Reactions of a Cyclodimethylsiloxane (Me₂SiO)₆ with Silver Salts of Weakly Coordinating Anions; Crystal Structures of [Ag(Me₂SiO)₆][Al] ([Al] = [FAl{OC(CF₃)₃}₃], [Al{OC(CF₃)₃}₄]) and Their Comparison with [Ag(18-Crown-6)]₂[SbF₆]₂

Abstract

Two silver-cyclodimethylsiloxane cation salts [AgD₆]­[Al] ([Al] = [Al­(OR_F)₄]­(<b>1</b>) or [FAl­(OR_F)₃]­(<b>2</b>), R_F = C­(CF₃)₃, D = Me₂SiO) were prepared by the reactions of Ag­[Al] with D₆ in SO₂(l). For a comparison the [Ag­(18-crown-6)]₂[SbF₆]₂(<b>3</b>) salt was prepared by the reaction of Ag­[SbF₆] and 18-crown-6 in SO₂(l). The compounds were characterized by IR, multinuclear NMR, and single crystal X-ray crystallography. The structures of <b>1</b> and <b>2</b> show that D₆ acts as a pseudo crown ether toward Ag⁺. The stabilities and bonding of [MD_<i>n</i>]⁺ and [M­(18-crown-6)]⁺ (M = Ag, Li, <i>n</i> = 4–8) complexes were studied with theoretical calculations. The calculations predicted that D₆ adopts a puckered <i>C</i>_<i>i</i> symmetric structure in the gas phase in contrast to previous reports. 18-Crown-6 was calculated to bind more strongly to Li⁺ and Ag⁺ than D₆. ²⁹Si­[¹H] NMR results in solution, and calculations in the gas phase established that a hard Lewis acid Li⁺ binds more strongly to D₆ than Ag⁺. A comparison of the [MD_<i>n</i>]⁺ complex stabilities showed D₇ to form the most stable metal complexes in the gas phase and the solid state and explained why [AgD₇]­[SbF₆] was isolated in a previous reaction where ring transformations resulted in an equilibrium of [AgD_<i>n</i>]⁺ complexes. In contrast, the isolations of <b>1</b> and <b>2</b> were possible because the corresponding equilibrium of [AgD_<i>n</i>]⁺ complexes was not observed with [Al]⁻ anions. The formation of the dinuclear complex salt <b>3</b> instead of the corresponding mononuclear complex salt was shown to be driven by the gain in lattice enthalpy in the solid state. The bonding to Li⁺ in D₆ and 18-crown-6 metal complexes was described by a quantum theory of atoms in molecules (QTAIM) analysis to be mostly electrostatic while the bonding to Ag⁺ also had a significant charge transfer component. The charge transfer from both D₆ and 18-crown-6 to Ag⁺ and Li⁺ metal ions was depicted by the QTAIM analysis to be of similar strength, and the difference in the stabilities of the complexes was attributed mostly to more attractive electrostatic interactions between 18-crown-6 and the metal ions despite the more negative oxygen atomic charges calculated for D₆