Perfectly ordered two-dimensional layer structures found in some endohedral metallofullerenes

A two-dimensional (2D) arrangement of metallofullerenes for both M@C ₈₂ (isomer I) (M = Y, La, Ce, Pr) and (M₂C _y)@C₈₂ (isomer III) (M = Er, y = 0, 2; M = Sc, y = 2) is crystallized with a 1:2 fullerene/toluene molecular ratio. The determined crystal is a 2D layered structure, which is composed of metallofullerene layers sandwiched by the toluene layers. Although the M@C₈₂(I) arrangement in a 1:1 crystal is almost identical to that of (M₂C _y)@C₈₂(III), the M@C₈₂(I) arrangement in a 1:2 crystal is different from (M₂C_y)@C₈₂(III). It is found that the difference of the molecular arrangement strongly correlates with an orientation of metallofullerenes in a 2D layer. Our findings suggest the existence of a subtle but important interaction between the confined atoms/molecules and the solvent molecules/toluene through the carbon cage, which leads to a guiding principle of crystallization control with a solvent ingredient for metallofullerenes.