Ab Initio Study of the Hypercloso Boron Hydrides BnHn and BnHn-. Exceptional Stability of Neutral B13H13

Abstract

The neutral hypercloso boron hydrides BnHn (n = 5−13, 16, 19, 22) and the boron hydride radical monanion BnHn- (n = 5−13) have been studied at the B3LYP/6-31G* and higher levels of density functional theory. The pairing and capping principles help rationalize the lowest energy structures. Neutral boron hydrides BnHn with n = 3p + 1 (p = 2−7) all have one boron atom on a 3-fold axis. Loss of one electron from BnHn2- (n = 5−13) is predicted to be exothermic except for B12H122-. This, the most stable and aromatic dianion, has a 10.9 kcal/mol (B3LYP/6-311+G**//B3LYP/6-31G*) adiabatic ionization potential. The relative stabilities of the neutral and monanion boron hydrides were evaluated using the equation B2H2x + (n − 2)BHinc → BnHnx, in which x = 0 and −1 and BHinc is the energy difference between B3H5 (C2v, planar) and B2H4 (D2h). Removing two electrons from aromatic dianions changes the nucleus-independent chemical shifts (NICS) from negative (aromatic) to positive (antiaromatic) except for neutral B6H6, B7H7, B12H12 (C2v), and B13H13 (C3v, C2v, Cs). The NICS values (ca. −21.0 ppm) for the three neutral B13H13 forms are comparable with the B13H132- dianion value (−23.3 ppm). Interestingly, for all these species, the NICS values at points 1.0 Å above the centers of the unique triangles are all negative, which is in contrast to the case for benzene with well-known shielding and deshielding regions. On the basis of the stabilization energy and other aromatic criteria, the neutral B13H13 boron hydride appears to be a good candidate for synthesis.