Siloles and germoles modified by partial hypercoordination through aminoxysubstituents

1-Dimethylaminoxy-2,3,4,5-tetraphenyl-1-silacyclopentadiene (1), 1,2-bis(di methylaminoxy)-2,3,4,5-tetraphenyl-1-silacyclopentadiene (2) and 1,2-bis(di methylaminoxy)-2,3,4,5-tetraphenyl-1-germacyclopentadiene (3) have been pre pared by the reactions of the corresponding chloro-2,3,4,5-tetraphenyl-1-(s ila/germa)cyclopentadienes with LiONMe2. They are yellow crystalline materi als, which have been identified by multinuclear NMR spectroscopy (H-1, C-13 , N-15, O-17, Si-29), mass spectrometry and elemental analysis. The UV-VIS spectra of 1, 2 and 3 show the absorption bands of the silole/germole to be only slightly affected by the Si ... N interaction, which indicates that o rbital interactions of the type lp(N)-->sigma*(SiC) do not play a significa nt role and thus the Si ... N attraction is better interpreted as a dipole interaction. The crystal structures of 2 and 3 reveal planar C4Si and C4Ge rings, with a propeller-like arrangement of the phenyl groups. The SiON and GeON groups contain short Si ... N distances [2.473(3) and 2.503(3) Angstr om in 2, and 2.535(7) to 2.608(7) Angstrom in 3]. 2 adopts a gauche-gauche conformation for the NOSiON backbone, while in 3, two independent molecules are found in the asymmetric unit, one with an anti-gauche conformation and one with a gauche-gauche conformation.