Cooperative Magnetic Phase Evolution via Oxygen Spin Coupling in a Layered Metal–Organic Framework

Metal–organic frameworks (MOFs) constructed from appropriate building units can exhibit both porosity and long-range magnetic order, enabling the modulation of magnetic states via mass transport. In most guest-responsive MOF magnets, the guest inclusions perturb the framework electronically or structurally and trigger abrupt phase transitions. In contrast, spin-active guests can directly mediate exchange pathways, enabling the continuous and controllable evolution of the magnetic order; however, the kinetic pathways of guest adsorption and spin mediation remain poorly understood. In this study, we demonstrated that the entrapment of O2 dimers in bottlenecked isolated pores between ferrimagnetic layers triggered a gradual evolution from ferromagnetism to antiferromagnetism in an isostructurally layered MOF. Systematic O2 sorption studies revealed a time-resolved shift in the Néel temperature (TN) from 17 to 28 K, correlating with the extent of O2 loading and indicating a cooperative growth of antiferromagnetic (AFM) domains. In contrast, the insertion of diamagnetic CO2 dimers preserved the original ferromagnetic (FM) ground state. Density functional theory calculations showed that the antiferromagnetically coupled O2 dimer acted as an efficient superexchange bridge between adjacent ferrimagnetic layers, stabilizing the AFM ground state. These findings provide direct evidence of the guest-induced, spin-mediated evolution of the magnetic phase in porous magnets and establish the entrapment of molecular oxygen as a versatile strategy for finely tuning the magnetic order in MOF magnets.