One of the goals of materials chemistry is the synthesis of crystalline compounds known as porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) that are built from organic ligands and metal ions.[1] Their frameworks are composed of a variety of structures ranging from onedimensional (1D), 2D, to 3D motifs depending on how the coordination bonds form. These compounds, which are lowdensity solids, can be tailored, and have potential use in gas storage, molecular sensing, and other materials applications. One of the most important discoveries in the area of PCPs is their flexible and dynamic properties, which are characteristic of the cooperative action of organic and inorganic moieties. [2, 3] Guest-responsive changes between the solid phases are particularly intriguing. Although various cases of structural transformation (single crystal to amorphous phase) are known, examples of single-crystal to single-crystal phase (SCSC) transformation [4][5][6] are still lacking. A low-dimensional (1D or 2D) framework has a structural freedom in regards to the metal ions, while a 3D framework is sufficiently rigid to prevent recombination of the coordination bonds around the metal ions. As the structures and properties of SCSC structural transformations are well characterized, they are highly suitable for study to gain a further understanding of their dynamic behavior. In particular, structural transformations involving 2D and 3D frameworks are most important because most of the coordination compounds with important properties-such as gas storage, separation, sensor, magnetism, and catalysis-are generally 3D frameworks, whereas 2D structures containing spaces between the layers have more dynamic properties. The exploration of such a transformation is significant because it can provide not only unique properties characteristic of 3D frameworks, but also external stimulus-responsive properties based on the 2D-3D transformation (Figure 1).Herein, we describe the synthesis of a 2D compound using tetramethylammonium chloride as a blocking agent [7] (to stop the formation of the 3D structure), a tridentate ligand having highly flexible arms, and lanthanide metal ions, which have variable and high coordination numbers, which can provide more structural freedom to facilitate the transformation to a 3D structure.Two isomorphous compounds, {[Ce(tci)(H 2 O) 2 ]·2 H 2 O} n (1) and {[Pr(tci)(H 2 O) 2 ]·2 H 2 O} n (2), were prepared by the hydrothermal technique using tris(2-carboxyethyl) isocyanurate (tciH 3 ) with Ln(NO 3 ) 3 ·x H 2 O in the presence of tetramethylammonium chloride. The compounds once formed are insoluble in most solvents, including H 2 O. The structure of 1 is shown in Figure 2. The asymmetric unit consists of one Ce III center, one molecule of tci, and two types of H 2 O moleculestwo bound to Ce ions and two free in the channel. Each Ce III ion is surrounded by eight oxygen atoms, two of the oxygen atoms being from the water molecules and the rest come from the tci unit to give an overall distorted sq...
