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

Abstract

The neutral hypercloso boron hydrides B n H n ( n = 5−13, 16, 19, 22) and the boron hydride radical monanion B n H n - ( 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 B n H n with n = 3 p + 1 ( p = 2−7) all have one boron atom on a 3-fold axis. Loss of one electron from B n H n 2 - ( n = 5−13) is predicted to be exothermic except for B 12 H 12 2 - . 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 B 2 H 2 x + ( n − 2)BH inc → B n H n x, in which x = 0 and −1 and BH inc is the energy difference between B 3 H 5 ( C 2 v, planar) and B 2 H 4 ( D 2 h ). Removing two electrons from aromatic dianions changes the nucleus-independent chemical shifts (NICS) from negative (aromatic) to positive (antiaromatic) except for neutral B 6 H 6, B 7 H 7, B 12 H 12 ( C 2 v ), and B 13 H 13 ( C 3 v, C 2 v, C s ). The NICS values (ca. −21.0 ppm) for the three neutral B 13 H 13 forms are comparable with the B 13 H 13 2 - 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 B 13 H 13 boron hydride appears to be a good candidate for synthesis.