Reversible Single-Crystal-to-Single-Crystal Transformation and Highly Selective Adsorption Property of Three-Dimensional Cobalt(II) Frameworks

A three-dimensional (3D) coordination polymer, [Co3(L)2(BTEC)(H2O)2]·2H2O [1, HL = 3,5-di(imidazol-1-yl)benzoic acid, H4BTEC = 1,2,4,5-benzenetetracarboxylic acid], with tfz-d topology has been hydrothermally synthesized. The framework of 1 has high thermal stability and exhibits single-crystal-to-single-crystal (SCSC) transformations upon removing and rebinding the noncoordinated and coordinated water molecules. X-ray crystallographic analyses revealed that the coordination geometry of Co(II) changes from octahedral to square pyramid upon dehydration, accompanying the appearance of one-dimensional (1D) open channels with dimensions of 2.0 × 2.8 Å. The dehydrated form [Co3(L)2(BTEC)] (2) exhibits highly selective adsorption of water molecules over N2, CH3OH, and CH3CH2OH, which could be used as sensors for water molecules. Furthermore, the magnetic properties of 1 and 2 were investigated, showing the existence of ferromagnetic interaction between the Co(II) atoms within the trinuclear subunit.