Theoretical Studies On The π Electron Effect Of Group 13 Olefin Compounds

Group 13 olefins have been widely used in the fields of organic synthesis,chemical catalysis and drug design.In recent years,more and more attention has been paid to these compounds.Exploring the bonding nature of group 13 olefin compounds can help us deeply understand their electronic structural properties and provide theoretical basis for chemical synthesis and catalytic applications.Valence bond theory is one of the important modern chemical bond theories,which can provide an intuitive description of chemical bond properties for the research system.In this paper,theπelectron effect in group 13 olefin compounds is studied by means of valence bond（VB）theory,block localized wave function（BLW）method and density functional theory（DFT）.The details are as follows:1.Given the extraordinary versatility in chemical reactions and applications,boron compounds have gained increasing attentions in the past two decades.One of the remarkable advances is the unprecedented preparation of unsaturated boron species.Notably,Braunschweig et al.found that the cyclic（alkyl）（amino）carbenes（CAACs）stabilized diboron molecules（CAAC）₂B₂（SR）₂host unpaired electrons and exist in the 90°-twisted diradical form,while other analogues such as N-heterocyclic carbenes（NHCs）stabilized diboron molecules prefer a conventional B=B double bond.Since previous studies recognized the differences in the steric effect between CAAC and NHC carbenes,here we focused on the particular role of thiol substituents in（CAAC）₂B₂（SR）₂by gradually localizing involved electrons.The co-planarity of the thiol groups and the consequent captodative effect were found to be the culprit for the 90°-twisted diradical form of（CAAC）₂B₂（SR）₂.Computational analyses identified two forces contributing to theπelectron movements.One is the“push”effect of lone pairs on the sulfur atoms which boosts theπelectron delocalization between the BB center and CAACs.The other is theπelectron delocalization within each（CAAC）B（SR）fragment.There are two such independent and orthogonal push-pull channels which function mainly in individual（CAAC）B（SR）fragments.This enhancedπpush-pull effect in the triplet state facilitates the electronic excitation in（CAAC）₂B₂（SR）₂by reducing the singlet-triplet gap.2.There have been remarkable advances in the syntheses and applications of groups 13and 14 homonuclear ethene analogues.However,successes are largely limited to aryl-and/or silyl-substituted species.Analogues bearing two or more heteroatoms are still scarce.To theoretically understand the likely causes for this scarcity,we employed the block-localized wavefunction（BLW）method at the density functional theory（DFT）level to study dialumene（NH₂）（PMe₃）Al=Al（PMe₃）（NH₂）and disilene（NH₂）（Si H₃）Si=Si（Si H₃）（NH₂）bearing two amino substituents whose optimal geometries exhibit significantly stretched central M=M（M=Al or Si）double bonds.Computational analyses showed that the repulsion between the lone electron pairs of amino substituents and M=Mπbond plays a critical role in the elongation of the M-M bonds.As evidence,replacing the substituent groups-NH₂with-BH₂can enhance the planarity and shorten the central double bonds due to the absence of lone pair electrons in BH₂.