Theoretical Study On The Aromaticity Of Metallacycles

Metallacycles,recognised as one important class of organometallic compounds,widely used,especially played a significant role in multiple catalytic processes for their multi-faceted structural chemistry.In comparison with traditional developed metallacycles,such as metallabenzenes,metallapyridines and metallafurans,the chemical properties of new aromatic metallapentalynes,metallapentalenes and their derivatives which have been synthesized in recent five years are much unknown.Hence,we performed thorough density functional theory(DFT)calculations on the aromaticity of different metallacycles to help understand and promote synthesis of more such kinds of compounds.This thesis mainly consists of the following four chapters:In Chapter 1,a brief review is given on the the development and criteria of aromaticity.In addition,the a-and M?bius aromaticity,the advance of aromaticity and reactivity of metallacycles are discussed.Finally,we proposed the motivations of the projects.In chapter 2,DFT calculations were carried out to investigate the aromaticity of ruthenapentalocyclobutadienes and osmapentalocyclobutadienes.The results reveal unexpected higher stabilisation achieved with a 4d ruthenium fragment compared to the 5d osmium counterpart.Moreover,the interaction for the direct 1-3 metal-carbon bonding in the metallabutadiene unit of these two complexes is negligible.In chapter 3,planar antiaromatic cyclopropaosmapentalenes have been synthesized via an aromaticity-to-antiaromaticity switch in experiment.According to theoretical analysis,the results demonstrate that the products are the first example of[4n+2]π electrons Craig-type M?bius antiaromatic compounds.Further theoretical calculations reveal a vital role of phosphonium substituents in such a novel conversion.In chapter 4,valence bond(VB)and molecular orbital(MO)theory were utilized to evaluate the σ-aromaticity in an unsaturated three-membered ring(3MR)of cyclopropene.The results indicate that the gain of σ-aromaticity in cyclopropene is attributed to the shortened C=C bond length.More importantly,switching the sp3-hybridised carbon in the 3 MR to a sp2-hybridised one did not affect σ-electron delocalization significantly,leading to the first σ-aromatic ring dominating in a fully unsaturated methylenecyclopropene and methylenecyclopropeosmapentalene.All these findings could enlarge the fundamental scope of the concept of σ-aromaticity.