Beyond Intrinsic Oxidation Activity: Charge-Distributed Reaction Fields in Polyoxometalate-Based Porous Ionic Crystals

Abstract

Molecular catalysts assembled into solids can exhibit catalytic properties beyond those of their individual components, yet the origin of this enhancement is often unclear. Here, we address this issue using porous ionic crystals (PICs) based on polyoxometalates (POMs). Three isostructural PICs containing [W6O19]2–, [V2W4O19]4–, and [V3W3O19]5– were synthesized by assembling the nitrate salt of the Cr(III) benzoate cation [Cr3O(OOCC6H5)6(H2O)3]+ (CrBz) with the corresponding potassium salts K2[W6O19] (W6), K4[V2W4O19] (V2W4), and K5[V3W3O19] (V3W3), and evaluated as solid catalysts for cyclooctene epoxidation with tert-butyl hydroperoxide. PIC_W6 exhibited substantially higher catalytic activity than its constituent components, demonstrating that PIC formation creates a favorable environment for catalysis. The catalytic activity increased in the order PIC_W6 < PIC_V2W4 < PIC_V3W3. Experimental and theoretical analyses reveal electronic modulation of both the POM and CrBz units upon PIC formation, including charge redistribution between the two components.