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(H2biim)3][M(btc)(Hbiim)]·2H2O (M = Co, (1); M = Ni, (2)) (btc = 1,3,5-benzenetricarboxylate; H2biim = 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(H2biim)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(H2biim)3]2+ cations act as pillars and link the anionic layers via robust heteromeric
hydrogen-bonded synthons

(9) and

(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 Å × 11.85 Å for 1 and 11.43 Å × 11.43 Å 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 gcat = 2.296, gNi = 2.564, J = −13.30 cm-1, and zJ‘ = −0.017 cm-1.
The microporous frameworks are stable at ca. 350 °C. They do not collapse after removal of the guest water
molecules in the channels, and they adsorb methanol molecules selectively.