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}n (1a) and {(NH4)0.6(H3O)0.4[Cu7(Hmesox)5(H2O)7]·11H2O}n (1b) 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 Tc 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.