N,N-Dimethylaminopropylsilane: A Case Study on the Nature of Weak Intramolecular Si[sdot][sdot][sdot]N Interactions

Organosilicon compounds have attracted attention for a number of decades, because they have found applications in both industry and everyday life. Examples include silicones in the form of the materials themselves and as cross-linkers; additives for paints, inks, adhesives (adhesion promoters) and cosmetics; and water-repellent agents. Some of the silanes already in use are also hypervalent silicon compounds, which are gaining in importance. In recent years so called a-silanes, that is, silanes with a donor function in geminal position relative to the silicon centre, are replacing and complementing the industrially used organofunctional silanes with donor functions in the d position due to their higher reactivity. The fact that the properties of silanes with donor functions in the b position differ strongly from those with a donor in d position was first established in the 1950s. Later Kostyanovskii et al. examined the basicity of amines with a group 4 element in geminal position and found that amines with geminal acceptor atoms showed reduced basicity. This was explained by the formation of a geminal donor–acceptor interaction between the group 4 and nitrogen atoms and was contrasted in this respect to the behaviour of the g-silanes. This interaction between silicon and nitrogen leads to pentacoordinate Si atoms, which were established to undergo much faster nucleophilic substitution reactions than tetraor hexacoordinate derivatives. Abstract: N,N-Dimethylaminopropylsilane H3Si ACHTUNGTRENNUNG(CH2)3NMe2 was synthesised by the reaction of (MeO)3SiACHTUNGTRENNUNG(CH2)3NMe2 with lithium aluminium hydride. Its solid-state structure was determined by X-ray diffraction, which revealed a five-membered ring with an Si···N distance of 2.712(2) 7. Investigation of the structure by gas-phase electron diffraction (GED), ab initio and density functional calculations and IR spectroscopy revealed that the situation in the gas phase is more complicated, with at least four conformers present in appreciable quantities. Infrared spectra indicated a possible Si···N interaction in the Si H stretching region (2000–2200 cm ), as the approach of the nitrogen atom in the five-membered ring weakens the bond to the hydrogen atom in the trans position. Simulated gas-phase IR spectra generated from ab initio calculations (MP2/TZVPP) exhibited good agreement with the experimental spectrum. A method is proposed by which the fraction of the conformer with a fivemembered ring can be determined by a least-squares fit of the calculated to experimental absorption intensities. The abundance of this conformer was determined as 23.7(6)%, in good agreement with the GED value of 24(6)%. The equilibrium Si···N distance predicted by theory for the gas-phase structure was highly variable, ranging from 2.73 (MP2) to 3.15 7 (HF). The value obtained by GED is 2.91(4) 7, which could be confirmed by a scan of the potential-energy surface at the DFLCCSD[T] level of theory. The nature of the weak dative bond in H3SiACHTUNGTRENNUNG(CH2)3NMe2 can be described in terms of attractive inter-electronic correlation forces (dispersion) and is also interpreted in terms of the topology of the electron density.