The synthesis and structural aspects of the perbromo-functionalised thiaboranes closo-SB_nBr_n (n = 5, 9, 11): the solid-state structure of the octahedral closo-SB₅Br₅, governed by strong dihalogen contacts

Abstract

Co-pyrolysis reactions of B₂Br₄ with S₂Br₂ at 350 °C in vacuo yielded the brominated thiaboranes closo-SB₅Br₅ (1), closo-1-SB₉Br₉ (2) and closo-SB₁₁Br₁₁ (3), confirmed by high-resolution mass spectrometry, experimental and computational ¹¹B NMR spectroscopy. The strong Br^δ+(σ-hole)⋯Br^δ−(ring) attraction has been the decisive energy contribution in the crystal of 1.

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Small polyhedral borane and heteroborane clusters of octahedral shape have so far been limited to the borate dianions [B₆X₆]²⁻ (X = H, Cl or Br),¹ the carboranes² 1,2- or 1,6-C₂B₄H₆ and the perhalogenated heteroboranes P₂B₄X₄ (X = Cl^3a or Br^3b), As₂B₄X₄ (X = Cl^3c or Br^3d), SB₅Cl₅,^3e SeB₅Cl₅^3f,g and TeB₅Cl₅.^3h In particular, recent studies of the thermal reactions of B₂Cl₄ with Se₂Cl₂ or TeCl₄, producing the octahedral SeB₅Cl₅^3f,g and TeB₅Cl₅^3h have shown surprising results: chalcogens heavier than sulphur apparently induce stronger perturbations to the corresponding cluster frameworks, as manifested by the long body diagonal in chlorinated octahedral selena- and telluraboranes, which may prevent their accommodation in octahedral moieties. There are two known pnictogenaboranes, P₂B₄Br₄ and As₂B₄Br₄, but no brominated octahedral chalcogenaborane as yet, which has prompted us to carry out the thiaborane reaction^3e with S₂Br₂. On that basis, we wish to report the synthesis and characterisation of closo-SB₅Br₅ (1) and other new brominated thiaboranes.

We have studied the co-pyrolysis reaction of B₂Br₄ with S₂Br₂ at temperatures above 350 °C. In comparison with chlorinated thiaboranes, prepared at 280 °C, this reaction required significantly higher temperatures because the bromo compounds are less volatile than the Cl-synthons.^3e

The procedure described yielded three new brominated thiaboranes, closo-SB₅Br₅ (1), closo-1-SB₉Br₉ (2) and closo-SB₁₁Br₁₁ (3) (Scheme 1), of which we were able to isolate only compound 1 by repeated vacuum-fractionation and crystallisation processes. Compound 1 was obtained in 5% yield based on the idealised eqn (1). The non-separable compounds 2 and 3 (eqn (2) and (3)) were formed in approximately 1% and 3% yields, respectively.

11B₂Br₄ + S₂Br₂ → 2SB₅Br₅ (1) + 12BBr₃ (1)

19B₂Br₄ + S₂Br₂ → 2SB₉Br₉ (2) + 20BBr₃ (2)

23B₂Br₄ + S₂Br₂ → 2SB₁₁Br₁₁ (3) + 24BBr₃ (3)

Scheme 1 Molecular diagrams of closo-SB₅Br₅ (1), closo-1-SB₉Br₉ (2) and closo-SB₁₁Br₁₁ (3).

The new thiaboranes 1–3 are thermally stable under inert atmosphere at least up to their formation in two experiments at 350 °C and 430 °C. In the very high vacuum applied in EI-MS (10⁻⁵ mbar), 1 starts to sublime already at 25 °C, 2 at 50 °C and 3 at 85 °C. They are soluble in common aprotic solvents such as benzene, chloroform and methylene chloride.

The corresponding reaction of B₂Cl₄ with S₂Cl₂ has resulted in a much higher number of individual chlorinated thiaboranes closo-SB_nCl_n (with n = 4, 5, 6, 9, 10, 11, 12) than in the present study. This may be attributed to the fact that the isodesmic heats of formation of the bromo-products are less negative than those of the chloro-products.^3e However, computational studies have confirmed in both cases that octahedral, bicapped square-antiprismatic and icosahedral cages are the most stable in the entire series. The computed isodesmic heats of formation in the origination of closo-SB_nBr_n from closo-SB_n−1Br_n−1 (n = 5, 9, 11) at the B3LYP/def2-QZVP//BP86/DZVP-DFT level (in kcal mol⁻¹) are as follows:

closo-1-SB₄Br₄ + B₂Br₄ → closo-SB₅Br₅ (1) + BBr₃ −15.6

closo-4-SB₈Br₈ + B₂Br₄ → closo-1-SB₉Br₉ (2) + BBr₃ −19.6

closo-2-SB₁₀Br₁₀ + B₂Br₄ → closo-SB₁₁Br₁₁ (3) + BBr₃ −34.3

The new thiaboranes have been characterised by ¹¹B NMR spectroscopy, high- and low-resolution mass spectrometry, X-ray structure determination for 1 and SO-DFT/ZORA/NMR computations (taking spin–orbit coupling (SO) into account), which have confirmed the experimental ¹¹B NMR chemical shifts. With SO not included in ¹¹B NMR chemical-shift computations, the values have shifted upfield by about 5 ppm⁴ with respect to the SO-DFT/ZORA/NMR results. Based on these results, we have derived the molecular structures of 1, 2 and 3 in their solutions (see also Table 1).

Table 1 The computed^a and experimental ¹¹B NMR chemical shifts of 1, 2 and 3

1 Octahedral motif
	B(2–5)	B(6)
a SOC-PBE0/T2ZP//B3LYP/6-311 + G**.
Comp.	−1.2	20.5
Exp.	−2.3	19.4

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2 Bicapped square-antiprismatic motif
	B(2–5)	B(6–9)	B(10)
Comp.	8.9	−4.7	53.5
Exp.	7.5	−3.7	49.6

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3 Icosahedral motif
	B(2–6)	B(7–11)	B(12)
Comp.	−1.7	−6.0	14.6
Exp.	−1.4	−3.7	14.2

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The ¹¹B chemical shifts of 1, 2 and 3 are within the normal range for halogenated heteroborane clusters^3a–h and closely follow the trends established for non-halogenated heteroboranes.^5a They also reflect the symmetries of C_4v, C_4v and C_5v, respectively.

As expected, 1 shows two broad signals in a 4 : 1 ratio at −2.3 ppm (h_1/2 ≈ 106 Hz, B(2–5)) and 19.4 ppm (h_1/2 ≈ 113 Hz, B(6)), which are in agreement with the signals for closo-SB₅Cl₅ at −0.3 ppm and 19.4 ppm.^3e With the application of the Lorentz–Gaussian transformation with a ¹J(¹¹B–¹¹B) of 25 Hz, the signals for B(2–5) are split into a quartet, indicating the coupling with B(6) and showing the expected cross-peak in the COSY ¹¹B¹¹B NMR with the low-field signal.

The ¹¹B NMR of 2 consists of three signals in a 4 : 4 : 1 ratio at −3.7 ppm (B6–9), 7.5 ppm (B2–6) and 49.6 ppm (B10), which are in good agreement with the signals for closo-SB₉Cl₉ at −0.3 ppm, 13.1 ppm and 48.8 ppm.^3e The signal at −3.7 ppm shows ¹¹B¹¹B cross-peak correlation with the other two signals and thus can be assigned to B(6–9), the signal at 7.5 ppm to B(2–5), and the poorly-resolved intensity-one signal at 49.6 ppm can be connected with the antipodal downfield shift⁵ of B(10).

The ¹¹B NMR of 3 consists of three signals in a 5 : 5 : 1 ratio at −3.7 ppm (B7–11), −1.4 ppm (B2–6) and 14.2 ppm (B12), with expected cross-peaks between B(7–11)/B(2–6) and B(7–11)/B(12). The signals are in agreement with the chemical shifts established for closo-SB₁₁Cl₁₁ at −3.9 ppm, 1.0 ppm and 14.4 ppm.^3e

The 70 eV EI mass spectra of all compounds show a strong molecular ion. In the case of thiaborane 1, the major cut-off indicates the abstraction of a SBBr edge from SB₅Br₅ to leave a [B₄Br₄]⁺ fragment. The formation of such a [B₄Hal₄]⁺ fragment has been analogously found in the mass spectra of closo-SB₅Cl₅^3e and closo-1,2-P₂B₄Cl₄.^3a The MS of thiaborane 2 shows the stepwise abstraction of S, Br, BBr₂ and BBr₃ fragments, whereas 3 exhibits only minimal fragmentation with the abstraction of a single Br atom, which indicates the high stability of the icosahedral cluster framework.

All mass-spectroscopy patterns are consistent with the computed spectra based on natural isotopic abundances. On the basis of simple skeletal electron-counting rules,⁶1, 2 and 3 should adopt octahedral, bicapped square-antiprismatic and icosahedral geometries, respectively, with sulphur contributing four electrons and each BBr unit two electrons to the cluster bonding. The employment of intrinsic-bond orbitals (IBOs)^3e,g,h has revealed the nature of bonding in 1, 2 and 3. According to the expansion coefficients (ECs: contributions from individual atoms to a particular IBO to reveal the nature of such orbitals), the sulphur involvement in 1 consists of two IBOs with the ECs (the contributing atom in parentheses) of 1.15 (S), 0.51 (B) and 0.25 (B) and one IBO with the ECs of 1.20 (S) and 0.53 (B). Whereas the first pair of IBOs can be assigned to 3c–2e bonding, the second IBO is of 2c–2e type. The rest of the octahedral cage is assembled through 3c–2e IBOs (Fig. 1). The bonding schemes in 2 and 3 are shown in ESI† (Fig. S2).

Fig. 1 Visualised IBOs for SB₅Br₅ (1). The colour coding is as follows: blue classical 2c–2e bonding, pink 3c–2e bonding.

The ESP (electrostatic potential molecular surface) of 1 is similar to that of its chlorine analogue,^3e but the bromine atoms of 1 have more positive σ-holes than the chlorines of SB₅Cl₅ (see Fig. 2 and Table 2).

Fig. 2 The ESP molecular surfaces of 1 (a) and SB₅Cl₅ (b) computed at the HF/cc-pVDZ level.

Table 2 The minimum and maximum values of the ESP molecular surfaces computed at the HF/cc-pVDZ level

Compound	V s,max [kcal mol^−1]		V s,min [kcal mol^−1]
	Sulphur atom	Halogen atoms	Halogen atoms
SB5Br5(1)	24.5	4 × 11.6; 10.2	4 × −6.0; −5.8
SB5Cl5	24.8^3e	4 × 6.6; 5.7	4 × −5.6; −5.1

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To confirm the octahedral structure of 1, single crystals were grown via vacuum sublimation and subjected to X-ray diffraction (see Fig. 3). Compound 1 crystallises in P2₁/c with three crystallographically independent closo-SB₅Br₅ octahedrons present in the corresponding asymmetric unit. As a consequence of symmetry, a total of twelve molecules are present in the centrosymmetric monoclinic cell (see ESI,† for details). Crystal packing is dominated by short contacts between the bromine atoms of neighbouring molecules, indicative of dihalogen bonding (d(Br⋯Br) = 339 to 368 pm, see also Fig. 4). Intermolecular bonding interactions between the sulphur and the bromine atoms, however, seem to be absent because no short contacts (<the sum of vdW radii) have been observed.

Fig. 3 ORTEP plot (thermal ellipsoids drawn at the 50% probability level) of the molecular structure of closo-SB₅Br₅ (1). The asymmetric unit contains three independent molecules with very similar metrical parameters; only one of these molecules is shown. Selected averaged bond lengths (pm) and angles (°): B_eq–Br_eq 189.4(5), B_ax–Br_ax 190.4(5), S–B_eq 196.8(2), B_eq–B_eq 177.8(7), B_eq–B_ax 170.1(3), B_eq–S–B_eq 53.7(2), B_eq–B_eq–B_eq 90.0(3), B_eq–B_ax–B_eq 63.0(3), S–B_ax–Br_ax 179.1(2).

Fig. 4 The short contacts below the sum of vdW radii in the crystal packing of 1 (a) and SB₅Cl₅^3e (b) are indicated by dot lines (in pm). An example of Br^δ+⋯Br^δ− is depicted in term of the corresponding molecular ESP surfaces. The color coding of the ESP is in the range of −26 to 26 kcal mol⁻¹.

All the interaction motifs present in the crystal structure of 1 have been investigated using a cluster model. Two-body interaction energies have been computed for the selected crystallographic molecule using the SAPT method,⁷ which enables energy decomposition into individual terms. For comparison, the same analysis has also been performed for SB₅Cl₅. Both crystals are dominated by dispersion as expected. Surprisingly, the contribution of dispersion to crystal packing was slightly less pronounced in 1 than in SB₅Cl₅ (i.e. the dispersion in them formed 63 and 67%, respectively, of the sum of all the attractive terms in the SAPT decomposition). The explanation is that there are considerably more positive σ-holes on Br atoms in 1 than on Cl atoms in SB₅Cl₅. Compound 1 thus has a better balance of positive and negative sites for crystal packing – it has six highly positive σ-holes and five negative rings around B–Br bonds, unlike SB₅Cl₅, which has only one highly positive σ-hole on the S atom to match the five negative rings around B–Cl bonds. Consequently, the electrostatic contribution is more pronounced in the crystal packing of 1, which is dominated by dihalogen bonds, than in SB₅Cl₅, dominated by chalcogen bonds (see Fig. 4 and Table 3). This enhanced electrostatic contribution may be ascribed to the fact that the crystallisation process of 1 is more straightforward than that of SB₅Cl₅.

Table 3 The interaction energies (ΔE²) computed at the SAPT0/jun-cc-pVDZ level have been decomposed into electrostatic (E_elec), exchange (E_exch), induction (E_ind) and dispersion (E_disp) contributions using the SAPT methodology. All the energies are in kcal mol⁻¹. The relative values in round brackets indicate the contribution to the sum of all the attractive terms

Compound	ΣSAPT0	ΣEelec	ΣEexch	ΣEind	ΣEdisp
SB5Br5 (1)	−41.5	−38.8 (30%)	86.4	−8.7 (7%)	−80.4 (63%)
SB5Cl5	−32.2	−24.0 (26%)	59.4	−6.1 (7%)	−61.5 (67%)

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In a thermodynamically controlled reaction at 350 °C, simple inorganic synthons such as B₂Br₄ and S₂Br₂ have provided octahedral, bicapped square-antiprismatic and icosahedral structural motifs referred to as closo-SB₅Br₅, closo-1-SB₉Br₉ and closo-SB₁₁Br₁₁, respectively. Structural features have been elucidated through a combination of experimental and computational approaches including NMR spectroscopy together with MS spectrometry and DFT/ZORA/NMR model chemistry. These methods have confirmed that these three structural arrangements are good representations of the molecular geometries in solutions. In addition, crystal-structure determination of the octahedral brominated thiaborane shows that the Br⋯Br contacts are strong enough to be able to overcome the repulsion between the σ-holes located on the bromines and the sulphur. As a consequence, the crystal-packing forces are dominated by strong Br^δ+(σ-hole)⋯Br^δ−(ring) attraction, i.e. the crystal is formed without chalcogen bonding detected in the chlorine analogue of 1.

We thank Dr Jürgen Conrad and Mr Mario Wolf (Institut für Chemie, Universität Hohenheim) for recording NMR spectra and Dipl.-Ing. (FH) J. Trinkner (Institut für Organische Chemie, Universität Stuttgart) for recording MS. Dr M-B. Sárosi (Universität Würzburg) is thanked for the assistance with the ADF computations. Financial support from the Czech Science Foundation, grant no. 23-05083S, is gratefully acknowledged.

Data availability

The data supporting of this article have been included as part of the ESI.† The crystallographic data for compound 1 (closo-SB₅Br₅) were deposited with the Cambridge Crystallographic Data Centre with CCDC no. 2403282.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. For selected surveys on polyhedral boranes, see: (a) W. N. Lipscomb, Boron Hydrides, Benjamin, New York, 1963; (b) E. L. Muetterties, Boron Hydride Chemistry, Academic Press, New York, 1975; (c) J. F. Liebman, A. Greenberg and R. E. Williams, Advances in Boron and the Boranes, VCH Verlagsgesellschaft, Weinheim New York, 1988; (d) R. B. King, Chem. Rev., 2001, 101, 1119–1152.
2. For selected surveys on carboranes, see: (a) Electron Deficient Boron and Carbon Clusters, ed. G. A. Olah, K. Wade and R. E. Williams, John Wiley and Sons, New York, 1991; (b) T. Onak, Polyhedral Carbaboranes, Comprehensive Organometallic Chemistry II, ed. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon, Oxford, UK, 1995, Vol. 1, 217–255; (c) M. A. Fox, Polyhedral Carboranes, Comprehensive Organometallic Chemistry III, ed. R. H. Crabtree and D. M. P. Mingos, Elsevier, 2007, Vol. 3, 49–112; (d) R. N. Grimes, Carboranes, ed., Elsevier, Amsterdam, The Netherlands, 2016.
3. (a) W. Haubold, W. Keller and G. Sawitzki, Angew. Chem., Int. Ed. Engl., 1988, 27, 925–926; (b) W. Keller, L. G. Sneddon, W. Einholz and A. Gemmler, Chem. Ber., 1992, 125, 2343–2346; (c) R. Schäfer, W. Einholz, W. Keller, G. Eulenberger and W. Haubold, Chem. Ber., 1995, 128, 735–736; (d) W. Keller, W. Einholz, D. Rudolph and T. Schleid, Z. Anorg. Allg. Chem., 2017, 643, 664–668; (e) W. Keller, F. Lissner, J. Ballmann, J. Fanfrlík, D. Hnyk and T. Schleid, Angew. Chem., Int. Ed., 2024, 63, e202406751; (f) W. Keller and M. Hofmann, Z. Anorg. Allg. Chem., 2017, 643, 729–731; (g) W. Keller, M. Hofmann, H. Wadepohl, M. Enders, J. Fanfrlík and D. Hnyk, Dalton Trans., 2023, 52, 16886–16893; (h) W. Keller, J. Ballmann, M. B. Sárosi, J. Fanfrlík and D. Hnyk, Angew. Chem., Int. Ed., 2023, 62, e202219018.
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6. (a) R. E. Williams, Inorg. Chem., 1971, 10, 210–214; (b) K. Wade, Adv. Inorg. Chem. Radiochem., 1976, 18, 1–66; (c) R. E. Williams, Adv. Inorg. Chem. Radiochem., 1976, 18, 67; (d) R. W. Rudolph, Acc. Chem. Res., 1976, 9, 446–452; (e) R. E. Williams in Electron Deficient Boron and Carbon Clusters, ed. G. A. Olah, K. Wade and R. E. Williams, Wiley and Sons, New York, 1991, 11–93; (f) R. E. Williams, Chem. Rev., 1992, 92, 177–207.
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Footnote

† Electronic supplementary information (ESI) available. CCDC 2403282. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc06334a

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