Alkali metal-sulfur dioxide complexes stabilized by halogenated closo-dodecaborate anions

Abstract

The alkali metal salts (M = Li, Na, K, Rb, Cs) of the perchlorinated closo-dodecaborate [B₁₂Cl₁₂]²⁻ were prepared by reaction of [NEt₃H]₂[B₁₂Cl₁₂] with the corresponding alkali metal hydroxide. Crystallization of M₂[B₁₂Cl₁₂] from liquid sulfur dioxide gave the sulfur dioxide complexes [Li₂(SO₂)₈][B₁₂Cl₁₂], Na₂[B₁₂Cl₁₂]·4SO₂, K₂[B₁₂Cl₁₂]·8SO₂, Rb₂[B₁₂Cl₁₂]·4SO₂, and Cs₂[B₁₂Cl₁₂]·SO₂, which were characterized by single crystal X-ray diffraction. In this work structurally characterized SO₂ complexes of the alkali metal cations K⁺ and Rb⁺ are reported for the first time. The structure of [Li₂(SO₂)₈][B₁₂Cl₁₂] contains discrete [Li₂(SO₂)₈]²⁺ dications and [B₁₂Cl₁₂]²⁻ dianions. Born-Haber cycles based on quantum chemical calculations and estimations of lattice enthalpies for the solid state explain the stability of the discrete dication [Li₂(SO₂)₈]²⁺ in the solid state. Heavier alkali metals form three-dimensional networks containing metal-anion and metal-sulfur dioxide contacts. The crystal structures of Na₂[B₁₂Br₁₂]·8SO₂ and Na₂[B₁₂I₁₂]·8SO₂ were determined to investigate the influence of the halogen substituent on the anion. They contain similar three-dimensional network structures. Na₂[B₁₂Br₁₂]·8SO₂ is isostructural to K₂[B₁₂Cl₁₂]·8SO₂. In addition the crystal structures of the complexes Na₂[B₁₂I₁₂]·8SO₂·H₂O and Na₂[B₁₂H₁₂]·6SO₂·2H₂O, which contain water ligands, are reported as well. A comparison of halogenated dodecaborates [B₁₂X₁₂]²⁻ (X = F, Cl, Br, I) based on _N–H stretching frequencies of the corresponding [Oct₃NH]₂[B₁₂X₁₂] (X = F – I) salts shows that the fluorinated anion [B₁₂F₁₂]²⁻ is the least basic and the iodinated anion [B₁₂I₁₂]²⁻ is the most basic anion in this series. These findings are in agreement with those for the corresponding series of perhalogenated carboranes and are explained by the polarizability of the halogen substituent.