Facile Proton Conduction via Ordered Water Molecules in a Phosphonate Metal−Organic Framework

A new phosphonate metal−organic framework (MOF) with a layered motif but not that of the classical hybrid inorganic−organic solid is presented. Zn3(L)(H2O)2·2H2O (L = [1,3,5-benzenetriphosphonate]6−), henceforth denoted as PCMOF-3, contains a polar interlayer lined with Zn-ligated water molecules and phosphonate oxygen atoms. These groups serve to anchor free water molecules into ordered chains, as observed by X-ray crystallography. The potential for proton conduction via the well-defined interlayer was studied by 2H solid-state NMR spectroscopy and AC impedance spectroscopy. The proton conductivity in H2 was measured as 3.5 × 10−5 S cm−1 at 25 °C and 98% relative humidity. More interestingly, an Arrhenius plot gave a low activation energy of 0.17 eV for proton transfer, corroborating the solid-state NMR data that showed exchange between all deuterium sites in the D2O analogue of PCMOF-3, even at −20 °C.