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Cite this: DOI: 10.1039/c3cc47573e Received 4th October 2013, Accepted 16th October 2013

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Design of aromatic heteropolycyclics containing borole frameworks† Truong Ba Tai,* Vu Thi Thu Huong and Minh Tho Nguyen*

DOI: 10.1039/c3cc47573e www.rsc.org/chemcomm

The heteropolycyclic compounds containing borole units were theoretically designed. The presence of electron deficient boron atoms results in full electron delocalization and remarkably affects their aromaticity. While molecules 1 and 2a exhibit antiaromaticity for inner rings and non-aromaticity for outer rings, 2b and 2c are completely aromatic.

In recent years, borole, a five-membered ring containing boron, and its derivatives have emerged as one of the most exciting subjects in boron chemistry. Some intriguing compounds such as perfluoroaryl boranes and ligand-stabilized boroles were successfully synthesized and their structures and reactivity were characterized in several experimental studies.1–5 However, due to the presence of electron-deficient boron atoms, all these cyclic compounds exhibit an antiaromatic or non-aromatic character with four valence p-electrons distributed on a five-membered ring. As a consequence, the search for, and preparation of, aromatic compounds containing borole frameworks constitutes an interesting and actual challenge. In 2010, Braunschweig and co-workers marked an important breakthrough when they successfully synthesized a carbene stabilized p-boryl anion whose boron atom exhibits unprecedented p-nucleophilic character.6 Owing to the presence of two excess p-electrons on the borole ring, one from the negatively charged state of the anionic radical and the other from the electron-donating carbene, this species is a rare borole-based compound which exhibits aromaticity with six valence p-electrons.7 An aromatic dianionic borole system was also recently synthesized by Braunschweig et al.8 In this context, a legitimate question arises as to whether aromatic molecules based on borole frameworks exist as stable compounds. In a different context, [8]circulene and its analogues are of current interest for chemists. The polycyclic compounds, such as tetraxo[8]circulene,9 tetracyclopenta[def, jkl,pqr,vwx]tetraphenylenes

Department of Chemistry, University of Leuven, Celestijnenlaan 200F, Leuven, Belgium. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Computational methods. Selected MOs of 1, 2a, 2b and 2c. See DOI: 10.1039/c3cc47573e

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(TCT),10 [8.5]coronene,11 octathio[8]circulene12 and its derivatives,13,14 were intriguing subjects for many recent experimental and theoretical studies. In spite of the earlier theoretical report on the highly strained structure and instability of [8]circulene,15 this molecule was successfully synthesized by Wu et al.16 A common characteristic of these compounds is that they possess antiaromatic inner eight-membered carbon rings. As far as we are aware, aromatic eight-membered rings are not common. We now report a method of designing aromatic eight-membered cycles using boroles as building blocks. We examine the structural and aromatic characteristics of some polycyclic compounds containing the borole frameworks. Compound 1 is constructed by replacing –CH groups of TCT by –B-R groups. Compounds 2a–2c are composed of an inner eight-membered carbon ring which is surrounded by fivemembered rings (Fig. 1). We found that compounds 1, 2a and 2b have planar and highly symmetrical geometries, and compound 2c has a bowl-shaped structure. More interestingly, while the designed molecules 1 and 2a exhibit antiaromaticity for inner rings and non-aromaticity for outer rings, the remaining molecules 2b and 2c are completely aromatic. The presence of electron deficient boron atoms results in strong electron delocalization over the entire cyclic structures and remarkably enhances their aromaticity. These results not only give more insight into new aromatic borole compounds, but they also show the emergence

Fig. 1

Structures of polycyclic compounds based on borole frameworks.

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of a new type of boron compounds, the aromatic boron polycyclics. Geometry optimizations and vibrational frequency calculations of all compounds considered were performed using density functional theory at the B3LYP/6-31G(d) level of theory. The optimal structures were used in further calculations of the nuclear independent chemical shift (NICS) and other properties. Geometrical parameters of structures considered (see the ESI†) reveal that compound 1 in which four –CH groups of five-membered rings of TCT are replaced by four –B-R groups has a planar geometry similar to that of TCT. The B–C distances of 1 are the same and equal to 1.542 Å. This value is somewhat greater than the value of 1.451 Å of corresponding C–C bonds of TCT. Compounds 2a and 2b are also found to have planar structures whose B–C distances are the same and equal to 1.588 Å. However, owing to much shorter C–C bond lengths of fivemembered rings (dC–C = 1.425 Å), compound 2c has a bowl-shaped structure similar to that of [8,5]coronene.11 Aromaticity is one of the most popular but intriguing characteristics of polycyclic compounds and widely discussed in the literature. In the present work, we examine aromatic characteristics of the designed molecules using the magnetic criteria of the nucleus independent chemical shift (NICS),17 and the model of disk-aromaticity which was successfully applied in evaluating the aromaticity of boron clusters.18 Calculations of the nucleus independent chemical shift (NICSzz) along the zz-component of the shielding tensors are carried out at the centers of outer and inner rings and also at positions of 1 Å above these centers. Computed results given in Table 1 reveal that the inner eight-membered rings of 1 and 2a have positive NICSzz(1) values and consequently exhibit antiaromaticity. The NICSzz(1) values of borole rings of 1 are found to be less positive than those of 2a (NICSzz(1) = +19.9 for 1 and +42.1 for 2a). These results are consistent with earlier studies showing that the heteroarene-fused boroles have enhanced antiaromatic character, whereas the benzene-fused boroles have reduced antiaromatic character.19 More interesting is perhaps the non-aromatic feature of benzene and thiophene units of these compounds. Our calculations showed that the NICSzz(1) values of benzene and thiophene units of 1 and 2a are close to zero. Surprisingly, we found that 2b and 2c are completely aromatic systems. The NICSzz(1) values of all inner and outer rings of these molecules are highly negative (Table 1). The presence of electron deficient boron atoms results in an enhanced electron delocalization and thereby aromaticity. To obtain a better understanding about the aromatic features of the molecules designed, we now examine them in some oligomers 3–10 containing borole units. In good agreement with previous reports,19 borole rings of molecules 3 and 4 exhibit Table 1

antiaromatic characteristics which are reduced in the benzene fused structure 3 and increased in the thiophene fused structure 4. The antiaromatic character of borole in 5 and 6 is considerably reduced. In contrast, both benzene (in compound 5) and thiophene (in compound 6) moieties are non-aromatic with NICSzz(1) values close to zero. These predictions are consistent with the aromatic features of 1 and 2a revealed above. We also found that the aromatic character of the borole unit in polyboroles increases with the increasing chain length. As shown in Fig. 2, borole rings in 7 and 8 turn out to be antiaromatic. In 9, two outer borole rings (B) exhibit antiaromaticity, whereas two remaining rings (A) are aromatic. All cyclic units of 10 contain aromatic features with negative NICSzz(1) values. These results are consistent with our observations stated above that both 2b and 2c are aromatic. In order to gain more insight into the electron distribution of these novel compounds, their characteristics of disk-aromaticity are considered. The concept of disk-aromaticity was recently proposed by us and successfully applied to homoatomic systems such as B19 , B202 and B30.18 An ultimate question thus arises as to whether it can be applied to heterocyclic systems. A detailed description of this concept can be found elsewhere.18b Generally, it is proposed on the basis of a simple model of a particle in a circular box. The eigenstates in the box model are characterized by radial (n = 1, 2, 3,. . .) and rotational (l = 0, 1, 2,. . . or s, p, d,. . .) quantum numbers and the energies are determined by the zeros of cylindrical Bessel functions. In an ideal model, the eigenstates having the same rotational quantum number (l ) will be degenerate in energy levels. As a consequence, the eigenstates with a non-zero value for the rotational quantum number will be twofold degenerate. The lowest-lying eigenstates in ascending order are 1s, 1p, 1d, 2s,. . . Accordingly, a molecule exhibits disk-aromaticity when its valence electrons fully occupy the degenerate eigenstates. Otherwise, it will be antiaromatic. The shapes of p orbitals depicted in Fig. 3 reveal that 2b has 16 p-valence electrons which in turn occupy 8 p-MOs. Although a rearrangement of energy levels occurs where 1F-MOs are occupied earlier than 2s-MO and 2p-MO, their shapes match well with those of the lowest-lying eigenstates of the model, and they are in ascending order: 1s, 1p, 1d, 1F and 2s. The reverse of some energy levels is probably due to the presence of

NICSzz values of compounds obtained at the B3LYP/6-31G(d) level

Inner ring

Rings 1,3,5,7

Rings 2,4,6,8

Comp

(0)

(1)

(0)

(1)

(0)

(1)

1 2a 2b 2c TCT Coronene

+41.1 +35.5 15.0 35.7 +97.0 +221.9

+25.9 +20.8 22.5 51.2 +76.7 +213.0

+46.6 +72.5 +20.1 +6.5 +95.0 +67.7

+19.9 +42.1 7.7 32.6 +65.3 +108.5

+19.4 +25.1 +20.3 1.0 +140.5 +67.7

1.8 0.3 7.7 34.5 +98.3 +108.5

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Fig. 2 Structures and NICSzz(1) values of some oligomers containing borole units. The R substituent is para-methyl phenyl.

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Communication the future. More importantly, the concept of disk-aromaticity is proven to be effective for evaluating the aromaticity of not only homoatomic, but also heteroatomic polycyclic compounds. We are indebted to the KU Leuven Research Council. We would like to thank the FWO-Vlaanderen for a postdoctoral fellowship for TBT and PhD scholarship for VTTH.

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Notes and references

Fig. 3

Shapes of the lowest eigenstates of model and p-MOs of 2b.

different elements, whereas the model of box is established from a single particle. As a consequence, molecule 2b can be proposed as a disk-aromatic species. More interestingly, there is a consistency between our model of disk-aromaticity and the ipsocentric model that was successfully applied to aromaticity of polycyclic compounds.20 HOMO of 2b has no vertical node (L = 0), whereas each of its LUMO and LUMO + 1 has one vertical node (L = 1). Consequently, the virtual excitation from HOMO to either LUMO or LUMO + 1 of 2b results in a diatropic ring current with DL = 1 which makes 2b aromatic. Similar observation was found for 2c whose 24 p-valence electrons are distributed in 12 p-MOs with an energy order of (1s)2(1p)4(1d)4(1F)4(2s)2(2p)4(1g)4 (see ESI†). Thus, molecule 2c also exhibits disk-aromaticity. It is worth noting that in the ¨ckel’s rule of (4N + 2) electrons, these comframework of Hu pounds will be antiaromatic with N = 4 for 2b and N = 5 for 2c. These predictions confirm that the concept of disk-aromaticity can be applied to both homoatomic compounds such as boron clusters18 and heteroatomic polycyclic compounds, whereas the ¨ckel’s rule of counting electrons cannot. Consisclassical Hu tently, valence p electrons of 1 and 2a do not fully occupy the degenerate eigenstates of model (see ESI†). They consequently behave as disk antiaromatic systems. In summary, we theoretically designed a new type of heteropolycyclic compounds containing borole units. We found that the presence of electron deficient boron atoms results in full electron delocalization. While molecules 1 and 2a exhibit antiaromaticity for inner rings and non-aromaticity for outer rings, both 2b and 2c are aromatic. Such aromatic B-compounds containing eight-membered rings have not been found yet. Since the analogous compounds such as TCT, C16S8, tetraxo[8]circulene, etc. were successfully synthesized in the laboratory, we hope that the designed molecules will be the subject for experimental studies in

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Chem. Commun.

Design of aromatic heteropolycyclics containing borole frameworks.

The heteropolycyclic compounds containing borole units were theoretically designed. The presence of electron deficient boron atoms results in full ele...
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