building block, and their structure, stability, and reactivity toward small molecules have been explored by density functional theory (DFT) calculations. It is found that the low-energy isomers of (B2S3)n clusters are of rich bonding characteristics for their structural constituents, such as [S−B−S], [B2S3], [B3S3], [S], etc. The planar or non-planar B2S2 rings, as the basic structural units, are bridged together by the S atoms to form the most stable structures of (B2S3)n clusters with n ≥ 2. These low-energy (B2S3)n clusters are predicted to be stable both structurally and electronically, and their boron centers show relatively high activity to the binding of small molecules NH3 and CO, until all boron atoms are ligated by NH3 or CO. The present results provide new insights into the structure and properties of (B2S3)n clusters, which are beneficial to the development of boron chemistry and the application of boron-based materials.
In the 1960s and 1970s, White and Gilles et al. confirmed the existence of boron sulfides, including BS2+,44 B2S2+,45,46 B2S3+,47,48 and so on, making a significant advancement in boosting the development of boron sulfide chemistry. The existing boron sulfide clusters can be categorized into three main types: proportional, sulfur-rich, and boron-rich boron sulfide clusters. In 2015, Li and co-workers presented a set of perfectly planar dicyclic B6S60/−/2− clusters featuring a twin B3S2 five-membered ring as the core. It was found that the closed-shell B6S62− exhibits a delocalized characteristic of 10π-electrons, closely analogous to the bonding pattern of the aromatic naphthalene.49 Subsequently, Zheng and co-workers investigated the structural, electronic, and bonding properties of BS0/− and BS30/− by combining experimental and theoretical methods.50 The results reveal that the ground state of BS3− has a planar structure with C2v symmetry, whereas BS3 presents an S−B−S−S bending structure. Recently, Chen and co-workers explored the structural evolution of BnS20/− (n = 2–13) clusters through the DFT calculations combined with particle swarm optimization algorithm software, and the predicted global minima for these clusters suggest a conformational evolution from a linear-chain structure to planar or quasi-planar ones.51 Obviously, while the above theoretical and experimental studies show that boron sulfide clusters exhibit very rich chemical bonding modes, depending on the B/S atomic ratio and the charged state, our understanding of B2S3-type clusters remains insufficient.
Notably, B2O3-type clusters were proved to play a crucial role in boron chemistry and oxidative dehydrogenation of propane.52–57 In 2013, Li and Cheng theoretically studied the structure evolution and electronic properties of (B2O3)n (n = 1–6) clusters, and found that the global minima structures of these clusters are planar up to n = 3, and cages at n = 4–6.58 Given the similarity in the valence electron configurations of sulfur and oxygen atoms, (B2S3)n and (B2O3)n may exhibit similar structural and electronic features. This also implies that B2S3-type clusters could display analogous chemical properties and stability, making them ideal candidates for organic catalysis and materials science.
Recently, B2S3-type two-dimensional (2D) materials were predicted to be promising catalysts for splitting water59,60 and suppressing the shuttle effect in lithium–sulfur batteries.61 However, our knowledge of B2S3-type clusters in terms of their structural evolution and chemical properties is still very limited. Further investigation into the growth of B2S3-type clusters could lead to the development of more stable and efficient materials for these applications.
Under this background, B2S3 has been chosen as a fundamental structural unit to search for the stable structures of a series of (B2S3)n (n = 1–6) clusters by using the artificial bee colony algorithm.62,63 Subsequent DFT calculations were used to determine the structural and electronic properties of the corresponding low-energy structures of (B2S3)n. Additionally, the interaction of selected clusters with small molecules was discussed to explore their potential applications in small molecule activation and capture. The present work aims to further understand the structures, properties, and potential applications of boron sulfide clusters, and to provide a theoretical basis for the development of 2D boron-based materials.
000 depending on the number of atoms in the studied clusters, are auto-generated and pre-optimized by the semiempirical PM6 and GFN2-xTB method with ABCluster.64–67 Then 80 searched structures with relatively low energy for the studied clusters are selected and fully optimized with the more accurate DFT method at the M06-2X-D368,69/6-31G(d,p)70,71 level of theory, implemented in the Gaussian 16 package.72 The final energy ranking of low-energy structures is obtained through a combination of PM6 and GFN2-xTB methods, effectively reducing the risk of missing important structures. All the optimized cluster structures underwent harmonic vibrational analysis at the same level, confirming that all the related species are stable without imaginary frequency. In addition, the composite quantum chemical method G4(MP2)-6X73 is adopted to evaluate the reliability of the M06-2X-D3/6-31G(d,p) level. Mayer bond order,74 adaptive natural density partitioning (AdNDP),75 and orbital composition analyses are performed by the Multiwfn program.76 Geometric structures and molecular orbitals were visualized by using the CYLview,77 IboView,78 and VMD programs.79
As shown in Fig. 1, the most stable isomer I of the B2S3 cluster exhibits a V-shaped structure with alternating bonding between B and S atoms. In contrast, isomers II (T-shaped) and III (Y-shaped) with a B−B motif are less stable than I by 22.5 and 29.0 kcal mol−1, respectively. For the B4S6 cluster, the most stable isomer I features a B2S2 rhombic core with two linear [S−B−S] arms in a trans-configuration, and its corresponding cis-configuration II is almost isoenergetic with I. The isomer III of B4S6 with two non-planar B2S2 units linked by S atoms is lying at 12.8 kcal mol−1 above I. Notably, both isomers IV and V without the B2S2 moiety, with the boron-centered structure connecting three linear [S−B−S] motifs, are higher in energy than I by 19.4 and 22.3 kcal mol−1, respectively. For relatively large clusters of B6S9, B8S12, B10S15, and B12S18, their most stable isomers generally consist of several planar or non-planar B2S2 rings as the main structural units, connected by S atoms to form a closed structure. Obviously, their higher-energy structures tend to feature six-membered [B3S3] rings or linear [S−B−S] structural units. Overall, the low-energy isomers of (B2S3)n clusters with n = 1 and 2 mainly exhibit a planar configuration, whereas a non-planar or cage configuration is adopted for the large-sized clusters with n = 3–6.
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| Fig. 1 The optimized low-energy structures and relative free energies (in kcal mol−1) of the studied boron sulfide clusters, where B and S atoms are in pink and yellow, respectively. | ||
Interestingly, it was found that the most stable isomers of B2S3 and B4S6 have the same configuration as that of the corresponding B2O3 and B4O6 clusters.25,58 Conversely, for relatively large clusters such as B6S9, B8S12, B10S15, and B12S18, the isomers with structures similar to the corresponding boron oxide clusters are less stable than the most stable species by more than 20.0 kcal mol−1. For instance, VI in B6S9, V in B8S12, III in B10S15, and II in B12S18 are higher in energy than their respective most stable isomers by 45.8, 63.0, 47.8, and 23.9 kcal mol−1, respectively. Presumably, there may be significant differences in structural and electronic properties between the boron sulfide (B2S3)n and boron oxide (B2O3)n clusters.
Fig. 2a presents key bond lengths, Mayer bond orders, and bond angles for the most stable isomers of (B2S3)n (n = 1–6) clusters. For planar B2S3 and B4S6 featuring linear [S−B−S] arms, the bonding between the terminal S and B atoms is intermediate between a double and a triple bond, with a Mayer bond order of 2.453 and bond lengths around 1.605 Å. For relatively large clusters, their most stable isomers are composed of planar or non-planar B2S2 cyclic structures bonded together by the sulfur atom, such as B4S6, B6S9, B8S12, B10S15, and B12S18, where the B−S single bonds vary from 1.809 to 1.832 Å. Furthermore, the natural adaptive orbital (NAdO) analyses are performed to describe the distribution, character, and intensity of the chemical bonding.80,81 The NAdO analysis reveals that the terminal B−S bond in B4S6 is characterized by three major NAdOs, one σ-type (0.914) and two π-type (0.621 and 0.601) bonding categories, and the occupancy sum of σ and π NAdOs is 2.116, indicating that the terminal B−S bond is between double and triple bonds, as shown by the Mayer bond order (2.453) analysis.
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| Fig. 3 AdNDP delocalized bonding patterns and the corresponding occupied numbers for (a) B2S3, (b) B4S6, (c) B6S9, and (d) B8S12. The color of atoms: B-pink; S-yellow. | ||
Additionally, the bonding strength between (B2S3)n clusters and the B2S3 monolayer52 are compared under the same theoretical level (see more details in Table S3, ESI†). The results show that the bonding energy of the B−S bonds in (B2S3)n clusters is generally stronger than that of the B2S3 monolayer (except B6S9 cluster). The B6S9 cluster has a lower bonding energy than the B2S3 monolayer, indicating its lower stability.
The key frontier molecular orbitals and the corresponding energy levels of the most stable clusters are shown in Fig. 4. It was found that the HOMO of B2S3 originates from the terminal B−S π bonding and the non-bonding 3p orbital of the meso S atoms, whereas the LUMO is characterized by the B−S antibonding (π*). Similarly, the HOMO of B4S6 is derived from the terminal B−S π bonding and the 3p orbitals of the connected S atoms, and its LUMO is mainly contributed by the S atoms of the B2S2 moiety. For the large-sized clusters, such as B6S9, B8S12, B10S15, and B12S18, their HOMOs and LUMOs are primarily composed of the 3p orbitals of S atoms and the 2p orbitals of B atoms, respectively. As shown in Fig. 4b, these boron sulfide clusters generally have relatively large HOMO–LUMO gaps (EGAP = 4.36–5.14 eV), indicating that all the studied (B2S3)n (n = 1–6) clusters may possess highly stable electronic structures. As the number of B2S3 basic units increases, the EGAP value decreases first and then maintains at about 4.4 eV.
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| Fig. 4 (a) The key molecular orbitals (isosurface = 0.05) and (b) the corresponding energy levels (in eV) of the studied (B2S3)n clusters. | ||
In order to understand the electronic gain and loss characteristics as well as the relative stability of these studied boron sulfide clusters, their adiabatic/vertical electron affinities (EAa and EAv) and adiabatic/vertical ionization potentials (IPa and IPv), average binding energies (Eb), and second-order energy difference (Δ2E) were calculated as follows:
| EAa = E0therm − E−1therm |
| EAv = E0 − E−1 |
| IPa = E1therm − E0therm |
| IPv = E1 − E0 |
| Etherm = Eelec + Ecorr |
| Δ2E([B2S3]n) = E([B2S3]n−1) + E([B2S3]n+1) − 2E([B2S3]n) |
As shown in Table 1, as the cluster size increases, the EAv values gradually increase overall, while the IPa and IPv values correspondingly decrease first and then remain less varied. Additionally, the EAa value of B12S18 is exceptionally low at 1.25 eV, primarily attributed to significant structural reorganization upon the formation of the anion cluster B12S18− (see more details in Fig. S16, ESI†). Notably, the relatively low EAa and EAv values suggest a diminished capability of the molecule to accommodate additional electrons, whereas the relatively large IPa and IPv values indicate that the corresponding clusters are highly stable in their neutral state. These observations collectively underscore the high chemical stability of the studied boron sulfide clusters. Note that the evaluated EAs and IPs are closely correlated with the frontier molecular orbital properties of the studied molecules. There is a sharp drop in IPs from B4S6 to B6S9, which can be ascribed to the transition of the HOMO composition from predominant π orbitals associated with B−S bonds to the 3p orbitals of S atoms.
| Cluster | EAa (eV) | EAv (eV) | IPa (eV) | IPv (eV) | E b (eV) | Δ2E |
|---|---|---|---|---|---|---|
| a Adiabatic electron affinities EAa, vertical electron affinities EAv, adiabatic ionization potentials IPa, vertical ionization potentials IPv, average binding energies Eb, and second-order energy difference Δ2E. | ||||||
| B2S3 | 1.79 | 0.69 | 9.60 | 9.73 | 0.1860 | — |
| B4S6 | 1.63 | 1.17 | 9.13 | 9.23 | 0.1922 | 0.2720 |
| B6S9 | 1.77 | 1.18 | 8.08 | 8.51 | 0.1936 | −0.9400 |
| B8S12 | 1.87 | 1.30 | 8.26 | 8.42 | 0.1961 | 0.2335 |
| B10S15 | 2.00 | 1.77 | 8.50 | 8.45 | 0.1972 | 0.2345 |
| B12S18 | 1.25 | 2.03 | 8.51 | 8.42 | 0.1976 | — |
The slightly increasing Eb suggests that the stability of (B2S3)n (n = 1–6) clusters gradually enhances with the increase in cluster size. Furthermore, the second-order differences (Δ2E) reveal a different perspective on stability. Although most isomers have positive Δ2E values, B6S9 stands out with a significantly negative Δ2E value of −0.9400, suggesting a higher reactivity structurally, compared to the other clusters. Overall, these studied clusters show relatively high stability both structurally and electronically.
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| Fig. 5 Predicted infrared spectra of the studied (B2S3)n clusters at the B3LYP/def2-TZVP level of theory. | ||
| ΔE = EB2S3Xm − (EB2S3 + m × EX) |
As shown in Fig. 6, main interaction modes between (B2S3)n (n = 1–6) clusters and NH3 are explored. Note that the complexation between NH3 and the B atom of (B2S3)n is actually a Lewis acid–base interaction. As shown in Fig. 6b, the complex of NH3 and B2S3 exhibits two configurations, arising from the nucleophilic attack of NH3 on the B site from both inside and outside the V-shaped structure, while the overall structure remains planar. Energetically, [B2S3][NH3]-I is a more stable configuration. According to the relative energies of various binding modes between B4S6 and NH3 in Fig. 6c, it was found that NH3 more readily binds to the cyclic B2S2 moiety rather than the linear [S−B−S] unit, with a difference in ΔE exceeding 10.0 kcal mol−1. The binding interaction of NH3 with the global minimum structure of clusters B6S9, B8S12, B10S15, and B12S18, forms a similar stable configuration, with the B atom in the B2S2 moiety as the main active site. Notably, the N−H bond (1.015 Å) and the H−N−H bond angle (106°) of NH3 increase to some extent, upon binding to the selected clusters, as shown in Fig. 6. Overall, the studied clusters can stably adsorb NH3 molecules through the Lewis acidity of their B atoms, which may be used for capture and separation of small molecules.
Subsequently, the interactions of (B2S3)n (n = 1–6) clusters with carbon monoxide (CO) are further explored, as depicted in Fig. 7. We note that the bond length of the CO adsorbed at the B atom of the [S−B−S] unit is almost unchanged (1.132∼1.134 Å), compared to the free molecule, but the CO molecule bound to the B atom of the cyclic B2S2 moiety has a shortened C
O bond with the bond length of about 1.27 Å, suggesting that the formation of a B−CO dative bond is beneficial to the dispersion of excess electrons on the carbon atom and thus further enhances the CO bonding interaction. The predicted binding energies (ΔE) range from −13.3 to −4.7 kcal mol−1, where the B6S9 cluster has the strongest interaction toward CO among these clusters considered here.
As Fig. 6 and 7 show, B6S9 has the strongest binding interaction with both NH3 and CO molecules among the boron sulfide clusters here. In addition, we investigated the interactions of (B2S3)n clusters with multiple NH3 or CO molecules, and the optimized structures and corresponding bind energies of these complexes [B2S3]n[NH3]m and [B2S3]n[CO]m, are presented in Fig. S8 and S9 (ESI†). Overall, all B atoms of these boron sulfide clusters can bind these small molecules until the boron sites are saturated.
Overall, the present results suggest that B2S3-type clusters exhibit potential applications in organic catalysis and materials engineering, specifically reflected in the following aspects: (a) the strong Lewis acidity of electron-deficient boron centers may facilitate the catalytic activation of small molecules, analogous to established B–N catalytic systems;87,88 (b) the structural stability and delocalized electron characteristics observed in these clusters provide design principles for developing novel 2D boron–sulfide materials with graphene-like architectures; (c) the bonding energy of the B−S bonds in (B2S3)n clusters is generally stronger than that in the reported B2S3 monolayer,52 suggesting that the predicted clusters may offer broader application prospects compared to the B2S3 monolayer materials due to their enhanced stability.
Footnote |
| † Electronic supplementary information (ESI) available: Functional testing; low-energy structures of charged clusters; AdNDP analysis; optimized structures of studied molecular compounds; key interaction analysis between [B2S3] with NH3 and CO molecules; absolute free energies and electronic energies; and Cartesian coordinates. See DOI: https://doi.org/10.1039/d4cp04699d |
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