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Article

Direct Synthesis of C-Substituted [RC(O)CH2-CB11H11]− Carborate Anions

by
Vanessa C. Barra
,
Eduard Bernhardt
*,
Sarah Fellinger
,
Carsten Jenne
* and
Shiomi S. Langenbach
Anorganische Chemie, Fakultät für Mathematik und Naturwissenschaften, Bergische Universität Wuppertal, Gaußstr. 20, 42119 Wuppertal, Germany
*
Authors to whom correspondence should be addressed.
Inorganics 2024, 12(6), 173; https://doi.org/10.3390/inorganics12060173
Submission received: 8 May 2024 / Revised: 12 June 2024 / Accepted: 13 June 2024 / Published: 19 June 2024
(This article belongs to the Special Issue State-of-the-Art Inorganic Chemistry in Germany)
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Abstract

:
A new synthetic method for the synthesis of C-substituted [RC(O)CH2-CB11H11]− carborate anions has been developed. The reaction of [closo-B11H11]2− with terminal alkynes in the presence of a copper catalyst leads to insertion into the boron cluster, and C-substituted [RC(O)CH2-CB11H11]− carborate anions are formed. These reactions are strongly dependent on the reaction conditions, the solvents, and the alkynes used. The alkynes HCCCO2Et, HCCCO2Me, and HCCCONH2 lead to the formation of [NH2C(O)CH2-CB11H11]− as the final product in aqueous ammonia solution. In contrast, the reaction using the alkyne HCCCOMe yields [MeC(O)CH2-CB11H11]−. The products have been fully characterized by multinuclear NMR and IR spectroscopy as well as mass spectrometry. The crystal structures of K[NH2C(O)CH2-CB11H11] and [NEt3CH2Cl][NH2C(O)CH2-CB11H11] have been determined.

Graphical Abstract

1. Introduction

The 1-carba-closo-dodecaborate anion [HCB11H11]− and its derivatives have gained increasing interest in chemistry [1,2,3,4]. In particular, methylated and halogenated derivatives show superior properties as weakly coordinating anions due to their low nucleophilicity and high chemical stability [1,3,5,6,7,8,9,10,11,12]. The increasing interest in these anions leads to a demand for simple and cheap syntheses of the parent 1-carba-closo-dodecaborate anion [HCB11H11]− (in the following, abbreviated as carborate) and its derivatives.
Original multi-step syntheses of the carborate anion make use of the commercially available, but expensive, decaborane(14) B10H14 as the starting material, while modern syntheses build up the carborate cluster in only one step beginning with nido-undecaborate [nido-B11H14]−. The latter is significantly cheaper and can be prepared more easily [13,14]. Scheme 1 summarizes the known synthetic routes for the preparation of the carborate anion.
The first synthesis of the carborate [HCB11H11]− from B10H14 was published by Knoth in 1967 [15,16]. He received the product in a five-step reaction cascade in an overall yield of 25% (reactions e, h, i, j, and k in Scheme 1). Later on, this method was modified and improved (reactions f, g, h, i, l, m, and n in Scheme 1), and the yield was increased up to 60% [17]. To overcome the disadvantage of several consecutive reaction steps, a simpler two-step method was reported in 2016 (reactions c and d in Scheme 1), leading to an overall yield of 40 % [18]. Nevertheless, all these methods still used expensive decaborane as a starting material. That is why attempts were made to replace decaborane by cheaper and more easily accessible boron cluster compounds. In 2001, Michl et al. were successful and prepared [HCB11H11]− from [nido-B11H14]−. The reported overall yield was 40% (reaction p in Scheme 1) [19,20,21], which, however, was later corrected to 10% [20]. A recently published improvement in this method provides the carborate in a 40% yield (reaction p in Scheme 1) [22]. The latter two methods are based on the insertion of in situ generated CCl2 into the B11 cage. Much higher yields (66–68%) can be obtained, when CHCl3 is replaced by the Ruppert–Prakasch reagent Me3SiCF3 (reactions q and s in Scheme 1) [23,24,25].
While the C-substitution is well known for neutral dicarbaboranes C2B10H12, only a few derivatives have been reported for the 1-carba-closo-dodecaborate anion [26]. Typically, the syntheses of C-substituted 1-carba-closo-dodecaborates start from the [HCB11H11]− anion [1,27,28,29,30,31,32], though some reports also exist where C-aryl-substituted carborates were prepared directly from B10H14 [33] (reactions a and b in Scheme 1) or [B11H14]− [20] (reaction o in Scheme 1; 12–43 % yield depending on the aryl substituent). The aim of this study is to present a new method for the synthesis of C-substituted 1-carba-closo-dodecaborates [RCB11H11]− using the easily accessible [closo-B11H11]2− as a precursor.

2. Results and Discussion

2.1. Reaction Design

Nido-undecaborate [nido-B11H14]− is a cheap and easily accessible boron cluster [13,14,34,35]. Only one carbon atom is missing for completion of the targeted 1-carba-closo-dodecaborate [closo-HCB11H11]− cluster [19,20,21,23,24]. Nido-undecaborate can be easily deprotonated to give the [B11H13]2− dianion [36,37]. However, the nucleophilicity of [B11H13]2− is still too low to react with weak electrophiles such as formic acid ester (HCO2R) or aldehydes (RCHO). In contrast, reactions of aldehydes with [B10H13]− and some other boron clusters are known (cf. Brellochs’ reaction in a and c in Scheme 1) [18,33,38]. Carbenes (CF2, CCl2, CBr2), which only exist as short-living intermediates, turned out to be suitable electrophiles for the reaction with the [B11H13]2− dianion [19,20,21,23,24]. The carbene provides the carbon atom for the insertion and oxidizes [B11H13]− to an intermediary [closo-B11H11]2−. The latter incorporates the carbon atom in a subsequent step [19,20,21]. The intermediate [closo-B11H11]2− is well known and can be obtained directly by oxidation of [nido-B11H14]− under basic conditions [34,35,39]. In analogy to B10H14 and [B10H13]2− and in contrast to [B11H14]− and other closo-borates [closo-BnHn]− (n = 6–10, 12 [40,41,42]), the closo-undecaborate [closo-B11H11]2− is able to react as a weak electrophile.
It has been established that B10H14 and [B10H13]− undergo a reaction with cyanide under the formation of [arachno-B10H13(CN)]2− [43], which carries the -CN group in the endo position [44]. Subsequent protonation leads to the incorporation of the carbon atom into the boron cluster, giving nido-H3NCB10H12 [15]. Similarly, the [closo-B11H11]2− anion reacts with cyanide in water, yielding [nido-7-NC-B11H12]2− [45,46], which, in contrast to [arachno-B10H13(CN)]2−, does not rearrange to [closo-H2N-CB11H11]−, because the -CN group is in an exo position [45,46].
For B10H14 and some other boron clusters, it is known that their reactions with alkynes lead to the formation of carboranes [47,48,49]. Analogues reactions of [nido-B11H14]− and [closo-B11H11]2− have not yet been reported. Due to the exceptional stability of [closo-B12H12]2− compared to [closo-B11H11]2− and [closo-B13H13]2− [50], it can be expected that in case of the reaction of acetylenes with [nido-B11H14]− or [closo-B11H11]2−, only one carbon atom will be incorporated into the boron cluster. Consequently, monosubstituted acetylenes should be more reactive than disubstituted acetylenes. In analogy to the known reaction with cyanide, it can be assumed that [closo-B11H11]2− would also react with acetylenes. Monosubstituted acetylenes can be deprotonated to the corresponding acetylides but, in water, they are immediately protonated, and the corresponding acetylenes are returned (pKa = 25 (HCCH), 14 (H2O), 9.3 (HCN)) [51]. This fact prevents the application of these acetylides in aqueous media [52]. However, some acetylides, in particular acetylides of the coinage metals copper [53,54,55,56,57,58,59,60,61,62], silver [63,64,65,66,67], and gold [68,69,70,71,72,73], and of mercury [74,75] can be generated in water.
In this work, we describe the reaction of closo-[B11H11]− with HCCCO2Et in aqueous ammonia solution in the presence of copper salts, yielding [NH2C(O)CH2-CB11H11]2− (Scheme 2). The yield in an NMR experiment is essentially quantitative, while the isolated yield of K[NH2C(O)CH2-CB11H11] is approximately 45% due to product loss during work up. The detailed reaction conditions are discussed in the next section.

2.2. Synthesis of [NH2C(O)CH2-CB11H11]−

The synthesis of [NH2C(O)CH2-CB11H11]− starts from K2[B11H11]. Reactions with terminal alkynes, such as HCCCO2Et, HCCCO2Me, and HCCCONH2, in aqueous ammonia solution in the presence of a copper catalyst proceed within minutes. A mixture of Cu(I)Cl and sodium ascorbate as a catalyst in aqueous ammonia solution turned out to be a good combination. Other copper salts work as well. For instance, Cu powder, CuCN, Cu2O, and CuO with copper in the oxidation states 0, +I, and +II, respectively, in acetonitrile solution and Cu(I)Cl, Cu[SO4], and Cu(II)Cl2 in aqueous ammonia solution were tested in reactions with ethyl propriolate. In the latter reactions, the blue tetraammine copper complex [Cu(NH3)4]− forms first [76]. After the addition of ethyl or methyl propriolate, respectively, the blue complex [Cu(NH3)4]− decomposes and the copper cation binds to the alkyne. When the reaction is performed in aqueous ammonia solution, an ammonolysis of methyl and ethyl propriolate occurs and CuCCCONH2 is yielded. The copper acetylide precipitates as a yellow solid. Due to its low solubility in water and other solvents, NMR spectroscopic investigations cannot be performed, but IR spectra show signals at 3431, 3310, and 3168 cm−1 in accordance with the presence of the -NH2 group [77].
For the syntheses of [NH2C(O)CH2-CB11H11]−, up to 50 % Cu(I)Cl was used. Excess copper, which does not bind to the terminal alkyne, reacts with the starting material [closo-B11H11]2− under formation of the stable complex [Cu(B11H11)2]3−, which remains in solution and can be detected by NMR (see section S3 in the Supplementary Material ) [78]. Therefore, it is crucial to inhibit this side reaction, which already occurs at low copper concentrations. The unwanted formation of [Cu(B11H11)2]3− significantly lowers the yield. This can be avoided by adding [closo-B11H11]2− only after the formation of CuCCC(O)NH2 is complete. The easily observable color of the reaction mixture helps to monitor the reaction progress. After complete dissolution of Cu(I)Cl in the presence of sodium ascorbate, the aqueous ammonia solution shows a deep-blue color. Addition of the immiscible terminal alkyne leads to a reaction on the phase boundary, giving a green emulsion and, finally, a colorless to light-yellow solution and a yellow precipitate. At this point, it can be anticipated that all the copper is bound to the alkyne and [closo-B11H11]2− can be added, leading to a brownish solution and a yellow-to-brown solid. [Closo-B11H11]2− should be added immediately after the formation of CuCCCONH2 is complete.
Terminal alkynes tend to polymerize, in particular in the presence of a base [79,80]. Propiolamide is a solid and shows the lowest tendency to polymerize, both in storage and in solution. In contrast, ethyl propiolate and methyl propiolate slowly polymerize, as indicated by an increasing yellow color. Since the alkyne should always be in excess, a two- to threefold excess compared to [closo-B11H11]2− ensures the presence of enough alkyne in solution. Note that all reactions should be performed at room temperature. Cooling (reaction is too slow) and heating (the alkyne polymerizes) lower the isolated yield.
For the isolation of the product from the reaction mixture, two different methods were developed (S2.1. in the Supplementary Material). Method 1: After filtration, the product can be extracted from the filtrate using either diethyl ether or ethyl acetate. After drying under vacuum, the product K[NH2C(O)CH2-CB11H11] is isolated as an orange solid, which still contains some copper compounds and ethyl acetate. Method 2: Acidifying using hydrochloric acid followed by the addition of triethylamine yield the triethylammonium salts, which can be extracted by dichloromethane. After drying under vacuum, the product is isolated in about 85 % yield as an orange solid, also containing traces of copper compounds. Subsequent metathesis using KOH in water leads to pure K[NH2C(O)CH2-CB11H11] as a colorless solid in an approximately 45 % yield.

2.3. Characterization and Properties of K[NH2C(O)CH2-CB11H11]

NMR Spectroscopy. Figure 1 shows the 1H{11B}, 11B, and 13C NMR spectra of K[NH2C(O)CH2-CB11H11] in D2O as an example. The chemical shifts in CD3CN are given in Section 3. For all spectra in different solvents and full data, see Section S2 in the Supplementary Material. In the 1H{11B} NMR spectrum (Figure 1a), the cluster hydrogen atoms appear as two resonances at 2.0 and 2.4 ppm in a 5:4 ratio. The methylene group is detected at 3.3 ppm. The resonances shift significantly, depending on the solvent. The NH2 protons are not always visible but, in some spectra, appear at about 5.5 and 6.0 ppm as two broad singlets (Figure S2 in the Supplementary Material). This indicates that the NH2 protons are chemically inequivalent, and the rotation around the C-N bond is slow. In the coupled 11B NMR spectrum (Figure 1b), the cluster boron atoms result in two doublets in a 1:10 ratio. The three different carbon atoms lead to resonances at approximately 48 (CH2 group), 67 (ipso-C), and 176 ppm (carboxamide) in the 13C NMR spectrum (Figure 1c).
X-ray diffraction. The constitution of the [NH2C(O)CH2-CB11H11]− anion was also confirmed by X-ray diffraction analyses (Section S4 in the Supplementary Material). The crystal structures of K[NH2C(O)CH2-CB11H11] and [NEt3CH2Cl][NH2C(O)CH2-CB11H11] were determined (Figure 2). Single crystals of K[NH2C(O)CH2-CB11H11] were obtained from water, while [NEt3CH2Cl][NH2C(O)CH2-CB11H11] was isolated as a minor by-product from a work up using NEt3 and CH2Cl2 [81,82,83]. The anion in [NEt3CH2Cl][NH2C(O)CH2-CB11H11] forms dimers via N-H…O hydrogen bridges, while in the potassium salt, no hydrogen bonds are detected. This, however, has almost no influence on the bond lengths in the acetamide group. A comparison with literature data for acetamides shows that the bond lengths within the functional group are in the expected range (Table 1). Only the Cipso- CH2 (C1-C2) bond is 3 to 5 pm longer than in related aryl derivatives. The B-B, B-C, and Cipso- C bond lengths (Table 1) are in the expected range (cf. Cipso-C in [Ph-CB11H11]− (151.2(3) pm) [33] and [Et-CB11H11] (153.3 pm) [84]) compared to other C-substituted 1-carba-closo-dodecaborates.
Thermogravimetric Analysis. K[NH2C(O)CH2-CB11H11] shows a melting point at 212 °C in a thermogravimetric DSC/TGA experiment (Figure S19 in the Supplementary Material). At 300 °C, dehydration of the carboxamide group starts, accompanied by rearrangement [87], yielding the corresponding nitrile K[NCCH2-CB11H11] (Figure S19). It was identified by multinuclear NMR spectroscopic (Figures S20–S24) and mass spectrometric investigations (Figure S25). The 1H NMR spectrum of the isolated product shows the absence of NH2 protons and a low field shift in the CH2 resonance. In the 13C NMR spectrum, the resonances resulting from the ipso-C and the CH2 group are shifted to a higher field compared to the carboxamide. The carboxamide resonance is missing and a new signal for the nitrile group at 118.6 ppm has appeared.

2.4. Reactions with Other Terminal Alkynes

The copper-catalyzed reaction of [closo-B11H11]2− with HCCCO2Et can be transferred to other alkynes. Depending on the alkyne and the solvent, different products are obtained. Alkynes carrying electron-withdrawing groups form the copper acetylide more easily. The formation of the copper acetylide has been observed with HCCCO2Me, HCCCONH2, and HCCCOMe. Reactions starting from HCCCO2Me or HCCCONH2 with [closo-B11H11]2− in aqueous ammonia solution exclusively yield [NH2C(O)CH2-CB11H11]−. In contrast, a reaction starting from HCCCOMe leads to [MeC(O)CH2-CB11H11]−, because the keto function cannot undergo ammonolysis (Section S2.5 in the Supplementary Material).

3. Experimental Section

Experimental Details. All compounds are stable to air and moisture. The reactions were carried out under atmospheric air and pressure. Two exemplifying reactions are presented below. Full synthetic details and all actual spectra are included in the Supplementary Material. Nuclear magnetic resonance measurements were performed on either a BRUKER (Ettlingen, Germany) Avance 400 spectrometer or a BRUKER (Ettlingen, Germany) Avance III 600 spectrometer using acetonitrile-d3 and water-d2 as solvents. A Netzsch (Selb, Germany) STA 449 F5 Jupiter allowed for thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. Vibrational spectroscopic measurements were performed on a BRUKER (Ettlingen, Germany) VERTEX 70 spectrometer with a diamond ATR unit. Mass spectra were analysed on a Bruker Daltonics (Bremen, Germany) micrOTOF mass spectrometer with Agilent 1100 Series liquid chromatograph (LC) in negative mode by electrospray ionisation (ESI).
Synthesis of K[NH2C(O)CH2-CB11H11] from K2[B11H11] and Ethyl Propiolate. HCCCO2Et (0.5 mL, 4.934 mmol, 2.06 eq.) was mixed with 10 mL of a 25% aqueous NH3 solution. CuCl (0.237 g, 2.394 mmol, 1.00 eq.) was added to the stirred solution, and the color of the solution changed from blue to green. After stirring for 15 min at room temperature, sodium ascorbate (100 mg) was added. The green solution turned brown, and a yellow solid precipitated. After stirring for 10 min, K2[B11H11] (0.497 g, 2.387 mmol, 1.00 eq.) was added. After stirring for another 3 h, concentrated hydrochloric acid (14 mL) and triethylamine (0.7 mL) were added carefully to the mixture to precipitate the triethylammonium salt. Subsequent extraction (three times, each with 50 mL dichloromethane), drying of the organic phase with sodium sulphate, and removal of the solvent under reduced pressure yielded [Et3NH][NH2C(O)CH2-CB11H11] (0.367 g, 1.214 mmol, 51 %) as an orange solid. The potassium salt was obtained by performing a metathesis reaction of [Et3NH][NH2C(O)CH2-CB11H11] (0.357 g, 1.181 mmol, 1.00 eq.) with KOH (0.485 g, 8.644 mmol, 7.32 eq.) in deionised water at room temperature. The aqueous solution was reduced and extracted three times with 50 mL diethyl ether. The combined organic phases were dried with potassium carbonate, and the solvents were removed under reduced pressure. The final product K[NH2C(O)CH2-CB11H11] was obtained as a white solid (0.215 g, 0.899 mmol, 76 %). The total yield based on the amount of K2[B11H11] used was 39 %. 1H{11B} NMR (400.13 MHz, CD3CN, 300 K): δ = 1.50 (s, 6H, [NH2C(O)CH2-CB11H11]−), 1.78 (s, 5H, [NH2C(O)CH2-CB11H11]−), 2.61 (s, 2H, [NH2C(O)CH2-CB11H11]−), 5.50 (s, 1H, [NH2C(O)CH2-CB11H11]−), 5.86 (s, 1H, [NH2C(O)CH2-CB11H11]−). 11B NMR (128.38 MHz, CD3CN, 300 K): δ = −13.2 (d, 1JBH = 144 Hz, 10B, B(2-11)-H), −9.6 (d, 1JBH = 138 Hz, 1B, B12-H). 13C{1H} NMR (100.62 MHz, CD3CN, 300 K): δ = 172.0 (s, [NH2C(O)CH2-CB11H11]−), 65.3 (s, [NH2C(O)CH2-CB11H11]−), 47.3 (s, [NH2C(O)CH2-CB11H11]−). ESI-MS [m/z]: found = 200.22 (calc. = 200.23, [NH2C(O)CH2-CB11H11]−). IR (diamond ATR): ν [cm−1] = 3619 (m), 3474 (m), 3380 (m), 3292 (w), 3229 (m), 3205 (m), 2941 (w), 2563 (s), 2525 (s), 2495 (s), 2361 (w), 2341 (w), 1995 (w), 1665 (s), 1609 (s), 1443 (m), 1394 (m), 1305 (w), 1215 (m), 1176 (w), 1112 (m), 1045 (m), 966 (m), 946 (m), 818 (w), 720 (m), 683 (w), 649 (w), 584 (m), 526 (m), 456 (m), 434 (m).
Synthesis of K[CH3C(O)CH2-CB11H11] with K2[B11H11] and 3-Butyn-2-one. CuCl (0.078 g, 0.788 mmol, 0.23 eq) and sodium ascorbate (200 mg) were dissolved in 12 mL of a 25% aqueous NH3 solution, and 3-butyn-2-one (0.6 mL, 7.668 mmol, 2.28 eq.) was added. After stirring for 30 min, K2[B11H11] (0.699 mg, 3.357 mmol, 1.00 eq.) was added. The reaction suspension was stirred for 1 h at room temperature, and, subsequently, potassium carbonate was added until the solution was saturated. The precipitate formed was filtered off, and the solution was extracted three times with 80 mL ethyl acetate. The combined organic phase was dried with sodium sulphate and removed under reduced pressure. The obtained light-yellow solid was dried at high vacuum (38 %, 0.305 g, 1.280 mmol). 1H{11B} NMR (400.13 MHz, CD3CN, 300 K): δ = 1.50 (s, 6H, [CH3C(O)CH2-CB11H11]−), 1.74 (s, 5H, [CH3C(O)CH2-CB11H11]−), 2.11 (s, 3H, [CH3C(O)CH2-CB11H11]−), 2.79 (s, 2H, [CH3C(O)CH2-CB11H11]−). 11B NMR (128.38 MHz, CD3CN, 300 K): δ = 13.2 (d, 1JBH = 145 Hz, 10B, B(2-11)-H), −9.3 (d, 1JBH = 136 Hz, 1B, B12-H). 13C{1H} NMR (150.95 MHz, CD3CN, 300 K): δ = 172.7 (s, [CH3C(O)CH2-CB11H11]−), 64.3 (s, [CH3C(O)CH2-CB11H11]−), 52.8 (s, [CH3C(O)CH2-CB11H11]−), 31.8 (s, [CH3C(O)CH2-CB11H11]−). ESI-MS (negative-mode): m/z = 199.23 (calc. = 199.23, [CH3C(O)CH2-CB11H11]−).
Crystal structure determinations. The data collection was performed using an Oxford Diffraction (Yarnton, UK) Gemini E Ultra diffractometer with a 2K × 2K EOS CCD camera, a four-circle goniometer with κ geometry, a sealed-tube Mo radiation source, and an Oxford Cryosystems (Long Hanborough, UK) Cobra Cryojet cooling unit. Processing of the raw data, scaling of diffraction data, and the application of an empirical absorption correction were performed with the CrysAlisPro 171.40.68a program [88]. The structures were solved by direct methods and refined against F2 [89,90,91]. The graphics were prepared with the program Diamond 3.2f [92]. Experimental details for each diffraction experiment are given in Section S4 (Supplementary Material). CCDC-2177597 and -2345230 contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre, https://www.ccdc.cam.ac.uk/structures/ (accessed on 12 June 2024).

4. Conclusions

A direct synthesis of C-substituted [RC(O)CH2-CB11H11]− carborate anions was developed. While previous carborate syntheses used decaborane or [nido-B11H14]− as the starting material, we showed for the first time the use of [closo-B11H11]2− as a precursor. The boron cluster [closo-B11H11]2− reacts with terminal alkynes in the presence of a copper catalyst to C-substituted [RC(O)CH2-CB11H11]− carborate anions. The mechanism is still an open question. It seems to be important to synthesize the copper alkyne complex prior to the addition of [closo-B11H11]2−. Otherwise, the very stable [Cu(B11H11)2]3− complex is formed and no insertion reaction into the cluster occurs. The reaction strongly depends on several reaction parameters but, in principle, can be transferred to other alkynes. In contrast to other C-functionalization methods, which require the synthesis of the 1-carba-closo-dodecaborate [HCB11H12]−, first, the herein presented method directly yields a C-functionalized carborate. In conclusion, we expect this new method to be a valuable contribution to the recent developments of modern and more efficient carborate syntheses.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics12060173/s1, S1. Numbering scheme for the [RC(O)CH2-CB11H11]− anions, S2. Experimental details and spectroscopic data, S2.1. Synthesis of K[NH2C(O)CH2-CB11H11] with K2[B11H11] and ethyl propiolate, S2.2. Pyrolysis of K[NH2C(O)CH2-CB11H11], S2.3. Synthesis of K[NH2C(O)CH2-CB11H11] with K2[B11H11] and methyl propiolate, S2.4. Synthesis of K[NH2C(O)CH2-CB11H11] with K2[B11H11] and propiolamide, S2.5. Synthesis of K[CH3C(O)CH2-CB11H11] with K2[B11H11] and 3-butyn-2-one, S3. NMR data for [Cu(B11H11)2]3−, S4. Crystal structure data.

Author Contributions

Conceptualization, E.B. and C.J.; investigation, V.C.B., E.B., S.F., S.S.L. and C.J.; writing—original draft preparation, E.B., S.F. and C.J.; writing—review and editing, C.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Acknowledgments

We are grateful to Björn Beele for performing thermogravimetric analyses and Marion Litz for proofreading.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Inorganics 12 00173 sch001
Scheme 1. (a) KOH, PhCHO; (b) BH3NEt3, 200 °C; (c) KOH, HCHO; (d) BH3SMe2; (e) NaCN; (f) OH−; (g) CN−; (h) H−, ion exchange or conc. HCl, 0 °C; (i) Me2SO4, OH−; (j) Na; (k) BH3NEt3, 180 °C; (l) BH3NEt3, 200 °C; (m) Me2SO4, OH−; (n) Na (NH3); (o) NaH, EtOH, PhCHCl2; (p) NaH, EtOH, CHCl3; (q) NaH, EtOH, Me3SiCF3; (r) NaOH/K2CO3, CHCl3; (s) NaHMDS, NaH, Me3SiCF3, 60 °C; (t) n-BuLi, MeI. The yields refer to overall yields for the different methods. Carbon atoms are labeled with C. All other cluster vertices correspond to B–H groups. Additional bridging and terminal hydrogen atoms are shown.
Scheme 1. (a) KOH, PhCHO; (b) BH3NEt3, 200 °C; (c) KOH, HCHO; (d) BH3SMe2; (e) NaCN; (f) OH−; (g) CN−; (h) H−, ion exchange or conc. HCl, 0 °C; (i) Me2SO4, OH−; (j) Na; (k) BH3NEt3, 180 °C; (l) BH3NEt3, 200 °C; (m) Me2SO4, OH−; (n) Na (NH3); (o) NaH, EtOH, PhCHCl2; (p) NaH, EtOH, CHCl3; (q) NaH, EtOH, Me3SiCF3; (r) NaOH/K2CO3, CHCl3; (s) NaHMDS, NaH, Me3SiCF3, 60 °C; (t) n-BuLi, MeI. The yields refer to overall yields for the different methods. Carbon atoms are labeled with C. All other cluster vertices correspond to B–H groups. Additional bridging and terminal hydrogen atoms are shown.
Inorganics 12 00173 sch001
Inorganics 12 00173 sch002
Scheme 2. Reaction scheme of the reaction of K2[B11H11] with ethyl propiolate in aqueous ammonia solution and in the presence of CuCl.
Scheme 2. Reaction scheme of the reaction of K2[B11H11] with ethyl propiolate in aqueous ammonia solution and in the presence of CuCl.
Inorganics 12 00173 sch002
Inorganics 12 00173 g001
Figure 1. 1H{11B} (a, left), 11B (b, middle) and 13C (c, right) NMR spectra (400.13 MHz, 128.38 MHz, 100.62, D2O, 300 K) of K[NH2C(O)CH2-CB11H11].
Figure 1. 1H{11B} (a, left), 11B (b, middle) and 13C (c, right) NMR spectra (400.13 MHz, 128.38 MHz, 100.62, D2O, 300 K) of K[NH2C(O)CH2-CB11H11].
Inorganics 12 00173 g001
Inorganics 12 00173 g002
Figure 2. Part of the crystal structure of [NEt3CH2Cl][NH2C(O)CH2-CB11H11] ((top), structure code ′ = 1 − x, y, z) and K[NH2C(O)CH2-CB11H11] (bottom). Ellipsoids are drawn at 50% probability, hydrogen atoms are drawn with arbitrary radii, and counter cations are not shown. A figure illustrating the packing in the crystal structure of [NEt3CH2Cl][NH2C(O)CH2-CB11H11] is included in the Supplementary Material (Figure S41).
Figure 2. Part of the crystal structure of [NEt3CH2Cl][NH2C(O)CH2-CB11H11] ((top), structure code ′ = 1 − x, y, z) and K[NH2C(O)CH2-CB11H11] (bottom). Ellipsoids are drawn at 50% probability, hydrogen atoms are drawn with arbitrary radii, and counter cations are not shown. A figure illustrating the packing in the crystal structure of [NEt3CH2Cl][NH2C(O)CH2-CB11H11] is included in the Supplementary Material (Figure S41).
Inorganics 12 00173 g002
Table 1. Comparison of selected bond lengths in pm.
Table 1. Comparison of selected bond lengths in pm.
Bond[NEt3CH2Cl]
[NH2C(O)CH2-CB11H11] a		K[NH2C(O)CH2-CB11H11] aNH2C(O)CH2-C6F5 bNH2C(O)CH2-C6H5 c,d
C1-C2155.0(12)153.43(12)150.43(17)150.2
C2-C3150.5(12)152.10(13)151.92(18)151.5
C3-O1124.8(9)123.74(12)124.30(15)123.9
C3-N1133.6(9)133.09(12)132,72(17)131.8
a This work. b Ref. [85]. c Ref. [86]. d Averaged over four independent molecules in the asymmetric unit.
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Barra, V.C.; Bernhardt, E.; Fellinger, S.; Jenne, C.; Langenbach, S.S. Direct Synthesis of C-Substituted [RC(O)CH2-CB11H11]− Carborate Anions. Inorganics 2024, 12, 173. https://doi.org/10.3390/inorganics12060173

AMA Style

Barra VC, Bernhardt E, Fellinger S, Jenne C, Langenbach SS. Direct Synthesis of C-Substituted [RC(O)CH2-CB11H11]− Carborate Anions. Inorganics. 2024; 12(6):173. https://doi.org/10.3390/inorganics12060173

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Barra, Vanessa C., Eduard Bernhardt, Sarah Fellinger, Carsten Jenne, and Shiomi S. Langenbach. 2024. "Direct Synthesis of C-Substituted [RC(O)CH2-CB11H11]− Carborate Anions" Inorganics 12, no. 6: 173. https://doi.org/10.3390/inorganics12060173

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