Solvent-Controlled Phase Transition of a Co^II -Organic Framework: From Achiral to Chiral and Two to Three Dimensions

An unprecedented reversible dynamic transformation is reported in a metal-organic framework involving bond formation, which is accompanied by two important structural changes; achiral to chiral and two- to three-dimensions. Using two bent organic ligands (diimpym=4,6-di(1H-imidazol-1-yl)pyrimidine; H₂ npta=5-nitroisophthalic acid) and Co^II (NO₃ )₂ ⋅6 H₂ O the coordination polymer Co(diimpym)(npta)⋅CH₃ OH, (1⋅CH₃ OH), was obtained solvothermally. Its structure consists of knitted pairs of square layers (4⁴ -sql net) of five-coordinated Co and disordered methanol, and it crystallized in the achiral Pbca space group at room temperature. It undergoes a single crystal to single crystal (SC-SC) transformation to a 3D interpenetrated framework (α-polonium-type net, pcu) of six-coordinated Co and ordered methanol in the chiral P2₁ 2₁ 2₁ space group below 220 K. Most unusual is the dynamic temperature-dependent shortening of a Co⋅⋅⋅O connection from a non-bonded 2.640 Å (298 K) to a bonded 2.347 Å distance (100 K) transforming the square pyramidal cobalt polyhedron to a distorted octahedron. The desolvated crystals (1) obtained at 480 K retain the full crystallinity and crystallize in the achiral Pbca space group between 100 and 298 K but the dynamic shortening of the Co⋅⋅⋅O distance connecting the layers into the 3D pcu framework structure is observed. Following post-synthetic insertion of ethanol (1⋅CH₃ CH₂ OH) it does not exhibit the transformation and retains the knitted 2D achiral Pbca structure for all temperatures (100-298 K) and the ethanol is always disordered. The structural analyses thus conclude that the ordering of the methanol induces the chirality while the available space controls the dynamic motion of the knitted 2D networks into the 3D interpenetrated framework. Consequently, 1 selectively adsorbs CO₂ to N₂ and exhibits Type-III isotherms indicating dynamic motion of the 2D networks to accommodate the CO₂ at 273 and 298 K in contrast to the rigidity of the 3D framework at 77 K preventing N₂ from penetrating the solid. The magnetic properties are also reported.