Hepta- and octacoordinate boron in molecular wheels of eight- and nine-atom boron clusters: observation and confirmation.

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 8and 9-atom boron clusters are perfectly planar molecular wheels, with a heptaor octacoordinated central boron atom, respectively, despite the predominance of three-dimensional structures normally found in bulk boron and its compounds. Heptaand octacoordinated planar boron compounds are highly unusual. The radii of the miniature molecular wheels are found to be 1.8 and 2.0 ' for the 8and 9-atom clusters, respectively. Analyses of their chemical bonding reveal that they possess double (s and p) 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. Figure 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 1b). 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 1a), 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 . 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 1a). 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.