PUB - Publikationen an der Universität Bielefeld

(F3C)F2SiONMe2 was prepared from LiONMe2 and F3CSiF3- It was characterized by gas IR and multinuclear solution NMR spectroscopy and by mass spectrometry. Its structure was elucidated by single crystal X-ray crystallography and by gas electron diffraction. (It exists as a conformer mixture.) Important findings were extremely acute SiON angles [solid 74.1 (1)degrees, gas anti 84.4(32)degrees and gauche 87.8(20)degrees] and short (SiN)-N-... distances [solid 1.904(2) angstrom]. The bending potential of the SiON unit was calculated at the MP2/6-311++G(3df,2dp) level of theory and appears very flat and highly asymmetric. The calculated atomic charges (NPA) are counterintuitive to the expected behavior for a classical Si-N dative bond, as upon formation of the (SiN)-N-... bond electron density is transferred mainly from oxygen to nitrogen, while the silicon charge is almost unaffected. Despite the molecular topology of a three-membered ring, the topology of the electron density shows neither a bond critical point between Si and N atoms nor a ring critical point, but the electron density and Laplacian values are related to other hypercoordinate Si compounds. The electronic properties of (F3C)F2SiONMe2 were compared to those of the adduct (F3C)F-2(MeO)Si-NMe3, whose properties and structure were also calculated. The charge distribution and Laplacian values along the Si-N vectors in both molecules are similar but not equivalent. (F3C)F2SiONMe2 contains thus a nonclassical (SiN)-N-... bond, and its properties can be regarded as a new model for the explanation of the old postulate of an a-effect in silicon chemistry, explaining the behavior of compounds with geminal Si and N atoms.