Synthetic Strategy for Incorporating Carboxylate Ligands into Coordination Polymers under a Solvent-Free Reaction

Solvent-free synthesis is a powerful approach for obtaining elusive crystal structures of coordination polymers (CPs) and metal–organic frameworks (MOFs). However, carboxylic acids were hardly employed in solvent-free synthesis due to their high melting points (Tm's). To employ carboxylates for the construction of CP/MOFs via solvent-free synthesis, we propose utilizing the melt state of organic cocrystals containing carboxylates as a reaction media. We prepared three organic cocrystals consisting of 1,4-benzenedicarboxylate (1,4-bdc2–) and azolates (imidazole, 2-methylimidazole, and 2-ethylimidazole). All of them have Tm's below 200 °C, which is far lower than the Tm of 1,4-H2bdc (402 °C) studied by differential scanning calorimetry (DSC). The solvent-free reaction of corresponding azoles, 1,4-bdc2–, and ferrocene (Tm = 173 °C) in a sealed tube reactor provided new Fe2+-based CP structures constructed from 1,4-bdc2– and corresponding azolates with high purity.