Intramolecular N-donor-stabilized silenes: An ab initio MO study of 1-methylene-5-methyl-5-aza-2,8-dioxa-1-silacyclooctane

Ab initio calculations with full geometry optimization were performed to predict the structure of l-methylene-5-methyl-5-aza-2,8-dioxa-1-silacyclooctane (MADS), a compound with an intramolecular Si<--N donor-acceptor bond between a sp(2)-hybridized silicon and a nitrogen atom. Calculations were carried out at the RHF/6-31G(d)//6-31G(d) and MP2/ 6-31G(d)//MP2/6-31G(d) levels of theory. The MADS structure with the shortest intramolecular Si...N distance (2.105 Angstrom) appears to be of C-s symmetry, whereas that with a long (3.695 Angstrom) Si...N distance belongs to the symmetry group C-1. Both structures are represented by a boat-boat conformation of the eight-membered cycle and correspond to local minima on the potential energy surface, the former being the global minimum. The energy of the intramolecular Si<--N donor-acceptor bond (19.7 kcal/mol) was calculated as the difference between the total energies of the two MADS structures with short and long Si...N distances representing the structures with and without Si<--N bonding, respectively. The NBO indices and the electron density maps of the HOMOs support the conclusion that the intramolecular Si<--N donor-acceptor bond is characteristic for MADS with the short Si...N distance. The results obtained for MADS are compared with data calculated for some model compounds, such as 1,1-dimethylsilene, 1,1-dimethoxysilene, their hydrogenated derivatives, the corresponding silanes, and Lewis base/acid complexes of the silenes and silanes with ammonia possessing intermolecular Si<--N donor-acceptor bonds. On the basis of the energy of the dehydrogenation of 1,5-dimethyl-5-aza-2,8-dioxa-1-silacyclooctane into MADS, which was calculated at the MP2/6-31G(d)//MP2/6-31G(d) level, the stabilization for the Si=C double bond was estimated to be 18.0 kcal/mol due to the formation of an intramolecular Si<--N bond. For complexes of ammonia with silenes, in which the silicon atom remains in the sp(2) valence state, the complex formation energy is considerably higher compared to complexes of NH3 with the corresponding silanes.