Assembly of Face Decorated Cuboidal Cages into Ultraporous Structures with Hierarchical Porosity: Accessing MOFs with the Awaited red-a Topology

The rational design of ultraporous metal-organic frameworks (MOFs) with hierarchical pore systems is of great significance but remains highly challenging. MOFs based on the reo-e or red topologies offer such pore architectures through face-shared cuboidal, cuboctahedral, and rhombicuboctahedral cages. Although hypothesized and computationally explored over the past two decades, these solids had not been experimentally realized. Here, we report the first MOFs based on the long-awaited red-a net, denoted as M-red-MOF-1 (M = Fe, Cr). Combining the nearly square yet rectangularly connected 4-c organic linker 4,4',″,4‴-([1,1':4',1″-terphenyl]-3,3'',5,5''-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid, denoted as H₄TCEPT, with FeCl₃·6H₂O under solvothermal conditions yielded Fe-red-MOF-1 as cubic-like single crystals. Extensive characterization using SCXRD, PXRD, SEM, TEM, gas sorption, TGA, and in-silico structure modeling, confirmed the red-a topology. Argon sorption at 87 K revealed three distinct S-type steps, consistent with the hierarchical pore network and demonstrated an ultrahigh pore volume (3.56 cm³ g^-1) and BET area (5081 m² g^-1). Owing to its hierarchical porosity, Fe-red-MOF-1 exhibits excellent hydrogen storage performance with high gravimetric (13.5 wt %) and volumetric (39.5 g·L^-1) working capacities under temperature and pressure swing conditions (77 K/100 bar → 160 K/5 bar), placing it among the top-performing MOFs. The isostructural Cr-red-MOF-1, obtained postsynthetically, showed a remarkable water uptake of 2.81 g g^-1 at 298 K, surpassing the current top-ranking Cr-soc-MOF-1 (1.95 g g^-1). Isoreticular analogues, denoted as M-red-MOF-2 (M = Fe, Cr), were also synthesized using the anthracene-based linker H₄TEBDA. The present work opens new directions for designing ultraporous, hierarchical MOFs based on the red-a net.