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 signifi-cance but remains highly challenging. MOFs based on the reo-e or red topologies offer this type of pore system originat-ed from face-shared cuboidal, cuboctahedral, and rhombicuboctahedral cages. For this reason, they have been hypothe-sized and computationally constructed over the past 20 years but have not yet been realized experimentally. We report herein the first examples of MOFs based on the awaited red-a net, denoted as M-red-MOF-1 (M: Fe3+, Cr3+). In particular, the use of the nearly square but with a rectangular connectivity 4-c linker 4,4',4'',4'''-([1,1':4',1''-terphenyl]-3,3'',5,5''-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid, denoted as H4TCEPT, combined with FeCl3.6H2O under solvothermal reaction conditions, readily afforded Fe-red-MOF-1 as cubic-like single crystals. Extensive structural characterization using SCXRD for unit cell determination, PXRD, SEM, TEM imaging, detailed gas sorption isotherms and TGA, coupled with in-silico materials construction, confirmed that Fe-red-MOF-1 displays the red-a topology. Argon sorption iso-therm recorded at 87 K revealed the presence of three well defined S-type steps, in full agreement with the hierarchical pore network of the red-a net, with an ultrahigh pore volume of 3.56 cm3 g-1 and a BET surface area of 5080.6 m2 g-1. Pore size distribution analysis by fitting the Argon isotherm in the range 0.1-0.99 p/p0 revealed two peaks centered at 29.6 and 41.2 Å, associated with the cuboctahedral and rhombicuboctahedral cages in Fe-red-MOF-1. Notably, the isotherm below 0.1 p/p0 shows a concave shape rather than a convex one as expected for rigid frameworks. The formation of a flexible framework in Fe-red-MOF-1 is supported by SEM and TEM images, revealing that the faces of the cubic particles are curved inwards. These results are consistent with the observed reduced atomic periodicity in Fe-red-MOF-1, as revealed from high-resolution PXRD data. Because of its hierarchical ultrahigh porosity, the hydrogen storage properties of Fe-red-MOF-1 were investigated, revealing an exceptional gravimetric working capacity of 13.5 wt% and a high volumetric ca-pacity (39.5 g·L⁻¹) under temperature and pressure swing conditions (77 K/100 bar → 160 K/5 bar), placing this material among the top-performing MOFs. Finally, the isostructural Cr-red-MOF-1 was successfully synthesized by post-synthetic metal exchange reaction. The water sorption properties of this material were investigated by recording the corresponding isotherm at 298 K up to saturation, revealing an astonishing uptake of 2.81 g g-1, which is significantly larger than that of the current top-ranking MOF, Cr-soc-MOF-1 (1.95 g g-1). The present work opens new directions to apply reticular chem-istry for the construction of novel, ultraporous MOFs with hierarchical porosity, displaying the red-a net.