The high electronegativity of halogens sees them form binary compounds with virtually every element. Such compounds populate many a chemical inventory because they exhibit diverse reactivity. For example, boron trihalides BX3 are classic Lewis acids that readily form tetrahedral adducts with suitable Lewis bases, including halides. Despite this, only scant evidence exists for the hexahalodiborates [B2X6]2−, which were notable absentees from an otherwise well-chartered boron–halogen chemical space. Writing in Angewandte Chemie International Edition, Guillaume Bélanger-Chabot and Holger Braunschweig have now filled this gap by reporting the structures of four such hexahalodiborates.
“We were wondering whether the Lewis acid–base reaction between BX3 and halides X− to give the classical weakly coordinating anions BX4− could be applied to diboron analogues B2X4,” recalls Guillaume Bélanger-Chabot. For example, the two negative charges in [B2X6]2− would not be as delocalized as they are in the pseudohalides [B2(NCS)6]2− and [B2(CN)6]2− — known anions in which Coulomb repulsion is minimized by extended resonance stabilization. Moreover, the pseudohalides are not even prepared by an addition reaction, so Bélanger-Chabot understandably had doubts about the reaction occurring with B2X4 analogues. Furthermore, the possibility to reductively couple two BX3 molecules to afford the dianions [B2X6]2− was viewed as being potentially too unselective to be useful.
Despite the challenges involved in boron–halogen chemistry — not least the high propensity of compounds to undergo hydrolysis — the team had previously developed an expeditious route to brominate B2(OMe)4 with BBr3 and get B2Br4, from which the congeners B2F4, B2Cl4 and B2I4 were readily available. With the diboron tetrahalides B2X4 in hand, Bélanger-Chabot was pleased to find that simple treatment with the halide source [Ph4P]X affords the [B2X6]2− targets. Although this is a bulletproof method to give [Ph4P]2[B2Cl6] and [Ph4P]2[B2Br6], which readily crystallize from CH2Cl2, the story is not so simple for [B2F6]2− and [B2I6]2−. The putative equilibrium [B2I5]− + I− ⇌ [B2I6]2− does not lie completely to the right, such that the hexaiodide, while isolable as [Ph4P]2[B2I6], exists along with the pentaiodide in solution. The hexafluoro congener required more exotic conditions: the condensation of B2F4 gas at −196 °C onto a CH2Cl2 solution containing two equivalents of [nBu4N][Ph3SiF2], a convenient and H2O-free F− source.
The compounds [nBu4N]2[B2F6], [Ph4P]2[B2Cl6], [Ph4P]2[B2Br6] and [Ph4P]2[B2I6] feature dianions that are almost isostructural, with each existing in a staggered conformation. The hexahalodiborates [B2X6]2−, of which only [B2F6]2− and [B2Cl6]2− were previously observed (albeit without full characterization), are isoelectronic to the carbon binary halides C2X6. Thus, there is hope that the present dianions might find applications in photodissociation and halogenation reactions. It is important to note that the bonds in the dianions [B2X6]2− are each formally 2e− interactions, to afford electron-precise structures that contrast with the electron-deficient species B2H6, which does not have a B–B bond and features 3-centred–2-electron bonding.
“We were wondering whether the Lewis acid–base reaction … could be applied to diboron analogues”
The counterintuitive finding that hexahalodiborates [B2X6]2− are not inherently unstable has inspired the group to pursue their boron–halogen chemistry further. For example, although one would expect [B2X5]− to be an intermediate in the formation of [B2X6]2−, “nobody has ever observed [B2X5]− monoanions,” notes Bélanger curiously. “And so the nature of solutions containing 1:1 ratios of B2X4 and X− remains somewhat conjectural.”
Bélanger-Chabot, G. & Braunschweig, H. Hexahalodiborate dianions: a new family of binary boron halides. Angew. Chem. Int. Ed. https://doi.org/10.1002/anie.201906666 (2019)