Crystal structures of metal-organic frameworks sustained by π-π interactions between triple-helices

Metal–organic frameworks [(Fe2L3)·(C4H10O)] 1, [(Fe2L3)·(C5H5N)] (2), [(Fe2L3)·(C6H6)] (3) and [(Fe2L3)·(C4H8O)4] (4), achieved from triple metal helical iron complex Fe2L3, were assembled in the solvents of diethyl ether/DMF, pyridine, benzene and THF, respectively, where H2L is one of the simplest imine-based ligands {[(C6H4)(OH)]CHN–NCH[(OH)(C6H4)]}. Compounds 1, 2, 3 and 4 all crystallize in a centro-symmetric space group, consequently the molecules occur as a racemic mixture of Δ–Δ and Λ–Λ configuration enantiomers, wherein the equivalent fragments are related by the C2 axis. The phenyl rings of the helical units contact the neighbors via π–π (face-to-face) and C–H⋯π interactions to form two-dimensional channeled frameworks in which solvent molecules are absorbed in the channels in each of the structures. Thermogravimetric analyses reveal that the solvent molecules can be evacuated from the pores without loss of the framework periodicity. The crystal lattice is thermally stable up to 350 °C, and diethyl ether can be re-adsorbed by putting the heated material in diethyl ether solvent. It is also interesting to find that upon addition of other solvents such as n-hexane and cyclohexane, thermal gravimetric measurement and elemental microanalysis do not support the absorbance of these guests, indicating that the inclusion can be selectively re-absorbed. All the results indicate that although C–H⋯π interactions have energies that are only in the range of 2–20 kJ mol−1, these interactions are significant enough in combination that the orientation of molecules in the solid can be predicted with a reasonable degree of accuracy.