Orthogonalization of Polyaryl Linkers as a Route to More Porous Phosphonate Metal‐Organic Frameworks

Abstract

The coordinative pliancy of the phosphonate functional group means that metal-phosphonate materials often self-assemble as well-packed structures with minimal porosity, as efficient inter-ligand packing is enabled. Here, we report a multistep synthesis of a novel aryl-phosphonate linker with an orthogonalized ligand core, 1,3,5-tris(4'-phosphonophenyl)-2,4,6-trimethylbenzene (H 6 L2) designed to form more open structures. A series of crystalline metal-phosphonate frameworks (CALF-35 to -39) have been assembled by coordinating to divalent metals (Ba, Sr, Ca, Mg, Zn). H 6 L2 is unable to pack efficiently and, as a consequence, yields several distinct microporous structures. The resulting structures are discussed in detail, with a focus on the solid-state packing of the sterically rigidified linker. Combined with larger cations (Sr, and Ba), H 6 L2 packs in a parallel-offset manner, yielding isomorphous and microporous metal-organic frameworks , and (Ba)). When coordinated to smaller metals (Ca, Mg, Zn), H 6 L2 forms four new structures. Two Ca MOFs of different stoichiometry, (CALF-36 and 37) and a Mg MOF CALF-38 show narrow pores and have high selectivities for CO 2 over N 2 and CH 4 . Finally, in CALF-39 (Zn), H 6 L2 linkers pack in a herringbone fashion, resulting in a material with 10.9 × 10.1 Å 2 square channels. The stability of all structures was tested, and the most porous structure, CALF-39 (Zn), was found to retain its structure and gas adsorption after immersion in water over pH 3-11.

A Molecular Insight into the Dehydration of a Metal–Organic Framework and its Impact on the CO2 Capture

“…This finding indicates that the aromatic cores of the OP1 ligand strongly contributes to the stabilization of CO 2 in CALF‐37, whereas the stabilization of N 2 is minor. This agrees with the high selectivity of CALF‐37 toward CO 2 over N 2 , observed experimentally [16] . In the PP region, the interactions consist of a subtle interplay between electrostatic and LJ, which depends on the distance between the adsorbate (CO 2 or N 2 ) and the ligands.…”

“…It's counter‐intuitive that the CO 2 molecules cannot diffuse between the pores, since the CO 2 molecules are expected to diffuse across the network in order to be adsorbed. Like the diffusion of water molecules during the activation, our CO 2 diffusion calculation can indicate that the adsorption of CO 2 in CALF‐37 is a very slow process, as observed during the adsorption isotherms measurements [16] . As a matter of fact, the authors revealed that 2–3 days were necessary for the maximum CO 2 uptake with CALF‐37, which confirmed the relatively slow kinetics of the process when compared to the other CALFs.…”

“…Indeed, the CA strongly interact with the phosphonate groups, which is responsible for the cohesion between the OP1 residues, and the stability of the network (see Figure S11, Supporting Information). Most importantly, the strong interaction between CO 2 and CA (up to 50 kJ/mol) confirms that the CA sites effectively act as high‐affinity metal sites for strong CO 2 binding [16] …”

“…For a comparison, we also investigated the adsorption of N 2 molecules by replacing all the CO 2 molecules in the system that has the maximum capacity of CO 2 molecules (100 %), although the experiments by Glavinovic et al [16] . show that the N 2 uptake in CALF‐37 is almost zero.…”

“…This MOF has a 16‐fold multiplicity, leading to a rigid microporous network and shows a high selectivity for CO 2 over N 2 and CH 4 . The CALF‐37 has been shown to have the highest CO 2 uptake capability among several new frameworks that have been recently synthesized [16] . CALF‐37 was first synthesized in an inactivated form, which means that the structure contains template molecules such as water and methanol from the synthesis process (HCALF‐37) which prevent CO 2 gas adsorption.…”