Isoreticular Relationships in Two Anthraquinone Tetracarboxylate Metal–Organic Frameworks

Abstract

The synthesis and structural characterization of two metal–organic frameworks constructed from tetratopic polycarboxylate ligands are described. The acid forms of the ligands, 5,5′-(9,10-dioxo-9,10-dihydroanthracene-1,5-diyl)diisophthalic acid (H₂(1,5-AQ-DIPA)) and 5,5′-(9,10-dioxo-9,10-dihydroanthracene-1,4-diyl)diisophthalic acid (H₂(1,4-AQ-DIPA)) are based on a polycyclic anthraquinone core, not previously considered for the crystallization of coordination polymers, and synthesis routes to both are reported. Subsequent solvothermal synthesis between each acid form of the ligand and copper(II) nitrate in a DMF/water mixture yields distinct frameworks whose structures are determined using single-crystal X-ray diffraction: MIL-179 and MIL-180. Both frameworks contain the copper paddle-wheel dimer {Cu₂(COO)₄}, but the distinct molecular structures of the two ligands lead to different framework topologies. MIL-179 has the nbo topology, while MIL-180 possesses the ssa topology. The three-dimensionally connected structures are compared with other frameworks reported in the literature for the same secondary building units linked by other tetratopic linkers, and isoreticular relationships are identified.