Pillared-layer microporous metal-organic frameworks constructed by robust hydrogen bonds.: Synthesis, characterization, and magnetic and adsorption properties of 2,2′-biimidazole and carboxylate complexes

Two new isostructural complexes [M(H(2)biim)(3)][M(btc)(Hbiim)]center dot 2H(2)O (M = Co, (1); M = Ni, (2)) (btc = 1,3,5-benzenetricarboxylate; H(2)biim = 2,2'-biimidazole) have been synthesized and characterized by single-crystal X-ray diffraction. They present a unique structure consisting of two distinct units: the monomeric cations [M(H(2)biim)(3)](2+) and the two-dimensional (2D) anionic polymer [M(Hbiim)(btc)](2-). In the anionic moiety, the Hbiim(-) monoanion is simultaneously coordinated to one metal atom in a bidentate mode and further to another metal atom in a monodentate mode. The imidazolate groups bridge the two adjacent metal ions into a helical chain which is further arranged in left- and right-handed manners. These chains are bridged by btc ligands into a 2D brick wall structure. The most interesting aspect is that the [M(H(2)biiM)(3)](2+) cations act as pillars and link the anionic layers via robust heteromeric hydrogen-bonded synthons R-2(2)(9) and R-2(1)(7) formed by the uncoordinated oxygen atoms of carboxylate groups and the H2biim ligands, resulting in a microporous metal-organic framework with one-dimensional (1D) channels (ca. 11.85 angstrom x 11.85 angstrom for 1 and 11.43 angstrom x 11.43 angstrom for 2). Magnetic properties of these two complexes have also been studied in the temperature range of 2-300 K, and their magnetic susceptibilities obey the Curie-Weiss law in the temperature range of 20-300 K (for 1) and 2-300 K (for 2), respectively, showing anti-ferromagnetic coupling through imidazolate bridging, Taking into consideration the Heisenberg infinite chain model as well as the possibility of chain-to-chain and chain-to-cation interactions, the anti-ferromagnetic exchange of 2 is analyzed via a correction for the molecular field, giving the values of g(cat) = 2.296, g(Ni) = 2.564, J = -13.30 cm(-1), and zJ' = -0.017 cm(-1). The microporous frameworks are stable at ca. 350 degrees C. They do not collapse after removal of the guest water molecules in the channels, and they adsorb methanol molecules selectively.