Bifunctional pyrazolate-carboxylate ligands for isoreticular cobalt and zinc MOF-5 analogs with magnetic analysis of the {Co-4(mu(4)-O)} node

The ditopic ligands 3,5-dimethyl-pyrazolate-4-carboxylate, (-)Me(2)pzCO(2)(-), and 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate, (-)Me(2)pzC(6)H(4)CO(2)(-), 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 {M-4(mu(4)-O)} nodes (M = Co, Zn) as MOF-5 and other IRMOFs. The microporous networks [M-4(mu(4)-O)(Me(2)pzCO(2))(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 angstrom diameter together with small amounts of high-boiling solvent remaining in the activated samples. For [Co-4(mu(4)-O)(Me(2)pzCO(2))(3)], the low-pressure H-2 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 m(2) g(-1). The analysis of the magnetic properties of [Co-4(mu 4-O)(Me(2)pzCO(2))(3)] takes into account the distribution of tetrahedra resulting from the disorder of the pyrazolate-carboxylate linker. An antiferromagnetic coupling observed for [Co-4(mu(4)-O)(Me(2)pzCO(2))(3)] arises from the interactions of the cobalt(II) ions through the combined mu(4)-O + syn-syn carboxylate and mu(4)-O + pyrazolate bridges.