Relevance of π‐Backbonding for the Reactivity of Electrophilic Anions [B₁₂X₁₁]⁻ (X=F, Cl, Br, I, CN)

Abstract

Electrophilic anions of type [B 12 X 11 ] À posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B 12 X 11 ] À with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N 2 as σ-donor-π-acceptors. Temperature-dependent formation of [B 12 X 11 NG] À 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 [B 12 X 11 ] À and steric effects. The binding of CO and N 2 to [B 12 X 11 ] À 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 [B 12 X 11 CO] À and [B 12 X 11 N 2 ] À were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N 2 stretching frequencies n CO and n N 2 , respectively. Observed shifts of n CO and n N 2 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 [B 12 X 11 ] À reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo-borate anions.

B12X11(H2)−: exploring the limits of isotopologue selectivity of hydrogen adsorption

“…Finally, the case of X = F is characterized by a substantially stronger orbital interaction with H 2 . This is unlike what has been observed for the attraction of noble gas atoms, which is weakest for X = F. This striking difference is in line with results from a recent publication, 43 which compared the binding of noble gases, N 2 and CO at B 12 X 11 − . It was found that X = F shows the weakest binding of noble gas atoms but the strongest binding of N 2 and CO due to π-like backbonding from B 0 into the antibonding σ* orbitals of the attached molecule (something which is not possible for noble gases).…”

Stable Noble Gas Compounds Based on Superelectrophilic Anions [B12(BO)11]− and [B12(OBO)11]−

“…The large ratio of covalent interaction in these heavier Ng-containing compounds in Table 3 demonstrates that ] À is often regarded as σ donor-acceptor interaction. [27] The deformation density is used to visualize the charge transfer between two fragments. Figure S4 shows the deformation density with the largest contribution of dative bond and electron-sharing bond for [B 12 (OBO) 11 ] À and Ar.…”

“…Recently, a systematical study on the reactivity of [B 12 X 11 ] À (X = F, Cl, Br, I, CN) towards Ngs were carried out and the results show that Ngindependent reactivity trend follows the sequence: CN 11 ] À . [27] The reactivity of those monoanions towards Ngs can be attributed to bindingsite boron atom in [B 12 X 11 ] À and steric effects. [B 12 (BO) 12 ] 2À has been studied theoretically with a second electron binding energy of 5.89 eV, which is even higher than that of [B 12 (CN) 12 ] 2À (ca.…”

Catalytic Potential of [B12X11]2– (X = F, Cl, Br, I, CN) Dianions

“…The optimized geometries of [B 12 We should point out that Mayer et al 25 had studied the interaction of CO and N 2 with [B 12 X 11 ] − (X = F, Cl, Br, I, and CN) both experimentally and theoretically. The binding of N 2 with [B 12 F 11 ] − , namely 1.73 eV, is slightly larger than that to [B 12 (CN) 11 ] − , namely 1.71 eV.…”