Porous metal-organic frameworks based on an anthracene derivative: syntheses, structure analysis, and hydrogen sorption studies.

Solvothermal assembly of 5,5'-(9,10-anthracenediyl)di-isophthalate (H(4)adip) with in situ-generated dicopper and diiron paddlewheel secondary building units (SBUs) afforded two porous metal-organic frameworks (MOFs), designated as PCN-14 and PCN-15, respectively. The two MOFs crystallize in different structures, characterized by a difference in the dihedral angles between the anthracene and the phenyl rings of the adip ligand. PCN-14 retains permanent porosity under dehydration and contains nanoscopic cages while PCN-15 contains only one-dimensional hexagonal channels along the (0 0 1) direction which require solvent stabilization. The aromaticity of the anthracene rings of the adip ligand in conjunction with the nanoscopic cages grants PCN-14 high excess hydrogen adsorption capacity of 2.70 wt % at 77 K, 760 Torr (4.42 wt % at saturation), as well as high hydrogen affinity of 8.6 kJ/mol at low H(2) coverage. These values are compared to other tetracarboxylate-derived MOFs to better understand the role of the aromatic rings in hydrogen adsorption.