Theoretical Studies On The Structures And Chemical Bonds Of Dithorium Endohedral Metallofullerenes

Endohedral metallofullerenes(EMFs),as a new type of nanomaterial,have shown great potential on the applications in many fields,especially in biomedicine.EMFs have important applications in anti-cancer and medical imaging,and now they have become star molecules in nanomedicine.The configurations and electronic structures of EMFs have attracted wide attention due to the diversity of encapsulated species and special charge transfer phenomena.Previous studies on EMFs are mainly based on the transition metals and lanthanides,while the studies of actinide-based EMFs are relatively limited.Due to the complexity and unique electronic properties of actinides,the electronic structures and physicochemical properties of actinide-based EMFs may be significantly different from other previous studied EMFs.Expanding the study of actinide-based EMFs is of great significance for both applications and fundamental research.In this work,we predicted a stable dimetallic actinide endohedral metallofullerene Th2@Ih-C80,using density functional theory of quantum chemistry.The calculated thermodynamic parameters of Th2@Ih-C80 are comparable to those of experimentally observed La2@Ih-C80,Ce2@Ih-C80 and U2@Ih-C80,confirming its stability.The optimized geometry of Th2@Ih-C80 is similar to the well-known La2@Ih-C80,and slightly different from Ce2@Ih-C80 and U2@Ih-C80.The analysis of the electronic structure show that each Th transfers three electrons to the Ih-C80 and the valence state of Th2@Ih-C80 is Th26+@Ih-C806-,this is equivalent to each Th being a trivalent cation.In previous studied Th-based mono-EMFs cases,Th is invariably in the most stable Th4+oxidation state.Th2@Ih-C80 has a triplet ground state with a Th-Th distance of 3.803 A.The two remaining valence electrons of Th26+ form open-shell two-fold OETC bonds with the electronic configuration of(6d7s7p)?1(5f6d)?1.However,if two Th atoms are placed in an open confined environment,where the coordination environment is similar to that in fullerene,the bond length of Th26+is extended to 4.234 A,and the Th-Th bonds are restored to a normal closed-shell ?2 single bond.Therefore,the spin polarization of the formal single bond can only be obtained by squeezing two metal cations through carbon cages.Based on this discovery,we obtained Th3+-Th3+single bonds with different electronic configurations by changing the size of the carbon cages.The Th3+-Th3+bond can further evolve into a 5f6d dominated spin-polarized ?2 configuration by compressing the Th-Th distance further down to 2.843 A within a smaller Ih-C60 cage.On the other hand,elongating the Th-Th distance to 4.063 A by encapsulating Th2 into a long diametric D3h-C78 fullerene returns the Th3+-Th3+bond to the normal close-shell(6d7s7p)?2 form.Hence,we proposed a new rule of carbon cage induced spin-polarization of M-M bonds,the key point of this rule is the size of the carbon cage.The squeezed effect of the carbon cage can conduce to the effective overlap of the 5f-5f and 6d-6d orbitals which significantly lowers the orbital energy of the ? bond,thus reducing and further reversing the original large singlet-triplet energy gap of the Th26+ unit.Governed by this mechanism,the formal single Th3+-Th3+covalent bond can be transformed from the initial close-shell ?2 configuration to the open-shell?1?1 and ?2 configurations.In addition,we calculated various indices of the bond critical points of metal-metal bonds and metal-carbon bonds based on the quantum theory of atoms in molecules(QTAIM),and investigated the metal-metal and metal-carbon interactions.And we explored the charge transfer from metals to carbon cages based on charge population analysis.The interaction between the inner Th26+ and the outer Ih-C806-is a mixture of ionic and covalent interactions.The calculation results of transition state show that the energy barrier for the geometric transformation of Th2@Ih-C80 is about 1.0 kcal/mol,we predict that The can freely rotate inside Ih-C80 cage.For our predicted Th2@h-C80,which is not only the first theoretically predictable stable dithorium EMF,but also the first case that trivalent Th cations are stabilized by fullerene.This work offers theoretical guidance for the follow-up synthesis and separation of Th2@C80,and provides a theoretical model and basic guidance for the development of new fullerene materials and the regulation of valences and covalent bonds of actinides.