Novel Metal−Organic Frameworks Derived from Group II Metal Cations and Aryldicarboxylate Anionic Ligands

Reaction of magnesium, calcium, strontium, and barium salts with a range of dicarboxylic acids [benzene-1,4-dicarboxylic acid (H2BDC), naphthalene-2,6-dicarboxylic acid (H2NDC), 4,5,9,10-tetrahydropyrene-2,7-dicarboxylic acid (H2TPDC), pyrene-2,7-dicarboxylic acid (H2PDC), 5,10-dihydroanthracene-2,7-dicarboxylic acid (H2DADC)] in N,N′-dimethylformamide (DMF) or N,N′-diethylformamide (DEF) in Teflon-lined stainless steel autoclaves produces a range of metal−organic framework materials. Single crystal X-ray analysis has confirmed that the predominant building block in these materials is a chain of metal centers bridged either by carboxylate moieties alone as in [M(DMF)(μ-BDC)]∞ (M = Mg or Sr), [Ca1.5(DEF)(μ-BDC)1.5]∞, and [Sr(DEF)(OH2)(μ-BDC)]∞ or bridged by both carboxylate ligands and DMF/DEF molecules as in [M(μ-DMF)(μ-NDC)]∞ (M = Ca, Sr, or Ba), [M(μ-DEF)(μ-TPDC)]∞ (M = Ca or Sr), [M(μ-DMF)(μ-DADC)]∞ (M = Ca or Sr), and [Sr(μ-DEF)(μ-PDC)]∞. In contrast, the isomorphous complexes [Mg3(DMF)4(μ-NDC)3]∞ and [Mg3(DEF)4(μ-NDC)3]∞ contain centrosymmetric trinuclear moieties in which each pair of cations is bridged by three carboxylate anions with two pendant solvent molecules coordinated to each of the terminal Mg2+ cations. These trinuclear building blocks act as six-connected nodes and generate a tilted α-Po type structure. Eleven of the 12 structures based upon cationic chains adopt a common extended architecture in which the aryldicarboxylate anions link the chains to generate diamond-shaped channels. However, in the material [Sr(DMF)(μ-BDC)]∞, the chains are linked to generate a hexagonal motif of triangular channels. In all 12 compounds based on cationic chains, the space within the channels is occupied by coordinated solvent molecules, leading to nonporous materials.