Liaosa Xu

M2@C79N (M = Y, Tb): Isolation and Characterization of Stable Endohedral Metallofullerenes Exhibiting M-M Bonding Interactions Inside Aza[80]Fullerene Cages

Y2@C79N and Tb2@C79N have been prepared by conducting the Kratschmer-Huffman electric-arc process under 20 Torr of N2 and 280 Torr of He with metal oxide-doped graphite rods. These new heterofullerenes were separated from the resulting mixture of empty cage fullerenes and endohedral fullerenes by chemical separation and a two-stage chromatographic process. Crystallographic data for Tb2@C79N x Ni(OEP) x 2 C6H6 demonstrate the presence of an 80-atom cage with idealized I(h) symmetry and two, widely separated Tb atoms inside with a Tb-Tb separation of 3.9020(10) A for the major terbium sites. The EPR spectrum of the odd-electron Y2@C79N indicates that the spin density largely resides on the two equivalent yttrium ions. Computational studies on Y2@C79N suggest that the nitrogen atom resides at a 665 ring junction in the equator on the fullerene cage and that the unpaired electron is localized in a bonding orbital between the two yttrium ions of this stable radical. Thus, the Tb-Tb bond length of the single-electron bond is an exceedingly long metal-metal bond.

Selective Formation of a Symmetric Sc3N@C78 Bisadduct: Adduct Docking Controlled by an Internal Trimetallic Nitride Cluster

In this paper we report a regioselective Bingel−Hirsch reaction of Sc3N@C78 yielding a single mono- and a dominate bis-ethyl malonate derivative for the first time. The Cs-symmetric monoadduct 1 and C2v-symmetric bisadduct 2 were isolated by HPLC and characterized by MS, UV−vis, and NMR spectroscopy. The symmetric bisadduct 2 clearly demonstrate the remarkable regioselectivity control exerted by the encapsulated Sc3N cluster. We have employed a LUMO electron density surface computational approach to predict multiadduct docking sites on the ellipsoidal fullerene cage surface.

89Y and 13C NMR Cluster and Carbon Cage Studies of an Yttrium Metallofullerene Family, Y3N@C2n (n = 40−43)

The members of a new family of yttrium trimetallic nitride-templated (TNT) endohedral metallofullerenes (EMFs), Y(3)N@C(2n) (n = 40-43), have been synthesized and purified. On the basis of experimental and computational (13)C NMR studies, we propose cage structures for Y(3)N@I(h)-C(80) (IPR allowed), Y(3)N@D(5h)-C(80) (IPR allowed), Y(3)N@C(s)-C(82) (non-IPR), Y(3)N@C(s)-C(84) (non-IPR), and Y(3)N@D(3)-C(86) (IPR allowed). A significant result is the limited number of isomers found for each carbon cage. For example, there are 24 isolated pentagon rule (IPR) and 51 568 non-IPR structures possible for the C(84) cage, but only one major isomer of Y(3)N@C(s)-C(84) was found. The current study confirms the unique role of the trimetallic nitride (M(3)N)(6+) cluster template in the Kratschmer-Huffman electric-arc process for fullerene cage size and high symmetry isomer selectivity. This study reports the first (89)Y NMR results for Y(3)N@I(h)-C(80,) Y(3)N@C(s)(51365)-C(84), and Y(3)N@D(3)(19)-C(86), which reveal a progression from isotropic to restricted (Y(3)N)(6+) cluster motional processes. Even more surprising is the sensitivity of the (89)Y NMR chemical shift parameter to subtle changes in the electronic environment at each yttrium nuclide in the (Y(3)N)(6+) cluster (more than 200 ppm for these EMFs). This (89)Y NMR study suggests that (89)Y NMR will evolve as a powerful tool for cluster motional studies of EMFs.

Highly Regioselective Derivatization of Trimetallic Nitride Templated Endohedral Metallofullerenes via a Facile Photochemical Reaction

Photochemically generated benzyl radicals react with Sc(3)N@C(80)-I(h) to produce a dibenzyl adduct [Sc(3)N@C(80)(CH(2)C(6)H(5))(2)] in 82% yield and high regioselectivity. The adducts (1)H spectrum revealed high symmetry: only one AB pattern was observed for the methylene protons. The (13)C NMR spectrum suggested a C(2)-symmetrical structure. DFT calculations reveal that a 1,4-adduct is more favorable than a 1,2-adduct by >10 kcal/mol. The 1,4-structure on [566] ring junctions was unambiguously confirmed by X-ray crystallographic analysis. UV-vis spectra revealed that the removal of two p orbitals from the pi system of the cage together with the benzylic substituents change the electronic properties of the metallofullerene in a manner similar to those reported for disilirane and trifluoromethyl moieties. Under the same conditions from Lu(3)N@C(80)-I(h) we prepared (63% yield) Lu(3)N@C(80)(CH(2)C(6)H(5))(2), which demonstrated properties similar to the 1,4-dibenzyl adduct of Sc(3)N@C(80)-I(h).

Enhanced dipole moments in trimetallic nitride template endohedral metallofullerenes with the pentalene motif.

Although not found to date in empty-cage fullerenes, the fused pentagon motifs (pentalenes) are allowed in endohedral metallofullerenes (EMFs). We have found that members of the trimetallic nitride template (TNT) EMF Y3N@C2n (n = 39-44) family that contain pentalene motifs exhibit significant dipole moments. This finding is predicted to be significant for other EMFs with a metal atom orientated toward the pentalene motif. Chromatographic retention data and computational results for Y3N@C2-C78, Y3N@Cs-C82, and Y3N@Cs-C84 are examples that pentalene groups lead to a significant induced dipole moment (∼1D). A special case is the Y3N@C2-C78 that contains two pentalenes in a relatively small cage. The (13)C NMR spectrum for Y3N@C2-C78 exhibits strongly deshielded signals for the fullerene cage (155-170 ppm) supporting the presence of the pentalene motif. In addition, a lengthening of the covalent M-N bond in the internal M3N cluster is found for all reported TNT EMFs that contain one or two pentalene motifs.