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 (T_N) 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.