Ca²⁺ Stabilized by Carborate Anions: Synthesis, Structure, and Reactivity

The syntheses of calcium carborates, Ca[HexCB₁₁Cl₁₁]₂, 1, Ca[HCB₁₁Cl₁₁]₂, 2, and Ca[MeCB₁₁Cl₁₁]₂·2(o-DFB), 3·2(o-DFB), (o-DFB = 1,2-difluorobenzene) including the structure of 2·4(o-DFB) are described. The large size of the Ca²⁺ cation is reflected by the coordination geometry with the Ca center being coordinated by three Cl donors from each of the carborate anions and one F donor from each of the two coordinated solvent molecules. Based on ¹¹B NMR and DOSY NMR studies, the salts 1, 2 and 2·2(o-DFB) form close ion pairs in PhBr and o-DFB solutions. The Fluoride Ion Affinities (FIA) in PhBr for the salts 1 and 2·2(o-DFB) were DFT-calculated (ωB97XD/6-31 + G**) at 211.3 kJ mol^-1 and 217.5 kJ mol^-1, respectively, which is below that of the landmark Lewis acid B(C₆F₅)₃ (241.5 kJ mol^-1). The Ca²⁺ center in salt 1 is sufficiently Lewis acidic to coordinate 1,5-cyclooctadiene (1,5-COD) and 2,6-octadiyne and to catalyze the hydrosilylation of 1-hexene and several alkynes, albeit slowly. Compound 1 also displays moderate activity as a catalyst for transfer hydrogenation catalysis and carbonyl-olefin metathesis (COM), a first for Ca compounds.