Lattice-Matched Polyoxometalates Direct Crystallization of Cationic Covalent Organic Frameworks

Cationic covalent organic frameworks (COFs) hold great potential for ion conduction and catalysis, yet poor crystallinity from interlayer electrostatic repulsion severely limits their performance. Herein, we demonstrate that lattice-matched polyoxometalates (POMs) as multifunctional counterions can direct the crystallization of cationic COFs and introduce functional sites. In situ incorporation of Keggin-type POMs affords highly crystalline honeycomb frameworks (POM@vCOF-CR), whereas post-synthetic ion exchange yields amorphous materials. Further investigations reveal that POMs play multiple roles during the crystallization process: in-plane, they modulate monomer release to promote ordered growth; out-of-plane, they act as electrostatic anchoring centers to facilitate ordered stacking. Crucially, lattice-matching between the POM sublattice and the COF framework eliminates configurational degeneracy and dictates long-range ordering. This principle is further validated in a square-lattice pvCOF, where only the larger Preyssler-type POM can form a matched square sublattice that enables crystallization, whereas smaller Keggin-type POMs lead to orientational disorder and amorphous products. Benefiting from the ordered functional sites and transport pathways, the crystalline POM@vCOF-CR exhibits significantly enhanced electrocatalytic activity for ethylbenzene oxidation and improved lithium-ion conductivity compared to its amorphous counterpart. This work establishes a rational solution for constructing crystalline cationic COFs by using lattice-matched POMs, thereby enabling functional frameworks with enhanced performance.