Homometallic 2D Cd2+ and Heterometallic 3D Cd2+/Ca2+, Cd2+/Sr2+ Metal–Organic Frameworks Based on an Angular Tetracarboxylic Ligand

This study reports on the synthesis, structural characterization and gas sorption studies of a homometallic 2D Cd²⁺ MOF and two heterometallic 3D Cd²⁺/Ca²⁺ and Cd²⁺/Sr²⁺ -MOFs based on the angular tetracarboxylic ligand 3,3',4,4'-sulfonyltetracarboxylic acid (H₄STBA). The homometallic 2D Cd²⁺ MOF with the formula [NH₂(CH₃)₂]⁺₂[Cd(STBA)]^2-_n·nDMF·1.5nH₂O-(<b>1</b>)_n·nDMF·1.5nH₂O was synthesized from the reaction of CdCl₂·H₂O and 3,3',4,4'-diphthalic sulfonyl dianhydride (3,3',4,4'-DPSDA) with stoichiometric ratio of 1:1.3 in DMF/H₂O (5/2 mL) at 100 °C. The two heterometallic Cd²⁺/Ca²⁺ and Cd²⁺/Sr²⁺ compounds were prepared from analogous reactions to this afforded (<b>1</b>)_n·nDMF·1.5nH₂O with the difference that the reaction mixture also contained AE(NO₃)₂ (AE²⁺ = Ca²⁺ or Sr²⁺) and, in particular, from the reaction of AE(NO₃)₂, CdCl₂·H₂O and 3,3',4,4'-DPSDA with stoichiometric ratio 1:1.1:1.4 in DMF/H₂O (5/2 mL) at 100 °C. Notably, compounds [CdCa(STBA)(H₂O)₂]_n·0.5nDMF-(<b>2</b>)_n·0.5nDMF and [CdSr(STBA)(H₂O)₂]_n·0.5nDMF-(<b>3</b>)_n·0.5nDMF are the first heterometallic compounds Mⁿ⁺/AE²⁺ (M = any metal ion) reported containing ligand H₄STBA. The structure of (<b>1</b>)_n·nDMF·1.5nH₂O comprises a 2D network based on helical 1D chain secondary building unit (SBU) [Cd²⁺(STBA)^4-)]^2-. The 2D sheets are linked through hydrogen bonding interactions, giving rise to a pseudo-3D structure. On the other hand, compounds (<b>2</b>)_n·1.5nH₂O and (<b>3</b>)_n·1.5nH₂O display 3D microporous structures consisting of a helical 1D chain SBU [Cd²⁺AE²⁺(STBA)^4-)]. All three compounds contain rhombic channels along c axes. The three MOFs exhibit an appreciable thermal stability, up to 350-400 °C. Gas sorption measurements on activated materials (<b>2</b>)_n and (<b>3</b>)_n revealed moderate BET surface areas of 370 m²/g and 343 m²/g, respectively, along with CO₂ uptake capacity of 2.58 mmol/g at 273 K.