Polymorphous Al-MOFs Based on V-Shaped Linker Molecules: Synthesis, Properties, and in Situ Investigation of Their Crystallization

The in situ and systematic high-throughput investigation of the system Al³⁺/4,4′-benzophenonedicarboxylic acid (H₂BPDC)/DMF/H₂O in the presence of various additives was carried out, and a new Al-MOF of composition [Al(OH)(BPDC)], denoted as CAU-21-BPDC, was obtained. Its crystal structure was determined from single-crystal X-ray diffraction data (space group I422, a = b = 17.2528(7) Å, c = 23.864(1) Å). The structure is built up by octanuclear rings of cis corner-sharing AlO₆ polyhedra forming the inorganic building unit (IBU). These {Al₈O₈} IBUs are arranged in a bcu packing and connected via BPDC^2- ions in a way that each IBU is linked via two linker molecules to each of the eight adjacent IBUs. Thus, accessible, one-dimensional modulated pores with a diameter between 3.6 and 6.5 Å are formed. In addition, tetrahedral cavities are formed by the BPDC^2- linker molecules. The framework of CAU-21-BPDC is polymorphous with that of CAU-8-BPDC, which contains one-dimensional chains of trans corner-sharing AlO₆ polyhedra connected by BPDC^2- ions. Replacing H₂BPDC by 4,4′-oxydibenzoic acid (H₂ODB), which contains an oxygen atom between the phenyl rings instead of a keto group, leads to the synthesis of Al-MOFs isoreticular with CAU-8-BPDC and CAU-21-BPDC. In addition, a coordination polymer, [Al(HODB)₂(OH)], was discovered and structurally characterized. The structure of CAU-8-ODB was refined from powder X-ray diffraction data, while a Pawley refinement was carried out for CAU-21-ODB to determine the lattice parameters and confirm phase purity. The structure of CAU-21-ODB was confirmed using density functional theory (DFT) calculations. A thorough characterization shows that the CAU-8 and CAU-21-type structures are stable up to 350 and 300 °C in air, respectively, almost independent of the linker molecules incorporated. The former MOFs are porous toward N₂ and CO₂, while the latter only adsorb CO₂.