Guest‐Induced Reversible Phase Conversion via Spin Frustration Relief in Spin‐Intercalated Layered Antiferromagnets

A spin-intercalated layered metal-organic framework (MOF) magnetic system, [MCp*₂][{Ru₂(2,3,5-F₃ArCO₂)₄}₂(TCNQ)]·solv (M = Co, Fe; [MCp*₂]⁺ = decamethylmetallocenium; 2,3,5-F₃ArCO₂ ^- = 2,3,5-trifluorobenzoate; solv = crystallization solvent) is reported, which enables reversible magnetic phase switching by controlling spin frustration. In this system, paramagnetic spins ([FeCp*₂]⁺ with S = 1/2) are intercalated into a strongly correlated layered antiferromagnet, leading to competition between the interlayer antiferromagnetic coupling (J_LL < 0) and another coupling between the host and intercalated spins (J_LS). The balance between these interactions governs the emergence and nature of spin frustration. When |J_LS| ≤ |J_LL|, the spin frustration is "evident," resulting in a magnetic order accompanied by spin reorientation, whereas when |J_LS| >> |J_LL|, the frustration becomes "hidden," and the system exhibits apparent ferromagnetic or ferrimagnetic behavior despite underlying interlayer antiferromagnetic interactions. Importantly, for the first time, a reversible transition between these two magnetic regimes is demonstrated, by controlling the solvation/desolvation of materials, which modulates the spin frustration degree without altering the intrinsic spin states. This controllable switching highlights the unique potential of spin-intercalated molecular layered magnets as tunable platforms for studying correlated spin systems. These findings provide fundamental insights into frustration-driven magnetic phase transitions and open new avenues for developing switchable functional materials.