Properties of gaseous closo-[B6X6]2− dianions (X = Cl, Br, I)

Abstract

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.