Hidden Transformations of a Crystalline Sponge: Elucidating the Stability of a Highly Porous Three-Dimensional Metal-Organic Framework

We present our investigations on the stability of a cobalt thiocyanate-based metal organic framework (MOF), coordinated by the 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) ligand, which is commonly employed for single-crystal-to single-crystal guest inclusion. The selected crystalline solid, {[(Co(NCS)(2))(3)(TPT)(4)]center dot x(solvent)}(n), is one emerging MOF currently being utilized as a host for the crystalline sponge method. Our experimental results shed light on two critical structural transformations that should be considered when attempting the revolutionary crystalline sponge method. We demonstrate that when the crystalline sponge is removed from the solution, a remarkable color change from orange to green is observed, yielding a semiamorphous material. Diffuse reflectance and X-ray absorption spectroscopy studies were carried out on this resulting green compound in order to examine the nature of this transformation. Notably, our analyses indicate a change in the coordination environment of the Co-II metal centers from octahedral to tetrahedral at the surface of the material. We further report a second structural transformation of the crystalline sponge, which arises from an increase in the local concentration of Co(NCS)(2) and leads to a significant change in the composition and structure of the material, giving a two-dimensional planar sheet network.