Tris(trimethylsilyl)silanides of the heavier alkali metals - A structural study

Transmetalation reactions between bis(hypersilyl)zinc Zn[Si(SiMe3)(3)](2) and alkali metals have already been established as a Facile route to powders of the solvent-free potassium, rubidium, and cesium derivatives of tris(trimethylsilyl)silane (hypersilane, (Me3Si)(3)SiH).([1,2]) By the use of boiling n-heptane as the solvent, the hitherto unknown NaSi(SiMe3)(3) (1) along with the previously synthesized KSi(SiMe3)(3) (2) have now been obtained as colorless crystalline materials. Information from NMR and Raman spectra in conjunction with the acute Si-Si-Si angles found in their molecular structures indicate mainly ionic bonding, This was verified by population analyses of suitable model systems. Both hypersilanides([2])consist of cyclic dimers [MSi(SiMe3)(3)](2) (1a, M = Na; 2a, M = K) with almost planar M2Si2 rings (Na-Si=299 pm (av); K - SI = 339 pm (av)), which are linked up into coordination polymers. In a similar manner to the related rubidium and cesium derivatives, a pentane suspension of the potassium compound afforded a yellow solution on addition of benzene, from which the crystalline, bright yellow tris(benzene) solvate 2.(benzene)(3) (2b) was isolated. Complex 2b consists of monomers containing short K-Si bonds (332-334 pm) and three eta(6)-bonded benzene molecules, No solvate of 1 was obtained under these conditions. However, on crystallization from pure benzene, crystals of (1)(2).benzene (1b) were isolated (Na-Si = 302 pm (av)). Benzene was found to be intercalated between rods of coordination polymers of (1)(2). In addition, the influence of pi or sigma donors on the molecular and crystal structures of hypersilylrubidium (3) and cesium (4) was investigated. The structures of the tetrahydrofuran solvate (4)(2).THF (4c), the biphenylene/pentane complex (4)(2).biphen.(pentane)(0.5) (4b) along with the known toluene solvates (3)(2).toluene (3a) and (4)(2).(toluene)(3) (4a)([1a]) are compared. Finally, an example is presented of how the alkali metal hypersilanides could be utilized in preparing extremely bulky stannanide anions.