Reversible On–Off Switching of a Single-Molecule Magnet via a Crystal-to-Crystal Chemical Transformation

Dynamic molecular crystals are of high interest due to their potential applications. Herein we report the reversible on-off switching of single-molecule magnet (SMM) behavior in a [Mo(CN)₇]^4- based molecular compound. Upon dehydration and rehydration, the trinuclear Mn₂Mo molecule [Mn(L)(H₂O)]₂[Mo(CN)₇]·2H₂O (1) undergoes reversible crystal-to-crystal transformation to a hexanuclear Mn₄Mo₂ compound [Mn(L)(H₂O)]₂[Mn(L)]₂[Mo(CN)₇]₂ (2). This structural transformation involves the breaking and reforming of coordination bonds which leads to significant changes in the color and magnetic properties. Compound 1 is an SMM with an energy barrier of 44.9 cm^-1, whereas 2 behaves as a simple paramagnet despite its higher ground state spin value. The distortion of the pentagonal bipyramidal geometry of [Mo(CN)₇]^4- in 2 disrupts the anisotropic exchange interactions that lead to SMM behavior in 1.