Molecules with planar hexacoordinate carbons (phC) are exciting prospects. They violate both the usual maximum tetracoordination of carbon and its proclivity for three-dimensional bonding. Two independent research groups discovered phC examples computationally in 2000. Exner and Schleyer's CB₆^2- (Figure 1, 1) and the isoelectronic C₃B₄ isomers (e.g., 2) have six aromatic electrons.¹ Minyaev and Gribanova proposed heteroatomic extensions of 1 with eight-membered ring perimeters (Figure 1, 3, X = NH and O).² We confirm these to be minima, despite having eight, rather than six electrons (see Figure 14S for the MO's of 3a).

Figure 1 Previously reported phC minima.1 Bond lengths in Å, the lowest frequency (min) in cm-1, HOMO-LUMO energy separation (Gap) in eV. See Supporting Information Figure 5S for the geometries of 3a and 3b.

In contrast to this limited number of phC's, Wang and Schleyer predicted numerous molecules with planar pentacoordinate carbons (ppC) in 2001. They showed how ppC chemistry might offer unlimited possibilities for generalization and experimental realization.³ Appropriate ppC structural units can be grafted onto virtually any arene or unsaturated ring having three adjacent CH groups. Likewise, generally applicable construction principles allow the incorporation of phC structural units, based on simple elaborations of 1 and 3. The preparation of numerous planar tetracoordinate carbon compounds following theoretical predictions⁴ offers hope for the realization of phC's.

Akin to principles devised for ppC's,³ building block strategies facilitate the design of numerous molecules with phC's. The initial idea was based on the finding that C_2v CH₂B₆^2- (4), the 1,2-dihydro derivative of 1, retains the phC despite its ruptured BB ring bond³ (albeit with somewhat elongated CB bond lengths). As in the design of ppC molecules,³ the opened edge of 4 can be bridged and the ring closed by appropriate atoms and groups. The selection of phC examples in Figure 2 retains the planar hexacoordinate carbon CB₆ unit. Like 3, more than one bridge is possible. Consequently, phC's can be grafted onto myriad systems.

Figure 2 Examples of phC minima optimized at B3LYP/6-311+G**. Bond distances are shown in Å, the lowest frequency (min) in cm-1, HOMO-LUMO energy separation (Gap) in eV.

The planar hypercoordinate bonding principles for phCs, like those for ppC's,³ are general and are easily extended to other combinations of atoms, with elements other than carbon in the centers.² Thus, Minkin et al. computed examples of planar hexacoordinate boron species in 2001,⁵ and the boron analogue of 4, C_2v H₂B₇^- (S-4a, see Figure 5S), was reported recently by Boldyrev and co-workers.⁶ As in earlier studies of planar hypercoordination,^1,4 we focus here on carbon as the central element since violations of the conventional tetrahedral tetracoordinate bonding of carbon seem more unusual. While all our new phC molecules are local minima,⁷ their isomers (e.g., with boron in the center and carbon on the outside) can be lower in energy. However, such species are no less interesting inherently and illustrate the generality of the bonding principles.

The two hydrogens in 4 provide substitution sites. Ring closure, for example, by replacing both the H's, as well as the two negative charges, of 4 with a CH=CH bridge, results in the neutral six electron C₃H₂B₆ (5). The appreciable vertical ionization potentials (VIP) of 5 and other neutral species imply stability (Table 1S). The six Wiberg bond indices (WBI) to the central carbon in 5, ranging from 0.46 to 0.80, document the hexacoordination to the B₆ ring. The total WBI (3.86) of the central carbon does not violate octet rule expectations. Exchanging the central carbon with the three unique borons resulted in isomers (with hexacoordinate borons in the center and carbons in the perimeter) ranging between 18.5 and 30.8 kcal/mol lower in energy (see Figure 4S). However, a Born-Oppenheimer molecular dynamics simulation⁸ (BOMD, Figure 3S for the trajectory), using the deMon 2004 program,⁹ demonstrates the viability of 5 as a local minimum and its resistance toward isomerization. Searches only located high-energy transition states for isomerization.

Benzannulation of 5 gives 6, the 10 electron analogue of naphthalene. Similarly, the replacement of vicinal hydrogens in benzene and essentially all arenes by a CB₆ group (i.e., 4 without its H's) can yield new phC candidates. The tropylium ion derivative, C₈H₅B₆⁺ (7), exemplifies other bicyclic systems. The tricyclic minimum, C_2v C₈H₂B₁₂ (8), has two CB₆ units, each with a phC, grafted meta onto benzene.

Elaborations of 4 with single-atom bridges result in phC minima with seven-membered ring perimeters. For example, a methylene group can replace the two H's in 4 and bridge the CB₆^2- unit to give C_s C₂H₂B₆^2- (9). While the methylene carbon in 9 deviates slightly from the CB₆ plane, the planarity of the CB₆ moiety is nearly exact. Replacing the two H's in 4 with CH^- results in C_2v C₂HB₆^- (10); planarity is retained. Heteroatomic bridging groups such as NH can result in favorable neutral phC minima, such as CHNB₆ (11).

The phC units in 4-11 have 4n + 2 electrons. Their aromaticity is demonstrated by refined nucleus-independent chemical shift (NICS) indices, such as the perpendicular tensor components, 1 Å above the central carbons (NICS(1)_zz).¹³ The NICS(1)_zz grid of 5 and the large negative value (-45.0) above the ring center indicate the presence of a strong induced diatropic ring current, with a shielding zone inside and a deshielding zone outside (see Figure 7S). Dissection of NICS(1)_zz shows that the shielding tensor contributions from the MO and from the in-plane radial MO sets are both diatropic (NICS(1)_zz = -18.5; NICS(1)_radialzz = -14.3. See Supporting Information Figure 10S and Table 1S for NICS(1) and dissected NICS(1) results). Schleyer et al. coined the term "doubly aromatic"¹⁰ for such situations. The NICS(1)_zz results for 5 indicate that both and radial MO's help to achieve the planar geometry with a phC. Planar hypercoordination may benefit even more from MO stabilization than from the MO system. S-4a,⁶ with only four electrons, also illustrates this point.

Two-fold bridging of 1 on opposite edges results in an eight-membered ring perimeter exemplified by Minyaev's phC molecules, 3 (X = NH and O), which have eight electrons (see Figure 15S). The isoelectronic 12 (i.e., D_2h C₃H₂B₆^2- where X = CH^- in 3) also has eight electrons, but two of these (in HOMO-1) are nonbonding due to remoteness of the CH units; the remaining six electrons are delocalized in the central CB₆ moiety (see Figure 11S; neutral D_2h C₃H₂B₆ is a triplet).

Replacing two H's in 4 with an exocyclic C=CH₂ results in C_2v C₃H₂B₆ (13), a phC minimum. Although 13 has a total of six electrons, two of these are localized as a C=C bond (HOMO-4; see Figure 10S). Four electrons remain for the CB₆ moiety, and a +66.2 paratropic NICS(1)_zz value for 13 results. The C_2v C₃H₂B₆^2- dianion (14), with six of the eight electrons on the CB₆ unit, results in a -53.4 diatropic NICS(1)_zz value, thus 14 may be more viable than 13. Double C=CH₂ bridging gives the eight electron phC minimum, D_2h C₅H₄B₆ (S-15; see Figure 5S for details). However, akin to 13, S-15 has four electrons localized on exocyclic C=C bonds (HOMO-3 and HOMO-5; see Figure 14S); only four electrons remain on the CB₆ unit. As expected, the 10 electron dianion D_2h C₅H₄B₆^2- (S-16; see Figures 5S and 15S) has six electrons on the CB₆ unit and a diatropic NICS(1)_zz (-55.8). Molecules like 3 (X = NH and O), 12, and 14 demonstrate that the total electron count is less important than the number of electrons associated with the phC moiety. Evidently, delocalized bonding of the system is needed to achieve planar hypercoordination.

CB₆ units with planar hexacoordinate carbons (or borons) can be grafted onto olefins, arenes, or saturated carbon systems. Most of these new phC candidates are doubly aromatic with delocalized as well as systems, but a total of 4n + 2 electrons is not required. All these phC derivatives provide additional examples of deltahedral bonding involving carbon and offer many opportunities for experimental realization.

Acknowledgment

This work was supported by the National Science Foundation Grant CHE-0209857 in Georgia, the 111 Project (B07012) in China, and Swiss National Science Foundation (Grant: PA002-115322 to C.C.). We thank Abdolreza Sadjadi for helpful discussions, and Frank Weinhold for an earlier version of the NBO 5.0g program.