Theoretical Study On The Structure Of Bimetallic Paddlewheel Complexes M₂L₄ And Its Catalytic Mechanism For The Addition Reaction Of CCl₄ To 1-hexene

The bimetallic complexes have drawn more and more attention because of their special electronic structure and physical chemical properties,and play important roles in coordination chemistry and material chemistry.Among these bimetallic compounds,four bridging ligands ligated dinuclear complexes with the directly bonded metal-metal bond are referred to as bimetallic paddlewheel complexes.Due to the special metal-metal bond and the paddle wheel shape geometry,these bimetallic paddlewheel complexes show the unique redox,electron transfer,catalysis,photoelectricity and magnetism performance.And they have very important applications in the fields of organic synthesis and catalysis.Therefore,it is of great significant to study the nature of chemical bond and catalytic performance of these bimetallic paddlewheel complexes,which can further understand their properties of such catalysts,expand their applications in organic synthesis,and provide theoretical guidance for improving their physical and chemical properties.In this paper,the nature of metal-metal bond in the bimetallic paddlewheel complexes M₂L₄(M=Cr,Mo,W,V,Nb,Ta;L=OCOCH₃,HNCNHCH₃),as well as the catalytic mechanism of Mo₂(CH₃NCHNCH₃)₃(OCOCH₃)on the atom transfer radical addition of CCl₄ to 1-hexene,are investigated based on density functional theory(DFT).The geometry optimizations,frequency calculations,single point energy corrections,as well as intrinsic reaction coordinates(IRC)calculation from transition states were all performed using Gaussian 09 program package.The nature of chemical bond was analyzed within the frame of molecular orbital theory(MOT),the quantum theory of atoms in molecule(AIM),the electron location function(ELF),and natural bond orbital(NBO)theory,using AIMALL,Topmod,and Multiwfn programs.This thesis mainly includes the following two parts:1.Two same VB and VIB transition metals could form bimetallic paddlewheel complexes with carboxylic acid,amide,pyrrolidine ester ligands.In this part,the stable geometries of M₂L₄(M=Cr,Mo,W,V,Nb,Ta;L=OCOCH₃,HNCNHCH₃)are optimized at B3LYP/LANL2DZ level.The results of MOT,AIM,ELF,and NBO analysis show that the M-M bonds are quadruply bond as M=Cr,Mo,and W,which contain one?bond?two?bond and one?bond.As M=V,Nb,and Ta,the M-M bond belong to triple bond,consisting one?and two?bond.With the increasing of element period number,the covalency of M-M bond is weakened and the metallic property is enhanced.According to the calculation of redox potential,the catalytic effects of the compounds with L=OCOCH₃ has better than those with L=HNCNHCH₃.2.The radical addition reaction catalyzed by transition metal is an efficient catalytic process.The experimental results show that Mo₂(CH₃NCHNCH₃)₃(OCOCH₃)was an efficient catalyst for the addition reaction of CCl₄with 1-hexene.Moreover,the catalytic effects of bimetallic paddlewheel complexes with mixed ligand were higher than those of the same ligand.In this part,the catalytic mechanism of Mo₂(CH₃NCHNCH₃)₃(OCOCH₃)on the radical addition of CCl₄ to 1-hexene to get 1,1,1,3-tetrachloroheptane are investigated at the PBEPBE/Def2SVP level.The calculated results show that Mo₂(CH₃NCHNCH₃)₃(OCOCH₃)is a good catalyst for the addition reaction of CCl₄ to 1-hexene radical,and the reaction can proceed smoothly under mild conditions.The entire catalytic reaction involves four steps,three of which are mediated by metals.Firstly,the C-Cl bond of first CCl₄ is activated by[Mo₂L₄]catalyst,[Mo₂L₃Cl]and CH₃COOCCl₃are obtained;Then the second CCl₄ adds to[Mo₂L₃Cl]to produce[Mo₂L₃Cl₂]and CCl₃radical;CCl₃radical interacts with 1-hexene to get an addition,the addition product reacts with one Cl atom of[Mo₂L₃Cl₂]to get the last product ⁿBu CHCl CH₂CCl₃ and[Mo₂L₃Cl]is regenerated.The addition of the first CCl₄ to[Mo₂L₄]catalyst is the rate-determining step of the whole reaction.Because this step is not in the catalytic cycle,the reaction would speed up after a certain period.The catalytic activity of dimolybdenum paddlewheel complex can be controlled by tuning the ligand,which have influences on the natural population analysis(NPA)charge of Mo and the redox potential E(Mo₂⁴⁺/Mo₂⁵⁺).The higher NPA of Mo atom and higher E(Mo₂⁴⁺/Mo₂⁵⁺)of the catalyst,the higher catalytic activity it has.Our calculated results provide an explanation for experimental observations and useful insights for further development of bimetallic catalysts in radical addition reactions.