Thiol
groups (−SH) offer versatile reactivity for functionalizing
metal–organic frameworks, and yet thiol-equipped MOF solids
remain underexplored due to synthetic challenges. Building on the
recent breakthrough using benzyl mercaptan as the sulfur source and
AlCl3 for uncovering the thiol function, we report on the
thiol-equipped linker 3,3′-dimercaptobiphenyl-4,4′-dicarboxylic
acid and its reaction with Zr(IV) ions to form a UiO-67-type MOF solid
with distinct functionalities. The thiol-equipped UiO-67 scaffold
shows substantial stability toward oxidation, e.g., it can be treated
with 30% H2O2 to afford oxidation of the thiol
to the strongly acidic sulfonic function while maintaining the ordered
porous MOF structure. The thiol groups also effectively take up palladium(II)
ions from solutions to allow for comparative studies on catalytic
activities and to help elucidate how the spatial configuration of
the thiol groups can be engineered to impact the performance of heterogeneous
catalysis in the solid state. Comparative studies on the stability
in the solventless (activated) state also help to highlight the steric
factor in stabilizing UiO-67-type frameworks.
We employ a two-step strategy for accessing crystalline porous covalent networks of highly conjugated π-electron systems. For this, we first assembled a crystalline metal-organic framework (MOF) precursor based on Zr(iv) ions and a linear dicarboxyl linker molecule featuring backfolded, highly unsaturated alkyne backbones; massive thermocyclization of the organic linkers was then triggered to install highly conjugated, fused-aromatic bridges throughout the MOF scaffold while preserving the crystalline order. The formation of cyclized carbon links not only greatly strengthens the precursor coordination scaffold, but also, more importantly, enhances electroactivity and charge transport throughout the polycyclic aromatic grid.
The cruciform linker molecule here features two designer functions: the pyrazole donors for framework construction, and the vicinal alkynyl units for benzannulation to form nanographene units into the Ni 8 -pyrazolate scaffold. Unlike the full 12 connections of the Ni 8 (OH) 4 (H 2 O) 2 clusters in other Ni 8pyrazolate networks, significant linker deficiency was observed here, leaving about half of the Ni(II) sites capped by acetate ligands, which can be potentially removed to open the metal sites for reactivity. The crystalline Ni 8 -pyrazolate scaffold also retains the crystalline order even after thermal treatments (up to 300 °C) that served to partially graphitize the neighboring alkyne units. The resultant nanographene components enhance the electroactive properties of the porous hosts, achieving hydrogen evolution reaction (HER) activity that rivals that of topical nickel/palladium-enabled materials.
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