title: Silicon-containing formal 4π-electron four-membered ring systems: antiaromatic, aromatic, or nonaromatic?
abstract: Density functional theory calculations (B3LYP) have been carried out to investigate the 4π-electron systems of 2,4-disila-1,3-diphosphacyclobutadiene (compound 1) and the tetrasilacyclobutadiene dication (compound 2). The calculated nucleus-independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4π-electron four-membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4π-electron system. The monoanionic bidentate ligand interacts with the conjugated π system to cause π-orbital splitting. This ligand-induced π-orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied π orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated π ring system and does not fulfill the requirements of a Hückel system.
authors.affiliation: Lab of Computational Chemistry and Drug Design, Laboratory of Chemical Genomics, Peking University Shenzhen Graduate School, Shenzhen 518055, PR China.
authors.forename: Yun-Fang
authors.initials: YF
authors.lastname: Yang
authors.forename: Gui-Juan
authors.initials: GJ
authors.lastname: Cheng
authors.forename: Jun
authors.initials: J
authors.lastname: Zhu
authors.forename: Xinhao
authors.initials: X
authors.lastname: Zhang
authors.forename: Shigeyoshi
authors.initials: S
authors.lastname: Inoue
authors.forename: Yun-Dong
authors.initials: YD
authors.lastname: Wu
mesh_terms: D004583:Electrons; D008962:Models, Theoretical; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D012825:Silicon / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D012825:Silicon