Ni₈ Carboxylate Metal-Organic Frameworks

Octanuclear nickel cube (Ni₈) is considered as a highly attractive inorganic building block in the field of metal-organic frameworks (MOFs) due to its high symmetry (O_h), 12 connection sites, and excellent chemical stability of the constructed MOFs. In this study, we report the synthesis of Ni₈-based MOFs constructed exclusively from carboxylic acid ligands, breaking from the conventional use of pyrazolate in Ni₈-containing MOFs. Two carboxylate-based MOFs featuring fcu topology (NNM-2 and NNM-3) were successfully synthesized. Both materials are stable across a wide pH range. In addition, they exhibit C₂H₂/CO₂ separation properties. For a C₂H₂/CO₂ (v:v = 50:50) mixture, NNM-2 showed a retention time difference of 36.4 min g^-1 between C₂H₂ and CO₂ in a packed column. Furthermore, because of its hydrophobic pore environment, NNM-2 maintained its separation performance even under near-saturated humidity (∼100% relative humidity).