Metal–organic frameworks (MOFs) are a class of crystalline molecular materials constructed from metal ions and organic ligands, and they produce two types of functional component groups, the “framework” and the “space,” which is surrounded by frameworks. The framework mediates superexchange coupling between electron spins for magnetic ordering; achieving control of the framework is a longstanding subject in molecular magnetism. The space can house interstitial guest molecules, that can potentially act as porous magnets or MOF magnets¹. These functions, magnetic ordering, and guest accommodation can be coupled MOFs that are structurally, electronically, and/or magnetically flexible, which will allow guest-induced switchable magnets^2,3. Common gases such as carbon dioxide (CO₂), nitrogen (N₂), and oxygen (O₂) are intriguing targets for adsorption because of, for example, CO₂ is a current global environmental issues, and contrast between paramagnetic O₂ and diamagnetic N₂ despite their similar diatomic molecules. Nevertheless, large variations in magnetism responding to these gas adsorptions have rarely been reported, mainly because these gases only provide weak interactions with the host frameworks. This can be achieved in two ways, a chemical event of 1) generation/annihilation of spin at a part of the framework, or 2) change of orbital overlap mode (1↔︎0) in MOFs (Fig. 1). To realize this big challenge of chemical events, one of the most efficient ways is to control in-host or host–guest electron transfer in MOFs as well as the construction of new magnetic pathways via guests⁴.

In our recent work, published in Nature Chemistry⁵, we succeeded in controlling in-host electron transfer using CO₂ adsorption/desorption treatment in a magnetic layered MOF. Consequently, the MOF material that is originally a ferrimagnet, converts to a paramagnet on CO₂ adsorption, and returns to the pristine ferrimagnetic state on CO₂ desorption. The MOF material has a charge-variable framework composed of electron-donor (D) and -acceptor (A) subunits. This type of MOF is called D/A-MOF⁶, in which the charge distribution between the D and A subunits can be varied by external perturbations, such as temperature, pressure, light, and electric field. One of the chemical perturbations, that is, a trigger for the electron transfer transition that is focused on in this study, is a type of structural modulation sterically induced by CO₂ adsorption as well as host–guest electronic interactions that stabilize the material after CO₂ accommodation. This simple strategy is useful for gaining guest-induced electronic/magnetic modifications in MOFs, is not it?

Fig. 1. How we can realize “magnet change” in porous magnets.

Read more about our work at Nature Chemistry: https://www.nature.com/articles/s41557-020-00577-y

References

1. Thorarinsdottir, A. E. & Harris, T. D. Metal−organic framework magnets. Chem. Rev. 120, 8716–8789 (2020).
2. Zhang, J. et al. Magnetic sponge behavior via electronic state modulations. J. Am. Chem. Soc. 140, 5644–5652 (2018).
3. Zhang, J. et al. Host-guest hydrogen bonding varies the charge-state behavior of magnetic sponges. Angew. Chem. Int. Ed. 58, 7351–7356 (2019).
4. Kosaka, W. et al. Gas-responsive porous magnet distinguishes the electron spin of molecular oxygen. Nature Commun. 9, 5420 (2018).
5. Zhang, J. et al. A metal–organic framework that exhibits CO₂-induced transitions between paramagnetism and ferrimagnetism. Nature Chem. DOI: 10.1038/s41557-020-00577-y
6. Miyasaka, H. Control of Charge Transfer in Donor/Acceptor Metal–Organic Frameworks. Acc. Chem. Res. 45, 248–257 (2013).