Synthesis and characterization of synthetically useful salts of the weakly-coordinating dianion [B12Cl12]2−

Abstract

The closo-dodecahydrododecaborate [NEt₃H]₂[B₁₂H₁₂] has been prepared on a lab scale by an improved synthesis from cheap and readily available starting materials Na[BH₄] and I₂ in diglyme (diethylene glycol dimethyl ether). Subsequent chlorination with elemental chlorine in aqueous solution at normal pressure yielded the per-chlorinated weakly coordinating [B₁₂Cl₁₂]²⁻ anion. By simple metathesis reaction a variety of useful salts [cation]₂[B₁₂Cl₁₂] (cation = [NEt₃H]⁺, [NBu₄]⁺, Li⁺, Na⁺, K⁺, Cs⁺) is available. These salts are useful starting materials, which have the potential to open up the chemistry of [B₁₂Cl₁₂]²⁻ as a weakly coordinating dianion. Exemplarily, they were used in further reactions to prepare [NO]₂[B₁₂Cl₁₂], [PPN]₂[B₁₂Cl₁₂], and [CPh₃]₂[B₁₂Cl₁₂]. The crystal structures of Cs₂[B₁₂Cl₁₂]·SO₂, [CPh₃]₂[B₁₂Cl₁₂]·2C₂H₄Cl₂, and [CPh₃]₂[B₁₂Cl₁₂]·2SO₂ and preliminary crystal structures of [NO]₂[B₁₂Cl₁₂]·SO₂ and [PPN]₂[B₁₂Cl₁₂]·CH₂Cl₂ were determined. The crystal structure of the SO₂ solvate Cs₂[B₁₂Cl₁₂]·SO₂ is related to the crystal structure of solvent free Cs₂[B₁₂Cl₁₂]. [CPh₃]₂[B₁₂Cl₁₂]·2C₂H₄Cl₂ and [CPh₃]₂[B₁₂Cl₁₂]·2SO₂ have very similar structures in the solid state. In both cases the [CPh₃]⁺ cations form only very weak contacts to the [B₁₂Cl₁₂]²⁻ anion and SO₂ or C₂H₄Cl₂solvent molecules respectively. The averaged experimental B–B (178.7 pm) and B–Cl (178.9 pm) bond lengths within [B₁₂Cl₁₂]²⁻ are essentially unchanged in all determined structures and are reproduced well by PBE0/TZVPP quantum chemical calculations (B–B 178.6 pm, B–Cl 179.3 pm). All results indicate that [B₁₂Cl₁₂]²⁻ is a readily accessible weakly-coordinating dianion.