1
Description of Diamondoids

Shortly after the 3D structure of diamond was determined, the German chemist Hermann Decker recognized the connection between diamond and saturated hydrocarbons with "the 6-ring system built out into the third dimension" and suggested the term "diamondoid" for such molecules [1]. As the spatial arrangement of the carbon atoms resembles the diamond crystal lattice, diamondoids can therefore be viewed as hydrogen-terminated nanometer-sized diamonds with distinctive properties determined by their sizes and topologies. The smallest diamondoid is adamantane (AD, Figure 1.1), which has a cage skeleton consisting of ten carbons. Formal addition of further isobutyl fragments to the AD parent structure in a cyclohexane ring-forming manner results in higher diamondoid homologues. Diamondoids are classified as lower and higher homologues: lower diamondoids have only one isomeric form and include AD (C10H16), diamantane (DIA, C14H20), and triamantane (TRIA, C18H24), while higher diamondoids start with tetramantane (TET, C22H28) and possess isomers. Among three possible TET isomers, one is chiral (123TET) and is viewed as the parent of a new family of s-helicenes [2]. As the cage grows, the number of isomers increases and beginning from pentamantane (PENT) spreads into different molecular weight subgroups, i.e., PENT has nine isomers with the C26H32 formula and one isomer with the C25H30 formula. Some other hydrocarbons also satisfy the structural criteria of partial or complete superposition on the diamond lattice, e.g., cyclohexane and decalin, so diamondoids are more precisely defined as "hydrocarbons containing at least one adamantane unit wholly or largely superimposable on the diamond lattice" [3]. Due to this definition, higher diamondoids bridge the gap between saturated hydrocarbons and diamond and are sometimes called nanodiamonds (in plural form to differentiate them from heterogeneous mixtures of nanodiamond material obtained by chemical vapor deposition, detonation, or shock-wave techniques [4]).

The molecular symmetry of diamondoids also plays a role in their self-assembly, readily producing crystals or serving as nucleation centers for bigger nanomaterial architectures. The smallest diamondoid AD is highly symmetric (Td point group), whereas symmetry is generally (but not always) reduced as the diamondoids become larger, e.g., DIA and TRIA belong to the D3d and C2v point groups, respectively. Isomers of the first higher diamondoid TET display C2h (121TET), C2 (123TET), and C3v (1(2)3TET) symmetry (Figure 1.1).

Figure 1.1 Structures and symmetry of diamondoid homologues up to cyclohexamantane (12312HEX).

Note that TETs exemplify a common occurrence for higher diamondoids: different isomers have markedly different symmetries, which can be useful in material design by tailoring both the solubility of the material and the shape of the used building blocks. Moreover, diamondoids have one important advantage over bulk diamonds: they are "knowable," that is, their shapes are precisely determined by their molecular structure (rods, disks, helices, prisms, pyramids, cubes, etc.; Figure 1.2) and stoichiometry, and they can be obtained in homogeneous forms because they are single-molecule, nanometer-scale-sized building blocks. For instance, DIA and 121TET are rod-shaped nanodiamond particles, TRIA and 1212PENT have triangular shape, 1(2,3)4PENT and 1231241(2)3DEC are tetrahedron and cube, respectively, and 1(2)3TET and 12312HEX are prisms.

According to the definition of polymantanes, face-fused AD cage structures that are the focus of this book are not the only existing diamondoids. For example, AD dimers and higher single-bonded oligomers also belong to the class of diamondoid hydrocarbons (Figure 1.3). The first step in determining whether a saturated hydrocarbon belongs to the class of polymantane compounds is to check whether it has at least one AD subunit. If yes, then the next condition is that all cage atoms of the molecule need to be part of an AD unit; if that is also true, then the final condition is that two or more AD cages need to have at least six common carbon atoms, meaning that they share one face. When these conditions are met, a structure can be classified as a true diamondoid (Figure 1.3) [3].

1.1 Nomenclature

Before going further, we first define the diamondoid classification and nomenclature. As can be anticipated, von Baeyer's IUPAC names for these polycyclic compounds become quite cumbersome, and precise structural assignments require representing such molecular structures in terms of planar graphs [6]. Note that some programs, such as ChemDoodle, are quite useful for automatic IUPAC naming. As the cages grow larger and become more complex, the need to develop a special nomenclature for diamondoids emerges [3]. The initially proposed graph-theory-based diamondoid classification and nomenclature [7] is still in use today and is termed the Balaban-Schleyer nomenclature (vide infra). As for the naming, the smallest representative AD is the basis: numerical multipliers indicate the number of fused AD subunits and are followed by adding the -amantane suffix, e.g., DIA, TRIA, and TET. Note, however, that starting from TET different isomers emerge, and they also need to be defined unambiguously. For this purpose, a dualist graph construction is used that gives the codes for specific stereoisomers and avoids confusing and non-systematic designations. For example, three possible isomers of TET are sometimes called anti-TET (C2h-symmetry), skew-TET (chiral, enantiomeric pair, and C2-symmetry), and iso-TET (C3v-symmetry). However, this naming is based on their apparent geometrical shape and can hardly be transferred to higher homologues. In contrast, when applying the dualist graph convention, the naming becomes 121TET (former anti, now [121]tetramantane), 123TET (former skew, now [123]tetramantane), and 1(2)3TET (former iso, now [1(2)3]tetramantane) and is a system applicable for all cage sizes. Essentially, this Balaban-Schleyer system uses four-digit codes (1, 2, 3, and 4) for the tetrahedral directions of covalent bonds around an imaginary center (Figure 1.4). These code descriptors are generated as follows: the center of the first AD cage is connected with the adjacent AD moieties in one of the four possible directions (AD has four faces), center-to-center. This direction is assigned number 1; the process is repeated until all AD subunits are accounted for and the whole molecule is traced with such vectors.

Figure 1.2 Structures of selected diamondoids linked to their geometrical representations.

Source: Adapted from Ref. [5].

Figure 1.3 Classification of polymantanes.

The digits emerge from taking different directions along the cage scaffold and, in the end, give a unique code characteristic for the isomer in question. This code is placed in brackets before the name of the stereoisomer. For more complicated geometries, when the diamondoid structure contains a branch, the digit of the corresponding vector is placed in parentheses, and if there are more branches, they are separated by commas inside the parentheses. In the case of longer branches, the chains of the branch are placed inside parentheses but without comma separation. One immediately notices the elegance of the Balaban-Schleyer approach as we have [123]tetramantane (123TET) instead of nonacyclo [11.8.1.01,20.02,7.04,21.06,19.09,18.011,16.015,20]docosane, [121]tetramantane (121TET) instead of nonacyclo[11.7.1.16,18.01,16.02,11.03,8.04,19.08,17.010,15]docosane, [1212]pentamantane (1212PENT) instead of undecacyclo[11.11.1.15,21.01,16.02,11.03,8.04,23.06,19.08,17.010,15.018,23]hexacosane, and so on. Note that in the older literature, DIA is sometimes called "congressane" since it was proposed [8] as a synthetic challenge for the participants of the XIXth 1963 IUPAC meeting in London.

Figure 1.4 Examples of dualist graph construction for diamondoid naming suggested by Balaban and Schleyer.

The dualist graph convention for the nomenclature of diamondoids enables a more straightforward way to designate diamondoid cages but as the cage size increases even such a naming system encounters complications. With increasing cage fusion, it becomes difficult to account for all possible stereoisomers and one way to resolve this type of complexity is to generate partitioned-formula tables based on the distribution of all the present carbon atoms according to them being quaternary (Q), tertiary (T), or secondary (S) [7a, 9]. By following this procedure, one obtains valence isomers of the same molecular formula CQ(CH)T(CH2)S that are then shortened and denoted as...