Effects of halogen substitution on the properties of eight- and nine-vertex closo-boranes

Abstract

The symmetry constrained geometries of the eight- and nine-vertex polyhedral boranes and haloboranes B_nX_n^z (n = 8 and 9; X = H, F and Cl; z = −2, −1 and 0) were optimized at the B3LYP/6-311+G(d) level and their nucleus-independent chemical shifts (NICS) were calculated using the GIAO method with Kohn–Sham orbitals. Substitution of halogens on borane cages was found to significantly impact not only the geometric but also magnetic properties. Multiple fluorine substituents cause a deviation from the Wade–Mingos skeletal electron rules in B₈F₈²⁻, resulting in a distortion from the expected D_2d bisdisphenoid to a C_2vnido type bicapped trigonal prism. However, all of the nine-vertex cages B₉X₉^z retain the D_3h tricapped trigonal prismatic structure of B₉H₉²⁻. The presence of halogen substituents was found to enhance the three-dimensional diatropic ring currents within the dianionic borane cages B₈X₈²⁻ and B₉X₉²⁻. For the neutral structures the NICS values indicate B_nF_n to be aromatic, B_nCl_n to be essentially non-aromatic, and B_nH_n to be antiaromatic (n = 8, 9).