On the gas-phase dimerization of negatively charged closo-dodecaborates: a theoretical study.

We have studied the intriguing gas-phase dimerization of the B12In(-) (n = 9, 8) anions to B24I2n(2-) dianions by means of density functional theory calculations. The dimerization of B12I9(-) to B24I18(2-) has been detected experimentally in a previous study (Phys. Chem. Chem. Phys., 2011, 13, 5712) utilizing electrospray ionization ion trap mass spectrometry (ESI-IT-MS), whereas the formation of B24I16(2-) from B12I8(-) is modeled here prior to experiment. Calculations are carried out to determine the molecular and electronic structures, the bonding situation and the stability of the dimers relative to the respective monomers. The dimerization process is found to be thermodynamically favorable, and the stability of the lowest-energy structures is substantiated by molecular dynamics simulations. The calculations imply that the experimentally observed B24I18(2-) and the hypothetical B24I16(2-) species are formed through dimerization of the respective B12In(-) (n = 9, 8) monomers, rather than through loss of two I radicals from B24I2n+2(2-) dimers. Electronic properties such as natural charges, vertical detachment energies (VDEs), frontier molecular orbitals (FMOs), and HOMO-LUMO gaps are computed and analyzed in detail for all monomers and dimers. The analysis shows that the most stable B24I2n(2-) dimers are formed through two 2c-2e B-B and two 3c-2e B-I-B bridges between the parent B12In(-) (n = 9, 8) monomers. These new bridging bonds engage the deiodinated (bare) faces of the two B12 icosahedra, as well as one (per monomer) of the nearest boron neighbors and its iodine substituent.