M2@C79N (M = Y, Tb): isolation and characterization of stable endohedral metallofullerenes exhibiting M-M bonding interactions inside aza[80]fullerene cages

Abstract

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.

Figure 1

A drawing of the crystallographically determined structure of Tb₂@C₇₉N, Only the symmetrical molecular site with 0.050 fractional occupancy is shown. At this site there is a crystallographic mirror plane which lies perpendicular to the plane of the paper and bisects the line between the two terbium atoms. The Tb1-Tb1A separation in 3.9020(10) and the occupancy of Tb1 corresponds to 0.43 of the total Tb content.

Figure 2

A drawing of the crystallographically determined structure of Tb₂@C₇₉N showing the unsymmetrical orientation of the heterofullerene cage. The orientation shown here has 0.25 fractional occupancy and a symmetry-related orientation generated by the crystallographic mirror plane has 0.25 occupancy. All of the terbium atom positions are shown along with those generated by the crystallographic mirror plane. The sites Tb2 through Tb11 have fractional occupancies that range from 0.102 to 0.026. When summed, these represent 0.57 of the total terbium content, while Tb1 (shown in red and hidden behind another terbium site) represents 0.43 of the terbium content in the asymmetric unit. The shortest contact (2.805(3) Å) between a cage atom and the nickel ion in the porphyrin is indicated by a dashed line.

Figure 3

ESR spectra of Y₂@C₇₉N samples in toluene solution, as a solid, and as a solid solution with C_84.

Figure 4

Illustrations of the computed structures of Y₂@C₇₉N with the nitrogen atom replacing: (A) a carbon atom in a pentagon (a 665 junction) and (B) a carbon atom that is not in a pentagon (a 666 junction).

Figure 5

Kohn-Sham UB3LYP/DZVP(Y)+6-31G*(C,N) molecular orbitals for the optimized structures of Y₂@ I_h-C₈₀, [Y₂@ I_h-C₈₀]⁻, and Y₂@C₇₉N molecules. Orbital energies are shown at the bottom (energies of beta LUMO of [Y₂@ I_h-C₈₀]⁻ and Y₂@C₇₉N molecules are omitted), while drawings of the critical yttrium based orbitals that contains the free spin in the paramagnetic molecules are shown at the top along with drawings of the distribution of the spin density in [Y₂@ I_h-C₈₀]⁻ and Y₂@C₇₉N.

Scheme 1

Azafullerenes