On the Oxidation of the Three‐Dimensional Aromatics [B₁₂X₁₂]²⁻ (X=F, Cl, Br, I)

Abstract

The perhalogenated closo-dodecaborate dianions [B12 X12 ](2-) (X=H, F, Cl, Br, I) are three-dimensional counterparts to the two-dimensional aromatics C6 X6 (X=H, F, Cl, Br, I). Whereas oxidation of the parent compounds [B12 H12 ](2-) and benzene does not lead to isolable radicals, the perhalogenated analogues can be oxidized by chemical or electrochemical methods to give stable radicals. The chemical oxidation of the closo-dodecaborate dianions [B12 X12 ](2-) with the strong oxidizer AsF5 in liquid sulfur dioxide (lSO2 ) yielded the corresponding radical anions [B12 X12 ](⋅-) (X=F, Cl, Br). The presence of radical ions was proven by EPR and UV/Vis spectroscopy and supported by quantum chemical calculations. Use of an excess amount of the oxidizing agent allowed the synthesis of the neutral perhalogenated hypercloso-boranes B12 X12 (X=Cl, Br). These compounds were characterized by single-crystal X-ray diffraction of dark blue B12 Cl12 and [Na(SO2 )6 ][B12 Br12 ]⋅B12 Br12 . Sublimation of the crude reaction products that contained B12 X12 (X=Cl, Br) resulted in pure dark blue B12 Cl12 or decomposition to red B9 Br9 , respectively. The energetics of the oxidation processes in the gas phase were calculated by DFT methods at the PBE0/def2-TZVPP level of theory. They revealed the trend of increasing ionization potentials of the [B12 X12 ](2-) dianions by going from fluorine to bromine as halogen substituent. The oxidation of all [B12 X12 ](2-) dianions was also studied in the gas phase by mass spectrometry in an ion trap. The electrochemical oxidation of the closo-dodecaborate dianions [B12 X12 ](2-) (X=F, Cl, Br, I) by cyclic and Osteryoung square-wave voltammetry in liquid sulfur dioxide or acetonitrile showed very good agreement with quantum chemical calculations in the gas phase. For [B12 X12 ](2-) (X=F, Cl, Br) the first and second oxidation processes are detected. Whereas the first process is quasi-reversible (with oxidation potentials in the range between +1.68 and +2.29 V (lSO2 , versus ferrocene/ferrocenium (Fc(0/+) ))), the second process is irreversible (with oxidation potentials ranging from +2.63 to +2.71 V (lSO2 , versus Fc(0/+) )). [B12 I12 ](2-) showed a complex oxidation behavior in cyclic voltammetry experiments, presumably owing to decomposition of the cluster anion under release of iodide, which also explains the failure to isolate the respective radical by chemical oxidation.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…Thesuccess of the closely related carborane anions prompted renewed interest in [B 12 X 12 ] 2À ,with updated synthetic procedures being reported as well as the fluorinated analogue. [110][111][112] Replacement of ah alogen atom in [B 12 X 12 ] 2À ions for an ammonium group and subsequent alkylation gives rise to the related monoanions of the type [R 3 N-B 12 X 11 ] À (R = alkyl;X= Fo rC l). [109] Furthermore,t he oxidation of [B 12 X 12 ] 2À to the corresponding radical monoanions [B 12 X 12 ]C À (where X = F, Cl, Br) and even to the neutral B 12 X 12 (for X = Cl or Br)with potential synthetic interest in introducing these WCAsin oxidative routes-has also been recently demonstrated.…”

“…[B 12 X 12 ]C À , where X = F, Cl, Br), [110][111][112] both of which result in the formation of the WCA salt of an oxidized species.H ere we again make reference to Figure 1, which delineates the independent requirements for stability that can be placed on agiven WCA.Apertinent example is the incompatability of the very common [B(C 6 F 5 ) 4 ] À WCA with the comparatively mildly oxidizing [NO] + because of the aromatic rings. [ Ag] + or [NO] + )o rinc ertain cases through the use of an oxidized form of the WCA itself (e.g.…”

“…[312] This represents an elegant alternative approach to the established hydride abstraction [97,313,314] and allyl leaving group methods [315] for the formation of heavier Group 14 analogues of carbenium ions.F urthermore,t he coupling of two [CB 11 Me 12 ] À ions through an ethene or ethyne linker provides access to the corresponding dianions,which can also be oxidized to give the biradicals. [110][111][112] Therelated neutral radical Me 3 NB 12 Cl 11 C has been used for the direct oxidation of thianthrene,C 6 Br 6 ,a nd I 2 to give the radical cations [C 12 H 8 S 2 ]C + and [C 6 Br 6 ]C + as well as the homopolyatomic iodine cation [I 5 ] + ,a ll supported by the [Me 3 NB 12 Cl 11 ] À anion (Scheme 13). [317] Additionally,the synthesis of arange of CB 11 (CH 3 ) n (CD 3 ) 12Àn C radicals featuring fivefold rotation symmetry have also been reported.…”

Synthesis and Applications of Perfunctionalized Boron Clusters

“…The perhalogenated derivatives B 9 X 9 (X = Cl, Br, I) were found to have two highly reversible, well-separated one-electron steps between the dianionic and neutral states, with significantly lower oxidation potentials compared to the B 12 X 12 species (−0.63 V to −0.74 V vs Fc/Fc + for the 2- to 1- oxidation step for B 9 X 9 clusters, compared to 1.68 V to 2.27 V vs Fc/Fc + for the same redox event with B 12 X 12 ). 87,88 …”

The chemical bonding and spectral assignments of rhodium(III)-catalyzedcloso-dodecaborate complexes: Ab initio study

“…Recently, Shen et al studied the iridium‐promoted B‐H activation of the CB 11 cage using the NMR spectroscopy and density functional theory (DFT) calculations, and revealed that the B‐Ir interaction between two fragments is direct presence toward the ligand. In addition, some aromatic complexes involving the closo ‐[B 12 H 12 ] 2− anion were also reported …”