Stable Alkali-Metal Complexes of Hybrid Disila-Crown Ethers

The complexation ability of hybrid disilane and ethylene containing crown ether ring systems was analyzed using 1,2-disila[12]­crown-4 (<b>1</b>), 1,2-disila­[15]­crown-5 (<b>2</b>), 1,2-disila[18]­crown-6 (<b>3</b>), and 1,2,7,8-tetrasila[12]­crown-4 (<b>7</b>). Alkali-metal complexes (Li⁺, Na⁺, K⁺) were obtained and analyzed via X-ray diffraction. The complex stability of [Li­(1,2-disila[12]­crown-4)]⁺ and [Li­(1,2,7,8-tetrasila[12]­crown-4)]⁺ was determined, in relation to the lithium complex of [12]­crown-4, by density functional theory (DFT) calculations employing the BP86/def2-TZVP level of theory. In solution, the exchange of lithium cations between pure [12]­crown-4 and hybrid [12]­crown-4 is on even terms, as has been shown from the relative binding affinity of compounds <b>1</b> and <b>7</b> by means of dynamic proton nuclear magnetic resonance (NMR) spectroscopy

Alkali and Alkaline Earth Metal Derivatives of Disila-Bridged Podands: Coordination Chemistry and Structural Diversity

Within this study, the synthesis and coordination chemistry of open-chain ligands bearing disila-units is presented. Instead of basic 1:1 complexes, structural diversity was discovered in the variety of ligand and salt. Stable complexes of alkali and alkaline earth metal complexes were obtained by equimolar reactions of different salts with the disila-bridged podands 8,9-disila-EO5 (<b>1</b>) and 11,12-disila-EO7 (<b>2</b>) (EO5 = pentaethylene glycol; EO7 = heptaethylene glycol). The respective alkaline earth metal complexes of the type [Ca­(8,9-disila-EO5)­(OTf)₂] (<b>3</b>), [Sr­(8,9-disila-EO5)­I₂] (<b>5</b>), [Sr­(11,12-disila-EO7)­I]I (<b>6</b>), and [Ba­(11,12-disila-EO7)­OTf₂] (<b>7</b>) (OTf = CF₃SO₃^–) were characterized via single-crystal X-ray diffraction analyses. Within the reaction of the alkali metal salt NaPF₆ with <b>1</b>, the sodium ion acts as a template during the complexation process. Under elimination of one molecule of diethylene glycol, the dinuclear species [Na₂(8,9,17,18-tetrasila-EO8)­(PF₆)₂]·EO2 (<b>4</b>) (EO8 = octaethylen glycol, EO2 = diethylene glycol) is obtained, in which the sodium cations are 7-fold coordinated within a disilane-bearing framework. The reaction of <b>2</b> with CsOTf failed, leading to recrystallization of anhydrous CsOTf. By means of DFT calculations it was shown that the disila-bearing ligands are burdened with negative hyperconjugation interactions between the silicon and the oxygen atoms, but the coordination by sufficiently hard cations can easily overcompensate the competing polarization. In contrast, soft Lewis acids barely share interactions with silicon-bonded oxygen atoms. All findings are consistent with observations made in solution according to ²⁹Si NMR spectroscopical studies