Perfect Planar Tetracoordinate Carbon in Neutral

Unsaturated Hydrocarbon Cages: A New Strategy

Utilizing Three-Dimensional Electron Delocalization

YANG WANG

Departamento de Quımica, C-9, Universidad Autonoma de Madrid, Madrid 28049, Spain

Received 7 November 2008; Revised 13 December 2008; Accepted 16 December 2008DOI 10.1002/jcc.21213

Published online 25 February 2009 in Wiley InterScience (www.interscience.wiley.com).

Abstract: A new series of unsaturated pure and boron-substituted hydrocarbons containing a perfect planar tetra-

coordinate carbon (ptC) have been proposed by performing density functional computations. The ptC is effectively

stabilized through three-dimensional delocalization of ptC’s lone pair into p-conjugated systems, by utilizing a new

strategy opening a brand new way of designing ptC structures. Compared to previously proposed ptC-containing

hydrocarbon cage compound, a neutral hydrocarbon designed here might be a more viable target for synthetic

attempts.

q 2009 Wiley Periodicals, Inc. J Comput Chem 30: 2122–2126, 2009

Key words: planar tetracoordinate carbon; hydrocarbon; cage compound; electron delocalization; density functional

calculation

Introduction

Over the last four decades, considerable efforts have been made

to search for planar tetracoordinate carbon (ptC), which chal-

lenges to van’t Hoff’s concept of tetrahedral carbon and is open-

ing a new world of ‘‘flat carbon’’ chemistry.1,2 Recent success of

experimental verifications3–5 of ptC6,7 has stimulated a continu-

ously increasing number of theoretical studies on this exciting

topic. In particular, great interest has been arisen for achieving

ptC(C)4 substructure, a ptC surrounded only by carbons.8–11

The ptC(C)4 is challenging in that so far it has never been syn-

thesized in experiment and no theoretical achievement has

been made until the first example was found in a computation-

ally designed neutral saturated hydrocarbon cage (1, D2h, see

Fig. 1) by Rasmussen and Radom in 1999.9 Incorporated into a

strained doubly bridged octaplane cage, a central ptC is per-

fectly achieved in 1.9 However, because of high steric strain, it

is rather difficult to synthesize such a compound as 1 with ptC

stabilized by utilizing solely ‘‘mechanical’’ strategy.2 An alter-

native theoretical strategy to stabilize ptC is the ‘‘electronic’’

approach. As proposed in the first article on ptC by Hoffmann

et al.,12 ptC can be stabilized by p-acceptor and r-donor sub-

stituents. More recently, Merino et al.13 have presented the

simplest hypothetical cluster C522 with a ptC(C)4 stabilized

exclusively by electronic factor, namely, the delocalization of

the lone pair on ptC’s pz-orbital. By using C522 as a building

block, ptC(C)4 substructures have also been designed in

extended systems.14

From a purely theoretical point of view, more structures con-

taining ptC(C)4 have been further achieved by combining both

‘‘mechanical’’ and ‘‘electronic’’ strategies. By replacing carbon

substituents with borons, Wang and Schleyer15 obtained a per-

fect ptC in a boraplane cage (2, D4h) in which the lone pair of

ptC is delocalized into the vacant boron orbitals parallel to ptC’s

pz-orbital. With similar framework of 2, they employed a

‘‘charge-compensation’’ strategy and obtained ptC(C)4 in neutral

molecules by replacing two carbons far away from the central

ptC by borons.16 Later on, some planar hydrocarbons were pro-

posed by incorporating ptC into cyclic p-delocalized rings.17,18

A very recent study shows that the incorporation of ptC into a

saturated cyclic ring can also help to delocalize ptC’s lone

pair.19 However, in all the known approaches so far, the delocal-

ization of ptC’s lone pair occurs only on the two-dimensional

plane. A question may be raised: Why not stabilize ptC through

three-dimensional electron delocalization?

This work reports a novel series of unsaturated pure and

boron-substituted hydrocarbons (3–9, see Fig. 1) containing a

ptC(C)4 substructure designed by DFT computations. Compared

with the first computationally designed saturated hydrocarbon

1,9 two proposed neutral compounds (3 and 7) are unsaturated

hydrocarbons. In particular, 7 is of great interest since it will be

Additional Supporting Information may be found in the online version of

this article.

Correspondence to: Y. Wang; e-mail: yang.wang@uam.es

q 2009 Wiley Periodicals, Inc.

shown that the syntheses might be more viable than that of the

first ptC-containing hydrocarbon 1. At the same time, for the

design of these new structures, a new electronic strategy is

applied for the stabilization of ptC(C)4. Different from all previ-

ous electronic strategies in which the delocalization of ptC’s

lone pair takes place on the same plane as the p-conjugated

Figure 1. B3LYP/6-3111G** (1–422, 7, 9), B3LYP/6-31G* (661 and 8) and B3LYP/6-311G* (542)

optimized geometries (in A).

2123Perfect Planar Tetracoordinate Carbon in Neutral Unsaturated Hydrocarbon Cages

Journal of Computational Chemistry DOI 10.1002/jcc

plane,1,2 the new strategy here suggests that ptC’s lone pair can

be effectively delocalized into perpendicular p-conjugated sys-

tems from the top and the bottom of ptC’s pz-orbital. For

instance, in the case of 3, all carbons above and below the equa-

tor are unsaturated and thus form p-conjugated rings. As will be

demonstrated below, the ptC’s lone pair is delocalized into the

p-conjugated systems from both sides of the pz-orbital. This is

an unprecedented finding, since it opens a brand new way of

constructing ptC (or even with planar higher coordination) struc-

tures, extended from two-dimensional to three-dimensional.

Computational Methods

All computations were carried out by using the Gaussian 03

package.20 Basically, geometric structures were fully optimized

at the B3LYP/6-3111G** level which has been applied to

explore planar tetracoordinate16,21 and other nonclassical carbon

systems.22,23 The stabilities of wavefunctions have been con-

firmed for the main compounds such as 1, 3, 7, and anions 422

and 542. Frequency computations, nucleus-independent chemical

shift (NICS)24 calculations, and natural bond orbital (NBO)25

analyses were performed at the same level of theory with the

optimized geometries. Ionization energies and energy differences

between singlet and triplet were computed at the MP2/6-

3111G**//B3LYP/6-3111G** level. All the optimized geome-

tries are provided as Cartesian coordinates in the Supporting In-

formation. As shown in Table 1, all the singlets are more stable

than the corresponding triplets for the calculated compounds.

Therefore, all discussions are based on the singlet states.

Results and Discussion

The perfect ptC(C)4 is stabilized in the neutral unsaturated

hydrocarbon cage 3 with D2h symmetry. No imaginary vibration

frequency is found and the smallest frequency value is 95.9

cm21. As a comparison, its saturated counterpart (with D2 sym-

metry), as reported by Rasmussen and Radom,9 is not able to

hold a perfect ptC and the central carbon undergoes a slight dis-

tortion from the planar conformation. Therefore, the role of un-

saturated systems in 3 in stabilizing ptC is obvious. Molecular

orbital analysis (see Fig. 2) demonstrates that there is a bonding

interaction between ptC’s pz-orbital and LUMO 11 of the

empty cage, leading to further stabilization of the system and

the electron transfer from ptC’s pz-orbital to vacant p-orbitals ofthe unsaturated carbons on the cage. This electron transfer is fur-

ther confirmed by the positive natural charge on the central ptC

(10.081 j e j, see Table 1). In the ptC(C)4 substructure, the car-

bon substituents offer electrons to the central carbon to form 3c-

2e bond.12 Hence, the central ptC should carry negative charge

if the lone pair in pz-orbital is not delocalized to anywhere, as

in the case of 1 (20.420 j e j on ptC, see Table 1). Furthermore,

the pz-orbital occupancy of ptC is only 1.07, as a result of the

delocalization of the lone pair. As a comparison, ptC’s pz-orbital

occupancy of 1 is much larger (1.64), since the ptC’s lone pair

is not effectively delocalized in 1.

Compared to 2 and other boron-substituted spiroalkaplanes

proposed by Wang and Schleyer,15,16 the ptC in neutral hydro-

carbon 3 is stabilized without introducing any heteroatom. More-

over, 3 makes use of electrons even more effectively. As shown

in Table 1, the Wiberg bond index (WBI)26 sum of the ptC’s for

3 is 4.0, much larger than the values (�3.2)12 for the boron-sub-

stituted compounds. Although the rigid cage framework of 3

also plays some ‘‘mechanical’’ role in stabilizing ptC, the effi-

ciency of utilizing electronic effect alleviates significantly the

stress suffered by ptC in 3. The bond length (1.54 A) between

two carbons surrounding ptC on the equator of 3 becomes more

‘‘normal’’ as a typical C��C single bond, while such bond

lengths are much shorter in 1 (1.44 A)9 and in boron-substituted

spiroalkaplanes (\1.50 A).12

To further investigate the strategy here that utilizes three-

dimensional delocalization of ptC’s lone pair, several derivatives

of 3 have been considered (see Fig. 1). Firstly, it is observed

that the p-conjugated system is not necessary to be closed

cyclic, but the electron counting rule should be obeyed to get a

stabilized ptC. For example, if we saturate two of the CH groups

in 3 to break the p-conjugated circle on each ring, the resulting

structure 422 (C2v) is still an energy minimum without any

imaginary frequency, as long as two more electrons are added to

Table 1. Smallest Imaginary or Real Vibration Frequency (Freq, in

cm21), the pz-Orbital Occupancy of ptC (Occ), Natural Charge on ptC

(Q, in a.u.), Wiberg Bond Index Sum of the ptC’s (WBI), Ionization

Energy (IP, in eV), and Vertical Energy Difference of Singlet Minus

Triplet at the Singlet Geometry (DES–T, in kcal mol21) for Calculated

Neutral Compounds Containing ptC.

Freq Occ Q WBI IP DES2T

1 226i 1.64 20.420 3.6 4.8 248.0

2 119 0.61 20.680 3.4 6.8 221.5

3 96 1.07 10.081 4.0 9.0 264.3

7 168 1.14 20.004 4.0 7.8 242.2

9 97 0.52 20.132 3.7 7.9 257.2

Figure 2. Schematic orbital-correlation diagram for 3.

2124 Wang • Vol. 30, No. 13 • Journal of Computational Chemistry

Journal of Computational Chemistry DOI 10.1002/jcc

keep the same number of p-electrons as before. Nevertheless,

the stabilization effect becomes weaker as the size of the p-delo-calized system decreases. If we saturate two more CH groups

from 4, the resulting tetraanion 542 (D2) cannot afford a perfect

ptC and the central carbon is slightly distorted from planar

arrangement, according to B3LYP/6-311G* results. Moreover,

the p-conjugated system can also be extended but without

changing the total number of delocalized electrons, as in the

case of 661 (C2h) which corresponds to an energy minimum at

B3LYP/6-31G* level. This advantage makes the new strategy

versatile for the design of ptC.

According to the knowledge that we have obtained above,

another neutral hydrocarbon with a perfect ptC is designed as 7

(D2h), which has been confirmed to be an energy minimum

without imaginary frequency. The top and bottom rings in 7 are

bigger than those in 3, but the total number of p-electrons is thesame since four carbons are saturated on the rings in 7. The

ptC’s lone pair in 7 is also effectively delocalized into the p-conjugated system. As listed in Table 1, the pz-orbital occu-

pancy, natural charge, and WBI sum for ptC in 7 are similar to

the corresponding values in 3. As reported by Rasmussen and

Radom,9 1 has an ionization energy (4.8 eV) comparable to that

of alkali metals. However, owing to the delocalization of ptC’s

lone pair, the ionization energies of 3 and 7 are much higher

(9.0 and 7.8 eV, respectively), as shown in Table 1.

The delocalization of ptC’s pz-electrons can also be demon-

strated by the NICS values at the center of the top and bottom

rings. The NICS(1) value from 1 A above the center of the ring

can represent the main contribution of the p orbitals.27 As

shown in Table 2, the NICS(1) values are slightly diatropic for

the saturated rings of cage-1 and the unsaturated rings of cage-3

and cage-7, compared to the aromatic benzene ring (NICS(1) 510.2, at the same level of theory). The corresponding NICS(1)

values become strongly diatropic upon the insertion of the cen-

tral ptC in 3 and 7, indicating the three-dimensional delocaliza-

tion of ptC’s pz-electrons in these systems. In contrast, the intro-

duction of ptC only leads to a small increase of the NICS(1)

value for the saturated rings of 1 in which no significant electron

delocalization is expected.

It is noteworthy to emphasize that, compared to the first

ptC(C)4-containing neutral hydrocarbon 1, hydrocarbon 7 here is

particularly remarkable, since, it is simpler in structure and,

apart from strain effect, the ptC inside is also further stabilized

by significant electron delocalization. In fact, it has been pointed

out that 1 is actually an energy minimum at MP2 but not at

B3LYP/6-3111G (3df,2p) level.16 At the B3LYP/6-3111G**

level as calculated here, 1 has a 226i cm21 imaginary frequency

(see Table 1). At the same time, thanks to the bigger ring size

in 7, the strain in 7 should be much less than in 3. Indeed, this

is confirmed by obviously lower strain energy of 7. By using

homodesmotic reactions28 (see Supp. Info.), the estimated strain

energies for 1, 3, and 7 are 245, 315, and 264 kcal mol21,

respectively, as shown in Table 2. The strain energy in 7 is a lit-

tle higher than in 1, which may be due to the fact that unsatu-

rated rings are involved in the cage structure of 7. However, the

strain energy induced by the insertion of ptC for 7 is smaller

than that for 1 (see Table 2), which can be ascribed to the fur-

ther stabilization through the p electron delocalization in 7.

Moreover, the smallest vibration frequency of 7 is much higher

than those of 3 (see Table 1), indicating that 7 may be also

kinetically more stable than 3. Because of the simplicity, perfec-

tion, and stability, hydrocarbon 7 should be of more interest for

synthetic challenges. It is expected that the strain can be further

alleviated by increasing the ring size from 7. However, the

resulting compound 8 (D2) only has a quasi-ptC in the center (at

the B3LYP/6-31G* level). Therefore, with a ptC achieved by

the delicate balance of strain and electronic effects, 7 might be

so far one of the best targets for the synthesis of the first hydro-

carbon containing ptC.

The combination of the new strategy and other previous strat-

egies (such as boron coordination and charge compensation)15,16

can help us explore more ptC structures. For instance, by

expanding the unsaturated rings in 3 and replacing the equatorial

carbons with borons, one can get a neutral unsaturated boraplane

compound containing a perfect ptC (9, D2h). 9 is of special in-

terest since it utilizes at the same time three different kinds of

electronic stabilization factors: r-donators and p-acceptors(borons) on the same plane as well as p-acceptors (unsaturated

hydrocarbon rings) in the perpendicular direction. As shown in

Table 1, the pz-orbital occupancy of ptC in 9 is only 0.52, even

obviously lower than that in 2 (0.61)11 which lacks the three-

dimensional delocalization of ptC’s lone pair.

Conclusions

In summary, a new series of compounds containing a perfect

ptC have been designed by means of a combined new strategy:

mechanically through the strain of rigid cage framework and

electronically through the electron delocalization in three-dimen-

sional p-conjugated system. This new strategy would have con-

siderable potential in predicting more intriguing planar tetra or

hypercoordinate carbon structures in the future. In addition, hav-

ing advantages over the first theoretically predicted ptC-contain-

ing hydrocarbon 1, the proposed neutral hydrocarbon 7 might be

a relatively more viable target for synthetic attempts.

Acknowledgments

Appreciation is expressed to the CCC-UAM for allocation of

computer time.

Table 2. Estimated Strain Energy Including Zero-Point Energy

Correction (Estr, in kcal mol21), Strain Energy Induced by the Insertion

of ptC (DEstr, in kcal mol21), and Nucleus-Independent Chemical Shift

Value at the Center (NICS(0), in ppm) and 1 A Above the Center

(NICS(1), in ppm) of the Top or Bottom Ring.

Estr DEstr NICS(0) NICS(1)

1 245 187 27.1 23.3

Cage-1 58 – 21.9 21.1

3 314 122 218.1 27.3

Cage-3 192 – 1.1 20.2

7 264 168 218.7 213.4

Cage-7 96 – 28.2 22.4

2125Perfect Planar Tetracoordinate Carbon in Neutral Unsaturated Hydrocarbon Cages

Journal of Computational Chemistry DOI 10.1002/jcc

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Journal of Computational Chemistry DOI 10.1002/jcc