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From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding
Published on: March 24, 2018
Synthesis, characterization, and crystal structures of silylium compounds of the weakly coordinating dianion
Mathias Kessler1, Carsten Knapp, Vanessa Sagawe
1Institut für Anorganische und Analytische Chemie, Albert-Ludwigs Universität Freiburg, Albertstrasse 21, 79104 Freiburg i. Br., Germany.
Abstract:
The reaction of [Ph(3)C](2)[B(12)Cl(12)] with R(3)SiH (R = Me, Et, iPr) in 1,2-difluorobenzene yielded the corresponding silylium compounds (R(3)Si)(2)B(12)Cl(12) containing the weakly coordinating dianion [B(12)Cl(12)](2-). The products were fully characterized by IR and Raman spectroscopy and by multinuclear ((1)H, (11)B, (13)C, (29)Si) NMR spectroscopy in solution and the solid state (magic angle spinning). (Et(3)Si)(2)B(12)Cl(12) and (iPr(3)Si)(2)B(12)Cl(12) were characterized by X-ray diffraction. In the solid state, the silylium cations are coordinated to the [B(12)Cl(12)](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 [Et(3)Si](+) cations are coordinated to chlorine atoms of [B(12)Cl(12)](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 (Me(3)Si)(2)B(12)Cl(12)] 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)[B(12)Cl(12)]. The presence of traces of water leads to crystals of [iPr(3)Si(OH(2))](2)[B(12)Cl(12)] containing the protonated silanol [iPr(3)Si(OH(2))](+), 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 [Ph(3)C](2)[B(12)Cl(12)].2SO(2) with an excess of iPr(3)SiH in 1,2-difluorobenzene. [iPr(3)SiOS(H)OSiiPr(3)](2)[B(12)Cl(12)] and [iPr(3)SiOS(H)OSiiPr(3)][(iPr(3)Si)B(12)Cl(12)] 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 [B(12)Cl(12)](2-) with the widely applied corresponding chlorinated carboranes based on several criteria including the nu(N-H) scale established the dianion [B(12)Cl(12)](2-) to be as weakly coordinating as the single negatively charged carboranes.
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