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.

Reversible controlling and switching of magnetic bistability remains relatively difficult. Here, reversible on−off switching of a hysteretic spin transition in a Co II complex via a single-crystal to single-crystal (SC-SC) transformation during dehydration and rehydration was reported. Upon dehydration, a switching from a basically low spin state to an abrupt and hysteretic spin crossover (SCO) with broad hysteresis loops was achieved. Hysteretic and anisotropic crystal lattice expansion or contraction in the spin transition temperature range was also observed in the dehydrated complex. The magneto−structural relationship in this system was established on the basis of detailed structure analyses on both the hydrated and dehydrated examples over a wide range of temperatures. The elimination of guest internal pressure, the tuning of the supramolecular interactions, and the strong electron−lattice coupling should be responsible for the hysteretic SCO in the dehydrated complex.

We herein report the syntheses, structures, and magnetic properties of two isostructural two-dimensional (2D) coordination polymers based on a pentagonal bipyramidal Co unit [Co(TODA)] and two hexacyanometallates, namely [M(CN)][Co(TODA)]·9HO (M = Cr (1), Co (2), TODA = 1,4,10-trioxa-7,13-diazacyclopentadecane). Structure analyses show that both complexes have 2D honeycomb structures where the [Co(TODA)] units are bridged by the [M(CN)] groups through three cyano groups in the facial positions. Magnetic investigation reveals ferromagnetic coupling between the Cr and Co centres through cyanides in 1. Due to the antiferromagnetic interaction between the layers, compound 1 exhibits an antiferromagnetic ordering below 11.4 K, and shows a metamagnetic phase transition under an external dc field. Due to the disorder of the TODA ligands, compound 1 shows a spin glass behavior, which leads to slow magnetic relaxation in 1. A butterfly-shaped hysteresis loop at 1.8 K can be observed with a coercive field of 720 Oe, which is quite large for cyano-bridged Cr-Co molecular magnets. For compound 2 containing the diamagnetic [Co(CN)] unit, field-induced slow magnetic relaxation was also verified, which makes compound 2 a rare example of an SIM assembled in a 2D network. An easy-plane magnetic anisotropy with a positive D value (29.9 cm by PHI and 26.5 cm by Anisofit2.0) was deduced for hepta-coordinated Co centers. These results show the efficiency of the strategy of combining cyanometallates and pentagonal bipyramidal precursors for novel molecular magnetic materials.