Chuanbao Chen

An improbable monometallic cluster entrapped in a popular fullerene cage: YCN@C(s)(6)-C82.

Since the first proposal that fullerenes are capable of hosting atoms, ions, or clusters by the late Smalley in 1985, tremendous examples of endohedral metallofullerenes (EMFs) have been reported. Breaking the dogma that monometallofullerenes (mono-EMFs) always exist in the form of M@C2n while clusterfullerenes always require multiple (two to four) metal cations to stabilize a cluster that is unstable as a single moiety, here we show an unprecedented monometallic endohedral clusterfullerene entrapping an yttrium cyanide cluster inside a popular C82 cage—YCN@Cs(6)-C82. X-ray crystallography and 13C NMR characterization unambiguously determine the cage symmetry and the endohedal cyanide structure, unexpectedly revealing that the entrapped YCN cluster is triangular. The unprecedented monometallic clusterfullerene structure unveiled by YCN@Cs(6)-C82 opens up a new avenue for stabilizing a cluster by a single metal cation within a carbon cage, and will surely stimulate further studies on the stability and formation mechanism of EMFs.

Spin-flow vibrational spectroscopy of molecules with flexible spin density: electrochemistry, ESR, cluster and spin dynamics, and bonding in TiSc2N@C80.

The recently isolated TiSc(2)N@C(80) was used to study the spin state of a Ti(3+) ion in a mixed metal nitride cluster in a fullerene cage. The electronic state of the new clusterfullerene is characterized starting with the redox behavior of this structure. It differs markedly from that of homometallic nitride clusterfullerenes in giving reversible one-electron transfers even on the cathodic scale. Both oxidation and reduction of TiSc(2)N@C(80) occur at the endohedral cluster changing the valence state of Ti from Ti(II) in anion to Ti(IV) in cation. The unpaired electron in TiSc(2)N@C(80) is largely fixed at the Ti ion as shown by low temperature ESR measurements. Isotropic g-factor 1.9454 points to the significant spin-orbit coupling with an unquenched orbital momentum of the 3d electron localized on Ti. Measurements with the frozen solution also point to the strong anisotropy of the g-tensor. DFT computations show that the cluster can adopt several nearly isoenergetic configurations. DFT-based Born-Oppenheimer molecular dynamics (BOMD) simulations reveal that, unlike in Sc(3)N@C(80), the cluster dynamics in TiSc(2)N@C(80) cannot be described as a 3D rotation. The cluster rotates around the Ti-N axis, while the Ti atom oscillates in one position around the pentagon/hexagon edge. Evolution of the spin populations along the BOMD trajectory has shown that the spin distribution in the cluster is very flexible, and both an intracluster and cluster-cage spin flows take place. Fourier transformation of the time dependencies of the spin populations results in the spin-flow vibrational spectra, which reveal the major spin-flow channels. It is shown that the cluster-cage spin flow is selectively coupled to one vibrational mode, thus, pointing to the utility of the clusterfullerene for the molecular spin transport. Spin-flow vibrational spectroscopy is thus shown to be a useful method for characterization of the spin dynamics in radicals with flexible spin density distribution.

Titanium/yttrium mixed metal nitride clusterfullerene TiY2N@C80: synthesis, isolation, and effect of the group-III metal.

Titanium/yttrium mixed metal nitride clusterfullerene (MMNCF) TiY(2)N@C(80) has been successfully synthesized, representing the first Ti-containing non-scandium MMNCF. TiY(2)N@C(80) has been isolated by multistep HPLC and characterized by various spectroscopies in combination with DFT computations. The electronic absorption property of TiY(2)N@C(80) was characterized by UV-vis-NIR spectroscopy, indicating the resemblance to that of TiSc(2)N@C(80) with broad shoulder absorptions. The optical band gap of TiY(2)N@C(80) (1.39 eV) is very close to that of TiSc(2)N@C(80) (1.43 eV) but much smaller than that of Y(3)N@C(80)(I(h), 1.58 eV). Such a resemblance of the overall absorption feature of TiY(2)N@C(80) to TiSc(2)N@C(80) suggests that TiY(2)N@C(80) has a similar electronic configuration to that of TiSc(2)N@C(80), that is, (TiY(2)N)(6+)@C(80)(6-). FTIR spectroscopic study and DFT calculations accomplish the assignment of the C(80):I(h) isomer to the cage structure of TiY(2)N@C(80), with the C(1) conformer being the lowest energy structure, which is different from the C(s) conformer assigned to TiSc(2)N@C(80). The electrochemical properties of TiY(2)N@C(80) were investigated by cyclic voltammetry, revealing the reversible first oxidation and first reduction step with E(1/2) at 0.00 and -1.13 V, respectively, both of which are more negative than those of TiSc(2)N@C(80), while the electrochemical energy gap of TiY(2)N@C(80) (1.11 V) is almost the same as that of TiSc(2)N@C(80) (1.10 V). Contrary to the reversible first reduction step, the second and third reduction steps of TiY(2)N@C(80) are irreversible, and this redox behavior is dramatically different from that of TiSc(2)N@C(80), which shows three reversible reduction steps, indicating the strong influence of the encaged group-III metal (Y or Sc) on the electronic properties of TiM(2)N@C(80) (M = Y, Sc).

The Cycloaddition Reaction of Ih‐Sc3N@C80 with 2‐Amino‐4,5‐diisopropoxybenzoic Acid and Isoamyl Nitrite to Produce an Open‐Cage Metallofullerene

The unique structural and electronic properties of endohedral metallofullerenes (EMFs) make them candidates for applications in nanoscience and biomedicine, and the functionalization of EMFs has attracted increasing attention. Various types of transformations, such as Diels–Alder reactions, 1,3dipolar cycloadditions, photochemical silylation, alkylation and carbene additions, Bingel reactions, and free-radical reactions have been reported to take place on the outer surface of EMFs. Icosahedral (Ih) Sc3N@C80, the most abundant EMF, can undergo most, but not all, of the abovementioned reactions. For example, the Bingel reaction of IhY3N@C80 [3a,c] and Ih-Gd3N@C80 [3b] has been reported to yield methanofullerene derivatives, but the same attempted cyclopropanation reaction with Ih-Sc3N@C80 was not successful. [3a]

Synthesis, Structure, and Theoretical Study of Trifluoromethyl Derivatives of C84(22) Fullerene

Trifluoromethylation of higher fullerene mixtures with CF(3)I was performed in ampoules at 400 to 420 and 550 to 560 °C. HPLC separation followed by crystal growth and X-ray diffraction studies allowed the structure elucidation of nine CF(3) derivatives of D(2)-C(84) (isomer 22). Molecular structures of two isomers of C(84)(22)(CF(3))(12), two isomers of C(84)(22)(CF(3))(14), four isomers of C(84)(22)(CF(3))(16), and one isomer of C(84)(22)(CF(3))(20) were discussed in terms of their addition patterns and relative formation energies. DFT calculations were also used to predict the most stable molecular structures of lower CF(3) derivatives, C(84)(22)(CF(3))(2-10). It was found that the addition of CF(3) groups to C(84)(22) is governed by two rules: additions can only occur at para positions of C(6)(CF(3))(2) hexagons and no additions can occur at triple-hexagon-junction positions on the fullerene cage.

Synthesis, isolation, and addition patterns of trifluoromethylated D5h and I(h) isomers of Sc3N@C80: Sc3N@D5h-C80(CF3)18 and Sc3N@I(h)-C80(CF3)14.

Sc(3)N@D(5h)-C(80) and Sc(3)N@I(h)-C(80) were trifluoromethylated with CF(3)I at 400 °C, affording mixtures of CF(3) derivatives. After separation with HPLC, the first multi-CF(3) derivative of Sc(3)N@D(5h)-C(80), Sc(3)N@D(5h)-C(80)(CF(3))(18), and three new isomers of Sc(3)N@I(h)-C(80)(CF(3))(14) were investigated by X-ray crystallography. The Sc(3)N@D(5h)-C(80)(CF(3))(18) molecule is characterized by a large number of double C-C bonds and benzenoid rings within the D(5h)-C(80) cage and a fully different position of the Sc(3)N unit compared to that in the pristine Sc(3)N@D(5h)-C(80). A detailed comparison of five Sc(3)N@I(h)-C(80)(CF(3))(14) isomers reveals a strong influence of the exohedral additions on the behavior of the Sc(3)N cluster inside the I(h)-C(80) cage.

X-ray Crystallographic Proof of the Isomer D2-C84(5) as Trifluoromethylated and Chlorinated Derivatives, C84(CF3)16, C84Cl20, and C84Cl32

Minor isomer comes forward: Minor isomer C(84)(5) has been captured by high temperature trifluoromethylation with CF(3)I and chlorination with VCl(4). The compounds C(84)(CF(3))(16), C(84)Cl(20), and C(84)(5)Cl(32) were investigated by X-ray crystallography providing the first direct proof of the cage connectivity of D(2)-C(84)(5). The D(2)-C(84)(5)Cl(32) molecule (see figure; C grey, Cl green) contains two flattened, pyrene-like substructures on opposite poles of the cage resulting in its drum-like shape.

New isomers of trifluoromethylated derivatives of metal nitride cluster fullerene: Sc3N@C80(CF3)n (n=14 and 16).

Sc(3)N@C(80) (I(h)) was trifluoromethylated with CF(3)I at 400 °C affording a mixture of CF(3) derivatives. Two isomers of Sc(3)N@C(80)(CF(3))(14) and Sc(3)N@C(80)(CF(3))(16) were separated by HPLC and investigated by X-ray crystallography. Detailed comparison of the four isomers revealed a strong influence of the exohedral CF(3) addition pattern on the behavior of the Sc(3)N cluster inside the C(80) fullerene cage.

Urea as a New and Cheap Nitrogen Source for the Synthesis of Metal Nitride Clusterfullerenes: The Role of Decomposed Products on the Selectivity of Fullerenes

By using urea as the new nitrogen source, for the first time, Sc-based metal nitride clusterfullerenes (NCFs), Sc(3)N@C(2n) (2n=80, 78, 70, 68), have been synthesized successfully. The optimum molar ratio of Sc(2)O(3)/CO(NH(2))(2)/C for the synthesis of Sc NCFs is 1:3:15. The yield of Sc(3)N@C(80)(I(h) +D(5h)) per gram of Sc(2)O(3), using CO(NH(2))(2) as the new nitrogen source, was quantitatively compared to those obtained when using the reported nitrogen sources, including N(2), NH(3), and guanidinium thiocyanate. We find that there is a clear difference on the selectivity of Sc-based NCFs within the extract mixture obtained from one rod and accumulative two rods. According to discharging experiments and XRD analysis, we conclude that NH(3) generated in situ from the decomposition of CO(NH(2))(2) is mainly responsible for the formation of Sc-based NCFs when using only one rod, whereas in the second rod CO(NH(2))(2) would decompose into melamine during discharging of the first rod. Thus, the selectivity of fullerenes is clearly dependent on the decomposed product of CO(NH(2))(2). Finally the difference in the decomposition behavior of CO(NH(2))(2) and melamine was studied in detail and a possible decomposition process of CO(NH(2))(2) during discharging was proposed. Accordingly, the difference in the selectivity and yield of Sc NCFs for CO(NH(2))(2) and melamine was interpreted.

Finely Dispersed Au Nanoparticles on SiO2 Achieved by the C60 Additive and Their Catalytic Activity

We have investigated in detail the effect of buckminsterfullerene (C60) additive on the structure and catalytic activity of Au/SiO2 catalysts prepared by a routine deposition–precipitation method employing HAuCl4 as the gold precursor. The structures of various catalysts have been characterized by using N2 adsorption–desorption isotherms, powder X‐ray diffraction, X‐ray photoelectron spectroscopy, transmission electron microscopy, photoluminescence, and Raman spectroscopy. The C60 additive was found to greatly enhance the dispersion of Au nanoparticles supported on SiO2. Supported Au nanoparticles that are about 3–5 nm in size can be synthesized without difficulty on C60/SiO2, whereas those sized about 7–10 nm are usually acquired on bare SiO2. Strong Au–C60 interaction with the charge transfer from Au nanoparticles to C60 has been observed in Au/C60/SiO2 and proven to suppress the agglomeration of supported Au nanoparticles and enhance their dispersion on SiO2. The catalyst Au/C60/SiO2‐10 (Au:C60 molar ratio of 10) exhibits a much better catalytic performance for CO oxidation than Au/SiO2, but is not active for CO oxidation at room temperature, which demonstrates that the intrinsic activity of supported Au nanoparticles increases with decreasing particle size, but supported Au 3–5 nm nanoparticles cannot activate oxygen for CO oxidation at room temperature. We propose that 3 nm is the critical size for Au nanoparticles to exhibit an intrinsic catalytic activity in CO oxidation at room temperature without additional contributions. These results provide novel and important insights into the fundamental understanding of intrinsic structure–activity relation of Au nanoparticles.