Structure determination and study of structural nonrigidity of pentaarylcyclopentadienyl halides are of great importance since these compounds are precursors to a wide range of cyclopentadiene derivatives with practically useful properties, including complexes that are prototypes of molecular motors [1–5]. In addition, numerous metal complexes with pentaarylcyclopentadienyl ligands are widely used to catalyze asymmetric cycloadditions and a number of other important reactions [1, 6] and are highly efficient luminophores and compounds with nonlinear optical properties [7–9]. Pentaphenylcyclopentadiene itself is a fluorescent molecular rotor whose emission is induced by intra- and intermolecular interactions between phenyl rings [10].

By using 1H and 13C NMR techniques we previously revealed and studied intramolecular migrations of halogens in halocyclopentadiene derivatives. It was found that chlorine and bromine atoms migrate around the five-membered ring of tolyltetraphenylcyclopentadiene with energy barriers ΔG≠298 of 25.9 and 18.1 kcal/mol, respectively [11–13]. The migrations of chlorine and bromine around pentamethyl cyclopentadienepentacarboxylate and tetramethyl alkylcyclopentadienetetracarboxylates were characterized by barriers of 25.7–27.3 and 16.2–22.9 kcal/mol, respectively [14]. Circumambulatory rearrangements of 5-iodocyclopentadiene are much faster, and the corresponding energy barrier is 14 kcal/mol [15]. According to DFT calculations, 5-iodocyclopentadiene in a rotating electric field behaves as a molecular rotor in which the iodine atom rapidly moves in one direction around the five-membered ring (k298 = 630 s–1) [16].

Rearrangements involving halogen migration in cyclopolyenes and other systems, in addition to sigmatropic shifts of hydrogen and some other organic, organoelement, and organometallic groups, are widely used in organic and organometallic chemistry [12, 13, 17–21].

In this work we examined possible halogen migration paths in 5-halo-1,2,3,4,5-pentaphenylcyclopentadienes 1a–1d [Hlg = F (a), Cl (b), Br (c), I (d)] and structures of the corresponding transition states by DFT quantum chemical calculations at the CAM-B3LYP/Def2TZVP level of theory. The available experimental data do not allow appropriate selection of one of the alternative paths for fluctuating behavior of compounds 1a–1d. Halogen migration around the cyclopentadiene ring of 1a–1d can be mediated by 1,5- or 1,3-sigmatropic shifts, as well as by randomization through the formation of tight ion pairs. Furthermore, there are no published data on the effects of all halogens on the activation barrier and mode of their migration in the pentaphenylcyclopentadiene system, nor conformations of these compounds in the ground and transition states were studied. In order to elucidate these problems, migration of halogens around the five-membered ring of pentaphenylcyclopentadiene was simulated by quantum chemical calculations, and the calculated activation barriers for these processes were compared with available experimental data.

The calculations showed that structures of 1a–1d corresponding to minima on the potential energy surfaces for the gas phase have chiral propeller [22] conformation (Table 1, Scheme 1, Fig. 1). Alternative structures 2a–2d where the phenyl groups are oriented in a nonpropeller fashion and are turned to different sides with respect to the five-membered ring plane were not localized on the PESs.

Table 1. Calculated (CAM-B3LYP/Def2TZVP) total and relative energies of the ground states of 5-halo-1,2,3,4,5-pentaphenylcyclopentadienes 1a–1d and transition states for halogen migration therein in the gas phasea–c
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Scheme
scheme 1

1.

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Fig. 1.
figure 1

Calculated (CAM-B3LYP/Def2TZVP) geometric parameters of chiral structures of compounds 1a–1d (P) in the ground state in the gas phase. Hereinafter, the bond lengths are given in Å.

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The four phenyl groups on C1–C4 of the cyclopentadiene ring of chiral propeller structures 1a–1d (P) are turned clockwise with respect to the five-membered ring through the following dihedral angles: C2C1C6C7 137.3° (1a), 131.3° (1b), 141.8° (1c), 149.9° (1d); C3C2C8C9 125.7° (1a), 128.6° (1b), 126.9° (1c), 125.4° (1d); C4C3C10C11 127.0° (1a), 126.6° (1b), 126.0° (1c), 124.2° (1d); and C5C4C12C13 123.6° (1a), 119.9° (1b), 115.8° (1c), 117.0° (1d). The phenyl group on C5 is almost orthogonal to the cyclopentadiene ring plane. The benzene rings on C1–C4 of the correspondingM enantiomers of 1a–1d are turned counterclockwise. The C5–Hlg bond lengths in 1a–1d are C5–F 1.397, C5–Cl 1.817, C5–Br 1.999, and C5–I 2.218 Å. The charge on the halogen atom in the most favorable structures 1a–1d changes from negative on the fluorine atom to positive on the iodine atom: –0.357 (F), –0.043 (Cl), 0.011 (Br), 0.111e (I).

Transition states (TS) 3a–3d and 4a–4d with C1 symmetry, which correspond to sigmatropic 1,5- and 1,3-halogen shifts around the five-membered ring, were localized on the PESs for halocyclopentadienes 1a–1d (Table 1; Schemes 1, 2; Figs. 2, 3). No structures corresponding to the randomization mechanism involving tight ion pairs were identified, which indicated that this mechanism is not operative in the migration of halogens around the five-membered ring of halopentaphenylcyclopentadienes 1a–1d.

Scheme
scheme 2

2.

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Fig. 2.
figure 2

Calculated (CAM-B3LYP/Def2TZVP) geometric parameters of chiral structures of transition states 3a–3d (P) for 1,5-halogen shift in the gas phase.

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Fig. 3.
figure 3

Calculated (CAM-B3LYP/Def2TZVP) geometric parameters of chiral structures of transition states 4a–4d (P) for 1,3-halogen shift in the gas phase.

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The distances between the migrating halogen atom and two nearest carbon atoms of the cyclopentadiene ring in asymmetric propeller-like transition states 3a–3d (P) for 1,5-shifts (Scheme 1, Fig. 2) increase in going from fluorine to iodine: 1.896 and 1.910 (F), 2.335 and 2.356 (Cl), 2.480 and 2.503 (Br), and 2.632 and 2.655 Å (I). The cyclopentadiene ring in TS 3a–3d is planar, and the π-electron density is delocalized over the entire ring, as follows from similarity of the C5–C1, C1–C2, C2–C3, and C3–C4 bond lengths. All five benzene rings are turned clockwise with respect to the five-membered ring; the torsion angles are C1C5C14C15 143.1° (1a), 150.7° (1b), 151.9° (1c), 153.6° (1d); C2C1C6C7 125.6° (1a), 126.9° (1b), 128.2° (1c), 130.4° (1d); C3C2C8C9 129.8° (1a), 128.4° (1b), 127.4° (1c), 126.2° (1d); C4C3C10C11 115.8° (1a), 113.5° (1b), 112.5° (1c), 113.2° (1d); C5C4C12C13 132.0° (1a), 135.9° (1b), 137.7° (1c), 139.1° (1d). The energy minima on the PESs corresponding to 1,5-halogen shift through TS 3a–3d (P), i.e., isomeric structures1a–1d and1a′–1d′, retain propeller conformation of the phenyl rings (P).

The calculated charges on the halogen atom in TS 3a–3d are negative and are fairly large for fluorine, chlorine, and bromine atoms [–0.514 (F), –0.301 (Cl), –0.221 (Br), –0.063 (I)], which suggests considerable charge separation between the migrating atom and the rest of the system. The energy barriers to 1,5-halogen shifts around the five-membered ring of 1a–1d through TS 3a–3d were estimated at ΔE≠ZPE = 42.5 (F), 26.2 (Cl), 20.2 (Br), and 15.2 kcal/mol (I) (Table 1).

The rearrangement of 1c was also simulated with inclusion of solvent effect using the PCM model. It was shown that the barrier to 1,5-shift of bromine in chlorobenzene is lower by 0.7 kcal/mol than in the gas phase and is ΔE≠ZPE = 19.5 kcal/mol. This value is very consistent with the barrier to bromine migration in 1c determined experimentally by dynamic NMR spectroscopy [11].

Halogen atoms are capable of migrating in each of the two enantiomers 1a–1d (P) and 1a–1d (M). According to the calculations, these enantiomers are readily interconvertible with energy barriers ΔE≠ZPE of 3.7 (F), 2.4 (Cl), 2.1 (Br), and 1.7 kcal/mol (I) via synchronous flips [22] (half-turns) of four phenyl substituents on C1–C4 through TS 5a–5d with Cs symmetry (Scheme 3, Fig. 4, Table 1). This leads to fast racemization of 1a–1d.

Scheme
scheme 3

3.

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Fig. 4.
figure 4

Calculated (CAM-B3LYP/Def2TZVP) geometric parameters of transition state 5c for the interconversion of theP and M enantiomers of 1c and transition state 6a (Cs symmetry) for 1,5-shift of fluorine in the gas phase.

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It should be noted that, unlike structures 1a–1d, the P and M enantiomers of pentaphenylcyclopentadienyl metal complexes are stable. According to the X-ray diffraction data, a unit cell of the complex [Fe(η5–C5Ph5)(CO){C(=O)H}PMe3] contains two molecules in which five phenyl groups are turned either clockwise or counterclockwise [1].

A competing mechanism of sigmatropic 1,5-fluorine shift through TS 6a with Cs symmetry was revealed for compound 1a. Its energy barrier ΔE≠ZPE = 45.3 kcal/mol (Scheme 4, Fig. 4, Table 1) is higher by 2.8 kcal/mol than that for TS 3a, which suggests higher probability of the path involving chiral TS 3a. The C5–F and C4–F distances in TS6a are equal to each other (1.884 Å). The C5–C1 and C1–C2 (C4–C3 and C3–C2) bond lengths are also very similar (1.387 and 1.414 Å). The benzene rings on C5, C1 and C4, C3 in TS 6a are turned in opposite directions (torsion angles C1C5C14C15 131.1°, C2C1C6C7 125.1°, C4C3C10C11 47.3°, C5C4C12C13 67.7°), and the benzene ring on C2 is orthogonal to the five-membered ring plane.

Scheme
scheme 4

4.

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The migrating halogen atom in transition states TS 4a–4d (Scheme 3, Fig. 3) for 1,3-sigmatropic shifts is more distant from the cyclopentadiene ring than in TS3a–3d; the distances between the halogen atom and two nearest cyclopentadiene carbon atoms are 2.397 and 2.399 (F), 3.042 and 3.044 (Cl), 3.222 and 3.224 (Br), and 3.457 and 3.459 Å (I). The C4 atom in TS 4a (Hlg = F) deviates from the plane formed by the four other ring atoms so that the torsion angle C1C2C3C4 is –8.3°. The cyclopentadiene ring in TS 4b–4d (Hlg = Cl, Br, I) is almost planar. The π-electron density in TS 4a–4d is delocalized over the C5C4C3 fragment, as follows from similar lengths of the C5–C4 and C4–C3 bonds, while the C1–C2 bond is double. Charge separation between the halogen atom and pentaphenylcyclopentadiene system in TS4a–4d is significantly greater than in TS 3a–3d: all halogen atoms possess a large negative charge, –0.720 (F), –0.719 (Cl), –0.691 (Br), and –0.621 e (I)].

The calculated energy barriers to 1,3-halogen shift in compounds 1a–1d through TS 4a–4d are significantly higher than those to 1,5-shift through TS 3a–3d and are ΔE≠ZPE = 60.7 (F), 38.6 (Cl), 32.0 (Br), and 27.9 kcal/mol (I) (Table 1). The differences between the barriers to 1,5- and 1,3-halogen shifts in 1a–1d are fairly large, ΔΔE≠ZPE = 18.2 (F), 12.4 (Cl), 11.8 (Br), and 12.7 kcal/mol (I). Therefore, the circumambulatory rearrangements of 5-halo-1,2,3,4,5-pentaphenylcyclopentadienes 1a–1d should proceed via sigmatropic 1,5-halogen shifts around the five-membered ring in keeping with the Woodward–Hofmann orbital symmetry conservation rules. The results of calculations are in good agreement with the barriers to migration of chlorine and bromine in pentaphenyl- and tolyltetraphenylcyclopentadiene systems, determined experimentally by dynamic NMR spectroscopy [11].

Our calculations revealed peculiar features of halogen migration paths around the five-membered ring of pentaphenylcyclopentadiene. In particular, energetic preference of chiral propeller conformations of molecules 1a–1d in the ground state and sigmatropic 1,5-halogen shifts through asymmetric transition states [1a–1d TS 3a–3d 1a′–1d′ …] have been established. The halogen migration processes involve synchronous rotations of the phenyl groups, leading to stable propeller conformations with the same helical chirality. Likewise, as we have recently shown by DFT calculations, sigmatropic 1,5-hydrogen shifts in 1,2,3,4,5-pentaphenylcyclopentadiene also involve propeller conformation and asymmetric transition state [23]. The experimental and theoretical energy barriers to halogen migration in the pentamethyl cyclopentadienepentacarboxylate and unsubstituted cyclopentadiene systems [14, 16, 24] are similar to those found for halopentaphenylcyclopentadienes.

It should be noted that, according to theoretical calculations, sigmatropic 1,5-shifts of chlorine and bromine in (Z,Z)-penta-1,3-dienes should be suprafacial, whereas antarafacial pathway is energetically more favorable for 1,5-sigmatropic migration of fluorine [25]. Halogen migration in related cyclopropene and cycloheptatriene derivatives, whose ionization is favored by the formation of stable aromatic cations, may follow both ionization–recombination mechanism through tight ion pairs and sigmatropic shifts, depending on the solvent polarity [12, 26, 27].

EXPERIMENTAL

Quantum chemical calculations were performed using CAM-B3LYP functional and Def2TZVP basis set included in Gaussian 09 software package both for the gas phase and with account taken of solvent effect in terms of the polarizable continuum model (PCM). Stationary points on the potential energy surfaces were identified by calculating the corresponding Hessian matrices. The charges on atoms were calculated by the natural bond orbital (NBO) method.

CONCLUSIONS

In summary, DFT CAM-B3LYP/Def2TZVP calculations have shown that halogen migration around the five-membered ring of 5-halo-1,2,3,4,5-pentaphenylcyclopentadienes occurs via 1,5-sigmatropic shifts in chiral propeller conformers through asymmetric transition states; the P and M enantiomers are readily interconvertible by way of synchronous flips of phenyl substituents. Energetic preference of 1,5-halogen shift around the five-membered ring in comparison to 1,3-shift has been revealed. The migrating ability of halogens increases in the series F < Cl < Br < I in parallel with their atomic radii.

Fast 1,5-shifts of chlorine, bromine, and iodine around the five-membered ring of pentaphenylcyclopentadiene give rise to five degenerate isomers; however, similar processes in unsymmetrical pentaarylcyclopentadienyl halides could lead to isomer mixtures, which should be taken into account while planning syntheses with the use of such reagents.