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)] based molecular compound. Upon dehydration and rehydration, the trinuclear MnMo molecule [Mn(L)(HO)][Mo(CN)]·2HO (1) undergoes reversible crystal-to-crystal transformation to a hexanuclear MnMo compound [Mn(L)(HO)][Mn(L)][Mo(CN)] (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, whereas 2 behaves as a simple paramagnet despite its higher ground state spin value. The distortion of the pentagonal bipyramidal geometry of [Mo(CN)] in 2 disrupts the anisotropic exchange interactions that lead to SMM behavior in 1.

A molecule complex exhibiting reversible on–off switching between spin crossover and single-molecule magnet behaviours via a single-crystal-to-single-crystal transformation is reported.

We herein reported the synthetic, structural, computational, and magnetic studies of four air-stable heptacoordinated mononuclear cobalt(II) complexes, namely, [Co(tdmmb)(HO)][BF] (1), [Co(tdmmb)(CN)]·2HO (2), [Co(tdmmb)(NCS)] (3), and [Co(tdmmb)(SPh)] (4) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[[3](2,6)pyridino[3](2,9)-1,10-phenanthrolinophane-2,10-diene; SPh = thiophenol anion). Constrained by the rigid pentadentate macrocyclic ligand tdmmb, the Co centers in all of these complexes are in the heptacoordinated pentagonal-bipyramidal geometry. While the equatorial environments of these complexes remain very similar to each other, the axial ligands are systematically modified from C to N to O to S atoms. Analyses of the magnetic data and the ab initio calculations both reveal large easy-plane magnetic anisotropy (D > 0) for all four complexes. While the experimentally obtained D values do not show any clear tendency when the axial coordinated atoms change from C to N to O atoms (complexes 1-3), the largest value is for the heavier and softer S-atom-coordinated complex 4. Because of significant magnetic anisotropy, all four complexes are field-induced single-ion magnets. This work represents a delicate modification of the magnetic anisotropy by tuning the chemical environment of the metal centers.