Computational and Experimental Assessment of CO₂ Uptake in Phosphonate Monoester Metal–Organic Frameworks

Phosphonate monoesters (PMEs) as ligands for metal–organic frameworks can potentially direct topology, enhance water stability, and modify pore chemistry. Here, we show, experimentally and computationally, not only that is the ratio of phosphonate to phosphonate monoester significant, but also that gas sorption depends on the distribution of the monoesters in the structure. A phosphonate monoester ligand, 1,3,5-tri(4-phosphonato)benzene-tris(monoethylester), was coordinated to copper(II) to form two different frameworks based on the same copper–phosphonate chain building units, one dense ( 1 ) and the other with an experimental surface area over 1000 m 2 g –1 ( CALF-33-Et 3 ). One of the three phosphonate monoesters in CALF-33-Et 3 can be hydrolyzed to make an isostructural material, CALF-33-Et 2 H, with approximately the same surface areas but vastly superior CO 2 sorption. Controlling the hydrolysis at this site allowed the partially hydrolyzed variants, CALF-33-Et 3– x H x (where 0 < x < 1), to be prepared and their gas sorption studied by experiment and simulation to determine CO 2 binding sites and binding energies. These results show that each PME group can impact multiple gas sorption sites meaning that clustering versus random distributions of ester groups gives very different gas uptake. Finally, an algorithm is put forward that allows the CO 2 uptake of the hydrolyzed MOF to be simulated by algebraically combining the isotherms of the nonhydrolyzed and fully hydrolyzed forms. This method can be used to assess both the degrees of ester hydrolysis and the distribution of ester groups in the solid.