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

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.