Synthesis,Separation And Crystallographic Structures Of Eu/Ti-based Metallofullerenes

Fullerenes are the only carbon nanomaterials with definite molecular structures.The inner space of fullerenes could accommodate metal atom(s)or metallic cluster forming new metal-carbon hybrid molecules,namely endohedral metallofullerenes(EMFs).Such species are particularly attractive because EMFs not only feature dual properties of both fullerene cages and encapsulated species but also may exhibit some unique physical and chemical properties due to the electron transfer from the inner metal to the outer cage.So far,in-depth research has been conducted on the trivalent rare-earth metals(Sc,Y,La,etc.)based EMFs.Diverse molecular structures and potential applications in biology and electronics of those EMFs have been reported.Divalent rare-earth metals and adjacent transition metals(Group IV or V)may transfer different electron numbers compared with trivalent metals when forming EMFs,which can generate new carbon cage structures and encapsulated species.This could not only expand the fullerene family,but also provide an ideal platform to study the composition of the entrapped metallic units and the evolution of the new carbon cages.Herein,synthesis,extraction,separation,and characterization of EMFs based on lanthanide metal Eu(II)and Group IVB transition metal Ti have been systematically studied in this paper,especially the crystal structures of EMFs determined by X-ray diffraction,which greatly expands the fullerene family.These results are summarized as follows:(1)Soot containing Eu-EMFs was prepared by arc-discharge method and fifteen Eu EMFs were isolated by high performance liquid chromatography(HPLC).They were Eu C_2n(2n=74-90),which were fully characterized by analytical HPLC,laser desorption/ionization time of flight mass spectrometry(LDI-TOF-MS),and Vis-NIR spectroscopy.Fifteen Eu-EMFs were co-crystallized with Ni ^II(OEP)and their molecular structures were identified by X-ray crystallography.Remarkably,Eu@C₇₆represents the first Eu-containing EMF with a cage that violated the isolated-pentagon-rule and Eu@C₇₈is the first C₇₈-based EMF stabilized by merely one metal atom.Besides,C₂(37)-C₉₀,C₁(7)-C₈₆,and C₂(27)-C₈₈are the newly discovered carbon cages.These results indicate that bivalent metal Eu(II)transfers different number of electrons to the carbon cage when forming mono-EMFs,resulting in new carbon cages,which greatly expands the family of metallofullerenes.(2)Based on the crystal structures of Eu@C_2n(2n=74-90),five Eu@C₉₀isomers can interconvert through Stone-Wales transformation.Besides,a transformation map from C₂(27)-C₈₈to D_3h(1)-C₇₄carbon cages with C₂(27)-C₈₈as the starting point was completed,covering up to 98%of reported fullerenes with carbon cage size between C₈₆and C₇₄,which is valuable for understanding the formation mechanism of fullerenes.In addition,C₂(27)-C₈₈is directly transformable from a graphene fragment in the raw materials,providing a solid support for the formation mechanism of fullerenes from the top-down formation mechanism.(3)Fifteen Eu-EMFs identified by X-ray crystallography are all mono-metallofullerenes and high-resolution mass spectroscopy reveals that mono-metallofullerenes prevail in the Eu-containing soot,which is different from the coexistence of various EMF species in soot for rare-earth metals.The density functional theory calculation was adopted to explain this preferential formation of mono-metallofullerenes.It shows that that the encapsulation energy of encapsulated metal determines the preferential formation of this kind of metallofullerene and can also explain a similar phenomenon when other metals such as Ca,Sr,Ba,and Yb form metallofullerenes.(4)Soot containing Ti-EMFs was prepared by arc-discharge method.Ti C serving as the metal source was used for the first time.And Lewis acid was innovatively used to treat the produced raw soot to enrich the Ti-EMFs.Four Ti EMFs were successfully obtained,which are Ti₃C₈₃,Ti₂C₈₀,and Ti₂C₈₄(I,II),fully characterized by analytical HPLC,laser desorption/ionization time of flight mass spectrometry(LDI-TOF-MS),and Vis-NIR spectroscopy.Also,the electrochemical properties of Ti₃C₈₃,Ti₂C₈₄(I,II)were characterized by cyclic voltammetry for the first time.(5)The obtained Ti₃C₈₃was cocrystallized and its molecular structure was determined by X-ray crystallography.Its molecular structure is assigned to be Ti₃C₃@I_h(7)-C₈₀,which is the first report on the crystal structure of metallofullerenes with three carbon atoms inside the carbon cage.Moreover,the crystallographic results show that the internal Ti₃C₃unit can be divided into approximately vertical cyclopropane-like C₃unit and outer coordinated Ti₃unit.This unique metal coordination mode has not been reported in organometallic compounds.Theoretical calculation supports the encapsulation of the Ti₃C₃cluster in the I_h(7)-C₈₀cage with high stability.Our results provide a new research perspective in the field of coordination chemistry.(6)We determined the molecular structures of Ti₂C₈₀and Ti₂C₈₄(I,II)by single crystal X-ray diffraction.The crystallographic results show that they are carbide cluster fullerenes,namely Ti₂C₂@C_s(6)-C₈₂,Ti₂C₂@C_3v(8)-C₈₂,and Ti₂C₂@D_3h(5)-C₇₈.Furthermore,the structural information shows an unusual phenomenon about the configuration of the M₂C₂cluster in the carbon cage.In the smaller D_3h(5)-C₇₈,the Ti₂C₂cluster has“Z”-shaped configuration,but in the two C_s(6)/C_3v(8)-C₈₂carbon cages,the Ti₂C₂cluster presents a compressed butterfly configuration.Theoretical calculation shows that the shape of the carbon cage affects the configurations of the internal cluster.This leads to an unexpected result,that is,in the field of M₂C₂@C_2nmetallofullerenes,the metal in Ti₂C₂@C_2n(2n=78,82)shows a variable valence state for the first time.