Fine-Tuning the Anchoring Strategies of Palladium Nanoparticles in Covalent Organic Frameworks for Enhanced Activity in the Multicomponent Reaction of Carbon Dioxide

Covalent organic frameworks (COFs) are promising platforms for immobilizing metal catalysts through varied anchoring strategies (ASs). While ASs exert a crucial influence on the properties of metals, a systematic investigation of their interactions is still lacking, which, however, may offer valuable insights for the precise fabrication of highly efficient catalysts toward specific transformations. Herein, three COFs, namely, TMT-DFB-PPh3, TMT-DFB, and BDA-HTA, were intentionally designed to immobilize palladium nanoparticles (Pd NPs) through well-defined PPh3 coordination, physical adsorption within pore spaces, and imine interlayer coordination, respectively. Such different ASs of Pd NPs in these COFs give rise to prominently distinct properties. Specifically, Pd@TMT-DFB-PPh3 with well-defined PPh3 moieties results in the formation of Pd NPs with a uniform dispersion, higher loadings, ultrafine sizes, different lattice planes, and enhanced electronegativity compared to the other two COFs with different ASs. Consequently, Pd@TMT-DFB-PPh3 exhibited outstanding catalytic performance in the one-pot three-component reaction between ortho-iodoaniline, CO2, and tert-butyl isocyanide for the preparation of quinazolinones, greatly surpassing the performance of Pd@BDA-HTA, Pd@TMT-DFB, and other Pd catalysts. The present work highlights the critical role of ASs within COFs in tuning the properties of metals and thereby the catalytic efficiency, offering valuable insights for the rational design of COF-supported metal nanocatalysts.