Coulombic Metal-Organic Frameworks Assembled from π-Stacked Organic Nodes and Polyoxometalate Inorganic Linkers

Metal-organic frameworks (MOFs) are typically constructed by forming coordination bonds between inorganic nodes and organic linkers. Here we present an alternative framework chemistry that does not rely on coordination bonding but instead exploits proton-transfer-facilitated Coulombic and hydrogen-bonding interactions. Protonation of basic amine molecules by acidic Keggin-type polyoxometalates (POMs) generates complementary organic cations and inorganic anions that rapidly self-assemble into crystalline frameworks at room temperature upon simple solution mixing. Solvent acidity modulates proton transfer and charge balance, thereby controlling amine-POM stoichiometry and packing modes and driving a structural evolution from π-stacked organic arrays (CouMOF-1) to a MOF-like node-linker network (CouMOF-2) and ultimately to a continuous POM-stacked phase (CouMOF-3). The intermediate phase, CouMOF-2, comprises a primitive cubic (pcu) net in which π-stacked organic molecules serve as 6-connected supramolecular organic nodes bridged by 2-connected POM clusters, producing a MOF-like topology without coordination bonding and representing a rare inversion of the conventional inorganic-node/organic-linker architecture of MOFs. These materials exhibit different sulfide oxidation activities and selectivities, highlighting solvent-controlled assembly as a route to tuning structure-function relationships. This assembly strategy extends to diverse amine monomers and polyoxometalates, and the synthetic simplicity together with the large chemical space of amine-POM combinations highlights organic-inorganic acid-base assembly as a versatile route to crystalline ionic frameworks that use nondirectional Coulombic interactions to generate ordered framework architectures, without resorting to coordination chemistry.