On the molecular architectures of siloxane coordination compounds: (re-)investigating the coordination of the cyclodimethylsiloxanes Dn (n = 5–8) towards alkali metal ions

Abstract

In this study we present a (re-)investigation into cyclodimethylsiloxanes in relation to silyl-ether bonding towards alkali metal ions. We demonstrate that all (non-radioactive) alkali metal ions can be incorporated into D_n cyclosiloxane frameworks (D = (SiMe₂O), n = 5–8), employing appropriate cation–anion combinations. Starting with the Li⁺ cation we were able to observe the coordination of D₅ with Li⁺ based on a suitable X-ray structure after reacting D₅ with LiI and GaI₃. Due to template effects, the dinuclear coordination compound [Li₂(D₅)(D₆)(GaI₄)₂] (1) was obtained. The direct reaction of D₆ with LiI and GaI₃ yields [Li(D₆)GaI₄] (2) in quantitative yield. Na⁺ ions could be trapped in D₆ and D₇ ligand moieties after the conversion of NaI, GaI₃, and the respective siloxane. The molecular structure of [Na(D₆)GaI₄] (3) reveals a six-fold coordinated Na⁺ ion, which is located on top of the siloxane D₆. In the case of [Na(D₇)(DCM)GaI₄] (4), the larger ligand D₇ provides 15-crown-5-like geometry in which the sodium ion is coordinated by the ligand in a coplanar fashion and further saturated by the solvent DCM (DCM = dichloromethane). The K⁺ ion was bound within the D₇ ligand in a similar manner and [K(D₇)(DCM)GaI₄] (5) could be characterized. Due to the resemblance of NH₄⁺ to K⁺, this cation was also employed for complex formation. Counterintuitively, we were able to synthesize and characterize the first ever non-metal-cyclosiloxane coordination compound. After the conversion of D₆ with NH₄I and GaI₃, the compound [NH₄(D₇)][Ga₂I₇] (6) was obtained. The ammonium cation favors D₇ coordination over D₆, and the willing formation of hydrogen bonding in such a siloxane moiety was realized. As these compounds could be obtained, we also tested the limits of silyl-ether bonding. Therefore, we reacted D₈ with in situ generated Rb[GaI₄] and Cs[GaI₄]. In the case of Rb⁺, we could cumbersomely characterize [Rb(D₈)(DCM)GaI₄] (7) via an X-ray structure, as well as by means of mass spectrometry, but the compound starts decomposing readily in solution. The reaction with the Cs⁺ salt failed. To obtain meaningful spectroscopic data from a Rb⁺ compound and to somehow obtain a Cs⁺ complex, we employed the weakly coordinating anion [Al_F]⁻ (Al_F⁻ = [Al{OC(CF₃)₃}₄]⁻). The conversion of M[Al_F] then yielded ¹_∞[M(D₈)AL_F] (M = Rb: 8; M = Cs⁺: 9) in the solid state. Both compounds 8 and 9 were fully characterized. Finally, we aimed at synthesizing 2 : 1 complexes of such siloxanes. The reactions of excess D₅ with K[Al_F] and D₆ with Cs[Al_F] turned out to be expedient and, in the forms of [K(D₅)₂][Al_F] (10) and [Cs(D₆)₂][Al_F] (12), the first ever sandwich-type complexes observed bearing a cyclosiloxane ligand were characterized.