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 M-2[B12Cl12] from liquid sulfur dioxide gave the sulfur dioxide complexes [Li-2(SO2)(8)][B12Cl12], Na-2[B12Cl12]center dot 4SO(2), K-2[B12Cl12]center dot 8SO(2), Rb-2[B12Cl12]center dot 4SO(2), and Cs-2[B12Cl12]center dot 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 [Li-2(SO2)(8)][B12Cl12] contains discrete [Li-2(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 [Li-2(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 Na-2[B12Br12]center dot 8SO(2) and Na-2[B12I12]center dot 8SO(2) were determined to investigate the influence of the halogen substituent on the anion. They contain similar three-dimensional network structures. Na-2[B12Br12]center dot 8SO(2) is isostructural to K-2[B12Cl12]center dot 8SO(2). In addition the crystal structures of the complexes Na-2[B12I12]center dot 8SO(2)center dot H2O and Na-2[B12H12]center dot 6SO(2)center dot 2H(2)O, which contain water ligands, are reported as well. A comparison of halogenated dodecaborates [B12X12](2-) (X = F, Cl, Br, I) based on (nu) over tilde (N-H) stretching frequencies of the corresponding [Oct(3)NH](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.