In situ construction of phenanthroline-based cationic radical porous hybrid polymers for metal-free heterogeneous catalysis

Abstract

Rational design of multifunctional radical porous polymers with redox activity for targeted metal-free heterogeneous catalysis is an important research topic. In this work, we reported a new class of phenanthroline-based cationic radical porous hybrid polymers (Phen˙⁺-PHPs), which were constructed from the Heck reaction between a newly designed dibromo-substituted phenanthroline ionic monomer (iDBPhen) and a rigid building block, octavinylsilsesquioxane (VPOSS). For the first time, the stable phenanthroline-based radical cation was unexpectedly discovered in these polyhedral oligomeric silsesquioxane (POSS)-based porous hybrid polymers, probably undergoing in situ reduction of the dicationic monomer iDBPhen during the alkaline reagent K₂CO₃-involved Heck reaction. The radical characters of the typical porous polymers Phen˙⁺-PHP-2 and Phen˙⁺-PHP-2Br were confirmed from the electron paramagnetic resonance (EPR) spectra and X-ray photoelectron spectra (XPS). The chemical structures and porous geometry were fully characterized by a series of advanced technologies. Surprisingly, the metal-free cationic radical polymer Phen˙⁺-PHP-2 exhibited high heterogeneous catalytic efficiency in the H₂O₂-mediated selective oxidation of various sulfides to sulfoxides with high yields under mild conditions, owing to the electron-accepting and redox ability of Phen-based dications and radical cations. Moreover, the extended sample Phen˙⁺-PHP-2Br prepared by post-treatment of Phen˙⁺-PHP-2 with aqueous HBr was also employed as a metal-free efficient heterogeneous catalyst in the conversion of CO₂ with epoxides into cyclic carbonates under atmospheric pressure and low temperatures. The remarkable catalytic performance in CO₂ conversion should be assigned to the synergistic catalysis of POSS-derived Si–OH groups and nucleophilic Br⁻ anions and N active atom-involved Phen cationic radical moieties within Phen˙⁺-PHP-2Br. These two catalysts can be facilely recovered and reused, also with stable recyclability in the above catalytic reaction systems, achieving the heterogeneous catalytic demands for multipurpose reactions.