New Heterometallic Carboxylate Frameworks: Synthesis, Structure, Robustness, Flexibility, and Porosity

Solvothermal reactions of cobalt acetate, alkali metal hydroxide, and isophthalic acid (H2ipa) in H2O/alcohol produce five new three-dimensional (3D) heterometallic carboxylate frameworks, namely, [Na2Co(ipa)2(MeOH)2]n (1), [Na2Co(ipa)2(EtOH)2]n (2), [Na2Co(ipa)2(H2O)(EtOH)]n (3), [Li2Co(ipa)2(EtOH)]n (4), and {[Li2Co(ipa)2(H2O)]·H2O}n (5). Compounds 1−5 are all based on anionic [Co(ipa)2]n square grids, which are linked by alkali metal ions in various manners to generate closely related but quite different 3D architectures. In addition to the carboxylate oxygen donors of the anionic [Co(ipa)2]n square grids, the alkali metal ions are also coordinated by the MeOH, EtOH, or H2O ligands. The prototypical compound 1 shows interesting solid-state structural transformations and sorption properties. The coordinated MeOH molecules in 1 can be easily substituted by moist H2O, accompanying a structural transformation from the chiral phase 1 to centrosymmetric phase 3. Further, thermal liberation of all MeOH ligands causes significant structural change and gives rise to a non-porous framework [CoNa2(ipa)2]n (1′), which adsorbs MeOH, EtOH, and MeCN but not acetone based on the sizes of solvent molecules. Moreover, 1′ can also selectively absorb CO2 over N2, H2, and C2H2, demonstrating the importance of oxophilic Na(I) ion in controlling the gate-opening behavior.