PUB - Publikationen an der Universität Bielefeld

The compounds Cl3SiOCH2CH2NMe2 (1) and Cl2HSiOCH2CH2NMe2 (2) were prepared by reactions of lithium 2-(dimethylamino)ethanolate with SiCl4 and HSO3. The analogous reaction with H2SiCl2 gave ClH2SiOCH2CH2NMe2 (3), but only in a mixture with Cl2HSiOCH2CH2NMe2 (2), from which it could not be separated. All compounds were characterized by IR and NMR (1(H), C-13, Si-29) spectroscopy, 1 and 2 by elemental analyses and by determination of their crystal structures. Cl3SiOCH2CH2NMe2 (1) and Cl2HSiOCH2CH2NMe2 (2) crystallize as monomeric ring compounds with pentacoordinate silicon atoms participating in intramolecular Si-N bonds [2.060(2) angstrom (1), 2.037(2) A (2)]. The dative bonds in 1 and 2 between the silicon and nitrogen atoms could also be proven to exist at low temperatures in solution in H-1, Si-29-HMBC-NMR experiments by detection of the scalar coupling between the 29Si and the protons of the NCH2 and NCH3 groups. A function describing the chemical shift delta Si-29(exp) dependent on the chemical shifts of the individual equilibrium components, the temperature, and the free enthalpy of reaction was worked out and fitted to the experimental VT-NMR data of 1 and 2. This provided values of the free reaction enthalpies of Delta G = -28.8 +/- 3.9 kJ . mol(-1) for 1 and Delta G = -22.3 +/- 0.4 kJ . mol(-1) for 2 and estimates for the chemical shifts of open-chain (index o) and ring conformers (index r) for 1 of delta(r) = -94 +/- 2 ppm and delta(o) = -36 +/- 5 ppm and for 2 of delta(r) = -82 +/- 1 ppm and delta(o) = -33 +/- 4 ppm. The value of delta(r) for 1 is very close to that obtained from a solid-state Si-29 MAS NMR spectrum. Quantumchemical calculations (up to MP2/TZVPP) gave largely differing geometries for 1 (with a Si...N distance of 3.072 angstrom), but well reproduced the geometry of 2. These differences are due to Cl...H and Cl...C repulsions and solid state effects, which can be modeled by conductor-like screening model calculations and also rationalized in terms of the topology of the electron density, which was analyzed in terms of the quantum theory of atoms in molecules.