In Situ Metal Chelation for Topology Regulation and Single-Site Construction in Covalent Organic Frameworks

Precise regulation over the dimensionality and topology of covalent organic frameworks (COFs) through metal coordination remains a fundamental challenge. Herein, we report an in situ metal-chelation strategy for topology regulation and single-site construction in COFs using a bifunctional β-hydroxy ketone-functionalized HDBC monomer. Metal-free Schiff-base condensation of HDBC with TAPy affords a one-dimensional covalent ribbon framework, named COF-HDBC. Strikingly, in situ Cu²⁺ chelation during solvothermal crystallization redirects the assembly pathway toward a two-dimensional bex-type Cu-COF-HDBC containing framework-embedded Cu─O coordination sites. Post-synthetic metalation of preassembled COF-HDBC fails to trigger this transformation, verifying the indispensable structure-directing effect of in situ chelation during nucleation and growth. This strategy is extendable to Ni²⁺ and Co²⁺, affording isostructural metal-chelated COFs. Multiscale characterizations and Cu K-edge XAFS collectively support the proposed framework structures and the predominantly isolated nature of the Cu centers. As a functional validation, Cu-COF-HDBC enables selective CO₂/CO electroreduction to CH₄. This work establishes in situ metal chelation as a synthetic handle for programming COF topology and constructing framework-embedded single-metal sites, offering a versatile route to crystalline porous materials with complex architectures and tunable functions.