Bonding with Boron Long Ago, a Global Search for Borane Superfuels Led Fortuitously to the Discovery of Carboranes

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in your element Bonding with boron Long ago, a global search for borane superfuels led fortuitously to the discovery of carboranes. Ken Wade recalls his own undistinguished part in the space race, and notes how carboranes revitalized boron hydride chemistry and modified our ideas of chemical bonding.

n the 1950s, during the space race, the carboranes (C2Bn–2Hn), mixed hydrides of communist east and capitalist west built boron and carbon with novel triangular- Iever bigger and more powerful rockets faced (deltahedral) structures that packed to threaten to lob nuclear payloads at each Closo their n boron and carbon atoms closely on other. Progress was marked by how far down spherical surfaces, held together by only test ranges nose cones were recovered, until (n+1) electron pairs, too few to allocate bleeps emitted by the first Sputnik showed a pair to each of their (3n–6) atom–atom who had won the race into orbit. Down below, links. Dubbed ‘electron-deficient’, they engineers continued designing and building were really ‘electron-sufficient’, as adding better rocket motors, while chemists sought electrons caused their structures to open higher-energy fuels. Boron attracted attention Nido up into fragments of bigger deltahedra. for a while because burning its hydrides Plotting their shapes against formula types (boranes) generated more heat per kilogram or electron numbers creates a pattern of than burning hydrocarbons. Although the complete deltahedra or fragments thereof, thermally unstable boranes seemed far from first reported by carborane pioneer ideal as fuels or fuel additives, the USA and Robert E. Williams in 1971, now routinely USSR invested millions of dollars and roubles, featured in textbooks. Many other cluster respectively, in borane research. The UK Arachno compounds conform to this pattern, as government, not to be outdone, allocated I found by teaching cluster chemistry to £600 for similar work. That paid me to work Durham students. Boranes, apparent rule- on boranes with Professor Harry Emeleus at breakers, had become pattern-makers. Cambridge (the alternative was to be drafted Now a mature field, carborane into the army). Borane‑type clusters held together by 13 electron chemistry attracts many talented Professor Emeleus was a delightful pairs have shapes based on the icosahedron. researchers, in organic, organometallic person to work with, kind, and with a dry This is complete (closo) with 12 skeletal atoms, and inorganic chemistry as well as 2– sense of humour. He provided me with a as in C2B10H12 or [B12H12] : nest‑like (nido, one materials science. It has provided new 2– face shield, asbestos gloves and a soft leather vacancy) with 11 skeletal atoms, as in [C2B9H11] ; substituents, ligands, reagents and apron, suggesting, with a twinkle, that, in an and cobweb‑like (arachno, two vacancies) with catalysts, thermally stable polymers, 2– explosion, the apron might not stop all the 10 skeletal atoms, as in [B10H14] . ceramic materials and anticancer drugs. glass fragments, but those that got through Carborane structures, once hotly disputed, should be big enough for the surgeons can now be computed with a precision (plural!) to pick out. Thus encouraged, I a greenish flash and a loud bang, the vessel rivalling that of X-ray crystallography.

studied assorted reactions of diborane (B2H6), disappeared, most of it so finely pulverized Boranes have featured in the work of condensing small samples with potential that no one was hurt. The jaws of the clamp such Nobel Laureates as William N. oxidants in glass vessels and allowing them to that had held it grinned vacantly at me. Lipscomb (1976; structures and bonding); react as liquids or gases as they warmed up. Only one glass fragment remained. The Herbert C. Brown (1979; hydroboration Most did so in a controllable manner. sealed tube end had shot sideways, creating studies, so apt for his initials); Roald One day I rashly sealed together a bullet hole in the reinforced glass of the Hoffmann (1981; structures, bonding and diborane and a volatile oxidant that I had fume cupboard, yet falling back intact, like reactions); and indeed George A. Olah felt sure would react immediately at low a rocket nose cone awaiting recovery. Was (1994; carbocations, analogues of boranes temperature. It didn’t, so hours later my this to be Britain’s fuel–oxidant mix? No. and carboranes). They and many other reaction vessel, clamped behind a safety Boranes were too expensive to prepare, and chemists have been in their element in the screen in the fume cupboard, contained proved unsuitable for satellite- or missile- fascinating world of boron chemistry. ❐ a potentially explosive mixture of the two launching. Other fuel–oxidant systems were gases at room temperature. Suddenly, with developed instead. Ken WAde is emeritus Professor in the However, the money spent globally department of Chemistry at the University on boranes was not wasted. It led to the of durham, UK. 2– discovery of new borane anions [BnHn] and e‑mail: [email protected] He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti

92 nature chemistry | VOL 1 | APRIL 2009 | www.nature.com/naturechemistry