Spin Crossover as an Efficient Strategy for Controllable Gas Molecule Capturing on Open Metal Sites in Ni-BTC and Cu-BTC

The development of efficient sorbent materials for capturing harmful gases, in particular CO2, is one of the most important research topics due to climate change concerns. In this regard, metal–organic frameworks (MOFs) incorporating open metal sites (OMSs) have emerged as promising materials. In this article, we propose the spin crossover as an experimentally feasible strategy for controllable gas molecule capturing on the OMS-containing MOFs. To this end, we have selected the neutral and cationic forms of M2(BTC)4 with M = Ni and Cu as prototypes of the paddlewheel secondary building unit of HKUST-1 and DUT-8, respectively. The interplay between magnetic properties and the adsorption behaviors of M2(BTC)4 toward six small gas molecules (CH4, H2, N2, CO2, CO, and H2S) is investigated using first-principle calculations. It is found that the adsorption of gas molecules on the low spin (LS) state of copper and high spin (HS) state of nickel is thermodynamically more favored. Whereas the differences between the adsorption enthalpies on the LS and HS states for cupric systems are not significant, in the case of nickel centers, the differences can reach more than 150 kJ mol–1. Our results indicate that all gas molecules are strongly adsorbed at the HS state of nickel centers and change from being chemisorbed to being physisorbed at the LS state. Taking inspiration from these results, we introduce Ni2(BTC)4 paddlewheels as controllable CO2 capture materials via spin crossover at the Ni center. To our knowledge, this is the first study that suggests the spin transition as a promising approach for CO2 capture/release technology.