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 [Mg₃(O₂CH)₆⊃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 α-[Mg₃(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 °C, with decomposition of the sample at approximately 417 °C. Gas sorption studies using both N₂ and H₂ 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 [Mg₃(O₂CH)₆⊃THF], 3; [Mg₃(O₂CH)₆⊃Et₂O], 4; [Mg₃(O₂CH)₆⊃Me₂CO], 5; [Mg₃(O₂CH)₆⊃C₆H₆], 6; [Mg₃(O₂CH)₆⊃EtOH], 7; and [Mg₃(O₂CH)₆⊃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.