Structure and Stability of the Heteroannulated [8–10]Circulenes: a Quantum-Chemical Study*

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Pure Appl. Chem., Vol. 82, No. 4, pp. 1011–1024, 2010. doi:10.1351/PAC-CON-09-10-36 © 2010 IUPAC, Publication date (Web): 19 March 2010 Structure and stability of the heteroannulated [8–10]circulenes: A quantum-chemical study*

Tatyana N. Gribanova1, Nikolay S. Zefirov2, and Vladimir I. Minkin1,‡

1Institute of Physical and Organic Chemistry at Southern Federal University, pr. Stachki 194/2, Rostov-on-Don, 344090, Russian Federation; 2Moscow State University, Leninskiye Gory, Moscow, 119899, Russian Federation

Abstract: The structure and stability of a new family of heterocyclic [n]circulenes CnNnYn (n = 8–10; Y = N, P, As, BF2, AlF2, GaF2) have been computationally studied using density functional theory (DFT) B3LYP/6-311G* calculations. The geometry of the compounds is determined by the balance of steric factors and effects of aromatic π-electron delocalization depending on the electronegativity of heteroatoms. An increase in the bulkiness of peripheral atoms Y and enlargement of the inner Cn cycle leads to the gradual transformation of the bowl-shaped circulene structure (Y = N, BF2) to planar and then saddle-shaped forms (Y = P, As, AlF2, GaF2). The calculations performed manifest stabilization of the heteroannulated circulenes ensured by the strong aromatic character of the conjugated five-membered rings fringing the central non-aromatic or antiaromatic [8–10] annulene rings.

Keywords: annulation; aromaticity; [n]circulenes; quantum-chemical calculations.

INTRODUCTION The concept of aromaticity continues to be a topic of many studies concerned with its quantification and classification of the structural types of the relevant compounds [1–3]. In general, a (4n + 2) π-electronic ring framework serves as a reliable indicator of the aromatic character of a molecule or an ion. At the same time, [6]radialenes 1c belonging to hydrocarbons of this type do not exhibit aromatic properties. Their molecules have a nonplanar chair conformation [4,5] and tend to rapidly polymerize at room tem- perature.

*Pure Appl. Chem. 82, 757–1063 (2010). An issue of reviews and research papers based on lectures presented at the 13th International Symposium on Novel Aromatic Compounds (ISNA-13), 19–24 July 2009, Luxembourg City, Luxembourg on the theme of aromaticity. ‡Corresponding author: E-mail: [email protected]

1011 1012 T. N. GRIBANOVA et al.

Radialenes can be stabilized by incorporating the exocyclic double bonds into a cyclic (heterocyclic) core. Thus, triphenylene 2, perylene 3, and trithiophene 4 [6] related to the radialene type pos- sess a planar central six-membered ring, which evidences the enhancement of the aromatic character of a composite system by the framing of the central radialene ring with a peripheral aromatic system. At the same time, mere extension of the conjugated system, as, for example, in [6]radialene de- rivative 5, does not warrant enhancement of aromaticity. Indeed, 5 has a quasi-cyclohexane chair conformation of the central ring and the central ring of coranullene 6, which is [5]radialene, has a boat-like conformation, whereas both coronene 7 and kekulene 8 have D6h symmetric planar structures [7].

It may, therefore, be assumed that the key structural factor ensuring flattening of the central six- membered ring in [6]radialene derivatives 2–4 and coronene 7 is the spatial angular strain caused by annulation of this ring. The role and relative significance of the electronic and spatial factors are illustrated by comparison of the ruffled D2d structure of [8]circulene 9, where the central antiaromatic cyclooctatetraene ring retains its typical tube-shaped conformation [8], and [8]circulene derivatives 10–14. Annulation of cyclobutene substituents (10a–c) stabilizes planar species of symmetry D4h [9]. Theoretical studies point to strong orbital interaction between the substituents and the central ring, the occurrence of which favors planarization of the central eight-membered ring [10,11]. Attachment to the antiaromatic 8π-electron ring of peripheral aromatic rings is exemplified by structure 11 and its heteroanalogs 12 and 13 [12–14]. The formation of the planar central ring in these compounds is facilitated by favoring angular strain conditions depending on the type of annulated fragment [14]. At the same time, [8,5]-coronene 14 has a boat conformation [14,15].

Here we report on the results of the quantum-chemical study by the density functional theory (DFT) B3LYP/6-311G* method of the structure and stability of a series of neutral nitrogen [n,5]-coronene analog 15–23 (n = 8–10), which can be treated as [n]circulene derivatives in which the inner ring is bonded and conjugated with the aromatic peripheral environment composed of atoms of Period 2–4 elements.

COMPUTATIONAL METHODS Calculations were carried out using Gaussian 03 [16] by B3LYP/6-311G* DFT approach [17–20], which is considered to be a reliable theoretical tool for accurate prediction of molecular geometry and properties of diverse molecules including circulene derivatives [6,14,21]. Analytic harmonic frequencies at the same level of theory were used to characterize the nature of the structures studied herein. All the structures corresponding to the stationary points on the respective potential energy surfaces (PESs)

RESULTS AND DISCUSSION According to the calculations performed for nitrogen-containing systems 15a–17a, the structures of λ λ symmetry Dnh correspond to the stationary points with the rank > 5 (hereinafter, denotes the number of negative eigenvalues of the Hessian at a given stationary point) on the corresponding PESs. The structures 15a–17a (Fig. 1) have stable bowl-shaped Cnv conformations with a planar central ring Cn and the equalized C–C bonds (1.394, 1.388, and 1.386 Å for 15a, 16a, and 17a, respectively) close in length to typical double C=C bonds (1.34 Å [23]). The increase in cycle size is accompanied by shortening of the C–C bonds and by flattening of the bowl-shaped structures: the magnitude of the “convex” angle ϕ (Scheme 1, a) being predicted in the increasing order of 15a (133º) < 16a (139º) < 17a (147º). The degenerate bowl-to-bowl inversion of 15a–17a through a planar transition state is in- hibited by high-energy barriers (45–150 kcal/mol), decreasing upon increasing in cycle size.

Fig. 1 Geometric characteristics (bond lengths are in ångströms) and the lowest harmonic vibrational frequencies ω –1 ( 1, in cm ) of the compounds 15a–17a corresponding to minima of the PESs calculated by the B3LYP/6-311G* method.

Scheme 1 Convex (a) and sector (b) angles.

The distortion of planar structures of the nitrogen-containing derivatives is caused by strong angular strain. The degree of this strain is determined by the sum of “sector” angles φ (Scheme 1, b) of peripheral heterocycles [13,24]. If the sum of the angles is close to 360º, a circulene is flat. A stronger deviation from 360º leads to the increase in the strain and distortion of the molecule to the bowl-shaped form (Σ << 360º) or to the saddle-shaped form (Σ >> 360º). The increase in the bulkiness of peripheral atoms Y is accompanied by increasing of the magnitude of the “sector” angle φ in the order of Y = N (φ = 29º; sum = 232º) < Y = P (φ = 41º; sum = 328º) < Y = As (φ = 45º; sum = 360º). The replacement of the peripheral nitrogen atoms in 15a by bulkier phosphorus atoms results in an increase of the sector corresponding to the peripheral five-membered heterocycle and in pronounced flattening of the polycyclic system (15b, Fig. 2). Thus, compound 15b acquires the structure of D4d symmetry with very insignificant out-of-plane distortions. The substitution of peripheral phosphorus atoms by bulkier arsenic centers (15c, Fig. 2) leads to the optimal steric conditions for stabilization of a planar D8h structure. The first harmonic vibrational frequencies of the compounds 15a–c correspond to the mode relating to the transformation towards the less energy favorable isomeric forms 15/, which are higher in energy than 15a–c by 13.2, 94.1, and 68.9 kcal/mol, respectively.

The eight-, nine-, and ten-membered rings in antiaromatic [8]annulene (CH)8 and [9]annulene + (CH)9 cation and formally aromatic [10]annulene (CH)10, that constitute the core cycles of hetero- coronenes 15–17, are distorted from planarity. The eight-membered ring of cyclooctatetraene (CH)8 corresponding to the central cycle of 15a has a nonplanar tub-shaped D2d-conformation 24 [25–28] and + cyclononatetraenyl cation (CH)9 adopts a twisted Möbius aromatic conformation 25 [29–31]. A molecule of cyclodecapentaene (CH)10 26, whose ten-membered ring is in the center of the structure 15a, is not planar because the steric strain overshadows the aromatic stabilization [7,32–36]. Hence, the results of the calculations give a clear evidence for that annulation of the central ring of [8–10]circulenes 15–17 with a peripheral aromatic system provides for the stabilization of their Cn planar structures.

The phosphorus derivatives 15b and 16b contain totally planar inner cycles, but for the next mem- ber of this family of compounds, 17b, the central ten-membered cycle undergoes a saddle-shaped deformation. The same trend is characteristic for the arsenic-containing compounds 15c–17c with the dif- ference that the distortion of the central ring starts already for [9]circulene 16c.

Fig. 2 Geometric characteristics (bond lengths are in ångströms) and the lowest harmonic vibrational frequencies ω –1 ( 1, in cm ) of the compounds 15b,c–17b,c corresponding to minima of the PESs calculated by the B3LYP/6- 311G* method.

Thus, gradual increase in the bulkiness of peripheral atoms Y and enlargement of the inner Cn cycle leads to the transformation of the bowl-shaped structure of the nitrogen-containing circulenes 15a–17a to flat structures of the phosphorus-containing compounds 15b, 16b and further to the saddle- shaped conformation of the central ring in 17b. For the arsenic-containing circulenes 15c–17c, deformation of the central ring starts begins in the [9]circulene 16c. These findings are in accord with the results of the previous studies of heterocirculenes 12 and 13 [12,13]. The use of magnetic indices of aromaticity, such as nuclear independent chemical shift (NICS [37–39]) values, allows one to judge on aromatic characteristic of each cyclic fragment of the composite system. The calculated NICS(0) values (Table 1) manifest non-aromatic or modestly antiaromatic character of the eight- to ten-membered inner rings in compounds 15–17 (NICS values are in the range of –3.3–6.6 ppm). At the same time for all the compounds, except for 16c and 17c, the five-membered rings display substantial aromatic character, which is weaker in the P/As substituted circulenes as com- pared with the analogous nitrogen-containing compounds. Because paratropic in-plane local effects can severely perturb NICS(0) values [31,39], NICS(1) values (1 Å above the plane) were also computed and found not to influence the qualitative conclusion on the aromatic properties of the compounds under study. For instance, for the inner cycles of planar systems 15b, 16b, and 15c, NICS(1) values are 2.1, 0.3, and 3.2 ppm, respectively.

Table 1 The B3LYP/6-311G* calculated characteristicsa of compounds 15–17. Δ Structure EHOMO–LUMO NICS(0) 15a 3.2 –3.3,b –14.0c 16a 3.4 –0.3,b –14.2c 17a 4.0 3.8,b –14.5c 15b 2.9 4.9,b –8.9c 16b 2.6 1.7,b –7.6c 17b 2.4 5.5,b –6.9c 15c 2.5 6.6,b –6.4c 16cd 2.3 3.4,b –4.1c 17cd 1.9 4.2,b –2.8c

aΔ EHOMO–LUMO (eV): energy gap between the frontier orbitals; NICS(0) (ppm): NICS index calculated at centers of the central (b) and peripheral five-membered (c) rings. dFor the nonplanar inner cycles, NICS(0) values were calculated at the geometrical center of ring.

All the compounds have significantly broad highest occupied molecular orbital–lowest unoccu- pied molecular orbital (HOMO–LUMO) energy gaps (Table 1), which increase with widening of the inner cycle for nitrogen derivatives 15a–17a, but exhibit an opposite tendency in the case of the circulenes framed by heavier atomic centers, 15b,c–17b,c. Decrease in the aromatic character of the peripheral environment is accompanied by lowering of the HOMO–LUMO energy gaps. The isoelectronic substitution of peripheral nitrogen centers in the systems 15a–17a by BF2, AlF2, and GaF2 groups generates a new series of heteroannulated circulenes 18–23 shown in Figs. 3–6. Structures 18–20 are formed by a partial substitution in the peripheral crown of 15–17. Alike their nitrogen analogs 15a, 16a, the boron-containing compounds 18a, 19a, have stable bowl-shaped Cnv structures containing planar Cn rings with alternating C–C bonds (Fig. 3). As a consequence of the larger size of the boron-containing centers conducing to relief of the steric strain of a polycyclic system, the energy barriers against the bowl-to-bowl inversion of these compounds are considerably lower (less than 20 kcal/mol) than those computed for 15a–17a. As in the case

Fig. 3 Geometric characteristics (bond lengths are in ångströms) and the lowest harmonic vibrational frequencies ω –1 ( 1, in cm ) of the compounds 18a–20a corresponding to minima of the PESs calculated by the B3LYP/6-311G* method. of nitrogen compounds 15a–17a, the increase in the size of the central cycle is accompanied by flattening structure, the tendency of which is illustrated by the magnitudes of angle ϕ: 147º and 153º for 18a and 19a, respectively. Further enlargement of this cycle in 20a leads to stabilization of the planar circulene structure (Fig. 3). Substitution of boron centers in 18a–20a by bulkier and less electronegative Al and Ga atoms gives rise to lower steric strain. This structural modification provides for stabilization of the planar species (18b,c, 19b,c, and 20b). The insignificant saddle-shaped distortion is observed for [10]circulene 20c (Fig. 4). Complete substitution of BF2 groups for the peripheral N atoms in systems 15a, 16a likewise cre- ates more favorable steric conditions and leads to stabilization of planar structures of Dnh symmetry 21a, 22a, while the subsequent enlargement of the inner cycle in 23a is accompanied by the saddle- shaped distortion (Fig. 5). Same-type deformation and destabilization of the planar structure are characteristic of the compounds 21b and 21c containing voluminous Al- and Ga-centered groups (Fig. 6). The saddle-shaped distortion of the structure increases with enlargement of the central cycle and by passing from AlF2 to GaF2 substituents. Therefore, it may be concluded that the geometry of compounds 15–23 is determined

Fig. 4 Geometric characteristics (bond lengths are in ångströms) and the lowest harmonic vibrational frequencies ω –1 ( 1, in cm ) of the compounds 18b,c–20b,c corresponding to minima of the PESs calculated by the B3LYP/6- 311G* method.

Fig. 5 Geometric characteristics (bond lengths are in ångströms) and the lowest harmonic vibrational frequencies ω –1 ( 1, in cm ) of the compounds 21a–23a corresponding to minima of the PESs calculated by the B3LYP/6-311G* method. by both steric factors related to the volume of peripheral heteroatoms and the effects of aromatic π-electron delocalization depending on the electronegativity of heteroatoms. In summary, we have computationally designed a new family of potentially viable [40] hetero - analogs of [8–10]circulenes. Their structure is determined by the balance of steric factors and aromatic π-electron delocalization effects. An increase in the bulkiness of peripheral atoms Y and enlargement of the inner Cn cycle leads to the gradual transformation of the bowl-shaped circulene structure to planar and then saddle-shaped forms. These compounds can form a basis for designing more complicated derivatives with interesting structural and electronic characteristics. This is demonstrated by our find- ing that stacking dimerization of monomer fragments yields stable heterofullerene structures 27, 28 [41]. As exemplified by the clusters 29, 30, a number of stable cage structures possessing interesting chemical properties can be designed by interlaying heteroannulated circulenes with bridging groups. Thus, the calculated proton affinity of the double-deck structure 29 significantly exceeds that of proton sponge. As seen from Fig. 7, the compound 29 binds up to eight protons (31). Protonation of cluster 29 occurs at the fluorine centers. Proton affinity (for attaching eight protons) is equal to 180 kcal mol–1.

Fig. 6 Geometric characteristics (bond lengths are in ångströms) and the lowest harmonic vibrational frequencies ω –1 ( 1, in cm ) of the compounds 21b,c–23b,c corresponding to minima of the PES calculated by the B3LYP/6- 311G* method.

Fig. 7 Geometric characteristics (bond lengths are in ångströms) and the lowest harmonic vibrational frequencies ω –1 ( 1, in cm ) of the compounds 27–31 corresponding to minima of the PES calculated by the B3LYP/6-311G* method.

ACKNOWLEDGMENTS This work was supported by CRDF (Grant RUC1-2889-RO-07), Russian Foundation for Basic Research (Grant 07-03-00223) and the Council for Grants of the President of the Russian Federation for Support of Young Scientists (Grant MD–5906.2008.3) and Leading Scientific Schools (Grant NSh- 3233.2010.3).

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