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Novel approach to boron-10 enriched decaborane(14): an important advance in synthetic boron hydride chemistry
Inorganic Chemistry Communications 5 (2002) 765–767 www.elsevier.com/locate/inoche
Novel approach to boron-10 enriched decaborane(14): an important advance in synthetic boron hydride chemistry Luqman Adams a, Susan Tomlinson a, Jianhui Wang a,b, Sumathy N. Hosmane a, John A. Maguire b, Narayan S. Hosmane a,* a
Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA b Department of Chemistry, Southern Methodist University, Dallas, TX 7527-0314, USA Received 18 June 2002; accepted 05 July 2002
Abstract The oxidative polyhedral cage-fusion reaction of the sodium or lithium salt of the B-10 enriched pentaborate anion, ½10 B5 H8 , with anhydrous FeCl3 , NiCl2 , Br2 , or C5 H11 Br, followed by room temperature sublimation of the product, produced 10 B10 H14 (3) in 52–65% yields. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Boron-10 enriched; Pentaborane(9); Decaborane(14); Anti-B18 H22 , Synthesis, BNCT; Boron hydrides;
Some of the principal bottlenecks in the study of the chemistry of both the small- and large-cage boranes and carboranes have been the restricted supply and high price of the boron hydride starting materials, such as B5 H9 ; B10 H14 , etc. [1,2]. Some of these, such as B4 H10 , and B5 H9 , are prepared by hot–cold gas phase reactor techniques or thermolysis reactions [3–8]. All of these methods suffer from some experimental drawbacks, not the least of which is dealing with large quantities of hazardous B2 H6 at elevated temperatures and high pressures. With these concerns in mind, we have recently developed a convenient, safe and practical route to prepare either natural or boron-10 enriched pentaborane (B5 H9 Þ [9]. Since the synthesis can be successfully done in heavy mineral oil, it makes this method most attractive, not only to the laboratories that are involved in the chemistry of small cage C2 B4 carboranes and metallacarboranes, but also to those who work with large-cage (C2 B9 Þ carborane systems. The convenient and safe reaction conditions of this new method now make it possible to routinely prepare large-cage boranes using B5 H9 as the starting material. *
Corresponding author. Tel.: +1-815-753-3556; fax: +1-815-7534802. E-mail address: [email protected] (N.S. Hosmane).
10
B-NMR
The importance of B10 H14 as a synthon for the production of both large-cage boranes and carboranes is well established in the literature [10]. The corresponding 10 B-enriched species can be used as precursors for a number of potential boron drugs involved in boron neutron capture therapy (BNCT). Although the natural B10 H14 is commercially available [11], the corresponding 10 B-enriched species is difficult to make, and both compounds are well known as some of the most expensive boron hydrides on the market; even the widespread use of this compound has not resulted in a reduction in price. It was this incentive that prompted us to seek an alternative, safer laboratory method for the preparation of decaborane(14). Herein, we report an unprecedented one-pot synthesis of boron-10 enriched decaborane(14) under mild reaction conditions that could lead to the routine synthesis of this compound. We believe that this report constitutes an important synthetic advance in the chemistry of boron hydrides. Boron-10 enriched pentaborane (1), prepared by the reaction of Na½10 B3 H8 3ðC4 H8 O2 Þ with NiCl2 in heavy mineral oil [9], was converted to solid Na10 B5 H8 (or Li10 B5 H8 Þ (2) [12] and then immediately subjected to redox reactions with metal reagents, FeCl3 or NiCl2 , in benzene at room temperature or with Br2 in dioxane at )78 °C, or with bromopentane in dioxane at 100 °C to
1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 7 0 0 3 ( 0 2 ) 0 0 5 5 3 - 1
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L. Adams et al. / Inorganic Chemistry Communications 5 (2002) 765–767
Scheme 1. Synthesis of nido-10 B10 H14 and anti-10 B18 H22 from
produce the corresponding 10 B-enriched decaborane, 10 B10 H14 (3), in 52–65% yields, as shown in Scheme 1 [13,14]. It is important to note that neither Na10 B5 H8 nor Li10 B5 H8 (2) are stable at room temperature in solution but dimerize quickly to form Na10 B9 H14 or Li10 B9 H14 that have a tendency to form anti-10 B18 H22 with oxidizing agents under similar reaction conditions shown in Scheme 1. Therefore, immediate reaction of the [B5 H8 is critical for the formation of 3, The higher yields of the decaborane resulting from Na10 B5 H8 is probably due to its greater stability when compared to the corresponding lithium salt. Nevertheless, the reactions shown in Scheme 1 are the most convenient and straightforward method for the synthesis of both the
10
B5 H9 .
natural and B-10 enriched decaborane(14). Compound 3 was the only solid borane among the products that is volatile at room temperature and, hence, it can be easily isolated and purified by repeated vacuum sublimations. Since the natural analogue of decaborane(14) is a wellcharacterized borane species [15], it is necessary only to point out the unique spectroscopic features of 3. Although the chemical shifts in 10 B-NMR spectra of 3 (see Fig. 1) are almost identical to those found in the 11 BNMR spectra, the most significant difference is found in their coupling constants. For example, the 10 B 1 H coupling constants (1 J) of 43–55 Hz in 3 are significantly smaller than the 146–160 Hz observed in the 11 B-NMR spectra of its natural analogue [15]. A similar magnitude
Fig. 1. Proton-coupled boron-10 NMR of nido-10 B10 H14 .
L. Adams et al. / Inorganic Chemistry Communications 5 (2002) 765–767
of coupling constants was observed in the 10 B-enriched pentaborane(9) [9]. There are no noteworthy features in the IR spectrum of 3 that deserve special comments. The synthetic route presented in Scheme 1 is of special interest in that the 10 B-enriched decaborane(14) can be prepared in sufficient quantities in a laboratory settings so as to provide ready access to this most expensive and valuable borane reagent, thereby allowing its use in preparing boron cage compounds that are currently being investigated in BNCT for cancer treatment. Detailed investigations on the related boron hydrides, carboranes, and metallacarboranes of C2 B10 -cage systems are currently underway in our laboratories.
Acknowledgements This work was supported by grants from the National Science Foundation (CHE-9988045), the Robert A. Welch Foundation (N-1322), the donors of the Petroleum Research Fund, administered by the American Chemical Society, and Northern Illinois University through Presidential Research Professorship.
References [1] T. Onak, S.G. Shore, M. Yamauchi, in: Gmelin Handbuch der Anorganischen Chemie, vol. 52, Springer-Verlag, Berlin, 1978, pp. 206–280 (Chapter 6). [2] T. Onak, S.G. Shore, M. Yamauchi, in: Gmelin Handbuch der Anorganischen Chemie, vol. 54, Springer-Verlag, Berlin, 1979, pp. 1–4 (Chapter 1). [3] M.J. Klein, B.C. Harrsion, I. Solomon, J. Am. Chem. Soc. 80 (1958) 4149–4151. [4] W.V. Kotlensky, R.J. Shaeffer, J. Am. Chem. Soc. 80 (1958) 4517– 4519. [5] R. Schaeffer, F. Tebbe, J. Am. Chem. Soc. 84 (1962) 3974–3975. [6] D.F. Gaines, R. Schaeffer, Inorg. Chem. 3 (1964) 438–440. [7] A.C. Bond, M.L. Pinsky, J. Am. Chem. Soc. 92 (1970) 32–36. [8] R.J. Remmel, H.D. Johnson II, I.S. Jaworiwsky, S.G. Shore, J. Am. Chem. Soc. 97 (1975) 5395–5403. [9] L.A. Adams, S.N. Hosmane, J.E. Eklund, J. Wang, N.S. Hosmane, J. Am. Chem. Soc. 124 (2002) 7292–7293. [10] M.A. Toft, J.B. Leach, F.L. Himpsl, S.G. Shore, Inorg. Chem. 21 (1982) 1952–1957. [11] Natural decaborane (B10 H14 ) are commercially available by KATCHEM, Czech Republic. [12] Synthesis of Li10 B5 H8 (or Na10 B5 H8 ) (2). A 11.83 mmol (0.70 g) sample of 10 B5 H9 was condensed into a 250 ml flask containing 11.85 mmol t-BuLi (6.97 ml of 1.7 M in n-hexane) or NaH (0.29 g), 24 ml THF, and a magnetic stirring bar. The resulting solution was stirred constantly at )78 °C for 3 h and then slowly warmed to room temperature. At this point, the reaction mixture was quickly filtered and the solvents from the filtrate were removed in vacuo, and the resulting solid was washed with n-hexane, then dried under vacuum to collect an off-white solid, identified as Li10 B5 H8 (0.61 g, 9.46 mmol) or Na10 B5 H8 (0.84 g, 10.41 mmol), in 79 or 88 % yield. [13] Synthesis of 10 B-enriched 10 B10 H14 : (a) with NiCl2 oxidizing agent: A 20 ml anhydrous benzene was transferred to the flask containing Li10 B5 H8 (0.61 g, 9.46 mmol) or Na10 B5 H8 (0. 84 g, 10.41 mmol)
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and the resulting mixture was quickly poured onto anhydrous NiCl2 (1.46 g, 11.42 mmol) in15 ml dry benzene at 0 °C in a reaction flask equipped with a magnetic stirring bar. The resulting heterogeneous mixture was stirred for 4 h at this temperature. All of the volatiles including the solvent were removed in vacuo at 0 °C to collect a dark-brown solid. The room temperature sublimation of the residue in vacuo over a period of 4–5 h, gave a colorless crystalline solid, identified as 10 B10 H14 (3) (mp ¼ 99–100 °C), that was collected in a detachable U-trap held at 0 °C [57% yield (0.31 g, 2.70 mmol) or 62% (0.37 g, 3.23 mmol)] based on the lithium or sodium salt consumed. The dark residue in the flask, containing metallic nickel, LiCl or NaCl, and some polymeric solid, was discarded. (b) with FeCl3 oxidizing agent: In a procedure, identical to that described above, 9.00 mmol or 10.16 mmol sample of Li10 B5 H8 (0.58 g) or Na10 B5 H8 (0. 82 g) was allowed to react with 11.40 mmol of FeCl3 (1.85 g, 11.40 mmol) to produce 2.70 mmol (0.32 g) or 3.38 mmol (0.39 g) of 3 (mp ¼ 98–99 °C, collected in a detachable U-trap held at 0 °C) in 60 or 65% yield. A trace quantity of anti-10 B18 H22 (identified, but not measured) was also obtained when the residue was sublimed at 100 °C in vacuo. The dark residue in the flask, containing metallic iron, LiCl or NaCl along with some polymeric solid, was discarded. (c) with C5 H11 Br oxidizing agent: A 100 ml two-necked highvacuum flask was charged with Li10 B5 H8 (0.58 g, 9.00 mmol) or Na10 B5 H8 (0.80 g, 10.16 mmol), magnetic stirring bar and 20 ml anhydrous dioxane in a dry box and then attached to a vacuum/ Schlenk line. A condenser and a dropping funnel, containing C5 H11 Br (1.59 g, 10.50 mmol) in 15 ml dry dioxane, were attached to this flask under N2 . The mixture in the flask was first heated to 100 °C and then the dioxane solution of C5 H11 Br was added dropwise into the flask with constant stirring over a period of 2 h. The flask was then cooled to room temperature, and then attached to a high vacuum line. All of the volatiles, including the solvent, were removed in vacuo at 0 °C to collect a yellow residue that was later sublimed at room temperature in vacuo over a period of 4–5 h to isolate a colorless crystalline solid of 3 in a detachable U-trap held at 0 °C in 53% (0.27 g, 2.39 mmol) or 60% (0.35 g, 3.05 mmol) yield based on the lithium or sodium salt. The dark residue in the flask, containing LiBr or NaBr, and some polymeric solid, was discarded. (d) with Br2 oxidizing agent: A 100 ml two-necked high-vacuum flask was charged with Li10 B5 H8 (0.58 g, 9.00 mmol) or Na10 B5 H8 (0.80 g, 10.16 mmol), a magnetic stirring bar and 20 ml dry dioxane in a drybox which was then attached to a vacuum/Schlenk line. A dropping funnel containing Br2 (1.59 g, 10.50 mmol) in 15 ml anhydrous dioxane was attached to the reaction flask under N2 . The mixture in the flask was cooled to )78 °C and then the Br2 solution was added drop-wise into the flask with constant stirring. After complete addition, the reaction mixture was stirred for an additional hour at this temperature and for 4 h at room temperature. At this point, the flask was attached to a highvacuum line and all of the volatiles were removed from the flask at 0 °C to collect a yellow residue that was later sublimed in vacuo at room temperature over a period of 4–5 h to isolate 3 in a detachable U-trap held at 0 °C in 52% (0.27 g, 2.34 mmol) or 60% (0.35 g, 3.05 mmol) yield based on Li or Na salt. [14] The spectroscopic and analytical data for 3: 1 H NMR (C6 D6 , relative to external Me4 Si) d 4.05 [overlapping heptet, 6 H], 0.67 [heptet, 4 H, 1 J ð1 H–10 BÞ ¼ 52:25 Hz], )2.09 [s, 4 H, bridge H]; 10 B NMR (C6 D6 , relative to external BF3 OEt2 ) d 11.45 [d, 2B, 1 J ð10 B–1 HÞ ¼ 46:3 Hz], 9.73 [d, 2B, 1 J ð10 B–1 HÞ ¼ 43:2 Hz], 0.00 [d, 4B, 1 J ð10 B–1 HÞ ¼ 54:9 Hz], )36.6 [d, 2B, 1 J ð10 B–1 HÞ ¼ 52:6 Hz]. IR (KBr pellet, cm1 ) spectral data: 2609 (s, s), 2580 (s, s), 2566 (s, br), 2529 (br, sh), [m(B–H)]; Anal. Calcd for 10 B10 H14 : B, 87.63; H, 12.37. Found: B, 87.64; H, 12.34. [15] R.E. Williams, Inorg. Chem. 4 (1965) 1504–1508.
Novel approach to boron-10 enriched decaborane(14): an important advance in synthetic boron hydride chemistry Luqman Adams a, Susan Tomlinson a, Jianhui Wang a,b, Sumathy N. Hosmane a, John A. Maguire b, Narayan S. Hosmane a,* a
Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA b Department of Chemistry, Southern Methodist University, Dallas, TX 7527-0314, USA Received 18 June 2002; accepted 05 July 2002
Abstract The oxidative polyhedral cage-fusion reaction of the sodium or lithium salt of the B-10 enriched pentaborate anion, ½10 B5 H8 , with anhydrous FeCl3 , NiCl2 , Br2 , or C5 H11 Br, followed by room temperature sublimation of the product, produced 10 B10 H14 (3) in 52–65% yields. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Boron-10 enriched; Pentaborane(9); Decaborane(14); Anti-B18 H22 , Synthesis, BNCT; Boron hydrides;
Some of the principal bottlenecks in the study of the chemistry of both the small- and large-cage boranes and carboranes have been the restricted supply and high price of the boron hydride starting materials, such as B5 H9 ; B10 H14 , etc. [1,2]. Some of these, such as B4 H10 , and B5 H9 , are prepared by hot–cold gas phase reactor techniques or thermolysis reactions [3–8]. All of these methods suffer from some experimental drawbacks, not the least of which is dealing with large quantities of hazardous B2 H6 at elevated temperatures and high pressures. With these concerns in mind, we have recently developed a convenient, safe and practical route to prepare either natural or boron-10 enriched pentaborane (B5 H9 Þ [9]. Since the synthesis can be successfully done in heavy mineral oil, it makes this method most attractive, not only to the laboratories that are involved in the chemistry of small cage C2 B4 carboranes and metallacarboranes, but also to those who work with large-cage (C2 B9 Þ carborane systems. The convenient and safe reaction conditions of this new method now make it possible to routinely prepare large-cage boranes using B5 H9 as the starting material. *
Corresponding author. Tel.: +1-815-753-3556; fax: +1-815-7534802. E-mail address: [email protected] (N.S. Hosmane).
10
B-NMR
The importance of B10 H14 as a synthon for the production of both large-cage boranes and carboranes is well established in the literature [10]. The corresponding 10 B-enriched species can be used as precursors for a number of potential boron drugs involved in boron neutron capture therapy (BNCT). Although the natural B10 H14 is commercially available [11], the corresponding 10 B-enriched species is difficult to make, and both compounds are well known as some of the most expensive boron hydrides on the market; even the widespread use of this compound has not resulted in a reduction in price. It was this incentive that prompted us to seek an alternative, safer laboratory method for the preparation of decaborane(14). Herein, we report an unprecedented one-pot synthesis of boron-10 enriched decaborane(14) under mild reaction conditions that could lead to the routine synthesis of this compound. We believe that this report constitutes an important synthetic advance in the chemistry of boron hydrides. Boron-10 enriched pentaborane (1), prepared by the reaction of Na½10 B3 H8 3ðC4 H8 O2 Þ with NiCl2 in heavy mineral oil [9], was converted to solid Na10 B5 H8 (or Li10 B5 H8 Þ (2) [12] and then immediately subjected to redox reactions with metal reagents, FeCl3 or NiCl2 , in benzene at room temperature or with Br2 in dioxane at )78 °C, or with bromopentane in dioxane at 100 °C to
1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 7 0 0 3 ( 0 2 ) 0 0 5 5 3 - 1
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L. Adams et al. / Inorganic Chemistry Communications 5 (2002) 765–767
Scheme 1. Synthesis of nido-10 B10 H14 and anti-10 B18 H22 from
produce the corresponding 10 B-enriched decaborane, 10 B10 H14 (3), in 52–65% yields, as shown in Scheme 1 [13,14]. It is important to note that neither Na10 B5 H8 nor Li10 B5 H8 (2) are stable at room temperature in solution but dimerize quickly to form Na10 B9 H14 or Li10 B9 H14 that have a tendency to form anti-10 B18 H22 with oxidizing agents under similar reaction conditions shown in Scheme 1. Therefore, immediate reaction of the [B5 H8 is critical for the formation of 3, The higher yields of the decaborane resulting from Na10 B5 H8 is probably due to its greater stability when compared to the corresponding lithium salt. Nevertheless, the reactions shown in Scheme 1 are the most convenient and straightforward method for the synthesis of both the
10
B5 H9 .
natural and B-10 enriched decaborane(14). Compound 3 was the only solid borane among the products that is volatile at room temperature and, hence, it can be easily isolated and purified by repeated vacuum sublimations. Since the natural analogue of decaborane(14) is a wellcharacterized borane species [15], it is necessary only to point out the unique spectroscopic features of 3. Although the chemical shifts in 10 B-NMR spectra of 3 (see Fig. 1) are almost identical to those found in the 11 BNMR spectra, the most significant difference is found in their coupling constants. For example, the 10 B 1 H coupling constants (1 J) of 43–55 Hz in 3 are significantly smaller than the 146–160 Hz observed in the 11 B-NMR spectra of its natural analogue [15]. A similar magnitude
Fig. 1. Proton-coupled boron-10 NMR of nido-10 B10 H14 .
L. Adams et al. / Inorganic Chemistry Communications 5 (2002) 765–767
of coupling constants was observed in the 10 B-enriched pentaborane(9) [9]. There are no noteworthy features in the IR spectrum of 3 that deserve special comments. The synthetic route presented in Scheme 1 is of special interest in that the 10 B-enriched decaborane(14) can be prepared in sufficient quantities in a laboratory settings so as to provide ready access to this most expensive and valuable borane reagent, thereby allowing its use in preparing boron cage compounds that are currently being investigated in BNCT for cancer treatment. Detailed investigations on the related boron hydrides, carboranes, and metallacarboranes of C2 B10 -cage systems are currently underway in our laboratories.
Acknowledgements This work was supported by grants from the National Science Foundation (CHE-9988045), the Robert A. Welch Foundation (N-1322), the donors of the Petroleum Research Fund, administered by the American Chemical Society, and Northern Illinois University through Presidential Research Professorship.
References [1] T. Onak, S.G. Shore, M. Yamauchi, in: Gmelin Handbuch der Anorganischen Chemie, vol. 52, Springer-Verlag, Berlin, 1978, pp. 206–280 (Chapter 6). [2] T. Onak, S.G. Shore, M. Yamauchi, in: Gmelin Handbuch der Anorganischen Chemie, vol. 54, Springer-Verlag, Berlin, 1979, pp. 1–4 (Chapter 1). [3] M.J. Klein, B.C. Harrsion, I. Solomon, J. Am. Chem. Soc. 80 (1958) 4149–4151. [4] W.V. Kotlensky, R.J. Shaeffer, J. Am. Chem. Soc. 80 (1958) 4517– 4519. [5] R. Schaeffer, F. Tebbe, J. Am. Chem. Soc. 84 (1962) 3974–3975. [6] D.F. Gaines, R. Schaeffer, Inorg. Chem. 3 (1964) 438–440. [7] A.C. Bond, M.L. Pinsky, J. Am. Chem. Soc. 92 (1970) 32–36. [8] R.J. Remmel, H.D. Johnson II, I.S. Jaworiwsky, S.G. Shore, J. Am. Chem. Soc. 97 (1975) 5395–5403. [9] L.A. Adams, S.N. Hosmane, J.E. Eklund, J. Wang, N.S. Hosmane, J. Am. Chem. Soc. 124 (2002) 7292–7293. [10] M.A. Toft, J.B. Leach, F.L. Himpsl, S.G. Shore, Inorg. Chem. 21 (1982) 1952–1957. [11] Natural decaborane (B10 H14 ) are commercially available by KATCHEM, Czech Republic. [12] Synthesis of Li10 B5 H8 (or Na10 B5 H8 ) (2). A 11.83 mmol (0.70 g) sample of 10 B5 H9 was condensed into a 250 ml flask containing 11.85 mmol t-BuLi (6.97 ml of 1.7 M in n-hexane) or NaH (0.29 g), 24 ml THF, and a magnetic stirring bar. The resulting solution was stirred constantly at )78 °C for 3 h and then slowly warmed to room temperature. At this point, the reaction mixture was quickly filtered and the solvents from the filtrate were removed in vacuo, and the resulting solid was washed with n-hexane, then dried under vacuum to collect an off-white solid, identified as Li10 B5 H8 (0.61 g, 9.46 mmol) or Na10 B5 H8 (0.84 g, 10.41 mmol), in 79 or 88 % yield. [13] Synthesis of 10 B-enriched 10 B10 H14 : (a) with NiCl2 oxidizing agent: A 20 ml anhydrous benzene was transferred to the flask containing Li10 B5 H8 (0.61 g, 9.46 mmol) or Na10 B5 H8 (0. 84 g, 10.41 mmol)
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and the resulting mixture was quickly poured onto anhydrous NiCl2 (1.46 g, 11.42 mmol) in15 ml dry benzene at 0 °C in a reaction flask equipped with a magnetic stirring bar. The resulting heterogeneous mixture was stirred for 4 h at this temperature. All of the volatiles including the solvent were removed in vacuo at 0 °C to collect a dark-brown solid. The room temperature sublimation of the residue in vacuo over a period of 4–5 h, gave a colorless crystalline solid, identified as 10 B10 H14 (3) (mp ¼ 99–100 °C), that was collected in a detachable U-trap held at 0 °C [57% yield (0.31 g, 2.70 mmol) or 62% (0.37 g, 3.23 mmol)] based on the lithium or sodium salt consumed. The dark residue in the flask, containing metallic nickel, LiCl or NaCl, and some polymeric solid, was discarded. (b) with FeCl3 oxidizing agent: In a procedure, identical to that described above, 9.00 mmol or 10.16 mmol sample of Li10 B5 H8 (0.58 g) or Na10 B5 H8 (0. 82 g) was allowed to react with 11.40 mmol of FeCl3 (1.85 g, 11.40 mmol) to produce 2.70 mmol (0.32 g) or 3.38 mmol (0.39 g) of 3 (mp ¼ 98–99 °C, collected in a detachable U-trap held at 0 °C) in 60 or 65% yield. A trace quantity of anti-10 B18 H22 (identified, but not measured) was also obtained when the residue was sublimed at 100 °C in vacuo. The dark residue in the flask, containing metallic iron, LiCl or NaCl along with some polymeric solid, was discarded. (c) with C5 H11 Br oxidizing agent: A 100 ml two-necked highvacuum flask was charged with Li10 B5 H8 (0.58 g, 9.00 mmol) or Na10 B5 H8 (0.80 g, 10.16 mmol), magnetic stirring bar and 20 ml anhydrous dioxane in a dry box and then attached to a vacuum/ Schlenk line. A condenser and a dropping funnel, containing C5 H11 Br (1.59 g, 10.50 mmol) in 15 ml dry dioxane, were attached to this flask under N2 . The mixture in the flask was first heated to 100 °C and then the dioxane solution of C5 H11 Br was added dropwise into the flask with constant stirring over a period of 2 h. The flask was then cooled to room temperature, and then attached to a high vacuum line. All of the volatiles, including the solvent, were removed in vacuo at 0 °C to collect a yellow residue that was later sublimed at room temperature in vacuo over a period of 4–5 h to isolate a colorless crystalline solid of 3 in a detachable U-trap held at 0 °C in 53% (0.27 g, 2.39 mmol) or 60% (0.35 g, 3.05 mmol) yield based on the lithium or sodium salt. The dark residue in the flask, containing LiBr or NaBr, and some polymeric solid, was discarded. (d) with Br2 oxidizing agent: A 100 ml two-necked high-vacuum flask was charged with Li10 B5 H8 (0.58 g, 9.00 mmol) or Na10 B5 H8 (0.80 g, 10.16 mmol), a magnetic stirring bar and 20 ml dry dioxane in a drybox which was then attached to a vacuum/Schlenk line. A dropping funnel containing Br2 (1.59 g, 10.50 mmol) in 15 ml anhydrous dioxane was attached to the reaction flask under N2 . The mixture in the flask was cooled to )78 °C and then the Br2 solution was added drop-wise into the flask with constant stirring. After complete addition, the reaction mixture was stirred for an additional hour at this temperature and for 4 h at room temperature. At this point, the flask was attached to a highvacuum line and all of the volatiles were removed from the flask at 0 °C to collect a yellow residue that was later sublimed in vacuo at room temperature over a period of 4–5 h to isolate 3 in a detachable U-trap held at 0 °C in 52% (0.27 g, 2.34 mmol) or 60% (0.35 g, 3.05 mmol) yield based on Li or Na salt. [14] The spectroscopic and analytical data for 3: 1 H NMR (C6 D6 , relative to external Me4 Si) d 4.05 [overlapping heptet, 6 H], 0.67 [heptet, 4 H, 1 J ð1 H–10 BÞ ¼ 52:25 Hz], )2.09 [s, 4 H, bridge H]; 10 B NMR (C6 D6 , relative to external BF3 OEt2 ) d 11.45 [d, 2B, 1 J ð10 B–1 HÞ ¼ 46:3 Hz], 9.73 [d, 2B, 1 J ð10 B–1 HÞ ¼ 43:2 Hz], 0.00 [d, 4B, 1 J ð10 B–1 HÞ ¼ 54:9 Hz], )36.6 [d, 2B, 1 J ð10 B–1 HÞ ¼ 52:6 Hz]. IR (KBr pellet, cm1 ) spectral data: 2609 (s, s), 2580 (s, s), 2566 (s, br), 2529 (br, sh), [m(B–H)]; Anal. Calcd for 10 B10 H14 : B, 87.63; H, 12.37. Found: B, 87.64; H, 12.34. [15] R.E. Williams, Inorg. Chem. 4 (1965) 1504–1508.