Properties of gaseous closo-[B6X6]2− dianions (X = Cl, Br, I)†
Electronic structure, collision-induced dissociation (CID) and bond properties of closo-[B6X6]2− (X = Cl–I) are investigated in direct comparison with their closo-[B12X12]2− analogues. Photoelectron spectroscopy (PES) and theoretical investigations reveal that [B6X6]2− dianions are electronically significantly less stable than the corresponding [B12X12]2− species. Although [B6Cl6]2− is slightly electronically unstable, [B6Br6]2− and [B6I6]2− are intrinsically stable dianions. Consistent with the trend in the electron detachment energy, loss of an electron (e− loss) is observed in CID of [B6X6]2− (X = Cl, Br) but not for [B6I6]2−. Halogenide loss (X− loss) is common for [B6X6]2− (X = Br, I) and [B12X12]2− (X = Cl, Br, I). Meanwhile, X˙ loss is only observed for [B12X12]2− (X = Br, I) species. The calculated reaction enthalpies of the three competing dissociation pathways (e−, X− and X˙ loss) indicated a strong influence of kinetic factors on the observed fragmentation patterns. The repulsive Coulomb barrier (RCB) determines the transition state for the e− and X− losses. A significantly lower RCB for X− loss than for e− loss was found in both experimental and theoretical investigations and can be rationalized by the recently introduced concept of electrophilic anions. The positive reaction enthalpies for X− losses are significantly lower for [B6X6]2− than for [B12X12]2−, while enthalpies for X˙ losses are higher. These observations are consistent with a difference in bond character of the B–X bonds in [B6X6]2− and [B12X12]2−. A complementary bonding analysis using QTAIM, NPA and ELI-D based methods suggests that B–X bonds in [B12X12]2− have a stronger covalent character than in [B6X6]2−, in which X has a stronger halide character.