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

## TL;DR

Researchers created new metal-organic frameworks using cadmium and other metals with a specific tetracarboxylic acid ligand, which showed good thermal stability and CO2 absorption.

## Contribution

The first heterometallic MOFs containing Cd2+/Ca2+ and Cd2+/Sr2+ with the H4STBA ligand are synthesized and characterized.

## Key findings

- The homometallic 2D Cd2+ MOF and two heterometallic 3D MOFs were successfully synthesized.
- The new MOFs have rhombic channels and moderate CO2 uptake capacity of 2.58 mmol/g.
- The materials exhibit thermal stability up to 350–400 °C.

## Abstract

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—(1)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 (1)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—(2)n·0.5nDMF and [CdSr(STBA)(H2O)2]n·0.5nDMF—(3)n·0.5nDMF are the first heterometallic compounds Mn+/AE2+ (M = any metal ion) reported containing ligand H4STBA. The structure of (1)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 (2)n·1.5nH2O and (3)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 (2)n and (3)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.