Reversible Solvatomagnetic Switching in a Spongelike Manganese(II)-Copper(II) 3D Open Framework with a Pillared Square/Octagonal Layer Architecture

The concept of “molecular magnetic sponges” was introduced for the first time in 1999 by the creative imagination of the late Olivier Kahn. It refers to the exotic spongelike behavior of certain molecule‐based materials that undergo a dramatic change of their magnetic properties upon reversible dehydration/rehydration processes. Here we report a unique example of a manganese(II)–copper(II) mixed‐metal–organic framework of formula [Na(H2O)4]4[Mn4{Cu2(mpba)2(H2O)4}3]⋅ 56.5 H2O (1) (mpba=N,N′‐1,3‐phenylenebis(oxamate)). Compound 1 possesses a 3D MnII4CuII6 pillared layer structure with mixed square and octagonal pores of approximate dimensions 1.2×1.2 nm and 2.1×3.0 nm, respectively, hosting a large amount of crystallization H2O molecules and hydrated NaI countercations as guests. It reversibly switches from a crystalline hydrated phase with long‐range ferromagnetic ordering at a rather high critical temperature (Tc) of 22.5 K to an amorphous dehydrated phase with Tc as low as 2.3 K, which is accompanied by a breathing‐type dynamic effect involving a large crystal volume (ca. 45 %) and color changes after water desorption/adsorption. The combination of both the open‐framework structure and the spongelike optical, mechanical, and magnetic switching behavior in this new class of oxamato‐based porous magnets offers fascinating possibilities in designing multifunctional materials for host–guest molecular sensing. Molecular magnetic sponges: A new oxamato‐based mixed‐metal–organic framework (shown here) reversibly switches from a crystalline hydrated phase with long‐range ferromagnetic ordering at a critical temperature (Tc) of 22.5 K to an amorphous dehydrated phase at Tc=2.3 K. The dehydration/rehydration process is accompanied by large changes in crystal volume (ca. 45 %) and color.