Syntheses and structural characterization of the heavier alkali-metal stannates bearing the tripodal triamino framework:: Structural trends in the series of [MeSi{SiMe2N(4-CH3C6H4)}3Sn][M] (M = Na, K, Rb)

Treatment of the lithium stannate [MeSi{SiMe2N(4-CH3C6H4)}(3)SnLi(OEt2)] (1) with AgCl yielded the corresponding distannane [MeSi{SiMe2N(4-CH3C6H4)}(3)Sn](2) (2) as the product of an oxidative coupling in good yield. The distannane (2) underwent reductive cleavage with the heavier alkali metals (sodium, potassium, rubidium) in a non-coordinating solvent (toluene or cyclohexane), to afford the solvate-free stannates(II) [MeSi{SiMe2N(4-CH3C6H4)}(3)Sn][M] (M = Na (3), K (4), and Rb (5)), which were structurally characterized. The Na atom in 3 is directly bonded to two of the three amido-N atoms of the stannate cage and additionally solvated by pi-coordination of a tolyl substituent of a neighboring stannate cage. In contrast, the group 1 metal cations in the two isomorphous compounds 4 and 5 are further removed from the nitrogen functions and are pi-coordinated by three tolyl arene rings. The Sn-K distances in 4 were found to be 3.635 and 3.870 Angstrom, whereas the Sn-Rb contacts in 5 are 3.708 and 3.908 Angstrom: these are the first such Sn-Rb bonding contacts to be characterized in a molecular compound by X-ray diffraction.