Diverse chemistry of the dianion [closo-B₉H₉]²⁻: synthesis and reactivity of its mono-anionic derivative [arachno-B₉H₁₂-4,8-Cl₂]⁻

Abstract

Attempted protonation of the dianion [closo-B₉H₉]²⁻ under moisture-free conditions did not afford its mono-protonated form [closo-B₉H₁₀]⁻. The reaction of the former closo-borate with CH₃COOH in dichloromethane yielded a monoanionic product [B₂O(MeCO₂)₅]⁻. The treatment of [closo-B₉H₉]²⁻ with HCl in dichloromethane afforded its arachno-derivative [arachno-B₉H₁₂-4,8-Cl₂]⁻ in a high yield. The experimental solution and quantum-chemically calculated ¹¹B and ¹H NMR spectra of the latter monoanion were found to be in a good agreement; its structure in the solid state was studied by the single crystal X-ray diffraction experiment for the crystal (PPh₄)[arachno-B₉H₁₂-4,8-Cl₂]·0.04HCl. The reaction of [arachno-B₉H₁₂-4,8-Cl₂]⁻ with liquid ammonia caused its quantitative conversion into the parent [closo-B₉H₉]²⁻.

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Borates [closo-B_nH_n]²⁻ (n > 4) are fundamental building blocks of the boron cluster chemistry (see for example^1–4) and these borates with n = 6–12 were synthesized in 1959–1967.^5–9 [closo-B_nH_n]²⁻ have three-dimensional aromaticity, while the organic aromatic compounds have two-dimensional aromaticity^10,11 (and literature cited therein). Among the [closo-B_nH_n]²⁻ (n = 6–12), those with n = 7–9 are the least explored.

Borates with an arachno nine-vertex are described in literature since 1960.^12–16 The existence of the monoanion [arachno-B₉H₁₄]⁻ has been postulated by Lipscomb,¹ while the first X-ray structural characterization in the form of its cesium(I) salt has been performed by Greenwood et al.;^4,5 two possible conformations of this monoanion (a and b) are shown in Scheme 1.

Scheme 1 Known nine-vertex conformations of the monoanion arachno-B₉H₁₄⁻ (a and b) and that of nido-B₁₀H₁₄ (c).¹⁷

Its i-configuration appears to be a deviation from Williams theory. The Williams theory postulates that for the generation of the arachno-boranes or carboranes from the nido-clusters, a vertex with the highest connectivity should be removed.^18,19 The examples of the n-configuration of the monoanion [arachno-B₉H₁₄]⁻ (in contrast to those of its i-configuration) are very rarely reported in literature. They include the dianion [n-B₉H₁₅]^2− 11 as well as the organometallic and coordination compounds, such as [η⁶-(C₆Me₆)RuB₈H₁₄],²⁰ [(dppe)Pt₂B₇H₁₁]²¹ and [(PMe₂Ph)₂PtB₇H₁₀NHEt].¹⁴ The structure with i-geometry, on the other hand, is obtained by removing a corner with a lower connectivity of nido-B₁₀H₁₄ (Scheme 1c). Such geometry has been however observed for numerous members of the borates, including the dianion [i-B₉H₁₅]^2− 11 and a series of neutral coordination compounds [arachno-B₉H₁₃L].¹⁵ The borate nido-B₁₀H₁₄ has been also assigned to this geometry (see ref. 17 and references therein). The n-configuration has been observed¹⁷ only for one of its halogen-containing derivative, viz., [arachno-B₉H₁₂-4,8-Br₂]⁻, which is a by-product of the synthesis of anti-B₁₈H₂₂ by the reaction of [nido-B₉H₁₂]⁻ with HgBr₂ in dichloromethane.^17,22 In the present communication, we describe the products of the reactions of [closo-B₉H₉]²⁻ with various acids and the reactivity of these products.

In the first stage of our investigation, we attempted to protonate [closo-B₉H₉]²⁻ with acetic acid under anhydrous conditions. Dry acetic acid was added to the red-orange solution of (PPh₄)₂[closo-B₉H₉] in dichloromethane and then, the reaction mixture was left under pentane atmosphere for 3 days. This reaction resulted in the complete destruction of a polyhedral boron cluster, thus affording the monoanion [B₂O(MeCO₂)₅]⁻ as a major product, which crystallizes as (PPh₄)[B₂O(MeCO₂)₅] in colorless crystals.²³ The heteroleptic products [B₂O(MeCO₂)₅]⁻ and B₂O(MeCO₂)₄ were already detected spectroscopically few years ago.^24–27 It should be noted that the homoleptic compounds [B(MeCO₂)₄]⁻ and B(MeCO₂)₃ were not described in literature till date.

We also studied the reaction of (PPh₄)₂[closo-B₉H₉] with HCl in dichloromethane. Indeed, the bromine-containing compound [arachno-B₉H₁₂-4,8-Br₂]⁻ has been prepared⁶ using the reaction of the [nido-B₉H₁₂]⁻ with HgBr₂. The treatment of (PPh₄)₂[closo-B₉H₉] with purified gaseous HCl (the oxygen impurities were removed using three condensation–argon saturation cycles) in dichloromethane at −78 °C (the latter makes it possible to remove the remaining moisture impurities) under inert atmosphere afforded the chlorine-containing monoanion [arachno-B₉H₁₂-4,8-Cl₂]⁻ as per the following eqn (1):

(PPh₄)₂[closo-B₉H₉] + 3HCl → (PPh₄)[arachno-B₉H₁₂-4,8-Cl₂] + (PPh₄)Cl (1)

The calculated structure of this monoanion with a labeling scheme of its atoms is shown in Fig. 1.

Fig. 1 Labeling scheme of the atoms in the anion [arachno-B₉H₁₂-4,8-Cl₂]⁻ and its quantum-chemically calculated structure.

The initial red-orange solution of (PPh₄)₂[closo-B₉H₉] in dichloromethane underwent an immediate decolouration after the addition of HCl. The resultant reaction mixture was carefully mixed with a five-fold volume of pentane and colorless crystals were formed after 3 days with high yield (83%).

The reaction of a suspension of (PPh₄)[arachno-B₉H₁₂-4,8-Cl₂] with liquid ammonia at r.t.¹⁹ resulted in the re-closing of this arachno-compound into the initial [closo-B₉H₉]²⁻: after 10 min, the initially colorless reaction mixture turned yellow, following which NH₃ escaped from the mixture and the intensity of its color increased, thus indicating the formation of the closo-borate dianion according to eqn (2):

2(PPh₄)[arachno-B₉H₁₂-4,8-Cl₂] + 6NH₃ → (PPh₄)₂[closo-B₉H₉] + (NH₄)₂[closo-B₉H₉] + 4NH₄Cl (2)

The yield of the orange solid product, based on its NMR spectra, was quantitative.

The initial closo-borate compound and the products of its transformations were characterized using multinuclear NMR spectroscopy and by the single crystal X-ray diffraction study as; their ¹¹B NMR spectra are shown in Fig. 2 and 3.

Fig. 2 ¹¹B NMR spectra of the CD₃CN solutions of the initial [closo-B₉H₉]²⁻ (a), the chlorine-containing derivative [arachno-B₉H₁₂-4,8-Cl₂]⁻ (b) and the product of its reaction with liquid ammonia (c).

Fig. 3 General view of the monoanion [arachno-B₉H₁₂-4,8-Cl₂]⁻ in the crystal (PPh₄)[arachno-B₉H₁₂-4,8-Cl₂]. Thermal ellipsoids are shown with 50% probability.

In the spectrum b of new compound (PPh₄)[arachno-B₉H₁₂-4,8-Cl₂], the signals of its ¹¹B nuclei split in the integral ratio 2 : 1 : 2 : 2 : 1 : 1; the two chlorine-substituted boron atoms have a singlet character. An assignment of the above signals was made using the theoretically calculated ¹¹B NMR values, which are also collected in Table 1.

Table 1 Experimental NMR data for the CD₃CN solution of the chlorine-containing monoanion [arachno-B₉H₁₂-4,8-Cl₂]⁻ in CD₃CN and those quantum-chemically calculated using GIAO//B3LYP/6-311G²⁰

Assignment	δ ^11B (ppm)	^1 J ^11B–^1H (Hz)	δ ^1H (ppm)
a In parentheses: the calculated NMR parameters; ^11B NMR data relative to BF3·OEt2 with δ^11B = 0 ppm; δ^11B = 101.63 − σ^11B; ^1H NMR data relative to Me4Si with δ^1H = 0 ppm; δ^1H = 31.97 − σ^1H. b The averaged calculated δ^11B. c The averaged calculated δ^1H for the atoms H2, H4 and H8.
B5, B7	2.2 (−1.3)^a^b	149	3.05 (2.53)^b^c
B6	−4.9 (−14.5)	153	2.86 (2.14)
B4, B8	−8.6 (−10.0)^b		−0.41 (1.13)^b^c
B1, B3	−10.9 (−17.3)^b	142	2.04 (1.58) ^b
B9	−31.4 (−35.5)	145	0.82 (0.32)
B2	−49.1 (−56.3)	152	−0.41 (1.13)^c
H10			(−2.35)
H11			(−3.20)
H12			(−4.22)

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The bridging hydrogen atoms of [arachno-B₉H₁₂-4,8-Cl₂]⁻ were not detected at room temperature using ¹H NMR method due to substantial broadening (due to the dynamic behavior)of the peaks. The same effect is also observed⁶ in its bromine-containing analog [arachno-B₉H₁₂-4,8-Br₂]⁻. The calculated values of δ_¹¹B for the boron-based framework of [arachno-B₉H₁₂-4,8-Cl₂]⁻ were found to be in a good agreement with those experimentally observed (except that for the atom B₆).

The colorless single crystals of the salt (PPh₄)[arachno-B₉H₁₂-4,8-Cl₂] were also characterized using the single crystal X-ray diffraction method;²¹ the molecular structure of its arachno-anion is shown in Fig. 3.

This structure is very similar to that of the parent anion [arachno-B₉H₁₄]⁻, having the same arrangement of both the backbone and the bridging protons. Two of the residual electron peaks observed in this spectrum were assigned to the HCl molecule with about 4% occupancy. In addition, peaks attributed to 3 μ-H and 2 endo-H could be found. The bridging proton H10 was evidenced to be deviated from the centrum of the corresponding boron–boron bond, while the same was not observed for the two other bridging protons. The bond distance B9–H10 (1.126 (12) Å) is smaller than that of B4–H10 (1.508 (13) Å). The quantum-chemical B3LYP/6-311G calculations²⁰ showed that such arrangement of the bridging hydrogen atoms in the crystal (PPh₄)[arachno-B₉H₁₂-4,8-Cl₂] is energetically preferable by 5.83 kJ mol⁻¹ as compared with its C_s-symmetric arrangement (Fig. 4).

Fig. 4 Quantum-chemically calculated structure of the monoanion [arachno-B₉H₁₂-4,8-Cl₂]⁻ having C_s symmetric arrangement.

Such calculated C_s-symmetric structure contains two μ-H and three endo-H atoms and it was not experimentally detected using the NMR method.

Thus, the dianion [closo-B₉H₉]²⁻ was found to react with gaseous HCl in dichloromethane under anhydrous conditions, affording its halogen-containing derivative [arachno-B₉H₁₂-4,8-Cl₂]⁻, which undergoes re-closing with NH₃ to form an initial closo-borate dianion.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1569828. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7nj04251e

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