The Various Packing Structures of Tb@C82 (I, II) Isomers in Their Cocrystals with Ni(OEP)

Abstract

Soot-containing terbium (Tb)-embedded fullerenes were prepared by evaporation of Tb₄O₇-doped graphite rods in an electric arc discharge chamber. After 1,2,4-trichlorobenzene extraction of the soot and rotary evaporation of the extract, a solid product was obtained and then dissolved into toluene by ultrasonication. Through a three-stage high-pressure liquid chromatographic (HPLC) process, Tb@C₈₂ (I, II) isomers were isolated from the toluene solution of fullerenes and metallofullerenes. With the success of the growth of cocrystals of Tb@C₈₂ (I, II) with Ni(OEP), the molecular structures of Tb@C₈₂ (I) and Tb@C₈₂ (II) were confirmed to be Tb@C_2v(9)-C₈₂ and Tb@Cs(6)-C₈₂, respectively, based on crystallographic data from X-ray single-crystal diffraction. Moreover, it was found that Tb@C₈₂ (I, II) isomers demonstrated different packing behaviors in their cocrystals with Ni(OEP). Tb@C_2v(9)-C₈₂ forms a 1:1 cocrystal with Ni(OEP), in which Tb@C_2v(9)-C₈₂ is aligned diagonally between the Ni(OEP) bilayers to form zigzag chains. In sharp contrast, Tb@Cs(6)-C₈₂ forms a 2:2 cocrystal with Ni(OEP), in which Tb@Cs(6)-C₈₂ forms a centrosymmetric dimer that is aligned linearly with Ni(OEP) pairs to form one-dimensional structures in the a-c lattice plane. In addition, the distance of a Ni atom in Ni(OEP) to the Cs(6)-C₈₂ cage is much shorter than that to the C_2v(9)-C₈₂ one, indicative of a stronger π-π interaction between Ni(OEP) and the C₈₂ carbon cage in the cocrystal of Tb@C_S(6)-C₈₂ and Ni(OEP). Density functional theory calculations reveal that the regionally selective dimerization of Tb@C_S(6)-C₈₂ is the result of a dominant unpaired spin existing on a particular C atom of the C_S(6)-C₈₂ cage.

Keywords: centrosymmetric dimer; cocrystals; metallofullerene; terbium; unpaired spin.

Figure 1

The (a) first, (c) second and (e) third stages of HPLC isolation of Tb@C₈₂ (I, II) isomers (column size: Ф20 mm × 250 mm; injection volume: 20 mL; eluent: toluene with a flow rate of 10 mL/min; detection wavelength: 290 nm at 298 K). The MALDI-TOF mass spectra of (b) fraction 1 and (d) fraction 1-2.

Figure 2

(a) Analytical liquid chromatography chromatograms on a Buckyprep column (Φ = 4.6 × 250 mm, 1 mL/min in toluene flow) and (b) MALDI-TOF mass spectra of Tb@C₈₂ (I, II) isomers.

Figure 3

The vis-NIR absorption spectra of Tb@C₈₂ (I, II) isomers in CS₂ at 298 K.

Figure 4

The orientations of (a) Tb@C_2v(9)-C₈₂ and (b) Tb@C_S(6)-C₈₂ to Ni(OEP) in the cocrystals of Tb@C_2v(9)-C₈₂]∙[Ni(OEP)] and 2[Tb@C_S(6)-C₈₂]∙2[Ni(OEP)], respectively. Only the major Tb site and one carbon cage orientation are shown. Solvent molecules and H atoms are omitted for clarity.

Figure 5

The interaction of the metal cation (major metal site) with the nearest cage carbon atoms for (a) Tb@C_2v(9)-C₈₂ and (b) Tb@C_S(6)-C₈₂.

Figure 6

The packing structures of (a) Tb@C_2v(9)-C₈₂ and (b) Tb@C_S(6)-C₈₂ in their cocrystals with Ni(OEP). Only one cage orientation and the major terbium site are illustrated, with solvent molecules omitted for clarity.

Figure 7

(a) Visualized spin density distributions and (b) corresponding spin population values of unequivalent cage carbon atoms of Tb@C_S(6)-C₈₂ and Tb@C_2v(9)-C₈₂. The absolute values are used for easy comparison. Please refer to Figure S3 for the numbering schemes. The C44 site of Tb@C_S(6)-C₈₂, with the largest spin density among all cage carbons, is highlighted by the red circle.

Figure 8

CV curves of (a) Tb@C_2v(9)-C₈₂ and (b) Tb@C_S(6)-C₈₂ isomers. Conditions: working electrode, platinum plate; counter electrode, platinum plate; reference electrode, Ag wire; supporting electrolyte, 0.05 M n-Bu₄NPF₆ in o-dichlorobenzene; scan rate, 20 mV/s.