Understanding the Pseudoflexibility of a Co-btc Coordination Polymer

A new functionalized coordination polymer (CP), [Co₂(DMA)₃(btc)Cl]n (CP 1) was developed starting from 1,3,5-trimesic acid (H₃btc), cobalt chloride, and N,N-dimethylacetamide (DMA). In the presence of moisture, the highly active blue-colored CP 1 was transformed into a solid pink-colored CP, [Co₃(btc)₂·12H₂O]_n (CP 2). The structures of CP 1 and CP 2 were confirmed by single-crystal X-ray diffraction (SCXRD) studies. Remarkably, CP 2 can be transformed back into CP 1 in the presence of DMA. The drastic reversible structural transformation between CP 1 and CP 2 involves the breakage and reformation of metal-carboxylate bonds, amendments in the coordination geometry around Co(II) centers, as well as changes in the coordination mode of the btc^3- ligand. A plausible mechanistic study reveals that, instead of a solvent-induced flexible structural transformation between these CPs, CP 1 actually degrades to CP 2 along with hydrated cobalt chloride. Here, we have shown that even after the degradation of a CP, the CP can be recovered by knowing the proper mechanistic pathway of the reaction. Besides that, a new strategy was developed to make the active and functionalized CP (CP 1) from the stable, well-ordered CP (CP 2). This unique phenomenon of CP transformation can be termed as "pseudoflexibility" of CPs.