Sila-polyethers as innocent crystallization reagents for heavy alkali metal compounds

• Published in: CrystEngComm 2018, Vol.20 (36), p.5370-5376
• Main Authors: Dankert, Fabian, Heine, Johanna, Rienmüller, Julia, von Hänisch, Carsten
• Format: Article
• Language: English
• Subjects: Alkali metal compounds; Cesium; Coordination compounds; Crystal structure; Crystallization; Dichloromethane; Diffraction; Metal compounds; Polyethers; Reagents; Rubidium; Single crystals
• ISSN: 1466-8033; 1466-8033
• DOI: 10.1039/C8CE01097H

In this study, we report on a novel approach for the crystallization of previously elusive rubidium and caesium salts. Stirring alkali metal salts in dichloromethane together with sila-polyethers such as 8,9-disila-EO5 ( 1 ) (EO5 = pentaethylene glycol), 11,12-disila-EO7 ( 2 ) (EO7 = heptaethylene glycol) and 1,2,10,11-tetrasila[18]crown-6 ( 3 ) under an inert-gas atmosphere yields crystalline materials of the respective salts suitable for single crystal diffraction analysis. This allowed the single crystal structure determination of the salts MOTf (M = Rb + , Cs + and OTf = CF 3 SO 3 − ) and Cs 2 Ox (Ox = C 2 O 4 2− ) whose structures had previously been solved from powder diffraction data. In contrast, the use of all-carbon polyethers like [18]crown-6 and EO5 under similar reaction conditions yields coordination compounds of the respective salts such as [M([18]crown-6)OTf], [Rb 3 (EO5)OTf 3 ] ( 5 ) and [Cs(EO5)OTf] ( 6 ). This lets us conclude that it is the sila group in the polyethers that allows these ligands to act as a very efficient crystallization aid without being included in the final compound.