Published on Web 02/02/2008 2- B2(BO)2 s Diboronyl Diborene: A Linear Molecule with a Triple Boron-Boron Bond Si-Dian Li,*,†,‡ Hua-Jin Zhai,§,| and Lai-Sheng Wang*,§,| Institute of Molecular Sciences, Shanxi UniVersity, Taiyuan 030006, People’s Republic of China, Xinzhou Teachers’ UniVersity, Xinzhou 034000, Shanxi, People’s Republic of China, Department of Physics, Washington State UniVersity, 2710 UniVersity DriVe, Richland, Washington 99354, and Chemical and Materials Sciences DiVision, Pacific Northwest National Laboratory, MS K8-88, P.O. Box 999, Richland, Washington 99352 Received September 13, 2007; E-mail: [email protected]; [email protected] - Abstract: We have produced and investigated an unique boron oxide cluster, B4O2 , using photoelectron spectroscopy and ab initio calculations. Relatively simple and highly vibrationally resolved PES spectra were obtained at two photon energies (355 and 193 nm). The electron affinity of neutral B4O2 was measured to be 3.160 ( 0.015 eV. Two excited states were observed for B4O2 at excitation energies of 0.48 and 0.83 eV above the ground state. Three vibrational modes were resolved in the 355 nm spectrum for the ground -1 state of B4O2 with frequencies of 350 ( 40, 1530 ( 30, and 2040 ( 30 cm . Ab initio calculations showed 3 - - 2 that neutral B4O2 (D∞h, Σg ) and anionic B4O2 (D∞h, Πu) both possess highly stable linear structures (OtBsBdBsBtO), which can be viewed as a B2 dimer bonded to two terminal boronyl groups. The lowest nonlinear structures are at least 1.5 eV higher in energy. The calculated electron detachment energies - from the linear B4O2 and the vibrational frequencies agree well with the experimental results. The three observed vibrational modes are due to the BsB, BdB, and BtO symmetric stretching vibrations, respectively, in the linear B2(BO)2. Chemical bonding analyses revealed that the HOMO of B2(BO)2, which is half-filled, is a bonding π orbital in the central B2 unit. Thus, adding two electrons to B2(BO)2 leads to a 2- 2- BtB triple bond in [OtBsBtBsBtO] . Possibilities for stabilizing B2(BO)2 in the form of B2(BO)2Li2 are considered computationally and compared with other valent isoelectronic, triple bonded species, B2H2- 2- 2- Li2,B2H2 , and C2H2. The high stability of B2(BO)2 suggests that it may exist as a viable building block in the condensed phase. 1. Introduction antiaromaticity. Boron-boron multiple bonds are rare due to boron’s electron deficiency but have been studied both Boron is an electron-deficient element and possesses interest- experimentally7-13 and theoretically.14-17 Partial BB π-bonding ing chemical bonding properties.1,2 The chemical bonding in was induced through one-electron reduction of a B R precur- boranes3 and, more recently, in elemental boron clusters4-6 is 2 4 sor.7,8 Further reduction led to a B R 2- dianion with the first dominated by three-center two-electron bonds and aromaticity/ 2 4 reported BdB double bond (1.62-1.64 Å) in 1992.9 The d † Shanxi University. diborene B2H2 is probably the simplest molecule with a B B ‡ Xinzhou Teachers’ University. § Washington State University. (6) For a recent review on all-boron aromatic clusters, see: Alexandrova, A. | Pacific Northwest National Laboratory. N.; Boldyrev, A. I.; Zhai, H. J.; Wang, L. S. Coord. Chem. ReV. 2006, (1) Greenwood, N. N.; Earnshaw, A. 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SOC. 2008, 130, 2573-2579 9 2573 ARTICLES Li et al. double bond, and it was characterized via ESR spectroscopy in laser vaporization and obtained its PES spectra at 355 nm (3.496 inert matrices at4Kin1995,12 constituting the simplest eV) and 193 nm (6.424 eV) photon energies. The 355 nm 3 - tetraatomic radical with a triplet ( Σg ) ground state. A boron- spectrum was vibrationally resolved, revealing three vibrational boron triple bond is extremely rare, which was first reported modes with frequencies of 350 ( 40, 1530 ( 30, and 2040 ( by Zhou et al. in 2002.13,18 They observed a linear molecule 30 cm-1. An accurate electron affinity of 3.160 ( 0.015 eV OCBBCO in an argon matrix at 8 K and showed that it possesses was obtained for the B4O2 neutral. Extensive DFT and ab initio - some boron-boron triple bond character. calculations were performed for B4O2 and B4O2 , which were Boron does form a strong triple bond with oxygen in the found to possess highly stable linear structures, OtBsBdBs 3 - - 2 diatomic BO molecule (boronyl), which is isoelectronic with BtO for B4O2 (D∞h, Σg ) and [OtBsBdBsBtO] (D∞h, Πu) - - CN. Boronyl is known as a σ-radical19 21 and was speculated for B4O2 . These structures are confirmed by the excellent to exist as network terminals in liquid B2O3 at high tempera- agreement between the calculated electron detachment energies tures.22 However, the chemistry of boronyl is relatively un- and vibrational frequencies and the experimental results. Mo- known, in contrast to CN, which is an important inorganic lecular orbital (MO) and bond order analyses showed that the - ligand. The electronic structure and bond strength of BO are BdB bond in B4O2 is of the order 2.5. Addition of one more - similar to those of CN,20,21 suggesting that it may be a robust electron to B4O2 leads to a unprecedented closed-shell, triple 2- 1 + chemical unit and may maintain its structural integrity in certain bonded B2 species, [OtBsBtBsBtO] (D∞h, Σg ), i.e., chemical compounds. Boron oxide clusters are important species diboronyl diborene. relevant to the development of highly energetic boron-based 2. Experimental and Computational Methods propellants.23 But our knowledge about these species is surpris- ingly limited beyond the BO molecule.24 We are interested in 2.1. Photoelectron Spectroscopy. The experiment was carried out characterizing the electronic structure and chemical bonding of using a magnetic-bottle-type PES apparatus equipped with a laser boron oxide clusters using photoelectron spectroscopy (PES) vaporization supersonic cluster source, details of which have been described previously.30,31 Briefly, the boron oxide cluster anions were and theoretical calculations.21,25 In a recent communication, we produced by laser vaporization of a pure disk target made of enriched reported a PES and density-functional theory (DFT) study of 10 - - B isotope (99.75%) in the presence of a helium carrier gas seeded two boron oxide clusters, B3O2 and B4O3 . They are found with 0.01% O . The resulting B O - clusters were analyzed using a - 2 m n ∞ 3 - surprisingly to possess a linear B(BO)2 (D h, Σg) and triangular time-of-flight mass spectrometer.