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 pr edict the structure of l-methylene-5-methyl-5-aza-2,8-dioxa-1-silacycloocta ne (MADS), a compound with an intramolecular Si<--N donor-acceptor bond bet ween a sp(2)-hybridized silicon and a nitrogen atom. Calculations were carr ied out at the RHF/6-31G(d)//6-31G(d) and MP2/ 6-31G(d)//MP2/6-31G(d) level s of theory. The MADS structure with the shortest intramolecular Si...N dis tance (2.105 Angstrom) appears to be of C-s symmetry, whereas that with a l ong (3.695 Angstrom) Si...N distance belongs to the symmetry group C-1. Bot h structures are represented by a boat-boat conformation of the eight-membe red cycle and correspond to local minima on the potential energy surface, t he former being the global minimum. The energy of the intramolecular Si<--N donor-acceptor bond (19.7 kcal/mol) was calculated as the difference betwe en the total energies of the two MADS structures with short and long Si...N distances representing the structures with and without Si<--N bonding, res pectively. The NBO indices and the electron density maps of the HOMOs suppo rt the conclusion that the intramolecular Si<--N donor-acceptor bond is cha racteristic for MADS with the short Si...N distance. The results obtained f or MADS are compared with data calculated for some model compounds, such as 1,1-dimethylsilene, 1,1-dimethoxysilene, their hydrogenated derivatives, t he 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 esti mated 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 rema ins in the sp(2) valence state, the complex formation energy is considerabl y higher compared to complexes of NH3 with the corresponding silanes.