Crystal Engineering of a 4,6‑c fsc Platform That Can Serve as a Carbon Dioxide Single-Molecule Trap

We report herein a crystal engineering strategy that affords a new and versatile metal–organic material (MOM) platform that is tunable in terms of both pore size and functionality. This platform is comprised of two long-known molecular building blocks (MBBs) that alternate to form a cationic square grid lattice. The MBBs, [Cu­(AN)₄]²⁺ (AN = aromatic nitrogen donor), and [Cu₂(CO₂R)₄] square paddlewheel moieties are connected by five different fs, L1–L5, that contain both AN and carboxylate moieties. The resulting square grid nets formed from alternating [Cu­(AN)₄]²⁺ and [Cu₂(CO₂R)₄] moieties are pillared at the axial sites of the [Cu­(AN)₄]²⁺ MBBs with dianionic pillars to form neutral 3D 4,6-connected fsc (four, six type c) nets. Pore size control in this family of fsc nets was exerted by varying the length of the ligand, whereas pore chemistry was defined by the presence of unsaturated metal centers (UMCs) and either inorganic or organic pillars. 1,5-Naphthalenedisulfonate (NDS) anions pillar in an angular fashion to afford fsc-1-NDS, fsc-2-NDS, fsc-3-NDS, fsc-4-NDS, and fsc-5-NDS from L1-L5, respectively. Experimental CO₂ sorption studies revealed higher isosteric heat of adsorption (Q_st) for the smallest pore size material (fsc-1-NDS). Computational studies revealed that there is higher CO₂ occupancy about the UMCs in fsc-1-NDS compared to other extended variants that were synthesized with NDS. SiF₆^2– (SIFSIX) anions in fsc-2-SIFSIX form linear pillars that result in eclipsed [Cu₂(CO₂R)₄] moieties at a distance of just 5.86 Å. The space between the [Cu₂(CO₂R)₄] moieties affords a strong CO₂ binding site that can be regarded as being an example of a single-molecule trap; this finding has been supported by modeling studies. Gas sorption studies on this new family of fsc nets reveal stronger affinity toward CO₂ for fsc-2-SIFSIX vs fsc-2-NDS along with higher Q_st and CO₂/N₂ selectivity. The fsc platform reported herein offers a plethora of possible porous structures that are amenable to tuning of both pore size and pore chemistry.