Isolation and Structure of Higher Diamondoids, Nanometer-Sized Diamond Molecules

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Science  03 Jan 2003:
Vol. 299, Issue 5603, pp. 96-99
DOI: 10.1126/science.1078239


We exploited the exceptional thermal stability and diverse molecular shapes of higher diamondoids (C22 and higher polymantanes) to isolate them from petroleum. Molecules containing 4 to 11 diamond-crystal cages were isolated and crystallized, and we obtained single-crystal x-ray structures for representatives from three families. Rigidity, strength, remarkable assortments of three- dimensional shapes, including resolvable chiral forms, and multiple, readily derivatizable attachment sites make them valuable nanometer-size molecular building blocks.

Diamond is the archetypal “macrosopic molecule.” However, hydrogen-terminated diamonds and nanometer-sized diamondoid hydrocarbons form a continuous structural series (1) that includes lower diamondoids (<1 nm), higher diamondoids (∼1 to 2 nm), nanocrystalline and chemical vapor deposition (CVD) diamonds (∼2 nm to μm), and macroscopic diamonds (2, 3). Larger members of the series have smaller surface-to-volume ratios and correspondingly lower hydrogen-to-carbon (H/C) ratios. Property data exists for all members of this series (1–4) except the higher diamondoids, and it is in their nanometer-size range that interesting transitions in properties are likely to be found.

The first diamondoid isolated from petroleum, adamantane, has been synthesized, and this molecule and its derivatives show a number of unusual chemical and physical properties (4). Adamantane derivatives have shown promise in pharmaceutical applications (5) and have been used as templates for crystallization of zeolite catalysts (6) and as monomers for the synthesis of high-temperature polymers (7), so interest in this molecule and higher diamondoids comes from both pure and applied sciences. Recently, interest in higher diamondoids has been renewed by molecular simulation studies, suggesting possible applications in nanotechnology (8–10) and use as seed crystals in CVD diamond production (11).

Diamondoids are hydrocarbons that have a carbon framework that is superimposable on the diamond lattice. The smallest diamondoid, adamantane (C10H16), has a single cage-shaped subunit excised from the diamond crystal lattice and the dangling carbon bonds are terminated with hydrogen (Fig. 1A). Each successive diamondoid family contains one additional diamond crystal lattice cage. Diamantane contains two face-fused cages and triamantane contains three (Fig. 1A). Tetramantane has four isomers that result from different face-fusing of the four cages and is the first unsubstituted diamondoid to exhibit chirality. Each successive higher diamondoid family shows greatly increasing structural complexity and varieties of molecular geometries (Fig. 1B).

Figure 1

The relation between the face-centered cubic diamond lattice and diamondoid structures. (A) Adamantane (yellow) is shown superimposed upon the lattice of a 455-carbon octaghedral diamond (some carbons removed for clarity). Adamantane, diamantane, and triamantane (all in yellow) are also shown separate from the diamond lattice. (B) Screw-shaped higher diamondoid P [1234] pentamantane (red), pyramidal [1(2,3)4] pentamantane (blue), and rhombus-shaped [121321] heptamantane (green) superimposed upon the diamond lattice and also separate from the lattice. (C) The 455-carbon diamond with superimposed diamondoid structures viewed along the (110), (100), and (111) lattice planes showing the diamond lattice faces of the pentamantanes and heptamantane. Views of the P [1234] pentamantane screw axis are indicated by straight arrows. The clockwise helical arrow indicates the groove in the red P [1234] pentamantane molecule.

Beginning with the pentamantanes, multiple molecular weight classes also appear because of the variable number of carbon atoms that can be shared by face-fusing diamond cages. Thus, for pentamantane, there are nine isomers with the formula C26H32 and molecular weight (MW) 344, and one isomer with the formula C25H30 and MW 330. The number of molecular weight classes increases with each additional diamond cage, as does the number of isomers. For instance, there are hundreds of octamantanes distributed across five molecular weight classes. The octamantane class with formula C34H38 and MW 446 shows 18 isomeric structures with both chiral and achiral forms. All of these higher diamondoids correspond to nanometer-sized H-terminated diamonds of diverse shapes and sizes.

The diamond face-fused cage structure of diamondoids gives them their stability, strength, and rigidity, but it also makes them extraordinarily difficult to synthesize (12). Only one of the four possible four-cage diamondoids (tetramantanes) has been synthesized, and that only with great difficulty and low yields (13). All other attempts to synthesize higher diamondoids have failed. Attempts at synthesis of the higher diamondoids via carbocation rearrangements similar to those successfully devised by von Schleyer (14) to prepare the lower diamondoids appear to have been thwarted by the astronomical number of potential intermediates, reaction pathways, and complex reaction kinetics (12, 14).

The largest diamondoid previously isolated from petroleum was diamantane (Fig. 1A) (15). We have found that higher diamondoids, from the tetramantanes to at least undecamantane, occur in petroleum. Although present in only trace concentrations in typical oils, due to their great thermodynamic stability, diamondoids are naturally concentrated by catagenesis, becoming important constituents of some natural gas condensates including those from the Norphlet Formation, U.S. Gulf of Mexico, and the Western Canada Basin (16).

We have isolated and crystallized all four tetramantanes, nine pentamantanes, one hexamantane, two heptamantanes, two octamantanes, one nonamantane, one decamantane, and one undecamantane (Fig. 2). We determined single-crystal x-ray structures for: [121] tetramantane (4 in Fig. 3), [1(2,3)4] pentamantane (Fig. 1B;Fig. 2, A and C; 7 in Fig. 3), P and M [1213] pentamantane (Fig. 2D; 5 and 6, Fig. 3), [12(3)4] pentamantane (8 in Fig. 3), [1212] pentamantane (11 in Fig. 3), and [121321] heptamantane (Fig. 1B; 20 in Fig. 3). Higher diamondoid nomenclature is that of Balaban and von Schleyer (17). In addition, we determined the x-ray crystal structure of a methylpentamantane, specifically, 3-methylundecacyclo[,5.13,19.15,15.17,15.113,17.09,14.013,20.09,22.019,24] hexacosane (25, Fig. 3); nomenclature according to the von Baeyer system (18) is required for numbering carbon atoms in the structure.

Figure 2

(A and B) GC-MS total ion chromatograms (TIC) and mass spectra of selected higher diamondoids crystallized from high-purity HPLC fractions. GC-MS used a HP-MS5 capillary GC5 column (30 m by 0.25 mm inner diameter (I.D.), 0.25 μ = micron or μm phase thickness). (A) [1(2,3)4] pentamantane, 7 (Fig. 3). (B) [1231241(2)3] decamantane, (24, Fig. 3). (C) Stereo view of the unit cell and packing of crystalline [1(2,3)4] pentaman- tane, 7 (orthorhombic, Pnma space group, 1.257 g/cm3 calculated density; unit cell: a = 11.4706(8) Å, b = 12.6418(8) Å, and c = 12.5169(8) Å; α = β = γ = 90°). (D) Stereo view of the unit cell and packing of crystalline [1213] pentamantane,5 and 6 [triclinic, P-1 space group, 1.332 g/cm3 calculated density; unit cell: a = 7.6728(9) Å, b = 10.1511(12) Å and c = 12.3117(17) Å; α = 87.101(4)° , β = 72.009(3)°, and γ = 70.613(3)°].

Figure 3

Examples of higher diamondoid molecular structures. The following higher diamondoid names are in Balaban-von Schleyer nomenclature (17). Enantiomer structure numbers are listed together in the order P and M in front of their Balaban-von Schleyer name. Tetramantanes: 1, [1(2)3];2 and 3, [123]; 4, [121]. Pentamantanes: 5 and 6, [1213];7, [1(2,3)4]; 8, [12(3)4]; 9and 10, [1234]; 11, [1212]; 12 and 13, [12(1)3]. Hexamantanes:14, [12(1,3)4]; 15 and 16, [12341]; 17, [12121]; 18 and19, [12123]. Heptamantanes: 20, [121321];21, [123124]; Octamantane: 22, [1213](1)21]; Nonamantane: 23, [121(2)32(1)3]. Decamantane: 24, [1231241(2)3]. Alkyl-pentamantane:25, 3-methyl-[1(2,3)4] pentamantane.

The presence of higher diamondoids in petroleum has been suspected (19), but neither isolations nor conclusive structural assignments have been previously reported. We concentrated higher diamondoids from condensates produced from the Norphlet Formation, Gulf of Mexico. Higher diamondoid–containing distillate fractions obtained by vacuum distillation above 345°C (atmospheric equivalent boiling point) were pyrolyzed at 400° to 450°C to remove non-diamondoids. Aromatic and polar compounds were removed from pyrolysis products by argentic silica gel liquid chromatography. Higher diamondoids were then isolated by a combination of reversed-phase high-performance liquid chromatography (HPLC) on octadecyl silane columns and highly shape-selective Hypercarb HPLC columns. Isolated higher diamondoids were recrystallized to high purity from acetone (Fig. 2) (20).

We found all of the theoretically possible unalkylated parent isomers (21) of the following: 292 MW tetramantanes (3), 344 MW pentamantanes (6), 396 MW hexamantanes (17), 394 MW heptamantanes (2), 446 MW octamantanes (12), and 456 MW decamantane (1) (Fig. 2B); we also determined gas chromatographic (GC) retention times and mass spectra. In addition, we found various 448 MW heptamantanes, 500 MW octamantanes, 498 MW nonamantanes, 496 MW decamantane, and 506 MW undecamantane, some of which co-elute in gas chromatography-mass spectrometry (GC-MS) but are separable using the HPLC systems described. Mass spectra of unalkylated higher diamondoids are nearly featureless (Fig. 2, A and B) except for strong molecular-ion (M+) base peaks and peaks at M+/2 (corresponding to the doubly charged molecular ion). Isolations with the use of pyrolysis at 450°C demonstrated that all of these higher diamondoids have thermal stabilities consistent with diamondoid structures. Selected structural representatives are shown in Fig. 3.

The great variety of rigid shapes of higher diamondoids (Fig. 3) makes them attractive structural components for molecular-design applications. A series of rod-shaped higher diamondoids have long axes perpendicular to their diamond (110) lattice planes, the shortest being [121] tetramantane at a length of 1.0 nm (Fig. 4, A and B). Each added diamond cage increases the length of the rod by 0.10 to 0.15 nm following the sequence [1212] pentamantane, [12121] hexamantane, etc. (Fig. 4, A and B). Another of the hexamantanes, [12312] hexamantane or cyclohexamantane, is a disc-shaped molecule (22). Additionally, two series of screw-shaped higher diamondoids have different helical pitches and diameters and helical axes that are parallel to different diamond crystal planes (Fig. 4, C and D). Examples in these screw-shaped series are the right- and left-handed [12341] and [12324] hexamantanes shown perpendicular and parallel to their helical axes in Fig. 4, C and D, respectively. The helical axis of [12341] hexamantane is parallel to the (100) diamond crystal lattice plane, whereas that of [12324] hexamantane is parallel to the (111) plane (Fig. 4D). Figure 4E shows the screw-shaped molecular series that includes [12341] hexamantane. Molecules of both series are rare primary helical structures (23), where the helicity is inherent in the backbone of the molecule (rather than arising from steric effects, such as in the helicenes).

Figure 4

Diversity of higher diamondoid molecular geometries. (A and B) A series of rod-shaped higher diamondoid molecules in carbon framework and space filling (CPK) forms. (C) Representative structures for two series of screw-shaped higher diamondoids: [12341] and [12324] hexamantanes. Axes and arrows indicating the direction of helical twist are in green. (D) Hexamantane structures from (C) shown parallel to their helical axes. (E) A series of screw-shaped higher diamondoids with a clockwise helical groove indicated by enlarged red-colored hydrogens along the groove. (F) Chiral gas chromatographic separation of the two enantiomeric tetramantanes, P and M [123] tetramantanes (2 and 3, Fig. 3) using a CP CycloDex B 2,3,6, M 0.25 μm stationary phase, 25 m by 250 μm ID chiral capillary GC column (FID detection). The sample, included achiral [121] tetramantane (4, Fig. 3) (20). (G) Chiral gas chromatographic analysis [as in (F)] of the 6.8- to 7.2-ml fraction from a chiral HPLC separation of the tetramantane mixture showing isolation of the first-eluting [123] tetramantane enantiomer. An ASTEC (Advanced Separation Technologies, Inc., Whippany, NJ) Cyclobond I-2000 column with acetonitrile mobile phase at 0.4 ml/min was used. (H) Chiral gas chromatographic analysis [as in (F)] of the 7.6- to 11.6-ml fraction from the same chiral HPLC separation (20).

We were able to resolve P and M helical enantiomers of [123] tetramantane using chiral capillary gas chromatography (Fig. 4F), and we isolated (Fig. 4, G and H) individual enantiomers by chiral HPLC (20). These may be the only primary helical enantiomers to have been resolved. Numerous other chiral molecular series are present among the higher diamondoids (Fig. 3). Chirality broadens potential applications of higher diamondoids in many fields.

Diamondoid molecules can assemble into macroscopic crystals with unusual characteristics. For example, the crystal structure (Fig. 2C) of pyramidal [1(2,3)4] pentamantane (7, Fig. 3) has an exceptionally low packing coefficient, 0.67 compared with >0.7 for other molecular crystals (24), indicating unexpected voids in the crystal lattice. These crystals visually resemble macroscopic diamond crystals (fig. S1) but show very different properties and are held together by weak intermolecular van der Waals forces. However, it is at the intramolecular sp3 carbon framework level, not the intermolecular level, that diamond-like properties, e.g., strength, rigidity, and stability, emerge.

The diverse geometries and varieties of attachment sites among higher diamondoids provide an extraordinary potential for the production of shape-targeted derivatives. Properties of diamond-like hydrocarbons can also be tuned by the addition of various functional groups. We have already prepared a number of functionalized tetramantanes, including bromo, hydroxyl, acetoamino, amino, oxa, and aza derivatives and have characterized them by GC-MS (25). Predictable and diverse derivatizable geometries are important features for molecular self-assembly and pharmacophore-based drug design (26,27). Incorporation of higher diamondoids in solid-state systems and polymers should provide high-temperature stability, a property already found for polymers synthesized from lower diamondoids (7). Particularly interesting may be the electronic properties of higher diamondoids because H-terminated diamond is the only semiconductor that shows a negative electron affinity, and nanocrystalline diamonds are being studied as field emitters (2).

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