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.