Node-Accessible\nZirconium MOFs

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