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 (Oh), 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).