Photoluminescence and photoredox catalytic properties of cationic Ru(<scp>ii</scp>) polypyridine complexes encapsulated within an InTATB metal–organic framework

The nanoscale environment within the void spaces of metal-organic frameworks (MOFs) can significantly influence the photoredox catalytic activity of encapsulated visible-light photoredox catalysts (PCs). To compare two isostructural PC@In-MOF systems, three cationic Ru(II) polypyridine complexes were successfully encapsulated within the mesoscale channels of the anionic framework of InTATB (H₃TATB = 4,4',4''-<i>s</i>-triazine-2,4,6-triyltribenzoic acid), which features a doubly interpenetrated framework structure. This encapsulation yielded three heterogenized visible-light PCs, RuL₃@InTATB, where L = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), or 2,2'-bipyrazine (bpz). A simple cation-exchange process partially replaced the NEt₄⁺ counter-cations in the as-prepared InTATB with the cationic Ru(II) polypyridine complexes. The TATB^3- linker contains a central triazine unit, a strong electron acceptor, which can engage in intermolecular π-π interactions with one of the three ligands of the [RuL₃]²⁺ complexes. This electronic interaction makes InTATB a more favorable MOF host for stabilizing the excited state of Ru(II)* polypyridine complexes, compared to the previously studied isostructural MOF, InBTB (H₃BTB = 1,3,5-benzenetribenzoic acid), which features a central phenyl donor. The photoluminescence (PL) spectra and PL lifetimes of the three RuL₃@InTATB systems were investigated. In all cases, the PL lifetimes were significantly extended compared to those of the corresponding free [RuL₃]²⁺ complexes. The photoredox catalytic activity was evaluated using the aza-Henry reaction of 2-phenyl-1,2,3,4-tetrahydroisoquinoline (THIQ) under visible light at room temperature (RT). The RuL₃@InTATB systems exhibited higher conversion rates and enhanced product selectivity compared to their RuL₃@InBTB counterparts, likely due to their extended PL lifetimes. These heterogenized systems also demonstrated strong catalytic performance in the visible-light-driven aerobic oxidation of benzyl halides in the presence of an auxiliary organocatalyst, also under mild conditions at RT.