The synthesis, structures and reactions of zinc and cobalt metal–organic frameworks incorporating an alkyne-based dicarboxylate linker

Abstract

The reaction of zinc(II) nitrate and 4,4′-ethynylenedibenzoic acid (H₂edb) in DMF at 80 °C gave the metal–organic framework material [Zn₄O(edb)₃(H₂O)₂]·6DMF 1 in which edb ligands connect Zn₄O centres into a doubly-interpenetrated cubic network with a similar topology to observed with other linear dicarboxylates in the IRMOF series. Analysis of the nitrogen isotherm revealed the material to have a BET surface area of 1088 m² g⁻¹, which is approximately one-third of the value calculated from GCMC simulations, suggesting incomplete activation or pore blocking in the activated material. The reaction of cobalt(II) nitrate and H₂edb in DMF gave [Co₃(edb)₃(DMF)₄]·2.6DMF 2. The structure of 2 is based on Co₃(O₂CR)₆ linear secondary building units that are linked by the edb ligands into a two-dimensional network. When 2 was placed under vacuum, a colour change from pale pink to deep blue was observed, which is consistent with loss of the coordinated DMF molecules. When treated with [Co₂(CO)₈], crystals of 1 turned dark red, and IR analysis is consistent with coordination of Co₂(CO)₆ fragments to the alkyne groups. However, the colour change was restricted to the external crystal surfaces. This is a likely consequence of partial framework collapse, which occurs following coordination of Co₂(CO)₆ to the alkyne groups. Coordination changes the preferred angle between carboxylate groups in the edb ligand, which in turn introduces strain into the network.