Theoretical Studies On The Exohedral Cage Functionalization Of Endohedral Metallofullerenes M_xSc_3-xN@C₈₀（M=Sc,Ti,V;x=1-2）

Endohedral Metallofullerenes(EMFs)have a wide range of applications because of their unique cavity structure.Among them,paramagnetic EMFs have unpaired electrons and can play a potential role in areas such as nuclear magnetic resonance imaging and quantum computing.In recent years,the derivatives of paramagnetic EMFs are also of great significance in materials science and medicine,so the study of the exohedral cage functionalization of paramagnetic EMFs has attracted great interest.So far,there are scarce experimental and theoretical studies on the exohedral cage functionalization of paramagnetic EMFs.Therefore,further research,may see more uses created for these materials in the future.In ordinary chemical reactions,the hydrogenation reaction process is relatively simple,and the Diels-Alder(DA)cycloaddition reaction also provides a powerful means for the exohedral cage functionalization of paramagnetic EMFs,so they are important reactions of EMFs.In this work,the regioselectivity of hydrogenation of three paramagnetic EMFs is introduced,and the DA reaction mechanism,reactivity and regioselectivity of three paramagnetic EMFs are compared.Firstly,we study the exohedral cage functionalization of paramagnetic fullerene TiSc2N@Ih(7)-C80 by density functional theory(DFT).In the third chapter of this work,the monomer structure of TiSc2N@C80 is determined,the regioselectivity of the hydrogenation reaction is studied comprehensively,and the hydride model is established to predict the stability of various derivatives.The stability of Bingel-Hirsch(BH)single bond products of TiSc2N@C80 are predicted by using the stability of single hydrogenation products,and the stability of DA cycloaddition products of TiSc2N@C80 are predicted by using the stability of the hydrogenated products where two hydrogen atoms are added two adjacent carbon atoms.Due to the influence of the spin density of the cluster Ti atom,there is poor correlation between he reaction free energy barrier and reaction free energy for the addition of s-cis-1,3-butadiene(BD)to TiSc2N@C80,which does not conform to the BEP(Bell-Evans-Polanyi)principle.Therefore,the reaction site between TiSc2N@C80 and BD should be fully considered to find the kinetic dominant reaction path.In Chapter 4,we consider all the addition sites of the reaction between TiSc2N@C80 and BD,and find the optimal reaction path.The results show that compared with the DA reaction of non-magnetic system Sc3N@Ih(7)-C80,the activity of the DA cycloaddition reaction of TiSc2N@C80 is significantly enhanced.Moreover,the reaction mechanism no longer follows the traditional[4+2]concerted mechanism,but tends to the[4+3]stepwise addition reaction mechanism,and the Gibbs free energy barrier of this process is 14.7 kcal/mol(in the gas phase).The main reason of the stepwise reaction mechanism is the formation of relatively stable intermediates.We give a reasonable explanation of the stepwise reaction mechanism by analyzing the change of the spin density of Ti atoms in the whole reaction process.In the reactants,the spin density is mainly distributed on the Ti atom,which is transferred to BD when the single bond intermediate is formed,and then returned to the Ti atom after the cycloaddition product is formed.By adding π electron-withdrawing groups to BD,the delocalization effect of free radical electrons in the intermediate is enhanced and further stabilized,thus improving the reactivity.We also consider the effect of toluene and orthodichlorobenzene(o-DCB)on the reaction between TiSc2N@C80 and BD,and finally concluded that these two solvents are beneficial to the reaction.In chapter 5,the hydrogenation and DA reactions of the paramagnetic metallofullerene VxSc3-xN@Ih(7)-C80(x=1,2)are studied.The reaction mechanism of DA cycloaddition of VxSc3-xN@C80(x=1,2)is consistent with that of TiSc2N@C80,which is also taken place according to the stepwise reaction mechanism,but the reactivity is higher.In the gas phase,the Gibbs free energy barrier of the reaction between VSc2N@C80 and BD is 11.0 kcal/mol,the Gibbs free energy barrier of the reaction between V2ScN@C80 and BD is 8.8 kcal/mol.The increase of the reactivity is mainly related to the electronegativity of the clusters.In VxSc3-xN@C80(x=1,2),the electronegativity of cluster is greater than that of TiSc2N.The DFT calculation results show that in the VxSc3-xN@C80(x=1,2)system,there are several radical centers involved in the reaction due to spin polarization,which is conducive to the reaction.The DA activity of VxSc3-xN@C80(x=1,2)is further improved by adding π electronwithdrawing groups to BD.We compare the reactivity of TiSc2N@C80,VSc2N@C80 and V2ScN@C80 with the derivatives of BD in gas phase,toluene and o-DCB solutions.In the gas phase,the reactivity of the three systems with derivatives of BD is similar.The non-polar toluene solvent is favorable to the DA reaction of TiSc2N@C80,unfavorable to the DA reaction of VSc2N@C80,and has little influence on the DA reaction of V2ScN@C80.The polar solvent o-DCB solution is more favorable to the DA reaction of TiSc2N@C80,but not to the DA reaction of VxSc3-xN@C80(x=1,2).In this work,we have obtained insightful conclusions that may deepen our understanding of the DA reaction about conventional metallofullerenes,and explain the reaction mechanism from the perspective of spin density.We also provide two kinds of solvents that are beneficial to the experiment,so as to facilitate the experimentalist to carry out related work.