Hepta‐ and Octacoordinate Boron in Molecular Wheels of Eight‐ and Nine‐Atom Boron Clusters: Observation and Confirmation

Two types of aromaticity: Experimental and theoretical evidence shows that eight- and nine-atom boron clusters are perfectly planar molecular wheels, with a hepta- or octacoordinated central boron atom, respectively (see picture, B green). The radii of the miniature molecular wheels are found to be 1. 8 and 2. 0 Å. Analyses of their chemical bonding reveal that they possess double (σ and π) aromaticity, which is responsible for the wheel structures and the extreme coordination environments. Clusters of atoms can adopt different atomic arrangements from bulk materials. They often exhibit novel structures and properties, which provide opportunities for new types of chemical bonding and stoichiometry. Herein we report experimental and theoretical evidence that 8- and 9-atom boron clusters are perfectly planar molecular wheels, with a hepta- or octacoordinated central boron atom, respectively, despite the predominance of three-dimensional structures normally found in bulk boron and its compounds. 1–3 Hepta- and octacoordinated planar boron compounds are highly unusual. The radii of the miniature molecular wheels are found to be 1. 8 and 2. 0 Å for the 8- and 9-atom clusters, respectively. Analyses of their chemical bonding reveal that they possess double (σ and π) aromaticity, which is responsible for the novel molecular structures and the extreme coordination environments. We produced the B8− and B9− clusters with a laser vaporization cluster source and probed their electronic structure with photoelectron spectroscopy (PES), analogous to our previous study on the smaller B5− and B6− clusters. 4, 5 Figure 1 1 shows the PES spectra of B8− at three photon energies and those of B9− at two photon energies. The 193 nm spectrum of B8− revealed four distinct features labeled X, A, B, C (Figure 1 b). A fifth band (D) can be tentatively identified, but the signal-to-noise ratios are poor at the high-binding-energy side. At 266 nm (Figure 1 a), fine vibrational features were observed for the X band, which were resolved more clearly in the 355 nm spectrum. Two vibrational modes were observed with frequencies of 510±50 and 1160±60 cm−1. A sharp peak (B′), which appeared as a shoulder in the 193 nm spectrum, was also resolved in the 266 nm spectrum of B8− (Figure 1 a). All the vertical detachment energies (VDEs) are given in Table 1 and compared with theoretical calculations (see below). The sharp peak of the X band in the spectrum of B8− defines the electron affinity (EA) of neutral B8 to be 3. 02±0. 02 eV. Photoelectron spectra of a) B8− at 266 nm (4. 661 eV), the inset shows the spectrum taken at 355 nm (3. 496 eV), and b) at 193 nm (6. 424 eV). c) The photoelectron spectrum of B9− at 266 nm and d) at 193 nm. Observed VDE Molecular Final VDE VDE features (exp. ) a orbital state (calcd) b (calcd) c B8− (C2v, 2B1) X 3. 02 (2) HOMO-1 (1a2) 3B2 2. 99d A 3. 35 (2) HOMO (2b1) 1A1 3. 45d 3. 41 HOMO-1 (1a2) 1B2 3. 45 B′ 4. 55 (4) HOMO-2 (6a1) 3B1 4. 74d B 4. 68 (6) HOMO-3 (4b2) 3A2 4. 78d HOMO-3 (4b2) 1A2 4. 79 C 4. 98 (6) HOMO-2 (6a1) 1B1 5. 15 HOMO-4 (1b1) 3A1 5. 86d B9− (D8h, 1A1g) X 3. 46 (6) HOMO (1e1g) 2E1g 3. 47e (3. 38) e A 5. 04 (6) HOMO-1 (2e1u) 2E1u 5. 07e (4. 84) e HOMO-2 (1a2u) 2A2u (5. 78) e The spectral features of B9− (Figure 1 c, d1) seemed to be broader and are dominated by three main bands (X, X′′, A). The weak signals at the low-binding-energy side (X′, Figure 1 c) depended on source conditions and could be due to either hot band or isomer contributions. The X′′ band around 4. 4 eV is very broad and not well defined even under the higher resolution condition at 266 nm (Figure 1 c). The X and A bands are sharper and more intense, each with a short vibrational progression (vibrational spacing ≈1300–1400 cm−1). From the threshold of the X band, we estimated an EA of 3. 39±0. 06 eV for B9. The VDEs of the X and A bands are listed in Table 1, and compared with theoretical calculations. Combining PES and computational chemistry is a powerful means to elucidate the electronic structure and chemical bonding of novel clusters. 6–8 We performed an extensive computational search for the global minimum structures for B8, B8−, B82−, B9, B9− by using a hybrid (density functional and Hartree–Fock) method. We found that all five species have planar wheel structures (Figure 2): B8 and B82− are perfect heptagons and B9− is a perfect octagon, whereas B8− and B9 have slight in-plane distortions within the heptagonal and octagonal structures, respectively. B8, B9, and B9− have been calculated previously, 9–13 but there is no consensus among the different studies. Our heptagon triplet structure for B8 agrees with the theoretical predictions by Bonacic-Koutecky et al. 9 and Boustani. 12 Our octagon structure for B9− agrees with a date by Wang and Schleyer. 13 Optimized global minimum structures. a) D7h B8, b) D7h B82−, c) C2v B8−, d) D2h B9, e) D8h B9−. Selected bond lengths given in Å. To confirm our computational results and facilitate comparison with the experimental data, we further calculated the low-lying VDEs for B8− and B9− using sophisticated ab initio methods (Table 1). To interpret the B8− spectra, it is convenient to start from the high symmetry B82− structure (Figure 2 b). The B82− ion is closed-shell and has a degenerate highest occupied molecular orbital (HOMO; 1e1′′, Figure 3 a). Upon one-electron detachment, the resulting doublet B8− undergoes a slight Jahn–Teller distortion to C2v symmetry (Figure 2 c) with the following valence electron configuration: 1a122a121b222b223a124a123b225a121b124b226a121a222b11. The doubly degenerate 1e1′′-HOMO is now split into 2b1 (HOMO) and 1a2 (HOMO-1) in the C2v structure. When a further electron is removed from B8−, the final state can be either a singlet (detachment from the singly occupied 2b1 MO) or a triplet (detachment from the doubly occupied 1a2 MO). The lowest energy peak at 3. 02 eV in the B8− spectra (Figure 1) corresponds to electron detachment from the 1a2 orbital resulting in a triplet state (Table 1), which upon geometry optimization relaxes to the perfect heptagon triplet ground state of B8 (Figure 2 a). The calculated VDE of 2. 99 eV is in excellent agreement with the experimental value of 3. 02 eV. According to our calculations two vibrational modes (481 and 1243 cm−1) can be responsible for the C2v–D7h transformation and these frequencies agree well with the experimentally observed frequencies at 510±50 and 1160±60 cm−1. Molecular orbital pictures. a) D7h B82−, b) D8h B9−. The second feature A of the B8− spectrum is observed at 3. 35 eV (Figure 1 a), in excellent agreement with the calculated VDE from the 2b1 orbital (Table 1). Our calculated VDE for the singlet final state from detachment from the 1a2 orbital is nearly degenerate with that of the 2b1 orbital and may also contribute to the A band. The next calculated VDE is at 4. 74 eV for the triplet final state from detachment from the 6a1 orbital (HOMO-2), which is assigned to the B′ band at 4. 55 eV (Table 1). Two other detachment channels, the triplet and singlet final states from the 4b2 orbital (HOMO-3), have VDEs very close to each other and are assigned to the intense B band at 4. 68 eV. The triplet final state from the 6a1 orbital has a calculated VDE of 5. 15 eV and is assigned to the C band at 4. 98 eV. There are no single-electron detachment channels between 3. 4 and 4. 7 eV. This calculated spectral pattern is in excellent agreement with the PES spectra of B8− (Figure 1 a, b1). The weak feature around 4 eV in the PES spectra is likely to be due to a two-electron transition. The next calculated VDE is at 5. 86 eV from HOMO-4 (1b1). Although the signal-to-noise ratio is poor in the high-binding-energy side of the B8− spectrum (Figure 1 b), we can tentatively identify a detachment feature (D) around 5. 9 eV. The overall agreement between the calculated VDEs and the PES spectra of B8− is excellent, confirming unequivocally its heptagonal structure. The D8h B9− has closed shell with a highly degenerate electronic configuration: 1a1g21e1u4 1e2g41e3u42a1g21b2g21a2u22e1u41e1g4. Detachment from the 1e1g HOMO results in a 2E1g state for B9. The calculated VDE for this detachment channel is 3. 47 eV, in excellent agreement with the measured VDE of the X band at 3. 46 eV (Table 1). However, the 2E1g state is expected to undergo a Jahn–Teller distortion to the D2h ground state of B9 (2B1g). Our calculated adiabatic detachment energy (ADE) or the EA of B9 is 3. 43 eV, which is very similar to the calculated VDE and agrees well with the experimental ADE of 3. 39±0. 06 eV. The similarity between the VDE and ADE suggests that the Jahn–Teller effect in B9 is small, consistent with the minor geometry changes between B9− and B9 (Figure 2 d, e2) and the sharp spectral pattern of the X band. The next detachment channel is from the 2e1u orbital (HOMO-1) with a calculated VDE at 5. 07 eV. The experimental VDE of the A band is in excellent agreement with this detachment channel. This calculated spectral pattern gives a large energy gap between 3. 5 and 5 eV, which suggests that there are no single-electron transitions in this energy range. Thus, the broad X′′ band may have two origins: either owing to two-electron transitions or arising from a strong Jahn–Teller splitting of the detachment from the degenerate 2e1u HOMO-1. We suspect that both of these mechanisms may contribute to the X′′ band, but at this point we are not able to definitely resolve this issue because it is challenging to calculate the Jahn–Teller distortions for the excited states. The next detachment is from the 1a2u orbital with a calculated VDE of 5. 78 eV at the ROVGF level of theory (Table 1). The ROVGF method may have significantly underestimated the VDE because no intense detachment channels were observed between 5. 08 and 6. 4 eV in the PES spectrum of B9− (Figure 1 d). The overall excellent agreement between the predicted detachment energies and the PES spectra of B8− and B9− lend considerable credibility to the wheel structures found for B8, B8−, B82−, B9, and B9−. Now the question is why these clusters adopt such unusual and beautiful structures? Insight can be gained from a detailed analysis of their valence MOs, as shown in Figure 3 for the closed shell B82− (Figure 3 a) and B9− (Figure 3 b). For each molecule, we can identify two sets of MOs, one set responsible for the peripheral BB bonding and the other set for bonding between the central atom and the peripheral B atoms. The latter set is the same in both molecules: the 1e1′′, 2e1′, 1a2′′, and 2a1′ for B82−; 1e1g, 2e1u, 1a2u, and 2a1g for B9−. To confirm this classification of the MOs, we performed a model MO calculation of a cyclic Be8, which would have similar peripheral bonding as that in B9−. Indeed we found that the eight MOs of the cyclic Be8 are very similar to the eight MOs responsible for the peripheral BB bonding in B9−. Among the six MOs responsible for the central B atom and peripheral B atom bonding, three are π orbitals (1e1′′ and 1a2′′ in B82−; 1e1g and 1a2u in B9−). These π-orbital patterns are very similar to those in the prototypical aromatic molecule, benzene. Thus, the B82− and B9− molecules can be considered to be aromatic, each with six π electrons conforming to the (4n+2) Hückel rule. The π aromaticity in SiB8, which is valence isoelectronic to B9−, was discussed by Minyaev et al. 14 However, we believe that the B82− and B9− molecules are not only π aromatic, but also σ aromatic. The three σ orbitals in each molecule (2e1′ and 2a1′ in B82−; 2e1u and 2a1g in B9−), which describe bonding between the central atom and the ring, are also highly delocalized and look very similar to the π MOs, thus conferring σ aromaticity on these clusters. We believe that the double aromatic characters in the chemical bonding of B82− and B9− are responsible for their planarity and their unique coordination environments. When one electron is detached from B82− or B9−, the π aromaticity is reduced and the Jahn–Teller effect leads to a slightly distorted B8− or B9, but they are still strongly bonded systems because the σ bonding framework remains the same. When two electrons are removed from B82−, the degenerate 1e1′′ HOMO becomes half-filled, which resulting in the triplet B8 ground state with very little structural change (Figure 2 a and b). The triplet ground state (3Ag) of B8 is highly π aromatic according to the alteration of the Hückel rule for triplet states. 15, 16 To further confirm aromaticity in the molecular wheels, we calculated nucleus-independent chemical shifts (NICS) 17 for the two doubly aromatic closed-shell boron clusters: B82− and B9−. In Table 2 we presented the NICS values calculated above the center of the cluster and above the B3 triangular ring in B82− and B9−. The very high NICS values (compared to benzene with NICS=−8. 0 ppm at B3LYP6-311++G**) clearly show the presence of σ and π aromaticity in the two clusters. R Å NICS above the center NICS above the center of the cluster of the B3 ring B82− 0. 5 −27. 0 −35. 7 1. 0 −24. 8 −20. 8 1. 5 −14. 0 −11. 3 2. 0 −8. 5 −6. 9 2. 5 −5. 5 −4. 5 3. 0 −3. 7 −3. 1 B9− 0. 5 −23. 2 −33. 0 1. 0 −25. 6 −21. 0 1. 5 −15. 3 −12. 1 2. 0 −9. 2 −7. 3 2. 5 −5. 8 −4. 7 3. 0 −3. 8 −3. 1 The chemical bonding between the central boron and the ring can be viewed as spokes of the molecular wheels. However, the delocalized nature of MOs describing the center-to-ring bonding suggests that individual spokes cannot be identified, much like a bicycle wheel spinning at a high speed. Hence, instead of spokes, the molecular wheels are strengthened by a “disk”. Recently, two aromatic planar pentacoordinate iron cations (FeSb5+ and FeBi5+) have been predicted, 18 which exhibit similar delocalized bonding between the central Fe and the Sb5 or Bi5 ring. This is an unprecedented delocalization pattern, which is different from the familiar cyclic delocalization in benzene. It may be appropriate to call this delocalization in the molecular wheels “disk delocalization”. Tetracoordinate planar carbon was proposed 30 years ago by Hoffmann et al. 19 and has been confirmed experimentally. 20, 21 More highly coordinated planar carbon and other main-group atoms have been proposed. 13, 14 Our work is the first confirmation of hepta- and octacoordinated boron atoms and demonstrates that σ and π aromaticity are responsible for the highly unusual coordination environments. It should be emphasized that detailed understanding of novel molecular structures using concepts of chemistry will be increasingly important in nanotechnology as device features rapidly approach the atomic and molecular scale. Photoelectron Spectroscopy: Details of the experimental apparatus have been published elsewhere. 22, 23 Briefly, a compressed disk made with pure 10B isotope (98 % enriched) was used as the laser vaporization target with helium carrier gas. Clusters from the source underwent a supersonic expansion and collimated with a skimmer. Negatively charged clusters were extracted from the cluster beam and were analyzed by a time-of-flight (TOF) mass spectrometer. The B8− and B9− cluster ions were mass-selected before photodetachment with one of the three photon energies: 355 nm (3. 496 eV), 266 nm (4. 661 eV), and 193 nm (6. 424 eV). The use of a single isotope target greatly simplified the mass spectra and allowed clean mass selections without any contamination. Photoelectron spectra were measured using a magnetic-bottle TOF photoelectron analyzer with an electron kinetic energy resolution of ΔEk/Ek≈2. 5 %. The spectrometer was calibrated with the known spectrum of Rh−. Theoretical Calculations: Theoretical methods used in this work were described in more detail elsewhere. 4, 5 The initial search for the global minima of B8, B8−, B9, and B9− and vibrational frequency calculations were performed using analytical gradients with polarized split-valence basis sets (6-311+G*) and the hybrid method, known as B3LYP. Some geometries and vibrational frequencies were further refined with the multiconfigurational method CASSCF with various active spaces. The coupled-cluster method CCSD (T) with 6-311+G* basis set was used for additional geometry optimizations, and for refinement of energies with the 6-311+G (2df) and 6-311+G (2d) basis sets. Vertical electron detachment energies were calculated using the restricted and unrestricted coupled-cluster methods R (U) CCSD (T) /6-311+G (2df), at the equation-of-motion method based on RCCSD (T) EOM-RCCSD (T) with the same 6-311+G (2df) basis set and at the outer-valence Green Function method (ROVGF). Molecular orbitals were calculated at the U (R) HF/6-311+G* levels of theory. U (R) HF, B3LYP, CASSCF, ROVGF, and CCSD (T) calculations were performed using Gaussian 98. 24 RCCSD (T) and EOM-RCCSD (T) calculations—using MOLPRO 1999 program. 25 Molecular orbital pictures were made using MOLDEN3. 4 program. 26 All calculations were performed on a 63-nodes Birch-Retford Beowulf cluster computer built at Utah State by K. A. Birch and B. P. Retford. Supporting information for this article is available on the WWW under http: //www. wiley-vch. de/contents/jc₂002/2003/z51874ₛ. pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.