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. 2025 Apr 24;64(18):8845–8850. doi: 10.1021/acs.inorgchem.5c00469

An Electrochemical Strategy for Chalcogenation of closo-Dodecaborate (B12H12)2– Anion

Tyler A Kerr †, Yessica A Nelson †, Nicholas A Bernier †, Alexander M Spokoyny †,#,*
PMCID: PMC12076541  PMID: 40272989

Abstract

graphic file with name ic5c00469_0005.jpg

Advancements in thermal neutron generation technologies within clinical environments have led to a renewed interest in developing boron-containing compounds for boron neutron capture therapy (BNCT). Previous syntheses of several key boron cluster-based therapeutics with clinical relevance are low-yielding and have complicated workup procedures. Using electrolytic methods, we report the in situ oxidation of pseudohalides, [SCN]− and [SeCN]−, to synthesize pseudohalogenated products, B12H11YCN2– (Y = S or Se). Further, these compounds can be reduced to their respective thiol or selenol, [B12H11SH]2– (BSH) or [B12H11SeH]2– (BSeH), which are exceedingly nucleophilic and able to form zwitterionic sulfonium and selenonium compounds using alkyl-based electrophiles. The newly reported preparation of BSH and BSeH provides an efficient and convenient route to the preparation of key chalcogenated boron cluster building blocks for the biomedical and materials science communities.

Short abstract

This work demonstrates the in situ oxidation of pseudohalides ([SCN]− and [SeCN]−) to generate pseudohalogenated boron clusters, B12H11YCN2− (Y = S or Se), and their subsequent reduction to nucleophilic thiol and selenol derivatives. These reactive intermediates facilitate the preparation of zwitterionic sulfonium and selenonium compounds, providing a streamlined and high-yielding approach to chalcogenated boron cluster derivatives.

Boron-containing compounds are one of two classes of molecular Trojan horses that have been studied in vivo for neutron-capture therapy and are the only candidates that have been tested in humans.1,2 Of these boron-containing compounds, boron clusters make up a significant area of research due to their synthetic versatility, stability, and high boron density.3,4 [B12H11SH]2– (BSH) emerged early as a candidate for BNCT5,6 and has been used in pilot clinical studies in humans as recently as 2011.7 Until recently, the need for a nuclear reactor drastically limited the production of neutrons for therapeutic purposes.8 Developments in the production of thermal and epithermal neutrons in the clinical setting have garnered renewed interest in BNCT applications and relevant boron-based compounds.9

BSH has been previously synthesized in relatively low yields, requiring tedious workups, as summarized in Scheme 1. With the increasing demand for BSH and few viable synthetic options, an improved synthesis for this valuable target is warranted. Closo-Dodecaborate has been shown to react with halogens and mixed halogens, forming anywhere from mono- to persubstituted species depending on reaction conditions.10,11 Pseudohalogens like (SCN)2 and (SeCN)2 were also shown to react with closo-dodecaborate, forming the corresponding chalcogenated boron clusters.12−14 Unlike molecular halogens, (SeCN)2 and (SCN)2 are not commercially available and are unstable under ambient conditions, making wide adoption of these synthetic approaches difficult.

Scheme 1. Top: Current Synthetic Routes for BSH from 1 (Na2B12H12);15−19 Bottom: Electrochemical Synthesis of 2a and 2b, and Subsequent Reduction to BSH and BSeH.

Scheme 1

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Recently, electrochemical techniques have emerged as scalable, tunable, and versatile tools in synthesis.20−24 With increasing access to user-friendly electrolytic methods, we sought to develop an electrosynthetic methodology to complement the current mild chemical reactions for boron cluster thiocyanation under aqueous conditions. In the process, we discovered that these electrochemical strategies are also competent for selenocyanation even in aqueous media, and the reduction of [B12H11SCN]2– to BSH can be achieved using mild conditions. A mention of nonaqueous electrochemical thio- and selenocyanation currently exists in a conference proceeding from 199725 and is cited in several subsequent review articles.26−28 Counter to Morris et al.,25 we show successful cleavage of the Se–C bond in [B12H11SeCN]2– under Birch reduction conditions and further report the nucleophilic addition of the fully reduced selenide and sulfide into alkyl bromides, forming sulfonium and selenonium compounds underpinning the highly nucleophilic nature of trianionic boron clusters containing exopolyhedral B–Y– (Y= S or Se) moieties.

Previous reports of boron cluster thiocyanation use a freshly prepared solution of the pseudohalogen13 or in situ chemical oxidation of [SCN]− in an aqueous solution.12 We elected to pursue electrochemical techniques, using an electrode rather than a chemical oxidant, according to Figure 1A. Initial cyclic voltammetric studies were performed on individual solutions of 5 mM NaSCN, KSeCN, and Na2B12H12 (1) in unbuffered 1 M NaClO4 (Figure 1B). Each showed irreversible oxidations with peak potentials of 1.05, 0.52, and ∼2.0 V vs SCE, respectively. Importantly, each pseudohalide salt oxidizes at a potential that is more negative than that of [B12H12]2– (∼2.0 V), indicating that under highly oxidizing conditions, the pseudohalides will be oxidized before the boron cluster.

Figure 1.

Figure 1

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(A) Oxidation of pseudohalide to pseudohalogen, which reacts with 1, forming pseudohalogenated products 2a and 2b. (B) Cyclic voltammograms of KSeCN (red), NaSCN (orange), and 1 (black) in unbuffered 1 M NaClO4 in H2O (gray) on a 3 mm glassy carbon disk electrode.

Leveraging the reactivity of the pseudohalogens with boron clusters (Figure 1A), we reasoned that the electrolytically generated (SCN)2 and (SeCN)2 would react with 1 to form the desired products 2a and 2b. Using a divided cell, solutions of 1 and NaSCN or KSeCN in water were electrolyzed at 100 mA constant current using a carbon cloth working electrode and copper wire counter electrode. The counter electrode compartment contained a 4 M solution of CuCl2 in 1 M KCl, and a Nafion membrane separated the solutions. The reactions were monitored by 11B NMR spectroscopy and showed complete conversion to single products after about three molar equivalents of electrons were passed per equivalent of 1. Products 2a and 2b were isolated in 90% and 82% yields, respectively. The 11B and 13C{1H} NMR spectra of 2a were consistent with the presence of a single exopolyhedral −SCN substituent on a closo-dodecaborate cluster.12 In the case of 2b, the product similarly showed a diagnostic 11B NMR spectrum for a singly substituted boron cluster. The 77Se NMR spectrum for this compound shows a broad resonance at −20 ppm with a fwhm (full width at half maximum) of 90 Hz, which is significantly broader than the 77Se resonance for KSeCN in D2O at –325 ppm with a fwhm of 4.5 Hz. The chemical shift of the 2b is upfield shifted compared to other selenocyanates with PhSeCN found at 328 ppm.29 An attempt was made to further substitute 1 with more than one -SCN substituent by treating it with 36 mol equiv of NaSCN and electrolyzing with 72 mol equiv of electrons, and 11B NMR spectra of the reaction mixture suggested the presence of a mixture of ortho-, meta-, and para-disubstituted [B12H10(SCN)2]2– (Figures S55–S56). A small amount of para-substituted isomer was recrystallized from the reaction mixture, and a single crystal X-ray structure is shown in Figure S95.

Reductions of alkyl and aryl thiocyanates have previously been shown to convert to the corresponding thiols using strong hydride donors30 and other strong reducing conditions, including the classical Birch reduction conditions with elemental sodium in liquid ammonia.31 More recently, phosphorus pentasulfide (P2S5) was shown to facilitate the reduction of thiocyanates under mild conditions.32 Thus, we explored the reduction of 2a and 2b using both P2S5 and Birch reduction conditions. Using P2S5, we were able to show quantitative conversion of 2a to 3a on a 100 mg scale under microwave irradiation. Characterization of BSH product by 11B{1H} NMR spectroscopy in D2O showed resonances at −8.8, −13.3, −15.5, and −19.1 ppm, consistent with previous literature.33 The mechanism of the conversion of thiocyanates to thiols using P2S5 is thought to involve the formation of a dithiocarbamate, which hydrolyzes on aqueous workup to form the thiol and thiocarbamic acid. In our attempts to perform the same reaction on the selenocyanated boron cluster, the desired 3b product was not formed. 13C NMR spectroscopy suggested the selenothiocarbamate intermediate that is formed in the reaction with P2S5 remained present after aqueous workup.

While using P2S5 is an effective method for the reduction of a thiocyanate, differing conditions were deemed necessary to reduce 2b. We found that sodium metal in liquid ammonia was a general method of decyanation of both the thiocyanate and the selenocyanate (Figure 2A). After condensation of liquid ammonia using a dry ice/acetone-filled condenser into a flask containing 2b cooled by a dry ice/acetone bath, pieces of sodium metal were added until the solution remained a deep blue color (∼3 equiv). After 2 h, the mixture was quenched by dropwise addition of a CsCl solution in water and allowed to warm to room temperature, affording a light gray powder, which was filtered in air. After filtration, the powder showed a color change to light orange. 11B{1H} NMR spectroscopy of this compound (Figure 2B) showed predominantly a 1-substituted boron cluster that is distinct from the 2b with a minor impurity. We believe the potently oxygen-sensitive product formed under Birch reduction conditions34 is 4b with an impurity of the oxidized diselenide, 7b. Further characterization of this selenide product by 77Se NMR spectroscopy was inconclusive, and we were unable to locate a resonance attributable to 4b between −2000 and 2000 ppm. Acidification of a suspension of 4b in water under rigorously oxygen-free conditions formed a new compound with a diagnostic 1H NMR resonance at −3.07 ppm assigned to the selenol hydrogen atom (Figure S25).

Figure 2.

Figure 2

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(A) Reaction of 2b under Birch reduction conditions to form 4b, which is oxidized by air to form 7b. (B) 11B{1H} of 4b in D2O and 11B{1H} and 77Se NMR spectra of 2b and 7b in D2O. Countercations and solvent molecules removed for clarity.

Exposure of a colorless aqueous solution of 4b to air led to an orange solution of the impurity seen in the 11B{1H} NMR spectrum of 4b and the appearance of a new 77Se NMR resonance at 40 ppm (Figure 2B). The chemical shift of this resonance is distinct from most diselenides that fall between 300 and 500 ppm in 77Se NMR. Single crystal X-ray analysis of red-orange crystals grown from DMF and Et2O showed the diselenide product, 7b, crystallized in the less common trans configuration (dihedral = 179.8(7)°) due to weakly bound Cs+ cations on each side of the diselenide bond. The Se–Se bond length was found to be 2.353(2) Å, similar to the 2.338 Å found in 9,9′-m-carboranyl diselenide.35

To confirm the presence of 4b, we turned to its reactivity, as shown in Figure 3. Using 4b as a nucleophile with iBuBr (isobutyl bromide), we found that after 4 days in DMF, the 11B NMR showed conversion to a single product that showed a 77Se resonance at 150 ppm. Integrations of the resonances in the 1H NMR spectrum suggested that the iBuBr substituted twice onto the boron cluster. Single crystals of the Cs+ salt of this compound were grown from CH3CN and Et2O, which showed that the selenide formed an air-stable zwitterionic selenonium compound, 5b. Using cyclopropylmethyl bromide as an electrophile, we synthesized 6b and support that this reaction proceeds through a simple SN2-like displacement reaction. Notably, the treatment of 4a (which is a sulfur-based congener of 4b, see SI) with iBuBr under the same conditions also forms sulfonium 5a, which has previously been observed.36

Figure 3.

Figure 3

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(A) Reaction scheme showing the disubstitution of selenide 4b forming selenonium products 5b and 6b with R-Br = iBuBr and cyclopropylmethyl bromide, respectively. (B) 11B and 77Se NMR spectra of 5b. (C) Single crystal X-ray structure of anionic 5b. Cs+ countercation and hydrogen atoms on carbons removed for clarity. Ellipsoids shown at 50%.

Alkyl and aryl/alkyl sulfonium and selenonium salts can be prepared from the corresponding thioethers and selenoethers using high temperatures or halide scavengers like Ag+.37−41 Because our conditions contain no halide scavengers and use mild heat, we support that the dianionic boron cluster increases the nucleophilicity of the sulfide and selenide. To test this, we subjected 9-m-carboranylselenol to the same conditions as before with iBuBr as the electrophile with CsOAc as a base, and integrations of the 1H resonances in the 1H NMR spectra are consistent with only single substitution forming the selenoether with iBuBr rather than a selenonium compound as with 4b (Figures S50–S52). A single nucleophilic substitution of the 9-m-carboranyl selenol with the alkyl bromide confirms that 4b is significantly more nucleophilic due to its charge rather than electronic effects imparted by the boron cluster.

Finally, we determined the pKa values of 3a and 3b through potentiometric titrations in aqueous solutions. We found pKa values of 13.3 for 3a and 10.7 for 3b (Figures S51 and S52). The value found for 3a is consistent with the previously found value of 13.4 by spectrophotometric titration.36 The pKa value of 10.7 for selenol 3b is significantly more basic than other reported selenols, making this the most basic selenol reported to date.42,43 These observations further reinforce the notion that polyhedral boron clusters can feature extreme electronic properties when attached to heteroatom sites.44−51

Using electrochemical methods, we have synthesized pseudohalide-substituted closo-dodecaborate clusters. We propose that these reactions proceed through direct electrophilic addition of thiocyanogen or selenocyanogen to the cage as with other halides, but we cannot rule out reactions with other species associated with aqueous (SCN)2 and (SeCN)2 decomposition52,53 that nevertheless form the desired pseudohalogenated product. The pseudohalogenated compounds undergo reduction to form BSH and its selenolated analog, BSeH, which is the first anionic polyhedral boron cluster-based selenol. The basicity and further reactivity of these compounds underpin the highly nucleophilic nature of the associated chalcogenides. Overall, this work provides an effective and high-yielding method to synthesize BSH and other chalcogenated boron clusters that may appeal to the medical community as potential BNCT agents.54 Furthermore, the work highlights how boron clusters can exhibit an extreme electron-donating character to the heteroatoms attached directly to a boron vertex, as exemplified by the newly synthesized BSeH compound which is the most basic selenol reported to date.55

Acknowledgments

A. M. S. thanks NIGMS (R35GM124746) for funding. We thank T. C. Ong (UCLA Molecular Instrumentation Core) for help with NMR spectroscopy experiments, and Veronica Carta (UC Riverside X-ray Facility) for instrument time on the diffractometer. Y. A. N. thanks the NSF for the Predoctoral (GRFP) Fellowship (DGE-2034835). A. M. S. thanks Dreyfus Foundation for the Camille Dreyfus Teacher-Scholar Award.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c00469.

  • Detailed experimental procedures, characterization data, crystallographic, and spectroscopy (PDF)

Author Contributions

‡ T. A. K. and Y. A. N. contributed equally.

The authors declare no competing financial interest.

Supplementary Material

ic5c00469_si_001.pdf (6.2MB, pdf)

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