Polyhedral boron clusters, for example, dodecaborate (B₁₂H₁₂^2–), can be stable under harsh conditions such as strong acidity or high temperatures. By functionalizing such compounds, their properties could be tuned for a range of applications, e.g., as photoactive materials, catalysts, or diagnostic agents. The smaller hexaborate cluster B₆H₆^2– is less well studied than the B₁₂-based clusters. It has a nucleophilic character and has previously been partially substituted using carbon-based electrophiles. However, it had not been peralkylated so far.

Alexander M. Spokoyny, University of California, Los Angeles, USA, and colleagues have synthesized the first perfunctionalized hexaborate cluster featuring B−C bonds. The team reacted [NBu₄][B₆H₆H^fac] (H^fac = proton on the cluster face) with benzyl bromide and Hünig’s base (N,N-diisopropylethylamine) in acetonitrile under microwave heating. The product was the peralkylated cluster [NBu₄][B₆(CH₂C₆H₅)₆H^fac], which was characterized using NMR spectroscopy. The team also synthesized the analogous cluster [NBu₄][B₆(CH₂C₆H₄Br)₆H^fac] (pictured) using 4-bromobenzyl bromide, which formed crystals that could be characterized using X-ray crystallography.

The synthesized compounds are stable under ambient conditions. The team suggests that this metal-free alkylation reaction is driven by the nucleophilic character of the hexaborate anion. Based on this approach, hexaborate clusters could potentially be functionalized using a range of other electrophiles.

• Metal-Free Peralkylation of the closo-Hexaborate Anion,
  Jonathan C. Axtell, Kent O. Kirlikovali, Dahee Jung, Rafal M. Dziedzic, Arnold L. Rheingold, Alexander M. Spokoyny,
  Organometallics 2017.
  DOI: 10.1021/acs.organomet.7b00078