Theoretical Study On The Stability And Adaptive Aromaticity Of Silicon-containing Species

Silicon has attracted considerable interest because its structure and bonding are significantly different from those of carbon analogues.For instance,a difference between disilene and alkene is its trans-bending of the substituents.Also,unlike the carbon atom,silicon atom is reluctant to participate in π bonding.The ground state of silylene is usually a singlet state,where the ground state of carbene could be the singlet or triplet state,depending on the substituents.In addition,aromaticity is one of the most important concepts in chemistry.Here,we employ theoretical calculations to demonstrate the stability and adaptive aromaticity of silicon-containing species.The main contents are summarized as following:1.Through theoretical calculations,the stability of disilenes,disilabenzenes and their isomeric silylenes have been examined.The results reveal that the relative energies of disilenes and their isomeric silylsilylenes can be tuned by different substituents and be rationalized by the Bent’s rule.Disilabenzenes are thermodynamically more stable than corresponding monocyclic silylene isomers due to the aromaticity.When the σdonating NHC coordinates to the silicon atom in disilabenzenes and their monocyclic isomers,the relative stabilities become reversed due to the loss of aromaticity in the disilabenzenes and stabilizing the isomeric silylenes by a donor-acceptor interaction.The kinetic effect of the tautomerization with several typical substituents(F,Me and OMe)has also been investigated.2.Bicyclo[2.1.1]hexane(BCH)silylene has been reported to be the global minima on the C4Si2H6 potential energy surface.In this work,we demonstrate the relative thermodynamic stability between BCH-silylene and isomeric ortho-disilabenzene can be reversed by substituent groups.3.Cyclic molecules with 4n+2 or 4n π electrons are aromatic in the lowest singlet state(S0)or the lowest triplet state(T1)according to Hückel and Baird’s rules.Thus,the design of aromatic species in both the S0 and T1 states(termed as adaptive aromaticity)is particularly challenging.In this work,we demonstrate that metallasilapentalynes show adaptive aromaticity supported by structural,magnetic,and electronic indices,in sharp contrast to metallapentalynes,which exhibit aromaticity in the S0 state only.The origin of the difference has been probed to be the excitation pattern,that is,πσ*excitation rather than ππ*which stated in Baird’s rule is necessary to achieve adaptive aromaticity.