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.