Synthesis, structural characterization, gas sorption and guest-exchange studies of the lightweight, porous metal-organic framework α-[Mg3(O2CH)6]

Unsolvated magnesium formate crystallizes upon reaction of the metal nitrate with formic acid in DMF at elevated temperatures. Single-crystal XRD studies reveal the formation of [Mg-3(O2CH)(6) superset of DMF], 1, a metal-organic framework with DMF molecules filling the channels of an extended diamondoid lattice. The DMF molecules in 1 can be entirely removed without disruption to the framework, giving the guest-free material alpha-[Mg-3(O2CH)(6)], 2. Compound 2 has been characterized by both powder and single-crystal XRD studies. Thermogravimetric analyses of 1 show guest loss from 120 to 190 degrees C, with decomposition of the sample at approximately 417 degrees C. Gas sorption studies using both N-2 and H-2 indicate that the framework displays permanent porosity. The porosity of the framework is further demonstrated by the ability of 2 to uptake a variety of small molecules upon soaking. Single-crystal XRD studies have been completed on the six inclusion compounds [Mg-3(O2CH)(6) superset of THF], 3; [Mg-3(O2CH)(6) superset of Et2O], 4; [Mg-3(O2CH) (6) superset of Me2CO], 5; [Mg-3(O2CH)(6) superset of C6H6], 6; [Mg-3(O2CH)(6) superset of EtOH], 7; and [Mg-3(O2CH)(6) superset of MeOH], 8. Analyses of the metrical parameters of 1-8 indicate that the framework has the ability to contract or expand depending on the nature of the guest present.