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

Abstract

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 ]·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 semi-amorphous 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 2D planar sheet network.

Sequestration of orange G and methylene blue from aqueous solutions using a Co(ii) coordination polymer

“…21 The nal step at 450 C corresponds to the decomposition of the SCN À ligands. 19 Both i-CoSP and a-CoSP showed mass loss as the samples were heated to 150 C; however, the inactivated sample underwent a larger percent change, consistent with the loss of a greater amount of solvent molecules relative to the activated cage, in which many solvents had already been removed. As the temperature exceeded $160 C, a steep drop in % weight was observed attributed to a loss of o-C 6 H 4 Cl 2 .…”

“…Murugesu and co-workers recently reported microcrystals of CoSP belonging to the trigonal space group R3m with the same overall structure as the framework reported by Fujita and co-workers, with different solvents of crystallization. 19 It was observed that the orange crystals (i-CoSP; inactive form) transformed to a green semi-amorphous material (a-CoSP; activated form) upon removal from the mother liquor, attributed to the removal of solvent and loss of crystallinity of the framework. In this activated form, guest molecules can occupy the void spaces that once housed solvents of crystallization.…”

“…Synthesis of inactive Co sponge powder (i-CoSP). {[(Co(NCS) 2) 3 (k 3 -TPT) 4 ]$a(H 2 O)$b(MeOH)} n (i-CoSP),19 was synthesized following a published procedure 17. A solution of TPT (315 mg, 1.0 mmol) in 250 mL of 1 : 4 (v/v) MeOH : o-C 6 H 4 Cl 2 was placed in a 500 mL round bottom ask and stirred at 600 rpm.…”

Crystallography of encapsulated molecules

“…Very recently a publication about the hidden transformations of the [(ZnI2)3(tpt)2x(solvent)] n crystalline sponge system has been published. 69 The initial difficulty to reproduce the soaking experiments led both Fujita and others to improve the protocols how the sponge crystals should be grown and selected, optimizing the soaking conditions and solvents used, and giving Figure 6. The Hirshfeld 24 surface plot (top) of the C70 guest in the cavity of the dimeric capsule.…”

“…Solvent molecules are excluded for clarity. Please do not adjust margins Please do not adjust margins guidelines about the actual data collection strategies with an in-house instrument 67,[69][70][71][72][73] or synchrotron radiation.…”