Metal-Organic Frameworks Derived from Calcium and Strontium Complexes of a Redox-Active Ligand

The reactions of monomeric [(dpp-Bian)M(thf)₄] (M = Ca (1a), Sr (1b); dpp-Bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with 4,4'-bipyridyl (4,4'-bipy) proceed with electron transfer from dpp-Bian^2- to 4,4'-bipy⁰ to afford calcium and strontium complexes containing simultaneously radical-anionic dpp-Bian^- and 4,4'-bipy^- ligands. In tetrahydrofuran (thf) the reactions result in 1D coordination polymers [{(dpp-Bian)M(4,4'-bipy)(thf)₂}·4thf] (M = Ca (2a), Sr (2b)), while in a thf/benzene mixture the reaction between 1a and 4,4'-bipy affords the 2D metal-organic framework [{(dpp-Bian)Ca(4,4'-bipy)₂}·2thf·2C₆H₆] (3). The structures of compounds 2a,b and 3 have been determined by single-crystal X-ray analyses. The presence of the ligand-localized unpaired electrons allows the use of ESR spectroscopy for characterization of the compounds 2a,b and 3. DFT calculations of model calcium complexes with the dpp-Bian, 4,4'-bipy, and thf ligands confirm the energetically favorable open-shell configurations of the molecules bearing the 4,4'-bipy fragments. The magnetic susceptibility measurements confirm the presence of two unpaired electrons per monomeric unit in 2a,b and 3. The thermal stability of compounds 2a,b and 3 was studied by thermogravimetric analysis (TGA). To the best of our knowledge, 3 is the first MOF simultaneously containing two different paramagnetic bridging ligands inside the framework.