Understanding the exact position of metal atom(s)within a cage is critical for elucidating the structural characteristics,metal–cage interplay and stability mechanism of endohedral metallofullerenes(EMFs).But it beco...Understanding the exact position of metal atom(s)within a cage is critical for elucidating the structural characteristics,metal–cage interplay and stability mechanism of endohedral metallofullerenes(EMFs).But it becomes rather challenging for the emerging actinide EMFs due to the variable oxidation state and complex valence orbitals of the encapsulated metals.Herein,density functional theory calculations were carried out for all 25 reported Th/U-based mono-metallofullerenes(mono-EMFs)with well-defined single crystal structures.The electronic ground states of Th-and U-contained mono-EMFs are singlet and triplet/quintet,respectively.Th always formally assumes the IV oxidation state inside the cage,whereas U has the Ⅲ or IV valence state highly depending on the cage structure.Due to the considerable intramolecular charge transfer(Th:4e;U:3e or 4e),their metal–cage interactions are mainly ionic,but with non-negligible covalent components largely contributed by the spatially extended Th/U-5f orbitals.Importantly,all the internal Th^(4+)/U^(3+)/^(4+)cations can be located by only considering the frontier molecular orbital distributions of the corresponding empty cages rather than the electrostatic potentials of cage anions commonly used for the classic lanthanide mono-EMFs.Such an obvious deviation from the simple ionic model(M^(q+)@C_(2n)^(q−))is ascribed to the strong actinide–cage covalency,which leads to much lower actual metal charge than the formal one.Our work unprecedently finds the overlooked important role of cage orbitals in determining the internal metal positions of all actinide mono-EMFs.It not only demonstrates the uniqueness of actinide EMFs,but also could help understand the structural characteristics of more EMFs with multiple possible internal metal locations.展开更多
Endohedral metalloazafullerenes(EMAFs)/metalloborafullerenes(EMBFs)are formed by encapsulating various metallic species inside the fullerene cages,of which some carbon atoms are replaced by nitrogen and boron,respecti...Endohedral metalloazafullerenes(EMAFs)/metalloborafullerenes(EMBFs)are formed by encapsulating various metallic species inside the fullerene cages,of which some carbon atoms are replaced by nitrogen and boron,respectively.Currently,the characterization of their exact heteroatom positions on the cage mainly depends on the calculated relative energies at T=0 K for isomers with varied heteroatom locations.In this work,density functional theory calculations were carried out to systematically investigate the structural characteristics,electronic properties,aromaticity and chemical reactivities of the recently synthesized U@C_(27)B borafullerene.Interestingly,besides the reported major U@Td-C_(27)B-[5,5,6]isomer,calculations considering the entropy effect at cage formation temperatures suggest that it may have another major U@Td-C_(27)B-[5,5,6]isomer and a minor U@Td-C_(27)B-[5,5,5]isomer with changed boron positions.The reported U@Td-C_(27)B-[5,5,6]and new U@Td-C_(27)B-[5,5,5]isomers could interconvert via a single bond rotation at elevated temperatures with energy barriers comparable to that of the classical Stone-Wales rearrangement in fullerene transformation.The internal U atom formally assumes the highest VI oxidation state regardless of the cage topology or B location.Unprecedentedly,metal-cage bonding also shows strong covalency with the U atom and the C_(27)B cage of U@Td-C_(27)B sharing the same 32 valence electrons to obey the 32-electron principle and Hirsch’s 2(N+1)^(2) rule(N=3 with a 1S^(2)1P^(6)1D^(10)1F^(14) electronic configuration)for spherical aromaticity,respectively.Such a novel mutual stabilization mechanism is similar to that of the classical organometallic compound,ferrocene.The neutral U@Td-C_(27)B isomers are radicals with one unpaired electron on the cage and thus may spontaneously form dimers with closed-shell structures.We propose that one could obtain the U@Td-C_(27)B monomer via radical addition reactions such as trifluoromethylation.Our work demonstrates the intriguing structures and properties as well as their correlations of the emerging EMBFs.The possible coexistence of multiple isomers and their interconversion are unprecedented and could also guide more in-depth studies on EMAFs as well as other types of heterofullerenes.展开更多
基金support from the National Natural Science Foundation of China(No.22171068)the Natural Science Foundation of Hebei Province(B2022202036)。
文摘Understanding the exact position of metal atom(s)within a cage is critical for elucidating the structural characteristics,metal–cage interplay and stability mechanism of endohedral metallofullerenes(EMFs).But it becomes rather challenging for the emerging actinide EMFs due to the variable oxidation state and complex valence orbitals of the encapsulated metals.Herein,density functional theory calculations were carried out for all 25 reported Th/U-based mono-metallofullerenes(mono-EMFs)with well-defined single crystal structures.The electronic ground states of Th-and U-contained mono-EMFs are singlet and triplet/quintet,respectively.Th always formally assumes the IV oxidation state inside the cage,whereas U has the Ⅲ or IV valence state highly depending on the cage structure.Due to the considerable intramolecular charge transfer(Th:4e;U:3e or 4e),their metal–cage interactions are mainly ionic,but with non-negligible covalent components largely contributed by the spatially extended Th/U-5f orbitals.Importantly,all the internal Th^(4+)/U^(3+)/^(4+)cations can be located by only considering the frontier molecular orbital distributions of the corresponding empty cages rather than the electrostatic potentials of cage anions commonly used for the classic lanthanide mono-EMFs.Such an obvious deviation from the simple ionic model(M^(q+)@C_(2n)^(q−))is ascribed to the strong actinide–cage covalency,which leads to much lower actual metal charge than the formal one.Our work unprecedently finds the overlooked important role of cage orbitals in determining the internal metal positions of all actinide mono-EMFs.It not only demonstrates the uniqueness of actinide EMFs,but also could help understand the structural characteristics of more EMFs with multiple possible internal metal locations.
基金financial support from the National Natural Science Foundation of China(No.22171068)the Natural Science Foundation of Hebei Province(B2022202036).
文摘Endohedral metalloazafullerenes(EMAFs)/metalloborafullerenes(EMBFs)are formed by encapsulating various metallic species inside the fullerene cages,of which some carbon atoms are replaced by nitrogen and boron,respectively.Currently,the characterization of their exact heteroatom positions on the cage mainly depends on the calculated relative energies at T=0 K for isomers with varied heteroatom locations.In this work,density functional theory calculations were carried out to systematically investigate the structural characteristics,electronic properties,aromaticity and chemical reactivities of the recently synthesized U@C_(27)B borafullerene.Interestingly,besides the reported major U@Td-C_(27)B-[5,5,6]isomer,calculations considering the entropy effect at cage formation temperatures suggest that it may have another major U@Td-C_(27)B-[5,5,6]isomer and a minor U@Td-C_(27)B-[5,5,5]isomer with changed boron positions.The reported U@Td-C_(27)B-[5,5,6]and new U@Td-C_(27)B-[5,5,5]isomers could interconvert via a single bond rotation at elevated temperatures with energy barriers comparable to that of the classical Stone-Wales rearrangement in fullerene transformation.The internal U atom formally assumes the highest VI oxidation state regardless of the cage topology or B location.Unprecedentedly,metal-cage bonding also shows strong covalency with the U atom and the C_(27)B cage of U@Td-C_(27)B sharing the same 32 valence electrons to obey the 32-electron principle and Hirsch’s 2(N+1)^(2) rule(N=3 with a 1S^(2)1P^(6)1D^(10)1F^(14) electronic configuration)for spherical aromaticity,respectively.Such a novel mutual stabilization mechanism is similar to that of the classical organometallic compound,ferrocene.The neutral U@Td-C_(27)B isomers are radicals with one unpaired electron on the cage and thus may spontaneously form dimers with closed-shell structures.We propose that one could obtain the U@Td-C_(27)B monomer via radical addition reactions such as trifluoromethylation.Our work demonstrates the intriguing structures and properties as well as their correlations of the emerging EMBFs.The possible coexistence of multiple isomers and their interconversion are unprecedented and could also guide more in-depth studies on EMAFs as well as other types of heterofullerenes.
