Hypercoordinated Carbon in 2,8,9-Sila- and Thia-Substituted Carbatranes

The structures of carbatranes XC(SiH₂CH₂CH₂)₃N (8) and XC(SCH₂CH₂)₃N (9), where X = H, F, and their in and out protonated forms were studied at the MP2 level of theory using the 6-31G*, 6-311G**, and 6-311++G** basis sets. Compounds 8 and 9 are the first representatives of the derivatives of 5-aza-1-carbabicyclo[3.3.3]undecanes, whose endo-isomer contains a nitrogen lone electron pair directed inside the cage. The geometrical, AIM, and NBO peculiarities of sila- (8) and thiacarbatranes (9) are indicative of the existence of a weak coordination XC←N interaction in these compounds, which is enhanced with the increasing of electronegativity of the axial substituent X. The MP2 and MP4/SDQ(T) (single-point calculations) methods demonstrate that the in protonated forms of compounds 8 are significantly (>6 kcal/mol) more favorable than their out forms. The quantum-topological AIM approach suggests that the main reason for this is the formation of a medium-strength (4−15 kcal/mol) hydrogen C¹···H⁺N bond unknown previously in the in silacarbanyl cations. That bond provides a high pentacoordination degree (>60%) of the bridged carbon atom C¹ in the compounds.