Isolation and structural proof of the large diamond molecule, cyclohexamantane (C26H30).

In this report, we describe the first isolation and structural verification of the diamondoid cyclohexamantane, C26H30, a material that has probably never previously existed anywhere in crystalline form. Since the diamond surface can terminate in hydrogen,[1] cyclohexamantane may be thought of as a nanometer-sized diamond of approximately 10 21 carats. In fact, the cyclohexamantane structure has been used to represent a small diamond in theoretical investigations.[2–4] Here we present the experimentally determined properties of cyclohexamantane isolated from petroleum, which includes its single-crystal X-ray structure, and analysis by NMR and laser Raman spectroscopy, as well as mass spectrometry. Diamondoids of the adamantane series are hydrocarbons composed of fused cyclohexane rings, all in stable chair conformations, which form interlocking cage structures that can be superimposed on the diamond crystal lattice.[5] The lower diamondoids have chemical formulas of C4n+6H4n+12, where n equals the number of diamond-cage subunits. The stability of diamondoids is illustrated by the heat of formation of adamantane (the smallest diamondoid) which is far lower than any other hydrocarbons of comparable carbon quantity and ring number. While there is only one form of adamantane, diamantane, and triamantane, there are four possible isomeric tetramantanes (iso-, anti-, and two enantiomeric skew-tetramantanes). There are ten possible pentamantanes,[6,7] nine isomers with the formula C26H32, and one with formula C25H30. Likewise, there are 39 postulated hexamantanes, 28 of which are isomers of C30H36, and ten of which are C29H34 isomers.[6, 7] The pericondensed hexamantane, also named cyclohexamantane, is the only diamondoid structure with the formula C26H30. The most efficient syntheses of all of the lower diamondoids (from adamantane through to triamantane) involve the carbocation-mediated superacid equilibration reaction discovered by von R. Schleyer.[8] However, this route to the higher diamondoids (that is, tetramantane and higher) is blocked by severe kinetic (mechanistic) constraints, and all attempts to synthesize the higher diamondoids have proven futile. Only one of the tetramantanes (anti-tetramantane) has been prepared by an elegant, complex, but low-yielding alternative synthetic pathway devised by McKervey's research group.[9] Diamondoids occur naturally in virtually all petroleum.[10] In most crude oils, diamondoid concentrations are in the order of 1–100 ppm, and occur predominantly as substituted and unsubstituted adamantanes and diamantanes.[10] Although it is not clear how diamondoids are formed in the geosphere, we believe they may be created through carbocation-mediated rearrangements arising when newly generated petroleum, which contains functionalized molecular species (for example, double bonds), reacts with superacid sites on naturally occurring clay minerals, such as montmorillonite, in petroleum source rocks. Although macroscopic diamonds are less stabile than graphite under oil reservoir conditions, this is not true of nanometer-sized hydrogen-terminated diamonds, such as cyclohexamantane.[11–13] Therefore, the formation of diamondoids from petroleum precursors is thermodynamically favored and this helps to explain their surprising existence. The number of different carbocation-mediated pathways to adamantane from only one starting material (endotetrahydrodicyclopentadiene) has been calculated to be 2897.[14] Similar treatments for diamantane yield numbers orders of magnitude higher.[15] With this increasing number of [*] Dr. J. E. P. Dahl, Dr. R. M. K. Carlson ChevronTexaco Energy Reseach & Technology Co. P.O. Box 1627, Richmond CA 94802 (USA) E-mail: [email protected]