| Transition-metal-containing aromatic compounds,which exhibit both aromatic properties and the characteristics of organometallic compounds,have attracted great attention due to their unique structure,electronic properties and fascinating reactivities.Aromaticity has always been an intriguing research area in contemporary chemistry,and it is account for the stability,reactivity,molecular structures and other properties of many unsaturated organic compounds.Transition metal-catalyzed atom transfer radical addition?TMC ATRA?reaction is an effective and versatile strategy for constructing carbon-carbon bonds in organic synthesis.Nowadays,most research groups reported about the ATRA process using late transition metals such as Cu,Ni,Ru,and Fe.However,early transition-metal-catalyzed ATRA reactions were less explored due to the difficulty to control the reversible redox processes.M2[N?CH3?CHN?CH3?]2?M=Cr,Mo,W?is a newly synthesized heterocyclic compound containing M-M quintuple bond.Therefore,studying its aromaticity and catalytic activity can better understand the properties of such catalysts,expand their applications in organic synthesis,and provide theoretical guidance for improving the physicochemical properties of such compounds.A detailed theoretical study on aromaticity and catalytic activity of the quintuply-bonded dinuclear complex M2[N?CH3?CHN?CH3?]2?M=Cr,Mo,W?was investigated using density functional theory.All geometry optimizations and single-point calculations were performed with Gaussian 09 package.Aromaticity analysis was performed using Multiwfn 3.6 program(chemical shift(?zz)map?the multi-center bond order(Iring)?ICSS map),AICD 2.0 program?ring current map?and NBO 6.0 program(NICSzz,CMO-NICS).The thermodynamic and kinetic analysis in the catalytic reaction were calculated by using the Transition State Theory?TST?.The research content of this paper mainly includes two parts:1.M2[N?CH3?CHN?CH3?]2?M=Cr,Mo,W?is a new type of heterocyclic compound with M-M quintuple bond.In this work,the aromaticity of the five-membered heterocyclic ring?CM2N2?in such compounds and their charged derivatives?-2?were studied.The electron delocalization of these compounds were confirmed by NICSzz-scan profiles,multicenter aromatic index,Iring,current density maps,and other indicators on the M06-L/Def2-TZVP level,and the origin of their aromaticity was further determined.The results show that the molecular aromaticity is enhanced by the increasing atomic numbers?Z?in M=Cr,Mo and W;The aromaticity of Cr2[N?CH3?CHN?CH3?]2 is weak,the five-membered ring in M2[N?CH3?CHN?CH3?]2?M=Cr,Mo,W?exhibit obvious?aromatic character.The contribution of?aromaticity and?aromaticity can be neglected;The addiditon of two electrons in M2[N?CH3?CHN?CH3?]22-?M=Cr,Mo,W?does not led to a contrary aromatic character,which due to the addition of 2 electrons locates in a?-type orbital,not the?orbital.2.Transition metal-catalyzed radical addition reaction is an effective way to build new C–C bond.Typically,the metal center in this metal-assisted radical transformation undergoes a reversible redox process.Based on the first part,we further investigated the catalytic mechanism of Mo2[N?CH3?CHN?CH3?]2 as potential catalyst for radical addition of CCl4 to1-hexene on the?B97XD/Def2-TZVP level.The distribution of atomic spin density during the reaction was analyzed.The study shows that the Mo2[N?CH3?CHN?CH3?]2 mediated radical addition are computationally predicted to occur with acceptable activation energies,indicating that the Mo-Mo quintuple bond can be applied as an effective catalyst for this transformation under mild conditions.The whole reaction involves 3 steps,two of which are metal-mediated.Firstly,the C-Cl bond activiation catalyzed by Mo2[N?CH3?CHN?CH3?]2 to obtain a metal-halide complex,Mo2[N?CH3?CHN?CH3?]2Cl and·CCl3 radical.The next step is a radical addition reaction of·CCl3 with 1-hexene to form an addition product,which further reacts with the Mo2[N?CH3?CHN?CH3?]2Cl to produce the last product and regenerate the catalyst Mo2[N?CH3?CHN?CH3?]2.Both the thermodynamic and kinetic study show that the second step is the rate-determine step.When coordinating solvent pyridine is added to the catalytic reaction,the reaction is suppressed due to their high energies barriers,which is consistent with experimental results. |
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