Simple solution-phase syntheses of tetrahalodiboranes(4) and their labile dimethylsulfide adducts

Merle Arrowsmith ab, Julian Böhnke ab, Holger Braunschweig *ab, Andrea Deißenberger ab, Rian D. Dewhurst ab, William C. Ewing a, Christian Hörl a, Jan Mies a and Jonas H. Muessig ab
aInstitute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. E-mail:
bInstitute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany

Received 24th April 2017 , Accepted 25th May 2017

First published on 31st May 2017

Convenient solution-phase syntheses of tetrahalodiboranes(4) B2F4, B2Cl4 and B2I4 are presented herein from common precursor B2Br4. In addition, the dimethylsulfide adducts B2Cl4(SMe2)2 and B2Br4(SMe2)2 are conveniently prepared in one-step gram and multigram scale syntheses from the commercially-available starting material B2(NMe2)4. The results provide simple access to the full range of tetrahalodiboranes(4) for the exploration of their untapped synthetic potential.

Diboranes(4), compounds of the form B2X4, are of immense interest for organic synthesis, given their position as reagents of choice for installing boron functional groups into organic molecules for further synthetic transformations.1 Consequently, attention paid to the reactivity of diboranes(4) is focused overwhelmingly on the B–B bond, and the use of diboranes(4) with reactive B–X bonds is negligible in comparison.1c

Tetrahalodiboranes(4) (B2X4; X = halide) have been known since Stock's 1925 synthesis of B2Cl4 by reduction of BCl3 with a zinc arc discharge.2 With their highly reactive B–X bonds, tetrahalodiboranes(4) represent intriguing alternatives to the diboranes in frequent use in organic synthesis (e.g. dicatechol- and dipinacoldiborane(4)). However, tetrahalodiboranes(4) are little-used in contemporary main-group chemistry and their synthetic possibilities have only been cursorily explored. The gas-phase synthesis of a number of tetrahalodiboranes3 in the mid-20th century saw a flurry of research activity into their properties and reactivity. This peaked in the early 1960s, impelled in part by the discovery that tetrahalodiboranes(4) could be used as reagents for the diboration of olefins and aromatic organic compounds (A, Fig. 1).4 The groups of Nöth5 and Haubold6 reported relatively convenient solution-phase syntheses of B2Br4 and B2I4, respectively, in the 1980s, and around the same time a range of unusual alkyne diboration reactions with B2Cl4 were reported by Siebert.7 However, the research interest in tetrahalodiboranes(4) never again reached the early heights attained in the 1960s.

image file: c7cc03148c-f1.tif
Fig. 1 Selected synthetic uses of tetrahalodiboranes(4) (L = neutral Lewis base; Cp = η5-C5H5).

Given the obvious synthetic value of tetrahalodiboranes(4) in a number of fields, and the high interest in these compounds in the 1950s and 1960s, the current non-commercial use of these reagents remains remarkably low, with fewer than five publications per year. In fact, over the past five years, the filing of patents involving tetrahalodiboranes(4) has outpaced their appearance in journal articles at a rate of two to one.8 In particular, B2F4 has attracted intense industrial interest over the past few years as a reagent for implantation of B+ ions into silicon for semiconductor device fabrication. B2F4 offers more facile ionization and fragmentation than BF3, the current primary feed gas for this application, while also being compatible with conventional ion implant platforms.9

In recent years, interest in the condensed-phase reactivity of tetrahalodiboranes(4) has come almost exclusively from our laboratories, where B2Br4 has been used as a precursor for Lewis base adducts and the construction of boron-boron multiple bonds (B, Fig. 1) by our group and that of Kinjo,1a,b,f,10 as well as precursors to bis(boratabenzene) ligands in transition metal complexes (C, Fig. 1).11 Beyond these applications, however, tetrahalodiboranes(4) offer a wide range of intriguing possibilities in organic, main-group, transition-metal, and materials chemistry that we have only begun to explore.

The hesitance of the scientific community to adopt these versatile reagents clearly stems from their synthetic routes, which are tedious, temperamental, and require special apparatus in some cases. Given these hurdles, we set out to develop simple, solution-phase syntheses of all four tetrahalodiboranes(4) (B2X4; X = F (1a), Cl (1b), Br (1c), I (1d)), in order to open up these reagents for widespread use. These reactions are presented herein, in addition to facile synthetic routes to tetrahalodiborane bis(dimethylsulfide) adducts (B2X4(SMe2)2; X = Cl (2b), Br (2c), I (2d)), two of which in a single step from commercially available precursor B2(NMe2)4. These adducts are more conveniently handled compounds that in many cases can play the same synthetic role as their tetrahalodiborane(4) analogues.

In our laboratories, Nöth's 1981 synthesis of B2Br4 (1c)5 from B2(OMe)4 using tribromoborane (Fig. 2) still provides the most convenient entry to tetrahalodiboranes(4), and reliable solution-phase routes exist to B2F4 (1a) and B2I4 (1d) from B2Cl4 (1b) from Schlesinger3d and Haubold,6 respectively. Although Timms was able to prepare B2Cl4 on a ca. 10 g scale by metal vapour deposition of molten copper with trichloroborane,12 we are aware of no convenient solution-phase route to the compound, making the synthesis of B2F4 and B2I4 difficult by extension. We reasoned that with judicious choice of reagents, we may be able to connect B2Br4 with the remaining tetrahalodiboranes 1a,b,d.

image file: c7cc03148c-f2.tif
Fig. 2 Old and new syntheses of tetrahalodiboranes(4).

Treatment of B2Br4 with 1.3 equiv. of either trifluorostibane, trichlorogallane, or triiodoborane, led to solutions of B2F4, B2Cl4 or B2I4, respectively. The lighter tetrahalodiboranes(4) B2F4 and B2Cl4 could be vacuum distilled from the heavy Sb/Ga salts, providing hexane solutions that can be used for further reactions by assuming complete conversion. In contrast, B2I4 was isolated as a colourless solid by vacuum distillation of the lighter byproducts out of the reaction mixture. The 11B NMR spectra of the tetrahalodiboranes(4) match the reported data,6 with a relatively high-field signal being observed for B2F4 (δB 22.1), and low-field signals for B2Cl4 (δB 61.9) and B2I4 (δB 70). A single resonance at δF −55 was also found in the 19F NMR spectrum of B2F4.

While these synthetic routes improve markedly upon the previous routes, and offer practical solution-phase conditions, the syntheses of 1a–c still comprise three- or four-step protocols from the commercially-available precursor B2(NMe2)4. This prompted us to seek a more direct route to the target compounds 1a–d. In general, treatment of B2(NMe2)4 with trihalotriels fails to produce 1a–d. However, we found that combining B2(NMe2)4 with excesses of the dimethylsulfide monoborane adducts BX3(SMe2) (X = Cl, Br) led to complete conversion to the bis-adducts B2X4(SMe2)2 (2b: X = Cl; 2c: X = Br; Fig. 3), which could be isolated as colourless solids in good yields by removal of monoborane byproducts ([BCl2(NMe2)]2 and BBrn(NMe2)3−n (n = 1, 2), respectively) by fractional crystallisation and vacuum distillation. The synthesis of the bis-adduct 2c could also be performed on a multi-gram scale, providing more than 18 g of product in 62% yield. Adducts 2b and 2c were identified by their sharp, high-field signals (2b: δB 7.3; 2c: δB −0.3), in addition to single-crystal X-ray diffractometry (vide infra, Fig. 4) and elemental analysis. It should be noted that 2b can also be prepared by addition of 2 equiv. of BCl3(SMe2) to B2Cl2(NMe2)2, or alternatively by simple addition of an excess of dimethylsulfide to B2Cl4 (Fig. 3). Likewise, 2c can be prepared by addition of excess dimethylsulfide to B2Br4.

image file: c7cc03148c-f3.tif
Fig. 3 Syntheses of dimethylsulfide adducts of tetrahalodiboranes(4).

image file: c7cc03148c-f4.tif
Fig. 4 Crystallographically-derived structures of 2b–d. Thermal ellipsoids set at the 50% probability level. Hydrogen atoms have been removed for clarity. Selected bond lengths [Å] and angles [°] for 2b: B–B 1.719(2), B–S 1.976(1), B–Cl 1.861(1), 1.868(1). For 2c: B–B 1.715(4), B–S 1.961(2), B–Br 2.026(2), 2.051(2). For 2d: B–B 1.714(7), B–S 1.955(4), B-I 2.250(3), 2.280(3).

Unfortunately, all attempts to prepare the iodo analogue B2I4(SMe2)2 (2d: X = I; Fig. 3) by a similarly direct route were unsuccessful. However, the tetraiodo derivative was found to be accessible by addition of dimethylsulfide to B2I4, providing 2d as colourless crystals in excellent yield. This compound showed an 11B NMR signal (δB −20) even further upfield of those of 2b and 2c.

The crystallographically-derived structures of 2b–d (Fig. 4) show equivalent B–B bond lengths within experimental error (2b: 1.719(2); 2c: 1.715(4); 2d: 1.714(7) Å), despite steadily increasing B–X distances (2b: B–Cl 1.861(1), 1.868(1); 2c: B–Br 2.026(2), 2.051(2); 2d: B–I 2.250(3), 2.280(3) Å) as the halides become larger. A moderate contraction of the B–S bonds is observed along this series (2b: B–S 1.976(1); 2c: B–S 1.961(2); 2d: B–S 1.955(4) Å), presumably due to lengthening B–X bonds and increasing Lewis acidity of the attached boron atoms.

The results presented herein provide convenient solution-phase access to all four tetrahalodiboranes, as well as one-step syntheses of B2Cl4(SMe2)2 and B2Br4(SMe2)2 from the commercially-available precursor B2(NMe2)4 (the latter being suitable on at least a multigram scale). We present these protocols in the hope that these reagents can find widespread use throughout the community, and their promising synthetic potential can be fully realised. We wish readers the best of luck in their synthetic endeavours.

This project was funded by the European Research Council (ERC) under the European Union Horizon 2020 Research and Innovation Program (grant agreement no. 669054, to H. B.), and the Alexander von Humboldt foundation (postdoctoral research fellowship to M. A.).

Notes and references

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Electronic supplementary information (ESI) available: Synthetic and crystallographic details. CCDC 1545629–1545631. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc03148c
Current address: Boron Specialties, 2301 Duss Ave, Ambridge PA 15003, USA.

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