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. Zn-3(L)(H2O)(2 center dot)2H(2)O (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 H-2 solid-state NMR spectroscopy and AC impedance spectroscopy. The proton conductivity in H-2 was measured as 3.5 x 10(-5) S cm(-1) at 25 degrees 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 degrees C.