Proton Conduction and Long-Range Ferrimagnetic Ordering in Two Isostructural Copper(II) Mesoxalate Metal–Organic Frameworks

Two compounds of formula {(H₃O)­[Cu₇(Hmesox)₅(H₂O)₇]·9H₂O}_<i>n</i> (<b>1a</b>) and {(NH₄)_0.6(H₃O)_0.4[Cu₇(Hmesox)₅(H₂O)₇]·11H₂O}_<i>n</i> (<b>1b</b>) were prepared and structurally characterized by single-crystal X-ray diffraction (H₄mesox = mesoxalic acid, 2-dihydroxymalonic acid). The compounds are crystalline functional metal–organic frameworks exhibiting proton conduction and magnetic ordering. Variable-temperature magnetic susceptibility measurements reveal that the copper­(II) ions are strongly ferro- and antiferromagnetically coupled by the alkoxide and carboxylate bridges of the mesoxalate linker to yield long-range magnetic ordering with a <i>T</i>_c of 17.6 K, which is reached by a rare mechanism known as topologic ferrimagnetism. Electric conductivity, measured by impedance methods, shows values as high as 6.5 × 10^–5 S cm^–1 and occurs by proton exchange among the hydronium/ammonium and water molecules of crystallization, which fill the voids left by the three-dimensional copper­(II) mesoxalate anionic network