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Synthesis, Reactivity, and Structural Characterization of a 14‐Vertex Carborane

2005; Wiley; Volume: 44; Issue: 14 Linguagem: Inglês

10.1002/anie.200462708

1521-3773

Liang Deng, Hoi‐Shan Chan, Zuowei Xie,

Radioactive element chemistry and processing

The largest known carborane, which has 14 vertices, was prepared by the reaction of arachno-carborane tetraanion 1 with HBBr2⋅SMe2 (see scheme). The reactivity of both 13- (2) and 14-vertex carboranes (3 and 4) has also been examined. The key to the successful preparation of supracarboranes is to block the redox reactions between carborane anions and RBX2 reagents. closo-Boranes and carboranes, which have received much attention, are among the simplest cluster systems; their chemistry is dominated by icosahedral molecules with 12 vertices.1 Carboranes with 13 vertices were unknown until 20032 although a series of metallacarboranes of s-, p-, d-, and f-block elements with 13 vertices have been prepared and structurally characterized3 since the first one was reported in 1971.4 A few metallacarboranes with 14 vertices are also known.5 Thus, the question arises as to why the development of supracarborane chemistry is well behind that of the metallacarboranes. One may attribute this fact to the thermodynamic difficulties that arise in building up large carboranes beyond the icosahedron, as suggested by the theoretical calculations on [BnHn]2− systems.6 We believe, on the basis of our own work, that the energies of BH (BR) group additions are not the major issue, rather, the reducing power of the parent nido-carborane dianions (nido-[R2C2B10H10]2−) is the key factor for the successful preparation of C2BnHn+2 when n>10. It has been well documented that "carbon-atoms-apart" (CAp) nido-[R2C2B10H10]2− ions are very strong reducing agents that can reduce M4+ (M=Group 4 metal atom) and Ln3+ (Ln=Sm, Eu, Yb) to the corresponding divalent species,7, 8 but are inert toward Group 1 metals.9 Conversely, "carbon-atoms-adjacent" (CAd) nido-[R2C2B10H10]2− ions do react with lithium metal to generate CAd arachno-[R2C2B10H10]4− ions,10 which suggests that CAp nido-[R2C2B10H10]2− ions are more powerful reducing agents than their CAd counterparts. In fact, there are two competitive reactions between nido-[R2C2B10H10]2− ions and R′BX2 (R′=H, aryl; X=halide): redox and capping reactions. Lowering the reducing power of nido-[R2C2B10H10]2− ions and the oxidizing ability of R′BX2 is critical for the preparation of supracarboranes. In this regard, we suggest that the isolation of the first carborane with 13 vertices2a should be attributed to the relatively weaker reducing power of CAd nido-[1,2-{o-C6H4(CH2)2}-1,2-C2B10H10]2−.10 We wondered if such a cage-opening and boron-insertion would be applicable to the CAd arachno-carborane tetraanions bearing both six- and five-membered bonding faces that were prepared recently in our laboratory.10 Such a methodology may lead to the formation of carboranes with 14 vertices in one reaction. Herein we report the synthesis, reactivity, and structural characterization of carboranes with 13 and 14 vertices. Treatment of 1,2-(CH2)3-1,2-C2B10H10 (1)11 with excess lithium metal in THF at room temperature gave {[(CH2)3C2B10H10][Li4(thf)5]}2 (2) in 85 % yield (Scheme 1). This reaction was closely monitored by the 11B NMR spectroscopy as 1 and 2 had distinct splitting patterns. A single-crystal X-ray analysis showed that 2 is a centrosymmetric dimer in which each CAd arachno-carborane tetraanion contains both hexagonal and pentagonal bonding faces (Figure 1).12 The structural motif is the same as in {[1,2-{o-C6H4(CH2)2}-1,2-C2B10H10][Li4(thf)6]}210 except for the bridging unit, which offers the possibility of adding two new vertices. Left: molecular structure of 2. Right: bonding interactions between Li+ ions and the open faces of 2. Selected bond lengths [Å]: C1C2 1.560(4), B12B7 1.898(5), B12B8 1.888(5), B7C1 1.625(4), B8C2 1.619(4), C1B6 1.640(4), B6B5 1.761(5), B5B4 1.768(5), B4B3 1.753(5), B3C2 1.635(4), av Li3–cage atom 2.257(6), av Li1–cage atom 2.298(6). Synthesis of closo-, nido-, and arachno-carboranes. The reaction of 2 with 2.5 equivalents of HBBr2⋅SMe2 in toluene at −78 to 25 °C gave, after chromatographic separation, a 13-vertex carborane (CH2)3C2B11H11 (3; 32 %), a 14-vertex carborane (CH2)3C2B12H12 (4 a; 7 %) and a 12-vertex species (1; 2 %; Scheme 1). Both 3 and 4 a were characterized by 1H, 13C, and 11B NMR spectroscopies as well as high-resolution mass spectrometry. The 11B NMR spectrum of 3 displayed a 1:5:5 splitting pattern whereas that of 4 a showed a 1:2:2:1:2:1:2:1 splitting pattern in the range 7.8 to −24.7 ppm. The unique resonance at −24.7 ppm could be attributed to the newly added BH vertex, which is less connected. The isolation of 1 and 3 indicated that HBBr2⋅SMe2 can oxidize 2 to nido-[(CH2)3C2B10H10]2− and finally to closo-(CH2)3C2B10H10 (1). The resulting nido-[(CH2)3C2B10H10]2− reacts with HBBr2⋅SMe2 to form a new 13-vertex carborane closo-(CH2)3C2B11H11 (3). These results also clearly showed that CAd arachno-carborane tetraanions can react with HBBr2⋅SMe2 to afford carboranes with 14 vertices. We are uncertain, however, if the two BH vertexes are added simultaneously or sequentially. It is assumed that the addition of the first BH vertex to 2 results in the formation of nido-[(CH2)3C2B11H11]2− ion, which can either host the second BH vertex to form 4 a or be oxidized by HBBr2⋅SMe2 to generate closo-carborane 3. Such an assumption prompted us to examine the property of new 13- and 14-vertex carboranes. Both 3 and 4 a are stable in air, in moisture, and in refluxing toluene as indicated by the 11B NMR spectroscopy, which suggests that they are thermodynamic products. Compounds 3 and 4 a react readily with excess sodium metal at room temperature to give the corresponding nido-carborane dianions, {nido-[(CH2)3C2B11H11][Na2(thf)4]}n (5) and presumably nido-[(CH2)3C2B12H12][Na2(thf)x], respectively. Treatment of 5 with 1.5 equivalents of HBBr2⋅SMe2 produced 4 a and 3 in 30 % and 12 % yield, respectively. The resulting nido-[(CH2)3C2B12H12][Na2(thf)x] reacted with HBBr2⋅SMe2 to afford a new 14-vertex carborane [closo-1,2-(CH2)3-1,2-C2B12H12] (4 b) in 37 % yield, which indicates that the redox reaction was favored (Scheme 1). These results show that 4 a and 4 b are not thermally interchangeable but 4 a can be converted into 4 b through a redox reaction. Compounds 4 a and 4 b have identical molecular masses but the splitting pattern of the 11B NMR spectrum of 4 b is much simpler than that of 4 a, which suggests that they are isomers and 4 b has a much higher symmetry than 4 a. X-ray analyses revealed that 5 is a coordination polymer in which [Na(thf)2]+ ions link nido-carborane cages to form zigzag polymeric chains (Figure 2).12 After careful examination of the cage structures in 2 and 5, we conclude that if the formation of 4 a proceeds by a stepwise mechanism, the first BH vertex adds to the open six-membered bonding face and the second BH vertex then adds to the five-membered bonding face to form 4 a, which has a long BC bond as depicted in Figure 3. The molecular structure of 3 is similar to that of the anion in 5, but the B2B2 A bond length of 3 is within the expected range for a normal BB bond and is the same as that previously reported for a carborane with 13 vertices.2 Compound 3 accepts two electrons from Na metal to break the B2B2A bond to form 5 Left: a portion of the infinite polymeric chains in 5. Right: structure of the anion in 5. Selected bond lengths [Å]: C1C1A 1.529(8), C1B2 1.557(6), B3B2 1.903(6), B3B2A 1.903(6), B2AC1A 1.557(6), B2⋅⋅⋅B2A 2.677(6). Proposed molecular structure of 4 a. Single-crystal X-ray analyses showed that 4 b is a bicapped hexagonal antiprism (Figure 4),12 with a geometry similar to that predicted for [B14H14]2− by computation.6c All twenty-four faces are triangulated, with B4 and B4A being seven-coordinated. The average separation of B4 from its connecting vertexes is 1.902(3) Å, the apical B4 is 0.84 Å above the hexagonal plane formed by C1, B3, B6, B7, B5A, and B2A, which is less than the corresponding distance of 0.94 Å observed in icosahedral o-carborane.13 The distance between the two hexagonal planes (1.51 Å) is comparable with that found in icosahedral o-carborane (1.50 Å).13 Molecular structure of 4 b. Selected bond lengths [Å]: C1C1A 1.599(3), B4C1 1.963(2), B4B3 1.911(3), B4B6 1.852(3), B4B7 1.910(3). In conclusion, we have demonstrated that two BH vertexes can be added to CAd arachno-carborane tetraanions in one reaction to give the first 14-vertex closo-carborane, the largest carborane presently known. The reactivity of the 13- and 14-vertex closo-carboranes has also been examined for the first time. This work suggests that the energies of BH group additions are overstated, and the key issue for the synthesis of supracarboranes is how to block the redox reactions between carborane anions and RBX2 reagents. It is anticipated that carboranes C2BnHn+2 with n>12 may be prepared as long as these redox reactions can be suppressed. 2: Finely cut Li metal (1.80 g, 200 mmol) was added to a solution of 1 (5.52 g, 30.0 mmol) in THF (100 mL)11, and the mixture was stirred at room temperature for one day to give a red solution. Removal of excess Li and THF yielded a pale-yellow solid. Recrystallization from THF afforded 2 as colorless crystals (14.6 g, 85 %). 1H NMR (300 MHz, [D5]pyridine): δ=3.64 (m, 20 H), 1.62 (m, 20 H, THF), 2.99 (m, 2 H), 2.84 (m, 2 H, CH2CH2CH2), 1.94 ppm (m, 2 H, CH2CH2CH2); 13C NMR (75 MHz, [D5]pyridine): δ=67.19, 25.15 (THF), 45.05 (CH2CH2CH2), 28.98 ppm (CH2CH2CH2), the cage carbons were not observed; 11B NMR (128 MHz, [D5]pyridine): δ=7.71 (2), 3.15 (2), −1.63 (3), −12.93 (1), −16.54 (1), −18.00 ppm (1); IR (KBr): =2510 (vs), 2427 (vs), 2355 cm−1 (s) (BH) ; elemental analysis calcd (%) for C21H48B10Li4O4 (2−THF): C 50.40, H 9.67; found: C 50.01, H 9.86. 3 and 4 a: HBBr2⋅SMe2 (1.0 M in dichloromethane, 75.0 mL, 75.0 mmol) at −78 °C was slowly added to a suspension of 2 (17.2 g, 30.0 mmol) in toluene (100 mL), and the mixture was stirred at this temperature for 1 h and at room temperature for a further 6 h. Removal of the precipitate by filtration and solvents by evaporation gave a brown sticky solid. Chromatographic separation (SiO2, 300–400 mesh, n-hexane) afforded 1 (0.12 g, 2 %), 3 (1.88 g, 32 %), and 4 a (0.44 g, 7 %) as white solids. 3: 1H NMR (300 MHz, CDCl3): δ=3.26 (t, J=7.5 Hz, 4 H, CH2CH2CH2), 2.18 ppm (m, 2 H, CH2CH2CH2); 13C NMR (100 MHz, CDCl3): δ=49.11 (CH2CH2CH2), 25.55 ppm (CH2CH2CH2), resonances from the cage carbon atoms were not observed; 11B NMR (128 MHz, CDCl3): δ=3.52 (1), 0.96 (5), −1.19 ppm (5); IR (KBr): =2570 cm−1 (vs) (BH); HRMS: calcd for [C5H17B11]+, m/z: 195.2457; found: 195.2455. For 4 a: 1H NMR (300 MHz, CDCl3): δ=3.15 (m, 4 H, CH2CH2CH2), 2.31 ppm (m, 2 H, CH2CH2CH2); 13C NMR (100 MHz, CDCl3): δ=40.73 (CH2CH2CH2), 25.88 ppm (CH2CH2CH2), resonances from the cage carbon atoms were not observed; 11B NMR (128 MHz, CDCl3): δ=7.80 (1), 5.64 (2), 2.87 (2), −4.23 (1), −6.39 (2), −9.29 (1), −12.34 (2), −24.73 ppm (1); IR (KBr): =2566 cm−1 (vs) (BH) ; HRMS: calcd for [C5H18B12]+, m/z: 208.2592; found: 208.2583. Treatment of 5 (3.20 g, 4.0 mmol) with HBBr2⋅SMe2 (6.0 mL of 1.0 M in dichloromethane, 6.0 mmol) in toluene (20 mL) by using the above procedure gave 3 (0.09 g, 12 %) and 4 a (0.26 g, 30 %). 4 b: Finely cut Na metal (0.20 g, 8.70 mmol) was added to a solution of 4 a (0.30 g, 1.44 mmol) in THF (10 mL) and the mixture was stirred at room temperature for one day to give a yellow solution. Removal of excess Na metal by filtration and THF by evaporation afforded a pale yellow solid, presumably nido-[(CH2)3C2B12H12][Na2(thf)x]. Toluene (10 mL) was then added to this solid to give a yellow suspension. HBBr2⋅SMe2 (3.0 mL of 1.0 M in dichloromethane, 3.0 mmol) was slowly added to the suspension at −78 °C, and the mixture was stirred at this temperature for 1 h, and then at room temperature for 6 h. Chromatographic separation gave 4 b as a white solid (0.11 g, 37 %). Single crystals suitable for X-ray analysis were obtained by recrystallization from n-hexane. 1H NMR (300 MHz, CDCl3): δ=2.89 (br, 4 H, CH2CH2CH2), 2.25 ppm (br, 2 H, CH2CH2CH2); 13C NMR (75 MHz, CDCl3): δ=42.96 (CH2CH2CH2), 28.86 ppm (CH2CH2CH2), the cage carbons were not observed; 11B NMR (128 MHz, CDCl3): δ=4.48 (2), 0.14 (2), −2.85 (4), −4.17 (2), −16.78 ppm (2); IR (KBr): =2555 cm−1 (vs) (BH); HRMS: calcd for [C5H18B12]+, m/z: 207.2629; found: 207.2636. 5: Finely cut Na metal (0.40 g, 17.4 mmol) was added to a solution of 3 (1.00 g, 5.10 mmol) in THF (10 mL) and the mixture was stirred at room temperature for one day to give a clear yellow solution. After removal of excess Na metal by filtration, the resulting yellow solution was concentrated to about 5 mL, to which 4 mL of toluene was added. 5 was obtained as colorless crystals from this solution after one week at room temperature (2.17 g, 80 %). 1H NMR (300 MHz, [D5]pyridine): δ=3.63 (m, 16 H), 1.59 (m, 16 H, THF), 2.46 (t, J=6.3 Hz, 4 H, CH2CH2CH2), 2.25 ppm (m, 2 H) (CH2CH2CH2); 13C NMR (100 MHz, [D5]pyridine): δ=67.15, 25.11 (THF), 42.93 (CH2CH2CH2), 29.24 ppm (CH2CH2CH2), the cage carbons were not observed; 11B NMR (128 MHz, [D5]pyridine): δ=9.01 (1), −14.67 (5), −24.64 ppm (5); IR (KBr): =2501 cm−1 (vs) (BH); elemental analysis calcd (%) for C17H41B11Na2O3 (5−THF): C 44.54, H 9.02; found: C 44.20, H 9.03.