Tianming Zuo

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.

New egg-shaped fullerenes: non-isolated pentagon structures of Tm3N@Cs(51365)-C84 and Gd3N@Cs(51365)-C84

Although there are 51 568 non-IPR and 24 IPR structures for C84, the egg-shaped endohedral fullerenes Tm3N@C(s)(51 365)-C84 and Gd3N@C(s)(51 365)-C84 utilize the same non-IPR cage structure as found initially for Tb3N@C(s)(51 365)-C84.

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.

Isolation and Structural Characterization of Two Very Large, and Largely Empty, Endohedral Fullerenes: Tm@C3v-C94 and Ca@C3v-C94

The structures of two newly synthesized endohedral fullerenes, Tm@C(3v)-C(94) and Ca@C(3v)-C(94), have been determined by single crystal X-ray diffraction on samples cocrystallized with Ni(II)(octaethylporphyrin). Both compounds exhibit the same cage geometry and conform to the isolated pentagon rule (IPR). The metal ions within these rather large cages are localized near one end and along the C(3) axis. While the calcium ion is situated over a C-C bond at a 6:6 ring junction, the thulium ion is positioned above a six-membered ring of the fullerene.

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.

Electronic Properties and 13C NMR Structural Study of Y3N@C88

In this paper, we report the synthesis, purification, (13)C NMR, and other characterization studies of Y(3)N@C(88). The (13)C NMR, UV-vis, and chromatographic data suggest an Y(3)N@C(88) having an IPR-allowed cage with D(2)(35)-C(88) symmetry. In earlier density functional theory (DFT) computational and X-ray crystallographic studies, it was reported that lanthanide (A(3)N)(6+) clusters are stabilized in D(2)(35)-C(88) symmetry cages and have reduced HOMO-LUMO gaps relative to other trimetallic nitride endohedral metallofullerene cage systems, for example, A(3)N@C(80). In this paper, we report that the nonlanthanide (Y(3)N)(6+) cluster in the D(2)(35)-C(88) cage exhibits a HOMO-LUMO gap consistent with other lanthanide A(3)N@C(88) molecules based on electrochemical measurements and DFT computational studies. These results suggest that the reduced HOMO-LUMO gap of A(3)N@C(88) systems is a property dominated by the D(2)(35)-C(88) carbon cage and not f-orbital lanthanide electronic metal cluster (A(3)N)(6+) orbital participation.

Isolation and Crystallographic Characterization of Tm3N@D2 (35)-C88

Tm3N@D2(35)-C88 has been prepared by vaporization of graphite rods doped with Tm2O3, graphite powder, and iron nitride in a Krätschmer-Huffman arc-discharge fullerene generator and isolated by high pressure liquid chromatography. A single crystal X-ray diffraction study reveals the dimensions of the D2(35)-C88 cage and the positioning of the Tm3N unit inside.