Microporous metal-organic frameworks: Structures, in situ formation of ligand, and crystal-to-crystal transformations

Among these new microporous metal-organic frameworks (MOFs) {[Cu ^I₁₇(L1)₁₂X₂]X₃(H ₂O)_n}_∞ (1, X = Cl, 1a, n = 6; 1b, 1c, n = 5, 1d, n = 3; 2, X = Br, 2a, n = 5; 2b, n = 3; L1 = 3,5-bis(4-aminophenyl)-1,2, 4-triazolate), 1a and 2a were obtained through in situ copper-mediated ring conversion of 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole into L1 under hydrothermal conditions, while the others were generated through interesting single-crystal-to-single-crystal transformations with an accompanying color change from red to black. In the process of synthesis, when Cu^II salt was used as starting reactant, the Cu^I complex 1a was obtained; so the redox reaction clearly took place under this condition. The fascinating features of these structures are the double-stranded helical chains and the largest metallamacrocycle based on triazolate ever reported. These isomorphous complexes adopt the rare binodal six-connected networks. In the solid state at low temperature (∼273 K), the red crystal sample 1b displays an emission band at ∼585 nm upon photoexcitation at 467 nm.