Quantum Chemistry Study On The Aromaticity Of Transition Metal Heterocycle Conjugated Systems

In this thesis, the aromaticity of transition metal heterocycle conjugated systems has been investigated firstly with the aid of quantum chemical calculations and the fragment orbital interaction analysis. There are three main parts:In the first part, conventional criteria and indices of aromaticity, including electronic, geometric, energetic and magnetic aspects, have been applied to examine the aromaticity of five typical transition metal heterocyclic complexes, i.e. six-membered osmabezene 1 and iridabenzene 2, five-membered cobaltacyclopentadiene 3 and iridacyclopentadiene 4, and four-membered tungstacyclobutadiene 5. The results show that the cyclic, planar, conjugated and Hückel 4n+2 rule's criteria in the transition-metal-containing heterocycles of the five complexes studied are all met. Four quantitative aromaticity indices, including Bird aromatic index (In), homodesmotic reaction aromatic stabilization energy (HASE), diamagnetic susceptibility exaltation (Λ) and nucleus-independent chemical shifts (NICS), qualitatively lead to a consistent and affirmative conclusion that all of them are aromatic. However, they fail to draw a common conclusion for their relative magnitudes of aromaticity, which primarily originates from the different strains along with different cycles included in the index of HASE.The second part aims at the six-membered transition metallabenzene and discusses the influence from the transition metal heteroatom and its ligand coplanar with the heterocycle on the aromaticity of the metallabenzene. The two magnetic criteria, Λand NICS, agree with In that the condition benefiting d-p ??back-bonding interaction can enhance the aromaticity of the metallabenzene except for model 1 with M = Fe and model 2 with L = H. However, the energetic index HASE gives a conclusion contrary to that from In when some effects are mentioned. Such inconsistency shows once again the shortcoming of HASE in the comparing of complex systems. Nevertheless, the inner discrepancy among each of the four criteria consistently reveal that the effect from different race metal heteroatom is stronger than the effect from different periodical metal heteroatom and the ligand effect.In the third part, the condensed Fukui function (CFF) has been used to predict the reactive sites for electrophilic aromatic substitution in osmabenzene and mono substituted osmabenzene. The systems studied are a non-substituted osmabenzene and a series of substituted osmabenzenes with substitutions located in ortho, meta or para with respect to the osmium atom. The calculated results show that the heteroatom group can be regarded as a meta conductor and the substituents jointed to the heterocycle keep well their orientation effects as substitutions of benzene for an electrophilic attack.