Early Carboranes and Their Structural Legacy

Robert E. Williams    Loker Hydrocarbon Research Institute, University of Southern California, University Park, Los Angeles, California, 90089-1661

I BACKGROUND AND INTRODUCTION

Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron deficient polyhedral molecular skeleton. Fulfilling an invitation to chronicle the evolution of their chemistry has not been without problems. However, a number of pleasant surprises have been brought into focus in retracing the last 40 years. Prior to the 1950s there were no recognized carboranes; however, by the end of the 1950s, examples of two or three kinds of carboranes had been discovered. Our small group on the west coast of the United States (1) discovered the first carboranes, i.e., the closo-carboranes, 1,5-C2B3H5 (1), 1,2-C2B4H6 (2), 1,6-C2B4H6 (3), and 2,4-C2B5H7 (4) (2–5) although in a low, synthetically useless, combined yield of less than 2%. The correct structures, deduced (3–5) from their “B NMR spectra, are illustrated in Fig. 1 along with selected, but incorrect, classical alternatives (A, B, C, D, and E) favored at the time by traditionalists to avoid five coordinate carbons and multicenter bonding. Throughout this article, with a few obvious exceptions, the sticks in the ball and stick illustrations represent connections rather than bonds. As multicenter bonds are invariably present there are more connections than electron pair bonds.

Shortly thereafter, other groups of investigators (6–18) in the Eastern United States, as well as a group in Russia (19–21), reported the discovery of the much larger (10 boron) icosahedral closo-carborane, 1,2-C2B10H12 (6) (Fig. 2), in much higher yields. Somewhat more reasonable, but still incorrect, suggested structures (F, G, H, and I) are also displayed in Fig. 2. Interestingly, nonclassical multicentered bonding was not rejected in the B10 portion of the molecule as the structure of B10H14 was well known.

The majority of the structural patterns, the emphasis of this article, became evident from structural studies on the more illuminating smaller closo-carboranes (Fig. 1), while the overwhelming majority of carborane synthetic chemistry was subsequently derived from reactions involving the more important icosahedral closo-carboranes (Figs. 2 and 3). As this account concentrates on structural patterns, as opposed to synthesis, the smaller closo-carboranes get top billing. We cover first the entire range of closo-carboranes (Figs. 1 through 7) and return for a more detailed discussion later.

Once the nonclassical structures, (1) to (8), had been established and accepted for the closo-C2BnHn + 2 series, wherein n = 3, 4, 5, and 10, it seemed reasonable that an intermediate sized species could be prepared, namely a derivative of C2B6H8 (9) (22), wherein n = 6 (Fig. 4). By this time classical structural alternatives had been abandoned and only nonclassical cluster structures with multicenter bonds were considered (see Fig. 4).

The remaining medium sized closo-carboranes, C2B7H9 (10), 1,2-C2B8H10 (11), 1,6-C2B8H10 (12), 1,10-C2B8H10 (13) (all in Fig. 5), and C2B9H11 (14) (in Fig. 6) were prepared by Hawthorne and co-workers (23) by the degradation of the icosahedral compounds. The incorrect structure for the monoanion closo-CB10H11] – is illustrated as Q, in Fig. 6, while the correct structure (20) is shown in Fig. 7 (23,24), along with the other monocarba-closo-carborane species.

Long before humans crossed the Bering Straits from Asia into North America, the Los Angeles basin was home to exposed pools of hydrocarbons, the La Brea Tar Pits. Oil deposits, deep under what is now Long Beach in Southern California, leaked to the surface and appeared as small lakes of hydrocarbons amidst Cypress and Pine groves on a sage brush plain in Los Angeles over 20 miles to the north. The volatile components evaporated and a sticky asphaltic, optically smooth mass remained. At times, water, sand, or dust collected over the surface and grasses masked the edges; a more certain death trap is difficult to imagine. The pursued and their pursuers encountering the tar were quickly ensnared, entombed, and embalmed. In this century, the bones of many predators, extinct for over 10 thousand years, such as 11-ft-tall flat-faced bears, giant American lions, saber-toothed tigers (Smilodon fatalis), dire wolves and their prey, horses, camels, enlarged versions of ground sloths, and the Antique bison, as well as mammoths and mastodons, have been recovered from the long congealed tar and reassembled, their soft tissues incorporated into the asphalt.

Most of the early paleontology was directed by Chester Stock (1892–1950) (28) of the University of California, Berkeley, and the California Institute of Technology. His original collections are on display in the Page Museum at the La Brea Tar Pits.

Chemists of today, gazing into these tar pits, cannot help but wonder how many different types and kinds of compounds, primarily hydrocarbons, must exist in each handful of such tar; surely hundreds, perhaps tens of thousands. Today, DNA, 13C, and 14C analyses are furnishing information on the faunal relationships between ancient and modern species, their normal temperatures during their lifetime, and the years since their death.

Of more recent vintage and enshrined at the University of Southern California is Anton B. Burg, the first American born, exclusively American educated, hydroboron chemist and boron chemistry’s link with it’s classical past. It was Anton Burg who, seeking pure boron by the Weintraub electric-arc method, was able to modify it into the first reasonable-yield synthesis of diborane, B2H6 (29). He subsequently obtained the support of his mentor, Hermann I. Schlesinger, for a critical restudy and extension of the works of the great German chemist, Professor Alfred Stock, the father of boron hydride chemistry (28). Indeed, Alfred Stock then adopted for his own use the Burg–Schlesinger diborane synthesis.

Although Alfred Stock reported in 1923 (30) and reviewed in his 1933 book (31) that diborane and ethylene reacted to produce aromatic smelling substances (no doubt the products of hydroboration), it was Burg (29) who first systematically hydroborated double bonds by reacting diborane, B2H6, with the carbonyl moieties of aldehydes, ketones, and esters, i.e., CH3CHO, (CH3)2CO, and HC(O)OCH3, and by identifying the dialkoxyborane products, (RO)2BH. Hurd (32) quantified the reactions of diborane with olefins (hydroboration of CC double bonds) by identifying the trialkylborane products,...