Planar Tetracoordinate Silicons in Organic Molecules As Carbenoid-Type Amphoteric Centers: A Computational Study.

Designing and synthesizing a stable compound with a planar tetracoordinate silicon (ptSi) center is a challenging goal for chemists. Here, we theoretically designed and computationally verified a series of potential aromatic ptSi compounds composed of four conjugated rings shared by a centrally embedded Si atom. Both Born-Oppenheimer molecular dynamics (BOMD) simulations and computations of decomposition pathways indicate the high stability and likely existence of these compounds particularly Si-5555 (SiN 4 C 8 H 8 ) with 16 π electrons under normal temperature and pressure. Notably, the Huckel aromaticity rule which works well for single rings seems inconsistent with the high stability of Si-5555 where the 16 π electrons are spread over four rings fused together. Bonding analyses show that the strong electron donation from the peripheral conjugated ring of 16 π electrons to the vacant central atomic orbital Si 3p z leads to the stabilization for both the ptSi coordination and planar aromaticity. The partial occupation of Si 3p z results in the peculiar carbenoid-type behaviors of the amphoteric center. By modulating the electron density on the ring with substituent groups, we can regulate the nucleophilic and electrophilic properties of the central Si.