PUB - Publikationen an der Universität Bielefeld

The three compounds H3SiCH2NMe2, Me8GeCH2NMe2, and Me3SnCH2NMe2 have been prepared for the purpose of the determination of their molecular structures. The gas-phase structure of H3SiCH2NMe2 has been established by electron diffraction (GED) and ab initio calculations up to the MP2/6-311G** level of theory. The geometry of Me3GeCH2NMe2 could be studied only by theoretical methods, while the structure of Me3SnCH2NMe2 has been determined by single-crystal X-ray diffraction (XRD) and computational methods. The E-C-N units (E = Si, Ge, Sn) in all three compounds adopt angles that are larger than the tetrahedral angle (H3SiCH2NMe2 GED 114.7(3)degrees, MP2/6-311G** 111.4 degrees; Me3GeCH2NMe2 SCF/6-31G* 116.1 degrees; Me3SnCH2NMe2 XRD 113.0(2)degrees, SCF/dzp 115.4 degrees), and all three compounds adopt gauche conformations; that is, the lone pairs at nitrogen are declined away from the plane E-C-N. These facts are interpreted in terms of the absence of an earlier postulated alpha-effect, which should lead to an attractive interaction between E and N centers. The structures, energies, and orbital interactions in the natural bond orbital (NBO) model for three possible conformations of the SiCH2NC2 skeleton (lone pairs at N vs Si atom) of H3SiCH2NMe2 have been calculated ab initio and show the gauche conformer to be the ground state and the syn form (+6.6 kJ mol(-1)) as well as the anti form (+10.2 kJ mol(-1)) to be transition states to rotation about the N-C(H-2) bond. The NBO calculations do not confirm the earlier postulated lp(N) --> d(E), sigma(NC) --> d(E), or lp(N) --> sigma*(EX) interactions as important contributors to electron delocalization. Calculations on the model system FH2SiCH2NMe2 predict a tendency of SiCN units to adopt small bond angles if very electronegative groups are bound to silicon. The conformational preferences of the unsubstituted H3SiCH2NMe2 are overridden in favor of a conformation bringing the lone pair of electrons closer to the electrophilic Si center. This compound is predicted to have significant lp(N) --> sigma*(SiF) interactions.