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.
Relevance of π-Backbonding for the Reactivity of Electrophilic Anions [B12X11]− (X=F, Cl, Br, I, CN)
Electrophilic anions of type [B12X11]− posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B12X11]− with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N2 as σ-donor-π-acceptors. Temperature-dependent formation of [B12X11NG]− indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B12X11]− and steric effects. The binding of CO and N2 to [B12X11]− is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B12X11CO]− and [B12X11N2]− were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N2 stretching frequencies (Formula presented.) and (Formula presented.), respectively. Observed shifts of (Formula presented.) and (Formula presented.) are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π-backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B12X11]− reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo-borate anions. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH