Are There Structurally Relevant Attractive Interactions between Nitrogen Atoms and Group 14 Elements in Their Aminomethyl Compounds?

The three compounds H 3 SiCH 2 NMe 2, Me 3 GeCH 2 NMe 2, and Me 3 SnCH 2 NMe 2 have been prepared for the purpose of the determination of their molecular structures. The gas-phase structure of H 3 SiCH 2 NMe 2 has been established by electron diffraction (GED) and ab initio calculations up to the MP2/6-311G** level of theory. The geometry of Me 3 GeCH 2 NMe 2 could be studied only by theoretical methods, while the structure of Me 3 SnCH 2 NMe 2 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 (H 3 SiCH 2 NMe 2 GED 114.7(3)°, MP2/6-311G** 111.4°; Me 3 GeCH 2 NMe 2 SCF/6-31G* 116.1°; Me 3 SnCH 2 NMe 2 XRD 113.0(2)°, SCF/dzp 115.4°), 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 α-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 SiCH 2 NC 2 skeleton (lone pairs at N vs Si atom) of H 3 SiCH 2 NMe 2 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), σ(NC) → d(E), or lp(N) → σ*(EX) interactions as important contributors to electron delocalization. Calculations on the model system FH 2 SiCH 2 NMe 2 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 H 3 SiCH 2 NMe 2 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) → σ*(SiF) interactions.