Structural flexibility is a remarkable characteristic of coordination polymers and significant for the attainment of environmental responsivity. We have prepared a 2D cyanide-bridged MnIICrIII coordination polymer, [Mn(NNdmenH)(H2O)][Cr(CN)6].H2O (1; NNdmen = N,N-dimethylethylenediamine), with sophisticatedly arranged removable water coligands. The compound clearly showed a reversible single-crystal-to-single-crystal transformation between the 2D sheet and a 3D pillared-sheet framework of dehydrated [Mn(NNdmenH)][Cr(CN)6] (1a). The structural change was reversible and accompanied with generation/cleavage of CN-Mn bonds between 2D sheets by dehydration/hydration. Compounds 1 and 1a also exhibited a ferrimagnetic ordering at 35.2 and 60.4 K, respectively, and the magnetic characteristics were reversibly converted by guest adsorption/desorption. In addition, the dehydrated 1a demonstrated a size-selective solvent adsorption linking chemi- and physisorption processes and shrinkage/expansion of its framework. The flexible magnetic framework incorporating removable coligands delivered multifunctions with chemical response.

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In recent years, porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) with flexible structures have attracted considerable attention, owing to their potential as functional materials.[1] Unlike rigid frameworks, where, in most cases, structure and properties remain unchanged after removal of guest molecules, flexible frameworks are very sensitive to the presence of guests and undergo structural variations depending upon the amount and nature of the guest molecules inside the framework.[2] Such compounds are capable of forming bistable phases and are expected to exhibit not only structural variations but also modulation of their physical properties, such as chirality as well as optical and magnetic behavior, to achieve multiple functions that are not observed in robust frameworks or other conventional solids. Structural flexibility has also been observed in inorganic frameworks, although the detected changes are not as drastic as those of PCPs. To create such systems, we have focused on modulation of the crystal structure by reversible removal of guest molecules, to produce multifunctional bistable coordination polymers. If the host framework undergoes singlecrystal-to-single-crystal (SCSC) transformations, [3] crystallographic analysis is a very useful tool for deepening our understanding of the dynamic behavior, and correlating the bistable structures with physical properties, such as magnetism.[1d, 4] PCPs containing flexible ligands and metal ions with variable coordination numbers have great potential for this purpose, as such flexibility allows for stability in various structural forms.Porous crystals based on 2D frameworks have several useful characteristics with regard to dynamic guest-responsive phenomena: 1) interlayer separations, which play a major role in guest-inclusion; 2) framework flexibility, as a result of sliding of the 2D layers and closing/opening of the channel spaces by the guest molecules; 3) rearrangement of the frameworks by cleavage and formation of coordination bonds between the layers, leading to expansion/shrinkage of the 2D layered structures by the guest molecules. Most importantly, the transformation between the 2D and 3D frameworks, with cleavage and generation of new bonds, is expected to lead to large differences in their structural and functional behaviors. These unique materials have a high potential for realizing new functions, such as switching and sensing. To make bistable compounds with drastic structural differences, 2D frameworks containing a flexible ligand and a metal ion with variable coordination numbers may easily undergo transformation to a 3D structure upon guest removal by contracting/expanding and concurrent sliding of the 2D layers, using the bond-switching mechanism of the coordination bonds around the metal centers (Figure 1). [5][6][7][8] In this regard, the Cu II metal ion is very promising, as it has versatile coordination chemistry and can readily adopt octahedral, square-pyramidal, trigonal bipyramidal, and square-planar geometries. H...

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Effects of pressure on the structures and magnetic properties of three types of 3-D cyanide-bridged bimetallic coordination polymer magnets, MnIICrIII ferrimagnet [Mn(en)]3[Cr(CN)6]2.4H2O (1; en = ethylenediamine), NiIICrIII ferromagnet [Ni(dipn)]3[Cr(CN)6]2.3H2O (2; dipn = N,N-di(3-aminopropyl)amine), and NiIIFeIII ferromagnet [Ni(dipn)]2[Ni(dipn)(H2O)][Fe(CN)6]2.11H2O (3), were systematically examined under hydrostatic pressure up to 19.8 GPa using a piston-cylinder-type pressure cell and a diamond anvil cell. The ferrimagnet 1 showed the reversible crystalline-to-amorphous-like phase change, and the magnetic phase transition temperature (TC) was reversibly changed from 69 K at 0 GPa to 126 K at 4.7 GPa. At higher pressure, the net magnetization was suppressed with increasing pressure, and the magnetic state at 19.8 GPa was assumed to be paramagnetic. The initial ferrimagnetic phase of 1 was not recovered after releasing the pressure from 19.8 GPa. The magnetic phase of 2 was reversibly converted between ferromagnetic and paramagnetic-like phase in the range 0