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Crystals 2022, 12, 1339 4 of 7 11H). 1H{11B} NMR (400 MHz, DMSO-d6) δ 2.36 (s, 1H), 1.55 (s, 6H), 1.40 (s, 5H). 13C NMR (101 MHz, DMSO-d6) δ 50.69 (s) (in Supplementary Materials). 3. Results and Discussion The [CB11H12]− anion was discovered by Knoth using toxic decaborane B10H14 as starting material in 1967 [2]. Since B10H14 was synthesized from NaBH4 via the [B11H14]− ion, to obviate the toxicity and tedious synthetic steps, the group of Michl [19] developed an alternative approach, using [B11H14]− as a starting material and dichlorocarbene as the carbon source, but in a poor yield. On the basis of this innovative method, Kütt’s group reported an improved method to obtain [Me3NH][CB11H12] from boron cluster [B11H14]− in up to 95% yield using difluorocarbene as the carbon source [21]. In 2021, the research group of Mark Paskevicius reported a cost-effective method to synthesize the [CB11H12]− anion in 40% yield from [B11H14]− using common laboratory reagents [22]. Generally speaking, the synthetic procedure of [CB11H12]− from [B11H14]− includes four steps represented by Equations (1)–(4), as reported in the literature, in which the cation exchange reactions are carried out (Equations (1) and (4)) considering the solubility of different salts. It is known that Na2[B11H13] can be formed by the deprotonation of [Me3NH][B11H14] with NaOH or NaH. The formed Me3N must be removed because the presence of NMe3 has previously been found to cause the formation of an excessive amount of 2-Me3NCB11H11 byproduct (Equation (2)) [19]. The Na2[B11H13] can then be used to synthesize the [CB11H12]− anion, by insertion of :CCl2 or :CF2 sourced from CHCl3 or CF3SiMe3 (Equation (3)). However, because of the limitation in the availability of suitable purification, the ion exchange through the metathesis reaction of Na[CB11H12] with Me3N·HCl is carried out at room temperature in the aqueous medium (Equation (4)). Na[B11H14] (aq) + Me3N·HCl (s) → [Me3NH][B11H14] (s) + NaCl (s) (1) [Me3NH][B11H14] (s) + NaH (s) or NaOH (s) → Na2[B11H13] (aq) + H2 or H2O + NMe3 (2) Na2[B11H13] (aq) + NaH (s) or NaOH (s) + :CX2 → Na[CB11H12] (aq) (3) Na[CB11H12] (aq) + Me3N·HCl (s) → [Me3NH][CB11H12] (s) + NaCl (s) (4) Considering the great importance of the alkali metal salts of the [CB11H12]− anion in the fields of catalysis and all-solid-state batteries, we developed a simple, quick, and one-step procedure to directly synthesize unsolvated M[CB11H12] using M[B11H14] (M = K, Na) as starting materials. The overall yields of M[CB11H12] were up to 66–68%. M[B11H14] is commercially available and we first screened different bases with K[B11H14] as substrate. The results showed that KH was the best choice. Then, we examined different molar ratios of K[B11H14] to KH (Table 1). It was found that the optimized ratio of K[B11H14] to KH to CF3SiMe3 was 1:3:3 in the overall preparation of K[CB11H12]. On the other hand, we attempted to synthesize Na[CB11H12] under similar conditions but failed. By using the same molar ratio of Na[B11H14]:NaH:CF3SiMe3 = 1:3:3, no reaction was observed and the starting material was monitored by 11B NMR spectroscopy. The reaction could not be observed even if we increased the amount of NaH up to six times. These results indicated that NaH could not abstract hydrogen from Na[B11H14] to form the Na2[B11H13] intermediate in this reaction. The observation further revealed that the reaction of Na[B11H14] with NaH was different from that of [Me3NH][B11H14] with NaH in which the deprotonation occurred. Thus, it was concluded that the counter cation influenced the reactions of the salts of the [B11H14]− anion. Then, we examined the reactivity of NaHMDS in this reaction in consideration with its strong basicity and good solubility in organic solvent. However, when three equivalent NaHMDS were used instead of NaH in the reaction, the decomposition of the boron cage was observed but no product was obtained. If only one equivalent NaHMDS was used in the reaction, Na2[B11H13] was monitored by 11B NMR. Based on these results, we speculated that NaHMDS could abstract hydrogen from Na[B11H14] to form Na2[B11H13], but NaHMDS would continually react with the formed Na2[B11H13], resulting in a decomposition to unidentified small boron cages. In considering that NaH can react with CF3SiMe3 to provide carbene:CF2, thus, we selected the combination of NaHMDS and NaH, and screened the different molar ratios of Na[B11H14] to NaHMDS to NaH (Table 2). After a series of screenings, the 1:1:3:3 molar ratio of Na[B11H14] to NaHMDS to NaH to CF3SiMe3 was used in the overall preparation of Na[CB11H12]. Furthermore, it is worth noting that when we examined DME as a solvent, the yields were relatively higher than those in THF, probably because the slight polarityPDF Image | Efficient Way to Directly Synthesize Unsolvated Alkali Metal
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