Search Results
Synthesis, Electronic Properties and Reactivity of [B12X11(NO2)]2− (X=F–I) Dianions
Nitro-functionalized undecahalogenated closo-dodecaborates [B12X11(NO2)]2− were synthesized in high purities and characterized by NMR, IR, and Raman spectroscopy, single crystal X-diffraction, mass spectrometry, and gas-phase ion vibrational spectroscopy. The NO2 substituent leads to an enhanced electronic and electrochemical stability compared to the parent perhalogenated [B12X12]2− (X=F–I) dianions evidenced by photoelectron spectroscopy, cyclic voltammetry, and quantum-chemical calculations. The stabilizing effect decreases from X=F to X=I. Thermogravimetric measurements of the salts indicate the loss of the nitric oxide radical (NO.). The homolytic NO. elimination from the dianion under very soft collisional excitation in gas-phase ion experiments results in the formation of the radical [B12X11O]2−.. Theoretical investigations suggest that the loss of NO. proceeds via the rearrangement product [B12X11(ONO)]2−. The O-bonded nitrosooxy structure is thermodynamically more stable than the N-bonded nitro structure and its formation by radical recombination of [B12X11O]2−. and NO. is demonstrated. © 2020 The Authors. Published by Wiley-VCH GmbH
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.