Bifunctional pyrazolate–carboxylate ligands for isoreticular cobalt and zinc MOF-5 analogs with magnetic analysis of the Co4 (μ4-O) node

The ditopic ligands 3, 5-dimethyl-pyrazolate-4-carboxylate, –Me2pzCO2–, and 4- (3, 5-dimethyl-1H-pyrazol-4-yl) benzoate, –Me2pzC6H4CO2–, combine a pyrazolate and carboxylate functionality in axial orientation and lead to porous cobalt or zinc azolate–carboxylate frameworks that have the same cubic pcu-a topology and M4 (μ4-O) nodes (M = Co, Zn) as MOF-5 and other IRMOFs. The microporous networks M4 (μ4-O) (Me2pzCO2) 3 (M = Co, Zn) with the short linker exhibit a solvent-induced gate effect, evidenced by gas desorption hysteresis due to small pore apertures of 2. 8 Å diameter together with small amounts of high-boiling solvent remaining in the activated samples. For Co4 (μ4-O) (Me2pzCO2) 3, the low-pressure H2 storage capacity (1. 7 wt%, 1 bar, 77 K) is higher than for MOF-5, and the CO2 uptake of 20. 8 wt% puts it among the top MOFs for low-pressure CO2 sorption even though the BET surface is less than 1000 m2 g−1. The analysis of the magnetic properties of Co4 (μ4-O) (Me2pzCO2) 3 takes into account the distribution of tetrahedra resulting from the disorder of the pyrazolate–carboxylate linker. An antiferromagnetic coupling observed for Co4 (μ4-O) (Me2pzCO2) 3 arises from the interactions of the cobalt (II) ions through the combined μ4-O + syn–syn carboxylate and μ4-O + pyrazolate bridges.