Post-Synthetic Modification of Metal-Organic Frameworks Bearing Phenazine Radical Cations for aza-Diels-Alder Reactions

Metal-organic frameworks (MOFs) consisting of organic radicals are of great interest because they have exhibited unique and intriguing optical, electronic, magnetic, and chemo-catalytic properties, and thus have demonstrated great potential applications in optical, electronic, and magnetic devices, and as catalysts. However, the preparation of MOFs bearing stable organic radicals is very challenging because most organic radicals are highly reactive and difficult to incorporate into the framework of MOFs. Herein we reported a post-synthetic modification strategy to prepare a novel MOF containing phenazine radical cations, which was used as heterogeneous catalyst for aza-Diels-Alder reaction. The zinc-based metal-organic framework Zn₂(PHZ)₂(dabco) (N) was successfully synthesized from 5,10-di(4-benzoic acid)-5,10-dihydrophenazine (PHZ), triethylene diamine (dabco) with Zn(NO₃)₂ ⋅ 6H₂O by solvothermal method. The as-synthesized MOF N was partially oxidized by AgSbF₆ to form MOF R containing ∼10% phenazine radical cation species. The resultant MOF R was found to keep the original crystal type of N and very persistent under ambient conditions. Consequently, MOF R was successfully employed in radical cation-catalyzed aza-Diels-Alder reactions with various imine substrates at room temperature with high reaction conversion. Moreover, heterogeneous catalyst MOF R was reusable up to five times without much loss of catalytic activity, demonstrating its excellent stability and recyclability. Therefore, the post-synthetic modification developed in this work is expected to become a versatile strategy to prepare radical-based MOFs for the application of heterogeneous catalysts in organic synthesis.