Si analogs of crown ethers

Over the years, chemists have reported that when they carried out reactions involving alkali-metal cations in the presence of silicone grease, they isolated unexpected salts containing cyclic dimethylsiloxane cations such as Li[(CH₃)₂SiO]₆⁺. This happened in Jack Passmore's lab at the University of New Brunswick. So he and his coworkers decided to see if they could produce such salts directly from an alkali-metal cation and a cyclic dimethylsiloxane, a reaction that apparently had not been reported. They succeeded in preparing high yields of Li[(CH₃)₂SiO]_n⁺ salts, where n = 5 or 6 and the anion is a weakly coordinating polyfluoroalkoxyaluminate (Angew. Chem. Int. Ed., published online March 20, dx.doi.org/10.1002/anie.200504262). Their synthetic work and calculations "imply the existence of a new class of host-guest complexes for the cyclic siloxanes that is similar to, but less extensive than, that for the crown ethers," they write. The structures of these complexes (such as the one shown, where Li is light blue; Si, dark blue; and O, red) imply that the cyclic dimethylsiloxanes act as "pseudo crown ethers and provide rare examples of silicon ethers behaving as Lewis bases," they note.