Synthesis, Characterization, and Crystal Structures of Silylium Compounds of the Weakly Coordinating Dianion [B12Cl12]2-

The reaction of [Ph3C](2)[B12Cl12] with R3SiH (R = Me, Et, iPr) in 1,2-difluorobenzene yielded the corresponding silylium compounds (R3Si)(2)B12Cl12 containing the weakly coordinating dianion [B12Cl12](2-). The products were fully characterized by IR and Raman spectroscopy and by multinuclear (H-1, B-11, C-13, Si-29) NMR spectroscopy in solution and the solid state (magic angle spinning). (Et3Si)(2)B12Cl12 and (iPr(3)Si)(2)B12Cl12 were characterized by X-ray diffraction. In the solid state, the silylium cations are coordinated to the [B12Cl12](2-) anion via silicon-chlorine contacts, which are significantly shorter than the sum of the van der Waals radii. Two different coordination patterns were found. The [Et3Si](+) cations are coordinated to chlorine atoms of [B12Cl12](2-) in the 1 and 12 positions, while the [iPr(3)Si](+) cations coordinate to chlorine atoms in the 1 and 7 positions. The 1,12 regioisomer is calculated [for (Me3Si)(2)B12Cl12] to be favored over the 1,7 isomer by only 8 kJ mol(-1), indicating that packing effects may cause the difference. The silylium cations are very reactive and bind to every Lewis base, being stronger than the aromatic solvent (e.g., benzene, 1,2-difluorobenzene, etc.) used. Consequently, three different crystal structures containing cationic Lewis acid-base complexes [iPr(3)Si-donor](+) were obtained from preparations of (iPr(3)Si)(2)[B12Cl12]. The presence of traces of water leads to crystals of [iPr(3)Si(OH2)](2)[B12Cl12] containing the protonated silanol [iPr(3)Si(OH2)](+), which is only the second example of its kind. Structures containing the [iPr(3)SiOS(H)OSiiPr(3)](+) cation were obtained from the reaction of [Ph3C](2)[B12Cl12] center dot 2SO(2) with an excess of iPr(3)SiH in 1,2-difluorobenzene. [iPr(3)SiOS(H)OSiiPr(3)](2)[B12Cl12] and [iPr(3)SiOS(H)OSiiPr(3)][(iPr(3)Si)B12Cl12] were structurally characterized by X-ray diffraction. On the basis of the structural data and quantum chemical calculations, the crystallographically invisible hydrogen atom bound to the sulfur atom was identified. A comparison of the weakly coordinating dianion [B12Cl12](2-) with the widely applied corresponding chlorinated carboranes based on several criteria including the nu(N-H) scale established the dianion [B12Cl12](2-) to be as weakly coordinating as the single negatively charged carboranes.