A simple and efficient method to synthesize metal salts of M₂[trans-B₂₀H₁₈] (M = Li, Na, K, and Cu)

Abstract

The synthesis of [trans-B₂₀H₁₈]²⁻ from [closo-B₁₀H₁₀]²⁻ based on redox principles has been developed but is still limited. Using different oxidants often leads to various by-products and affects yields. This paper presents a facile and efficient method for synthesizing [trans-B₂₀H₁₈]²⁻ by directly using the alkali metal salt of [closo-B₁₀H₁₀]²⁻ as the precursor. This approach enables the synthesis of the metal salts of [trans-B₂₀H₁₈]²⁻. When Cu(OTf)₂ is used as the oxidant, Cu₂[trans-B₂₀H₁₈] is formed with a yield exceeding 94%. On the other hand, when H₃P(Mo₃O₁₀)₄·xH₂O is used as the oxidant, alkali metal salts of [trans-B₂₀H₁₈]²⁻ (M = Li, Na, and K) are obtained with a yield of up to 85%. All synthesized compounds were characterized by IR, TGA-DSC, and NMR spectroscopy, and M₂[trans-B₂₀H₁₈] (M = K, Cu) was characterized by X-ray single crystal diffraction. Both methods allow for the rapid completion of the reaction in a short time. This synthesis method offers promising potential for further applications, particularly as an all-solid-state electrolyte in battery technology.

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Polyhedral borane anions, with a general formula of [B_nH_n]²⁻ (n = 6–12), are a type of typical boron/hydrogen compound,^1–3 and have been widely applied in materials, medicine, and catalysis fields because their closo-structural features, resulting in their σ-aromaticity and high stability, compared with the small borane Lewis base adducts.^4–7 Among the polyhedral borane compounds, the [closo-B₁₀H₁₀]²⁻ anion derivatives have relatively low stability, low symmetry, and higher reactivity. The B–H of the [closo-B₁₀H₁₀]²⁻ anions could be substituted with various functional groups, leading to a wide range of derivatives. Due to the presence of highly reactive “cap” B atoms and low stability in the double-capped square antiprism structure, the conjuncto-hydroborate anion, [B₂₀H₁₈]²⁻,^8–10 could be synthesized by performing oxidative dehydrogenation coupling of the [closo-B₁₀H₁₀]²⁻ anion derivatives. The high symmetry and stability of the icosahedral structure of B₁₂H₁₂²⁻ prevent it from undergoing dimerization reactions.

The synthesis of polyhedral borane polymers centers on utilizing the active sites of boron clusters (such as B–H bonds and heteroatom substituents) to react with oxidants, bridging agents, and metal ions, thereby achieving inter-cluster linkages through covalent or coordination bonds. These methods are potentially applicable to the expansion of highly reactive boron cages, and provide a reference pathway for the design of functionalized borane materials (e.g., ion conductors and catalyst supports).

In particular, the conjuncto-hydroborate anion, [B₂₀H₁₈]²⁻, exists as various isomers:^11–18 [trans-B₂₀H₁₈]²⁻, [cis-B₂₀H₁₈]²⁻, [iso-B₂₀H₁₈]²⁻, and [cross-B₂₀H₁₈]²⁻, in which [trans-B₂₀H₁₈]²⁻ is the most easily obtained and stable conformer (Fig. 1). Compared to the parent ten-boron [closo-B₁₀H₁₀]²⁻ anion, each [B₂₀H₁₈]²⁻ molecular formula unit contains twice the number of boron atoms. Such a {B₂₀} family, as a high-boron content and tumor-targeting agent, is particularly important in boron neutron capture therapy. On the other hand, the aromatic behavior in such species should be noted, as it is due to the formation of the fused {B₂₀} core. However, the inherent spherical aromatic characteristics of the parent closo-[B₁₀H₁₀]²⁻ cluster is persistent in both iso- and trans-[B₂₀H₁₈]²⁻ isomers as an interplay between the spherical aromatic properties from both B10 motifs, leading to an overlap of the shielding regions from shielding cone properties, ascribed to a dual spherical–spherical aromatic cluster.¹⁹ The variation of the B₁₀–B₁₀ backbone provided by the intercluster B-interaction, thus, results in a more effective aggregation connecting such building units, towards boron-based cluster materials.^20–23

Fig. 1 Structure of [B₁₀H₁₀]²⁻ and four types of [B₂₀H₁₈]²⁻ anions.

Although the synthesis of the [trans-B₂₀H₁₈]²⁻ anion derivatives has been extensively studied based on oxidative dehydrogenation coupling of the [closo-B₁₀H₁₀]²⁻ anion derivatives since the 1960s, the synthetic methods are still limited. The most examined starting materials are organic ammonium salts of the [closo-B₁₀H₁₀]²⁻ anion. In 1962, the first ammonium salt, [Et₃NH]₂[trans-B₂₀H₁₈], was synthesized by reacting Fe³⁺ with [Et₃NH]₂[closo-B₁₀H₁₀] under high-temperature conditions.¹⁰ However, this method only worked for the ammonium salt and did not apply to alkali metal salts such as M₂[trans-B₂₀H₁₈] (M = Li, Na, and K). Therefore, the various salts of the [trans-B₂₀H₁₈]²⁻ anions with different counteractions and organic ligands synthesized by metathesis reactions and their coordination chemistry have been widely explored. Several silver(I) salts with PPh₃ ligands were isolated and reported.²⁴ The lead(II) salts with Bipy ligands of [B₂₀H₁₈]²⁻ were synthesized and characterized by IR spectroscopy and X-ray diffraction.²⁵ A tris-chelate manganese(II) complex with a Bipy ligand [Mn(Bipy)₃][B₂₀H₁₈]²⁶ and an iron(II) complex with a cyclopentadienyl ligand [CpFe(Cp-CH₂-NMe₂Et₂)][B₂₀H₁₈]²⁷ were isolated and characterized. In addition, iron(II), cobalt(II), and nickel(II) complexes with 1,10-phenanthroline and 2,2′-bipyridyl in the presence of [B₂₀H₁₈]²⁻, with varying reagent ratios and solvents, were obtained. Several tris-chelate and binuclear complexes were isolated and studied by IR spectroscopy, elemental analysis, and X-ray diffraction.²⁸ The gold(III) salts of a [B₂₀H₁₈]²⁻ anion, [Au(L)Cl₂]₂[B₂₀H₁₈] (L = bipy, phen), were isolated and characterized.²⁹ Moreover, the synthesis of a nickel(II) complex [Ni(Phen)₃][B₂₀H₁₈] was briefly reported, indicating Ni(II) complexes with [B₂₀H₁₈]²⁻ formed by the oxidation of the complex [Ni(Phen)₃][B₁₀H₁₀] with cerium(IV).²⁵

The oxidative dehydrogenation coupling of the [closo-B₁₀H₁₀]²⁻ anion derivatives was generally carried out in an aqueous solution with Fe³⁺ or Ce⁴⁺ as an oxidizer (Fig. 2). Only one attempt was made to use acetonitrile as a solvent⁸ and an electrochemical method used CH₂Cl₂, THF, or CH₃CN.³⁰ Another approach involved the high-temperature pyrolysis of [NMe₄][N₂B₁₀H₉] to obtain [NMe₄]₂[trans-B₂₀H₁₈], but it suffered from low yield, long reaction time, and the need for high temperature.³¹

Fig. 2 Methods for the synthesis of [trans-B₂₀H₁₈]²⁻ anions.

To solve the limitations of these methods, especially for synthesizing alkali metal-based advanced polyhedral boron cluster compounds, we found that by using H₃P(Mo₃O₁₀)₄·xH₂O as the oxidant, alkali metal salts of the [trans-B₂₀H₁₈]²⁻ anion could be directly synthesized in non-aqueous solvent, based on our group's work on the synthesis of polyhedral boranes and functionalization of carborane.^32–35 M₂[trans-B₂₀H₁₈] (M = Na, K) can be obtained as solvent-free salts, while Li₂[trans-B₂₀H₁₈]·0.23CH₃CN is formed with a small amount of acetonitrile solvent. The yield ranges from 83% to 86%. Interestingly, when Cu(OTf)₂ is used as the oxidant, oxidative dehydrogenation coupling of M₂[closo-B₁₀H₁₀] (M = Li, Na, and K) leads to the formation of Cu₂[trans-B₂₀H₁₈]·1.2CH₃CN, different from the reaction carried out in aqueous solution.¹⁰ Both methods achieve rapid reactions under simple and mild conditions.

The primary synthetic methods of [trans-B₂₀H₁₈]²⁻ anions³⁶ reported so far are shown in eqn (1)–(3). Using Fe³⁺ as an oxidant, at 100 °C, [Et₃NH]₂[closo-B₁₀H₁₀] could convert a salt of [Et₃NH]₂[trans-B₂₀H₁₈] (eqn (1)) with a yield of up to 95%.¹⁰ Using Ce⁴⁺ as the oxidizing agent, [NiL₃][closo-B₁₀H₁₀] was converted to [NiL₃][trans-B₂₀H₁₈] at room temperature with a yield of 80% (eqn (2)).³⁷ By the electrochemical synthesis, the [trans-B₂₀H₁₈]²⁻ anion was obtained with a reaction time of 4 hours and a yield of 85%.³⁰ Another method involves the pyrolysis of [N₂B₁₀H₉]⁻, which was discovered accidentally (eqn (3)), with a lower yield, a longer reaction time, and high temperatures.³¹ As reported in the literature, the direct synthesis of the alkali metal salts of [trans-B₂₀H₁₈]²⁻ remained unexplored.³⁸

2 [B₁₀H₁₀]²⁻ + 4 Fe³⁺ → [trans-B₂₀H₁₈]²⁻ + 4 Fe²⁺ + 2 H⁺ (1)

NMe₄[N₂B₁₀H₉] → (NMe₄)₂[B₂₀H₁₈] + 2 N₂ (3)

\scale97%{2 [B₁₀H₁₀]^{2−} + 4 Cu²⁺ → [trans-B₂₀H₁₈]^{2{−}} + 4 Cu⁺ + 2 H⁺ (5)

We have developed an efficient one-step direct synthesis method of M₂[trans-B₂₀H₁₈] under mild conditions using M₂[closo-B₁₀H₁₀] (M = Li, Na, and K) as the starting material. Yields of M₂[trans-B₂₀H₁₈] can reach an impressive 83–86%. The mechanism of the reaction is similar to that of the ammonium salt of [trans-B₂₀H₁₈]²⁻, in which an appropriate oxidant was used to induce oxidative dehydrogenation coupling of M₂B₁₀H₁₀ to form M₂[trans-B₂₀H₁₈]. We initially used Na₂B₁₀H₁₀ as the substrate and employed the established method reported in the literature to screen various Lewis acid oxidants (Table 1). The experimental results show that [trans-B₂₀H₁₈]²⁻ can be obtained from both H₃P(Mo₃O₁₀)₄·xH₂O and Cu(OTf)₂. Both [P(Mo₃O₁₀)₄]³⁺ (eqn (4)) and Cu²⁺ (eqn (5)) could act as oxidizers and exhibit high compatibility with the polyhedral borane [closo-B₁₀H₁₀]²⁻. When Cu(OTf)₂ is used as the oxidant, Cu₂[trans-B₂₀H₁₈] is produced with a yield exceeding 94%. It is worth noting that the solvents play a crucial role in these reactions. Cu²⁺ does not play an oxidant role to initiate such oxidative dehydrogenation coupling of M₂[closo-B₁₀H₁₀] under similar conditions in aqueous solution, where only a CuB₁₀H₁₀ precipitate was formed.¹⁰ On the other hand, when H₃P(Mo₃O₁₀)₄·xH₂O is used as the oxidant, alkali metal salts of [trans-B₂₀H₁₈]²⁻ (M = Li, Na, and K) are obtained, with a yield of up to 85%.

Table 1 Reaction of Na₂B₁₀H₁₀ with different types of oxidants at 30 °C^a

Entry	Oxidant	t/h	Yield^b (%)
a Reaction conditions: Na2B10H10 (5 mmol) and different oxidants (10 mmol) in CH3CN (25 mL) at 30 °C. b Isolated yield as Na^+ salts.
1	PMA	1.5	83
2	Cu(OTf)2	1.5	91
3	Al(OTf)3	2	Trace
4	FeCl3	2	43
5	Fe(OTf)3	2	46
6	Ni(OTf)2	2	Trace
7	Ce(OTf)3	2	Trace
8	Ce(OTf)4	2	38
9	Ce(NH3)2(NO3)6	2	41
10	CF3COOCu	2	32

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We then examined the effects of different molar ratios of Na₂B₁₀H₁₀ to H₃P(Mo₃O₁₀)₄·xH₂O or Cu(OTf)₂ and found that the optimized molar ratio for the reaction was different when the oxidants were H₃P(Mo₃O₁₀)₄·xH₂O or Cu(OTf)₂. The ideal ratio of Na₂B₁₀H₁₀ to H₃P(Mo₃O₁₀)₄·xH₂O is 3 : 1 (Table 2). The optimal ratio of Na₂B₁₀H₁₀ to Cu(OTf)₂ is 1 : 3 (Table 3). The excess usage of H₃P(Mo₃O₁₀)₄·xH₂O might have been due to every Mo⁶⁺ in PMA being an efficient oxidant.

Table 2 Reaction of Na₂B₁₀H₁₀ with PMA in CH₃CN (25 mL) at 30 °C^a

Entry	PMA/Na2B10H10	t/h	Yield^b (%)
a Reaction conditions: Na2B10H10 (5 mmol) and PMA (x mmol) in CH3CN (25 mL) at 30 °C. b Isolated yield as Na^+ salts.
1	1 : 2	2	83
2	1 : 3	2	83
3	1 : 4	2	62
4	1 : 5	2	38
5	1 : 10	2	27

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Table 3 Reaction of Na₂B₁₀H₁₀ with Cu(OTf)₂ in CH₃CN (25 mL) at 30 °C^a

Entry	Cu(OTf)2/Na2B10H10	t/h	Yield^b (%)
a Reaction conditions: Na2B10H10 (5 mmol) and Cu(OTf)2 (x mmol) in CH3CN (25 mL) at 30 °C. b Isolated yield as Cu^+ salts.
1	1 : 1	2	48
2	2 : 1	2	84
3	3 : 1	2	91
4	4 : 1	2	82
5	5 : 1	2	75

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It is worth noting that when H₃P(Mo₃O₁₀)₄·xH₂O is used as the oxidant and M₂[closo-B₁₀H₁₀] (M = Li, Na, and K) as the substrate, the corresponding alkali metal advanced polyhedral boranes M₂[trans-B₂₀H₁₈] (M = Li, Na, and K) are obtained, with yields ranging from 83% to 86%. Alkali metal salts, Na₂[trans-B₂₀H₁₈] and K₂[trans-B₂₀H₁₈], can be obtained without acetonitrile coordination, while Li₂[trans-B₂₀H₁₈]·0.23CH₃CN is obtained with a small amount of solvent coordination because of the stronger coordination ability of Li⁺. What is more interesting is that when Cu(OTf)₂ is used as the oxidant and M₂[closo-B₁₀H₁₀] (M = Li, Na, and K) as the substrate, the final product is Cu₂[trans-B₂₀H₁₈]·1.2CH₃CN, with a yield as high as 91–94%, rather than the alkali metal salts. This result indicates that the formed Cu⁺ cations after reaction have a stronger interaction ability with the [B₂₀H₁₈]²⁻ anions in the presence of acetonitrile solvent.

These M₂[trans-B₂₀H₁₈] (M = Cu, Li, and K) complexes were fully characterized by NMR spectroscopy (¹H, ¹H{¹¹B}, ¹¹B, and ¹¹B{¹H} NMR) (Fig. S1−S24), XRD, XPS, and TGA (Fig. S25–29). To further prove the role of Mo⁶⁺ as an oxidizer in reactions, the valence state of Mo in H₃P(Mo₃O₁₀)₄·xH₂O was detected by XPS before and after reactions. After the reactions, the valence state of a part of Mo in H₃P(Mo₃O₁₀)₄·xH₂O was transformed from +6 to +5 (Fig. S29).

These M₂[trans-B₂₀H₁₈] (M = Cu, Li, and K) complexes were fully characterized using XRD patterns and infrared spectra (Fig. S25−S27). Thermogravimetric analysis (TGA) results reveal the temperatures at which the remaining solvent molecules are removed from the samples: for Li₂B₂₀H₁₈·1.5CH₃CN (Fig. S27), K₂B₂₀H₁₈·0.3CH₃CN (Fig. S28), and hydrous Cu₂B₂₀H₁₈·1.2CH₃CN (Fig. S29) the temperatures are approximately 200, 290, and 190 °C, respectively. This viewpoint has been verified through computational analysis.

The single-crystal structures of the salts [Cu·CH₃CN]₂[trans-B₂₀H₁₈] and [K·CH₃CN]₂[trans-B₂₀H₁₈] were successfully determined (Fig. 3). While the structure of the octadecahydroeicosaborate anion is similar to those reported in the literature, the interactions between the cation and anion are different.^23–29 In the structure of [Cu·CH₃CN]₂[trans-B₂₀H₁₈], each Cu(I) cation is coordinated to one CH₃CN molecule with the Cu–N distance of 1.933 Å, in which CH₃CN acts as a monodentate ligand. Each Cu(I) also coordinated to two B–H bonds with the distances of 2.305 and 2.328 Å for Cu-B and 1.890 and 1.628 Å for Cu–H, in which Cu adopts a three-coordination, based on which the interactions between Cu(I) and the B–H bond are partly covalent. Moreover, each B₂₀H₁₈²⁻ anion also interacts with four Cu(I) cations via two Cu⋯H⋯B interactions. A similar interaction between cations and H atoms also exists in the compound [Au(L)Cl₂] [trans-B₂₀H₁₈] (L = bipy, phen).²⁹ In contrast, [K·CH₃CN]₂[trans-B₂₀H₁₈] seems to be an ion pair structure, which is different from [Cu·CH₃CN]₂[trans-B₂₀H₁₈]. Each CH₃CN, as a η²-ligand, bonds with two K cations with K–N distances of 2.872 and 3.029 Å, similar to the coordination geometry of K₂[B₂₀H₁₈]·4CH₃CN ³⁹ and K[B(CN)₄]·CH₃CN, resulting in the formation of an extended cation chain with the alternating linkages of K⁺ ions and CH₃CN molecules. Each K cation interacts with 5 B–H bonds in its other half sphere with distances of 2.868–3.014 Å, indicating an electrostatic attraction. A similar behavior was observed for [Fe(Bipy)₃][B₂₀H₁₈]·Bipy·2CH₃CN, [Ni(Phen)₃][B₂₀H₁₈]·2DMF, and [Co(Phen)₃][B₂₀H₁₈]·4CH₃CN complexes.²⁸ Each [B₂₀H₁₈]²⁻ anion is surrounded by eight K, with the K–B distances of 3.417–4.513 Å, consisting of a rectangular prism. Such weak interactions between K and B–H may be why Cu(I) could replace the alkali metals, Li, Na, and K, in the reaction to afford the u(I) salt.

Fig. 3 Molecular structures of [Cu·CH₃CN]₂[trans-B₂₀H₁₈] (a) and [K·CH₃CN]₂[trans-B₂₀H₁₈], (b) both showing 50% probability thermal ellipsoids.

In conclusion, we have developed facile methods for directly synthesizing M₂[trans-B₂₀H₁₈] (M = Li, Na, K, and Cu) by the reactions of M₂[B₁₀H₁₀] (M = Li, Na, and K) with an oxidizer, such as H₃P(Mo₃O₁₀)₄·xH₂O or Cu(OTf)₂. It was found that when H₃P(Mo₃O₁₀)₄·xH₂O was used as an oxidizer with its molar ratio to M₂[B₁₀H₁₀] (M = Li, Na, and K) of 1 : 3, M₂[trans-B₂₀H₁₈] (M = Li, Na, and K) was obtained with yields of 83–86%. When Cu(OTf)₂ was used as an oxidizer with its molar ratio to M₂[B₁₀H₁₀] of 3 : 1, Cu₂[trans-B₂₀H₁₈] was obtained with yields of 91–94%. This work not only provides an efficient synthetic method for [trans-B₂₀H₁₈]²⁻ but also paves the way for the practical application of M₂[trans-B₂₀H₁₈] (M = Li, Na, and K) derivatives in various fields, including solid ionic conductors.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI: experimental procedure, supplementary figures and NMR spectra. See DOI: https://doi.org/10.1039/d5dt01718a.

CCDC 2449540 and 2415748 contain the supplementary crystallographic data for this paper.^40a,b

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22171246, U23A2078, and 22479039).

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