Node-Accessible Zirconium MOFs

High-stability, zirconium-based metal–organic frameworks are attractive as heterogeneous catalysts and as model supports for uniform arrays of subsequently constructed heterogeneous catalystsfor example, MOF-node-grafted metal–oxy and metal–sulfur clusters. For hexa-Zr­(IV)-MOFs characterized by nodes that are less than 12-connected, sites not used for linkers are ideally occupied by reactive and displaceable OH/H2O pairs. The desired pairs are ideal for grafting the aforementioned catalytic clusters, while aqua-ligand lability renders them effective for exposing highly Lewis-acidic Zr­(IV) sites (catalytic sites) to candidate reactants. New single-crystal X-ray studies of an eight-connected Zr-MOF, NU-1000, reveal that conventional activation fully removes modulator ligands, but replaces them with three node-blocking formate ligands (from solvent decomposition) and only one OH/H2O pair, not foura largely overlooked complication that now appears to be general for Zr-MOFs. Here we describe an alternative activation protocol that effectively removes modulators, avoids formate, and installs the full complement of terminal OH/H2O pairs. It does so via an unusual isolatable intermediate featuring eight aqua ligands and four non-ligated chloridesagain as supported by single-crystal X-ray data. We find that complete replacement of node-blocking modulators/formate with the originally envisioned OH/OH2 pairs has striking consequences; here we touch upon just three. First, elimination of unrecognized formate renders aqua ligands much more thermally labile, enabling open Zr­(IV) sites to be obtained at lower temperature. Second, in the absence of formate, which otherwise links and locks pairs of node Zr­(IV) ions, reversible removal of aqua ligands engenders reversible contraction of MOF meso- and micropores, as evidenced by X-ray diffraction. Third, formate replacement with OH/OH2 pairs renders NU-1000 ca.10× more active for catalytic hydrolytic degradation of a representative simulant of G-type chemical warfare agents.